One document matched: draft-denis-udp-transport-00.txt
Internet Engineering Task Force R. Denis-Courmont
Internet-Draft Nokia
Intended status: Experimental July 04, 2008
Expires: January 5, 2009
UDP-Encapsulated Transport Protocols
draft-denis-udp-transport-00
Status of This Memo
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Abstract
This memo defines modified formats for conveyance of TCP and SCTP
packets within UDP datagrams, to ease traversal of network address
translators.
1. Introduction
The widespread deployment of network address and port translators
(NATs) across the Internet constitutes a major impediment to the
transmission of end-to-end traffic, especially when both ends of a
communication channel are located behind (distinct) NATs.
NATs are typically designed in such a manner, that the connection-
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oriented transport protocols, such as TCP, will only operate if:
o the passive end of the connection is not hindered by a NAT (i.e.
NATs can only be on the active end side),
o any NAT on the path must explicitly support the transport protocol
used (in practice, only TCP is commonly supported).
Several experiments have consistently showed that, when both sides of
a communication channel are behind NATs, the transmission of UDP
datagrams gives a much higher success rate.
This memo proposes modified packet formatting rules for use of the
TCP and SCTP transport protocols through UDP datagram flows, with
optimizations to avoid having to shrink the maximum segment sizes,
nor require the use of IP-level packet fragmentation.
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].
3. Applicability statement
UDP encapsulation is not backward compatible with normal TCP and SCTP
implementations. They also require application-layer changes, though
any affected applications would likely not operate at all without
such modifications.
Futhermore, middleboxes typically implement short binding expiration
timers for UDP flows (commonly 30 seconds to a few minutes). As a
consequence, it is necessary to send keep-alive packets in both
direction rather frequently. That precludes the use of UDP
encapsulation in scenarios where the sending of frequent keep-alive
is not acceptable (e.g. battery-powered device with a cellular access
network).
Because of these major limitations, the proposed mechanism SHOULD
only be used when normal packet formats would not work, such as in
NAT-to-NAT scenarios.
4. Packet formats
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4.1. Encapsulated TCP
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 | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Data | |C|E|A|P|R|S|F| |
|1| Offset| Res. |W|C|C|S|S|Y|I| Window |
| | | |R|E|K|H|T|N|N| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
UDP-encapsulated TCP header
The UDP-encapsulated TCP format is defined as follow:
Source, Destination Ports: TCP/UDP source and destination port
numbers.
Length: Length in octets of the datagram, including this entire
header and the data.
Checksum: UDP checksum (as specified in [RFC0768]).
1: Bit always set to 1, to differentiate STUN/UDP datagrams from TCP
frames.
Data Offset: TCP Data Offset, the number of 32-bits words from
Source Port (included) to data (excluded). MUST be at least 5.
Other fields: The other have the same semantics as with the TCP
protocol (see [RFC0793]).
Note that the URG bit and the Urgent pointer field are suppressed.
Support for TCP urgent data is left for further study.
The TCP checksum is removed, the UDP checksum provides the same level
of protection.
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4.2. Encapsulated SCTP
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 Number | Destination Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
UDP-encapsulated SCTP common header
Source, Destination Port Numbers: UDP/SCTP source and destination
port numbers.
Length: Length in octets of the datagram, including this entire
header and the data.
Checksum: UDP checksum (as specified in [RFC0768]).
1: Bit always set to 1, to differentiate STUN/UDP datagrams from
encapsulated SCTP packets.
Verification Tag: 31 lower order bits of the SCTP verification tag
defined in [RFC4960].
5. Usage with STUN
The above packet formats are defined such that the first bit of UDP
payload data is always set to 1. This allows for sending STUN
packets multiplexed through the same UDP flow as either a UDP-
encapsulated TCP or SCTP session. Indeed STUN packets always have
their first bit set to 0, as per [I-D.ietf-behave-rfc3489bis].
5.1. Usage with ICE
Because of this, it is possible to establish a UDP-encapsulated TCP
or SCTP flow using Interactive Connection Establishment
[I-D.ietf-mmusic-ice] as for any other ICE-negotiated UDP flow. In
that case, STUN packets are first exchanged to probe end-to-end
connectivity, and mutually authenticate endpoints. Once a flow is
successfully established, UDP-encapsulated TCP or SCTP packets can be
exchanged, in accordance with the respective transport protocol state
machines.
