Network Working Group D. Perkins
Request for Comments: 1547 Carnegie Mellon University
Category: Informational December 1993
Requirements for an Internet Standard Point-to-Point Protocol
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
This document discusses the evaluation criteria for an Internet
Standard Data Link Layer protocol to be used with point-to-point
links. Although many industry standard protocols and ad hoc
protocols already exist for the data link layer, none are both
complete and sufficiently versatile to be accepted as an Internet
Standard. In preparation to designing such a protocol, the features
necessary to qualify a point-to-point protocol as an Internet
Standard are discussed in detail. An analysis of the strengths and
weaknesses of several existing protocols on the basis of these
requirements demonstrates the failure of each to address key issues.
Historical Note: This was the design requirements document dated
June 1989, which was followed for RFC-1134 through the present.
It is now published for completeness and future guidance.
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Table of Contents
1. Introduction ................................................31.1 Definitions of Terms ........................................42. Required Features ...........................................62.1 Simplicity ..................................................72.2 Transparency ................................................72.3 Packet Framing ..............................................72.4 Bandwidth Efficiency ........................................82.5 Protocol Processing Efficiency ..............................82.6 Protocol Multiplexing .......................................82.7 Multiple Physical and Data Link Layer Protocols..............82.8 Error Detection .............................................92.9 Standardized Maximum Packet Length (MTU) ....................92.10 Switched and Non-Switched Media .............................92.11 Symmetry ....................................................92.12 Connection Liveness .........................................102.13 Loopback Detection ..........................................102.14 Misconfiguration Detection ..................................112.15 Network Layer Address Negotiation ...........................112.16 Data Compression Negotiation ................................112.17 Extensibility and Option Negotiation ........................123. Features Not Required .......................................123.1 Error Correction ............................................123.2 Flow Control ................................................133.3 Sequencing ..................................................133.4 Backward Compatibility ......................................133.5 Multi-Point Links ...........................................133.6 Half-Duplex or Simplex Links ................................143.7 7-bit Asynchronous RS-232 Links .............................144. Prior Work On PPP Protocols .................................144.1 Internet Protocols ..........................................144.1.1RFC 891 - DCN Local-Network Protocols, Appendix A............144.1.2RFC 914 - Thinwire Protocols ................................144.1.3RFC 916 - Reliable Asynchronous Transfer Protocol............154.1.4RFC 935 - Reliable Link Layer Protocols .....................154.1.5RFC 1009 - Requirements for Internet Gateways ...............154.1.6RFC 1055 - Serial Line IP ...................................164.2 International Protocols .....................................164.2.1 ISO 3309 - HDLC Frame Structure .............................164.2.2 ISO 6256 - HDLC Balanced Class of Procedures.................164.2.3 CCITT X.25 and X.25 LAPB ....................................174.2.4 CCITT I.441 LAPD ............................................174.3 Other Protocols .............................................174.3.1 Cisco Systems point-to-point protocols ......................174.3.2 MIT PC/IP framing protocol ..................................184.3.3 Proteon p4200 point-to-point protocol .......................184.3.4 Ungermann Bass point-to-point protocol ......................18
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4.3.5 Wellfleet point-to-point protocol ...........................194.3.6 XNS Synchronous Point-to-Point Protocol .....................19
REFERENCES ........................................................20
SECURITY CONSIDERATION.............................................21
CHAIR'S ADDRESS ...................................................21
AUTHOR'S ADDRESS ..................................................21
EDITOR'S ADDRESS ..................................................21
The Internet has seen explosive growth in the number of hosts
supporting IP [1]. The vast majority of these hosts are connected to
Local Area Networks (LANs) of various types, Ethernet being the most
common. Most of the other hosts are connected through Wide Area
Networks (WANs), such as X.25 style Public Data Networks (PDNs).
In the past, relatively few of these hosts were connected with simple
point-to-point links. Yet, point-to-point serial links are among the
oldest methods of data communications, and almost every host supports
point-to-point connections. For example, asynchronous RS-232
interfaces are essentially ubiquitous.
One reason for the small number of point-to-point IP links was the
lack of a single established encapsulation protocol. There were
plenty of non-standard (and at least one de facto standard)
encapsulation protocols available, but there was not one which was
agreed upon as an Internet Standard.
