Network Working Group M. Maher

Network Working Group M. Maher
Request for Comments: 2331 USC/ISI
Category: Standards Track April 1998

ATM Signalling Support for IP over ATM - UNI Signalling 4.0 Update

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

This memo describes how to efficiently use the ATM call control
signalling procedures defined in UNI Signalling 4.0 [SIG40] to
support IP over ATM environments as described in RFC 2225 [LAUB98]
and in RFC 2332 [LUC98]. Among the new features found in UNI
Signalling 4.0 are Available Bit Rate signalling and traffic
parameter negotiation. This memo highlights the features of UNI
Signalling 4.0 that provide IP entities capabilities for requesting
ATM service in sites with SVC support, whether it is private ATM or
publicly provisioned ATM, in which case the SVC support is probably
configured inside PVPs.

This document is only relevant to IP when used as the well known
"best effort" connectionless service. In particular, this means that
this document does not pertain to IP in the presence of implemented
IP Integrated Services. The topic of IP with Integrated Services
over ATM will be handled by a different specification or set of
specifications being worked on in the ISSLL WG.

This specification is a follow-on to RFC 1755, "ATM Signaling Support
for IP over ATM", which is based on UNI 3.1 signalling [UNI95].
Readers are assumed to be familiar with RFC 1755.

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RFC 2331 IP over ATM Signalling - SIG 4.0 Update April 1998

Table of Contents

1. Conventions ............................................... 2
2. Overview .................................................. 2
3. Use of Protocol Procedures ................................ 3
3.1 VC Teardown........................................... 3
4. Overview of Call Establishment Message Content ............ 3
5. Description of Information Elements ....................... 4
5.1 ATM Adaptation Layer Parameters ...................... 4
5.2 Broadband Low Layer Information ..................... 5
5.3 Traffic Management Issues and Related IEs............. 5
5.3.1 ATM Traffic Descriptor ........................ 6
5.3.1.1 Tagging vs. Dropping ................. 7
5.3.2 Traffic Parameter Negotiation .................. 7
5.3.3 Broadband Bearer Capability .................... 8
5.3.4 QoS Parameter .................................. 8
5.3.4.1 Signalling of Individual QoS Parameters 8
5.4 ATM Addressing Information ........................... 9
6. ABR Signalling In More Detail ............................ 9
7. Frame Discard Capability .................................. 10
8. Security Considerations ................................... 10
9. Acknowledgements........................................... 10
10. References ................................................ 10
11. Author's Address .......................................... 12
Appendix A Sample Signalling Messages ........................ 13
Appendix B ABR and nrt-VBR Signalling Guidelines for IP Routers 15
Appendix C Combinations of Traffic Related Parameters ........ 18
Full Copyright Statement ...................................... 26

1. Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [BRA97].

2. Overview

UNI Signalling version 4.0 (SIG 4.0) is the ATM Forum follow-on
specification to UNI 3.1 signalling (UNI 3.1). Among the new features
in SIG 4.0, those of particular interest to IP over ATM environments
are:

o Available Bit Rate (ABR) Signalling for Point-to-Point Calls
o Traffic Parameter Negotiation
o Frame Discard Support
o Leaf Initiated Join (LIJ) Capability
o ATM Anycast Capability
o Switched Virtual Path (VP) Service

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This memo highlights the first three capabilities listed above. The
last three capabilities are not discussed because models for their
use in IP over ATM environments have not yet been defined. The ION
WG is considering the applicability of LIJ and Group Addressing to
the RFC2022 problem space. Furthermore, Anycast addressing is being
explored as a technique for supporting server discovery in ATM
networks.

3. Use of Protocol Procedures

Section 3 in RFC 1755 introduces requirements of virtual circuit (VC)
management intended to prevent VC thrashing, excessive VC
consumption, and other related problems. This section updates RFC
1755's requirements related to VC teardown.

3.1. VC Teardown

In environments running layer 3 (L3) signalling protocols, such as
RSVP [RSVP], over ATM, data VCs might correspond to L3 reserved flows
(even if the VC is a 'best effort' VC). In such environments it is
beneficial for VCs to be torn down only when the L3 reservation has
expired. In other words, it is more efficient for the sender of a L3
reserved flow to initiate VC tear-down when the receiver(s) has
ceased refreshing the reservation. To support such L3 behavior,
systems implementing a Public ATM UNI interface and serving as the
_called_ party of a VCC MUST NOT use an inactivity timer on such a
VCC by default. A system MAY use an inactivity timer on such a VCC
if configured to do so.

4. Overview of Call Establishment Message Content

Signalling messages are structured to contain mandatory and optional
variable length information elements (IEs). 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:

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Calling Party Subaddress
Called Party Subaddress
Transit Network Selection

(New in SIG 4.0:)
Minimum Acceptable ATM Traffic Descriptor
Alternative ATM Traffic Descriptor
ABR Setup Parameters
ABR Additional Parameters
Connection Scope Selection
Extended QoS Parameters
End-to-End Transit Delay

In SIG 4.0, like 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 a sample setup message.

