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rfc:rfc1755

Network Working Group M. Perez Request for Comments: 1755 ISI Category: Standards Track F. Liaw

                                                    FORE Systems, Inc.
                                                             A. Mankin
                                                            E. Hoffman
                                                                   ISI
                                                           D. Grossman
                                                        Motorola Codex
                                                              A. Malis
                                                  Ascom Timeplex, Inc.
                                                         February 1995
               ATM Signaling Support for IP over ATM

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.

Abstract

 This memo describes the ATM call control signaling exchanges needed
 to support Classical IP over ATM implementations as described in RFC
 1577 [LAUB94]. ATM endpoints will incorporate ATM signaling services
 as specified in the ATM Forum User-Network Interface (UNI)
 Specification Version 3.1 [ATMF94]. IP over ATM implementations
 utilize the services of local ATM signaling entities to establish and
 release ATM connections. This memo should be used to define the
 support required by IP over ATM implementations from their local ATM
 signaling entities.
 This document is an implementors guide intended to foster
 interoperability among RFC 1577, RFC 1483, and UNI ATM signaling.  It
 applies to IP hosts and routers which are also ATM endsystems and
 assumes ATM networks that completely implement the ATM Forum UNI
 Specification Version 3.1. Unless explicitly stated, no distinction
 is made between the Private and Public UNI.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 1] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 UNI 3.1 is considered an erratum to the UNI 3.0 specification. It has
 been produced by the ATM Forum, largely for reasons of alignment with
 Recommendation Q.2931. Although UNI 3.1 is based on UNI 3.0 there are
 several changes that make the two versions incompatible. A
 description of how to support IP over ATM using UNI 3.0 is found in
 Appendix B.

Table of Contents

   1.  Conventions ...............................................   3
   2.  Overview ..................................................   3
   3.  Use of Protocol Procedures ................................   4
       3.1  VC Establishment .....................................   4
       3.2  Multiprotocol Support on VCs  ........................   4
       3.3  Support for Multiple VCs .............................   5
       3.4  VC Teardown...........................................   6
   4.  Overview of UNI Call Setup Signaling ......................   6
   5.  Overview of Call Establishment Message Content ............   7
   6.  Information Elements with Endpoint Significance ...........   8
       6.1  ATM Adaptation Layer Parameters ......................   8
       6.2  Broadband Low Layer Information  .....................   8
            6.2.1  Framework for Protocol Layering ...............   9
   7.  Information Elements with Significance to the ATM Network .  11
       7.1  ATM Traffic Descriptor ...............................  11
       7.2  Broadband Bearer Capability ..........................  15
       7.3  QoS Parameter.........................................  16
       7.4  ATM Addressing Information ...........................  16
   8.  Dealing with Failure of Call Establishment.................  18
   9. Security Considerations ....................................  18
   10. Open Issues ...............................................  19
   11. Acknowledgements...........................................  19
   12. References ................................................  19
   13. Authors' Addresses ........................................  20
   Appendix A  Sample Signaling Messages .........................  22
   Appendix B  IP over ATM using UNI 3.0 Signaling ...............  25
   Appendix C  Combinations of Traffic Related Parameters ........  27
   Appendix D  Frame Relay Interworking ..........................  28

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 2] RFC 1755 ATM Signaling Support for IP over ATM February 1995

1. Conventions

The following language conventions are used in the items of specification in this document:

 o   MUST, SHALL, or MANDATORY -- the item is an absolute requirement
     of the specification.
 o   SHOULD or RECOMMEND -- this item SHOULD generally be followed for
     all but exceptional circumstances.
 o   MAY or OPTIONAL -- the item is truly optional and MAY be followed
     or ignored according to the needs of the implementor.

2. Overview

 In a Switched Virtual Connection (SVC) environment, ATM virtual
 channel connections (VCCs) are dynamically established and released
 as needed. This is accomplished using the ATM call/connection control
 signaling protocol, which operates between ATM endsystems and the ATM
 network.  The signaling entities use the signaling protocol to
 establish and release calls (association between ATM endpoints) and
 connections (VCCs).  Signaling procedures include the use of
 addressing to locate ATM endpoints and allocation of resource in the
 network for the connection.  It also provides indication and
 negotiation between ATM endpoints for selection of end-to-end
 protocols and their parameters.  This memo describes how the
 signaling protocol is used in support of IP over ATM, and, in
 particular, the information exchanged in the signaling protocol to
 effect this support.
 IP address to ATM address resolution and routing issues are not in
 the scope of this memo, and are treated as part of IP in figure 1.
            +--------------+     +------+     +----------+
            |              |     |      |<--->| IP / ARP |
            |              |<--->| This |     | RFC 1577 |
            |    ATM       |     | Memo |     +----------+
            |  signaling   |     |      |<--->| RFC 1483 |
            |              |     +------+     +----------+
            |              |   -------------> |  AAL 5   |
            |              |                  +----------+
            |              |   -------------> |   ATM    |
            +--------------+                  +----------+
                                Figure 1.
               Relationship of this memo to IP, RFC 1483,
                       ATM signaling, ATM and AAL5

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 3] RFC 1755 ATM Signaling Support for IP over ATM February 1995

3. Use of Protocol Procedures

 The following requirements are motivated to provide implementation
 guidelines on how multiple ATM connections between peer systems
 SHOULD be managed, to prevent connection thrashing and related
 problems.

3.1. VC Establishment

 The owner of an existing VCC is defined to be the entity within the
 ATM endsystem that establishes the connection.  An ATM endsystem MAY
 establish an ATM call when it has a datagram to send and either there
 is no existing VCC that it can use for this purpose, it chooses not
 to use an existing VCC, (e.g., for reasons of route optimization or
 quality of service), or the VCC owner does not allow sharing.
 To reduce the latency of the address resolution procedure at the
 called station, the following procedure MAY be used:
 If a VCC is established using the LLC/SNAP encapsulation, the calling
 endstation of the VCC MAY send an InARP_REQUEST to the called
 endstation after the connection is established (i.e. received a
 CONNECT message) and before the calling endstation sends the first
 data packet.  In addition, the calling endstation MAY send its data
 packets without waiting for the InARP_REPLY. An endstation MAY
 respond, generate, and manage its ATMARP table according to the
 procedures specified in RFC1293 [BRAD92], Section 7, "Protocol
 Operation", during the life time of the VCC.
 To avoid establishing multiple VCCs to the same endstation, a called
 endstation MAY associate the calling party number in the SETUP
 message with the established VCC. This VCC MAY be used to transmit
 data packets destined to a endstation whose ATMARP resolution results
 in an ATM address that is the same as the associated calling party
 number.  Sharing of VCCs is subject to the policies configured at the
 endstation as described in section 4.3 of this recommendation.

3.2. Multiprotocol Support on VCs

 When two ATM endsystems run multiple protocols, an ATM connection MAY
 be shared among two or more datagram protocol entities, as long as
 the VCC owner allows sharing and if the encapsulation allows proper
 multiplexing and demultiplexing (i.e. the LLC/SNAP encapsulation).
 This indication of sharing a VCC MAY be by configuration or via an
 API.  Similarly, the Internet layer supports multiplexing of multiple
 end-to-end transport sessions.  To properly detect idle connections
 while sharing a VCC among more than one higher layer protocol
 entities, the ATM endsystem MUST monitor the traffic at the lowest

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 4] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 multiplexing layer.

