Datagram Congestion Control Protocol (DCCP) Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal
draft-ietf-dccp-simul-open-08
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 5596.
|
|
|---|---|---|---|
| Author | Gorry Fairhurst | ||
| Last updated | 2015-10-14 (Latest revision 2009-05-02) | ||
| Replaces | draft-fairhurst-dccp-behave-update | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 5596 (Proposed Standard) | |
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(None)
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||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Lars Eggert | ||
| Send notices to | (None) |
draft-ietf-dccp-simul-open-08
DCCP Working Group G. Fairhurst
Internet-Draft University of Aberdeen
Updates: 4340 (if approved) May 02, 2009
Intended status: Standards Track
Expires: November 3, 2009
DCCP Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal
draft-ietf-dccp-simul-open-08
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This Internet-Draft will expire on November 3, 2009.
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Abstract
This document specifies an update to the Datagram Congestion Control
Protocol (DCCP), a connection-oriented and datagram-based transport
protocol. The update adds support for the DCCP-Listen packet. This
assists DCCP applications to communicate through middleboxes (e.g. a
DCCP server behind a firewall, or a Network Address Port Translator),
where peering endpoints need to initiate communication in a near-
simultaneous manner to establish necessary middlebox state.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope of this Document . . . . . . . . . . . . . . . . . . 3
1.2. DCCP NAT Traversal . . . . . . . . . . . . . . . . . . . . 3
1.3. Structure of this Document . . . . . . . . . . . . . . . . 4
2. Procedure for Near-Simultaneous Open . . . . . . . . . . . . . 5
2.1. Conventions and Terminology . . . . . . . . . . . . . . . 5
2.2. Protocol Method . . . . . . . . . . . . . . . . . . . . . 5
2.2.1. DCCP-Listen Packet Format . . . . . . . . . . . . . . 5
2.2.2. Protocol Method for DCCP Server Endpoints . . . . . . 8
2.2.3. Protocol Method for DCCP Client Endpoints . . . . . . 11
2.2.4. Processing by Routers and Middleboxes . . . . . . . . 12
2.3. Examples of Use . . . . . . . . . . . . . . . . . . . . . 13
2.3.1. Repetition of DCCP-Listen . . . . . . . . . . . . . . 13
2.3.2. Optional Triggered Retransmission of DCCP-Request . . 15
2.4. Backwards Compatibility with RFC 4340 . . . . . . . . . . 16
3. Discussion of Design Decisions . . . . . . . . . . . . . . . . 17
3.1. Rationale for a New Packet Type . . . . . . . . . . . . . 17
3.1.1. Use of sequence numbers . . . . . . . . . . . . . . . 18
3.2. Generation of Listen Packets . . . . . . . . . . . . . . . 18
3.3. Repetition of DCCP-Listen Packets . . . . . . . . . . . . 18
4. Security Considerations . . . . . . . . . . . . . . . . . . . 20
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
6. Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.1. Normative References . . . . . . . . . . . . . . . . . . . 26
7.2. Informative References . . . . . . . . . . . . . . . . . . 26
Appendix A. Discussion of Existing NAT Traversal Techniques . . . 28
A.1. NAT traversal Based on a Simultaneous-Request . . . . . . 29
A.2. Role Reversal . . . . . . . . . . . . . . . . . . . . . . 30
Appendix B. Change Log - to be removed by RFC-Ed . . . . . . . . 31
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 34
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1. Introduction
The Datagram Congestion Control Protocol (DCCP) [RFC4340] is both
datagram-based and connection-oriented. According to RFC 4340, DCCP
servers establish connections by passively listening for incoming
connection requests that are actively transmitted by DCCP clients.
These asymmetric roles can cause problems when the server is 'inside'
a middlebox, such as a Network Address and Port Translator (NAPT)
that only allows connection requests to be initiated from inside
(e.g. due to port overloading) [ID-BEHAVE-DCCP]. Host-based and
network firewalls can also implement policies that lead to similar
problems. This behaviour is currently the default for many
firewalls.
UDP can support middlebox traversal using various techniques
[RFC4787] that leverage the connectionless nature of UDP and are
therefore not suitable for DCCP. TCP supports middlebox traversal
through the use of its simultaneous open procedure [RFC5382]. The
concepts of the TCP solution are applicable to DCCP, but DCCP can not
simply reuse the same methods (see Appendix A).
After discussing the problem space for DCCP, this document specifies
an update to the DCCP state machine to offer native support to allow
a DCCP client to establish a connection to a DCCP server that is
inside one or more middleboxes. This reduces dependence on external
aids such as data relay servers [ID-BEHAVE-TURN] by explicitly
supporting a widely used principle known as 'hole punching'.
The method requires only a minor change to the standard DCCP
operational procedure. The use of a dedicated DCCP packet type ties
usage to a specific condition, ensuring the method is inter-operable
with hosts that do not implement this update, or choose to disable it
(see Section 4).
1.1. Scope of this Document
This method is useful in scenarios when a DCCP server is located
inside the perimeter controlled by a middlebox. It is relevant to
both client/server and peer-to-peer applications, such as VoIP, file
sharing, or online gaming and assists connections that utilise prior
out-of-band signaling (e.g. via a well-known rendezvous server
([RFC3261], [H.323])) to notify both endpoints of the connection
parameters ([RFC3235], [NAT-APP]).
