Network Working Group M. Nottingham
Internet-Draft October 1, 2013
Intended status: Standards Track
Expires: April 4, 2014
Encryption for HTTP URIs Using Alternate Services
draft-nottingham-http2-encryption-00
Abstract
This document proposes a way to optimistically encrypt HTTP/2.0 using
TLS for HTTP URIs.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 4, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Goals and Non-Goals . . . . . . . . . . . . . . . . . . . 3
1.2. Notational Conventions . . . . . . . . . . . . . . . . . . 4
2. Alternate Services . . . . . . . . . . . . . . . . . . . . . . 4
2.1. The Alt-Svc HTTP Header Field . . . . . . . . . . . . . . 5
2.2. HTTP-related Protocol Identifiers . . . . . . . . . . . . 6
2.2.1. The "http2-tls-relaxed" Protocol . . . . . . . . . . . 7
3. Security Considerations . . . . . . . . . . . . . . . . . . . 8
3.1. Alt-Svc Header Field Downgrade Attacks . . . . . . . . . . 8
3.2. Poor Client Profile Choices . . . . . . . . . . . . . . . 8
3.3. Alt-Svc Header Field Hijacking . . . . . . . . . . . . . . 8
3.4. Alt-Svc Header Field "Gap" . . . . . . . . . . . . . . . . 9
4. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Normative References . . . . . . . . . . . . . . . . . . . 9
4.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Appendix B. Recent History and Background . . . . . . . . . . . . 11
Appendix C. Next Steps . . . . . . . . . . . . . . . . . . . . . 12
Appendix D. Frequently Asked Questions . . . . . . . . . . . . . 13
D.1. Will this make encryption mandatory in HTTP/2.0? . . . . . 13
D.2. No certificate checks? Really? . . . . . . . . . . . . . . 13
D.3. Why do this if a downgrade attack is so easy? . . . . . . 14
D.4. What about using DNS? . . . . . . . . . . . . . . . . . . 14
D.5. Doesn't Alt-Svc make it easy to hijack a Web server? . . . 14
D.6. What about using Upgrade? . . . . . . . . . . . . . . . . 14
D.7. Why not 305 Use Proxy? . . . . . . . . . . . . . . . . . . 14
D.8. Will this make negotiation too "chatty"? . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
In discussion at IETF87, it was proposed that the current means of
bootstrapping encryption in HTTP [I-D.ietf-httpbis-p1-messaging] -
using the "HTTPS" URI scheme - unintentionally gives the server
disproportionate power in determining whether encryption is used.
Furthermore, HTTP's current use of TLS [RFC5246] for "https://" URIs
is inflexible; it is difficult to introduce new trust roots, for
example.
This document proposes changes to HTTP that decouple the URI scheme
from the use and configuration of underlying encryption, as well as
other aspects of the protocol.
In particular, it defines the concept of an "alternate service" that
allows an origin to advertise when its resources are available at a
separate location, using a different configuration of protocols.
This allows a "http://" URI to be upgraded to use TLS optimistically.
Because deploying TLS requires acquiring and configuring a valid
certificate, some deployments may find supporting it difficult.
Therefore, this document also specifies a "relaxed" profile of
HTTP/2.0 over TLS that does not require strong server authentication,
specifically for use with "http://" URIs.
Note: This is a preliminary draft that attempts to capture the state
of relevant discussion to this point. It has not be reviewed for
security, deployability, or effectiveness, and is only intended to
serve as the basis of further discussion in the HTTPbis Working
Group.
1.1. Goals and Non-Goals
This proposal attempts to de-couple a HTTP URI's scheme from the
specific wire protocol in use, as well as that protocol's layering
onto the network.
The immediate goal is to make HTTP URIs more robust in the face of
passive monitoring.
Such passive attacks are often opportunistic; they rely on sensitive
information being available in the clear. Furthermore, they are
often broad, where all available data is collected en masse, being
analyzed separately for relevant information.
