Out With the Old and In With the New: Planning for Protocol Transitions
draft-iab-protocol-transitions-01.txt

Abstract

Over the many years since the introduction of the Internet Protocol, we have seen a number of transitions, throughout the protocol stack, from one protocol or technology to another. Many protocols and technologies were not designed to enable smooth transition to alternatives or to easily deploy extensions, and thus some transitions, such as the introduction of IPv6, have been difficult. This document attempts to summarize some basic principles to enable future transitions, and also summarizes what makes for a good transition plan.

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Table of Contents

• 1. Introduction
• 2. Transition vs. Co-existence
• 3. Translation/Adaptation Location
• 4. Translation Plans
• 5. Security Considerations
• 6. IANA Considerations
• 7. IAB Members at the Time of This Writing
• 8. Informative References
• Appendix A. Case Studies
• A.1. Explicit Congestion Notification
• Author's Address

1. Introduction

A "transition" is "the process or period of changing from one state or condition to another. There are several types of such transitions, including both technical transitions (e.g., changing protocols or deploying an extension) and organizational transitions (e.g., changing what organization manages the IETF web site, or the RFC production center). This document focuses solely on technical transitions, although some principles might apply to other types as well.

There have been many IETF and IAB RFCs and IAB statements discussing transitions of various sorts. Most are protocol-specific documents about specific transitions. For example, some relevant ones in which the IAB has been involved include:

• IAB RFC 3424 [RFC3424] recommended that any technology for so-called "unilateral self-address fixing (UNSAF)" across NATs include an exit strategy to transition away from such a mechanism. Since the IESG, not the IAB, approves IETF documents, the IESG thus became the body to enforce (or not) such a requirement.
• IAB RFC 4690 [RFC4690] gave recommendations around internationalized domain names. It discussed issues around the process of transitioning to new versions of Unicode, and this resulted in the creation of the IETF Precis WG to address this problem.
• The IAB statement on "Follow-up-work on NAT-PT" [IabIpv6TransitionStatement] pointed out gaps at the time in transitioning to IPv6, and this resulted in the rechartering of the IETF Behave WG to solve this problem.

More recently, the IAB has done work on more generally applicable principles, including two RFCs.

IAB RFC 5218 [RFC5218] on "What Makes for a Successful Protocol?" studied specifically what factors contribute to, and detract from, the success of a protocol and it made a number of recommendations. It discussed two types of transitions: "initial success" (the transition to the technology) and extensibility (the transition to updated versions of it). The principles and recommendations in that document are generally applicable to all technical transitions. Some important principles included:

1. Incentive: Transition is easiest when the benefits come to those bearing the costs. That is, the benefits should outweigh the costs at *each* entity. Some successful cases did this by providing incentives (e.g., tax breaks), or by reducing costs (e.g., freely available source), or by imposing costs of not transitioning (e.g., regulation), or even by narrowing the scenarios of applicability to just the cases where benefits do outweigh costs at all relevant entities.
2. Incremental Deployability: Backwards compatibility makes transition easier. Furthermore, transition is easiest when changing only one entity still benefits that entity. In the easiest case, the benefit immediately outweighs the cost and so entities are naturally incented to transition. More commonly, the benefits only outweigh the costs once a significant number of other entities also transition. Unfortunately, in such cases, the natural incentive is often to delay transitioning.
3. Total Cost: Don't underestimate the cost of things other than the hardware/software itself. For example, operational tools and processes, personnel training, business model (accounting/billing) dependencies, and legal (regulation, patents, etc.) costs all add up.
4. Extensibility: Design for extensibility so that things can be fixed up later.

IAB RFC 7305 [RFC7305] reported on a IAB workshop on Internet Technology Adoption and Transition (ITAT). Like RFC 5218, this workshop also discussed economic aspects of transition, not just technical aspects. Some important observations included:

1. Early-Adopter Incentives: Part of Bitcoin's strategy was extra incentives for early adopters compared to late adopters. That is, providing a long-term advantage to early adopters can help stimulate transition even when the initial costs outweigh the initial benefit.
2. Policy Partners: Policy-making organizations of various sorts (RIRs, ICANN, etc.) can be important partners in enabling and facilitating transition.

The remainder of this document continues the discussion in those two RFCs and provides some additional thoughts on the topic of transition strategies and plans.

