| DCCP Working Group | T. Phelan |
| Internet-Draft | Sonus |
| Intended status: Standards Track | G. Fairhurst |
| Expires: September 11, 2011 | University of Aberdeen |
| March 10, 2011 |
Datagram Congestion Control Protocol (DCCP) Encapsulation for NAT Traversal (DCCP-UDP)
draft-ietf-dccp-udpencap-07
This document specifies an alternative encapsulation of the Datagram Congestion Control Protocol (DCCP), referred to as DCCP-UDP. This encapsulation allows DCCP to be carried through the current generation of Network Address Translation (NAT) middleboxes without modification of those middleboxes. This document also updates the SDP information for DCCP defined in RFC 5762.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 11, 2011.
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The Datagram Congestion Control Protocol (DCCP), specified in [RFC4340], is a transport-layer protocol that provides upper layers with the ability to use non-reliable congestion-controlled flows. The current specification for DCCP [RFC4340] specifies a direct encapsulation in IPv4 or IPv6 packets.
[RFC5597] specifies how DCCP should be handled by devices that use Network Address Translation (NAT) or Network Address and Port Translation (NAPT). However, there is a significant installed base of NAT/NAPT devices that do not support [RFC5597]. In the short term, it would be useful to have an encapsulation for DCCP that is compatible with this installed base of NAT/NAPT devices that support [RFC4787], but do not support [RFC5597]. This document specifies that encapsulation, which is referred to as DCCP-UDP. For convenience, the standard encapsulation for DCCP [RFC4340] (including [RFC5596] as required) is referred to as DCCP-STD.
The encapsulation described in this document may also be used as a transition mechanism to enable support for DCCP in devices that support UDP, but do not yet natively support DCCP. This therefore also allows the DCCP transport to be implemented within an application using DCCP-UDP.
The document also updates the SDP specification for DCCP to convey the encapsulation type. In this respect only, it updates the method in [RFC5762].
The DCCP-UDP encapsulation specified in this document supports all of the features contained in DCCP-STD, but with limited functionality for partial checksums.
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].
The basic approach is to insert a UDP [RFC0768] header between the IP header and the DCCP packet. Note that this is not a tunneling approach. The IP addresses of the communicating end systems are carried in the IP header. The method does not embed additional IP addresses.
The method is designed to support use when these addresses are modified by a device that implements NAT/NAPT. A NAT translates the IP addresses, which impacts the transport-layer checksum. A NAPT device may also translate the port values (usually the source port). In both cases, the outer transport header that includes these values would need to be updated by the NAT/NAPT.
A device offering or using DCCP services via DCCP-UDP encapsulation listens on a UDP port (default port, XXX IANA PORT XXX), or may bind to a specified port utilising out-of-band signalling, such as the Session Description Protocol (SDP). The DCCP-UDP server accepts incoming packets over the UDP transport and passes the received packets to the DCCP protocol module, after removing the UDP encapsulation.
A DCCP implementation MAY allow services to be simultaneously offered over any or all combinations of DCCP-STD and DCCP-UDP encapsulations with IPv4 and IPv6.
The basic format of a DCCP-UDP packet is:
+-----------------------------------+ | IP Header (IPv4 or IPv6) | Variable length +-----------------------------------+ | UDP Header | 8 bytes +-----------------------------------+ | DCCP Generic Header | 12 or 16 bytes +-----------------------------------+ | Additional (type-specific) Fields | Variable length (could be 0) +-----------------------------------+ | DCCP Options | Variable length (could be 0) +-----------------------------------+ | Application Data Area | Variable length (could be 0) +-----------------------------------+
Section 3.8 describes usage of UDP ports. This includes implementation of a DCCP-UDP encapsulation service as a daemon that listens on a well-known port, allowing multiplexing of different DCCP applications over the port.
The format of the UDP header is specified in [RFC0768]:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Dest Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For DCCP-UDP, the fields are interpreted as follows:
Source and Dest(ination) Ports: 16 bits each
Length: 16 bits
Checksum: 16 bits
The DCCP Generic Header [RFC4340] takes two forms, one with long sequence numbers (48 bits) and the other with short sequence numbers (24 bits).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Dest Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Offset | CCVal | CsCov | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |X| | .
| Res | Type |=| Reserved | Sequence Number (high bits) .
| | |1| | .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (low bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Generic DCCP Header with long sequence numbers [RFC4340]
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Dest Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Offset | CCVal | CsCov | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |X| |
| Res | Type |=| Sequence Number (low bits) |
| | |0| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Generic DCCP Header with short sequence numbers [RFC4340]
All generic header fields, except for the Checksum field, have the meaning specified in [RFC4340] updated by [RFC5596].