1.2. DCCP NAT Traversal
The behavioural requirements for NAT devices supporting DCCP are
described in [ID-BEHAVE-DCCP]. A "traditional NAT" [RFC3022], that
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directly maps an IP address to a different IP address does not
require the simultaneous open method described in this document.
The method is required when the DCCP server is positioned behind one
or more NAPT devices in the path (hierarchies of nested NAPT devices
are possible). This document refers to DCCP hosts located inside the
perimeter controlled by one or more NAPT devices as having "private"
addresses, and to DCCP hosts located in the global address realm as
having "public" addresses.
DCCP NAT traversal is considered for the following scenarios:
1. Private client connects to public server.
2. Public client connects to private server.
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 possible using the protocol defined in
[RFC4340]. A pre-configured, static NAT address map would allow
outside hosts to connections in cases (2) and (3).
A DCCP implementation conforming to [RFC4340] and a NAT device
conforming to [ID-BEHAVE-DCCP] would require a DCCP relay server to
perform NAT traversal for cases (2) and (3).
This document describes a method to support cases (2) and (3) without
the aid of a DCCP relay server. This method updates RFC 4340 and
requires the DCCP server to discover the IP address and the DCCP port
that correspond to the DCCP client. Such signalling may be performed
out-of-band (e.g. using SDP [RFC4566]).
1.3. Structure of this Document
For background information on existing NAT traversal techniques,
please consult Appendix A.
The normative specification of the update is presented in Section 2.
An informative discussion of underlying design decisions then
follows, in Section 3. Security considerations are provided in
Section 4 and IANA considerations in the concluding Section 5.
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2. Procedure for Near-Simultaneous Open
This section is normative and specifies the simultaneous-open
technique for DCCP. It updates the connection-establishment
procedures of [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. Protocol Method
The term "session" is used 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, which can be used to
identify different services available via the same port [ID-DCCP-SC].
2.2.1. DCCP-Listen Packet Format
This document adds a new DCCP packet type, DCCP-Listen, whose format
is shown below.
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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| Reserved | Sequence Number High Bits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number Low Bits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format of a DCCP-Listen Packet
o The Source Port is the port on which the DCCP server is listening
for a connection from the IP address that appears as the
destination IP address in the packet.
o The Destination Port is the port selected by a DCCP client to
identify a connection by the DCCP client. In this technique, this
value must be communicated out-of-band to the server.
o The value of X MUST be set to 1. A DCCP-Listen packet is sent
before a connection is established, therefore there is no way to
negotiate use of short sequence numbers ([RFC4340], 5.1).
o The value of the sequence number field in a DCCP-Listen packet is
not related to the DCCP sequence number used in normal DCCP
messages (see Section 3 for a description of the use of the DCCP
sequence number). Thus, for DCCP-Listen packets:
* A DCCP server SHOULD set the high and low bits of the Sequence
Number field to 0.
* A DCCP client MUST ignore the value of the Sequence Number
field.
* Middleboxes MUST NOT interpret sequence numbers on DCCP-Listen
packets.
o The Service Code field contains the Service Code value for which
the server is listening for a connection ([RFC4340], 8.1.2,
[ID-DCCP-SC]). This value MUST correspond to a Service Code that
the server is actually offering for a connection identified by the
same source IP address and the same Source Port as that of the
DCCP-Listen packet. Since the server may use multiple Service
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Codes, the specific value of the Service Code field needs to be
communicated out-of-band, from client to server, prior to sending
the DCCP-Listen packet, e.g. described using the Session
Description Protocol, SDP [RFC4566].
o At the time of writing, there are no known uses of header options
([RFC4340] , sec. 5.8) with a DCCP-Listen packet. Clients MUST
ignore all options in received DCCP-Listen packets. Therefore,
feature values can not be negotiated using a DCCP-Listen packet.
o At the time of writing, there are no known uses of payload data
with a DCCP-Listen packet. Since DCCP-Listen packets are issued
before an actual connection is established, endpoints MUST ignore
any payload data encountered in DCCP-Listen packets.
o The following protocol fields are required to have specific
values:
* Data Offset MUST have a value of five or more (i.e. at least 20
bytes).
* CCVal MUST be zero (a connection has not been established).
* CsCov MUST be zero (use of the CsCov feature can not be
negotiated).
* Type has the value 10, assigned by IANA to denote a DCCP-Listen
packet.
* X MUST be 1 (the Generic header must be used).
The remaining fields, including the "Res" and "Reserved" fields are
specified by [RFC4340] and its successors. The interpretation of
these fields is not modified by this document.
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Note to the RFC Editor:
This value assigned to the DCCP-Listen packet needs to be confirmed
by IANA when this document is published. Please then remove this
note.
==> End of note to the RFC Editor. <==
2.2.2. Protocol Method for DCCP Server Endpoints
This document updates section 8.1 of [RFC4340] for the case of a
fully specified DCCP server endpoint. The update modifies the way
the server performs a passive-open.
