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Versions: 00 01 02 03 04 05                                             
SIP                                                          C. Jennings
Internet-Draft                                             Cisco Systems
Expires: January 13, 2006                                    N. Modadugu
                                                     Stanford University
                                                           July 12, 2005


                   Using DTLS as a Transport for SIP
                       draft-jennings-sip-dtls-01

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   This Internet-Draft will expire on January 13, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This draft specifies how to use Datagram Transport Layer Security
   (DTLS) as a transport for SIP.  DTLS is a new protocol for providing
   TLS security over a datagram protocol.

   This draft is being discussed on the sip@ietf.org mailing list.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Transport Parameters . . . . . . . . . . . . . . . . . . . . .  3
   4.  DTLS Usage . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Locating DTLS SIP Servers  . . . . . . . . . . . . . . . . . .  4
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  5
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     9.1   Normative References . . . . . . . . . . . . . . . . . . .  5
     9.2   Informational References . . . . . . . . . . . . . . . . .  6
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  6
       Intellectual Property and Copyright Statements . . . . . . . .  7




































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1.  Introduction

   Datagram Transport Layer Security (DTLS) [7] provides communication
   privacy similar to TLS for datagram packets.  SIP can run over both
   stream and datagram transports, including UDP and TCP.  SIP already
   defines how to use TLS with stream oriented transports.  This
   specification extends SIP to use DTLS with datagram oriented
   transports.

   There has been considerable discussion of why SIP needs DTLS when we
   have TLS.  This is the wrong question.  The right question is why SIP
   has UDP and TCP (not to mention SCTP).  There are two reasons for
   believing that UDP is likely to be an important protocol in SIP for
   the foreseeable future.

   o  In theory, there is no problem building systems that terminate a
      million TCP connections on a single host.  In practice, the common
      operating systems used for building SIP aggregation devices make
      this impossible.  To date, no one has demonstrated terminating
      over 100k SIP TCP connections to a single host.  Doing that many
      connections with UDP has not been difficult.
   o  If we want to talk about "running code" for SIP, it's UDP.  Unless
      UDP is deprecated for SIP, it is important to provide a reasonable
      level of security for it.

   The dominant technique for providing security for SIP/TCP is DTLS.
   Although in principle S/MIME can be used to provide security for both
   SIP/UDP and SIP/TCP, deployment has been minimal.  Given that SIP/UDP
   is not likely to disappear any time soon, this leaves us with two
   options:

   1.  Leave UDP users without a channel-level security mechanism; or
   2.  Provide SIP/UDP users with an alternate security mechanism.

   We don't consider the first option to be acceptable.  The obvious
   approach to the second option is to use DTLS, which provides UDP with
   a level of channel security equivalent to that which TLS provides for
   TCP.

2.  Terminology

   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 [3].

3.  Transport Parameters

   SIP URIs can carry a transport parameter indicating the transport



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   protocol to be used.  This specification defines two new values for
   the transport parameter: "dtls-udp" for the SIP URI transport
   parameter to be used for messages sent using DTLS over UDP, and
   "dtls-dccp" for messages sent using DTLS over DCCP.  The update to
   the ABNF in RFC 3261 for this parameter is the following:

   transport-param   =  "transport="
                      ( "udp" / "tcp" / "sctp" / "tls" / "tls-sctp"
                        "dtls-dccp" / "dtls-udp"
                         / other-transport)

   The following is an example of SIP URIs using "dtls-udp":

   sip:alice@example.com;transport=dtls-udp

   Via header fields also carry a transport protocol identifier.  This
   specification extends RFC 3261 to define the value "DTLS-UDP" for
   DTLS over UDP and "DTLS-DCCP" for DTLS over DCCP.  The update to the
   ABNF in RFC 3261 for this parameter is the following:

   transport         =  "UDP" / "TCP" / "TLS" / "SCTP" / "TLS-SCTP"
                        "DTLS-DCCP" / "DTLS-UDP"
                        / other-transport

   The following is an example Via header field:

   Via: SIP/2.0/DTLS-UDP atlanta.example.com:5060


4.  DTLS Usage

   The normal rules for sending a request over UDP in RFC 3261 apply to
   sending over DTLS.  Note that the congestion safety rules for UDP do
   not apply to DCCP.  In addition, the normal rules for validating a
   TLS connection in RFC 3261 apply to DTLS connections.  Requests with
   a SIPS URI can be sent over DTLS as well as TLS.

5.  Locating DTLS SIP Servers

   The normal rules from RFC 3263 [4] apply when locating a SIP server
   that supports DTLS.  The following new NAPTR[5] service values are
   defined: "SIPS+D2U" for UDP, and "SIPS+D2D" for DCCP.  In addition,
   the service value "SIP+D2D" should be used for SIP without DTLS over
   DCCP.

   The default port for DTLS over UDP is 5061.





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6.  Security Considerations

   The security issues with SIP using DTLS are equivalent to the issues
   of using SIP with TLS.  All the security considerations in RFC 3261
   relevant to TLS apply to DTLS.

7.  IANA Considerations

   The IANA is requested to update the following entry to the "SIP/SIPS
   URI Parameters" registry.  The reference to this RFC should appear in
   double-brackets and be appended to the list of references already
   listed on for the transport parameter, as indicated in RFC 3969 [6].
   The result is shown below:

   Parameter Name     Predefined Values     Reference
   --------------     -----------------     ---------
   transport          Yes                   [RFC3261] [[RFCXXXX]]

   This document also defines new NAPTR service field values.  The IANA
   is requested to register these values under the "Registry for the SIP
   SRV Resource Record Services Field".  The resulting entries should
   be:

    Services Field        Protocol  Reference
    --------------------  --------  ---------
    SIPS+D2U              UDP       [RFCXXXX]
    SIPS+D2D              DCCP      [RFCXXXX]
    SIP+D2D               DCCP      [RFCXXXX]

   [Note to IANA: Please replace XXXX with the RFC number of this
   specification.]

8.  Acknowledgments

   Much of text and outline for this specification came from [8]
   authored by Jonathan Rosenberg, Henning Schulzrinne, and Gonzalo
   Camarillo.  Eric Rescorla provided helpful comments and text.

9.  References

9.1  Normative References

   [1]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
        Security", draft-rescorla-dtls-05 (work in progress), June 2005.

   [2]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.



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

   [4]  Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
        (SIP): Locating SIP Servers", RFC 3263, June 2002.

   [5]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        Three: The Domain Name System (DNS) Database", RFC 3403,
        October 2002.

9.2  Informational References

   [6]  Camarillo, G., "The Internet Assigned Number Authority (IANA)
        Uniform Resource Identifier (URI) Parameter Registry for the
        Session Initiation Protocol (SIP)", BCP 99, RFC 3969,
        December 2004.

   [7]  Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
        draft-ietf-dccp-spec-09 (work in progress), November 2004.

   [8]  Rosenberg, J., "The Stream Control Transmission Protocol (SCTP)
        as a Transport for the Session Initiation Protocol (SIP)",
        draft-ietf-sip-sctp-06 (work in progress), February 2005.


Authors' Addresses

   Cullen Jennings
   Cisco Systems
   170 West Tasman Drive
   MS: SJC-21/2
   San Jose, CA  95134
   USA

   Phone: +1 408 902-3341
   Email: fluffy@cisco.com


   Nagendra Modadugu
   Stanford University
   353 Serra Mall
   Stanford, CA  94305
   USA

   Email: Nagendra@cs.stanford.edu






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