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Versions: (draft-fairhurst-dccp-behave-update) 00 01 02 03 04 05 06 07 08 RFC 5596

DCCP Working Group                                          G. Fairhurst
Internet-Draft                                                 G. Renker
Updates: 4340 (if approved)                       University of Aberdeen
Intended status: Standards Track                           June 17, 2008
Expires: December 19, 2008

 DCCP Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The IETF Trust (2008).

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   This document specifies an update to the Datagram Congestion Control
   Protocol (DCCP), a connection-oriented and datagram-based transport

   The update assists DCCP applications which need to communicate
   through one or more middleboxes (e.g.  Network Address Translators or
   firewalls), where establishing necessary middlebox state requires
   peering endpoints to initiate communication in a near-simultaneous

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Scope of this Document . . . . . . . . . . . . . . . . . .  3
     1.2.  Scope of the Problem to be Tackled . . . . . . . . . . . .  4
     1.3.  Structure of this Document . . . . . . . . . . . . . . . .  4
   2.  Procedure for Near-Simultaneous Open . . . . . . . . . . . . .  5
     2.1.  Conventions and Terminology  . . . . . . . . . . . . . . .  5
     2.2.  DCCP-Listen Packet Format  . . . . . . . . . . . . . . . .  5
     2.3.  Protocol Method  . . . . . . . . . . . . . . . . . . . . .  7
       2.3.1.  Protocol Method for DCCP-Server Endpoints  . . . . . .  7
       2.3.2.  Protocol Method for DCCP-Client Endpoints  . . . . . .  9
       2.3.3.  Processing by Routers and Middleboxes  . . . . . . . .  9
     2.4.  Examples of Use  . . . . . . . . . . . . . . . . . . . . .  9
     2.5.  Backwards Compatibility with RFC 4340  . . . . . . . . . . 10
   3.  Discussion of Design Decisions . . . . . . . . . . . . . . . . 12
     3.1.  Rationale for a New Packet Type  . . . . . . . . . . . . . 12
     3.2.  Generation of Listen Packets . . . . . . . . . . . . . . . 13
     3.3.  Repetition of Listen Packets . . . . . . . . . . . . . . . 13
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 18
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 18
   Appendix A.  Discussion of Existing NAT Traversal Techniques . . . 20
     A.1.  NAT traversal Based on Simultaneous-Open . . . . . . . . . 21
     A.2.  Role Reversal  . . . . . . . . . . . . . . . . . . . . . . 21
   Appendix B.  Change Log  . . . . . . . . . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
   Intellectual Property and Copyright Statements . . . . . . . . . . 26

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1.  Introduction

   UDP Network Address Translator (NAT) traversal is well understood and
   widely implemented.  NAT traversal for connection-oriented protocols
   (e.g.  TCP) uses similar principles, but in some cases requires more
   complex and expensive solutions, such as data relay servers [TURN].

   DCCP [RFC4340] is both datagram-based and connection-oriented.  As
   such it faces faces the same problems as TCP NAT traversal, without
   the ability to simply reuse traversal solutions which work for UDP.
   An additional issue is that DCCP can not perform a simultaneous-open,
   a TCP-inherent characteristic which greatly simplifies NAT traversal.

   After discussing the problem space for DCCP, this document specifies
   a DCCP extension to facilitate DCCP NAT traversal, by explicitly
   supporting a widely used principle known as 'hole punching'.  This
   extension produces the same outward effect as an simultaneous-open,
   but without internal changes to the standard DCCP operational
   procedure.  The extension uses a dedicated indicator message, whose
   usage is tied to a specific condition, can thus be turned off, and is
   inter-operable with non-extended hosts.

   The object of this extension is DCCP native support for middlebox
   traversal, reducing dependence on external aids such as data relay

1.1.  Scope of this Document

   This document is specifically targeted at NAT traversal.  However,
   due to the similarity of involved principles, the technique and
   presented extension of DCCP may also be of similar use to the
   traversal of other types of middlebox, such as firewalls.

   The technique described by this document applies to scenarios where
   one or both DCCP peers are located behind a middlebox.

   The proposed extension is relevant to both client/server and peer-to-
   peer applications, such as VoIP, file sharing, or online gaming.  It
   assists connections that utilise prior out-of-band signaling (e.g.
   via a well-known rendezvous server ([RFC3261], [H.323])) to notify
   both endpoints of the connection parameters ([RFC3235], [NAT-APP]).

