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RFC 3108

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Conventions for the use of the Session Description Protocol (SDP) for ATM Bearer Connections

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Network Working Group                                           R. Kumar
Request for Comments: 3108                                    M. Mostafa
Category: Standards Track                                  Cisco Systems
                                                                May 2001


   Conventions for the use of the Session Description Protocol (SDP)
                       for ATM Bearer Connections

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 (2001).  All Rights Reserved.

Abstract

   This document describes conventions for using the Session Description
   Protocol (SDP) described in RFC 2327 for controlling ATM Bearer
   Connections, and any associated ATM Adaptation Layer (AAL).  The AALs
   addressed are Type 1, Type 2 and Type 5.  This list of conventions is
   meant to be exhaustive.  Individual applications can use subsets of
   these conventions.  Further, these conventions are meant to comply
   strictly with the SDP syntax as defined in RFC 2327.

Table of Contents

   1. Introduction...................................................  3
   1.1  Key words to indicate Requirement Levels.....................  5
   2. Representation of Certain Fields within SDP description lines..  5
   2.1  Representation of Extension Attributes.......................  5
   2.2  Representation of Parameter Values...........................  5
   2.3  Directionality Convention....................................  6
   2.4 Case convention...............................................  7
   2.5 Use of special characters in SDP parameter values.............  8
   3. Capabilities Provided by SDP conventions.......................  8
   4. Format of the ATM Session Description..........................  9
   5.  Structure of the Session Description Lines.................... 11
   5.1  The Origin Line.............................................. 11
   5.2  The Session Name Line........................................ 12
   5.3  The Connection Information Line.............................. 13
   5.4  The Timestamp Line........................................... 15

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   5.5  Media Information Line for ATM connections................... 16
   5.5.1  The Virtual Connection ID.................................. 16
   5.5.2  The Transport Parameter.................................... 19
   5.5.3  The Format List for AAL1 and AAL5 applications............. 21
   5.5.4  The Format List for AAL2 applications...................... 21
   5.5.5  Media information line construction........................ 22
   5.6  The Media Attribute Lines.................................... 27
   5.6.1  ATM bearer connection attributes........................... 28
   5.6.1.1  The 'eecid' attribute.................................... 30
   5.6.1.2  The 'aalType' attribute.................................. 31
   5.6.1.3  The 'capability' attribute............................... 32
   5.6.1.4  The 'qosClass' attribute................................. 33
   5.6.1.5  The 'bcob' attribute..................................... 34
   5.6.1.6  The 'stc' attribute...................................... 34
   5.6.1.7  The 'upcc' attribute..................................... 35
   5.6.1.8  The 'atmQOSparms' attribute.............................. 35
   5.6.1.9  The 'atmTrfcDesc'  attribute............................. 37
   5.6.1.10 The 'abrParms' attribute................................. 39
   5.6.1.11 The 'abrSetup' attribute................................. 40
   5.6.1.12 The 'bearerType' attribute............................... 41
   5.6.1.13 The 'lij' attribute...................................... 42
   5.6.1.14 The 'anycast' attribute.................................. 43
   5.6.1.15 The 'cache' attribute.................................... 43
   5.6.1.16 The 'bearerSigIE' attribute.............................. 44
   5.6.2  ATM Adaptation Layer (AAL) attributes...................... 45
   5.6.2.1  The 'aalApp' attribute................................... 46
   5.6.2.2  The 'cbrRate' attribute.................................. 48
   5.6.2.3  The 'sbc' attribute...................................... 49
   5.6.2.4  The 'clkrec' attribute................................... 51
   5.6.2.5  The 'fec' attribute...................................... 51
   5.6.2.6  The 'prtfl' attribute.................................... 51
   5.6.2.7  The 'structure' attribute................................ 52
   5.6.2.8  The 'cpsSDUsize' attribute............................... 53
   5.6.2.9  The 'aal2CPS' attribute.................................. 53
   5.6.2.10 The 'aal2CPSSDUrate' attribute........................... 54
   5.6.2.11 The 'aal2sscs3661unassured' attribute.................... 54
   5.6.2.12 The 'aal2sscs3661assured' attribute...................... 55
   5.6.2.13 The 'aal2sscs3662' attribute............................. 56
   5.6.2.14 The 'aal5sscop' attribute................................ 58
   5.6.3  Service attributes......................................... 58
   5.6.3.1  The 'atmmap' attribute................................... 60
   5.6.3.2  The 'silenceSupp' attribute.............................. 63
   5.6.3.3  The 'ecan' attribute..................................... 65
   5.6.3.4  The 'gc' attributes...................................... 66
   5.6.3.5  The 'profileDesc' attribute.............................. 67
   5.6.3.6  The 'vsel' attribute..................................... 68
   5.6.3.7  The 'dsel' attribute..................................... 70
   5.6.3.8  The 'fsel' attribute..................................... 72

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   5.6.3.9  The 'onewaySel' attribute................................ 73
   5.6.3.10 The 'codecconfig' attribute.............................. 75
   5.6.3.11 The 'isup_usi' attribute................................. 76
   5.6.3.12 The 'uiLayer1_Prot' attribute............................ 76
   5.6.4  Miscellaneous media attributes............................. 77
   5.6.4.1 The 'chain' attribute..................................... 77
   5.6.5  Use of the second media-level part in H.323 Annex C
          applications............................................... 78
   5.6.6  Use of the eecid media attribute in call establishment
          procedures................................................. 78
   6. List of Parameters with  Representations....................... 83
   7. Examples of ATM session descriptions using SDP................. 93
   8. Security Considerations........................................ 94
   8.1  Bearer Security.............................................. 94
   8.2  Security of the SDP description.............................. 95
   9. ATM SDP Grammar................................................ 95
   References........................................................104
   Acknowledgements..................................................109
   Authors' Addresses................................................109
   Full Copyright Statement..........................................110

1. Introduction

   SDP will be used in conjunction with a connection handling /device
   control protocol such as Megaco (H.248) [26], SIP [18] or MGCP [25]
   to communicate the information needed to set up ATM and AAL2 bearer
   connections.  These connections include voice connections, voiceband
   data connections, clear channel circuit emulation connections, video
   connections and baseband data connections (such as fax relay, modem
   relay, SSCOP, frame relay etc.).

