Network Working Group M. Perez

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.

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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

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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

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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

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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.

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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.

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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.

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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

----------------------------------------------------------
| aal_parameters |
----------------------------------------------------------
| 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

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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

----------------------------------------------------------
| bb_low_layer_information |
----------------------------------------------------------
| 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.

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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)

----------------------------------------------------------
| bb_low_layer_information |
----------------------------------------------------------
| 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.

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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].

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RFC 1755 ATM Signaling Support for IP over ATM February 1995

Format and field values of ATM Traffic Descriptor IE

----------------------------------------------------------
| traffic_descriptor |
----------------------------------------------------------
| 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).

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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.

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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.

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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

----------------------------------------------------------
| 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 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.

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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

----------------------------------------------------------
| qos_parameter |
----------------------------------------------------------
| 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.

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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

----------------------------------------------------------
| called_party_number |
----------------------------------------------------------
| type_of_number (international number / unknown) |
| addr_plan_ident (ISDN / ATM Endsystem Address) |
| addr_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 & passed) |
| addr_number (E.164 / ATM Endsystem Address |
----------------------------------------------------------

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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: [email protected]

Fong-Ching Liaw
FORE Systems, Inc.
174 Thorn Hill Road
Warrendale, PA 15086-7535

Phone: (412) 772-8668
EMail: [email protected]

Allison Mankin
USC/Information Sciences Institute
4350 N. Fairfax Drive Suite 400
Arlington, VA 22203

Phone: 703-807-0132
EMail: [email protected]

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: [email protected]

Dan Grossman
Motorola Codex

Phone: 617-821-7333
EMail: [email protected]

Andrew G. Malis
Ascom Timeplex, Inc.
Advanced Products Business Unit
289 Great Road Suite 205
Acton, MA 01720

Phone: (508) 266-4522
EMail: [email protected]

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)

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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]

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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

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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.

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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).

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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

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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

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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.

-------- --------
------- | | | | -------
| 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.

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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.

-------- --------
------- | | | | -------
| 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.

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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.)

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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]