wdiff rfc5597.original rfc5597.txt

Behavior Engineering for Hindrance
Network Working Group R. Denis-Courmont
Avoidance
Request for Comments: 5597 VideoLAN project

Intended status: BCP
Expires: May 31,
BCP: 150 July 2009
Category: Best Current Practice

Network Address Translation (NAT) Behavioral Requirements for the
Datagram Congestion Control Protocol
draft-ietf-behave-dccp-05.txt

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This Internet-Draft will expire on May 31, 2009. English.

Abstract

This document defines a set of requirements for NATs handling the
Datagram Congestion Control Protocol (DCCP). Those These requirements
allow DCCP applications, such as streaming applications applications, to operate consistently.
These requirements
consistently, and they are very similar to the TCP requirements for NATs
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.

Table of Contents

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

1. Introduction

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

This draft memo discusses the technical issues involved, 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 model of DCCP will not work. A
simultaneous open
simultaneous-open must be performed, as in
[I-D.ietf-dccp-simul-open]. [RFC5596]. Also, a separate
separate, unspecified mechanism may be needed, such as Unilateral
Self Address Fixing
(UNSAF)[RFC3424] (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 documentation 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 "endpoint-
independent mapping", "address-dependent mapping", "address and port-dependent port-
dependent mapping", "filtering behavior", "endpoint-independent
filtering", "address-dependent filtering", "address and port-dependent port-
dependent filtering", "port assignment", "port overloading",
"hairpinning", and "external source IP address and port" as defined
in [RFC4787].

3. Applicability statement 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 particular, the
IP addresses), NAT devices need to mangle:

o the DCCP source port, port for outgoing packets, depending on the NAT
mapping
mapping,

o the DCCP destination port, port for incoming packets, depending on the
NAT mapping 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 a "generic" transport protocol 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 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
[I-D.ietf-dccp-simul-open].
[RFC5596]. That mode updates DCCP by adding a new packet type, DCCP-Listen. 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, SCTP. and SCTP (Stream Control Transmission
Protocol).

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

6. Application Level 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 sane any application-layer state
needed for an ALG Application Level Gateway (ALG) to function. function properly.
Additionally, there are no known DCCP-capable Application Level Gateways (ALGs) 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 UDP-specific NAT behavioral requirements are
described in [RFC4787]. A list of ICMP specific ICMP-specific NAT behavioral
requirements are described in [I-D.ietf-behave-nat-icmp]. [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
[I-D.ietf-behave-nat-icmp]. [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" "hairpinning" for DCCP. Furthermore, A a
NAT's Hairpinning 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 Specific to DCCP

8.1. Partial checksum coverage Checksum Coverage

DCCP supports partial checksum coverage. A NAT will usually need to
perform incremental changes to the packet checksum 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 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 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 Service Code value.

Further guidance on the use of Service Codes by middleboxes,
including NATs, can be found in [I-D.ietf-dccp-serv-codes]. [RFC5595].

9. DCCP without NAT support Support

If the NAT device cannot be updated to support DCCP, DCCP datagrams
can be encapsulated within an 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,

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 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, 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 to
respond to unsolicited 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 minutes, depending
on the state of the connection. To protect against denial-of-service
attack
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. IANA Considerations

This document raises no IANA considerations.

12. 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 Eggert, and Christian Vogt for their comments
and help on this document.

This memo borrows heavily from draft-ietf-behave-tcp-07, [RFC5382] by S. Guha (editor), K.
Biswas, B. Ford, S. Sivakumar Sivakumar, and P. Srisuresh.

13.

12. References

13.1.

12.1. Normative References

[I-D.ietf-behave-nat-icmp] Srisuresh, P., Ford, B., Sivakumar, S.,
and S. Guha, "NAT Behavioral Requirements
for ICMP protocol",
draft-ietf-behave-nat-icmp-11 (work in
progress), November 2008.

[I-D.ietf-dccp-simul-open] Fairhurst, G., "DCCP Simultaneous-Open
Technique to Facilitate NAT/Middlebox
Traversal", draft-ietf-dccp-simul-open-05
(work in progress), October 2008.

[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.ietf-dccp-serv-codes]

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

[RFC5596] Fairhurst, G., "The DCCP Service Code",
draft-ietf-dccp-serv-codes-08 (work in
progress), September 2008. "Datagram Congestion Control Protocol
(DCCP) Simultaneous-Open Technique to Facilitate NAT/
Middlebox Traversal", RFC 5596, July 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, July 2009.

Author's Address

Remi Denis-Courmont
VideoLAN project

EMail: [email protected]
URI: http://www.videolan.org/

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