One document matched: draft-ietf-dccp-simul-open-00.txt
DCCP Working Group G. Fairhurst
Internet-Draft G. Renker
Intended status: Standards Track University of Aberdeen
Expires: August 21, 2008 February 18, 2008
DCCP Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal
draft-ietf-dccp-simul-open-00
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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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 assists DCCP applications which need to communicate
through one or more middleboxes (e.g. Network Address Translators or
firewalls), where establishing necessary middlebox state requires
peering endpoints to initiate communication in a near-simultaneous
manner.
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 Existing NAT Traversal Techniques . . . . . 4
1.3.1. Near Simultaneous-Open of Connections . . . . . . . . 5
1.3.2. Role Reversal . . . . . . . . . . . . . . . . . . . . 6
2. Procedure for Near-Simultaneous Open . . . . . . . . . . . . . 8
2.1. Conventions and Terminology . . . . . . . . . . . . . . . 8
2.2. DCCP-Listen Packet Format . . . . . . . . . . . . . . . . 8
2.3. Protocol Method . . . . . . . . . . . . . . . . . . . . . 10
2.3.1. Protocol Method for DCCP-Server Endpoints . . . . . . 10
2.3.2. Protocol Method for DCCP-Client Endpoints . . . . . . 12
2.3.3. Processing by Routers and Middleboxes . . . . . . . . 12
2.4. Examples of Use . . . . . . . . . . . . . . . . . . . . . 12
2.5. Backwards Compatibility with RFC 4340 . . . . . . . . . . 14
3. Discussion of Design Decisions . . . . . . . . . . . . . . . . 15
3.1. Rationale for a New Packet Type . . . . . . . . . . . . . 15
3.2. Generation of Listen Packets . . . . . . . . . . . . . . . 16
3.3. Repetition of Listen Packets . . . . . . . . . . . . . . . 16
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 20
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.1. Normative References . . . . . . . . . . . . . . . . . . . 21
6.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 25
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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 data relay servers [TURN].
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 simply reuse UDP-based NAT traversal
techniques. In addition, DCCP has the disadvantage of not being able
to perform a simultaneous-open, a TCP-inherent characteristic which
greatly simplifies NAT traversal.
After discussing the problem space for DCCP, this document specifies
an extension to facilitate DCCP NAT traversal, by explicitly
supporting a widely implemented traversal principle known as 'hole
punching'. This extension produces the same outward effect as an
simultaneous-open, but without internal changes to the standard
operational procedure of DCCP. The extension uses a dedicated
indicator message, whose usage is tied to a specific condition, can
thus be turned off, and is inter-operable with non-extended hosts.
The object of this extension is in built-in support for middlebox
traversal, to reduce reliance on external aids such as data relay
servers.
1.1. Scope of this Document
The technique described by this document applies to scenarios where
one or both DCCP peers are located behind a middlebox.
This document is specifically targeted at NAT traversal. However,
due to the similarity of involved principles, the technique and
presented extension of DCCP may also be of similar use to the
traversal of other types of middlebox, such as firewalls.
The proposed extension is relevant to both client/server and peer-to-
peer applications, such as VoIP, file sharing, or online gaming. It
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]).
For the scope of this document we assume traditional (outbound) types
of NAT as defined in [RFC2663] and further discussed in [RFC3022].
We understand NAT traversal as involving 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
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are BEHAVE-compliant [NAT-APP].
This memo does not discuss behavioural requirements of NAT devices to
support DCCP traversal. These may be described by a separate
document. We further assume that NAT devices provide only a minimum
of DCCP protocol support, in that layer-4 checksums are updated to
account for changes in the pseudo-header.
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 an external data relay server. The
extension described in this document facilitates NAT traversal
without indirection via relay servers.
1.3. Discussion of Existing NAT Traversal Techniques
This section is a brief review of 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
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allow a reverse connection back to the host in the private address
realm.
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 technique. 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.
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
separation between client and server has been one of the initial
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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
server would be required to change its role temporarily to 'client'.
This requires modification of settings, in particular service codes
and perhaps Init Cookies.
Further, the the server 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).
Lastly, the server needs additional processing to ensure that the
connection coming through the listening socket matches the one for
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which it previously opened an active connection.
We therefore do not recommend this approach for DCCP.
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2. Procedure for Near-Simultaneous Open
This section presents the packet-processing details of the
simultaneous-open technique for DCCP. The technique does not employ
role reversal - both endpoints start out with their designated roles,
as specified in [RFC4340]. Neither does it require protocol support
for a genuinely simultaneous handshake.
The presented extension 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. 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| Reserved | Sequence Number High Bits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number Low Bits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DCCP-Listen Packet Format
A DCCP-Listen Packet MUST NOT include any DCCP options (since this
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packet does not modify the receiver protocol state) and also MUST NOT
include application data. Therefore Data Offset MUST be set to 5,
the length of the DCCP-Listen packet in 32-bit words.
