Considerations in the development of a QoS Architecture for CCNx-like ICN protocols

Versions: 00 01 02 03 04 05 06                             Informational
ICNRG                                                            D. Oran
Internet-Draft                       Network Systems Research and Design
Intended status: Informational                            24 August 2020
Expires: 25 February 2021

 Considerations in the development of a QoS Architecture for CCNx-like
                             ICN protocols


   This is a position paper.  It documents the author's personal views
   on how Quality of Service (QoS) capabilities ought to be accommodated
   in ICN protocols like CCNx or NDN which employ flow-balanced
   Interest/Data exchanges and hop-by-hop forwarding state as their
   fundamental machinery.  It argues that such protocols demand a
   substantially different approach to QoS from that taken in TCP/IP,
   and proposes specific design patterns to achieve both classification
   and differentiated QoS treatment on both a flow and aggregate basis.
   It also considers the effect of caches in addition to memory, CPU and
   link bandwidth as a resource that should be subject to explicitly
   unfair resource allocation.  The proposed methods are intended to
   operate purely at the network layer, providing the primitives needed
   to achieve both transport and higher layer QoS objectives.  It
   explicitly excludes any discussion of Quality of Experience (QoE)
   which can only be assessed and controlled at the application layer or

   This document is not a product of the IRTF Information-Centric
   Networking Research Group (ICNRG) but has been through formal last
   call and has the support of the participants in the research group
   for publication as an individual submission.

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   This Internet-Draft will expire on 25 February 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Applicability Assessment by ICNRG Chairs  . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Background on Quality of Service in network protocols . . . .   4
     3.1.  Basics on how ICN protocols like NDN and CCNx work  . . .   6
     3.2.  Congestion Control basics relevant to ICN . . . . . . . .   7
   4.  What can we control to achieve QoS in ICN?  . . . . . . . . .   9
   5.  How does this relate to QoS in TCP/IP?  . . . . . . . . . . .  10
   6.  Why is ICN Different?  Can we do Better?  . . . . . . . . . .  12
     6.1.  Equivalence class capabilities  . . . . . . . . . . . . .  12
     6.2.  Topology interactions with QoS  . . . . . . . . . . . . .  12
     6.3.  Specification of QoS treatments . . . . . . . . . . . . .  13
     6.4.  ICN forwarding semantics effect on QoS  . . . . . . . . .  14
     6.5.  QoS interactions with Caching . . . . . . . . . . . . . .  15
   7.  Strawman principles for an ICN QoS architecture . . . . . . .  15
     7.1.  Can Intserv-like traffic control in ICN provide richer QoS
           semantics?  . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     10.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

   The TCP/IP protocol suite used on today's Internet has over 30 years
   of accumulated research and engineering into the provision of Quality
   of Service machinery, employed with varying success in different
   environments.  ICN protocols like Named Data Networking (NDN [NDN])
   and Content-Centric Networking (CCNx [RFC8569],[RFC8609]) have an
   accumulated 10 years of research and very little deployment.  We
   therefore have the opportunity to either recapitulate the approaches

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   taken with TCP/IP (e.g.  Intserv [RFC2998] and Diffserv [RFC2474]) or
   design a new architecture and associated mechanisms aligned with the
   properties of ICN protocols which differ substantially from those of
   TCP/IP.  This position paper advocates the latter approach and
   comprises the author's personal views on how Quality of Service (QoS)
   capabilities ought to be accommodated in ICN protocols like CCNx or
   NDN.  Specifically, these protocols differ in fundamental ways from
   TCP/IP.  The important differences are summarized in the following

   |            TCP/IP           |            CCNx or NDN             |
   |     Stateless forwarding    |        Stateful forwarding         |
   |        Simple Packets       | Object model with optional caching |
   |     Pure datagram model     |       Request-response model       |
   |      Asymmetric Routing     |         Symmetric Routing          |
   | Independent flow directions |            Flow balance            |
   |  Flows grouped by IP prefix |    Flows grouped by name prefix    |
   |           and port          |                                    |
   |    End-to-end congestion    |   Hop-by-hop congestion control    |
   |           control           |                                    |

   Table 1: Differences between IP and ICN relevant to QoS architecture

   This document proposes specific design patterns to achieve both flow
   classification and differentiated QoS treatment for ICN on both a
   flow and aggregate basis.  It also considers the effect of caches in
   addition to memory, CPU and link bandwidth as a resource that should
   be subject to explicitly unfair resource allocation.  The proposed
   methods are intended to operate purely at the network layer,
   providing the primitives needed to achieve both transport and higher
   layer QoS objectives.  It does not propose detailed protocol
   machinery to achieve these goals; it leaves these to supplementary
   specifications, such as [I-D.moiseenko-icnrg-flowclass] and
   [I-D.anilj-icnrg-dnc-qos-icn].  It explicitly excludes any discussion
   of Quality of Experience (QoE) which can only be assessed and
   controlled at the application layer or above.

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   Much of this document is derived from presentations the author has
   given at ICNRG meetings over the last few years that are available
   through the IETF datatracker (see, for example [Oran2018QoSslides]).

1.1.  Applicability Assessment by ICNRG Chairs

   QoS in ICN is an important topic with a huge design space.  ICNRG has
   been discussing different specific protocol mechanisms as well as
   conceptual approaches.  This document presents architectural
   considerations for QoS, leveraging ICN properties instead of merely
   applying IP-QoS mechanisms - without defining a specific architecture
   or specific protocols mechanisms yet.  However, there is consensus in
   ICNRG that this document, clarifying the author's views, could
   inspire such work and should hence be published as a position paper.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Background on Quality of Service in network protocols

   Much of this background material is tutorial and can be simply
   skipped by readers familiar with the long and checkered history of
   quality of service in packet networks.  Other parts of it are
   polemical yet serve to illuminate the author's personal biases and
   technical views.