A number of protocols have been proposed to the Internet community,
but no consensus was reached as to which protocol should be adopted
as a standard. The reason may be that these proposals often
addressed specific problems rather than providing general purpose
service.
For example, one of the most successful protocols to-date was Rick
Adam's SLIP protocol for BSD UNIX [9]. SLIP provides only the most
rudimentary support for sending IP datagrams over asynchronous serial
lines, and ignores issues such as the use of protocols other than IP
and the use of synchronous links.
This document proposes a set of requirements for an Internet Standard
point-to-point protocol (ISPPP). Its purpose is not to propose any
one design for the standard; any solutions outlined in the text are
intended only as examples, and do not preclude other implementations.
The document is divided into four major sections. The first section
defines a number of technical terms used in this document. The
second section lists the proposed requirements and details some
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issues that are ignored by other protocols. The third section
attempts to clarify a number of non-requirements. The fourth section
analyzes existing protocols in light of the proposed requirements and
discusses the failure of each to address key issues.
This section defines many of the terms which will be used in further
sections of this document. The terms "layer" and "level" are used
extensively and refer to protocol layers as defined by the
International Organization For Standardization's Reference Model
(ISORM) standard. In particular, the terms Physical Layer, Data Link
Layer and Network Layer refer to layers one, two and three
respectively of the ISORM. A "higher layer" refers to one with a
numerically larger layer number.
datagram
The unit of transmission in the network layer (such as IP). A
datagram may be encapsulated in one or more packets (q.v.) passed
to the data link layer.
data link layer
Layer two in the ISO reference model. Defines how bits
transmitted and received by the physical layer are recognized as
bytes and frames. May also define procedures for error detection
and correction, sequencing and flow control.
fragment
The result of fragmentation. Fragmentation at the network layer
breaks large datagrams into multiple parts less than or equal to
the size of the packets passed to the data link layer.
Fragmentation at the data link layer breaks large packets into
multiple frames.
frame
The unit of transmission at the data link layer. A frame may
include a header and/or a trailer along with some number of units
of data.
framing protocol
A protocol at the data link level for marking the beginning and
end of a frame transmitted across a link.
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internet
An interconnected system of networks tied together by a common
"internet protocol" providing a common and consistent network
address structure.
Internet
Specifically refers to the IP Internet.
Internet Standard Point-to-Point Protocol (ISPPP)
A point-to-point protocol which is declared an official Internet
Standard. This protocol does not yet exist, but its proposed
characteristics are presented in this paper.
Maximum Transmission Unit (MTU)
The maximum allowable length for a packet (q.v.) transmitted over
a point-to-point link without incurring network layer
fragmentation.
network layer
Layer three in the ISO reference model. Responsible for routing
packets (q.v) between physical networks.
octet
A unit of transmission consisting of 8 bits. On most machines an
octet is the same as a byte or a character, but this need not be
true.
packet
The unit of transmission passed across the interface between the
network layer and the data link layer. A packet is usually mapped
to a frame (q.v); the exception is when data link layer
fragmentation is being performed.
physical layer
The first layer in the ISO reference model. Describes electrical,
mechanical and timing characteristics of a link.
point-to-point protocol (ppp)
A data link layer protocol for the transmission of packets (q.v.)
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over a point-to-point link. In the following discussion, the
acronym "ppp" refers to any generic point-to-point protocol.
serial line IP (slip)
Often incorrectly used as a synonym for "point-to-point protocol",
"slip" specifically refers to any protocol for the transmission of
IP datagrams over a serial point-to-point line.
SLIP
Although many proposed protocols are named "SLIP", this document
will use SLIP (uppercase) to refer to Rick Adam's slip (q.v.) for
BSD UNIX [9].
In order for a point-to-point protocol to be accepted by the Internet
community it must adequately address many requirements. This section
itemizes and discusses the proposed requirements. Although the main
emphasis of the discussion is on protocol architecture requirements,
implementation requirements are sometimes discussed as well.
These particular requirements were chosen to assure that the ISPPP
adequately serves the needs of its users. Some of these needs are
universal and dictate clear requirements for the protocol; for
example, a packet framing protocol is a fundamental necessity. Other
needs are more specific and may even be conflicting. Connection
liveness determination is very important on some links but can be
very expensive on others. A standard protocol must address all of
these needs; in particular, it must be able to resolve conflicts
effectively.
