HTTP Working Group                                           B. Schwartz
Internet-Draft                                                    Google
Intended status: Standards Track                               M. Bishop
Expires: July 20, 2018                               Akamai Technologies
                                                        January 16, 2018

     Finding HTTP Alternative Services via the Domain Name Service


   The HTTP Alternative Services (Alt-Svc) mechanism allows an HTTP
   origin to be served from multiple network endpoints, and over
   multiple protocols.  However, the client must first contact the
   origin server, in order to learn of the alternative services.  This
   draft proposes a straightforward mapping of Alt-Svc into DNS,
   allowing clients to learn of these services before their first
   contact with the origin.  This arrangement offers potential benefits
   to both performance and privacy.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on July 20, 2018.

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   document authors.  All rights reserved.

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  The ALTSVC record type  . . . . . . . . . . . . . . . . . . .   3
     2.1.  Comparison with alternatives  . . . . . . . . . . . . . .   4
       2.1.1.  Differences from the SRV RRTYPE . . . . . . . . . . .   4
       2.1.2.  Differences from the TXT RRTYPE . . . . . . . . . . .   4
   3.  Differences from Alt-Svc as transmitted over HTTP . . . . . .   5
     3.1.  Omitting Max Age  . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Interaction with other standards  . . . . . . . . . . . .   5
     3.3.  Granularity and lifetime control  . . . . . . . . . . . .   5
   4.  Client behaviors  . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Cache interaction . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Optimizing for performance  . . . . . . . . . . . . . . .   6
     4.3.  Optimizing for privacy  . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The HTTP Alternative Services standard [AltSvc] defines

   o  an extensible data model for describing alternative network
      endpoints that are authoritative for an origin

   o  the "Alt-Svc Field Value", a text format for representing this

   o  standards for sending information in this format from a server to
      a client over HTTP/1.1 and HTTP/2.

   Together, these components provide a toolkit that has proven useful
   and effective for informing a client of alternative services for an
   origin.  However, making use of an alternative service requires
   contacting the origin server first.  This creates an obvious
   performance cost: users wait for a full HTTP connection initiation
   (multiple roundtrips) before learning of an alternative service that
   is preferred by the origin.  The first connection also publicly

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   reveals the user's intended destination to all entities along the
   network path.

   This draft proposes a straightforward mechanism to distribute the
   Alt-Svc Field Value, in its standard text format, through the DNS.
   If a client receives this information during DNS resolution, it can
   skip the initial connection and proceed directly to an alternative

1.1.  Terminology

   For consistency with [AltSvc], we adopt the following definitions

   o  An "origin" is an information source as in [RFC6454].

   o  The "origin server" is the server that the client would reach when
      accessing the origin in the absence of Alt-Svc.

   o  An "alternative service" is a different server that can serve the

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  The ALTSVC record type

   The ALTSVC DNS resource record (RR) type (RRTYPE ???) is used to
   associate an Alternative Service Field Value with an origin.
   Abstractly, the origin consists of a scheme (typically "https"), a
   host name, and a port (typically "443").

   In the case of the ALTSVC RR, the origin is represented by prefixing
   the scheme and port with "_", then concatenating them with the host,
   resulting in a domain name like "".

   The RDATA portion of an ALTSVC resource record contains an Alt-Svc
   Field Value, exactly as defined in Section 4 of [AltSvc].

   For example, if the operator of intends to
   include an HTTP response header like

   Alt-Svc: h2=":8000"; ma=60

   They would also publish an ALTSVC DNS record like

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Internet-Draft               Alt-Svc via DNS                January 2018 60S IN ALTSVC "h2=\":8000\""

   This data type can be represented as an Unknown RR as described in
   [RFC3597]: 60S IN TYPE??? \# 10

   This construction is intended to be extensible in two ways.  First,
   any extensions that are made to the Alt-Svc format for transmission
   over HTTPS are also applicable here, unless expressly mentioned
   otherwise.  Second, including the scheme in the DNS name allows for
   ALTSVC to serve schemes other than HTTPS, such as HTTP with
   Opportunistic Security [RFC8164] and any future schemes for which
   Alt-Svc may be defined.

