Network Working Group K. Nichols
Request for Comments: 2638 V. Jacobson
Category: Informational Cisco
L. Zhang
UCLA
July 1999
A Two-bit Differentiated Services Architecture for the Internet
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
This document was originally submitted as an internet draft in
November of 1997. As one of the documents predating the formation of
the IETF's Differentiated Services Working Group, many of the ideas
presented here, in concert with Dave Clark's subsequent presentation
to the December 1997 meeting of the IETF Integrated Services Working
Group, were key to the work which led to RFCs 2474 and 2475 and the
section on allocation remains a timely proposal. For this reason, and
to provide a reference, it is being submitted in its original form.
The forwarding path portion of this document is intended as a record
of where we were at in late 1997 and not as an indication of future
direction.
The postscript version of this document includes Clark's slides as an
appendix. The postscript version of this document also includes many
figures that aid greatly in its readability.
This document presents a differentiated services architecture for the
internet. Dave Clark and Van Jacobson each presented work on
differentiated services at the Munich IETF meeting [2,3]. Each
explained how to use one bit of the IP header to deliver a new kind
of service to packets in the internet. These were two very different
kinds of service with quite different policy assumptions. Ensuing
discussion has convinced us that both service types have merit and
that both service types can be implemented with a set of very similar
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mechanisms. We propose an architectural framework that permits the
use of both of these service types and exploits their similarities in
forwarding path mechanisms. The major goals of this architecture are
each shared with one or both of those two proposals: keep the
forwarding path simple, push complexity to the edges of the network
to the extent possible, provide a service that avoids assumptions
about the type of traffic using it, employ an allocation policy that
will be compatible with both long-term and short-term provisioning,
make it possible for the dominant Internet traffic model to remain
best-effort.
The major contributions of this document are to present two distinct
service types, a set of general mechanisms for the forwarding path
that can be used to implement a range of differentiated services and
to propose a flexible framework for provisioning a differentiated
services network. It is precisely this kind of architecture that is
needed for expedient deployment of differentiated services: we need a
framework and set of primitives that can be implemented in the
short-term and provide interoperable services, yet can provide a
"sandbox" for experimentation and elaboration that can lead in time
to more levels of differentiation within each service as needed.
At the risk of belaboring an analogy, we are motivated to provide
services tiers in somewhat the same fashion as the airlines do with
first class, business class and coach class. The latter also has
tiering built in due to the various restrictions put on the purchase.
A part of the analogy we want to stress is that best effort traffic,
like coach class seats on an airplane, is still expected to make up
the bulk of internet traffic. Business and first class carry a small
number of passengers, but are quite important to the economics of the
airline industry. The various economic forces and realities combine
to dictate the relative allocation of the seats and to try to fill
the airplane. We don't expect that differentiated services will
comprise all the traffic on the internet, but we do expect that new
services will lead to a healthy economic and service environment.
This document is organized into sections describing service
architecture, mechanisms, the bandwidth allocation architecture, how
this architecture might interoperate with RSVP/int-serv work, and
gives recommendations for deployment.
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The current internet delivers one type of service, best-effort, to
all traffic. A number of proposals have been made concerning the
addition of enhanced services to the Internet. We focus on two
particular methods of adding a differentiated level of service to IP,
each designated by one bit [1,2,3]. These services represent a
radical departure from the Internet's traditional service, but they
are also a radical departure from traditional "quality of service"
architectures which rely on circuit-based models. Both these
proposals seek to define a single common mechanism that is used by
interior network routers, pushing most of the complexity and state of
differentiated services to the network edges. Both use bandwidth as
the resource that is being requested and allocated. Clark and
Wroclawski defined an "Assured" service that follows "expected
capacity" usage profiles that are statistically provisioned [3]. The
assurance that the user of such a service receives is that such
traffic is unlikely to be dropped as long as it stays within the
expected capacity profile. The exact meaning of "unlikely" depends on
how well provisioned the service is. An Assured service traffic flow
may exceed its Profile, but the excess traffic is not given the same
assurance level. Jacobson defined a "Premium" service that is
provisioned according to peak capacity Profiles that are strictly not
oversubscribed and that is given its own high-priority queue in
routers [2]. A Premium service traffic flow is shaped and hard-
limited to its provisioned peak rate and shaped so that bursts are
not injected into the network. Premium service presents a "virtual
wire" where a flow's bursts may queue at the shaper at the edge of
the network, but thereafter only in proportion to the indegree of
each router. Despite their many similarities, these two approaches
result in fundamentally different services. The former uses buffer
management to provide a "better effort" service while the latter
creates a service with little jitter and queueing delay and no need
for queue management on the Premium packets's queue.
An Assured service was introduced in [3] by Clark and Wroclawski,
though we have made some alterations in its specification for our
architecture. Further refinements and an "Expected Capacity"
framework are given in Clark and Fang [10]. This framework is
focused on "providing different levels of best-effort service at
times of network congestion" but also mentions that it is possible to
have a separate router queue to implement a "guaranteed" level of
assurance. We believe this framework and our Two-bit architecture
are compatible but this needs further exploration. As Premium
service has not been documented elsewhere, we describe it next and
follow this with a description of the two-bit architecture.
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In [2], a Premium service was presented that is fundamentally
different from the Internet's current best effort service. This
service is not meant to replace best effort but primarily to meet an
emerging demand for a commercial service that can share the network
with best effort traffic. This is desirable economically, since the
same network can be used for both kinds of traffic. It is expected
that Premium traffic would be allocated a small percentage of the
total network capacity, but that it would be priced much higher. One
use of such a service might be to create "virtual leased lines",
saving the cost of building and maintaining a separate network.
Premium service, not unlike a standard telephone line, is a capacity
which the customer expects to be there when the receiver is lifted,
although it may, depending on the household, be idle a good deal of
the time. Provisioning Premium traffic in this way reduces the
capacity of the best effort internet by the amount of Premium
allocated, in the worst case, thus it would have to be priced
accordingly. On the other hand, whenever that capacity is not being
used it is available to best effort traffic. In contrast to normal
best effort traffic which is bursty and requires queue management to
deal fairly with congestive episodes, this Premium service by design
creates very regular traffic patterns and small or nonexistent
queues.
