Network Working Group S. Friedl
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track October 16, 2012
Expires: April 19, 2013
Transport Layer Security (TLS) Application Layer Protocol Negotiation
Extension
draft-friedl-tls-applayerprotoneg-00
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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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 19, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 4
1.2. Application Layer Protocol Negotiation Extension . . . . . 4
1.3. Protocol Selection . . . . . . . . . . . . . . . . . . . . 7
1.4. Design Considerations . . . . . . . . . . . . . . . . . . . 7
1.5. Security Considerations . . . . . . . . . . . . . . . . . . 7
1.6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7
2. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
3. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Normative References . . . . . . . . . . . . . . . . . . . 8
3.2. Informative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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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. To do
this, NPN introduces a non-symmetric and slightly idiosyncratic
extension to the TLS handshake. 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.
History demonstrates that there exist a multiplicity of compelling
needs for TCP/IP networks to provide differentiated network treatment
based on protocol. This differentiated treatment may include QOS
and/or firewalling to permit enterprises and carriers to manage the
flows within their networks. QOS requirements may be driven by the
needs of real-time protocols or service provider SLAs or service
tiers. Firewalling is required to meet a variety of regulatory,
compliance, appropriate use and security mandates.
This draft proposes an alternative formulation of the NPN protocol
which 1) brings the offer/answer negotiation into alignment with the
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majority of other application layer protocol negotiation standards,
2) improves the symmetry of the offer/answer exchange and 3) makes
the definitive protocol selection answer from the server visible to
the network.
1.1. 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].
1.2. 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 "ProtocolIdentifierList" where:
struct {
IdentifierType id_type;
select (id_type) {
case IANA_application_id: ApplicationID;
} id;
} ProtocolIdentifier;
enum {
IANA_application_id(0), (255)
} IdentifierType;
opaque ApplicationID<2^32-1>;
struct {
ProtocolIdentifier protocol_id_list<1..2^16-1>
} ProtocolIdentifierList;
"ApplicationID" contains the base 10 IANA registered application
number associated with the requested protocol. This new IANA
controlled value is 32 bits in length and each application layer
protocol negotiated within the ALPN handshake must have an entry in
this IANA registry. "ApplicationID" will be serialized as a fixed
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length 10 byte string using UTF-8 encoding [UTF8]. Leading zeros
SHALL be used to pad the identifiers to 10 digits. Spaces SHALL be
used to separate port numbers in a "ProtocolIdentifierList".
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 "ProtocolIdentifier" that it does not recognize. A new
ServerHello extension type ("connection_protocol(TBD)") MAY be
returned to the client within the extended ServerHello message. The
"extension_data" field of the ("connection_protocol(TBD)") extension
SHALL contain a single protocol identifier serialized as an
"ApplicationID" as described for the client "extension_data" protocol
list.
The additional content associated with this extension MUST be
included in the hash calculations associated with the "Finished"
messages.
enum {
connection_protocol(TBD), (65535)
} ExtensionType;
struct {
IdentifierType id_type;
select (id_type) {
case IANA_application_id: ApplicationID;
} id;
} ProtocolIdentifier;
opaque ApplicationID<2^32-1>;
Therefore, a full handshake with the
"application_layer_protocol_negotiation" and "connection_protocol"
extensions has the following flow (contrast with section 7.3 of RFC
5246 [RFC5246]):
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Client Server
ClientHello --------> ServerHello
(ALPN extension & (connection protocol
list of protocols) extension &
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" and "connection_protocol"
extensions the following flow:
Client Server
ClientHello --------> ServerHello
(ALPN extension & (connection protocol
list of protocols) extension &
selected protocol)
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
Figure 2
Unlike many other TLS extension, 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.
For the same reasons, after a handshake has been performed for a
given connection, renegotiations on the same connection MUST NOT
include the "application_layer_protocol_negotiation" or
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"connection_protocol" extensions.
1.3. 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 SHOULD select the first protocol
in its ordered list.
The protocol identified in the "connection_protocol" 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.
1.4. Design Considerations
The ALPN extension is intended to follow the typical design of TLS
protocol extensions. Specifically, the negotiation is performed
entirely within the hello messages and the ClientHello and
ServerHello extensions conform to the same general pattern used by
other TLS extensions. The "connection_protocol" 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.
1.5. 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.
1.6. IANA Considerations
This document requires the IANA to update its registry of TLS
extensions to assign two entries referred to here as
"application_layer_protocol_negotiation" for extended ClientHello
messages and "connection_protocol" for extended ServerHello messages.
This document requires the IANA to create a new registry of
application identifiers to serve as protocol identifiers for ALPN.
It is suggested this identifier be 32 bit numeric value with the
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bottom 16 bits associated with current IANA port numbers when the
upper 16 bits are all set to zero.
2. 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.
3. References
3.1. 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.
3.2. 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.
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Author's Address
Stephan Friedl
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Phone: (720)562-6785
Email: sfriedl@cisco.com
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