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RFC Index
rfc3108
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
<networkType> 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 <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.
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=<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.
Kumar & Mostafa Standards Track [Page 11]
RFC 3108 ATM SDP May 2001
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=-
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=<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:
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 "$" or a "-", independently of whether other
parameters on this line are wildcarded or not. Not all syntactically
legal wildcard combinations are meaningful in a particular
application.
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Examples of meaningful wildcard combinations in the ATM context are:
c=- - -
c=$ $ $
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>
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* <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
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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.
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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:
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RFC 3108 ATM SDP May 2001
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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5.6 The Media Attribute Lines
In an SDP line sequence, the media information line 'm' is followed
by one or more media attribute or 'a' lines. Media attribute lines
are per the format below:
a=<attribute>:<value>
or
a=<value>
In general, media attribute lines are optional except when needed to
qualify the media information line. This qualification is necessary
when the "m" line for an AAL1 or AAL5 session specifies a payload
type that needs to be dynamically mapped. The 'atmmap' media
attribute line defined below is used for this purpose.
In attribute lines, subparameters that are meant to be left
unspecified are set to a "-". These are generally inapplicable or,
if applicable, are known by other means such as provisioning. In
some cases, a media attribute line with all parameters set to "-"
carries no information and should be preferably omitted. In other
cases, such as the 'lij' media attribute line, the very presence of
the media attribute line conveys meaning.
There are no restrictions placed by RFC 2327 [1] regarding the order
of 'a' lines with respect to other 'a' lines. However, these lines
must not contradict each other or the other SDP lines.
Inconsistencies are not to be ignored and should be flagged as
errors. Repeated media attribute lines can carry additional
information. These should not be inconsistent with each other.
Applications will selectively use the optional media attribute lines
listed below. This is meant to be an exhaustive list for describing
the general attributes of ATM bearer networks.
The base specification for SDP, RFC 2327 [1], allows the definition f
new attributes. In keeping with this spirit, some of the attributes
defined in this document can also be used in SDP descriptions of IP
nd other non-ATM sessions. For example, the 'vsel', 'dsel' and
'fsel' attributes defined below refer generically to codec-s. These
can be bed for service-specific codec negotiation and assignment in
non-ATM s well as ATM applications.
SDP media attributes defined in this document for use in the ATM
context are classified as:
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RFC 3108 ATM SDP May 2001
* ATM bearer connection attributes (Section 5.6.1)
* AAL attributes (Section 5.6.2)
* Service attributes (Section 5.6.3).
* Miscellaneous media attributes, that cannot be classified as
ATM, AAL or service attributes (Section 5.6.4).
In addition to these, the SDP attributes defined in [1] can also be
used in the ATM context. Examples are:
* The attributes defined in RFC 2327 which allow indication of
the direction in which a session is active. These are
a=sendonly, a=recvonly, a=sendrecv, a=inactive.
* The 'Ptime' attribute defined in RFC 2327. It indicates the
packet period. It is not recommended that this attribute be
used in ATM applications since packet period information is
provided with other parameters (e.g., the profile type and
number in the 'm' line, and the 'vsel', 'dsel' and 'fsel'
attributes). Also, for AAL1 applications, 'ptime' is not
applicable and should be flagged as an error. If used in AAL2
and AAL5 applications, 'ptime' should be consistent with the
rest of the SDP description.
* The 'fmtp' attribute used to designate format-specific
parameters.
5.6.1 ATM bearer connection attributes
The following is a summary list of the SDP media attributes that can
be used to describe ATM bearer connections. These are detailed in
subsequent subsections.
* The 'eecid' attribute. This stands for 'end-to-end connection
identifier'. It provides a means of correlating service-level
connections with underlying ATM bearer connections. In the
Q.1901 [36] context, the eecid is synonymous with the bnc-id
(backbone network connection identifier).
* The 'aalType' attribute. This is used to indicate the nature
of the ATM adaptation layer (AAL).
* The 'capability' attribute, which indicates the ATM transfer
capability (ITU nomenclature), synonymous with the ATM Service
Category (ATMF nomenclature).
* The 'qosClass' attribute, which indicates the QoS class of the
ATM bearer connection.
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* The 'bcob' attribute, which indicates the broadband connection
oriented bearer class, and whether end-to-end timing is
required.
* The 'stc' attribute, which indicates susceptibility to
clipping.
* The 'upcc' attribute, which indicates the user plane connection
configuration.
* The 'atmQOSparms' attribute, which is used to describe certain
key ATM QoS parameters.
* The 'atmTrfcDesc' attribute, which is used to describe ATM
traffic descriptor parameters.
* The 'abrParms' attribute, which is used to describe ABR-
specific parameters. These parameters are per the UNI 4.0
signaling specification [5].
* The 'abrSetup' attribute, which is used to indicate the ABR
parameters needed during call/connection establishment.
* The 'bearerType' attribute, which is used to indicate whether
the underlying bearer is an ATM PVC/SPVC, an ATM SVC, or a
subchannel within an existing ATM SVC/PVC/SPVC.
* The 'lij' attribute, which is used to indicate the presence of
a connection that uses the Leaf-initiated-join capability
described in UNI 4.0 [5], and to optionally describe parameters
associated with this capability.
* The 'anycast' attribute, which is used to indicate the
applicability of the anycast function described in UNI 4.0 [5],
and to optionally qualify it with certain parameters.
* The 'cache' attribute, which is used to enable SVC caching and
to specify an inactivity timer for SVC release.
* The 'bearerSigIE' attribute, which can be used to represent ITU
Q-series information elements in bit-map form. This is useful
in describing parameters that are not closely coupled to the
ATM and AAL layers. Examples are the B-HLI and B-LLI IEs
specified in ITU Q.2931 [15], and the user-to-user information
element described in ITU Q.2957 [48].
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5.6.1.1 The 'eecid' attribute
The 'eecid' attribute is synonymous with the 4-byte 'bnc-id'
parameter used by T1SI, the ATM forum and the ITU (Q.1901)
standardization effort. The term 'eecid' stands for 'end-to-end
connection identifier', while 'bnc-id' stands for 'backbone network
connection identifier'. The name "backbone" is slightly misleading
since it refers to the entire ATM network including the ATM edge and
ATM core networks. In Q.1901 terminology, an ATM "backbone" connects
TDM or analog edges.
