The Negative-acknowledgment (NACK) Oriented Reliable Multicast (NORM)
protocol is designed to provide reliable transport of data from one
or more sender(s) to a group of receivers over an IP multicast
network. The primary design goals of NORM are to provide efficient,
scalable, and robust bulk data (e.g., computer files, transmission of
persistent data) transfer across possibly heterogeneous IP networks
and topologies. The NORM protocol design provides support for
distributed multicast session participation with minimal coordination
among senders and receivers. NORM allows senders and receivers to
dynamically join and leave multicast sessions at will with minimal
overhead for control information and timing synchronization among
participants. To accommodate this capability, NORM protocol message
headers contain some common information allowing receivers to easily
synchronize to senders throughout the lifetime of a reliable
multicast session. NORM is designed to be self-adapting to a wide
range of dynamic network conditions with little or no pre-
configuration. The protocol is purposely designed to be tolerant of
inaccurate timing estimations or lossy conditions that may occur in
many networks including mobile and wireless. The protocol is also
designed to exhibit convergence and efficient operation even in
situations of heavy packet loss and large queuing or transmission
delays.
This document is a product of the IETF RMT WG and follows the
guidelines provided in RFC 3269 [1]. The key words "MUST", "MUST
NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in BCP 14, RFC 2119 [2].
Statement of Intent
This memo contains part of the definitions necessary to fully specify
a Reliable Multicast Transport protocol in accordance with RFC 2357.
As per RFC 2357, the use of any reliable multicast protocol in the
Internet requires an adequate congestion control scheme.
While waiting for such a scheme to be available, or for an existing
scheme to be proven adequate, the Reliable Multicast Transport
working group (RMT) publishes this Request for Comments in the
"Experimental" category.
It is the intent of RMT to re-submit this specification as an IETF
Proposed Standard as soon as the above condition is met.
Adamson, et al. Experimental [Page 3]
RFC 3940 NORM Protocol November 2004
A NORM protocol instance (NormSession) is defined within the context
of participants communicating connectionless (e.g., Internet Protocol
(IP) or User Datagram Protocol (UDP)) packets over a network using
pre-determined addresses and host port numbers. Generally, the
participants exchange packets using an IP multicast group address,
but unicast transport may also be established or applied as an
adjunct to multicast delivery. In the case of multicast, the
participating NormNodes will communicate using a common IP multicast
group address and port number that has been chosen via means outside
the context of the given NormSession. Other IETF data format and
protocol standards exist that may be applied to describe and convey
the required "a priori" information for a specific NormSession (e.g.,
Session Description Protocol (SDP) [7], Session Announcement Protocol
(SAP) [8], etc.).
The NORM protocol design is principally driven by the assumption of a
single sender transmitting bulk data content to a group of receivers.
However, the protocol MAY operate with multiple senders within the
context of a single NormSession. In initial implementations of this
protocol, it is anticipated that multiple senders will transmit
independent of one another and receivers will maintain state as
necessary for each sender. However, in future versions of NORM, it
is possible that some aspects of protocol operation (e.g., round-trip
time collection) may provide for alternate modes allowing more
efficient performance for applications requiring multiple senders.
NORM provides for three types of bulk data content objects
(NormObjects) to be reliably transported. These types include:
1) static computer memory data content (NORM_OBJECT_DATA type),
2) computer storage files (NORM_OBJECT_FILE type), and
3) non-finite streams of continuous data content (NORM_OBJECT_STREAM
type).
The distinction between NORM_OBJECT_DATA and NORM_OBJECT_FILE is
simply to provide a "hint" to receivers in NormSessions serving
multiple types of content as to what type of storage should be
allocated for received content (i.e., memory or file storage). Other
than that distinction, the two are identical, providing for reliable
transport of finite (but potentially very large) units of content.
These static data and file services are anticipated to be useful for
multicast-based cache applications with the ability to reliably
provide transmission of large quantities of static data. Other types
of static data/file delivery services might make use of these
Adamson, et al. Experimental [Page 4]
RFC 3940 NORM Protocol November 2004
transport object types, too. The use of the NORM_OBJECT_STREAM type
is at the application's discretion and could be used to carry static
data or file content also. The NORM reliable stream service opens up
additional possibilities such as serialized reliable messaging or
other unbounded, perhaps dynamically produced content. The
NORM_OBJECT_STREAM provides for reliable transport analogous to that
of the Transmission Control Protocol (TCP), although NORM receivers
will be able to begin receiving stream content at any point in time.
The applicability of this feature will depend upon the application.
The NORM protocol also allows for a small amount of "out-of-band"
data (sent as NORM_INFO messages) to be attached to the data content
objects transmitted by the sender. This readily-available "out-of-
band" data allows multicast receivers to quickly and efficiently
determine the nature of the corresponding data, file, or stream bulk
content being transmitted. This allows application-level control of
the receiver node's participation in the current transport activity.
This also allows the protocol to be flexible with minimal pre-
coordination among senders and receivers. The NORM_INFO content is
designed to be atomic in that its size MUST fit into the payload
portion of a single NORM message.
NORM does _not_ provide for global or application-level
identification of data content within in its message headers. Note
the NORM_INFO out-of-band data mechanism could be leveraged by the
application for this purpose if desired, or identification could
alternatively be embedded within the data content. NORM does
identify transmitted content (NormObjects) with transport identifiers
that are applicable only while the sender is transmitting and/or
repairing the given object. These transport data content identifiers
(NormTransportIds) are assigned in a monotonically increasing fashion
by each NORM sender during the course of a NormSession. Each sender
maintains its NormTransportId assignments independently so that
individual NormObjects may be uniquely identified during transport
with the concatenation of the sender session-unique identifier
(NormNodeId) and the assigned NormTransportId. The NormTransportIds
are assigned from a large, but fixed, numeric space in increasing
order and may be reassigned during long-lived sessions. The NORM
protocol provides mechanisms so that the sender application may
terminate transmission of data content and inform the group of this
in an efficient manner. Other similar protocol control mechanisms
(e.g., session termination, receiver synchronization, etc.) are
specified so that reliable multicast application variants may
construct different, complete bulk transfer communication models to
meet their goals.
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RFC 3940 NORM Protocol November 2004
To summarize, the NORM protocol provides reliable transport of
different types of data content (including potentially mixed types).
The senders enqueue and transmit bulk content in the form of static
data or files and/or non-finite, ongoing stream types. NORM senders
provide for repair transmission of data and/or FEC content in
response to NACK messages received from the receiver group.
Mechanisms for "out-of-band" information and other transport control
mechanisms are specified for use by applications to form complete
reliable multicast solutions for different purposes.
Group communication scalability requirements lead to adaptation of
negative acknowledgment (NACK) based protocol schemes when feedback
for reliability is required [9]. NORM is a protocol centered around
the use of selective NACKs to request repairs of missing data. NORM
provides for the use of packet-level forward error correction (FEC)
techniques for efficient multicast repair and optional proactive
transmission robustness [10]. FEC-based repair can be used to
greatly reduce the quantity of reliable multicast repair requests and
repair transmissions [11] in a NACK-oriented protocol. The principal
factor in NORM scalability is the volume of feedback traffic
generated by the receiver set to facilitate reliability and
congestion control. NORM uses probabilistic suppression of redundant
feedback based on exponentially distributed random backoff timers.
The performance of this type of suppression relative to other
techniques is described in [12]. NORM dynamically measures the
group's roundtrip timing status to set its suppression and other
protocol timers. This allows NORM to scale well while maintaining
reliable data delivery transport with low latency relative to the
network topology over which it is operating.
Feedback messages can be either multicast to the group at large or
sent via unicast routing to the sender. In the case of unicast
feedback, the sender "advertises" the feedback state to the group to
facilitate feedback suppression. In typical Internet environments,
it is expected that the NORM protocol will readily scale to group
sizes on the order of tens of thousands of receivers. A study of the
quantity of feedback for this type of protocol is described in [13].
NORM is able to operate with a smaller amount of feedback than a
single TCP connection, even with relatively large numbers of
receivers. Thus, depending upon the network topology, it is possible
that NORM may scale to larger group sizes. With respect to computer
resource usage, the NORM protocol does _not_ require that state be
kept on all receivers in the group. NORM senders maintain state only
for receivers providing explicit congestion control feedback. NORM
receivers must maintain state for each active sender. This may
constrain the number of simultaneous senders in some uses of NORM.
Adamson, et al. Experimental [Page 6]
RFC 3940 NORM Protocol November 2004
All of the environmental requirements and considerations that apply
to the RMT NORM Building Block [4] and the RMT FEC Building Block [5]
also apply to the NORM protocol.
The NORM protocol SHALL be capable of operating in an end-to-end
fashion with no assistance from intermediate systems beyond basic IP
multicast group management, routing, and forwarding services. While
the techniques utilized in NORM are principally applicable to "flat"
end-to-end IP multicast topologies, they could also be applied in the
sub-levels of hierarchical (e.g., tree-based) multicast distribution
if so desired. NORM can make use of reciprocal (among senders and
receivers) multicast communication under the Any-Source Multicast
(ASM) model defined in RFC 1112 [3], but SHALL also be capable of
scalable operation in asymmetric topologies such as Source Specific
Multicast (SSM) [14] where there may only be unicast routing service
from the receivers to the sender(s).
NORM is compatible with IPv4 and IPv6. Additionally, NORM may be
used with networks employing Network Address Translation (NAT)
providing the NAT device supports IP multicast and/or can cache UDP
traffic source port numbers for remapping feedback traffic from
receivers to the sender(s).
A NormSession is comprised of participants (NormNodes) acting as
senders and/or receivers. NORM senders transmit data content in the
form of NormObjects to the session destination address and the NORM
receivers attempt to reliably receive the transmitted content using
negative acknowledgments to request repair. Each NormNode within a
NormSession is assumed to have a preselected unique 32-bit identifier
(NormNodeId). NormNodes MUST have uniquely assigned identifiers
within a single NormSession to distinguish between possible multiple
senders and to distinguish feedback information from different
receivers. There are two reserved NormNodeId values. A value of
0x00000000 is considered an invalid NormNodeId value and a value of
0xffffffff is a "wildcard" NormNodeId. While the protocol does not
preclude multiple sender nodes concurrently transmitting within the
context of a single NORM session (i.e., many-to-many operation), any
type of interactive coordination among NORM senders is assumed to be
controlled by the application or higher protocol layer. There are
some optional mechanisms specified in this document that can be
leveraged for such application layer coordination.
Adamson, et al. Experimental [Page 7]
RFC 3940 NORM Protocol November 2004
As previously noted, NORM allows for reliable transmission of three
different basic types of data content. The first type is
NORM_OBJECT_DATA, which is used for static, persistent blocks of data
content maintained in the sender's application memory storage. The
second type is NORM_OBJECT_FILE, which corresponds to data stored in
the sender's non-volatile file system. The NORM_OBJECT_DATA and
NORM_OBJECT_FILE types both represent "NormObjects" of finite but
potentially very large size. The third type of data content is
NORM_OBJECT_STREAM, which corresponds to an ongoing transmission of
undefined length. This is analogous to the reliable stream service
provide by TCP for unicast data transport. The format of the stream
content is application-defined and may be byte or message oriented.
The NORM protocol provides for "flushing" of the stream to expedite
delivery or possibly enforce application message boundaries. NORM
protocol implementations may offer either (or both) in-order delivery
of the stream data to the receive application or out-of-order (more
immediate) delivery of received segments of the stream to the
receiver application. In either case, NORM sender and receiver
implementations provide buffering to facilitate repair of the stream
as it is transported.
All NormObjects are logically segmented into FEC coding blocks and
symbols for transmission by the sender. In NORM, an FEC encoding
symbol directly corresponds to the payload of NORM_DATA messages or
"segment". Note that when systematic FEC codes are used, the payload
of NORM_DATA messages sent for the first portion of a FEC encoding
block are source symbols (actual segments of original user data),
while the remaining symbols for the block consist of parity symbols
generated by FEC encoding. These parity symbols are generally sent
in response to repair requests, but some number may be sent
proactively at the end each encoding block to increase the robustness
of transmission. When non-systematic FEC codes are used, all symbols
sent consist of FEC encoding parity content. In this case, the
receiver must receive a sufficient number of symbols to reconstruct
(via FEC decoding) the original user data for the given block. In
this document, the terms "symbol" and "segment" are used
interchangeably.
Transmitted NormObjects are temporarily yet uniquely identified
within the NormSession context using the given sender's NormNodeId,
NormInstanceId, and a temporary NormObjectTransportId. Depending
upon the implementation, individual NORM senders may manage their
NormInstanceIds independently, or a common NormInstanceId may be
agreed upon for all participating nodes within a session if needed as
a session identifier. NORM NormObjectTransportId data content
identifiers are sender-assigned and applicable and valid only during
a NormObject's actual _transport_ (i.e., for as long as the sender is
transmitting and providing repair of the indicated NormObject). For
Adamson, et al. Experimental [Page 8]
RFC 3940 NORM Protocol November 2004
a long-lived session, the NormObjectTransportId field can wrap and
previously-used identifiers may be re-used. Note that globally
unique identification of transported data content is not provided by
NORM and, if required, must be managed by the NORM application. The
individual segments or symbols of the NormObject are further
identified with FEC payload identifiers which include coding block
and symbol identifiers. These are discussed in detail later in this
document.
A NORM sender primarily generates messages of type NORM_DATA. These
messages carry original data segments or FEC symbols and repair
segments/symbols for the bulk data/file or stream NormObjects being
transferred. By default, redundant FEC symbols are sent only in
response to receiver repair requests (NACKs) and thus normally little
or no additional transmission overhead is imposed due to FEC
encoding. However, the NORM implementation MAY be optionally
configured to proactively transmit some amount of redundant FEC
symbols along with the original content to potentially enhance
performance (e.g., improved delay) at the cost of additional
transmission overhead. This option may be sensible for certain
network conditions and can allow for robust, asymmetric multicast
(e.g., unidirectional routing, satellite, cable) [15] with reduced
receiver feedback, or, in some cases, no feedback.
A sender message of type NORM_INFO is also defined and is used to
carry OPTIONAL "out-of-band" context information for a given
transport object. A single NORM_INFO message can be associated with
a NormObject. Because of its atomic nature, missing NORM_INFO
messages can be NACKed and repaired with a slightly lower delay
process than NORM's general FEC-encoded data content. NORM_INFO may
serve special purposes for some bulk transfer, reliable multicast
applications where receivers join the group mid-stream and need to
ascertain contextual information on the current content being
transmitted. The NACK process for NORM_INFO will be described later.
When the NORM_INFO message type is used, its transmission should
precede transmission of any NORM_DATA message for the associated
NormObject.
The sender also generates messages of type NORM_CMD to assist in
certain protocol operations such as congestion control, end-of-
transmission flushing, round trip time estimation, receiver
synchronization, and optional positive acknowledgment requests or
application defined commands. The transmission of NORM_CMD messages
from the sender is accomplished by one of three different procedures.
These procedures are: single, best effort unreliable transmission of
the command; repeated redundant transmissions of the command; and
Adamson, et al. Experimental [Page 9]
RFC 3940 NORM Protocol November 2004
positively-acknowledged commands. The transmission technique used
for a given command depends upon the function of the command.
Several core commands are defined for basic protocol operation.
Additionally, implementations MAY wish to consider providing the
OPTIONAL application-defined commands that can take advantage of the
transmission methodologies available for commands. This allows for
application-level session management mechanisms that can make use of
information available to the underlying NORM protocol engine (e.g.,
round-trip timing, transmission rate, etc.).
NORM receivers generate messages of type NORM_NACK or NORM_ACK in
response to transmissions of data and commands from a sender. The
NORM_NACK messages are generated to request repair of detected data
transmission losses. Receivers generally detect losses by tracking
the sequence of transmission from a sender. Sequencing information
is embedded in the transmitted data packets and end-of-transmission
commands from the sender. NORM_ACK messages are generated in
response to certain commands transmitted by the sender. In the
general (and most scalable) protocol mode, NORM_ACK messages are sent
only in response to congestion control commands from the sender. The
feedback volume of these congestion control NORM_ACK messages is
controlled using the same timer-based probabilistic suppression
techniques as for NORM_NACK messages to avoid feedback implosion. In
order to meet potential application requirements for positive
acknowledgment from receivers, other NORM_ACK messages are defined
and available for use. All sender and receiver transmissions are
subject to rate control governed by a peak transmission rate set for
each participant by the application. This can be used to limit the
quantity of multicast data transmitted by the group. When NORM's
congestion control algorithm is enabled the rate for senders is
automatically adjusted. In some networks, it may be desirable to
establish minimum and maximum bounds for the rate adjustment
depending upon the application even when dynamic congestion control
is enabled. However, in the case of the general Internet, congestion
control policy SHALL be observed that is compatible with coexistent
TCP flows.
The operation of the NORM protocol is based primarily upon the
concepts presented in the Nack-Oriented Reliable Multicast (NORM)
Building Block document [4]. This includes the basic NORM
architecture and the data transmission, repair, and feedback
strategies discussed in that document. Additional reliable multicast
building blocks are applied in creating the full NORM protocol
instantiation [16]. NORM also makes use of Forward Error Correction
encoding techniques for repair messaging and optional transmission
robustness as described in [10]. NORM uses the FEC Payload ID as
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RFC 3940 NORM Protocol November 2004
specified by the FEC Building Block Document [5]. Additionally, for
congestion control, this document includes a baseline congestion
control mechanism (NORM-CC) based on the TCP-Friendly Multicast
Congestion Control (TFMCC) scheme described in [19].
While the various features of NORM are designed to provide some
measure of general purpose utility, it is important to emphasize the
understanding that "no one size fits all" in the reliable multicast
transport arena. There are numerous engineering tradeoffs involved
in reliable multicast transport design and this requires an increased
awareness of application and network architecture considerations.