5. Description of Information Elements

This section describes the coding of, and procedures surrounding,
information elements in SETUP and CONNECT messages. The first two IEs
described, ATM Adaptation Layer Parameters and Broadband Low Layer
Information, are categorized as having significance only to the end-
points of an ATM call supporting IP. That is, the network does not
process these IEs.

5.1. ATM Adaptation Layer (AAL) Parameters

The AAL Parameters IE carries information about the ATM adaptation
layer to be used on the connection. The parameters specified in this
IE are the same as specified in [PER95].

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) |
----------------------------------------------------------

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This shows maximum size MTUs. In practice, most sites have used 9180
IP MTUs for ATM [RFC1626].

5.2. 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. B-LLI
negotiation is described in [PER95] in Appendix D. The procedures
remain the same for this SIG 4.0 based specification.

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) |
----------------------------------------------------------

5.3. Traffic Management Issues and Related IEs

The ATM Forum Traffic Management Sub-working group has completed
version 4.0 of their specification [TMGT40]. This latest version
focuses primarily on the definition of the ABR service category. As
opposed to the Unspecified Bit Rate (UBR) traffic class, ABR uses a
rate-based flow control mechanism to assure certain traffic
guarantees (bandwidth and delay). There has been much debate on
whether IP benefits from ABR, and if so, how IP should use ABR. The
IP Integrated Services (IIS) and RSVP models in IP add complexity to
this issue because mapping IIS traffic classes to ATM traffic classes
is not straightforward.

This document attempts only to present the required IP to ATM
signaling interface for IP over ATM systems that do not support IIS
as yet. It is an attempt to cause IP over ATM vendors to support
enough options for signalling the traffic characteristics of VCs
serving non-IIS IP datagrams. This specification also aims to give
guidance to ATM system administrators so that they can configure
their IP over ATM entities to conform to the varied services that
their ATM provider may have sold to them. By definition, IP without
IIS cannot be expected to provide a signalling interface that is
flexible and allows application specific traffic descriptors. The
topic of IP over ATM signalling for IP _with_ IIS is to be presented
in other specifications being produced by the ISSLL WG of the IETF.

An IP over ATM interface may be configured to support all the defined
ATM Service Categories (ASC). They are:

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- CBR
- CBR with CLR specified (loss-permitting CBR)
- ABR
- UBR
- real time VBR
- non-real time VBR

The ATM Traffic Descriptor IE, Broadband Bearer Capability IE, and
the QoS Parameter IE together define the signalling view of ATM
traffic management. Additionally, the Extended QoS parameters IE and
the End-to-end Transit Delay IE may be used to provide more specifics
about traffic requirements, however this note does not provide
explicit recommendations on their use. Annex 9 of [SIG40] describes
a set of allowable combinations of traffic and QoS related
paramenters defined for SIG 4.0. This set includes all forms of
non-IIS IP signaling configurations that MUST be implemented in ATM
endsystems to accommodate varied sites' needs. The principle is that
IP over ATM service may be available in different sites by different
types of procured ATM service; for one site, a CBR PVP might be
cost-effective and then the SVCs that IP over ATM without IIS must
establish must be CBR. Similarly, VBR or ABR PVPs could be
provisioned. The intent of this document is to specify the use of
the most sensible parameters within this non-IIS configuration. For
instance, for non-IIS VBR, the SCR value may need to be hand-
configured for IP users, or for ABR, the PCR value may be link-rate
with a 0 MCR.

For the reader's convenience, we have replicated the tables found in
Annex 9 of [SIG40] in Appendix C of this document. Ideally this
document could recommend specific values for the various table
parameters that would offer the most sensible IP over ATM service.
Nevertheless, it is not possible to mandate specific values given the
varied scenarios of procured ATM service.

5.3.1. ATM Traffic Descriptor

Even with the newly defined ABR ASC, the most convenient model for
supporting IP still corresponds to the best effort capability, the
UBR ASC. The rationale for this assertion stems from the fact that a
non-IIS IP service has no notion of the performance requirements of
the higher layers it supports. Therefore, if a site's configuration
allows use of UBR, users SHOULD signal for it using the IE's and
parameters pertaining to the UBR ATC. See Appendix C for the list of
those IE's and parameters.

Although we consider the UBR ASC the most natural ASC for best-effort
IP, ATM vendors that implement VBR and ABR services could possibly
create hooks for convenient use of these services. If this is the

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case, IP routers may perhaps have the most to gain from use of VBR or
ABR services because of the large aggregated traffic volume they are
required to forward. See Appendix B for detailed suggestions on VBR
and ABR signalling for IP routers. We simply note here that, in
support of ABR service, two new subfields have been added in SIG 4.0
to the Traffic Descriptor IE. These fields are the forward and
backward 'Minimum Cell Rate' fields.

5.3.1.1. Tagging vs. Dropping

The Traffic Descriptor IE contains a 'tagging' subfield used for
indicating whether the network is allowed to tag the source's data
cells. Tagging in the network may occur during periods of congestion
or when the source's traffic has violated the traffic contract for
the connection. See Section 4 of [TMGT40] for an explanation of ATM
connection conformance and the Usage Parameter Control (UPC)
function.