While the term 'bnc-id' might be used in the bearer signaling plane
and in an ISUP (Q.1901) call control plane, SDP session descriptors
use the neutral term 'eecid'. This provides a common SDP baseline
for applications that use ISUP (Q.1901) and applications that use
SIP/SIP+.
Section 5.6.6 depicts the use of the eecid in call establishment
procedures. In these procedures, the eecid is used to correlate
service-level calls with SVC set-up requests.
In the forward SVC establishment model, the call-terminating gateway
selects an eecid and transmits it via SDP to the call-originating
gateway. The call originating gateway transmits this eecid to the
call terminating gateway via the bearer set-up message (SVC set-up or
Q.2630.1 establish request).
In the backward SVC establishment model, the call-originating gateway
selects an eecid and transmits it via SDP to the call-terminating
gateway. The call terminating gateway transmits this eecid to the
call originating gateway via the bearer set-up message (SVC set-up or
Q.2630.1 establish request).
The value of the eecid attribute values needs to be unique within the
node terminating the SVC set-up but not across multiple nodes.
Hence, the SVC-terminating gateway has complete control over using
and releasing values of this parameter. The eecid attribute is used
to correlate, one-to-one, received bearer set-up requests with
service-level call control signaling.
Within an SDP session description, the eecid attribute is used as
follows:
a=eecid:<eecid>
where <eecid> consists of up to 8 hex digits (equivalent to 4
octets). Since this is always represented in hex, the "0x" prefix
shall not be used.
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Within the text representation of the <eecid> parameter, hex digits
to the left are more significant than hex digits to the right
(Section 2.2).
This SDP document does not specify how the eecid (synonymous with
bnc-id) is to be communicated through bearer signaling (Q.931, UNI,
PNNI, AINI, IISP, proprietary signaling equivalent, Q.2630.1). This
is a task of these bearer signaling protocols. However, the
following informative statements are made to convey a sense of the
interoperability that is a goal of current standardization efforts:
- ITU Q.2941.3 and the ATMF each recommend the use of the GIT IE for
carrying the eecid (synonymous with bnc-id) in the set-up message
of ATM signaling protocols (Q.2931, UNI 4.0, PNNI, AINI, IISP).
The coding for carrying the eecid (bnc-id) in the GIT IE is
defined in ITU Q.2941.3 and accepted by the ATM forum.
- Another alternate method is to use the called party subaddress IE.
In some networks, this might be considered a protocol violation
and is not the recommended means of carrying the eecid (bnc-id).
The GIT IE is the preferred method of transporting the eecid
(bnc-id) in ATM signaling messages.
- The establish request (ERQ) message of the Q.2630.1 [37] signaling
protocol can use the SUGR (Served User Generated Reference) IE to
transport the eecid (bnc-id).
The node assigning the eecid can release and re-use it when it
receives a Q.2931 [15] set-up message or a Q.2630.1 [37] establish
request message containing the eecid.
However, in both cases (backward and forward models), it is
recommended that this eecid be retained until the connection
terminates. Since the eecid space is large enough, it is not
necessary to release it as soon as possible.
5.6.1.2 The 'aalType' attribute
When present, the 'aalType' attribute is used to indicate the ATM
adaptation layer. If this information is redundant with the 'm'
line, it can be omitted. The format of the 'aalType' media attribute
line is as follows:
a=aalType: <aalType>
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Here, <aalType> can take on the following string values: "AAL1",
"AAL1_SDT", "AAL1_UDT", "AAL2", "AAL3/4", "AAL5" and
"USER_DEFINED_AAL". Note that "AAL3/4" and "USER DEFINED AAL" are
not addressed in this document.
5.6.1.3 The 'capability' attribute
When present, the 'capability' attribute indicates the ATM Transfer
Capability described in ITU I.371 [28], equivalent to the ATM Service
Category described in the UNI 4.1 Traffic Management specification
[6].
The 'capability' media attribute line is structured in one of the
following ways:
a=capability:<asc> <subtype>
a=capability:<atc> <subtype>
Possible values of the <asc> are enumerated below:
"CBR", "nrt-VBR", "rt-VBR", "UBR", "ABR", "GFR"
Possible values of the <atc> are enumerated below:
"DBR","SBR","ABT/IT","ABT/DT","ABR"
Some applications might use non-standard <atc> and <asc> values not
listed above. Equipment designers will need to agree on the meaning
and implications of non-standard transfer capabilities / service
capabilities.
The <subtype> field essentially serves as a subscript to the <asc>
and <atc> fields. In general, it can take on any integer value, or
the "-" value indicating that it does not apply or that the
underlying data is to be known by other means, such as provisioning.
For an <asc> value of CBR and an <atc> value of DBR, the <subtype>
field can be assigned values from Table 4-6 of ITU Q.2931 [15].
These are:
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<asc>/<atc> <subtype> Meaning
"CBR"/"DBR" 1 Voiceband signal transport
(ITU G.711, G.722, I.363)
"CBR"/"DBR" 2 Circuit transport (ITU I.363)
"CBR"/"DBR" 4 High-quality audio signal transport
(ITU I.363)
"CBR"/"DBR" 5 Video signal transport (ITU I.363)
Note that [15] does not define a <subtype> value of 3.
For other values of the <asc> and <atc> parameters, the following
values can be assigned to the <subtype> field, based on [6] and [28].
<asc>/<atc> <subtype> Meaning
nrt-VBR 1 nrt-VBR.1
nrt-VBR 2 nrt-VBR.2
nrt-VBR 3 nrt-VBR.3
rt-VBR 1 rt-VBR.1
rt-VBR 2 rt-VBR.2
rt-VBR 3 rt-VBR.3
UBR 1 UBR.1
UBR 2 UBR.2
GFR 1 GFR.1
GFR 2 GRR.2
SBR 1 SBR1
SBR 2 SBR2
SBR 3 SBR3
It is beyond the scope of this specification to examine the
equivalence of some of the ATMF and ITU definitions. These need to
be recognized from the ATMF and ITU source specifications and
exploited, as much as possible, to simplify ATM node design.
When the bearer connection is a single AAL2 CID connection within a
multiplexed AAL2 VC, the 'capability' attribute does not apply.
5.6.1.4 The 'qosClass' attribute
When present, the 'qosClass' attribute indicates the QoS class
specified in ITU I.2965.1 [34].