Performance requirements affecting design can include: group size,
heterogeneity (e.g., capacity and/or delay), asymmetric delivery,
data ordering, delivery delay, group dynamics, mobility, congestion
control, and transport across low capacity connections. NORM
contains various parameters to accommodate many of these differing
requirements. The NORM protocol and its mechanisms MAY be applied in
multicast applications outside of bulk data transfer, but there is an
assumed model of bulk transfer transport service that drives the
trade-offs that determine the scalability and performance described
in this document.
The ability of NORM to provide reliable data delivery is also
governed by any buffer constraints of the sender and receiver
applications. NORM protocol implementations SHOULD be designed to
operate with the greatest efficiency and robustness possible within
application-defined buffer constraints. Buffer requirements for
reliability, as always, are a function of the delay-bandwidth product
of the network topology. NORM performs best when allowed more
buffering resources than typical point-to-point transport protocols.
This is because NORM feedback suppression is based upon randomly-
delayed transmissions from the receiver set, rather than immediately
transmitted feedback. There are definitive tradeoffs between buffer
utilization, group size scalability, and efficiency of performance.
Large buffer sizes allow the NORM protocol to perform most
efficiently in large delay-bandwidth topologies and allow for longer
feedback suppression backoff timeouts. This yields improved group
size scalability. NORM can operate with reduced buffering but at a
cost of decreased efficiency (lower relative goodput) and reduced
group size scalability.
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RFC 3940 NORM Protocol November 2004
This Protocol Instantiation document, in conjunction with the RMT
Building Block documents of [4] and [5], completely specifies a
working reliable multicast transport protocol that conforms to the
requirements described in RFC 2357 [17].
This document specifies the following message types and mechanisms
which are REQUIRED in complying NORM protocol implementations:
+--------------------+-----------------------------------------------+
| Message Type | Purpose |
+--------------------+-----------------------------------------------+
|NORM_DATA | Sender message for application data |
| | transmission. Implementations must support |
| | at least one of the NORM_OBJECT_DATA, |
| | NORM_OBJECT_FILE, or NORM_OBJECT_STREAM |
| | delivery services. The use of the NORM FEC |
| | Object Transmission Information header |
| | extension is OPTIONAL with NORM_DATA |
| | messages. |
+--------------------+-----------------------------------------------+
|NORM_CMD(FLUSH) | Sender command to excite receivers for repair |
| | requests in lieu of ongoing NORM_DATA |
| | transmissions. Note the use of the |
| | NORM_CMD(FLUSH) for positive acknowledgment |
| | of data receipt is OPTIONAL. |
+--------------------+-----------------------------------------------+
|NORM_CMD(SQUELCH) | Sender command to advertise its current valid |
| | repair window in response to invalid requests |
| | for repair. |
+--------------------+-----------------------------------------------+
|NORM_CMD(REPAIR_ADV)| Sender command to advertise current repair |
| | (and congestion control state) to group when |
| | unicast feedback messages are detected. Used |
| | to control/suppress excessive receiver |
| | feedback in asymmetric multicast topologies. |
+--------------------+-----------------------------------------------+
|NORM_CMD(CC) | Sender command used in collection of round |
| | trip timing and congestion control status |
| | from group (this may be OPTIONAL if |
| | alternative congestion control mechanism and |
| | round trip timing collection is used). |
+--------------------+-----------------------------------------------+
|NORM_NACK | Receiver message used to request repair of |
| | missing transmitted content. |
+--------------------+-----------------------------------------------+
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RFC 3940 NORM Protocol November 2004
+--------------------+-----------------------------------------------+
|NORM_ACK | Receiver message used to proactively provide |
| | feedback for congestion control purposes. |
| | Also used with the OPTIONAL NORM Positive |
| | Acknowledgment Process. |
+--------------------+-----------------------------------------------+
This document also describes the following message types and
associated mechanisms which are OPTIONAL for complying NORM protocol
implementations:
+----------------------+----------------------------------------------+
| Message Type | Purpose |
+----------------------+----------------------------------------------+
|NORM_INFO | Sender message for providing ancillary |
| | context information associated with NORM |
| | transport objects. The use of the NORM FEC |
| | Object Transmission Information header |
| | extension is OPTIONAL with NORM_INFO |
| | messages. |
+----------------------+----------------------------------------------+
|NORM_CMD(EOT) | Sender command to indicate it has reached |
| | end-of-transmission and will no longer |
| | respond to repair requests. |
+----------------------+----------------------------------------------+
|NORM_CMD(ACK_REQ) | Sender command to support application- |
| | defined, positively acknowledged commands |
| | sent outside of the context of the bulk data |
| | content being transmitted. The NORM Positive|
| | Acknowledgment Procedure associated with this|
| | message type is OPTIONAL. |
+----------------------+----------------------------------------------+
|NORM_CMD(APPLICATION) | Sender command containing application-defined|
| | commands sent outside of the context of the |
| | bulk data content being transmitted. |
+----------------------+----------------------------------------------+
|NORM_REPORT | Optional message type reserved for |
| | experimental implementations of the NORM |
| | protocol. |
+----------------------+----------------------------------------------+
As mentioned in Section 2.1, there are two primary classes of NORM
messages: sender messages and receiver messages. NORM_CMD,
NORM_INFO, and NORM_DATA message types are generated by senders of
data content, and NORM_NACK and NORM_ACK messages generated by
receivers within a NormSession. An auxiliary message type of
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RFC 3940 NORM Protocol November 2004
NORM_REPORT is also provided for experimental purposes. This section
describes the message formats used by the NORM protocol. These
messages and their fields are referenced in the detailed functional
description of the NORM protocol given in Section 5. Individual NORM
messages are designed to be compatible with the MTU limitations of
encapsulating Internet protocols including IPv4, IPv6, and UDP. The
current NORM protocol specification assumes UDP encapsulation and
leverages the transport features of UDP. The NORM messages are
independent of network addresses and can be used in IPv4 and IPv6
networks.
There are some common message fields contained in all NORM message
types. Additionally, a header extension mechanism is defined to
expand the functionality of the NORM protocol without revision to
this document. All NORM protocol messages begin with a common header
with information fields as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type | hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM Common Message Header Format
The "version" field is a 4-bit value indicating the protocol version
number. NORM implementations SHOULD ignore received messages with
version numbers different from their own. This number is intended to
indicate and distinguish upgrades of the protocol which may be non-
interoperable. The NORM version number for this specification is 1.
The message "type" field is a 4-bit value indicating the NORM
protocol message type. These types are defined as follows:
Message Value
NORM_INFO 1
NORM_DATA 2
NORM_CMD 3
NORM_NACK 4
NORM_ACK 5
NORM_REPORT 6
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RFC 3940 NORM Protocol November 2004
The 8-bit "hdr_len" field indicates the number of 32-bit words that
comprise the given message's header portion. This is used to
facilitate header extensions that may be applied. The presence of
header extensions are implied when the "hdr_len" value is greater
than the base value for the given message "type".
The "sequence" field is a 16-bit value that is set by the message
originator as a monotonically increasing number incremented with each
NORM message transmitted to a given destination address. A
"sequence" field number space SHOULD be maintained for messages sent
to the NormSession group address. This value can be monitored by
receiving nodes to detect packet losses in the transmission from a
sender and used in estimating raw packet loss for congestion control
purposes. Note that this value is NOT used in the NORM protocol to
detect missing reliable data content and does NOT identify the
application data or FEC payload that may be attached. With message
authentication, the "sequence" field may also be leveraged for
protection from message "replay" attacks, particularly of NORM_NACK
or other feedback messages. In this case, the receiver node should
maintain a monotonically increasing "sequence" field space for each
destination to which it transmits (this may be multiple destinations
when unicast feedback is used). The size of this field is intended
to be sufficient to allow detection of a reasonable range of packet
loss within the delay-bandwidth product of expected network
connections.
The "source_id" field is a 32-bit value identifying the node that
sent the message. A participant's NORM node identifier (NormNodeId)
can be set according to application needs but unique identifiers must
be assigned within a single NormSession. In some cases, use of the
host IP address or a hash of it can suffice, but alternative
methodologies for assignment and potential collision resolution of
node identifiers within a multicast session need to be considered.
For example, the "source identifier" mechanism defined in the Real-
Time Protocol (RTP) specification [18] may be applicable to use for
NORM node identifiers. At this point in time, the protocol makes no
assumptions about how these unique identifiers are actually assigned.
NORM Header Extensions
When header extensions are applied, they follow the message type's
base header and precede any payload portion. There are two formats
for header extensions, both of which begin with an 8-bit "het"
(header extension type) field. One format is provided for variable-
length extensions with "het" values in the range from 0 through 127.
The other format is for fixed length (one 32-bit word) extensions
with "het" values in the range from 128 through 255. These formats
are given here:
Adamson, et al. Experimental [Page 15]
RFC 3940 NORM Protocol November 2004
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het <=127 | hel | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Header Extension Content |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM Variable Length Header Extension Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het >=128 | reserved | Header Extension Content |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM Fixed Length (32-bit) Header Extension Format
The "Header Extension Content" portion of these header extension
format is defined for each header extension type defined for NORM
messages. Some header extensions are defined within this document
for NORM baseline FEC and congestion control operations.
NORM sender messages include the NORM_DATA type, the NORM_INFO type,
and the NORM_CMD type. NORM_DATA and NORM_INFO messages contain
application data content while NORM_CMD messages are used for various
protocol control functions.
The NORM_DATA message is expected to be the predominant type
transmitted by NORM senders. These messages are used to encapsulate
segmented data content for objects of type NORM_OBJECT_DATA,
NORM_OBJECT_FILE, and NORM_OBJECT_STREAM. NORM_DATA messages may
contain original or FEC-encoded application data content.
The format of NORM_DATA messages is comprised of three logical
portions: 1) a fixed-format NORM_DATA header portion, 2) a FEC
Payload ID portion with a format dependent upon the FEC encoding
used, and 3) a payload portion containing source or encoded
application data content. Note for objects of type
NORM_OBJECT_STREAM, the payload portion contains additional fields
used to appropriately recover stream content. NORM implementations
MAY also extend the NORM_DATA header to include a FEC Object
Adamson, et al. Experimental [Page 16]
RFC 3940 NORM Protocol November 2004
Transmission Information (EXT_FTI) header extension. This allows
NORM receivers to automatically allocate resources and properly
perform FEC decoding without the need for pre-configuration or out-
of-band information.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=2| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header_extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_reserved* | payload_len* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_offset* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_data* |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_DATA Message Format
*NOTE: The "payload_reserved", "payload_len" and "payload_offset"
fields are present only for objects of type NORM_OBJECT_STREAM. The
"payload_len" and "payload_offset" fields allow senders to
arbitrarily vary the size of NORM_DATA payload segments for streams.
This allows applications to flush transmitted streams as needed to
meet unique streaming requirements. For objects of types
NORM_OBJECT_FILE and NORM_OBJECT_DATA, these fields are unnecessary
since the receiver can calculate the payload length and offset
information from the "fec_payload_id" using the algorithm described
in Section 5.1.1. The "payload_reserved" field is kept for
anticipated future NORM stream control functions. When systematic
FEC codes (e.g., "fec_id" = 129) are used, the "payload_len" and
"payload_offset" fields contain actual length and offset values for
the encapsulated application data segment for those NORM_DATA
messages containing source data symbols. In NORM_DATA messages that
contain parity information, these fields are not actual length or
Adamson, et al. Experimental [Page 17]
RFC 3940 NORM Protocol November 2004
offset values, but instead are values computed from FEC encoding the
"payload_len" and "payload_offset" fields of the _source_ data
symbols of the corresponding applicable coding block.
The "version", "type", "hdr_len", "sequence", and "source_id" fields
form the NORM Common Message Header as described in Section 4.1. The
value of the NORM_DATA "type" field is 2. The NORM_DATA _base_
"hdr_len" value is 4 (32-bit words) plus the size of the
"fec_payload_id" field. The "fec_payload_id" field size depends upon
the FEC encoding used for the referenced NormObject. The "fec_id"
field is used to indicate the FEC coding type. For example, when
small block, systematic codes are used, a "fec_id" value of 129 is
indicated and the size of the "fec_payload_id" is two 32-bit words.
In this case the NORM_DATA base "hdr_len" value is 6. The cumulative
size of any header extensions applied is added into the "hdr_len"
field.
The "instance_id" field contains a value generated by the sender to
uniquely identify its current instance of participation in the
NormSession. This allows receivers to detect when senders have
perhaps left and rejoined a session in progress. When a sender
(identified by its "source_id") is detected to have a new
"instance_id", the NORM receivers SHOULD drop their previous state on
the sender and begin reception anew.
The "grtt" field contains a non-linear quantized representation of
the sender's current estimate of group round-trip time (GRTT) (this
is also referred to as R_max in [19]). This value is used to control
timing of the NACK repair process and other aspects of protocol
operation as described in this document. The algorithm for encoding
and decoding this field is described in the RMT NORM Building Block
document [4].
The "backoff" field value is used by receivers to determine the
maximum backoff timer value used in the timer-based NORM NACK
feedback suppression. This 4-bit field supports values from 0-15
which is multiplied by the sender GRTT to determine the maximum
backoff timeout. The "backoff" field informs the receiver set of the
sender's backoff factor parameter "Ksender". Recommended values and
their use are described in the NORM receiver NACK procedure
description in Section 5.3. The "gsize" field contains a
representation of the sender's current estimate of group size. This
4-bit field can roughly represent values from ten to 500 million
where the most significant bit value of 0 or 1 represents a mantissa
of 1 or 5, respectively and the three least significant bits
incremented by one represent a base 10 exponent (order of magnitude).
For examples, a field value of "0x0" represents 1.0e+01 (10), a value
of "0x8" represents 5.0e+01 (50), a value of "0x1" represents 1.0e+02
Adamson, et al. Experimental [Page 18]
RFC 3940 NORM Protocol November 2004
(100), and a value of "0xf" represents 5.0e+08. For NORM feedback
suppression purposes, the group size does not need to be represented
with a high degree of precision. The group size may even be
estimated somewhat conservatively (i.e., overestimated) to maintain
low levels of feedback traffic. A default group size estimate of
10,000 ("gsize" = 0x4) is recommended for general purpose reliable
multicast applications using the NORM protocol.
The "flags" field contains a number of different binary flags
providing information and hints regarding how the receiver should
handle the identified object. Defined flags in this field include:
+--------------------+-------+-----------------------------------------+
| Flag | Value | Purpose |
+--------------------+-------+-----------------------------------------+
|NORM_FLAG_REPAIR | 0x01 | Indicates message is a repair |
| | | transmission |
+--------------------+-------+-----------------------------------------+
|NORM_FLAG_EXPLICIT | 0x02 | Indicates a repair segment intended to |
| | | meet a specific receiver erasure, as |
| | | compared to parity segments provided by |
| | | the sender for general purpose (with |
| | | respect to an FEC coding block) erasure |
| | | filling. |
+--------------------+-------+-----------------------------------------+
|NORM_FLAG_INFO | 0x04 | Indicates availability of NORM_INFO for |
| | | object. |
+--------------------+-------+-----------------------------------------+
|NORM_FLAG_UNRELIABLE| 0x08 | Indicates that repair transmissions for |
| | | the specified object will be unavailable|
| | | (One-shot, best effort transmission). |
+--------------------+-------+-----------------------------------------+
|NORM_FLAG_FILE | 0x10 | Indicates object is "file-based" data |
| | | (hint to use disk storage for |
| | | reception). |
+--------------------+-------+-----------------------------------------+
|NORM_FLAG_STREAM | 0x20 | Indicates object is of type |
| | | NORM_OBJECT_STREAM. |
+--------------------+-------+-----------------------------------------+
|NORM_FLAG_MSG_START | 0x40 | Marks the first segment of application |
| | | messages embedded in |
| | | NORM_OBJECT_STREAMs. |
+--------------------+-------+-----------------------------------------+
NORM_FLAG_REPAIR is set when the associated message is a repair
transmission. This information can be used by receivers to help
observe a join policy where it is desired that newly joining
receivers only begin participating in the NACK process upon receipt
Adamson, et al. Experimental [Page 19]
RFC 3940 NORM Protocol November 2004
of new (non-repair) data content. NORM_FLAG_EXPLICIT is used to mark
repair messages sent when the data sender has exhausted its ability
to provide "fresh" (previously untransmitted) parity segments as
repair. This flag could possibly be used by intermediate systems
implementing functionality to control sub-casting of repair content
to different legs of a reliable multicast topology with disparate
repair needs. NORM_FLAG_INFO is set only when optional NORM_INFO
content is actually available for the associated object. Thus,
receivers will NACK for retransmission of NORM_INFO only when it is
available for a given object. NORM_FLAG_UNRELIABLE is set when the
sender wishes to transmit an object with only "best effort" delivery
and will not supply repair transmissions for the object. NORM
receivers SHOULD NOT execute repair requests for objects marked with
the NORM_FLAG_UNRELIABLE flag. Note that receivers may inadvertently
request repair of such objects when all segments (or info content)
for those objects are not received (i.e., a gap in the
"object_transport_id" sequence is noted). In this case, the sender
should invoke the NORM_CMD(SQUELCH) process as described in Section
4.2.3. NORM_FLAG_FILE can be set as a "hint" from the sender that
the associated object should be stored in non-volatile storage.
NORM_FLAG_STREAM is set when the identified object is of type
NORM_OBJECT_STREAM. When NORM_FLAG_STREAM is set, the
NORM_FLAG_MSG_START can be optionally used to mark the first data
segments of application-layer messages transported within the NORM
stream. This allows NORM receiver applications to "synchronize" with
NORM senders and to be able to properly interpret application layer
data when joining a NORM session already in progress. In practice,
the NORM implementation MAY set this flag for the segment transmitted
following an explicit "flush" of the stream by the application.
