Network Working Group J. Goldberg
Internet-Draft Cisco
Intended status: Standards Track M. Westerlund
Expires: January 15, 2009 Ericsson
T. Zeng
Nextwave Wireless, Inc.
July 14, 2008
An Network Address Translator (NAT) Traversal mechanism for media
controlled by Real-Time Streaming Protocol (RTSP)
draft-ietf-mmusic-rtsp-nat-07
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Abstract
This document defines a solution for Network Address Translation
(NAT) traversal for datagram based media streams setup and controlled
with Real-time Streaming Protocol version 2 (RTSP 2.0). It uses
Interactive Connectivity Establishment (ICE) adapted to use RTSP as a
signalling channel, defining the necessary extra RTSP extensions and
procedures.
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Requirements Language
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 RFC 2119 [RFC2119].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 4
3. RTSP Extensions . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. ICE Transport Lower Layer . . . . . . . . . . . . . . . . 7
3.2. ICE Candidate Transport Header Parameter . . . . . . . . . 8
3.3. ICE Password and Username Transport Header Parameters . . 10
3.4. ICE Feature Tag . . . . . . . . . . . . . . . . . . . . . 11
3.5. Status Codes . . . . . . . . . . . . . . . . . . . . . . . 11
3.5.1. 150 ICE connectivity checks in progress . . . . . . . 12
3.5.2. 480 ICE Processing Failed . . . . . . . . . . . . . . 12
3.6. New Reason for PLAY_NOTIFY . . . . . . . . . . . . . . . . 12
3.7. Server Side SDP Attribute for ICE Support . . . . . . . . 12
3.8. ICE Features Not Required in RTSP . . . . . . . . . . . . 13
3.8.1. ICE-Lite . . . . . . . . . . . . . . . . . . . . . . . 13
3.8.2. ICE-Mismatch . . . . . . . . . . . . . . . . . . . . . 13
3.8.3. ICE Remote Candidate Transport Header Parameter . . . 13
4. Detailed Solution . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Session description and RTSP DESCRIBE (optional) . . . . . 14
4.2. Setting up the Media Resources . . . . . . . . . . . . . . 15
4.3. RTSP SETUP Request . . . . . . . . . . . . . . . . . . . . 15
4.4. Gathering Candidates . . . . . . . . . . . . . . . . . . . 16
4.5. RTSP Server Response . . . . . . . . . . . . . . . . . . . 17
4.6. Server to Client ICE Connectivity Checks . . . . . . . . . 17
4.7. Client to Server ICE Connectivity Check . . . . . . . . . 18
4.8. Client Connectivity Checks Complete . . . . . . . . . . . 18
4.9. Server Connectivity Checks Complete . . . . . . . . . . . 18
4.10. Releasing Candidates . . . . . . . . . . . . . . . . . . . 18
4.11. Steady State . . . . . . . . . . . . . . . . . . . . . . . 19
4.12. re-SETUP . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.13. Server Side Changes After Steady State . . . . . . . . . . 19
5. ICE and Proxies . . . . . . . . . . . . . . . . . . . . . . . 21
5.1. Media Handling Proxies . . . . . . . . . . . . . . . . . . 21
5.2. Signalling Only Proxies . . . . . . . . . . . . . . . . . 22
5.3. Non-supporting Proxies . . . . . . . . . . . . . . . . . . 22
6. RTP and RTCP Multiplexing . . . . . . . . . . . . . . . . . . 23
7. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 24
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
8.1. RTSP Feature Tags . . . . . . . . . . . . . . . . . . . . 24
8.2. Transport Protocol Specifications . . . . . . . . . . . . 24
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8.3. RTSP Transport Parameters . . . . . . . . . . . . . . . . 25
8.4. RTSP Status Codes . . . . . . . . . . . . . . . . . . . . 25
8.5. Notify-Reason value . . . . . . . . . . . . . . . . . . . 25
8.6. SDP Attribute . . . . . . . . . . . . . . . . . . . . . . 25
9. Security Considerations . . . . . . . . . . . . . . . . . . . 26
9.1. ICE and RTSP . . . . . . . . . . . . . . . . . . . . . . . 26
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
11.1. Normative References . . . . . . . . . . . . . . . . . . . 26
11.2. Informative References . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . . . 29
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1. Introduction
Real-time Streaming Protocol (RTSP)
[RFC2326][I-D.ietf-mmusic-rfc2326bis] is a protocol used to setup and
control one or more media streams delivering media to receivers. It
is RTSP's functionality of setting up media streams that get into
serious issues with Network Address Translators (NAT) [RFC3022].
Commonly the media will be totally blocked by the NAT unless extra
provisions are taken by the protocol. There is a clear and present
need for NAT traversal mechanism for the media setup using RTSP.
RTSP 1.0 [RFC2326] has suffered from the lack of a standardized NAT
traversal mechanism for a long time, however due to quality of the
RTSP 1.0 specification, the work has had to wait on the recently
defined RTSP 2.0 [I-D.ietf-mmusic-rfc2326bis]. RTSP 2.0 is similar
to RTSP 1.0 in many respects but significantly for this work, it
contains a well defined extension mechanism so allowing a NAT
traversal extension to be defined that is backwards compatible with
RTSP 2.0 peers not supporting the extension. This extension
mechanism was not possible in RTSP 1.0 as it would break RTSP 1.0
syntax so causing compatibility issues.
There have been a number of suggested ways of resolving the NAT-
traversal of media for RTSP of which a large number are already used
in implementations. The evaluation of these NAT traversal solutions
in[I-D.ietf-mmusic-rtsp-nat-evaluation] has shown that there are many
issues to consider, so after extensive evaluation, we selected a
mechanism based on Interactive Connectivity Establishment (ICE).