   For the scope of this document we assume traditional (outbound) types
   of NAT as defined in [RFC2663] and further discussed in [RFC3022].
   We understand NAT traversal as involving one or more NAT devices of
   this type in the path (i.e. hierarchies of nested NAT devices are
   possible).  It is assumed that all NATs in the path between endpoints
   are BEHAVE-compliant [NAT-APP].

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   This document does not discuss specific behavioural requirements of
   devices to support DCCP NAT traversal.  These may be described by a
   separate document.  We further limit our assumptions regarding NAT
   devices to a minimum of DCCP protocol support, in that layer-4
   checksums are updated to account for changes in the pseudo-header.

1.2.  Scope of the Problem to be Tackled

   This document refers to DCCP hosts located behind one or more NAT
   devices as having "private" addresses, and to DCCP hosts located in
   the global address realm as having "public" addresses.

   We consider DCCP NAT traversal for the following scenarios:

   1.  Private client connects to public server.

   2.  Public server connects to private client.

   3.  Private client connects to private server.

   A defining characteristic of traditional NAT devices [RFC3022] is
   that private hosts can connect to external hosts, but not vice versa.
   Hence the case (1) is always possible, whereas cases (2) and (3)
   require NAT traversal techniques.

   In this document we do not consider use of pre-configured, static NAT
   address maps, which would also allow outside hosts to connect to the
   private network in cases (2) and (3).

   A DCCP implementation conforming to [RFC4340] requires a relay server
   to perform NAT traversal.  The extension specified by this document
   enables DCCP NAT traversal without the aid of relay servers.

1.3.  Structure of this Document

   For background information on existing NAT traversal techniques,
   please consult Appendix A.

   The normative specification of the extension is presented in the next
   section.  An informative discussion of underlying design decisions
   then follows in Section 3.  Security considerations are provided in
   Section 4 and IANA considerations in the concluding Section 5.

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2.  Procedure for Near-Simultaneous Open

   This section is normative and specifies the simultaneous-open
   technique for DCCP.

   The presented extension updates the connection-establishment
   procedures of [RFC4340].

2.1.  Conventions and Terminology

   The document uses the terms and definitions provided in [RFC4340].
   Familiarity with this specification is assumed.  In particular, the
   following convention from ([RFC4340], 3.2) is used:

      "Each DCCP connection runs between two hosts, which we often name
      DCCP A and DCCP B. Each connection is actively initiated by one of
      the hosts, which we call the client; the other, initially passive
      host is called the server."

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

2.2.  DCCP-Listen Packet Format

   This document updates DCCP by adding a new packet type, DCCP-Listen,
   whose format is shown below.

   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            |
   | Res | Type  |X|   Reserved    |  Sequence Number High Bits    |
   |                    Sequence Number Low Bits                   |
   |                         Service Code                          |

                    Figure 1: DCCP-Listen Packet Format

   DCCP-Listen Packets MAY include header options ([RFC4340], sec. 5.8).
   Note however that, at the time of this writing, there are no known
   uses of header options for the DCCP-Listen packet.

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   Since DCCP-Listen packets are issued before an actual connection is
   established, they SHALL NOT carry payload data, and endpoints MUST
   ignore any payload data encountered on DCCP-Listen packets.

   Servers SHOULD set both Sequence Number fields to 0; clients MUST
   ignore the value of the Sequence Number fields; and middleboxes MUST
   NOT interpret sequence numbers on DCCP-Listen packets.

   Furthermore, the following protocol fields MUST all be set to zero:

      CCVal (a connection has not been established),

      CsCov (there is no payload).

   The "Res" and "Reserved" fields are specified by [RFC4340] and its
   successors.  The interpretation of these fields is not modified by
   this document.

   The Type field has the value XX-IANA-assigned-XX, which indicates
   that this is a DCCP-Listen packet.

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Note to the RFC Editor:

   Please replace XX-IANA-assigned-XX in the above paragraph with the
   value assigned in the registry and remove this note.

   ==> End of note to the RFC Editor. <==

   Since the use of short sequence numbers ([RFC4340], 5.1) depends on
   the value of the Allow Short Seqno feature ([RFC4340], 7.6.1) and
   since DCCP-Listen packets are sent before a connection is
   established, there is no way of negotiating the use of short sequence
   numbers.  Consequently, the default value of 0 for the Allow Short
   Seqno feature ([RFC4340], 6.4) SHALL be used, X MUST be set to 1, and
   DCCP-Listen packets with X=0 MUST be ignored.

