INTERNET-DRAFT          DCCP-NAT Encapsulation       February 14, 2008

DCCP-NAT Encapsulation
Internet Draft                                                T. Phelan
Document: draft-phelan-dccp-natencap-00.txt              Sonus Networks
Expires: August 2008                                  February 14, 2008
Intended status: Proposed Standard

                Datagram Congestion Control Protocol (DCCP)
                Encapsulation for NAT Traversal (DCCP-NAT)

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

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   Table of Contents

   1. Introduction...................................................3
   2. Terminology....................................................3
   3. DCCP-NAT.......................................................3
      3.1 UDP Header.................................................4
      3.2 DCCP-NAT Generic Header....................................5
         3.2.1 DCCP-RAW Checksum and CsCov Fields....................5
      3.3 Other DCCP Headers and Options.............................6
      3.4 Service Codes and the DCCP Port Registry...................6
   4. Security Considerations........................................6
   5. IANA Considerations............................................7
   6. References.....................................................7
      6.1 Normative References.......................................7
      6.2 Informative References.....................................7
   7. Author's Address...............................................7

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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 work is underway to specify the necessary operations.

   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-NAT.  For convenience, the [RFC4340]
   encapsulation is referred to as DCCP-RAW.

2. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].


   The basic approach here is to insert a UDP ([RFC768]) "shim" layer
   between the IP header and a DCCP packet with a modified generic
   header (modified to eliminate redundancies between UDP and DCCP).
   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-NAT encapsulation
   listen on a UDP port (normally 2210) for incoming packets and pass
   received packets along to the DCCP protocol.  DCCP implementations
   MAY allow services to be simultaneously offered over all combinations
   of DCCP-RAW and DCCP-NAT encapsulations with IPv4 and IPv6.

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   The basic format of a DCCP-NAT packet is:

    |     IP Header (IPv4 or IPv6)      |  Variable length
    |            UDP Header             |  8 bytes
    |     DCCP-NAT Generic Header       |  12 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 [RFC768]:

       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-NAT, 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-NAT packets (normally both are port 2210).  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-NAT is the DCCP-NAT
     datagram).  For DCCP-NAT, it MUST be at least the size of the UDP
     header (8 bytes) plus the minimum size of a DCCP-NAT header (12
     bytes), for a total minimum value of 20 bytes.  Received packets
     with a UDP Length of less than 20 bytes MUST be ignored.

   Checksum: 16 bits
     This field is the Internet checksum of a network-layer pseudoheader
     and the entire UDP packet.  For DCCP-NAT, a packet with a checksum
     field equal to 0 MUST be ignored as incorrect checksum.

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3.2 DCCP-NAT Generic Header

   Unlike the DCCP-RAW generic header, the DCCP-NAT generic header takes
   only one form; it does not support short sequence numbers.  Its
   format is as follows:

       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 | Type  |  Sequence Number (high bits)  |
      .                  Sequence Number (low bits)                   |

   All DCCP-NAT generic header fields function as specified in

3.2.1 DCCP-RAW Checksum and CsCov Fields

   For DCCP-NAT, the function of the DCCP-RAW generic header field
   Checksum is performed by the UDP Checksum field.

   If the UDP Checksum field in a received packet is invalid or has a
   value of 0, that packet MUST be ignored as per the invalid checksum
   procedures of DCCP-RAW (i.e., the options in the packet MUST NOT be

   If the UDP Length field in a received packet is less than the length
   of the UDP header plus the entire DCCP-NAT header (including the
   generic header, type-specific fields and options), or the UDP Length
   field is greater than the length of the packet from the beginning of
   the UDP header to the end of the packet, that packet MUST also be
   ignored as per the invalid checksum procedures.

   Since the UDP Checksum can only cover the entire packet, there is no
   ability to apply checksum coverage to only part of the user data as
   with DCCP-RAW, and therefore there is no need for the CsCov (Checksum
   Coverage) field in DCCP-NAT.  DCCP-NAT implementations SHOULD include
   the ability to negotiate the DCCP Minimum Checksum Coverage feature,
   but SHOULD NOT accept any value other than 0 for this field.

   Note that if UDP-Lite ([RFC3828]) were used as the shim layer it
   would be possible to provide functionality equivalent to the DCCP
   partial checksum coverage.  However, UDP-Lite uses a different
   protocol number than UDP and would therefore defeat the purpose of
   DCCP-NAT in passing through NAT devices.

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3.3 Other DCCP Headers and Options

   All type-specific DCCP headers are as in DCCP-RAW, except that the
   short sequence number version of the acknowledgement header is not
   supported.  All option and feature encodings are as in DCCP-RAW.

3.4 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
   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. Security Considerations

   DCCP-NAT provides all of the security risk-mitigation measures
   present in DCCP-RAW, and also all of the security risks, except those
   associated with short sequence numbers and partial checksum coverage
   (since DCCP-NAT does not support those features).

   The purpose of DCCP-NAT 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-NAT 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.

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5. IANA Considerations

   This document requests allocation of UDP port 2210 for the DCCP-NAT
   service.  The allocation should have the form of:

   dccp-nat      2210/udp      DCCP-NAT Encapsulation

6. References

6.1 Normative References

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

   [RFC768]    Postel, J., "User Datagram Protocol", RFC 768, August

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

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

6.2 Informative References

   [RFC3828]   Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E.,
               Fairhurst, G., "The Lightweight User Datagram Protocol
               (UDP-Lite)", RFC 3828, July 2004.

7. Author's Address

   Tom Phelan
   Sonus Networks
   7 Technology Park Dr.
   Westford, MA USA 01886
   Phone: 978-614-8456

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