Prior to connection setup, it is common for a DCCP server endpoint to
not be fully specified: before the connection is established, a
server usually specifies only the destination port, and Service Code.
(Sometimes the destination address is also specified.) This leaves
the source address and source port unspecified. The endpoint only
becomes fully specified after performing the handshake for an
incoming connection. For such cases, this document does not update
[RFC4340], i.e. the server adheres to the existing state transitions
in the left half of Figure 2 (CLOSED => LISTEN => RESPOND).
A fully specified DCCP server endpoint permits exactly one client,
identified by source IP-address:port, destination IP-address:port,
plus a single Service Code, to set up the connection. Such a server
SHOULD perform the actions and state transitions shown in the right
half of Figure 2 in section 8.4, and specified below.
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unspecified remote +--------+ fully specified remote
+---------------------| CLOSED |---------------------+
| +--------+ send DCCP-Listen |
| |
v v
+--------+ timeout +---------+
| LISTEN | +---+-----------| INVITED |
+--------+ | | +---------+
| | | 1st / 2nd ^ |
| more than 2 | | retransm. | | receive
| retransmissions | +-------------+ | Request
| | resend Listen v
| | +---------+
| +-------------->| LISTEN1 |
| +---------+
| |
| receive Request +---------+ receive Request* |
+------------------->| RESPOND |<--------------------+
send Response +---------+ send Response
* Note: A server that responds a DCCP-Request in the INVITED state,
transitions to the LISTEN1 state and then immediately transitions
to the RESPOND state. This does not require reception of an
additional DCCP-Request packet.
Figure 2: Updated state transition diagram for DCCP-Listen
This diagram introduces two additional DCCP server states in addition
to those listed in section 4.3 of [RFC4340]:
o INVITED The INVITED state is associated with a specific DCCP
connection and represents a fully-specified server socket in the
listening state that is generating DCCP-Listen packets towards the
client.
o LISTEN1 The LISTEN1 state is associated with a specific DCCP
connection and represents a fully-specified server socket in the
passive listening state that will not generate further DCCP-Listen
packets towards the client.
A fully specified server endpoint performs a passive-open from the
CLOSED state by inviting the remote client to connect. This is
performed by sending a single DCCP-Listen packet to the specified
remote IP-address:port, using the format specified in Section 2.2.1.
The figure below provides pseudocode describing the packet processing
in the INVITED state. This processing step follows Step 2 in section
8,5 of [RFC4340]).
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The INVITED state is, like LISTEN, a passive state, characterised by
waiting in the absence of an established connection. If the server
endpoint in the INVITED state receives a DCCP-Request that matches
the set of bound ports and addresses, it transitions to the LISTEN'
state and then immediately transitions to the RESPOND state, where
further processing resumes as specified in [RFC4340].
The server SHOULD repeat sending a DCCP-Listen packet while in the
INVITED state, at a 200 millisecond interval with up to at most 2
repetitions (Section 3 discusses this choice of time interval). If
the server is still in the INVITED state after a further period of
200ms following transmission of the third DCCP-Listen packet, it
SHOULD progress to the LISTEN1state.
Fully specified server endpoints SHOULD treat ICMP error messages
received in response to a DCCP-Listen packet as "soft errors" that do
not cause a state transition. Reception of an ICMP error message as
a result of sending a DCCP-Listen packet does not necessarily
indicate a failure of the following connection request, and therefore
should not result in a server state change. This reaction to soft
errors exploits the valuable feature of the Internet that for many
network failures, the network can be dynamically reconstructed
without any disruption of the endpoints.
Server endpoints SHOULD ignore any incoming DCCP-Listen packets. A
DCCP server in the LISTEN, INVITED, or LISTEN1states MAY generate a
DCCP-Reset packet (Code 7, "Connection Refused") in response to a
received DCCP-Listen packet. This DCCP-Reset packet is an indication
that two servers are simultaneously awaiting connections on the same
port.
Further details on the design rationale are discussed in Section 3.
The figure below provides pseudocode describing the packet processing
in the INVITED state. This processing step follows Step 2 in section
8.5 of RFC 4340 [RFC4340]
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Step 2a: Process INVITED state
If S.state == INVITED,
/* State only entered for fully-specified server endpoints */
/* on entry S will have been set to a socket */
If P.type == Request
/* Exit INVITED state and continue to process the packet */
S.state = LISTEN1
Continue with S.state := LISTEN1
Otherwise,
If P.type == Listen
/* The following line is optional */
Generate Reset(Connection Refused)
/* otherwise Drop packet and return */
Otherwise,
Generate Reset(No Connection) unless P.type == Reset
Step 2b: Process LISTEN1 state
If S.state == LISTEN1,
/* State only entered for fully-specified server endpoints */
/* Follows receipt of a Response packet */
/* or sending third Listen packet (after timer expiry) */
If P.type == Request,
S.state = RESPOND
Choose S.ISS (initial seqno) or set from Init Cookies
Initialize S.GAR := S.ISS
Set S.ISR, S.GSR, S.SWL, S.SWH from packet or Init Cookies
Continue with S.state == RESPOND
/* A Response packet will be generated in Step 11 */
Otherwise,
If P.type == Listen
/* The following line is optional */
Generate Reset(Connection Refused)
/* otherwise Drop packet and return */
Otherwise,
Generate Reset(No Connection) unless P.type == Reset
Figure 3: Updated DCCP pseudocode for INVITED and LISTEN' states
2.2.3. Protocol Method for DCCP Client Endpoints
This document updates section 8.1.1 of [RFC4340], by adding the
following rule for the reception of DCCP-Listen packets by clients:
A client in any state MUST silently discard any received DCCP-Listen
packet, unless it implements the optional procedure defined in the
following section.