   The Service Code field contains the service code ([RFC4340], 8.1.2)
   that the client peer wants to use for this connection.  This value
   MUST correspond to a service code that the server is actually
   offering for connections identified by the same source IP address and
   the same Source Port as that of the DCCP-Listen packet.  Since the
   server may use multiple service codes, the value of the Service Code
   field needs to be communicated out-of-band, from client to server,
   prior to sending the DCCP-Listen packet (e.g. using SDP).

2.3.  Protocol Method

   We use the term "session" as defined in ([RFC2663], 2.3): DCCP
   sessions are uniquely identified by the tuple of <source IP-address,
   source port, target IP-address, target port>.

   DCCP, in addition, introduces service codes which can be used to
   identify different services available via the same port [Fai08].

   We call the five-tuple <source IP-address, source port, service code,
   target IP-address, target port> a fully specified DCCP connection,
   and refer to an endpoint that has been assigned all five parameters
   as a "fully specified endpoint".  DCCP-Listen packets are only sent
   for the specific case of fully specified DCCP-server endpoints.

2.3.1.  Protocol Method for DCCP-Server Endpoints

   This document updates [RFC4340] for the case of fully specified DCCP-
   server endpoints.  The update is normative and applies to the way the
   server performs passive-open.

   Prior to connection setup, it is common for DCCP-server endpoints to
   not be fully specified: before the connection is established, a
   server usually sets the target IP-address:port to wildcard values

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   (i.e. leaves these unspecified); the endpoint only becomes fully
   specified after performing the handshake with an incoming connection.
   For such cases, this document does not update [RFC4340], i.e. the
   server adheres to the existing state transitions in the left half of
   Figure 2 (CLOSED => LISTEN => RESPOND).

   A fully specified DCCP-server endpoint permits exactly one client,
   identified by target IP-address:port plus service code, to set up the
   connection.  Such a server SHOULD perform the actions and state
   transitions shown in the right half of Figure 2, and specified below.

           unspecified remote   +--------+   fully specified remote
          +---------------------| CLOSED |---------------------+
          |                     +--------+   send DCCP-Listen  |
          |                                                    |
          |                                                    |
          v                                                    v
     +--------+                                  timeout  +---------+
     | LISTEN |<------------------------------+-----------| INVITED |
     +--------+  more than 2 retransmissions  |           +---------+
          |                                   |  1st / 2nd  ^  |
          |                                   |  retransm.  |  |
          |                                   +-------------+  |
          |                                    resend Listen   |
          |                                                    |
          |                                                    |
          |  receive Request   +---------+    receive Request  |
          +------------------->| RESPOND |<--------------------+
             send Response     +---------+    send Response

        Figure 2: Updated state transition diagram for DCCP-Listen

   A fully-specified server endpoint performs passive-open from CLOSED
   by inviting the remote client to connect.  This is done by sending a
   single DCCP-Listen packet to the specified remote IP-adress:port,
   using the format specified in Section 2.2.  The server then
   transitions to INVITED.

   The INVITED state is, like LISTEN, a passive state, characterised by
   waiting in the absence of an established connection.  If the server
   endpoint in state INVITED receives a DCCP-Request, it transitions to
   RESPOND, where further processing resumes as specified in [RFC4340].

   The server SHOULD repeat sending a DCCP-Listen packet while in state
   INVITED, at a 200 millisecond interval and up to at most 2
   repetitions (Section 3 discusses this choice of timer interval).  The
   retransmission timer is restarted with the same 200ms interval after
   the second retransmission.  When, upon the next timeout, the server

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   is still in the INVITED state, it SHOULD progress to LISTEN, and
   resume processing as per [RFC4340].

   Fully-specified server endpoints SHOULD treat ICMP error messages
   received in response to a DCCP-Listen packet as "soft errors" that do
   not cause a state transition.

   Server endpoints SHOULD in general ignore any incoming DCCP-Listen
   packets.  As an exception to this rule, a DCCP-Server in state LISTEN
   MAY generate a Reset (Code 7, "Connection Refused") in response to a
   DCCP-Listen packet.

   Further details on the rationale are discussed in Section 3.

2.3.2.  Protocol Method for DCCP-Client Endpoints

   This document updates [RFC4340], by adding the following normative
   rule for the reception of DCCP-Listen packets by clients.

