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RFC 2638

A Two-bit Differentiated Services Architecture for the Internet

Pages: 26

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Network Working Group                                          K. Nichols
Request for Comments: 2638                                    V. Jacobson
Category: Informational                                             Cisco
                                                                 L. Zhang
                                                                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.


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.

1. Introduction

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

   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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2. Architecture

2.1 Background

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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2.2 Premium service

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

   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

2.3 Two-bit differentiated services architecture

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
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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

3. Mechanisms

3.1 Forwarding Path Primitives

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

   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.

3.2 Passing configuration information

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.

3.3 Discussion

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.

4. The Architectural Framework for Marked Traffic Allocation

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

4.1 Bandwidth Brokers: Allocating and Controlling Bandwidth Shares

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

   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.

4.2 Examples

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
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   Figure 8. End-to-end static allocation example with no remaining

   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.

5. RSVP/int-serv and this architecture

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.

5.1 Providing Controlled-Load and Guaranteed Service

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.

5.2 RSVP and BBs

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.

6. Recommendations

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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   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.

7. Acknowledgments

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.

8. Security Considerations

There are no security considerations associated with this document.

9. References

[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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   [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://

Authors' Addresses

Kathleen Nichols Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134-1706 Phone: 408-525-4857 EMail: Van Jacobson Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134-1706 EMail: Lixia Zhang UCLA 4531G Boelter Hall Los Angeles, CA 90095 Phone: 310-825-2695 EMail:
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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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