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Path Aware Networking: Obstacles to Deployment (A Bestiary of Roads Not Taken)

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This is an older version of an Internet-Draft that was ultimately published as RFC 9049.
Author Spencer Dawkins
Last updated 2021-02-08 (Latest revision 2020-12-29)
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PANRG                                                    S. Dawkins, Ed.
Internet-Draft                                           Tencent America
Intended status: Informational                          29 December 2020
Expires: 2 July 2021

Path Aware Networking: Obstacles to Deployment (A Bestiary of Roads Not


   At the first meeting of the Path Aware Networking Research Group, the
   research group agreed to catalog and analyze past efforts to develop
   and deploy Path Aware techniques, most of which were unsuccessful or
   at most partially successful, in order to extract insights and
   lessons for path-aware networking researchers.

   This document contains that catalog and analysis.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 2 July 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  What Does "Path Awareness" Mean in this Document? . . . .   3
     1.2.  Note to RFC Editor  . . . . . . . . . . . . . . . . . . .   4
   2.  A Perspective On This Document  . . . . . . . . . . . . . . .   4
     2.1.  Notes for the Reader  . . . . . . . . . . . . . . . . . .   5
     2.2.  A Note About Path-Aware Techniques Included In This
           Document  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Venue for Discussion of this Document . . . . . . . . . .   5
     2.4.  Architectural Guidance  . . . . . . . . . . . . . . . . .   5
     2.5.  Terminology Used in this Document . . . . . . . . . . . .   6
     2.6.  Methodology for Contributions . . . . . . . . . . . . . .   6
   3.  Applying the Lessons We've Learned  . . . . . . . . . . . . .   7
   4.  Summary of Lessons Learned  . . . . . . . . . . . . . . . . .  10
     4.1.  Justifying Deployment . . . . . . . . . . . . . . . . . .  10
     4.2.  Providing Benefits for Early Adopters . . . . . . . . . .  10
     4.3.  Providing Benefits During Partial Deployment  . . . . . .  11
     4.4.  Outperforming End-to-end Protocol Mechanisms  . . . . . .  11
     4.5.  Paying for Path Aware Techniques  . . . . . . . . . . . .  11
     4.6.  Impact on Operational Practices . . . . . . . . . . . . .  11
     4.7.  Per-connection State  . . . . . . . . . . . . . . . . . .  11
     4.8.  Keeping Traffic on Fast-paths . . . . . . . . . . . . . .  12
     4.9.  Endpoints Trusting Intermediate Nodes . . . . . . . . . .  12
     4.10. Intermediate Nodes Trusting Endpoints . . . . . . . . . .  12
     4.11. Reacting to Distant Signals . . . . . . . . . . . . . . .  12
     4.12. Support in Endpoint Protocol Stacks . . . . . . . . . . .  13
   5.  Future Work . . . . . . . . . . . . . . . . . . . . . . . . .  13
   6.  Contributions . . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Stream Transport (ST, ST2, ST2+)  . . . . . . . . . . . .  14
       6.1.1.  Reasons for Non-deployment  . . . . . . . . . . . . .  15
       6.1.2.  Lessons Learned.  . . . . . . . . . . . . . . . . . .  15
     6.2.  Integrated Services (IntServ) . . . . . . . . . . . . . .  16
       6.2.1.  Reasons for Non-deployment  . . . . . . . . . . . . .  16
       6.2.2.  Lessons Learned.  . . . . . . . . . . . . . . . . . .  17
     6.3.  Quick-Start TCP . . . . . . . . . . . . . . . . . . . . .  17
       6.3.1.  Reasons for Non-deployment  . . . . . . . . . . . . .  18
       6.3.2.  Lessons Learned . . . . . . . . . . . . . . . . . . .  19
     6.4.  ICMP Source Quench  . . . . . . . . . . . . . . . . . . .  20
       6.4.1.  Reasons for Non-deployment  . . . . . . . . . . . . .  20
       6.4.2.  Lessons Learned . . . . . . . . . . . . . . . . . . .  20
     6.5.  Triggers for Transport (TRIGTRAN) . . . . . . . . . . . .  21
       6.5.1.  Reasons for Non-deployment  . . . . . . . . . . . . .  22
       6.5.2.  Lessons Learned.  . . . . . . . . . . . . . . . . . .  23
     6.6.  Shim6 . . . . . . . . . . . . . . . . . . . . . . . . . .  23
       6.6.1.  Reasons for Non-deployment  . . . . . . . . . . . . .  24
       6.6.2.  Lessons Learned . . . . . . . . . . . . . . . . . . .  25
       6.6.3.  Addendum on MultiPath TCP . . . . . . . . . . . . . .  25

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     6.7.  Next Steps in Signaling (NSIS)  . . . . . . . . . . . . .  26
       6.7.1.  Reasons for Non-deployment  . . . . . . . . . . . . .  26
       6.7.2.  Lessons Learned . . . . . . . . . . . . . . . . . . .  28
     6.8.  IPv6 Flow Label . . . . . . . . . . . . . . . . . . . . .  28
       6.8.1.  Reasons for Non-deployment  . . . . . . . . . . . . .  29
       6.8.2.  Lessons Learned . . . . . . . . . . . . . . . . . . .  30
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  31
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  31
   10. Informative References  . . . . . . . . . . . . . . . . . . .  32
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  39

1.  Introduction

   This document describes the lessons that IETF participants have
   learned (and learned the hard way) about Path Aware Networking over a
   period of several decades, and provides an analysis of reasons why
   various Path Aware Networking techniques have seen limited or no

1.1.  What Does "Path Awareness" Mean in this Document?

   The current definition of "Path Awareness", used by the Path Aware
   Networking Research Group, appears in Section 1.1 ("Definition") in
   [I-D.irtf-panrg-questions].  That definition is included here as a
   convenience to the reader.

   |  For purposes of this document, "path aware networking" describes
   |  endpoint discovery of the properties of paths they use for
   |  communication, and endpoint reaction to these properties that
   |  affects routing and/or transmission; note that this can and
   |  already does happen to some extent in the current Internet
   |  architecture.  Expanding on this definition, a "path aware
   |  internetwork" is one in which endpoint discovery of path
   |  properties and endpoint selection of paths used by traffic
   |  exchanged by the endpoint are explicitly supported, regardless of
   |  the specific design of the protocol features which enable this
   |  discovery and selection.

   Because this document reflects work performed over several decades,
   some technologies described in Section 6 may not reflect the current
   definition, but these technologies were considered "path aware" by
   their contributors, so these contributions are included in this
   retrospective document.

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1.2.  Note to RFC Editor

   If the "Definition" in Section 1.1 ("Definition") of
   [I-D.irtf-panrg-questions] changes, the text in Section 1.1 of this
   document should be should be changed as well.

   Whether that happens or not, the RFC Editor is requested to remove
   this section.

2.  A Perspective On This Document

   At the first meeting of the Path Aware Networking Research Group
   [PANRG], at IETF 99 [PANRG-99], Olivier Bonaventure led a discussion
   of "A Decade of Path Awareness" [PATH-Decade], on attempts, which
   were mostly unsuccessful for a variety of reasons, to exploit Path
   Aware techniques and achieve a variety of goals over the past decade.
   At the end of that discussion, two things were abundantly clear.

   *  The Internet community has accumulated considerable experience
      with many Path Aware techniques over a long period of time, and

   *  Although some path aware techniques have been deployed (for
      example, Differentiated Services, or DiffServ [RFC2475]), most of
      these techniques haven't seen widespread adoption and deplyment.
      Even "successful" techniques like DiffServ can face obstacles that
      prevents wider usage.  The reasons for non-adoption and limited
      adoption and deployment are many, and are worthy of study.

   The meta-lessons from that experience were

   *  Path aware networking has been more Research than Engineering, so
      establishing an IRTF Research Group for Path Aware Networking is
      the right thing to do [RFC7418].

   *  Analyzing a catalog of past experience to learn the reasons for
      non-adoption would be a great first step for the Research Group.

   Allison Mankin, as IRTF Chair, officially chartered the Path Aware
   Networking Research Group in July, 2018.

   This document contains the analysis performed by that research group
   (Section 4), based on that catalog (Section 6).

   This document represents the consensus of the Path Aware Networking
   Research Group.

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2.1.  Notes for the Reader

   This Informational document discusses Path Aware protocol mechanisms
   considered, and in some cases standardized, by the Internet
   Engineering Task Force (IETF), and considers Lessons Learned from
   those mechanisms.  The intention is to inform the work of protocol
   designers, whether in the IRTF, the IETF, or elsewhere in the
   Internet ecosystem.