This was mainly two reasons: Firstly the mechanism supports RTSP
servers behind NATs and secondly the mechanism solves the security
threat that uses RTSP servers as Distributed Denial of Service (DDoS)
attack tools.
This document specifies an ICE based solution that is optimized for
media delivery server to client. If in the future extensions are
specified for other delivery modes than PLAY, then the optimizations
in regards to when PLAY request are sent needs to be reconsidered.
The NAT problem for RTSP signalling traffic itself is beyond the
scope of this document and is left for future study should the need
arise, because it is a less prevalent problem than the NAT problem
for RTSP media streams.
2. Solution Overview
This overview assumes that the reader has some familiarity with how
ICE [I-D.ietf-mmusic-ice] works, as it primarily points out how the
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different ICE steps are accomplished in RTSP.
1. RTSP server can indicate it has support for ICE via an SDP
[RFC4566] attribute in, for example, the SDP returned in RTSP
DESCRIBE message. This allows RTSP clients to only send the new
ICE interchanges with servers that support ICE so as to limit
the overhead on current non-ICE supporting RTSP servers. If
RTSP DESCRIBE is used the normal capability determination
mechanism can be used, i.e. "Supported" header and the defined
feature tag.
2. RTSP client reviews the session description returned, for
example by an RTSP DESCRIBE message, to determine what media
resources that need to be setup. For each of these media
resources where the transport protocol supports Session
Traversal Utilities for (NAT) (STUN)
[I-D.ietf-behave-rfc3489bis] based connectivity checks, the
client gathers candidate addresses. See section 4.1.1 in
[I-D.ietf-mmusic-ice]. The client also installs the STUN
servers on each of the local candidates.
3. RTSP client sends a SETUP request with both a transport
specification with a lower layer indicating ICE and a new RTSP
Transport header parameter listing the ICE candidates for each
media resource. RTSP proxies in non-ICE transport
specifications should be treated at lower priority than those
transport specifications supporting ICE.
4. After receiving the list of candidates from a client, the RTSP
server gathers its own candidates. If the server has a public
IP address then a single candidate per address family (e.g.
IPv4 and IPv6) can be included to reduce the number of
combinations and speed up the completion.
5. The server sets up the media and if successful responds to the
SETUP request with a 200 OK response. In that response the
server selects the transport specification using ICE and
includes its candidates in the server candidate parameter.
6. If the server is behind a NAT then it starts the connectivity
checks following the procedures described in Section 5.7 and 5.8
of [I-D.ietf-mmusic-ice]. If the server has a public IP address
with a single candidate and the setup address and port pair is
reachable from any public address then it can refrain from
server initiated connectivity checks and rely on triggered
checks.
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7. The client receives the SETUP response and learns the candidate
address to use for the connectivity checks, and then initiates
its connectivity check, following the procedures in Section 6 of
[I-D.ietf-mmusic-ice].
8. When a connectivity check from the client reaches the server it
will result in a triggered check from the server. This is why
servers with a public IP address can wait until this triggered
check to send out any checks for itself so saving resources and
mitigating the DDoS potential from server connectivity checks.
9. When the client has concluded its connectivity checks and has
corresponding received the server connectivity checks on the
promoted candidates for all components of all media streams, it
can issue a PLAY request. If the connectivity checks have not
concluded successfully then the client may send a new SETUP
request assuming it has any new information or believes the
server may be able to do more that can result in successful
checks.
10. When the RTSP servers receives a PLAY request it checks to see
the connectivity checks has concluded successfully and only then
can play the stream. If there is a problem with the checks then
the server sends to the client either a 150 (ICE connectivity
checks in progress) response to show that it is still working on
the connectivity checks or a 480 (ICE Processing Failed)
response to indicate a failure of the checks. If the checks are
successful then the server sends a 200 OK response and starts
delivering media.
The client may release unused candidates when the ICE processing has
concluded and a single candidate per component has been promoted.
The client shall continue to use STUN to send keep-alive for the used
bindings. This is important as often RTSP media sessions only
contain media traffic from the server to the client so the bindings
in the NAT needs to be refreshed by the client to server traffic
provided by the STUN keep-alive.
3. RTSP Extensions
This section defines the necessary RTSP extensions for performing ICE
with RTSP. Note that these extensions are based on the SDP
attributes in the ICE specification unless expressly indicated.
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3.1. ICE Transport Lower Layer
A new lower layer "D-ICE" for transport specifications is defined.
This lower layer is datagram clean except that the protocol used must
be demultiplexiable with STUN messages (see STUN
[I-D.ietf-behave-rfc3489bis]). With datagram clean we mean that it
must be capable of describing the length of the datagram, transport
that datagram (as a binary chunk of data) and provide it at the
receiving side as one single item. This lower layer can be any
transport type defined for ICE which does provide datagram transport
capabilities. Though only UDP is defined at present, however TCP
with framing may be specified and used in the future.
This lower layer uses ICE to determine which of the different
candidates shall be used and then when the ICE processing has
concluded, uses the selected candidate to transport the datagrams
over this transport.
This lower layer transport can be combined with all upper layer media
transport protocols that are possible to demultiplex with STUN and
which use datagrams. This specification defines the following
combinations:
o RTP/AVP/D-ICE
o RTP/AVPF/D-ICE
o RTP/SAVP/D-ICE
o RTP/SAVPF/D-ICE
This list can easily be extended with more transport specifications
after having performed the evaluation that they are compatible with
D-ICE as lower layer.