Resolving these conflicts requires that a protocol feature have both
enabled and disabled modes and that these modes must be compatible
with each other. The enabled mode allows the protocol to solve
problems in environments where they exist. The disabled mode allows
problems to be ignored in environments where they do not exist. To
assure interoperabilty, implementations are required to support both
modes and allow the user (not necessarily human) to dynamically
choose which is appropriate.
This is essentially the same solution used in the User Datagram
Protocol (UDP) [2]. The UDP datagram checksum may be computed
(enabled mode) or it may not (disabled mode). Compatibility is
maintained by requiring the checksum to be transmitted as zero in
disabled mode and ignored when received as zero in either mode.
Implementations of UDP are generally encouraged to support both modes
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but allow the application to choose modes.
The ISPPP must be simple. The Internet architecture very carefully
places the most complexity in the transport layer (that is, TCP).
The internetwork layer (IP) is a fairly simple, almost stateless
protocol providing an unreliable datagram service. The data link
layer need provide no more capability than the IP protocol; no error
correction, sequencing or flow control is necessary. Including these
would in most cases needlessly duplicate the capabilities of the
transport layer, and might possibly decrease efficiency. This is not
to say that these capabilities must never be included; there are some
cases which may warrant them. For instance, very noisy links may be
more efficiently handled using a more complex data link layer
protocol such as CCITT's LAPB. Nevertheless, the watchword for a
point-to-point protocol should be simplicity.
A simple design also decreases the incidence of programming errors,
thereby increasing the likelihood of interoperability among different
implementations. Since interoperability is a primary goal of
standardization, this is another strong argument for simplicity.
The ISPPP must be transparent to higher layers. The protocol must
not place any constraints on transmitted data. All ISPPP data,
including higher level headers as well as data, must be transported
unmodified end-to-end. No restrictions are placed on how the ISPPP
accomplishes this. For example, if the ISPPP uses a particular
character for framing, it must also provide some way of
disambiguating higher level data containing that character from a
framing character (such as escaping or bit-stuffing). This is mainly
an issue for the data link and physical layer protocols incorporated
into the ISPPP.
The ISPPP must be able to correctly and efficiently frame packets. A
receiver must be able to locate correctly the beginning and end of
each transmitted packet. Within each packet, the receiver must be
able to identify the boundaries of each octet. Finally, within each
octet, each bit must be located and identified. No restrictions
other than those specified in this document are placed on the packet
framing protocol.
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The processing of the ISPPP headers must typically be very fast and
efficient. The format for data packets should be very simple in the
normal case, without complex field checking.
The ISPPP must support multiplexing of many higher level protocols.
Although the Internet community is interested mainly in IP, co-
existence of other protocols is frequently required. IP networks
must often support additional protocols such as AppleTalk, DECnet,
IPX, and XNS. For point-to-point links to connect gateways on
geographically separated Local Area Networks (LANs), the ISPPP must
simultaneously support all protocols implemented on both the LANs and
the gateways. This suggests that the ISPPP must include a protocol
type field or other multiplexing scheme. Given the large number of
protocols, the potential use of the protocol type field as a data
compression aid, and the experimental nature of the Internet, eight
bits of type field are not sufficient. Sixteen bits of type field
are suggested, although twelve bits (4096 protocols) should suffice.
The ISPPP must support a multiplicity of physical and data link layer
protocols. Many types of point-to-point links exist. Links can be
serial or parallel, synchronous or asynchronous, low speed or high
speed, electrical or optical. Standards are required for the
transmission of IP datagrams over each type of commonly used link.
The ISPPP must not inhibit the use of any type of link. This
includes, but is not limited to, asynchronous, bit-oriented
synchronous (HDLC [10] and X.25 LAPB [11]), and byte-oriented
synchronous (BISYNC and DDCMP [15]) links.
The ISPPP must initially provide support for at least the following
types of links:
Full duplex asynchronous RS-232 [3] links with 8 bits of data and
no parity, ranging in speeds from 300 to 19.2k bps or more.
Full duplex bit-oriented synchronous links including RS-422, RS-
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423, V.35 and T1.
Other links should be standardized as the need arises.
The ISPPP must provide some form of basic error detection. Most
network and transport layer protocols provide mechanisms to detect
corrupted packets. However, some network protocols expect error
free transmission and either provide error detection only on a
conditional basis or do not provide it at all. It is the
consensus of the Internet community that error correction should
always be implemented in the end-to-end transport, but that link
error detection in the form of a checksum, Cyclic Redundancy Check
(CRC) or other frame check mechanism is useful to prevent wasted
bandwidth from propagation of corrupted packets. Link level error
correction is not required.