SIG 4.0 and TMGT 4.0 define two modes of UBR, UBR.1 which disables
tagging and UBR.2 which enables tagging (see Appendix C). In some
network environments there is no potential for UBR traffic sources to
violate the connection traffic contract because, either the user's
terminal equipment supports traffic shaping, or the network does not
enforce PCR. In such environments, the user SHOULD specify 'no
tagging' in the SETUP message (UBR.1). Specifying 'no tagging'
indicates to the network that cells should be dropped during periods
of congestion instead of being randomly marked/tagged as low
priority. Cells of packets that the source itself has marked as low
priority are dropped first, thereby preserving the source's
characterization of the traffic.

On the other hand, when the network applies PCR to the UPC function,
meaning it enforces PCR, and traffic shaping is not enabled at the
source, the source has the potential to violate the traffic contract
and SHOULD therefore signal for tagging (UBR.2). Tagging allows the
source's non-conforming cells to be tagged and forwarded instead of
dropped.

5.3.2. Traffic Parameter Negotiation

SIG 4.0 allows certain traffic parameters to be negotiated during the
call establishment phase Traffic parameters cannot be 'renegotiated'
after the call is active. Two new IEs make negotiation possible:

- the Minimum Acceptable ATM Traffic Descriptor IE allows
negotiation of PCR parameters

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- the Alternative ATM Traffic Descriptor IE allows negotiation of
other traffic parameters

A SETUP or CONNECT message may include ONLY one of the above IEs.
That is, the calling party may only offer an 'alternative' or
'minimum' to the requested traffic parameters. (See Section 8 of
[SIG40].) IP over ATM entities SHOULD take advantage of this
capability whenever possible. In order to do so, IP over ATM entities
SHOULD specify PCR _equal_ to the link rate in the ATM Traffic
Descriptor IE of the SETUP message and a minimum of zero PCR in the
Minimum Acceptable ATM Traffic Descriptor IE.

5.3.3. Broadband Bearer Capability

A new field in UNI signalling 4.0 called, 'ATM Transfer Capability'
(ATC), has been defined in the Broadband Bearer Capability IE for the
purpose of explicitly specifying the desired ATM traffic category.
The figure below shows the allowable ATC values.

Format and field values of Broadband Bearer Capability IE

-------------------------------------------------------------
| bb_bearer_capability |
------------------------------------------------------------|
| spare 0 |
| bearer_class bcob-x,c,a or VP |
| transfer_capability cbr, rt-vbr, nrt-vbr, abr |
| susceptibility_to_clipping 0 (not suscept) |
| spare 0 |
| user_plane_configuration pt-to-pt, pt-to-mpt |
-------------------------------------------------------------

5.3.4. QoS Parameter

Inclusion of the QoS Parameter IE is not mandatory in SIG 4.0. It
may be omitted from a SETUP message _if and only if_ the Extended QoS
Parameters IE is included (see next section). This specification
makes no explicit recommendation on the use of the QoS related IEs.

5.3.4.1. Two IEs for Signalling of Individual QoS Parameters

SIG 4.0 allows for signalling of individual QoS parameters for the
purpose of giving the the network and called party a more exact
description of the desired delay and cell loss characteristics. The
two individual QoS related IEs, Extended QoS Parameters IE and End-
to-End Transit Delay IE, can be used in the SETUP and CONNECT
signaling messages in place of the 'generic' QoS Parameter IE. Note
that inclusion of these two IEs depends on the type of ATM service

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category requested (see Annex 9 in [SIG40]).

5.4. 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. The ATM Forum ILMI
Specification 4.0 [ILMI40] 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.

Format and field values of Called 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) |
----------------------------------------------------------

6. ABR Signaling In More Detail

The IEs and procedures pertaining to ABR signalling are briefly
described in this section. Nevertheless, this document makes no
specific recommendation on when to use the ABR service category for
IP VCCs or give suggestions on appropriate values for the various
parameters in the ABR related IEs.

Two new IEs have been defined for ABR signaling:

o ABR Setup Parameters
o ABR Additional Parameters

These IEs may be optionally included in a SETUP or CONNECT message.
The ABR Setup Parameters IE contains the following subfields:

- Forward/Backward ABR Initial Cell Rate
- Forward/Backward ABR Transient Buffer Exposure
- Cumulative RM Fixed Round Trip Time
- Forward/Backward Rate Increment Factor
- Forward/Backward Rate Decrease Factor

The ABR Additional Parameters IE contains one subfield:

- Forward/Backward Additional Parameters Record

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The Additional Parameters Record value is a compressed encoding of a
set of ABR parameters (see [SIG40] and [ABRS]).

7. Frame Discard Capability

The frame discard capability in SIG 4.0 is primarily based on the
'Partial and Early Packet Discard' strategy [ROM94]. Its use is
defined for any of the ATM services, except for loss-less CBR. Frame
discard signaling MUST be supported by all IP over ATM entities and
it is RECOMMENDED that frame discard be signaled for all IP SVCs
because it has been proven to increase throughput under network
congestion. Signaling for frame discard is done by setting the frame
discard bit in the 'Traffic Management Options' subfield in the
Traffic Descriptor IE. It is possible that not all network entities
in the SVC path support frame discard, but it is required that they
all forward the signaling.

