Behavior Engineering for Hindrance                     R. Denis-Courmont
Avoidance (if taken)                                    VideoLAN project
Internet-Draft                                         February 17, 2008
Intended status: Standards Track
Expires: August 20, 2008

   Network Address Translation (NAT) Behavioral Requirements for DCCP

Status of This Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   This document defines a set of requirements for NATs that handle

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Applicability statement . . . . . . . . . . . . . . . . . . . . 3
   4.  DCCP Connection Initiation  . . . . . . . . . . . . . . . . . . 4
   5.  NAT Session Refresh . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Application Level Gateways  . . . . . . . . . . . . . . . . . . 5
   7.  Other Requirements Applicable to DCCP . . . . . . . . . . . . . 6
   8.  DCCP without NAT support  . . . . . . . . . . . . . . . . . . . 6
   9.  DCCP simultaneous open  . . . . . . . . . . . . . . . . . . . . 7
   10. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     13.1.  Normative References . . . . . . . . . . . . . . . . . . . 7
     13.2.  Informative References . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   For historical reasons, NAT devices are not typically capable of
   handling datagrams and flows for application using the Datagram
   Congestion Control Protocol (DCCP)[RFC4340].

   This draft discusses the technical issues involved, and proposes a
   set of requirements for NAT devices to handle DCCP in a way that
   enables when either or both of the DCCP endpoints are located behind
   one or more NAT devices.  All definitions and requirements in
   [RFC4787] are inherited here.  The requirements are otherwise
   designed similarly to those in [I-D.ietf-behave-tcp], from which this
   memo borrows its structure and much of its content.

   Note however that, if both endpoints are hindered by NAT devices, the
   normal model of asymmetric connection model of DCCP will not work.  A
   simultaneous open must be performed, as in
   [I-D.fairhurst-dccp-behave-update].  Also, a separate unspecified
   mechanism may be needed, such as UNSAF protocols, if an endpoint
   needs to learn its own external NAT mappings.

2.  Definitions

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

   This documentation uses the term "DCCP connection" to refer to
   invidual DCCP flows, as uniquely identified by the the 4-tuple
   (source and destination IP addresses and DCCP ports) at a given time.

   This document uses the term "NAT mapping" to refer to state at the
   NAT necessary for network address and port translation of DCCP
   connections.  This document also uses the terms "endpoint independent
   mapping", "address dependent mapping", "address and port dependent
   mapping", "filtering behavior", "endpoint independent filtering",
   "address dependent filtering", "address and port dependent
   filtering", "port assignment", "port overloading", "hairpinning", and
   "external source IP address and port" as defined in [RFC4787].

3.  Applicability statement

   This document applies to NAT devices that want to handle DCCP
   datagrams.  It is not the intent of this document to deprecate the
   overwhelming majority of deployed NAT devices.  These NATs are simply
   not expected to handle DCCP, so this memo is not applicable to them.

   Expected NAT behaviors applicable to DCCP connections are very

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   similar to those applicable to TCP connections (with the exception or
   REQ-6 below).  The following requirements are discussed and justified
   extensively in [I-D.ietf-behave-tcp].  These justifications are not
   reproduced here for the sake of brevity.

   In addition to the usual changes to the IP header (in particular the
   IP addresses), NAT devices need to mangle:
   o  the DCCP source port, for outgoing packets, depending on the NAT
   o  the DCCP destination port, for incoming packets, depending on the
      NAT mapping
   o  the DCCP checksum, to compensate for IP address and port number

   Because changing the the source or destination IP address of a DCCP
   packet will normally invalidate the DCCP checksum, it is not possible
   to use DCCP through a NAT without dedicated support.  Some NAT
   devices are known to provide a "generic" transport protocol support,
   whereby only the IP header is mangled.  That scheme is not sufficient
   to support DCCP in any case.