It is not a goal of this document to address active or targeted
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attacks, although future solutions may be complementary.
Other goals include ease of implementation and deployment, with
minimal impact upon performance (in keeping with the goals of
HTTP/2.0).
Furthermore, since this proposal is designed as an alternate
negotiation mechanism for HTTP/2.0, it is expected that it is useful
for that use case as well.
1.2. 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].
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.
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")
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.
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
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options for that protocol.
Practically speaking, 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 the security context of the connection; by
default, 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.
The changes SHOULD, however, be made visible in debugging tools,
consoles, etc.
Clients MUST NOT use alternate services on a host other than the
origin's without strong server authentication; 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.
Formally, an alternate service is identified by the combination of:
o An ALPN protocol, as per [I-D.ietf-tls-applayerprotoneg]
o A host, as per [RFC3986]
o A port, as per [RFC3986]
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.
2.1. The Alt-Svc HTTP Header Field
A HTTP server can advertise the availability of alternate services to
HTTP/1.1 and HTTP/2.0 clients by adding an Alt-Svc header field to
responses. For example:
Alt-Svc: http2-tls-relaxed=:443
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This indicates that the "http2tls-relaxed" protocol on the same host
using the indicated port (in this case, 443).
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-tls=:443, 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.
When an alternate service is advertised using Alt-Svc, it is
considered to be valid for all resources associated with the origin,
and by default is valid 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
which indicates the number of seconds since the response was
generated the policy is considered fresh for. See
[I-D.ietf-httpbis-p6-cache] Section 4.2.3 for details of determining
response age.
[[TODO: ABNF]]
2.2. HTTP-related Protocol Identifiers
To accommodate this approach, HTTP/2.0 will need to nominate several
protocol negotiation strings (a.k.a. "profiles") to allow negotiation
for the desired properties in alternate services.
This might include:
o http1 - http/1.x over TCP
o http1-tls - http/1.x over TLS over TCP (as per [RFC2818])
o http2 - http/2.x over TCP
o http2-tls - http/2.x over TLS over TCP (as per [RFC2818])
o http2-tls-relaxed - http/2.x over TLS over TCP (see below)
Most of these are already latently defined by HTTP/2.0, with the
exception being http2-tls-relaxed, defined below.
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These profiles are expected to be used not only in the Alt-Svc header
field, but also in other HTTP/2.0 negotiation mechanisms; e.g., ALPN,
the "Upgrade dance" and so forth.
Note that, as discussed in Security Considerations, there may be
situations (e.g,. ALPN) where advertising some of these profiles are
inapplicable or inadvisable.
For example, in an ALPN negotiation for a "https://" URI, it is only
sensible to offer http1-tls and http2-tls.
2.2.1. The "http2-tls-relaxed" Protocol
Servers that support the "http2-tls-relaxed" protocol indicate that
they support TLS for access to URIs with the "http" URI scheme using
HTTP/2.0 or greater.
Servers MAY advertise the "http2-tls-relaxed" profile for resources
with a "http" origin scheme; they MUST NOT advertise it for resources
with a "https" origin.
When a client connects to an "http2-tls-relaxed" alternate service,
it MUST use TLS1.1 or greater, and MUST use HTTP/2.x. HTTP/2.0
SHOULD be used as soon as TLS negotiation is completed; i.e., the
"Upgrade dance" SHOULD NOT be performed.
When connecting to an "http2-tls-relaxed" service, the algorithm for
authenticating the server described in [RFC2818] Section 3.1 changes;
the client does not necessarily validate its certificate for expiry,
hostname match or relationship to a known certificate authority (as
it would with "normal" HTTPS).
However, the client MAY perform additional checks on the certificate
and make a decision as to its validity before using the server.
Definition of such additional checks are out of scope for this
specification.
Upon initial adoption of this proposal, it is expected that no such
additional checks will be performed. Therefore, the client MUST NOT
use the "http2-tls-relaxed" profile to connect to alternate services
whose host does not match that of the origin, unless additional
checks are performed.