The ISPPP must have a standardized default maximum packet length
for each type of point-to-point link. This standardization helps
to promote interoperable implementations. Higher layer protocols
must not attempt to transmit packets longer than the MTU. If a
higher layer protocol does try to transmit a packet which is too
long, the ISPPP must drop the packet and return an error. The MTU
may potentially be changed from the default via some sort of
explicit negotiation or private agreement, but the default must be
enforced in all other cases. The default should be at least 1500
bytes, to efficiently carry common LAN traffic.
The ISPPP must be able to support both switched (dynamic) and non-
switched (static) point-to-point links. A common example of a
non- switched link is a 3-wire asynchronous RS-232 cable which
might connect a host to a particular gateway. Switched media may
be exemplified by connections over a standard voice network or an
Integrated Services Digital Network (ISDN). Links over ISDN are
currently rare, but are expected to become increasingly
commonplace. To be a viable standard, the ISPPP must be able to
effectively support both types of links. Procedures for
establishing switched connections are beyond the scope of this
document.
The ISPPP should operate symmetrically to maximize flexibility.
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The ISPPP must allow communications among any combination of
gateways and hosts. One host may need to communicate directly
with another host, or it may be connected to a gateway to gain
access to a whole network. A gateway may establish a connection
to a single host in order to deliver a packet, or it may connect
to another gateway on a permanent or transient basis. Symmetry is
destroyed by pre-assigned static roles, such as master and slave
or gateway and host. If necessary, roles may be dynamically
determined on a per connection basis.
The ISPPP must include a mechanism to automatically determine when
a link is functioning properly and when it is defunct. This
mechanism should be enabled by default, but the protocol and all
implementations must allow this mechanism to be disabled.
When enabled, this mechanism should discover changes in a link's
status in a timely fashion -- no more than a few minutes.
Continuing to utilize a link which is down often causes routing
problems commonly referred to as "black holes". These problems
can be hard to find and diagnose. By automatically detecting a
failing link, a point-to-point protocol can avoid such problems,
and also provide a powerful tool for a network manager trying to
locate and remedy the fault.
When a point-to-point connection is not functioning properly, it
must be declared "down" for the purposes of routing packets for
higher level protocols. In order to certify a link "up", the
systems on either end of the link must be able to successfully
exchange packets. In other words, the systems at both ends must
be able both to transmit and to receive packets, and the link must
be able to transport packets in both directions. Links are
defined to be "down" at initialization, their liveness must be
verified before they may be declared "up".
This feature may be disabled in situations where connection status
determination is "expensive". For example, a link may traverse a
Public Data Network (such as TELENET or TYMNET) which accounts for
bandwidth utilization. Constant pinging would result in charges
being accrued even in the absence of useful communications.
The ISPPP must be capable of automatically detecting a looped-back
link without operator assistance. Modems and other communications
gear are often placed in a loopback mode to aid in diagnosis of
circuit failures. Detection of this condition must take no longer
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than one period of the liveness protocol. While the link is in
loopback mode, each end of the link must declare the other end to be
unreachable. However, to aid in diagnosis, each end of the link may
declare itself reachable for any higher-level protocol which
distinguishes between the two ends of the link.
The ISPPP must be able to quickly detect misconfigured point-to-point
connections. A connection which is misconfigured must never be
declared to be up. Many systems, gateways in particular, have more
than one point-to-point connection. When many cables terminate
within a small area, the possibility for confusion abounds. It
becomes very easy to mistakenly plug a cable into the wrong
connector, or even to swap cables. The protocol should do its best
to provide protection against these errors by verifying the remote
end's identity whenever possible before marking an interface as
operational. The purpose of this verification is not rigorous
authentication but the detection of simple errors.
The ISPPP must allow network layer (such as IP) addresses to be
negotiated. The negotiation algorithm should be as simple as
possible and must be guaranteed to terminate in all cases. Many
network layer protocols and implementations are required to know the
addresses at both ends of a point-to-point link before packets may be
routed. These addresses may be statically configured, but it may
sometimes be necessary or convenient for these addresses be
dynamically ascertained at connection establishment. This is
especially important when switched media are used. For example, a
dial-up IP gateway must know the IP address of its peer before
packets can be successfully routed. This address can be either
statically or dynamically configured. In the former case, the
gateway's peer must therefore learn the static address (static with
respect to the gateway). In the latter situation, the gateway must
dynamically learn the address used by its peer.