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2.2.3.1. Optional generation of Triggered Requests
This section describes an optional optimisation at the client that
can avoid clients having to wait for a timeout following a dropped
DCCP-Request. The operation requires clients to respond to reception
of DCCP-Listen packets when received in the REQUEST state. DCCP-
Listen packets MUST be silently discarded in all other states.
A client implementing this optmisation MAY immediately perform a
retransmission of a DCCP-Request following the reception of the first
DCCP-Listen packet. The retransmission is performed in the same
manner as a timeout in the REQUEST state [RFC4340]. A triggered
retransmission SHOULD result in the client increasing the
exponential-backoff timer interval.
Note that a path delay greater than 200ms will result in multiple
DCCP-Listen packets arriving at the client before a DCCP-Response is
received. Clients MUST therefore perform only one such
retransmission for each DCCP connection. This requires maintaining
local state (e.g. one flag per connection)
Implementors and users of this optional method need to be aware that
host timing or path reordering can result in a server receiving two
DCCP-Requests (i.e., the server sending one unnecessary packet).
This would, in turn, trigger a client to send a second corresponding
DCCP-Response (also unnecessary). These additional packets are not
expected to modify or delay the DCCP open procedure [RFC4340].
Section 2.3.2 provides examples of the use of triggered
retransmission.
2.2.4. Processing by Routers and Middleboxes
DCCP-Listen packets do not require special treatment and should thus
be forwarded end-to-end across Internet paths, by routers and
middleboxes alike.
Middleboxes may utilise the connection information (address, port,
Service Code) to establish local forwarding state. The DCCP-Listen
packet carries the necessary information to uniquely identify a DCCP
session in combination with the source and destination addresses
(found in the enclosing IP-header), including the DCCP Service Code
value [ID-DCCP-SC]. The processing of the DCCP-Listen packet by NAT
devices is specified in [ID-BEHAVE-DCCP].
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2.3. Examples of Use
In the examples below, DCCP A is the client and DCCP B is the server.
A middlebox device (NAT/Firewall), NA is placed before DCCP A, and
another middlebox, 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 out for the same connection.
In the figure below, DCCP A and DCCP B decide to communicate using an
out-of-band mechanism (in this case labelled SDP), whereupon the
client and server are started. DCCP B actively indicates its
listening state by sending a DCCP-Listen message. This fulfils the
requirement of punching a hole in NB (also creating the binding to
the external address and port). This message is dropped by NA since
no hole exists there yet. DCCP A initiates a connection by entering
the REQUEST state and sending a DCCP-Request. (It is assumed that if
NA were a NAT device, then this would also result in a binding that
maps the pinhole to the external address and port.) The DCCP-Request
is received by DCCP B, via the binding at NB. DCCP B transmits the
DCCP-Response and connects through the bindings now in place at NA
and NB.
DCCP A DCCP B
------ NA NB ------
+-----------------+ +-+ +-+ +-----------------+
| | | | | | | | State = CLOSED
| SDP --> |--+-+----+-+->| | State = INVITED
| | | |X---+-+--|<-- DCCP-Listen |
|(State=REQUEST) | | | | | | |
|DCCP-Request --> |--+-+----+-+->| |
|(State=PARTOPEN) | <+-+----+-+--|<-- DCCP-Response| State = RESPOND
|DCCP-Ack --> |--+-+----+-+> | |
| | | | | | | |
| | | | | | | |
|DCCP-Data --> |--+-+----+-+->| | State = OPEN
+-----------------+ +-+ +-+ +-----------------+
Figure 4: Event sequence when the server is started before the client
2.3.1. Repetition of DCCP-Listen
This section examines the effect of not receiving the DCCP-Request.
The figure below shows the sequence of packets where the DCCP server
enters the INVITED state after reception of out-of-band signaling
(e.g. SDP). The key timer operations at the client and server are
respectively shown on the left and right of the diagram. It
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considers the case when the server does not receive a DCCP-Request
within the first 600 ms (often the request would be received within
this interval).
The repetition of DCCP-Listen packets may be implemented using a
timer. The timer is restarted with an interval of 200ms when sending
each DCCP-Listen packet. It is cancelled when the server leaves the
INVITED state. If the timer expires after the first and second
transmission, it triggers a transmission of another DCCP-Listen
Packet. If it expires after sending the third DCCP-Listen packet,
the server leaves the INVITED state, to enter the LISTEN1state (where
it passively waits for a DCCP-Request).
