One document matched: draft-phelan-dccp-natencap-03.txt
Differences from draft-phelan-dccp-natencap-02.txt
INTERNET-DRAFT DCCP-NAT Encapsulation November 18, 2009
DCCP-NAT Encapsulation
Internet Draft T. Phelan
Document: draft-phelan-dccp-natencap-03.txt Sonus Networks
Expires: May 2010 November 18, 2009
Intended status: Proposed Standard
Datagram Congestion Control Protocol (DCCP)
Encapsulation for NAT Traversal (DCCP-NAT)
Status of this Memo
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Abstract
This document specifies an alternative encapsulation of the Datagram
Congestion Control Protocol (DCCP), referred to as DCCP-NAT. This
encapsulation will allow DCCP to be carried through the current
generation of Network Address Translation (NAT) middleboxes without
modification of those middleboxes.
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Table of Contents
1. Introduction...................................................4
2. Terminology....................................................4
3. DCCP-NAT.......................................................4
3.1 UDP Header.................................................5
3.2 DCCP-NAT Generic Header....................................6
3.2.1 DCCP-RAW Checksum Field...............................6
3.3 Partial Checksum Extension Header..........................7
3.4 Minimum Checksum Coverage Feature..........................7
3.5 Other DCCP Headers and Options.............................8
3.6 Service Codes and the DCCP Port Registry...................8
4. DCCP-NAT and Higher-Layer Protocols............................8
5. Signaling the Use of DCCP-NAT..................................9
5.1 SDP for RTP over DCCP......................................9
6. Security Considerations.......................................10
7. IANA Considerations...........................................10
8. References....................................................10
8.1 Normative References......................................10
9. Author's Address..............................................11
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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.
In order for the [RFC4340] encapsulation to pass through Network
Address Translation (NAT) devices, these devices must be updated to
recognize and properly modify DCCP. This is the long-term objective
for DCCP, and work is underway to specify the necessary operations.
However, in the short term it would be useful to have an
encapsulation for DCCP that would be compatible with NAT devices
conforming to [RFC4787]. This document specifies that encapsulation,
which is referred to as DCCP-NAT. For convenience, the [RFC4340]
encapsulation is referred to as DCCP-RAW.
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-NAT
The basic approach here is to insert a UDP ([RFC768]) "shim" layer
between the IP header and a DCCP packet with a modified generic
header (modified to eliminate redundancies between UDP and DCCP).
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-NAT 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
all combinations of DCCP-RAW and DCCP-NAT encapsulations with IPv4
and IPv6.
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The basic format of a DCCP-NAT packet is:
+-----------------------------------+
| IP Header (IPv4 or IPv6) | Variable length
+-----------------------------------+
| UDP Header | 8 bytes
+-----------------------------------+
| DCCP-NAT Generic Header | 12 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 [RFC768]:
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-NAT, 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-NAT 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-NAT is the DCCP-NAT
datagram). For DCCP-NAT, when the UDP Checksum is non-zero, Length
MUST be at least the size of the UDP header (8 bytes) plus the
minimum size of a DCCP-NAT header (12 bytes), for a total minimum
value of 20 bytes. When the UDP Checksum is zero, the DCCP-NAT
header MUST also contain a Partial Checksum Extension Header,
therefore the minimum DCCP-NAT header is 16 bytes and the total
minimum is 24 bytes. Received packets with a UDP Length of less
than the applicable minimum length MUST be ignored.
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Checksum: 16 bits
This field is the Internet checksum of a network-layer pseudoheader
and the entire UDP packet. For DCCP-NAT, a packet with a checksum
field equal to 0 that does not contain a Partial Checksum Extension
Header, or contains an invalid Partial Checksum Extension Header
MUST be ignored as incorrect checksum.
3.2 DCCP-NAT Generic Header
Unlike the DCCP-RAW generic header, the DCCP-NAT generic header takes
only one form; it does not support short sequence numbers. Its
format is as follows:
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 | Type | Sequence Number (high bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Sequence Number (low bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All DCCP-NAT generic header fields function as specified in
[RFC4340].
3.2.1 DCCP-RAW Checksum Field
For DCCP-NAT, the function of the DCCP-RAW generic header field
Checksum is performed by the UDP Checksum field.
If the UDP Checksum field in a received packet is non-zero and is
invalid, that packet MUST be ignored as per the invalid checksum
procedures of DCCP-RAW (i.e., the options in the packet MUST NOT be
processed).
If the UDP Length field in a received packet is less than the length
of the UDP header plus the entire DCCP-NAT header (including the
generic header, Partial Checksum Extension Header if present, type-
specific fields and options), 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, that packet MUST also be ignored as per the
invalid checksum procedures.
If the UDP Checksum field is zero, then the Partial Checksum
Extension Header procedures apply. See section 3.3 for more
information.
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3.3 Partial Checksum Extension Header
If the UDP Checksum field is zero, the DCCP-NAT generic header MUST
be immediately followed by a Partial Checksum Extension Header.
Additional type-specific header fields and DCCP Options would then
follow the Partial Checksum Extension Header. If the UDP Checksum
field is non-zero the Partial Checksum Extension Header MUST NOT be
included. The format of the header is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum Coverage | PChecksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are defined as follows:
Checksum Coverage: 16 bits
This is the number of bytes of user data that are covered by the
partial checksum. It MUST NOT be greater than the entire length of
the user data (from the end of the DCCP-NAT header, including
options, to the end of the packet). Packets whose Checksum
Coverage fields are greater than the length of the user data MUST
be ignored as incorrect checksum.
