DCCP NAT requirements: Network Address Translation (NAT) Behavioral Requirements for the Datagram Congestion Control Protocol
Thirteen rules that tell NATs how to carry DCCP connections so applications keep working.
Denis-Courmont, R., "Network Address Translation (NAT) Behavioral Requirements for the Datagram Congestion Control Protocol", BCP 150, RFC 5597, DOI 10.17487/RFC5597, September 2009, <https://www.rfc-editor.org/info/rfc5597>.
@misc{rfc5597,
series = {Request for Comments},
number = 5597,
howpublished = {RFC 5597},
publisher = {RFC Editor},
doi = {10.17487/RFC5597},
url = {https://www.rfc-editor.org/info/rfc5597},
author = {R. Denis-Courmont},
title = {{Network Address Translation (NAT) Behavioral Requirements for the Datagram Congestion Control Protocol}},
pagetotal = 9,
year = 2009,
month = sep,
} RFC text
Contents
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.
Copyright and License Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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/
ISO Plain Language
On this page
§1Introduction
For historical reasons, NAT devices are not typically capable of handling datagrams and flows for applications that use the Datagram Congestion Control Protocol (DCCP). 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 RFC 4787 are inherited here. The requirements are otherwise designed similarly to those in RFC 5382, from which this memo borrows its structure and much of its content.
The memo notes one important limit up front. If both endpoints are hindered by NAT devices, the normal model for DCCP of asymmetric connection will not work, and a simultaneous-open must be performed, as described in RFC 5596. Also, a separate, unspecified mechanism may be needed, such as Unilateral Self Address Fixing protocols, if an endpoint needs to learn its own external NAT mappings.
A NAT rewrites addresses and ports as traffic passes through, and it has to check the checksum that DCCP carries. Most NATs cannot do that, so DCCP applications such as streaming often fail to connect. This document lists what a NAT has to do if it wants to carry DCCP, and it is deliberately modelled on the rules the IETF already wrote for TCP.
- REQ-1 MappingThe mapping must not depend on the endpoint it talks to.
- REQ-2 HandshakeThe NAT must handle both the three-way handshake and the simultaneous open.
- REQ-3 FilteringEndpoint-independent filtering is recommended for application transparency.
- REQ-4 Delayed ICMPWait at least 6 seconds before answering an unsolicited inbound packet.
- REQ-5 TimeoutsKeep established sessions 124 minutes and transitory ones 4 minutes.
- REQ-6 to REQ-10No ALG changes, no port overloading, hairpinning must work, ICMP must not end a connection.
- REQ-11 to REQ-13Fix the checksum, leave the checksum coverage and the Service Code alone.
§3Applicability Statement
This document applies to NAT devices that want to handle DCCP datagrams. It is not the intent of the document to deprecate the overwhelming majority of deployed NAT devices: these NATs are simply not expected to handle DCCP, so the memo is not applicable to them.
Expected NAT behaviours applicable to DCCP connections are very similar to those applicable to TCP connections, with the exception of REQ-6. The requirements are discussed and justified extensively in RFC 5382, and those 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, and the DCCP checksum, to compensate for the 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, and that scheme is not sufficient to support DCCP.
If the NAT device cannot be updated to support DCCP, DCCP datagrams can be encapsulated within a UDP transport header, since most NAT devices are already capable of handling UDP. That is however beyond the scope of this document.
The rules apply only to NATs that choose to handle DCCP; NATs that do not are not broken by this document. A NAT that does handle DCCP has to change more than the IP header: the DCCP ports and the DCCP checksum change too, because the checksum covers the addresses and ports. Rewriting only the IP header is not enough.
| Field | What the NAT does |
|---|---|
| IP addresses | Changed, as for any other protocol |
| DCCP source port | Changed for outgoing packets, depending on the mapping |
| DCCP destination port | Changed for incoming packets, depending on the mapping |
| DCCP checksum | Changed to compensate for the address and port changes |
§4DCCP Connection Initiation
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 behaviour regarding when a mapping can be reused differs for different NATs, as described in RFC 4787.