2.1.  Comparison with alternatives

   The ALTSVC record type closely resembles some existing record types.

2.1.1.  Differences from the SRV RRTYPE

   An SRV record can perform a similar function to the ALTSVC record,
   informing a client to look in a different location for a service.
   However, there are several differences:

   o  SRV records are typically mandatory, whereas clients will always
      continue to function correctly without making use of Alt-Svc.

   o  SRV records cannot instruct the client to switch or upgrade
      protocols, whereas Alt-Svc can signal such an upgrade (e.g. to

   o  SRV records are not extensible, whereas Alt-Svc can be extended
      with new parameters.  For example, this is what allows the privacy
      improvements related to SNI selection in [AltSvcSNI].

   o  Using SRV records would not allow a client to skip processing of
      the Alt-Svc information in a subsequent connection, so it does not
      confer a performance advantage.

2.1.2.  Differences from the TXT RRTYPE

   The ALTSVC record uses an identical format to a TXT record, and could
   be implemented as such.  However, we define a new record type for
   clarity, and to respect the use of TXT for human-readable notes as
   recommended in [RFC5507].

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3.  Differences from Alt-Svc as transmitted over HTTP

   Publishing an ALTSVC record in DNS is intended to be equivalent to
   transmitting this field value over HTTP, and receiving an ALTSVC
   record is intended to be equivalent to receiving this field value
   over HTTP.  However, there are some small differences in the intended
   client and server behavior.

3.1.  Omitting Max Age

   When publishing an ALTSVC record in DNS, server operators MUST omit
   the "ma" parameter, which encodes the "max age" (i.e. expiration
   time) of an Alt-Svc Field Value.  Instead, server operators SHOULD
   encode the expiration time in the DNS TTL, and MUST NOT set a TTL
   longer than the intended "max age".

   Server operators MAY publish multiple ALTSVC records as an RRSET,
   with semantics equivalent to other mechanisms of providing multiple
   Alt-Svc values to the client.  When publishing an RRSET with multiple
   ALTSVC records, the server operator MUST set the overall TTL to the
   minimum of the "max age" values (following Section 5.2 of [RFC2181]).

   When receiving an ALTSVC record, clients MAY synthesize a new "ma"
   parameter from the DNS TTL, in order to interoperate with Alt-Svc
   processing subsystems.

3.2.  Interaction with other standards

   The purpose of this standard is to reduce connection latency and
   improve user privacy.  Server operators implementing this standard
   SHOULD also implement TLS 1.3 [I-D.ietf-tls-tls13] and OCSP Stapling
   [RFC6066], both of which confer substantial performance and privacy
   benefits when used in combination with ALTSVC records.

   To realize the greatest privacy benefits, this proposal is intended
   for use with a privacy-preserving DNS transport (like DNS over TLS
   [RFC7858] or DNS over HTTPS [DOH]), and with the "SNI" Alt-Svc
   Parameter [AltSvcSNI].  However, performance improvements, and some
   modest privacy improvements, are possible without the use of those

3.3.  Granularity and lifetime control

   Sending Alt-Svc over HTTP allows the server to tailor the Alt-Svc
   Field Value specifically to the client.  When using an ALTSVC DNS
   record, groups of clients will necessarily receive the same Alt-Svc
   Field Value.  Therefore, this standard is not suitable for servers
   that require single-client granularity in Alt-Svc.  Server operators

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   that want to serve different Alt-Svc Field Values to different
   geographic or network regions SHOULD configure their authoritative
   DNS server to respect the EDNS0 Client Subnet extension [RFC7871].

   Some DNS caching systems incorrectly extend the lifetime of DNS
   records beyond the stated TTL.  Server operators MUST NOT rely on
   ALTSVC records expiring on time, and MAY shorten the TTL to

4.  Client behaviors

4.1.  Cache interaction

   If the client has an Alt-Svc cache, and a usable Alt-Svc value is
   present in that cache, then the client SHOULD NOT issue an ALTSVC DNS
   query.  Instead, the client SHOULD proceed with alternative service
   connection as usual.