   Clients MUST silently discard any received DCCP-Listen packets,
   regardless of their current state.

2.3.3.  Processing by Routers and Middleboxes

   DCCP-Listen packets do not require special treatment and should thus
   be forwarded end-to-end across Internet paths, by routers and
   middleboxes alike.

   Middleboxes may utilise the connection information (address, port,
   service code) to establish local forwarding state.  This has been the
   main motivation for adding the Service Code field: in combination
   with the source and destination addresses (found in the enclosing IP-
   header), the DCCP-Listen packet carries all necessary information to
   uniquely identify a DCCP session.

2.4.  Examples of Use

   In the examples below, DCCP A is the client and DCCP B is the server.
   NAT/Firewall device NA is placed before DCCP A, and NAT/Firewall
   device NB is placed before DCCP B.

   Both NA and NB use a policy that permits DCCP packets to traverse the
   device for outgoing links, but only permit incoming DCCP packets when
   a previous packet has been sent out for the same connection.

   DCCP A and DCCP B decide to communicate using some out-of-band
   mechanism, whereupon the client and server are started.  DCCP A
   initiates a connection by sending a DCCP-Request.  DCCP B actively

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   indicates its listening state by sending a DCCP-Listen message.  This
   fulfils the requirement of punching a hole in NB, so that DCCP A can
   retransmit the DCCP-Request and connect through it.

          DCCP A                                       DCCP B
          ------               NA      NB              ------
          +------------------+  +-+    +-+  +-----------------+
          |(1) Initiation    |  | |    | |  |                 |
          |DCCP-Request -->  +--+-+---X| |  |                 |
          |                  |<-+-+----+-+--+<-- DCCP-Listen  |
          |                  |  | |    | |  |                 |
          |DCCP-Request -->  +--+-+----+-+->|                 |
          |                  |<-+-+----+-+--+<-- DCCP-Response|
          |DCCP-Ack -->      +--+-+----+-+->|                 |
          |                  |  | |    | |  |                 |
          |(2) Data transfer |  | |    | |  |                 |
          |DCCP-Data -->     +--+-+----+-+->|                 |
          +------------------+  +-+    +-+  +-----------------+

   Figure 3: Event sequence when the client is started before the server

   The diagram below shows the reverse sequence of events, where the
   server sends the DCCP-Listen before the client sends a DCCP-Request:

          DCCP A                                       DCCP B
          ------               NA      NB              ------
          +------------------+  +-+    +-+  +-----------------+
          |(1) Initiation    |  | |    | |  |                 |
          |                  |  | |X---+-+--+<-- DCCP-Listen  |
          |DCCP-Request -->  +--+-+----+-+->|                 |
          |                  | <+-+----+-+--+<-- DCCP-Response|
          |DCCP-Ack -->      +--+-+----+-+> |                 |
          |                  |  | |    | |  |                 |
          |(2) Data transfer |  | |    | |  |                 |
          |DCCP-Data -->     +--+-+----+-+> |                 |
          +------------------+  +-+    +-+  +-----------------+

   Figure 4: Event sequence when the server is started before the client

2.5.  Backwards Compatibility with RFC 4340

   There are no changes if a client conforming to this document
   communicates with a server conforming to [RFC4340].

   If a client implements only [RFC4340], incoming DCCP-Listen packets
   would be ignored due to step 1 in [RFC4340], 8.1, which at the same
   time also conforms to the behaviour specified by this document.

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   This document further does not modify communication for any server
   that implements a passive-open without fully binding the addresses,
   ports and service codes to be used.

   The authors therefore do not expect practical deployment problems.

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3.  Discussion of Design Decisions

3.1.  Rationale for a New Packet Type

   The DCCP-Listen packet specified in Section 2.2 has the same format
   as the DCCP-Request packet ([RFC4340], 5.1), the only difference is
   in the value of the Type field.

   The usage, however, differs.  The DCCP-Listen packet serves as
   advisory message, not as part of the actual connection setup:
   sequence numbers have no meaning, and no payload may be present.

   It is important to point out that a DCCP-Request packet could in
   theory also be used for the same purpose.  The following arguments
   were against this.

   The first problem is that of semantic overloading: the Request is
   defined in [RFC4340] to serve a well-defined purpose, being the
   initial packet of the 3-way handshake.  Additionally using it in the
   manner of a DCCP-Listen packet would require DCCP processors to have
   two different processing paths: one where a Request is interpreted as
   part of the initial handshake, and another where the same packet is
   interpreted as indicator message.  This complicates packet processing
   in hosts and in particular stateful middleboxes (which may have
   restricted computational resources).

