Network Working Group M. Nottingham
Internet-Draft Akamai
Intended status: Standards Track P. McManus
Expires: June 14, 2014 Mozilla
December 11, 2013
HTTP Alternate Services
draft-nottingham-httpbis-alt-svc-01
Abstract
This document introduces "alternate services" to allow an HTTP
origin's resources to be available at a separate network location,
possibly accessed with a different protocol configuration.
It also specifies one means of discovering alternate services, the
"Alt-Svc" header field.
Status of this Memo
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This Internet-Draft will expire on June 14, 2014.
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Copyright (c) 2013 IETF Trust and the persons identified as the
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3
2. Alternate Services . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Client Handling for Alternate Services . . . . . . . . . . 5
2.1.1. Host Authentication . . . . . . . . . . . . . . . . . 5
2.1.2. Alternate Service Caching . . . . . . . . . . . . . . 5
2.1.3. Alternate Service Priorities . . . . . . . . . . . . . 6
2.1.4. Requiring Server Name Indication . . . . . . . . . . . 6
2.1.5. Using Alternate Services . . . . . . . . . . . . . . . 6
3. The Alt-Svc HTTP Header Field . . . . . . . . . . . . . . . . 7
3.1. Caching Alt-Svc Header Field Values . . . . . . . . . . . 7
3.2. Indicating Alt-Svc Header Field Priority . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
4.1. Changing Ports . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Changing Hosts . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Changing Protocols . . . . . . . . . . . . . . . . . . . . 10
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Normative References . . . . . . . . . . . . . . . . . . . 10
5.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Appendix B. TODO . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
[I-D.ietf-httpbis-http2] specifies a few ways to negotiate the use of
HTTP/2.0 without changing existing URIs. However, several
deficiencies in using the "upgrade dance" for "http://" URIs have
become apparent. While that mechanism is still being investigated,
some have expressed interest in an alternate approach.
Furthermore, some implementers have expressed a strong desire utilize
HTTP/2 only in conjunction with TLS. Alternate-Services provides a
potential mechanism for achieving that for "http://" URIs; see
[I-D.nottingham-http2-encryption] for details.
Finally, HTTP/2.0 is designed to have longer-lived, fewer and more
active TCP connections. While these properties are generally
"friendlier" for the network, they can cause problems for servers
that currently exploit the short-lived flow characteristics of
HTTP/1.x for load balancing, session affinity and maintaining
locality to the user.
This document specifies a new concept in HTTP, the "alternate
service," to address these use cases. An alternate service can be
used to interact with the resources on an origin server at a separate
location on the network, possibly using a different protocol
configuration.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document uses the Augmented BNF defined in [RFC5234] along with
the "OWS", "DIGIT", "parameter", "uri-host", "port" and "delta-
second" rules from [I-D.ietf-httpbis-p1-messaging], and uses the
"#rule" extension defined in Section 7 of that document.
2. Alternate Services
On the Web, a resource is accessed through a scheme (e.g., "https" or
"http") on a nominated host / port combination.
These three pieces of information collectively can be used to
establish the authority for ownership of the resource (its "origin";
see [RFC6454]), as well as providing enough information to bootstrap
access to it.
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This document introduces the notion of an "Alternate Service"; when
an origin's resources are accessible through a different protocol /
host / port combination, it is said to have an alternate service.
For example, an origin:
("http", "www.example.com", "80")
might declare that its resources are also accessible at the alternate
service:
("http2-tls", "new.example.com", "443")
By their nature, alternate services are explicitly at the granularity
of an origin; i.e., they cannot be selectively applied to resources
within an origin.
Alternate services do not replace or change the origin for any given
resource; in general, they are not visible to the software "above"
the access mechanism. The alternate service is essentially alternate
routing information that can also be used to reach the origin in the
same way that DNS CNAME or SRV records define routing information at
the name resolution level.
