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rfc:rfc1946

Network Working Group S. Jackowski Request for Comments: 1946 NetManage Incorporated Category: Informational May 1996

                    Native ATM Support for ST2+

Status of This Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.

Abstract

 As the demand for networked realtime services grows, so does the need
 for shared networks to provide deterministic delivery services. Such
 deterministic delivery services demand that both the source
 application and the network infrastructure have capabilities to
 request, setup, and enforce the delivery of the data. Collectively
 these services are referred to as bandwidth reservation and Quality
 of Service (QoS).
 The IETF is currently working on an integrated services model to
 support realtime services on the Internet  The IETF has not yet
 focused on the integration of ATM and its inherent QoS and bandwidth
 allocation mechanisms for delivery of realtime traffic over shared
 wires. (ATM hardware and interfaces provide the network
 infrastructure for the determinitic data delivery, however the host
 resident protocol stacks and applications need more attention.)
 Current IETF efforts underway in the IP over ATM (ipatm) working
 group rely on intserv, rsvp and ST2 to address QoS issues for ATM. As
 such, RFC 1577 and the ATM Forum's Lan Emulation do not provide
 direct QoS and bandwidth allocation capabilities to  network
 applications. Without providing a mapping of reservations-style QoS
 to ATM signalling, ATM will remain a 'wire' rather than a shared
 media infrastructure component.
 This memo describes a working implementation which enables
 applications to directly invoke ATM services in the following
 environments:
  1. ATM to internet,
  2. internet to ATM, and
  3. internet to internet across ATM.

Jackowski Informational [Page 1] RFC 1946 Native ATM Support for ST2+ May 1996

Table of Contents

 1.0     Introduction...............................................2
 2.0     ST-2 and ST-2+.............................................5
 3.0     Implementation Issues for Reservations over ATM............6
 3.1     Addressing.................................................6
 3.2     Changes to Bandwidth and QoS...............................6
 3.3     Multicasting...............................................7
 3.4     Receiver Initiated JOIN Requests to Multicast Groups.......8
 3.5     Computation of QoS Parameters..............................8
 3.6     Use of HELLOs..............................................9
 4.0     Reservation Signalling with ATM............................9
 4.1     Embedded Reservation Signalling within Q.2931.............10
 4.2     In-Band Reservation Signalling............................11
 4.3     Dedicated Virtual Circuits for Reservation Signalling.....12
 4.4     Reservation Signalling via IP over ATM or LAN Emulation...13
 4.5     Summary of Reservation Signalling Options.................14
 5.0     Mapping Reservation QoS to ATM QoS........................15
 5.1     CPCS-SDU Size Computation.................................16
 5.2     PCR Computation...........................................17
 5.3     Maximum End to End Transit Delay..........................17
 5.4     Maximum Bit Error Rate....................................18
 5.5     Accumulated Mean Delay....................................18
 5.6     Accumulated Delay Variance (jitter).......................18
 6.0     Data Stream Transmission..................................18
 7.0     Implementation Considerations and Conclusions.............19
 8.0     Security Considerations...................................20
 9.0     References................................................20
 10.0    Author's Address..........................................21

1.0 Introduction

 The ATM Forum and the IETF seem to approach ATM networking
 differently.
 The ATM forum appeaars to believe that host systems require no
 protocols beyond OSI layer 2 to deal with ATM.  They define a layer 2
 API and Q.2931 signaling for all new applications.
 LAN Emulation, a mechanism to make the ATM interface appear to be a
 LAN/internet, is intended to support 'legacy' network applications.
 LAN emulation does not provide applications any visibility of the ATM
 features, nor does it provide a mechanism to allow applications to
 request specific ATM services. With LAN Emulation, application
 traffic shares virtual circuits with no policing or guarantees of
 service. LAN Emulation simply extends LAN characteristics to ATM.

