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

Internet Engineering Task Force                                T. Phelan
Internet-Draft                                                     Sonus
Intended status: Standards Track                            G. Fairhurst
Expires: August 6, 2011                           University of Aberdeen
                                                        February 2, 2011


   Datagram Congestion Control Protocol (DCCP) Encapsulation for NAT
                          Traversal (DCCP-UDP)
                      draft-ietf-dccp-udpencap-04

Abstract

   This document specifies an alternative encapsulation of the Datagram
   Congestion Control Protocol (DCCP), referred to as DCCP-UDP.  This
   encapsulation allows DCCP to be carried through the current
   generation of Network Address Translation (NAT) middleboxes without
   modification of those middleboxes.  This document also updates the
   SDP information for DCCP defined in RFC 5762.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 6, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  DCCP-UDP . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  The UDP Header . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  The DCCP Generic Header  . . . . . . . . . . . . . . . . .  5
     3.3.  DCCP-UDP Checksum Procedures . . . . . . . . . . . . . . .  6
       3.3.1.  Partial Checksums and the Minimum Checksum
               Coverage Feature . . . . . . . . . . . . . . . . . . .  7
     3.4.  Network Layer Options  . . . . . . . . . . . . . . . . . .  7
     3.5.  Explicit Congestion Notification . . . . . . . . . . . . .  7
     3.6.  ICMP handling for messages relating to DCCP-UDP  . . . . .  7
     3.7.  Path Maximum Transmission Unit Discovery . . . . . . . . .  8
     3.8.  Usage of the UDP port by DCCP-UDP  . . . . . . . . . . . .  8
     3.9.  Service Codes and the DCCP Port Registry . . . . . . . . .  9
   4.  DCCP-UDP and Higher-Layer Protocols  . . . . . . . . . . . . .  9
   5.  Signaling the Use of DCCP-UDP  . . . . . . . . . . . . . . . .  9
     5.1.  SDP support for DCCP-UDP . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12














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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 ability to use non-reliable congestion-controlled flows.
   The current specification for DCCP [RFC4340] specifies a direct
   encapsulation in IPv4 or IPv6 packets.

   [RFC5597] specifies how DCCP should be handled by devices that use
   Network Address Translation (NAT) or Network Address and Port
   Translation (NAPT).  However, there is a significant installed base
   of NAT/NAPT devices that do not support [RFC5597].  In the short
   term, it would be useful to have an encapsulation for DCCP that is
   compatible with this installed base of NAT/NAPT devices that support
   [RFC4787], but do not support [RFC5597].  This document specifies
   that encapsulation, which is referred to as DCCP-UDP.  For
   convenience, the standard encapsulation for DCCP [RFC4340] (including
   [RFC5596] as required) is referred to as DCCP-STD.

   The document also provides an updated SDP specification for DCCP, to
   convey the encapsulation type and, in this respect only, it updates
   the method in [RFC5762].

   The DCCP-UDP encapsulation specified in this document supports all of
   the features contained in DCCP-STD, but with limited functionality
   for partial checksums.


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 is to insert a UDP [RFC0768] header between the IP
   header and the DCCP packet.  Note that this is not a tunneling
   approach.  The IP addresses of the communicating end systems are
   carried in the IP header.  The method does not embed additional IP
   addresses.

   The method is designed to support use when these addresses are
   modified by a device that implements NAT/NAPT.  A NAT translates the
   IP addresses, which impacts the transport-layer checksum.  A NAPT
   device may also translate the port values (usually the source port).
   In both cases, the outer transport header that includes these values



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   would need to be updated by the NAT/NAPT.

   A devoce offering or using DCCP services via DCCP-UDP encapsulation
   listens on a UDP port (default port, XXX IANA PORT XXX), or may bind
   to a specified port utilising out-of-band signalling, such as the
   Session Description Protocol (SDP).  The DCCP-UDP server accepts
   incoming packets over the UDP transport and passes the received
   packets to the DCCP protocol module, after removing the UDP
   encapsulation.

   A DCCP implementation 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:

    +-----------------------------------+
    |     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.  The UDP Header

   The format of the UDP header is specified in [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






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      These fields identify the UDP ports on which the source and
      destination (respectively) of the packet are listening for
      incoming DCCP-UDP packets (both may be the default port assigned
      by IANA).  The UDP port values 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 (for DCCP-UDP, the payload is a DCCP-UDP
      datagram).

   Checksum: 16 bits

      This field is the Internet checksum of a network-layer
      pseudoheader and Length bytes of the UDP packet [RFC0768].  The
      UDP checksum must not be zero for a UDP packet that carries DCCP-
      UDP.

