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

For historical reasons, NAT devices are not typically capable of handling datagrams and flows for applications using the Datagram Congestion Control Protocol (DCCP). This draft 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 are inherited here. The requirements are otherwise designed similarly to those in , 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 . Also, a separate unspecified mechanism may be needed, such as Unilateral Self Address Fixing (UNSAF) protocols, if an endpoint needs to learn its own external NAT mappings.

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 . This documentation 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 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 .

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 . 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: the DCCP source port, for outgoing packets, depending on the NAT mapping the DCCP destination port, for incoming packets, depending on the NAT mapping 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 a "generic" transport protocol support, whereby only the IP header is mangled. That scheme is not sufficient to support DCCP.

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

REQ-2: A NAT MUST support all valid sequences of DCCP packets (defined in 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 . 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.

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. The filtering behavior MAY be an option configurable by the administrator of the NAT. The filtering behavior for DCCP MAY be independent of the filtering behavior for any other transport-layer protocol, such as UDP, UDP-Lite, TCP, SCTP. 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.

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 TIMEWAIT state is left unspecified.

Contrary to TCP, DCCP is a loss-tolerant protocol. Therefore, modifying the payload of DCCP packets may present a significant additional challenge in maintaining sane any application-layer state needed for an ALG to function. Additionally, there are no known DCCP-capable Application Level Gateways (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 .

A list of general and UDP specific NAT behavioral requirements are described in . A list of ICMP specific NAT behavioral requirements are described in . The requirements listed below reiterate the requirements from these two documents that directly affect DCCP. The following requirements do not relax any requirements in or .

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

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".

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.

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 . 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.

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 . 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 .

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

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 and . Similar to , REQ-4 of this document recommends a NAT to 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 attack 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.

This document raises no IANA considerations.

The author would like to thank Gorry Fairhurst, Eddie Kohler, Dan Wing, Alfred Hönes, 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 draft-ietf-behave-tcp-07, by S. Guha (editor), K. Biswas, B. Ford, S. Sivakumar and P. Srisuresh.