REQ-1: a NAT MUST have an endpoint-independent mapping behaviour for DCCP.
REQ-2: a NAT MUST support all valid sequences of DCCP packets for connections initiated both internally and externally when the connection is permitted by the NAT. In addition to handling the DCCP three-way handshake mode of connection initiation, a NAT MUST handle the DCCP simultaneous-open mode of connection initiation, defined in RFC 5596. That mode updates DCCP by adding a new packet type, DCCP-Listen, which communicates the information necessary to uniquely identify a DCCP session. NATs may use the connection information, the address, port and 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 behaviour for DCCP. If a more stringent filtering behaviour is most important, it is RECOMMENDED that a NAT have an address-dependent filtering behaviour. The filtering behaviour MAY be an option configurable by the administrator of the NAT, and MAY be independent of the filtering behaviour for any other transport-layer protocol, such as UDP, UDP-Lite, TCP and 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.
Four rules cover the start of a connection. The mapping must not depend on the endpoint it talks to. The NAT must handle both ways of opening a DCCP connection, including the simultaneous open that works when both ends are behind NAT. Filtering should let any peer in, unless the operator wants something stricter. And an unsolicited packet must not be answered straight away: the NAT waits at least six seconds before sending an error, so that it does not cut off a connection that is still starting.
§5NAT 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 is 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 124 minutes or longer, and the default value of the transitory connection idle-timeout MUST be 4 minutes or longer. A NAT that implements session timeouts may be configurable to use smaller values for the NAT idle-timeouts.
NAT behaviour for handling DCCP-Reset packets or connections in the TIMEWAIT state is left unspecified.
A NAT eventually forgets idle sessions. The rule sets the floor: an open session must be kept for at least 124 minutes of silence, and one that is being closed for at least 4 minutes. An operator may choose shorter times, but the defaults must not be shorter than those.
§6Application-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 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. The RFC notes that this is not consistent with REQ-6 of RFC 5382, the corresponding rule for TCP.
An application-level gateway reads the traffic inside a connection and changes it. For DCCP the RFC forbids that: if a NAT has such gateways for other protocols, they must leave DCCP alone.
§7Other Requirements Applicable to DCCP
A list of general and UDP-specific NAT behavioural requirements is described in RFC 4787, and a list of ICMP-specific NAT behavioural requirements is described in RFC 5508. The requirements below reiterate the ones from those two documents that directly affect DCCP, and they do not relax any requirement in either.
REQ-7: a NAT MUST NOT have a port assignment behaviour of port overloading for DCCP.
REQ-8: a NAT MUST support hairpinning for DCCP, and its hairpinning behaviour MUST be of the 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.
Four more rules come from the general NAT and ICMP documents. A NAT must not share one port between different connections. It must be able to send a packet from inside back out to the same inside host, which is called hairpinning. It should rewrite the addresses inside ICMP error messages that it produces. And an ICMP error must never end the DCCP connection it was about.
§8Requirements Specific to DCCP
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 RFC 4340.
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, is given in RFC 5595.
Two DCCP fields need careful handling. The checksum has to be fixed after the addresses and ports change, and the checksum coverage, which says how much of the packet the checksum protects, must be left exactly as it was. The Service Code, which names the application the client wants to reach, must also be left alone.
§10Security Considerations
RFC 4787 discusses security considerations for NATs that handle IP and unicast UDP traffic, and all of them apply equally to this document. Security concerns specific to handling DCCP packets are discussed here.
REQ-1 and REQ-6 through REQ-13 introduce no new known security concerns. REQ-2 introduces no new known security concerns either: while a NAT may elect to keep track of some DCCP-specific per-flow state, compared with 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, which is already addressed in RFC 4787 and RFC 5382.
REQ-4 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 of 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.
The security rules for NATs apply unchanged. Two of the requirements carry a trade-off the RFC spells out. Sending an ICMP error straight back can tell an outsider that a NAT is there, but staying silent makes network problems harder to diagnose. And keeping idle sessions alive protects long-lived connections but uses up the NAT's memory, which is what denial-of-service attacks aim to fill.