DCCP Working Group                                          G. Fairhurst
Internet-Draft                                    University of Aberdeen
Updates: 4340 (if approved)                                  Oct 4, 2008
Intended status: Standards Track
Expires: April 7, 2009

 DCCP Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal

Status of this Memo

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   This document specifies an update to the Datagram Congestion Control
   Protocol (DCCP), a connection-oriented and datagram-based transport
   protocol.  The update adds support for the DCCP-Listen packet.  This
   assists DCCP applications to communicate through middleboxes (e.g. a
   DCCP server behind a firewall, or Network Address Port Translators),
   where establishing necessary middlebox state requires peering
   endpoints to initiate communication in a near-simultaneous manner.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Scope of this Document . . . . . . . . . . . . . . . . . .  3
     1.2.  DCCP NAT Traversal . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Structure of this Document . . . . . . . . . . . . . . . .  4
   2.  Procedure for Near-Simultaneous Open . . . . . . . . . . . . .  5
     2.1.  Conventions and Terminology  . . . . . . . . . . . . . . .  5
     2.2.  Protocol Method  . . . . . . . . . . . . . . . . . . . . .  5
       2.2.1.  Protocol Method for DCCP-Client Endpoints  . . . . . .  5
       2.2.2.  Processing by Routers and Middleboxes  . . . . . . . .  5
       2.2.3.  DCCP-Listen Packet Format  . . . . . . . . . . . . . .  6
     2.3.  Protocol Method for DCCP-Server Endpoints  . . . . . . . .  8
     2.4.  Examples of Use  . . . . . . . . . . . . . . . . . . . . . 10
     2.5.  Backwards Compatibility with RFC 4340  . . . . . . . . . . 11
   3.  Discussion of Design Decisions . . . . . . . . . . . . . . . . 13
     3.1.  Rationale for a New Packet Type  . . . . . . . . . . . . . 13
       3.1.1.  Use of sequence numbers  . . . . . . . . . . . . . . . 14
     3.2.  Generation of Listen Packets . . . . . . . . . . . . . . . 14
     3.3.  Repetition of DCCP-Listen Packets  . . . . . . . . . . . . 14
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Appendix A.  Discussion of Existing NAT Traversal Techniques . . . 22
     A.1.  NAT traversal Based on a Simultaneous-Request  . . . . . . 23
     A.2.  Role Reversal  . . . . . . . . . . . . . . . . . . . . . . 23
   Appendix B.  Change Log  . . . . . . . . . . . . . . . . . . . . . 25
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 28
   Intellectual Property and Copyright Statements . . . . . . . . . . 29

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1.  Introduction

   DCCP [RFC4340] is both datagram-based and connection-oriented.  As
   such, it faces the same problems as TCP in sending packets through
   devices that require all connections to be initiated by a 'trusted'
   host.  This is true of host-based firewalls, the default policy on
   many firewalls, and (due to port overloading) Network Address Port
   Translators, NAPTs [ID-Behave-DCCP].  DCCP can not simply reuse
   traversal solutions that work for UDP.  In addition, the original
   specification of DCCP did not allow a server to perform a
   simultaneous-open, an inherent characteristic of TCP that greatly
   simplifies TCP Network Address Translator (NAT) traversal.

   After discussing the problem space for DCCP, this document specifies
   an update to the DCCP state machine to offer native DCCP support for
   middlebox traversal.  This reduces dependence on external aids such
   as data relay servers ) [TURN] by explicitly supporting a widely used
   principle known as 'hole punching'.

   The method requires only a minor change to the standard DCCP
   operational procedure.  The use of a dedicated DCCP packet type ties
   usage to a specific condition, ensuring the method is inter-operable
   with hosts that do not implement this update, or disable it (see
   Section 4).

1.1.  Scope of this Document

   This method is useful in scenarios when a DCCP server is located
   behind a middlebox.  It is relevant to both client/server and peer-
   to-peer applications, such as VoIP, file sharing, or online gaming
   and 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],

1.2.  DCCP NAT Traversal

   The behavioural requirements for NAT devices supporting DCCP are
   described in [ID-Behave-DCCP].  A "traditional NAT" [RFC3022], that
   directly maps an IP address to a different IP address does not
   require the simultaneous open method described in this document.