   All networking systems provide some degree of "quality of service" in
   that they exhibit non-zero utility when offered traffic to carry.  In
   other words, the network is totally useless if it never delivers any
   of the traffic injected by applications.  The term QoS is therefore
   more correctly applied in a more restricted sense to describe systems
   that control the allocation of various resources in order to achieve
   _managed unfairness_.  Absent explicit mechanisms to decide what
   traffic to be unfair to, most systems try to achieve some form of
   "fairness" in the allocation of resources, optimizing the overall
   utility delivered to all offered load under the constraint of
   available resources.  From this it should be obvious that you cannot
   use QoS mechanisms to create or otherwise increase resource capacity!
   In fact, all known QoS schemes have non-zero overhead and hence may
   (albeit slightly) decrease the total resources available to carry
   user traffic.

   Further, accumulated experience seems to indicate that QoS is helpful
   in a fairly narrow range of network conditions:

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   *  If your resources are lightly loaded, you don't need it, as
      neither congestive loss nor substantial queueing delay occurs

   *  If your resources are heavily oversubscribed, it doesn't save you.
      So many users will be unhappy that you are probably not delivering
      a viable service

   *  Failures can rapidly shift your state from the first above to the
      second, in which case either:

      -  your QoS machinery cannot respond quickly enough to maintain
         the advertised service quality continuously, or

      -  resource allocations are sufficiently conservative to result in
         substantial wasted capacity under non-failure conditions

   Nevertheless, though not universally deployed, QoS is advantageous at
   least for some applications and some network environments.  Some
   examples include:

   *  applications with steep utility functions [Shenker2006], such as
      real-time multimedia

   *  applications with safety-critical operational constraints, such as
      avionics or industrial automation

   *  dedicated or tightly managed networks whose economics depend on
      strict adherence to challenging service level agreements (SLAs)

   Another factor in the design and deployment of QoS is the scalability
   and scope over which the desired service can be achieved.  Here there
   are two major considerations, one technical, the other economic/

   *  Some signaled QoS schemes, such as RSVP (Resource reSerVation
      Protocol) [RFC2205], maintain state in routers for each flow,
      which scales linearly with the number of flows.  For core routers
      through which pass millions to billions of flows, the memory
      required is infeasible to provide.

   *  The Internet is comprised of many minimally cooperating autonomous
      systems [AS].  There are practically no successful examples of QoS
      deployments crossing the AS boundaries of multiple service
      providers.  This in almost all cases limits the applicability of
      QoS capabilities to be intra-domain.

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   While this document adopts the narrow definition of QoS as _managed
   unfairness_, much of the networking literature uses the term more
   colloquially as applying to any mechanism that improves overall
   performance.  Readers assuming this broader context will find a large
   class of proven techniques to be ignored.  This is intentional.
   Among these are seamless producer mobility schemes like MAPME
   [Auge2018], and network coding of ICN data as discussed in

   Finally, the relationship between QoS and either accounting or
   billing is murky.  Some schemes can accurately account for resource
   consumption and ascertain to which user to allocate the usage.
   Others cannot.  While the choice of mechanism may have important
   practical economic and political consequences for cost and workable
   business models, this document considers none of those things and
   discusses QoS only in the context of providing managed unfairness.

   For those unfamiliar with ICN protocols, a brief description of how
   NDN and CCNx operate as a packet network is below in Section 3.1.
   Some further background on congestion control for ICN follows in
   Section 3.2.

3.1.  Basics on how ICN protocols like NDN and CCNx work

   The following is intended as a brief summary of the salient features
   of the NDN and CCnx ICN protocols relevant to congestion control and
   QoS.  Quite extensive tutorial information may be found in a number
   of places including material available from [NDNTutorials].

   In NDN and CCNx, all protocol interactions operate as a two-way
   handshake.  Named content is requested by a _consumer_ via an
   _Interest message_ which is routed hop-by-hop through a series of
   _forwarders_ until it reaches a node that stores the requested data.
   This can be either the _producer_ of the data, or a forwarder holding
   a cached copy of the requested data.  The content matching the name
   in the Interest is returned to the requester over the _inverse_ of
   the path traversed by the corresponding Interest.

   Forwarding in CCNx and NDN is _per-packet stateful_. Routing
   information to select next-hops for an Interest is obtained from a
   _Forwarding Information Base (FIB)_ which is similar in function to
   the FIB in an IP router, except that it holds name prefixes rather
   than IP address prefixes.  Conventionally a _Longest Name Prefix
   Match (LNPM)_ is used for lookup, although other algorithms are
   possible including controlled flooding and adaptive learning based on
   prior history.

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   Each Interest message leaves a trail of "breadcrumbs" as state in
   each forwarder.  This state, held in a data structure known as a
   _Pending Interest Table (PIT)_ is used to forward the returning Data
   message to the consumer.  Since the PIT constitutes per-packet state
   it is therefore a large consumer of memory resources especially in
   forwarders carrying high traffic loads over long Round Trip Time
   (RTT) paths, and hence plays a substantial role as a QoS-controllable
   resource in ICN forwarders.

   In addition to its role in forwarding Interest messages and returning
   the corresponding Data messages, an ICN forwarder can also operate as
   a cache, optionally storing a copy of any Data messages it has seen
   in a local data structure known as a _Content Store (CS)_. Data in
   the Content Store may be returned in response to a matching Interest
   rather than forwarding the Interest further through the network to
   the original Producer.  Both CCNx and NDN have a variety of ways to
   configure caching, including mechanisms to avoid both cache pollution
   and cache poisoning (these are clearly beyond the scope of this brief

3.2.  Congestion Control basics relevant to ICN

   In any packet network that multiplexes traffic among multiple sources
   and destinations, congestion control is necessary in order to:

   1.  Prevent collapse of utility due to overload, where the total
       offered service declines as load increases, perhaps
       precipitously, rather than increasing or remaining flat.

   2.  Avoid starvation of some traffic due to excessive demand by other

   3.  Beyond the basic protections against starvation, achieve
       "fairness" among competing traffic.  Two common objective
       functions are [minmaxfairness] and [proportionalfairness] both of
       which have been implemented and deployed successfully on packet
       networks for many years.