Premium service levels are specified as a desired peak bit-rate for a
specific flow (or aggregation of flows). The user contract with the
network is not to exceed the peak rate. The network contract is that
the contracted bandwidth will be available when traffic is sent.
First-hop routers (or other edge devices) filter the packets entering
the network, set the Premium bit of those that match a Premium
service specification, and perform traffic shaping on the flow that
smooths all traffic bursts before they enter the network. This
approach requires no changes in hosts. A compliant router along the
path needs two levels of priority queueing, sending all packets with
the Premium bit set first. Best-effort traffic is unmarked and queued
and sent at the lower priority. This results in two "virtual
networks": one which is identical to today's Internet with buffers
designed to absorb traffic bursts; and one where traffic is limited
and shaped to a contracted peak-rate, but packets move through a
network of queues where they experience almost no queueing delay.
In this architecture, forwarding path decisions are made separately
and more simply than the setting up of the service agreements and
traffic profiles. With the exception of policing and shaping at
administrative or "trust" boundaries, the only actions that need to
be handled in the forwarding path are to classify a packet into one
of two queues on a single bit and to service the two queues using
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simple priority. Shaping must include both rate and burst parameters;
the latter is expected to be small, in the one or two packet range.
Policing at boundaries enforces rate compliance, and may be
implemented by a simple token bucket. The admission and set-up
procedures are expected to evolve, in time, to be dynamically
configurable and fairly complex while the mechanisms in the
forwarding path remain simple.
A Premium service built on this architecture can be deployed in a
useful way once the forwarding path mechanisms are in place by making
static allocations. Traffic flows can be designated for special
treatment through network management configuration. Traffic flows
should be designated by the source, the destination, or any
combination of fields in the packet header. First-hop (of leaf)
routers will filter flows on all or part of the header tuple
consisting of the source IP address, destination IP address, protocol
identifier, source port number, and destination port number. Based on
this classification, a first-hop router performs traffic shaping and
sets the designated Premium bit of the precedence field. End-hosts
are thus not required to be "differentiated services aware", though
if and when end-systems become universally "aware", they might do
their own shaping and first-hop routers merely police.
Adherence to the subscribed rate and burst size must be enforced at
the entry to the network, either by the end-system or by the first-
hop router. Within an intranet, administrative domain, or "trust
region" the packets can then be classified and serviced solely on the
Premium bit. Where packets cross a boundary, the policing function is
critical. The entered region will check the prioritized packet flow
for conformance to a rate the two regions have agreed upon,
discarding packets that exceed the rate. It is thus in the best
interests of a region to ensure conformance to the agreed-upon rate
at the egress. This requirement means that Premium traffic is burst-
free and, together with the no oversubscription rule, leads directly
to the observation that Premium queues can easily be sized to prevent
the need to drop packets and thus the need for a queue management
policy. At each router, the largest queue size is related to the in-
degree of other routers and is thus quite small, on the order of ten
packets.
Premium bandwidth allocations must not be oversubscribed as they
represent a commitment by the network and should be priced
accordingly. Note that, in this architecture, Premium traffic will
also experience considerably less delay variation than either best
effort traffic or the Assured data traffic of [3]. Premium rates
might be configured on a subscription basis in the near-term, or on-
demand when dynamic set-up or signaling is available.
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Figure 1 shows how a Premium packet flow is established within a
particular administrative domain, Company A, and sent across the
access link to Company A's ISP. Assume that the host's first-hop
router has been configured to match a flow from the host's IP address
to a destination IP address that is reached through ISP. A Premium
flow is configured from a host with a rate which is both smaller than
the total Premium allocation Company A has from the ISP, r bytes per
second, and smaller than the amount of that allocation has been
assigned to other hosts in Company A. Packets are not marked in any
special way when they leave the host. The first-hop router clears the
Premium bit on all arriving packets, sets the Premium bit on all
packets in the designated flow, shapes packets in the Premium flow to
a configured rate and burst size, queues best-effort unmarked packets
in the low priority queue and shaped Premium packets in the high
priority queue, and sends packets from those two queues at simple
priority. Intermediate routers internal to Company A enqueue packets
in one of two output queues based on the Premium bit and service the
queues with simple priority. Border routers perform quite different
tasks, depending on whether they are processing an egress flow or an
ingress flow. An egress border router may perform some reshaping on
the aggregate Premium traffic to conform to rate r, depending on the
number of Premium flows aggregated. Ingress border routers only need
to perform a simple policing function that can be implemented with a
token bucket. In the example, the ISP accepts all Premium packets
from A as long as the flow does not exceed r bytes per second.
Figure 1. Premium traffic flow from end-host to organization's ISP
Clark's and Jacobson's proposals are markedly similar in the location
and type of functional blocks that are needed to implement them.
Furthermore, they implement quite different services which are not
incompatible in a network. The Premium service implements a
guaranteed peak bandwidth service with negligible queueing delay that
cannot starve best effort traffic and can be allocated in a fairly
straightforward fashion. This service would seem to have a strong
appeal for commercial applications, video broadcasts, voice-over-IP,
and VPNs. On the other hand, this service may prove both too
restrictive (in its hard limits) and overdesigned (no overallocation)
for some applications. The Assured service implements a service that
has the same delay characteristics as (undropped) best effort packets
and the firmness of its guarantee depends on how well individual
links are provisioned for bursts of Assured packets. On the other
hand, it permits traffic flows to use any additional available
capacity without penalty and occasional dropped packets for short
congestive periods may be acceptable to many users. This service
might be what an ISP would provide to individual customers who are
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willing to pay a bit more for internet service that seems unaffected
by congestive periods. Both services are only as good as their
admission control schemes, though this can be more difficult for
traffic which is not peak-rate allocated.
There may be some additional benefits of deploying both services. To
the extent that Premium service is a conservative allocation of
resources, unused bandwidth that had been allocated to Premium might
provide some "headroom" for underallocated or burst periods of
Assured traffic or for best effort. Network elements that deploy both
services will be performing RED queue management on all non-Premium
traffic, as suggested in [4], and the effects of mixing the Premium
streams with best effort might serve to reduce burstiness in the
latter. A strength of the Assured service is that it allows bursts to
happen in their natural fashion, but this also makes the
provisioning, admission control and allocation problem more difficult
so it may take more time and experimentation before this admission
policy for this service is completely defined. A Premium service
could be deployed that employs static allocations on peak rates with
no statistical sharing.
