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rfc:rfc4145

Network Working Group D. Yon Request for Comments: 4145 Tactical Software, LLC Category: Standards Track G. Camarillo

                                                              Ericsson
                                                        September 2005
TCP-Based Media Transport in the Session Description Protocol (SDP)

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 This document describes how to express media transport over TCP using
 the Session Description Protocol (SDP).  It defines the SDP 'TCP'
 protocol identifier, the SDP 'setup' attribute, which describes the
 connection setup procedure, and the SDP 'connection' attribute, which
 handles connection reestablishment.

Yon & Camarillo Standards Track [Page 1] RFC 4145 Connection-Oriented Media September 2005

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
 2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
 3.  Protocol Identifier  . . . . . . . . . . . . . . . . . . . . .  3
 4.  Setup Attribute  . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  The Setup Attribute in the Offer/Answer Model. . . . . .  4
 5.  The Connection Attribute . . . . . . . . . . . . . . . . . . .  5
     5.1.  Offerer Behaviour. . . . . . . . . . . . . . . . . . . .  6
     5.2.  Answerer Behaviour . . . . . . . . . . . . . . . . . . .  7
 6.  Connection Management  . . . . . . . . . . . . . . . . . . . .  8
     6.1.  Connection Establishment . . . . . . . . . . . . . . . .  8
     6.2.  Connection Reestablishment . . . . . . . . . . . . . . .  8
     6.3.  Connection Termination . . . . . . . . . . . . . . . . .  8
 7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Passive/Active . . . . . . . . . . . . . . . . . . . . .  9
     7.2.  Actpass/Passive. . . . . . . . . . . . . . . . . . . . .  9
     7.3.  Existing Connection Reuse. . . . . . . . . . . . . . . . 10
     7.4.  Existing Connection Refusal. . . . . . . . . . . . . . . 10
 8.  Other Connection-Oriented Transport Protocols. . . . . . . . . 11
 9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
 10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     12.1. Normative References . . . . . . . . . . . . . . . . . . 13
     12.2. Informative References . . . . . . . . . . . . . . . . . 13

Yon & Camarillo Standards Track [Page 2] RFC 4145 Connection-Oriented Media September 2005

1. Introduction

 The Session Description Protocol [4] provides a general-purpose
 format for describing multimedia sessions in announcements or
 invitations.  SDP uses an entirely textual data format (the US-ASCII
 subset of UTF-8 [11]) to maximize portability among transports.  SDP
 does not define a protocol; it defines the syntax to describe a
 multimedia session with sufficient information to participate in that
 session.  Session descriptions may be sent using arbitrary existing
 application protocols for transport (e.g., SAP [9], SIP [10], RTSP
 [6], email, HTTP [8], etc.).
 SDP [4] defines two protocol identifiers: RTP/AVP and UDP, both of
 which represent unreliable, connectionless protocols.  While these
 transports are appropriate choices for multimedia streams, there are
 applications for which TCP is more appropriate.  This document
 defines a new protocol identifier, 'TCP', to describe TCP connections
 in SDP.
 TCP introduces two new factors when describing a session: how and
 when should endpoints perform the TCP connection setup procedure.
 This document defines two new attributes to describe TCP connection
 setups: 'setup' and 'connection'.

2. Terminology

 In this document, the key words "MUST", "MUST NOT", "REQUIRED",
 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
 RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
 described in BCP 14, RFC 2119 [3], and they indicate requirement
 levels for compliant implementations.

3. Protocol Identifier

 The following is the ABNF for an 'm' line, as specified by RFC 2327
 [4].
  media-field =         "m=" media space port ["/" integer]
                        space proto 1*(space fmt) CRLF
 This document defines a new value for the proto field: 'TCP'.
 The 'TCP' protocol identifier is similar to the 'UDP' protocol
 identifier in that it only describes the transport protocol, and not
 the upper-layer protocol.  An 'm' line that specifies 'TCP' MUST

Yon & Camarillo Standards Track [Page 3] RFC 4145 Connection-Oriented Media September 2005

 further qualify the application-layer protocol using an fmt
 identifier.  Media described using an 'm' line containing the 'TCP'
 protocol identifier are carried using TCP [1].