The "fec_id" field corresponds to the FEC Encoding Identifier
described in the FEC Building Block document [5]. The "fec_id" value
implies the format of the "fec_payload_id" field and, coupled with
FEC Object Transmission Information, the procedures to decode FEC
encoded content. Small block, systematic codes ("fec_id" = 129) are
expected to be used for most NORM purposes and the NORM_OBJECT_STREAM
requires systematic FEC codes for most efficient performance.
The "object_transport_id" field is a monotonically and incrementally
increasing value assigned by the sender to NormObjects being
transmitted. Transmissions and repair requests related to that
object use the same "object_transport_id" value. For sessions of
very long or indefinite duration, the "object_transport_id" field may
be repeated, but it is presumed that the 16-bit field size provides
an adequate enough sequence space to avoid object confusion amongst
receivers and sources (i.e., receivers SHOULD re-synchronize with a
server when receiving object sequence identifiers sufficiently out-
of-range with the current state kept for a given source). During the
Adamson, et al. Experimental [Page 20]
RFC 3940 NORM Protocol November 2004
course of its transmission within a NORM session, an object is
uniquely identified by the concatenation of the sender "source_id"
and the given "object_transport_id". Note that NORM_INFO messages
associated with the identified object carry the same
"object_transport_id" value.
The "fec_payload_id" identifies the attached NORM_DATA "payload"
content. The size and format of the "fec_payload_id" field depends
upon the FEC type indicated by the "fec_id" field. These formats are
given in the FEC Building Block document [5] and any subsequent
extensions of that document. As an example, the format of the
"fec_payload_id" format small block, systematic codes ("fec_id" =
129) given here:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_len | encoding_symbol_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Small Block, Systematic Code ("fec_id" = 129) "fec_payload_id" Format
The FEC payload identifier "source_block_number", "source_block_len",
and "encoding_symbol_id" fields correspond to the "Source Block
Number", "Source Block Length, and "Encoding Symbol ID" fields of the
FEC Payload ID format given by the IETF FEC Building Block document
[5]. The "source_block_number" identifies the coding block's
relative position with a NormObject. Note that, for NormObjects of
type NORM_OBJECT_STREAM, the "source_block_number" may wrap for very
long lived sessions. The "source_block_len" indicates the number of
user data segments in the identified coding block. Given the
"source_block_len" information of how many symbols of application
data are contained in the block, the receiver can determine whether
the attached segment is data or parity content and treat it
appropriately. The "encoding_symbol_id" identifies which specific
symbol (segment) within the coding block the attached payload
conveys. Depending upon the value of the "encoding_symbol_id" and
the associated "source_block_len" parameters for the block, the
symbol (segment) referenced may be a user data or an FEC parity
segment. For systematic codes, encoding symbols numbered less than
the source_block_len contain original application data while segments
greater than or equal to source_block_len contain parity symbols
calculated for the block. The concatenation of
Adamson, et al. Experimental [Page 21]
RFC 3940 NORM Protocol November 2004
object_transport_id::fec_payload_id can be viewed as a unique
transport protocol data unit identifier for the attached segment with
respect to the NORM sender's instance within a session.
Additional FEC Object Transmission Information (as described in the
FEC Building Block document [5]) is required to properly receive and
decode NORM transport objects. This information MAY be provided as
out-of-band session information. However, in some cases, it may be
useful for the sender to include this information "in band" to
facilitate receiver operation with minimal preconfiguration. For
this purpose, the NORM FEC Object Transmission Information Header
Extension (EXT_FTI) is defined. This header extension MAY be applied
to NORM_DATA and NORM_INFO messages to provide this necessary
information. The exact format of the extension depends upon the FEC
code in use, but in general it SHOULD contain any required details on
the FEC code in use (e.g., FEC Instance ID, etc.) and the byte size
of the associated NormObject (For the NORM_OBJECT_STREAM type, this
size corresponds to the stream buffer size maintained by the NORM
sender). As an example, the format of the EXT_FTI for small block
systematic codes ("fec_id" = 129) is given here:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 64 | hel = 4 | object_length (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_length (lsb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_instance_id | segment_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_max_block_len | fec_num_parity |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FEC Object Transmission Information Header Extension (EXT_FTI) for
Small Block Systematic Codes ("fec_id" = 129)
The header extension type "het" field value for this header extension
is 64. The header extension length "hel" depends upon the format of
the FTI for FEC code type identified by the "fec_id" field. In this
example (for "fec_id" = 129), the "hel" field value is 4.
The 48-bit "object_length" field indicates the total size of the
object (in bytes) for the static object types of NORM_OBJECT_FILE and
NORM_OBJECT_DATA. This information is used by receivers to determine
storage requirements and/or allocate storage for the received object.
Receivers with insufficient storage capability may wish to forego
reliable reception (i.e., not NACK for) of the indicated object. In
the case of objects of type NORM_OBJECT_STREAM, the "object_length"
Adamson, et al. Experimental [Page 22]
RFC 3940 NORM Protocol November 2004
field is used by the sender to indicate the size of its stream buffer
to the receiver group. In turn, the receivers SHOULD use this
information to allocate a stream buffer for reception of
corresponding size.
The "fec_instance_id" corresponds to the "FEC Instance ID" described
in the FEC Building Block document [5]. In this case, the
"fec_instance_id" SHALL be a value corresponding to the particular
type of Small Block Systematic Code being used (e.g., Reed-Solomon
GF(2^8), Reed-Solomon GF(2^16), etc). The standardized assignment of
FEC Instance ID values is described in [5]. The "segment_size" field
indicates the sender's current setting for maximum message payload
content (in bytes). This allows receivers to allocate appropriate
buffering resources and to determine other information in order to
properly process received data messaging.
The "fec_max_block_len" indicates the current maximum number of user
data segments per FEC coding block to be used by the sender during
the session. This allows receivers to allocate appropriate buffer
space for buffering blocks transmitted by the sender.
The "fec_num_parity" corresponds to the "maximum number of encoding
symbols that can be generated for any source block" as described in
for FEC Object Transmission Information for Small Block Systematic
Codes in the FEC Building Block document [5]. For example, Reed-
Solomon codes may be arbitrarily shortened to create different code
variations for a given block length. In the case of Reed-Solomon
(GF(2^8) and GF(2^16)) codes, this value indicates the maximum number
of parity segments available from the sender for the coding blocks.
This field MAY be interpreted differently for other systematic codes
as they are defined.
The payload portion of NORM_DATA messages includes source data or FEC
encoded application content.
The "payload_reserved", "payload_len" and "payload_offset" fields are
present ONLY for transport objects of type NORM_OBJECT_STREAM. These
fields indicate the size and relative position (within the stream) of
the application content represented by the message payload. For
senders employing systematic FEC encoding, these fields contain
_actual_ length and offset values (in bytes) for the payload of
messages which contain original data source symbols. For NORM_DATA
messages containing calculated parity content, these fields will
actually contain values computed by FEC encoding of the "payload_len"
and "payload_offset" values of the NORM_DATA data segments of the
corresponding FEC coding block. Thus, the "payload_len" and
"payload_offset" values of missing data content can be determined
upon decoding a FEC coding block. Note that these fields do NOT
Adamson, et al. Experimental [Page 23]
RFC 3940 NORM Protocol November 2004
contribute to the value of the NORM_DATA "hdr_len" field. These
fields are NOT present when the "flags" portion of the NORM_DATA
message indicate the transport object if of type NORM_OBJECT_FILE or
NORM_OBJECT_DATA. In this case, the length and offset information
can be calculated from the "fec_payload_id" using the methodology
described in Section 5.1.1. Note that for long-lived streams, the
"payload_offset" field can wrap.
The "payload_data" field contains the original application source or
parity content for the symbol identified by the "fec_payload_id".
The length of this field SHALL be limited to a maximum of the
sender's NormSegmentSize bytes as given in the FTI for the object.
Note the length of this field for messages containing parity content
will always be of length NormSegmentSize. When encoding data
segments of varying sizes, the FEC encoder SHALL assume ZERO value
padding for data segments with length less than the NormSegmentSize.
It is RECOMMENDED that a sender's NormSegmentSize generally be
constant for the duration of a given sender's term of participation
in the session, but may possibly vary on a per-object basis. The
NormSegmentSize is expected to be configurable by the sender
application prior to session participation as needed for network
topology maximum transmission unit (MTU) considerations. For IPv6,
MTU discovery may be possibly leveraged at session startup to perform
this configuration. The "payload_data" content may be delivered
directly to the application for source symbols (when systematic FEC
encoding is used) or upon decoding of the FEC block. For
NORM_OBJECT_FILE and NORM_OBJECT_STREAM objects, the data segment
length and offset can be calculated using the algorithm described in
Section 5.1.1. For NORM_OBJECT_STREAM objects, the length and offset
is obtained from the segment's corresponding "payload_len" and
"payload_offset" fields.
The NORM_INFO message is used to convey OPTIONAL, application-
defined, "out-of-band" context information for transmitted
NormObjects. An example NORM_INFO use for bulk file transfer is to
place MIME type information for the associated file, data, or stream
object into the NORM_INFO payload. Receivers may use the NORM_INFO
content to make a decision as whether to participate in reliable
reception of the associated object. Each NormObject can have an
independent unit of NORM_INFO associated with it. NORM_DATA messages
contain a flag to indicate the availability of NORM_INFO for a given
NormObject. NORM receivers may NACK for retransmission of NORM_INFO
when they have not received it for a given NormObject. The size of
the NORM_INFO content is limited to that of a single NormSegmentSize
Adamson, et al. Experimental [Page 24]
RFC 3940 NORM Protocol November 2004
for the given sender. This atomic nature allows the NORM_INFO to be
rapidly and efficiently repaired within the NORM reliable
transmission process.
When NORM_INFO content is available for a NormObject, the
NORM_FLAG_INFO flag SHALL be set in NORM_DATA messages for the
corresponding "object_transport_id" and the NORM_INFO message shall
be transmitted as the first message for the NormObject.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=1| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header_extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_data |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_INFO Message Format
The "version", "type", "hdr_len", "sequence", and "source_id" fields
form the NORM Common Message Header as described in Section 4.1. The
value of "hdr_len" field when no header extensions are present is 4.
The "instance_id", "grtt", "backoff", "gsize", "flags", "fec_id", and
"object_transport_id" fields carry the same information and serve the
same purpose as with NORM_DATA messages. These values allow the
receiver to prepare appropriate buffering, etc, for further
transmissions from the sender when NORM_INFO is the first message
received.
As with NORM_DATA messages, the NORM FTI Header Extension (EXT_FTI)
may be optionally applied to NORM_INFO messages. To conserve
protocol overhead, some NORM implementations may wish to apply the
EXT_FTI when used to NORM_INFO messages only and not to NORM_DATA
messages.
Adamson, et al. Experimental [Page 25]
RFC 3940 NORM Protocol November 2004
The NORM_INFO "payload_data" field contains sender application-
defined content which can be used by receiver applications for
various purposes as described above.
NORM_CMD messages are transmitted by senders to perform a number of
different protocol functions. This includes functions such as
round-trip timing collection, congestion control functions,
synchronization of sender/receiver repair "windows", and notification
of sender status. A core set of NORM_CMD messages is enumerated.
Additionally, a range of command types remain available for potential
application-specific use. Some NORM_CMD types may have dynamic
content attached. Any attached content will be limited to maximum
length of the sender NormSegmentSize to retain the atomic nature of
commands. All NORM_CMD messages begin with a common set of fields,
after the usual NORM message common header. The standard NORM_CMD
fields are:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor | |
+-+-+-+-+-+-+-+-+ NORM_CMD Content +
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD Standard Fields
The "version", "type", "hdr_len", "sequence", and "source_id" fields
form the NORM Common Message Header as described in Section 4.1. The
value of the "hdr_len" field for NORM_CMD messages without header
extensions present depends upon the "flavor" field.
The "instance_id", "grtt", "backoff", and "gsize" fields provide the
same information and serve the same purpose as with NORM_DATA and
NORM_INFO messages. The "flavor" field indicates the type of command
to follow. The remainder of the NORM_CMD message is dependent upon
the command type ("flavor"). NORM command flavors include:
Adamson, et al. Experimental [Page 26]
RFC 3940 NORM Protocol November 2004
+----------------------+-------------+---------------------------------+
| Command |Flavor Value | Purpose |
+----------------------+-------------+---------------------------------+
|NORM_CMD(FLUSH) | 1 | Used to indicate sender |
| | | temporary end-of-transmission. |
| | | (Assists in robustly initiating |
| | | outstanding repair requests from|
| | | receivers). May also be |
| | | optionally used to collect |
| | | positive acknowledgment of |
| | | reliable reception from subset |
| | | of receivers. |
+----------------------+-------------+---------------------------------+
|NORM_CMD(EOT) | 2 | Used to indicate sender |
| | | permanent end-of-transmission. |
+----------------------+-------------+---------------------------------+
|NORM_CMD(SQUELCH) | 3 | Used to advertise sender's |
| | | current repair window in |
| | | response to out-of-range NACKs |
| | | from receivers. |
+----------------------+-------------+---------------------------------+
|NORM_CMD(CC) | 4 | Used for GRTT measurement and |
| | | collection of congestion control|
| | | feedback. |
+----------------------+-------------+---------------------------------+
|NORM_CMD(REPAIR_ADV) | 5 | Used to advertise sender's |
| | | aggregated repair/feedback state|
| | | for suppression of unicast |
| | | feedback from receivers. |
+----------------------+-------------+---------------------------------+
|NORM_CMD(ACK_REQ) | 6 | Used to request application- |
| | | defined positive acknowledgment |
| | | from a list of receivers |
| | | (OPTIONAL). |
+----------------------+-------------+---------------------------------+
|NORM_CMD(APPLICATION) | 7 | Used for application-defined |
| | | purposes which may need to |
| | | temporarily preempt data |
| | | transmission (OPTIONAL). |
+----------------------+-------------+---------------------------------+
The NORM_CMD(FLUSH) command is sent when the sender reaches the end
of all data content and pending repairs it has queued for
transmission. This may indicate a temporary or permanent end of data
transmission, but the sender is still willing to respond to repair
requests. This command is repeated once per 2*GRTT to excite the
Adamson, et al. Experimental [Page 27]
RFC 3940 NORM Protocol November 2004
receiver set for any outstanding repair requests up to and including
the transmission point indicated within the NORM_CMD(FLUSH) message.
The number of repeats is equal to NORM_ROBUST_FACTOR unless a list of
receivers from which explicit positive acknowledgment is expected
("acking_node_list") is given. In that case, the "acking_node_list"
is updated as acknowledgments are received and the NORM_CMD(FLUSH) is
repeated according to the mechanism described in Section 5.5.3. The
greater the NORM_ROBUST_FACTOR, the greater the probability that all
applicable receivers will be excited for acknowledgment or repair
requests (NACKs) _and_ that the corresponding NACKs are delivered to
the sender. If a NORM_NACK message interrupts the flush process, the
sender will re-initiate the flush process after any resulting repair
transmissions are completed.
Note that receivers also employ a timeout mechanism to self-initiate
NACKing (if there are outstanding repair needs) when no messages of
any type are received from a sender. This inactivity timeout is
related to 2*GRTT*NORM_ROBUST_FACTOR and will be discussed more
later. With a sufficient NORM_ROBUST_FACTOR value, data content is
delivered with a high assurance of reliability. The penalty of a
large NORM_ROBUST_FACTOR value is potentially excess sender
NORM_CMD(FLUSH) transmissions and a longer timeout for receivers to
self-initiate the terminal NACK process.
For finite-size transport objects such as NORM_OBJECT_DATA and
NORM_OBJECT_FILE, the flush process (if there are no further pending
objects) occurs at the end of these objects. Thus, FEC repair
information is always available for repairs in response to repair
requests elicited by the flush command. However, for
NORM_OBJECT_STREAM, the flush may occur at any time, including in the
middle of an FEC coding block if systematic FEC codes are employed.
In this case, the sender will not yet be able to provide FEC parity
content as repair for the concurrent coding block and will be limited
to explicitly repairing stream data content for that block.
Applications that anticipate frequent flushing of stream content
SHOULD be judicious in the selection of the FEC coding block size
(i.e., do not use a very large coding block size if frequent flushing
occurs). For example, a reliable multicast application transmitting
an on-going series of intermittent, relatively small messaging
content will need to trade-off using the NORM_OBJECT_DATA paradigm
versus the NORM_OBJECT_STREAM paradigm with an appropriate FEC coding
block size. This is analogous to application trade-offs for other
transport protocols such as the selection of different TCP modes of
operation such as "no delay", etc.
Adamson, et al. Experimental [Page 28]
RFC 3940 NORM Protocol November 2004
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 1 | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| acking_node_list (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(FLUSH) Message Format
In addition to the NORM common message header and standard NORM_CMD
fields, the NORM_CMD(FLUSH) message contains fields to identify the
current status and logical transmit position of the sender.
The "fec_id" field indicates the FEC type used for the flushing
"object_transport_id" and implies the size and format of the
"fec_payload_id" field. Note the "hdr_len" value for the
NORM_CMD(FLUSH) message is 4 plus the size of the "fec_payload_id"
field when no header extensions are present.
The "object_transport_id" and "fec_payload_id" fields indicate the
sender's current logical "transmit position". These fields are
interpreted in the same manner as in the NORM_DATA message type.
Upon receipt of the NORM_CMD(FLUSH), receivers are expected to check
their completion state _through_ (including) this transmission
position. If receivers have outstanding repair needs in this range,
they SHALL initiate the NORM NACK Repair Process as described in
Section 5.3. If receivers have no outstanding repair needs, no
response to the NORM_CMD(FLUSH) is generated.
