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Versions: (draft-phelan-dccp-natencap) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 6773

Datagram Congestion Control                                    T. Phelan
Protocol                                                           Sonus
Internet-Draft                                             June 24, 2010
Intended status: Experimental
Expires: December 26, 2010


Datagram Congestion Control Protocol (DCCP) Encapsulation in UDP for NAT
                          Traversal (DCCP-UDP)
                      draft-ietf-dccp-udpencap-01

Abstract

   This document specifies an alternative encapsulation of the Datagram
   Congestion Control Protocol (DCCP), referred to as DCCP-UDP.  This
   encapsulation will allow DCCP to be carried through the current
   generation of Network Address Translation (NAT) middleboxes without
   modification of those middleboxes.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 26, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  DCCP-UDP . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  UDP Header . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  DCCP Generic Header  . . . . . . . . . . . . . . . . . . .  5
     3.3.  DCCP-UDP Checksum Procedures . . . . . . . . . . . . . . .  5
       3.3.1.  Minimum Checksum Coverage Feature  . . . . . . . . . .  6
     3.4.  Explicit Congestion Notification . . . . . . . . . . . . .  6
     3.5.  Path Maximum Transmission Unit Discovery . . . . . . . . .  6
     3.6.  Other DCCP Headers and Options . . . . . . . . . . . . . .  6
     3.7.  Service Codes and the DCCP Port Registry . . . . . . . . .  6
   4.  DCCP-UDP and Higher-Layer Protocols  . . . . . . . . . . . . .  7
   5.  Signaling the Use of DCCP-UDP  . . . . . . . . . . . . . . . .  7
     5.1.  SDP for RTP over DCCP  . . . . . . . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10



























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1.  Introduction

   The Datagram Congestion Control Protocol (DCCP), specified in
   [RFC4340], is a transport-layer protocol that provides upper layers
   with the capability of using unreliable but congestion controlled
   flows.  According to [RFC4340], DCCP packets are directly
   encapsulated in IPv4 or IPv6 packets.

   In order for the [RFC4340] encapsulation to pass through Network
   Address Translation (NAT) devices, these devices must be updated to
   recognize and properly modify DCCP.  This is the long-term objective
   for DCCP, and is specified in [RFC5597].

   However, in the short term it would be useful to have an
   encapsulation for DCCP that would be compatible with NAT devices
   conforming to [RFC4787].  This document specifies that encapsulation,
   which is referred to as DCCP-UDP.  For convenience, the [RFC4340]
   encapsulation is referred to as DCCP-STD.

   The DCCP-UDP encapsulation specified here supports all of the
   features contained in DCCP-STD except for partial checksums.  Also,
   support for ECN might be impractical for some implementations.  Those
   implementations MAY choose to not support ECN.


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 [RFC2119].


3.  DCCP-UDP

   The basic approach here is to insert a UDP ([RFC0768]) header between
   the IP header and a DCCP packet.  Note that this is not strictly a
   tunneling approach.  The IP addresses of the communicating end
   systems are carried in the IP header (which could be modified by NAT
   devices) and there are no other IP addresses embedded.

   Devices offering or using DCCP services via DCCP-UDP encapsulation
   listen on a UDP port (default port awaiting IANA action) for incoming
   packets and pass received packets along to the DCCP protocol.  DCCP
   implementations MAY allow services to be simultaneously offered over
   any or all combinations of DCCP-STD and DCCP-UDP encapsulations with
   IPv4 and IPv6.

   The basic format of a DCCP-UDP packet is:



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    +-----------------------------------+
    |     IP Header (IPv4 or IPv6)      |  Variable length
    +-----------------------------------+
    |            UDP Header             |  8 bytes
    +-----------------------------------+
    |       DCCP Generic Header         |  12 or 16 bytes
    +-----------------------------------+
    | Additional (type-specific) Fields |  Variable length (could be 0)
    +-----------------------------------+
    |           DCCP Options            |  Variable length (could be 0)
    +-----------------------------------+
    |      Application Data Area        |  Variable length (could be 0)
    +-----------------------------------+

3.1.  UDP Header

   The format of the UDP header is taken from [RFC0768]:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Source Port          |           Dest Port           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Length            |           Checksum            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   For DCCP-UDP, the fields are interpreted as follows:

   Source and Dest(ination) Ports: 16 bits each

      These fields identify the UDP ports on which the source and
      destination (respectively) of the packet are listening for
      incoming DCCP-UDP packets (normally both are the default port to
      be assigned by IANA).  Note that they do not identify the DCCP
      source and destination ports.

   Length: 16 bits

      This field is the length of the UDP datagram, including the UDP
      header and the payload (which for DCCP-UDP is the DCCP-UDP
      datagram).

