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rfc:rfc2638

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.

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

Nichols, et al. Informational [Page 1] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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.

Nichols, et al. Informational [Page 2] RFC 2638 Two-bit Differentiated Services Architecture July 1999

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.

Nichols, et al. Informational [Page 3] RFC 2638 Two-bit Differentiated Services Architecture July 1999

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

Nichols, et al. Informational [Page 4] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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.

Nichols, et al. Informational [Page 5] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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

Nichols, et al. Informational [Page 6] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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.

Nichols, et al. Informational [Page 7] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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-

Nichols, et al. Informational [Page 8] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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.

Nichols, et al. Informational [Page 9] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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

Nichols, et al. Informational [Page 10] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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.

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,

Nichols, et al. Informational [Page 11] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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

Nichols, et al. Informational [Page 12] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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

Nichols, et al. Informational [Page 13] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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.

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

Nichols, et al. Informational [Page 14] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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

Nichols, et al. Informational [Page 15] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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.

Nichols, et al. Informational [Page 16] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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.

Nichols, et al. Informational [Page 17] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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.

Nichols, et al. Informational [Page 18] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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.

Nichols, et al. Informational [Page 19] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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.

Nichols, et al. Informational [Page 20] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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

Nichols, et al. Informational [Page 21] RFC 2638 Two-bit Differentiated Services Architecture July 1999

 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

Nichols, et al. Informational [Page 22] 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.

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.

Nichols, et al. Informational [Page 23] 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

Nichols, et al. Informational [Page 24] RFC 2638 Two-bit Differentiated Services Architecture July 1999

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.

Nichols, et al. Informational [Page 25] RFC 2638 Two-bit Differentiated Services Architecture July 1999

Full Copyright Statement

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 This document and translations of it may be copied and furnished to
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Nichols, et al. Informational [Page 26]

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