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 Kumar & Mostafa Standards Track [Page 1] RFC 3108 ATM SDP May 2001 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 Kumar & Mostafa Standards Track [Page 2] RFC 3108 ATM SDP May 2001 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 field in an SDP descriptor is set to "ATM" should preclude the misinterpretation of extended parameter values by RFC 2327-compliant SDP parsers. Kumar & Mostafa Standards Track [Page 3] RFC 3108 ATM SDP May 2001 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. Kumar & Mostafa Standards Track [Page 4] RFC 3108 ATM SDP May 2001 - 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. Kumar & Mostafa Standards Track [Page 5] RFC 3108 ATM SDP May 2001 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 Kumar & Mostafa Standards Track [Page 6] RFC 3108 ATM SDP May 2001 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 subparameter within the 'aal2sscs3661unassured' media attribute line, the , and subparameters within the 'aal2sscs3661assured' media attribute line, the and subparameters within the 'aal5sscop' media attribute line, and the parameter within the 'aal2sscs3662' media attribute line. Examples for the backward direction are the subparameter within the 'aal2sscs3661unassured' media attribute line, the , and subparameters within the 'aal2sscs3661assured' media attribute line, the and subparameters within the 'aal5sscop' media attribute line, and the 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. Kumar & Mostafa Standards Track [Page 7] RFC 3108 ATM SDP May 2001 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. Kumar & Mostafa Standards Track [Page 8] RFC 3108 ATM SDP May 2001 * 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) Kumar & Mostafa Standards Track [Page 9] RFC 3108 ATM SDP May 2001 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. Kumar & Mostafa Standards Track [Page 10] RFC 3108 ATM SDP May 2001 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=
The is set to "-". The 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. Kumar & Mostafa Standards Track [Page 11] RFC 3108 ATM SDP May 2001 The field is represented as a decimal or hex number of up to 32 digits. A "0x" prefix is used before the hex representation. The 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 timestamp will be the same as the timestamp, if any. As with the , only the integer part of the NTP timestamp is used. When equated to the integer part of an NTP timestamp, the 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 in SDP session descriptions for ATM applications should be assigned the string value "ATM" or wildcarded to a "$" or "-". The and
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 with ,
pairs. The and
need to be consistent with each other. 5.2 The Session Name Line In general, the session name line is structured as follows: s= For ATM-based sessions, the parameter is set to a "-". The resulting line is: s=- Kumar & Mostafa Standards Track [Page 12] RFC 3108 ATM SDP May 2001 5.3 The Connection Information Line In general, the connection information line [1] is structured as follows: c=
For ATM networks, additional values of , and
are defined, over and above those listed in [1]. The ABNF syntax (Section 9) for ATM SDP does not limit the ways in which can be combined with ,
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 and
can be paired. For ATM networks, the value of should be set to "ATM". Further, this may be wildcarded to "$" or "-". If this is done, an node using ATM as the basic transport mechanism will select a value of "ATM". A node that interfaces with multiple network types ("IN", "ATM" etc.) that include ATM can also choose a value of "ATM". When the SDP description is built by a node such as a media gateway, the
refers to the address of the node building the SDP description. When this description is forwarded to another node, it still contains the original node's address. When the media gateway controller builds part or all of the SDP description, the local descriptor contains the address of the local node, while the remote descriptor contains the address of the remote node. If the
and/or are irrelevant or are known by other means, they can be set to a "$" or a "-", as described below. Additionally, in all contexts, the 'm' line can have an ATM address in the subparameter which, if present, is the remote address if the 'c' line address is local, and vice versa. For ATM networks, the can be NSAP, E164 or GWID (ALIAS). For ATM networks, the
syntax depends on the syntax of the . SDP parsers should check the consistency of with
. 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: Kumar & Mostafa Standards Track [Page 13] RFC 3108 ATM SDP May 2001 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 "$" or a "-", independently of whether other parameters on this line are wildcarded or not. Not all syntactically legal wildcard combinations are meaningful in a particular application. Kumar & Mostafa Standards Track [Page 14] RFC 3108 ATM SDP May 2001 Examples of meaningful wildcard combinations in the ATM context are: c=- - - c=$ $ $ c=ATM - - c=ATM $ $ c=ATM - c=ATM $ 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=- - c=$ $ c=-