This requirement bounds the risk of a service being hijacked and
redirected to another host; see Security Considerations for details.
[[TODO: define "match"]]
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Servers SHOULD use the same certificate consistently over time, to
aid future extensions for building trust and adding other services.
[[TODO: define "same"; likely not the same actual certificate. ]]
When the http2-tls-relaxed protocol is in use, User Agents MUST NOT
indicate the connection has the same level of security as https://
(e.g. using a "lock device").
3. Security Considerations
3.1. Alt-Svc Header Field Downgrade Attacks
Because the Alt-Svc header field appears in the clear (for "http://"
URIs), it is subject to downgrade by attackers that are able to Man-
in-the-Middle the network connection; in its simplest form, an
attacker that wants the connection to remain in the clear need only
strip the Alt-Svc header from responses.
This proposal does not offer a remedy for this risk. However, it's
important to note that it is no worse than current use of unencrypted
HTTP in the face of such active attacks.
3.2. Poor Client Profile Choices
Furthermore, because different protocols can have different security
properties, clients ought not blindly use alternate services, but
instead they option(s) presented for conformance to implementation
policy, user preferences, and general security.
For example, in theory the header field could be used to advertise a
downgrade to plain TCP from a TLS-protected connection, or to relax
certificate checks for a HTTPS URI; opting to use such an alternate
would seldom be desirable.
3.3. Alt-Svc Header Field Hijacking
An attacker local to the Web server who can inject response header
fields can redirect HTTP traffic to a different port on the same host
using the "http2-tls-relaxed" protocol; if it is under their control,
the can masquerade as the HTTP server.
An attacker local to the Web server who can inject response header
fields can redirect HTTP traffic to an arbitrary host and port using
the "http2-tls" protocol; if they can present a certificate which
validates for the host under attack, they can masquerade as that
server.
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Both of these risks can be mitigated by appropriate controls to
setting response header fields, as well as control over who can open
a port for listening (in the former case) and good certificate
hygiene (in the latter case).
An attacker who can Man-in-the-Middle the network connection and
inject response header fields directly can redirect HTTP traffic to a
different port and (presumably) masquerade as that server.
As with HTTP today, it is not possible to mitigate this latter risk
without cryptographic solutions.
3.4. Alt-Svc Header Field "Gap"
When the Alt-Svc header field is used in "http://" URIs, the client
needs to send an unencrypted HTTP request to the server to discover
the alternates. In doing so, it potentially exposes sensitive
information (e.g., cookies [RFC6265]) to surveillance.
This risk can be mitigated if the client is willing to send a
separate request (e.g., OPTIONS *) to the origin to discover policy
before making requests containing potentially sensitive information.
However, doing so adds a round-trip of latency to such requests.
Likewise, if the Alt-Svc is cacheable for a long period (using a
large ma parameter), it reduces the window for such attacks (but does
not eliminate it).
Alternatively, this risk can be mitigated by using an out-of-band
discovery mechanism (e.g., DNS).
4. References
4.1. Normative References
[I-D.ietf-httpbis-http2]
Belshe, M., Peon, R., Thomson, M., and A. Melnikov,
"Hypertext Transfer Protocol version 2.0",
draft-ietf-httpbis-http2-06 (work in progress),
August 2013.
[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-24 (work in progress),
September 2013.
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[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-24 (work in progress),
September 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-02
(work in progress), September 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011.
4.2. Informative References
[I-D.mbelshe-httpbis-spdy]
Belshe, M. and R. Peon, "SPDY Protocol",
draft-mbelshe-httpbis-spdy-00 (work in progress),
February 2012.
[RFC2804] IAB and IESG, "IETF Policy on Wiretapping", RFC 2804,
May 2000.
[RFC3365] Schiller, J., "Strong Security Requirements for Internet
Engineering Task Force Standard Protocols", BCP 61,
RFC 3365, August 2002.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.