The 'qosClass' media attribute line is structured as follows:
a=qosClass:<qosClass>
Here, <qosClass> is an integer in the range 0 - 5.
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<qosClass> Meaning
0 Default QoS
1 Stringent
2 Tolerant
3 Bi-level
4 Unbounded
5 Stringent bi-level
5.6.1.5 The 'bcob' attribute
When present, the 'bcob' attribute represents the broadband
connection oriented bearer class defined in [5], [15] and [33]. It
can also be used to indicate whether end-to-end timing is required.
The 'bcob' media attribute line is structured as follows:
a=bcob:<bcob> <eetim>
Here, <bcob> is the decimal or hex representation of a 5-bit field.
The following values are currently defined:
<bcob> Meaning
0x01 BCOB-A
0x03 BCOB-C
0x05 Frame relaying bearer service
0x10 BCOB-X
0x18 BCOB-VP (transparent VP service)
The <eetim> parameter can be assigned a value of "on" or "off"
depending on whether end-to-end timing is required or not (Table 4-8
of [15]).
Either of these parameters can be left unspecified by setting it to a
"-". A 'bcob' media attribute line with all parameters set to "-"
carries no information and should be omitted.
5.6.1.6 The 'stc' attribute
When present, the 'stc' attribute represents susceptibility to
clipping. The 'stc' media attribute line is structured as follows:
a=stc:<stc>
Here, <stc> is the decimal equivalent of a 2-bit field. Currently,
all values are unused and reserved with the following exceptions:
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<stc> value Binary Equivalent Meaning
0 00 Not susceptible to clipping
1 01 Susceptible to clipping
5.6.1.7 The 'upcc' attribute
When present, the 'upcc' attribute represents the user plane
connection configuration. The 'upcc' media attribute line is
structured as follows:
a=upcc:<upcc>
Here, <upcc> is the decimal equivalent of a 2-bit field. Currently,
all values are unused and reserved with the following exceptions:
<upcc> value Binary Equivalent Meaning
0 00 Point to point
1 01 Point to multipoint
5.6.1.8 The 'atmQOSparms' attribute
When present, the 'atmQOSparms' attribute is used to describe certain
key ATM QoS parameters.
The 'atmQOSparms' media attribute line is structured as follows:
a=atmQOSparms:<directionFlag><cdvType><acdv><ccdv><eetd><cmtd><aclr>
The <directionFlag> can be assigned the following string values: "f",
"b" and "fb". "f" and "b" indicate the forward and backward
directions respectively. "fb" refers to both directions (forward and
backward). Conventions for the forward and backward directions are
per section 2.3.
The <cdvType> parameter can take on the string values of "PP" and
"2P". These refer to the peak-to-peak and two-point CDV as defined
in UNI 4.0 [5] and ITU Q.2965.2 [35] respectively.
The CDV parameters, <acdv> and <ccdv>, refer to the acceptable and
cumulative CDVs respectively. These are expressed in units of
microseconds and represented as the decimal equivalent of a 24-bit
field. These use the cell loss ratio, <aclr>, as the "alpha"
quantiles defined in the ATMF TM 4.1 specification [6] and in ITU
I.356 [47].
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The transit delay parameters, <eetd> and <cmtd>, refer to the end-
to-end and cumulative transit delays respectively in milliseconds.
These are represented as the decimal equivalents of 16-bit fields.
These parameters are defined in Q.2965.2 [35], UNI 4.0 [5] and Q.2931
[15].
The <aclr> parameter refers to forward and backward acceptable cell
loss ratios. This is the ratio between the number of cells lost and
the number of cells transmitted. It is expressed as the decimal
equivalent of an 8-bit field. This field expresses an order of
magnitude n, where n is an integer in the range 1-15. The Cell Loss
Ratio takes on the value 10 raised to the power of minus n.
The <directionFlag> is always specified. Except for the
<directionFlag>, the remaining parameters can be set to "-" to
indicate that they are not specified, inapplicable or implied.
However, there must be some specified parameters for the line to be
useful in an SDP description.
There can be several 'atmQOSparms' lines in an SDP description.
An example use of these attributes for an rt-VBR, single-CID AAL2
voice VC is:
a=atmQOSparms:f PP 8125 3455 32000 - 11
a=atmQOSparms:b PP 4675 2155 18000 - 12
This implies a forward acceptable peak-to-peak CDV of 8.125 ms, a
backward acceptable peak-to-peak CDV of 4.675 ms, forward cumulative
peak-to-peak CDV of 3.455 ms, a backward cumulative peak-to-peak CDV
of 2.155 ms, a forward end-to-end transit delay of 32 ms, a backward
end-to-end transit delay of 18 ms, an unspecified forward cumulative
transit delay, an unspecified backward cumulative transit delay, a
forward cell loss ratio of 10 raised to minus 11 and a backward cell
loss ratio of 10 to the minus 12.
An example of specifying the same parameters for the forward and
backward directions is:
a=atmQOSparms:fb PP 8125 3455 32000 - 11
This implies a forward and backward acceptable peak-to-peak CDV of
8.125 ms, a forward and backward cumulative peak-to-peak CDV of 3.455
ms, a forward and backward end-to-end transit delay of 32 ms, an
unspecified cumulative transit delay in the forward and backward
directions, and a cell loss ratio of 10 raised to minus 11 in the
forward and backward directions.
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5.6.1.9 The 'atmTrfcDesc' attribute
When present, the 'atmTrfcDesc' attribute is used to indicate ATM
traffic descriptor parameters. There can be several 'atmTrfcDesc'
lines in an SDP description.
The 'atmTrfcDesc' media attribute line is structured as follows:
a=atmTrfcDesc:<directionFlag><clpLvl>
<pcr><scr><mbs><cdvt><mcr><mfs><fd><te>
The <directionFlag> can be assigned the following string values: "f",
"b" and "fb". "f" and "b" indicate the forward and backward
directions respectively. "fb" refers to both directions (forward and
backward). Conventions for the forward and backward directions are
per section 2.3.
The <directionFlag> is always specified. Except for the
<directionFlag>, the remaining parameters can be set to "-" to
indicate that they are not specified, inapplicable or implied.
However, there must be some specified parameters for the line to be
useful in an SDP description.
The <clpLvl> (CLP level) parameter indicates whether the rates and
bursts described in these media attribute lines apply to CLP values
of 0 or (0+1). It can take on the following string values: "0",
"0+1" and "-". If rates and bursts for both <clpLvl> values are to
be described, then it is necessary to use two separate media
attribute lines for each direction in the same session descriptor.