   These conventions use standard SDP syntax as defined in RFC 2327 [1]
   to describe the ATM-level and AAL-level connections, addresses and
   other parameters.  In general, parameters associated with layers
   higher than the ATM adaptation layer are included only if they are
   tightly coupled to the ATM or AAL layers.  Since the syntax conforms
   to RFC 2327, standard SDP parsers should react in a well-defined and
   safe manner on receiving session descriptions based on the SDP
   conventions in this document.  This is done by extending the values
   of fields defined in RFC 2327 rather than by defining new fields.
   This is true for all SDP lines except the of the media attribute
   lines, in which case new attributes are defined.  The SDP protocol
   allows the definition of new attributes in the media attribute lines
   which are free-form.  For the remaining lines, the fact that the
   <networkType> field in an SDP descriptor is set to "ATM" should
   preclude the misinterpretation of extended parameter values by RFC
   2327-compliant SDP parsers.

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   These conventions are meant to address the following ATM
   applications:

      1. Applications in which a new SVC is set-up for each service
         connection.  These SVCs could be AAL1 or AAL5 SVCs or single-
         CID AAL2 SVCs.

      2. Applications in which existing path resources are assigned to
         service connections.  These resources could be:

         *  AAL1/AAL5 PVCs, SPVCs or cached SVCs,
         *  AAL2 single-CID PVCs, SPVCs or cached SVCs,
         *  CIDs within AAL2 SVCs/PVCs/SPVCs that multiplex multiple
            CIDs.
         *  Subchannels (identified by CIDs) within AAL1 [8] or AAL2
            [11] SVCs/PVCs/SPVCs.

   Note that the difference between PVCs and SPVCs is in the way the
   bearer virtual circuit connection is set up.  SPVCs are a class of
   PVCs that use bearer signaling, as opposed to node-by-node
   provisioning, for connection establishment.

   This document is limited to the case when the network type is ATM.
   This includes raw RTP encapsulation [45] or voice sample
   encapsulation [46] over AAL5 with no intervening IP layer.  It does
   not address SDP usage for IP, with or without ATM as a lower layer.

   In some cases, IP connection set-up is independent of lower layers,
   which are configured prior to it.  For example, AAL5 PVCs that
   connect IP routers can be used for VoIP calls.  In other cases, VoIP
   call set-up is closely tied to ATM-level connection set-up.  This
   might require a chaining of IP and ATM descriptors, as described in
   section 5.6.4.1.

   This document makes no assumptions on who constructs the session
   descriptions (media gateway, intermediate ATM/AAL2 switch, media
   gateway controller etc.).  This will be different in different
   applications.  Further, it allows the use of one session description
   for both directions of a connection (as in SIP and MGCP applications)
   or the use of separate session descriptions for different directions.
   It also addresses the ATM multicast and anycast capabilities.

   This document makes no assumptions about how the SDP description will
   be coded.  Although the descriptions shown here are encoded as text,
   alternate codings are possible:

   -  Binary encoding such as ASN.1.  This is an option (in addition to
      text encoding) in the Megaco context.

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   -  Use of extended ISUP parameters [36] to encode the information in
      SDP descriptors, with conversion to/from binary/text-based SDP
      encoding when needed.

1.1 Key words to indicate Requirement Levels

   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 [62].

2. Representation of Certain Fields within SDP description lines

   This document conforms to the syntactic conventions of standard SDP
   as defined in RFC 2327 [1].

2.1  Representation of Extension Attributes

   The SDP protocol [1] requires that non-standard attributes and codec
   names use an "X-" prefix.

   In this internet document, the "X-" prefix is used consistently for
   codec names (Table 2) that have not been registered with the IANA.
   The IANA-registered codec names listed in [31] do not use this
   prefix, regardless of  whether they are statically or dynamically
   assigned payload types.

   However, this prefix is not used for the extension SDP attributes
   defined in this document.  This has been done to enhance legibility.

   This document suggests that parsers be flexible in the use of the
   "X-" prefix convention.  They should accept codec names and attribute
   names with or without the "X-" prefix.

2.2 Representation of Parameter Values

   Depending on the format of their representation in SDP, the
   parameters defined in this document fall into the following classes:

   (1) Parameters always represented in a decimal format.
   (2) Parameters always represented in a hexadecimal format.
   (3) Parameters always represented as character strings.
   (4) Parameters that can be represented in either decimal or
       hexadecimal format.

   No prefixes are needed for classes 1 - 3, since the format is fixed.
   For class 4, a "0x" prefix shall always be used to differentiate the
   hexadecimal from the decimal format.

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   For both decimal and hex representations, if the underlying bit field
   is smaller or larger than the binary equivalent of the SDP
   representation, then leading 0 bits should be added or removed as
   needed.  Thus, 3 and 0x3 translate into the following five-bit
   pattern: 0 0011.  The SDP representations 0x12 and 18 translate into
   the following five-bit pattern: 1 0010.

   Leading 0 digits shall not be used in decimal representations.
   Generally, these are also not used in hexadecimal representations.
   Exceptions are when an exact number of hex digits is expected, as in
   the case of NSAP addresses.  Parsers shall not reject leading zeros
   in hex values.

   Both single-character and multi-character string values are enclosed
   in double quotes (i.e., ").  By contrast, single quotes (i.e., ') are
   used for emphasizing keywords rather than to refer to characters or
   strings.

   In the text representation of decimal and hex numbers, digits to the
   left are more significant than digits to the right.

2.3 Directionality Convention

   This section defined the meaning of the terms 'forward' and
   'backward' as used in this document.  This is specially applicable to
   parameters that have a specific direction associated with them.

   In this document, 'forward' refers to the direction away from the ATM
   node under consideration, while 'backward' refers to the direction
   towards the ATM node.  This convention must be used in all SDP-based
   session descriptions regardless of whether underlying bearer is an
   SVC, a dynamically allocated PVC/SPVC or a dynamically allocated CID.
   This is regardless of which side originates the service connection.
   If ATM SVC or AAL2 Q.2630.1 signaling is used, the directionality
   convention is independent of which side originates the SVC or AAL2
   connection.

   This provides a simple way of identifying the direction in which a
   parameter is applicable, in a manner that is independent of the
   underlying ATM or AAL2 bearer.  This simplicity comes at a price,
   described below.

   The convention used by all ATM/AAL2 signaling specifications (e.g.,
   Q.2931 Section 1.3.3 and Q.2630.1) mandates that forward direction is
   from the end initiating setup/establishment via bearer signaling
   towards the end receiving the setup/establishment request.  The
   backward direction is in the opposite direction.  In some cases, the
   'forward' and 'backward' directions of the ATM signaling convention

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   might be the exact opposite of the SDP convention described above,
   requiring the media gateway to perform the necessary translation.  An
   example case in which this is needed is described below.