Furthermore, it is important to note that the first member of an
alternate service tuple is different from the "scheme" component of
an origin; it is more specific, identifying not only the major
version of the protocol being used, but potentially communication
options for that protocol.
This means that clients using an alternate service will change the
host, port and protocol that they are using to fetch resources, but
these changes MUST NOT be propagated to the application that is using
HTTP; from that standpoint, the URI being accessed and all
information derived from it (scheme, host, port) are the same as
before.
Importantly, this includes its security context; in particular, when
TLS [RFC5246] is in use, the alternate server will need to present a
certificate for the origin's host name, not that of the alternate.
Likewise, the Host header is still derived from the origin, not the
alternate service (just as it would if a CNAME were being used).
The changes MAY, however, be made visible in debugging tools,
consoles, etc.
Formally, an alternate service is identified by the combination of:
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o An ALPN protocol, as per [I-D.ietf-tls-applayerprotoneg]
o A host, as per [RFC3986]
o A port, as per [RFC3986]
Additionally, each alternate service MUST have:
o A freshness lifetime, expressed in seconds; see Section 2.1.2
o A numeric priority; see Section 2.1.3
Potentially, there are many ways that a client could discover the
alternate service(s) associated with an origin; this document
currently defines one, the Alt-Svc HTTP Header Field (Section 3).
2.1. Client Handling for Alternate Services
2.1.1. Host Authentication
Clients MUST NOT use alternate services with a host other than the
origin's without strong server authentication; this mitigates the
attack described in Section 4.2. One way to achieve this is for the
alternate to use TLS with a certificate that is valid for that
origin.
For example, if the origin's host is "www.example.com" and an
alternate is offered on "other.example.com" with the "http2-tls"
protocol, and the certificate offered is valid for "www.example.com",
the client can use the alternate. However, if "other.example.com" is
offered with the "http2" protocol, the client cannot use it, because
there is no mechanism in that protocol to establish strong server
authentication.
2.1.2. Alternate Service Caching
Mechanisms for discovering alternate services can associate a
freshness lifetime with them; for example, the Alt-Svc header field
uses the "ma" parameter.
Clients MAY choose to use an alternate service instead of the origin
at any time when it is considered fresh; see Section 2.1.5 for
specific recommendations.
To mitigate risks associated with caching compromised values (see
Section 4.2 for details), user agents SHOULD examine cached alternate
services when they detect a change in network configuration, and
remove any that could be compromised (for example, those whose
association with the trust root is questionable). UAs that do not
have a means of detecting network changes SHOULD place an upper bound
on their lifetime.
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2.1.3. Alternate Service Priorities
Mechanisms for discovering alternate services can associate a
priority with them; for example, the Alt-Svc header field uses the
"pr" parameter.
Priorities are numeric, with a range of 1-64, and are relative to the
origin server, which has a static priority of 32. Higher values are
preferable.
Therefore, an alternate with a priority of 48 will be used in
preference to the origin server, whereas one with a priority of 10
will be used only when the origin server becomes unavailable.
Note that priorities are not specific to the mechanism that an
alternate was discovered with; i.e., there is only one "pool" of
priorities for an origin.
2.1.4. Requiring Server Name Indication
A client must only use a TLS based alternate service if the client
also supports TLS Server Name Indication (SNI) [RFC6066]. This
supports the conservation of IP addresses on the alternate service
host.
2.1.5. Using Alternate Services
By their nature, alternate services are optional; clients are not
required to use them. However, it is advantageous for clients to
behave in a predictable way when they are used by servers (e.g., for
load balancing).
Therefore, if a client becomes aware of an alternate service that has
a higher priority than a connection currently in use, the client
SHOULD use that alternate service as soon as it is available,
provided that the security properties of the alternate service
protocol are desirable, as compared to the existing connection.
For example, if an origin advertises a "http2-tls" alternate service
using an "Alt-Svc" response header field, the client ought to
immediately establish a connection to the most preferable alternate
service, and use it in preference to the origin connection once
available.
