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Network Working Group                                          K. NicholsRequest for Comments: 2638                                    V. JacobsonCategory: Informational                                             Cisco                                                                 L. Zhang                                                                     UCLA                                                                July 1999A Two-bit Differentiated Services Architecture for the InternetStatus 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 similarNichols, 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 19992. Architecture2.1 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 19992.2 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 usingNichols, 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 ISP2.3 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 areNichols, 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 insection 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 inRFC 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 insection 4.   Figure 5. Border router input interface Profile Meters3. Mechanisms3.1 Forwarding Path PrimitivesSection 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 keptNichols, 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 serviceNichols, 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 thinkNichols, 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 thanNichols, 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 insection 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 andNichols, 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 insection 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 insection 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. FromRFC 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 insection 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 fullNichols, 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 weNichols, 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 includingsection 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.pdfAuthors' 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.eduNichols, et al.              Informational                     [Page 24]

RFC 2638      Two-bit Differentiated Services Architecture     July 1999Appendix: A Combined Approach to Differential Service in the Internet by          David D. Clark   After thedraft-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 1999Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Nichols, et al.              Informational                     [Page 26]