[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]
Versions: (draft-dawkins-panrg-what-not-to-do)
00 01 02 03 04 05 06 07 08 09 10 11
12 13
PANRG S. Dawkins, Ed.
Internet-Draft Tencent America
Intended status: Informational 1 September 2020
Expires: 5 March 2021
Path Aware Networking: Obstacles to Deployment (A Bestiary of Roads Not
Taken)
draft-irtf-panrg-what-not-to-do-13
Abstract
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 https://datatracker.ietf.org/drafts/current/.
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 5 March 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 (https://trustee.ietf.org/
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.
Dawkins Expires 5 March 2021 [Page 1]
Internet-Draft What Not To Do September 2020
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Purpose of This Document . . . . . . . . . . . . . . . . 4
1.2. What Does "Path Awareness" Mean in this Document? . . . . 4
1.3. Note to RFC Editor . . . . . . . . . . . . . . . . . . . 4
1.4. A Note About Path-Aware Techniques Included In This
Document . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5. Venue for Discussion of this Document . . . . . . . . . . 5
1.6. Architectural Guidance . . . . . . . . . . . . . . . . . 5
1.7. Terminology Used in this Document . . . . . . . . . . . . 6
2. Summary of Lessons Learned . . . . . . . . . . . . . . . . . 6
2.1. Justifying Deployment . . . . . . . . . . . . . . . . . . 6
2.2. Providing Benefits for Early Adopters . . . . . . . . . . 6
2.3. Providing Benefits During Partial Deployment . . . . . . 7
2.4. Outperforming End-to-end Protocol Mechanisms . . . . . . 7
2.5. Paying for Path Aware Techniques . . . . . . . . . . . . 7
2.6. Impact on Operational Practices . . . . . . . . . . . . . 7
2.7. Per-connection State . . . . . . . . . . . . . . . . . . 7
2.8. Keeping Traffic on Fast-paths . . . . . . . . . . . . . . 8
2.9. Endpoints Trusting Intermediate Nodes . . . . . . . . . . 8
2.10. Intermediate Nodes Trusting Endpoints . . . . . . . . . . 8
2.11. Reacting to Distant Signals . . . . . . . . . . . . . . . 8
2.12. Support in Endpoint Protocol Stacks . . . . . . . . . . . 9
3. Do We Understand the Lessons We've Learned? . . . . . . . . . 9
4. Template for Contributions . . . . . . . . . . . . . . . . . 12
5. Contributions . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Stream Transport (ST, ST2, ST2+) . . . . . . . . . . . . 12
5.1.1. Reasons for Non-deployment . . . . . . . . . . . . . 13
5.1.2. Lessons Learned. . . . . . . . . . . . . . . . . . . 13
5.2. Integrated Services (IntServ) . . . . . . . . . . . . . . 13
5.2.1. Reasons for Non-deployment . . . . . . . . . . . . . 14
5.2.2. Lessons Learned. . . . . . . . . . . . . . . . . . . 15
5.3. Quick-Start TCP . . . . . . . . . . . . . . . . . . . . . 15
5.3.1. Reasons for Non-deployment . . . . . . . . . . . . . 16
5.3.2. Lessons Learned . . . . . . . . . . . . . . . . . . . 17
5.4. ICMP Source Quench . . . . . . . . . . . . . . . . . . . 18
5.4.1. Reasons for Non-deployment . . . . . . . . . . . . . 18
5.4.2. Lessons Learned . . . . . . . . . . . . . . . . . . . 18
5.5. Triggers for Transport (TRIGTRAN) . . . . . . . . . . . . 19
5.5.1. Reasons for Non-deployment . . . . . . . . . . . . . 20
5.5.2. Lessons Learned. . . . . . . . . . . . . . . . . . . 21
5.6. Shim6 . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.6.1. Reasons for Non-deployment . . . . . . . . . . . . . 22
5.6.2. Lessons Learned . . . . . . . . . . . . . . . . . . . 23
5.6.3. Addendum on MultiPath TCP . . . . . . . . . . . . . . 23
5.7. Next Steps in Signaling (NSIS) . . . . . . . . . . . . . 23
5.7.1. Reasons for Non-deployment . . . . . . . . . . . . . 24
Dawkins Expires 5 March 2021 [Page 2]
Internet-Draft What Not To Do September 2020
5.7.2. Lessons Learned . . . . . . . . . . . . . . . . . . . 25
5.8. IPv6 Flow Label . . . . . . . . . . . . . . . . . . . . . 26
5.8.1. Reasons for Non-deployment . . . . . . . . . . . . . 27
5.8.2. Lessons Learned . . . . . . . . . . . . . . . . . . . 28
6. Security Considerations . . . . . . . . . . . . . . . . . . . 29
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
9. Informative References . . . . . . . . . . . . . . . . . . . 30
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
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 2), based on that catalog (Section 5).