DCCP A DCCP B
------ NA NB ------
+----+ +-+ +-+ +-----------------+
| | | | | | | | State = CLOSED
| -->|--+-+----+-+--|--> SDP |
| | | | | | | | State = INVITED
| | | | | | | |
| | | |X---+-+--|<-- DCCP-Listen | Timer Starts
| | | | | | | | |
DCCP-Request | -->|--->+--X | | | (dropped) | |
Timer Starts | | | | | | | | |
| | | | | | | | | 1st Timer Expiry
| | |<-+-+----+++--|<-- DCCP-Listen |
| | | | | | | | | Timer Starts
| | | | | | | | | |
| | | | | | | | | 2nd Timer Expiry
| | | | | | | | |
| | |<-+-+----+-+--|<-- DCCP-Listen | Timer Starts
| | | | | | | | | |
| | | | | | | | | 3rd Timer Expiry
| | | | | | | | |
| | | | | | | | | State = LISTEN1
| ~ ~ ~ ~ ~ ~ ~ ~
| | | | | | | | |
Timer Expiry | -->|--+-+----+-+--|--> DCCP-Request |
| | | | | | | | State = RESPOND
| <--|--+-+----+-+--|<-- DCCP-Response|
+----+ +-+ +-+ +-----------------+
Figure 5: Repetition of the DCCP-Listen Packet
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2.3.2. Optional Triggered Retransmission of DCCP-Request
The following figure illustrates a triggered retransmission. In this
figure, the first DCCP-Listen is assumed to be lost in the network
(e.g. does not open a pin-hole at NB). A later DCCP-Request is also
not received (perhaps as a side-effect of the first loss). After
200ms, the DCCP-Listen packet is retransmitted and correctly
received. This triggers the retransmission of the DCCP-Request,
which, when received, results in a corresponding DCCP-Response.
DCCP A DCCP B
------ NA NB ------
+-----------------+ +-+ +-+ +-----------------+
| | | | | | | | State = CLOSED
|SDP |--+-+----+-+->| | State = INVITED
|(State= REQUEST) | | | | | | |
| | | | | |X-|<-- DCCP-Listen |
|DCCP-Request --> |--+-+---X| | | |
| | <+-+----+-+--|<-- DCCP-Listen |(retransmit)
| | | | | | | |
|DCCP-Request --> |--+-+----+-+->| | State = RESPOND
| (Triggered) | | | | | | |
| |<-+-+----+-+--|<-- DCCP-Response|
|(State= PARTOPEN)| | | | | | |
|DCCP-Ack --> |--+-+----+-+->| | State = OPEN
+-----------------+ +-+ +-+ +-----------------+
Figure 6: Example showing a triggered DCCP-Request
The figure below illustrates the sequence of packets exchanged when a
DCCP-Listen and DCCP-Request are processed out of order. Reception
of the DCCP-Listen packet by the client triggers retransmission of
the DCCP-Request. The server responds to the first DCCP-Request, and
enters the RESPOND state. The server subsequently responds to the
second DCCP-Request with another DCCP-Response, which is ignored by
the client (already in the PARTOPEN state).
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DCCP A DCCP B
------ NA NB ------
+-----------------+ +-+ +-+ +-----------------+
| | | | | | | | State = CLOSED
|SDP |--+-+----+-+->| | State = INVITED
|(State = REQUEST)| | | | | | |
|DCCP-Request --> |--+-+- -+-+--|<-- DCCP-Listen |
| | | | \/ | | | |
| | | | /\ | | | |
| |<-+-+- -+-+->| |
|DCCP-Request --> |--+-+- -+-+--|<-- DCCP-Response| State = RESPOND
| (Triggered) | | | \/ | | | |
| | | | /\ | | | |
| |<-+-+- -+-+->| |
|(State= PARTOPEN)| | | | | | |
|DCCP-Ack --> |--+-+- -+-+--|<-- DCCP-Response|
| (Triggered) | | | \/ | | | |
| | | | /\ | | | |
| (Ignored) |<-+-+- -+-+->| | State = OPEN
| | | | | | | |
+-----------------+ +-+ +-+ +-----------------+
Figure 7: Example showing an unnecessary triggered DCCP-Request
2.4. Backwards Compatibility with RFC 4340
No changes are required if a DCCP client conforming to this document
communicates with a DCCP server conforming to [RFC4340].
If a client implements only [RFC4340], an incoming DCCP-Listen packet
would be ignored due to step 1 in [RFC4340], 8.1, which at the same
time also conforms to the behaviour specified by this document.
This document further does not modify communication for any DCCP
server that implements a passive-open without fully binding the
addresses, ports and Service Codes to be used. The authors therefore
do not expect practical deployment problems with existing conformant
DCCP implementations.
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3. Discussion of Design Decisions
This is an informative section that reviews the rationale for the
design of this method.
3.1. Rationale for a New Packet Type
The DCCP-Listen packet specified in Section 2.2.1 has the same format
as the DCCP-Request packet ([RFC4340], 5.1), the only difference is
in the value of the Type field. The usage, however, differs. The
DCCP-Listen packet serves as an advisory message, not as part of the
actual connection setup: sequence numbers have no meaning, and no
payload can be communicated.