   Consider an SDP description sent by a media gateway controller to the
   gateway originating a service-level call.  In the backward SVC call
   set-up model, this gateway terminates (rather than originates) an SVC
   call.  The media gateway refers to the traffic descriptor (and hence
   the PCR) in the direction away from this gateway as the forward
   traffic descriptor and forward PCR.  Clearly, this is at odds with
   ATM SVC signaling which refers to this very PCR as the backward PCR.
   The gateway needs to be able to perform the required swap of
   directions.  In this example, the media gateway terminating the
   service level call (and hence originating the SVC call) does not need
   to perform this swap.

   Certain parameters within attributes are defined exclusively for the
   forward or  backward directions.  Examples for the forward direction
   are the <fsssar> subparameter within the 'aal2sscs3661unassured'
   media attribute line, the <fsssar>, <fsscopsdu> and <fsscopuu>
   subparameters within the 'aal2sscs3661assured' media attribute line,
   the <fsscopsdu> and <fsscopuu> subparameters within the 'aal5sscop'
   media attribute line, and the <fmaxFrame> parameter within the
   'aal2sscs3662' media attribute line.  Examples for the backward
   direction are the <bsssar> subparameter within the
   'aal2sscs3661unassured' media attribute line, the <bsssar>,
   <bsscopsdu> and <bsscopuu> subparameters within the
   'aal2sscs3661assured' media attribute line, the <bsscopsdu> and
   <bsscopuu> subparameters within the 'aal5sscop' media attribute line,
   and the <bmaxFrame> parameter within the 'aal2sscs3662' media
   attribute line.

2.4 Case convention

   As defined in RFC 2327 [1], SDP syntax is case-sensitive.  Since
   these ATM conventions conform strictly with SDP syntax, they are
   case-sensitive.  SDP line types (e.g., "c", "m", "o", "a") and fields
   in the SDP lines should be built according to the case conventions in
   [1] and in this document.  It is suggested, but not required, that
   SDP parsers for ATM applications be case-tolerant where ignoring case
   does not result in ambiguity.  Encoding names, which are defined
   outside the SDP protocol, are case-insensitive.

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2.5 Use of special characters in SDP parameter values

   In general, RFC 2327-conformant string values of SDP parameters [1]
   do not include special characters that are neither alphabets nor
   digits.  An exception is the "/" character used in the value
   "RTP/AVP" of transport sub-field of the 'm' line.

   String values used in SDP descriptions of ATM connections retain this
   convention, while allowing the use of the special character "/" in a
   manner commensurate with [1].  In addition, the special characters
   "$" and "-" are used in the following manner.  A "$" value is a
   wildcard that allows the recipient of the SDP description to select
   any permitted value of the parameter.  A "-" value indicates that it
   is not necessary to specify the value of the parameter in the SDP
   description because this parameter is irrelevant for this
   application, or because its value can be known from another source
   such as provisioning, defaults, another protocol, another SDP
   descriptor or another part of the same SDP descriptor.  If the use of
   these special characters is construed as a violation of RFC 2327 [1]
   syntax, then reserved string values can be used.  The string "CHOOSE"
   can be used in lieu of "$".  The string "OMIT" can be used in lieu of
   "-" for an omitted parameter.

3. Capabilities Provided by SDP conventions

   To support the applications listed in section 1, the SDP conventions
   in this document provide the following session control capabilities:

      *  Identification of the underlying bearer network type as ATM.

      *  Identification by an ATM network element of its own address, in
         one of several possible formats.  A connection peer can
         initiate SVC set-up to this address.  A call agent or
         connection peer can select an pre-established bearer path to
         this address.

      *  Identification of the ATM bearer connection that is to be bound
         to the service-level connection.  Depending on the application,
         this is either a VCC or a subchannel (identified by a CID)
         within a VCC.

      * Identification of media type: audio, video, data.

      *  In AAL1/AAL5  applications, declaration of a set of payload
         types that can be bound to the ATM bearer connection.  The
         encoding names and payload types defined for use in the RTP
         context [31] are re-used for AAL1 and AAL5, if applicable.

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      *  In AAL2 applications, declaration of a set of profiles that can
         be bound to the ATM bearer connection.  A mechanism for
         dynamically defining custom profiles within the SDP session
         description is included.  This allows the use of custom
         profiles for connections that span multi-network interfaces.

      *  A means of correlating service-level connections with
         underlying ATM bearer connections.  The backbone network
         connection identifier or bnc-id specified in ITU Q.1901 [36]
         standardization work is used for this purpose.  In order to
         provide a common SDP base for applications based on Q.1901 and
         SIP/SIP+, the neutral term 'eecid' is used in lieu of 'bnc-id'
         in the SDP session descriptor.

      *  A means of  mapping codec types and packetization periods into
         service types (voice, voiceband data and facsimile).  This is
         useful in determining the encoding to use when the connection
         is upspeeded in response to modem or facsimile tones.

      *  A means of describing the adaptation type, QoS class, ATM
         transfer capability/service category, broadband bearer class,
         traffic parameters, CPS parameters and SSCS parameters related
         the underlying bearer connection.

      *  Means for enabling or describing special functions such as
         leaf- initiated-join, anycast and SVC caching.

      *  For H.323 Annex C applications, a means of specifying the IP
         address and port number on which the node will receive RTCP
         messages.

      *  A means of chaining consecutive SDP descriptors so that they
         refer to different layers of the same connection.

4. Format of the ATM Session Description

   The sequence of lines in the session descriptions in this document
   conforms to RFC 2327 [1].  In general, a session description consists
   of a  session-level part followed by zero or more media-level parts.
   ATM session descriptions consist of a session-level part followed by
   one or two media-level parts.  The only two media applicable are the
   ATM bearer medium and RTCP control (where applicable).

   The session level part consists of the following lines:

   v=  (protocol version, zero or one line)
   o=  (origin, zero or one line)
   s=  (session name, zero or one line)

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   c=  (connection information, one line)
   b=  (bandwidth, zero or more lines)
   t=  (timestamp, zero or one line)
   k=  (encryption key, zero or one line)

   In ATM session descriptions, there are no media attribute lines in
   the session level part.  These are present in the media-level parts.