For NORM_OBJECT_STREAM objects using systematic FEC codes, receivers
MUST request "explicit-only" repair of the identified
"source_block_number" if the given "encoding_symbol_id" is less than
the "source_block_len". This condition indicates the sender has not
yet completed encoding the corresponding FEC block and parity content
is not yet available. An "explicit-only" repair request consists of
NACK content for the applicable "source_block_number" which does not
include any requests for parity-based repair. This allows NORM
Adamson, et al. Experimental [Page 29]
RFC 3940 NORM Protocol November 2004
sender applications to "flush" an ongoing stream of transmission when
needed, even if in the middle of an FEC block. Once the sender
resumes stream transmission and passes the end of the pending coding
block, subsequent NACKs from receivers SHALL request parity-based
repair as usual. Note that the use of a systematic FEC code is
assumed here. Normal receiver NACK initiation and construction is
discussed in detail in Section 5.3. The OPTIONAL "acking_node_list"
field contains a list of NormNodeIds for receivers from which the
sender is requesting explicit positive acknowledgment of reception up
through the transmission point identified by the
"object_transport_id" and "fec_payload_id" fields. The length of the
list can be inferred from the length of the received NORM_CMD(FLUSH)
message. When the "acking_node_list" is present, the lightweight
positive acknowledgment process described in Section 5.5.3 SHALL be
observed.
The NORM_CMD(EOT) command is sent when the sender reaches permanent
end-of-transmission with respect to the NormSession and will not
respond to further repair requests. This allows receivers to
gracefully reach closure of operation with this sender (without
requiring any timeout) and free any resources that are no longer
needed. The NORM_CMD(EOT) command SHOULD be sent with the same
robust mechanism as used for NORM_CMD(FLUSH) commands to provide a
high assurance of reception by the receiver set.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 2 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(EOT) Message Format
The value of the "hdr_len" field for NORM_CMD(EOT) messages without
header extensions present is 4. The "reserved" field is reserved for
future use and MUST be set to an all ZERO value. Receivers MUST
ignore the "reserved" field.
Adamson, et al. Experimental [Page 30]
RFC 3940 NORM Protocol November 2004
The NORM_CMD(SQUELCH) command is transmitted in response to outdated
or invalid NORM_NACK content received by the sender. Invalid
NORM_NACK content consists of repair requests for NormObjects for
which the sender is unable or unwilling to provide repair. This
includes repair requests for outdated objects, aborted objects, or
those objects which the sender previously transmitted marked with the
NORM_FLAG_UNRELIABLE flag. This command indicates to receivers what
content is available for repair, thus serving as a description of the
sender's current "repair window". Receivers SHALL not generate
repair requests for content identified as invalid by a
NORM_CMD(SQUELCH).
The NORM_CMD(SQUELCH) command is sent once per 2*GRTT at the most.
The NORM_CMD(SQUELCH) advertises the current "repair window" of the
sender by identifying the earliest (lowest) transmission point for
which it will provide repair, along with an encoded list of objects
from that point forward that are no longer valid for repair. This
mechanism allows the sender application to cancel or abort
transmission and/or repair of specific previously enqueued objects.
The list also contains the identifiers for any objects within the
repair window that were sent with the NORM_FLAG_UNRELIABLE flag set.
In normal conditions, it is expected the NORM_CMD(SQUELCH) will be
needed infrequently, and generally only to provide a reference repair
window for receivers who have fallen "out-of-sync" with the sender
due to extremely poor network conditions.
The starting point of the invalid NormObject list begins with the
lowest invalid NormTransportId greater than the current "repair
window" start from the invalid NACK(s) that prompted the generation
of the squelch. The length of the list is limited by the sender's
NormSegmentSize. This allows the receivers to learn the status of
the sender's applicable object repair window with minimal
transmission of NORM_CMD(SQUELCH) commands. The format of the
NORM_CMD(SQUELCH) message is:
Adamson, et al. Experimental [Page 31]
RFC 3940 NORM Protocol November 2004
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 3 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 3 | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| invalid_object_list |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(SQUELCH) Message Format
In addition to the NORM common message header and standard NORM_CMD
fields, the NORM_CMD(SQUELCH) message contains fields to identify the
earliest logical transmit position of the sender's current repair
window and an "invalid object list" beginning with the index of the
logically earliest invalid repair request from the offending NACK
message which initiated the squelch transmission.
The "object_transport_id" and "fec_payload_id" fields are
concatenated to indicate the beginning of the sender's current repair
window (i.e., the logically earliest point in its transmission
history for which the sender can provide repair). The "fec_id" field
implies the size and format of the "fec_payload_id" field. This
serves as an advertisement of a "synchronization point" for receivers
to request repair. Note, that while an "encoding_symbol_id" may be
included in the "fec_payload_id" field, the sender's repair window
SHOULD be aligned on FEC coding block boundaries and thus the
"encoding_symbol_id" SHOULD be zero.
The "invalid_object_list" is a list of 16-bit NormTransportIds that,
although they are within the range of the sender's current repair
window, are no longer available for repair from the sender. For
example, a sender application may dequeue an out-of-date object even
though it is still within the repair window. The total size of the
"invalid_object_list" content is can be determined from the packet's
payload length and is limited to a maximum of the NormSegmentSize of
the sender. Thus, for very large repair windows, it is possible that
a single NORM_CMD(SQUELCH) message may not be capable of listing the
entire set of invalid objects in the repair window. In this case,
Adamson, et al. Experimental [Page 32]
RFC 3940 NORM Protocol November 2004
the sender SHALL ensure that the list begins with a NormObjectId that
is greater than or equal to the lowest ordinal invalid NormObjectId
from the NACK message(s) that prompted the NORM_CMD(SQUELCH)
generation. The NormObjectIds in the "invalid_object_list" MUST be
greater than the "object_transport_id" marking the beginning of the
sender's repair window. This insures convergence of the squelch
process, even if multiple invalid NACK/ squelch iterations are
required. This explicit description of invalid content within the
sender's current window allows the sender application (most notably
for discrete "object" based transport) to arbitrarily invalidate
(i.e., dequeue) portions of enqueued content (e.g., certain objects)
for which it no longer wishes to provide reliable transport.
The NORM_CMD(CC) messages contains fields to enable sender-to-
receiver group greatest round-trip time (GRTT) measurement and to
excite the group for congestion control feedback. A baseline NORM
congestion control scheme (NORM-CC), based on the TCP-Friendly
Multicast Congestion Control (TFMCC) scheme of [19] is described in
Section 5.5.2 of this document. The NORM_CMD(CC) message is usually
transmitted as part of NORM-CC congestion control operation. A NORM
header extension is defined below to be used with the NORM_CMD(CC)
message to support NORM-CC operation. Different header extensions
may be defined for the NORM_CMD(CC) (and/or other NORM messages as
needed) to support alternative congestion control schemes in the
future. If NORM is operated in a private network with congestion
control operation disabled, the NORM_CMD(CC) message is then used for
GRTT measurement only and may optionally be sent less frequently than
with congestion control operation.
Adamson, et al. Experimental [Page 33]
RFC 3940 NORM Protocol November 2004
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 4 | reserved | cc_sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send_time_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send_time_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_node_list (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(CC) Message Format
The NORM common message header and standard NORM_CMD fields serve
their usual purposes.
The "reserved" field is for potential future use and should be set to
ZERO in this version of the NORM protocol.
The "cc_sequence" field is a sequence number applied by the sender.
For NORM-CC operation, it is used to provide functionality equivalent
to the "feedback round number" (fb_nr)described in [19]. The most
recently received "cc_sequence" value is recorded by receivers and
can be fed back to the sender in congestion control feedback
generated by the receivers for that sender. The "cc_sequence" number
can also be used in NORM implementations to assess how recently a
receiver has received NORM_CMD(CC) probes from the sender. This can
be useful instrumentation for complex or experimental multicast
routing environments.
The "send_time" field is a timestamp indicating the time that the
NORM_CMD(CC) message was transmitted. This consists of a 64-bit
field containing 32-bits with the time in seconds ("send_time_sec")
and 32-bits with the time in microseconds ("send_time_usec") since
some reference time the source maintains (usually 00:00:00, 1 January
1970). The byte ordering of the fields is "Big Endian" network
order. Receivers use this timestamp adjusted by the amount of delay
Adamson, et al. Experimental [Page 34]
RFC 3940 NORM Protocol November 2004
from the time they received the NORM_CMD(CC) message to the time of
their response as the "grtt_response" portion of NORM_ACK and
NORM_NACK messages generated. This allows the sender to evaluate
round-trip times to different receivers for congestion control and
other (e.g., GRTT determination) purposes.
To facilitate the baseline NORM-CC scheme described in Section 5.5.2,
a NORM-CC Rate header extension (EXT_RATE) is defined to inform the
group of the sender's current transmission rate. This is used along
with the loss detection "sequence" field of all NORM sender messages
and the NORM_CMD(CC) GRTT collection process to support NORM-CC
congestion control operation. The format of this header extension is
as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 128 | reserved | send_rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM-CC Rate Header Extension Format (EXT_RATE)
The "send_rate" field indicates the sender's current transmission
rate in bytes per second. The 16-bit "send_rate" field consists of
12 bits of mantissa in the most significant portion and 4 bits of
base 10 exponent (order of magnitude) information in the least
significant portion. The 12-bit mantissa portion of the field is
scaled such that a floating point value of 0.0 corresponds to 0 and a
floating point value of 10.0 corresponds to 4096. Thus:
send_rate = (((int)(Value_mantissa * 4096.0 / 10.0 + 0.5)) << 4) |
Value_exponent;
For example, to represent a transmission rate of 256kbps (3.2e+04
bytes per second), the lower 4 bits of the 16-bit field contain a
value of 0x04 to represent the exponent while the upper 12 bits
contain a value of 0x51f as determined from the equation given above:
send_rate = (((int)((3.2 * 4096.0 / 10.0) + 0.5)) << 4) | 4;
= (0x51f << 4) | 0x4
= 0x51f4
To decode the "send_rate" field, the following equation can be used:
value = (send_rate >> 4) * 10.0 / 4096.0 *
power(10.0, (send_rate & x000f))
Adamson, et al. Experimental [Page 35]
RFC 3940 NORM Protocol November 2004
Note the maximum transmission rate that can be represented by this
scheme is approximately 9.99e+15 bytes per second.
When this extension is present, a "cc_node_list" may be attached as
the payload of the NORM_CMD(CC) message. The presence of this header
extension also implies that NORM receivers should respond according
to the procedures described in Section 5.5.2. The "cc_node_list"
consists of a list of NormNodeIds and their associated congestion
control status. This includes the current limiting receiver (CLR)
node, any potential limiting receiver (PLR) nodes that have been
identified, and some number of receivers for which congestion control
status is being provided, most notably including the receivers'
current RTT measurement. The maximum length of the "cc_node_list"
provides for at least the CLR and one other receiver, but may be
configurable for more timely feedback to the group. The list length
can be inferred from the length of the NORM_CMD(CC) message.
Each item in the "cc_node_list" is in the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_node_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Congestion Control Node List Item Format
The "cc_node_id" is the NormNodeId of the receiver which the item
represents.
The "cc_flags" field contains flags indicating the congestion control
status of the indicated receiver. The following flags are defined:
Adamson, et al. Experimental [Page 36]
RFC 3940 NORM Protocol November 2004
+------------------+-------+------------------------------------------+
| Flag | Value | Purpose |
+------------------+-------+------------------------------------------+
|NORM_FLAG_CC_CLR | 0x01 | Receiver is the current limiting |
| | | receiver (CLR). |
+------------------+-------+------------------------------------------+
|NORM_FLAG_CC_PLR | 0x02 | Receiver is a potential limiting |
| | | receiver (PLR). |
+------------------+-------+------------------------------------------+
|NORM_FLAG_CC_RTT | 0x04 | Receiver has measured RTT with respect |
| | | to sender. |
+------------------+-------+------------------------------------------+
|NORM_FLAG_CC_START| 0x08 | Sender/receiver is in "slow start" phase |
| | | of congestion control operation (i.e., |
| | | The receiver has not yet detected any |
| | | packet loss and the "cc_rate" field is |
| | | the receiver's actual measured receive |
| | | rate). |
+------------------+-------+------------------------------------------+
|NORM_FLAG_CC_LEAVE| 0x10 | Receiver is imminently leaving the |
| | | session and its feedback should not be |
| | | considered in congestion control |
| | | operation. |
+------------------+-------+------------------------------------------+
The "cc_rtt" contains a quantized representation of the RTT as
measured by the sender with respect to the indicated receiver. This
field is valid only if the NORM_FLAG_CC_RTT flag is set in the
"cc_flags" field. This one byte field is a quantized representation
of the RTT using the algorithm described in the NORM Building Block
document [4]. The "cc_rate" field contains a representation of the
receiver's current calculated (during steady-state congestion control
operation) or twice its measured (during the "slow start" phase)
congestion control rate. This field is encoded and decoded using the
same technique as described for the NORM_CMD(CC) "send_rate" field.
The NORM_CMD(REPAIR_ADV) message is used by the sender to "advertise"
its aggregated repair state from NORM_NACK messages accumulated
during a repair cycle and/or congestion control feedback received.
This message is sent only when the sender has received NORM_NACK
and/or NORM_ACK(CC) (when congestion control is enabled) messages via
unicast transmission instead of multicast. By "echoing" this
information to the receiver set, suppression of feedback can be
achieved even when receivers are unicasting that feedback instead of
multicasting it among the group [13].
Adamson, et al. Experimental [Page 37]
RFC 3940 NORM Protocol November 2004
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 5 | flags | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| repair_adv_payload |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(REPAIR_ADV) Message Format
The "instance_id", "grtt", "backoff", "gsize", and "flavor" fields
serve the same purpose as in other NORM_CMD messages. The value of
the "hdr_len" field when no extensions are present is 4.
The "flags" field provide information on the NORM_CMD(REPAIR_ADV)
content. There is currently one NORM_CMD(REPAIR_ADV) flag defined:
NORM_REPAIR_ADV_FLAG_LIMIT = 0x01
This flag is set by the sender when it is unable to fit its full
current repair state into a single NormSegmentSize. If this flag is
set, receivers should limit their NACK response to generating NACK
content only up through the maximum ordinal transmission position
(objectId::fecPayloadId) included in the "repair_adv_content".
When congestion control operation is enabled, a header extension may
be applied to the NORM_CMD(REPAIR_ADV) representing the most limiting
(in terms of congestion control feedback suppression) congestion
control response. This allows the NORM_CMD(REPAIR_ADV) message to
suppress receiver congestion control responses as well as NACK
feedback messages. The field is defined as a header extension so
that alternative congestion control schemes may be used with NORM
without revision to this document. A NORM-CC Feedback Header
Extension (EXT_CC) is defined to encapsulate congestion control
feedback within NORM_NACK, NORM_ACK, and NORM_CMD(REPAIR_ADV)
messages. If another congestion control technique (e.g., Pragmatic
General Multicast Congestion Control (PGMCC) [20]) is used within a
Adamson, et al. Experimental [Page 38]
RFC 3940 NORM Protocol November 2004
NORM implementation, an additional header extension MAY need to be
defined to encapsulate any required feedback content. The NORM-CC
Feedback Header Extension format is:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 3 | hel = 3 | cc_sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_loss |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_rate | cc_reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM-CC Feedback Header Extension (EXT_CC) Format
The "cc_sequence" field contains the current greatest "cc_sequence"
value receivers have received in NORM_CMD(CC) messages from the
sender. This information assists the sender in congestion control
operation by providing an indicator of how current ("fresh") the
receiver's round-trip measurement reference time is and whether the
receiver has been successfully receiving recent congestion control
probes. For example, if it is apparent the receiver has not been
receiving recent congestion control probes (and thus possibly other
messages from the sender), the sender may choose to take congestion
avoidance measures. For NORM_CMD(REPAIR_ADV) messages, the sender
SHALL set the "cc_sequence" field value to the value set in the last
NORM_CMD(CC) message sent.
The "cc_flags" field contains bits representing the receiver's state
with respect to congestion control operation. The possible values
for the "cc_flags" field are those specified for the NORM_CMD(CC)
message node list item flags. These fields are used by receivers in
controlling (suppressing as necessary) their congestion control
feedback. For NORM_CMD(REPAIR_ADV) messages, the NORM_FLAG_CC_RTT
should be set only when all feedback messages received by the sender
have the flag set. Similarly, the NORM_FLAG_CC_CLR or
NORM_FLAG_CC_PLR should be set only when no feedback has been
received from non-CLR or non-PLR receivers. And the
NORM_FLAG_CC_LEAVE should be set only when all feedback messages the
sender has received have this flag set. These heuristics for setting
the flags in NORM_CMD(REPAIR_ADV) ensure the most effective
suppression of receivers providing unicast feedback messages.
The "cc_rtt" field SHALL be set to a default maximum value and the
NORM_FLAG_CC_RTT flag SHALL be cleared when no receiver has yet
received RTT measurement information. When a receiver has received
RTT measurement information, it shall set the "cc_rtt" value
accordingly and set the NORM_FLAG_CC_RTT flag in the "cc_flags"
field.
Adamson, et al. Experimental [Page 39]
RFC 3940 NORM Protocol November 2004
For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_rtt"
field value to the largest non-CLR/non-PLR RTT it has measured from
receivers for the current feedback round.
The "cc_loss" field represents the receiver's current packet loss
fraction estimate for the indicated source. The loss fraction is a
value from 0.0 to 1.0 corresponding to a range of zero to 100 percent
packet loss. The 16-bit "cc_loss" value is calculated by the
following formula:
"cc_loss" = decimal_loss_fraction * 65535.0
For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_loss"
field value to the largest non-CLR/non-PLR loss estimate it has
received from receivers for the current feedback round.