   The second problem is that a client receiving a DCCP-Request from a
   server could generate a Reset if it has not yet entered the REQUEST
   state (step 7 in [RFC4340], 8.5).  This document lets client
   endpoints ignore DCCP-Listen packets.  Adding a similar rule for the
   Request packet is more cumbersome: clients can not distinguish
   between a Request meant to be an indicator message and a genuinely
   erratic connection initiation.

   The third problem is similar and refers to a client receiving the
   DCCP-Listen after having itself sent a (connection-initiation)
   Request.  Step 7 in section 8.5 of [RFC4340] requires the client to
   reply to an "indicator message" Request from the server with a Sync.
   Since sequence numbers are ignored for this type of message,
   additional and complex processing becomes necessary: either to ask
   the client not to respond to a Request when the Request is of type
   "indicator message"; or ask middleboxes and servers to ignore Sync
   packets generated in response to "indicator message" Requests.
   Furthermore, since no initial sequence numbers have been negotiated
   at this stage, sending a SyncAck would not be meaningful.

   Using a separate packet type allows simpler and clearer processing.

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   The rationale for ignoring the Sequence Number fields on DCCP-Listen
   packets is that endpoints have not yet entered connection setup: the
   Listen packet is sent out while the server is still in the passive-
   open (INVITED) state, i.e. it has not yet allocated state other than
   binding to the client's IP-address:port and service code.

   Although the DCCP-Listen Sequence Number fields are ignored, they
   have been retained to reuse the generic header format from section
   5.1 of [RFC4340].

3.2.  Generation of Listen Packets

   Since DCCP-Listen packets solve a particular problem (NAT and/or
   firewall traversal), the generation of DCCP-Listen packets on passive
   sockets is tied to a condition (binding to an a priori known remote
   address and service code), so as to not interfere with the general
   case of "normal" DCCP connections (where client addresses are
   generally not known in advance).

   In the TCP world, the analogue is a transition from LISTEN to
   SYN_SENT by virtue of sending data: "A fully specified passive call
   can be made active by the subsequent execution of a SEND" ([RFC0793],

   Unlike TCP, this proposal does not perform a role-change from passive
   to active.

   Like TCP, we require that DCCP-Listen packets are only sent by a
   DCCP-server when the endpoint is fully specified (Section 2.3).

3.3.  Repetition of Listen Packets

   Repetition is a necessary requirement, to increase robustness and the
   chance of successful connection establishment: in case a Listen
   packet is lost due to congestion, link loss or some other reason.

   Recommending a maximum number of 3 timeouts (2 repetitions) is due to
   the following considerations.  The repeated copies need to be spaced
   sufficiently far apart in time to avoid suffering from correlated
   loss.  The interval of 200ms has been chosen to accommodate a wide
   range of wireless and wired network paths.

   Another constraint is given by the retransmission interval for the
   DCCP-Request ([RFC4340], 8.1.1).  To establish state, intermediate
   systems need to receive a (retransmitted) DCCP-Listen packet before
   the DCCP-Request times out (1 second).  With three timeouts, each
   spaced 200 milliseconds apart, the overall time is still below one
   second.  On the other hand, the sum of 600 milliseconds is

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   sufficiently large to provide for longer one-way delays, such as e.g.
   found on some wireless links.

   The rationale behind transitioning to the LISTEN state after two
   retransmissions is that other problems, independent of establishing
   middlebox state, may occur (such as delay or loss of the initial
   DCCP-Request).  Any late or retransmitted DCCP-Request packets will
   then still reach the server, so that connection establishment
   completes successfully.

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4.  Security Considerations

   The method specified in this document exposes the state of a DCCP
   server that has been explicitly pre-configured to accept a connection
   from a known client.  Establishing this state requires prior out-of-
   band signalling between the client and server (e.g. via the Session
   Initiation Protocol [RFC3261]).

   The technique generates a packet addressed to the expected client.
   This increases the vulnerability of the DCCP server, by revealing
   which ports are in a passive listening state (the information is not
   encrypted and therefore could be seen on the path to the client
   through the network).

   Servers that do not wish to disclose this information may refrain
   from generating DCCP-Listen packets, without impacting subsequent
   DCCP state transitions.