   The media-level part for the ATM bearer consists of the following
   lines:

   m=  (media information and transport address, one line)
   b=  (bandwidth, zero or more lines)
   k=  (encryption key, zero or more lines)
   a=  (media attribute, zero or more lines)

   The media-level part for RTCP control consists of the following
   lines:

   m=  (media information and transport address, one line)
   c=  (connection information for control only, one line)

   In general, the 'v', 'o', 's', and 't' lines are mandatory.  However,
   in the Megaco [26] context, these lines have been made optional.  The
   'o', 's', and 't' lines are omitted in most MGCP [25] applications.

   Note that SDP session descriptors for ATM can contain bandwidth (b=)
   and encryption key (k=) lines.  Like all other lines, these lines
   should strictly conform to the SDP standard [1].

   The bandwidth (b=) line is not necessarily redundant in the ATM
   context since, in some applications, it can be used to convey
   application-level information which does not map directly into the
   atmTrfcDesc media attribute line.  For instance, the 'b' line can be
   used in SDP descriptors in RTSP commands to describe content
   bandwidth.

   The encryption key line (k=) can be used to indicate an encryption
   key for the bearer, and a method to obtain the key.  At present, the
   encryption of ATM and AAL2 bearers has not been conventionalized,
   unlike the encryption of RTP payloads.  Nor has the authentication or
   encryption of ATM or AAL2 bearer signaling.  In the ATM and AAL2
   contexts, the term 'bearer' can include 'bearer signaling' as well as
   'bearer payloads'.

   The order of lines in an ATM session description is exactly in the
   RFC 2327-conformant order depicted above.  However, there is no order
   of the media attribute ('a') lines with respect to other 'a' lines.

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   The SDP protocol version for session descriptions using these
   conventions is 0.  In conformance with standard SDP, it  is strongly
   recommended that the 'v' line be included at the beginning of each
   SDP session description.  In some contexts such as Megaco, the
   'v' line is optional and may be omitted unless several session
   descriptions are provided in sequence, in which case the 'v' line
   serves as a delimiter.  Depending on the application, sequences of
   session descriptions might refer to:

   -  Different connections or sessions.
   -  Alternate ways of realizing the same connection or session.
   -  Different layers of the same session (section 5.6.4.1).

   The 'o', 's' and 't' lines are included for strict conformance with
   RFC 2327.  It is possible that these lines might not carry useful
   information in some ATM-based applications.  Therefore, some
   applications might omit these lines, although it is recommended that
   they not do so.  For maximum interoperability, it is preferable that
   SDP parsers not reject session descriptions that do not contain these
   lines.

5. Structure of the Session Description Lines

5.1 The Origin Line

   The origin line for an ATM-based session is structured as follows:

         o=<username> <sessionID> <version> <networkType>
           <addressType> <address>

   The <username> is set to "-".

   The <sessionID> can be  set to one of the following:

      *  an NTP timestamp referring to the moment when the SDP session
         descriptor was created.
      *  a Call ID, connection ID or context ID that uniquely identifies
         the session within the scope of the ATM node.  Since calls can
         comprise multiple connections (sessions), call IDs are
         generally not suitable for this purpose.

   NTP time stamps can be represented as decimal or hex integers.  The
   part of the NTP timestamp that refers to an integer number of seconds
   is sufficient.  This is a 32-bit field

   On the other hand, call IDs, connection IDs and context IDs can be
   can be 32 hex digits long.

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   The <sessionID> field is represented as a decimal or hex number of up
   to 32 digits.  A "0x" prefix is used before the hex representation.
   The <version> refers to the version of the SDP session descriptor
   (not that of the SDP protocol).  This is can be set to one of the
   following:

      *  0.
      *  an NTP timestamp referring to the moment when the SDP session
         descriptor was modified.  If the SDP session descriptor has not
         been modified by an intermediate entity (such as an MGC), then
         the <version> timestamp will be the same as the <sessionId>
         timestamp, if any.  As with the <sessionId>, only the integer
         part of the NTP timestamp is used.

   When equated to the integer part of an NTP timestamp, the <version>
   field is 10 digits wide.  This is more restricted than [1], which
   allows unlimited size.  As in [1], the most significant digit is
   non-zero when an NTP timestamp is used.

   The <networkType> in SDP session descriptions for ATM applications
   should be assigned the string value "ATM" or wildcarded to a "$" or
   "-".

   The <addressType> and <address>  parameters are identical to those
   for the connection information ('c') line (Section 5.3).  Each of
   these parameters can be wildcarded per the conventions described for
   the 'c' line in Section 5.3.  These parameters should not me omitted
   since this would violate SDP syntax [1].

   As with the 'c' line, SDP parsers are not expected to check the
   consistency of <networkType> with <addressType>, <address> pairs.
   The <addressType> and <address> need to be consistent with each
   other.

5.2 The Session Name Line

   In general, the session name line is structured as follows:

        s=<sessionName>

   For ATM-based sessions, the <sessionName> parameter is set to a "-".
   The resulting line is:

        s=-

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5.3 The Connection Information Line

   In general, the connection information line [1] is structured as
   follows:

        c=<networkType> <addressType> <address>

   For ATM networks, additional values of <networkType>, <addressType>
   and <address> are defined, over and above those listed in [1].  The
   ABNF syntax (Section 9) for ATM SDP does not limit the ways in which
   <networkType> can be combined with <addressType>, <address> pairs.
   However, some combinations will not be valid in certain applications,
   while others will never be valid.  Invalid combinations should be
   rejected by application-specific functions, and not by generic
   parsers.  The ABNF syntax does limit the ways in which <addressType>
   and <address> can be paired.

   For ATM networks, the value of <networkType> should be set to "ATM".
   Further, this may be wildcarded to "$" or "-".  If this is done, an
   node using ATM as the basic transport mechanism will select a value
   of "ATM".  A node that interfaces with multiple network types ("IN",
   "ATM" etc.) that include ATM can also choose a value of "ATM".

   When the SDP description is built by a node such as a media gateway,
   the <address> refers to the address of the node building the SDP
   description.  When this description is forwarded to another node, it
   still contains the original node's address.  When the media gateway
   controller builds part or all of the SDP description, the local
   descriptor contains the address of the local node, while the remote
   descriptor contains the address of the remote node.  If the <address>
   and/or <addressType> are irrelevant or are known by other means, they
   can be set to a "$" or a "-", as described below.