Jackowski Informational [Page 2] RFC 1946 Native ATM Support for ST2+ May 1996

 Thus far, the IETF, through  RFC 1577[1]  treats an ATM network as a
 wire.  The ipatm working group has explicitly left issues of specific
 QoS handling out of their specifications and working documents.
 Current approaches do not give the application access to individual
 virtualcircuits and their associated guaranteed bandwidth and QoS.
 Instead, all IP traffic between two hosts shares virtual circuits
 with no granularity assigned to application-specific traffic or QoS
 requirements.
 Thus, neither LAN Emulation nor RFC 1577 (IP over ATM) uses the
 features of ATM that make it a unique and desirable technology.  RFC
 1821 (Integration of Realtime Services in an IP-ATM Network
 Architecture) [2] raises many of the issues associated with current
 IETF efforts towards integrating ATM into the Internet, but it does
 not propose any solutions.
 This document offers a  framework for provision of native ATM
 circuits for applications which require bandwidth guarantees and QoS.
 It identifies  the requirements of  a native ATM protocol which is
 complementary to standard IP and describes one working
 implementation.
 This document recognizes  the fact that it is critical that such a
 native ATM  protocol  is consistent in the four topologies described
 in [2]:
  • Communication across an ATM-only network between two hosts

directly connected to the ATM network,

  • Communication between ATM connected hosts which involves some

non-ATM subnets,

  • Communication between a host on a non-ATM subnet and a host

directly connected to ATM,

  • Communication between two hosts, neither of which has a direct

ATM connection, but which may make use of one or more ATM

         networks for some part of the path.
 That is, to the host systems, the underlying type of network remains
 transparent even when QoS is involved in internet, ATM, and mixed
 networking environments.  To make this consistency possible, the
 'native ATM' protocol must also be:
  • Multicast capable, to optimize transmission overhead and

support ATM multipoint facilities,

  • Routable, to enable transmissions across subnets and

internets,

  • QoS knowledgeable, to take advantage of ATM QoS facilities,
  • Capable of Bandwidth/QoS Reservation to allocate proper

facilities for application traffic as it travels across

Jackowski Informational [Page 3] RFC 1946 Native ATM Support for ST2+ May 1996

         different types of networks: to effectively extend virtual
         circuits across internets, and
 *       Capable of policing to ensure proper packet scheduling
         behavior and to protect guaranteed services at merge points.
 Clearly the protocol should support reservations.  Reservation
 protocols enable creation of  'virtual circuits'  with guaranteed
 bandwidth and QoS on the LAN or internet, and simultaneously can act
 as signaling mechanisms to routers or ATM interfaces to request
 provisioning of circuits. Use of a reservation protocol makes
 characteristics of  mixed networks (LANs, internet, ATM, ISDN)
 transparent to the host systems.   That is, a reservation will allow
 the host or router to provision ATM circuits which match the
 reservation, but in mixed networks, will allow routers and host to
 provide bandwidth reservation and QoS across the non-ATM interfaces
 as well.  Effectively, the reservation maps ATM virtual circuits to
 reservations on subnets and internets.
 This creates a consistent End-to-End, QoS-guaranteed service for
 mixed network topologies.
 While it is beyond the scope of this document, the same requirements
 apply to mixed ISDN networks and are currently being explored by the
 ITU for their H.323, H.223, and T.123 standards.
 Arguably, the reservation protocol that provides this end-to-end
 guaranteed service should be connection-oriented to facilitate
 mapping of real connections (ATM or ISDN) with virtual connections on
 the LAN/internet.  [2] points out the shortcomings of IP and RSVP [3]
 in the ATM environment. Most notable among these are the difficulty
 of mapping connectionless traffic to ATM connections, the constant
 softstate refreshes of RSVP (and merging of RESV messages), the
 receiver orientation of  RSVP, and the dependence on IP multicast.
 [6] is an excellent document that proposes solutions to many of the
 issues raised in [2], but the solutions recommend modifications to
 the current RSVP and ATM implementations.  Recently, issues of
 incompatibility with the current IP over ATM model, VC explosions due
 to use of multicast groups and VC explosions due to features
 associated with heterogeneous receivers suggest that the current
 version of RSVP may be inappropriate for ATM implementations.
 Since ATM is connection-oriented, hard state, and origin-oriented for
 transmission, signaling, and multicast, and is bandwidth and QoS
 knowledgeable, perhaps the simplest and most elegant approach to a
 native protocol for ATM would include a protocol that shares these
 characteristics.