   Before moving on to QoS, it is useful to consider how congestion
   control works in NDN or CCNx.  Unlike the IP protocol family, which
   relies exclusively on end-to-end congestion control (e.g.
   TCP[RFC0793], DCCP[RFC4340], SCTP[RFC4960],
   QUIC[I-D.ietf-quic-transport]), CCNx and NDN can employ hop-by-hop
   congestion control.  There is per-Interest/Data state at every hop of
   the path and therefore outstanding Interests provide information that
   can be used to optimize resource allocation for data returning on the
   inverse path, such as bandwidth sharing, prioritization and overload
   control.  In current designs, this allocation is often done using

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   Interest counting.  By accepting one Interest packet from a
   downstream node, implicitly this provides a guarantee (either hard or
   soft) that there is sufficient bandwidth on the inverse direction of
   the link to send back one Data packet.  A number of congestion
   control schemes have been developed for ICN that operate in this
   fashion, for example [Wang2013], [Mahdian2016], [Song2018],
   [Carofiglio2012].  Other schemes, like [Schneider2016] neither count
   nor police Interests, but instead monitor queues using AQM (active
   queue management) to mark returning Data packets that have
   experienced congestion.  This later class of schemes is similar to
   those used on IP in the sense that they depend on consumers
   adequately reducing their rate of Interest injection to avoid Data
   packet drops due to buffer overflow in forwarders.  The former class
   of schemes is (arguably) more robust against mis-behavior by

   Given the stochastic nature of round trip times, and the ubiquity of
   wireless links and encapsulation tunnels with variable bandwidth, a
   simple scheme that admits interests only based on a time-invariant
   estimate of the returning link bandwidth will perform poorly.
   However, two characteristics of NDN and CCNx-like protocols can help
   substantially to improve the accuracy and responsiveness of the
   bandwidth allocation:

   1.  RTT is bounded by the inclusion of an _Interest Lifetime_ in each
       Interest message, which puts an upper bound on the RTT
       uncertainty for any given Interest/Data exchange.  If Interest
       lifetimes are kept reasonably short (a few RTTs) the allocation
       of local forwarder resources do not have to deal with an
       arbitrarily long tail.  One could in fact do a deterministic
       allocation on this basis, but the result would be highly
       pessimistic.  Nevertheless, having a cut-off does improve the
       performance of an optimistic allocation scheme.

   2.  Returning Data packets can be congestion marked by an ECN-like
       marking scheme if the inverse link starts experiencing long queue
       occupancy or other congestion indication.  Unlike TCP/IP, where
       the rate adjustment can only be done end-to-end, this feedback is
       usable immediately by the downstream ICN forwarder and the
       Interest shaping rate lowered after a single link RTT.  This may
       allow less pessimistic rate adjustment schemes than the Additive
       Increase, Multiplicative Decrease (AIMD) with .5 multiplier that
       is used on TCP/IP networks.  It also allows the rate adjustments
       to be spread more accurately among the Interest/Data flows
       traversing a link sending congestion signals.

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   A useful discussion of these properties and how they demonstrate the
   advantages of ICN approaches to congestion control can be found in

4.  What can we control to achieve QoS in ICN?

   QoS is achieved through managed unfairness in the allocation of
   resources in network elements, particularly in the routers doing
   forwarding of ICN packets.  So, a first order question is what
   resources need to be allocated, and how to ascertain which traffic
   gets what allocations.  In the case of CCNx or NDN the important
   network element resources are:

     |    Resource   | ICN Usage                                     |
     | Communication | buffering for queued packets                  |
     | Link capacity |                                               |
     | Content Store | to hold cached data                           |
     |    capacity   |                                               |
     |   Forwarder   | for the Pending Interest Table (PIT)          |
     |     memory    |                                               |
     |    Compute    | for forwarding packets, including the cost of |
     |    capacity   | Forwarding Information Base (FIB) lookups.    |

               Table 2: ICN-related Network Element Resources

   For these resources, any QoS scheme has to specify two things:

   1.  How do you create _equivalence classes_ (a.k.a. flows) of traffic
       to which different QoS treatments are applied?

   2.  What are the possible treatments and how are those mapped to the
       resource allocation algorithms?

   Two critical facts of life come into play when designing a QoS
   scheme.  First, the number of equivalence classes that can be
   simultaneously tracked in a network element is bounded by both memory
   and processing capacity to do the necessary lookups.  One can allow
   very fine-grained equivalence classes, but not be able to employ them
   globally because of scaling limits of core routers.  That means it is
   wise to either restrict the range of equivalence classes, or allow
   them to be _aggregated_, trading off accuracy in policing traffic
   against ability to scale.

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   Second, the flexibility of expressible treatments can be tightly
   constrained by both protocol encoding and algorithmic limitations.
   The ability to encode the treatment requests in the protocol can be
   limited (as it is for IP - there are only 6 of the Type of Service
   (TOS) bits available for Diffserv treatments), but as or more
   important is whether there are practical traffic policing, queuing,
   and pacing algorithms that can be combined to support a rich set of
   QoS treatments.

   The two considerations above in combination can easily be
   substantially more expressive than what can be achieved in practice
   with the available number of queues on real network interfaces or the
   amount of per-packet computation needed to enqueue or dequeue a

5.  How does this relate to QoS in TCP/IP?

   TCP/IP has fewer resource types to manage than ICN, and in some cases
   the allocation methods are simpler, as shown in the following table:

     | Resource      | IP Relevant | TCP/IP Usage                   |
     | Communication |     YES     | buffering for queued packets   |
     | Link capacity |             |                                |
     | Content Store |      NO     | no content store in IP         |
     | capacity      |             |                                |
     | Forwarder     |    MAYBE    | not needed for output-buffered |
     | memory        |             | designs^(*)                    |
     | Compute       |     YES     | for forwarding packets, but    |
     | capacity      |             | arguably much cheaper than ICN |