The lower-layer "D-ICE" has the following rules for the inclusion of
transport parameters:
unicast: As ICE only supports unicast operations, thus it is
REQUIRED that one include the unicast indicator parameter, see
section 16.46 in [I-D.ietf-mmusic-rfc2326bis].
candidates: The "candidates" parameter SHALL be included as this
specify at least one candidate to try to establish a working
transport path with.
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dest_addr: This parameter SHALL NOT be included as "candidates" is
used instead to provide the necessary address information.
ICE-Password: This parameter SHALL be included.
ICE-Userfrag: This parameter SHALL be included.
3.2. ICE Candidate Transport Header Parameter
This section defines a new RTSP transport parameter for carrying ICE
candidates related to the transport specification they appear within,
which may then be validated with an end-to-end connectivity check
using STUN [I-D.ietf-behave-rfc3489bis]. Transport parameters may
only occur once in each transport specification. For transport
specification using "D-ICE" as lower layer, this parameter needs to
be present. The parameter can contain one or more ICE candidates.
In the SETUP response there is only a single transport specification,
and if that uses the "D-ICE" lower layer this parameter also needs to
present including the server side candidates.
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tr-parameter =/ SEMI ice-trn-par
ice-trn-par = "candidates" EQUAL DQ SWS ice-candidate
*(SEMI ice-candidate) SWS DQ
ice-candidate = foundation SP
component-id SP
transport SP
priority SP
connection-address SP
port SP
cand-type
[SP rel-addr]
[SP rel-port]
*(SP extension-att-name SP extension-att-value)
foundation = <See section 15.1 of [I-D.ietf-mmusic-ice]>
component-id = <See section 15.1 of [I-D.ietf-mmusic-ice]>
transport = <See section 15.1 of [I-D.ietf-mmusic-ice]>
transport-extension = <See section 15.1 of [I-D.ietf-mmusic-ice]>
priority = <See section 15.1 of [I-D.ietf-mmusic-ice]>
cand-type = <See section 15.1 of [I-D.ietf-mmusic-ice]>
candidate-types = <See section 15.1 of [I-D.ietf-mmusic-ice]>
rel-addr = <See section 15.1 of [I-D.ietf-mmusic-ice]>
rel-port = <See section 15.1 of [I-D.ietf-mmusic-ice]>
extension-att-name = <See section 15.1 of [I-D.ietf-mmusic-ice]>
extension-att-value = <See section 15.1 of [I-D.ietf-mmusic-ice]>
ice-char = <See section 15.1 of [I-D.ietf-mmusic-ice]>
connection-address = <See [RFC4566]>
port = <See [RFC4566]>
EQUAL = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
DQ = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
SWS = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
SEMI = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
<connection-address>: is the IP address of the candidate, allowing
for IPv4 addresses, IPv6 addresses and Fully qualified domain names
(FQDN), taken from [RFC4566]. The connection address SHOULD be on
the same format (explicit IP or FQDN) as in the dest_addr parameter
used to express fallbacks. An IP address SHOULD be used, but an FQDN
MAY be used in place of an IP address. In that case, when receiving
an offer or answer containing an FQDN in an a=candidate attribute,
the FQDN is looked up in the DNS first using an AAAA record (assuming
the agent supports IPv6), and if no result is found or the agent only
supports IPv4, using an A. If the DNS query returns more than one IP
address, one is chosen, and then used for the remainder of ICE
processing.
<port>: is the port of the candidate taken from RFC 4566 [RFC4566].
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<transport>: indicates the transport protocol for the candidate. The
ICE specification only defines UDP. However, extensibility is
provided to allow for future transport protocols to be used with ICE,
such as TCP or the Datagram Congestion Control Protocol (DCCP)
[RFC4340].
<foundation>: is an identifier that is equivalent for two candidates
that are of the same type, share the same base, and come from the
same STUN server, and is composed of one to thirty two <ice-char>.
The foundation is used to optimize ICE performance in the Frozen
algorithm.
<component-id>: identifies the specific component of the media stream
for which this is a candidate and os a positive integer between 1 and
256. It MUST start at 1 and MUST increment by 1 for each component
of a particular candidate. For media streams based on RTP,
candidates for the actual RTP media MUST have a component ID of 1,
and candidates for RTCP MUST have a component ID of 2. Other types
of media streams which require multiple components MUST develop
specifications which define the mapping of components to component
IDs. See Section 14 for additional discussion on extending ICE to
new media streams.
<priority>: is a positive integer between 1 and (2**31 - 1).
<cand-type>: encodes the type of candidate. The ICE specification
defines the values "host", "srflx", "prflx" and "relay" for host,
server reflexive, peer reflexive and relayed candidates,
respectively. The set of candidate types is extensible for the
future.
<rel-addr> and <rel-port>: convey transport addresses related to the
candidate, useful for diagnostics and other purposes. <rel-addr> and
<rel-port> MUST be present for server reflexive, peer reflexive and
relayed candidates. If a candidate is server or peer reflexive,
<rel-addr> and <rel-port> is equal to the base for that server or
peer reflexive candidate. If the candidate is relayed, <rel-addr>
and <rel-port> is equal to the mapped address in the Allocate
Response that provided the client with that relayed candidate (see
Appendix B.3 for a discussion of its purpose). If the candidate is a
host candidate <rel-addr> and <rel-port> MUST be omitted.
3.3. ICE Password and Username Transport Header Parameters
The ICE password and username for each agent needs to be transported
using RTSP. For that purpose new transport header parameters are
defined.
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There MUST be an "ICE-Password" and "ICE-Userfrag" parameter for each
media stream. If two SETUP requests in the same RTSP session have
identical ICE-Userfrag's, they MUST have identical ICE-Password's.