PChecksum: 16 bits
This is the Internet checksum of the DCCP-NAT header plus Checksum
Coverage bytes of the user data. Using the TCP/IP checksum
algorithm, the PChecksum field is first set to zero. If the
Checksum Coverage field is odd, the data to be summed is extended
by one byte set to zero. This byte does not overwrite the
corresponding byte in the DCCP-NAT packet, and is not transmitted.
The PChecksum field is then set to the one's complement of the
one's complement sum of the sixteen-bit words covered (DCCP header
plus Checksum Coverage bytes of user data plus one zero byte if
Checksum Coverage is odd). Note that PChecksum does not include an
IP pseudoheader. Packets with invalid PChecksum fields MUST be
ignored as incorrect checksum.
3.4 Minimum Checksum Coverage Feature
The Minimum Checksum Coverage Feature lets a DCCP endpoint determine
whether its peer is willing to accept packets with partial checksum
coverage. It takes values from 0 to 15. For DCCP-NAT the feature
values are interpreted as follows:
o Minimum Checksum Coverage = 0, the peer will not accept packets
with partial checksum. All UDP Checksum fields should be non-
zero and the Partial Checksum Extension Header is never included.
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o Minimum Checksum Coverage > 0, the peer will accept packets with
partial checksum as long as the Checksum Coverage field is at
least (Minimum Checksum Coverage - 1)*4.
As in DCCP-RAW, peers may refuse to process packets with unacceptable
Checksum Coverage. Such packets SHOULD be reported using Data
Dropped options with Drop Code 0, Protocol Constraints.
3.5 Other DCCP Headers and Options
All type-specific DCCP headers are as in DCCP-RAW, except that the
short sequence number version of the acknowledgement header is not
supported. All option and feature encodings are as in DCCP-RAW.
3.6 Service Codes and the DCCP Port Registry
There is one Service Code registry and one DCCP port registry and
they 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.
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. Since
the port registry supports multiple applications registering the same
port (as long as the Service Codes are different), other applications
MAY register on the same port, but those registrations are also
applicable to all combinations of encapsulation and IP version.
4. DCCP-NAT and Higher-Layer Protocols
In general, the encapsulation of a higher-layer protocol within DCCP
SHOULD be the same in both DCCP-RAW and DCCP-NAT. At this time,
encapsulations of DTLS over DCCP, defined in [RFC5238] and RTP over
DCCP, defined in [RTP-DCCP], have been already defined. The
encapsulations of those protocols in DCCP-NAT 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-RAW and DCCP-NAT. If a
document does not specify different encapsulations for DCCP-RAW and
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DCCP-NAT, the specified encapsulation SHALL apply to both DCCP-RAW
and DCCP-NAT.
5. Signaling the Use of DCCP-NAT
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
[RTP-DCCP] 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 [RTP-DCCP] 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-encap:" dccp-encap-type
dccp-encap-type = dccp-raw-encap / dccp-nat-encap
dccp-raw-encap = "dccp-raw"
dccp-nat-encap = "dccp-nat" [":" udp-port-num]
udp-port-num = *DIGIT
where *DIGIT is as defined in [RFC5234].
For example:
o To specify a connection that will use DCCP-RAW encapsulation use:
a=dccp-encap:dccp-raw
o To specify a connection that will use DCCP-NAT encapsulation,
with the DCCP-NAT service running on the default UDP port for
DCCP-NAT defined in section 7 use:
a=dccp-encap:dccp-nat
o To specify a connection that will use DCCP-NAT encapsulation,
with the DCCP-NAT service running UDP port 50,000 use:
a=dccp-encap=dccp-nat:50000
The absence of an "a=dccp-encap" attribute SHALL be interpreted as
"a=dccp-encap:dccp-raw".
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Note that the port number in the media description (m=) always
specifies the DCCP port number.
6. Security Considerations
DCCP-NAT provides all of the security risk-mitigation measures
present in DCCP-RAW, and also all of the security risks, except those
associated with short sequence numbers (since DCCP-NAT does not
support that feature).
The purpose of DCCP-NAT 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-NAT 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 has been placed with IANA for the dccp-nat
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-encap
Long-form attribute name in English: DCCP encapsulation type
Type of attribute: Media level
Subject to charset attribute? No
Purpose of the attribute: See this document section 5.1
Allowed attribute values: See this document section 5.1
8. References
8.1 Normative References
[RFC4340] Kohler, E., Handley, M., Floyd, S., "Datagram Congestion
Control Protocol (DCCP)", RFC 4340, March 2006.
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[RFC768] Postel, J., "User Datagram Protocol", RFC 768, August
1980.
[RFC4787] Audet, F., Jennings, C., "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", RFC 4787,
January 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RTP-DCCP] Perkins, C., "RTP and the Datagram Congestion Control
Protocol (DCCP)", draft-ietf-dccp-rtp-07.txt, June 2007.
[RFC5238] Phelan, T., "Datagram Transport Layer Security (DTLS)
over the Datagram Congestion Control Protocol (DCCP)",
RFC 5238, May 2008.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", RFC 5234, October 2005.
9. Author's Address
Tom Phelan
Sonus Networks
7 Technology Park Dr.
Westford, MA USA 01886
Phone: 978-614-8456
Email: tphelan@sonusnet.com
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