Alt-Svc Fixes and Feature Candidates

Document Type Active Internet-Draft (individual)
Author Erik Nygren 
Last updated 2021-06-11
Stream (None)
Intended RFC status (None)
Formats pdf htmlized bibtex
Stream Stream state (No stream defined)
Consensus Boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date
Responsible AD (None)
Send notices to (None)
TODO Working Group                                             E. Nygren
Internet-Draft                                                    Akamai
Intended status: Informational                              11 June 2021
Expires: 13 December 2021

                  Alt-Svc Fixes and Feature Candidates


   HTTP Alternative Services has become the primary mechanism for HTTP/3
   upgrade, but overlaps with and disagrees with other developing
   standards, such as the HTTPS resource record in DNS.  This document
   explores a set of potential fixes and/or additional features for Alt-
   Svc. It is used to record and share thoughts, and is not expected to
   progress on its own.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the mailing list
   (, which is archived at

   Source for this draft and an issue tracker can be found at

Status of This Memo

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

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

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

   This Internet-Draft will expire on 13 December 2021.

Nygren                  Expires 13 December 2021                [Page 1]
Internet-Draft                Alt-Svc Fixes                    June 2021

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Potential Scope Items . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Incorporating errata and Editorial improvements . . . . .   3
     2.2.  Fix ALPN handling . . . . . . . . . . . . . . . . . . . .   3
     2.3.  Address concerns about Alt-Svc lifetime bounding  . . . .   4
     2.4.  Support ECH . . . . . . . . . . . . . . . . . . . . . . .   4
     2.5.  Better Interactions with HTTPS Record . . . . . . . . . .   4
     2.6.  HTTP/3 Frame Definition . . . . . . . . . . . . . . . . .   5
     2.7.  Accept-Alt-Svc Request Header . . . . . . . . . . . . . .   5
     2.8.  Improve/Replace Alt-Used Header . . . . . . . . . . . . .   5
     2.9.  Path-Scoped Alt-Svc . . . . . . . . . . . . . . . . . . .   6
     2.10. Persist and Caching Concerns  . . . . . . . . . . . . . .   7
     2.11. Radical Simplification  . . . . . . . . . . . . . . . . .   8
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Overview

   Alt-Svc [AltSvc] was published in April of 2016.  Since then, it has
   become the primary mechanism to upgrade connections to HTTP/3, at
   least until HTTPS RRs [SVCB] are standardized and widely supported.

   This brainstorms a set of potential fixes and feature candidates for
   an Alt-Svc BIS.  In the spirit of brainstorming, some of the things
   in this list may be bad ideas.  One of the points of this document is
   to judge interest in each of these items to determine what potential
   authors may be interested in including in a draft.

Nygren                  Expires 13 December 2021                [Page 2]
Internet-Draft                Alt-Svc Fixes                    June 2021

   A number of these items were deferred out of the SVCB/HTTPS draft.
   Others are based on regrets from implementation experience with
   AltSvc.  A few are potentially valuable features.

   It is not yet defined whether this should be a new header or can be
   done via extensions to Alt-Svc as it exists today.

   A number of major clients have yet to implement Alt-Svc to other
   hostnames fully, and some of this is due to concerns that they have
   with the current specification.

   It may make sense to split this list into batches, but another option
   would be to try and get all of these done at once even if as separate
   but cooperating drafts.

2.  Potential Scope Items

2.1.  Incorporating errata and Editorial improvements

   (Hopefully this is non-controversial.)


   *  Incorporate errata

   *  Reference RFC 8336 "ORIGIN Frames" (regarding disabling connection

   *  Incorporate [I-D.pardue-httpbis-dont-be-clear]

   *  ...

2.2.  Fix ALPN handling

   The ALPN semantics in [AltSvc] are ambiguous, and problematic in some
   interpretations.  We should update [AltSvc] to give it well-defined
   semantics that match the HTTPS RRs.  For example, specify that the
   ALPN [ALPN] negotiated via the TLS handshake does not need to be the
   same as the ALPN indicated in the AltSvc.

   (From HTTPS RR #246 (
   issues/246) and other discussion threads during HTTPS RR.  David
   Benjamin also has strong opinions on this topic.)

   One option would be to pull in the text we landed on for the HTTPS/
   SVCB draft, see Section 6.1 of [SVCB].  See also

Nygren                  Expires 13 December 2021                [Page 3]
Internet-Draft                Alt-Svc Fixes                    June 2021

2.3.  Address concerns about Alt-Svc lifetime bounding

   Some people have expressed concerns that Alt-Svc allows a compromised
   Origin to hold onto clients forever by continuing to offer updated
   Alt-Svc entries.  There may be ways to reduce the vulnerability
   exposure here, such as by periodic reconfirmation with the "real"
   origin or something it controls.  This is of particular concern when
   an Alt-Svc record has a much longer lifetime than an HTTPS RR.