8. Security Considerations

The ATM Forum Security sub-working group is currently defining
security mechanisms in ATM. The group has yet to produce a
specification, therefore it is premature to begin defining IP over
ATM signalling's use of ATM security. The ATM Forum is working on
authentication mechanisms for signalling and on mechanisms for
providing data integrity and confidentiality (e.g encryption). Lack
of these ATM security mechanisms prevents the authentication of the
originator of signalling messages, such as, connection setup request
or connection teardown request. IP Security (RFC1825) can be applied
to IP datagrams over ATM VCs to overcome the lack of security at the
ATM layer.

9. Acknowledgements

The authors would like to thank the members of the ION working group
for their input. Special thanks to K.K. Ramakrishnan and Kerry
Fendick who contributed Appendix B of this document.

REFERENCES

[ABRS] ATM Forum, "Addendum to UNI Signalling v4.0 for ABR Parameter
Negotiation", af-sig-0076.000; available at
ftp://ftp.atmforum.com/pub.

[ABRT] ATM Forum, "Addendum to Traffic Management v4.0 for ABR
Parameter Negotiation", af-tm-0077.000; available at
ftp://ftp.atmforum.com/pub.

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RFC 2331 IP over ATM Signalling - SIG 4.0 Update April 1998

[RFC1122] Braden, R., Editor, "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC 1122, October 1989.

[RFC1633] Braden, R., Clark, D., and S. Shenker, "Integrated Service
in the Internet Architecture: An Overview", RFC 1633, June 1994.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC1483] Heinanen, J., "Multiprotocol Encapsulation over ATM
Adaptation Layer 5", RFC 1483, July 1993.

[ILMI40] ATM Forum, "Integrated Local Management Interface (ILMI)
Specification Version 4.0", af-ilmi-0065.000, finalized September
1996; available at ftp://ftp.atmforum.com/pub.

[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)

[LAUB98] Laubach, M., and J. Halpern, "Classical IP and ARP over
ATM", RFC 2225, April 1998.

[LUC98] Luciani, J., Katz, D., Piscitello, D., Cole, B., and N.
Doraswamy, "NBMA Next Hop Resolution Protocol (NHRP)", RFC 2332,
April 1998.

[RFC1755] Perez*, M., et. al., "ATM Signaling Support for IP over
ATM", RFC 1755, February 1995. (* see author's information below)

[ROM94] Romanow, A., and Floyd, S., Dynamics of TCP Traffic over ATM
Networks. IEEE JSAC, V. 13 N. 4, May 1995, p. 633-641. Abstract. An
earlier version appeared in SIGCOMM '94, August 1994, pp. 79-88.

[RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) - Version 1 Functional
Specification", RFC 2205, September 1997.

[SIG40] ATM Forum, "ATM User-Network Interface (UNI) Signalling
Specification Version 4.0", af-sig-0061.000, finalized July 1996;
available at ftp://ftp.atmforum.com/pub.

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[TMGT40] ATM Forum, "Traffic Management Specification Version 4.0",
af-tm-0056.000, finalized April 1996; available at
ftp://ftp.atmforum.com/pub.

[UNI95] ATM Forum, "ATM User-Network Interface Specification Version
3.1", Prentice Hall, Upper Saddle River, NJ, 1995.

Author's Address

Maryann P. Maher (formerly Maryann Perez)
USC/ISI
4350 N. Fairfax Drive, Suite 620
Arlington VA 22203

EMail: [email protected]

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Appendix A. A Sample SIG 4.0 SETUP Message

+--------------------------------------------------------------------+
SETUP

Information Elements/
Fields Value/(Meaning)
-------------------- ---------------

aal_parameters
aal_type 5 (AAL 5)
fwd_max_sdu_size_ident 140
fwd_max_sdu_size (xmit IP MTU value)
bkw_max_sdu_size_ident 129
bkw_max_sdu_size (recv IP MTU, 0 for disallowing return traffic)
sscs_type identifier 132
sscs_type 0 (null SSCS)

traffic_descriptor
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)
traff_mngt_options_ident 191
fwd_frame_discard 1 (on)
bkw_frame_discard 1 (on if return traffic indicated)
spare 0
tagging_bkw 1 (on)
tagging_fwd 1 (on if return traffic indicated)
best_effort_indication 190 (on)

minimum_acceptable_traffic_descriptor
fwd_peak_cell_rate_0_1_ident 132
fwd_peak_cell_rate_0_1 0
bkw_peak_cell_rate_0_1_ident 133
bkw_peak_cell_rate_0_1 0

bb_bearer_capability /* a coding for specifying UBR like service */
spare 0
bearer_class 16 (BCOC-X)
spare 0
atm_transfer_capability 10 (nrt-vbr)
susceptibility_to_clipping 0 (not susceptible to clipping)
spare 0
user_plane_configuration 0 (point_to_point)

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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)

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

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Appendix B. ABR and VBR Signaling Guidelines for IP Routers

When ATM is used to interconnect routers that are supporting a best
effort service, the ATM connection typically carries an aggregation
of IP flows, e.g., all best effort IP traffic between a pair of
routers. With the efforts undertaken by ATM to be more "packet
friendly" (e.g., frame discard), it is useful to examine ways that a
VC can provide service comparable to or better than that of a
dedicated or leased "link" in terms of performance and packet loss.