This document represents the consensus of the Path Aware Networking
Research Group.
Dawkins Expires 5 March 2021 [Page 3]
Internet-Draft What Not To Do September 2020
1.1. Purpose of This Document
This Informational document discusses 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
contains.
1.2. 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 5 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.
1.3. Note to RFC Editor
If the "Definition" in Section 1.1 ("Definition") of
[I-D.irtf-panrg-questions] changes, the text in Section 1.2 of this
document should be should be changed as well.
Whether that happens or not, the RFC Editor is requested to remove
this section.
Dawkins Expires 5 March 2021 [Page 4]
Internet-Draft What Not To Do September 2020
1.4. 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 2 to inform researchers and protocol
engineers in their work.
No shame is intended for the techniques included in this document.
As shown in Section 2, 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.
1.5. 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.
1.6. 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].
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 2 reflect considerations
described in [RFC5218], [RFC7305], and [RFC8170].
Dawkins Expires 5 March 2021 [Page 5]
Internet-Draft What Not To Do September 2020
1.7. Terminology Used in this Document
The terms Node and Element in this document have the meaning defined
in [PathProp].
2. Summary of Lessons Learned
This section summarizes the Lessons Learned from the contributed
subsections in Section 5.
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 5.6 and Section 5.6.3 as one example of this
shifting boundary.
2.1. Justifying Deployment
The benefit of Path Awareness must be great enough to justify making
changes in an operational network. The colloquial American English
expression, "If it ain't broke, don't fix it" is a "best current
practice" on today's Internet. (See Section 5.3, Section 5.5, and
Section 5.4).
2.2. 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 5.2 and Section 5.3).
Dawkins Expires 5 March 2021 [Page 6]
Internet-Draft What Not To Do September 2020
2.3. 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 5.2 and Section 5.3).
2.4. 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 5.3 and Section 5.5).
2.5. 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 5.1, Section 5.2, and Section 5.5).
2.6. 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. These impacts may make deployment more likely, but often
discourage deployment. (See Section 5.6, including Section 5.6.3).
2.7. Per-connection State
Per-connection state in intermediate nodes has been an impediment to
adoption and deployment in the past. This is especially true as we
move from the edge of the network, further into the routing core (See
Section 5.1 and Section 5.2).
Dawkins Expires 5 March 2021 [Page 7]
Internet-Draft What Not To Do September 2020
2.8. 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 5.7).
2.9. 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 5.5, Section 5.4). 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.
2.10. 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 5.2 and Section 5.7. We
also note this still remains a factor hindering deployment of
DiffServ).
2.11. 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 5.3).
Dawkins Expires 5 March 2021 [Page 8]
Internet-Draft What Not To Do September 2020
2.12. 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.
TAPS is exploring such an API [TAPS-WG], yet even with such an API,
policy is needed to bind the application expectations to the network
characteristics. (See Section 5.1 and Section 5.2).