   This document requires endpoint nodes to ignore reception of DCCP-
   Listen packets (in any state other than LISTEN).

   We do not believe these changes significantly increase the complexity
   or vulnerability of a DCCP implementation that conforms to [RFC4340].

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5.  IANA Considerations

   This document requires IANA action by allocation of a new Packet Type
   from the IANA DCCP Packet Types Registry.  The name of the Packet
   Type is the "DCCP-Listen" packet, and its type field is set to XX-

   The Registry entry is to reference this document.

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Note to the RFC Editor:

   Please replace XX-IANA-assigned-XX throughout this document with the
   value assigned in the registry and remove this note.

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6.  References

6.1.  Normative References

   [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.

6.2.  Informative References

   [Epp05]    Eppinger, J-L., "TCP Connections for P2P Apps: A Software
              Approach to Solving the NAT Problem", Carnegie Mellon
              University/ISRI Technical Report CMU-ISRI-05-104,
              January 2005.

   [FSK05]    Ford, B., Srisuresh, P., and D. Kegel, "Peer-to-Peer
              Communication Across Network Address Translators",
              Proceedings of USENIX-05, pages 179-192, 2005.

   [Fai08]    Fairhurst, G., "The DCCP Service Code", Work In
              Progress, draft-ietf-dccp-serv-codes-06, June 2008.

   [GBF+07]   Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", Work In
              Progress, draft-ietf-behave-tcp-07, April 2007.

   [GF05]     Guha, S. and P. Francis, "Characterization and Measurement
              of TCP Traversal through NATs and Firewalls", Proceedings
              of Internet Measurement Conference (IMC-05), pages 199-
              211, 2005.

   [GTF04]    Guha, S., Takeda, Y., and P. Francis, "NUTSS: A SIP based
              approach to UDP and TCP connectivity", Proceedings of
              SIGCOMM-04 Workshops, Portland, OR, pages 43-48, 2004.

   [H.323]    ITU-T, "Packet-based Multimedia Communications Systems",
              Recommendation H.323, July 2003.

   [NAT-APP]  Ford, B., Srisuresh, P., and D. Kegel, "Application Design
              Guidelines for Traversal through Network Address
              Translators", Work In Progress, draft-ford-behave-app-05,
              March 2007.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

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   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC3235]  Senie, D., "Network Address Translator (NAT)-Friendly
              Application Design Guidelines", RFC 3235, January 2002.

   [RFC3261]  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.

   [Ros08]    Rosenberg, J., "TCP Candidates with Interactive
              Connectivity Establishment (ICE)", Work In
              Progress, draft-ietf-mmusic-ice-tcp-06, February 2008.

   [TURN]     Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", Work In
              Progress, draft-ietf-behave-turn-07, February 2008.

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Appendix A.  Discussion of Existing NAT Traversal Techniques

   This Appendix is a brief review of existing techniques to establish
   connectivity across NAT devices, with the aim of providing background

   We first consider TCP NAT traversal based on simultaneous-open, and
   then discuss a second technique based on role reversal.  Further
   information can be found in [GTF04] and [GF05].

   A central idea shared by these techniques is to make peer-to-peer
   sessions look like "outbound" sessions on each NAT device.

   Often a rendezvous server, located in the public address realm, is
   used to enable clients to discover their NAT topology and the
   addresses of peers.

   The term 'hole punching' was coined in [FSK05] and refers to creating
   soft state in a traditional NAT device by initiating an outbound
   connection.  A well-behaved NAT can subsequently exploit this to
   allow a reverse connection back to the host in the private address

   UDP and TCP hole punching use nearly the same technique.  The
   adaptation of the basic UDP hole punching principle to TCP NAT
   traversal was introduced in section 4 of [FSK05] and relies on the
   simultaneous-open feature of TCP [RFC0793].  A further difference
   between UDP and TCP lies in the way the clients perform connectivity
   checks, after obtaining suitable address pairs for connection
   establishment.  Whereas in UDP a single socket is sufficient, TCP
   clients require several sockets for the same address / port tuple:

   o  a passive socket to listen for connectivity tests from peers and

   o  multiple active connections from the same address to test
      reachability of other peers.