   As an Informational document published in the IRTF stream, this
   document has no authority beyond the quality of the analysis it

2.2.  A Note About Path-Aware Techniques Included In This Document

   This document does not catalog every proposed path aware networking
   technique that was not adopted and deployed.  Instead, we limited our
   focus to technologies that passed through the IETF community, and
   still identified enough techniques to provide background for the
   lessons included in Section 4 to inform researchers and protocol
   engineers in their work.

   No shame is intended for the techniques included in this document.
   As shown in Section 4, the quality of specific techniques had little
   to do with whether they were deployed or not.  Based on the
   techniques cataloged in this document, it is likely that when these
   techniques were put forward, the proponents were trying to engineer
   something that could not be engineered without first carrying out
   research.  Actual shame would be failing to learn from experience,
   and failing to share that experience with other networking
   researchers and engineers.

2.3.  Venue for Discussion of this Document

   (RFC Editor: please remove this section before publication)

   Discussion of specific contributed experiences and this document in
   general should take place on the PANRG mailing list.

2.4.  Architectural Guidance

   As background for understanding the Lessons Learned contained in this
   document, the reader is encouraged to become familiar with the
   Internet Architecture Board's documents on "What Makes for a
   Successful Protocol?"  [RFC5218] and "Planning for Protocol Adoption
   and Subsequent Transitions" [RFC8170].

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   Although these two documents do not specifically target path-aware
   networking protocols, they are helpful resources for readers seeking
   to improve their understanding of considerations for successful
   adoption and deployment of any protocol.  For example, the Basic
   Success Factors described in Setion 2.1 of [RFC5218] are helpful for
   readers of this document.

   Because there is an economic aspect to decisions about deployment,
   the IAB Workshop on Internet Technology Adoption and Transition
   [ITAT] report [RFC7305] also provides food for thought.

   Several of the Lessons Learned in Section 4 reflect considerations
   described in [RFC5218], [RFC7305], and [RFC8170].

2.5.  Terminology Used in this Document

   The terms Node and Element in this document have the meaning defined
   in [PathProp].

2.6.  Methodology for Contributions

   This document grew out of contributions by various IETF participants
   with experience with one or more Path Aware Networking techniques.

   There are many things that could be said about the Path Aware
   networking techniques that have been developed.  For the purposes of
   this document, contributors were requested to provide

   *  the name of a technique, including an abbreviation if one was used

   *  if available, a long-term pointer to the best reference describing
      the technique

   *  a short description of the problem the technique was intended to

   *  a short description of the reasons why the technique wasn't

   *  a short statement of the lessons that researchers can learn from
      our experience with this technique.

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3.  Applying the Lessons We've Learned

   The initial scope for this document was roughly "what mistakes have
   we made in the decade prior to [PANRG-99], that we shouldn't make
   again".  Some of the contributions in Section 6 predate the initial
   scope.  The earliest Path-Aware Networking technique referred to in
   Section 6 is Section 6.1, published in the late 1970s.  Given that
   the networking ecosystem has evolved continuously, it seems
   reasonable to consider how to apply these lessons.

   The PANRG Research Group reviewed the Lessons Learned (Section 4)
   contained in the May 23, 2019 version of this document at IETF 105
   [PANRG-105-Min], and carried out additional discussion at IETF 106
   [PANRG-106-Min].  Table 1 provides the "sense of the room" about each
   lesson after those discussions.  The intention is to capture whether
   a specific lesson seems to be

   *  "Invariant" - well-understood and is likely to be applicable for
      any proposed Path Aware Networking solution.

   *  "Variable" - has impeded deployment in the past, but might not be
      applicable in a specific technique.  Engineering analysis to
      understand whether the lesson is applicable is prudent.

   *  "Not Now" - this characteristic tends to turn up a minefield full
      of dragons, and prudent network engineers will wish to avoid
      gambling on a technique that relies on this, until something
      significant changes

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    | Lesson                                              | Category  |
    | Justifying Deployment (Section 4.1)                 | Invariant |
    | Providing Benefits for Early Adopters (Section 4.2) | Invariant |
    | Providing Benefits during Partial Deployment        | Invariant |
    | (Section 4.3)                                       |           |
    | Outperforming End-to-end Protocol Mechanisms        | Variable  |
    | (Section 4.4)                                       |           |
    | Paying for Path Aware Techniques (Section 4.5)      | Invariant |
    | Impact on Operational Practices (Section 4.6)       | Invariant |
    | Per-connection State (Section 4.7)                  | Variable  |
    | Keeping Traffic on Fast-paths (Section 4.8)         | Variable  |
    | Endpoints Trusting Intermediate Nodes (Section 4.9) | Not Now   |
    | Intermediate Nodes Trusting Endpoints               | Not Now   |
    | (Section 4.10)                                      |           |
    | Reacting to Distant Signals (Section 4.11)          | Variable  |
    | Support in Endpoint Protocol Stacks (Section 4.12)  | Variable  |

                                  Table 1

   "Justifying Deployment", "Providing Benefits for Early Adopters",
   "Paying for Path Aware Techniques", and "Impact on Operational
   Practice" were considered to be invariant - the sense of the room was
   that these would always be considerations for any proposed Path Aware

   "Providing Benefits During Partial Deployment" was added after IETF
   105, during research group last call, and is also considered to be

   For "Outperforming End-to-end Protocol Mechanisms", there is a trade-
   off between improved performance from Path Aware Techniques and
   additional complexity required by some Path Aware Techniques.

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   *  For example, if you can obtain the same understanding of path
      characteristics from measurements obtained over a few more round
      trips, endpoint implementers are unlikely to be eager to add
      complexity, and many attributes can be measured from an endpoint,
      without assistance from intermediate nodes.

   For "Per-connection State", the key questions discussed in the
   research group were "how much state" and "where state is maintained".

   *  IntServ (Section 6.2) required state at every intermediate node
      for every connection between two endpoints.  As the Internet
      ecosystem has evolved, carrying many connections in a tunnel that
      appears to intermediate nodes as a single connection has become
      more common, so that additional end-to-end connections don't add
      additional state to intermediate nodes between tunnel endpoints.
      If these tunnels are encrypted, intermediate nodes between tunnel
      endpoints can't distinguish between connections, even if that were

   For "Keeping Traffic on Fast-paths", we noted that this was true for
   many platforms, but not for all.

   *  For backbone routers, this is likely an invariant, but for
      platforms that rely more on general-purpose computers to make
      forwarding decisions, this may not be a fatal flaw for Path Aware
      Networking techniques.

   For "Endpoints Trusting Intermediate Nodes" and "Intermediate Nodes
   Trusting Endpoints", these lessons point to the broader need to
   revisit the Internet Threat Model.

   *  We noted with relief that discussions about this were already
      underway in the IETF community at IETF 105 (see the Security Area
      Open Meeting minutes [SAAG-105-Min] for discussion of
      [I-D.arkko-arch-internet-threat-model] and [I-D.farrell-etm]), and
      the Internet Architecture Board has created a mailing list for
      continued discussions ([model-t]), but we recognize that there are
      Path Aware Networking aspects of this effort, requiring research.

   For "Reacting to Distant Signals", we noted that not all attributes
   are equal.

   *  If an attribute is stable over an extended period of time, is
      difficult to observe via end-to-end mechanisms, and is valuable,
      Path Aware Techniques that rely on that attribute to provide a
      significant benefit become more attractive.

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   *  Analysis to help identify attributes that are useful enough to
      justify deployment of Path Aware techniques that make use of those
      attributes would be helpful.

   For "Support in Endpoint Protocol Stacks", we noted that Path Aware
   applications must be able to identify and communicate requirements
   about path characteristics.

   *  The de-facto sockets API has no way of signaling application
      expectations for the network path to the protocol stack.

4.  Summary of Lessons Learned

   This section summarizes the Lessons Learned from the contributed
   subsections in Section 6.

   Each Lesson Learned is tagged with one or more contributions that
   encountered this obstacle as a significant impediment to deployment.
   Other contributed techniques may have also encountered this obstacle,
   but this obstacle may not have been the biggest impediment to
   deployment for those techniques.

   It is useful to notice that sometimes an obstacle might impede
   deployment, while at other times, the same obstacle might prevent
   adoption and deployment entirely.  The research group discussed
   distinguishing between obstacles that impede and obstacles that
   prevent, but it appears that the boundary between "impede" and
   "prevent" can shift over time - some of the Lessons Learned are based
   on both Path Aware techniques that were not deployed, and Path Aware
   techniques that were deployed, but were not deployed widely or
   quickly.  See Section 6.6 and Section 6.6.3 as one example of this
   shifting boundary.