A DCCP-Request packet could in theory also have been used for the
same purpose. The following arguments were against this:
The first problem was that of semantic overloading: the DCCP-Request
defined in [RFC4340] serves a well-defined purpose, being the initial
packet of the 3-way handshake. Additional use in the manner of a
DCCP-Listen packet would have required DCCP processors to have had
two different processing paths: one where a DCCP-Request was
interpreted as part of the initial handshake, and another where the
same packet was interpreted as an indicator message. This would
complicate packet processing in hosts and in particular stateful
middleboxes (which may have restricted computational resources).
The second problem is that a client receiving a DCCP-Request from a
server could generate a DCCP-Reset packet if it had not yet entered
the REQUEST state (step 7 in [RFC4340], 8.5). The method specified
in this document lets client endpoints ignore DCCP-Listen packets.
Adding a similar rule for the DCCP-Request packet would have been
cumbersome: clients would not have been able to distinguish between a
Request meant to be an indicator message and a genuinely erratic
connection initiation.
The third problem is similar, and refers to a client receiving the
indication after having itself sent a (connection-initiation) DCCP-
Request. Step 7 in section 8.5 of [RFC4340] requires the client to
reply to an "indicator message" Request from the server with a DCCP-
Sync. Since sequence numbers are ignored for this type of message,
additional and complex processing would become necessary: either to
ask the client not to respond to a DCCP-Request when the request is
of type "indicator message"; or ask middleboxes and servers to ignore
Sync packets generated in response to "indicator message" DCCP-
Requests. Furthermore, since no initial sequence numbers have been
negotiated at this stage, sending a DCCP-SyncAck would not be
meaningful.
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The use of a separate packet type therefore allows simpler and
clearer processing.
3.1.1. Use of sequence numbers
Although the DCCP-Listen sequence number fields are ignored, they
have been retained in the DCCP-Listen packet header to reuse the
generic header format from section 5.1 of [RFC4340].
DCCP assigns a random initial value to the sequence number when a
DCCP connection is established [RFC4340]. However, a sender is
required to set this value to zero for a DCCP-Listen packet. Both
clients and middleboxes are also required to ignore this value.
The rationale for ignoring the sequence number fields of DCCP-Listen
packets is that at the time the DCCP-Listen is exchanged, the
endpoints have not yet entered connection setup: the DCCP-Listen
packet is sent while the server is still in the passive-open
(INVITED) state, i.e. it has not yet allocated state, other than
binding to the client's IP-address:port and Service Code.
3.2. Generation of Listen Packets
A DCCP server SHOULD by default permit generation of DCCP-Listen
packets. Since DCCP-Listen packets solve a particular problem with
NAT and/or firewall traversal, the generation of DCCP-Listen packets
on passive sockets is tied to a condition (binding to an a priori
known remote address and Service Code) to ensure this does not
interfere with the general case of "normal" DCCP connections (where
client addresses are generally not known in advance).
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 update does not perform a role-change from
passive to active. Like TCP, DCCP-Listen packets are only sent by a
DCCP-server when the endpoint is fully specified (Section 2.2).
3.3. Repetition of DCCP-Listen Packets
Repetition is a necessary requirement, to increase robustness and the
chance of successful connection establishment when a DCCP-Listen
packet is lost due to congestion, link loss, or some other reason.
The decision to recommend a maximum number of 3 timeouts (2 repeated
copies of the original DCCP-Listen packet) results from the following
considerations: The repeated copies need to be spaced sufficiently
far apart in time to avoid suffering from correlated loss. The
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interval of 200 ms was chosen to accommodate a wide range of wireless
and wired network paths.
Another constraint is given by the retransmission interval for the
DCCP-Request ([RFC4340], 8.1.1). To establish state, intermediate
systems need to receive a (retransmitted) DCCP-Listen packet before
the DCCP-Request times out (1 second). With three timeouts, each
spaced 200 milliseconds apart, the overall time is still below one
second. On the other hand, the sum of 600 milliseconds is
sufficiently large to provide for longer one-way delays, such as e.g.
found on some wireless links.
The rationale behind transitioning to the LISTEN1 state after two
repetitions is that other problems, independent of establishing
middlebox state, may occur (such as delay or loss of the initial
DCCP-Request). Any late or retransmitted DCCP-Request packets will
then still reach the server allowing connection establishment to
successfully complete.
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4. Security Considerations
General security considerations for DCCP are described in [RFC4340].
Security considerations for Service Codes are further described in
[ID-DCCP-SC].
The method specified in this document generates a DCCP-Listen packet
addressed to a specific DCCP client. This exposes the state of a
DCCP server that is in a passive listening state (i.e. waiting to
accept a connection from a known client).
The exposed information is not encrypted and therefore could be seen
on the network path to the DCCP client. An attacker on this return
path could observe a DCCP-Listen packet and then exploit this by
spoofing a packet (e.g. DCCP-Request, DCCP-Reset) with the IP
addresses, DCCP ports, and Service Code that correspond to the values
to be used for the connection. As in other on-path attacks, this
could be used to inject data into a connection or to deny a
connection request. A similar on-path attack is also possible for
any DCCP connection, once the session is initiated by the client
([RFC4340], Section 18).