   Checksum: 16 bits

      This field is the Internet checksum of a network-layer
      pseudoheader and the UDP packet.





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3.2.  DCCP Generic Header

   As defined in [RFC4340], the DCCP Generic Header takes two forms, one
   with long sequence numbers (48 bits) and the other with short
   sequence numbers (24 bits)

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |           Dest Port           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data Offset  | CCVal | CsCov |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     |       |X|               |                               .
   | Res | Type  |=|   Reserved    |  Sequence Number (high bits)  .
   |     |       |1|               |                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Sequence Number (low bits)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   and

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |           Dest Port           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data Offset  | CCVal | CsCov |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     |       |X|                                               |
   | Res | Type  |=|          Sequence Number (low bits)           |
   |     |       |0|                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   For DCCP-UDP, all generic header fields except for Checksum function
   as specified in [RFC4340].

3.3.  DCCP-UDP Checksum Procedures

   For DCCP-UDP, the function of the DCCP Checksum field is performed by
   the UDP checksum field.  On transmit, the DCCP Checksum field SHOULD
   be set to zero.  On receive, the DCCP Checksum field MUST be ignored.

   If the UDP Length field is less than 20 (UDP Header length and
   minimum DCCP-NAT header length), the packet MUST be dropped.

   If the UDP Checksum field, computed using standard UDP methods,is
   invalid, the packet MUST be dropped.



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3.3.1.  Minimum Checksum Coverage Feature

   The Minimum Checksum Coverage Feature lets a DCCP endpoint determine
   whether its peer is willing to accept packets with partial checksum
   coverage.  This feature is not supported in DCCP-UDP.  A DCCP-UDP
   implementation MUST NOT send a "Change R(Minimum Checksum Coverage,
   value)" with any "value" other than 0, and MUST answer all "Change
   R(Minimum Checksum Coverage, value)" with "Confirm L(Minimum Checksum
   Coverage, 0)".

3.4.  Explicit Congestion Notification

   DCCP-NAT implementations SHOULD follow the procedures of DCCP-STD
   section 12 by setting the ECN fields in the IP Headers of outgoing
   packets and examining the values received in the ECN fields of
   incoming packets.

   However, some implementations might find it impractical to set or
   receive the ECN fields (e.g., user-space implementations using the
   socket interface).  These implementations MUST follow the procedures
   in DCCP-STD section 12.1 for implementations that are not ECN
   capable.

3.5.  Path Maximum Transmission Unit Discovery

   DCCP-NAT implementations SHOULD follow DCCP-STD section 14 with
   regard to maximum packet size and Path Maximum Transmission Unit
   Discovery (PMTUD).

3.6.  Other DCCP Headers and Options

   All type-specific DCCP headers are as in DCCP-STD.  All option and
   feature encodings are as in DCCP-STD.

3.7.  Service Codes and the DCCP Port Registry

   There is one Service Code registry and one DCCP port registry and
   they apply to all combinations of encapsulation and IP version.  A
   DCCP Service Code specifies an application using DCCP regardless of
   the combination of DCCP encapsulation and IP version.  An application
   MAY choose not to support some combinations of encapsulation and IP
   version, but its Service Code will remain registered for those
   combinations and MUST NOT be used by other applications.  An
   application SHOULD NOT register different Service Codes for different
   combinations of encapsulation and IP version.

   Similarly, a port registration is applicable to all combinations of
   encapsulation and IP version.  Again, an application MAY choose not



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   to support some combinations of encapsulation and IP version on its
   registered port, although the port will remain registered for those
   combinations.  Applications SHOULD NOT register different ports just
   for the purpose of using different encapsulation combinations.  Since
   the port registry supports multiple applications registering the same
   port (as long as the Service Codes are different), other applications
   MAY register on the same port, but those registrations are also
   applicable to all combinations of encapsulation and IP version.


4.  DCCP-UDP and Higher-Layer Protocols

   In general, the encapsulation of a higher-layer protocol within DCCP
   SHOULD be the same in both DCCP-STD and DCCP-UDP.  At this time,
   encapsulations of DTLS over DCCP, defined in [RFC5238] and RTP over
   DCCP, defined in [RFC5762], have been already defined.  The
   encapsulations of those protocols in DCCP-UDP SHALL be the same as
   specified in those documents.

   Higher-layer protocols that require different encapsulations for
   different DCCP modes MUST justify the reasons for the difference and
   MUST specify the encapsulations for both DCCP-STD and DCCP-UDP.  If a
   document does not specify different encapsulations for DCCP-STD and
   DCCP-UDP, the specified encapsulation SHALL apply to both DCCP-STD
   and DCCP-UDP.