4.1.  Justifying Deployment

   The benefit of Path Awareness must be great enough to justify making
   changes in an operational network.  The colloquial U.S.  American
   English expression, "If it ain't broke, don't fix it" is a "best
   current practice" on today's Internet.  (See Section 6.3,
   Section 6.5, and Section 6.4, in addition to [RFC5218]).

4.2.  Providing Benefits for Early Adopters

   Providing benefits for early adopters can be key - if everyone must
   deploy a technique in order for the technique to provide benefits, or
   even to work at all, the technique is unlikely to be adopted widely
   or quickly.  (See Section 6.2 and Section 6.3, in addition to

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4.3.  Providing Benefits During Partial Deployment

   Some proposals require that all path elements along the full length
   of the path must be upgraded to support a new technique, before any
   benefits can be seen.  This is likely to require coordination between
   operators who control a subset of path elements, and between
   operators and end users if endpoint upgrades are required.  If a
   technique provides benefits when only a part of the path has been
   upgraded, this is likely to encourage adoption and deployment.  (See
   Section 6.2 and Section 6.3, in addition to [RFC5218]).

4.4.  Outperforming End-to-end Protocol Mechanisms

   Adaptive end-to-end protocol mechanisms may respond to feedback
   quickly enough that the additional realizable benefit from a new Path
   Aware mechanism that tries to manipulate nodes along a path, or
   observe the attributes of nodes along a path, may be much smaller
   than anticipated (Section 6.3 and Section 6.5).

4.5.  Paying for Path Aware Techniques

   "Follow the money."  If operators can't charge for a Path Aware
   technique to recover the costs of deploying it, the benefits to the
   operator must be really significant.  Corollary: If operators charge
   for a Path Aware technique, the benefits to users of that Path Aware
   technique must be significant enough to justify the cost.  (See
   Section 6.1, Section 6.2, and Section 6.5).

4.6.  Impact on Operational Practices

   Impact of a Path Aware technique requiring changes to operational
   practices can affect how quickly or widely a promising technique is
   deployed.  The impacts of these changes may make deployment more
   likely, but often discourage deployment.  (See Section 6.6, including
   Section 6.6.3).

4.7.  Per-connection State

   Per-connection state in intermediate nodes has been an impediment to
   adoption and deployment in the past, because of added cost and
   complexity.  Often, similar benefits can be achieved with much less
   finely-grained state.  This is especially true as we move from the
   edge of the network, further into the routing core (See Section 6.1
   and Section 6.2).

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4.8.  Keeping Traffic on Fast-paths

   Many modern platforms, especially high-end routers, have been
   designed with hardware that can make simple per-packet forwarding
   decisions ("fast-paths"), but have not been designed to make heavy
   use of in-band mechanisms such as IPv4 and IPv6 Router Alert Options
   (RAO) that require more processing to make forwarding decisions.
   Packets carrying in-band mechanisms are diverted to other processors
   in the router with much lower packet processing rates.  Operators can
   be reluctant to deploy techniques that rely heavily on in-band
   mechanisms because they may significantly reduce packet throughput.
   (See Section 6.7).

4.9.  Endpoints Trusting Intermediate Nodes

   If intermediate nodes along the path can't be trusted, it's unlikely
   that endpoints will rely on signals from intermediate nodes to drive
   changes to endpoint behaviors.  (See Section 6.5, Section 6.4).  We
   note that "trust" is not binary - one, low, level of trust applies
   when a node issuing a message can confirm that it has visibility of
   the packets on the path it is seeking to control [RFC8085] (e.g., an
   ICMP message included a quoted packet from the source).  A higher
   level of trust can arise when an endpoint has established a short
   term, or even long term, trust relationship with network nodes.

4.10.  Intermediate Nodes Trusting Endpoints

   If the endpoints do not have any trust relationship with the
   intermediate nodes along a path, operators have been reluctant to
   deploy techniques that rely on endpoints sending unauthenticated
   control signals to routers.  (See Section 6.2 and Section 6.7.  We
   also note this still remains a factor hindering deployment of

4.11.  Reacting to Distant Signals

   Because the Internet is a distributed system, if the distance that
   information from distant path elements travels to a Path Aware host
   is sufficiently large, the information may no longer accurately
   represent the state and situation at the distant host or elements
   along the path when it is received locally.  In this case, the
   benefit that a Path Aware technique provides will be inconsistent,
   and may not always be beneficial.  (See Section 6.3).

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4.12.  Support in Endpoint Protocol Stacks

   Just because a protocol stack provides a new feature/signal does not
   mean that applications will use the feature/signal.  Protocol stacks
   may not know how to effectively utilize Path-Aware techniques,
   because the protocol stack may require information from applications
   to permit the technique to work effectively, but applications may not
   a-priori know that information.  Even if the application does know
   that information, the de-facto sockets API has no way of signaling
   application expectations for the network path to the protocol stack.
   In order for applications to provide these expectations to protocol
   stacks, we need an API that signals more than the packets to be sent.
   (See Section 6.1 and Section 6.2).

5.  Future Work

   By its nature, this document has been retrospective.  In addition to
   considering how the Lessons Learned to date apply to current and
   future Path Aware networking proposals, it's also worth considering
   whether there is deeper investigation left to do.

   *  We note that this work was based on contributions from experts on
      various Path Aware networking techniques, and all of the
      contributed techniques involved unicast protocols.  We didn't
      consider how these lessons might apply to multicast, and, given
      anecdotal reports at the IETF 109 MOPS working group meeting of IP
      multicast offerings within data centers at one or more cloud
      providers ([MOPS-109-Min]), it might be useful to think about path
      awareness in multicast, before we have a history of unsuccessful
      deployments to document.

   *  The question of whether a mechanism supports admission control,
      based on either endpoints or applications, is associated with Path
      Awareness.  One of the motivations of IntServ and a number of
      other architectures (e.g.  Deterministic Networking, [RFC8655]) is
      the ability to "say no" to an application based on resource
      availability on a path, before the application tries to inject
      traffic onto that path and discovers the path does not have the
      capacity to sustain enough utility to meet the application's
      minimum needs.  The question of whether admission control is
      needed comes up repeatedly, but we have learned a few useful
      lessons that, while covered implicitly in some of the lessons
      learned of the document, might be explained explicitly:

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      -  We have gained a lot of experience with application-based
         adaptation since the days where applications just injected
         traffic in-elastically into the network.  Such adaptations seem
         to work well enough that admission control is of less value to
         these applications

      -  There are end-to-end measurement techniques that can steer
         traffic at the application layer (Content Distribution
         Networks, multi-CDNs like Conviva [Conviva], etc.)

      -  We noted in Section 4.12 that applications often don't know how
         to utilize Path Aware techniques.  This includes not knowing
         enough about their admission control threshold to be able to
         ask accurately for the resources they need, whether this is
         because the application itself doesn't know, or because the
         application has no way to signal its expectations to the
         underlying protocol stack.  To date, attempts to help them
         haven't gotten anywhere (e.g. the multiple-TSPEC additions to
         RSVP to attempt to mirror codec selection by applications
         [I-D.ietf-tsvwg-intserv-multiple-tspec] expired in 2013).

   *  We note that this work took the then-current IP network
      architecture as given, at least at the time each technique was
      proposed.  It might be useful to consider aspects of the now-
      current IP network architecture that ease, or impede, Path Aware
      networking techniques.  For example, there is limited ability in
      IP to constrain bidirectional paths to be symmetric, and
      information-centric networking protocols such as Named Data
      Networking (NDN) and Content-Centric Networking (CCNx) ([RFC8793])
      must force bidirectional path symmetry using protocol-specific

6.  Contributions

   Contributions on these Path Aware networking techniques were analyzed
   to arrive at the Lessons Learned captured in Section 4.

   Our expectation is that most readers will not need to read through
   this section carefully, but we wanted to record these hard-fought
   lessons as a service to others who may revisit this document, so
   they'll have the details close at hand.

6.1.  Stream Transport (ST, ST2, ST2+)

   The suggested references for Stream Transport are:

   *  ST - A Proposed Internet Stream Protocol [IEN-119]

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   *  Experimental Internet Stream Protocol, Version 2 (ST-II) [RFC1190]

   *  Internet Stream Protocol Version 2 (ST2) Protocol Specification -
      Version ST2+ [RFC1819]

   The first version of Stream Transport, ST [IEN-119], was published in
   the late 1970's and was implemented and deployed on the ARPANET at
   small scale.  It was used throughout the 1980's for experimental
   transmission of voice, video, and distributed simulation.