3.3. Support for Multiple VCs

 An ATMARP server or client MAY establish an ATM call when it has a
 datagram to send and either there is no existing VCC that it can use
 for this purpose, it chooses not to use an existing VCC, or the owner
 of the VCC does not allow sharing. Note that there might be VCCs to
 the destination which are used for IP, but an ARP server might prefer
 to use a separate VCC for ARP only. The ATMARP server or client MAY
 maintain or release the call as specified in RFC 1577. However, if
 the VCC is shared among several protocol entities, the ATMARP client
 or server SHALL NOT disconnect the call as suggested in RFC 1577.
 Systems MUST be able to support multiple connections between peer
 systems (without regard to which peer system initiated each
 connection).  They MAY be configured to only allow one such
 connection at a time.
 If a receiver accepts more than one call from a single source, that
 receiver MUST then accept incoming PDUs on the additional
 connection(s), and MAY transmit on the additional connections.
 Receivers SHOULD NOT accept the incoming call, only to close the
 connection or ignore PDUs from the connection.
 Because opening multiple connections is specifically allowed,
 algorithms to prevent connection call collision, such as the one
 found in section 8.4.3.5 of ISO/IEC 8473 [ISO8473], MUST NOT be
 implemented.
 While allowing multiple connections is specifically desired and
 allowed, implementations MAY choose (by configuration) to permit only
 a single connection to some destinations.  Only in such a case, if a
 colliding incoming call is received while a call request is pending,
 the incoming call MUST be rejected.  Note that this MAY result in a
 failure to establish a connection.  In such a case, each system MUST
 wait at least a configurable collision retry time in the range 1 to
 10 seconds before retrying.  Systems MUST add a random increment,
 with exponential backoff.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 5] RFC 1755 ATM Signaling Support for IP over ATM February 1995

3.4. VC Teardown

 Either endsystem MAY close a connection. If the connection is closed
 or reset while a datagram is being transmitted, the datagram is lost.
 Systems SHOULD be able to configure a minimum holding time for
 connections to remain open as long as the endpoints are up.  (Note
 that holding time, the time the connection has been open, differs
 from idle time.)  A suggested default value for the minimum holding
 time is 60 seconds.
 Because some public networks MAY charge for connection holding time,
 and connections MAY be a scarce resource in some networks or
 endsystems, each system implementing a Public ATM UNI interface MUST
 support the use of a configurable inactivity timer to clear
 connections that are idle for some period of time.  The timer's range
 SHOULD include a range from a small number of minutes to "infinite".
 A default value of 20 minutes is RECOMMENDED. Systems which only
 implement a Private ATM UNI interface SHOULD support the inactivity
 timer.  If implemented, the inactivity timer MUST monitor traffic in
 both directions of the connection.

4. Brief Overview of UNI Call Setup Signaling Procedures and Messages

 This section provides a summary of point-to-point signaling
 procedures. Readers are referred to [ATMF93].
 UNI signaling messages used for point-to-point call/connection
 control are the following:
             Call Setup                       Call Release
             ----------                       ------------
               SETUP                             RELEASE
               CALL PROCEEDING                   RELEASE COMPLETE
               CONNECT
               CONNECT ACKNOWLEDGE
 An ATM endpoint initiates a call request by sending a SETUP message
 to the network. The network processes the call request to determine
 if the call can be progressed. If so, the network indicates the value
 of the newly allocated VPCI/VCI in its first response to the the
 SETUP message, which is either a CALL PROCEEDING or CONNECT message.
 If a call cannot be accepted, by the network or destination ATM end-
 point, a RELEASE COMPLETE is sent.  At the destination ATM endpoint,
 the network offers the call using the SETUP message.  If the
 destination endpoint is able to accept the call, it responds with a
 CONNECT message (which MAY be preceded by a CALL PROCEEDING);
 otherwise, it sends a RELEASE COMPLETE message.  See Appendix A,
 Section 2 for guidance on the use of the CALL PROCEEDING message.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 6] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 Call release can be initiated by either endpoint or (rarely) by the
 network.  When an endpoint wishes to release a call, it sends a
 RELEASE message to the network. The network responds with a RELEASE
 COMPLETE message, frees up resources associated with the call, and
 initiates clearing toward the other endpoint. The network initiates
 clearing by sending a RELEASE message to the ATM endpoint, which
 reponds by sending a RELEASE COMPLETE message.  Upon receipt of the
 RELEASE COMPLETE message, the network frees any resources associated
 with the call.

5. Overview of Call Establishment Message Content

 Signaling messages are structured to contain mandatory and optional
 variable length information elements (IEs).  IEs are further
 subdivided into octet groups, which in turn are divided into fields.
 IEs contain information related to the call, which is relevant to the
 network, the peer endpoint or both.  Selection of optional IEs and
 the content of mandatory and optional IEs in a call establishment
 message determines the parties to and nature of the communication
 over the ATM connection. For example, the call establishment message
 for a call which will be used for constant bitrate video over AAL 1
 will have different contents than a call which will be used for IP
 over AAL 5.
 A SETUP message which establishes an ATM connection to be used for IP
 and multiprotocol interconnection calls MUST contain the following
 IEs:
      AAL Parameters
      ATM Traffic Descriptor
      Broadband Bearer Capability
      Broadband Low Layer Information
      QoS Parameter
      Called Party Number
      Calling Party Number
 and MAY, under certain circumstance contain the following IEs:
      Calling Party Subaddress
      Called Party Subaddress
      Transit Network Selection
 In UNI 3.1, the AAL Parameters and the Broadband Low Layer
 Information IEs are optional in a SETUP message.  However, in support
 of IP over ATM these two IEs MUST be included. Appendix A shows an
 example SETUP message coded in the manner indicated in this memo.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 7] RFC 1755 ATM Signaling Support for IP over ATM February 1995

6. Information Elements with Endpoint to Endpoint Significance

 This section describes the coding of, and procedures surrounding,
 information elements in a SETUP message with significance only to the
 endpoints of an ATM call supporting IP.

6.1. ATM Adaptation Layer Parameters

 The AAL Parameters IE (see section 5.4.5.5 and Annex F of [ATMF93])
 carries information about the ATM Adaptation Layer (AAL) to be used
 on the connection. RFC 1483 specifies encapsulation of IP over AAL 5.
 Thus, AAL 5 MUST be indicated in the "AAL type" field.
 Coding and procedure related to the 'Forward and Backward Maximum
 CPCS-SDU Size' fields are discussed in [ATKI94]. Values may range
 from zero to 65,535. Although the default IP over AAL 5/ATM is 9188
 bytes, endstations are encouraged to support MTU sizes up to and
 including 64k.
 Ordinarily, no Service Specific Convergence Sublayer (SSCS) will be
 used for multiprotocol interconnect over AAL5.  Therefore, the SSCS
 'type' field SHOULD be absent or, if present, coded to Null SSCS.
        Format and field values of AAL Parameters IE
  1. ———————————————————

| aal_parameters |

  1. ———————————————————

| aal_type 5 (AAL 5) |

        |  fwd_max_sdu_size_identifier 140                       |
        |  fwd_max_sdu_size            65,535   (desired IP MTU) |
        |  bkw_max_sdu_size_identifier 129                       |
        |  bkw_max_sdu_size            65,535   (desired IP MTU) |
        |  sscs_type identifier        132                       |
        |  sscs_type                   0        (null SSCS)      |
        ----------------------------------------------------------