As there appear to be a number of advantages to an architecture that
permits these two types of service and because, as we shall see, they
can be made to share many of the same mechanisms, we propose
designating two bit-patterns from the IP header precedence field. We
leave the explicit designation of these bit-patterns to the standards
process thus we use the shorthand notation of denoting each pattern
by a bit, one we will call the Premium or P-bit, the other we call
the assurance or A-bit. It is possible for a network to implement
only one of these services and to have network elements that only
look at the one applicable bit, but we focus on the two service
architecture. Further, we assume the case where no changes are made
in the hosts, appropriate packet marking all being done in the
network, at the first-hop, or leaf, router. We describe the
forwarding path architecture in this section, assuming that the
service has been allocated through mechanisms we will discuss in
section 4.
In a more general sense, Premium service denotes packets that are
enqueued at a higher priority than the ordinary best-effort queue.
Similarly, Assured service denotes packets that are treated
preferentially with respect to the dropping probability within the
"normal" queue. There are a number of ways to add more service levels
within each of these service types [7], but this document takes the
position of specifying the base-level services of Premium and
Assured.
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The forwarding path mechanisms can be broken down into those that
happen at the input interface, before packet forwarding, and those
that happen at the output interface, after packet forwarding.
Intermediate routers only need to implement the post packet
forwarding functions, while leaf and border routers must perform
functions on arriving packets before forwarding. We describe the
mechanisms this way for illustration; other ways of composing their
functions are possible.
Leaf routers are configured with a traffic profile for a particular
flow based on its packet header. This functionality has been defined
by the RSVP Working Group in RFC 2205. Figure 2 shows what happens to
a packet that arrives at the leaf router, before it is passed to the
forwarding engine. All arriving packets must have both the A-bit and
the P-bit cleared after which packets are classified on their header.
If the header does not match any configured values, it is immediately
forwarded. Matched flows pass through individual Markers that have
been configured from the usage profile for that flow: service class
(Premium or Assured), rate (peak for Premium, "expected" for
Assured), and permissible burst size (may be optional for Premium).
Assured flow packets emerge from the Marker with their A-bits set
when the flow is in conformance to its Profile, but the flow is
otherwise unchanged. For a Premium flow, the Marker will hold packets
when necessary to enforce their configured rate. Thus Premium flow
packets emerge from the Marker in a shaped flow with their P-bits
set. (It is possible for Premium flow packets to be dropped inside of
the Marker as we describe below.) Packets are passed to the
forwarding engine when they emerge from Markers. Packets that have
either their P or A bits set we will refer to as Marked packets.
Figure 2. Block diagram of leaf router input functionality
Figure 3 shows the inner workings of the Marker. For both Assured and
Premium packets, a token bucket "fills" at the flow rate that was
specified in the usage profile. For Assured service, the token bucket
depth is set by the Profile's burst size. For Premium service, the
token bucket depth must be limited to the equivalent of only one or
two packets. (We suggest a depth of one packet in early deployments.)
When a token is present, Assured flow packets have their A-bit set to
one, otherwise the packet is passed to the forwarding engine. For
Premium-configured Marker, arriving packets that see a token present
have their P-bits set and are forwarded, but when no token is
present, Premium flow packets are held until a token arrives. If a
Premium flow bursts enough to overflow the holding queue, its packets
will be dropped. Though the flow set up data can be used to configure
a size limit for the holding queue (this would be the meaning of a
"burst" in Premium service), it is not necessary. Unconfigured
holding queues should be capable of holding at least two bandwidth-
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delay products, adequate for TCP connections. A smaller value might
be used to suit delay requirements of a specific application.
Figure 3. Markers to implement the two different services
In practice, the token bucket should be implemented in bytes and a
token is considered to be present if the number of bytes in the
bucket is equal or larger to the size of the packet. For Premium, the
bucket can only be allowed to fill to the maximum packet size; while
Assured may fill to the configured burst parameter. Premium traffic
is held until a sufficient byte credit has accumulated and this
holding buffer provides the only real queue the flow sees in the
network. For Assured, traffic, we just test if the bytes in the
bucket are sufficient for the packet size and set A if so. If not,
the only difference is that A is not set. Assured traffic goes into a
queue following this step and potentially sees a queue at every hop
along its path.
Each output interface of a router must have two queues and must
implement a test on the P-bit to select a packet's output queue. The
two queues must be serviced by simple priority, Premium packets
first. Each output interface must implement the RED-based RIO
mechanism described in [3] on the lower priority queue. RIO uses two
thresholds for when to begin dropping packets, a lower one based on
total queue occupancy for ordinary best effort traffic and one based
on the number of packets enqueued that have their A-bit set. This
means that any action preferential to Assured service traffic will
only be taken when the queue's capacity exceeds the threshold value
for ordinary best effort service. In this case, only unmarked packets
will be dropped (using the RED algorithm) unless the threshold value
for Assured service is also reached. Keeping an accurate count of the
number of A-bit packets currently in a queue requires either testing
the A-bit at both entry and exit of the queue or some additional
state in the router. Figure 4 is a block diagram of the output
interface for all routers.
Figure 4. Router output interface for two-bit architecture
The packet output of a leaf router is thus a shaped stream of packets
with P-bits set mingled with an unshaped best effort stream of
packets, some of which may have A-bits set. Premium service clearly
cannot starve best effort traffic because it is both burst and
bandwidth controlled. Assured service might rely only on a
conservative allocation to prevent starvation of unmarked traffic,
but bursts of Assured traffic might then close out best-effort
traffic at bottleneck queues during congestive periods.
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After [3], we designate the forwarding path objects that test flows
against their usage profiles "Profile Meters". Border routers will
require Profile Meters at their input interfaces. The bilateral
agreement between adjacent administrative domains must specify a peak
rate on all P traffic and a rate and burst for A traffic (and
possibly a start time and duration). A Profile Meter is required at
the ingress of a trust region to ensure that differentiated service
packet flows are in compliance with their agreed-upon rates. Non-
compliant packets of Premium flows are discarded while non-compliant
packets of Assured flows have their A-bits reset. For example, in
figure 1, if the ISP has agreed to supply Company A with r bytes/sec
of Premium service, P-bit marked packets that enter the ISP through
the link from Company A will be dropped if they exceed r. If instead,
the service in figure 1 was Assured service, the packets would simply
be unmarked, forwarded as best effort.