4. Setup Attribute

 The 'setup' attribute indicates which of the end points should
 initiate the TCP connection establishment (i.e., send the initial TCP
 SYN).  The 'setup' attribute is charset-independent and can be a
 session-level or a media-level attribute.  The following is the ABNF
 of the 'setup' attribute:
       setup-attr           =  "a=setup:" role
       role                 =  "active" / "passive" / "actpass"
                               / "holdconn"
    'active': The endpoint will initiate an outgoing connection.
    'passive': The endpoint will accept an incoming connection.
    'actpass': The endpoint is willing to accept an incoming
    connection or to initiate an outgoing connection.
    'holdconn': The endpoint does not want the connection to be
    established for the time being.

4.1. The Setup Attribute in the Offer/Answer Model

 The offer/answer model, defined in RFC 3264 [5], provides endpoints
 with a means to obtain shared view of a session.  Some session
 parameters are negotiated (e.g., codecs to use), while others are
 simply communicated from one endpoint to the other (e.g., IP
 addresses).  The value of the 'setup' attribute falls into the first
 category.  That is, both endpoints negotiate its value using the
 offer/answer model.
 The negotiation of the value of the 'setup' attribute takes places as
 follows.  The offerer states which role or roles it is willing to
 perform; and the answerer, taking the offerer's willingness into
 consideration, chooses which roles both endpoints will actually
 perform during connection establishment.  The following are the
 values that the 'setup' attribute can take in an offer/answer
 exchange:

Yon & Camarillo Standards Track [Page 4] RFC 4145 Connection-Oriented Media September 2005

          Offer      Answer
          ________________
          active     passive / holdconn
          passive    active / holdconn
          actpass    active / passive / holdconn
          holdconn   holdconn
 The active endpoint SHOULD initiate a connection to the port number
 on the 'm' line of the other endpoint.  The port number on its own
 'm' line is irrelevant, and the opposite endpoint MUST NOT attempt to
 initiate a connection to the port number specified there.
 Nevertheless, since the 'm' line must contain a valid port number,
 the endpoint using the value 'active' SHOULD specify a port number of
 9 (the discard port) on its 'm' line.  The endpoint MUST NOT specify
 a port number of zero, except to denote an 'm' line that has been or
 is being refused.
 The passive endpoint SHOULD be ready to accept a connection on the
 port number specified in the 'm' line.
 A value of 'actpass' indicates that the offerer can either initiate a
 connection to the port number on the 'm' line in the answer, or
 accept a connection on the port number specified in the 'm' line in
 the offer.  That is, the offerer has no preference as to whether it
 accepts or initiates the connection and, so, is letting the answerer
 choose.
 A value of 'holdconn' indicates that the connection should not be
 established for the time being.
 The default value of the setup attribute in an offer/answer exchange
 is 'active' in the offer and 'passive' in the answer.

5. The Connection Attribute

 The preceding description of the 'setup' attribute is placed in the
 context of using SDP to initiate a session.  Still, SDP may be
 exchanged between endpoints at various stages of a session to
 accomplish tasks such as terminating a session, redirecting media to
 a new endpoint, or renegotiating the media parameters for a session.
 After the initial session has been established, it may be ambiguous
 whether a subsequent SDP exchange represents a confirmation that the
 endpoint is to continue using the current TCP connection unchanged,
 or is a request to make a new TCP connection.  The media-level
 'connection' attribute, which is charset-independent, is used to
 disambiguate these two scenarios.  The following is the ABNF of the
 connection attribute:

Yon & Camarillo Standards Track [Page 5] RFC 4145 Connection-Oriented Media September 2005

       connection-attr        = "a=connection:" conn-value
       conn-value             = "new" / "existing"

5.1. Offerer Behaviour

 Offerers and answerers use the 'connection' attribute to decide
 whether a new transport connection needs to be established or, on the
 other hand, the existing TCP connection should still be used.  When
 an offerer generates an 'm' line that uses TCP, it SHOULD provide a
 connection attribute for the 'm' line unless the application using
 the 'm' line has other means to deal with connection reestablishment.
 After the initial offer/answer exchange, any of the endpoints can
 generate a new offer to change some characteristics of the session
 (e.g., the direction attribute).  If such an offerer wants to
 continue using the previously-established transport-layer connection
 for the 'm' line, the offerer MUST use a connection value of
 'existing' for the 'm' line.  If, on the other hand, the offerer
 wants to establish a new transport-layer connection for the 'm' line,
 it MUST use a connection value of 'new'.
    Note that, according to the rules in this section, an offer that
    changes the transport address (IP address or port number) of an
    'm' line will have a connection value of 'new'.  Similarly, the
    'connection' attribute in an initial offer (i.e., no transport
    connection has been established yet) takes the value of 'new'.
 The 'connection' value resulting from an offer/answer exchange is the
 'connection' value in the answer.  If the 'connection' value in the
 answer is 'new', the end-points SHOULD establish a new connection.
 If the connection value in the answer is 'existing', the end-points
 SHOULD continue using the exiting connection.
 Taking into consideration the rules in Section 5.2, the following are
 the values that the 'connection' attribute can take in an
 offer/answer exchange:
          Offer      Answer
          ________________
          new        new
          existing   existing / new
 If the connection value resulting from an offer/answer exchange is
 'existing', the end-points continue using the existing connection.
 Consequently, the port numbers, IP addresses, and 'setup' attributes
 negotiated in the offer/answer exchange are ignored because there is
 no need to establish a new connection.