3. Do We Understand 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 5 predate the initial
scope. The earliest Path-Aware Networking technique referred to in
Section 5 is Section 5.1, published in the late 1970s. Given that
the networking ecosystem has evolved continuously, it seemed
reasonable to ask whether the Lessons we've Learned are still true.
The PANRG Research Group reviewed the Lessons Learned 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" 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
Dawkins Expires 5 March 2021 [Page 9]
Internet-Draft What Not To Do September 2020
+-----------------------------------------------------+-----------+
| Lesson | Category |
+=====================================================+===========+
| Justifying Deployment (Section 2.1) | Invariant |
+-----------------------------------------------------+-----------+
| Providing Benefits for Early Adopters (Section 2.2) | Invariant |
+-----------------------------------------------------+-----------+
| Providing Benefits during Partial Deployment | Invariant |
| (Section 2.3) | |
+-----------------------------------------------------+-----------+
| Outperforming End-to-end Protocol Mechanisms | Variable |
| (Section 2.4) | |
+-----------------------------------------------------+-----------+
| Paying for Path Aware Techniques (Section 2.5) | Invariant |
+-----------------------------------------------------+-----------+
| Impact on Operational Practices (Section 2.6) | Invariant |
+-----------------------------------------------------+-----------+
| Per-connection State (Section 2.7) | Variable |
+-----------------------------------------------------+-----------+
| Keeping Traffic on Fast-paths (Section 2.8) | Variable |
+-----------------------------------------------------+-----------+
| Endpoints Trusting Intermediate Nodes (Section 2.9) | Not Now |
+-----------------------------------------------------+-----------+
| Intermediate Nodes Trusting Endpoints | Not Now |
| (Section 2.10) | |
+-----------------------------------------------------+-----------+
| Reacting to Distant Signals (Section 2.11) | Variable |
+-----------------------------------------------------+-----------+
| Support in Endpoint Protocol Stacks (Section 2.12) | 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
Technique.
"Providing Benefits During Partial Deployment" was added after IETF
105, during research group last call, and is also considered to be
invariant.
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.
Dawkins Expires 5 March 2021 [Page 10]
Internet-Draft What Not To Do September 2020
* 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 5.2) 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
desirable.
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
[draft-arkko-arch-internet-threat-model] and [draft-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.
Dawkins Expires 5 March 2021 [Page 11]
Internet-Draft What Not To Do September 2020
* 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 current work
in the TAPS working group ([TAPS-WG]) could provide an API that
allows applications to more easily make use of Path Aware techniques,
but this isn't guaranteed.
4. Template for Contributions
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
solve
* a short description of the reasons why the technique wasn't
adopted
* a short statement of the lessons that researchers can learn from
our experience with this technique.
5. Contributions
Contributions on these Path Aware networking techniques were analyzed
to arrive at the Lessons Learned captured in Section 2.
5.1. Stream Transport (ST, ST2, ST2+)
The suggested references for Stream Transport are:
* ST - A Proposed Internet Stream Protocol [IEN-119]
* Experimental Internet Stream Protocol, Version 2 (ST-II) [RFC1190]
* Internet Stream Protocol Version 2 (ST2) Protocol Specification -
Version ST2+ [RFC1819]
Dawkins Expires 5 March 2021 [Page 12]
Internet-Draft What Not To Do September 2020
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 protocol numbers (IP, at that
time, used protocol number 4, while ST2 used protocol 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.
5.1.1. 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.
5.1.2. 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 5.2.
5.2. Integrated Services (IntServ)
The suggested references for IntServ are:
* RFC 1633 Integrated Services in the Internet Architecture: an
Overview [RFC1633]
Dawkins Expires 5 March 2021 [Page 13]
Internet-Draft What Not To Do September 2020
* 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 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.
5.2.1. 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
* IntServ maintained per flow state
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 never be denied, so endpoints and Internet Service Providers
had little reason to enable IntServ.