c=$
Note that and/or
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= 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 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 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 parameter is set to 0 for ATM-based SDP descriptors. Kumar & Mostafa Standards Track [Page 15] RFC 3108 ATM SDP May 2001 5.5 Media Information Line for ATM connections The general format of the media information line adapted for AAL1 and AAL5 applications is: m= The general format of the media information line adapted for AAL2 applications is: m= ... Note that is equivalent to in [1]. The subparameter can take on all the values defined in [1]. These are: "audio", "video", "application", "data" and "control". When the parameter has more than one value in the 'm' line, the 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 can be in one of the following formats: * * -
/ *
/ * / * // * // * / * -
// *
// In applications in which the media-level part of a session descriptor is bound to a subchannel within an ATM virtual circuit, the can be in one of the following formats: * / * -
// *
// * // * /// * /// Kumar & Mostafa Standards Track [Page 16] RFC 3108 ATM SDP May 2001 * // * -
/// *
/// Here, = VCCI- = VPCI- = BCG- = PORT- = VPI- = VCI- = CID- The , , , and are decimal numbers or hexadecimal numbers. An "0x" prefix is used before their values when they are in the hex format. The is always a hexadecimal number. An "0x" prefix is not used with it. The and
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 , as defined here, is meant for use in ATM networks, the values of and
in the are limited to ATM-specific values. The , and are the Virtual Path Identifier, Virtual Circuit Identifier and Channel Identifier respectively. The is an 8 or 12 bit field. The is a 16-bit field. The is an 8-bit field ([8] and [11]). For AAL1 applications, it corresponds to the channel number defined in Annex C of [8]. The is a 16-bit field defined in Section 4.5.16 of ITU Q.2931 [Ref. 15]. The is similar to the , except for its width and the fact that it retains its value across VP crossconnects. In some applications, the size of the is the same as the size of the (8 or 12 bits). In this case, the most significant 8 or 4 bits are ignored. The is a 16-bit field defined in ITU Recommendation Q.2941.2 [32]. The is similar to the , except for the fact that it retains its value across VC crossconnects. In general, and 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 Kumar & Mostafa Standards Track [Page 17] RFC 3108 ATM SDP May 2001 address of the remote node. These parameters can be pre-provisioned or signaled. When signaled, the is encapsulated in the connection identifier information element of SVC signaling messages. The is encapsulated in the Generic Information Transport (GIT) information element of SVC signaling messages. In an ATM node pair, either node can assign 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 values from different ends of the range, using the lowest or highest available value as applicable. When and 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 values, then a glare reduction mechanism is needed. One such scheme for glare reduction is the assignment of values from different ends of the range, using the lowest or highest available value as applicable. The 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 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 parameter, or some elements of this parameter. The "$" wildcard character can be substituted for the entire parameter, or some of its terms. In the latter case, the constant strings that qualify the terms in the are retained. The concatenation -
can be wildcarded in the following ways: * The entire concatenation, -
, is replaced with a "$". *
is replaced with a "$", but is not. Kumar & Mostafa Standards Track [Page 18] RFC 3108 ATM SDP May 2001 Examples of wildcarding the in the AAL1 and AAL5 contexts are: $, VCCI-$, BCG-100/VPI-20/VCI-$. Examples of wildcarding the 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 parameter to a "-" indicating its irrelevance. 5.5.2 The Transport Parameter The 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 values are defined: Kumar & Mostafa Standards Track [Page 19] RFC 3108 ATM SDP May 2001 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/ | | | AAL2/ | | | AAL5/ | | | AAL1/IEEE: | Corporate document | | AAL2/IEEE: | | | AAL5/IEEE: | | +------------------------+--------------------------------------------+ | RTP/AVP | Annex C of H.323 [45] | +------------------------+--------------------------------------------+ In H.323 Annex C applications [45], the 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 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. Kumar & Mostafa Standards Track [Page 20] RFC 3108 ATM SDP May 2001 The use of values "AAL1/", "AAL2/", "AAL5/", "AAL1