   Additionally, in all contexts, the 'm' line can have an ATM address
   in the <virtualConnectionId> subparameter which, if present, is the
   remote address if the 'c' line address is local, and vice versa.

   For ATM networks, the <addressType> can be NSAP, E164 or GWID
   (ALIAS).  For ATM networks, the <address> syntax depends on the
   syntax of the <addressType>.  SDP parsers should check the
   consistency of <addressType> with <address>.

   NSAP: If the addressType is NSAP, the address is expressed in the
   standard dotted hex form.  This is a string of 40 hex digits, with
   dots after the 2nd, 6th, 10th, 14th, 18th, 22nd, 26th, 30th, 34th and
   38th digits.  The last octet of the NSAP address is the 'selector'
   field that is available for non-standard use.  An example of a line
   with an NSAP address is:

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       c=ATM NSAP 47.0091.8100.0000.0060.3e64.fd01.0060.3e64.fd01.00

   A "0x" prefix shall not be used in this case since this is always in
   hexadecimal format.

   E164: If the addressType is E164, the address is expressed as a
   decimal number  with up to 15 digits.  For example:

        c=ATM E164 9738294382

   The use of E.164 numbers in the B-ISDN context is defined in ITU
   E.191.  There is a disparity between the ATM forum and the ITU in the
   use of E.164 numbers for ATM addressing.  The ATM forum (e.g., UNI
   Signaling 4.0) allows only International Format E.164 numbers, while
   the ITU (e.g., Q.2931) allows private numbering plans.  Since the
   goal of this SDP specification is to interoperate with all bearer
   signaling protocols, it allows the use of numbers that do not conform
   to the E.164 International Format.  However, to maximize overall
   consistency, network administrators can restrict the provisioning of
   numbers to the E.164 International Format.

   GWID (ALIAS): If the addressType is GWID, it means that the address
   is a Gateway Identifier or Node Alias.  This may or may not be
   globally unique.  In this format, the address is expressed as an
   alphanumeric string ("A"-"Z", "a"-"z", "0" - "9",".","-","_").  For
   example:

       c=ATM GWID officeABCmgx101vism12

   Since these SDP conventions can be used for more than gateways, the
   string "ALIAS" can be used instead of "GWID" in the 'c' line.  Thus,
   the example above is equivalent to:

       c=ATM ALIAS officeABCmgx101vism12

   An example of a GWID (ALIAS)is the CLLI code used for telecom
   equipment.  For all practical purposes, it should be adequate for the
   GWID (ALIAS) to be a variable length string with a maximum size of 32
   characters.

   The connection information line is always present in an SDP session
   descriptor.  However, each of the parameters on this line can be
   wildcarded to a "$" or a "-", independently of whether other
   parameters on this line are wildcarded or not.  Not all syntactically
   legal wildcard combinations are meaningful in a particular
   application.

Top      ToC       Page 15 
   Examples of meaningful wildcard combinations in the ATM context are:

         c=- - -
         c=$ $ $
         c=ATM - -
         c=ATM $ $
         c=ATM <addressType> -
         c=ATM <addressType> $

   Specifying the ATM address type without specifying the ATM address is
   useful when the recipient is asked to select an ATM address of a
   certain type (NSAP, E.164 etc.).

   Examples of syntactically legal wildcard combinations of dubious
   utility are:

         c=- $ -
         c=- $ $
         c=- <addressType> -
         c=$ <addressType> $
         c=- <addressType> <address>
         c=$ <addressType> <address>

   Note that <addressType> and/or <address> should not omitted without
   being set to a "-" or "$" since this would violate basic SDP syntax
   [1].

5.4 The Timestamp Line

   The timestamp line for an SDP session descriptor is structured as
   follows:

         t= <startTime> <stopTime>

   Per Ref. [49], NTP time stamps use a 32 bit unsigned representation
   of seconds, and a 32 bit unsigned representation of fractional
   seconds.  For ATM-based sessions, the <startTime>parameter can be
   made equal to the NTP timestamp referring to the moment when the SDP
   session descriptor was created.  It can also be set to 0 indicating
   its irrelevance.  If it made equal to the NTP timestamp in seconds,
   the fractional part of the NTP timestamp is omitted.  When equated to
   the integer part of an NTP timestamp, the <startTime> field is 10
   digits wide.  This is more restricted than [1], which allows
   unlimited size.  As in [1], the most significant digit is non-zero
   when an NTP timestamp is used.

   The <stopTime> parameter is set to 0 for ATM-based SDP descriptors.

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5.5 Media Information Line for ATM connections

   The general format of the media information line adapted for AAL1 and
   AAL5 applications is:

   m=<media> <virtualConnectionId> <transport> <format list>

   The general format of the media information line adapted for AAL2
   applications is:

m=<media> <virtualConnectionId> <transport#1> <format list#1>
         <transport#2> <format list#2> ... <transport#M> <format list#M>

   Note that <virtualConnectionId> is equivalent to <port> in [1].

   The subparameter <media> can take on all the values defined in [1].
   These are: "audio", "video", "application", "data" and "control".

   When the <transport> parameter has more than one value in the 'm'
   line, the <transport> <format list> pairs can be arranged in
   preferential order.

5.5.1 The Virtual Connection ID

   In applications in which the media-level part of a session descriptor
   is bound to an ATM virtual circuit, the <virtualConnectionId> can be
   in one of the following formats:

         * <ex_vcci>
         * <addressType>-<address>/<ex_vcci>
         * <address>/<ex_vcci>
         * <ex_bcg>/<ex_vcci>
         * <ex_portId>/<ex_vpi>/<ex_vci>
         * <ex_bcg>/<ex_vpi>/<ex_vci>
         * <ex_vpci>/<ex_vci>
         * <addressType>-<address>/<ex_vpci>/<ex_vci>
         * <address>/<ex_vpci>/<ex_vci>

   In applications in which the media-level part of a session descriptor
   is bound to a subchannel within an ATM virtual circuit, the
   <virtualConnectionId> can be in one of the following formats:

         * <ex_vcci>/<ex_cid>
         * <addressType>-<address>/<ex_vcci>/<ex_cid>
         * <address>/<ex_vcci>/<ex_cid>
         * <ex_bcg>/<ex_vcci>/<ex_cid>
         * <ex_portId>/<ex_vpi>/<ex_vci>/<ex_cid>
         * <ex_bcg>/<ex_vpi>/<ex_vci>/<ex_cid>

Top      ToC       Page 17 
         * <ex_vpci>/<ex_vci>/<ex_cid>
         * <addressType>-<address>/<ex_vpci>/<ex_vci>/<ex_cid>
         * <address>/<ex_vpci>/<ex_vci>/<ex_cid>

   Here,

          <ex_vcci> = VCCI-<vcci>
          <ex_vpci> = VPCI-<vpci>
          <ex_bcg> = BCG-<bcg>
          <ex_portId> = PORT-<portId>
          <ex_vpi> = VPI-<vpi>
          <ex_vci> = VCI-<vci>
          <ex_cid> = CID-<cid>

   The <vcci>, <vpi>, <vci>, <vpci> and <cid> are decimal numbers or
   hexadecimal numbers.  An "0x" prefix is used before their values when
   they are in the hex format.