              Table 3: IP-related Network Element Resources

   ^(*)Output-buffered designs are where all packet buffering resources
   are associated with the output interfaces and there are no receiver
   interface or internal forwarding buffers that can be over-subscribed.
   Output-buffered switchs or routers are common but not universal, as
   they generally require an internal speed-up factor where forwarding
   capacity is greater than the sum of the input capacity of the

   For these resources, IP has specified three fundamental things, as
   shown in the following table:

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   |     What     | How                                                |
   | *Equivalence | subset+prefix match on IP                          |
   |   classes*   | 5-tuple {SA,DA,SP,DP,PT}                           |
   |              | SA=Source Address                                  |
   |              | DA=Destination Address                             |
   |              | SP=Source Port                                     |
   |              | DP=Desintation Port                                |
   |              | PT=IP Protocol Type                                |
   |  *Diffserv   | (very) small number of                             |
   | treatments*  | globally-agreed traffic                            |
   |              | classes                                            |
   |   *Intserv   | per-flow parameterized                             |
   | treatments*  | _Controlled Load_ and                              |
   |              | _Guaranteed_ service                               |
   |              | classes                                            |

      Table 4: Fundamental protocol elements to achieve QoS for TCP/IP

   Equivalence classes for IP can be pairwise, by matching against both
   source and destination address+port, pure group using only
   destination address+port, or source-specific multicast with source
   adress+port and destination multicast address+port.

   With Intserv, the Resource ReSerVation signaling protocol (RSVP)
   [RFC2205] carries two data structures, the Flow Specifier (FLOWSPEC)
   and the Traffic Specifier (TSPEC).  The former fulfills the
   requirement to identify the equivalence class to which the QoS being
   signaled applies.  The latter comprises the desired QoS treatment
   along with a description of the dynamic character of the traffic
   (e.g. average bandwidth and delay, peak bandwidth, etc.).  Both of
   these encounter substantial scaling limits, which has meant that
   Intserv has historically been limited to confined topologies, and/or
   high-value usages, like traffic engineering.

   With Diffserv, the protocol encoding (6 bits in the TOS field of the
   IP header) artificially limits the number of classes one can specify.
   These are documented in [RFC4594].  Nonetheless, when used with fine-
   grained equivalence classes, one still runs into limits on the number
   of queues required.

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6.  Why is ICN Different?  Can we do Better?

   While one could adopt an approach to QoS mirroring the extensive
   experience with TCP/IP, this would, in the author's view, be a
   mistake.  The implementation and deployment of QoS in IP networks has
   been spotty at best.  There are of course economic and political
   reasons as well as technical reasons for these mixed results, but
   there are several architectural choices in ICN that make it a
   potentially much better protocol base to enhance with QoS machinery.
   This section discusses those differences and their consequences.

6.1.  Equivalence class capabilities

   First and foremost, hierarchical names are a much richer basis for
   specifying equivalence classes than IP 5-tuples.  The IP address (or
   prefix) can only separate traffic by topology to the granularity of
   hosts, and not express actual computational instances nor sets of
   data.  Ports give some degree of per-instance demultiplexing, but
   this tends to be both coarse and ephemeral, while confounding the
   demultiplexing function with the assignment of QoS treatments to
   particular subsets of the data.  Some degree of finer granularity is
   possible with IPv6 by exploiting the ability to use up to 64 bits of
   address for classifying traffic.  In fact, the hICN project
   [I-D.muscariello-intarea-hicn], while adopting the request-response
   model of CCNx, uses IPv6 addresses as the available namespace, and
   IPv6 packets (plus "fake" TCP headers) as the wire format.

   Nonetheless, the flexibility of tokenized (i.e. strings treated as
   opaque tokens), variable length, hierarchical names allows one to
   directly associate classes of traffic for QoS purposes with the
   structure of an application namespace.  The classification can be as
   coarse or fine-grained as desired by the application.  While not
   _always_ the case, there is typically a straightforward association
   between how objects are named, and how they are grouped together for
   common treatment.  Examples abound; a number can be conveniently
   found in [I-D.moiseenko-icnrg-flowclass].

6.2.  Topology interactions with QoS

   In ICN, QoS is not pre-bound to network topology since names are non-
   topological, unlike unicast IP addresses.  This allows QoS to be
   applied to multi-destination and multi-path environments in a
   straightforward manner, rather than requiring either multicast with
   coarse class-based scheduling or complex signaling like that in RSVP-
   TE [RFC3209] that is needed to make point-to-multipoint Muti-Protocol
   Label Switching (MPLS) work.

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   Because of IP's stateless forwarding model, complicated by the
   ubiquity of asymmetric routes, any flow-based QoS requires state that
   is decoupled from the actual arrival of traffic and hence must be
   maintained, at least as soft-state, even during quiescent periods.
   Intserv, for example, requires flow signaling with state O(#flows).
   ICN, even worst case, requires state O(#active Interest/Data
   exchanges), since state can be instantiated on arrival of an
   Interest, and removed (perhaps lazily) once the data has been

6.3.  Specification of QoS treatments

   Unlike Intserv, Diffserv eschews signaling in favor of class-based
   configuration of resources and queues in network elements.  However,
   Diffserv limits traffic treatments to a few bits taken from the ToS
   field of IP.  No such wire encoding limitations exist for NDN or
   CCNx, as the protocol is completely TLV (Type-Length-Value) based,
   and one (or even more than one) new field can be easily defined to
   carry QoS treatment information.

   Therefore, there are greenfield possibilities for more powerful QoS
   treatment options in ICN.  For example, IP has no way to express a
   QoS treatment like "try hard to deliver reliably, even at the expense
   of delay or bandwidth".  Such a QoS treatment for ICN could invoke
   native ICN mechanisms, none of which are present in IP, such as:

   *  In-network retransmission in response to hop-by-hop errors
      returned from upstream forwarders

   *  Trying multiple paths to multiple content sources either in
      parallel or serially

   *  Assign higher precedence for short-term caching to recover from
      downstream^(*) errors

   *  Coordinating cache utilization with forwarding resources

      |  ^(*)_Downstream_ refers to the direction Data messages flow
      |  toward the consumer (the issuer of Interests).  Conversely,
      |  _Upstream_ refers to the direction Interests flow toward the
      |  producer of data.