   The method is required when the DCCP server is positioned behind one
   or more NAT devices in the path (i.e. hierarchies of nested NAT
   devices are possible).  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"

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   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 possible using the protocol defined in
   [RFC4340].  A pre-configured, static NAT address map would allow
   outside hosts to connect to the private network in cases (2) and (3).

   This document describes a method to support cases (2) and (3) that
   require NAT traversal techniques.  A DCCP implementation conforming
   to [RFC4340] and a NAT device conforming to [ID-Behave-DCCP] would
   require a DCCP relay server to perform NAT traversal for cases (2)
   and (3).

   The document updates RFC 4340 to enable DCCP NAT traversal without
   the aid of DCCP relay servers.  This method requires the DCCP server
   to discover the IP address and the DCCP port that correspond to the
   DCCP Client.  Such signalling may be performed out-of-band (e.g.
   using SDP [RFC4566]).

1.3.  Structure of this Document

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

   The normative specification of the update is presented in Section 2.
   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.  It 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.  Protocol Method

   The term "session" is used 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].

2.2.1.  Protocol Method for DCCP-Client Endpoints

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

   A client in any state MUST silently discard any received DCCP-Listen

2.2.2.  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.  The DCCP-Listen
   packet carries the necessary information to uniquely identify a DCCP
   session in combination with the source and destination addresses

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   (found in the enclosing IP-header), including the DCCP Service Code
   value.  The processing of the DCCP-Listen packet by NAT devices is
   specified in [ID-Behave-DCCP].

2.2.3.  DCCP-Listen Packet Format

   This document adds a new DCCP 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: Format of a DCCP-Listen Packet

   o  The Source Port is the port on which the server is listening for a
      connection from the IP address that appears as the destination IP
      address in the packet.

   o  The default value of 0 for the Allow Short Seqno feature MUST be
      used, X MUST be set to 1, and DCCP-Listen packets with X=0 MUST be
      ignored.  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.

   o  The sequence number of a DCCP-Listen packet is not related to the
      DCCP sequence number for normal DCCP messages Section 3.  Thus,
      for DCCP-Listen packets:

      *  DCCP servers should set both Sequence Number fields to 0.

      *  DCCP clients MUST ignore the value of the Sequence Number

      *  Middleboxes MUST NOT interpret sequence numbers on DCCP-Listen

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   o  The Service Code field contains the service code value for which
      the server is listening for a connections([RFC4340], 8.1.2).  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
      specific 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. described using the Session Description
      Protocol, SDP [RFC4566].

   o  At the time of writing, there are no known uses of the header
      option ([RFC4340] , sec. 5.8) with a DCCP-Listen packet.  Clients
      MUST ignore all options in received DCCP-Listen packets.
      Therefore, option values can not be negotiated using a DCCP-Listen

   o  There is no payload data.  Since DCCP-Listen packets are issued
      before an actual connection is established, they MUST NOT carry
      payload data.  Endpoints MUST ignore any payload data encountered
      in DCCP-Listen packets.  In addition, DCCP clients MUST ignore the
      value of the Sequence Number fields; and middleboxes MUST NOT
      interpret the sequence numbers of DCCP-Listen packets.

   o  The following protocol fields are required to have specific

      *  Data Offset MUST be zero (there is no payload).

      *  CCVal MUST be zero (a connection has not been established).

      *  CsCov MUST be zero (there is no payload).

      *  Type has the value 10 (assigned by IANA to denote a DCCP-Listen

      *  X MUST be 1 (the Generic header must be used).

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

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Note to the RFC Editor:

   This value assigned to the DCCP-Listen packet needs to be confirmed
   by IANA when this document is published.  Please then remove this

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

2.3.  Protocol Method for DCCP-Server Endpoints

   This document updates [RFC4340] for the case of a fully specified
   DCCP server endpoint.  The update modifies the way the server
   performs a 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
   (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 a single 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

           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

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        Figure 2: Updated state transition diagram for DCCP-Listen

   A fully specified server endpoint performs a passive-open from the
   CLOSED state by inviting the remote client to connect.  This is
   performed by sending a single DCCP-Listen packet to the specified
   remote IP-adress:port, using the format specified in Section 2.2.3.
   The server then transitions to the INVITED state.