   The <portId> is always a hexadecimal number.  An "0x" prefix is not
   used with it.

   The <addressType> and <address> are identical to their definitions
   above for the connection information line with the difference that
   this address refers to the remote peer in the media information line.
   Since the <virtualConnectionId>, as defined here, is meant for use in
   ATM networks, the values of <addressType> and <address> in the
   <virtualConnectionId> are limited to ATM-specific values.

   The <vpi>, <vci> and <cid> are the Virtual Path Identifier, Virtual
   Circuit Identifier and Channel Identifier respectively. The <vpi> is
   an 8 or 12 bit field. The <vci> is a 16-bit field. The <cid> is an
   8-bit field ([8] and [11]). For AAL1 applications, it corresponds to
   the channel number defined in Annex C of [8].

   The <vpci> is a 16-bit field defined in Section 4.5.16 of ITU Q.2931
   [Ref. 15].  The <vpci> is similar to the <vpi>, except for its width
   and the fact that it retains its value across VP crossconnects.  In
   some applications, the size of the <vpci> is the same as the size of
   the <vpi> (8 or 12 bits).  In this case, the most significant 8 or 4
   bits are ignored.

   The <vcci> is a 16-bit field defined in ITU Recommendation Q.2941.2
   [32].  The <vcci> is similar to the <vci>, except for the fact that
   it retains its value across VC crossconnects.

   In general, <vpci> and <vcci> values are unique between a pair of
   nodes.  When they are unique between a pair of nodes but not unique
   within a network, they need to be qualified, at any node, by the ATM

Top      ToC       Page 18 
   address of the remote node.  These parameters can be pre-provisioned
   or signaled.  When signaled, the <vpci> is encapsulated in the
   connection identifier information element of SVC signaling messages.
   The <vcci> is encapsulated in the Generic Information Transport (GIT)
   information element of SVC signaling messages.  In an ATM node pair,
   either node can assign <vcci> values and signal it to the other end
   via SVC signaling.  A glare avoidance scheme is defined in [32] and
   [44].  This mechanism works in SVC applications.  A different glare
   avoidance technique is needed when a pool of existing PVCs/SPVCs is
   dynamically assigned to calls.  One such scheme for glare reduction
   is the assignment of <vcci> values from different ends of the <vcci>
   range, using the lowest or highest available value as applicable.

   When <vpci> and <vcci> values are pre-provisioned, administrations
   have the option of provisioning them uniquely in a network.  In this
   case, the ATM address of the far end is not needed to qualify these
   parameters.

   In the AAL2 context, the definition of a VCC implies that there is no
   CID-level switching between its ends.  If either end can assign <cid>
   values, then a glare reduction mechanism is needed.  One such scheme
   for glare reduction is the assignment of <cid> values from different
   ends of the <cid> range, using the lowest or highest available value
   as applicable.

   The <portId> parameter is used to identify the physical trunk port on
   an ATM module.  It can be represented as a hexadecimal number of up
   to 32 hex digits.

   In some applications, it is meaningful to bundle a set of connections
   between a pair of ATM nodes into a bearer connection group.  The
   <bcg> subparameter is an eight bit field that allows the bundling of
   up to 255 VPCs or VCCs.

   In some applications, it is necessary to wildcard the
   <virtualConnectionId> parameter, or some elements of this parameter.
   The "$" wildcard character can be substituted for the entire
   <virtualConnectionId> parameter, or some of its terms.  In the latter
   case, the constant strings that qualify the terms in the
   <virtualConnectionId> are retained.  The concatenation
   <addressType>-<address> can be wildcarded in the following ways:

      *  The entire concatenation, <addressType>-<address>, is replaced
         with a "$".
      *  <address> is replaced with a "$", but <addressType> is not.

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   Examples of wildcarding the <virtualConnectionId> in the AAL1 and
   AAL5 contexts are: $, VCCI-$, BCG-100/VPI-20/VCI-$.  Examples of
   wildcarding the <virtualConnectionId> in the AAL2 context are: $,
   VCCI-40/CID-$, BCG-100/VPI-20/VCI-120/CID-$, NSAP-$/VCCI-$/CID-$,
   $/VCCI-$/CID-$.

   It is also permissible to set the entire <virtualConnectionId>
   parameter to a "-" indicating its irrelevance.

5.5.2 The Transport Parameter

   The <transport> parameter indicates the method used to encapsulate
   the service payload.  These methods are not defined in this document,
   which refers to existing ATMF and ITU-T standards, which, in turn,
   might refer to other standards.  For ATM applications, the following
   <transport> values are defined:

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   Table 1: List of Transport Parameter values used in SDP in the ATM
            context

+---------------------------------------------------------------------+
|                        |       Controlling Document for             |
|   Transport            |    Encapsulation of Service Payload        |
+------------------------+--------------------------------------------+
|    AAL1/ATMF           |          af-vtoa-0078.000 [7]              |
+------------------------+--------------------------------------------+
|    AAL1/ITU            |          ITU-T H.222.1 [51]                |
+------------------------+--------------------------------------------+
|    AAL5/ATMF           |          af-vtoa-0083.000 [46]             |
+------------------------+--------------------------------------------+
|    AAL5/ITU            |          ITU-T H.222.1 [51]                |
+------------------------+--------------------------------------------+
|    AAL2/ATMF           |          af-vtoa-0113.000 [44]  and        |
|                        |          af-vmoa-0145.000 [52]             |
+------------------------+--------------------------------------------+
|    AAL2/ITU            |          ITU-T I.366.2 [13]                |
+------------------------+--------------------------------------------+
|    AAL1/custom         |         Corporate document or              |
|    AAL2/custom         |   application-specific interoperability    |
|    AAL5/custom         |              statement.                    |
+------------------------+--------------------------------------------+
|  AAL1/<corporateName>  |                                            |
|  AAL2/<corporateName>  |                                            |
|  AAL5/<corporateName>  |                                            |
|  AAL1/IEEE:<oui>       |          Corporate document                |
|  AAL2/IEEE:<oui>       |                                            |
|  AAL5/IEEE:<oui>       |                                            |
+------------------------+--------------------------------------------+
|     RTP/AVP            |          Annex C of H.323 [45]             |
+------------------------+--------------------------------------------+

   In H.323 Annex C applications [45], the <transport> parameter has a
   value of "RTP/AVP".  This is because these applications use the RTP
   protocol [2] and audio/video profile [3].  The fact that RTP is
   carried directly over AAL5 per [45] can be indicated explicitly via
   the aalApp media attribute.