The ISPPP must provide a way to negotiate the use of data compression
algorithms. This mechanism should be as simple as possible and must
be guaranteed to terminate in all cases. The protocol is not
required to standardize any data compression algorithms; conforming
implementations of the protocol therefore may refuse to do data
compression when negotiating (refusal to do data compression always
takes precedence over an offer to do it). However, to allow the use
of data compression between consenting systems, the point-to-point
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protocol must not impede the use of data compression. In fact, it
should be possible to use multiple, independent data compression
schemes simultaneously. Because data compression algorithms are
still very experimental in the Internet environment, it is likely
that many different algorithms will be tried. The negotiation
protocol must distinguish between these different algorithms to
ensure that data compression is not enabled unless the same algorithm
or algorithms are used at both ends of the connection. The number of
such supported algorithms must be easily extensible.
The ISPPP must allow for future extensions in a flexible way. The
Internet will never cease to evolve. Changes in technology and user
demands create new requirements. To function effectively as a
standard, the protocol must have the ability to evolve along with its
environment.
To accomplish this, the ISPPP should be designed to be as extensible
as possible and to allow for experimentation within the guidelines of
the other requirements presented in this document. A proposed
solution is to specify an option negotiation protocol. The option
negotiation protocol could be used for the negotiation of network
layer addresses, data compression schemes, MTU, encryption, etc. The
option negotiation protocol must itself be extensible; it should
allow the negotiation of a large number of future options and it
should allow the use of other types of point-to-point links and
encapsulation schemes.
This section discusses functionality which is explicitly not
required. These functions may potentially be included in
implementations as long as the inclusion does not violate any of the
requirements itemized in the previous section.
As discussed above in the sections on Simplicity and Error Detection,
error correction is the responsibility of the transport layer and is
not required in a point-to-point protocol. However, on links with
high error rates, performance may be increased by adding error
correction at the data link level. Therefore, the ISPPP must not
prevent the addition of error correction by private agreement, even
though such mechanisms are not required in the basic implementation.
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Flow control (such as XON/XOFF) is not required. Any implementation
of the ISPPP is expected to be capable of receiving packets at the
full rate possible for the particular data link and physical layers
used in the implementation. If higher layers cannot receive packets
at the full rate possible, it is up to those layers to discard
packets or invoke flow control procedures. As discussed above, end-
to-end flow control is the responsibility of the transport layer.
Including flow control within a point-to-point protocol often causes
violation of the simplicity requirement.
Sequencing of packets is not required. The ISPPP need provide no
more service than the IP protocol, an unreliable datagram service
which is free to reorder packets. In fact, it is specifically
allowed to reorder packets based upon some type-of-service criteria
implemented in higher-level protocols.
There is no requirement for the ISPPP to provide backward
compatibility with any other point-to-point protocol. First, there
are no official Internet Standards with which backward compatibility
must be maintained. Second, attempting to maintain backward
compatibility may lead to needless restrictions on the new protocol.
However, there is no need for the designers of the ISPPP to go out of
their way to inhibit backward compatibility.
There is no requirement for supporting multi-point links. Many
features which are required are only valid between two peers. These
links are sufficiently rare that the benefits of supporting them are
outweighed by the added complexity their support would introduce into
the ISPPP.
Historical Note: The original rationale also stated: "Furthermore,
it is unlikely that many new types of multi-point links will be
introduced in the foreseeable future." Since this was written,
considerable effort has been expended in new multi-point links,
including Switched Multimegabit Data Service, Frame Relay, and
Asynchronous Transfer Mode. However, it is clear that these are
considerably more complex than ISPPP.
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Support for half-duplex or simplex links is not required. These
types of links are not in common use in the current Internet. Half-
duplex links require some method of turning the line around. The
ISPPP need not have an explicit mechanism for handling line turn-
around. Such support might possibly be added in the future via the
required extension mechanism.
The use of asynchronous RS-232 need not support 7-bit links. 8-bit
links are predominant in the Internet environment and supporting 7-
bit links introduces unnecessary complexity.
