One document matched: draft-fairhurst-dccp-behave-update-01.txt
Differences from draft-fairhurst-dccp-behave-update-00.txt
DCCP Working Group G. Fairhurst
Internet-Draft G. Renker
Intended status: Standards Track University of Aberdeen
Expires: May 17, 2008 November 16, 2007
An Update for DCCP Connection Establishment to Assist NAT & Firewall
Traversal
draft-fairhurst-dccp-behave-update-01
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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Abstract
This document proposes an update to the Datagram Congestion Control
Protocol (DCCP), a connection-oriented and datagram-based transport
protocol. The update assists DCCP applications that need to traverse
middleboxes (e.g. Network Address Translators or firewalls), where
peering endpoints need to nearly simultaneously initiate the
connection to establish middlebox state required for communication.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope of this Document . . . . . . . . . . . . . . . . . . 3
1.2. Scope of the Problem to be Tackled . . . . . . . . . . . . 4
1.3. Discussion of Traversal Techniques for Traditional NAT
Devices . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.1. Near Simultaneous-Open of Connections . . . . . . . . 5
1.3.2. Role Reversal . . . . . . . . . . . . . . . . . . . . 6
2. Solution for DCCP NAT Traversal . . . . . . . . . . . . . . . 7
2.1. Conventions and Terminology . . . . . . . . . . . . . . . 7
2.2. DCCP-Listen Packet Format . . . . . . . . . . . . . . . . 7
2.3. Protocol Method . . . . . . . . . . . . . . . . . . . . . 8
2.3.1. Protocol Method for DCCP-Server Endpoints . . . . . . 8
2.3.2. Protocol Method for DCCP-Client Endpoints . . . . . . 9
2.3.3. Processing by Routers and Middleboxes . . . . . . . . 9
2.4. Examples of Use . . . . . . . . . . . . . . . . . . . . . 9
2.5. Backwards Compatibility with RFC 4340 . . . . . . . . . . 10
3. Discussion of Design Decisions . . . . . . . . . . . . . . . . 12
3.1. Generation of Listen Packets . . . . . . . . . . . . . . . 12
3.2. Repetition of Listen Packets . . . . . . . . . . . . . . . 12
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Normative References . . . . . . . . . . . . . . . . . . . 15
6.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . 19
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1. Introduction
UDP Network Address Translator (NAT) traversal is well understood and
widely implemented. NAT traversal for connection-oriented protocols
(e.g. TCP) uses similar principles, but in some cases requires more
complex and expensive solutions, such as dedicated relay servers.
DCCP [RFC4340] is both datagram-based and connection-oriented; and
thus NAT traversal of DCCP faces the same problems as TCP NAT
traversal, without being able to reap the benefits of datagram-based
NAT traversal as in UDP. In addition, DCCP has the disadvantage of
not being able to perform a simultaneous-open, a TCP-inherent
characteristic which greatly simplifies TCP NAT traversal.
After discussing the problem space for DCCP NAT traversal, this
document proposes a solution to natively support NAT traversal in
DCCP. Among the discussed solutions, it requires the least changes
and is based on an indicator message. A new type of DCCP packet is
introduced, which is sent by the server and addressed to the expected
client. Since the use of this packet type is tied to an optional
condition, it facilitates robust and native support for NAT
traversal, while at the same time leaving the standard operational
procedure of DCCP untouched.
1.1. Scope of this Document
The solution proposed by this document assists in connection
establishment when one or both peers are located behind a middlebox.
While the solution is specifically targeted at NAT traversal, due to
the similarity of involved principles, it may also be of similar use
to the traversal of other types of middlebox, such as firewalls.
For the scope of this document we consider traditional (outbound)
types of NAT as defined in [RFC2663] and further discussed in
[RFC3022]. We consider NAT traversal to involve one or more NAT
devices of this type in the path (i.e. hierarchies of nested NAT
devices are possible). It is assumed that all NATs in the path
between endpoints are BEHAVE-compliant [NAT-APP].
We consider NAT devices which provide a minimum of DCCP protocol
support, in that layer-4 checksums can be updated to account for
changes in the pseudo-header. Since this document tackles an
inherent problem of DCCP NAT traversal, we do not specify any further
requirements for DCCP support in NAT devices. These may be described
by a separate document.