For ATM connections used to interconnect routers, a non-zero
bandwidth reservation may be required to achieve consistently
adequate performance for the aggregate set of flows. The support of
bandwidth commitments for an ATM connection carrying IP traffic helps
to assure that a certain fraction of each link's capacity is reserved
for the total IP traffic between the routers. Reserving bandwidth
for the aggregation of best-effort traffic between a pair of routers
is analogous to provisioning a particular link bandwidth between the
routers. There are at least 3 service classes defined in the ATM
Traffic Management specification that provide varying degrees of
capability that are suitable for interconnecting IP routers: UBR, ABR
and VBR non-real-time. Although the use of best-effort service (UBR)
at the ATM layer is the most straightforward and uncomplicated, it
lacks the capability to enforce bandwidth commitments.

Note that we are talking of providing a "virtual link" between
routers, for the aggregate traffic. The provisioning is for the
aggregate. It is therefore distinct from the per-flow bandwidth
reservations that might be appropriate for Integrated Services.

Even best-effort IP flows, when supported on an aggregate basis, have
some broad service goals. The primary one is that of keeping packet
loss rate reasonably small. A service class that strives to achieve
this, keeping in mind the tradeoff between complexity and adequate
service, is desirable. It has been recommended in this memo that UBR
be the default service for this. UBR with (some form of) packet
discard has the desirable goal of being simple in function, and it
appears that vendors will be supporting it. However, when available,
it may be quite worthwhile to consider ABR and VBR non-real-time
service classes.

Because AAL5 frames with missing cells are discarded by the receiver,
ATM bandwidth commitments are most useful if supported in the form of
a committed rate of cell delivery in complete, non-errored AAL5
frames delivered to the receiver. In addition, it is desirable for
the ATM connection to deliver additional complete frames, beyond this
commitment, on a best-effort basis.

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These characteristics can be achieved through the ABR service
category through the use of a Minimum Cell Rate, if the ABR service
is supported by the ATM endpoints and if efficient frame discard is
supported at the ABR source. The mechanisms put in place for the ABR
service strive to keep loss quite low within the ATM network.

The parameters that should be specified by the end system are (i) the
Peak Cell Rate (likely the link rate), (ii) the Minimum Cell Rate
(the committed rate), and (iii) the Cumulative RM Fixed Round-Trip
Time. The remaining parameter values, if left unspecified by the
calling party, are selected by the network or are chosen from the
default values specified in the ATM Forum Traffic Management
specification.

Parameters (i) and (ii) are contained in the mandatory Traffic
Descriptor IE, whereas parameter (iii) is contained in the mandatory
ABR Setup Parameters IE. Other paramenters in the ABR Setup
Parameters IE may be omitted. (Note that the third IE which pertains
to ABR signalling, the ABR Additional Parameters IE, is an optional
IE and therefore need not be included.) Parameter (iii) is dependent
on the hardware of the end system, so that the default value
specified for that hardware should be used. In the absense of such a
default, a value of zero MAY be specified by the end system. Entities
using ABR connections for IP over ATM SHOULD take advantage of
parameter negotiation by specifying Peak Cell Rate equal to the link
rate in the ATM Traffic Descriptor IE of the SETUP message. The value
selected for the Minimum Cell Rate is implementation specific. Note
that the MCR also MAY be negotiated if an MCR parameter is included
by the end system in the Minimum Acceptable ATM Traffic Descriptor
IE. The use of MCR negotiation by the end system is implementation
specific. Also, note that Frame Discard MAY be requested for ABR
connections as well as for UBR connections. Although the ABR service
attempts to minimize cell loss, the use of Frame Discard may improve
throughput when cell loss is not eliminated.

ATM recognizes in addition to the service class (UBR, ABR, etc.), a
notion of a QoS class. The QoS class specifies the type of guarantee
requested of the network when the call is setup. This is distinct
from the service class requested for the connection, and the
specification of the traffic parameters (which specify what the
source's traffic will look like). QoS class 0 is the "simplest", and
is called the Unspecified QoS class. In the context of ABR (and VBR
non-realtime below), we are only concerned with the QoS class
providing an assurance of acceptable loss behavior for the
connection.

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RFC 2331 IP over ATM Signalling - SIG 4.0 Update April 1998

The Unspecified QoS Class (QoS Class 0) MUST be requested for ABR
connections. In this context, QoS Class 0 corresponds to a network-
specific objective for the cell loss ratio. Networks in general are
expected to support a low Cell Loss Ratio for ABR sources that adjust
cell flow in response to control information.

The VBR-nrt service category provides an alternate means of achieving
these characteristics. These characteristics may be obtained with
VBR-nrt connections for which (i) the VBR.3 conformance definition is
used, (ii) a Sustainable Cell Rate (SCR) and Maximum Burst Size
(MBS), and Peak Cell Rate (PCR) are specified, and (iii) both tagging
and frame discard are requested. A request for tagging indicates
that best-effort delivery is desired for traffic offered in excess of
the SCR and MBS. A request for frame discard indicates to the
network that the user desires allocations of committed and excess
bandwidth to translate into corresponding throughputs at the frame
level.