The "cc_rate" field represents the receivers current local congestion
control rate. During "slow start", when the receiver has detected no
loss, this value is set to twice the actual rate it has measured from
the corresponding sender and the NORM_FLAG_CC_START is set in the
"cc_flags' field. Otherwise, the receiver calculates a congestion
control rate based on its loss measurement and RTT measurement
information (even if default) for the "cc_rate" field. For
NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_loss"
field value to the lowest non-CLR/non-PLR "cc_rate" report it has
received from receivers for the current feedback round.
The "cc_reserved" field is reserved for future NORM protocol use.
Currently, senders SHALL set this field to ZERO, and receivers SHALL
ignore the content of this field.
The "repair_adv_payload" is in exactly the same form as the
"nack_content" of NORM_NACK messages and can be processed by
receivers for suppression purposes in the same manner, with the
exception of the condition when the NORM_REPAIR_ADV_FLAG_LIMIT is
set.
The NORM_CMD(ACK_REQ) message is used by the sender to request
acknowledgment from a specified list of receivers. This message is
used in providing a lightweight positive acknowledgment mechanism
that is OPTIONAL for use by the reliable multicast application. A
range of acknowledgment request types is provided for use at the
application's discretion. Provision for application-defined,
positively-acknowledged commands allows the application to
automatically take advantage of transmission and round-trip timing
information available to the NORM protocol. The details of the NORM
Adamson, et al. Experimental [Page 40]
RFC 3940 NORM Protocol November 2004
positive acknowledgment process including transmission of the
NORM_CMD(ACK_REQ) messages and the receiver response (NORM_ACK) are
described in Section 5.5.3. The format of the NORM_CMD(ACK_REQ)
message is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 6 | reserved | ack_type | ack_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| acking_node_list |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(ACK_REQ) Message Format
The NORM common message header and standard NORM_CMD fields serve
their usual purposes. The value of the "hdr_len" field for
NORM_CMD(ACK_REQ) messages with no header extension present is 4.
The "ack_type" field indicates the type of acknowledgment being
requested and thus implies rules for how the receiver will treat this
request. The following "ack_type" values are defined and are also
used in NORM_ACK messages described later:
+---------------------+--------+---------------------------------+
| ACK Type | Value | Purpose |
+---------------------+--------+---------------------------------+
|NORM_ACK_CC | 1 | Used to identify NORM_ACK |
| | | messages sent in response to |
| | | NORM_CMD(CC) messages. |
+---------------------+--------+---------------------------------+
|NORM_ACK_FLUSH | 2 | Used to identify NORM_ACK |
| | | messages sent in response to |
| | | NORM_CMD(FLUSH) messages. |
+---------------------+--------+---------------------------------+
|NORM_ACK_RESERVED | 3-15 | Reserved for possible future |
| | | NORM protocol use. |
+---------------------+--------+---------------------------------+
|NORM_ACK_APPLICATION | 16-255 | Used at application's |
| | | discretion. |
+---------------------+--------+---------------------------------+
Adamson, et al. Experimental [Page 41]
RFC 3940 NORM Protocol November 2004
The NORM_ACK_CC value is provided for use only in NORM_ACKs generated
in response to the NORM_CMD(CC) messages used in congestion control
operation. Similarly, the NORM_ACK_FLUSH is provided for use only in
NORM_ACKs generated in response to applicable NORM_CMD(FLUSH)
messages. NORM_CMD(ACK_REQ) messages with "ack_type" of NORM_ACK_CC
or NORM_ACK_FLUSH SHALL NOT be generated by the sender.
The NORM_ACK_RESERVED range of "ack_type" values is provided for
possible future NORM protocol use.
The NORM_ACK_APPLICATION range of "ack_type" values is provided so
that NORM applications may implement application-defined,
positively-acknowledged commands that are able to leverage internal
transmission and round-trip timing information available to the NORM
protocol implementation.
The "ack_id" provides a sequenced identifier for the given
NORM_CMD(ACK_REQ) message. This "ack_id" is returned in NORM_ACK
messages generated by the receivers so that the sender may associate
the response with its corresponding request.
The "reserved" field is reserved for possible future protocol use and
SHALL be set to ZERO by senders and ignored by receivers.
The "acking_node_list" field contains the NormNodeIds of the current
NORM receivers that are desired to provide positive acknowledge
(NORM_ACK) to this request. The packet payload length implies the
length of the "acking_node_list" and its length is limited to the
sender NormSegmentSize. The individual NormNodeId items are listed
in network (Big Endian) byte order. If a receiver's NormNodeId is
included in the "acking_node_list", it SHALL schedule transmission of
a NORM_ACK message as described in Section 5.5.3.
This command allows the NORM application to robustly transmit
application-defined commands. The command message preempts any
ongoing data transmission and is repeated up to NORM_ROBUST_FACTOR
times at a rate of once per 2*GRTT. This rate of repetition allows
the application to observe any response (if that is the application's
purpose for the command) before it is repeated. Possible responses
may include initiation of data transmission, other
NORM_CMD(APPLICATION) messages, or even application-defined,
positively-acknowledge commands from other NormSession participants.
The transmission of these commands will preempt data transmission
when they are scheduled and may be multiplexed with ongoing data
transmission. This type of robustly transmitted command allows NORM
applications to define a complete set of session control mechanisms
Adamson, et al. Experimental [Page 42]
RFC 3940 NORM Protocol November 2004
with less state than the transfer of FEC encoded reliable content
requires while taking advantage of NORM transmission and round-trip
timing information.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 7 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Application-Defined Content |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(APPLICATION) Message Format
The NORM common message header and NORM_CMD fields are interpreted as
previously described. The value of the NORM_CMD(APPLICATION)
"hdr_len" field when no header extensions are present is 4.
The "Application-Defined Content" area contains information in a
format at the discretion of the application. The size of this
payload SHALL be limited to a maximum of the sender's NormSegmentSize
setting.
The NORM message types generated by participating receivers consist
of NORM_NACK and NORM_ACK message types. NORM_NACK messages are sent
to request repair of missing data content from sender transmission
and NORM_ACK messages are generated in response to certain sender
commands including NORM_CMD(CC) and NORM_CMD(ACK_REQ).
The principal purpose of NORM_NACK messages is for receivers to
request repair of sender content via selective, negative
acknowledgment upon detection of incomplete data. NORM_NACK messages
will be transmitted according to the rules of NORM_NACK generation
and suppression described in Section 5.3. NORM_NACK messages also
contain additional fields to provide feedback to the sender(s) for
purposes of round-trip timing collection and congestion control.
Adamson, et al. Experimental [Page 43]
RFC 3940 NORM Protocol November 2004
The payload of NORM_NACK messages contains one or more repair
requests for different objects or portions of those objects. The
NORM_NACK message format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=4| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| nack_payload |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_NACK Message Format
The NORM common message header fields serve their usual purposes.
The value of the "hdr_len" field for NORM_NACK messages without
header extensions present is 6.
The "server_id" field identifies the NORM sender to which the
NORM_NACK message is destined.
The "instance_id" field contains the current session identifier given
by the sender identified by the "server_id" field in its sender
messages. The sender SHOULD ignore feedback messages which contain
an invalid "instance_id" value.
The "grtt_response" fields contain an adjusted version of the
timestamp from the most recently received NORM_CMD(CC) message for
the indicated NORM sender. The format of the "grtt_response" is the
same as the "send_time" field of the NORM_CMD(CC). The
"grtt_response" value is _relative_ to the "send_time" the source
provided with a corresponding NORM_CMD(CC) command. The receiver
adjusts the source's NORM_CMD(CC) "send_time" timestamp by adding the
time differential from when the receiver received the NORM_CMD(CC)
Adamson, et al. Experimental [Page 44]
RFC 3940 NORM Protocol November 2004
to when the NORM_NACK is transmitted to calculate the value in the
"grtt_response" field. This is the
"receive_to_response_differential" value used in the following
formula:
"grtt_response" = NORM_CMD(CC) "send_time" +
receive_to_response_differential
The receiver SHALL set the "grtt_response" to a ZERO value, to
indicate that it has not yet received a NORM_CMD(CC) message from the
indicated sender and that the sender should ignore the
"grtt_response" in this message.
For NORM-CC operation, the NORM-CC Feedback Header Extension, as
described in the NORM_CMD(REPAIR_ADV} message description, is added
to NORM_NACK messages to provide feedback on the receivers current
state with respect to congestion control operation. Note that
alternative header extensions for congestion control feedback may be
defined for alternative congestion control schemes for NORM use in
the future.
The "reserved" field is for potential future NORM use and SHALL be
set to ZERO for this version of the protocol.
The "nack_content" of the NORM_NACK message specifies the repair
needs of the receiver with respect to the NORM sender indicated by
the "server_id" field. The receiver constructs repair requests based
on the NORM_DATA and/or NORM_INFO segments it requires from the
sender in order to complete reliable reception up to the sender's
transmission position at the moment the receiver initiates the NACK
Procedure as described in Section 5.3. A single NORM Repair Request
consists of a list of items, ranges, and/or FEC coding block erasure
counts for needed NORM_DATA and/or NORM_INFO content. Multiple
repair requests may be concatenated within the "nack_payload" field
of a NORM_NACK message. Note that a single NORM Repair Request can
possibly include multiple "items", "ranges", or "erasure_counts". In
turn, the "nack_payload" field may contain multiple repair requests.
A single NORM Repair Request has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form | flags | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| repair_request_items |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Adamson, et al. Experimental [Page 45]
RFC 3940 NORM Protocol November 2004
NORM Repair Request Format
The "form" field indicates the type of repair request items given in
the "repair_request_items" list. Possible values for the "form"
field include:
Form Value
NORM_NACK_ITEMS 1
NORM_NACK_RANGES 2
NORM_NACK_ERASURES 3
A "form" value of NORM_NACK_ITEMS indicates each repair request item
in the "repair_request_items" list is to be treated as an individual
request. A value of NORM_NACK_RANGES indicates that the
"repair_request_items" list consists of pairs of repair request items
that correspond to inclusive ranges of repair needs. And the
NORM_NACK_ERASURES "form" indicates that the repair request items are
to be treated individually and that the "encoding_symbol_id" portion
of the "fec_payload_id" field of the repair request item (see below)
is to be interpreted as an "erasure count" for the FEC coding block
identified by the repair request item's "source_block_number".
The "flags" field is currently used to indicate the level of data
content for which the repair request items apply (i.e., an individual
segment, entire FEC coding block, or entire transport object).
Possible flag values include:
+------------------+-------+-----------------------------------------+
| Flag | Value | Purpose |
+------------------+-------+-----------------------------------------+
|NORM_NACK_SEGMENT | 0x01 | Indicates the listed segment(s) or range|
| | | of segments are required as repair. |
+------------------+-------+-----------------------------------------+
|NORM_NACK_BLOCK | 0x02 | Indicates the listed block(s) or range |
| | | of blocks in entirety are required as |
| | | repair. |
+------------------+-------+-----------------------------------------+
|NORM_NACK_INFO | 0x04 | Indicates that NORM_INFO is required as |
| | | repair for the listed object(s). |
+------------------+-------+-----------------------------------------+
|NORM_NACK_OBJECT | 0x08 | Indicates the listed object(s) or range |
| | | of objects in entirety are required as |
| | | repair. |
+------------------+-------+-----------------------------------------+
When the NORM_NACK_SEGMENT flag is set, the "object_transport_id" and
"fec_payload_id" fields are used to determine which sets or ranges of
individual NORM_DATA segments are needed to repair content at the
Adamson, et al. Experimental [Page 46]
RFC 3940 NORM Protocol November 2004
receiver. When the NORM_NACK_BLOCK flag is set, this indicates the
receiver is completely missing the indicated coding block(s) and
requires transmissions sufficient to repair the indicated block(s) in
their entirety. When the NORM_NACK_INFO flag is set, this indicates
the receiver is missing the NORM_INFO segment for the indicated
"object_transport_id". Note the NORM_NACK_INFO may be set in
combination with the NORM_NACK_BLOCK or NORM_NACK_SEGMENT flags, or
may be set alone. When the NORM_NACK_OBJECT flag is set, this
indicates the receiver is missing the entire NormTransportObject
referenced by the "object_transport_id". This also implicitly
requests any available NORM_INFO for the NormObject, if applicable.
The "fec_payload_id" field is ignored when the flag NORM_NACK_OBJECT
is set.
The "length" field value is the length in bytes of the
"repair_request_items" field.
The "repair_request_items" field consists of a list of individual or
range pairs of transport data unit identifiers in the following
format.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id | reserved | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM Repair Request Item Format
The "fec_id" indicates the FEC type and can be used to determine the
format of the "fec_payload_id" field. The "reserved" field is kept
for possible future use and SHALL be set to a ZERO value and ignored
by NORM nodes processing NACK content.
The "object_transport_id" corresponds to the NormObject for which
repair is being requested and the "fec_payload_id" identifies the
specific FEC coding block and/or segment being requested. When the
NORM_NACK_OBJECT flag is set, the value of the "fec_payload_id" field
is ignored. When the NORM_NACK_BLOCK flag is set, only the FEC code
block identifier portion of the "fec_payload_id" is to be
interpreted.
The format of the "fec_payload_id" field depends upon the "fec_id"
field value.
Adamson, et al. Experimental [Page 47]
RFC 3940 NORM Protocol November 2004
When the receiver's repair needs dictate that different forms (mixed
ranges and/or individual items) or types (mixed specific segments
and/or blocks or objects in entirety) are required to complete
reliable transmission, multiple NORM Repair Requests with different
"form" and or "flags" values can be concatenated within a single
NORM_NACK message. Additionally, NORM receivers SHALL construct
NORM_NACK messages with their repair requests in ordinal order with
respect to "object_transport_id" and "fec_payload_id" values. The
"nack_payload" size SHALL NOT exceed the NormSegmentSize for the
sender to which the NORM_NACK is destined.
NORM_NACK Content Examples:
In these examples, a small block, systematic FEC code ("fec_id" =
129) is assumed with a user data block length of 32 segments. In
Example 1, a list of individual NORM_NACK_ITEMS repair requests is
given. In Example 2, a list of NORM_NACK_RANGES requests _and_ a
single NORM_NACK_ITEMS request are concatenated to illustrate the
possible content of a NORM_NACK message. Note that FEC coding block
erasure counts could also be provided in each case. However, the
erasure counts are not really necessary since the sender can easily
determine the erasure count while processing the NACK content.
However, the erasure count option may be useful for operation with
other FEC codes or for intermediate system purposes.
Adamson, et al. Experimental [Page 48]
RFC 3940 NORM Protocol November 2004
Example 1: NORM_NACK "nack_payload" for: Object 12, Coding Block 3,
Segments 2,5,8
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 1 | flags = 0x01 | length = 36 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Adamson, et al. Experimental [Page 49]
RFC 3940 NORM Protocol November 2004
Example 2: NORM_NACK "nack_payload" for: Object 18 Coding Block 6,
Segments 5, 6, 7, 8, 9, 10; and Object 19 NORM_INFO and Coding Block
1, segment 3
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 2 | flags = 0x01 | length = 24 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 10 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 1 | flags = 0x05 | length = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 19 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The NORM_ACK message is intended to be used primarily as part of NORM
congestion control operation and round-trip timing measurement. As
mentioned in the NORM_CMD(ACK_REQ) message description, the
acknowledgment type NORM_ACK_CC is provided for this purpose. The
generation of NORM_ACK(CC) messages for round-trip timing estimation
and congestion-control operation is described in Sections 5.5.1 and
5.5.2, respectively. However, some multicast applications may
benefit from some limited form of positive acknowledgment for certain
functions. A simple, scalable positive acknowledgment scheme is
defined in Section 5.5.3 that can be leveraged by protocol
implementations when appropriate. The NORM_CMD(FLUSH) may be used
for OPTIONAL collection of positive acknowledgment of reliable
reception to a certain "watermark" transmission point from specific
receivers using this mechanism. The NORM_ACK type NORM_ACK_FLUSH is
provided for this purpose and the format of the "nack_payload" for
this acknowledgment type is given below. Beyond that, a range of
Adamson, et al. Experimental [Page 50]
RFC 3940 NORM Protocol November 2004
application-defined "ack_type" values is provided for use at the NORM
application's discretion. Implementations making use of
application-defined positive acknowledgments may also make use the
"nack_payload" as needed, observing the constraint that the
"nack_payload" field size be limited to a maximum of the
NormSegmentSize for the sender to which the NORM_ACK is destined.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=5| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | ack_type | ack_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ack_payload (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_ACK Message Format
The NORM common message header fields serve their usual purposes.
The "server_id", "instance_id", and "grtt_response" fields serve the
same purpose as the corresponding fields in NORM_NACK messages. And
header extensions may be applied to support congestion control
feedback or other functions in the same manner.
The "ack_type" field indicates the nature of the NORM_ACK message.
This directly corresponds to the "ack_type" field of the
NORM_CMD(ACK_REQ) message to which this acknowledgment applies.
The "ack_id" field serves as a sequence number so that the sender can
verify that a NORM_ACK message received actually applies to a current
acknowledgment request. The "ack_id" field is not used in the case
of the NORM_ACK_CC and NORM_ACK_FLUSH acknowledgment types.
Adamson, et al. Experimental [Page 51]
RFC 3940 NORM Protocol November 2004
The "ack_payload" format is a function of the "ack_type". The
NORM_ACK_CC message has no attached content. Only the NORM_ACK
header applies. In the case of NORM_ACK_FLUSH, a specific
"ack_payload" format is defined:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id | reserved | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_ACK_FLUSH "ack_payload" Format
The "object_transport_id" and "fec_payload_id" are used by the
receiver to acknowledge applicable NORM_CMD(FLUSH) messages
transmitted by the sender identified by the "server_id" field.
The "ack_payload" of NORM_ACK messages for application-defined
"ack_type" values is specific to the application but is limited in
size to a maximum the NormSegmentSize of the sender referenced by the
"server_id".
Some additional message formats are defined for general purpose in
NORM multicast sessions whether the participant is acting as a sender
and/or receiver within the group.