The simplest border router input interface is a Profile Meter
constructed from a token bucket configured with the contracted rate
across that ingress link (see figure 5). Each type, Premium or
Assured, and each interface must have its own profile meter
corresponding to a particular class across a particular boundary.
(This is in contrast to models where every flow that crosses the
boundary must be separately policed and/or shaped.) The exact
mechanisms required at a border router input interface depend on the
allocation policy deployed; a more complex approach is presented in
section 4.
Figure 5. Border router input interface Profile Meters
Section 2.3 introduced the forwarding path objects of Markers and
Profile Meters. In this section we specify the primitive building
blocks required to compose them. The primitives are: general
classifier, bit-pattern classifier, bit setter, priority queues,
policing token bucket and shaping token bucket. These primitives can
compose a Marker (either a policing or a shaping token bucket plus a
bit setter) and a Profile Meter (a policing token bucket plus a
dropper or bit setter).
General Classifier: Leaf or first-hop routers must perform a
transport-level signature matching based on a tuple in the packet
header, a functionality which is part of any RSVP-capable router. As
described above, packets whose tuples match one of the configured
flows are conformance tested and have the appropriate service bit
set. This function is memory- and processing-intensive, but is kept
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at the edges of the network where there are fewer flows.
Bit-pattern classifier: This primitive comprises a simple two-way
decision based on whether a particular bit-pattern in the IP header
is set or not. As in figure 4, the P-bit is tested when a packet
arrives at a non-leaf router to determine whether to enqueue it in
the high priority output queue or the low priority packet queue. The
A-bit of packets bound for the low priority queue is tested to 1)
increment the count of Assured packets in the queue if set and 2)
determine which drop probability will be used for that packet.
Packets exiting the low priority queue must also have the A-bit
tested so that the count of enqueued Assured packets can be
decremented if necessary.
Bit setter: The A-bits and P-bits must be set or cleared in several
places. A functional block that sets the appropriate bits of the IP
header to a configured bit-pattern would be the most general.
Priority queues: Every network element must include (at least) two
levels of simple priority queueing. The high priority queue is for
the Premium traffic and the service rule is to send packets in that
queue first and to exhaustion. Recall that Premium traffic must never
be oversubscribed, thus Premium traffic should see little or no
queue.
Shaping token bucket:This is the token bucket required at the leaf
router for Premium traffic and shown in figure 3. As we shall see,
shaping is also useful at egress points of a trust region. An
arriving packet is immediately forwarded if there is a token present
in the bucket, otherwise the packet is enqueued until the bucket
contains tokens sufficient to send it. Shaping requires clocking
mechanisms, packet memory, and some state block for each flow and is
thus a memory and computation-intensive process.
Policing token bucket: This is the token bucket required for Profile
Meters and shown in figure 5. Policing token buckets never hold
arriving packets, but check on arrival to see if a token is available
for the packet's service class. If so, the packet is forwarded
immediately. If not, the policing action is taken, dropping for
Premium and reclassifying or unmarking for Assured.
Clearly, mechanisms are required to communicate the information
about the request to the leaf router. This configuration information
is the rate, burst, and whether it is a Premium or Assured type.
There may also need to be a specific field to set or clear this
configuration. This information can be passed in a number of ways,
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including using the semantics of RSVP, SNMP, or directly set by a
network administrator in some other way. There must be some
mechanisms for authenticating the sender of this information. We
expect configuration to be done in a variety of ways in early
deployments and a protocol and mechanism for this to be a topic for
future standards work.
The requirements of shapers motivate their placement at the edges of
the network where the state per router can be smaller than in the
middle of a network. The greatest burden of flow matching and shaping
will be at leaf routers where the speeds and buffering required
should be less than those that might be required deeper in the
network. This functionality is not required at every network element
on the path. Routers that are internal to a trust region will not
need to shape traffic. Border routers may need or desire to shape the
aggregate flow of Marked packets at their egress in order to ensure
that they will not burst into non-compliance with the policing
mechanism at the ingress to the other domain (though this may not be
necessary if the in-degree of the router is low). Further, the
shaping would be applied to an aggregation of all the Premium flows
that exit the domain via that path, not to each flow individually.
These mechanisms are within reach of today's technology and it seems
plausible to us that Premium and Assured services are all that is
needed in the Internet. If, in time, these services are found
insufficient, this architecture provides a migration path for
delivering other kinds of service levels to traffic. The A- and P-
bits would continue to be used to identify traffic that gets Marked
service, but further filter matching could be done on packet headers
to differentiate service levels further. Using the bits this way
reduces the number of packets that have to have further matching done
on them rather than filtering every incoming packet. More queue
levels and more complex scheduling could be added for P-bit traffic
and more levels of drop priority could be added for A-bit traffic if
experience shows them to be necessary and processing speeds are
sufficient. We propose that the services described here be considered
as "at least" services. Thus, a network element should at least be
capable of mapping all P-bit traffic to Premium service and of
mapping all A-bit traffic to be treated with one level of priority in
the "best effort" queue (it appears that the single level of A-bit
traffic should map to a priority that is equivalent to the best level
in a multi-level element that is also in the path).
On the other hand, what is the downside of deploying an architecture
for both classes of service if later experience convinces us that
only one of them is needed? The functional blocks of both service
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classes are similar and can be provided by the same mechanism,
parameterized differently. If Assured service is not used, very
little is lost. A RED-managed best effort queue has been strongly
recommended in [4] and, to the extent that the deployment of this
architecture pushes the deployment of RED-managed best effort queues,
it is clearly a positive. If Premium service goes unused, the two-
queues with simple priority service is not required and the shaping
function of the Marker may be unused, thus these would impose an
unnecessary implementation cost.