Dawkins Expires 5 March 2021 [Page 14]
Internet-Draft What Not To Do September 2020
* DiffServ [RFC2475] offered a more cost-effective, albeit less
fine-grained, solution to the QoS problem.
5.2.2. Lessons Learned.
The following lessons were learned:
* Any mechanism that requires every 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 user.
* 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].
5.3. 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]
* 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
Dawkins Expires 5 March 2021 [Page 15]
Internet-Draft What Not To Do September 2020
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.
5.3.1. Reasons for Non-deployment
However, experiments with Quick-Start in [Sch11] revealed several
challenges:
* 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.
Dawkins Expires 5 March 2021 [Page 16]
Internet-Draft What Not To Do September 2020
* 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
transports.
5.3.2. 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
support.
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
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.
Dawkins Expires 5 March 2021 [Page 17]
Internet-Draft What Not To Do September 2020
5.4. ICMP Source Quench
The suggested references for ICMP Source Quench are:
* INTERNET CONTROL MESSAGE PROTOCOL [RFC0792]
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
congestion.
5.4.1. 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.
5.4.2. Lessons Learned
This method had several problems:
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.
Dawkins Expires 5 March 2021 [Page 18]
Internet-Draft What Not To Do September 2020
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.
5.5. Triggers for Transport (TRIGTRAN)
The suggested references for TRIGTRAN are:
* TRIGTRAN BOF at IETF 55 [TRIGTRAN-55]
* TRIGTRAN BOF at IETF 56 [TRIGTRAN-56]
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,
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.
Dawkins Expires 5 March 2021 [Page 19]
Internet-Draft What Not To Do September 2020
5.5.1. 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 5.4, 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.
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.
Dawkins Expires 5 March 2021 [Page 20]
Internet-Draft What Not To Do September 2020
5.5.2. 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
equipment.
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.
5.6. Shim6
The suggested references for Shim6 are:
* Shim6: Level 3 Multihoming Shim Protocol for IPv6 [RFC5533]
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.
Dawkins Expires 5 March 2021 [Page 21]
Internet-Draft What Not To Do September 2020
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
Locator.
5.6.1. 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.
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].
Dawkins Expires 5 March 2021 [Page 22]
Internet-Draft What Not To Do September 2020
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.
5.6.2. 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
considerations.
5.6.3. 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.
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.
5.7. Next Steps in Signaling (NSIS)
The suggested references for Next Steps in Signaling (NSIS) are:
Dawkins Expires 5 March 2021 [Page 23]
Internet-Draft What Not To Do September 2020
* 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
traversal).
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].
5.7.1. Reasons for Non-deployment
The obstacles for deployment can be grouped into implementation-
related aspects and operational aspects.
* 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.
Dawkins Expires 5 March 2021 [Page 24]
Internet-Draft What Not To Do September 2020
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.,
[RFC5981]).
* 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.
5.7.2. 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
include:
Dawkins Expires 5 March 2021 [Page 25]
Internet-Draft What Not To Do September 2020
* the presence of asymmetric routes between endpoints and routers
* the lack of security and trust at large in the Internet
infrastructure
* the presence of different trust boundaries
* the effects of best-effort networks (e.g., robustness to packet
loss)
* divergence from the fate sharing principle (e.g., state within the
network).
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.
5.8. IPv6 Flow Label
The suggested references for IPv6 Flow Label are:
* IPv6 Flow Label Specification [RFC6437]
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].
Dawkins Expires 5 March 2021 [Page 26]
Internet-Draft What Not To Do September 2020
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)).
5.8.1. 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
stakeholders:
* 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].
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.
Dawkins Expires 5 March 2021 [Page 27]
Internet-Draft What Not To Do September 2020
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
field.
5.8.2. 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.
* 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
value.
Dawkins Expires 5 March 2021 [Page 28]
Internet-Draft What Not To Do September 2020
* [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.
6. 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.
7. IANA Considerations
This document makes no requests of IANA.