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   Such mechanisms are typically described in NDN and CCNx as
   _forwarding strategies_. However, little or no guidance is given for
   what application actions or protocol machinery is used to decide
   which forwarding strategy to use for which Interests that arrive at a
   forwarder.  See [BenAbraham2018] for an investigation of these
   issues.  Associating forwarding strategies with the equivalence
   classes and QoS treatments directly can make them more accessible and
   useful to implement and deploy.

   Stateless forwarding and asymmetric routing in IP limits available
   state/feedback to manage link resources.  In contrast, NDN or CCNx
   forwarding allows all link resource allocation to occur as part of
   Interest forwarding, potentially simplifying things considerably.  In
   particular, with symmetric routing, producers have no control over
   the paths their data packets traverse, and hence any QoS treatments
   intended to influence routing paths from producer to consumer will
   have no effect.

   One complication in the handling of ICN QoS treatments is not present
   in IP and hence worth mention.  CCNx and NDN both perform _Interest
   aggregation_ (See Section 2.3.2 of [RFC8569]).  If an Interest
   arrives matching an existing PIT entry, but with a different QoS
   treatment from an Interest already forwarded, it can be tricky to
   decide whether to aggregate the interest or forward it, and how to
   keep track of the differing QoS treatments for the two Interests.
   Exploration of the details surrounding these situations is beyond the
   scope of this document; further discussion can be found for the
   general case of flow balance and congestion control in
   [I-D.oran-icnrg-flowbalance], and specifically for QoS treatments in

6.4.  ICN forwarding semantics effect on QoS

   IP has three forwarding semantics, with different QoS needs (Unicast,
   Anycast, Multicast).  ICN has the single forwarding semantic, so any
   QoS machinery can be uniformly applied across any request/response
   invocation.  This applies whether the forwarder employs dynamic
   destination routing, multi-destination forwarding with next-hops
   tried serially, multi-destination with next-hops used in parallel, or
   even localized flooding (e.g. directly on L2 multicast mechanisms).
   Additionally, the pull-based model of ICN avoids a number of thorny
   multicast QoS problems that IP has ([Wang2000], [RFC3170],

   The Multi-destination/multi-path forwarding model in ICN changes
   resource allocation needs in a fairly deep way.  IP treats all
   endpoints as open-loop packet sources, whereas NDN and CCNx have
   strong asymmetry between producers and consumers as packet sources.

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6.5.  QoS interactions with Caching

   IP has no caching in routers, whereas ICN needs ways to allocate
   cache resources.  Treatments to control caching operation are
   unlikely to look much like the treatments used to control link
   resources.  NDN and CCNx already have useful cache control directives
   associated with Data messages.  The CCNx controls include:

   ExpiryTime:  time after which a cached Content Object is considered
      expired and MUST no longer be used to respond to an Interest from
      a cache.

   Recommended Cache Time:  time after which the publisher considers the
      Content Object to be of low value to cache.

   See [RFC8569] for the formal definitions.

   ICN flow classifiers, such as those in
   [I-D.moiseenko-icnrg-flowclass] can be used to achieve soft or hard
   partitioning^(*) of cache resources in the content store of an ICN
   forwarder.  For example, cached content for a given equivalence class
   can be considered _fate shared_ in a cache whereby objects from the
   same equivalence class can be purged as a group rather than
   individually.  This can recover cache space more quickly and at lower
   overhead than pure per-object replacement when a cache is under
   extreme pressure and in danger of thrashing.  In addition, since the
   forwarder remembers the QoS treatment for each pending Interest in
   its PIT, the above cache controls can be augmented by policy to
   prefer retention of cached content for some equivalence classes as
   part of the cache replacement algorithm.

      |  ^(*)With hard partitioning, there are dedicated cache resources
      |  for each equivalence class (or enumerated list of equivalence
      |  classes).  With soft partitioning, resources are at least
      |  partly shared among the (sets of) equivalence classes of
      |  traffic.

7.  Strawman principles for an ICN QoS architecture

   Based on the observations made in the earlier sections, this summary
   section captures the author's ideas for clear and actionable
   architectural principles for how to incorporate QoS machinery into
   ICN protocols like NDN and CCNx.  Hopefully, they can guide further
   work and focus effort on portions of the giant design space for QoS
   that have the best tradeoffs in terms of flexibility, simplicity, and

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   *Define equivalence classes using the name hierarchy rather than
   creating an independent traffic class definition*. This directly
   associates the specification of equivalence classes of traffic with
   the structure of the application namespace.  It can allow
   hierarchical decomposition of equivalence classes in a natural way
   because of the way hierarchical ICN names are constructed.  Two
   practical mechanisms are presented in [I-D.moiseenko-icnrg-flowclass]
   with different tradeoffs between security and the ability to
   aggregate flows.  Either prefix-based (EC3) or explicit name
   component based (ECNT) or both could be adopted as the part of the
   QoS architecture for defining equivalence classes.

   *Put consumers in control of Link and Forwarding resource
   allocation*. Do all link buffering and forwarding (both memory and
   CPU) resource allocations based on Interest arrivals.  This is
   attractive because it provides early congestion feedback to
   consumers, and allows scheduling the reverse link direction ahead of
   time for carrying the matching data.  It makes enforcement of QoS
   treatments a single-ended (i.e. at the consumer) rather than a
   double-ended problem and can avoid wasting resources on fetching data
   that will wind up dropped when it arrives at a bottleneck link.

   *Allow producers to influence the allocation of cache resources*.
   Producers want to affect caching decisions in order to:

   *  Shed load by having Interests served by content stores in
      forwarders before reaching the producer itself.

   *  Survive transient producer reachability or link outages close to
      the producer.

   For caching to be effective, individual Data objects in an
   equivalence class need to have similar treatment; otherwise well-
   known cache thrashing pathologies due to self-interference emerge.
   Producers have the most direct control over caching policies through
   the caching directives in Data messages.  It therefore makes sense to
   put the producer, rather than the consumer or network operator in
   charge of specifying these equivalence classes.