   The second version of the ST specification (ST2) [RFC1190] [RFC1819]
   was an experimental connection-oriented internetworking protocol that
   operated at the same layer as connectionless IP.  ST2 packets could
   be distinguished by their IP header version numbers (IP, at that
   time, used version number 4, while ST2 used version number 5).

   ST2 used a control plane layered over IP to select routes and reserve
   capacity for real-time streams across a network path, based on a flow
   specification communicated by a separate protocol.  The flow
   specification could be associated with QoS state in routers,
   producing an experimental resource reservation protocol.  This
   allowed ST2 routers along a path to offer end-to-end guarantees,
   primarily to satisfy the QoS requirements for realtime services over
   the Internet.

6.1.1.  Reasons for Non-deployment

   Although implemented in a range of equipment, ST2 was not widely used
   after completion of the experiments.  It did not offer the
   scalability and fate-sharing properties that have come to be desired
   by the Internet community.

   The ST2 protocol is no longer in use.

6.1.2.  Lessons Learned.

   As time passed, the trade-off between router processing and link
   capacity changed.  Links became faster and the cost of router
   processing became comparatively more expensive.

   The ST2 control protocol used "hard state" - once a route was
   established, and resources were reserved, routes and resources
   existing until they were explicitly released via signaling.  A soft-
   state approach was thought superior to this hard-state approach, and
   led to development of the IntServ model described in Section 6.2.

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6.2.  Integrated Services (IntServ)

   The suggested references for IntServ are:

   *  RFC 1633 Integrated Services in the Internet Architecture: an
      Overview [RFC1633]

   *  RFC 2211 Specification of the Controlled-Load Network Element
      Service [RFC2211]

   *  RFC 2212 Specification of Guaranteed Quality of Service [RFC2212]

   *  RFC 2215 General Characterization Parameters for Integrated
      Service Network Elements [RFC2215]

   *  RFC 2205 Resource ReSerVation Protocol (RSVP) [RFC2205]

   In 1994, when the IntServ architecture document [RFC1633] was
   published, real-time traffic was first appearing on the Internet.  At
   that time, bandwidth was still a scarce commodity.  Internet Service
   Providers built networks over DS3 (45 Mbps) infrastructure, and sub-
   rate (< 1 Mpbs) access was common.  Therefore, the IETF anticipated a
   need for a fine-grained QoS mechanism.

   In the IntServ architecture, some applications can require service
   guarantees.  Therefore, those applications use the Resource
   Reservation Protocol (RSVP) [RFC2205] to signal QoS reservations
   across network paths.  Every router in the network that participates
   in IntServ maintains per-flow soft-state to a) perform call admission
   control and b) deliver guaranteed service.

   Applications use Flow Specification (Flow Specs) [RFC2210] to
   describe the traffic that they emit.  RSVP reserves capacity for
   traffic on a per Flow Spec basis.

6.2.1.  Reasons for Non-deployment

   Although IntServ has been used in enterprise and government networks,
   IntServ was never widely deployed on the Internet because of its
   cost.  The following factors contributed to operational cost:

   *  IntServ must be deployed on every router that is on a path where
      IntServ is to be used.  Although it is possible to include a
      router that does not participate in IntServ along the path being
      controlled, if that router is likely to become a bottleneck,
      IntServ cannot be used to avoid that bottleneck along the path

   *  IntServ maintained per flow state

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   As IntServ was being discussed, the following occurred:

   *  For many expected uses, it became more cost effective to solve the
      QoS problem by adding bandwidth.  Between 1994 and 2000, Internet
      Service Providers upgraded their infrastructures from DS3 (45
      Mbps) to OC-48 (2.4 Gbps).  This meant that even if an endpoint
      was using IntServ in an IntServ-enabled network, its requests
      would rarely, if ever, be denied, so endpoints and Internet
      Service Providers had little reason to enable IntServ.

   *  DiffServ [RFC2475] offered a more cost-effective, albeit less
      fine-grained, solution to the QoS problem.

6.2.2.  Lessons Learned.

   The following lessons were learned:

   *  Any mechanism that requires every participating onpath router to
      maintain per-flow state is not likely to succeed, unless the
      additional cost for offering the feature can be recovered from the

   *  Any mechanism that requires an operator to upgrade all of its
      routers is not likely to succeed, unless the additional cost for
      offering the feature can be recovered from the user.

   In environments where IntServ has been deployed, trust relationships
   with endpoints are very different from trust relationships on the
   Internet itself, and there are often clearly-defined hierarchies in
   Service Level Agreements (SLAs), and well-defined transport flows
   operating with pre-determined capacity and latency requirements over
   paths where capacity or other attributes are constrained.

   IntServ was never widely deployed to manage capacity across the
   Internet.  However, the technique that it produced was deployed for
   reasons other than bandwidth management.  RSVP is widely deployed as
   an MPLS signaling mechanism.  BGP reuses the RSVP concept of Filter
   Specs to distribute firewall filters, although they are called Flow
   Spec Component Types in BGP [RFC5575].

6.3.  Quick-Start TCP

   The suggested references for Quick-Start TCP are:

   *  Quick-Start for TCP and IP [RFC4782]

   *  Determining an appropriate initial sending rate over an
      underutilized network path [SAF07]

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   *  Fast Startup Internet Congestion Control for Broadband Interactive
      Applications [Sch11]

   *  Using Quick-Start to enhance TCP-friendly rate control performance
      in bidirectional satellite networks [QS-SAT]

   Quick-Start [RFC4782] is an Experimental TCP extension that leverages
   support from the routers on the path to determine an allowed initial
   sending rate for a path through the Internet, either at the start of
   data transfers or after idle periods.  Without information about the
   path, a sender cannot easily determine an appropriate initial sending
   rate.  The default TCP congestion control therefore uses the safe but
   time-consuming slow-start algorithm [RFC5681].  With Quick-Start,
   connections are allowed to use higher initial sending rates if there
   is significant unused bandwidth along the path, and if the sender and
   all of the routers along the path approve the request.

   By examining the Time To Live (TTL) field in Quick-Start packets, a
   sender can determine if routers on the path have approved the Quick-
   Start request.  However, this method is unable to take into account
   the routers hidden by tunnels or other network nodes invisible at the
   IP layer.

   The protocol also includes a nonce that provides protection against
   cheating routers and receivers.  If the Quick-Start request is
   explicitly approved by all routers along the path, the TCP host can
   send at up to the approved rate; otherwise TCP would use the default
   congestion control.  Quick-Start requires modifications in the
   involved end-systems as well in routers.  Due to the resulting
   deployment challenges, Quick-Start was only proposed in [RFC4782] for
   controlled environments.

   The Quick-Start mechanism is a lightweight, coarse-grained, in-band,
   network-assisted fast startup mechanism.  The benefits are studied by
   simulation in a research paper [SAF07] that complements the protocol
   specification.  The study confirms that Quick-Start can significantly
   speed up mid-sized data transfers.  That paper also presents router
   algorithms that do not require keeping per-flow state.  Later studies
   [Sch11] comprehensively analyzes Quick-Start with a full Linux
   implementation and with a router fast path prototype using a network
   processor.  In both cases, Quick-Start could be implemented with
   limited additional complexity.

6.3.1.  Reasons for Non-deployment

   However, experiments with Quick-Start in [Sch11] revealed several

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   *  Having information from the routers along the path can reduce the
      risk of congestion, but cannot avoid it entirely.  Determining
      whether there is unused capacity is not trivial in actual router
      and host implementations.  Data about available capacity visible
      at the IP layer may be imprecise, and due to the propagation
      delay, information can already be outdated when it reaches a
      sender.  There is a trade-off between the speedup of data
      transfers and the risk of congestion even with Quick-Start.  This
      could be mitigated by only allowing Quick-Start to access a
      proportion of the unused capacity along a path.

   *  For scalable router fast path implementation, it is important to
      enable parallel processing of packets, as this is a widely used
      method e.g. in network processors.  One challenge is
      synchronization of information between packets that are processed
      in parallel, which should be avoided as much as possible.