This is an optional message generated by NORM participants. This
message could be used for periodic performance reports from receivers
in experimental NORM implementations. The format of this message is
currently undefined. Experimental NORM implementations may define
NORM_REPORT formats as needed for test purposes. These report
messages SHOULD be disabled for interoperability testing between
different NORM implementations.
This section describes the detailed interactions of senders and
receivers participating in a NORM session. A simple synopsis of
protocol operation is given here:
Adamson, et al. Experimental [Page 52]
RFC 3940 NORM Protocol November 2004
1) The sender periodically transmits NORM_CMD(CC) messages as needed
to initialize and collect roundtrip timing and congestion control
feedback from the receiver set.
2) The sender transmits an ordinal set of NormObjects segmented in
the form of NORM_DATA messages labeled with NormTransportIds and
logically identified with FEC encoding block numbers and symbol
identifiers. NORM_INFO messages may optionally precede the
transmission of data content for NORM transport objects.
3) As receivers detect missing content from the sender, they initiate
repair requests with NORM_NACK messages. Note the receivers track
the sender's most recent objectId::fecPayloadId transmit position
and NACK _only_ for content ordinally prior to that transmit
position. The receivers schedule random backoff timeouts before
generating NORM_NACK messages and wait an appropriate amount of
time before repeating the NORM_NACK if their repair request is not
satisfied.
4) The sender aggregates repair requests from the receivers and
logically "rewinds" its transmit position to send appropriate
repair messages. The sender sends repairs for the earliest
ordinal transmit position first and maintains this ordinal repair
transmission sequence. Previously untransmitted FEC parity
content for the applicable FEC coding block is used for repair
transmissions to the greatest extent possible. If the sender
exhausts its available FEC parity content on multiple repair
cycles for the same coding block, it resorts to an explicit repair
strategy (possibly using parity content) to complete repairs.
(The use of explicit repair is expected to be an exception in
general protocol operation, but the possibility does exist for
extreme conditions). The sender immediately assumes transmission
of new content once it has sent pending repairs.
5) The sender transmits NORM_CMD(FLUSH) messages when it reaches the
end of enqueued transmit content and pending repairs. Receivers
respond to the NORM_CMD(FLUSH) messages with NORM_NACK
transmissions (following the same suppression backoff timeout
strategy as for data) if they require further repair.
6) The sender transmissions are subject to rate control limits
determined by congestion control mechanisms. In the baseline
NORM-CC operation, each sender in a NormSession maintains its own
independent congestion control state. Receivers provide
congestion control feedback in NORM_NACK and NORM_ACK messages.
NORM_ACK feedback for congestion control purposes is governed
using a suppression mechanism similar to that for NORM_NACK
messages.
Adamson, et al. Experimental [Page 53]
RFC 3940 NORM Protocol November 2004
While this overall concept is relatively simple, there are details to
each of these aspects that need to be addressed for successful,
efficient, robust, and scalable NORM protocol operation.
Upon startup, the NORM sender immediately begins sending NORM_CMD(CC)
messages to collect round trip timing and other information from the
potential group. If NORM-CC congestion control operation is enabled,
the NORM-CC Rate header extension MUST be included in these messages.
Congestion control operation SHALL be observed at all times when
operating in the general Internet. Even if congestion control
operation is disabled at the sender, it may be desirable to use the
NORM_CMD(CC) messaging to collect feedback from the group using the
baseline NORM-CC feedback mechanisms. This proactive feedback
collection can be used to establish a GRTT estimate prior to data
transmission and potential NACK operation.
In some cases, applications may wish for the sender to also proceed
with data transmission immediately. In other cases, the sender may
wish to defer data transmission until it has received some feedback
or request from the receiver set indicating that receivers are indeed
present. Note, in some applications (e.g., web push), this
indication may come out-of-band with respect to the multicast session
via other means. As noted, the periodic transmission of NORM_CMD(CC)
messages may precede actual data transmission in order to have an
initial GRTT estimate.
With inclusion of the OPTIONAL NORM FEC Object Transmission
Information Header Extension, the NORM protocol sender message
headers can contain all information necessary to prepare receivers
for subsequent reliable reception. This includes FEC coding
parameters, the sender NormSegmentSize, and other information. If
this header extension is not used, it is presumed that receivers have
received the FEC Object Transmission Information via other means.
Additionally, applications may leverage the use of NORM_INFO messages
associated with the session data objects in the session to provide
application-specific context information for the session and data
being transmitted. These mechanisms allow for operation with minimal
pre-coordination among the senders and receivers.
The NORM sender begins segmenting application-enqueued data into
NORM_DATA segments and transmitting it to the group. The
segmentation algorithm is described in Section 5.1.1. The rate of
transmission is controlled via congestion control mechanisms or is a
fixed rate if desired for closed network operations. The receivers
participating in the multicast group provide feedback to the sender
as needed. When the sender reaches the end of data it has enqueued
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for transmission or any pending repairs, it transmits a series of
NORM_CMD(FLUSH) messages at a rate of one per 2*GRTT. Receivers may
respond to these NORM_CMD(FLUSH) messages with additional repair
requests. A protocol parameter "NORM_ROBUST_FACTOR" determines the
number of flush messages sent. If receivers request repair, the
repair is provided and flushing occurs again at the end of repair
transmission. The sender may attach an OPTIONAL "acking_node_list"
to NORM_CMD(FLUSH) containing the NormNodeIds for receivers from
which it expects explicit positive acknowledgment of reception. The
NORM_CMD(FLUSH) message may be also used for this optional function
any time prior to the end of data enqueued for transmission with the
NORM_CMD(FLUSH) messages multiplexed with ongoing data transmissions.
The OPTIONAL NORM positive acknowledgment procedure is described in
Section 5.5.3.
NORM senders and receivers must use a common algorithm for logically
segmenting transport data into FEC encoding blocks and symbols so
that appropriate NACKs can be constructed to request repair of
missing data. NORM FEC coding blocks are comprised of multi-byte
symbols which are transmitted in the payload of NORM_DATA messages.
Each NORM_DATA message contains one source or encoding symbol and the
NormSegmentSize sender parameter defines the maximum symbol size in
bytes. The FEC encoding type and associated parameters govern the
source block size (number of source symbols per coding block). NORM
senders and receivers use these FEC parameters, along with the
NormSegmentSize and transport object size to compute the source block
structure for transport objects. These parameters are provided in
the FEC Transmission Information for each object. The algorithm
given below is used to compute a source block structure such that all
source blocks are as close to being equal length as possible. This
helps avoid the performance disadvantages of "short" FEC blocks.
Note this algorithm applies only to the statically-sized
NORM_OBJECT_DATA and NORM_OBJECT_FILE transport object types where
the object size is fixed and predetermined. For NORM_OBJECT_STREAM
objects, the object is segmented according to the maximum source
block length given in the FEC Transmission Information, unless the
FEC Payload ID indicates an alternative size for a given block.
The NORM block segmentation algorithm is defined as follows. For a
transport object of a given length (L_obj) in bytes, a first number
of FEC source blocks (N_large) is delineated of a larger block size
(B_large), and a second number of source blocks (N_small) is
delineated of a smaller block size (B_small). Given the maximum FEC
source block size (B_max) and the sender's NormSegmentSize, the block
segmentation for a given NORM transport object is determined as
follows:
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Inputs:
B_max = Maximum source block length (i.e., maximum number of source
symbols per source block)
L_sym = Encoding symbol length in bytes (i.e., NormSegmentSize)
L_obj = Object length in bytes
Outputs:
N_total = The total number of source blocks into which the transport
object is partitioned.
N_large = Number of larger source blocks (first set of blocks)
B_large = Size (in encoding symbols) of the larger source blocks
N_small = Number of smaller source blocks (second set of blocks)
B_small = Size (in encoding symbols) of the smaller source blocks
L_final = Length (in bytes) of the last source symbol of the last
source block (All other symbols are of length L_sym).
Algorithm:
1) The total number of source symbols in the transport object is
computed as: S_total = L_obj/L_sym [rounded up to the nearest
integer]
2) The transport object is partitioned into N_total source blocks,
where: N_total = S_total/B_max [rounded up to the nearest
integer]
3) The average length of a source block is computed as: B_ave =
S_total/N_total (this may be non-integer)
4) The size of the first set of larger blocks is computed as:
B_large = B_ave [rounded up to the nearest integer] (Note it will
always be the case that B_large <= B_max)
5) The size of the second set of smaller blocks is computed as:
B_small = B_ave [rounded down to the nearest integer] (Note if
B_ave is an integer B_small = B_large; otherwise B_small = B_large
- 1)
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6) The fractional part of B_ave is computed as: B_fraction = B_ave -
B_small
7) The number of larger source blocks is computed as: N_large =
B_fraction * N_total (Note N_large is an integer in the range 0
through N_total - 1)
8) The number of smaller source blocks is computed as: N_small =
N_total - N_large
9) Each of the first N_large source blocks consists of B_large source
symbols. Each of the remaining N_small source blocks consists of
B_small source symbols. All symbols are L_sym bytes in length
except for the final source symbol of the final source block which
is of length (in bytes):
L_final = L_obj - (N_large*B_large + N_small*B_small - 1) * L_sym
The NORM protocol is designed such that receivers may join and leave
the group at will. However, some applications may be constrained
such that receivers need to be members of the group prior to start of
data transmission. NORM applications may use different policies to
constrain the impact of new receivers joining the group in the middle
of a session. For example, a useful implementation policy is for new
receivers joining the group to limit or avoid repair requests for
transport objects already in progress. The NORM sender
implementation may wish to impose additional constraints to limit the
ability of receivers to disrupt reliable multicast performance by
joining, leaving, and rejoining the group often. Different receiver
"join policies" may be appropriate for different applications and/or
scenarios. For general purpose operation, default policy where
receivers are allowed to request repair only for coding blocks with a
NormTransportId and FEC coding block number greater than or equal to
the first non-repair NORM_DATA or NORM_INFO message received upon
joining the group is RECOMMENDED. For objects of type
NORM_OBJECT_STREAM it is RECOMMENDED that the join policy constrain
receivers to start reliable reception at the current FEC coding block
for which non-repair content is received.
When the receiver detects it is missing data from a sender's NORM
transmissions, it initiates its NACKing procedure. The NACKing
procedure SHALL be initiated _only_ at FEC coding block boundaries,
NormObject boundaries, and upon receipt of a NORM_CMD(FLUSH) message.
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The NACKing procedure begins with a random backoff timeout. The
duration of the backoff timeout is chosen using the "RandomBackoff"
algorithm described in the NORM Building Block document [4] using
(Ksender*GRTTsender) for the "maxTime" parameter and the sender
advertised group size (GSIZEsender) as the "groupSize" parameter.
NORM senders provide values for GRTTsender, Ksender and GSIZEsender
via the "grtt", "backoff", and "gsize" fields of transmitted
messages. The GRTTsender value is determined by the sender based on
feedback it has received from the group while the Ksender and
GSIZEsender values may determined by application requirements and
expectations or ancillary information. The backoff factor "Ksender"
MUST be greater than one to provide for effective feedback
suppression. A value of K = 4 is RECOMMENDED for the Any Source
Multicast (ASM) model while a value of K = 6 is RECOMMENDED for
Single Source Multicast (SSM) operation.
Thus:
T_backoff = RandomBackoff(Ksender*GRTTsender, GSIZEsender)
To avoid the possibility of NACK implosion in the case of sender or
network failure during SSM operation, the receiver SHALL
automatically suppress its NACK and immediately enter the "holdoff"
period described below when T_backoff is greater than (Ksender-
1)*GRTTsender. Otherwise, the backoff period is entered and the
receiver MUST accumulate external pending repair state from NORM_NACK
messages and NORM_CMD(REPAIR_ADV) messages received. At the end of
the backoff time, the receiver SHALL generate a NORM_NACK message
only if the following conditions are met:
1) The sender's current transmit position (in terms of
objectId::fecPayloadId) exceeds the earliest repair position of
the receiver.
2) The repair state accumulated from NORM_NACK and
NORM_CMD(REPAIR_ADV) messages do not equal or supersede the
receiver's repair needs up to the sender transmission position at
the time the NACK procedure (backoff timeout) was initiated.
If these conditions are met, the receiver immediately generates a
NORM_NACK message when the backoff timeout expires. Otherwise, the
receiver's NACK is considered to be "suppressed" and the message is
not sent. At this time, the receiver begins a "holdoff" period
during which it constrains itself to not reinitiate the NACKing
process. The purpose of this timeout is to allow the sender worst-
case time to respond to the repair needs before the receiver requests
repair again. The value of this "holdoff" timeout (T_rcvrHoldoff)
as described in [4] is:
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T_rcvrHoldoff =(Ksender+2)*GRTTsender
The NORM_NACK message contains repair request content beginning with
lowest ordinal repair position of the receiver up through the coding
block prior to the most recently heard ordinal transmission position
for the sender. If the size of the NORM_NACK content exceeds the
sender's NormSegmentSize, the NACK content is truncated so that the
receiver only generates a single NORM_NACK message per NACK cycle for
a given sender. In summary, a single NACK message is generated
containing the receiver's lowest ordinal repair needs.
For each partially-received FEC coding block requiring repair, the
receiver SHALL, on its _first_ repair attempt for the block, request
the parity portion of the FEC coding block beginning with the lowest
ordinal _parity_ "encoding_symbol_id" (i.e., "encoding_symbol_id" =
"source_block_len") and request the number of FEC symbols
corresponding to its data segment erasure count for the block. On
_subsequent_ repair cycles for the same coding block, the receiver
SHALL request only those repair symbols from the first set it has not
yet received up to the remaining erasure count for that applicable
coding block. Note that the sender may have provided other
different, additional parity segments for other receivers that could
also be used to satisfy the local receiver's erasure-filling needs.
In the case where the erasure count for a partially-received FEC
coding block exceeds the maximum number of parity symbols available
from the sender for the block (as indicated by the NORM_DATA
"fec_num_parity" field), the receiver SHALL request all available
parity segments plus the ordinally highest missing data segments
required to satisfy its total erasure needs for the block. The goal
of this strategy is for the overall receiver set to request a lowest
common denominator set of repair symbols for a given FEC coding
block. This allows the sender to construct the most efficient repair
transmission segment set and enables effective NACK suppression among
the receivers even with uncorrelated packet loss. This approach also
requires no synchronization among the receiver set in their repair
requests for the sender.
For FEC coding blocks or NormObjects missed in their entirety, the
NORM receiver constructs repair requests with NORM_NACK_BLOCK or
NORM_NACK_OBJECT flags set as appropriate. The request for
retransmission of NORM_INFO is accomplished by setting the
NORM_NACK_INFO flag in a corresponding repair request.
The principle goal of the sender is to make forward progress in the
transmission of data its application has enqueued. However, the
sender must occasionally "rewind" its logical transmission point to
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satisfy the repair needs of receivers who have NACKed. Aggregation
of multiple NACKs is used to determine an optimal repair strategy
when a NACK event occurs. Since receivers initiate the NACK process
on coding block or object boundaries, there is some loose degree of
synchronization of the repair process even when receivers experience
uncorrelated data loss.
When a sender is in its normal state of transmitting new data and
receives a NACK, it begins a procedure to accumulate NACK repair
state from NORM_NACK messages before beginning repair transmissions.
Note that this period of aggregating repair state does _not_
interfere with its ongoing transmission of new data.
As described in [4], the period of time during which the sender
aggregates NORM_NACK messages is equal to:
T_sndrAggregate = (Ksender+1)*GRTT
where "Ksender" is the same backoff scaling value used by the
receivers, and "GRTT" is the sender's current estimate of the group's
greatest round-trip time.
When this period ends, the sender "rewinds" by incorporating the
accumulated repair state into its pending transmission state and
begins transmitting repair messages. After pending repair
transmissions are completed, the sender continues with new
transmissions of any enqueued data. Also, at this point in time, the
sender begins a "holdoff" timeout during which time the sender
constrains itself from initiating a new repair aggregation cycle,
even if NORM_NACK messages arrive. As described in [4], the value of
this sender "holdoff" period is:
T_sndrHoldoff = (1*GRTT)
If additional NORM_NACK messages are received during this sender
"holdoff" period, the sender will immediately incorporate these "late
messages" into its pending transmission state ONLY if the NACK
content is ordinally greater than the sender's current transmission
position. This "holdoff" time allows worst case time for the sender
to propagate its current transmission sequence position to the group,
thus avoiding redundant repair transmissions. After the holdoff
timeout expires, a new NACK accumulation period can be begun (upon
arrival of a NACK) in concert with the pending repair and new data
transmission. Recall that receivers are not to initiate the NACK
repair process until the sender's logical transmission position
exceeds the lowest ordinal position of their repair needs. With the
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new NACK aggregation period, the sender repeats the same process of
incorporating accumulated repair state into its transmission plan and
subsequently "rewinding" to transmit the lowest ordinal repair data
when the aggregation period expires. Again, this is conducted in
concert with ongoing new data and/or pending repair transmissions.
The NORM sender should leverage transmission of FEC parity content
for repair to the greatest extent possible. Recall that the
receivers use a strategy to request a lowest common denominator of
explicit repair (including parity content) in the formation of their
NORM_NACK messages. Before falling back to explicitly satisfying
different receivers' repair needs, the sender can make use of the
general erasure-filling capability of FEC-generated parity segments.
The sender can determine the maximum erasure filling needs for
individual FEC coding blocks from the NORM_NACK messages received
during the repair aggregation period. Then, if the sender has a
sufficient number (less than or equal to the maximum erasure count)
of previously unsent parity segments available for the applicable
coding blocks, the sender can transmit these in lieu of the specific
packets the receiver set has requested. Only after exhausting its
supply of "fresh" (unsent) parity segments for a given coding block
should the sender resort to explicit transmission of the receiver
set's repair needs. In general, if a sufficiently powerful FEC code
is used, the need for explicit repair will be an exception, and the
fulfillment of reliable multicast can be accomplished quite
efficiently. However, the ability to resort to explicit repair
allows the protocol to be reliable under even very extreme
circumstances.
