Datagram Congestion Control T. Phelan
Protocol Sonus
Internet-Draft August 26, 2010
Intended status: Experimental
Expires: February 27, 2011
Datagram Congestion Control Protocol (DCCP) Encapsulation in UDP for NAT
Traversal (DCCP-UDP)
draft-ietf-dccp-udpencap-02
Abstract
This document specifies an alternative encapsulation of the Datagram
Congestion Control Protocol (DCCP), referred to as DCCP-UDP. This
encapsulation will allow DCCP to be carried through the current
generation of Network Address Translation (NAT) middleboxes without
modification of those middleboxes.
Status of this Memo
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 February 27, 2011.
Copyright Notice
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DCCP-UDP . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. UDP Header . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. DCCP Generic Header . . . . . . . . . . . . . . . . . . . . 5
3.3. DCCP-UDP Checksum Procedures . . . . . . . . . . . . . . . 5
3.3.1. Partial Chescksums and the Minimum Checksum
Coverage Feature . . . . . . . . . . . . . . . . . . . 6
3.4. Explicit Congestion Notification . . . . . . . . . . . . . 6
3.5. Path Maximum Transmission Unit Discovery . . . . . . . . . 6
3.6. Service Codes and the DCCP Port Registry . . . . . . . . . 6
4. DCCP-UDP and Higher-Layer Protocols . . . . . . . . . . . . . . 7
5. Signaling the Use of DCCP-UDP . . . . . . . . . . . . . . . . . 7
5.1. SDP for RTP over DCCP . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
The Datagram Congestion Control Protocol (DCCP), specified in
[RFC4340], is a transport-layer protocol that provides upper layers
with the capability of using unreliable but congestion controlled
flows. According to [RFC4340], DCCP packets are directly
encapsulated in IPv4 or IPv6 packets.
[RFC5597] specifies how DCCP should be handled by NAT devices, but
there is a significant installed base of NAT devices that do not
support [RFC5597]. In the short term, it would be useful to have an
encapsulation for DCCP that is compatible with the installed base of
NAT devices that supports [RFC4787] but do not support [RFC5597].
This document specifies that encapsulation, which is referred to as
DCCP-UDP. For convenience, the [RFC4340] encapsulation (updated by
[RFC5596] as required) is referred to as DCCP-STD.
The DCCP-UDP encapsulation specified here supports all of the
features contained in DCCP-STD except for partial checksums.
2. Terminology
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].
3. DCCP-UDP
The basic approach here is to insert a UDP ([RFC0768]) header between
the IP header and the DCCP packet. Note that this is not strictly a
tunneling approach. The IP addresses of the communicating end
systems are carried in the IP header (which could be modified by NAT
devices) and there are no other IP addresses embedded.
Devices offering or using DCCP services via DCCP-UDP encapsulation
listen on a UDP port (default port awaiting IANA action) for incoming
packets and pass received packets along to the DCCP protocol. DCCP
implementations 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:
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+-----------------------------------+
| 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)
+-----------------------------------+
3.1. UDP Header
The format of the UDP header is taken from [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
These fields identify the UDP ports on which the source and
destination (respectively) of the packet are listening for
incoming DCCP-UDP packets (normally both are the default port to
be assigned by IANA). Note that they do not identify the DCCP
source and destination ports.
Length: 16 bits
This field is the length of the UDP datagram, including the UDP
header and the payload (which for DCCP-UDP is the DCCP-UDP
datagram).
Checksum: 16 bits
This field is the Internet checksum of a network-layer
pseudoheader and the UDP packet.
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3.2. DCCP Generic Header
As defined in [RFC4340], the DCCP Generic Header 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
and
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| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All generic header fields, except for Checksum field, have the same
meaning as specified in [RFC4340] and the update specified in
[RFC5596].
3.3. DCCP-UDP Checksum Procedures
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.
If the UDP Length field is less than 20 (UDP Header length and
minimum DCCP-NAT header length), the packet MUST be dropped.
If the UDP Checksum field, computed using standard UDP methods, is
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invalid, the packet MUST be dropped.
If the UDP Checksum field is zero, the packet MUST be dropped.
3.3.1. Partial Chescksums and the Minimum Checksum Coverage Feature
DCCP-UDP supports the syntax of partial checksums (it supports
negotiation of the Minimum Checksum Coverage feature and settings of
the CsCov field), but, since the UDP checksum field always covers the
entire packet, not the function.
3.4. Explicit Congestion Notification
DCCP-UDP implementations 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 packets.
Implementations that do not support ECN MUST follow the procedures in
DCCP-STD section 12.1 for implementations that are not ECN capable.
3.5. Path Maximum Transmission Unit Discovery
DCCP-UDP implementations SHOULD follow DCCP-STD section 14 with
regard to maximum packet size and Path Maximum Transmission Unit
Discovery (PMTUD).
3.6. Service Codes and the DCCP Port Registry
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 MUST NOT be used by other applications. An
application SHOULD NOT register different Service Codes for different
combinations of encapsulation and IP version. See [RFC5595] for more
information on 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 encapsulation combinations.
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4. DCCP-UDP and Higher-Layer Protocols
In general, the encapsulation of a higher-layer protocol within DCCP
SHOULD be the same in both DCCP-STD and DCCP-UDP. At this time,
encapsulations of DTLS over DCCP, defined in [RFC5238] and RTP over
DCCP, defined in [RFC5762], have been already defined. The
encapsulations of those protocols in DCCP-UDP SHALL be the same as
specified in those documents.
Higher-layer protocols that require different encapsulations for
different DCCP modes MUST justify the reasons for the difference and
MUST specify the encapsulations for both DCCP-STD and DCCP-UDP. If a
document does not specify different encapsulations for DCCP-STD and
DCCP-UDP, the specified encapsulation SHALL apply to both DCCP-STD
and DCCP-UDP.
5. Signaling the Use of DCCP-UDP
Applications often signal transport connection parameters through
outside means, such as the Session Description Protocol (SDP).
Applications that define such methods for DCCP MUST define how the
DCCP encapsulation is chosen, and MUST allow either type of
encapsulation to be signaled.
5.1. SDP for RTP over DCCP
[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 IANA allocated port for
the DCCP-UDP service if no port is included.
The absence of "a=dccp-in-udp" in an SDP offer indicates that the
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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.
6. Security Considerations
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 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 devices.
DCCP-UDP may also allow DCCP applications to pass through existing
firewall devices, if the administrators of the devices so choose.
The option is a binary one however; either allow all DCCP
applications or allow none. Proper control of DCCP application-by-
application will require enhancements to firewalls.
7. IANA Considerations
A port allocation request will be placed with IANA for the dccp-udp
service port in UDP.
The following new SDP attribute ("att-field") is to be registered:
Contact name: Tom Phelan <tphelan@sonusnet.com>
Attribute name: dccp-in-udp
Long-form attribute name in English: DCCP in UDP Encapsulation
Type of attribute: Media level
Subject to charset attribute? No
Purpose of the attribute: See this document section Section 5.1
Allowed attribute values: See this document section Section 5.1
8. References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
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[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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[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.
[RFC5762] Perkins, C., "RTP and the Datagram Congestion Control
Protocol (DCCP)", RFC 5762, April 2010.
Author's Address
Tom Phelan
Sonus Networks
7 Technology Dr.
Westford, MA 01886
US
Phone: +1 978 614 8456
Email: tphelan@sonusnet.com
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