If the <clpLvl> parameter is set to "-", then it implies that the CLP
parameter is known by other means such as default, MIB provisioning
etc.
The meaning, units and applicability of the remaining parameters are
per [6] and [28]:
PARAMETER MEANING UNITS APPLICABILITY
<pcr> PCR Cells/ CBR, rt-VBR, nrt-VBR,
second ABR, UBR, GFR;
CLP=0,0+1
<scr> SCR Cells/ rt-VBR, nrt-VBR;
second CLP=0,0+1
<mbs> MBS Cells rt-VBR, nrt-VBR,
GFR;
CLP=0,0+1
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<cdvt> CDVT Microsec. CBR, rt-VBR, nrt-VBR,
ABR, UBR, GFR;
CLP=0,0+1
<mcr> MCR Cells/ ABR,GFR;
second CLP=0+1
<mfs> MFS Cells GFR;
CLP=0,0+1
<fd> Frame "on"/"off" CBR, rt-VBR, nrt-VBR,
Discard ABR, UBR, GFR;
Allowed CLP=0+1
<te> CLP "on"/"off" CBR, rt-VBR, nrt-VBR,
tagging ABR, UBR, GFR;
Enabled CLP=0
<fd> indicates that frame discard is permitted. It can take on the
string values of "on" or "off". Note that, in the GFR case, frame
discard is always enabled. Hence, this subparameter can be set to
"-" in the case of GFR. Since the <fd> parameter is independent of
CLP, it is meaningful in the case when <clpLvl> = "0+1". It should
be set to "-" for the case when <clpLvl> = "0".
<te> (tag enable) indicates that CLP tagging is allowed. These can
take on the string values of "on" or "off". Since the <te> parameter
applies only to cells with a CLP of 0, it is meaningful in the case
when <clpLvl> = "0". It should be set to "-" for the case when
<clpLvl> = "0+1".
An example use of these media attribute lines for an rt-VBR, single-
CID AAL2 voice VC is:
a=atmTrfcDesc:f 0+1 200 100 20 - - - on -
a=atmTrfcDesc:f 0 200 80 15 - - - - off
a=atmTrfcDesc:b 0+1 200 100 20 - - - on -
a=atmTrfcDesc:b 0 200 80 15 - - - - off
This implies a forward and backward PCR of 200 cells per second all
cells regardless of CLP, forward and backward PCR of 200 cells per
second for cells with CLP=0, a forward and backward SCR of 100 cells
per second for all cells regardless of CLP, a forward and backward
SCR of 80 cells per second for cells with CLP=0, a forward and
backward MBS of 20 cells for all cells regardless of CLP, a forward
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RFC 3108 ATM SDP May 2001
and backward MBS of 15 cells for cells with CLP=0, an unspecified
CDVT which can be known by other means, and an MCR and MFS which are
unspecified because they are inapplicable. Frame discard is enabled
in both the forward and backward directions. Tagging is not enabled
in either direction.
The <pcr>, <scr>, <mbs>, <cdvt>, <mcr> and <mfs> are represented as
decimal integers, with range as defined in Section 6. See section
2.2 regarding the omission of leading zeros in decimal
representations.
5.6.1.10 The 'abrParms' attribute
When present, the 'abrParms' attribute is used to indicate the '
additional' ABR parameters specified in the UNI 4.0 signaling
specification [5]. There can be several 'abrParms' lines in an SDP
description.
The 'abrParms' media attribute line is structured as follows:
a=abrParms:<directionFlag><nrm><trm><cdf><adtf>
The <directionFlag> can be assigned the following string values: "f",
"b" and "fb". "f" and "b" indicate the forward and backward
directions respectively. "fb" refers to both directions (forward and
backward). Conventions for the forward and backward directions are
per section 2.3.
The <directionFlag> is always specified. Except for the
<directionFlag>, the remaining parameters can be set to "-" to
indicate that they are not specified, inapplicable or implied.
However, there must be some specified parameters for the line to be
useful in an SDP description.
These parameters are mapped into the ABR service parameters in [6] in
the manner described below. These parameters can be represented in
SDP as decimal integers, with fractions permitted for some. Details
of the meaning, units and applicability of these parameters are in
[5] and [6].
In SDP, these parameters are represented as the decimal or hex
equivalent of the binary fields mentioned below.
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RFC 3108 ATM SDP May 2001
+-----------+----------------------------------+-----------------------+
| PARAMETER | MEANING | FIELD SIZE |
+-----------+----------------------------------+-----------------------+
| <nrm> | Maximum number of cells per | 3 bits |
| | forward Resource Management cell | |
+-----------+----------------------------------+-----------------------+
| <trm> | Maximum time between | 3 bits |
| | forward Resource Management cells| |
+-----------+----------------------------------+-----------------------+
| <cdf> | Cutoff Decrease Factor | 3 bits |
+-----------+----------------------------------+-----------------------+
| <adtf> | Allowed Cell Rate Decrease | 10 bits |
| | Time Factor | |
+-----------+----------------------------------+-----------------------+
5.6.1.11 The 'abrSetup' attribute
When present, the 'abrSetup' attribute is used to indicate the ABR
parameters needed during call/connection establishment (Section
10.1.2.2 of the UNI 4.0 signaling specification [5]). This line is
structured as follows:
a=abrSetup:<ficr><bicr><ftbe><btbe><crmrtt><frif><brif><frdf><brdf>
These parameters are defined as follows:
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RFC 3108 ATM SDP May 2001
+-----------+----------------------------------+-----------------------+
| PARAMETER | MEANING | REPRESENTATION |
+-----------+----------------------------------+-----------------------+
| <ficr> | Forward Initial Cell Rate | Decimal equivalent |
| | (Cells per second) | of 24-bit field |
+-----------+----------------------------------+-----------------------+
| <bicr> | Backward Initial Cell Rate | Decimal equivalent |
| | (Cells per second) | of 24-bit field |
+-----------+----------------------------------+-----------------------+
| <ftbe> | Forward transient buffer | Decimal equivalent |
| | exposure (Cells) | of 24-bit field |
+-----------+----------------------------------+-----------------------+
| <btbe> | Backward transient buffer | Decimal equivalent |
| | exposure (Cells) | of 24-bit field |
+-----------+----------------------------------+-----------------------+
| <crmrtt> | Cumulative RM round-trip time | Decimal equivalent |
| | (Microseconds) | of 24-bit field |
+-----------+----------------------------------+-----------------------+
| <frif> | Forward rate increase factor | Decimal integer |
| | (used to derive cell count) | 0 -15 |
+-----------+----------------------------------+-----------------------+
| <brif> | Backward rate increase factor | Decimal integer |
| | (used to derive cell count) | 0 -15 |
+-----------+----------------------------------+-----------------------+
| <frdf> | Forward rate decrease factor | Decimal integer |
| | (used to derive cell count) | 0 -15 |
+-----------+----------------------------------+-----------------------+
| <brdf> | Backward rate decrease factor | Decimal integer |
| | (used to derive cell count) | 0 -15 |
+-----------+----------------------------------+-----------------------+
See Section 2.3 for a definition of the terms 'forward' and
'backward'.