Thus far we have focused on the service definitions and the
forwarding path mechanisms. We now turn to the problem of allocating
the level of Marked traffic throughout the Internet. We observe that
most organizations have fixed portions of their budgets, including
data communications, that are determined on an annual or quarterly
basis. Some additional monies might be attached to specific projects
for discretionary costs that arise in the shorter term. In turn,
service providers (ISPs and NSPs) must do their planning on annual
and quarterly bases and thus cannot be expected to provide
differentiated services purely "on call". Provisioning sets up static
levels of Marked traffic while call set-up creates an allocation of
Marked traffic for a single flow's duration. Static levels can be
provisioned with time-of-day specifications, but cannot be changed in
response to a dynamic message. We expect both kinds of bandwidth
allocation to be important. The purchasers of Marked services can
generally be expected to work on longer-term budget cycles where
these services will be accounted for similarly to many information
services today. A mail-order house may wish to purchase a fixed
allocation of bandwidth in and out of its web-server to give
potential customers a "fast" feel when browsing their site. This
allocation might be based on hit rates of the previous quarter or
some sort of industry-based averages. In addition, there needs to be
a dynamic allocation capability to respond to particular events, such
as a demonstration, a network broadcast by a company's CEO, or a
particular network test. Furthermore, a dynamic capability may be
needed in order to meet a precommitted service level when the
particular source or destination is allowed to be "anywhere on the
Internet". "Dynamic" covers the range from a telephoned or e-mailed
request to a signalling type model. A strictly statically allocated
scenario is expected to be useful in initial deployment of
differentiated services and to make up a major portion of the Marked
traffic for the forseeable future.
Without a "per call" dynamic set up, the preconfiguring of usage
profiles can always be construed as "paying for bits you don't use"
whether the type of service is Premium or Assured. We prefer to think
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of this as paying for the level of service that one expects to have
available at any time, for example paying for a telephone line. A
customer might pay an additional flat fee to have the privilege of
calling a wide local area for no additional charge or might pay by
the call. Although a customer might pay on a "per call" basis for
every call made anywhere, it generally turns out not to be the most
economical option for most customers. It's possible similar pricing
structures might arise in the internet.
We use Allocation to refer to the process of making Marked traffic
commitments anywhere along this continuum from strictly preallocated
to dynamic call set-up and we require an Allocation architecture
capable of encompassing this entire spectrum in any mix. We further
observe that Allocation must follow organizational hierarchies, that
is each organization must have complete responsibility for the
Allocation of the Marked traffic resource within its domain. Finally,
we observe that the only chance of success for incremental deployment
lies in an Allocation architecture that is made up of bilateral
agreements, as multilateral agreements are much too complex to
administer. Thus, the Allocation architecture is made up of
agreements across boundaries as to the amount of Marked traffic that
will be allowed to pass. This is similar to "settlement" models used
today.
The goal of differentiated services is controlled sharing of some
organization's Internet bandwidth. The control can be done
independently by individuals, i.e., users set bit(s) in their packets
to distinguish their most important traffic, or it can be done by
agents that have some knowledge of the organization's priorities and
policies and allocate bandwidth with respect to those policies.
Independent labeling by individuals is simple to implement but
unlikely to be sufficient since it's unreasonable to expect all
individuals to know all their organization's priorities and current
network use and always mark their traffic accordingly. Thus this
architecture is designed with agents called bandwidth brokers (BB)
[2], that can be configured with organizational policies, keep track
of the current allocation of marked traffic, and interpret new
requests to mark traffic in light of the policies and current
allocation.
We note that such agents are inherent in any but the most trivial
notions of sharing. Neither individuals nor the routers their
packets transit have the information necessary to decide which
packets are most important to the organization. Since these agents
must exist, they can be used to allocate bandwidth for end-to-end
connections with far less state and simpler trust relationships than
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deploying per flow or per filter guarantees in all network elements
on an end-to-end path. BBs make it possible for bandwidth allocation
to follow organizational hierarchies and, in concert with the
forwarding path mechanisms discussed in section 3, reduce the state
required to set up and maintain a flow over architectures that
require checking the full flow header at every network element.
Organizationally, the BB architecture is motivated by the observation
that multilateral agreements rarely work and this architecture allows
end-to-end services to be constructed out of purely bilateral
agreements. BBs only need to establish relationships of limited trust
with their peers in adjacent domains, unlike schemes that require the
setting of flow specifications in routers throughout an end-to-end
path. In practical technical terms, the BB architecture makes it
possible to keep state on an administrative domain basis, rather than
at every router and the service definitions of Premium and Assured
service make it possible to confine per flow state to just the leaf
routers.
BBs have two responsibilities. Their primary one is to parcel out
their region's Marked traffic allocations and set up the leaf routers
within the local domain. The other is to manage the messages that are
sent across boundaries to adjacent regions' BBs. A BB is associated
with a particular trust region, one per domain. A BB has a policy
database that keeps the information on who can do what when and a
method of using that database to authenticate requesters. Only a BB
can configure the leaf routers to deliver a particular service to
flows, crucial for deploying a secure system. If the deployment of
Differentiated Services has advanced to the stage where dynamically
allocated, marked flows are possible between two adjacent domains,
BBs also provide the hook needed to implement this. Each domain's BB
establishes a secure association with its peer in the adjacent domain
to negotiate or configure a rate and a service class (Premium or
Assured) across the shared boundary and through the peer's domain. As
we shall see, it is possible for some types of service and
particularly in early implementations, that this "secure association"
is not automatic but accomplished through human negotiation and
subsequent manual configuration of the adjacent BBs according to the
negotiated agreement. This negotiated rate is a capability that a BB
controls for all hosts in its region.
When an allocation is desired for a particular flow, a request is
sent to the BB. Requests include a service type, a target rate, a
maximum burst, and the time period when service is required. The
request can be made manually by a network administrator or a user or
it might come from another region's BB. A BB first authenticates the
credentials of the requester, then verifies there exists unallocated
bandwidth sufficient to meet the request. If a request passes these
tests, the available bandwidth is reduced by the requested amount and
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the flow specification is recorded. In the case where the flow has a
destination outside this trust region, the request must fall within
the class allocation through the "next hop" trust region that was
established through a bilateral agreement of the two trust regions.
The requester's BB informs the adjacent region's BB that it will be
using some of this rate allocation. The BB configures the appropriate
leaf router with the information about the packet flow to be given a
service at the time that the service is to commence. This
configuration is "soft state" that the BB will periodically refresh.