6.2. Broadband Low Layer Information

 Selection of an encapsulation to support IP over an ATM VCC is done
 using the Broadband Low Layer Information (B-LLI) IE, along with the
 AAL Parameters IE, and the B-LLI negotiation procedure.
 RFC 1577 specifies LLC/SNAP as the default encapsulation.  This
 encapsulation MUST be implemented by all endstations.  LLC
 encapsulation MUST be signaled in the B-LLI as shown below.
 Signaling indication of other encapsulations is discussed in Appendix
 D, Section 4.  Note that only LLC is indicated in the B-LLI. It is up

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 8] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 to the LLC layer to look into the encapsulation header of the packets
 following call setup. A B-LLI specifying both LLC and a layer_3_id
 SNAP layer is not recommended.  If in those packets, the SNAP header
 indicates IP, it is the LLC layer's job to hand the packets up to IP.
        Format of B-LLI IE indicating LLC/SNAP encapsulation
  1. ———————————————————

| bb_low_layer_information |

  1. ———————————————————

| layer_2_id 2 |

        |  user_information_layer     12  (lan_llc - ISO 8802/2) |
        ----------------------------------------------------------

6.2.1. Framework for Protocol Layering

 The support of connectionless services from a connection oriented
 link layer exposes general problems of connection management,
 specifically the problems of connection acceptance, assignment of
 quality of service, and connection shutdown. For a connection to be
 associated with the correct protocol on the called host, it is
 necessary for information about one or more layers of protocol
 identification to be associated with a connection "management entity"
 or "endpoint".  This association is what we call a binding in this
 memo.  In this section we attempt to describe a framework for a
 usable binding or service architecture given the available IEs in the
 ATM call control messages.
 It is important to distinguish between two basic uses of protocol
 identification elements present in the UNI setup message. The first
 is the description of the protocol encapsulation that will be used on
 the data packet over the virtual connection, the second is the entity
 that will be responsible for managing the call. All protocols present
 in various IEs MUST be used to encapsulate the call, but the most
 specific, or highest, layer specified SHOULD manage the call. This
 defines a hierarchy of services and provides a framework for
 applications, including LLC and IP, to terminate calls. This
 hierarchy provides a clear mechanism for support of higher level
 protocol and application bindings, when their use and specification
 is defined in the appropriate standards bodies.
 In general, it would be desirable to allow data packets to be stored
 directly into an application's address space after connection is
 established.  This is possible only if we have both encapsulation and
 managing entity indications in the signaling message.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 9] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 The B-LLI is the only information element currently available in UNI
 3.1 for designating protocol endpoints. It contains codepoints that
 describe layer 2 and layer 3 protocol entities associated with the
 call. There are other information elements under consideration in the
 ATM Forum and ITU, which could come to play a significant role in the
 description of application to connection binding, but their use is
 not yet defined, and they are not part of the framework described by
 RFC 1577. They include B-HLI, for containing information for a higher
 layer protocol, Network Layer Information (NLI) to contain
 information for the network layer, and UUI, which is meant to carry
 information for use by the top level application.
 The following figure shows a B-LLI that MAY be used for specifying in
 call setup that IP will manage the call and that this VC will be used
 only for IP traffic. Called parties MUST accept this B-LLI.  The
 caller using VC MUST use LLC-SNAP encapsulation on all IP datagrams,
 despite the fact that the caller views the VC as dedicated to IP.
 The reason for this requirement is that while we require receivers to
 accept this form of call setup, they may choose whether or not to
 multiplex the call through LLC, in other words to ignore the Layer 3
 information.  This choice is dependent on the receiver's
 implementation's  protocol architecture and is local to the receiver.
         Format of B-LLI IE indicating VC ownership by IP
           (NOTE: LLC/SNAP encapsulation is still used)
  1. ———————————————————

| bb_low_layer_information |

  1. ———————————————————

| layer_2_id 2 |

        |  user_information_layer     12  (lan_llc - ISO 8802/2) |
        |  layer_3_id                 3                          |
        |  ISO/IEC TR 9577 IPI        204 (0xCC)                 |
        ----------------------------------------------------------
 Null-encapsulated VCs are described in RFC 1483. Such a VC would
 result in the most direct form of binding a VC to IP.  However, the
 method of signaling for this type of VC has not yet been integrated
 into the IP over ATM context.  For completeness, we mention that the
 signaling would use a B-LLI containing the layer 3 identifier with
 the ISO/IEC TR-9577 protocol codepoint and omitting the layer 2
 identifier [ATMF93].  Since no layer 2 is specified, frames produced
 by AAL processing would be given directly to IP.  Processing of this
 B-LLI is not required at this time.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 10] RFC 1755 ATM Signaling Support for IP over ATM February 1995

7. Information Elements with Significance to the ATM Network

 This section describes the coding of, and procedures surrounding,
 information elements with significance to the ATM network, as well as
 the endpoints of an ATM call supporting multiprotocol operation.
 The standards, implementation agreements, research and experience
 surrounding such issues as traffic management, quality of service and
 bearer service description are still evolving.  Much of this material
 is cast to give the greatest possible latitude to ATM network
 implementation and service offerings.  ATM endsystems need to match
 the traffic contract and bearer service they request from the network
 to the capabilities offered by the network.  Therefore, this memo can
 only offer what, at the present time, are the most appropriate and
 efficient coding rules to follow for setting up IP and ATMARP VCCs.
 Future revisions of this memo may take advantage of ATM services and
 capabilities that are not yet available.

7.1. ATM Traffic Descriptor

 The ATM traffic descriptor characterizes the ATM virtual connection
 in terms of peak cell rate (PCR), sustainable cell rate (SCR), and
 maximum burst size.  This information is used to allocate resources
 (e.g., bandwidth, buffering) in the network.  In general, the ATM
 traffic descriptor for supporting multiprotocol interconnection over
 ATM will be driven by factors such as the capacity of the network,
 conformance definition supported by the network, performance of the
 ATM endsystem and (for public networks) cost of services.
 The most convenient model of IP behavior corresponds to the Best
 Effort Capability (see section 3.6.2.4 of [ATMF93]). If this
 capability is offered by the ATM network(s), it MAY be requested by
 including the Best Effort Indicator, the peak cell rate forward
 (CLP=0+1) and peak cell rate backward (CLP=0+1) fields in the ATM
 Traffic Descriptor IE. When the Best Effort Capability is used, no
 guarantees are provided by the network, and in fact, throughput may
 be zero at any time.  This type of behavior is also described by RFC
 1633 [BRAD94].