If any of these parameters in the 'abrSetup' media attribute line is
not specified, is inapplicable or is implied, then it is set to h "-
".
5.6.1.12 The 'bearerType' attribute
When present, the 'bearerType' attribute is used to indicate whether
the underlying bearer is an ATM PVC/SPVC, an ATM SVC, or a subchannel
within an existing ATM SVC/PVC/SPVC. Additionally, for ATM SVCs and
AAL2 CID connections, the 'bearerType' attribute can be used to
indicate whether the media gateway initiates connection set-up via
bearer signaling (Q.2931-based or Q.2630.1 based). The format of the
'bearerType' media attribute line is as follows:
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RFC 3108 ATM SDP May 2001
a=bearerType: <bearerType> <localInitiation>
The <bearerType> field can take on the following string values:
"PVC", "SVC", "CID", with semantics as defined above. Here, "PVC"
includes both the PVC and SPVC cases.
In the case when the underlying bearer is a PVC/SPVC, or a CID
assigned by the MGC rather than through bearer signaling, the
<localInitiation> flag can be omitted or set to "-". In the case
when bearer signaling is used, this flag can be omitted when it is
known by default or by other means whether the media gateway
initiates the connection set-up via bearer signaling. Only when this
is to be indicated explicitly that the <localInitiation> flag takes
on the values of "on" or "off". An "on" value indicates that the
media gateway is responsible for initiating connection set-up via
bearer signaling (SVC signaling or Q.2630.1 signaling), an "off"
value indicates otherwise.
5.6.1.13 The 'lij' attribute
When present, the 'lij' attribute is used to indicate the presence of
a connection that uses the Leaf-initiated-join capability described
in UNI 4.0 [5], and to optionally describe parameters associated with
this capability. The format of the 'lij' media attribute line is as
follows:
a=lij: <sci><lsn>
The <sci> (screening indication) is a 4-bit field expressed as a
decimal or hex integer. It is defined in the UNI 4.0 signaling
specification [5]. It is possible that the values of this field will
be defined later by the ATMF and/or ITU. Currently, all values are
reserved with the exception of 0, which indicates a 'Network Join
without Root Notification'.
The <lsn> (leaf sequence number) is a 32-bit field expressed as a
decimal or hex integer. Per the UNI 4.0 signaling specification [5],
it is used by a joining leaf to associate messages and responses
during LIJ (leaf initiated join) procedures.
Each of these fields can be set to a "-" when the intention is to not
specify them in an SDP descriptor.
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5.6.1.14 The 'anycast' attribute
When present, the 'anycast' attribute line is used to indicate the
applicability of the anycast function described in UNI 4.0 [5].
Optional parameters to qualify this function are provided. The format
of the 'anycast' attribute is:
a=anycast: <atmGroupAddress> <cdStd> <conScpTyp> <conScpSel>
The <atmGroupAddress> is per Annex 5 of UNI 4.0 [5]. Within an SDP
descriptor, it can be represented in one of the formats (NSAP, E.164,
GWID/ALIAS) described elsewhere in this document.
The remaining subparameters mirror the connection scope selection
information element in UNI 4.0 [5]. Their meaning and representation
is as shown below:
PARAMETER MEANING REPRESENTATION
<cdStd> Coding standard for the Decimal or hex
connection scope selection IE equivalent of
Definition: UNI 4.0 [5] 2 bits
<conScpTyp> Type of connection scope Decimal or hex
Definition: UNI 4.0 [5] equivalent of
4 bits
<conScpSel> Connection scope selection Decimal or hex
Definition: UNI 4.0 [5] equivalent of
8 bits
Currently, all values of <cdStd> and <conScpTyp> are reserved with
the exception of <cdStd> = 3 (ATMF coding standard) and <conScpTyp> =
1 (connection scope type of 'organizational').
Each of these fields can be set to a "-" when the intention is to not
specify them in an SDP descriptor.
5.6.1.15 The 'cache' attribute
This attribute is used to enable SVC caching. This attribute has the
following format:
a=cache:<cacheEnable><cacheTimer>
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The <cacheEnable> flag indicates whether caching is enabled or not,
corresponding to the string values of "on" and "off" respectively.
The <cacheTimer> indicates the period of inactivity following which
the SVC is to be released by sending an SVC release message into the
network. This is specified as the decimal or hex equivalent of a
32-bit field, indicating the timeout in seconds. As usual, leading
zeros can be omitted. For instance,
a=cache:on 7200
implies that the cached SVC is to be deleted if it is idle for 2
hours.
The <cacheTimer> can be set to "-" if it is inapplicable or implied.
5.6.1.16 The 'bearerSigIE' attribute
ATM signaling standards provide 'escape mechanisms' to represent,
signal and negotiate higher-layer parameters. Examples are the B-HLI
and B-LLI IEs specified in ITU Q.2931 [15], and the user-to-user
information element described in ITU Q.2957 [48].
The 'bearerSigIE'(bearer signaling information element) attribute is
defined to allow a similar escape mechanism that can be used with
these ATM SDP conventions. The format of this media attribute line
is as follows:
a=bearerSigIE: <bearerSigIEType> <bearerSigIELng> <bearerSigIEVal>
When an 'bearerSigIE' media attribute line is present, all its
subparameters are mandatory. The "0x" prefix is not used since these
are always represented in hex.
The <bearerSigIEType> is represented as exactly 2 hex digits. It is
the unique IE identifier as defined in the ITU Q-series standards.