As with UBR connections, entities using VBR-nrt connections for IP
over ATM should take advantage of parameter negotiation by specifying
PCR equal to the link rate in the ATM Traffic Descriptor IE of the
SETUP message and PCR equal to SCR in the Minimum Acceptable Traffic
descriptor. The selection of SCR, MBS, and CLR (cell loss ratio)
should be implementation specific. However, for IP over ATM, an MBS
value of N*(Maximum MTU) is RECOMMENDED, where N>=1 with a default of
2 and where Maximum MTU is equal to 192 cells (consistent with an IP
MTU size of 9180 bytes [RFC1626]).

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RFC 2331 IP over ATM Signalling - SIG 4.0 Update April 1998

Appendix C. Combinations of Traffic Related Parameters

This appendix contains a copy of the five tables found in Annex 9 of
[SIG40] which show the allowable combinations of traffic and QoS
related parameters in a SIG 4.0 SETUP message.

+--------------------------------------------------------------------+
|ATM Service Category| CBR |
|--------------------|---------------|---------------|---------------|
| Conformance |CBR.1 (note 10)| (note 4) | (note 4) |
|--------------------|---------------|---------------|---------------|
| Bearer Capability | | | |
|--------------------|---------------|---------------|---------------|
| BB Bearer Class | A | X | VP | A | X | VP^| A | X | VP^|
|--------------------|---------------|----|-----|----|----|-----|----|
| ATM Transfer | | | 4,5,| | | 4,5,| |
| Capability (note 1)| 7 | abs| or 6| 5 | abs| or 6| 5 |
|--------------------|---------------|---------------|---------------|
| Traffic Descriptor | | | |
| for a given dir. | | | |
|--------------------|---------------|---------------|---------------|
| PCR (CLP=0) | | | S |
|--------------------|---------------|---------------|---------------|
| PCR (CLP=0+1) | S | S | S |
|--------------------|---------------|---------------|---------------|
| SCR, MBS (CLP=0) | | | |
|--------------------|---------------|---------------|---------------|
| SCR, MBS (CLP=0+1) | | | |
|--------------------|---------------|---------------|---------------|
| Best Effort | | | |
|--------------------|---------------|---------------|---------------|
| Tagging | N | N | Y/N |
|--------------------|---------------|---------------|---------------|
| Frame Discard | Y/N | Y/N | Y/N |
|--------------------|---------------|---------------|---------------|
| QoS Classes | * | * | * |
|--------------------|---------------|---------------|---------------|
| Transit Delay | O | O | O |
|--------------------|---------------|---------------|---------------|
| Peak-to-Peak CDV | O | O | O |
|--------------------|---------------|---------------|---------------|
| CLR (CLP=0)~ | | O | O |
|--------------------|---------------|---------------|---------------|
| CLR (CLP=0+1)~ | O | | |
+--------------------------------------------------------------------+

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RFC 2331 IP over ATM Signalling - SIG 4.0 Update April 1998

+--------------------------------------------------------------------+
|ATM Service Category| Real Time VBR |
|--------------------|---------------|---------------|---------------|
| Conformance |VBR.1 (note 10)| VBR.2 | VBR.3 |
|--------------------|---------------|---------------|---------------|
| Bearer Capability | | | |
|--------------------|---------------|---------------|---------------|
| BB Bearer Class | C | X | VP | C | X | VP | C | X | VP |
|--------------------|---------------|----|-----|----|----|-----|----|
| ATM Transfer | | | 1 | | | 1 | |
| Capability | 19 | 9 | or 9| 9 | 9 | or 9| 9 |
|--------------------|---------------|---------------|---------------|
| Traffic Descriptor | | | |
| for a given dir. | | | |
|--------------------|---------------|---------------|---------------|
| PCR (CLP=0) | | | |
|--------------------|---------------|---------------|---------------|
| PCR (CLP=0+1) | S | S | S |
|--------------------|---------------|---------------|---------------|
| SCR, MBS (CLP=0) | | S | S |
|--------------------|---------------|---------------|---------------|
| SCR, MBS (CLP=0+1) | S | | |
|--------------------|---------------|---------------|---------------|
| Best Effort | | | |
|--------------------|---------------|---------------|---------------|
| Tagging | N | N | Y/N |
|--------------------|---------------|---------------|---------------|
| Frame Discard | Y/N | Y/N | Y/N |
|--------------------|---------------|---------------|---------------|
| QoS Classes | * | * | * |
|--------------------|---------------|---------------|---------------|
| Transit Delay(nt.2)| O | O | O |
|--------------------|---------------|---------------|---------------|
| Peak-to-Peak CDV | O | O | O |
|--------------------|---------------|---------------|---------------|
| CLR (CLP=0)~ | | O | O |
|--------------------|---------------|---------------|---------------|
| CLR (CLP=0+1)~ | O | | |
+--------------------------------------------------------------------+

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RFC 2331 IP over ATM Signalling - SIG 4.0 Update April 1998