4.  DCCP Connection Initiation

4.1.  Address and Port Mapping Behavior

   A NAT uses a mapping to translate packets for each DCCP connection.
   A mapping is dynamically allocated for connections initiated from the
   internal side, and potentially reused for certain subsequent
   connections.  NAT behavior regarding when a mapping can be reused
   differs for different NATs as described in [RFC4787].

   REQ-1: A NAT MUST have an "Endpoint Independent Mapping" behavior for

4.2.  Internally Initiated Connections

   FIXME/TBD: may change as DCCP simultaneous open progresses.

   REQ-2: A NAT MUST support all valid sequences of DCCP packets
   (defined in [RFC4340] and its updates) for connections initiated both
   internally as well as externally when the connection is permitted by
   the NAT.

   In particular, in addition to handling the DCCP 3-way handshake mode
   of connection initiation, A NAT MUST handle the DCCP simultaneous-
   open mode of connection initiation (FIXME: currently work-in-progress
   in the DCCP working group).

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4.3.  Externally Initiated Connections

   REQ-3: If application transparency is most important, it is
   RECOMMENDED that a NAT have an "Endpoint independent filtering"
   behavior for DCCP.  If a more stringent filtering behavior is most
   important, it is RECOMMENDED that a NAT have an "Address dependent
   filtering" behavior.
   o  The filtering behavior MAY be an option configurable by the
      administrator of the NAT.
   o  The filtering behavior for TCP MAY be independent of thefiltering
      behavior for UDP.

   REQ-4: A NAT MUST NOT respond to an unsolicited inbound DCCP-Request
   (TBD: add stuff for DCCP simultaneous open) packet for at least 6
   seconds after the packet is received.  If during this interval the
   NAT receives and translates an outbound DCCP packet (TBD: DCCP packet
   type) for the connection the NAT MUST silently drop the original
   unsolicited inbound DCCP-Request packet.  Otherwise the NAT SHOULD
   send an ICMP Port Unreachable error (Type 3, Code 3) for the original
   DCCP-Request, unless the security policy forbids it.

5.  NAT Session Refresh

   The "established connection idle-timeout" for a NAT is defined as the
   minimum time a DCCP connection in the established phase must remain
   idle before the NAT considers the associated session a candidate for
   removal.  The "transitory connection idle-timeout" for a NAT is
   defined as the minimum time a DCCP connection in the CLOSEREQ or
   CLOSING phases must remain idle before the NAT considers the
   associated session a candidate for removal.  DCCP connections in the
   TIMEWAIT state are not affected by the "transitory connection idle-

   REQ-5: If a NAT cannot determine whether the endpoints of a DCCP
   connection are active, it MAY abandon the session if it has been idle
   for some time.  In such cases, the value of the "established
   connection idle-timeout" MUST NOT be less than 2 hours 4 minutes.
   The value of the "transitory connection idle-timeout" MUST NOT be
   less than 4 minutes.  The value of the NAT idle-timeouts MAY be

   NAT behavior for handling DCCP-Reset packets, or connections in
   TIMEWAIT state is left unspecified.

6.  Application Level Gateways

   Contraty to TCP, DCCP is a loss-tolerant protocol.  Therefore,
   modifying the payload of DCCP packets present a significant

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   additionnal challenge in maintaining sane DCCP sequence numbers, if
   the size of the payload were altered.  Also, there are no known DCCP-
   capable Application Level Gateways (ALGs) at the time of writing this

   REQ-6: If a NAT includes ALGs, it MUST NOT affect DCCP.

   NOTE: This is not consistent with REQ-6 of [I-D.ietf-behave-tcp].

7.  Other Requirements Applicable to DCCP

   A list of general and UDP specific NAT behavioral requirements are
   described in [RFC4787].  A list of ICMP specific NAT behavioral
   requirements are described in [I-D.ietf-behave-nat-icmp].  The
   requirements listed below reiterate the requirements from these two
   documents that directly affect DCCP.  The following requirements do
   not relax anyrequirements in [RFC4787] or [I-D.ietf-behave-nat-icmp].