The method described by this document is relevant to both client/
server and peer-to-peer applications, such as VoIP, file sharing, or
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online gaming. This update assists connections that utilise prior
out-of-band signaling between the client and server (e.g. a well-
known rendezvous server, [RFC3261], [H.323]) to notify both endpoints
of the connection parameters ([RFC3235], [NAT-APP]).
1.2. Scope of the Problem to be Tackled
We refer to DCCP hosts located behind one or more NAT devices as
having "private" addresses, and to DCCP hosts located in the global
address realm as having "public" addresses.
We consider DCCP NAT traversal for the following scenarios:
1. Private client connects to public server.
2. Public server connects to private client.
3. Private client connects to private server.
A defining characteristic of traditional NAT devices [RFC3022] is
that private hosts can connect to external hosts, but not vice versa.
Hence the case (1) is always possible, whereas cases (2) and (3)
require NAT traversal techniques. In this document we do not
consider use of pre-configured static NAT address maps, which would
also allow outside hosts to connect to the private network in cases
(2) and (3).
A DCCP implementation conforming to [RFC4340] can perform NAT
traversal with the help of a public relay server. The update
described by this document allows simple NAT traversal, without
indirection via relay servers.
1.3. Discussion of Traversal Techniques for Traditional NAT Devices
This section is a brief review of some existing techniques to
establish connectivity across NAT devices, the basic idea being to
make peer-to-peer sessions look like "outbound" sessions on each NAT
device.
Often a rendezvous server, located in the public address realm, is
used to enable clients to discover their NAT topology and the
addresses of peers.
The term 'hole punching' was coined in [FSK05] and refers to creating
soft state in a traditional NAT device by initiating an outbound
connection. A well-behaved NAT can subsequently exploit this to
allow a reverse connection back to the host in the private address
realm.
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The adaptation of the basic hole punching principle to TCP NAT
traversal was introduced in section 4 of [FSK05] and relies on the
simultaneous-open feature of TCP [RFC0793]. UDP and TCP hole
punching use nearly the same protocol. The main difference lies in
the way the clients perform connectivity checks after obtaining the
address pairs from the server; whereas in UDP a single socket is
sufficient, TCP clients require several sockets for the same address/
port tuple:
o a passive socket to listen for connectivity tests from peers and
o multiple active connections from the same address to test
connectivity of other peers.
The SYN sent out by client A to its peer B creates soft state in A's
NAT. At the same time, B tries to connect to A:
o if the SYN from B has left B's NAT before the arrival of A's SYN,
both endpoints perform simultaneous-open (4-way handshake of SYN/
SYNACK);
o otherwise A's SYN may not enter B's NAT, which leads to B
performing a normal open (SYN_SENT => ESTABLISHED) and A
performing a simultaneous-open (SYN_SENT => SYN_RCVD =>
ESTABLISHED).
In the latter case it is necessary that the NAT does not interfere
with a RST segment (REQ-4 in [GBF+07]). The simultaneous-open
solution is convenient due to its simplicity, and is thus a preferred
mode of operation in the TCP extension for ICE (section 2 of
[Ros07]).
We note that a simultaneous-open is not the only existing solution
for TCP NAT traversal [GTF04], [GF05]; other approaches are reviewed
in the next subsection.
The considerations in this section and the following subsections
motivate a discussion as to whether and how the DCCP state machine
can be enhanced to natively facilitate easier middlebox traversal.
1.3.1. Near Simultaneous-Open of Connections
Among the various TCP NAT traversal approaches, simultaneous-open
suggests itself due to its simplicity [GF05], [NAT-APP]. A
characteristic of simultaneous-open is that the clear distinction
between client and server is erased: both sides enter through active
(SYN_SENT) as well as passive (SYN_RCVD) states. This characteristic
is in conflict with several ideas underlying DCCP, as a clear
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separation between client and server has been one of the initial
design decisions ([RFC4340], 4.6). Furthermore, several mechanisms
implicitly rely on clearly-defined client/server roles:
o Feature Negotiation: with few exceptions, almost all of DCCP's
negotiable features use the "server-priority" reconciliation rule
([RFC4340], 6.3.1), whereby peers exchange their preference lists
of feature values, and the server decides the outcome.
o Closing States: only servers may generate CloseReq packets (asking
the peer to hold timewait state), while clients are only permitted
to send Close or Reset packets to terminate a connection
([RFC4340], 8.3).
o Service Codes: servers may be associated with multiple service
codes, while clients must be associated with exactly one
([RFC4340], 8.1.2).
o Init Cookies: may only be used by the server and on DCCP-Response
packets ([RFC4340], 8.1.4).