Jackowski Informational [Page 4] RFC 1946 Native ATM Support for ST2+ May 1996

 In surveying protocols described in IETF RFCs and Internet Drafts,
 only two seem to meet these requirements: Experimental Internet
 Stream Protocol: Version 2 (RFC 1190) [4] and Internet STream
 Protocol Version 2+ (RFC 1819) [5]; ST2 and ST2+ respectively.

2.0 ST2 and ST2+

 Both ST2 and ST2+ have been given the Internet Protocol Version 5
 (IPv5) designation.  In fact, ST2+ is an updated version of ST2.
 Both protocols are origin-oriented reservation and multicast
 protocols that provide bandwidth and QoS guarantees through
 internets.  Unlike IPv4 or IPv6, ST2 and ST2+ are connection-
 oriented, subscribing to the philosophy that once a connection is
 established, protocol and routing overhead can be substantially
 reduced.  This carries forward to QoS and Bandwidth Reservation as
 well, simplifying the implementation of QoS guarantees. THESE
 PROTOCOLS WERE INTENDED TO COMPLEMENT STANDARD CONNECTIONLESS IP,
 RECOGNIZING THAT WHILE MOST INTERNET TRAFFIC BENEFITS FROM
 CONNECTIONLESS NETWORKING, PERFORMANCE AND QoS GUARANTEES COULD BE
 ACHIEVED MOST EASILY WITH INTERNET CONNECTIONS.
 Both ST2 and ST2+ really consist of two protocols: SCMP and ST.  SCMP
 is analogous to ICMP in that it is the control and signaling
 protocol, while ST is the low-overhead streaming protocol.   ST-2
 uses standard IP addresses during connection setup, but then reduces
 header overhead by including a stream identifier in each data packet.
 ST2+ includes simplification of many of the original ST2 features as
 well as clarification of the ST2 specification.  Among these
 simplifications and clarifications are:
 1) Much simpler connection setup.
 2) Flow Specification independence and consolidation of experimental
    Flow Specifications.
 3) Clarification on the implementation of Groups of Streams.
 4) Clarification of leaf-initiated JOINs in multicast trees (several
    ST2 implementations had done this).
 While there continues to be a  dramatic increase in the use of ST2
 for videoconferencing, video on demand, telemetry applications and
 networked virtual reality, ST2+  has no commercial implementations
 and is not yet supported by any router vendors.  This is because ST2+
 was released as an RFC late in the summer of 1995.  It is expected
 that several implementations will appear over the coming months.  As
 such, the approach described in this document applies to both
 protocols, and, in fact, would be valid for any other similar
 protocol used to establish 'native' ATM circuits.  Since ST2 and ST2+
 are so similar, this document will refer to  'the ST2 protocols'

Jackowski Informational [Page 5] RFC 1946 Native ATM Support for ST2+ May 1996

 generically in describing an implementation approach to both.  Where
 particular features of ST2+ are required or affect implementation,
 'ST2+ ' will be used specifically.

3.0 Implementation Issues for Reservations over ATM

 As described above, ST is a connection-oriented, hard state, origin-
 oriented multicast protocol and thus maps fairly well to ATM.
 However, ST-2 has several features that may be difficult to support
 in the current version of ATM signaling with Q.2931 and UNI 3.1.
 Among these are:
 1) Addressing.
 2) Changes to Bandwidth and QoS.
 3) Multicasting.
 4) Receiver initiated JOINs to multicast groups.
 5) Computation of certain QoS parameters.
 6) Use of HELLOs.
 The degree of difficulty in supporting these functions is dependent
 on the signaling mechanism chosen.  See Section 4 for descriptions of
 possible signaling approaches and their respective impact on the
 features listed above.

