DCCP NAT requirements: Network Address Translation (NAT) Behavioral Requirements for the Datagram Congestion Control Protocol

Network Working Group                                  R. Denis-Courmont
Request for Comments: 5597                              VideoLAN project
BCP: 150                                                  September 2009
Category: Best Current Practice


   Network Address Translation (NAT) Behavioral Requirements for the
                  Datagram Congestion Control Protocol

Abstract

   This document defines a set of requirements for NATs handling the
   Datagram Congestion Control Protocol (DCCP).  These requirements
   allow DCCP applications, such as streaming applications, to operate
   consistently, and they are very similar to the TCP requirements for
   NATs, which have already been published by the IETF.  Ensuring that
   NATs meet this set of requirements will greatly increase the
   likelihood that applications using DCCP will function properly.

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 2
   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 . . . . . . . . . . . . . 5
   8.  Requirements Specific to DCCP . . . . . . . . . . . . . . . . . 6
   9.  DCCP without NAT Support  . . . . . . . . . . . . . . . . . . . 7
   10. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.  Introduction

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

   This memo discusses the technical issues involved and proposes a set
   of requirements for NAT devices to handle DCCP in a way that enables
   communications 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 [RFC5382], 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 for DCCP of asymmetric connection will not work.  A
   simultaneous-open must be performed, as in [RFC5596].  Also, a
   separate, unspecified mechanism may be needed, such as Unilateral
   Self Address Fixing (UNSAF) [RFC3424] protocols, if an endpoint needs
   to learn its own external NAT mappings.

2.  Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This document uses the term "DCCP connection" to refer to individual
   DCCP flows, as uniquely identified by the quadruple (source and
   destination IP addresses and DCCP ports) at a given time.

   This document uses the term "NAT mapping" to refer to a state at the
   NAT that is 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
   similar to those applicable to TCP connections (with the exception of
   REQ-6 below).  The following requirements are discussed and justified
   extensively in [RFC5382].  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
      mapping,

   o  the DCCP destination port for incoming packets, depending on the
      NAT mapping, and

   o  the DCCP checksum, to compensate for IP address and port number
      modifications.

   Because changing 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 "generic" transport-protocol support,
   whereby only the IP header is mangled.  That scheme is not sufficient
   to support DCCP.

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 is 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
   DCCP.

4.2.  Established Connections

   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, defined in
   [RFC5596].  That mode updates DCCP by adding a new packet type: DCCP-
   Listen.  The DCCP-Listen packet communicates the information
   necessary to uniquely identify a DCCP session.  NATs may utilise the
   connection information (address, port, Service Code) to establish
   local forwarding state.

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 for DCCP.

   o  The filtering behavior MAY be an option configurable by the
      administrator of the NAT.

   o  The filtering behavior for DCCP MAY be independent of the
      filtering behavior for any other transport-layer protocol, such as
      UDP, UDP-Lite, TCP, and SCTP (Stream Control Transmission
      Protocol).

   REQ-4: A NAT MUST wait for at least 6 seconds from the reception of
   an unsolicited, inbound DCCP-Listen or DCCP-Sync packet before it may
   respond with an ICMP Port Unreachable error, an ICMP Protocol
   Unreachable error, or a DCCP-Reset.  If, during this interval, the
   NAT receives and translates an outbound DCCP-Request packet for the
   connection, the NAT MUST silently drop the original unsolicited,
   inbound DCCP-Listen packet.  Otherwise, the NAT SHOULD send an ICMP
   Port Unreachable error (Type 3, Code 3) for the original DCCP-Listen
   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-
   timeout".

   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.  Where a NAT implements session timeouts, the default
   value of the "established connection idle-timeout" MUST be of 124
   minutes or longer, and the default value of the "transitory
   connection idle-timeout" MUST be of 4 minutes or longer.  A NAT that
   implements session timeouts may be configurable to use smaller values
   for the NAT idle-timeouts.

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

6.  Application-Level Gateways

   Contrary to TCP, DCCP is a loss-tolerant protocol.  Therefore,
   modifying the payload of DCCP packets may present a significant
   additional challenge in maintaining any application-layer state
   needed for an Application Level Gateway (ALG) to function properly.
   Additionally, there are no known DCCP-capable ALGs at the time of
   writing this document.

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

   NOTE: This is not consistent with REQ-6 of [RFC5382].