   The SYN sent out by client A to its peer B creates soft state in A's
   NAT.  At the same time, B tries to connect to A:

   o  if the SYN from B has left B's NAT before the arrival of A's SYN,
      both endpoints perform simultaneous-open (4-way handshake of SYN/

   o  otherwise A's SYN may not enter B's NAT, which leads to B
      performing a normal open (SYN_SENT => ESTABLISHED) and A
      performing a simultaneous-open (SYN_SENT => SYN_RCVD =>

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   In the latter case it is necessary that the NAT does not interfere
   with a RST segment (REQ-4 in [GBF+07]).  The simultaneous-open
   solution is convenient due to its simplicity, and is thus a preferred
   mode of operation in the TCP extension for ICE ([Ros08], sec. 2).

A.1.  NAT traversal Based on Simultaneous-Open

   Among the various TCP NAT traversal approaches, simultaneous-open
   suggests itself due to its simplicity [GF05], [NAT-APP].

   A characteristic of simultaneous-open is that the clear distinction
   between client and server is erased: both sides enter through active
   (SYN_SENT) as well as passive (SYN_RCVD) states.  This characteristic
   is in conflict with several ideas underlying DCCP, as a clear
   separation between client and server has been one of the initial
   design decisions ([RFC4340], 4.6).  Furthermore, several mechanisms
   implicitly rely on clearly-defined client/server roles:

   o  Feature Negotiation: with few exceptions, almost all of DCCP's
      negotiable features use the "server-priority" reconciliation rule
      ([RFC4340], 6.3.1), whereby peers exchange their preference lists
      of feature values, and the server decides the outcome.

   o  Closing States: only servers may generate CloseReq packets (asking
      the peer to hold timewait state), while clients are only permitted
      to send Close or Reset packets to terminate a connection
      ([RFC4340], 8.3).

   o  Service Codes: servers may be associated with multiple service
      codes, while clients must be associated with exactly one
      ([RFC4340], 8.1.2).

   o  Init Cookies: may only be used by the server and on DCCP-Response
      packets ([RFC4340], 8.1.4).

   The latter two points are not obstacles per se, but hinder the
   transition from a passive to an active socket.  The assumption that
   "all DCCP hosts are clients", on the other hand, must be dismissed
   since it limits application programming.  As a consequence, retro-
   fitting simultaneous-open into DCCP does not seem a very sensible

A.2.  Role Reversal

   After the simultaneous-open, one of the simplest TCP NAT traversal
   schemes involves role traversal ([Epp05] and [GTF04]), where a peer
   first opens an active connection for the single purpose of punching a
   hole in the firewall; and then reverts to a listening socket,

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   accepting connections arriving via the new path.

   This solution has several disadvantages for DCCP.  First, a DCCP
   server would be required to change its role temporarily to 'client'.
   This requires modification of settings, in particular service codes
   and perhaps Init Cookies.

   Further, the the server must not yet have started feature
   negotiation, since its choice of initial options may rely on its role
   (i.e. if an endpoint knows it is the server, it can make a priori
   assumptions about the preference lists of features it is negotiating
   with the client, thereby enforcing a particular policy).

   Lastly, the server needs additional processing to ensure that the
   connection coming through the listening socket matches the one for
   which it previously opened an active connection.

   We therefore do not recommend this approach for DCCP.

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Appendix B.  Change Log

   Revision 00 retrieved from previous individual submission
   draft-fairhurst-dccp-behave-update-01 by the same authors.

   Revision 01:

   o  introduced many format changes to improve readability

   o  migrated background information into the Appendix

   o  added Section 1.3 to summarize the document structure

   o  updated introductory paragraph of Section 2 to account for new

   o  added captions to all figures

   o  updated the specification in Section 2 to (i) permit options on
      DCCP-Listen packets; (ii) explain why the presence of payload data
      is not useful; (iii) clarify that middleboxes must not interpret
      sequence numbers on DCCP-Listen packets

   o  clarified that the default value of the Allow Short Seqno feature
      is to be used

   o  added references to the service code draft [Fai08]

   o  clarified the processing of DCCP-Listen packets by server

   o  corrected the reaction of a client implementing [RFC4340] only -
      DCCP-Listen packets are treated as unknown and hence do not
      generate a Reset

   o  swapped order of IANA / Security-Considerations sections

   o  added a note in the Security Considerations section that servers
      may refrain from generating DCCP-Listen packets

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Note to the RFC Editor:

   Please remove this Change Log when done with the document.

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Authors' Addresses

   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen  AB24 3UE

   Email: gorry@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk

   Gerrit Renker
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen  AB24 3UE

   Email: gerrit@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk

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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

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