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 11] RFC 1755 ATM Signaling Support for IP over ATM February 1995

        Format and field values of ATM Traffic Descriptor IE
  1. ———————————————————

| traffic_descriptor |

  1. ———————————————————

| fwd_peak_cell_rate_0+1_identifier 132 |

        |  fwd_peak_cell_rate_0+1               (link rate)      |
        |  bkw_peak_cell_rate_0+1_identifier    133              |
        |  bkw_peak_cell_rate_0+1               (link rate)      |
        |  best_effort_indication               190              |
        ----------------------------------------------------------
 When the network does not support Best Effort Capability or more
 predictable ATM service is desired for IP, more specific traffic
 parameters MAY be specified and the Best Effort capability not used.
 Doing so includes use of two other traffic-related IEs and is
 discussed in the following paragraphs and sections.
 The Traffic Descriptor IE is accompanied by the Broadband Bearer
 Capability IE and the QoS Parameter IE.  Together these define the
 signaling view of ATM traffic management.  In this memo, we present
 an agreed-on, required subset of traffic management capabilities, as
 specified by using the three IEs. The figure immediately below shows
 the set of the allowable combinations of traffic parameters which all
 IP over ATM endsystems MUST support in their ATM signaling.  The
 subset includes Best Effort in the form of a non-guaranteed bitrate
 combination (the rightmost column of the table below); a type of
 traffic description that is intended for ATM "pipes", for example
 between two routers (the middle column); and a type of traffic
 description that will allow initial use of token-bucket style
 characterizations of the source, as presented in RFC 1363 [PART92]
 and RFC 1633, for example (the leftmost column).

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 12] RFC 1755 ATM Signaling Support for IP over ATM February 1995

               Combinations of Traffic Related Paramenters
               that MUST be supported in the SETUP message
                 |---------------------------------|
                 |Broadband Bearer                 |
                 |Capability                       |
                 |---------------------------------|
                 |Broadband Bearer     | C | X | X |
                 |---------------------|---|---|---|
                 |Traffic Type         |   |   |   |
                 |(CBR,VBR)            |   |CBR| & |
                 |---------------------|---|---|---|
                 |Timing Required      |   |YES| &&|
                 |---------------------------------|
                 |Traffic Descriptor               |
                 |Parameter                        |
                 |---------------------------------|
                 |PCR (CLP=0)          |   |   |   |
                 |---------------------|---|---|---|
                 |PCR (CLP=0+1)        | S | S | S |
                 |---------------------|---|---|---|
                 |SCR (CLP=0)          |   |   |   |
                 |---------------------|---|---|---|
                 |SCR (CLP=0+1)        | S |   |   |
                 |---------------------|---|---|---|
                 |MBS (CLP=0)          |   |   |   |
                 |---------------------|---|---|---|
                 |MBS (CLP=0+1)        | S |   |   |
                 |---------------------|---|---|---|
                 |Best Effort          |   |   | S |
                 |---------------------|---|---|---|
                 |Tagging              | NO| NO| NO|
                 |---------------------------------|
                 |---------------------------------|
                 |QOS Classes          | 0 | 0 | 0 |
                 -----------------------------------
 S = Specified
 & = Parameter is coded to either "no indication" or VBR or octet 5a
     (Traffic Type/Timing Required) is absent; these three codings are
     treated as equivalent
 && = Parameter is coded to either "no indication" or "No" or octet 5a
      is absent; these three codings are treated as equivalent
 Use of other allowable combinations of traffic parameters listed in
 the large table in Appendix C may work, since they are allowed by
 [ATMF94], but this will depend on the the calling endsystem, the
 network, and the called endsystem.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 13] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 If Best Effort service is not use, link rate SHOULD not be requested
 as the peak cell rate. Without any knowledge of the application, it
 is RECOMMENDED that a fraction, such as 1/10th, of the the link
 bandwidth be requested.
 [ATMF93] does not provide any capability for negotiation of the ATM
 traffic descriptor paramenters.  This means that:
   a) the calling endsystem SHOULD have some prior knowledge as to
      the traffic contract that will be acceptable to both the
      called endsystem and the network.
   b) if, in response to a SETUP message, a calling endsystem
      receive a RELEASE COMPLETE message, or a CALL PROCEEDING
      message followed by a RELEASE COMPLETE message, with cause
      #37, User Cell Rate Unavailable, it MAY examine the
      diagnostic field of the Cause IE and reattempt the call after
      selecting smaller values for the parameter(s) indicated.  If
      the RELEASE COMPLETE or RELEASE message is received with cause
      #73, Unsupported combination of traffic parameter, it MAY
      try other combinations from table 5-7 and 5-8 of [ATMF93].
   c) the called endsystem SHOULD examine the ATM traffic descriptor
      IE in the SETUP message.  If it is unable to process cells at
      the Forward PCR indicated, it SHOULD clear the call with cause
      #37, User Cell Rate Unavailable.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 14] RFC 1755 ATM Signaling Support for IP over ATM February 1995

7.2. Broadband Bearer Capability

 Broadband Bearer Connection Oriented Service Type X (BCOB-X) or Type
 C (BCOB-C) are both applicable for multiprotocol interconnection,
 depending on the service(s) provided by the ATM network and the
 capabilities (e.g., for traffic shaping) of the ATM endsystem. The
 table in the previous section showed the use of BCOB-X and BCOB-C
 with other parameters.  The figure below shows format and field
 values for a BCOB-X when the Traffic Descriptor IE indicates Best
 Effort.
        Format and field values of Broadband Bearer Capability IE
  1. ———————————————————

| bb_bearer_capability |

  1. ———————————————————

| spare 0 |

        |  bearer_class                16      (BCOC-X)          |
        |  spare                       0                         |
        |  traffic_type                0       (no indication)   |
        |  timing_reqs                 0       (no indication)   |
        |  susceptibility_to_clipping  0       (not suscept)     |
        |  spare                       0                         |
        |  user_plane_configuration    0       (point_to_point)  |
        ----------------------------------------------------------
 IP over ATM signaling MUST permit BCOB-C and BCOB-X, in the
 combinations shown in the previous section.  It MAY also permit one
 of the allowable combinations shown in Appendix C.
 Currently, there is no capability for negotiation of the broadband
 bearer capability.  This means that:
   a) the calling endsystem SHOULD have some prior knowledge as to
      the broadband bearer capability that will be acceptable to
      both the called endsystem and the network.
   b) if, in response to a SETUP message, a calling endsystem
      receives a RELEASE COMPLETE message, or a CALL PROCEEDING
      message followed by a RELEASE COMPLETE message, with cause
      #57, bearer capability not authorized or #58 bearer capability
      not presently available, it MAY reattempt the call after
      selecting another bearer capability.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 15] RFC 1755 ATM Signaling Support for IP over ATM February 1995

7.3. QoS Parameter

 The Unspecified QoS class (Class 0) is the only QoS class that must
 be supported by all networks and the only QoS class allowed when
 using the Best Effort service. The Specified QoS Class for Connection
 Oriented Data Transfer (Class 3) or the Specified QoS Class for
 Connectionless Data Transfer (Class 4) may be applicable to
 multiprotocol over ATM, but their use has to be negotiated with the
 network provider.  The combinations of QoS parameters with the ATM
 Traffic Descriptor and the Broadband Bearer Capability are detailed
 in the Traffic Descriptor section and in Appendix C.
        Format and field values of QoS Parameters IE
  1. ———————————————————

| qos_parameter |

  1. ———————————————————

| qos_class_fwd 0 (class 0) |

        |  qos_class_bkw              0         (class 0)        |
        ----------------------------------------------------------
 [ATMF93] does not provide any capability for negotiation of Quality
 of Service parameters.  This means that:
   a) the calling endsystem SHOULD have some prior knowledge as to
      the QoS classes offered by the ATM network in conjunction with
      the requested Broadband Bearer Service and Traffic Descriptor.
   b) if, in response to a SETUP message, a calling endsystem
      receives a RELEASE COMPLETE message, or a CALL PROCEEDING
      message followed by a RELEASE COMPLETE message, with cause
      #49, Quality of Service Unavailable, it MAY reattempt the call
      after selecting another QoS class.
 Note: The two-bit 'coding standard' field of the General Information
 octet in the IE header, SHOULD be set to '00' now that the ITU-T has
 standardized QoS class 0. Endsystems SHOULD treat either value ('11'
 or '00') as requesting the ITU-T QoS class.