Furthermore, the following protocol fields MUST all be set to zero:
CCVal (the connection has not been established),
CsCov (there is no payload).
A server conforming to this revision of the specification SHOULD set
both Sequence Number fields to 0; clients MUST ignore the value of
the Sequence Number fields; and middleboxes SHOULD NOT interpret
sequence numbers on DCCP-Listen packets.
The "Res" and "Reserved" fields are specified by [RFC4340] and its
successors. The interpretation of these fields is not modified by
this document.
The Type field has the value XX-IANA-assigned-XX, which indicates
that this is a DCCP-Listen packet.
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Note to the RFC Editor:
Please replace XX-IANA-assigned-XX in the above paragraph with the
value assigned in the registry and remove this note.
==> End of note to the RFC Editor. <==
DCCP-Listen packets use a single format only and therefore do not
support the alternative use of short sequence numbers defined in
section 5.1 of [RFC4340]. Hence X MUST be set to 1, and all DCCP-
Listen packets with X=0 MUST be ignored.
The Service Code field contains the service code ([RFC4340], 8.1.2)
that the client peer wants to use for this connection. This value
MUST correspond to a service code that the server is actually
offering for connections identified by the same source IP address and
the same Source Port as the DCCP-Listen packet. Since the server may
use multiple service codes, the value of the Service Code field needs
to be communicated out-of-band, from client to server, prior to
sending the DCCP-Listen packet.
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 which can be used to
identify different services that may be offered via 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 an endpoints that has 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. The update conditionally applies to the way the
server performs passive-open.
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 IP-address:port to wildcard numbers
(i.e. leaves these unspecified); the endpoint only becomes fully
specified after performing the handshake with an incoming connection.
For such cases, this document does not update [RFC4340], i.e. the
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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 target IP-address:port plus 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, and specified below.
unspecified remote +--------+ fully specified remote
+---------------------| CLOSED |---------------------+
| +--------+ send DCCP-Listen |
| |
| |
v v
+--------+ timeout +---------+
| LISTEN |<------------------------------+-----------| INVITED |
+--------+ more than 2 retransmissions | +---------+
| | 1st / 2nd ^ |
| | retransm. | |
| +-------------+ |
| resend Listen |
| |
| |
| receive Request +---------+ receive Request |
+------------------->| RESPOND |<--------------------+
send Response +---------+ send Response
Figure 2: Updated state transition diagram for DCCP-Listen
A fully-specified server endpoint performs passive-open from CLOSED
by inviting the remote client to connect, via a single DCCP-Listen
packet. The packet is sent to the specified remote IP-adress:port,
using the format specified in Section 2.2. The server then
transitions to INVITED. (The INVITED state is, like LISTEN, a
passive state, characterised by waiting in the absence of an
established connection.)
If the server endpoint in state INVITED receives a DCCP-Request, it
transitions to RESPOND; where further processing resumes as specified
in [RFC4340].
The server SHOULD repeat sending a DCCP-Listen packet while in state
INVITED, at a 200 millisecond interval and up to at most 2
repetitions. The retransmission timer is restarted with the same
200ms interval after the second retransmission. When, upon the next
timeout, the server is still in the INVITED state, it SHOULD progress
to LISTEN, and resume processing as per [RFC4340].
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Fully-specified server endpoints SHOULD treat ICMP error messages
received in reply to a DCCP-Listen packet as "soft errors" that do
not cause a state transition.
Any server receiving a DCCP-Listen packet in the LISTEN state MUST
reply with a Reset (using Reset Code 7, "Connection Refused"), which
is the expected behaviour with regard to [RFC4340]. Listen packets
received in any other state MUST be ignored (cf. next subsection).
Further details on the rationale are discussed in Section 3.
2.3.2. Protocol Method for DCCP-Client Endpoints
This document updates [RFC4340], by adding the following rules for
the reception of DCCP-Listen packets by clients.
Clients MUST silently discard any received DCCP-Listen packets,
regardless of their current state. The packet indicates only a
willingness to accept a connection: if the client has already
established a connection (OPEN or PARTOPEN), this has no meaning.
If a client is awaiting the response to a DCCP-Request, it does not
need to take specific action. While in state REQUEST, other than
ignoring DCCP-Listen packets, it MUST use the protocol method defined
in [RFC4340]. This ensures that retransmissions will happen in the
expected manner.
2.3.3. Processing by Routers and Middleboxes
Routers and middleboxes both act as forwarding agents for DCCP
packets. This document does not specify rules for forwarding DCCP
packets. We note, however, that DCCP-Listen packets do not require
special treatment, and should therefore be forwarded end-to-end
across Internet paths.
Middleboxes may utilise the connection information (address, port,
Service Code) to establish local forwarding state. This has been the
main motivation for adding the Service Code field to the DCCP-Listen
packet: in combination with the source and destination addresses
found in the enclosing IP-header, the DCCP-Listen packet thereby
communicates all the information necessary to uniquely identify a
DCCP session.