   *  Only some types of application traffic can benefit from Quick-
      Start.  Capacity needs to be requested and discovered.  The
      discovered capacity needs to be utilized by the flow, or it
      implicitly becomes available for other flows.  Failing to use the
      requested capacity may have already reduced the pool of Quick-
      Start capacity that was made available to other competing Quick-
      Start requests.  The benefit is greatest when senders use this
      only for bulk flows and avoid sending unnecessary Quick-Start
      requests, e.g. for flows that only send a small amount of data.
      Choosing an appropriate request size requires application-internal
      knowledge that is not commonly expressed by the transport API.
      How a sender can determine the rate for an initial Quick-Start
      request is still a largely unsolved problem.

   There is no known deployment of Quick-Start for TCP or other IETF

6.3.2.  Lessons Learned

   Some lessons can be learned from Quick-Start.  Despite being a very
   light-weight protocol, Quick-Start suffers from poor incremental
   deployment properties, both regarding the required modifications in
   network infrastructure as well as its interactions with applications.
   Except for corner cases, congestion control can be quite efficiently
   performed end-to-end in the Internet, and in modern stacks there is
   not much room for significant improvement by additional network

   After publication of the Quick-Start specification, there have been
   large-scale experiments with an initial window of up to 10 MSS
   [RFC6928].  This alternative "IW10" approach can also ramp-up data

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   transfers faster than the standard congestion control, but it only
   requires sender-side modifications.  As a result, this approach can
   be easier and incrementally deployed in the Internet.  While
   theoretically Quick-Start can outperform "IW10", the improvement in
   completion time for data transfer times can, in many cases, be small.
   After publication of [RFC6928], most modern TCP stacks have increased
   their default initial window.

6.4.  ICMP Source Quench

   The suggested references for ICMP Source Quench are:


   The ICMP Source Quench message [RFC0792] allowed an on-path router to
   request the source of a flow to reduce its sending rate.  This method
   allowed a router to provide an early indication of impending
   congestion on a path to the sources that contribute to that

6.4.1.  Reasons for Non-deployment

   This method was deployed in Internet routers over a period of time,
   the reaction of endpoints to receiving this signal has varied.  For
   low speed links, with low multiplexing of flows the method could be
   used to regulate (momentarily reduce) the transmission rate.
   However, the simple signal does not scale with link speed, or the
   number of flows sharing a link.

   The approach was overtaken by the evolution of congestion control
   methods in TCP [RFC2001], and later also by other IETF transports.
   Because these methods were based upon measurement of the end-to-end
   path and an algorithm in the endpoint, they were able to evolve and
   mature more rapidly than methods relying on interactions between
   operational routers and endpoint stacks.

   After ICMP Source Quench was specified, the IETF began to recommend
   that transports provide end-to-end congestion control [RFC2001].  The
   Source Quench method has been obsoleted by the IETF [RFC6633], and
   both hosts and routers must now silently discard this message.

6.4.2.  Lessons Learned

   This method had several problems:

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   First, [RFC0792] did not sufficiently specify how the sender would
   react to the ICMP Source Quench signal from the path (e.g.,
   [RFC1016]).  There was ambiguity in how the sender should utilize
   this additional information.  This could lead to unfairness in the
   way that receivers (or routers) responded to this message.

   Second, while the message did provide additional information, the
   Explicit Congestion Notification (ECN) mechanism [RFC3168] provided a
   more robust and informative signal for network nodes to provide early
   indication that a path has become congested.

   The mechanism originated at a time when the Internet trust model was
   very different.  Most endpoint implementations did not attempt to
   verify that the message originated from an on-path node before they
   utilized the message.  This made it vulnerable to denial of service
   attacks.  In theory, routers might have chosen to use the quoted
   packet contained in the ICMP payload to validate that the message
   originated from an on-path node, but this would have increased per-
   packet processing overhead for each router along the path, would have
   required transport functionality in the router to verify whether the
   quoted packet header corresponded to a packet the router had sent.
   In addition, section 5.2 of [RFC4443] noted ICMPv6-based attacks on
   hosts that would also have threatened routers processing ICMPv6
   Source Quench payloads.  As time passed, it became increasingly
   obvious that the lack of validation of the messages exposed receivers
   to a security vulnerability where the messages could be forged to
   create a tangible denial of service opportunity.

6.5.  Triggers for Transport (TRIGTRAN)

   The suggested references for TRIGTRAN are:



   TCP [RFC0793] has a well-known weakness - the end-to-end flow control
   mechanism has only a single signal, the loss of a segment, and TCP
   implementations since the late 1980s have interpreted the loss of a
   segment as evidence that the path between two endpoints may have
   become congested enough to exhaust buffers on intermediate hops, so
   that the TCP sender should "back off" - reduce its sending rate until
   it knows that its segments are now being delivered without loss
   [RFC5681].  More modern TCP stacks have added a growing array of
   strategies about how to establish the sending rate [RFC5681], but
   when a path is no longer operational, TCP would continue to retry
   transmissions, which would fail, again, and double their
   Retransmission Time Out (RTO) timers with each failed transmission,

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   with the result that TCP would wait many seconds before retrying a
   segment, even if the path becomes operational while the sender is
   waiting for its next retry.

   The thinking behind TRIGTRAN was that if a path completely stopped
   working because a link along the path was "down", somehow something
   along the path could signal TCP when that link returned to service,
   and the sending TCP could retry immediately, without waiting for a
   full retransmission timeout (RTO) period.

6.5.1.  Reasons for Non-deployment

   The early dreams for TRIGTRAN were dashed because of an assumption
   that TRIGTRAN triggers would be unauthenticated.  This meant that any
   "safe" TRIGTRAN mechanism would have relied on a mechanism such as
   setting the IPv4 TTL or IPv6 Hop Count to 255 at a sender and testing
   that it was 254 upon receipt, so that a receiver could verify that a
   signal was generated by an adjacent sender known to be on the path
   being used, and not some unknown sender which might not even be on
   the path (e.g., "The Generalized TTL Security Mechanism (GTSM)"
   [RFC5082]).  This situation is very similar to the case for ICMP
   Source Quench messages as described in Section 6.4, which were also
   unauthenticated, and could be sent by an off-path attacker, resulting
   in deprecation of ICMP Source Quench message processing [RFC6633].

   TRIGTRAN's scope shrunk from "the path is down" to "the first-hop
   link is down".

   But things got worse.

   Because TRIGTRAN triggers would only be provided when the first-hop
   link was "down", TRIGTRAN triggers couldn't replace normal TCP
   retransmission behavior if the path failed because some link further
   along the network path was "down".  So TRIGTRAN triggers added
   complexity to an already complex TCP state machine, and did not allow
   any existing complexity to be removed.

   There was also an issue that the TRIGTRAN signal was not sent in
   response to a specific host that had been sending packets, and was
   instead a signal that stimulated a response by any sender on the
   link.  This needs to scale when there are multiple flows trying to
   use the same resource, yet the sender of a trigger has no
   understanding how many of the potential traffic sources will respond
   by sending packets - if recipients of the signal back-off their
   responses to a trigger to improve scaling, then that immediately
   mitigates the benefit of the signal.

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   Finally, intermediate forwarding nodes required modification to
   provide TRIGTRAN triggers, but operators couldn't charge for TRIGTRAN
   triggers, so there was no way to recover the cost of modifying,
   testing, and deploying updated intermediate nodes.

   Two TRIGTRAN BOFs were held, at IETF 55 [TRIGTRAN-55] and IETF 56
   [TRIGTRAN-56], but this work was not chartered, and there was no
   interest in deploying TRIGTRAN unless it was chartered and
   standardized in the IETF.

6.5.2.  Lessons Learned.

   The reasons why this work was not chartered, much less deployed,
   provide several useful lessons for researchers.

   *  TRIGTRAN started with a plausible value proposition, but
      networking realities in the early 2000s forced reductions in scope
      that led directly to reductions in potential benefits, but no
      corresponding reductions in costs and complexity.

   *  These reductions in scope were the direct result of an inability
      for hosts to trust or authenticate TRIGTRAN signals they received
      from the network.

   *  Operators did not believe they could charge for TRIGTRAN
      signaling, because first-hop links didn't fail frequently, and
      TRIGTRAN provided no reduction in operating expenses, so there was
      little incentive to purchase and deploy TRIGTRAN-capable network

   It is also worth noting that the targeted environment for TRIGTRAN in
   the late 1990s contained links with a relatively small number of
   directly-connected hosts - for instance, cellular or satellite links.
   The transport community was well aware of the dangers of sender
   synchronization based on multiple senders receiving the same stimulus
   at the same time, but the working assumption for TRIGTRAN was that
   there wouldn't be enough senders for this to be a meaningful problem.
   In the 2010s, it is common for a single "link" to support many
   senders and receivers on a single link, likely requiring TRIGTRAN
   senders to wait some random amount of time before sending after
   receiving a TRIGTRAN signal, which would have reduced the benefits of
   TRIGTRAN even more.