7.4. ATM Addressing Information

 ATM addressing information is carried in the Called Party Number,
 Calling Party Number, and, under certain circumstance, Called Party
 Subaddress, and Calling Party Subaddress IE. Section 5.8 of [ATMF93]
 provides the procedure for an ATM endsystem to learn its own ATM
 address from the ATM network, for use in populating the Calling Party
 Number IE.  Section 5.4.5.14 [ATMF94] describes the syntax and
 semantics of the calling party subaddress IE.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 16] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 RFC 1577 RECOMMENDS that a router be able to provide multiple LIS
 support with a single physical ATM interface that may have one or
 more individual ATM endsystem addresses.  Use of the Selector field
 in the NSAPAs and E.164 addresses (in the NSAP format) is identified
 as a way to differentiate up to 256 different LISs for the same ESI.
 Therefore, an IP router MAY associate the IP addresses of the various
 LISs it supports with distinct ATM addresses differentiated only by
 the SEL field. If an IP router does this association, then its
 signaling entity MUST carry in the SETUP message the ATM addresses
 corresponding to the particular IP entity requesting the call, and
 the IP entity it is requesting a call to. These ATM addresses are
 carried in the Calling and Called Party Number IEs respectively.
 Native E.164 addresses do not support a SEL field.  For IP routers
 residing in a Public UNI where native E.164 addresses are used it is
 RECOMMENDED that multiple E.164 addresses be used to support multiple
 LISs.  Note: multiple LIS support is the only recommended use of the
 SEL field. Use of this field is not recommended for selection of
 higher level applications.
 Resolution of IP addresses to ATM addresses is required of hosts and
 routers which are ATM endsystems that use ATM SVCs. RFC 1577 provides
 a mechanism for doing IP to ATM address resolution in the classical
 IP model.
        Format and field values of Called and Calling Party Number IE
  1. ———————————————————

| called_party_number |

  1. ———————————————————

| type_of_number (international number / unknown) |

        |  addr_plan_ident     (ISDN / ATM Endsystem Address)    |
        |  addr_number         (E.164 / ATM Endsystem Address)   |
        ----------------------------------------------------------
  1. ———————————————————

| calling_party_number |

  1. ———————————————————

| type_of_number (international number / unknown) |

        |  addr_plan_ident     (ISDN / ATM Endsystem Address)    |
        |  presentation_indic  (presentation allowed)            |
        |  spare               0                                 |
        |  screening_indic     (user provided verified & passed) |
        |  addr_number         (E.164 / ATM Endsystem Address    |
        ----------------------------------------------------------

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 17] RFC 1755 ATM Signaling Support for IP over ATM February 1995

8. Dealing with Failure of Call Establishment

 If an ATM call attempt fails with any of the following causes, the
 situation SHOULD be treated as Network Unreachable (if the called ATM
 endsystem is a router) or Host Unreachable (if the called ATM
 endsystem is a host).  See the treatment of Network and Host
 Unreachable conditions in RFC 1122 [BRAD89].
      #  1  unallocated (unassigned) number
      #  3  no route to destination
      # 17  user busy
      # 18  no user reponding
      # 27  destination out of order
      # 38  network out of order
      # 41  temporary failure
      # 47  resource unavailable, unspecified
 If an ATM call attempt fails with any of the following causes, the
 ATM endsystem MAY retry the call, changing (or adding) the IE(s)
 indicated by the cause code and diagnostic.
         #  2  no route to specified transit network
         # 21  call rejected
         # 22  number changed
         # 23  user rejects call with CLIR
         # 37  user cell rate unavailable
         # 49  quality of service unavailable
         # 57  bearer capability not authorized
         # 58  bearer capability not presently available
         # 65  bearer capability not implemented
         # 73  unsupported combination of traffic parameter
         # 88  incompatible destination
         # 91  invalid transmit network selection
         # 78  AAL parameter cannot be supported

9. Security Considerations

 Not all of the security issues relating to IP over ATM are clearly
 understood at this time, due to the fluid state of ATM
 specifications, newness of the technology, and other factors.  Future
 revisions of this specification will address the security
 capabilities that future signaling standards may offer to IP over ATM
 signaling.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 18] RFC 1755 ATM Signaling Support for IP over ATM February 1995

10. Open Issues

 o   This document version is specifically an RFC 1577/RFC 1483
     implementation document. Although RFC 1577 and RFC 1483
     specify an LLC/SNAP encapsulation, which is inherently a
     multiprotocol encapsulation, it is beyond to scope of this
     document to go into any multiprotocol specifications other than
     to point out some examples (see Appendix D for an example of
     NLPID encapsulation).

11. Acknowledgments

 The authors wish to thank the work of their colleagues who attend the
 IP over ATM working group; the ATM Forum Technical Committee; the ATM
 Signaling Subworking Group in ANSI-Accredited Technical Subcommittee
 T1S1; the ATM Access Signaling experts in ITU-T (formerly CCITT)
 Study Group 11. Rao Cherukuri (IBM) and Jeff Kiel (formerly with
 Bellcore, presently with BellSouth) were particularly valuable in
 coordinating among T1S1, ITU-T and the ATM Forum to make sure that
 the needs of multiprotocol over ATM could be expressed in the ATM
 signaling protocol.

REFERENCES

 [ATKI94] Atkinson, R., "Default IP MTU over ATM AAL5", RFC 1626,
     Naval Research Laboratory, May 1994.
 [ATMF94] ATM Forum, "ATM User-Network Interface Specification Version
     3.1", 1994.
 [ATMF93] ATM Forum, "ATM User-Network Interface Specification Version
     3.0", (Englewood Cliffs, NJ: Prentice Hall, 1993).
 [BRAD89] Braden, R., Editor, "Requirements for Internet Hosts --
     Communication Layers", STD 3, RFC 1122, USC/Information Science
     Institute, October 1989.
 [BRAD94] Braden, R., Clark, D., and S. Shenker, "Integrated Service
     in the Internet Architecture:  An Overview", RFC 1633,
     USC/Information Science Institute, June 1994.
 [BRAD92] Bradley, T., and C. Brown, "Inverse Address Resolution
     Protocol", RFC 1293, Wellfleet Communications, Inc., January
     1992.
 [HEIN93] Heinanen, J., "Multiprotocol Encapsulation over ATM
     Adaptation Layer 5", RFC 1483, Telecom Finland, July 1993.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 19] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 [ISO8473] ISO/IEC 8473, Information processing systems - Data
     communications - Protocol for providing the connectionless-mode
     network service, 1988.
 [ISO9577] Information Technology - Telecommunication and information
     exchange between systems - Protocol identification in the network
     layer ISO/IEC TR9577 (International Standards Organization:
     Geneva, 1990).
 [LAUB93] Laubach, M., "Classical IP and ARP over ATM", RFC 1577,
     Hewlett-Packard Laboratories, December 1993.
 [PART92] Partridge, C., "A Proposed Flow Specification", RFC 1363,
     BBN, September 1992.
 [Q.2931] Broadband Integrated Service Digital Network (B-ISDN)
     Digital Subscriber Signaling System No.2 (DSS2) User Network
     Interface Layer 3 Specification for Basic Call/Connection Control
     ITU-T Recommendation Q.2931, (International Telecommunication
     Union: Geneva, 1994)