The DCCP-Listen packet is only sent in response to explicit prior
out-of-band signaling from a DCCP client to the DCCP server (e.g.
SDP [RFC4566]) information communicated via the Session Initiation
Protocol [RFC3261]), and will normally directly precede a DCCP-
Request sent by the client (which carries the same information).
This update does not significantly increase the complexity or
vulnerability of a DCCP implementation that conforms to [RFC4340]. A
DCCP server should therefore by default permit generation of DCCP-
Listen packets. A server that wishes to prevent disclosing this
information MAY refrain from generating DCCP-Listen packets, without
impacting subsequent DCCP state transitions, but possibly inhibiting
middlebox traversal.
The DCCP base specification in RFC 4340 defines an Init Cookie
option, which lets a DCCP server avoid having to hold any state until
the three-way connection setup handshake has completed. This
specification enables an out-of-band mechanism that forces the server
to hold state for a connection that has not yet been established.
This is a change in the security profile of DCCP, although the impact
is expected to be minimal and depends on the security features of the
out-of-band mechanism (SIP SDP is one such mechanism that provides
sufficient security features).
The method creates a new way for a client to set up a DCCP connection
to a server using out-of-band data, transported over a signaling
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connection. If the signaling connection is not encrypted, an
eavesdropper could see the client IP address and the port for the to-
be-established DCCP connection and generate a DCCP-Listen packet
towards the client using its own server-IP address and port.
However, a client will only respond to a received DCCP-Listen packet
if the server-IP address and port match an existing DCCP connection
that is in the REQUEST state (section 2.3.2). The method therefore
cannot be used to redirect the connection to a different server IP
address.
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5. IANA Considerations
The IANA should register a new Packet Type, "DCCP-Listen", in the
IANA DCCP Packet Types Registry. The decimal value 10 has been
assigned to this type. This registry entry must reference this
document.
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Note to the RFC Editor:
This value of 10 must be confirmed by IANA in the registry when this
document is published, please then remove this note.
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Acknowledgement
This update was originally co-authored by Dr Gerrit Renker,
University of Aberdeen, and the present author acknowledges his
insight in design of the protocol mechanism and in careful review of
the early revisions of the document text. Dan Wing assisted on
issues relating to the use of NAT and NAPT.
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6. Disclaimer
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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7. References
7.1. Normative References
[ID-DCCP-SC]
Fairhurst, G., "The DCCP Service Code", Work In
Progress, draft-ietf-dccp-serv-codes-09, June 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", March 2006.
7.2. 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.
[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.
[ID-BEHAVE-DCCP]
"Network Address Translation (NAT) Behavioral Requirements
for DCCP", Work in Progress draft-ietf-behave-dccp-05.txt,
2008.
[ID-BEHAVE-TURN]
Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", Work In
Progress, draft-ietf-behave-turn-14, February 2008.
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[ID-MMUSIC-ICE]
Rosenberg, J., "TCP Candidates with Interactive
Connectivity Establishment (ICE)", Work In
Progress, draft-ietf-mmusic-ice-tcp-07, February 2008.
[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",
September 1981.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
August 1999.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", January 2001.
[RFC3235] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", January 2002.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", June 2002.
[RFC4566] "SDP: Session Description Protocol", July 2006.
[RFC4787] "Network Address Translation (NAT) Behavioral Requirements
for Unicast UDP", January 2007.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP",
RFC5382, April 2007.
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Appendix A. Discussion of Existing NAT Traversal Techniques
This appendix provides a brief review of existing techniques to
establish connectivity across NAT devices, with the aim of providing
background information. This first considers TCP NAT traversal based
on simultaneous-open, and then discuss a second technique based on
role reversal. Further information can be found in [GTF04] and
[GF05].
A central idea shared by these techniques is 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.
UDP and TCP hole punching use nearly the same technique [RFC4787].
The adaptation of the basic UDP hole punching principle to TCP NAT
traversal [RFC5382] was introduced in section 4 of [FSK05] and relies
on the simultaneous-open feature of TCP [RFC0793]. A further
difference between UDP and TCP lies in the way the clients perform
connectivity checks, after obtaining suitable address pairs for
connection establishment. Whereas in UDP a single socket is
sufficient, TCP clients require several sockets for the same address
and port tuple:
o a passive socket to listen for connectivity tests from peers and
o multiple active connections from the same address to test
reachability 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).
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In the latter case, it is necessary that the NAT does not interfere
with a RST segment (REQ-4 in [RFC5382]). The simultaneous-open
solution is convenient due to its simplicity, and is thus a preferred
mode of operation in the TCP extension for ICE ([ID-MMUSIC-ICE], sec.
2).
A.1. NAT traversal Based on a Simultaneous-Request
Among the various TCP NAT traversal approaches, the one using a TCP
simultaneous-open suggests itself as a candidate for DCCP due to its
simplicity [GF05], [NAT-APP].
A characteristic of TCP simultaneous-open is that this erases the
clear distinction between client and server: both sides enter through
active (SYN_SENT) as well as passive (SYN_RCVD) states. This
characteristic conflicts with the DCCP design decision to provide a
clear separation between client and server functions ([RFC4340],
4.6).