   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 the
   INVITED state, at a 200 millisecond interval and up to at most 2
   retransmissions (Section 3 discusses this choice of time interval).
   If the server is still in the INVITED state after a further period of
   200ms following transmission of the third DCCP-Listen packet, it
   SHOULD progress to LISTEN, and resume processing as specified in
       DCCP A                           DCCP B
       ------  NA     NB                ------
       +----+  +-+    +-+  +-----------------+
       | -->+--+-+----+-+--+--> SDP          |
       |    |  | |    | |  |                 | State = INVITED
       |    |  | |    | |  |                 |
       |    |<-+-+----+-+--+<-- DCCP-Listen  |     Timer Starts
       |    |  | |    | |  |                 |          |
       |    |  | |    | |  |                 |     1st Timer Expiry
       |    |  | |    | |  |                 |
       |    |<-+-+----+++--+<-- DCCP-Listen  |     Timer Starts
       |    |  | |    | |  |                 |          |
       |    |  | |    | |  |                 |     2nd Timer Expiry
       |    |  | |    | |  |                 |
       |    |<-+-+----+-+--+<-- DCCP-Listen  |     Timer Starts
       |    |  | |    | |  |                 |          |
       |    |  | |    | |  |                 |     3rd Timer Expiry
       |    |  | |    | |  |                 |
       |    |  | |    | |  |                 | State = LISTEN
       ~    ~  ~ ~    ~ ~  ~                 ~
       |    |  | |    | |  |                 |
       | -->+--+-+----+-+--+--> DCCP-Request |
       |    |  | |    | |  |                 | State = RESPOND
       | <--+--+-+----+-+--+<-- DCCP-Response|
       +----+  +-+    +-+  +-----------------+

                 Figure 3: Retransmission of Listen Packet

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   The diagram above shows the sequence of packets where the DCCP server
   enters the INVITED state after reception of out-of-band signaling
   (SDP).  It considers the case when the server does not receive a
   DCCP-Request within the first 600 ms (often the request would be
   received within this interval).  DCCP A is the client and DCCP B is
   the server, and NA and NB are middlebox devices.  Retransmission is
   implemented using a timer.  The timer is restarted with an interval
   of 200ms when sending each DCCP-Listen packet.  It is cancelled when
   the server leaves the INVITED state.  If the timer expires after the
   first and second transmission, it triggers a retransmission of the
   DCCP-Listen Packet.  If it expires after sending the third DCCP-
   Listen packet, the server leaves the INVITED state, to enter the
   LISTEN state (where it passively waits for a DCCP-Request).

   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.  Reception of an ICMP error message as
   a result of sending a DCCP-Listen packet does not necessarily
   indicate a failure of the following connection request, and therefore
   should not result in a server state change.  This reaction to soft
   errors exploits the valuable feature of the Internet that for many
   network failures, the network can be dynamically reconstructed
   without any disruption of the endpoints.

   Server endpoints SHOULD ignore any incoming DCCP-Listen packets.  A
   DCCP Server in state LISTEN MAY generate a DCCP-Reset packet (Code 7,
   "Connection Refused") in response to a received DCCP-Listen packet.
   This DCCP-Reset packet is an indication that two servers are
   simultaneously awaiting connections on the same port.

   Further details on the design rationale are discussed in Section 3.

2.4.  Examples of Use

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

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

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          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 4: 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 5: Event sequence when the server is started before the client

2.5.  Backwards Compatibility with RFC 4340

   No changes are required if a DCCP Client conforming to this document
   communicates with a DCCP Server conforming to [RFC4340].

   If a client implements only [RFC4340], an incoming DCCP-Listen packet
   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.

   This document further does not modify communication for any DCCP
   server that implements a passive-open without fully binding the

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   addresses, ports and service codes to be used.  The authors therefore
   do not expect practical deployment problems with existing conformant
   DCCP implementations.

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3.  Discussion of Design Decisions

3.1.  Rationale for a New Packet Type

   This is an informative section that reviews the rationale for the
   design of this technique.  The DCCP-Listen packet specified in
   Section 2.2.3 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.