   For example, if the Alt-Svc records were signed with a key published
   in DNS.  Records remain valid so long as the key that signed them is
   still claimed by the domain.  An unsigned record has a very short
   lifetime bound.

2.4.  Support ECH

   The HTTPS RR [SVCB] is currently the only way to retrieve keys for
   Encrypted Client Hello [ECH].  To maintain security, it puts Alt-Svc
   out-of-scope, since Alt-Svc cannot deliver ECH keys.

   Two options (and there may be more) include:

   *  Add an ech= parameter to Alt-Svc

   *  Defining some better integration between Alt-Svc and HTTPS RRs.
      For example, allow an AltSvc server name to be treated as an
      "AliasMode" reference to an HTTPS record.

2.5.  Better Interactions with HTTPS Record

   This was deferred out of the HTTPS RR draft.  There are a number of
   design options here, but requirements and pros/cons will want to be
   discussed in-detail before proposing designs.  Supporting ECH and
   Alt-Svc together is a primary goal.

   An important item here is which takes precedence, providing safe and
   time-bounded ways to allow Alt-Svc to take precedence over HTTPS
   records.  Alt-Svc has the ability to be delivered in a user-specific
   manner -- useful operationally, but potentially problematic for
   privacy.  HTTPS records can be easily revalidated, which is more
   difficult with an Alt-Svc record.

   Providing a way for Alt-Svc to act as AliasMode references to HTTPS
   SvcMode records seems like one clean way for interaction in that it
   avoids needing to duplicate SVCB in Alt-Svc.  We would still need to
   address time-bounding and trust considerations.

Nygren                  Expires 13 December 2021                [Page 4]
Internet-Draft                Alt-Svc Fixes                    June 2021

2.6.  HTTP/3 Frame Definition

   The ALTSVC frame has not been defined for HTTP/3.  Perhaps it should
   be [I-D.bishop-httpbis-altsvc-quic].  Alternatively, if the frame has
   not been widely adopted, should it be deprecated from HTTP/2 instead?

2.7.  Accept-Alt-Svc Request Header

   There is significant variation in client support for the Alt-Svc
   specification, including some clients which only implement a subset
   of the specification.  Having an Accept-Alt-Svc request header that
   lists a set of supported Alt-Svc features allows for extension of
   Alt-Svc but also allows for deprecation.

   If we don't deprecate the frames, we'd also need a SETTINGS

   There are potential client fingerprinting concerns here, so we'll
   want to not go too far with this.

2.8.  Improve/Replace Alt-Used Header

   There is limited implementation support for Alt-Used out of privacy
   concerns.  It also only sends a subset of the Alt-Svc record being
   used, and there are unclear interactions between Alt-Used and HTTPS

   Daniel Stenberg points out:

      Alt-Used (RFC 7838 section 5) is a request header that only sends
      host name + port number, with no hint if that port number is TCP
      or UDP (or ALPN name), which makes at least one large HTTP/3
      deployment trigger its Alt-Svc loop detection when only switching
      protocols to h3.

   It is proposed that we replace or redefine Alt-Used and also define
   how it interacts with SVCB.  Note that hostile origins have many
   knobs for getting this information (e.g., encoding in hostnames,
   ports, or IPv6 addresses) so a goal would be to allow non-hostile
   origins to get information on which Alt-Svc or SVCB record is being
   used in a way that doesn't make things worse from a privacy

   Some options include:

   *  Just send the whole Alt-Svc or SVCB binding used in the header

Nygren                  Expires 13 December 2021                [Page 5]
Internet-Draft                Alt-Svc Fixes                    June 2021

   *  Have a param encoding an N-bit or N-character value for the
      record-id.  This value would be sent as Alt-Used.  How many bits
      to use is an open question.

      -  Allowing for a dynamic length where the client chooses how many
         bits or characters to include based on privacy budget is one
         attractive but complicated option.  Server implementers would
         put the most important info into earlier bits/characters.

   The goal here is to allow for virtual hosting of alternative
   services, allowing the server to know which alternative service was
   used (eg, for load feedback, diagnostics/debugging, loop detection,
   and other operational purposes), but without hacks like separate
   ports or IP addresses that leak information to passive network

   The usefulness of Alt-Used is currently limited by the fact that most
   servers simply send ":443" and some clients won't consider any other
   alternative offered.

   See some discussion and other options here

2.9.  Path-Scoped Alt-Svc

   The largest-scope, most disruptive, and perhaps most controversial
   item would be to allow Alt-Svc to be scoped to URL paths with a way
   to indicate that transitions to use the Alt-Svc should be done

   This is desired for use-cases of large content libraries where an
   Origin would like to have clients use different endpoints for
   different objects while sharing a single Origin.  This would also
   likely need negotiation.