   See [I-D.moiseenko-icnrg-flowclass] for specific mechanisms to
   achieve this.

   *Allow consumers to influence the allocation of cache resources*.
   Consumers want to affect caching decisions in order to:

   *  Reduce latency for retrieving data

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   *  Survive transient outages of either a producer or links close to
      the consumer

   Consumers can have indirect control over caching by specifying QoS
   treatments in their Interests.  Consider the following potential QoS
   treatments by consumers that can drive caching policies:

   *  A QoS treatment requesting better robustness against transient
      disconnection can be used by a forwarder close to the consumer (or
      downstream of an unreliable link) to preferentially cache the
      corresponding data.

   *  Conversely a QoS treatment together with, or in addition to a
      request for short latency, to indicate that new data will be
      requested soon enough that caching the current data being
      requested would be ineffective and hence to only pay attention to
      the caching preferences of the producer.

   *  A QoS treatment indicating a mobile consumer likely to incur a
      mobility event within an RTT (or a few RTTs).  Such a treatment
      would allow a mobile network operator to preferentially cache the
      data at a forwarder positioned at a _join point_ or _rendezvous
      point_ of their topology

   *Give network operators the ability to match customer SLAs to cache
   resource availability*. Network operators, whether closely tied
   administratively to producer or consumer, or constituting an
   independent transit administration, provide the storage resources in
   the ICN forwarders.  Therefore, they are the ultimate arbiters of how
   the cache resources are managed.  In addition to any local policies
   they may enforce, the cache behavior from the QoS standpoint emerges
   from how the producer-specified equivalence classes map onto cache
   space availability, including whether cache entries are treated
   individually, or fate-shared.  Forwarders also determine how the
   consumer-specified QoS treatments map to the precedence used for
   retaining Data objects in the cache.

   Besides utilizing cache resources to meet the QoS goals of individual
   producers and consumers, network operators also want to manage their
   cache resources in order to:

   *  Ameliorate congestion hotspots by reducing load converging on
      producers they host on their network.

   *  Improve Interest satisfaction rates by utilizing caches as short-
      term retransmission buffers to recover from transient producer
      reachability problems, link errors or link outages.

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   *  Improve both latency and reliability in environments when
      consumers are mobile in the operator's topology.

   *Re-think how to specify traffic treatments - don't just copy
   Diffserv*. Some of the Diffserv classes may form a good starting
   point, as their mapping onto queuing algorithms for managing link
   buffering are well understood.  However, Diffserv alone does not
   allow one to express latency versus reliability tradeoffs or other
   useful QoS treatments.  Nor does it permit "Traffic Specification
   (TSPEC)"-style traffic descriptions as are allowed in a signaled QoS
   scheme.  Here are some examples:

   *  A "burst" treatment, where an initial Interest gives an aggregate
      data size to request allocation of link capacity for a large burst
      of Interest/Data exchanges.  The Interest can be rejected at any
      hop if the resources are not available.  Such a treatment can also
      accommodate Data implosion produced by the discovery procedures of
      management protocols like [I-D.irtf-icnrg-ccninfo].

   *  A "reliable" treatment, which affects preference for allocation of
      PIT space for the Interest and Content Store space for the data in
      order to improve the robustness of IoT data delivery in
      constrained environment, as is described in

   *  A "search" treatment, which, within the specified Interest
      Lifetime, tries many paths either in parallel or serial to
      potentially many content sources, to maximize the probability that
      the requested item will be found.  This is done at the expense of
      the extra bandwidth of both forwarding Interests and receiving
      multiple responses upstream of an aggregation point.  The
      treatment can encode a value expressing tradeoffs like breadth-
      first versus depth-first search, and bounds on the total resource
      expenditure.  Such a treatment would be useful for instrumentation
      protocols like [I-D.mastorakis-icnrg-icntraceroute].

      |  As an aside, loose latency control (on the order of seconds or
      |  tens of milliseconds as opposed milliseconds or microseconds)
      |  can be achieved by bounding Interest Lifetime as long as this
      |  lifetime machinery is not also used as an application mechanism
      |  to provide subscriptions or to establish path traces for
      |  producer mobility.  See [Krol2018] for a discussion of the
      |  network versus application timescale issues in ICN protocols.

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7.1.  Can Intserv-like traffic control in ICN provide richer QoS

   Basic QoS treatments such as those summarized above may not be
   adequate to cover the whole range of application utility functions
   and deployment environments we expect for ICN.  While it is true that
   one does not necessarily need a separate signaling protocol like RSVP
   given the state carried in the ICN data plane by forwarders, there
   are some potentially important capabilities not provided by just
   simple QoS treatments applied to per- Interest/Data exchanges.
   Intserv's richer QoS capabilities may be of value, especially if they
   can be provided in ICN at lower complexity and protocol overhead than

   There are three key capabilities missing from Diffserv-like QoS
   treatments, no matter how sophisticated they may be in describing the
   desired treatment for a given equivalence class of traffic.  Intserv-
   like QoS provides all of these:

   1.  The ability to *describe traffic flows* in a mathematically
       meaningful way.  This is done through parameters like average
       rate, peak rate, and maximum burst size.  The parameters are
       encapsulated in a data structure called a "TSPEC" which can be
       placed in whatever protocol needs the information (in the case of
       TCP/IP Intserv, this is RSVP).

   2.  The ability to perform *admission control*, where the element
       requesting the QoS treatment can know _before_ introducing
       traffic whether the network elements have agreed to provide the
       requested traffic treatment.  An important side-effect of
       providing this assurance is that the network elements install
       state that allows the forwarding and queuing machinery to police
       and shape the traffic in a way that provides a sufficient degree
       of _isolation_ from the dynamic behavior of other traffic.
       Depending on the admission control mechanism, it may or may not
       be possible to explicitly release that state when the application
       no longer needs the QoS treatment.