The BB in the adjacent region is responsible for configuring the
border router to permit the allocated packet flow to pass and for any
additional configurations and negotiations within and across its
borders that will allow the flow to reach its final destination.
At DMZs, there must be an unambiguous way to determine the local
source of a packet. An interface's source could be determined from
its MAC address which would then be used to classify packets as
coming across a logical link directly from the source domain
corresponding to that MAC address. Thus with this understanding we
can continue to use figures illustrating a single pipe between two
different domains.
In this way, all agreements and negotiations are performed between
two adjacent domains. An initial request might cause communication
between BBs on several domains along a path, but each communication
is only between two adjacent BBs. Initially, these agreements will be
prenegotiated and fairly static. Some may become more dynamic as the
service evolves.
This section gives examples of BB transactions in a non-trivial,
multi-transit-domain Internet. The BB framework allows operating
points across a spectrum from "no signalling across boundaries" to
"each flow set up dynamically". We might expect to move across this
spectrum over time, as the necessary mechanisms are ubiquitously
deployed and BBs become more sophisticated, but the statically
allocated portions of the spectrum should always have uses. We
believe the ability to support this wide spectrum of choices
simultaneously will be important both in incremental deployment and
in allowing ISPs to make a wide range of offerings and pricings to
users. The examples of this section roughly follow the spectrum of
increasing sophistication. Note that we assume that domains contract
for some amount of Marked traffic which can be requested as either
Assured or Premium in each individual flow setup transaction. The
examples say "Marked" although actual transactions would have to
specify either Assured or Premium.
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A statically configured example with no BB messages exchanged: Here
all allocations are statically preallocated through purely bilateral
agreements between users (individual TCPs, individual hosts, campus
networks, or whole ISPs) [6]. The allocations are in the form of
usage profiles of rate, burst, and a time during which that profile
is to be active. Users and providers negotiate these Profiles which
are then installed in the user domain BB and in the provider domain
BB. No BB messages cross the boundary; we assume this negotiation is
done by human representatives of each domain. In this case, BBs only
have to perform one of their two functions, that of allocating this
Profile within their local domain. It is even possible to set all of
this suballocations up in advance and then the BB only needs to set
up and tear down the Profile at the proper time and to refresh the
soft state in the leaf routers. From the user domain BB, the Profile
is sent as soft state to the first hop router of the flow during the
specified time. These Profiles might be set using RSVP, a variant of
RSVP, SNMP, or some vendor-specific mechanism. Although this static
approach can work for all Marked traffic, due to the strictly not
oversubscribed requirement, it is only appropriate for Premium
traffic as long as it is kept to a small percentage of the bottleneck
path through a domain or is otherwise constrained to a well-known
behavior. Similar restrictions might hold for Assured depending on
the expectation associated with the service.
In figure 6, we show an example of setting a Profile in a leaf
router. A usage profile has been negotiated with the ISP for the
entire domain and the BB parcels it out among individual flows as
requested. The leaf router mechanism is that shown in figure 3, with
the token bucket set to the parameters from the usage profile. The
ISP's BB would configure its own Profile Meter at the ingress router
from that customer to ensure the Profile was maintained. This
mechanism was shown in figure 5. We assume that the time duration and
start times for any Profile to be active are maintained in the BB.
The Profile is sent to the ingress device or cleared from the ingress
device by messages sent from the BB. In this example, we assume that
van@lbl wants to talk to ddc@mit. The LBL-BB is sent a request from
Van asking that premium service be assigned to a flow that is
designated as having source address "V:4" and going to destination
address "D:8". This flow should be configured for a rate of 128kb/sec
and allocated from 1pm to 3pm. The request must be "signed" in a
secure, verifiable manner. The request might be sent as data to the
LBL-BB, an e-mail message to a network administrator, or in a phone
call to a network administrator. The LBL-BB receives this message,
verifies that there is 128kb/sec of unused Premium service for the
domain from 1-3pm, then sends a message to Leaf1 that sets up an
appropriate Profile Meter. The message to Leaf1 might be an RSVP
message, or SNMP, or some proprietary method. All the domains passed
must have sufficient reserve capacity to meet this request.
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Figure 6. Bandwidth Broker setting Profiles in leaf routers
A statically configured example with BB messages exchanged: Next we
present an example where all allocations are statically preallocated
but BB messages are exchanged for greater flexibility. Figure 7 shows
an end-to-end example for Marked traffic in a statically allocated
internet. The numbers at the trust region boundaries indicate the
total statically allocated Marked packet rates that will be accepted
across those boundaries. For example, 100kbps of Marked traffic can
be sent from LBL to ESNet; a Profile Meter at the ESNet egress
boundary would have a token bucket set to rate 100kbps. (There MAY be
a shaper set at LBL's egress to ensure that the Marked traffic
conforms to the aggregate Profile.) The tables inside the transit
network "bubbles" show their policy databases and reflect the values
after the transaction is complete. In Figure 7, V wants to transmit a
flow from LBL to D at MIT at 10 Kbps. As in figure 6, a request for
this profile is made of LBL's BB. LBL's BB authenticates the request
and checks to see if there is 10kbps left in its Marked allocation
going in that direction. There is, so the LBL-BB passes a message to
the ESNet-BB saying that it would like to use 10kbps of its Marked
allocation for this flow. ESNet authenticates the message, checks its
database and sees that it has a 10kbps Marked allocation to NEARNet
(the next region in that direction) that is being unused. The policy
is that ESNet-BB must always inform ("ask") NEARNet-BB when it is
about to use part of its allocation. NEARNET-BB authenticates the
message, checks its database and discovers that 20kbps of the
allocation to MIT is unused and the policy at that boundary is to not
inform MIT when part of the allocation is about to be used ("<50 ok"
where the total allocation is 50). The dotted lines indicate the
"implied" transaction, that is the transaction that would have
happened if the policy hadn't said "don't ask me". Now each BB can
pass an "ok" message to this request across its boundary. This allows
V to send to D, but not vice versa. It would also be possible for the
request to originate from D.
Figure 7. End-to-end example with static allocation.
Consider the same example where the ESNet-BB finds all of its Marked
allocation to NEARNet, 10 kbps, in use. With static allocations,
ESNet must transmit a "no" to this request back to the LBL-BB.
