PANRG S. Dawkins, Ed.
Internet-Draft Huawei Technologies
Intended status: Informational June 18, 2018
Expires: December 20, 2018

Path Aware Networking: A Bestiary of Roads Not Taken
draft-dawkins-panrg-what-not-to-do-01

Abstract

At the first meeting of the proposed Path Aware Networking Research Group, Oliver Bonaventure led a discussion of our mostly-unsuccessful attempts to exploit Path Awareness to achieve a variety of goals, over the past decade. At the end of that discussion, the research group agreed to catalog and analyze these ideas, to extract insights and lessons for path-aware networking researchers.

This document contains that catalog and analysis.

Status of This Memo

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This Internet-Draft will expire on December 20, 2018.

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Table of Contents

1. Introduction

At IETF 99, the proposed Path Aware Networking Research Group [PANRG] held its first meeting [PANRG-99], and the first presentation in that session was “A Decade of Path Awareness” [PATH-Decade]. At the end of this discussion, two things were abundantly clear.

The meta-lessons from this experience are

This document contains that catalog and analysis.

1.1. About this Document

This document is not intended to include every idea about Path Aware Networking that we can find. Instead, we include enough ideas to provide background for new lessons to guide researchers in their work, in order to add those lessons to Section 2.

1.2. A Note for Contributors (Consider removing after approval)

There is no shame to having your idea included in this document. When these proposals were made, we were trying to engineer something that was research. The document editor started with a subsection on his own idea. The only shame is not learning from experience, and not sharing that experience with other networking researchers and engineers.

This document is being built collaboratively. To contribute your experience, please send a Github pull request to https://github.com/panrg/draft-dawkins-panrg-what-not-to-do.

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

1.3. A Note for the Editor (Remove after taking these actions)

The to-do list for upcoming revisions includes

1.4. 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].

Although these two documents do not specifically target path-aware networking protocols, they are helpful resources on successful protocol adoption and deployment.

2. Summary of Lessons Learned

This section summarizes the Lessons Learned from the contributed sections in Section 4.

3. Template for Contributions

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

4. Contributions

The editor has added some suggested subsections as a starting place, but others are solicited and welcome.

4.1. Integrated Services (IntServ)

The suggested references for IntServ are:

In 1994, when the IntServ architecture document [RFC1633] was published, real-time traffic was first appearing on the Internet. At that time, bandwidth was 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 require service guarantees. Therefore, those applications use the Resource Reservation Protocol (RSVP) [RFC2205] to signal bandwidth reservations across the network. Every router in the network maintains per-flow state in order 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 bandwidth for traffic on a per Flow Spec basis.

4.1.1. Reasons for Non-deployment

IntServ was never widely deployed because of its cost. The following factors contributed to cost:

As IntServ was being discussed, the following occurred:

4.1.2. Lessons Learned.

The following lessons were learned:

IntServ was never widely deployed. However, the technology that it produced was deployed for reasons other than bandwidth management. RSVP is widely deployed as an MPLS signaling mechanism. BGP uses Flow Specs to distribute firewall filters.

4.2. Quick-Start TCP

Quick-Start [RFC4782] is an experimental TCP extension that leverages support from the routers on the path to determine an allowed sending rate, either at the start of data transfers or after idle periods. In these cases, a TCP sender cannot easily determine an appropriate sending rate, given the lack of information about the path. The default TCP congestion control therefore uses the time-consuming slow-start algorithm. With Quick-Start, connections are allowed to use higher 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 Time To Live (TTL) fields, a sender can determine if all routers have approved the Quick-Start request. 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 has been being proposed in [RFC4782] for controlled environments such as intranets only.

The Quick-Start protocol 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.

4.2.1. Reasons for Non-deployment

However, the experiments with Quick-Start in [Sch11] reveal several challenges:

After completion 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 TCP congestion control, but it only requires sender-side TCP modifications. As a result, this approach can be easier and incrementally deployed in the Internet. While theoretically Quick-Start can outperform “IW10”, the absolute improvement of data transfer times is rather small in many cases. After publication of [RFC6928], most modern TCP stacks have increased their default initial window. There is no known deployment of Quick-Start TCP.

4.2.2. Lessons Learned

There are some lessons 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 TCP stacks there is not much room for significant improvement by additional network support.