Yon & Camarillo Standards Track [Page 6] RFC 4145 Connection-Oriented Media September 2005

 The previous rule implies that an offerer generating an offer with a
 connection value of 'existing' and a setup value of 'passive' needs
 to be ready (i.e., needs to allocate resources) to receive a
 connection request from the answerer just in case the answerer
 chooses a connection value of 'new' for the answer.  However, if the
 answerer uses a connection value of 'existing' in the answer, the
 offerer would need to deallocate the previously allocated resources
 that were never used because no connection request was received.
 To avoid allocating resources unnecessarily, offerers using a
 connection value of 'existing' in their offers may choose to use a
 setup value of 'holdconn'.  Nevertheless, offerers using this
 strategy should be aware that if the answerer chooses a connection
 value of 'new', a new offer/answer exchange (typically initiated by
 the previous offerer) with setup value different than 'holdconn' will
 be needed to establish the new connection.  This may, of course,
 cause delays in the application using the TCP connection.
 The default value of the connection attribute in both offers and
 answers is 'new'.

5.2. Answerer Behaviour

 The connection value for an 'm' line is negotiated using the offer/
 answer model.  The resulting connection value after an offer/answer
 exchange is the connection value in the answer.  If the connection
 value in the offer is 'new', the answerer MUST also use a value of
 'new' in the answer.  If the connection value in the offer is
 'existing', the answerer uses a value of 'existing' in the answer if
 it wishes to continue using the existing connection and a value of
 'new' if it wants a new connection to be established.
    In some scenarios where third party call control [12] is used, an
    endpoint may receive an initial offer with a connection value of
    'existing'.  Following the previous rules, such an answerer would
    use a connection value of 'new' in the answer.
 If the connection value for an 'm' line resulting from an offer/
 answer exchange is 'new', the endpoints SHOULD establish a new TCP
 connection as indicated by the 'setup' attribute.  If a previous TCP
 connection is still up, the endpoints SHOULD close it as soon as the
 offer/answer exchange is completed.  It is up to the application to
 ensure proper data synchronization between the two TCP connections.
 If the connection value for an 'm' line resulting from an offer/
 answer exchange is 'existing', the endpoints SHOULD continue using
 the existing TCP connection.

Yon & Camarillo Standards Track [Page 7] RFC 4145 Connection-Oriented Media September 2005

6. Connection Management

 This section addresses connection establishment, connection
 reestablishment, and connection termination.

6.1. Connection Establishment

 An endpoint that according to an offer/answer exchange is supposed to
 initiate a new TCP connection SHOULD initiate it as soon as it is
 able to, even if the endpoint does not intend to immediately begin
 sending media to the remote endpoint.  This allows media to flow from
 the remote endpoint if needed.
    Note that some endpoints need to wait for some event to happen
    before being able to establish the connection.  For example, a
    wireless terminal may need to set up a radio bearer before being
    able to initiate a TCP connection.

6.2. Connection Reestablishment

 If an endpoint determines that the TCP for an 'm' line has been
 closed and should be reestablished, it SHOULD perform a new offer/
 answer exchange using a connection value of 'new' for this 'm' line.
    Note that the SDP direction attribute (e.g., 'a=sendonly') deals
    with the media sent over the TCP connection, but has no impact on
    the TCP connection itself.

6.3. Connection Termination

 Typically, endpoints do not close the TCP connection until the
 session has expired, been explicitly terminated, or a new connection
 value has been provided for the 'm' line.  Additionally, specific
 applications can describe further scenarios where an end-point may
 close a given TCP connection (e.g., whenever a connection is in the
 half-close state).  As soon as an end-point notices that it needs to
 terminate a TCP connection, it SHOULD do so.
 In any case, individual applications may provide further
 considerations on how to achieve a graceful connection termination.
 For example, a file application using TCP to receive a FIN from the
 remote endpoint may need to finish the ongoing transmission of a file
 before sending its own FIN.