6.6.  Shim6

   The suggested references for Shim6 are:

   *  Shim6: Level 3 Multihoming Shim Protocol for IPv6 [RFC5533]

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   The IPv6 routing architecture [RFC1887] assumed that most sites on
   the Internet would be identified by Provider Assigned IPv6 prefixes,
   so that Default-Free Zone routers only contained routes to other
   providers, resulting in a very small IPv6 global routing table.

   For a single-homed site, this could work well.  A multihomed site
   with only one upstream provider could also work well, although BGP
   multihoming from a single upstream provider was often a premium
   service (costing more than twice as much as two single-homed sites),
   and if the single upstream provider went out of service, all of the
   multihomed paths could fail simultaneously.

   IPv4 sites often multihomed by obtaining Provider Independent
   prefixes, and advertising these prefixes through multiple upstream
   providers.  With the assumption that any multihomed IPv4 site would
   also multihome in IPv6, it seemed likely that IPv6 routing would be
   subject to the same pressures to announce Provider Independent
   prefixes, resulting in a global IPv6 routing table that exhibited the
   same explosive growth as the global IPv4 routing table.  During the
   early 2000s, work began on a protocol that would provide multihoming
   for IPv6 sites without requiring sites to advertise Provider
   Independent prefixes into the IPv6 global routing table.

   This protocol, called Shim6, allowed two endpoints to exchange
   multiple addresses ("Locators") that all mapped to the same endpoint
   ("Identity").  After an endpoint learned multiple Locators for the
   other endpoint, it could send to any of those Locators with the
   expectation that those packets would all be delivered to the endpoint
   with the same Identity.  Shim6 was an example of an "Identity/Locator
   Split" protocol.

   Shim6, as defined in [RFC5533] and related RFCs, provided a workable
   solution for IPv6 multihoming using Provider Assigned prefixes,
   including capability discovery and negotiation, and allowing end-to-
   end application communication to continue even in the face of path
   failure, because applications don't see Locator failures, and
   continue to communicate with the same Identity using a different

6.6.1.  Reasons for Non-deployment

   Note that the problem being addressed was "site multihoming", but
   Shim6 was providing "host multihoming".  That meant that the decision
   about what path would be used was under host control, not under
   router control.

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   Although more work could have been done to provide a better technical
   solution, the biggest impediments to Shim6 deployment were
   operational and business considerations.  These impediments were
   discussed at multiple network operator group meetings, including
   [Shim6-35] at [NANOG-35].

   The technique issues centered around concerns that Shim6 relied on
   the host to track all the connections, while also tracking Identity/
   Locator mappings in the kernel, and tracking failures to recognize
   that a backup path has failed.

   The operator issues centered around concerns that operators were
   performing traffic engineering, but would have no visibility or
   control over hosts when they chose to begin using another path, and
   relying on hosts to engineer traffic exposed their networks to
   oscillation based on feedback loops, as hosts move from path to path.
   At a minimum, traffic engineering policies must be pushed down to
   individual hosts.  In addition, firewalls that expected to find a
   transport-level protocol header in the IP payload, would see a Shim6
   Identity header, and be unable to perform transport-protocol-based
   firewalling functions because its normal processing logic would not
   look past the Identity header.

   The business issues centered removing or reducing the ability to sell
   BGP multihoming service, which is often more expensive than single-
   homed connectivity.

6.6.2.  Lessons Learned

   It is extremely important to take operational concerns into account
   when a path-aware protocol is making decisions about path selection
   that may conflict with existing operational practices and business

6.6.3.  Addendum on MultiPath TCP

   During discussions in the PANRG session at IETF 103 [PANRG-103-Min],
   Lars Eggert, past Transport Area Director, pointed out that during
   charter discussions for the Multipath TCP working group [MP-TCP],
   operators expressed concerns that customers could use Multipath TCP
   to loadshare TCP connections across operators simultaneously and
   compare passive performance measurements across network paths in real
   time, changing the balance of power in those business relationships.
   Although the Multipath TCP working group was chartered, this concern
   could have acted as an obstacle to deployment.

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   Operator objections to Shim6 were focused on technical concerns, but
   this concern could have also been an obstacle to Shim6 deployment if
   the technical concerns had been overcome.

6.7.  Next Steps in Signaling (NSIS)

   The suggested references for Next Steps in Signaling (NSIS) are:

   *  the concluded working group charter [NSIS-CHARTER-2001]

   *  GIST: General Internet Signalling Transport [RFC5971]

   *  NAT/Firewall NSIS Signaling Layer Protocol (NSLP) [RFC5973]

   *  NSIS Signaling Layer Protocol (NSLP) for Quality-of-Service
      Signaling [RFC5974]

   *  Authorization for NSIS Signaling Layer Protocols [RFC5981]

   The NSIS Working Group worked on signaling techniques for network
   layer resources (e.g., QoS resource reservations, Firewall and NAT

   When RSVP [RFC2205] was used in deployments, a number of questions
   came up about its perceived limitations and potential missing
   features.  The issues noted in the NSIS Working Group charter
   [NSIS-CHARTER-2001] include interworking between domains with
   different QoS architectures, mobility and roaming for IP interfaces,
   and complexity.  Later, the lack of security in RSVP was also
   recognized ([RFC4094]).

   The NSIS Working Group was chartered to tackle those issues and
   initially focused on QoS signaling as its primary use case.  However,
   over time a new approach evolved that introduced a modular
   architecture using application-specific signaling protocols (the NSIS
   Signaling Layer Protocol (NSLP)) on top of a generic signaling
   transport protocol (the NSIS Transport Layer Protocol (NTLP)).

   The NTLP is defined in [RFC5971].  Two NSLPs are defined: the NSIS
   Signaling Layer Protocol (NSLP) for Quality-of-Service Signaling
   [RFC5974] as well as the NAT/Firewall NSIS Signaling Layer Protocol
   (NSLP) [RFC5973].

6.7.1.  Reasons for Non-deployment

   The obstacles for deployment can be grouped into implementation-
   related aspects and operational aspects.

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   *  Implementation-related aspects:

   Although NSIS provides benefits with respect to flexibility,
   mobility, and security compared to other network signaling
   techniques, hardware vendors were reluctant to deploy this solution,
   because it would require additional implementation effort and would
   result in additional complexity for router implementations.

   The NTLP mainly operates as path-coupled signaling protocol, i.e, its
   messages are processed at the intermediate node's control plane that
   are also forwarding the data flows.  This requires a mechanism to
   intercept signaling packets while they are forwarded in the same
   manner (especially along the same path) as data packets.  NSIS uses
   the IPv4 and IPv6 Router Alert Option (RAO) to allow for interception
   of those path-coupled signaling messages, and this technique requires
   router implementations to correctly understand and implement the
   handling of RAOs, e.g., to only process packet with RAOs of interest
   and to leave packets with irrelevant RAOs in the fast forwarding
   processing path (a comprehensive discussion of these issues can be
   found in [RFC6398]).  The latter was an issue with some router
   implementations at the time of standardization.

   Another reason is that path-coupled signaling protocols that interact
   with routers and request manipulation of state at these routers (or
   any other network element in general) are under scrutiny: a packet
   (or sequence of packets) out of the mainly untrusted data path is
   requesting creation and manipulation of network state.  This is seen
   as potentially dangerous (e.g., opens up a Denial of Service (DoS)
   threat to a router's control plane) and difficult for an operator to
   control.  Path-coupled signaling approaches were considered
   problematic (see also section 3 of [RFC6398]).  There are
   recommendations on how to secure NSIS nodes and deployments (e.g.,

   *  Operational Aspects:

   NSIS not only required trust between customers and their provider,
   but also among different providers.  Especially, QoS signaling
   techniques would require some kind of dynamic service level agreement
   support that would imply (potentially quite complex) bilateral
   negotiations between different Internet service providers.  This
   complexity was not considered to be justified and increasing the
   bandwidth (and thus avoiding bottlenecks) was cheaper than actively
   managing network resource bottlenecks by using path-coupled QoS
   signaling techniques.  Furthermore, an end-to-end path typically
   involves several provider domains and these providers need to closely
   cooperate in cases of failures.