3.2.  The DCCP Generic Header

   The DCCP Generic Header [RFC4340] 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)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Generic DCCP Header with long sequence numbers [RFC4340]











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       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|                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Generic DCCP Header with short sequence numbers [RFC4340]

   All generic header fields, except for Checksum field, have the
   meaning specified in [RFC4340] updated by [RFC5596].

3.3.  DCCP-UDP Checksum Procedures

   DCCP-UDP employs a checksum at the UDP level and eliminates the use
   of the DCCP checksum.  This approach was chosen to enable use of
   current NAT/NATP traversal methods developed for UDP.  Such methods
   will generally be unaware whether DCCP is being encapsulated and
   hence do not update the inner checksum in the DCCP header.  Although
   it was known to increase processing for lightweight systems use of a
   UDP checksum was mandated, since standard DCCP processing requires
   protection of the DCCP header fields.  In addition, UDP NAT traversal
   does not support partial checksums, and hence although this is still
   permitted end-to-end in the encapsulated DCCP datagram, links along
   the path will treat these as UDP packets and can not enable special
   partial checksum processing.

   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.

   The UDP checksum MUST NOT be zero for a UDP packet that is sent using
   DCCP-UDP.  If the received UDP Checksum field is zero, the packet
   MUST be dropped.

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

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

   If the UDP Length field in a received packet is less than the length
   of the UDP header plus the entire DCCP-UDP header (including the



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   generic header and type-specific fields and options, if present), 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, the packet
   MUST be dropped.

3.3.1.  Partial Checksums and the Minimum Checksum Coverage Feature

   DCCP-UDP supports the syntax of partial checksums.  It also supports
   negotiation of the Minimum Checksum Coverage feature and settings of
   the CsCov field.  However, since the UDP checksum field in DCCP-UDP
   always covers the entire DCCP datagram, an application that enables
   this feature will experience a service that is functionally identical
   to using full checksum coverage.

3.4.  Network Layer Options

   A DCCP-UDP implementations MAY transfer network-layer options
   intended for DCCP to the network-layer header of the encapsulating
   UDP packet.

   A DCCP-UDP endpoint that receives IP-options for the encapsulating
   UDP packet MAY forward these to the DCCP protocol module.  If the
   endpoint forwards a specific network layer option to the DCCP module,
   it MUST also forward all subsequent packets with this option.
   Consistent forwarding is essential for correct operation of many end-
   to-end options.

3.5.  Explicit Congestion Notification

   A DCCP-UDP endpoint 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 IP
   packets, relaying any packet markings to the DCCP module.

   Implementations that do not support ECN MUST follow the procedures in
   DCCP-STD section 12.1 with regard to implementations that are not ECN
   capable.

3.6.  ICMP handling for messages relating to DCCP-UDP

   To allow ICMP messages to be demultiplexed by the receiving endpoint,
   part of the original packet that resulted in the message is included
   in the payload of the ICMP error message.  The receiving endpoint can
   therefore use this information to associate the ICMP error with the
   transport protocol instance that resulted in the ICMP message.  When
   DCCP-UDP is used, the error message and the payload of the ICMP error
   message relate to the UDP transport.




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   DCCP-UDP endpoints SHOULD forward ICMP messages relating to a UDP
   packet that carries a DCCP-UDP to the DCCP module.  This may imply
   translation of the payload of the ICMP message into a form that is
   recognised by the DCCP stack.  [ICMP] describes precautions that are
   desirable before TCP acts on the receipt of an ICMP message.  Similar
   precautions are desirable prior to forwarding by DCCP-UDP to the DCCP
   module.

3.7.  Path Maximum Transmission Unit Discovery

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

3.8.  Usage of the UDP port by DCCP-UDP

   A DCCP-UDP endpoint MAY use any UDP port number, providing the active
   endpoint knows a valid UDP Destination Port on the passive endpoint.

   By default, the DCCP-UDP client sets the source and destination ports
   to the default port number.  UDP port number XXX IANA PORT XXX has
   been registered with IANA for this purpose.

   A DCCP-UDP server (that is, an initially passive endpoint that wishes
   to receive DCCP-Request packets [RFC4340] over DCCP-UDP) binds a UDP
   port number for all encapsulated DCCP connections.  If the DCCP-UDP
   server binds to this default port reserved for the DCCP-UDP service,
   it SHOULD accept datagrams from any UDP source port.  For example,
   this would be needed if a NAPT along the path had translated the
   original UDP source port.