   A value of "AAL1/custom", "AAL2/custom" or "AAL5/custom" for the
   <transport> parameter can indicate non-standard or semi-standard
   encapsulation schemes defined by a corporation or a multi-vendor
   agreement.  Since there is no standard administration of this
   convention, care should be taken to preclude inconsistencies within
   the scope of a deployment.

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   The use of <transport> values "AAL1/<corporateName>",
   "AAL2/<corporateName>", "AAL5/<corporateName>", "AAL1/IEEE:<oui>",
   "AAL2/IEEE:<oui>" and "AAL5/IEEE:<oui>" is similar.  These indicate
   non-standard transport mechanisms or AAL2 profiles which should be
   used consistently within the scope of an application or deployment.
   The parameter <corporateName> is the registered, globally unique name
   of a corporation (e.g., Cisco, Telcordia etc.).  The parameter <oui>
   is the hex representation of a three-octet field identical to the OUI
   maintained by the IEEE.  Since this is always represented in hex, the
   "0x" prefix shall not be used.  Leading zeros can be omitted.  For
   example, "IEEE:00000C" and "IEEE:C" both refer to Cisco Systems, Inc.

5.5.3 The Format List for AAL1 and AAL5 applications

   In the AAL1 and AAL5 contexts, the <format list> is a list of payload
   types:

      <payloadType#1> <payloadType#2>...<payloadType#n>

   In most AAL1 and AAL5 applications, the ordering of payload types
   implies a preference (preferred payload types before less favored
   ones).  The payload type can be statically assigned or dynamically
   mapped.  Although the transport is not the same, SDP in the ATM
   context leverages the encoding names and payload types registered
   with IANA [31] for RTP.  Encoding names not listed in [31] use a "X-"
   prefix.  Encodings that are not statically mapped to payload types in
   [31] are to be dynamically mapped at the time of connection
   establishment to payload types in the decimal range 96-127.  The SDP
   'atmmap' attribute (similar to 'rtpmap') is used for this purpose.

   In addition to listing the IANA-registered encoding names and payload
   types found in [31], Table 2 defines a few non-standard encoding
   names(with "X-" prefixes).

5.5.4 The Format List for AAL2 applications

   In the AAL2 context, the <format list> is a list of AAL2 profile
   types:

      <profile#1> <profile#2>...<profile#n>

   In most applications, the ordering of profiles implies a preference
   (preferred profiles before less favored ones).  The <profile>
   parameter is expressed as a decimal number in the range 1-255.

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5.5.5 Media information line construction

   Using the parameter definitions above, the 'm' for AAL1-based audio
   media can be constructed as follows:

      m=audio <virtualConnectionId> AAL1/ATMF  <payloadType#1>
                <payloadType#2>...<payloadType #n>

   Note that only those payload types, whether statically mapped or
   dynamically assigned, that are consistent with af-vtoa-78 [7] can be
   used in this construction.

   Backwards compatibility note: The transport value "AAL1/AVP" used in
   previous versions of this document should be considered equivalent to
   the value "AAL1/ATMF" defined above.  "AAL1/AVP" is unsuitable
   because the AVP profile is closely tied to RTP.

   An example 'm' line use for audio media over AAL1 is:

      m=audio VCCI-27 AAL1/ATMF 0

   This indicates the use of an AAL1 VCC with VCCI=24 to carry PCMU
   audio that is encapsulated according to ATMF's af-vtoa-78 [7].

   Another example of the use of the 'm' line use for audio media over
   AAL1 is:

      m=audio $  AAL1/ATMF 0 8

   This indicates that any AAL1 VCC may be used.  If it exists already,
   then its selection is subject to glare rules.  The audio media on
   this VCC is encapsulated according to ATMF's af-vtoa-78 [7].  The
   encodings to be used are either PCMU or PCMA, in preferential order.

   The 'm' for AAL5-based audio media can be constructed as follows:

      m=audio <virtualConnectionId> AAL5/ATMF  <payloadType#1>
                <payloadType#2>...<payloadType #n>

   An example 'm' line use for audio media over AAL5 is:

      m=audio PORT-2/VPI-6/$  AAL5/ITU  9  15

   implies that any VCI on VPI= 6 of trunk port #2 may be used.  The
   identities of the terms in the virtual connection ID are implicit in
   the application context.  The audio media on this VCC is encapsulated
   according to ITU-T H.222.1 [51].  The encodings to be used are either
   ITU-T G.722 or ITU-T G.728 (LD-CELP), in preferential order.

Top      ToC       Page 23 
   The 'm' for AAL5-based H.323 Annex C audio [45] can be constructed as
   follows:

      m=audio <virtualConnectionId> RTP/AVP <payloadType#1>
                    <payloadType#2>...<payloadType #n>

   For example:

         m=audio PORT-9/VPI-3/VCI-$  RTP/AVP 2 96
         a=rtpmap:96 X-G727-32
         a=aalType:AAL5
         a=aalApp:itu_h323c - -

   implies that any VCI on VPI= 3 of trunk port #9 may be used.  This VC
   encapsulates RTP packets directly on AAL5 per [45].  The 'rtpmap'
   (rather than the 'atmmap') attribute is used to dynamically map the
   payload type of 96 into the codec name X-G727-32 (Table 2).  This
   name represents 32 kbps EADPCM.