   A DCCP-Request packet could in theory also have been used for the
   same purpose.  The following arguments were against this:

   The first problem was that of semantic overloading: the DCCP-Request
   defined in [RFC4340] serves a well-defined purpose, being the initial
   packet of the 3-way handshake.  Additional use in the manner of a
   DCCP-Listen packet would have required DCCP processors to have had
   two different processing paths: one where a DCCP-Request was
   interpreted as part of the initial handshake, and another where the
   same packet was interpreted as an indicator message.  This would
   complicate 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 DCCP-Reset packet if it had not yet entered
   the REQUEST state (step 7 in [RFC4340], 8.5).  The method specified
   in this document lets client endpoints ignore DCCP-Listen packets.
   Adding a similar rule for the DCCP-Request packet would have been
   cumbersome: clients would not have been able to 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
   indication 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 DCCP-
   Sync.  Since sequence numbers are ignored for this type of message,
   additional and complex processing would become necessary: either to
   ask the client not to respond to a DCCP-Request when the request is
   of type "indicator message"; or ask middleboxes and servers to ignore
   Sync packets generated in response to "indicator message" DCCP-
   Requests.  Furthermore, since no initial sequence numbers have been
   negotiated at this stage, sending a DCCP-SyncAck would not be

   The use of a separate packet type therefore allows simpler and

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   clearer processing.

3.1.1.  Use of sequence numbers

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

   DCCP assigns a random initial value to the sequence number when a
   DCCP connection is established [RFC4340].  Howeve, a sender is
   required to set this value to zero for a DCCP-Listen packet.  Both
   clients and middleboxes are also required to ignore this value.

   The rationale for ignoring the Sequence Number fields on DCCP-Listen
   packets is that at the time the DCCP-Listen is exchanged, the
   endpoints have not yet entered connection setup: the DCCP-Listen
   packet is sent 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.

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) to ensure this does 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],
   3.8).  Unlike TCP, this update does not perform a role-change from
   passive to active.  Like TCP, DCCP-Listen packets are only sent by a
   DCCP-server when the endpoint is fully specified (Section 2.2).

3.3.  Repetition of DCCP-Listen Packets

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

   The decision to recommend a maximum number of 3 timeouts (2 repeated
   copies of the original DCCP-Listen packet) results from the following
   considerations: The repeated copies need to be spaced sufficiently
   far apart in time to avoid suffering from correlated loss.  The
   interval of 200 ms was chosen to accommodate a wide range of wireless
   and wired network paths.

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   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
   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 allowiing connection establishment to
   successfully complete.

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4.  Security Considerations

   The method specified in this document generates a DCCP-Listen packet
   addressed to a specific DCCP client.  This exposes the state of a
   DCCP server that is in a passive listening state (i.e. waiting to
   accept a connection from a known client).

   The exposed information is not encrypted and therefore could be seen
   on the network path to the DCCP client.  An attacker on this return
   path could observe a DCCP-Listen packet and then exploit this by
   spoofing a packet (e.g.  DCCP-Request, DCCP-Reset) with the IP
   addresses, DCCP ports, and service code that correspond to the values
   to be used for the connection.  As in other on-path attacks, this
   could be used to inject data into a connection or to deny a
   connection request.  A similar on-path attack is also possible for
   any DCCP connection, once the session is initiated by the client
   ([RFC4340], Section 18).

   The DCCP-Listen packet is only sent in response to explicit prior
   out-of-band signaling from a DCCP client to the DCCP server (e.g.
   [RFC4566]) information communicated via the Session Initiation
   Protocol [RFC3261]), and will normally directly precede a DCCP-
   Request sent by the client (which carries the same information).

   This update does not significantly increase the complexity or
   vulnerability of a DCCP implementation that conforms to [RFC4340].  A
   server SHOULD therefore by default permit generation of DCCP-Listen
   packets.  A server that wishes to prevent disclosing this information
   MAY refrain from generating DCCP-Listen packets, without impacting
   subsequent DCCP state transitions, but possibly inhibiting middlebox

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5.  IANA Considerations

   The IANA should register a new Packet Type, "DCCP-Listen", in the
   IANA DCCP Packet Types Registry.  The decimal value 10 has been
   assigned to this types.  This registry entry must reference this

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Note to the RFC Editor:

   This value must be confirmed by IANA in the registry when this
   document is published, please then remove this note.