The client is not required to block requests; the origin's connection
can be used until the alternate connection is established. However,
if the security properties of the existing connection are weak (e.g.
cleartext HTTP/1.1) then it might make sense to block until the new
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connection is fully available in order to avoid information leakage.
Furthermore, if the connection to the alternate service fails or is
unresponsive, the client MAY fall back to using the origin, or a less
preferable alternate service.
3. The Alt-Svc HTTP Header Field
A HTTP(S) origin server can advertise the availability of alternate
services to clients by adding an Alt-Svc header field to responses.
Alt-Svc = 1#( alternate *( OWS ";" OWS parameter ) )
alternate = protocol-id "=" [ uri-host ] ":" port
protocol-id = <ALPN protocol identifier>
For example:
Alt-Svc: http2=:8000
This indicates that the "http2" protocol on the same host using the
indicated port (in this case, 8000).
Alt-Svc can also contain a host:
Alt-Svc: http2-tls=other.example.com:443
This indicates that all resources on the origin are available using
the "http2-tls" profile on other.example.com port 443.
It can also have multiple values:
Alt-Svc: http2=:8000, http2-tls=other.example.com:443
The value(s) advertised by Alt-Svc can be used by clients to open a
new connection to one or more alternate services immediately, or
simultaneously with subsequent requests on the same connection.
Intermediaries MUST NOT change or append Alt-Svc values.
3.1. Caching Alt-Svc Header Field Values
When an alternate service is advertised using Alt-Svc, it is
considered fresh for 24 hours from generation of the message. This
can be modified with the 'ma' (max-age') parameter;
Alt-Svc: http2-tls=:443;ma=3600
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which indicates the number of seconds since the response was
generated the alternate service is considered fresh for.
ma = delta-seconds
See [I-D.ietf-httpbis-p6-cache] Section 4.2.3 for details of
determining response age. For example, a response:
HTTP/1.1 200 OK
Content-Type: text/html
Cache-Control: 600
Age: 30
Alt-Svc: http2=:8000; ma=60
indicates that an alternate service is available and usable for the
next 60 seconds. However, the response has already been cached for
30 seconds (as per the Age header field value), so therefore the
alternate service is only fresh for the 30 seconds from when this
response was received, minus estimated transit time.
When an Alt-Svc response header is received from an origin, its value
invalidates and replaces all cached alternate services for that
origin. This includes the empty Alt-Svc header, which clears all
cached alternate services for an origin.
See Section 2.1.2 for general requirements on caching alternate
services.
Note that the freshness lifetime for HTTP caching (here, 600 seconds)
does not affect caching of Alt-Svc values.
3.2. Indicating Alt-Svc Header Field Priority
Finally, an explicit priority can be associated with an Alt-Svc
header field value by using the "pr" parameter:
Alt-Svc: http2-tls:8000 ;pr=64
See Section 2.1.3 for details of the priority mechanism.
pr = 1*2DIGIT
If the "pr" parameter is not present or is invalid, the default
priority for alternate services discovered with the Alt-Svc header
field is 48.
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4. Security Considerations
4.1. Changing Ports
Using an alternate service implies accessing an origin's resources on
an alternate port, at a minimum. An attacker that can inject
alternate services and listen at the advertised port is therefore
able to hijack an origin.
For example, an attacker that can add HTTP response header fields can
redirect traffic to a different port on the same host using the Alt-
Svc header field; if that port is under the attacker's control, they
can thus masquerade as the HTTP server.
This risk can be mitigated by restricting the ability to set the Alt-
Svc response header field on the origin, and restricting who can open
a port for listening on that host.
4.2. Changing Hosts
When the host is changed due to the use of an alternate service, it
presents an opportunity for attackers to hijack communication to an
origin.
For example, if an attacker can convince a user agent to send all
traffic for "innocent.example.org" to "evil.example.com" by
successfully associating it as an alternate service, they can
masquerade as that origin. This can be done locally (see mitigations
above) or remotely (e.g., by an intermediary as a man-in-the-middle
attack).