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6.7.2.  Lessons Learned

   One goal of NSIS was to decrease the complexity of the signaling
   protocol, but a path-coupled signaling protocol comes with the
   intrinsic complexity of IP-based networks, beyond the complexity of
   the signaling protocol itself.  Sources of intrinsic complexity

   *  the presence of asymmetric routes between endpoints and routers

   *  the lack of security and trust at large in the Internet

   *  the presence of different trust boundaries

   *  the effects of best-effort networks (e.g., robustness to packet

   *  divergence from the fate sharing principle (e.g., state within the

   Any path-coupled signaling protocol has to deal with these realities.

   Operators view the use of IPv4 and IPv6 Router Alert Option (RAO) to
   signal routers along the path from end systems with suspicion,
   because these end systems are usually not authenticated and heavy use
   of RAOs can easily increase the CPU load on routers that are designed
   to process most packets using a hardware "fast path" and diverting
   packets containing RAO to a slower, more capable processor.

6.8.  IPv6 Flow Label

   The suggested references for IPv6 Flow Label are:

   *  IPv6 Flow Label Specification [RFC6437]

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   IPv6 specifies a 20-bit field Flow Label field [RFC6437], included in
   the fixed part of the IPv6 header and hence present in every IPv6
   packet.  An endpoint sets the value in this field to one of a set of
   pseudo-randomly assigned values.  If a packet is not part of any
   flow, the flow label value is set to zero [RFC3697].  A number of
   Standards Track and Best Current Practice RFCs (e.g., [RFC8085],
   [RFC6437], [RFC6438]) encourage IPv6 endpoints to set a non-zero
   value in this field.  A multiplexing transport could choose to use
   multiple flow labels to allow the network to independently forward
   its subflows, or to use one common value for the traffic aggregate.
   The flow label is present in all fragments.  IPsec was originally put
   forward as one important use-case for this mechanism and does encrypt
   the field [RFC6438].

   Once set, the flow label can provide information that can help inform
   network nodes about subflows present at the transport layer, without
   needing to interpret the setting of upper layer protocol fields
   [RFC6294].  This information can also be used to coordinate how
   aggregates of transport subflows are grouped when queued in the
   network and to select appropriate per-flow forwarding when choosing
   between alternate paths [RFC6438] (e.g. for Equal Cost Multipath
   Routing (ECMP) and Link Aggregation (LAG)).

6.8.1.  Reasons for Non-deployment

   Despite the field being present in every IPv6 packet, the mechanism
   did not receive as much use as originally envisioned.  One reason is
   that to be useful it requires engagement by two different

   *  Endpoint Implementation:

   For network nodes along a path to utilize the flow label there needs
   to be a non-zero value inserted in the field [RFC6437] at the sending
   endpoint.  There needs to be an incentive for an endpoint to set an
   appropriate non-zero value.  The value should appropriately reflect
   the level of aggregation the traffic expects to be provided by the
   network.  However, this requires the stack to know granularity at
   which flows should be identified (or conversely which flows should
   receive aggregated treatment), i.e., which packets carry the same
   flow label.  Therefore, setting a non-zero value may result in
   additional choices that need to be made by an application developer.

   Although the standard [RFC3697] forbids any encoding of meaning into
   the flow label value, the opportunity to use the flow label as a
   covert channel or to signal other meta-information may have raised
   concerns about setting a non-zero value [RFC6437].

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   Before methods are widely deployed to use this method, there could be
   no incentive for an endpoint to set the field.

   *  Operational support in network nodes:

   A benefit can only be realized when a network node along the path
   also uses this information to inform its decisions.  Network
   equipment (routers and/or middleboxes) need to include appropriate
   support so they can utilize the field when making decisions about how
   to classify flows, or to inform forwarding choices.  Use of any
   optional feature in a network node also requires corresponding
   updates to operational procedures, and therefore is normally only
   introduced when the cost can be justified.

   A benefit from utilizing the flow label is expected to be increased
   quality of experience for applications - but this comes at some
   operational cost to an operator, and requires endpoints to set the

6.8.2.  Lessons Learned

   The flow label is a general purpose header field for use by the path.
   Multiple uses have been proposed.  One candidate use was to reduce
   the complexity of forwarding decisions.  However, modern routers can
   use a "fast path", often taking advantage of hardware to accelerate
   processing.  The method can assist in more complex forwarding, such
   as ECMP and load balancing.

   Although [RFC6437] recommended that endpoints should by default
   choose uniformly-distributed labels for their traffic, the
   specification permitted an endpoint to choose to set a zero value.
   This ability of endpoints to choose to set a flow label of zero has
   had consequences on deployability:

   *  Before wide-scale support by endpoints, it would be impossible to
      rely on a non-zero flow label being set.  Network nodes therefore
      would need to also employ other techniques to realize equivalent
      functions.  An example of a method is one assuming semantics of
      the source port field to provide entropy input to a network-layer
      hash.  This use of a 5-tuple to classify a packet represents a
      layering violation [RFC6294].  When other methods have been
      deployed, they increase the cost of deploying standards-based
      methods, even though they may offer less control to endpoints and
      result in potential interaction with other uses/interpretation of
      the field.

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   *  Even though the flow label is specified as an end-to-end field,
      some network paths have been observed to not transparently forward
      the flow label.  This could result from non-conformant equipment,
      or could indicate that some operational networks have chosen to
      re-use the protocol field for other (e.g. internal purposes).
      This results in lack of transparency, and a deployment hurdle to
      endpoints expecting that they can set a flow label that is
      utilized by the network.  The more recent practice of "greasing"
      [GREASE] would suggest that a different outcome could have been
      achieved if endpoints were always required to set a non-zero

   *  [RFC1809] noted that setting the choice of the flow label value
      can depend on the expectations of the traffic generated by an
      application, which suggests an API should be presented to control
      the setting or policy that is used.  However, many currently
      available APIs do not have this support.

   A growth in the use of encrypted transports, (e.g.  QUIC [QUIC-WG])
   seems likely to raise similar issues to those discussed above and
   could motivate renewed interest in utilizing the flow label.

7.  Security Considerations

   This document describes Path Aware techniques that were not adopted
   and widely deployed on the Internet, so it doesn't affect the
   security of the Internet.

   If this document meets its goals, we may develop new techniques for
   Path Aware Networking that would affect the security of the Internet,
   but security considerations for those techniques will be described in
   the corresponding RFCs that specify them.

8.  IANA Considerations

   This document makes no requests of IANA.

9.  Acknowledgments

   Initial material for Section 6.1 on ST2 was provided by Gorry

   Initial material for Section 6.2 on IntServ was provided by Ron

   Initial material for Section 6.3 on Quick-Start TCP was provided by
   Michael Scharf, who also provided suggestions to improve this section
   after it was edited.

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   Initial material for Section 6.4 on ICMP Source Quench was provided
   by Gorry Fairhurst.

   Initial material for Section 6.5 on Triggers for Transport (TRIGTRAN)
   was provided by Spencer Dawkins.

   Section 6.6 on Shim6 builds on initial material describing obstacles
   provided by Erik Nordmark, with background added by Spencer Dawkins.

   Initial material for Section 6.7 on Next Steps In Signaling (NSIS)
   was provided by Roland Bless and Martin Stiemerling.

   Initial material for Section 6.8 on IPv6 Flow Labels was provided by
   Gorry Fairhurst.

   Our thanks to C.M.  Heard, David Black, Erik Auerswald, Gorry
   Fairhurst, Joe Touch, Joeri de Ruiter, Mohamed Boucadair, Roland
   Bless, Ruediger Geib, Theresa Enghardt, and Wes Eddy, who provided
   review comments on previous versions.

   Mallory Knodel reviewed this document for the Internet Engineering
   Steering Group, and provided many helpful suggestions.

   David Oran also provided helpful comments and text suggestions on
   this document during Internet Engineering Steering Group balloting.
   In particular, Section 5 reflects his review.

   Special thanks to Adrian Farrel for helping Spencer navigate the
   twisty little passages of Flow Specs and Filter Specs in IntServ,
   RSVP, MPLS, and BGP.  They are all alike, except when they are
   different [Colossal-Cave].