NORM_DATA messages sent as repair transmissions SHALL be flagged with
the NORM_FLAG_REPAIR flag. This allows receivers to obey any
policies that limit new receivers from joining the reliable
transmission when only repair transmissions have been received.
Additionally, the sender SHOULD additionally flag NORM_DATA
transmissions sent as explicit repair with the NORM_FLAG_EXPLICIT
flag.
Although NORM end system receivers do not make use of the
NORM_FLAG_EXPLICIT flag, this message transmission status could be
leveraged by intermediate systems wishing to "assist" NORM protocol
performance. If such systems are properly positioned with respect to
reciprocal reverse-path multicast routing, they need to sub-cast only
a sufficient count of non-explicit parity repairs to satisfy a
multicast routing sub-tree's erasure filling needs for a given FEC
coding block. When the sender has resorted to explicit repair, then
the intermediate systems should sub-cast all of the explicit repair
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packets to those portions of the routing tree still requiring repair
for a given coding block. Note the intermediate systems will be
required to conduct repair state accumulation for sub-routes in a
manner similar to the sender's repair state accumulation in order to
have sufficient information to perform the sub-casting.
Additionally, the intermediate systems could perform additional
NORM_NACK suppression/aggregation as it conducts this repair state
accumulation for NORM repair cycles. The detail of this type of
operation are beyond the scope of this document, but this information
is provided for possible future consideration.
If the sender receives a NORM_NACK message for repair of data it is
no longer supporting, the sender generates a NORM_CMD(SQUELCH)
message to advertise its repair window and squelch any receivers from
additional NACKing of invalid data. The transmission rate of
NORM_CMD(SQUELCH) messages is limited to once per 2*GRTT. The
"invalid_object_list" (if applicable) of the NORM_CMD(SQUELCH)
message SHALL begin with the lowest "object_transport_id" from the
invalid NORM_NACK messages received since the last NORM_CMD(SQUELCH)
transmission. Lower ordinal invalid "object_transport_ids" should be
included only while the NORM_CMD(SQUELCH) payload is less than the
sender's NormSegmentSize parameter.
When a NORM sender receives NORM_NACK messages from receivers via
unicast transmission, it uses NORM_CMD(REPAIR_ADV) messages to
advertise its accumulated repair state to the receiver set since the
receiver set is not directly sharing their repair needs via multicast
communication. The NORM_CMD(REPAIR_ADV) message is multicast to the
receiver set by the sender. The payload portion of this message has
content in the same format as the NORM_NACK receiver message payload.
Receivers are then able to perform feedback suppression in the same
manner as with NORM_NACK messages directly received from other
receivers. Note the sender does not merely retransmit NACK content
it receives, but instead transmits a representation of its aggregated
repair state. The transmission of NORM_CMD(REPAIR_ADV) messages are
subject to the sender transmit rate limit and NormSegmentSize
limitation. When the NORM_CMD(REPAIR_ADV) message is of maximum
size, receivers SHALL consider the maximum ordinal transmission
position value embedded in the message as the senders "current"
transmission position and implicitly suppress requests for ordinally
higher repair. For congestion control operation, the sender may also
need to provide information so that dynamic congestion control
feedback can be suppressed as needed among receivers. This document
specifies the NORM-CC Feedback Header Extension that is applied for
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baseline NORM-CC operation. If other congestion control mechanisms
are used within a NORM implementation, other header extensions may be
defined. Whatever content format is used for this purpose should
ensure that maximum possible suppression state is conveyed to the
receiver set.
In addition to the principal function of data content transmission
and repair, there are some other protocol mechanisms that help NORM
to adapt to network conditions and play fairly with other coexistent
protocols.
For NORM receivers to appropriately scale backoff timeouts and the
senders to use proper corresponding timeouts, the participants must
agree on a common timeout basis. Each NORM sender monitors the
round-trip time of active receivers and determines the group greatest
round-trip time (GRTT). The sender advertises this GRTT estimate in
every message it transmits so that receivers have this value
available for scaling their timers. To measure the current GRTT, the
sender periodically sends NORM_CMD(CC) messages that contain a
locally generated timestamp. Receivers are expected to record this
timestamp along with the time the NORM_CMD(CC) message is received.
Then, when the receivers generate feedback messages to the sender, an
adjusted version of the sender timestamp is embedded in the feedback
message (NORM_NACK or NORM_ACK). The adjustment adds the amount of
time the receiver held the timestamp before generating its response.
Upon receipt of this adjusted timestamp, the sender is able to
calculate the round-trip time to that receiver.
The round-trip time for each receiver is fed into an algorithm that
weights and smoothes the values for a conservative estimate of the
GRTT. The algorithm and methodology are described in the NORM
Building Block document [4] in the section entitled "One-to-Many
Sender GRTT Measurement". A conservative estimate helps feedback
suppression at a small cost in overall protocol repair delay. The
sender's current estimate of GRTT is advertised in the "grtt" field
found in all NORM sender messages. The advertised GRTT is also
limited to a minimum of the nominal inter-packet transmission time
given the sender's current transmission rate and system clock
granularity. The reason for this additional limit is to keep the
receiver somewhat "event driven" by making sure the sender has had
adequate time to generate any response to repair requests from
receivers given transmit rate limitations due to congestion control
or configuration.
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When the NORM-CC Rate header extension is present in NORM_CMD(CC)
messages, the receivers respond to NORM_CMD(CC) messages as described
in Section 5.5.2, "NORM Congestion Control Operation". The
NORM_CMD(CC) messages are periodically generated by the sender as
described for congestion control operation. This provides for
proactive, but controlled, feedback from the group in the form of
NORM_ACK messages. This provides for GRTT feedback even if no
NORM_NACK messages are being sent. If operating without congestion
control in a closed network, the NORM_CMD(CC) messages may be sent
periodically without the NORM-CC Rate header extension. In this
case, receivers will only provide GRTT measurement feedback when
NORM_NACK messages are generated since no NORM_ACK messages are
generated. In this case, the NORM_CMD(CC) messages may be sent less
frequently, perhaps as little as once per minute, to conserve network
capacity. Note that the NORM-CC Rate header extension may also be
used proactively solicit RTT feedback from the receiver group per
congestion control operation even though the sender may not be
conducting congestion control rate adjustment. NORM operation
without congestion control should be considered only in closed
networks.
This section describes baseline congestion control operation for the
NORM protocol (NORM-CC). The supporting NORM message formats and
approach described here are an adaptation of the equation-based TCP-
Friendly Multicast Congestion Control (TFMCC) approach described in
[19]. This congestion control scheme is REQUIRED for operation
within the general Internet unless the NORM implementation is adapted
to use another IETF-sanctioned reliable multicast congestion control
mechanism (e.g., PGMCC [20]). With this TFMCC-based approach, the
transmissions of NORM senders are controlled in a rate-based manner
as opposed to window-based congestion control algorithms as in TCP.
However, it is possible that the NORM protocol message set may
alternatively be used to support a window-based multicast congestion
control scheme such as PGMCC. The details of that alternative may be
described separately or in a future revision of this document. In
either case (rate-based TFMCC or window-based PGMCC), successful
control of sender transmission depends upon collection of sender-to-
receiver packet loss estimates and RTTs to identify the congestion
control bottleneck path(s) within the multicast topology and adjust
the sender rate accordingly. The receiver with loss and RTT
estimates that correspond to the lowest result transmission rate is
identified as the "current limiting receiver" (CLR).
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As described in [21], a steady-state sender transmission rate, to be
"friendly" with competing TCP flows can be calculated as:
S
Rsender = --------------------------------------------------------------
tRTT * (sqrt((2/3)*p) + 12 * sqrt((3/8)*p) * p *
(1 + 32*(p^2)))
where
S = Nominal transmitted packet size. (In NORM, the "nominal"
packet size can be determined by the sender as an
exponentially weighted moving average (EWMA) of transmitted
packet sizes to account for variable message sizes).
tRTT = The RTT estimate of the current "current limiting receiver"
(CLR).
p = The loss event fraction of the CLR.
To support congestion control feedback collection and operation, the
NORM sender periodically transmits NORM_CMD(CC) command messages.
NORM_CMD(CC) messages are multiplexed with NORM data and repair
transmissions and serve several purposes:
1) Stimulate explicit feedback from the general receiver set to
collect congestion control information.
2) Communicate state to the receiver set on the sender's current
congestion control status including details of the CLR.
3) Initiate rapid (immediate) feedback from the CLR in order to
closely track the dynamics of congestion control for that current
"worst path" in the group multicast topology.
The format of the NORM_CMD(CC) message is describe in Section 4.2.3
of this document. The NORM_CMD(CC) message contains information to
allow measurement of RTTs, to inform the group of the congestion
control CLR, and to provide feedback of individual RTT measurements
to the receivers in the group. The NORM_CMD(CC) also provides for
exciting feedback from OPTIONAL "potential limiting receiver" (PLR)
nodes that may be determined administratively or possibly
algorithmically based on congestion control feedback. PLR nodes are
receivers that have been identified to have potential for (perhaps
soon) becoming the CLR and thus immediate, up-to-date feedback is
beneficial for congestion control performance. The details of PLR
selection are not discussed in this document.
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The NORM_CMD(CC) message is transmitted periodically by the sender
along with its normal data transmission. Note that the repeated
transmission of NORM_CMD(CC) messages may be initiated some time
before transmission of user data content at session startup. This
may be done to collect some estimation of the current state of the
multicast topology with respect to group and individual RTT and
congestion control state.
A NORM_CMD(CC) message is immediately transmitted at sender startup.
The interval of subsequent NORM_CMD(CC) message transmission is
determined as follows:
1) By default, the interval is set according to the current sender
GRTT estimate. A startup GRTT of 0.5 seconds is recommended when
no feedback has yet been received from the group.
2) If a CLR has been identified (based on previous receiver
feedback), the interval is the RTT between the sender and CLR.
3) Additionally, if the interval of nominal data message transmission
is greater than the GRTT or RTT_clr interval, the NORM_CMD(CC)
interval is set to this greater value. This ensures that the
transmission of this control message is not done to the exclusion
of user data transmission.
The NORM_CMD(CC) "cc_sequence" field is incremented with each
transmission of a NORM_CMD(CC) command. The greatest "cc_sequence"
recently received by receivers is included in their feedback to the
sender. This allows the sender to determine the "age" of feedback to
assist in congestion avoidance.
The NORM-CC Rate Header Extension is applied to the NORM_CMD(CC)
message and the sender advertises its current transmission rate in
the "send_rate" field. The rate information is used by receivers to
initialize loss estimation during congestion control startup or
restart.
The "cc_node_list" contains a list of entries identifying receivers
and their current congestion control state (status "flags", "rtt" and
"loss" estimates). The list may be empty if the sender has not yet
received any feedback from the group. If the sender has received
feedback, the list will minimally contain an entry identifying the
CLR. A NORM_FLAG_CC_CLR flag value is provided for the "cc_flags"
field to identify the CLR entry. It is RECOMMENDED that the CLR
entry be the first in the list for implementation efficiency.
Additional entries in the list are used to provide sender-measured
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individual RTT estimates to receivers in the group. The number of
additional entries in this list is dependent upon the percentage of
control traffic the sender application is willing to send with
respect to user data message transmissions. More entries in the list
may allow the sender to be more responsive to congestion control
dynamics. The length of the list may be dynamically determined
according to the current transmission rate and scheduling of
NORM_CMD(CC) messages. The maximum length of the list corresponds to
the sender's NormSegmentSize parameter for the session. The
inclusion of additional entries in the list based on receiver
feedback are prioritized with following rules:
1) Receivers that have not yet been provided RTT feedback get first
priority. Of these, those with the greatest loss fraction receive
precedence for list inclusion.
2) Secondly, receivers that have previously been provided RTT are
included with receivers yielding the lowest calculated congestion
rate getting precedence.
There are "cc_flag" values in addition to NORM_FLAG_CC_CLR that are
used for other congestion control functions. The NORM_FLAG_CC_PLR
flag value is used to mark additional receivers from that the sender
would like to have immediate, non-suppressed feedback. These may be
receivers that the sender algorithmically identified as potential
future CLRs or that have been pre-configured as potential congestion
control points in the network. The NORM_FLAG_CC_RTT indicates the
validity of the "cc_rtt" field for the associated receiver node.
Normally, this flag will be set since the receivers in the list will
typically be receivers from which the sender has received feedback.
However, in the case that the NORM sender has been pre-configured
with a set of PLR nodes, feedback from those receivers may not yet
have been collected and thus the "cc_rtt" and "cc_rate" fields do not
contain valid values when this flag is not set.
Receivers explicitly respond to NORM_CMD(CC) messages in the form of
a NORM_ACK(RTT) message. The goal of the congestion control feedback
is to determine the receivers with the lowest congestion control
rates. Receivers that are marked as CLR or PLR nodes in the
NORM_CMD(CC) "cc_node_list" immediately provide feedback in the form
of a NORM_ACK to this message. When a NORM_CMD(CC) is received,
non-CLR or non-PLR nodes initiate random feedback backoff timeouts
similar to that used when the receiver initiates a repair cycle (see
Section 5.3) in response to detection of data loss. The backoff
timeout for the congestion control response is generated as follows:
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RFC 3940 NORM Protocol November 2004
T_backoff = RandomBackoff(K*GRTTsender, GSIZEsender)
The "RandomBackoff()" algorithm provides a truncated exponentially
distributed random number and is described in the NORM Building Block
document [4]. The same backoff factor K = Ksender MAY be used as
with NORM_NACK suppression. However, in cases where the application
purposefully specifies a very small Ksender backoff factor to
minimize the NACK repair process latency (trading off group size
scalability), it may still be desirable to maintain a larger backoff
factor for congestion control feedback, since there may often be a
larger volume of congestion control feedback than NACKs in many cases
and congestion control feedback latency may be tolerable where
reliable delivery latency is not. As previously noted, a backoff
factor value of K = 4 is generally recommended for ASM operation and
K = 6 for SSM operation. A receiver SHALL cancel the backoff timeout
and thus its pending transmission of a NORM_ACK(RTT) message under
the following conditions:
1) The receiver generates another feedback message (NORM_NACK or
other NORM_ACK) before the congestion control feedback timeout
expires,
2) A NORM_CMD(CC) or other receiver feedback with an ordinally
greater "cc_sequence" field value is received before the
congestion control feedback timeout expires (this is similar to
the TFMCC feedback round number),
3) When the T_backoff is greater than 1*GRTT. This prevents NACK
implosion in the event of sender or network failure,
4) "Suppressing" congestion control feedback is heard from another
receiver (in a NORM_ACK or NORM_NACK) or via a
NORM_CMD(REPAIR_ADV) message from the sender. The local
receiver's feedback is "suppressed" if the rate of the competing
feedback (Rfb) is sufficiently close to or less than the local
receiver's calculated rate (Rcalc). The local receiver's feedback
is canceled when:
Rcalc > (0.9 * Rfb)
Also note receivers that have not yet received an RTT measurement
from the sender are suppressed only by other receivers that have
not yet measured RTT. Additionally, receivers whose RTT estimate
has "aged" considerably (i.e., they haven't been included in the
NORM_CMD(CC) "cc_node_list" in a long time) may wish to compete as
a receiver with no prior RTT measurement after some expiration
period.
Adamson, et al. Experimental [Page 68]
RFC 3940 NORM Protocol November 2004
When the backoff timer expires, the receiver SHALL generate a
NORM_ACK(RTT) message to provide feedback to the sender and group.
This message may be multicast to the group for most effective
suppression in ASM topologies or unicast to the sender depending upon
how the NORM protocol is deployed and configured.
Whenever any feedback is generated (including this NORM_ACK(RTT)
message), receivers include an adjusted version of the sender
timestamp from the most recently received NORM_CMD(CC) message and
the "cc_sequence" value from that command in the applicable NORM_ACK
or NORM_NACK message fields. For NORM-CC operation, any generated
feedback message SHALL also contain the NORM-CC Feedback header
extension. The receiver provides its current "cc_rate" estimate,
"cc_loss" estimate, "cc_rtt" if known, and any applicable "cc_flags"
via this header extension.
During slow start (when the receiver has not yet detected loss from
the sender), the receiver uses a value equal to two times its
measured rate from the sender in the "cc_rate" field. For steady-
state congestion control operation, the receiver "cc_rate" value is
from the equation-based value using its current loss event estimate
and sender<->receiver RTT information. (The GRTT is used when the
receiver has not yet measured its individual RTT).
The "cc_loss" field value reflects the receiver's current loss event
estimate with respect to the sender in question.
When the receiver has a valid individual RTT measurement, it SHALL
include this value in the "cc_rtt" field. The NORM_FLAG_CC_RTT MUST
be set when the "cc_rtt" field is valid.
After a congestion control feedback message is generated or when the
feedback is suppressed, a non-CLR receiver begins a "holdoff" timeout
period during which it will restrain itself from providing congestion
control feedback, even if NORM_CMD(CC) messages are received from the
sender (unless the receive becomes marked as a CLR or PLR node). The
value of this holdoff timeout (T_ccHoldoff) period is:
T_ccHoldoff = (K*GRTT)
Thus, non-CLR receivers are constrained to providing explicit
congestion control feedback once per K*GRTT intervals. Note,
however, that as the session progresses, different receivers will be
responding to different NORM_CMD(CC) messages and there will be
relatively continuous feedback of congestion control information
while the sender is active.