4.1.1 RFC 891 - DCN Local-Network Protocols, Appendix A
In Appendix A of RFC 891, "DCN Local-Network Protocols" [4], D.L.
Mills describes the data link layer packet formats used by the
Fuzzball system for asynchronous, character-oriented synchronous,
DDCMP, HDLC, ARPANET 1822, X.25 LAPB and ethernet links. These
protocols meet the stated requirements for simplicity, transparency,
packet framing and efficiency, but fall short of many of the others.
Most of these protocols assume the use of the IP protocol, and do not
include any type of protocol demultiplexing field. No error
detection mechanism is provided except when necessary to comply with
another standard such as ethernet. RFC 891 does not mention the MTU
used for any of these links. Other requirements such as loopback
detection and misconfiguration detection are not discussed. Finally,
no option negotiation scheme is defined; without a protocol
demultiplexing field it would be difficult or impossible to include
one.
RFC 914, "Thinwire Protocols" [5], discusses the use of low speed
links in the Internet. This document places its main emphasis on
decreasing round-trip delay and increasing link efficiency with the
help of header compression (vs. data compression) techniques. Three
"Thinwire" protocols are discussed, Thinwire I, Thinwire II and
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Thinwire III. The latter two protocols require the use of a reliable
data link layer protocol; one such protocol, "SLIP" (not to be
confused with Rick Adams' SLIP), is proposed in Appendix D of the
RFC. As proposed, "SLIP" does not meet many of the stated
requirements. Although not terribly complex, as a reliable, error
detecting and correcting protocol, it is not "simple". The 32 octet
packet size makes it inefficient for large or uncompressed packets,
requiring complex fragmentation and reassembly. The use of other
than asynchronous links is not mentioned. The entire reliable link
layer would be redundant over LAPB links. There is no mechanism for
option negotiation or future extensibility.
4.1.3 RFC 916 - Reliable Asynchronous Transfer Protocol
RFC 916 [6] presents RATP, the Reliable Asynchronous Transfer
Protocol. RATP provides error detection and correction, sequencing
and flow control across a point-to-point connection. It is directed
towards full duplex RS-232 links although it is useful for other
point-to-point links. Although the author claims that RATP is not as
complex as some other protocols, it is far from simple. RATP solves
many of the problems which we have labeled non-requirements and fails
to solve many of our stated requirements. Specifically, RATP does
not support option negotiation and has no mechanism for future
extensibility. Since RFC 916 was published, no consensus has emerged
advocating RATP. For these reasons RATP is not recommended as the
ISPPP.
RFC 935 [7] is a rebuttal to the protocols proposed in RFCs 914 and
916. J. Robinson discusses existing and widely-used national and
international standards which meet the needs addressed by the two
prior RFCs. The standards reviewed include character-oriented
asynchronous and synchronous (bisynch) protocols and bit-oriented
synchronous protocols. RFC 935 does not present any higher level
issues such as option negotiation or extensibility.
Section 3 of RFC 1009, "Constituent Network Interfaces" [8], briefly
discusses requirements for transmission of IP datagrams over a number
of types of point-to-point links including X.25 LAPB, HDLC framed
synchronous links, Xerox Synchronous Point-to-Point synchronous lines
and the MIT Serial Line Framing Protocol for asynchronous lines. RFC
1009 merely mentions these as reasonable candidates and does not go
into depth on any of them. All are discussed further in this
document.
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Rick Adams' Serial Line IP (SLIP) protocol [9] has become something
of a de facto standard due to the popularity of the 4.2 and 4.3BSD
UNIX operating systems. SLIP is easily added to 4.2 systems and is
included with 4.3. Many other TCP/IP implementation have added SLIP
implementations in order to be compatible. Yet SLIP is not a real
standard; the protocol was only recently published in RFC form.
Before RFC 1055 it was specified in the SLIP source code. SLIP does
not meet most of the requirements set forth above. SLIP certainly
meets the requirement for simplicity, and also meets the requirements
for transparency and bandwidth efficiency. But SLIP only provides
for sending IP packets over asynchronous serial lines. Since it
provides no higher level protocol field for demultiplexing, SLIP
cannot support multiple concurrent higher level protocols. Providing
only a framing protocol, SLIP would be entirely redundant when used
with a LAPB synchronous link. SLIP includes absolutely no mechanism
for error detection, not even parity. Again due to its lack of a
protocol type field, SLIP does not support any type of option
negotiation or extensibility.