Yon & Camarillo Standards Track [Page 8] RFC 4145 Connection-Oriented Media September 2005

7. Examples

 The following examples show the most common usage of the 'setup'
 attribute combined with TCP-based media descriptions.  For the
 purpose of brevity, the main portion of the session description is
 omitted in the examples, which only show 'm' lines and their
 attributes (including 'c' lines).

7.1. Passive/Active

 An offerer at 192.0.2.2 signals its availability for a T.38 fax
 session at port 54111:
         m=image 54111 TCP t38
         c=IN IP4 192.0.2.2
         a=setup:passive
         a=connection:new
 An answerer at 192.0.2.1 receiving this offer responds with the
 following answer:
         m=image 9 TCP t38
         c=IN IP4 192.0.2.1
         a=setup:active
         a=connection:new
 The endpoint at 192.0.2.1 then initiates the TCP connection to port
 54111 at 192.0.2.2.

7.2. Actpass/Passive

 In another example, an offerer at 192.0.2.2 signals its availability
 for a T.38 fax session at TCP port 54111.  Additionally, this offerer
 is also willing to set up the media stream by initiating the TCP
 connection:
         m=image 54111 TCP t38
         c=IN IP4 192.0.2.2
         a=setup:actpass
         a=connection:new
 The endpoint at 192.0.2.1 responds with the following description:
         m=image 54321 TCP t38
         c=IN IP4 192.0.2.1
         a=setup:passive
         a=connection:new

Yon & Camarillo Standards Track [Page 9] RFC 4145 Connection-Oriented Media September 2005

 This will cause the offerer (at 192.0.2.2) to initiate a connection
 to port 54321 at 192.0.2.1.

7.3. Existing Connection Reuse

 Subsequent to the exchange in Section 7.2, another offer/answer
 exchange is initiated in the opposite direction.  The endpoint at
 192.0.2.1 wishes to continue using the existing connection:
          m=image 54321 TCP t38
          c=IN IP4 192.0.2.1
          a=setup:passive
          a=connection:existing
 The endpoint at 192.0.2.2 also wishes to use the existing connection
 and responds with the following description:
          m=image 9 TCP t38
          c=IN IP4 192.0.2.2
          a=setup:active
          a=connection:existing
 The existing connection from 192.0.2.2 to 192.0.2.1 will be reused.
    Note that the endpoint at 192.0.2.2 uses 'setup:active' in
    response to the offer of 'setup:passive', and uses port 9 because
    it is active.

7.4. Existing Connection Refusal

 Subsequent to the exchange in Section 7.3, another offer/answer
 exchange is initiated by the endpoint at 192.0.2.2, again wishing to
 reuse the existing connection:
          m=image 54111 TCP t38
          c=IN IP4 192.0.2.2
          a=setup:passive
          a=connection:existing
 However, this time the answerer is unaware of the old connection and
 thus wishes to establish a new one.  (This could be the result of a
 transfer via third-party call control.)  It is unable to act in the
 'passive' mode and thus responds as 'active':

Yon & Camarillo Standards Track [Page 10] RFC 4145 Connection-Oriented Media September 2005

          m=image 9 TCP t38
          c=IN IP4 192.0.2.3
          a=setup:active
          a=connection:new
 The endpoint at 192.0.2.3 then initiates the TCP connection to port
 54111 at 192.0.2.2, and the endpoint at 192.0.2.2 closes the old
 connection.
    Note that the endpoint at 192.0.2.2, while using a connection
    value of 'existing', has used a setup value of 'passive'.  Had it
    not done this and instead used a setup value of 'holdconn'
    (probably to avoid allocating resources as described in
    Section 5.1), a new offer/answer exchange would have been needed
    in order to establish the new connection.