10.  Informative References

              "Wikipedia Page for Colossal Cave Adventure", January

   [Conviva]  "Conviva Precision : Data Sheet", December 2020,

   [GREASE]   Thomson, M., "Long-term Viability of Protocol Extension
              Mechanisms", July 2019, <

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              Arkko, J., "Changes in the Internet Threat Model", Work in
              Progress, Internet-Draft, draft-arkko-arch-internet-
              threat-model-01, 8 July 2019, <

              Farrell, S., "We're gonna need a bigger threat model",
              Work in Progress, Internet-Draft, draft-farrell-etm-03, 6
              July 2019, <

              Polk, J. and S. Dhesikan, "Integrated Services (IntServ)
              Extension to Allow Signaling of Multiple Traffic
              Specifications and Multiple Flow Specifications in
              RSVPv1", Work in Progress, Internet-Draft, draft-ietf-
              tsvwg-intserv-multiple-tspec-02, 25 February 2013,

              Trammell, B., "Current Open Questions in Path Aware
              Networking", Work in Progress, Internet-Draft, draft-irtf-
              panrg-questions-08, 23 December 2020,

   [IEN-119]  Forgie, J., "ST - A Proposed Internet Stream Protocol",
              September 1979,

   [ITAT]     "IAB Workshop on Internet Technology Adoption and
              Transition (ITAT)", December 2013,

   [model-t]  "Model-t -- Discussions of changes in Internet deployment
              patterns and their impact on the Internet threat model",
              n.d., <>.

              "Media Operations Working Group - IETF-109 Minutes",
              November 2020,

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   [MP-TCP]   "Multipath TCP Working Group Home Page", n.d.,

   [NANOG-35] "North American Network Operators Group NANOG-35 Agenda",
              October 2005,

              "Next Steps In Signaling Working Group Charter", March

   [PANRG]    "Path Aware Networking Research Group (Home Page)", n.d.,

              "Path Aware Networking Research Group - IETF-103 Minutes",
              November 2018,

              "Path Aware Networking Research Group - IETF-105 Minutes",
              July 2019,

              "Path Aware Networking Research Group - IETF-106 Minutes",
              November 2019,

   [PANRG-99] "Path Aware Networking Research Group - IETF-99", July

              Bonaventure, O., "A Decade of Path Awareness", July 2017,

   [PathProp] Enghardt, T. and C. Kraehenbuehl, "A Vocabulary of Path
              Properties", November 2019, <

   [QS-SAT]   Secchi, R., Sathiaseelan, A., Potorti, F., Gotta, A., and
              G. Fairhurst, "Using Quick-Start to enhance TCP-friendly
              rate control performance in bidirectional satellite
              networks", 2009,

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   [QUIC-WG]  "QUIC Working Group Home Page", n.d.,

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,

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

   [RFC1016]  Prue, W. and J. Postel, "Something a Host Could Do with
              Source Quench: The Source Quench Introduced Delay
              (SQuID)", RFC 1016, DOI 10.17487/RFC1016, July 1987,

   [RFC1190]  Topolcic, C., "Experimental Internet Stream Protocol:
              Version 2 (ST-II)", RFC 1190, DOI 10.17487/RFC1190,
              October 1990, <>.

   [RFC1633]  Braden, R., Clark, D., and S. Shenker, "Integrated
              Services in the Internet Architecture: an Overview",
              RFC 1633, DOI 10.17487/RFC1633, June 1994,

   [RFC1809]  Partridge, C., "Using the Flow Label Field in IPv6",
              RFC 1809, DOI 10.17487/RFC1809, June 1995,

   [RFC1819]  Delgrossi, L., Ed. and L. Berger, Ed., "Internet Stream
              Protocol Version 2 (ST2) Protocol Specification - Version
              ST2+", RFC 1819, DOI 10.17487/RFC1819, August 1995,

   [RFC1887]  Rekhter, Y., Ed. and T. Li, Ed., "An Architecture for IPv6
              Unicast Address Allocation", RFC 1887,
              DOI 10.17487/RFC1887, December 1995,

   [RFC2001]  Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
              Retransmit, and Fast Recovery Algorithms", RFC 2001,
              DOI 10.17487/RFC2001, January 1997,

   [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, <>.

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   [RFC2210]  Wroclawski, J., "The Use of RSVP with IETF Integrated
              Services", RFC 2210, DOI 10.17487/RFC2210, September 1997,

   [RFC2211]  Wroclawski, J., "Specification of the Controlled-Load
              Network Element Service", RFC 2211, DOI 10.17487/RFC2211,
              September 1997, <>.

   [RFC2212]  Shenker, S., Partridge, C., and R. Guerin, "Specification
              of Guaranteed Quality of Service", RFC 2212,
              DOI 10.17487/RFC2212, September 1997,

   [RFC2215]  Shenker, S. and J. Wroclawski, "General Characterization
              Parameters for Integrated Service Network Elements",
              RFC 2215, DOI 10.17487/RFC2215, September 1997,

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,

   [RFC3697]  Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
              "IPv6 Flow Label Specification", RFC 3697,
              DOI 10.17487/RFC3697, March 2004,

   [RFC4094]  Manner, J. and X. Fu, "Analysis of Existing Quality-of-
              Service Signaling Protocols", RFC 4094,
              DOI 10.17487/RFC4094, May 2005,

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,

   [RFC4782]  Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-
              Start for TCP and IP", RFC 4782, DOI 10.17487/RFC4782,
              January 2007, <>.

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   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,

   [RFC5218]  Thaler, D. and B. Aboba, "What Makes for a Successful
              Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,

   [RFC5533]  Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
              Shim Protocol for IPv6", RFC 5533, DOI 10.17487/RFC5533,
              June 2009, <>.

   [RFC5575]  Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
              and D. McPherson, "Dissemination of Flow Specification
              Rules", RFC 5575, DOI 10.17487/RFC5575, August 2009,

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,

   [RFC5971]  Schulzrinne, H. and R. Hancock, "GIST: General Internet
              Signalling Transport", RFC 5971, DOI 10.17487/RFC5971,
              October 2010, <>.

   [RFC5973]  Stiemerling, M., Tschofenig, H., Aoun, C., and E. Davies,
              "NAT/Firewall NSIS Signaling Layer Protocol (NSLP)",
              RFC 5973, DOI 10.17487/RFC5973, October 2010,

   [RFC5974]  Manner, J., Karagiannis, G., and A. McDonald, "NSIS
              Signaling Layer Protocol (NSLP) for Quality-of-Service
              Signaling", RFC 5974, DOI 10.17487/RFC5974, October 2010,

   [RFC5981]  Manner, J., Stiemerling, M., Tschofenig, H., and R. Bless,
              Ed., "Authorization for NSIS Signaling Layer Protocols",
              RFC 5981, DOI 10.17487/RFC5981, February 2011,

   [RFC6294]  Hu, Q. and B. Carpenter, "Survey of Proposed Use Cases for
              the IPv6 Flow Label", RFC 6294, DOI 10.17487/RFC6294, June
              2011, <>.

   [RFC6398]  Le Faucheur, F., Ed., "IP Router Alert Considerations and
              Usage", BCP 168, RFC 6398, DOI 10.17487/RFC6398, October
              2011, <>.

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   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437,
              DOI 10.17487/RFC6437, November 2011,

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,

   [RFC6633]  Gont, F., "Deprecation of ICMP Source Quench Messages",
              RFC 6633, DOI 10.17487/RFC6633, May 2012,

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,

   [RFC7305]  Lear, E., Ed., "Report from the IAB Workshop on Internet
              Technology Adoption and Transition (ITAT)", RFC 7305,
              DOI 10.17487/RFC7305, July 2014,

   [RFC7418]  Dawkins, S., Ed., "An IRTF Primer for IETF Participants",
              RFC 7418, DOI 10.17487/RFC7418, December 2014,

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <>.

   [RFC8170]  Thaler, D., Ed., "Planning for Protocol Adoption and
              Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170,
              May 2017, <>.

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,

   [RFC8793]  Wissingh, B., Wood, C., Afanasyev, A., Zhang, L., Oran,
              D., and C. Tschudin, "Information-Centric Networking
              (ICN): Content-Centric Networking (CCNx) and Named Data
              Networking (NDN) Terminology", RFC 8793,
              DOI 10.17487/RFC8793, June 2020,

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              "Security Area Open Meeting - IETF-105 Minutes", July
              2019, <

   [SAF07]    Sarolahti, P., Allman, M., and S. Floyd, "Determining an
              appropriate sending rate over an underutilized network
              path", Computer Networking Volume 51, Number 7, May 2007.

   [Sch11]    Scharf, M., "Fast Startup Internet Congestion Control for
              Broadband Interactive Applications", Ph.D. Thesis,
              University of Stuttgart, April 2011.

   [Shim6-35] Meyer, D., Huston, G., Schiller, J., and V. Gill, "IAB
              IPv6 Multihoming Panel at NANOG 35", NANOG North American
              Network Operator Group, October 2005,

              "Triggers for Transport BOF at IETF 55", July 2003,

              "Triggers for Transport BOF at IETF 56", November 2003,

Author's Address

   Spencer Dawkins (editor)
   Tencent America


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