Internet DRAFT - draft-friedl-tls-applayerprotoneg
draft-friedl-tls-applayerprotoneg
Network Working Group S. Friedl
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track A. Popov
Expires: August 25, 2013 Microsoft Corp.
February 21, 2013
Transport Layer Security (TLS) Application Layer Protocol Negotiation
Extension
draft-friedl-tls-applayerprotoneg-02
Abstract
This document describes a Transport Layer Security (TLS) extension
for application layer protocol negotiation within the TLS handshake.
For instances in which the TLS connection is established over a well
known TCP/IP port not associated with the desired application layer
protocol, this extension allows the application layer to negotiate
which protocol will be used within the TLS session.
Status of this Memo
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Table of Contents
1. Introduction
2. Requirements Language
3. Application Layer Protocol Negotiation
3.1. The Application Layer Protocol Negotiation Extension
3.2. Protocol Selection
4. Design Considerations
5. Security Considerations
6. IANA Considerations
7. Acknowledgements
8. References
8.1. Normative References
8.2. Informative References
Authors' Addresses
1. Introduction
Currently, the Next Protocol Negotiation extension (NPN) is used to
establish a SPDY [spdy] protocol session within a TLS RFC 5246
[RFC5246] session on port 443. NPN is not specific to SPDY and can
be used to negotiate sessions for a wide variety of protocols within
the TLS handshake.
NPN seeks to provide a reliable mechanism for application developers
to establish secure sessions for arbitrary protocols without
interference from firewalls, HTTP proxies and MITM proxies. It
addresses this goal by introducing a protocol negotiation process
into the TLS handshake under the constraints that no additional
roundtrips be added to the handshake and that the final protocol
selection be opaque to the network carrying the TLS session. Within
the NPN extension, it is the server that first generates and
transmits an offer of supported protocols to the client. The offer
is sent as part of the TLS ServerHello message before the
[ChangeCipherSpec] subprotocol has been started, therefore the list
of protocols supported by the server is transmitted in plaintext.
The client chooses a protocol which may or may not appear in the
offer from the server and then responds with the definitive protocol
selection answer. The client response is sent after the
[ChangeCipherSpec] subprotocol has been initiated, so the protocol
selected is encrypted in the client response.
In many other application layer protocol negotiation processes, it is
the client that first sends an offer of protocols it supports to the
server. The server then selects the protocol to be used in the
session and includes this answer in the response. RFC 3264 [RFC3264]
describes a SDP based offer/answer model which is not proscriptive in
terms of which party generates the offer, however in practice it is
typically the client generating the offer and the server replying
with the answer. This permits the server to act as the definitive
entity for selection of the application layer protocol.
This draft proposes an alternative formulation of the NPN protocol
which 1) brings the offer/answer negotiation into alignment with the
majority of other application layer protocol negotiation standards,
2) allows certificate selection based on the application protocol and
3) makes the definitive protocol selection answer from the server
visible to the network, when the parties so desire.
2. Requirements Language
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 RFC 2119 [RFC2119].
3. Application Layer Protocol Negotiation
3.1. The Application Layer Protocol Negotiation Extension
A new extension type ("application_layer_protocol_negotiation(TBD)")
is defined and MAY be included by the client in its "ClientHello"
message.
enum {
application_layer_protocol_negotiation(TBD), (65535)
} ExtensionType;
The "extension_data" field of the
("application_layer_protocol_negotiation(TBD)") extension SHALL
contain a "ProtocolNameList" value.
opaque ProtocolName<1..2^8-1>;
struct {
ProtocolName protocol_name_list<2..2^16-1>
} ProtocolNameList;
"ProtocolNameList" contains the list of protocols advertised by the
client, in descending order of preference. Protocols are named by
IANA registered, opaque, non-empty byte strings. Implementations
MUST ensure that an empty string is not included and that no byte
strings are truncated.
Experimental protocol names, which are not registered by IANA, will
start with the following sequence of bytes: 0x65, 0x78, 0x70 ("exp").
Servers that receive a client hello containing the
"application_layer_protocol_negotiation" extension, MAY return a
suitable protocol selection response to the client. The server will
ignore any protocol name that it does not recognize. A new
ServerHello extension type
("application_layer_protocol_negotiation(TBD)") MAY be returned to
the client within the extended ServerHello message. The
"extension_data" field of the
("application_layer_protocol_negotiation(TBD)") extension SHALL be
structured the same as described above for the client
"extension_data", except that the "ProtocolNameList" MUST contain
exactly one "ProtocolName".