The latter two points are not obstacles per se, but hinder the
transition from a passive to an active socket. The assumption that
"all DCCP hosts are clients", on the other hand, must be dismissed
since it limits application programming. As a consequence, retro-
fitting simultaneous-open into DCCP does not seem a very sensible
idea.
1.3.2. Role Reversal
After the simultaneous-open, one of the simplest TCP NAT traversal
schemes involves role traversal ([Epp05] and [GTF04]), where a peer
first opens an active connection for the single purpose of punching a
hole in the firewall, and then reverts to a listening socket to
accept incoming connections arriving via the new path.
This solution has several disadvantages for DCCP. First, a DCCP
client would be required to change its role from initially 'client'
to 'server'. This makes common server processing difficult: it is
not clear how to assign multiple service codes (this would have to be
done after the transition); a similar issue arises with Init Cookies.
Further, the the client must not yet have started feature
negotiation, since its choice of initial options may rely on its role
(i.e. if an endpoint knows it is the server, it can make a priori
assumptions about the preference lists of features it is negotiating
with the client, thereby enforcing a particular policy). We
therefore do not recommend this approach for DCCP.
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2. Solution for DCCP NAT Traversal
This section revises packet processing for a DCCP-client and Server,
to assist in connection establishment when one or both peers are
located behind a middlebox. The method does not employ role
reversal; both endpoints start out with their designated roles, as
specified in [RFC4340].
2.1. Conventions and Terminology
The document uses the terms and definitions provided in [RFC4340].
Familiarity with this specification is assumed. In particular, the
following convention from ([RFC4340], 3.2) is used:
"Each DCCP connection runs between two hosts, which we often name
DCCP A and DCCP B. Each connection is actively initiated by one of
the hosts, which we call the client; the other, initially passive
host is called the server."
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].
2.2. DCCP-Listen Packet Format
The document updates DCCP by adding a new packet type: DCCP-Listen,
whose format is shown below
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Res | Type |X| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DCCP-Listen Packet Format
The Reserved (Res) field is specified by [RFC4340], and its
successors. This document does not modify that definition.
The Type field has the value XX-IANA-assigned-XX, which indicates
that this is a DCCP-Listen packet.
For a DCCP-Listen type of packet the following protocol fields MUST
be set to zero:
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Data Offset (the connection has not been established),
CCVal (the connection has not been established),
CsCov (there is no payload),
Sequence Number (no initial sequence number has been negotiated).
DCCP-Listen packets MUST set X to 1 (short sequence numbers are not
permitted), and endpoints MUST ignore any such packets with X set to
zero.
A DCCP-Listen Packet MUST NOT include any DCCP options (since this
packet does not modify the receiver protocol state) and MUST NOT
include application data.
2.3. Protocol Method
We use the term "session" as defined in ([RFC2663], 2.3): DCCP
sessions are uniquely identified by the tuple of <source IP-address,
source port, target IP-address, target port>.
DCCP, in addition, introduces service codes ([RFC4340], 8.1.2) which
can be used to identify different services that may be connected
using the same port.
We call the five-tuple <source IP-address, source port, service code,
target IP-address, target port> a fully specified DCCP connection,
and refer to endpoints that have been assigned all five parameters as
a "fully specified endpoint". DCCP-Listen packets are only sent for
the specific case of fully specified DCCP-server endpoints.
2.3.1. Protocol Method for DCCP-Server Endpoints
This document updates [RFC4340] for the case of fully specified DCCP-
server endpoints.
Prior to connection setup, it is common for DCCP-server endpoints to
not be fully specified: before the connection is established, a
server usually sets the target address / target port to wildcard
numbers (i.e. leaves these unspecified); the endpoint only becomes
fully specified after performing the handshake with an incoming, new
connection. For such cases, this document does not update the server
behaviour described in [RFC4340], i.e. no DCCP-Listen packet is sent.
A fully specified DCCP-server endpoint permits only one specific
client (matching the target IP-address / target port) to set up the
connection. When such a server is in the LISTEN state, it SHOULD use
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the method specified in this document, and send a single DCCP-Listen
packet to the remote endpoint.