3.1 Addressing

 Of course mapping an Internet address to ATM address is always
 problematic.  It would be possible to set up a well known ARP server
 to resolve the IP addresses of targets.  However, the widespread
 deployment of IP over ATM and LAN emulation in host-based ATM
 drivers, and the assumption that most host systems will be running
 some  IP applications that do not need specific QoS and bandwidth
 provisioning, suggests that  use of ARP facilities provided by IP
 over ATM and LAN Emulation  is the most obvious choice for address
 resolution.
 It should be noted that ATMARP returns the ATM address.  For some
 implementations (particularly kernel-based protocols), an NSAP
 address is also required.  Since these addresses are often difficult
 to get from the ATM network itself in advance of the connection, it
 may be necessary to invoke out-of-band signaling mechanisms to pass
 this address, or it may be better to create an NSAP address server.

3.2 Changes to Bandwidth and QoS

 Both ST-2 and ST-2+ allow the origin to dynamically change the QoS
 and Bandwidth of a particular stream.  At this time Q.2931 and UNI
 3.1 do not support this feature. Until this capability is available,

Jackowski Informational [Page 6] RFC 1946 Native ATM Support for ST2+ May 1996

 full support of the SCMP CHANGE message for dedicated ATM circuits
 (one reservation = one ATM circuit) can only be implemented  by
 tearing down the existing VC for a stream and establishing a new one
 if efficient use of ATM resources are to be preserved.
 Of course, the CHANGE message can simply be passed across the ATM
 virtual circuit to the hosts or routers. This would allow the hosts
 to relax resource requirements locally, and permit routers to relax
 access to downstream circuits, but the ATM VC itself, would still
 retain excessive bandwidth.
 In addition, if the implementation allows sharing of virtual circuits
 by multiple streams, the bandwidth/QoS of individual streams within
 the VC can be CHANGEd.

3.3 Multicasting

 ST-2 and ST-2+ support origin-oriented multicasting.  That is, the
 origin of a stream explicitly specifies the addresses of the targets
 it wants involved in the connection.  In addition, the origin can Add
 or drop targets as desired.  Aside from receiver-initiated JOINs
 (discussed in section 3.4), there is a one to one mapping between
 ST-2 multicast and ATM multipoint connections.  Origin-initiated
 additions can be accomplished through an ADDPARTY, and drops can be
 done through DROPPARTY.
 A key goal in implementation of a native ATM protocol is to ensure
 consistent implementation for unicast and multicast data transfers.
 One difficulty in doing this with ATM Virtual Circuits is the fact
 that point-to-point circuits are duplex, while multipoint circuits
 are simplex.  This means that for multicast connections to be mapped
 to multipoint ATM Virtual Circuits, any two-way, end-to-end signaling
 must be done out of band.  An alternative is to  let the local
 reservation agent act as a split/merge point for the connection by
 establishing point-to-point Virtual Circuits for each member of the
 multicast group directly connected to the ATM network.  For multicast
 group members not directly connected to the ATM network, traffic can
 be multicast to the router connected at the edge across a single
 virtual circuit associated with the reservation.
 Section 4 describes alternative mechanisms for implementing
 signaling.
 Included in each discussion is the optimal means for mapping
 multicast to ATM  point-to-point or multipoint circuits.
 Note that the fact that ST-2 does not rely on IP multicast is a
 strong advantage in implementation of a native protocol for ATM.  The

Jackowski Informational [Page 7] RFC 1946 Native ATM Support for ST2+ May 1996

 one-to-one mapping of ST-2 multicast connections to ATM multipoint
 virtual circuits minimizes the number of circuits required to support
 large multicast groups.

3.4 Receiver Initiated JOINs to Multicast Groups

 ST-2+ provides an in-band mechanism to permit receivers to join an
 existing stream.  Based on an origin-established authorization level,
 the JOIN can be refused immediately, can be allowed with notification
 of the origin, or can be allowed without notifying the origin.  This
 capability is made available through a new SCMP JOIN message.  If the
 receiver knows the IP address of the origin and the Stream ID, he can
 join the stream if authorized to do so.
 Note that since the JOIN flows from the receiver to the origin, there
 will be issues in trying to  support this feature with Q.2931 and UNI
 3.1. The JOIN may have to be sent out of band depending on the
 signaling mechanism chosen (section 4) because of the uni-directional
 flow for point to multipoint ATM connections.  This is supposed to
 change with availability of UNI 4.0.
 ST-2 did not support receiver initiated JOINs (unlike ST-2+).
 However, most implementations created an out-of-band, or SCMP
 extension to support this facility.  Again, depending on the SCMP
 signaling mechanism chosen, this feature may be difficult to support.