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.
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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.
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
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
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2.5. Backwards Compatibility with RFC 4340
This document updates the connection-establishment procedures of
[RFC4340].
There are no changes if a client implementing the extensions
described in this document communicates with a server conforming to
[RFC4340].
This document 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.
The receipt of a DCCP-Listen packet by a client that implements only
[RFC4340] would lead to a DCCP-Reset (likely using code 4, "Packet
Error" if the unknown packet type passes through). This would abort
the connection.
The authors do not however expect these compatibility issues to
introduce practical deployment problems.
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3. Discussion of Design Decisions
3.1. Rationale for a New Packet Type
The DCCP-Listen packet specified in Section 2.2 has the same format
as the DCCP-Request packet ([RFC4340], sec. 5.1), the only difference
is in the value of the Type field.
The usage however differs, since the DCCP-Listen serves as advisory
message, not as part of the actual connection setup: sequence numbers
have no meaning, and neither options nor payload are present.
It is important to point out that a DCCP-Request packet could in
theory also serve as indicator message, in the same way as the DCCP-
Listen packet. The following arguments were against this
alternative.
The first problem is that of semantic overloading: the Request is
defined in [RFC4340] to serve a well-defined purpose, as the first
packet of the 3-way handshake. Additionally using it in the way as
here specified for the DCCP-Listen packet would require DCCP
processors to have two different processing paths: one where a
Request is interpreted as part of the initial handshake, and one
where the same packet is interpreted as indicator message. This
complicates 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 Reset if it has not yet entered the REQUEST
state. This document addresses that issue by asking clients to
ignore DCCP-Listen packets in any state. Adding a similar rule for
the Request packet is more cumbersome: clients can not 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
DCCP-Listen after having itself sent a (connection-initiation)
Request. Step 7 in section 8.5 of [RFC4340] requires the client to
reply to an (indicator message) Request from the server with a Sync.
However, sequence numbers are ignored for this type of message, so
additional and complicating processing becomes necessary: either to
ask the client not to respond to a Request when the Request of type
"indicator message"; or ask middleboxes and servers to ignore Sync
packets generated in response to Request packets serving as indicator
message. Furthermore, since no initial sequence numbers have been
negotiated yet, sending a SyncAck would not be meaningful.
Using a separate packet type allows simpler and clearer processing.
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The rationale for ignoring the Sequence Number fields on DCCP-Listen
packets is that endpoints have not yet entered connection setup: the
Listen packet is sent out 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.
Although the Sequence Number fields thus do not serve a purpose, both
have been retained, to reuse the generic header format from section
5.1 of [RFC4340].
3.2. 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 (binding to an a priori known
remote address and service code), so as to 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 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).
3.3. Repetition of Listen Packets
Repetition is a necessary requirement to increase robustness and the
chance of successful connection establishment, in case a Listen
packet is lost due to congestion, link loss or some other reason.
Recommending a maximum number of 3 timeouts (2 repetitions) is due to
the following considerations. The repeated copies need to be spaced
sufficiently far apart in time to avoid suffering from correlated
loss. The interval of 200ms has been chosen to accommodate a wide
range of wired and wireless network paths.
Another constraint is given by the retransmission interval for the
DCCP-Request. To establish state, intermediate systems need to
receive a (retransmitted) DCCP-Listen packet before the DCCP-Request
times out (1 second, cf. section 8.1.1 of [RFC4340]). With three
timeouts, each spaced 200 milliseconds apart, the overall time is
still less than this value. On the other hand, the sum of 600
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milliseconds is sufficiently large to provide for large one-way
delays, such as e.g. found on some wireless links.
The rationale behind transitioning to the LISTEN state after two
retransmissions 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
in such cases still reach the server, thus allowing connection
establishment to succeed.
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4. IANA Considerations
This document requires IANA action by allocation of a new Packet Type
from the IANA DCCP Packet Types Registry. The name of the Packet
Type is "DCCP-Listen" packet, and its type field is set to XX-IANA-
assigned-XX.
The Registry entry is to reference this document.
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Note to the RFC Editor:
Please replace XX-IANA-assigned-XX in the above paragraph with the
value assigned in the registry and remove this note.
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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 known client. Establishing this state requires prior out-of-
band signaling between the client and server (e.g. via SIP). The
technique generates a packet addressed to the expected client.
This increases the vulnerability of the DCCP server, by revealing
which ports are in a passive listening state (the information is not
encrypted and therefore could be seen on the path to the client
through the network).
This document requires endpoint nodes to ignore reception of DCCP-
Listen packets in any state other than LISTEN.
We do not believe these changes significantly increase the complexity
or vulnerability of a DCCP implementation that conforms to [RFC4340].
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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-05, November 2007.
[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-06, January 2008.
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Appendix A. Change Log
Revision 00 retrieved from previous individual submission
draft-fairhurst-dccp-behave-update-01 by the same authors.
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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
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