8. Acknowledgments
Initial material for Section 5.1 on ST2 was provided by Gorry
Fairhurst.
Initial material for Section 5.2 on IntServ was provided by Ron
Bonica.
Initial material for Section 5.3 on Quick-Start TCP was provided by
Michael Scharf, who also provided suggestions to improve this section
after it was edited.
Initial material for Section 5.4 on ICMP Source Quench was provided
by Gorry Fairhurst.
Initial material for Section 5.5 on Triggers for Transport (TRIGTRAN)
was provided by Spencer Dawkins.
Section 5.6 on Shim6 builds on initial material describing obstacles
provided by Erik Nordmark, with background added by Spencer Dawkins.
Initial material for Section 5.7 on Next Steps In Signaling (NSIS)
was provided by Roland Bless and Martin Stiemerling.
Dawkins Expires 5 March 2021 [Page 29]
Internet-Draft What Not To Do September 2020
Initial material for Section 5.8 on IPv6 Flow Labels was provided by
Gorry Fairhurst.
Our thanks to C.M. Heard, David Black, 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.
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].
9. Informative References
[Colossal-Cave]
"Wikipedia Page for Colossal Cave Adventure", January
2019,
<https://en.wikipedia.org/wiki/Colossal_Cave_Adventure>.
[draft-arkko-arch-internet-threat-model]
Arkko, J., "Changes in the Internet Threat Model", n.d.,
<https://datatracker.ietf.org/doc/draft-arkko-arch-
internet-threat-model/>.
[draft-farrell-etm]
Farrell, S., "We're gonna need a bigger threat model",
n.d.,
<https://datatracker.ietf.org/doc/draft-farrell-etm/>.
[GREASE] Thomson, M., "Long-term Viability of Protocol Extension
Mechanisms", July 2019, <https://tools.ietf.org/html/
draft-iab-use-it-or-lose-it-00>.
[I-D.irtf-panrg-questions]
Trammell, B., "Current Open Questions in Path Aware
Networking", Work in Progress, Internet-Draft, draft-irtf-
panrg-questions-07, 29 August 2020, <http://www.ietf.org/
internet-drafts/draft-irtf-panrg-questions-07.txt>.
[IEN-119] Forgie, J., "ST - A Proposed Internet Stream Protocol",
September 1979,
<https://www.rfc-editor.org/ien/ien119.txt>.
[ITAT] "IAB Workshop on Internet Technology Adoption and
Transition (ITAT)", December 2013,
<https://www.iab.org/activities/workshops/itat/>.
Dawkins Expires 5 March 2021 [Page 30]
Internet-Draft What Not To Do September 2020
[model-t] "Model-t -- Discussions of changes in Internet deployment
patterns and their impact on the Internet threat model",
n.d., <https://www.iab.org/mailman/listinfo/model-t>.
[MP-TCP] "Multipath TCP Working Group Home Page", n.d.,
<https://datatracker.ietf.org/wg/mptcp/about/>.
[NANOG-35] "North American Network Operators Group NANOG-35 Agenda",
October 2005,
<https://www.nanog.org/meetings/nanog35/agenda>.
[NSIS-CHARTER-2001]
"Next Steps In Signaling Working Group Charter", March
2011,
<https://datatracker.ietf.org/doc/charter-ietf-nsis/>.
[PANRG] "Path Aware Networking Research Group (Home Page)", n.d.,
<https://irtf.org/panrg>.
[PANRG-103-Min]
"Path Aware Networking Research Group - IETF-103 Minutes",
November 2018,
<https://datatracker.ietf.org/doc/minutes-103-panrg/>.
[PANRG-105-Min]
"Path Aware Networking Research Group - IETF-105 Minutes",
July 2019,
<https://datatracker.ietf.org/doc/minutes-105-panrg/>.
[PANRG-106-Min]
"Path Aware Networking Research Group - IETF-106 Minutes",
November 2019,
<https://datatracker.ietf.org/doc/minutes-106-panrg/>.