Leading zeros are not omitted. Some pertinent values are 7E (User-
user IE per ITU Q.2957 [48]), 5F (B-LLI IE) and 5D (B-HLI IE). B-LLI
and B-HLI, which stand for Broadband Low-layer Information and
Broadband High-layer Information respectively, are defined in ITU
Q.2931 [15]. Both of these refer to layers above the ATM adaptation
layer.
The <bearerSigIELng> consists of 1-4 hex digits. It is the length of
the information element in octets. Leading zeros may be omitted.
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The <bearerSigIEVal> is the value of the information element,
represented as a hexadecimal bit map. Although the size of this bit
map is network/ service dependent, setting an upper bound of 256
octets (512 hex digits) is adequate. Since this a bit map, leading
zeros should not be omitted. The number of hex digits in this bit map
is even.
5.6.2 ATM Adaptation Layer (AAL) attributes
The following is a summary list of the SDP media attributes that can
be used to describe the ATM Adaptation Layer (AAL). These are
detailed in subsequent subsections.
* The 'aalApp' attribute, which is used to point to the
controlling standard for an application layer above the ATM
adaptation layer.
* The 'cbrRate' attribute, which represents the CBR rate octet
defined in Table 4-6 of ITU Q.2931 [15].
* The 'sbc' attribute, which denotes the subchannel count in the
case of n x 64 clear channel communication.
* The 'clkrec' attribute, which indicates the clock recovery
method for AAL1 unstructured data transfer (UDT).
* The 'fec' attribute, which indicates the use of forward error
correction.
* The 'prtfl' attribute, which indicates indicate the fill level
of partially filled cells.
* The 'structure' attribute, which is used to indicate the
presence or absence of AAL1 structured data transfer (SDT), and
the size of the SDT blocks.
* The 'cpsSDUsize' attribute, which is used to indicate the
maximum size of the CPCS SDU payload.
* The 'aal2CPS' attribute, which is used to indicate that an AAL2
CPS sublayer as defined in ITU I.363.2 [13] is associated with
the VCC referred to in the 'm' line. Optionally, it can be
used to indicate selected CPS options and parameter values for
this VCC.
* The 'aal2CPSSDUrate' attribute, which is used to place an upper
bound on the SDU bit rate for an AAL2 CID.
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* The 'aal2sscs3661unassured' attribute, which is used to
indicate the presence of an AAL2 SSCS sublayer with unassured
transmission as defined in ITU I.366.1 [12]. Optionally, it
can be used to indicate selected options and parameter values
for this SSCS.
* The 'aal2sscs3661assured' attribute, which is used to indicate
the presence of an AAL2 SSCS sublayer with assured transmission
as defined in ITU I.366.1 [12]. Optionally, it can be used to
indicate selected options and parameter values for this SSCS.
* The 'aal2sscs3662' attribute, which is used to indicate the
presence of an AAL2 SSCS sublayer as defined in ITU I.366.2.
Optionally, it can be used to indicate selected options and
parameter values for this SSCS.
* The 'aal5sscop' attribute, which is used to indicate the
existence of an SSCOP protocol layer over an AAL5 CPS layer,
and the parameters which pertain to this SSCOP layer.
5.6.2.1 The 'aalApp' attribute
When present, the 'aalApp' attribute is used to point to the
controlling standard for an application layer above the ATM
adaptation layer. The format of the 'aalApp' media attribute line is
as follows:
a=aalApp: <appClass> <oui> <appId>
If any of the subparameters, <appClass>, <oui> or <appId>, is meant
to be left, unspecified, it is set to "-". However, an 'aalApp'
attribute line with all subparameters set to "-" carries no
information and should be omitted.
The <appClass>, or application class, field can take on the string
values listed below.
This list is not exhaustive. An "X-" prefix should be used with
<appClass> values not listed here.
<appClass> Meaning
"itu_h323c" Annex C of H.323 which specifies direct
RTP on AAL5 [45].
"af83" af-vtoa-0083.001, which specifies
variable size AAL5 PDUs with PCM voice
and a null SSCS [46].
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"AAL5_SSCOP" SSCOP as defined in ITU Q.2110 [43]
running over an AAL5 CPS [21].
No information is provided regarding
any layers above SSCOP such as Service
Specific Coordination Function (SSCF)
layers.
"itu_i3661_unassured" SSCS with unassured transmission,
per ITU I.366.1 [12].
"itu_i3661_assured" SSCS with assured transmission,
per ITU I.366.1 [12]. This uses SSCOP [43].
"itu_i3662" SSCS per ITU I.366.2 [13].
"itu_i3651" Frame relay SSCS per ITU I.365.1 [39].
"itu_i3652" Service-specific coordination function,
as defined in ITU I.365.2, for Connection
Oriented Network Service (SSCF-CONS) [40].
This uses SSCOP [43].
"itu_i3653" Service-specific coordination function,
as defined in ITU I.365.3, for Connection
Oriented Transport Service (SSCF-COTS) [41].
This uses SSCOP [43].
"itu_i3654" HDLC Service-specific coordination function,
as defined in ITU I.365.4 [42].
"FRF5" Use of the FRF.5 frame relay standard [53],
which references ITU I.365.1 [39].
"FRF8" Use of the FRF.8.1 frame relay standard [54].
This implies a null SSCS and the mapping of
the frame relay header into the ATM header.
"FRF11" Use of the FRF.11 frame relay standard [55].
"itu_h2221" Use of the ITU standard H.222.1 for
audiovisual communication over AAL5 [51].
The <oui>, or Organizationally Unique Identifier, refers to the
organization responsible for defining the <appId>, or Application
Identifier. The <oui> is maintained by the IEEE. One of its uses is
in 802 MAC addresses. It is a three-octet field represented as one
to six hex digits. Since this is always represented in hex, the "0x"
prefix is not used. Leading zeros may be omitted.
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The <appId> subparameter refers to the application ID, a hex number
consisting of up to 8 digits. Leading zeros may be omitted. The
"0x" prefix is not used, since the representation is always
hexadecimal. Currently, the only organization that has defined
application identifiers is the ATM forum. These have been defined in
the context of AAL2 ([44], [52], Section 5 of [61]). Within SDP,
these can be used with <appClass> = itu_i3662. The <oui> value for
the ATM forum is 0x00A03E.