5.  Signaling the Use of DCCP-UDP

   Applications often signal transport connection parameters through
   outside means, such as the Session Description Protocol (SDP).
   Applications that define such methods for DCCP MUST define how the
   DCCP encapsulation is chosen, and MUST allow either type of
   encapsulation to be signaled.

5.1.  SDP for RTP over DCCP

   [RFC5762] defines SDP extensions for signaling RTP over DCCP
   connections.  Since it predates this document, it does not define a
   method for determining the DCCP encapsulation type.  This document
   updates [RFC5762] to add a method for determining the DCCP
   encapsulation type.

   A new SDP attribute "dccp-encap" is defined for signaling the DCCP
   encapsulation according to the following ABNF [RFC5234]:

       dccp-encap-attr = %x61 "=dccp-in-udp" [":" udp-port-num]




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       udp-port-num    = *DIGIT

   where *DIGIT is as defined in [RFC5234].

   The presence of "a=dccp-in-udp" in an SDP offer indicates that the
   offerer is listening for DCCP-UDP connections on the indicated UDP
   port (if udp-port-num is included) or on the IANA allocated port for
   the DCCP-UDP service if no port is included.

   The absence of "a=dccp-in-udp" in an SDP offer indicates that the
   offerer is listening for DCCP-STD connections.  The presence of
   "a=dccp-in-udp" conveys no information about whether or not the
   offerer is listening for DCCP-STD connections.

   For example (adapted from examples in [RFC5762]):

   An offerer at 192.0.2.47 signals its availability for an H.261 video
   session, using RTP/AVP over DCCP with service code "RTPV" (using the
   hexadecimal encoding of the service code in the SDP).  RTP and RTCP
   packets are multiplexed onto a single DCCP connection and DCCP-UDP
   encapsulation is supported:

          v=0
          o=alice 1129377363 1 IN IP4 192.0.2.47
          s=-
          c=IN IP4 192.0.2.47
          t=0 0
          m=video 5004 DCCP/RTP/AVP 99
          a=rtcp-mux
          a=rtpmap:99 h261/90000
          a=dccp-service-code:SC=x52545056
          a=setup:passive
          a=connection:new
          a=dccp-in-udp

   An answerer at 192.0.2.128 receives this offer and responds with the
   following answer:














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          v=0
          o=bob 1129377364 1 IN IP4 192.0.2.128
          s=-
          c=IN IP4 192.0.2.128
          t=0 0
          m=video 9 DCCP/RTP/AVP 99
          a=rtcp-mux
          a=rtpmap:99 h261/90000
          a=dccp-service-code:SC:RTPV
          a=setup:active
          a=connection:new
          a=dccp-in-udp

   The end point at 192.0.2.128 then initiates a DCCP-UDP connection to
   UDP port to-be-allocated and DCCP port 5004 at 192.0.2.47.  DCCP port
   5004 is used for both the RTP and RTCP data, and port 5005 is unused.
   The textual encoding of the service code is used in the answer, and
   represents the same service code as in the offer.


6.  Security Considerations

   DCCP-UDP provides all of the security risk-mitigation measures
   present in DCCP-STD, and also all of the security risks.

   The purpose of DCCP-UDP is to allow DCCP to pass through NAT devices,
   and therefore it exposes DCCP to the risks associated with passing
   through NAT devices.  It does not create any new risks with regard to
   NAT devices.

   DCCP-UDP may also allow DCCP applications to pass through existing
   firewall devices, if the administrators of the devices so choose.
   The option is a binary one however; either allow all DCCP
   applications or allow none.  Proper control of DCCP application-by-
   application will require enhancements to firewalls.


7.  IANA Considerations

   A port allocation request will be placed with IANA for the dccp-udp
   service port in UDP.

   The following new SDP attribute ("att-field") is to be registered:

      Contact name: Tom Phelan <tphelan@sonusnet.com>

      Attribute name: dccp-in-udp




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      Long-form attribute name in English: DCCP in UDP Encapsulation

      Type of attribute: Media level

      Subject to charset attribute?  No

      Purpose of the attribute: See this document section Section 5.1

      Allowed attribute values: See this document section Section 5.1


8.  References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5238]  Phelan, T., "Datagram Transport Layer Security (DTLS) over
              the Datagram Congestion Control Protocol (DCCP)",
              RFC 5238, May 2008.

   [RFC5597]  Denis-Courmont, R., "Network Address Translation (NAT)
              Behavioral Requirements for the Datagram Congestion
              Control Protocol", BCP 150, RFC 5597, September 2009.

   [RFC5762]  Perkins, C., "RTP and the Datagram Congestion Control
              Protocol (DCCP)", RFC 5762, April 2010.












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Author's Address

   Tom Phelan
   Sonus Networks
   7 Technology Dr.
   Westford, MA  01886
   US

   Phone: +1 978 614 8456
   Email: tphelan@sonusnet.com









































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