Adamson, et al. Experimental [Page 69]
RFC 3940 NORM Protocol November 2004
During steady-state operation, the sender will directly adjust its
transmission rate to the rate indicated by the feedback from its
currently selected CLR. As noted in [19], the estimation of
parameters (loss and RTT) for the CLR will generally constrain the
rate changes possible within acceptable bounds. For rate increases,
the sender SHALL observe a maximum rate of increase of one packet per
RTT at all times during steady-state operation.
The sender processes congestion control feedback from the receivers
and selects the CLR based on the lowest rate receiver. Receiver
rates are either determined directly from the slow start "cc_rate"
provided by the receiver in the NORM-CC Feedback header extension or
by performing the equation-based calculation using individual RTT and
loss estimates ("cc_loss") as feedback is received.
The sender can calculate a current RTT for a receiver (RTT_rcvrNew)
using the "grtt_response" timestamp included in feedback messages.
When the "cc_rtt" value in a response is not valid, the sender simply
uses this RTT_rcvrNew value as the receiver's current RTT (RTT_rcvr).
For non-CLR and non-PLR receivers, the sender can use the "cc_rtt"
value provided in the NORM-CC Feedback header extension as the
receiver's previous RTT measurement (RTT_rcvrPrev) to smooth
according to:
RTT_rcvr = 0.5 * RTT_rcvrPrev + 0.5 * RTT_rcvrNew
For CLR receivers where feedback is received more regularly, the
sender SHOULD maintain a more smoothed RTT estimate upon new feedback
from the CLR where:
RTT_clr = 0.9 * RTT_clr + 0.1 * RTT_clrNew
"RTT_clrNew" is the new RTT calculated from the timestamp in the
feedback message received from the CLR. The RTT_clr is initialized
to RTT_clrNew on the first feedback message received. Note that the
same procedure is observed by the sender for PLR receivers and that
if a PLR is "promoted" to CLR status, the smoothed estimate can be
continued.
There are some additional periods besides steady-state operation that
need to be considered in NORM-CC operation. These periods are:
1) during session startup,
2) when no feedback is received from the CLR, and
Adamson, et al. Experimental [Page 70]
RFC 3940 NORM Protocol November 2004
3) when the sender has a break in data transmission.
During session startup, the congestion control operation SHALL
observe a "slow start" procedure to quickly approach its fair
bandwidth share. An initial sender startup rate is assumed where:
Rinitial = MIN(NormSegmentSize / GRTT, NormSegmentSize) bytes/second.
The rate is increased only when feedback is received from the
receiver set. The "slow start" phase proceeds until any receiver
provides feedback indicating that loss has occurred. Rate increase
during slow start is applied as:
Rnew = Rrecv_min
where "Rrecv_min" is the minimum reported receiver rate in the
"cc_rate" field of congestion control feedback messages received from
the group. Note that during "slow start", receivers use two times
their measured rate from the sender in the "cc_rate" field of their
feedback. Rate increase adjustment is limited to once per GRTT
during slow start.
If the CLR or any receiver intends to leave the group, it will set
the NORM_FLAG_CC_LEAVE in its congestion control feedback message as
an indication that the sender should not select it as the CLR. When
the CLR changes to a lower rate receiver, the sender should
immediately adjust to the new lower rate. The sender is limited to
increasing its rate at one additional packet per RTT towards any new,
higher CLR rate.
The sender should also track the "age" of the feedback it has
received from the CLR by comparing its current "cc_sequence" value
(Seq_sender) to the last "cc_sequence" value received from the CLR
(Seq_clr). As the "age" of the CLR feedback increases with no new
feedback, the sender SHALL begin reducing its rate once per RTT_clr
as a congestion avoidance measure.
The following algorithm is used to determine the decrease in sender
rate (Rsender bytes/sec) as the CLR feedback, unexpectedly,
excessively ages:
Age = Seq_sender - Seq_clr;
if (Age > 4) Rsender = Rsender * 0.5;
This rate reduction is limited to the lower bound on NORM
transmission rate. After NORM_ROBUST_FACTOR consecutive NORM_CMD(CC)
rounds without any feedback from the CLR, the sender SHOULD assume
the CLR has left the group and pick the receiver with the next lowest
Adamson, et al. Experimental [Page 71]
RFC 3940 NORM Protocol November 2004
rate as the new CLR. Note this assumes that the sender does not have
explicit knowledge that the CLR intentionally left the group. If no
receiver feedback is received, the sender MAY wish to withhold
further transmissions of NORM_DATA segments and maintain NORM_CMD(CC)
transmissions only until feedback is detected. After such a CLR
timeout, the sender will be transmitting with a minimal rate and
should return to slow start as described here for a break in data
transmission.
When the sender has a break in its data transmission, it can continue
to probe the group with NORM_CMD(CC) messages to maintain RTT
collection from the group. This will enable the sender to quickly
determine an appropriate CLR upon data transmission restart.
However, the sender should exponentially reduce its target rate to be
used for transmission restart as time since the break elapses. The
target rate SHOULD be recalculated once per RTT_clr as:
Rsender = Rsender * 0.5;
If the minimum NORM rate is reached, the sender should set the
NORM_FLAG_START flag in its NORM_CMD(CC) messages upon restart and
the group should observer "slow start" congestion control procedures
until any receiver experiences a new loss event.
NORM provides options for the source application to request positive
acknowledgment (ACK) of NORM_CMD(FLUSH) and NORM_CMD(ACK_REQ)
messages from members of the group. There are some specific
acknowledgment requests defined for the NORM protocol and a range of
acknowledgment request types that are left to be defined by the
application. One predefined acknowledgment type is the
NORM_ACK_FLUSH type. This acknowledgment is used to determine if
receivers have achieved completion of reliable reception up through a
specific logical transmission point with respect to the sender's
sequence of transmission. The NORM_ACK_FLUSH acknowledgment may be
used to assist in application flow control when the sender has
information on a portion of the receiver set. Another predefined
acknowledgment type is NORM_ACK(CC), which is used to explicitly
provide congestion control feedback in response to NORM_CMD(CC)
messages transmitted by the sender for NORM-CC operation. Note the
NORM_ACK(CC) response does NOT follow the positive acknowledgment
procedure described here. The NORM_CMD(ACK_REQ) and NORM_ACK
messages contain an "ack_type" field to identify the type of
acknowledgment requested and provided. A range of "ack_type" values
is provided for application-defined use. While the application is
responsible for initiating the acknowledgment request and interprets
application-defined "ack_type" values, the acknowledgment procedure
Adamson, et al. Experimental [Page 72]
RFC 3940 NORM Protocol November 2004
SHOULD be conducted within the protocol implementation to take
advantage of timing and transmission scheduling information available
to the NORM transport.
The NORM positive acknowledgment procedure uses polling by the sender
to query the receiver group for response. Note this polling
procedure is not intended to scale to very large receiver groups, but
could be used in large group setting to query a critical subset of
the group. Either the NORM_CMD(ACK_REQ), or when applicable, the
NORM_CMD(FLUSH) message is used for polling and contains a list of
NormNodeIds for receivers that should respond to the command. The
list of receivers providing acknowledgment is determined by the
source application with "a priori" knowledge of participating nodes
or via some other application-level mechanism.
The ACK process is initiated by the sender that generates
NORM_CMD(FLUSH) or NORM_CMD(ACK_REQ) messages in periodic "rounds".
For NORM_ACK_FLUSH requests, the NORM_CMD(FLUSH) contain a
"object_transport_id" and "fec_payload_id" denoting the watermark
transmission point for which acknowledgment is requested. This
watermark transmission point is "echoed" in the corresponding fields
of the NORM_ACK(FLUSH) message sent by the receiver in response.
NORM_CMD(ACK_REQ) messages contain an "ack_id" field which is
similarly "echoed" in response so that the sender may match the
response to the appropriate request.
In response to the NORM_CMD(ACK_REQ), the listed receivers randomly
spread NORM_ACK messages uniformly in time over a window of (1*GRTT).
These NORM_ACK messages are typically unicast to the sender. (Note
that NORM_ACK(CC) messages SHALL be multicast or unicast in the same
manner as NORM_NACK messages).
The ACK process is self-limiting and avoids ACK implosion in that:
1) Only a single NORM_CMD(ACK_REQ) message is generated once per
(2*GRTT), and,
2) The size of the "acking_node_list" of NormNodeIds from which
acknowledgment is requested is limited to a maximum of the sender
NormSegmentSize setting per round of the positive acknowledgment
process.
Because the size of the included list is limited to the sender's
NormSegmentSize setting, multiple NORM_CMD(ACK_REQ) rounds may be
required to achieve responses from all receivers specified. The
content of the attached NormNodeId list will be dynamically updated
as this process progresses and NORM_ACK responses are received from
the specified receiver set. As the sender receives valid responses
Adamson, et al. Experimental [Page 73]
RFC 3940 NORM Protocol November 2004
(i.e., matching watermark point or "ack_id") from receivers, it SHALL
eliminate those receivers from the subsequent NORM_CMD(ACK_REQ)
message "acking_node_list" and add in any pending receiver
NormNodeIds while keeping within the NormSegmentSize limitation of
the list size. Each receiver is queried a maximum number of times
(NORM_ROBUST_FACTOR, by default). Receivers not responding within
this number of repeated requests are removed from the payload list to
make room for other potential receivers pending acknowledgment. The
transmission of the NORM_CMD(ACK_REQ) is repeated until no further
responses are required or until the repeat threshold is exceeded for
all pending receivers. The transmission of NORM_CMD(ACK_REQ) or
NORM_CMD(FLUSH) messages to conduct the positive acknowledgment
process is multiplexed with ongoing sender data transmissions.
However, the NORM_CMD(FLUSH) positive acknowledgment process may be
interrupted in response to negative acknowledgment repair requests
(NACKs) received from receivers during the acknowledgment period.
The NORM_CMD(FLUSH) positive acknowledgment process is restarted for
receivers pending acknowledgment once any the repairs have been
transmitted.
In the case of NORM_CMD(FLUSH) commands with an attached
"acking_node_list", receivers will not ACK until they have received
complete transmission of all data up to and including the given
watermark transmission point. All receivers SHALL interpret the
watermark point provided in the request NACK for repairs if needed as
for NORM_CMD(FLUSH) commands with no attached "acking_node_list".
NORM sender messages contain a "gsize" field that is a representation
of the group size and is used in scaling random backoff timer ranges.
The use of the group size estimate within the NORM protocol does not
require a precise estimation and works reasonably well if the
estimate is within an order of magnitude of the actual group size.
By default, the NORM sender group size estimate may be
administratively configured. Also, given the expected scalability of
the NORM protocol for general use, a default value of 10,000 is
recommended for use as the group size estimate.
It is possible that group size may be algorithmically approximated
from the volume of congestion control feedback messages which follow
the exponentially weighted random backoff. However, the
specification of such an algorithm is currently beyond the scope of
this document.
Adamson, et al. Experimental [Page 74]
RFC 3940 NORM Protocol November 2004
The same security considerations that apply to the NORM, and FEC
Building Blocks also apply to the NORM protocol. In addition to
vulnerabilities that any IP and IP multicast protocol implementation
may be generally subject to, the NACK-based feedback of NORM may be
exploited by replay attacks which force the NORM sender to
unnecessarily transmit repair information. This MAY be addressed by
network layer IP security implementations that guard against this
potential security exploitation. It is RECOMMENDED that such IP
security mechanisms be used when available. Another possible
approach is for NORM senders to use the "sequence" field from the
NORM Common Message Header to detect replay attacks. This can be
accomplished if the NORM packets are cryptographically protected and
the sender is willing to maintain state on receivers which are
NACKing. A cache of receiver state may provide some protection
against replay attacks. Note that the "sequence" field of NORM
messages should be incremented with independent values for different
destinations (e.g., group-addressed versus unicast-addressed messages
versus "receiver" messages). Thus, the congestion control loss
estimation function of the "sequence" field can be preserved for
sender messages when receiver messages are unicast to the sender.
The NORM protocol is compatible with the use of the IP security
(IPsec) architecture described in [22]. It is important to note that
while NORM does leverage FEC-based repair for scalability, this does
not alone guarantee integrity of received data. Application-level
integrity-checking of data content is highly RECOMMENDED.
No information in this specification is currently subject to IANA
registration. However, several Header Extensions are defined within
this document. If/when additional Header Extensions are developed,
the first RFC MUST establish an IANA registry for them, with a
"Specification Required" policy [6] and all Header Extensions,
including those in the present document, MUST be registered
thereafter. Additionally, building blocks components used by NORM
may introduce additional IANA considerations. In particular, the FEC
Building Block used by NORM does require IANA registration of the FEC
codecs used. The registration instructions for FEC codecs are
provided in [5].
The present NORM protocol is seen as useful tool for the reliable
data transfer over generic IP multicast services. It is not the
intention of the authors to suggest it is suitable for supporting
all envisioned multicast reliability requirements. NORM provides a
Adamson, et al. Experimental [Page 75]
RFC 3940 NORM Protocol November 2004
simple and flexible framework for multicast applications with a
degree of concern for network traffic implosion and protocol overhead
efficiency. NORM-like protocols have been successfully demonstrated
within the MBone for bulk data dissemination applications, including
weather satellite compressed imagery updates servicing a large group
of receivers and a generic web content reliable "push" application.
In addition, this framework approach has some design features making
it attractive for bulk transfer in asymmetric and wireless
internetwork applications. NORM is capable of successfully operating
independent of network structure and in environments with high packet
loss, delay, and misordering. Hybrid proactive/reactive FEC-based
repairing improve protocol performance in some multicast scenarios.
A sender-only repair approach often makes additional engineering
sense in asymmetric networks. NORM's unicast feedback capability may
be suitable for use in asymmetric networks or in networks where only
unidirectional multicast routing/delivery service exists. Asymmetric
architectures supporting multicast delivery are likely to make up an
important portion of the future Internet structure (e.g.,
DBS/cable/PSTN hybrids) and efficient, reliable bulk data transfer
will be an important capability for servicing large groups of
subscribed receivers.
The authors would like to thank Rick Jones, Vincent Roca, Rod Walsh,
Toni Paila, Michael Luby, and Joerg Widmer for their valuable input
and comments on this document. The authors would also like to thank
the RMT working group chairs, Roger Kermode and Lorenzo Vicisano, for
their support in development of this specification, and Sally Floyd
for her early input into this document.
[1] Kermode, R. and L. Vicisano, "Author Guidelines for Reliable
Multicast Transport (RMT) Building Blocks and Protocol
Instantiation documents", RFC 3269, April 2002.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
1112, August 1989.
Adamson, et al. Experimental [Page 76]
RFC 3940 NORM Protocol November 2004
[4] Adamson, B., Bormann, C., Handley, M., and J. Macker,
"Negative-Acknowledgment (NACK)-Oriented Reliable Multicast
(NORM) Building Blocks", RFC 3941, November 2004.
[5] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and
J. Crowcroft, "Forward Error Correction (FEC) Building Block",
RFC 3452, December 2002.
[6] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[7] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[8] Handley, M., Perkins, C., and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
[9] S. Pingali, D. Towsley, J. Kurose, "A Comparison of Sender-
Initiated and Receiver-Initiated Reliable Multicast Protocols",
In Proc. INFOCOM, San Francisco CA, October 1993.
[10] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and
J. Crowcroft, "The Use of Forward Error Correction (FEC) in
Reliable Multicast", RFC 3453, December 2002.
[11] Macker, J. and B. Adamson, "The Multicast Dissemination Protocol
(MDP) Toolkit", Proc. IEEE MILCOM 99, October 1999.
[12] Nonnenmacher, J. and E. Biersack, "Optimal Multicast Feedback",
Proc. IEEE INFOCOMM, p. 964, March/April 1998.
[13] J. Macker, B. Adamson, "Quantitative Prediction of Nack Oriented
Reliable Multicast (NORM) Feedback", Proc. IEEE MILCOM 2002,
October 2002.
[14] H.W. Holbrook, "A Channel Model for Multicast", Ph.D.
Dissertation, Stanford University, Department of Computer
Science, Stanford, California, August 2001.
[15] D. Gossink, J. Macker, "Reliable Multicast and Integrated Parity
Retransmission with Channel Estimation", IEEE GLOBECOMM 98',
September 1998.
[16] Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd, S.,
and M. Luby, "Reliable Multicast Transport Building Blocks for
One-to-Many Bulk-Data Transfer", RFC 3048, January 2001.
Adamson, et al. Experimental [Page 77]
RFC 3940 NORM Protocol November 2004
[17] Mankin, A., Romanow, A., Bradner, S., and V. Paxson, "IETF
Criteria for Evaluating Reliable Multicast Transport and
Application Protocols", RFC 2357, June 1998.
[18] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[19] J. Widmer and M. Handley, "Extending Equation-Based Congestion
Control to Multicast Applications", Proc ACM SIGCOMM 2001, San
Diego, August 2001.
[20] L. Rizzo, "pgmcc: A TCP-Friendly Single-Rate Multicast
Congestion Control Scheme", Proc ACM SIGCOMM 2000, Stockholm,
August 2000.
[21] J. Padhye, V. Firoiu, D. Towsley, and J. Kurose, "Modeling TCP
Throughput: A Simple Model and its Empirical Validation", Proc
ACM SIGCOMM 1998.
[22] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
Adamson, et al. Experimental [Page 78]
RFC 3940 NORM Protocol November 2004
Brian Adamson
Naval Research Laboratory
Washington, DC, USA, 20375
EMail: adamson@itd.nrl.navy.mil
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28334 Bremen, Germany
EMail: cabo@tzi.org
Mark Handley
Department of Computer Science
University College London
Gower Street
London
WC1E 6BT
UK
EMail: M.Handley@cs.ucl.ac.uk
Joe Macker
Naval Research Laboratory
Washington, DC, USA, 20375
EMail: macker@itd.nrl.navy.mil
Adamson, et al. Experimental [Page 79]
RFC 3940 NORM Protocol November 2004
Full Copyright Statement
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Adamson, et al. Experimental [Page 80]