   If the client has a cached Alt-Svc entry that is expiring, the client
   MAY perform an ALTSVC query to refresh the entry.

4.2.  Optimizing for performance

   Clients that are optimizing for performance (i.e. minimum connection
   setup time) SHOULD implement the following connection sequence:

   1.  Issue address (AAAA and/or A) queries, immediately followed by
       the ALTSVC query.

   2.  If an ALTSVC response is received first, proceed with alternative
       service connection and ignore the address responses if they are
       no longer relevant.

   3.  Otherwise, initiate connection to the origin server.

   4.  As soon as an Alt-Svc field value is received, through the DNS or
       over HTTP, proceed with alternative service connection.  Do not
       abort this connection if an Alt-Svc field value is received from
       the other source later.

   If the ALTSVC and address queries return approximately
   simultaneously, this process typically saves three roundtrips on a
   fresh connection that uses Alt-Svc: one each for TCP, TLS 1.3, and
   HTTP.  (On subsequent connections, the Alt-Svc information is
   expected to be cached, so this procedure does not apply.)

   If a client can cache Alt-Svc entries that were received over both
   HTTP and DNS, the client MAY prefer entries that were received over

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   HTTP.  These records may be more narrowly targeted for the specific

   As an additional optimization, when choosing among multiple Alt-Svc
   values, clients MAY prefer those that will not require an address
   query, either because the corresponding address record is already in
   cache or because the host is an IP address.

   Note that this procedure does not rely on recursive resolvers
   handling the ALTSVC record type correctly.  If ALTSVC queries receive
   spurious NXDOMAIN responses, or even no response at all, connections
   will proceed as usual without any delay.

4.3.  Optimizing for privacy

   Clients that are optimizing for privacy SHOULD implement [AltSvcSNI]
   and DNS over a secure transport (e.g.  [RFC7858] or [DOH]).  Use of a
   secure transport is important not only for privacy protection, but
   also to ensure that queries for the new ALTSVC RRTYPE are handled
   correctly.  Additionally, these clients SHOULD implement the
   following connection sequence:

   1.  Issue the ALTSVC DNS query first, immediately followed by the
       address queries.

   2.  Wait for the ALTSVC record response.

   3.  If the response is nonempty, proceed with alternative service
       connection and ignore the address query responses.

   4.  Otherwise, wait for the address queries and connect as usual.

   Note that this process is also expected to be faster than Alt-Svc
   over HTTP in the case of HTTP Opportunistic Upgrade Probing
   (Section 2 of [RFC8164]).

5.  Security Considerations

   Alt-Svc Field Values are intended for distribution over untrusted
   channels, and clients are REQUIRED to verify that the alternative
   service is authoritative for the origin (Section 2.1 of [AltSvc]).
   Therefore, DNSSEC signing and validation are OPTIONAL for publishing
   and using ALTSVC records.

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6.  IANA Considerations

   This draft requires assignment of a new DNS RRTYPE value.

7.  References

7.1.  Normative References

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

              Bishop, M., "The "SNI" Alt-Svc Parameter", draft-bishop-
              httpbis-sni-altsvc-01 (work in progress), January 2018.

   [DOH]      Hoffman, P. and P. McManus, "DNS Queries over HTTPS",
              draft-ietf-doh-dns-over-https-02 (work in progress),
              November 2017.

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-23 (work in progress),
              January 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
              2003, <>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,

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   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

7.2.  Informative References

   [RFC5507]  IAB, Faltstrom, P., Ed., Austein, R., Ed., and P. Koch,
              Ed., "Design Choices When Expanding the DNS", RFC 5507,
              DOI 10.17487/RFC5507, April 2009,

   [RFC7871]  Contavalli, C., van der Gaast, W., Lawrence, D., and W.
              Kumari, "Client Subnet in DNS Queries", RFC 7871,
              DOI 10.17487/RFC7871, May 2016,

   [RFC8164]  Nottingham, M. and M. Thomson, "Opportunistic Security for
              HTTP/2", RFC 8164, DOI 10.17487/RFC8164, May 2017,

Authors' Addresses

   Ben Schwartz


   Mike Bishop
   Akamai Technologies


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