   There is a risk that the same DCCP source port number will be used by
   two endpoints each behind a NAPT.  A DCCP-UDP endpoint SHOULD
   therefore demultiplex a DCCP-UDP flow using both the UDP source and
   destination port numbers in addition to processing of the DCCP ports
   by the DCCP module.

   XXX The next part on how to handle port conflicts is currently being
   considered by the DCCP WG XXX

   Hence, the endpoint identifier for a DCCP-UDP connection should be
   the 6-tuple <source address, UDP Source Port, DCCP Source Port,
   destination address, UDP Destination Port, DCCP Destination Port>,
   rather than a 4-tuple <source address, source port, destination
   address, destination port> defined by DCCP-STD.






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3.9.  Service Codes and the DCCP Port Registry

   This section clarifies the usage of DCCP Service Codes and the
   registration of server ports by DCCP-UDP.  The section is not
   intended to update the procedures for allocating Service Codes or
   server ports.

   There is one Service Code registry and one DCCP port registration
   that 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 the Service Code must not be used by other
   applications.  An application should not register different Service
   Codes for different combinations of encapsulation and IP version.
   [RFC5595] provides additional information about DCCP Service Codes.

   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 combinations of encapsulation.


4.  DCCP-UDP and Higher-Layer Protocols

   The encapsulation of a higher-layer protocol within DCCP MUST be the
   same for both DCCP-STD and DCCP-UDP.  Encapsulations of DTLS over
   DCCP is defined in [RFC5238] and RTP over DCCP is defined in
   [RFC5762].  This document therefore does not update these
   encapsulations when using DCCP-UDP.


5.  Signaling the Use of DCCP-UDP

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

   Procedures for handling DCCP-STD and/or DCCP-NAT with Interactive
   Connectivity Establishment (ICE) may need to be developed, but are
   left for further work.






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5.1.  SDP support for DCCP-UDP

   [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]

       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 default port for the
   DCCP-UDP service if no port is included.  This attribute MAY also be
   used in a declarative SDP file.

   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.

   The new SDP attribute specified in this section is expected to be
   useful when the offering party is on the public Internet, or in the
   same private addressing realm as the answering party.  Some other
   NAT/NAPT toplogies may result in the advertised port being
   unreachable via the NAT/NAPT.


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/NAPT
   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/NAPT devices.

   The tunnel encapsulation recommends processing of ICMP messages
   received for packets sent using DCCP-UDP and translation to allow use
   by DCCP.  [ICMP] describes precautions that are desirable before TCP



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   acts on receipt of ICMP messages.  Similar precautions are desirable
   for endpoints processing ICMP for DCCP-UDP.

   DCCP-UDP may also allow DCCP applications to pass through existing
   firewall devices, if the administrators of the devices so choose.  A
   simple use may either allow all DCCP applications or allow none.

   A firewall than interprets this specification could inspect the
   encapsualted DCCP header to filter based on DCCP information.  Full
   control of DCCP conenctions by application will require enhancements
   to firewalls, as discussed in [RFC4340] and related RFCs (e.g.
   [RFC5595]).


7.  IANA Considerations

   This document requests IANA to allocate a UDP port for the dccp-udp
   service.

   XXX Note: IANA is requested to replace all occurrences of "XXX IANA
   PORT XXX" by the allocated port value prior to publication.  XXX

   IANA is also requested to allocate the following new SDP attribute
   ("att-field"):

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

      Attribute name: dccp-in-udp

      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.  Acknowledgments

   This document was produced by the DCCP WG.  The following contributed
   during the working group last call:

   Andrew Lentvorski, Lloyd Wood, Pasi Sarolahti, Gerrit Renker, Eddie
   Kohler, Colin Perkins, Gorry Fairhurst and Tom Phelan.




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9.  References

9.1.  Normative 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.

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

   [RFC5762]  Perkins, C., "RTP and the Datagram Congestion Control
              Protocol (DCCP)", RFC 5762, April 2011.

9.2.  Informative References

   [ICMP]     Gont, ""ICMP attacks against TCP", IETF Work-in-
              Progress.".

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

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

   [RFC5595]  Fairhurst, G., "The Datagram Congestion Control Protocol
              (DCCP) Service Codes", RFC 5595, September 2009.

   [RFC5596]  Fairhurst, G., "Datagram Congestion Control Protocol
              (DCCP) Simultaneous-Open Technique to Facilitate NAT/
              Middlebox Traversal", RFC 5596, September 2009.

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









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Authors' Addresses

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

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


   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Aberdeen, Scotland  AB24 3UE
   UK

   Phone:
   Email: gorry@erg.abdn.ac.uk































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