Section 3.8 describes how a DCCP-UDP implementation treats UDP and DCCP ports.
DCCP-UDP employs a checksum at the UDP level and eliminates the use of the DCCP checksum. This approach was chosen to enable use of current NAT/NATP traversal methods developed for UDP. Such methods will generally be unaware whether DCCP is being encapsulated and hence do not update the inner checksum in the DCCP header. Standard DCCP requires protection of the DCCP header fields, this justifies any processing overhead incurred from calculating the UDP checksum.
In addition, UDP NAT traversal does not support partial checksums. Although this is still permitted end-to-end in the encapsulated DCCP datagram, links along the path will treat these as UDP packets and can not enable special partial checksum processing.
For DCCP-UDP, the function of the DCCP Checksum field is performed by the UDP checksum field. On transmit, the DCCP Checksum field SHOULD be set to zero. On receive, the DCCP Checksum field MUST be ignored.
The UDP checksum MUST NOT be zero for a UDP packet that is sent using DCCP-UDP. If the received UDP Checksum field is zero, the packet MUST be dropped.
If the UDP Length field is less than 20 (the UDP Header length and minimum DCCP-UDP header length), the packet MUST be dropped.
If the UDP Checksum field, computed using standard UDP methods, is invalid, the packet MUST be dropped.
If the UDP Length field in a received packet is less than the length of the UDP header plus the entire DCCP-UDP header (including the generic header and type-specific fields and options, if present), or the UDP Length field is greater than the length of the packet from the beginning of the UDP header to the end of the packet, the packet MUST be dropped.
This document describes an encapsulation for DCCP that uses the UDP transport. It requires the UDP checksum to be enabled. This checksum provides coverage of the entire encapsulated DCCP datagram.
DCCP-UDP supports the syntax of partial checksums. It also supports negotiation of the Minimum Checksum Coverage feature and settings of the CsCov field. However, the UDP checksum field in DCCP-UDP always covers the entire DCCP datagram and the DCCP checksum is ignored on receipt. An application that enables the partial checksums feature in the DCCP Module will therefore experience a service that is functionally identical to using full DCCP checksum coverage. This is also the service that the application would have received if it had used a network path that did not provide optimised processing for DCCP partial checksums.
A DCCP-UDP implementations MAY transfer network-layer options intended for DCCP to the network-layer header of the encapsulating UDP packet.
A DCCP-UDP endpoint that receives IP-options for the encapsulating UDP packet MAY forward these to the DCCP protocol module. If the endpoint forwards a specific network layer option to the DCCP module, it MUST also forward all subsequent packets with this option. Consistent forwarding is essential for correct operation of many end-to-end options.
A DCCP-UDP endpoint SHOULD follow the procedures of DCCP-STD section 12 by setting the ECN fields in the IP Headers of outgoing packets and examining the values received in the ECN fields of incoming IP packets, relaying any packet markings to the DCCP module.
Implementations that do not support ECN MUST follow the procedures in DCCP-STD section 12.1 with regard to implementations that are not ECN capable.
To allow ICMP messages to be demultiplexed by the receiving endpoint, part of the original packet that resulted in the message is included in the payload of the ICMP error message. The receiving endpoint can therefore use this information to associate the ICMP error with the transport protocol instance that resulted in the ICMP message. When DCCP-UDP is used, the error message and the payload of the ICMP error message relate to the UDP transport.
DCCP-UDP endpoints SHOULD forward ICMP messages relating to a UDP packet that carries a DCCP-UDP to the DCCP module. This may imply translation of the payload of the ICMP message into a form that is recognised by the DCCP stack. [ICMP] describes precautions that are desirable before TCP acts on the receipt of an ICMP message. Similar precautions are desirable prior to forwarding by DCCP-UDP to the DCCP module.
The minimal length ICMP error message generated in response to processing a UDP Datagram only identifies the Source UDP Port and Destination UDP Port. This ICMP message does not carry sufficient information to discover the encapsulated DCCP Port values. A DCCP-UDP endpoint that supports multiple DCCP connections over the same pair of UDP ports (see section Section 3.8) may not therefore be able to associate an ICMP message with a unique DCCP-UDP connection.
DCCP-UDP implementations SHOULD follow DCCP-STD section 14 with regard to determining the maximum packet size and the use of Path Maximum Transmission Unit Discovery (PMTUD).
The effect of encapsulation is to incur additional datagram overhead. This will reduce the Maximum Packet Size (MPS) at the DCCP level.