The ICE-Userfrag and ICE-Password attributes MUST be chosen randomly
at the beginning of a session. The ICE-Userfrag attribute MUST
contain at least 24 bits of randomness, and the ICE-Password
attribute MUST contain at least 128 bits of randomness. This means
that the ICE-Userfrag attribute will be at least 4 characters long,
and the ICE-Password at least 22 characters long, since the grammar
for these attributes allows for 6 bits of randomness per character.
The attributes MAY be longer than 4 and 22 characters respectively,
of course, up to 256 characters. The upper limit allows for buffer
sizing in implementations. Its large upper limit allows for
increased amounts of randomness to be added over time.
The ABNF [RFC5234] for these parameters are:
tr-parameter =/ SEMI ice-password-par
tr-parameter =/ SEMI ice-userfrag-par
ice-password-par = ICE-Password" EQUAL password
ice-userfrag-par = ICE-Userfrag" EQUAL ufrag
password = <Defined in [I-D.ietf-mmusic-ice]>
ufrag = <Defined in [I-D.ietf-mmusic-ice]>
EQUAL = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
SEMI = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
3.4. ICE Feature Tag
A feature tag is defined for usage in the RTSP capabilities mechanism
for ICE support for media transport using datagrams: "setup.ice-d-m".
This feature tag indicates that one support all the mandatory to
support functions of this specification. It is applicable to all
types of RTSP agents; clients, servers and proxies.
The RTSP client should send the feature tag "setup.ice-d-m" in the
"Supported" header in all SETUP requests that contain the "D-ICE"
lower layer transport.
3.5. Status Codes
ICE needs two new RTSP response codes to indicate correctly progress
and errors.
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+------+----------------------------------------------+-------------+
| Code | Reason | Method |
+------+----------------------------------------------+-------------+
| 150 | Server still working on ICE connectivity | PLAY |
| | checks | |
| 480 | ICE Connectivity check failure | PLAY, SETUP |
+------+----------------------------------------------+-------------+
Table 1: New Status codes and their usage with RTSP methods
3.5.1. 150 ICE connectivity checks in progress
The 150 response code indicates that ICE connectivity checks are
still in progress and haven't concluded. This response SHALL be sent
within 200 milliseconds of receiving a PLAY request that currently
can't be fulfilled because ICE connectivity checks are still running.
Subsequently, every 3 seconds after the previous sent one, a 150
reply shall be sent until the ICE connectivity checks conclude either
successfully or in failure, and a final response for the request can
be provided.
3.5.2. 480 ICE Processing Failed
The 480 client error response code is used in cases when the request
can't be fulfilled due to a failure in the ICE processing, such as
that all the connectivity checks have timed out. This error message
can appear either in response to a SETUP request to indicate that no
candidate pair can be constructed or to a PLAY request that the
server's connectivity checks resulted in failure.
3.6. New Reason for PLAY_NOTIFY
A new value used in the PLAY_NOTIFY methods Notify-Reason header is
defined: "ice-restart". This reason indicates that a ICE restart
needs to happen on the identified resource and session.
Notify-Reas-val =/ "ice-restart"
3.7. Server Side SDP Attribute for ICE Support
If the server supports the media NAT traversal for RTSP controlled
sessions, as described in this RFC, then the Server SHALL include the
"a=rtsp-ice-d-m" SDP attribute in any SDP (if used) describing
content served by the server. This is an session level attribute.
rtsp-ice-d-m-attr = "a=" "rtsp-ice-d-m"
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3.8. ICE Features Not Required in RTSP
A number of ICE signalling features are not needed with RTSP and are
discussed below.
3.8.1. ICE-Lite
The ICE-Lite attribute shall not be used in the context of RTSP. The
ICE specification describes two implementations of ICE: Full and
Lite, where hosts that are not behind a NAT are allowed to implement
only Lite. For RTSP, the Lite implementation is insufficient because
it does not cause the media server to send a connectivity check,
which are used to protect against making the RTSP server a denial of
service tool. This document defines another variation implementation
of ICE, called ICE-RTSP. It has its own set of simplifications
suitable to RTSP. Conceptually, this implementation of ICE-RTSP is
between ICE-FULL and ICE-LITE for a server and simpler than ICE-FULL
for clients.
3.8.2. ICE-Mismatch
The ice-mismatch parameter indicates that the offer arrived with a
default destination for a media component that didn't have a
corresponding candidate attribute. This is not needed for RTSP as
the ICE based lower layer transport specification either is supported
or another alternative transport is used. This is always explicitly
indicated in the SETUP request and response.
3.8.3. ICE Remote Candidate Transport Header Parameter
The Remote candidate attribute is not needed for RTSP for the
following reasons. Each SETUP results in a independent ICE
processing chain which either fails or results in promoting a single
candidate pair to usage. If a new SETUP request for the same media
is sent this needs to use a new userfragment and password to avoid
any race conditions or uncertainty for which processing round the
STUN requests relate to.
4. Detailed Solution
This section describes in detail how the interaction and flow of ICE
works with RTSP messages.
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4.1. Session description and RTSP DESCRIBE (optional)
The RTSP server should indicate it has support for ICE by sending the
"rtsp-ice-d-m" SDP attribute in the response to the RTSP DESCRIBE
message if SDP is used. This allows RTSP clients to only send the
new ICE interchanges with servers that support ICE so limiting the
overhead on current non-ICE supporting RTSP servers. When not using
RTSP DESCRIBE it is still recommended to use the SDP attribute for
session description.
A Client can also use the DESCRIBE request to determine explicitly if
both server and any proxies support ICE. The client includes the
"Supported" header with its supported feature tags, including
"setup.ice-d-m". Any proxy upon seeing the "Supported" header will
include the "Proxy-Supported" header with the feature tags it
supports. The server will echo back the "Proxy-Supported" header and
its own version of the Supported header so enabling a client to
determine if all involved parties support ICE or not. Note that even
if a proxy is present in the chain that doesn't indicate support for
ICE, it may still work.