   3.  The permissable *degree of divergence* in the actual traffic
       handling from the requested handling.  Intserv provided two
       choices here, the _controlled load_ service and the _guaranteed_
       service.  The former allows stochastic deviation equivalent to
       what one would experience on an unloaded path of a packet
       network.  The latter conforms to the TSPEC deterministically, at
       the obvious expense of demanding extremely conservative resource

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   Given the limited applicability of these capabilities in today's
   Internet, the author does not take any position as to whether any of
   these Intserv-like capabilities are needed for ICN to be succesful.
   However, a few things seem important to consider.  The following
   paragraphs speculate about the consequences to the CCNx or NDN
   protocol architectures of incorporating these features.

   Superficially, it would be quite straightforward to accommodate
   Intserv-equivalent traffic descriptions in CCNx or NDN.  One could
   define a new TLV for the Interest message to carry a TSPEC.  A
   forwarder encountering this, together with a QoS treatment request
   (e.g. as proposed in Section 6.3) could associate the traffic
   specification with the corresponding equivalence class derived from
   the name in the Interest.  This would allow the forwarder to create
   state that not only would apply to the returning Data for that
   Interest when being queued on the downstream interface, but be
   maintained as soft state across multiple Interest/Data exchanges to
   drive policing and shaping algorithms at per-flow granularity.  The
   cost in Interest message overhead would be modest, however the
   complications associated with managing different traffic
   specifications in different Interests for the same equivalence class
   might be substantial.  Of course, all the scalability considerations
   with maintaining per-flow state also come into play.

   Similarly, it would be equally straightforward to have a way to
   express the degree of divergence capability that Intserv provides
   through its controlled load and guaranteed service definitions.  This
   could either be packaged with the traffic specification or encoded

   In contrast to the above, performing admission control for ICN flows
   is likely to be just as heavy-weight as it turned out to be with IP
   using RSVP.  The dynamic multi-path, multi-destination forwarding
   model of ICN makes performing admission control particularly tricky.
   Just to illustrate:

   *  Forwarding next-hop selection is not confined to single paths (or
      a few ECMP equivalent paths) as it is with IP, making it difficult
      to know where to install state in advance of the arrival of an
      Interest to forward.

   *  As with point-to-multipoint complexities when using RSVP for MPLS-
      TE, state has to be installed to multiple producers over multiple
      paths before an admission control algorithm can commit the
      resources and say "yes" to a consumer needing admission control

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   *  Knowing when to remove admission control state is difficult in the
      absence of a heavy-weight resource reservation protocol.  Soft
      state timeout may or may not be an adequate answer.

   Despite the challenges above, it may be possible to craft an
   admission control scheme for ICN that achieves the desired QoS goals
   of applications without the invention and deployment of a complex
   separate admission control signaling protocol.  There have been
   designs in earlier network architectures that were capable of
   performing admission control piggybacked on packet transmission.

      |  (The earliest example the author is aware of is [Autonet]).

   Such a scheme might have the following general shape *(warning:
   serious hand waving follows!)*:

   *  In addition to a QoS treatment and a traffic specification, an
      Interest requesting admission for the corresponding equivalence
      class would so indicate via a new TLV.  It would also need to: (a)
      indicate an expiration time after which any reserved resources can
      be released, and (b) indicate that caches be bypassed, so that the
      admission control request arrives at a bone-fide producer.

   *  Each forwarder processing the Interest would check for resource
      availability and if not available, or the requested service not
      feasible, reject the Interest with an admission control failure.
      If resources are available, the forwarder would record the traffic
      specification as described above and forward the Interest.

   *  If the Interest successfully arrives at a producer, the producer
      returns the requested Data.

   *  Each on-path forwarder, on receiving the matching Data message, if
      the resources are still available, does the actual allocation, and
      marks the admission control TLV as "provisionally approved".
      Conversely, if the resource reservation fails, the admission
      control is marked "failed", although the Data is still passed

   *  Upon the Data message arriving, the consumer knows if admission
      succeeded or not, and subsequent Interests can rely on the QoS
      state being in place until either some failure occurs, or a
      topology or other forwarding change alters the forwarding path.
      To deal with this, additional machinery is needed to ensure
      subsequent Interests for an admitted flow either follow that path
      or an error is reported.  One possibility (also useful in many
      other contexts), is to employ a _Path Steering_ mechanism, such as
      the one described in [Moiseenko2017].

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8.  IANA Considerations

   This document does not require any IANA actions.

9.  Security Considerations

   There are a few ways in which QoS for ICN interacts with security and
   privacy issues.  Since QoS addresses relationships among traffic
   rather than the inherent characteristics of traffic, it neither
   enhances nor degrades the security and privacy properties of the data
   being carried, as long as the machinery does not alter or otherwise
   compromise the basic security properties of the associated protocols.
   The QoS approaches advocated here for ICN can serve to amplify
   existing threats to network traffic however:

   *  An attacker able to manipulate the QoS treatments of traffic can
      mount a more focused (and potentially more effective) denial of
      service attack by suppressing performance on traffic the attacker
      is targeting.  Since the architecture here assumes QoS treatments
      are manipulable hop-by-hop, any on-path adversary can wreak havoc.
      Note however, that in basic ICN, an on-path attacker can do this
      and more by dropping, delaying, or mis-routing traffic independent
      of any particular QoS machinery in use.

   *  By explicitly revealing equivalence classes of traffic via either
      names or other fields in packets, an attacker has yet one more
      handle to use to discover linkability of multiple requests.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8569]  Mosko, M., Solis, I., and C. Wood, "Content-Centric
              Networking (CCNx) Semantics", RFC 8569,
              DOI 10.17487/RFC8569, July 2019,

   [RFC8609]  Mosko, M., Solis, I., and C. Wood, "Content-Centric
              Networking (CCNx) Messages in TLV Format", RFC 8609,
              DOI 10.17487/RFC8609, July 2019,

10.2.  Informative References

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   [AS]       "Autonomous System (Internet)", no date,

   [Auge2018] Augé, J., Carofiglio, G., Grassi, G., Muscariello, L.,
              Pau, G., and X. Zeng, "MAP-Me: Managing Anchor-Less
              Producer Mobility in Content-Centric Networks", in IEEE
              Transactions on Network and Service Management (Volume: 15
              , Issue: 2 , June 2018), DOI 10.1109/TNSM.2018.2796720,
              June 2018, <>.