The server SHOULD treat ICMP error messages received in response to a
DCCP-Listen packet as "soft errors", that do not abort a connection.
2.3.2. Protocol Method for DCCP-Client Endpoints
A DCCP-client that implements [RFC4340] and receives a DCCP-Listen
would issue a DCCP-Reset (as it would for all unknown types of
packet).
This document updates [RFC4340], so that a DCCP-client in any state
that receives a DCCP-Listen packet, MUST silently discard the packet.
The packet indicates only a willingness to accept a connection; if
the client has already established a connection, this has no meaning.
If the client is awaiting the response to a DCCP-Request, the client
does not need to take specific action, and should continue to use the
protocol method defined in [RFC4340].
Receipt of a DCCP-Listen packet by a server in the LISTEN state must
necessarily lead to a Reset (Reset Code 7, "Connection Refused" is
suggested). This is the expected behaviour for [RFC4340].
2.3.3. Processing by Routers and Middleboxes
Routers and middleboxes both need to forward DCCP packets. This
document does not specify the rules for forwarding these packets, but
notes that DCCP-Listen packets do not require special treatment and
should be forwarded end-to-end across the internet path. Middleboxes
may utilise the connection information (address, port, Service Code)
to establish local forwarding state.
2.4. Examples of Use
In the examples below, DCCP A is the client and DCCP B is the server.
NAT/Firewall device NA is placed before DCCP A, and NAT/Firewall
device NB is placed before DCCP B.
Both NA and NB use a policy that permits DCCP packets to traverse the
device for outgoing links, but only permit incoming DCCP packets when
a previous packet has been sent for the same connection.
DCCP A and DCCP B decide to communicate using some out-of-band
mechanism, whereupon the client and server are started. DCCP A
initiates a connection by sending a DCCP-Request. DCCP B actively
indicates its state by sending a Listen message. This fulfils the
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requirement of punching a hole in NB so that DCCP A can retransmit
the DCCP-Request and connect through it.
DCCP A DCCP B
------ NA NB ------
+------------------+ +-+ +-+ +-----------------+
|(1) Initiation | | | | | | |
|DCCP-Request --> +--+-+---X| | | |
| |<-+-+----+-+--+<-- DCCP-Listen |
| | | | | | | |
|DCCP-Request --> +--+-+----+-+->| |
| |<-+-+----+-+--+<-- DCCP-Response|
|DCCP-Ack --> +--+-+----+-+->| |
| | | | | | | |
|(2) Data transfer | | | | | | |
|DCCP-Data --> +--+-+----+-+->| |
+------------------+ +-+ +-+ +-----------------+
Sequence of events when a client is started before the server
The diagram below reverses this sequencing:
DCCP A DCCP B
------ NA NB ------
+------------------+ +-+ +-+ +-----------------+
|(1) Initiation | | | | | | |
| | | |X---+-+--+<-- DCCP-Listen |
|DCCP-Request --> +--+-+----+-+->| |
| | <+-+----+-+--+<-- DCCP-Response|
|DCCP-Ack --> +--+-+----+-+> | |
| | | | | | | |
|(2) Data transfer | | | | | | |
|DCCP-Data --> +--+-+----+-+> | |
+------------------+ +-+ +-+ +-----------------+
Sequence of events when a server is started before the client
2.5. Backwards Compatibility with RFC 4340
This proposal does not modify communication between a server
conforming to [RFC4340] and a client that implements the update
described in this document.
This proposal also does not modify communication for any server that
implements a passive-open without fully binding the addresses, ports
and service codes to be used.
A change in the protocol exchange does occur when a server implements
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this update and binds to a client that implements [RFC4340].
If DCCP A and B were not behind NAT devices, the receipt of a DCCP-
Listen packet at A by a server that implements [RFC4340] would lead
to a DCCP-Reset (presumably Code 3, "No Connection", since Code 7,
"Connection Refused" applies to DCCP-Requests only). This would
abort the connection.
Since the outgoing DCCP-Listen packet is expected to open a hole in
the firewall, the same conclusion is expected when using NATs; as in
the examples presented in the previous section.
The authors do not however expect these issues to introduce practical
deployment problems.
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3. Discussion of Design Decisions
This section identifies a number of design decisions that were made
in defining the current approach.
