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PROPOSED STANDARD

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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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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   [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: maher@isi.edu




































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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.




Maher                       Standards Track                    [Page 15]

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


   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.






Maher                       Standards Track                    [Page 16]

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]).
























Maher                       Standards Track                    [Page 17]

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       |               |               |
+--------------------------------------------------------------------+






Maher                       Standards Track                    [Page 18]

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       |               |               |
+--------------------------------------------------------------------+












Maher                       Standards Track                    [Page 19]

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)~     |               |               |               |
+--------------------------------------------------------------------+












Maher                       Standards Track                    [Page 20]

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       |               |               |
+--------------------------------------------------------------------+












Maher                       Standards Track                    [Page 21]

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)~     |               |               |               |
+--------------------------------------------------------------------+












Maher                       Standards Track                    [Page 22]

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.






Maher                       Standards Track                    [Page 23]

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.






Maher                       Standards Track                    [Page 24]

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.































Maher                       Standards Track                    [Page 25]

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
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   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
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























Maher                       Standards Track                    [Page 26]


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