   The 'm' line for AAL5-based video media can be constructed as
   follows:

      m=video <virtualConnectionId> AAL5/ITU  <payloadType#1>
                <payloadType#2>...<payloadType #n>

   In this case, the use of AAL5/ITU as the transport points to H.222.1
   as the controlling standard [51].  An example 'm' line use for video
   media is:

      m=video PORT-9/VPI-3/VCI-$  AAL5/ITU  33

   This indicates that any VCI on VPI= 3 of trunk port #9 may be used.
   The video media on this VCC is encapsulated according to ITU-T
   H.222.1 [51].  The encoding scheme is an MPEG 2 transport stream
   ("MP2T" in Table 1).  This is statically mapped per [31] to a payload
   type of 33.

   Using the parameter definitions in the previous subsections, the
   media information line for AAL2-based audio media can be constructed
   as follows:

m=<media> <virtualConnectionId> <transport#1> <format list#1>
        <transport#2> <format list#2> ... <transport#M> <format list#M>

   where <format list#i> has the form <profile#i_1>...<profile#i_N>
   Unlike the 'm' line for AAL1 or AAL5 applications, the 'm' line for
   AAL2 applications can have multiple <transport> parameters, each
   followed by a <format list>.  This is because it is possible to

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   consider definitions from multiple sources (ATMF, ITU and non-
   standard documents) when selecting AAL2 profile to be bound to a
   connection.

   In most applications, the ordering of profiles implies a preference
   (preferred profiles before less favored ones).  Therefore, there can
   be multiple instances of the same <transport> value in the same 'm'
   line.

   An example 'm' line use for audio media over AAL2 is:

      m=audio VCCI-27/CID-19 AAL2/ITU 7 AAL2/custom 100 AAL2/ITU 1

   This indicates the use of CID #19 on VCCI #27 to carry audio.  It
   provides a preferential list of profiles for this connection: profile
   AAL2/ITU 7 defined in [13], AAL2/custom 100 defined in an
   application-specific or interoperability document and profile
   AAL2/ITU 1 defined in [13].

   Another example of the use of the 'm' line use for audio media over
   AAL2 is:

      m=audio VCCI-$/CID-$  AAL2/ATMF 6 8

   This indicates that any AAL2 CID may be used, subject to any
   applicable glare avoidance/reduction rules.  The profiles that can be
   bound to this connection are AAL2/ATMF 6 defined in af-vtoa-0113.000
   [44] and AAL2/ATMF 8 defined in af-vmoa-0145.000 [52].  These sources
   use non-overlapping profile number ranges.  The profiles they define
   fall under the <transport> category "AAL2/ATMF".  This application
   does not order profiles preferentially.  This rule is known a priori.
   It is not embedded in the 'm' line.

   Another example of the use of the 'm' line use for audio media over
   AAL2 is:

      m=audio VCCI-20/CID-$  AAL2/xyzCorporation 11

   AAL2 VCCs in this application are single-CID VCCs.  Therefore, it is
   possible to wildcard the CID.  The single-CID VCC with VCCI=20 is
   selected.  The AAL2 profile to be used is AAL2/xyzCorporation 11
   defined by xyzCorporation.

   In some applications, an "-" can be used in lieu of:

   -  <format list>
   -  <transport> and <format list>

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   This implies that these parameters are irrelevant or are known by
   other means (such as defaults).  For example:

         m=audio VCCI-234 - -
         a=aalType:AAL1

   indicates the use of VCCI=234 with AAL1 adaptation and unspecified
   encoding.

   In another example application, the 'aal2sscs3662' attribute can
   indicate <faxDemod> = "on" and any other competing options as "off",
   and the <aalType> attribute can indicate AAL2.  Thus:

      m=audio VCCI-123/CID-5 - -
      a=aalType:AAL2
      a=aal2sscs3662:audio off off on off on off off off - - -

   Besides indicating an audio medium, a VCCI of 123 and a CID of 5, the
   'm' line indicates an unspecified profile.  The media attribute lines
   indicate an adaptation layer of AAL2, and the use of the audio SAP
   [13] to carry demodulated facsimile.

   The media information line for "data" media has one of the following
   the following formats:

      m=data <virtualConnectionId> - -
      m=data - - -

   The data could be circuit emulation data carried over AAL1 or AAL2,
   or packet data carried over AAL5.  Media attribute lines, rather than
   the 'm' line, are used to indicate the adaptation type for the data
   media.  Examples of the representation of data media are listed
   below.

         m=data  PORT-7/VPI-6/VCI-$  - -
         a=aalApp:AAL5_SSCOP-                             -

   implies that any VCI on VPI= 6 of trunk port #7 may be used.  This VC
   uses SSCOP on AAL5 to transport data.

         m=data  PORT-7/VPI-6/VCI-50  - -
         a=aalType:AAL1_SDT
         a=sbc:6

   implies that VCI 50 on VPI 6 on port 7 uses structured AAL1 to
   transfer 6 x 64 kbps circuit emulation data.  This may be alternately
   represented as:

Top      ToC       Page 26 
         m=data  PORT-7/VPI-6/VCI-50  - -
         b=AS:384
         a=aalType:AAL1_SDT

   The following lines:

         m=data VCCI-123/CID-5 - -
         a=aalType:AAL2
         a=sbc:2

   imply that CID 5 of VCCI 123 is used to transfer 2 x 64 kbps circuit
   emulation data.

   In the AAL1 context, it is also permissible to represent circuit mode
   data as an "audio" codec.  If this is done, the codec types used are
   X-CCD or X-CCD-CAS.  These encoding names are dynamically mapped into
   payload types through the 'atmmap' attribute.  For example:

         m=audio VCCI-27 AAL1/AVP 98
         a=atmmap:98 X-CCD
         a=sbc:6

   implies that AAL1 VCCI=27 is used for 6 x 64 transmission.

   In the AAL2 context, the X-CCD codec can be assigned a profile type
   and number.  Even though it is not possible to construct a profile
   table as described in ITU I.366.2 for this "codec", it is preferable
   to adopt the common AAL2 profile convention in its case.  An example
   AAL2 profile mapping for the X-CCD codec could be as follows:

      PROFILE TYPE         PROFILE NUMBER        "CODEC" (ONLY ONE)
        "custom"                200                     X-CCD

   The profile does not identify the number of subchannels ('n' in
   nx64).  This is known by other means such as the 'sbc' media
   attribute line.

   For example, the media information line:

         m=audio $ AAL2/custom 200
         a=sbc:6

   implies 384 kbps circuit emulation using AAL2 adaptation.

   It is not necessary to define a profile with the X-CCD-CAS codec,
   since this method of CAS transport [7] is not used in AAL2
   applications.


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