7.1.  Port Assignment

   REQ-7: A NAT MUST NOT have a "Port assignment" behavior of "Port
   overloading" for DCCP.

7.2.  Hairpinning Behavior

   REQ-8: A NAT MUST support "Hairpinning" for DCCP.  Futhermore, A
   NAT's Hairpinning behavior MUST be of type "External source IP
   address and port".

7.3.  ICMP Responses to DCCP Packets

   REQ-9: If a NAT translates DCCP, it SHOULD translate ICMP Destination
   Unreachable (Type 3) messages.

   REQ-10: Receipt of any sort of ICMP message MUST NOT terminate the
   NAT mapping or DCCP connection for which the ICMP was generated.

8.  DCCP without NAT support

   If the NAT device cannot be updated to support DCCP, DCCP datagram
   could be encapsulated within an additionnal UDP transport header.
   Indeed, most NAT devices are already capable of handling UDP.

   There are significant disadvantages to this approach:
   o  Both sides of the DCCP session need must be updated to use
      tunnelling, even though only one side might be hindered with a

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   o  A method MUST be defined to negociate when to use tunnelling.
   o  The per-packet overhead is increased.

   A DCCP transport-specific solution is specified by
   [I-D.phelan-dccp-natencap].  Alternatively, existing IP tunneling
   protocols, such as ESP-in-UDP[RFC3948] (especially with the NULL
   cipher suite) or Teredo[RFC4380], could be used.

9.  DCCP simultaneous open

   When both parties to an intended DCCP session are located behind
   either a NAT device or a stateful firewall, neither can act as the
   paassive endpoint in the connection establishment.

   Unfortunately, at the time of writing, the DCCP connection state
   machine does not allow both peers to behave as active endpoint, as is
   the case in TCP simultaneous open.  It is expected that this issue
   will be tackled in the DCCP working group shortly (TODO: reference
   relevant I-D).

10.  Security Considerations


11.  IANA Considerations

   This document raises no IANA considerations.

12.  Acknowledgments

   The author would like to thank ... for their comments on this

   This memo borrows heavily from draft-ietf-behave-tcp-07, by S. Guha
   (editor), K. Biswas, B. Ford, S. Sivakumar and P. Srisuresh.

13.  References

13.1.  Normative References

   [I-D.ietf-behave-nat-icmp]          Srisuresh, P., Ford, B.,
                                       Sivakumar, S., and S. Guha, "NAT
                                       Behavioral Requirements for ICMP
                                       (work in progress),
                                       February 2008.

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

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

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

13.2.  Informative References

   [I-D.fairhurst-dccp-behave-update]  Fairhurst, G. and G. Renker, "An
                                       Update for DCCP Connection
                                       Establishment to Assist NAT &
                                       Firewall  Traversal", draft-
                                       (work in progress),
                                       November 2007.

   [I-D.ietf-behave-tcp]               Guha, S., "NAT Behavioral
                                       Requirements for TCP",
                                       draft-ietf-behave-tcp-07 (work in
                                       progress), April 2007.

   [I-D.phelan-dccp-natencap]          Phelan, T., "Datagram Congestion
                                       Control Protocol (DCCP)
                                       Encapsulation for NAT Traversal
                                       (work in progress),
                                       February 2008.

   [RFC3948]                           Huttunen, A., Swander, B., Volpe,
                                       V., DiBurro, L., and M. Stenberg,
                                       "UDP Encapsulation of IPsec ESP
                                       Packets", RFC 3948, January 2005.

   [RFC4380]                           Huitema, C., "Teredo: Tunneling
                                       IPv6 over UDP through Network
                                       Address Translations (NATs)",
                                       RFC 4380, February 2006.

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Author's Address

   Remi Denis-Courmont
   VideoLAN project


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