[PANRG-99] "Path Aware Networking Research Group - IETF-99", July
2017,
<https://datatracker.ietf.org/meeting/99/sessions/panrg>.
[PATH-Decade]
Bonaventure, O., "A Decade of Path Awareness", July 2017,
<https://datatracker.ietf.org/doc/slides-99-panrg-a-
decade-of-path-awareness/>.
[PathProp] Enghardt, T. and C. Kraehenbuehl, "A Vocabulary of Path
Properties", November 2019, <https://tools.ietf.org/html/
draft-enghardt-panrg-path-properties-03>.
Dawkins Expires 5 March 2021 [Page 31]
Internet-Draft What Not To Do September 2020
[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,
<https://dl.acm.org/citation.cfm?id=3160304.3160305>.
[QUIC-WG] "QUIC Working Group Home Page", n.d.,
<https://datatracker.ietf.org/wg/quic/about/>.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[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,
<https://www.rfc-editor.org/info/rfc1016>.
[RFC1190] Topolcic, C., "Experimental Internet Stream Protocol:
Version 2 (ST-II)", RFC 1190, DOI 10.17487/RFC1190,
October 1990, <https://www.rfc-editor.org/info/rfc1190>.
[RFC1633] Braden, R., Clark, D., and S. Shenker, "Integrated
Services in the Internet Architecture: an Overview",
RFC 1633, DOI 10.17487/RFC1633, June 1994,
<https://www.rfc-editor.org/info/rfc1633>.
[RFC1809] Partridge, C., "Using the Flow Label Field in IPv6",
RFC 1809, DOI 10.17487/RFC1809, June 1995,
<https://www.rfc-editor.org/info/rfc1809>.
[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,
<https://www.rfc-editor.org/info/rfc1819>.
[RFC1887] Rekhter, Y., Ed. and T. Li, Ed., "An Architecture for IPv6
Unicast Address Allocation", RFC 1887,
DOI 10.17487/RFC1887, December 1995,
<https://www.rfc-editor.org/info/rfc1887>.
Dawkins Expires 5 March 2021 [Page 32]
Internet-Draft What Not To Do September 2020
[RFC2001] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery Algorithms", RFC 2001,
DOI 10.17487/RFC2001, January 1997,
<https://www.rfc-editor.org/info/rfc2001>.
[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, <https://www.rfc-editor.org/info/rfc2205>.
[RFC2210] Wroclawski, J., "The Use of RSVP with IETF Integrated
Services", RFC 2210, DOI 10.17487/RFC2210, September 1997,
<https://www.rfc-editor.org/info/rfc2210>.
[RFC2211] Wroclawski, J., "Specification of the Controlled-Load
Network Element Service", RFC 2211, DOI 10.17487/RFC2211,
September 1997, <https://www.rfc-editor.org/info/rfc2211>.
[RFC2212] Shenker, S., Partridge, C., and R. Guerin, "Specification
of Guaranteed Quality of Service", RFC 2212,
DOI 10.17487/RFC2212, September 1997,
<https://www.rfc-editor.org/info/rfc2212>.
[RFC2215] Shenker, S. and J. Wroclawski, "General Characterization
Parameters for Integrated Service Network Elements",
RFC 2215, DOI 10.17487/RFC2215, September 1997,
<https://www.rfc-editor.org/info/rfc2215>.
[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,
<https://www.rfc-editor.org/info/rfc2475>.
[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,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
"IPv6 Flow Label Specification", RFC 3697,
DOI 10.17487/RFC3697, March 2004,
<https://www.rfc-editor.org/info/rfc3697>.
[RFC4094] Manner, J. and X. Fu, "Analysis of Existing Quality-of-
Service Signaling Protocols", RFC 4094,
DOI 10.17487/RFC4094, May 2005,
<https://www.rfc-editor.org/info/rfc4094>.