8. Other Connection-Oriented Transport Protocols

 This document specifies how to describe TCP-based media streams using
 SDP.  Still, some of the attributes defined here could possibly be
 used to describe media streams based on other connection-oriented
 transport protocols as well.  This section provides advice to authors
 of specifications of SDP extensions that deal with connection-
 oriented transport protocols other than TCP.
 It is recommended that documents defining new SDP protocol
 identifiers that involve extra protocol layers between TCP and the
 media itself (e.g., TLS [7] over TCP) start with the string 'TCP/'
 (e.g., 'TCP/TLS').
 The 'setup' and the 'connection' attributes are specified in
 Section 4 and Section 5 respectively.  While both attributes are
 applicable to 'm' lines that use the 'TCP' protocol identifier, they
 are general enough to be reused in 'm' lines with other connection-
 oriented transport protocols.  Therefore, it is recommended that the
 'setup' and 'connection' attributes are reused, as long as it is
 possible, for new proto values associated with connection-oriented
 transport protocols.
 Section 6 deals with TCP connection management.  It should be noted
 that while in TCP both end-points need to close a connection, other
 connection-oriented transport protocols may not have the concept of
 half-close connections.  In such a case, a connection would be
 terminated as soon as one of the end-points closed it, making it
 unnecessary for the other end-point to perform any further action to
 terminate the connection.  So, specifications dealing with such
 transport protocols may need to specify slightly different procedures
 regarding connection termination.

Yon & Camarillo Standards Track [Page 11] RFC 4145 Connection-Oriented Media September 2005

9. Security Considerations

 See RFC 2327 [4] for security and other considerations specific to
 the Session Description Protocol in general.
 An attacker may attempt to modify the values of the connection and
 setup attributes in order to have endpoints reestablish connections
 unnecessarily or to keep them from establishing a connection.  So, it
 is strongly RECOMMENDED that integrity protection be applied to the
 SDP session descriptions.  For session descriptions carried in SIP
 [10], S/MIME is the natural choice to provide such end-to-end
 integrity protection, as described in RFC 3261 [10].  Other
 applications MAY use a different form of integrity protection.

10. IANA Considerations

 This document defines two session- and media-level SDP attributes:
 setup and connection.  Their formats are defined in Section 4 and
 Section 5, respectively.  These two attributes should be registered
 by the IANA under "Session Description Protocol (SDP) Parameters"
 under "att-field (both session and media level)".
 This document defines a proto value: TCP.  Its format is defined in
 Section 3.  This proto value should be registered by the IANA under
 "Session Description Protocol (SDP) Parameters" under "proto".
 The SDP specification, RFC2327, states that specifications defining
 new proto values, like the TCP proto value defined in this RFC, must
 define the rules by which their media format (fmt) namespace is
 managed.  For the TCP protocol, new formats SHOULD have an associated
 MIME registration.  Use of an existing MIME subtype for the format is
 encouraged.  If no MIME subtype exists, it is RECOMMENDED that a
 suitable one is registered through the IETF process [2] by production
 of, or reference to, a standards-track RFC that defines the transport
 protocol for the format.

11. Acknowledgements

 Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul
 Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer
 Holmberg provided valuable insights and contributions.

Yon & Camarillo Standards Track [Page 12] RFC 4145 Connection-Oriented Media September 2005

12. References

12.1. Normative References

 [1]  Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
      September 1981.
 [2]  Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
      Mail Extensions (MIME) Part Four: Registration Procedures",
      BCP 13, RFC 2048, November 1996.
 [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [4]  Handley, M. and V. Jacobson, "SDP: Session Description
      Protocol", RFC 2327, April 1998.
 [5]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
      Session Description Protocol (SDP)", RFC 3264, June 2002.

12.2. Informative References

 [6]   Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
       Protocol (RTSP)", RFC 2326, April 1998.
 [7]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
       RFC 2246, January 1999.
 [8]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
       Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
       HTTP/1.1", RFC 2616, June 1999.
 [9]   Handley, M., Perkins, C., and E. Whelan, "Session Announcement
       Protocol", RFC 2974, October 2000.
 [10]  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.
 [11]  Yergeau, F., "UTF-8, a transformation format of ISO 10646",
       STD 63, RFC 3629, November 2003.
 [12]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
       "Best Current Practices for Third Party Call Control (3pcc) in
       the Session Initiation Protocol (SIP)", BCP 85, RFC 3725,
       April 2004.

Yon & Camarillo Standards Track [Page 13] RFC 4145 Connection-Oriented Media September 2005

Authors' Addresses

 David Yon
 Tactical Software, LLC
 1750 Elm St., Suite 803
 Manchester, NH  03104
 USA
 EMail: yon-comedia@rfdsoftware.com
 Gonzalo Camarillo
 Ericsson
 Hirsalantie 11
 Jorvas  02420
 Finland
 EMail: Gonzalo.Camarillo@ericsson.com

Yon & Camarillo Standards Track [Page 14] RFC 4145 Connection-Oriented Media September 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Yon & Camarillo Standards Track [Page 15]

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