[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, April 2012.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
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Transparency", RFC 6962, June 2013.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
July 2013.
[firesheep]
Butler, E., "Firesheep", 2010,
<http://codebutler.com/firesheep/>.
[streetview]
Kravets, D., "The Anatomy of Google's Wi-Fi Sniffing
Debacle", 2012, <http://www.wired.com/threatlevel/2012/05/
google-wifi-fcc-investigation/>.
[xkeyscore]
Greenwald, G., "NSA tool collects 'nearly everything a
user does on the internet'", 2013, <http://
www.theguardian.com/world/2013/jul/31/
nsa-top-secret-program-online-data>.
Appendix A. Acknowledgements
Thanks to Patrick McManus, Eliot Lear, Stephen Farrell, Guy Podjarny,
Stephen Ludin, Erik Nygren, Paul Hoffman and Adam Langley for their
feedback and suggestions.
Appendix B. Recent History and Background
One of the design goals for SPDY [I-D.mbelshe-httpbis-spdy] was
increasing the use of encryption on the Web, achieved by only
supporting the protocol over a connection protected by TLS [RFC5246].
This was done, in part, because sensitive information - including not
only login credentials, but also personally identifying information
(PII) and even patterns of access - are increasingly prevalent on the
Web, being evident in potentially every HTTP request made.
Attacks such as FireSheep [firesheep] showed how easy it is to gather
such information when it is sent in the clear, and incidents such as
Google's collection of unencrypted data by its StreetView Cars
[streetview] further illustrated the risks.
In adopting SPDY as the basis of HTTP/2 [I-D.ietf-httpbis-http2], the
HTTPbis Working Group agreed not to make TLS mandatory to implement
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(MtI) or mandatory to use (MtU) in our charter, despite an IETF
policy to prefer the "best security available" [RFC3365].
There were a variety of reasons for this, but most significantly,
HTTP is used for much more than the traditional browsing case, and
encryption is not needed for all of these uses. Making encryption
MtU or MtI was seen as unlikely to succeed because of the wide
deployment of HTTP URIs.
However, since making that decision, there have been developments
that have caused the Working Group to discuss these issues again:
1. Active contributors to some browser implementations have stated
that their products will not use HTTP/2 over unencrypted
connections. If this eventuates, it will prevent wide deployment
of the new protocol (i.e., it couldn't be used with those
products for HTTP URIs; only HTTPS URIs).
2. It has been reported that surveillance of HTTP traffic takes
place on a broad scale [xkeyscore]. While the IETF does not take
a formal, moral position on wiretapping, we do have a strongly
held belief "that both commercial development of the Internet and
adequate privacy for its users against illegal intrusion requires
the wide availability of strong cryptographic technology"
[RFC2804]. This requirement for privacy is further reinforced by
[RFC6973].
As a result, we decided to revisit the issue of how encryption is
used in HTTP/2.0 at IETF87.
Appendix C. Next Steps
There are three separable aspects to this proposal:
o The concept of alternate services
o The Alt-Svc header field
o The http2-tls-relaxed protocol
In evaluating it, they should be considered separately.
Depending on what aspects we decide to adopt, there are also a number
of related issues that should be discussed:
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 Upgrade: For some flows, it may be advantageous to do an "upgrade
dance" to the tls-relaxed protocol, a la STARTTLS. If there is
sufficient interest, a future revision may also include a proposal
for that.
o http1-tls-relaxed: If there is sufficient interest, it may also be
worthwhile defining a HTTP/1-based tls-relaxed protocol.
o Priority and Weight: It may be advantageous to include measures of
priority and weight in the Alternate Services model (similar to
SRV).
o Indicating Chosen Service: It's likely necessary for the server to
know which protocol the client has chosen, and perhaps even the
hostname (for load balancing). This could be conveyed as part of
the "magic", or as a request header. There are also security
implications here; for example, without this information, the
server doesn't know if the client has checked the certificate,
leading to a situation where an intermediary can downgrade a HTTPS
connection to relaxed HTTP.
o Client Behavior: Currently, this mechanism is completely
declarative, and clients have free reign as to how they use the
alternate services. It may be desirable to specify this more
closely.
o IPV6: The intersection between Alternate Services and IPV6 / Happy
Eyeballs [RFC6555] should be investigated.