ISO 3309 [10], the HDLC frame structure, is a simple data link layer
protocol which provides framing of packets transmitted over bit-
oriented synchronous links. Special flag sequences mark the
beginning and end of frames and bit stuffing allows data containing
flag characters to be transmitted. A 16-bit Frame Check Sequence
provides error detection.
By itself, the HDLC frame structure does not meet most of the
requirements. HDLC does not provide protocol multiplexing, standard
MTUs, fault detection or option negotiation. There is no mechanism
for future extensibility.
Given the HDLC frame structure's wide acceptance and simplicity, it
may be an ideal building block for the ISPPP.
ISO 6256, the HDLC Balanced Class of Procedures, specifies a data
link layer protocol which provides error correction, sequencing and
flow control. ISO 6256 builds on ISO 3309 and ISO 4335, HDLC
Elements of Procedures.
As far as meeting our requirements is concerned, ISO 6256 does not
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provide any more utility than does ISO 3309. The capabilities that
are provided are all considered unnecessary and overly complex.
CCITT recommendation X.25 [11] describes a network layer protocol
providing error-free, sequenced, flow controlled virtual circuits.
X.25 includes a data link layer, X.25 LAPB, which uses ISO 3309, 4335
and 6256. Neither X.25 LAPB or full LAPB meet any more of our
requirements than the ISO protocols.
CCITT I.441 LAPD [12] defines the Link Access Procedure on the ISDN
D-Channel. The data link layer of LAPD is very similar to that of
LAPB and fails to meet the same requirements.
The Cisco Systems gateway supports both asynchronous links using SLIP
and synchronous links using either simple HDLC framing, X.25 LAPB or
full X.25. The HDLC framing procedure includes a four byte header.
The first octet (address) is either 0x0F (unicast intent) or 0x8F
(multicast intent). The second octet (control byte) is left zero and
is not checked on reception. The third and fourth octets contain a
standard 16 bit Ethernet protocol type code.
A "keepalive" or "beaconing" protocol is used to ensure the two-way
connectivity of the serial line. Each end of the link periodically
sends two 32 bit sequence numbers to the other side. One sequence
number is the local side's sequence number, the other is the sequence
number received from the other side. Hearing the local sequence
number from the other side indicates that the link is working in both
directions.
The keepalive protocol is extensible. One extension is used to
default IP addresses on serial lines of systems without non-volatile
memory. A request for address is sent to the remote side. The
remote side responds with its own IP address and a subnet mask. When
the querying side receives the reply, it checks if the host portion
of the remote address is either 1 or 2. If so, the opposite address
is chosen for the local address. If not, the protocol cannot be used
and we must rely on other address resolution means. This protocol
assumes that each serial link uses one subnet or network number.
LAPB assuming IP is another possible encapsulation. A multi-protocol
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extension of LAPB (multi-LAPB) includes a 16 bit Ethernet type code
after the address and control bytes and in front of the actual
protocol data. DDN X.25 and Commercial X.25 encapsulations are also
supported. Multiple protocols are supported by making protocol
dependent CALL REQUEST's.
The MIT PC/IP framing protocol [13] provides a mechanism for the
transmission of IP datagrams over asynchronous links. The low-level
protocol (LLP) sublayer provides encapsulation while the local net
protocol provides multiplexing of IP datagrams and IP address request
packets. The protocol only allows host-to-gateway connections.
Host-to-gateway flow control is provided by requiring the host to
transmit request packets to the gateway until an acknowledgment is
received. Rudimentary IP address negotiation requires the host to
ascertain its IP address from the gateway.
The protocol does not implement error detection, connection status
determination, fault detection or option negotiation. Only
asynchronous links are supported.
The Proteon p4200 multi-protocol router supports transmission of
packets over bit-oriented synchronous links with a wide range of
speeds (zero to 2 Mb/sec). The p4200 point-to-point protocol
encapsulates packets inside HDLC frames but does not use the HDLC
address or control fields; these two octets are instead used for a
16-bit type field. The p4200 does use the HDLC frame check sequence
trailer. Protocol type numbers are ad hoc and do not correspond to
any existing standard. A simple liveness protocol detects dead
connections.
Although the Proteon protocol does meet many of our requirements, it
does not meet our requirements for option negotiation.