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For example:
C->S: DESCRIBE rtsp://server.example.com/fizzle/foo RTSP/2.0
CSeq: 312
User-Agent: PhonyClient 1.2
Accept: application/sdp, application/example
Supported: setup.ice-d-m
S->C: RTSP/2.0 200 OK
CSeq: 312
Date: 23 Jan 1997 15:35:06 GMT
Server: PhonyServer 1.1
Content-Type: application/sdp
Content-Length: 367
Supported: setup.ice-d-m
v=0
o=mhandley 2890844526 2890842807 IN IP4 192.0.2.46
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.example.com/lectures/sdp.ps
e=seminar@example.com (Seminar Management)
t=2873397496 2873404696
a=recvonly
a=rtsp-ice-d-m
a=control: *
m=audio 3456 RTP/AVP 0
a=control: /audio
m=video 2232 RTP/AVP 31
a=control: /video
4.2. Setting up the Media Resources
The RTSP client reviews the session description returned, for example
by an RTSP DESCRIBE message, to determine what media resources that
need to be setup. For each of these media resources where the
transport protocol supports ICE connectivity checks, the client shall
gather candidate addresses as described in section 4.1.1 in
[I-D.ietf-mmusic-ice] according to standard ICE rather than the ICE-
Lite implementation.
4.3. RTSP SETUP Request
The RTSP client will then send one or more SETUP requests to
establish the media streams required for the desired session. For
each media stream where it desires to use ICE it will include a
transport specification with "D-ICE" as the lower layer. This
transport specification SHOULD be placed first in the list to give it
highest priority. It is RECOMMENDED that additional transport
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specifications are provided as a fallback in case of non ICE
supporting proxies. For example (Note that some lines are broken in
contradiction with the defined syntax due to space restrictions in
the documenting format:
C->S: SETUP rtsp://server.example.com/fizzle/foo/audio RTSP/2.0
CSeq: 302
Transport: RTP/AVP/D-ICE; unicast; ICE-Username=8hhY;
ICE-Password=asd88fgpdd777uzjYhagZg; candidates="
1 1 UDP 2130706431 10.0.1.1 8998 typ host;
2 1 UDP 1694498815 192.0.2.3 45664 typ srflx
raddr 10.0.1.1 rport 9002",
RTP/AVP/UDP; unicast; dest_addr=":6970"/":6971",
RTP/AVP/TCP;unicast;interleaved=0-1
Accept-Ranges: NPT, UTC
User-Agent: PhonyClient/1.2
Supported: setup.ice-d-m
The client will be initiating and thus the controlling party in the
ICE processing.
4.4. Gathering Candidates
Upon receiving a SETUP request the server can determine what media
resource should be delivered and which transport alternatives that
the client supports. If one based on D-ICE is first on the list of
supported transports, the below applies, otherwise another transport
method is preferred and supported.
The transport specification will provide which media protocol is to
be used and based on this and the clients candidates, the server
determines the protocol and if it supports ICE with that protocol.
The server shall then gather its candidates according to section
4.1.1 in [I-D.ietf-mmusic-ice]. Servers that have an address that is
generally reachable by any clients within the address scope the
server intends to serve MAY be specially configured (high-
reachability configuration). This special configuration has the goal
of reducing the server side candidate to preferably a single one per
address family. Instead of gathering all possible addresses
including relayed and server reflexive addresses, the server uses a
single address per address family that it knows it should be
reachable by a client behind one or more NATs. The reason for this
special configuration is two fold: Firstly it reduces the load on the
server in address gathering and in ICE processing during the
connectivity checks. Secondly it will reduce the number of
permutations for candidate pairs significantly thus potentially
speeding up the conclusion of the ICE processing. Note however that
using this option on a server that doesn't fulfill the requirement of
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being reachable is counter-productive and it is important that this
is correctly configured.
4.5. RTSP Server Response
The server determines if the SETUP request is successful from the
other perspectives and will return a 200 OK response, otherwise
returning an error code from the list in Table 4 in
[I-D.ietf-mmusic-rfc2326bis]. At that point the server, having
selected a transport specification using the "D-ICE" lower layer,
will need to include that transport specification in the response
message. The transport specification shall include the candidates
gathered in SectionSection 4.4 in the "candidates" transport header
parameter as well as the server's username and password. In the case
that there are no valid candidate pairs with the combination of the
client and servers candidates, a 480 (ICE Processing Failed) error
response shall be returned which must include the servers'
candidates. The return of a 480 error may allow both the server and
client to release its candidates.
S->C: RTSP/2.0 200 OK
CSeq: 302
Session: 12345678
Transport: RTP/AVP/D-ICE; unicast; ICE-Username=MkQ3;
ICE-Password=pos12Dgp9FcAjpq82ppaF; candidates="
1 1 UDP 2130706431 192.0.2.56 50234 typ host"
Accept-Ranges: NPT
Date: 23 Jan 1997 15:35:06 GMT
Server: PhonyServer 1.1
Supported: setup.ice-d-m
4.6. Server to Client ICE Connectivity Checks
The server shall start the connectivity checks following the
procedures described in Section 5.7 and 5.8 of [I-D.ietf-mmusic-ice]
unless it is configured to use the high-reachability option. If it
is then it can suppress its own checks until the servers checks are
triggered by the client's connectivity checks.
The server SHALL use a single pacer for all STUN transactions within
a single RTSP session, i.e across all media streams that are part of
the same RTSP session.