3.1. Generation of Listen Packets
Since DCCP-Listen packets solve a particular problem (NAT and/or
firewall traversal), the generation of DCCP-Listen packets on passive
sockets has been tied to a condition (i.e. the server is aware of the
expected client connection request), so as to not interfere with the
general case of "normal" DCCP connections.
In the TCP world, the analogue is a transition from LISTEN to
SYN_SENT by virtue of sending data: "A fully specified passive call
can be made active by the subsequent execution of a SEND" ([RFC0793],
3.8).
Unlike TCP, this proposal does not perform a role-change from passive
to active.
Like TCP, we require that DCCP-Listen packets are only sent by a
DCCP-server when the endpoint is fully specified (Section 2.3).
If this condition were considered too weak, it could be coupled with
a socket option to trigger generation of DCCP-Listen packets on fully
specified passive sockets.
3.2. Repetition of Listen Packets
Section 4.3 of [RFC4340] defines the LISTEN state as:
"Represents server sockets in the passive listening state. LISTEN
and CLOSED are not associated with any particular DCCP
connection."
This document revises this definition with regard to the protocol
method described in Section 2.3. In the current revision of this
document, this is represented as an additional transition (to
the LISTEN state), but the authors also acknowledge the possibility
of introducing a new state.
This state would allow a DCCP-Listen packet to be re-issued
periodically, to refresh firewall state. This approach would add
robustness to the proposed mechanism, but has the disadvantage that
the DCCP-Listen packets could then contribute unwanted load in a mis-
configured network and procedures may be needed to limit the rate and
number of packets sent.
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4. IANA Considerations
This document requires IANA action by allocation of a new Packet Type
from the IANA Packet Types Registry. The Registry entry is to
reference this document. This allocation requires IESG review and
approval and standards-track IETF RFC publication.
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5. Security Considerations
The method specified in this document exposes the state of a DCCP
server that has been explicitly configured to accept a connection
from a client. This requires prior out-of-band signaling between the
client and server (e.g. via SIP) to establish this state. The method
generates a packet addressed to the expected client.
This increases the vulnerability of the DCCP host by revealing which
ports are in a passive LISTEN state to the expected client (this
information is not encrypted, and therefore could be seen on the path
to the client through the network). We do not believe this change
significantly increases the complexity or vulnerability of a DCCP
implementation that conforms to [RFC4340].
Reception of a DCCP-Listen packet would trigger a DCCP-Reset in
[RFC4340], but is ignored by the method described in this protocol.
This does not introduce new security concerns.
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6. References
6.1. Normative References
[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.
6.2. Informative References
[Epp05] Eppinger, J-L., "TCP Connections for P2P Apps: A Software
Approach to Solving the NAT Problem", Carnegie Mellon
University/ISRI Technical Report CMU-ISRI-05-104,
January 2005.
[FSK05] Ford, B., Srisuresh, P., and D. Kegel, "Peer-to-Peer
Communication Across Network Address Translators",
Proceedings of USENIX-05, pages 179-192, 2005.
[GBF+07] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", Work In
Progress, draft-ietf-behave-tcp-07, April 2007.
[GF05] Guha, S. and P. Francis, "Characterization and Measurement
of TCP Traversal through NATs and Firewalls", Proceedings
of Internet Measurement Conference (IMC-05), pages 199-
211, 2005.
[GTF04] Guha, S., Takeda, Y., and P. Francis, "NUTSS: A SIP based
approach to UDP and TCP connectivity", Proceedings of
SIGCOMM-04 Workshops, Portland, OR, pages 43-48, 2004.
[H.323] ITU-T, "Packet-based Multimedia Communications Systems",
Recommendation H.323, July 2003.
[NAT-APP] Ford, B., Srisuresh, P., and D. Kegel, "Application Design
Guidelines for Traversal through Network Address
Translators", Work In Progress, draft-ford-behave-app-05,
March 2007.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
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[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[RFC3235] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[Ros07] Rosenberg, J., "TCP Candidates with Interactive
Connectivity Establishment (ICE)", Work In
Progress, draft-ietf-mmusic-ice-tcp-04, July 2007.
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Appendix A. Change Log
This is the first public draft.
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Authors' Addresses
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
Scotland
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk
Gerrit Renker
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
Scotland
Email: gerrit@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk
Fairhurst & Renker Expires April 3, 2008 [Page 18]
Internet-Draft DCCP NAT Traversal November 2007
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