In the following example, the aalApp media attribute line is used to
indicate 'Loop Emulation Service using CAS (POTS only) without the
Emulated Loop Control Protocol (ELCP) [52]. The Application ID is
defined by the ATM forum [61]. The SSCS used is per ITU I.366.2
[13].
a=aalApp:itu_i3662 A03E A
If leading zeros are not dropped, this can be represented as:
a=aalApp:itu_i3662 00A03E 0000000A
Since application identifiers have been specified only in the context
of the AAL2 SSCS defined in ITU I.366.2 [13],the <appClass> can be
set to '-' without ambiguity. The aalApp media attribute line can be
reduced to:
a=aalApp:- A03E A
or
a=aalApp:- 00A03E 0000000A
5.6.2.2 The 'cbrRate' attribute
When present, the 'cbrRate' attribute is used to represent the CBR
rate octet defined in Table 4-6 of ITU Q.2931 [15]. The format of
this media attribute line is:
a=cbrRate: <cbrRate>
Here, <cbrRate> is represented as exactly two hex digits. The "0x"
prefix is omitted since this parameter is always represented in hex.
Values currently defined by the ITU are:
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+------------+-----------------------------------------------+
| VALUE | MEANING |
| (hex) | |
+------------+-----------------------------------------------+
| 01 | 64 kbps |
+------------+-----------------------------------------------+
| 04 | 1544 kbps |
+------------+-----------------------------------------------+
| 05 | 6312 kbps |
+------------+-----------------------------------------------+
| 06 | 32064 kbps |
+------------+-----------------------------------------------+
| 07 | 44736 kbps |
+------------+-----------------------------------------------+
| 08 | 97728 kbps |
+------------+-----------------------------------------------+
| 10 | 2048 kbps |
+------------+-----------------------------------------------+
| 11 | 8448 kbps |
+------------+-----------------------------------------------+
| 12 | 34368 kbps |
+------------+-----------------------------------------------+
| 13 | 139264 kbps |
+------------+-----------------------------------------------+
| 40 | n x 64 kbps |
+------------+-----------------------------------------------+
| 41 | n x 8 kbps |
+------------+-----------------------------------------------+
It is preferable that the cbrRate attribute be omitted rather than
set to an unspecified value of "-", since it conveys no information
in the latter case.
5.6.2.3 The 'sbc' attribute
The 'sbc' media attribute line denotes the subchannel count and is
meaningful only in the case of n x 64 clear channel communication. A
clear n x 64 channel can use AAL1 (ATM forum af-vtoa-78) or AAL2
adaptation (ITU I.366.2). Although no such standard definition
exists, it is also possible to use AAL5 for this purpose. An n x 64
clear channel is represented by the encoding names of "X-CCD" and
"X-CCD-CAS" in Table 2.
The format of the 'sbc' media attribute line is as follows:
a=sbc:<sbc>
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Here, <sbc> can be expressed as a decimal or hex integer. This
attribute indicates the number of DS0s in a T1 or E1 frame that are
aggregated for transmitting clear channel data. For T1-based
applications, it can take on integral values in the inclusive range
[1...24]. For E1-based applications, it can take on integral values
in the inclusive range [1...31]. When omitted, other means are to be
used to determine the subchannel count.
Use of the 'sbc' attribute provides a direct way to indicate the
number of 64 kbps subchannels bundled into an n x 64 clear channel.
An alternate mechanism to indicate this exists within the SDP
bandwidth information, or 'b', line [1]. In this case, instead of
specifying the number of subchannels, the aggregate bandwidth in kbps
is specified. The syntax of the 'b' line, copied verbatim from [1],
is as follows:
b=<modifier>:<bandwidth-value>
In the case of n x 64 clear channels, the <modifier> is assigned a
text string value of "AS", indicating that the 'b' line is
application-specific. The <bandwidth-value> parameter, which is a
decimal number indicating the bandwidth in kbps, is limited to one of
the following values in the n x 64 clear channel application context:
64, 128, 192, 256, 320, 384, 448, 512, 576, 640, 704, 768, 832,
896, 960, 1024, 1088, 1152, 1216, 1280, 1344, 1408, 1472, 1600,
1664, 1728, 1792, 1856, 1920, 1984
Thus, for n x 64 circuit mode data service,
a=sbc:6
is equivalent to
b=AS:384
The media attribute line
a=sbc:2
is equivalent to
b=AS:128
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5.6.2.4 The 'clkrec' attribute
When present, the 'clkrec' attribute is used to indicate the clock
recovery method. This attribute is meaningful in the case of AAL1
unstructured data transfer (UDT). The format of the 'clkrec' media
attribute line is as follows:
a=clkrec:<clkrec>
The <clkrec> field can take on the following string values: "NULL",
"SRTS" or "ADAPTIVE". SRTS and adaptive clock recovery are defined
in ITU I.363.1 [10]. "NULL" indicates that the stream (e.g., T1/E1)
encapsulated in ATM is synchronous to the ATM network or is retimed,
before AAL1 encapsulation, via slip buffers.
5.6.2.5 The 'fec' attribute
When present, the 'fec' attribute is used to indicate the use of
forward error correction. Currently, there exists a forward error
correction method defined for AAL1 in ITU I.363.1 [10]. The format
of the 'fec' media attribute line is as follows:
a=fec:<fecEnable>
The <fecEnable> flag indicates the presence of absence of Forward
Error Correction. It can take on the string values of "NULL",
"LOSS_SENSITIVE" and "DELAY_SENSITIVE". An "NULL" value implies
disabling this capability. FEC can be enabled differently for
delay-sensitive and loss-sensitive connections.
5.6.2.6 The 'prtfl' attribute
When present, the 'prtfl' attribute is used to indicate the fill
level of cells. When this attribute is absent, then other means
(such as provisionable defaults) are used to determine the presence
and level of partial fill.
This attribute indicates the number of non-pad payload octets, not
including any AAL SAR or convergence sublayer octets. For example,
in some AAL1 applications that use partially filled cells with
padding at the end, this attribute indicates the number of leading
payload octets not including any AAL overhead.
The format of the 'prtfl' media attribute line is as follows:
a=prtfl:<partialFill>
Here, <partialFill> can be expressed as a decimal or a hex integer.
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In general, permitted values are integers in the range 1 - 48
inclusive. However, this upper bound is different for different
adaptations since the AAL overhead, if any, is different. If the
specified partial fill is greater than or equal to the maximum fill,
then complete fill is used. Using a 'partial' fill of 48 always
disables partial fill.