Authors' Addresses

 Maryann Perez Maher
 USC/Information Sciences Institute
 4350 N. Fairfax Drive Suite 400
 Arlington, VA 22203
 Phone: 703-807-0132
 EMail: perez@isi.edu
 Fong-Ching Liaw
 FORE Systems, Inc.
 174 Thorn Hill Road
 Warrendale, PA 15086-7535
 Phone: (412) 772-8668
 EMail: fong@fore.com
 Allison Mankin
 USC/Information Sciences Institute
 4350 N. Fairfax Drive Suite 400
 Arlington, VA 22203
 Phone:  703-807-0132
 EMail: mankin@isi.edu

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 20] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 Eric Hoffman
 USC/Information Sciences Institute
 4350 N. Fairfax Drive Suite 400
 Arlington, VA 22203
 Phone:  703-807-0132
 EMail: hoffman@isi.edu
 Dan Grossman
 Motorola Codex
 Phone: 617-821-7333
 EMail: dan@merlin.dev.cdx.mot.com
 Andrew G. Malis
 Ascom Timeplex, Inc.
 Advanced Products Business Unit
 289 Great Road   Suite 205
 Acton, MA  01720
 Phone: (508) 266-4522
 EMail: malis@maelstrom.timeplex.com

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 21] RFC 1755 ATM Signaling Support for IP over ATM February 1995

Appendix A. Sample Signaling Messages

1. SETUP and CONNECT messages

 This appendix shows sample codings of the SETUP and CONNECT signaling
 messages. The fields in the IE header are not shown.
+--------------------------------------------------------------------+
                              SETUP
  Information Elements/
    Fields                         Value/(Meaning)
  --------------------             ---------------
  aal_parameters
    aal_type                       5        (AAL 5)
    fwd_max_sdu_size_ident         140
    fwd_max_sdu_size               (send IP MTU value)
    bkw_max_sdu_size_ident         129
    bkw_max_sdu_size               (recv IP MTU value)
    sscs_type identifier           132
    sscs_type                      0        (null SSCS)
  user_cell_rate
    fwd_peak_cell_rate_0_1_ident   132
    fwd_peak_cell_rate_0_1         (link rate)
    bkw_peak_cell_rate_0_1_ident   133
    bkw_peak_cell_rate_0_1         (link rate)
    best_effort_indication         190
  bb_bearer_capability
    spare                          0
    bearer_class                   16       (BCOC-X)
    spare                          0
    traffic_type                   0        (no indication)
    timing_reqs                    0        (no indication)
    susceptibility_to_clipping     0        (not susceptible to
                                            clipping)
    spare                          0
    user_plane_configuration       0        (point_to_point)
  bb_low_layer_information
    layer_2_id                     2
    user_information_layer         12       (lan_llc (ISO 8802/2)
  qos_parameter
    qos_class_fwd                  0        (class 0)
    qos_class_bkw                  0        (class 0)

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 22] RFC 1755 ATM Signaling Support for IP over ATM February 1995

  called_party_number
    type_of_number                 (international number / unknown)
    addr_plan_ident                (ISDN / ATM Endsystem Address)
    number                         (E.164 / ATM Endsystem Address)
  calling_party_number
    type_of_number                 (international number / unknown)
    addr_plan_ident                (ISDN / ATM Endsystem Address)
    presentation_indic             (presentation allowed)
    spare                          0
    screening_indic                (user_provided verified and passed)
    number                         (E.164 / ATM Endsystem Address)
+--------------------------------------------------------------------+
                            Figure 1.
                    Sample contents of SETUP message
[* : optional, ignored if present]

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 23] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 In IP over ATM environments the inclusion of the "AAL parameters" IE
 is *mandatory* to allow for MTU size negotiation between the source
 and destination. The "Broadband Low Layer Information" IE is also
 mandatory for specifying the IP encapsulation scheme.
+--------------------------------------------------------------------+
                              CONNECT
  Information Elements/
    Fields                         Value
  --------------------             -----
  aal_parameters
    aal_type                       5        (AAL 5)
    fwd_max_sdu_size_ident         140
    fwd_max_sdu_size               (send IP MTU value)
    bkw_max_sdu_size_ident         129
    bkw_max_sdu_size               (recv IP MTU value)
    sscs_type identifier           132
    sscs_type                      0        (null SSCS)
  bb_low_layer_information
    layer_2_id                     2
    user_information_layer         12       (lan_llc (ISO 8802/2)
  connection identifier
    spare                          0
    vp_assoc_signaling             1        (explicit indication of VPCI)
    preferred_exclusive            0        (exclusive vpci/vci)
    vpci                           (assigned by network)
    vci                            (assigned by network)
+--------------------------------------------------------------------+
                             Figure 2.
                  Sample contents of CONNECT message
 As in the SETUP message, IP over ATM environments demand the
 inclusion of the "AAL parameters" IE so that the destination may
 specify the MTU size that it is willing to receive.
 2.  Hints on Use of CALL PROCEEDING Message
 Use of the CALL PROCEEDING message is beneficial in implementations
 where the called party's ATM signaling entity and AAL Users are
 decoupled. An arriving SETUP may result in an immediate CALL
 PROCEEDING response from the called party's ATM signaling entity,
 while it locally queries the called IP-ATM entity to see if the
 SETUP's conditions are acceptable. The acceptance of the SETUP's
 conditions would then cause the ATM signaling entity to issue a
 CONNECT back to the switch. The two possible refusal modes at the

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 24] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 called party then become:
         a) Called party has no IP-ATM entity resident. Issue RELEASE
            COMPLETE in response to SETUP.
         b) Called party has a resident IP-ATM entity, so CALL PROCEEDING
            was issued. The IP-ATM entity rejects the call request, so a
            RELEASE is issued instead (to be acknowledged by the network
            with RELEASE COMPLETE).