This is the reason for the requirement in Section 2.1.1 that any
alternate service with a host different to the origin's be strongly
authenticated with the origin's identity; i.e., presenting a
certificate for the origin proves that the alternate service is
authorized to serve traffic for the origin.
However, this authorization is only as strong as the method used to
authenticate the alternate service. In particular, there are well-
known exploits to make an attacker's certificate appear as
legitimate.
Alternate services could be used to persist such an attack; for
example, an intermediary could man-in-the-middle TLS-protected
communication to a target, and then direct all traffic to an
alternate service with a large freshness lifetime, so that the user
agent still directs traffic to the attacker even when not using the
intermediary.
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As a result, there is a requirement in Section 2.1.2 to examine
cached alternate services when a network change is detected.
4.3. Changing Protocols
When the ALPN protocol is changed due to the use of an alternate
service, the security properties of the new connection to the origin
can be different from that of the "normal" connection to the origin,
because the protocol identifier itself implies this.
For example, if a "https://" URI had a protocol advertised that does
not use some form of end-to-end encryption (most likely, TLS), it
violates the expectations for security that the URI scheme implies.
Therefore, clients cannot blindly use alternate services, but instead
evaluate the option(s) presented to assure that security requirements
and expectations (of specifications, implementations and end users)
are met.
5. References
5.1. Normative References
[I-D.ietf-httpbis-p1-messaging]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing",
draft-ietf-httpbis-p1-messaging-25 (work in progress),
November 2013.
[I-D.ietf-httpbis-p6-cache]
Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
Transfer Protocol (HTTP/1.1): Caching",
draft-ietf-httpbis-p6-cache-25 (work in progress),
November 2013.
[I-D.ietf-tls-applayerprotoneg]
Friedl, S., Popov, A., Langley, A., and S. Emile,
"Transport Layer Security (TLS) Application Layer Protocol
Negotiation Extension", draft-ietf-tls-applayerprotoneg-03
(work in progress), October 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
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[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011.
5.2. Informative References
[I-D.ietf-httpbis-http2]
Belshe, M., Peon, R., Thomson, M., and A. Melnikov,
"Hypertext Transfer Protocol version 2.0",
draft-ietf-httpbis-http2-08 (work in progress),
November 2013.
[I-D.nottingham-http2-encryption]
Nottingham, M., "Opportunistic Encryption for HTTP URIs",
draft-nottingham-http2-encryption-01 (work in progress),
October 2013.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, April 2012.
Appendix A. Acknowledgements
Thanks to Eliot Lear, Stephen Farrell, Guy Podjarny, Stephen Ludin,
Erik Nygren, Paul Hoffman, Adam Langley and Will Chan for their
feedback and suggestions.
The Alt-Svc header field was influenced by the design of the
Alternate-Protocol header in SPDY.
Appendix B. TODO
o GOAWAY: A GOAWAY-like frame (or just a GOAWAY modification) that
allows an alternate service to be switched to might be suggested
in a future revision.
o DNS: Alternate services are also amenable to DNS-based discovery.
If there is sufficient interest, a future revision may include a
proposal for that.
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o Indicating Chosen Service: It's likely necessary for the server to
know which protocol the user agent has chosen, and perhaps even
the hostname (for load balancing). This could be conveyed as part
of the "magic", or as a request header.
o IPV6: The intersection between Alternate Services and Happy
Eyeballs [RFC6555] should be investigated.
o ALPN strings: all of the ALPN strings in this document are
fictional; they need to be updated based upon that specification's
progress (and the registry, eventually).
o Advice for setting headers: guidelines for servers that use the
Alt-Svc header field.
Authors' Addresses
Mark Nottingham
Akamai
Email: mnot@mnot.net
URI: http://www.mnot.net/
Patrick McManus
Mozilla
Email: mcmanus@ducksong.com
URI: https://mozillians.org/u/pmcmanus/
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