In the AAL1 context, this media attribute line applies uniformly to
both P and non-P cells. In AAL1 applications that do not distinguish
between P and non-P cells, a value of 47 indicates complete fill
(i.e., the absence of partial fill). In AAL1 applications that
distinguish between P and non-P cells, a value of 46 indicates no
padding in P-cells and a padding of one in non-P cells.
If partial fill is enabled (i.e there is padding in at least some
cells), then AAL1 structures must not be split across cell
boundaries. These shall fit in any cell. Hence, their size shall be
less than or equal to the partial fill size. Further, the partial
fill size is preferably an integer multiple of the structure size.
If not, then the partial fill size stated in the SDP description
shall be truncated to an integer multiple (e.g., a partial fill size
of 40 is truncated to 36 to support six 6 x 64 channels).
5.6.2.7 The 'structure' attribute
This attribute applies to AAL1 connections only. When present, the '
structure' attribute is used to indicate the presence or absence of
structured data transfer (SDT), and the size in octets of the SDT
blocks. The format of the 'structure' media attribute line is as
follows:
a=structure: <structureEnable> <blksz>
where the <structureEnable> flag indicates the presence of absence of
SDT. It can take on the values of "on" or "off". An "on" value
implies AAL1 structured data transfer (SDT), while an "off" value
implies AAL1 unstructured data transfer (UDT).
The block size field, <blksz>, is an optional 16-bit field [15] that
can be represented in decimal or hex. It is set to a "-" when not
applicable, as in the case of unstructured data transfer (UDT). For
SDT, it can be set to a "-" when <blksz> is known by other means.
For instance, af-vtoa-78 [7] fixes the structure size for n x 64
service, with or without CAS. The theoretical maximum value of
<blksz> is 65,535, although most services use much less.
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5.6.2.8 The 'cpsSDUsize' attribute
When present, the 'cpsSDUsize' attribute is used to indicate the
maximum size of the CPCS SDU payload. There can be several '
cpsSDUsize' lines in an SDP description.
The format of this media attribute line is as follows:
a=cpsSDUsize:<directionFlag><cpcs>
The <directionFlag> can be assigned the following string values: "f",
"b" and "fb". "f" and "b" indicate the forward and backward
directions respectively. "fb" refers to both directions (forward and
backward). Conventions for the forward and backward directions are
per section 2.3.
The <cpcs> fields is a 16-bit integer that can be represented in
decimal or in hex. The meaning and values of these fields are as
follows:
Application Field Meaning Values
AAL5 <cpcs> Maximum CPCS-SDU size 1- 65,535
AAL2 <cpcs> Maximum CPCS-SDU size 45 or 64
5.6.2.9 The 'aal2CPS' attribute
When present, the 'aal2CPS' attribute is used to describe parameters
associated with the AAL2 CPS layer.
The format of the 'aal2CPS' media attribute line is as follows:
a=aal2CPS:<cidLowerLimit><cidUpperLimit><timerCU> <simplifiedCPS>
Each of these fields can be set to a "-" when the intention is to not
specify them in an SDP descriptor.
The <cidLowerLimit> and <cidUpperLimit> can be assigned integer
values between 8 and 255 [11], with the limitation that
<cidUpperLimit> be greater than or equal to <cidLowerLimit>. For
instance, for POTS applications based on [52], <cidLowerLimit> and
<cidUpperLimit> can have values of 16 and 223 respectively.
The <timerCU> integer represents the "combined use" timerCU defined
in ITU I.363.2. This timer is represented as an integer number of
microseconds. It is represented as the decimal integer equivalent of
32 bits.
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The <simplifiedCPS> parameter can be assigned the values "on" or
"off". When it is "on", the AAL2 CPS simplification described in
[52] is adopted. Under this simplification, each ATM cell contains
exactly on AAL2 packet. If necessary, octets at the end of the cell
are padded with zeros. Since the <timerCU> value in this context is
always 0, it can be set to "-".
5.6.2.10 The 'aal2CPSSDUrate' attribute
When present, the 'aal2CPSSDUrate' attribute is used to place an
upper bound on the SDU bit rate for an AAL2 CID. This is useful for
limiting the bandwidth used by a CID, specially if the CID is used
for frame mode data defined in [13], or with the SSSAR defined in
[12]. The format of this media attribute line is as follows:
a=aal2CPSSDUrate: <fSDUrate><bSDUrate>
The fSDUrate and bSDUrate are the maximum forward and backward SDU
rates in bits/second. These are represented as decimal integers,
with range as defined in Section 6. If any of these parameters in
these media attribute lines is not specified, is inapplicable or is
implied, then it is set to "-".
5.6.2.11 The 'aal2sscs3661unassured' attribute
When present, the 'aal2sscs3661unassured' attribute is used to
indicate the options that pertain to the unassured transmission SSCS
defined in ITU I.366.1 [12]. This SSCS can be selected via the
aalApp attribute defined below, or by virtue of the presence of the '
aal2sscs3661unassured' attribute. The format of this media attribute
line is as follows:
a=aal2sscs3661unassured: <ted> <rastimer> <fsssar> <bsssar>
Each of these fields can be set to a "-" when the intention is to not
specify them in an SDP descriptor.
The <ted> flag indicates the presence or absence of transmission
error detection as defined in I.366.1. It can be assigned the values
of "on" or "off". An "on" value indicates presence of the
capability.
The <rastimer> subparameter indicates the SSSAR reassembly timer in
microseconds. It is represented as the decimal equivalent of 32
bits.
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The <fsssar> and <bsssar> fields are 24-bit integers that can be
represented in decimal or in hex. The meaning and values of the
<fsssar> and <bsssar> fields are as follows:
Field Meaning Values
<fsssar> Maximum SSSAR-SDU size 1- 65,568
forward direction
<bsssar> Maximum SSSAR-SDU size 1- 65,568
backward direction
If present, the SSTED (Service-Specific Transmission Error Detection)
sublayer is above the SSSAR (Service-Specific Segmentation and
Reassembly) sublayer [12]. Since the maximum size of the SSTED-SDUs
can be derived from the maximum SSSAR-SDU size, it need not be
specified separately.
5.6.2.12 The 'aal2sscs3661assured' attribute
When present, the 'aal2sscs3661assured' attribute is used to indic | |