+--------------------------------------------------------------------+
|ATM Service Category| Real Time VBR |
|--------------------|---------------|---------------|---------------|
| Conformance | (note 4,7) | (note 4,8) | (note 4) |
|--------------------|---------------|---------------|---------------|
| Bearer Capability | | | |
|--------------------|---------------|---------------|---------------|
| BB Bearer Class | X | X | X | C or VP^|
|--------------------|---------------|---------------|-----|---------|
| ATM Transfer | | | | |
| Capability | 1 or 9 | 1 or 9 | 1or9| 9 |
|--------------------|---------------|---------------|---------------|
| Traffic Descriptor | | | |
| for a given dir. | | | |
|--------------------|---------------|---------------|---------------|
| PCR (CLP=0) | S | | |
|--------------------|---------------|---------------|---------------|
| PCR (CLP=0+1) | S | S | S |
|--------------------|---------------|---------------|---------------|
| SCR, MBS (CLP=0) | | | |
|--------------------|---------------|---------------|---------------|
| SCR, MBS (CLP=0+1) | | | S |
|--------------------|---------------|---------------|---------------|
| Best Effort | | | |
|--------------------|---------------|---------------|---------------|
| Tagging | Y/N | N | N |
|--------------------|---------------|---------------|---------------|
| Frame Discard | Y/N | Y/N | Y/N |
|--------------------|---------------|---------------|---------------|
| QoS Classes | * | * | * |
|--------------------|---------------|---------------|---------------|
| Transit Delay(nt.2)| O | O | O |
|--------------------|---------------|---------------|---------------|
| Peak-to-Peak CDV | O | O | O |
|--------------------|---------------|---------------|---------------|
| CLR (CLP=0)~ | O | O | O |
|--------------------|---------------|---------------|---------------|
| CLR (CLP=0+1)~ | | | |
+--------------------------------------------------------------------+

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RFC 2331 IP over ATM Signalling - SIG 4.0 Update April 1998

+--------------------------------------------------------------------+
|ATM Service Category| Non-Real Time VBR |
|--------------------|---------------|---------------|---------------|
| Conformance |VBR.1 (note 10)| VBR.2 | VBR.3 |
|--------------------|---------------|---------------|---------------|
| Bearer Capability | | | |
|--------------------|---------------|---------------|---------------|
| BB Bearer Class | C | X | VP |C | X | VP|C | X | VP|
|--------------------|---------------|--|--------|---|--|--------|---|
| ATM Transfer | | |abs,0,2,|abs| |abs,0,2,|abs|
| Capability | 11 |ab| 8,10 |10 |ab| 8,10 |10 |
|--------------------|---------------|---------------|---------------|
| Traffic Descriptor | | | |
| for a given dir. | | | |
|--------------------|---------------|---------------|---------------|
| PCR (CLP=0) | | | |
|--------------------|---------------|---------------|---------------|
| PCR (CLP=0+1) | S | S | S |
|--------------------|---------------|---------------|---------------|
| SCR, MBS (CLP=0) | | S | S |
|--------------------|---------------|---------------|---------------|
| SCR, MBS (CLP=0+1) | S | | |
|--------------------|---------------|---------------|---------------|
| Best Effort | | | |
|--------------------|---------------|---------------|---------------|
| Tagging | N | N | Y |
|--------------------|---------------|---------------|---------------|
| Frame Discard | Y/N | Y/N | Y/N |
|--------------------|---------------|---------------|---------------|
| QoS Classes | * | * | * |
|--------------------|---------------|---------------|---------------|
| Transit Delay(nt.2)| (note 3) | (note 3) | (note 3) |
|--------------------|---------------|---------------|---------------|
| Peak-to-Peak CDV | | | |
|--------------------|---------------|---------------|---------------|
| CLR (CLP=0)~ | | O | O |
|--------------------|---------------|---------------|---------------|
| CLR (CLP=0+1)~ | O | | |
+--------------------------------------------------------------------+

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RFC 2331 IP over ATM Signalling - SIG 4.0 Update April 1998

+--------------------------------------------------------------------+
|ATM Service Category| Non-Real Time VBR |
|--------------------|---------------|---------------|---------------|
| Conformance | (note 4,7) | (note 4,8) | (note 4) |
|--------------------|---------------|---------------|---------------|
| Bearer Capability | | | |
|--------------------|---------------|---------------|---------------|
| BB Bearer Class | C | X | C | X |C | X |VP^|
|--------------------|-------|-------|-------|-------|--|--------|---|
| ATM Transfer | |abs,0,2| |abs,0,2| |abs,0,2,|abs|
| Capability | abs |8 or 10| |8 or 10|ab| 8 or10 |10 |
|--------------------|---------------|---------------|---------------|
| Traffic Descriptor | | | |
| for a given dir. | | | |
|--------------------|---------------|---------------|---------------|
| PCR (CLP=0) | S | | |
|--------------------|---------------|---------------|---------------|
| PCR (CLP=0+1) | S | S | S |
|--------------------|---------------|---------------|---------------|
| SCR, MBS (CLP=0) | | | |
|--------------------|---------------|---------------|---------------|
| SCR, MBS (CLP=0+1) | | | S |
|--------------------|---------------|---------------|---------------|
| Best Effort | | | |
|--------------------|---------------|---------------|---------------|
| Tagging | Y/N | N | N |
|--------------------|---------------|---------------|---------------|
| Frame Discard | Y/N | Y/N | Y/N |
|--------------------|---------------|---------------|---------------|
| QoS Classes | * | * | * |
|--------------------|---------------|---------------|---------------|
| Transit Delay(nt.2)| (note 3) | (note 3) | (note 3) |
|--------------------|---------------|---------------|---------------|
| Peak-to-Peak CDV | | | |
|--------------------|---------------|---------------|---------------|
| CLR (CLP=0)~ | O | O | O |
|--------------------|---------------|---------------|---------------|
| CLR (CLP=0+1)~ | | | |
+--------------------------------------------------------------------+