When a connectivity check from the client reaches the server it will
result in a triggered check from the server as specified in section
7.2.1.4 of [I-D.ietf-mmusic-ice]. This is why servers with a high
reachability address can wait until this triggered check to send out
any checks for itself so saving resources and mitigating the DDoS
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potential.
4.7. Client to Server ICE Connectivity Check
The client receives the SETUP response and learns the candidate
address to use for the connectivity checks. The client shall
initiate its connectivity check, following the procedures in Section
6 of [I-D.ietf-mmusic-ice].
Aggressive nomination SHALL be used with RTSP. This doesn't have the
negative impact that it has in offer/answer as media playing only
starts after issuing a PLAY request.
4.8. Client Connectivity Checks Complete
When the client has concluded its connectivity checks and have
nominated its desired candidate pairs, it can issue a PLAY request.
Note, that due to the aggressive nomination, there is a risk that any
outstanding check may nominate another pair than what was already
nominated. If the client has locally determined that its checks have
failed it may try providing an extended set of candidates and update
the server candidate list by issuing a new SETUP request for the
media stream.
If the client concluded its connectivity checks succesfully and
therefore sent a PLAY request but the server can not concluded
successfully, the server will respond with a 480 (ICE Processing
Failed). Upon receiving the 480 (ICE Processing Failed) response,
then the client may send a new SETUP request assuming it has any new
information that can be included in the candidate list. If the
server is still performing the checks it will respond with a 150 (CE
connectivity checks in progress) response to indicate this.
4.9. Server Connectivity Checks Complete
When the RTSP server receives a PLAY request, it checks to see that
the connectivity checks have concluded successfully and only then
will it play the stream. If there is a problem with the checks then
the server sends to the client either a 150 (ICE connectivity checks
in progress) response to show that it is still working on the
connectivity checks or a 480 response to indicate a failure of the
checks. If the checks are successful then the server sends a 200 OK
response and starts delivering media.
4.10. Releasing Candidates
Both server and client may release its non nominated candidates as
soon as a 200 PLAY response has been issued/received and no
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outstanding connectivity checks exist.
4.11. Steady State
The client will continue to use STUN to send keep-alive for the used
bindings. This is important as normally RTSP play mode sessions only
contain traffic from the server to the client so the bindings in the
NAT needs to be refreshed by the cleint to server traffic provided by
the STUN keep-alive.
4.12. re-SETUP
The server SHALL support SETUP requests in PLAYING state if
supporting this specification, as long as the SETUP changes only ICE
parameters, i.e. ICE-Password, ICE-Username and the content of ICE
candidates.
If the client decides to change any parameter related to the media
stream SETUP it will send a new SETUP request. In this new SETUP
request the client SHALL include a new different username and
password to use in the ICE processing. This request will also cause
the ICE processing to start from the beginning again.
If the RTSP session is in playing state at the time of sending the
SETUP request, the ICE connectivity checks SHALL use Regular
nomination. Any ongoing media delivery continues on the previously
nominated candidate pairs until the new pairs have been nominated for
the individual candidate. Once the nomination of the new candidate
pair has completed, all unused candidates may be released.
4.13. Server Side Changes After Steady State
A Server may require an ICE restart because of server side load
balancing or a failure resulting in an IP address and a port number
change. It shall use the PLAY_NOTIFY method to inform the client
(draft-ietf-mmusic-rfc2326bis-18#section-13.5) with a new Notify-
Reason header: ice-restart. The server will identify if the change
is for a single media or for the complete session by including the
corresponding URI in the PLAY_NOTIFY request.
Upon receiving and responding to this PLAY_NOTIFY with ice-restart
reason the client SHALL gather new ICE candidates, send SETUP
requests for each media stream part of the session. The server
provides its candidates in the SETUP response the same way as for the
first time ICE processing. Both server and client shall provide new
ICE usernames and passwords. The client MAY issue the SETUP request
while the session is in PLAYING state.
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If the RTSP session is in PLAYING state when the client issues the
SETUP request the client SHALL use regular nomination. If not the
client will use the same procedures as for when first creating the
session.
Note that keepalives on the previous set of candidate pairs should
continue until all new candidate pairs have been nominated. After
having nominated a new set of candidate pairs, the client may
continue to receive media for some additional time. Even if the
server stops delivering media over that candidate pair at the time of
nomination media may arrive for up to one maximum segement lifetime
as defined in TCP (2 minutes). Unfortuntately if the RTSP server is
decomposited with separete controller and media transmitters then a
failure may result in continued media delivery for longer time than
that. Thus possibility to filter on the source is recommended.