3.5 Computation of QoS Parameters

 The recommended flow specifications (flowspecs) for ST-2 and ST-2+
 include parameters that are not currently available to ATM virtual
 circuits through Q.2931 and  UNI 3.1.  The mapping of packet rate to
 cell rate,  packet delay to cell delay, and other translatable QoS
 parameters is described in section 5.  However,  the ST-2 flowspecs
 also include parameters like accumulated end-to-end delay and
 accumulated jitter.  These parameters assume that the SCMP messages
 follow the same path as the data.  Depending on the signaling
 mechanism chosen, this may not be true with ATM and thus certain QoS
 parameters may be rendered useless.
 It should also be noted that since ST-2 connections are simplex, all
 QoS parameters are specified separately for each direction of data
 transfer.  Thus two connections and two QoS negotiations are required
 for a duplex connection.  To take advantage of the full duplex nature
 of point-to-point ATM connections, special multiplexing of ST
 connections would be required by ST-2 agents.

Jackowski Informational [Page 8] RFC 1946 Native ATM Support for ST2+ May 1996

3.6 Use of HELLOs

 Both ST-2 and ST-2+ support HELLO messages.  HELLOs are intended to
 assure that the neighboring agent is alive.  Failure to respond to a
 HELLO indicates that the connection is down and that the reservation
 for that particular link should be freed.
 While the ATM network will notify an ST-2 agent if the network
 connection is down, there is still the possibility that the
 connection is intact but that the ST-2 agent itself is down.
 Knowledge of the neighboring agent's status is increasingly important
 when multiple ST-2 connections share virtual circuits, when the
 neighboring agents are routers, and when there are multiple dedicated
 virtual circuits between agents.
 As such, HELLO is a desirable feature.  Note that some signaling
 schemes (section 4), provide less than optimal support for HELLO.

4.0 Reservation Signaling with ATM

 Use of Permanent Virtual Circuits (PVCs) for reservation signaling
 presents no problem for ST-2, ST-2+, or RSVP.  Each circuit is
 considered to be a dedicated link to the next hop.  If the PVCs are
 to be shared, reservation protocols can divide and regulate the
 bandwidth just as they would with any other link type.
 Where ATM connections become more interesting is when the ATM network
 takes on the role of an extended LAN or internet.  To do this,
 Switched Virtual Circuits are used to establish dynamic connections
 to various endpoints and routers.  The ITU-TS Q.2931 SETUP message is
 used to request a connection from the network with specific bandwidth
 and QoS requirements, and a CONNECT message is received by the origin
 to indicate that connection establishment is complete.
 For IP over ATM and LAN Emulation, SVCs are established between
 endpoints and data traffic for a given destination shares the SVCs.
 There is no mechanism to allow specific QoS guarantees for the
 traffic, nor is there a mechanism to set up virtual circuits with
 specific bandwidth and QoS for a particular type of traffic.  This is
 what reservation protocols will attempt to do.  The goal is to use
 reservations to request establishment of individual virtual circuits
 with matching bandwidth and QoS for each reservation.  This will
 guarantee the requirements of the application while taking full
 advantage of the ATM network's capabilities.
 There are four possible mechanisms to perform reservation signaling
 over ATM:

Jackowski Informational [Page 9] RFC 1946 Native ATM Support for ST2+ May 1996

 1) Embedding  reservation signaling equivalents within the ATM Q.2931
    controls.
 2) Signaling in-band with the data.
 3) Signaling over dedicated signaling VCs.
 4) Implicitly sharing existing VCs for IP over ATM or LAN Emulation.
 Note that ATM circuits are not necessarily reliable.  As such, the
 reliability mechanisms provided by SCMP must be maintained to assure
 delivery of all reservation signaling messages.