   [Autonet]  Schroeder, M., Birrell, A., Burrows, M., Murray, H.,
              Needham, R., Rodeheffer, T., Satterthwaite, E., and C.
              Thacker, "Autonet: a High-speed, Self-configuring Local
              Area Network Using Point-to-point Links", in IEEE Journal
              on Selected Areas in Communications ( Volume: 9, Issue: 8,
              Oct 1991), DOI 10.1109/49.105178, October 1991,

              Ben Abraham, H., Parwatikar, J., DeHart, J., Dresher, A.,
              and P. Crowley, ""Decoupling Information and Connectivity
              via Information-Centric Transport", in ICN '18:
              Proceedings of the 5th ACM Conference on Information-
              Centric Networking September 21-23, 2018, Boston, MA, USA,
              DOI 10.1145/3267955.3267963, September 2018,

              Carofiglio, G., Gallo, M., and L. Muscariello, "Joint hop-
              by-hop and receiver-driven Interest control protocol for
              content-centric networks", in ACM SIGCOMM Computer
              Communication Review, September 2012,
              DOI 10.1016/j.comnet.2016.09.012, September 2012,

              Carofiglio, G., Gallo, M., and L. Muscariello, "Optimal
              multipath congestion control and request forwarding in
              information-centric networks: Protocol design and
              experimentation", in Computer Networks, Vol. 110 No. 9,
              December 2016, DOI 10.1145/2377677.2377772, December 2016,

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              Jangam, A., suthar, P., and M. Stolic, "QoS Treatments in
              ICN using Disaggregated Name Components", Work in
              Progress, Internet-Draft, draft-anilj-icnrg-dnc-qos-icn-
              01, 11 September 2019, <

              Gundogan, C., Schmidt, T., Waehlisch, M., Frey, M., Shzu-
              Juraschek, F., and J. Pfender, "Quality of Service for ICN
              in the IoT", Work in Progress, Internet-Draft, draft-
              gundogan-icnrg-iotqos-01, 8 July 2019,

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-27, 21 February 2020,

              Asaeda, H., Ooka, A., and X. Shao, "CCNinfo: Discovering
              Content and Network Information in Content-Centric
              Networks", Work in Progress, Internet-Draft, draft-irtf-
              icnrg-ccninfo-02, 8 July 2019,

              Matsuzono, K., Asaeda, H., and C. Westphal, "Network
              Coding for Content-Centric Networking / Named Data
              Networking: Requirements and Challenges", Work in
              Progress, Internet-Draft, draft-irtf-nwcrg-nwc-ccn-reqs-
              02, 20 September 2019, <

              Mastorakis, S., Gibson, J., Moiseenko, I., Droms, R., and
              D. Oran, "ICN Traceroute Protocol Specification", Work in
              Progress, Internet-Draft, draft-mastorakis-icnrg-
              icntraceroute-06, 13 February 2020,

              Moiseenko, I. and D. Oran, "Flow Classification in
              Information Centric Networking", Work in Progress,

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              Internet-Draft, draft-moiseenko-icnrg-flowclass-05, 20
              January 2020, <

              Muscariello, L., Carofiglio, G., Auge, J., and M.
              Papalini, "Hybrid Information-Centric Networking", Work in
              Progress, Internet-Draft, draft-muscariello-intarea-hicn-
              03, 30 October 2019, <

              Oran, D., "Maintaining CCNx or NDN flow balance with
              highly variable data object sizes", Work in Progress,
              Internet-Draft, draft-oran-icnrg-flowbalance-02, 3
              February 2020, <

   [Krol2018] Król, M., Habak, K., Oran, D., Kutscher, D., and I.
              Psaras, "RICE: Remote Method Invocation in ICN", in
              ICN'18: Proceedings of the 5th ACM Conference on
              Information-Centric Networking September 21-23, 2018,
              Boston, MA, USA, DOI 10.1145/3267955.3267956, September
              2018, <

              Mahdian, M., Arianfar, S., Gibson, J., and D. Oran,
              "MIRCC: Multipath-aware ICN Rate-based Congestion
              Control", in Proceedings of the 3rd ACM Conference on
              Information-Centric Networking,
              DOI 10.1145/2984356.2984365, September 2016,

              "Max-min Fairness", no date,

              Moiseenko, I. and D. Oran, "Path Switching in Content
              Centric and Named Data Networks", in ICN '17: Proceedings
              of the 4th ACM Conference on Information-Centric
              Networking, DOI 10.1145/3125719.3125721, September 2017,

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   [NDN]      "Named Data Networking", various,

              "NDN Tutorials", various,

              Oran, D., "Thoughts on Quality of Service for NDN/CCN-
              style ICN protocol architectures", presented at ICNRG
              Interim Meeting, Cambridge MA, 24 September 2018,

              "Proportionally Fair", no date,

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC2205]  Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
              Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
              Functional Specification", RFC 2205, DOI 10.17487/RFC2205,
              September 1997, <>.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,

   [RFC2998]  Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L.,
              Speer, M., Braden, R., Davie, B., Wroclawski, J., and E.
              Felstaine, "A Framework for Integrated Services Operation
              over Diffserv Networks", RFC 2998, DOI 10.17487/RFC2998,
              November 2000, <>.

   [RFC3170]  Quinn, B. and K. Almeroth, "IP Multicast Applications:
              Challenges and Solutions", RFC 3170, DOI 10.17487/RFC3170,
              September 2001, <>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,

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   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              DOI 10.17487/RFC4594, August 2006,

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,

              Schneider, K., Yi, C., Zhang, B., and L. Zhang, ""A
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Author's Address

   Dave Oran
   Network Systems Research and Design
   4 Shady Hill Square
   Cambridge, MA 02138
   United States of America


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