Appendix B. IP over ATM using UNI 3.0 Signaling

 This appendix describes how to support IP over ATM using UNI 3.0
 signalling.  Differences in the coding or semantics of each relevant
 IE is given.
 1. AAL parameter
 Values for maximum SDU size may range from one (not zero) to 64K.
 A 'mode' field is an allowable field in UNI 3.0. Nevertheless, this
 'mode' field SHOULD be omitted from the AAL Parameters IE and MUST be
 ignored by the destination endsystem.
 2. Traffic Management Related IEs
 In UNI 3.0 issues of traffic management were less understood than in
 UNI 3.1. UNI 3.0 does not contain a guide to coordinating the use of
 the User Cell Rate IE (Traffic Descriptor IE in UNI 3.1), Broadband
 Bearer Capability IE, and QoS parameters IE. Therefore, the
 recommendation for specifying parameters in these IEs is the same as
 that given above when using UNI 3.1.  The following section merely
 describes relevant differences in names and code values.
 2.1 ATM User Cell Rate (instead of ATM Traffic Descriptor)
 The ATM Traffic Descriptor IE is refered to as 'ATM User Cell Rate'
 IE in UNI 3.0. Also, the value for the cause 'user cell rate
 unavailable' is #51.
 2.3 QoS parameters
 The two-bit 'coding standard' field of the General Information octet
 in the IE header, should be set to '11' inidicating that the IE is a
 standard defined for the network (as opposed to an ITU-TS standard)
 present on the network side of the interface.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 25] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 3. ATM Addressing Information
 In UNI 3.1, the 'ATM Endsystem Address' type was introduced to
 differentiate ATM addresses from OSI NSAPs. In UNI 3.0, 'ATM
 Endsystem Address' is not a valid type. Therefore, in the called and
 calling party subaddress IEs the three-bit 'type of subaddress' field
 MUST specify 'NSAP' (value = 001) when using the subaddress IE to
 carry ATM addresses.
 4. Dealing with Failure of Call Establishment
 In UNI 3.0 the there are certain cause values which are different
 than UNI 3.1. Two relevant differences are the following:
    'AAL Parameter Cannot Be Supported' is #93 (#78 in UNI 3.1), and
    'User Cell Rate Unavailable' is #51 (#37 in UNI 3.1).

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 26] RFC 1755 ATM Signaling Support for IP over ATM February 1995

Appendix C.

               Combinations of Traffic Related Parameters
               tha MAY be supported in the SETUP message
  |-----------------------------------------------------------------|
  |Broadband Bearer                                                 |
  |Capability                                                       |
  |-----------------------------------------------------------------|
  |Broadband Bearer     |A,C| X |X  |C  | X |C| X |A,C| X | X |C| X |
  |---------------------|---|---|---|---|---|-|---|---|---|---|-|---|
  |Traffic Type         |   |   |   |   |   | |   |   |   |   | |   |
  |(CBR,VBR)            |   |CBR| & |   |&  | |&  |   |CBR|&  |&| & |
  |---------------------|---|---|---|---|---|-|---|---|---|---|-|---|
  |Timing Required      |   | Y |&& |   |&& | |&& |   | Y |&& | |&& |
  |-----------------------------------------------------------------|
  |Traffic Descriptor                                               |
  |Parameter                                                        |
  |-----------------------------------------------------------------|
  |PCR (CLP=0)          | S | S | S |   |   | |   |   |   |   | |   |
  |---------------------|---|---|---|---|---|-|---|---|---|---|-|---|
  |PCR (CLP=0+1)        | S | S | S | S | S |S| S | S | S | S |S| S |
  |---------------------|---|---|---|---|---|-|---|---|---|---|-|---|
  |SCR (CLP=0)          |   |   |   |   | S |S|   |   |   |   | |   |
  |---------------------|---|---|---|---|---|-|---|---|---|---|-|---|
  |SCR (CLP=0+1)        |   |   |   |   |   | | S | S |   |   | |   |
  |---------------------|---|---|---|---|---|-|---|---|---|---|-|---|
  |MBS (CLP=0)          |   |   |   |   | S |S|   |   |   |   | |   |
  |---------------------|---|---|---|---|---|-|---|---|---|---|-|---|
  |MBS (CLP=0+1)        |   |   |   |   |   | | S | S |   |   | |   |
  |---------------------|---|---|---|---|---|-|---|---|---|---|-|---|
  |Best Effort          |   |   |   |   |   | |   |   |   |   |S| S |
  |---------------------|---|---|---|---|---|-|---|---|---|---|-|---|
  |Tagging              |Y/N|Y/N|Y/N|Y/N|Y/N|N| N | N | N | N |N| N |
  |-----------------------------------------------------------------|
  |-----------------------------------------------------------------|
  |QOS Classes          | * | * | * | * | * |*| * | * | * | * |0| 0 |
  |-----------------------------------------------------------------|
  (Table 2 is a reproduction of Table F-1 of Appendix F in [ATMF 94].)
  PCR = Peak Cell Rate, SCR = Sustainable Cell Rate,
  MBS = Maximum Burst Size
  Y = Yes, N = No, S = Specified
  Y/N = either "Yes" or "No" is allowed

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 27] RFC 1755 ATM Signaling Support for IP over ATM February 1995

  • = allowed QoS class values are a network option. Class 0 is

always supported for alignment with ITU-T

  & = parameter is coded to either "no indication" or VBR or
      octet 5a(Traffic Type/Timing Required) is absent; these three
      codings are treated as equivalent
  && = parameter is coded to either "no indication" or "No" or
      octet 5a(Traffic Type/Timing Required) is absent; these three
      codings are treated as equivalent
  A blank entry in the table indicates that the parameter is not
  present.

Appendix D. Frame Relay Interworking

1. RFC 1490 over FR-SSCS vs. RFC 1483 over null-SSCS

 Procedures for Frame Relay to ATM signaling interworking have not yet
 been specified by ITU-T, the ATM Forum, or the Frame Relay Forum. If
 an ATM endsystem wishes to use FR-SSCS, FR-SSCS and RFC 1490
 encapsulation must both be be specified in the SETUP message.
 Nevertheless, since neither LLC encapsulation nor VC-multiplexing
 will interoperate when used over FR-SSCS, these two encapsulations
 cannot be negotiated as alternatives to RFC 1490 encapsulation (see
 Section 4, Encapsulation Negotiation).
 In ATM environments the SSCS layer is part of the AAL functionality.
 The SSCS serves to coordinate the needs of a protocol above with the
 requirements of next lower layer, the Common Part Convergence
 Sublayer (CPCS). For example, the UNI ATM signaling protocol runs on
 top of a signaling SSCS which among other things provides an assured
 transfer service for signaling messages. Since the SSCS is considered
 part of the AAL, the SSCS type is specified as one of the parameters
 in the AAL Parameters IE.  To date there has not been an SSCS defined
 for data transmission in ATM and this type field is usually set to
 'null'.
 The exception occurs when doing FR interworking where an ATM
 endsystem may choose to use the FR-SSCS over AAL 5 in order to
 communicate with a FR endsystem.  In that case the SSCS type in the
 AAL Parameters IE of the SETUP message is set to 'FR-SSCS'.
 Also included in a SETUP message is an indication in the B-LLI IE of
 the protocol layers to be used above the AAL. In particular, ATM
 connections established to carry connectionless network interconnect
 traffic require a layer above the AAL for multiplexing multiple
 protocols over a single VC [HEIN 93]. As mentioned above, RFC 1577

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 28] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 defines LLC as default multiplexing layer for IP over AAL5.
 Specification of the SSCS restricts the encapsulation protocol used
 over it, since RFC 1483 (in addition to applicable ITU standards)
 defines the use of RFC 1490 encapsulation over the FR-SSCS, and LLC
 or null encapsulation otherwise.  The fact that it is not possible,
 in the UNI 3.0 signaling specification, to negotiate between the FR-
 SSCS and null-SSCS can result in interoperability restrictions
 between stations that implement and wish to use the FR-SSCS and those
 that do not, even though they both are using IP. The guidelines in
 the following section were developed to decrease the chance that such
 interoperability restrictions occur.