4.3. Triggers for Transport (TRIGTRAN)

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 semi-modern TCPs (since the late 1980s) have interpreted the loss of a segment as evidence that the path between two endpoints has 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 [RFC2581]. More modern TCPs have added a growing array of strategies about how to establish the sending rate [RFC5681], but when a path is no longer operational, TCPs can wait many seconds before retrying a segment, even if the path becomes operational while the sender is waiting to retry.

The thinking in Triggers for Transport was that if a path completely stopped working because its first-hop link was “down”, that somehow TCP could be signaled when the first-hop link returned to service, and the sending TCP could retry immediately, without waiting for a full Retransmission Time Out (RTO).

4.3.1. Reasons for Non-deployment

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 in the IETF.

4.3.2. Lessons Learned.

The reasons why this work was not chartered provide several useful lessons for researchers.

4.4. Shim6

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 routing table.

For a single-homed site, this could work well. A multi-homed 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 multi-homed 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 problems as the global IPv4 routing table. During the early 2000s, work began on a protocol that would provide the same benefits for multihomed IPv6 sites without requiring sites to advertise Provider Independent prefixes into the 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.

4.4.1. 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].

The technology issues centered around scaling concerns that Shim6 relied on the host to track all the TCP connections and the file descriptions with associated HTTP state, 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, the usual concerns about firewalls that expected to find a transport-level protocol header in the IP payload, and won’t be able to perform firewalling functions because its processing logic would have to 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.

4.4.2. 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.

We also note that some path-aware networking ideas recycle. Although Shim6 did not achieve significant deployment, the IETF chartered a working group to specify “Multipath TCP” [MP-TCP] in 2009, and Multipath TCP allows TCP applications to control path selection, with many of the same advantages and disadvantages of Shim6.

4.5. Next Steps in Signaling (NSIS)

Write-up of Next Steps in Signaling (NSIS) [RFC5974]

Your description could be here.

5. Security Considerations

This document describes ideas 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 ideas for Path Aware Networking that would affect the security of the Internet, but security considerations for those ideas will be described in the corresponding RFCs that propose them.

6. IANA Considerations

This document makes no requests of IANA.

7. Acknowledgements

The section on IntServ was provided by Ron Bonica.

The section on Quick-Start TCP was provided by Michael Scharf.

The section on Shim6 builds on input provided by Erik Nordmark, with background added by Spencer Dawkins.

The section on Triggers for Transport (TRIGTRAN) was provided by Spencer Dawkins.

Review comments were provided by (your name could be here).

8. Informative References

[MP-TCP] "Multipath TCP Working Group Home Page", n.d..
[NANOG-35] "North American Network Operators Group NANOG-35 Agenda", October 2005.
[PANRG] "Path Aware Networking Research Group (Home Page)", n.d..
[PANRG-99] "Path Aware Networking Research Group - IETF-99", July 2017.
[PATH-Decade] Bonaventure, O., "A Decade of Path Awareness", July 2017.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981.
[RFC1633] Braden, R., Clark, D. and S. Shenker, "Integrated Services in the Internet Architecture: an Overview", RFC 1633, DOI 10.17487/RFC1633, June 1994.
[RFC1887] Rekhter, Y. and T. Li, "An Architecture for IPv6 Unicast Address Allocation", RFC 1887, DOI 10.17487/RFC1887, December 1995.
[RFC2205] Braden, R., 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.
[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.
[RFC2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion Control", RFC 2581, DOI 10.17487/RFC2581, April 1999.
[RFC4782] Floyd, S., Allman, M., Jain, A. and P. Sarolahti, "Quick-Start for TCP and IP", RFC 4782, DOI 10.17487/RFC4782, January 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.
[RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009.
[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.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y. and M. Mathis, "Increasing TCP's Initial Window", RFC 6928, DOI 10.17487/RFC6928, April 2013.
[RFC7418] Dawkins, S., "An IRTF Primer for IETF Participants", RFC 7418, DOI 10.17487/RFC7418, December 2014.
[RFC8170] Thaler, D., "Planning for Protocol Adoption and Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170, May 2017.
[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.
[TRIGTRAN-55] "Triggers for Transport BOF at IETF 55", July 2003.
[TRIGTRAN-56] "Triggers for Transport BOF at IETF 56", November 2003.

Author's Address

Spencer Dawkins (editor) Huawei Technologies EMail: spencerdawkins.ietf@gmail.com