Presumably, the LBL-BB would record this information to complain to
ESNet about the overbooking at the end of the month! One solution to
this sort of "busy signal" is for ESNet to get better at anticipating
its customers needs or require long advance bookings for every flow,
but it's also possible for bandwidth brokerage decisions to become
dynamic.
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Figure 8. End-to-end static allocation example with no remaining
allocation
Dynamic Allocation and additional mechanism: As we shall see, dynamic
allocation requires more complex BBs as well as more complex border
policing, including the necessity to keep more state. However, it
enables an important service with a small increase in state.
The next set of figures (starting with figure 9) show what happens in
the case of dynamic allocation. As before, V requests 10kbps to talk
to D at MIT. Since the allocation is dynamic, the border policers do
not have a preset value, instead being set to reflect the current
peak value of Marked traffic permitted to cross that boundary. The
request is sent to the LBL-BB.
Figure 9. First step in end-to-end dynamic allocation example.
In figure 10, note that ESNet has no allocation set up to NEARNet.
This system is capable of dynamic allocations in addition to static,
so it asks NEARNet if it can "add 10" to its allocation from ESNet.
As in the figure 7 example, MIT's policy is set to "don't ask" for
this case, so the dotted lines represent "implicit transactions"
where no messages were exchanged. However, NEARNet does update its
table to indicate that it is now using 20kbps of the Marked
allocation to MIT.
Figure 10. Second step in end-to-end dynamic allocation example
In figure 11, we see the third step where MIT's "virtual ok" allows
the NEARNet-BB to tell its border router to increase the Marked
allocation across the ESNet-NEARNet boundary by 10 kbps.
Figure 11. Third step in end-to-end dynamic allocation example
Figure 11 shows NEARNet-BB's "ok" for that request transmitted back
to ESNet-BB. This causes ESNet-BB to send its border router a message
to create a 10 kbps subclass for the flow "V->D". This is required in
order to ensure that the 10kpbs that has just been dynamically
allocated gets used only for that connection. Note that this does
require that the per flow state be passed from LBL-BB to ESNet-BB,
but this is the only boundary that needs that level of flow
information and this further classification will only need to be done
at that one boundary router and only on packets coming from LBL. Thus
dynamic allocation requires more complex Profile Metering than that
shown in figure 5.
Figure 12. Fourth step in end-to-end dynamic allocation example.
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In figure 12, the ESNet border router gives the "ok" that a subclass
has been created, causing the ESNet-BB to send an "ok" to the LBL-BB
which lets V know the request has been approved.
Figure 13. Final step in end-to-end dynamic allocation example
For dynamic allocation, a basic version of a CBQ scheduler [5] would
have all the required functionality to set up the subclasses. RSVP
currently provides a way to move the TSpec for the flow.
For multicast flows, we assume that packets that are bound for at
least one egress can be carried through a domain at that level of
service to all egress points. If a particular multicast branch has
been subscribed to at best-effort when upstream branches are Marked,
it will have its bit settings cleared before it crosses the boundary.
The information required for this flow identification is used to
augment the existing state that is already kept on this flow because
it is a multicast flow. We note that we are already "catching" this
flow, but now we must potentially clear the bit-pattern.
Much work has been done in recent years on the definition of related
integrated services for the internet and the specification of the
RSVP signalling protocol. The two-bit architecture proposed in this
work can easily interoperate with those specifications. In this
section we first discuss how the forwarding mechanisms described in
section 3 can be used to support integrated services. Second, we
discuss how RSVP could interoperate with the administrative structure
of the BBs to provide better scaling.
We believe that the forwarding path mechanisms described in section 3
are general enough that they can also be used to provide the
Controlled-Load service [8] and a version of the Guaranteed Quality
of Service [9], as developed by the int-serv WG. First note that
Premium service can be thought of as a constrained case of
Controlled-Load service where the burst size is limited to one packet
and where non-conforming packets are dropped. A network element that
has implemented the mechanisms to support premium service can easily
support the more general controlled-load service by making one or
more minor parameter adjustments, e.g. by lifting the constraint on
the token bucket size, or configuring the Premium service rate with
the peak traffic rate parameter in the Controlled-Load specification,
and by changing the policing action on out-of-profile packets from
dropping to sending the packets to the Best-effort queue.
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It is also possible to implement Guaranteed Quality of Service using
the mechanisms of Premium service. From RFC 2212 [9]: "The definition
of guaranteed service relies on the result that the fluid delay of a
flow obeying a token bucket (r, b) and being served by a line with
bandwidth R is bounded by b/R as long as R is no less than r.
Guaranteed service with a service rate R, where now R is a share of
bandwidth rather than the bandwidth of a dedicated line approximates
this behavior." The service model of Premium clearly fits this model.
RFC 2212 states that "Non-conforming datagrams SHOULD be treated as
best-effort datagrams." Thus, a policing Profile Meter that drops
non-conforming datagrams would be acceptable, but it's also possible
to change the action for non-compliant packets from a drop to sending
to the best-effort queue.
In this section we discuss how RSVP signaling can be used in
conjunction with the BBs described in section 4 to deliver a more
scalable end-to-end resource set up for Integrated Services. First we
note that the BB architecture has three major differences with the
original RSVP resource set up model:
1. There exist apriori bilateral business relations between BBs of
adjacent trust regions before one can set up end-to-end resource
allocation; real-time signaling is used only to activate/confirm the
availability of pre-negotiated Marked bandwidth, and to dynamically
readjust the allocation amount when necessary. We note that this
real-time signaling across domains is not required, but depends on
the nature of the bilateral agreement (e.g., the agreement might
state "I'll tell you whenever I'm going to use some of my allocation"
or not).
2. A few bits in the packet header, i.e. the P-bit and A-bit, are
used to mark the service class of each packet, therefore a full
packet classification (by checking all relevant fields in the header)
need be done only once at the leaf router; after that packets will be
served according to their class bit settings.
3. RSVP resource set up assumes that resources will be reserved hop-
by-hop at each router along the entire end-to-end path.