2. Scenarios for Interworking

 The following discussion uses the terms "network interworking" and
 "service interworking".  "Network interworking" uses FR-SSCS over
 AAL5 between the InterWorking Unit (IWU) and the ATM endsystem, and
 the ATM endsystem is aware that the other endpoint is a FR/ATM
 Network IWU.  "Service interworking" aims to make the operation
 transparent to the ATM endsystem by adding encapsulation translation
 and other payload processing in the FR/ATM Service IWU to allow the
 ATM endsystem to operate as if it were talking to another ATM
 endsystem.
 The most common scenario where FR-SSCS could be negotiated is between
 an ATM endsystem and a FR/ATM network IWU to allow connectivity among
 an ATM endsystem and a FR endsystem residing behind a FR/ATM network
 IWU.
  1. ——- ——–
  2. —— | | | | ——-

| A | | FR/ATM | | ATM | | B |

   |  (FR) |----->|  IWU   |----->| switch |----->| (ATM) |
    -------       |        |      |        |       -------
                   --------        --------
           |      |        |                      |
            ----->          --------------------->
           FR call                 ATM call
 A network IWU can place a call to an ATM host (on behalf of a FR
 host) by signaling for FR-SSCS and assuming that the ATM endsystem
 supports FR-SSCS. The B-LLI IE SHALL be encoded to indicate RFC 1490
 encapsulation and the SSCS type field of the AAL Parameters IE SHALL
 be coded to indicate FR-SSCS.  If the FR-SSCS negotiation fails
 because the called ATM host does not support FR-SSCS, the IWU can
 retry the call negotiating for LLC encapsulation or VC-multiplexing.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 29] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 However, the IWU can only attempt the retry if it is able to do FR-
 ATM service interworking. Such service interworking adds extra
 processing overhead during the call.
 The even more problematic case occurs when a call is requested in the
 opposite direction, i.e. when an ATM host places a call to a host
 residing behind an IWU.
  1. ——- ——–
  2. —— | | | | ——-

| B | | FR/ATM | | ATM | | A |

   |  (FR) |<-----|  IWU   |<-----| switch |<-----| (ATM) |
    -------       |        |      |        |       -------
                   --------        --------
           |      |        |                      |
            <-----          <---------------------
           FR call                 ATM call
 Not knowing that the destination resides behind an IWU, the calling
 host will negotiate for the default LLC encapsulation (possibly
 requesting VC-multiplexing as an alternative).  In this situation the
 IWU can accept the call and do the necessary service interworking or
 reject the call specifying 'AAL Parameters not supported'. If the IWU
 rejects the call it risks the possibility that calling host does not
 support FR-SSCS or simply does not retry and the call will never be
 established.

3. Possible Alternatives

 While Frame Relay interworking is possible, it is not possible to
 negotiate FR-SSCS with LLC encapsulation or VC-multiplexing, which
 decreases the chances of completing an ATM call.  However,
 interoperability can be increased using the following alternatives:
 1. Maintaining external knowledge that a particular destination uses
 FR-SSCS.  This knowledge can be configured, or in the future added to
 some network host database.
 2. In the absence of such external knowledge, an ATM endsystem is
 required to negotiate for the default LLC encapsulation (possibly
 requesting VC-multiplexing as an alternative).  There are three sub-
 cases:
 2a. The IWU supports service interworking and network interworking,
 and prefers service interworking.  The IWU simply accepts the call
 using LLC encapsulation.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 30] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 2b. The IWU supports service interworking and network interworking,
 and prefers network interworking.  The IWU simply accepts the call,
 but attempts to open a parallel connection back to the original ATM
 endsystem negotiating the FR-SSCS use.  If the connection is
 accepted, the IWU closes the service interworking connection.
 2c. The IWU supports network interworking only.  The IWU rejects the
 call specifying 'AAL Parameters not supported', and then attempts to
 open a connection back to the original ATM endsystem negotiating the
 FR-SSCS use.

4. Encapsulation negotiation

 The call/connection control signaling protocol includes a mechanism
 to support negotiation of encapsulation for endsystems that support
 more than one. This section describes the procedures for negotiation
 of an encapsulation.
 The B-LLI negotiation procedures (see Annex C of [ATMF93]) are
 initiated by the calling ATM endsystem by including up to three
 instances of the B-LLI IE in the SETUP message in descending order of
 preference (following the rule for repeating IE in section 5.4.5.1 of
 [ATMF93]).
 The following is the list of the three possible combinations that B-
 LLI IE instances MAY be included in the SETUP message.  Each instance
 is referred to by its encapsulation name as it appears in RFC 1483,
 and corresponding section labels from Appendix D of the ATM Forum UNI
 3.0 specification.
   a) LLC/SNAP encapsulation (D.3.1)
 In this case, the calling ATM endsystem can only send and receive
 packets preceded by an LLC/SNAP identification. This memo requires
 that hosts and routers which are ATM endsystems implement LLC/SNAP
 encapsulation.
   b) VC-multiplexing (D.3.2) and LLC/SNAP (D.3.1)
 The calling ATM endsystem prefers to use VC multiplexing, but is
 willing to agree to use LLC/SNAP encapsulation instead, if the called
 ATM endsytem only supports LLC/SNAP.
   c) RFC 1490 encapsulation (NLPID multiplexing) over FRSSCS
     (D.3.3, omitting octets 7a and 7b and MUST have FR-SSCS in SSCS
     type of AAL Parameters IE.)

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 31] RFC 1755 ATM Signaling Support for IP over ATM February 1995

 The calling ATM endsystem can only send and receive packets using RFC
 1490 encapsulation (NLPID multiplexing) over FRSSCS.  Use of RFC 1490
 encapsulation presently cannot be negotiated as an alternative to LLC
 encapsulation or VC-multiplexing.  If the B-LLI IE is encoded to
 indicate RFC 1490 encapsulation, the SSCS type field of the AAL
 Parameters IE SHALL coded to indicate FRSSCS.  Note that the AAL
 Parameters IE can not be coded to indicate both NULL and FR-SSCS and
 neither LLC encapsulation nor VC-multiplexing will be interoperable
 when used over FR-SSCS.
 The called ATM endsystem SHALL select the encapsulation method it is
 able to support from the B-LLI IE present in SETUP message.  If it
 supports more than one of the encapsulations indicated in the SETUP
 message, it MUST select the one which appears first in the SETUP
 message.  The called ATM endsystem then includes the B-LLI IE content
 corresponding to the selected encapsulation in the CONNECT message.
 If the called endsystem does not support any encapsulation indicated
 in the incoming SETUP message, it SHALL clear the call with cause
 #88, incompatible destination.  If the received SETUP message does
 not include the B-LLI IE, the call SHALL be cleared with cause #21,
 "call rejected", with diagnostics indicating rejection reason =
 information element missing and the B-LLI IE identifier.  As
 described in Annex C of [ATMF93], if the calling ATM endpoint
 receives a CONNECT message that does not contain a B-LLI IE, it SHALL
 assume the encapsulation indicated in the first BLLI IE that it
 included in the SETUP message.

Perez, Liaw, Mankin, Hoffman, Grossman & Malis [Page 32]

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