The additional content associated with this extension MUST be
included in the hash calculations associated with the "Finished"
messages.
Therefore, a full handshake with the
"application_layer_protocol_negotiation" extension in the ClientHello
and ServerHello messages has the following flow (contrast with
section 7.3 of RFC 5246 [RFC5246]):
Client Server
ClientHello --------> ServerHello
(ALPN extension & (ALPN extension &
list of protocols) selected protocol)
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
Figure 1
An abbreviated handshake with the
"application_layer_protocol_negotiation" extension has the following
flow:
Client Server
ClientHello --------> ServerHello
(ALPN extension & (ALPN extension &
list of protocols) selected protocol)
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
Figure 2
Unlike many other TLS extensions, this extension does not establish
properties of the session, only of the connection. When session
resumption or session tickets RFC 5077 [RFC5077] are used, the
previous contents of this extension are irrelevant and only the
values in the new handshake messages are considered.
3.2. Protocol Selection
It is expected that a server will have a list of protocols that it
supports, in preference order, and will only select a protocol if the
client supports it. In that case, the server SHOULD select the most
highly preferred protocol it supports which is also advertised by the
client. In the event that the server supports no protocols that the
client advertises, then the server SHALL respond with a fatal
"no_application_protocol" alert.
enum {
no_application_protocol(120),
(255)
} AlertDescription;
The "no_application_protocol" fatal alert is only defined for the
"application_layer_protocol_negotiation" extension and MUST NOT be
sent unless the server has received a ClientHello message containing
this extension.
The protocol identified in the
"application_layer_protocol_negotiation" extension type in the
ServerHello SHALL be definitive for the connection. The server SHALL
NOT respond with a selected protocol and subsequently use a different
protocol for application data exchange.
4. Design Considerations
The ALPN extension is intended to follow the typical design of TLS
protocol extensions. Specifically, the negotiation is performed
entirely within the client/server hello exchange in accordance with
established TLS architecture. The
"application_layer_protocol_negotiation" ServerHello extension is
intended to be definitive for the connection and is sent in plaintext
to permit network elements to provide differentiated service for the
connection when the TCP/IP port number is not definitive for the
application layer protocol to be used in the connection. By placing
ownership of protocol selection on the server, ALPN facilitates
scenarios in which certificate selection or connection rerouting may
be based on the negotiated protocol.
Finally, by managing protocol selection in the clear as part of the
handshake, ALPN avoids introducing false confidence with respect to
the the ability to hide the negotiated protocol in advance of
establishing the connection. If hiding the protocol is required,
then renegotiation after connection establishment, which would
provide true TLS security guarantees, would be a preferred
methodology.
5. Security Considerations
The ALPN extension does not impact the security of TLS session
establishment or application data exchange. ALPN serves to provide
an externally visible marker for the application layer protocol
associated with the TLS connection. Historically, the application
layer protocol associated with a connection could be ascertained from
the TCP/IP port number in use.
Encrypting the selected application protocol information and sending
it before the Finished messages are exchanged, as done in NPN, does
not provide confidentiality guarantees due to the possibility of man-
in-the-middle attacks.
6. IANA Considerations
This document requires the IANA to update its registry of TLS
extensions to assign an entry referred to here as
"application_layer_protocol_negotiation" for extended ClientHello and
ServerHello messages.
This document also requires the IANA to create a registry of
Application Layer Protocol Negotiation protocol byte strings,
initially containing the following entries:
- "http/1.1": HTTP/1.1 [RFC2616];
- "http/2.0": HTTP/2.0;
- "spdy/1": (obsolete) SPDY version 1;
- "spdy/2": SPDY version 2;
- "spdy/3": SPDY version 3.
A namespace will be assigned for experimental protocols, comprising
byte strings which start with the following sequence of bytes: 0x65,
0x78, 0x70 ("exp"). Assignments in this namespace do not need IANA
registration.
7. Acknowledgements
This document benefitted specifically from the NPN extension draft
authored by Adam Langley of Google and from discussions with Tom
Wesselman and Cullen Jennings both of Cisco.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
8.2. Informative References
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
[spdy] Belshe, M. and R. Peon, "SPDY Protocol (Internet Draft)",
2012.
Authors' Addresses
Stephan Friedl
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Phone: (720)562-6785
Email: sfriedl@cisco.com
Andrei Popov
Microsoft Corp.
One Microsoft Way
Redmond, WA 98052
USA
Email: andreipo@microsoft.com