In DCCP 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 a peer exchanges its preference lists
of feature values, and the server decides the outcome.
o Closing States: only a server may generate DCCP-CloseReq packets
(asking the peer to hold timewait state), while a client is only
permitted to send DCCP-Close or DCCP-Reset packets to terminate a
connection ([RFC4340], 8.3).
o Service Codes [ID-DCCP-SC]: a server may be associated with
multiple Service Codes, while a client must be associated with
exactly one ([RFC4340], 8.1.2).
o Init Cookies: may only be used by a server and on DCCP-Response
packets ([RFC4340], 8.1.4).
The latter two points are not obstacles per se, but would have
hindered the transition from a passive to an active socket. In DCCP,
a DCCP-Request is only generated by a client. The assumption that
"all DCCP hosts may be clients", was dismissed, since it would
require undersirable changes to the state machine and would limit
application programming. As a consequence, the retro-fitting a TCP-
style simultaneous-open into DCCP to allow simulatenous exchange of
DCCP-Connect packets was not recommended.
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A.2. Role Reversal
Another simple TCP NAT traversal scheme uses 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, accepting connections arriving via the new
path.
This solution would have had several disadvantages if used with DCCP.
First, a DCCP server would be required to change its role to
temporarily become a 'client'. This would have required modification
to the state machine, in particular the treatment of Service Codes
and perhaps Init Cookies. Further, the method must follow feature
negotiation, since an endpoint's choice of initial options can 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).
Finally, the server would have needed additional processing to ensure
that the connection arriving at the listening socket matches the
previously opened active connection.
This approach was therefore not recommend for DCCP.
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Appendix B. Change Log - to be removed by RFC-Ed
Revision 00 was based on a previous individual submission
draft-fairhurst-dccp-behave-update-01 by the same authors.
Revision 01:
o introduced many format changes to improve readability
o migrated background information into the Appendix
o added Section 1.3 to summarize the document structure
o updated introductory paragraph of Section 2 to account for new
structure
o added captions to all figures
o updated the specification in Section 2 to (i) permit on DCCP-
Listen packets; (ii) explain why the presence of payload data is
not useful; (iii) clarify that middleboxes must not interpret
sequence numbers on DCCP-Listen packets
o clarified that the default value of the Allow Short Seqno feature
is to be used
o added references to the Service Code draft [ID-DCCP-SC]
o clarified the processing of DCCP-Listen packets by server
endpoints
o corrected the reaction of a client implementing [RFC4340] only -
DCCP-Listen packets are treated as unknown and hence do not
generate a DCCP-Reset
o swapped order of IANA / Security-Considerations sections
o added a note in the Security Considerations section that servers
may refrain from generating DCCP-Listen packets
Revision 02:
o minor edits following WG feedback at IETF meeting
o updated to reflect ID.Behave-DCCP
o update to reflect comments from Colin Perkins
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o added a tentative IANA code point (as suggested at IETF-73)
o DRAFT -02
o Edits following editorial corrections and suggestions from Tom
Phelan
o Edits following comments from Dan Wing on role of NAT,
retransmission, and other issues.
o Revised authors list
o Reworded abstract, reworded appendices to clarify what was not
done
o Checked spelling
o Although this version includes significant changes to format and
text it does not seek to modify the intended procedure for a
server.
o
o DRAFT - 03
o Comments by Dan Wing
o DRAFT -04
o Corrections by Dan Wing, and new diagram Figure 5 to and text to
clarify the retransmission algorithm.
o DRAFT -05
o Sections re-ordered to bring the packet type definition to the
front, and to correct a section mis-order in draft-04. References
linked to IETF WGs and updated to satisfy IDNiTs.
o A number of Typos were fixed.
o DRAFT -06
o This draft follows a completed WGLC.
o It includes responses to comments from Eddie Kohler, during WGLC.
o Magnus Westerlund requested two updates:
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o A triggered join, based on reception of a DCCP-Listen causing a
retransmission of a DCCP-Request.
o Updated figure 5.
o Updated language to differentiate the loss-prevention DCCP-Listen
repetition being confused with the DCCP-Request retransmission.
o Added pseudocode and redrew state diagram, to match the
pseudocode, including a transitory state transition to the
LISTEN1state to process the received DCCP-Request. This is
intended to be the same processing as in draft -05, even though
the diagram now looks less pretty, and another state was created.
o This version circulated in draft form to WG on 29/11/08 - no
additional comments received.
o Updated references.
o
o DRAFT -07
o Fixed Section heading (missing in rev -06)
o Updated figure with a note to clarify the Response (from Magnus
Westerlund).
o Editorial corrections proposed by T.Phelan.
o
o DRAFT -08
o Fixed text in response to Sec-Dir review (Chris Newman) and GEN-
ART review (Miguel A. Garcia)
o SC draft made normative (draft currently with IESG)
o Listen' changed to LISTEN1 in all places - to make this label
clearer in the text
o Security considerations updated
o Addressed editorial comments received during LC.
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Author's Address
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
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