Appendix D. Frequently Asked Questions
D.1. Will this make encryption mandatory in HTTP/2.0?
Not in the sense that this proposal would have it required (with a
MUST) in the specification.
What might happen, however, is that some browser implementers will
take the flexibility that this approach grants and decide to not
negotiate for HTTP/2.0 without one of the encryption profiles. That
means that servers would need to implement one of the encryption-
enabling profiles to interoperate using HTTP/2.0 for HTTP URIs.
D.2. No certificate checks? Really?
http2-tls-relaxed has the effect of relaxing certificate checks on
"http://" - but not "https://" - URIs when TLS is in use. Since TLS
isn't in use for any "http://" URIs today, there is no net loss of
security, and we gain some privacy from passive attacks.
In the future, if the certificate trust system can be improved such
that it's both more reliable and has a lower barrier to entry (e.g.,
see [RFC6962]), it may be possible to modify or even drop the http2-
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tls-relaxed profile (even before HTTP/2 ships, depending on progress
there).
D.3. Why do this if a downgrade attack is so easy?
There are many attack scenarios (e.g., third parties in coffee shops)
where active attacks are not feasible, or much more difficult.
Furthermore, active attacks can be more easily detected. Future
infrastructure (again, along similar lines to [RFC6962]) might be
able to detect them and mitigate the risk.
D.4. What about using DNS?
Using DNS for discovery of alternate services has attractive
performance characteristics, and also avoids the "gap" vulnerability.
However, it is significantly more difficult to deploy, compared to a
HTTP header.
If there is implementer interest, a future revision might include a
DNS approach.
D.5. Doesn't Alt-Svc make it easy to hijack a Web server?
In introducing Alt-Svc, we are taking a bounded risk, in that anyone
who has access to write a response header for an origin can
effectively take over the Web site.
To mitigate this, we require the alternate server to either a) be a
port on the same hostname (as the Alternate-Protocol header from SPDY
did), or if it's on another host b) present a certificate that's
valid for the origin server.
D.6. What about using Upgrade?
While it's possible that the HTTP Upgrade header could be used in a
STARTTLS-like connection upgrade, that's more difficult to deploy
with existing infrastructure, and is constrained to upgrading the
same connection, leading to possible latency issues. Alt-Svc offers
a more flexible and less intrusive approach.
That said, if there is sufficient interest, we'll look at defining an
Upgrade-based mechanism.
D.7. Why not 305 Use Proxy?
While it's possible to use a HTTP response code to redirect the
client to an alternate service, this would unavoidably introduce a
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round trip (at least) before the new connection is established, which
violates the performance focus of HTTP/2.0.
D.8. Will this make negotiation too "chatty"?
Putting more information into the protocol string implies that more
protocols will be created, to cover the possible space of
identifiers. In turn, this brings the risk that the negotiation
phase could become bloated by a mass of identifiers that can impact
performance, much as HTTP content negotiation has become in some
cases.
There are a few factors that should mitigate this. First, as
discussed above, it's not necessary to advertise every protocol you
support; only those that are applicable to the current context need
to be sent.
Moreover, we expect that the protocol mechanism will be used to
negotiate coarse-grained, backwards-incompatible changes to the
protocol; this is one of the reasons the "http2-tls-relaxed" protocol
is so loosely defined, so that future mechanisms can be easily
layered upon it.
Nevertheless, the appropriate role of an ALPN protocol needs to be
scrutinized to make sure we have agreement upon what's in and out of
scope for its function.
Author's Address
Mark Nottingham
Email: mnot@mnot.net
URI: http://www.mnot.net/
Nottingham Expires April 4, 2014 [Page 15]