   This use-case is similar to that served by
   [I-D.reschke-http-oob-encoding], which is one possible solution.  In
   that model, the origin retains control of the entire namespace while
   delegating delivery of particular objects to other endpoints.

   Extending Alt-Svc is another approach which might allow more
   flexibility.  For example:

   *  Client indicates via a request header (eg, Accept-*) or a SETTING
      that it supports this feature

Nygren                  Expires 13 December 2021                [Page 6]
Internet-Draft                Alt-Svc Fixes                    June 2021

   *  Server's Alt-Svc indicates that the path="/movies/
      MurderOnTheExampleExpress/" should be accessed by a particular
      alternative service

   *  Server returns a new 3xx response header response header
      indicating that the Alt-Svc should be used synchronously to fetch
      the response

2.10.  Persist and Caching Concerns

   [AltSvc] defines the "persist" parameter.

      Alternative services that are intended to be longer lived (such as
      those that are not specific to the client access network) can
      carry the "persist" parameter with a value "1" as a hint that the
      service is potentially useful beyond a network configuration

      When alternative services are used to send a client to the most
      optimal server, a change in network configuration can result in
      cached values becoming suboptimal.  Therefore, clients SHOULD
      remove from cache all alternative services that lack the "persist"
      flag with the value "1" when they detect such a change, when
      information about network state is available.

   For some clients (e.g. cURL), detecting network changes is very
   tricky.  Certain clients default to behaving like persist=1 for all

   The RFC text today seems to imply that servers factor in client
   network properties when deciding what to advertise.  That is not true
   for all deployments.  The recommendation that a client invalidate
   Alt-Svc cache entries based on their own network state changes can
   seem mistaken today.  The situation can potentially get worse with
   protocol evolution (connection migration, multipath, etc).

   This feature was designed to address the "mistaken mapping" scenario,
   where either DNS mapping or anycast landed you at one POP but the
   server knows another one is closer to you: You're in Seattle, your
   VPN endpoint and DNS server is in Sacramento, and so DNS resolution
   gives you a CDN node in California.  The California endpoint gives
   you the unicast IP of the Seattle endpoint as a friendly shove.

   Similarly, it's one of the best work-around options for off-net DNS
   (via DoH, ODoH, etc.) if there is a CDN endpoint very close to the
   user's network, but doesn't work so well if the Alt-Svc record keeps
   being used after a network change.

Nygren                  Expires 13 December 2021                [Page 7]
Internet-Draft                Alt-Svc Fixes                    June 2021

   When you're no longer in the same network environment, we can trust
   mapping again.  What is the signal this redirect is no longer useful
   if clients can't reliably detect network changes?

2.11.  Radical Simplification

   If we are able to reach wide deployment and use of the HTTPS record,
   it may supersede many use cases for Alt-Svc.  We should reassess the
   needs from the new baseline to see whether Alt-Svc can be radically
   simplified.  (Chrome never fully implemented Alt-Svc redirection

3.  Security Considerations

   TODO Security

4.  IANA Considerations

   This document has no IANA actions.

5.  Informative References

   [ALPN]     Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <>.

   [AltSvc]   Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <>.

   [ECH]      Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-10, 8 March 2021,

              Bishop, M., "ALTSVC Frame in HTTP/QUIC", Work in Progress,
              Internet-Draft, draft-bishop-httpbis-altsvc-quic-01, 15
              May 2020, <

              Pardue, L. and A. Ramine, "Reserving the clear ALPN
              Protocol ID", Work in Progress, Internet-Draft, draft-
              pardue-httpbis-dont-be-clear-00, 15 March 2021,

Nygren                  Expires 13 December 2021                [Page 8]
Internet-Draft                Alt-Svc Fixes                    June 2021

              Reschke, J. F. and S. Loreto, "'Out-Of-Band' Content
              Coding for HTTP", Work in Progress, Internet-Draft, draft-
              reschke-http-oob-encoding-12, 24 June 2017,

              Thomson, M., "Secure Negotiation of Incompatible Protocols
              in TLS", Work in Progress, Internet-Draft, draft-thomson-
              tls-snip-01, 3 January 2021,

   [SVCB]     Schwartz, B., Bishop, M., and E. Nygren, "Service binding
              and parameter specification via the DNS (DNS SVCB and
              HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-
              dnsop-svcb-https-05, 21 April 2021,


   Martin Thomson, Lucas Pardue, and Mike Bishop reviewed and commented
   on an early version of this draft (

Author's Address

   Erik Nygren


Nygren                  Expires 13 December 2021                [Page 9]