The Ungermann Bass router supports synchronous links using simple
HDLC framing. Neither the HDLC address or control field are used, IP
datagrams are placed immediately after the HDLC flag.
The U-B protocol does not meet any of our requirements for fault
detection or option negotiation. No mechanism for future
extensibility is currently defined.
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The Wellfleet router supports synchronous links using simple HDLC
framing. The HDLC framing procedure uses the HDLC address and places
the Unnumbered Information (UI) command in all frames. A simple
header following the UI command provides a two octet type field using
the same values as Ethernet.
The Wellfleet protocol does not meet any of our requirements for
fault detection or option negotiation. No mechanism for future
extensibility is currently defined, although one could be added.
The Xerox Network Systems Synchronous Point-to-Point protocol (XNS
PPP) [14] was designed to address most of the same issues that an
ISPPP must address. In particular, it addresses the issues of
simplicity, transparency, efficiency, packet framing, protocol
multiplexing, error detection, standard MTUs, symmetry, switched and
non-switched media, connection status, network address negotiation
and future extensibility. However, the XNS SPPP does not meet our
requirements for multiple data link layer protocols, fault detection
and data compression negotiation. Although protocol multiplexing is
provided, the packet type field has only 8 bits which is too few.
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RFC 1547 Point-to-Point Protocol Requirements December 1993
References
[1] Postel, J., "Internet Protocol", STD 5, RFC 791, USC/Information
Sciences Institute, September 1981.
[2] Postel, J., "User Datagram Protocol", STD 6, RFC768, USC/Information
Sciences Institute, August 1980.
[3] Electronic Industries Association, EIA Standard RS-232-C,
"Interface Between Data Terminal Equipment and Data
Communications Equipment Employing Serial Binary Data
Interchange", August 1969.
[4] Mills, D. L., "DCN Local-Network Protocols", STD 44, RFC 891,
University of Delaware, December 1983.
[5] Farber, David J., Delp, Gary S., and Conte, Thomas M., "A
Thinwire Protocol for Connecting Personal Computers to the
Internet", RFC 914, University of Delaware, September 1984.
[6] Finn, G., "Reliable Asynchronous Transfer Protocol (RATP)",
RFC 916, USC/Information Sciences Institute, October 1984.
[7] Robinson, J., "Reliable Link Layer Protocols", RFC 935, BBN,
January 1985.
[8] Braden, R., and J. Postel, "Requirements for Internet
Gateways", STD 4, RFC1009, USC/Information Sciences Institute,
June 1987.
[9] Romkey, J., "A Nonstandard for the Transmission of IP Datagrams
Over Serial Lines: SLIP", STD 47, RFC 1055, June 1988. STD
4, RFC 1009, June 1987.
[10] ISO International Standard (IS) 3309, "Data Communications -
High-level Data Link Control Procedures - Frame Structure",
1979.
[11] CCITT Recommendation X.25, "Interface Between Data Terminal
Equipment (DTE) and Data Circuit Terminating Equipment (DCE)
for Terminals Operating in the Packet Mode on Public Data
Networks", Vol. VIII, Fascicle VIII.2, Rec. X.25.
[12] CCITT Recommendation Q.921 "ISDN User-Network Interface Data
Link Layer Specification".
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RFC 1547 Point-to-Point Protocol Requirements December 1993
[13] Romkey, J.L., "PC/IP Programmer's Manual", Massachussetts
Institute of Technology Laboratory for Computer Science,
January 1986.
[14] Xerox Corporation, "Synchronous Point-to-Point Protocol", Xerox
System Integration Standard, Stamford, Connecticut, XSIS
158412, December 1984.
[15] "Digital Data Communications Message Protocol", Digital
Equipment Corporation.
Security Consideration
Security issues are not discussed in this memo.
Chair's Address
The working group can be contacted via the current chair:
Fred Baker
Advanced Computer Communications
315 Bollay Drive
Santa Barbara, California 93117
EMail: fbaker@acc.com
Author's Address
Questions about this memo can also be directed to:
Drew Perkins
4015 Holiday Park Drive
Murrysville, PA 15668
EMail: perkins+@cmu.edu
Editor's Address
Typographic revision and historical notes by:
William Allen Simpson
1384 Fontaine
Madison Heights, Michigan 48071
EMail: Bill.Simpson@um.cc.umich.edu
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