For example:
S->C: PLAY_NOTIFY rtsp://example.com/fizzle/foo RTSP/2.0
CSeq: 854
Notify-Reason: ice-restart
Session: uZ3ci0K+Ld
Server: PhonyServer 1.1
C->S: RTSP/2.0 200 OK
CSeq: 854
User-Agent: PhonyClient/1.2
C->S: SETUP rtsp://server.example.com/fizzle/foo/audio RTSP/2.0
CSeq: 302
Session: uZ3ci0K+Ld
ICE-Password:
ICE-Username:
Transport: RTP/AVP/D-ICE; unicast; ICE-Username=Kl1C;
ICE-Password=H4sICGjBsEcCA3Rlc3RzLX; candidates = "
1 1 UDP 2130706431 10.0.1.1 8998 typ host;
2 1 UDP 1694498815 192.0.2.3 51456 typ srflx
raddr 10.0.1.1 rport 9002",
RTP/AVP/UDP; unicast; dest_addr=":6970"/":6971",
RTP/AVP/TCP;unicast;interleaved=0-1
Accept-Ranges: NPT, UTC
User-Agent: PhonyClient/1.2
C->S: SETUP rtsp://server.example.com/fizzle/foo/video RTSP/2.0
CSeq: 303
Session: uZ3ci0K+Ld
Transport: RTP/AVP/D-ICE; unicast; ICE-Username=hZv9;
ICE-Password=JAhA9myMHETTFNCrPtg+kJ; candidates="
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1 1 UDP 2130706431 10.0.1.1 9000 typ host;
2 1 UDP 1694498815 192.0.2.3 51576 typ srflx
raddr 10.0.1.1 rport 9004",
RTP/AVP/UDP; unicast; dest_addr=":6972"/":6973",
RTP/AVP/TCP;unicast;interleaved=0-1
Accept-Ranges: NPT, UTC
User-Agent: PhonyClient/1.2
S->C: RTSP/2.0 200 OK
CSeq: 302
Session: uZ3ci0K+Ld
Transport: RTP/AVP/D-ICE; unicast; ICE-Username=CbDm;
ICE-Password=OfdXHws9XX0eBr6j2zz9Ak; candidates="
1 1 UDP 2130706431 192.0.2.56 50234 typ host"
Accept-Ranges: NPT
Date: 23 Jan 1997 15:43:12 GMT
Server: PhonyServer 1.1
S->C: RTSP/2.0 200 OK
CSeq: 303
Session: uZ3ci0K+Ld
Transport: RTP/AVP/D-ICE; unicast; ICE-Username=jigs;
ICE-Password=Dgx6fPj2lsa2WI8b7oJ7+s; candidates="
1 1 UDP 2130706431 192.0.2.56 47233 typ host"
Accept-Ranges: NPT
Date: 23 Jan 1997 15:43:13 GMT
Server: PhonyServer 1.1
5. ICE and Proxies
RTSP allows for proxies which can be of two fundamental types
depending if they relay and potentially cache the media or not.
Their differing impact on the RTSP NAT traversal solution including
backwards compatibility is explained below.
5.1. Media Handling Proxies
An RTSP proxy that relays or caches the media stream for a particular
media session can be considered to split the media transport into two
parts: A media transport between the server and the proxy according
to the proxies need, and delivery from the proxy to the client. This
split means that the NAT traversal solution will need to be run on
each individual media leg according to need.
It is RECOMMENDED that any media handling proxy support the media NAT
traversal defined within this specification. This is for two
reasons: Firstly to enable clients to perform NAT traversal for the
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media between the proxy and itself and secondly to allow the proxy to
be topology independent so able to support performing NAT traversal
for non-NAT traversal capable clients present in the same address
domain.
For a proxy to support the media NAT traversal defined in this
specification a proxy will need to implement the solution fully and
be ready as both a controlling and a controlled ICE peer. The proxy
also SHALL include the "setup.ice-d-m" feature tag in any applicable
capability negotiation headers, such as "Proxy-Supported".
5.2. Signalling Only Proxies
A signalling only proxy handles only the RTSP signalling and does not
have the media relayed through proxy functions. This type of proxy
is not likely to work unless the media NAT traversal solution is in
place between the client and the server, because the DoS protection
measures usually prevent media delivery to other addresses other than
from where the RTSP signalling arrives at the server.
The solution for the Signalling Only proxy is that it must forward
the RTSP SETUP requests including any transport specification with
the "D-ICE" lower layer and the related transport parameters. A
proxy supporting this functionality SHOULD indicate its capability by
always including the "setup.ice-d-m" feature tag in the "Proxy-
Supported" header.
5.3. Non-supporting Proxies
A media handling proxy that doesn't support the ICE media NAT
traversal specified here is assumed to remove the transport
specification and use any of the lower prioritized transport
specifications if provided by the requester. The specification of
such a non ICE transport enables the negotiation to complete,
although with a less prefered method as a NAT between the proxy and
the client will result in failure of the media path.
A non-media handling transport proxy is expected to ignore and simply
forward all unknown transport specifications, however, this can only
be guaranteed for proxies following the published RTSP 2.0
specification.
Unfortunately the usage of the "setup.ice-d-m" feature tag in the
proxy-require will have contradicting results. For a non ICE
supporting media handling proxy, the inclusion of the feature tag
will result in aborting the setup and indicating that it isn't
supported, which is desirable if you want to provide other fallbacks
or other transport configurations to handle the situation. For non-
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supporting non-media handling proxies the result will also result in
aborting the setup, however, setup might have worked if the proxy-
require tag wasn't present. This variance in results makes usage of
proxy-require not recommended. We recommend instead the usage of the
Supported header to force proxies to include the feature tags they
support in the proxy-supported which will provide a positive
indication when all proxies in the chain between the client and
server support the functionality. Even if not explicitly indicating
support, any SETUP response including a transport specification with
"D-ICE" will be implicit indication that the proxy chain supports at
least passthrough of this media.
6. RTP and RTCP Multiplexing
[I-D.ietf-avt-rtp-and-rtcp-mux] specifies how and when RTP and RTCP
can be multiplexed on the same port. This multiplexing is highly
recommended to combine with ICE as it makes RTP and RTCP only need a
single component per media stream instead of two, so reducing the
load on the connectivity checks.
To enable signalling for the usage of RTP and RTCP multiplexing a new
RTSP transport header parameter is defined. The formal syntax (ABNF
[RFC5234]) of this parameter is the following:
tr-parameter =/ SEMI rtcp-mux-par
rtcp-mux-par = "rtp-rtcp-mux"
SEMI = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
EQUAL = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
The "rtp-rtcp-mux" parameter MAY be included in any transport
specification that use RTP where RTP and RTCP multiplexing is desired
and indicates in a SETUP request that multiplexing is requested. If
the SETUP response also includes the parameter then RTP and RTCP
multiplexing SHALL be used for that transport specification. A SETUP
request may indicate address information for both RTP and RTCP for
backwards compatibility reasons. If RTP and RTCP multiplexing is
used then only the information specified for RTP SHALL be used.