A DCCP-UDP server (that is, an initially passive endpoint that wishes to receive DCCP-Request packets [RFC4340] over DCCP-UDP) listens for connections on one or more UDP ports. UDP port number XXX IANA PORT XXX has been reserved as the default listening UDP port for a DCCP-UDP server. Some NAT/NAPT topologies may require using a non-default listening port.
The purpose of this IANA-assigned port is for the operating system or a framework to receive and process DCCP-UDP datagrams for delivery to the DCCP module. Because of this, the IANA-assigned port SHOULD NOT be used as the Destination UDP Port by a DCCP-UDP server listening for incoming DCCP-UDP packets and SHOULD NOT be used as a Source UDP Port by a client application sending DCCP-UDP packets.
A DCCP-UDP client provides UDP source and destination ports as well as DCCP source and destination ports at connection initiation time. A client SHOULD ensure that each DCCP connection maps to a single UDP connection by setting the UDP source port. Choosing a distinct source UDP port for each distinct DCCP connection ensures that UDP-based flow identifiers differ whenever DCCP-based flow identifiers differ. Specifically, two connections with different <source IP address, source DCCP port, destination IP address, destination DCCP port> DCCP 4-tuples will have different <source IP address, source UDP port, destination IP address, destination UDP port> UDP 4-tuples.
A DCCP-UDP server SHOULD accept datagrams from any UDP source port. There is a risk that the same DCCP source port number could be used by two endpoints each behind a NAPT. A DCCP-UDP server must therefore demultiplex a DCCP-UDP flow using both the UDP source and destination port numbers and the encapsulated DCCP ports. This ensures than an active DCCP connection is uniquely identified by the 6-tuple <source IP address, source UDP port, source DCCP port, destination IP address, destination UDP port, destination DCCP port>.
The demultiplexing at a DCCP-UDP endpoint occurs in two stages:
1) In the first stage, DCCP-UDP packets are demultiplexed using the UDP 4-tuple: <source IP address, source UDP port, destination IP address, destination UDP port>.
2) In the second stage, a receiving endpoint MUST ensure that two independent DCCP connections that were multiplexed to the same UDP 4-tuple are not associated with the same connection in the DCCP module. The endpoint therefore needs to keep state for the set of active DCCP-UDP endpoints using each combination of a UDP 4-tuple: <source IP address, source UDP port, destination IP address, destination UDP port>. A DCCP endpoint MUST implement one of the two methods:
This section clarifies the usage of DCCP Service Codes and the registration of server ports by DCCP-UDP. The section is not intended to update the procedures for allocating Service Codes or server ports.
There is one Service Code registry and one DCCP port registration that apply to all combinations of encapsulation and IP version. A DCCP Service Code specifies an application using DCCP regardless of the combination of DCCP encapsulation and IP version. An application may choose not to support some combinations of encapsulation and IP version, but its Service Code will remain registered for those combinations and the Service Code must not be used by other applications. An application should not register different Service Codes for different combinations of encapsulation and IP version. [RFC5595] provides additional information about DCCP Service Codes.
Similarly, a port registration is applicable to all combinations of encapsulation and IP version. Again, an application may choose not to support some combinations of encapsulation and IP version on its registered port, although the port will remain registered for those combinations. Applications should not register different ports just for the purpose of using different combinations of encapsulation.
The encapsulation of a higher-layer protocol within DCCP MUST be the same for both DCCP-STD and DCCP-UDP. Encapsulations of DTLS over DCCP is defined in [RFC5238] and RTP over DCCP is defined in [RFC5762]. This document therefore does not update these encapsulations when using DCCP-UDP.
Applications often signal transport connection parameters through outside means, such as SDP. Applications that define such methods for DCCP MUST define how the DCCP encapsulation is chosen, and MUST allow either encapsulation to be signaled. Where DCCP-STD and DCCP-UDP are both supported, DCCP-STD SHOULD be preferred.
Procedures for handling DCCP-STD and/or DCCP-UDP with Interactive Connectivity Establishment (ICE) may need to be developed, but are left for further work.
[RFC5762] defines SDP extensions for signaling RTP over DCCP connections. Since it predates this document, it does not define a method for determining the DCCP encapsulation type. This document updates [RFC5762] to add a method for determining the DCCP encapsulation type.
A new SDP attribute "dccp-encap" is defined for signaling the DCCP encapsulation according to the following ABNF [RFC5234]:
dccp-encap-attr = %x61 "=dccp-in-udp" [":" udp-port-num]
udp-port-num = *DIGIT
where *DIGIT is as defined in [RFC5234].