For this to work, both endpoints need to exchange their ICE candidate
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out-of-band, such as with SIP[RFC3261] or XMPP[RFC3920]. TBD: in
case SDP conveys the ICE parameters, a new media protocol or
attribute will be required.
5.2. Keep-alives packets
This multiplexing scheme also allow sending and receiving of ICE
keepalive packets, which may be required to refresh binding timers on
NATs and other middleboxes on the data path. It should be noted that
UDP flows are commonly associated with rather short bindings timeouts
(30 seconds to a few minutes). Therefore, keep-alives packets may
need to be sent frequently.
In principle, TCP keep-alive packets could also be used to refresh
NAT bindings. However, the typical TCP keep-alive period is way too
long. For instance, at the time of writing, the GNU/Linux operating
system will send TCP keep-alives only after 2 hours of TCP session
inactivity (assuming keep-alives are enabled for the session).
6. Alternative solutions
This non-normative section documents other potential solutions to
establishing TCP and SCTP sessions through a UDP flow.
6.1. Tunneling IP over UDP
The Teredo protocol[RFC4380] allows encapsulating IP (version 6)
packets into UDP/IPv4 datagrams. This potentially allows the use of
any IP-based transport protocol between two NATted IPv4 hosts,
provided that the operating system and applications support IPv6
(proper IPv6 connectivity is however not required).
Each Teredo client is automatically provisioned with its own unique
IPv6 address, which can be used as the rendez-vous mechanism, thus no
application-layer rendez-vous protocol are needed. For this to work,
clients must maintain a live UDP flow binding with their Teredo
server, however.
The Teredo protocol provides a fixed per-packet overhead of 48 bytes:
8 bytes for the UDP header and 40 bytes for the IPv6 header. In its
current state, Teredo limits the packet MTU to 1280 bytes (the
minimum IPv6 MTU), in order to avoid fragmentation. For TCP, this
translates to a maximum segment size of 1220 bytes.
6.2. Tunneling transport header over UDP
Another option would involve encapsulating the unmodified transport
protocol header into a UDP packet. draft-tuexen-sctp-udp-encaps and
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draft-phelan-dccp-natencap were example of this.
7. Security Considerations
TBD.
8. IANA Considerations
This document raises no IANA considerations.
9. References
9.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol",
STD 6, RFC 768, August 1980.
[RFC0793] Postel, J., "Transmission Control
Protocol", STD 7, RFC 793,
September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs
to Indicate Requirement Levels",
BCP 14, RFC 2119, March 1997.
[RFC4960] Stewart, R., "Stream Control
Transmission Protocol", RFC 4960,
September 2007.
9.2. Informative References
[I-D.ietf-behave-rfc3489bis] Rosenberg, J., Mahy, R., Matthews, P.,
and D. Wing, "Session Traversal
Utilities for (NAT) (STUN)",
draft-ietf-behave-rfc3489bis-16 (work
in progress), July 2008.
[I-D.ietf-mmusic-ice] Rosenberg, J., "Interactive
Connectivity Establishment (ICE): A
Protocol for Network Address
Translator (NAT) Traversal for Offer/
Answer Protocols",
draft-ietf-mmusic-ice-19 (work in
progress), October 2007.
[RFC3261] Rosenberg, J., Schulzrinne, H.,
Camarillo, G., Johnston, A., Peterson,
J., Sparks, R., Handley, M., and E.
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Schooler, "SIP: Session Initiation
Protocol", RFC 3261, June 2002.
[RFC3920] Saint-Andre, P., Ed., "Extensible
Messaging and Presence Protocol (XMPP):
Core", RFC 3920, October 2004.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6
over UDP through Network Address
Translations (NATs)", RFC 4380,
February 2006.
Author's Address
Remi Denis-Courmont
Nokia Corporation
P.O. Box 407
NOKIA GROUP 00045
FI
Phone: +358 50 487 6315
EMail: remi.denis-courmont@nokia.com
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