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 [RFC5508].  The requirements listed
   below reiterate the requirements from these two documents that
   directly affect DCCP.  The following requirements do not relax any
   requirements in [RFC4787] or [RFC5508].

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.  Furthermore, 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.  Requirements Specific to DCCP

8.1.  Partial Checksum Coverage

   DCCP supports partial checksum coverage.  A NAT will usually need to
   perform incremental changes to the packet Checksum field, as for
   other IETF-defined protocols.  However, if it needs to recalculate a
   correct checksum value, it must take the checksum coverage into
   account, as described in Section 9.2 of [RFC4340].

   REQ-11: If a NAT translates a DCCP packet with a valid DCCP checksum,
   it MUST ensure that the DCCP checksum is translated such that it is
   valid after the translation.

   REQ-12: A NAT MUST NOT modify the value of the DCCP Checksum
   Coverage.

   The Checksum Coverage field in the DCCP header determines the parts
   of the packet that are covered by the Checksum field.  This always
   includes the DCCP header and options, but some or all of the
   application data may be excluded as determined on a packet-by-packet
   basis by the application.  Changing the Checksum Coverage in the
   network violates the integrity assumptions at the receiver and may
   result in unpredictable or incorrect application behaviour.

8.2.  Services Codes

   DCCP specifies a Service Code as a 4-byte value (32 bits) that
   describes the application-level service to which a client application
   wishes to connect [RFC4340].

   REQ-13: If a NAT translates a DCCP packet, it MUST NOT modify its
   DCCP Service Code value.

   Further guidance on the use of Service Codes by middleboxes,
   including NATs, can be found in [RFC5595].

9.  DCCP without NAT Support

   If the NAT device cannot be updated to support DCCP, DCCP datagrams
   can be encapsulated within a UDP transport header.  Indeed, most NAT
   devices are already capable of handling UDP.  This is however beyond
   the scope of this document.

10.  Security Considerations

   [RFC4787] discusses security considerations for NATs that handle IP
   and unicast (UDP) traffic, all of which apply equally to this
   document.  Security concerns specific to handling DCCP packets are
   discussed in this section.

   REQ-1 and REQ-6 through REQ-13 do not introduce any new known
   security concerns.

   REQ-2 does not introduce any new known security concerns.  While a
   NAT may elect to keep track of some DCCP-specific, per-flow state
   (compared to UDP), it has no obligations to do so.

   REQ-3 allows a NAT to adopt either a more secure or a more
   application-transparent filtering policy.  This is already addressed
   in [RFC4787] and [RFC5382].

   Similar to [RFC5382], REQ-4 of this document recommends that a NAT
   respond to unsolicited, inbound Listen and Sync packets with an ICMP
   error delayed by a few seconds.  Doing so may reveal the presence of
   a NAT to an external attacker.  Silently dropping the Listen makes it
   harder to diagnose network problems and forces applications to wait
   for the DCCP stack to finish several retransmissions before reporting
   an error.  An implementer must therefore understand and carefully
   weigh the effects of not sending an ICMP error or rate-limiting such
   ICMP errors to a very small number.

   REQ-5 recommends that a NAT that passively monitors DCCP state keep
   idle sessions alive for at least 124 minutes or 4 minutes, depending
   on the state of the connection.  To protect against denial-of-service
   attacks filling its state storage capacity, a NAT may attempt to
   actively determine the liveliness of a DCCP connection, or the NAT
   administrator could configure more conservative timeouts.

11.  Acknowledgments

   The author would like to thank Gorry Fairhurst, Eddie Kohler, Dan
   Wing, Alfred Hoenes, Magnus Westerlund, Miguel Garcia, Catherine
   Meadows, Tim Polk, Lars Eggert, and Christian Vogt for their comments
   and help on this document.

   This memo borrows heavily from [RFC5382] by S. Guha (editor), K.
   Biswas, B. Ford, S. Sivakumar, and P. Srisuresh.

12.  References

12.1.  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.

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

   [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
              Behavioral Requirements for ICMP", BCP 148, RFC 5508,
              April 2009.

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

12.2.  Informative References

   [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
              Self-Address Fixing (UNSAF) Across Network Address
              Translation", RFC 3424, November 2002.

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

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

Author's Address

   Remi Denis-Courmont
   VideoLAN project

   EMail: rem@videolan.org
   URI:   http://www.videolan.org/