The presence of "a=dccp-in-udp" in an SDP offer indicates that the offerer is listening for DCCP-UDP connections on the indicated UDP port (if udp-port-num is included) or on the default port for the DCCP-UDP service if no port is included. This attribute MAY also be used in a declarative SDP file.
The absence of "a=dccp-in-udp" in an SDP offer indicates that the offerer is listening for DCCP-STD connections. The presence of "a=dccp-in-udp" conveys no information about whether or not the offerer is listening for DCCP-STD connections.
The new SDP attribute specified in this section is expected to be useful when the offering party is on the public Internet, or in the same private addressing realm as the answering party. In this case, the DCCP-UDP server has a public address. The client may either have a public address or be behind a NAT/NAPT. This is considered a scenario that has the potential to be an important use-case.
Some other NAT/NAPT topologies may result in the advertised port being unreachable via the NAT/NAPT.
The text below provides an example of SDP signalling, where an application signals support for both native DCCP and for DCCP-UDP:
v=0
o=alice 1129377363 1 IN IP4 192.0.2.47
s=-
c=IN IP4 192.0.2.47
t=0 0
m=video 5004 DCCP/RTP/AVP 99
a=rtcp-mux
a=rtpmap:99 h261/90000
a=dccp-service-code:SC=x5254505
a=dccp-in-udp:9999
a=setup:passive
a=connection:new
DCCP-UDP provides all of the security risk-mitigation measures present in DCCP-STD, and also all of the security risks.
The purpose of DCCP-UDP is to allow DCCP to pass through NAT/NAPT devices, and therefore it exposes DCCP to the risks associated with passing through NAT devices. It does not create any new risks with regard to NAT/NAPT devices.
The tunnel encapsulation recommends processing of ICMP messages received for packets sent using DCCP-UDP and translation to allow use by DCCP. [ICMP] describes precautions that are desirable before TCP acts on receipt of ICMP messages. Similar precautions are desirable for endpoints processing ICMP for DCCP-UDP.
DCCP-UDP may also allow DCCP applications to pass through existing firewall devices, if the administrators of the devices so choose. A simple use may either allow all DCCP applications or allow none.
A firewall than interprets this specification could inspect the encapsualted DCCP header to filter based on DCCP information. Full control of DCCP connections by application will require enhancements to firewalls, as discussed in [RFC4340] and related RFCs (e.g. [RFC5595]).
This document requests IANA to make the allocations described in the following sections.
IANA is requested to allocate a UDP port for the dccp-udp service. Use of this port is defined in section Section 3.8
XXX Note: IANA is requested to replace all occurrences of "XXX IANA PORT XXX" by the allocated port value prior to publication. XXX
IANA is requested to assign a new DCCP Reset Code in the DCCP Reset Codes Registry, with the short description "Encapsulated Port Reuse". This code applies to all DCCP congestion control IDs and should be allocated a value less than 120 decimal. Use of this reset code is defined in section Section 3.8
. Section 5.6 of RFC4340 defines three "Data" bytes that are carried by a DCCP Reset. For this Reset Code these are defined as below:
XXX Note: IANA is requested to replace all occurrences of "XXX IANA Port Reuse XXX" by the allocated DCCP reset code value prior to publication. XXX
IANA is requested to allocate the following new SDP attribute ("att-field"):
This document was produced by the DCCP WG. The following contributed during the working group last call:
Andrew Lentvorski, Lloyd Wood, Pasi Sarolahti, Gerrit Renker, Eddie Kohler, Colin Perkins, Dan Wing, Gorry Fairhurst and Tom Phelan.
| [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. |
| [RFC0768] | Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. |
| [RFC5234] | Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. |
| [RFC5762] | Perkins, C., "RTP and the Datagram Congestion Control Protocol (DCCP)", RFC 5762, April 2010. |
| [RFC4787] | Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, January 2007. |
| [RFC5238] | Phelan, T., "Datagram Transport Layer Security (DTLS) over the Datagram Congestion Control Protocol (DCCP)", RFC 5238, May 2008. |
| [RFC5595] | Fairhurst, G., "The Datagram Congestion Control Protocol (DCCP) Service Codes", RFC 5595, September 2009. |
| [RFC5596] | Fairhurst, G., "Datagram Congestion Control Protocol (DCCP) Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal", RFC 5596, September 2009. |
| [RFC5597] | Denis-Courmont, R., "Network Address Translation (NAT) Behavioral Requirements for the Datagram Congestion Control Protocol", BCP 150, RFC 5597, September 2009. |
| [ICMP] | Gont, , ""ICMP attacks against TCP", IETF Work-in-Progress.", . |