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   This update was originally co-authored by Dr Gerrit Renker,
   University of Aberdeen, and the present author acknowledges his
   insight in design of the protocol mechanism and in careful review of
   the early revisions of the document text.  Dan Wing assisted on
   issues relating to the use of NAT and NAPT.

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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-07, 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.

              "Network Address Translation (NAT) Behavioral Requirements
              for DCCP", Work in Progress draft-ietf-behave-dccp-02.txt,

   [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,

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              March 2007.

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

   [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.

   [RFC4566]  "SDP: Session Description Protocol", July 2006.

   [Ros08]    Rosenberg, J., "TCP Candidates with Interactive
              Connectivity Establishment (ICE)", Work In
              Progress, draft-ietf-mmusic-ice-tcp-07, 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-09, February 2008.

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Appendix A.  Discussion of Existing NAT Traversal Techniques

   This appendix provides a brief review of existing techniques to
   establish connectivity across NAT devices, with the aim of providing
   background information.  This first considers 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

   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

   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 =>

   In the latter case, it is necessary that the NAT does not interfere

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   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 a Simultaneous-Request

   Among the various TCP NAT traversal approaches, the one using
   simultaneous-open suggests itself as a candidate for DCCP due to its
   simplicity [GF05], [NAT-APP].

   A characteristic of TCP simultaneous-open is that this erases the
   clear distinction between client and server: both sides enter through
   active (SYN_SENT) as well as passive (SYN_RCVD) states. .  This
   characteristic conflicts with the DCCP design decision to provide a
   clear separation between client and server functions ([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 DCCP-CloseReq packets
      (asking the peer to hold timewait state), while clients are only
      permitted to send DCCP-Close or DCCP-Reset packets to terminate a
      connection ([RFC4340], 8.3).

   o  Service Codes [Fai08]: servers may be associated with multiple
      servi=ce 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 would have
   hindered the transition from a passive to an active socket.  In DCCP,
   a DCCP-Request is only generated by a client.  The assumption that
   "all DCCP hosts may be clients", was dismissed, since it would
   require undersirable changes to the state machine and would limit
   application programming.  As a consequence, the retro-fitting a TCP-
   style simultaneous-open into DCCP to allow simulatenous exchange of
   DCCP-Connect packets was not recommended.

A.2.  Role Reversal

   Another simple TCP NAT traversal schemes uses role traversal ([Epp05]
   and [GTF04]), where a peer first opens an active connection for the

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   single purpose of punching a hole in the firewall; and then reverts
   to a listening socket, accepting connections arriving via the new

   This solution would have had several disadvantages if used with DCCP.
   First, a DCCP server would be required to change its role to
   temporarily become a 'client'.  This would have required modification
   to the state machine, in particular the trearment of service codes
   and perhaps Init Cookies.  Further, the method must follow feature
   negotiation, since a host's choice of initial options can 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).
   Finally, the server would have needed additional processing to ensure
   that the connection arriving at the listening socket matches the
   previously opened active connection.

   This approach was therefore not recommend for DCCP.

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Appendix B.  Change Log

   Revision 00 was based on a 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 DCCP-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

   Revision 02:

   o  minor edits following WG feedback at IETF meeting

   o  updated to reflect ID.Behave-DCCP

   o  update to reflect comments from Colin Perkins

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   o  added a tentative IANA code point (as suggested at IETF-73)

   o  DRAFT -02

   o  Edits following editorial corrections and suggestions from Tom

   o  Edits following comments from Dan Wing on role of NAT,
      retransmision, and other issues.

   o  Revised authors list

   o  Reworded abstract, reworded appendices to clarify what was not

   o  Checked spelling

   o  Although this version includes significant changes to format and
      text it does not seek to modify the intended procedure for a


   o  DRAFT - 03

   o  Comments by Dan Wing

   o  DRAFT -04

   o  Corrections by Dan Wing, and new diagram Figure 3 to and text to
      clarify the retransmission algorithm.

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Note to the RFC Editor:

   Please remove this Change Log when done with the document.

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Author's Address

   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

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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

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