For capability exchange, an RTSP feature tag for RTP and RTCP
multiplexing is defined: "setup.rtp-mux".
RTSP servers and clients that supports "D-ICE" lower layer transport
in combination with RTP SHALL also implement RTP and RTCP
multiplexing as specified in this section and
[I-D.ietf-avt-rtp-and-rtcp-mux].
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7. Open Issues
Below is listed the known open issues and questions that needs to be
resolved:
1. Need a descriptive section on how ICE works for RTSP folks.
2. A solution has been proposed for server side change of IP address
but more works needs to be done on if this needs more
optimization and what happens if the checks fails for either
party.
3. Does we need to support multiple components?
4. Is the role and processing the most optimal one that can be used?
8. IANA Considerations
This document request registration in a number of registries, both
for RTSP and SDP.
8.1. RTSP Feature Tags
This document request that two RTSP feature tags are registered in
the "RTSP feature tag" registry:
setup.rtp-mux See Section Section 6.
setup.ice-d-m See Section Section 3.4.
8.2. Transport Protocol Specifications
This document needs to register a number of transport protocol
combinations are registered in RTSP's "Transport Protocol
Specifications" registry.
"RTP/AVP/D-ICE":
"RTP/AVPF/D-ICE":
"RTP/SAVP/D-ICE":
"RTP/SAVPF/D-ICE":
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8.3. RTSP Transport Parameters
This document requests that 4 transport parameters are registered in
RTSP's "Transport Parameters":
"candidates": See Section Section 3.2.
"ICE-Password": See Section Section 3.3.
"ICE-Userfrag": See Section Section 3.3.
"rtp-rtcp-mux": See Section Section 6.
8.4. RTSP Status Codes
This document requests that 2 assignments are done in the "RTSP
Status Codes" registry. The suggested values are:
150: See Section Section 3.5.1.
480: See Section Section 3.5.2.
8.5. Notify-Reason value
This document requests that one assignment is done in the Notify-
Reason header value registry. The suggested value is:
ice-restart: See section Section 3.6.
8.6. SDP Attribute
The registration of one SDP attribute is requested:
SDP Attribute ("att-field"):
Attribute name: rtsp-ice-d-m
Long form: ICE for RTSP datagram media NAT traversal
Type of name: att-field
Type of attribute: Session level only
Subject to charset: No
Purpose: RFC XXXX
Reference: RFC XXXX
Values: No values defined.
Contact: Magnus Westerlund
E-mail: magnus.westerlund@ericsson.com
phone: +46 8 404 82 87
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9. Security Considerations
ICE [I-D.ietf-mmusic-ice] provides an extensive discussion on
security considerations which applies here as well.
9.1. ICE and RTSP
A long-standing risk with transmitting a packet stream over UDP is
that the host may not be interested in receiving the stream. On
today's Internet many hosts are behind NATs or operate host firewalls
which do not respond to unsolicited packets with an ICMP port
unreachable error. Thus, an attacker can construct SDP with a
victim's IP address and cause a flood of media packets to be sent to
a victim. The addition of ICE, as described in this document,
provides protection from the attack described above. By performing
the ICE connectivity check, the media server receives confirmation
that the RTSP client wants the media. While this protection could
also be implemented by requiring the IP addresses in the SDP match
the IP address of the RTSP signaling packet, such a mechanism does
not protect other hosts with the same IP address (such as behind the
same NAT), and such a mechanism would prohibit separating the RTSP
controller from the media playout device (e.g., an IP-enabled remote
control and an IP-enabled television).
10. Acknowledgements
The authors would like to thank Remi Denis-Courmont for suggesting
the method of integrating ICE in RTSP signalling, Dan Wing for help
with the security section and numerous other issues.
11. References
11.1. Normative References
[I-D.ietf-avt-rtp-and-rtcp-mux]
Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port",
draft-ietf-avt-rtp-and-rtcp-mux-07 (work in progress),
August 2007.
[I-D.ietf-behave-rfc3489bis]
Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for (NAT) (STUN)",
draft-ietf-behave-rfc3489bis-16 (work in progress),
July 2008.
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[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[I-D.ietf-mmusic-rfc2326bis]
Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
and M. Stiemerling, "Real Time Streaming Protocol 2.0
(RTSP)", draft-ietf-mmusic-rfc2326bis-18 (work in
progress), May 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
11.2. Informative References
[I-D.ietf-mmusic-rtsp-nat-evaluation]
Westerlund, M. and T. Zeng, "The evaluation of different
NAT traversal Techniques for media controlled by Real-
time Streaming Protocol (RTSP)",
draft-ietf-mmusic-rtsp-nat-evaluation-01 (work in
progress), July 2008.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
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Authors' Addresses
Jeff Goldberg
Cisco
11 New Square, Bedfont Lakes
Feltham,, Middx TW14 8HA
United Kingdom
Phone: +44 20 8824 1000
Fax:
Email: jgoldber@cisco.com
URI:
Magnus Westerlund
Ericsson
Torshamsgatan 23
Stockholm, SE-164 80
Sweden
Phone: +46 8 719 0000
Fax:
Email: magnus.westerlund@ericsson.com
URI:
Thomas Zeng
Nextwave Wireless, Inc.
12670 High Bluff Drive
San Diego, CA 92130
USA
Phone: +1 858 480 3100
Fax:
Email: thomas.zeng@gmail.com
URI:
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Full Copyright Statement
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contained in BCP 78, and except as set forth therein, the authors
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