2. Transition vs. Co-existence

We need to distinguish between a strict "flag-day" style transition where an old mechanism is immediately replaced with a new mechanism, vs. a looser co-existence based approach where transition proceeds in stages where a new mechanism is first added alongside an existing one for some overlap period, and then the old mechanism is removed at a later stage.

When a new mechanism is backwards compatible with an existing mechanism, transition is easiest, and the difference between the two types of transition is not particularly significant. However, when no backwards compatibility exists (such as in the IPv4 to IPv6 transition), a transition plan must choose either a "flag day" or a period of co-existence. When a large number of entities are involved, a flag day becomes impractical. Coexistence, on the other hand, involves additional costs of maintaining two separate mechanisms during the overlap period which could be quite long. Furthermore, the longer the overlap period, the more the old mechanism might get further deployment and thus increase the overall pain of transition.

Often the decision between a "flag day" and a sustained co-existence period may be difficult, such as in the case of IDNA2008 [RFC5891] [RFC5895] and Unicode TR46 [TR46].

3. Translation/Adaptation Location

A translation or adaptation layer is often required if the old and new mechanisms are not interoperable. Care must be taken when determining where such a translator is best placed.

Requiring a translator in the middle of the path can hamper end-to-end security and reliability. For example, see the discussion of network-based filtering in [I-D.iab-filtering-considerations].

On the other hand, requiring a translation layer within an endpoint can be a resource issue in some cases, such as if the endpoint could be a constrained node [RFC7228].

Any transition strategy for a non-backward-compatible mechanism should include a discussion of where it is placed and a rationale.

4. Translation Plans

A good transition plan includes at least the following components:

1. An explanation of incentives for each entity involved
2. A description of transition phases. For example, there might be pilot, co-existence, deprecation, and removal phases for a transition from one technology to another incompatible one.
3. A proposed timeline
4. A way to effectively communicate the proposed plan to the entities affected, and incorporate their feedback

5. Security Considerations

This document discusses attributes of protocol transitions. Some types of transition can adversely affect security or privacy. For example, requiring a translator in the middle of the path may hamper end-to-end security and privacy, since it creates an attractive target. For further discussion of some of these issues, see Section 5 of [I-D.iab-filtering-considerations].

6. IANA Considerations

This document requires no actions by the IANA.

7. IAB Members at the Time of This Writing

Jari Arkko
Mary Barnes
Marc Blanchet
Ralph Droms
Ted Hardie
Joe Hildebrand
Russ Housley
Erik Nordmark
Robert Sparks
Andrew Sullivan
Dave Thaler
Brian Trammell
Suzanne Woolf
            

8. Informative References

Appendix A. Case Studies

A.1. Explicit Congestion Notification

Explicit Congestion Notification (ECN) is a mechanism to replace loss as the only signal for the detection of congestion with an explicit signal sent from a router to the recipient of a packet, then reflected back to the sender. It was standardized in 2000 in [RFC3168], and the mechanism consists of two parts: congestion detection in the IP layer, reusing two bits of the old IP Type of Service (TOS) field, and congestion feedback in the transport layer. Feedback in TCP uses two TCP flags, ECN Echo and Congestion Window Reduced. Together with a suitably configured active queue management (AQM), ECN can improve TCP performance on congested links.

The deployment of ECN is a case study in failed transition followed by possible redemption. Initial deployment of ECN in the early and mid 2000s led to severe problems with some network equipment, including home router crashes and reboots when packets with ECN IP or TCP flags was received [TSV2007]. This led to firewalls stripping ECN IP and TCP flags, or even dropping packets with these flags set. This stalled deployment. The need for both endpoints (to negotiate and support ECN) and on-path devices (to mark traffic when congestion occurs) to cooperate in order to see any benefits from ECN deployment was a further issue. The deployment of ECN had failed.

In the late 2000s, Linux and Windows servers defaulted to "passive ECN support", meaning they would negotiate ECN if asked by the client, but would not ask to negotiate ECN by default. This decision was regarded as without risk: only if a client were explicitly configured to negotiate ECN would any possible connectivity problems surface. Gradually, this has increased server support in the Internet from near zero in 2008, to 11% of the top million Alexa webservers in 2011, to 30% in 2012, to 65% in late 2014. In the meantime, the risk to connectivity of ECN negotiation has reduced dramatically [PAM2015], leading to ongoing work to make Windows, Apple iOS, OSX, and Linux clients negotiate ECN by default. It is hoped that a critical mass of clients and servers negotiating ECN will provide an incentive to mark congestion on ECN-enabled traffic, thus breaking the logjam.

Author's Address