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RFC 2331 IP over ATM Signalling - SIG 4.0 Update April 1998

+--------------------------------------------------------------------+
|ATM Service Category| ABR | UBR |
|--------------------|---------------|---------------|---------------|
| Conformance | ABR | UBR.1 | UBR.2 |
|--------------------|---------------|---------------|---------------|
| Bearer Capability | | | |
|--------------------|---------------|---------------|---------------|
| BB Bearer Class | C | X | VP |C | X | VP|C | X | VP|
|--------------------|---------------|--|--------|---|--|--------|---|
| ATM Transfer | | |abs,0,2,|abs| |abs,0,2,|abs|
| Capability | 12 |ab| 8,10 |10 |ab| 8,10 |10 |
|--------------------|---------------|---------------|---------------|
| Traffic Descriptor | | | |
| for a given dir. | | | |
|--------------------|---------------|---------------|---------------|
| PCR (CLP=0) | | | |
|--------------------|---------------|---------------|---------------|
| PCR (CLP=0+1) | S | S | S |
|--------------------|---------------|---------------|---------------|
| SCR, MBS (CLP=0) | | S | S |
|--------------------|---------------|---------------|---------------|
| SCR, MBS (CLP=0+1) | S | | |
|--------------------|---------------|---------------|---------------|
| ABR MCR | (note 6) | | |
|--------------------|---------------|---------------|---------------|
| Best Effort | | S (note 9) | S (note 9) |
|--------------------|---------------|---------------|---------------|
| Tagging | N | N | N |
|--------------------|---------------|---------------|---------------|
| Frame Discard | Y/N | Y/N | Y/N |
|--------------------|---------------|---------------|---------------|
| QoS Classes | 0 | 0 | 0 |
|--------------------|---------------|---------------|---------------|
| Transit Delay(nt.2)| | | |
|--------------------|---------------|---------------|---------------|
| Peak-to-Peak CDV | | | |
|--------------------|---------------|---------------|---------------|
| CLR (CLP=0)~ | | | |
|--------------------|---------------|---------------|---------------|
| CLR (CLP=0+1)~ | | | |
+--------------------------------------------------------------------+

ab, abs = absent.

Y/N = either "Yes" or "No" is allowed.

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RFC 2331 IP over ATM Signalling - SIG 4.0 Update April 1998

O = Optional. May be specified using:

- an additional QoS parameter encoded i the Extended QoS
parameters information element or the end-to-end transit
information element; or,

- objectives implied from the QoS class If an Extended
QoS Parameters IE is not present in the message, then any
value of this parameter is acceptable. If neither the
parameter nor the Extended QoS Parameters IE is present,
then the objective for this parameter is determined from
the QoS class in the QoS Parameter IE.

S = Specified.

(blank) = Unspecified.

* = allowed QoS class values are a network option. Class 0 is
always for alignment with ITU-T.

^ = (note 5).

~ = (note 11).

Note 1 - Values 0,1,2,4,6, and 8 are not used on transmission
but shall be understood on reception.

Note 2 - Maximum end-2-end transit delay objectives may only be
specified for the forward direction.

Note 3 - Maximum end-2-end transit delay objectives may be
specified for the ATM Service Category of Non-real
Time VBR for reasons of backward compatibility with
ITU-T Recommendations.

Note 4 - Included for reasons of backward compatibility with
UNI 3.1and ITU-T Recommendations. With these
conformance definitions, the CLR commitment is only
for the CLP=0 traffic stream.

Note 5 - Included to allow switched virtual paths to use the
UNI 3.1 conformance definitions.

Note 6 - Optional in the user-to-network direction. Specified
in the network-to-user direction.

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RFC 2331 IP over ATM Signalling - SIG 4.0 Update April 1998

Note 7 - This combination should be treated as if the received
PCR (CLP=0) parameter were a SCR (CLP=0) parameter and
a MBS (CLP=0) parameter with a value of 1.

Note 8 - This combination should be treated as if an additional
SCR (CLP=0) parameter were received with the same
value as a PCR (CLP=0+1) parameter and a MBS (CLP=0)
parameter with a value of 1.

Note 9 - The best effort parameter applies to both the forward
and backward directions.

Note 10 - This combination should only be used when the CLR
commitment on CLP=0+1 is required versus CLR
commitment on CLP=0 traffic, since these combinations
are not supported by UNI 3.0/3.1 nor ITU-T Q.2931.

Note 11 - In this table the CLR commitment is shown as two
entries to indicated explicitly whether the CLR
commitment is for the CLP=0 or the CLP=0+1 cells.

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RFC 2331 IP over ATM Signalling - SIG 4.0 Update April 1998

Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
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followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Maher Standards Track [Page 26]