Dawkins Expires 5 March 2021 [Page 33]
Internet-Draft What Not To Do September 2020
[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,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC4782] Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-
Start for TCP and IP", RFC 4782, DOI 10.17487/RFC4782,
January 2007, <https://www.rfc-editor.org/info/rfc4782>.
[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,
<https://www.rfc-editor.org/info/rfc5082>.
[RFC5218] Thaler, D. and B. Aboba, "What Makes for a Successful
Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,
<https://www.rfc-editor.org/info/rfc5218>.
[RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, DOI 10.17487/RFC5533,
June 2009, <https://www.rfc-editor.org/info/rfc5533>.
[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,
<https://www.rfc-editor.org/info/rfc5575>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[RFC5971] Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", RFC 5971, DOI 10.17487/RFC5971,
October 2010, <https://www.rfc-editor.org/info/rfc5971>.
[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,
<https://www.rfc-editor.org/info/rfc5973>.
[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,
<https://www.rfc-editor.org/info/rfc5974>.
Dawkins Expires 5 March 2021 [Page 34]
Internet-Draft What Not To Do September 2020
[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,
<https://www.rfc-editor.org/info/rfc5981>.
[RFC6294] Hu, Q. and B. Carpenter, "Survey of Proposed Use Cases for
the IPv6 Flow Label", RFC 6294, DOI 10.17487/RFC6294, June
2011, <https://www.rfc-editor.org/info/rfc6294>.
[RFC6398] Le Faucheur, F., Ed., "IP Router Alert Considerations and
Usage", BCP 168, RFC 6398, DOI 10.17487/RFC6398, October
2011, <https://www.rfc-editor.org/info/rfc6398>.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437,
DOI 10.17487/RFC6437, November 2011,
<https://www.rfc-editor.org/info/rfc6437>.
[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,
<https://www.rfc-editor.org/info/rfc6438>.
[RFC6633] Gont, F., "Deprecation of ICMP Source Quench Messages",
RFC 6633, DOI 10.17487/RFC6633, May 2012,
<https://www.rfc-editor.org/info/rfc6633>.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928,
DOI 10.17487/RFC6928, April 2013,
<https://www.rfc-editor.org/info/rfc6928>.
[RFC7305] Lear, E., Ed., "Report from the IAB Workshop on Internet
Technology Adoption and Transition (ITAT)", RFC 7305,
DOI 10.17487/RFC7305, July 2014,
<https://www.rfc-editor.org/info/rfc7305>.
[RFC7418] Dawkins, S., Ed., "An IRTF Primer for IETF Participants",
RFC 7418, DOI 10.17487/RFC7418, December 2014,
<https://www.rfc-editor.org/info/rfc7418>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8170] Thaler, D., Ed., "Planning for Protocol Adoption and
Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170,
May 2017, <https://www.rfc-editor.org/info/rfc8170>.
Dawkins Expires 5 March 2021 [Page 35]
Internet-Draft What Not To Do September 2020
[SAAG-105-Min]
"Security Area Open Meeting - IETF-105 Minutes", July
2019, <https://datatracker.ietf.org/meeting/105/materials/
minutes-105-saag-00>.
[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,
<https://www.youtube.com/watch?v=ji6Y_rYHAQs>.
[TAPS-WG] "Transport Services Working Group Home Page", n.d.,
<https://datatracker.ietf.org/wg/taps/about/>.
[TRIGTRAN-55]
"Triggers for Transport BOF at IETF 55", July 2003,
<https://www.ietf.org/proceedings/55/239.htm>.
[TRIGTRAN-56]
"Triggers for Transport BOF at IETF 56", November 2003,
<https://www.ietf.org/proceedings/56/251.htm>.
Author's Address
Spencer Dawkins (editor)
Tencent America
Email: spencerdawkins.ietf@gmail.com
Dawkins Expires 5 March 2021 [Page 36]
Html markup produced by rfcmarkup 1.129d, available from
https://tools.ietf.org/tools/rfcmarkup/