RSVP messages sent to leaf routers by hosts can be intercepted and
sent to the local domain's BB. The BB processes the message and, if
the request is approved, forwards a message to the leaf router that
sets up appropriate per-flow packet classification. A message should
also be sent to the egress border router to add to the aggregate
Marked traffic allocation for packet shaping by the Profile Meter on
outbound traffic. (Its possible that this is always set to the full
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allocation.) An RSVP message must be sent across the boundary to
adjacent ISP's border router, either from the local domain's border
router or from the local domain's BB. If the ISP is also implementing
the RSVP with a BB and diff-serv framework, its border router
forwards the message to the ISP's local BB. A similar process (to
what happened in the first domain) can be carried out in the ISP
domain, then an RSVP message gets forwarded to the next ISP along the
path. Inside a domain, packets are served solely according to the
Marked bits. The local BB knows exactly how much Premium traffic is
permitted to enter at each border router and from which border router
packets exit.
This document has presented a reference architecture for
differentiated services. Several variations can be envisioned,
particularly for early and partial deployments, but we do not
enumerate all of these variations here. There has been a great market
demand for differentiated services lately. As one of the many efforts
to meet that demand this memo sketches out the framework of a
flexible architecture for offering differential services, and in
particular defines a simple set of packet forwarding path mechanisms
to support two basic types of differential services. Although there
remain a number of issues and parameters that need further
exploration and refinement, we believe it is both possible and
feasible at this time to start deployment of differentiated services
incrementally. First, given that the basic mechanisms required in the
packet forwarding path are clearly understood, both Assured and
Premium services can be implemented today with manually configured
BBs and static resource allocation. Initially we recommend
conservative choices on the amount of Marked traffic that is admitted
into the network. Second, we plan to continue the effort started with
this memo and the experimental work of the authors to define and
deploy increasingly sophisticated BBs. We hope to turn the experience
gained from in-progress trial implementations on ESNet and CAIRN into
future proposals to the IETF.
Future revisions of this memo will present the receiver-based and
multicast flow allocations in detail. After this step is finished,
we believe the basic picture of an scalable, robust, secure resource
management and allocation system will be completed. In this memo, we
described how the proposed architecture supports two services that
seem to us to provide at least a good starting point for trial
deployment of differentiated services. Our main intent is to define
an architecture with three services, Premium, Assured, and Best
effort, that can be determined by specific bit- patterns, but not to
preclude additional levels of differentiation within each service. It
seems that more experimentation and experience is required before we
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RFC 2638 Two-bit Differentiated Services Architecture July 1999
could standardize more than one level per service class. Our base-
level approach says that everyone has to provide "at least" Premium
service and Assured service as documented. We feel rather strongly
about both 1) that we should not try to define, at this time,
something beyond the minimalist two service approach and 2) that the
architecture we define must be open-ended so that more levels of
differentiation might be standardized in the future. We believe this
architecture is completely compatible with approaches that would
define more levels of differentiation within a particular service, if
the benefits of doing so become well understood.
The authors have benefited from many discussions, both in person and
electronically and wish to particularly thank Dave Clark who has been
responsible for the genesis of many of the ideas presented here,
though he does not agree with all of the content this document. We
also thank Sally Floyd for comments on an earlier draft. A comment
from Jon Crowcroft was partially responsible for our including
section 5. Comments from Fred Baker made us try to make it clearer
that we are defining two base-level services, irrespective of the bit
patterns used to encode them.
[1] D. Clark, "Adding Service Discrimination to the Internet",
Proceedings of the 23rd Annual Telecommunications Policy Research
Conference (TPRC), Solomons, MD, October 1995.
[2] V. Jacobson, "Differentiated Services Architecture", talk in the
Int-Serv WG at the Munich IETF, August, 1997.
[3] Clark, D. and J. Wroclawski, "An Approach to Service Allocation
in the Internet", Work in Progress, also talk by D. Clark in the
Int-Serv WG at the Munich IETF, August, 1997.
[4] Braden, et al., "Recommendations on Queue Management and
Congestion Avoidance in the Internet", RFC 2309, April 1998.
[4] Braden, R., Zhang, L., Berson, S., Herzog, S. and S. Jamin,
"Resource Reservation Protocol (RSVP) - Version 1 Functional
Specification", RFC 2205, September 1997.
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RFC 2638 Two-bit Differentiated Services Architecture July 1999
[5] S. Floyd and V. Jacobson, "Link-sharing and Resource Management
Models for Packet Networks", IEEE/ACM Transactions on Networking,
pp 365-386, August 1995.
[6] D. Clark, private communication, October 26, 1997.
[7] "Advanced QoS Services for the Intelligent Internet", Cisco
Systems White Paper, 1997.
[8] Wroclawski, J., "Specification of the Controlled-Load Network
Element Service", RFC 2211, September 1997.
[9] Shenker, S., Partirdge, C. and R. Guerin, "Specification of
Guaranteed Quality of Service", RFC 2212, September 1997.
[10] D. Clark and W. Fang, "Explicit Allocation of Best Effort packet
Delivery Service", IEEE/ACM Transactions on Networking, August,
1998, Vol6, No 4, pp. 362-373. also at: http://
diffserv.lcs.mit.edu/Papers/exp-alloc-ddc-wf.pdf
Authors' Addresses
Kathleen Nichols
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
Phone: 408-525-4857
EMail: kmn@cisco.com
Van Jacobson
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
EMail: van@cisco.com
Lixia Zhang
UCLA
4531G Boelter Hall
Los Angeles, CA 90095
Phone: 310-825-2695
EMail: lixia@cs.ucla.edu
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Appendix: A Combined Approach to Differential Service in the Internet by
David D. Clark
After the draft-nichols-diff-svc-00 was submitted, the co-authors had
a discussion with Dave Clark and John Wroclawski which resulted in
Clark's using the presentation slot for the draft at the December
1997 IETF Integrated Services Working Group meeting. A reading of the
slides shows that it was Clark's proposal on "mechanisms",
"services", and "rules" and how to proceed in the standards process
that has guided much of the process in the subsequently formed IETF
Differentiated Services Working Group. We believe Dave Clark's talk
gave us a solid approach for bringing quality of service to the
Internet in a manner that is compatible with its strengths.
The slides presented at the December 1997 IETF Integrated Services
Working Group are included with the Postscript version.
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Full Copyright Statement
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