4.1 Embedded Reservation Signaling Equivalents within ATM Q.2931

  Controls
 The basic idea in embedding reservation signaling within the ATM
 controls is to use the Q.2931 SETUP and CONNECT messages to establish
 both reservations and dedicated data paths (virtual circuits) across
 the ATM network.  This eliminates the need for dedicated signaling
 channels, in-band signaling, or out of band mechanisms to communicate
 between endpoints.  Since SETUP and CONNECT include bandwidth and QoS
 information, the basic concept is sound.  In fact, this approach will
 speed network connection by preventing multiple passes at
 establishing a reservation and associated connection.  This normally
 results from the fact that most higher layer protocols (network and
 transport) first require a link to signal their connection
 requirements.  As such,  with ATM, the ATM virtual circuit must be
 established before the network  and/or transport protocols can do
 their own signaling.
 Embedded reservation signaling allows the reservation information to
 be carried in the SETUP and CONNECT messages, allowing the
 reservation protocol to do its signaling simultaneously with the ATM
 signaling.
 [7] describes a clever way of combining the reservation signaling
 with the ATM control plane signaling for ST-2.  This 'simultaneous
 connection establishment' process will optimize the establishment of
 circuits and minimize connection setup time while simultaneously
 eliminating unnecessary network layer signaling in ST-2.  To be
 effective, [7] requires enhancements to Q.2931 signaling and to the
 ST-2 protocol implementations.  In addition, it currently only
 applies to point-to-point connections and will not work with
 multipoint largely due to the simplex nature of multipoint
 communication in current ATM implementations.
 Implementation of multicast for Embedded Reservation Signaling is
 done as described above: the reservation agent at the edge of the ATM
 network must create point-to-point virtual circuits for each target
 that is directly connected to the ATM network, and for each router

Jackowski Informational [Page 10] RFC 1946 Native ATM Support for ST2+ May 1996

 that supports downstream targets.  This ensures two-way signaling
 between targets and the origin.
 Signaling itself is quite simple:
      CONNECT maps directly to one or more (multicast) Q.2931
              SETUPs and CONNECTs.
      ACCEPT maps directly to Q.2931 CONNECTACK.
      CHANGE/CHANGE REQUEST are  not supported.
      DISCONNECT maps directly to Q.2931 RELEASE.
      HELLOs are not needed.
 Unfortunately, the flowspec in the reservation protocol CONNECT
 message cannot be passed across the ATM network in the signaling
 messages and thus must be regenerated by the receiving agent.
 In addition, User Data, which can be sent in most SCMP messages
 cannot be supported without substantial changes to current Q.2931
 signaling.
 One of the additional complexities with embedding the reservation
 signaling occurs in heterogeneous networks.  Since ATM signaling only
 operates point to point across the ATM network itself, if the
 endpoints reside on other types of networks or subnets, the routers
 at the edge of the ATM networks must generate and regenerate
 endpoint-based signaling messages on behalf of the host reservation
 agents.  In particular, CONNECT and ACCEPT messages and their
 associated flowspecs must be regenerated.  Refer to Section 5 for
 details on the QoS mappings and on which QoS parameters can be
 recreated for the generated flowspecs.
 This approach is worth revisiting as an optimal signaling method in
 pure ATM network environments once ATM signaling capabilities expand.
 However, for heterogeneous networks,  other signaling mechanisms may
 be more appropriate.

4.2 In-Band Reservation Signaling

 In-Band Reservation Signaling is the easiest signaling mechanism to
 implement.  When the applications requests a reservation, the
 reservation agent simply sets up ATM virtual circuits to the
 endpoints with the   QoS specified in the CONNECT request.  When
 ACCEPTed, all subsequent data transmissions proceed  on the virtual
 circuits.
 Once again, to support multicast, the reservation agent must create
 individual point-to-point virtual circuits to the targets which are

Jackowski Informational [Page 11] RFC 1946 Native ATM Support for ST2+ May 1996

 directly connected to the ATM network, as well as to routers which
 can access downstream targets.
 Since signaling is done in-band, all reservation signaling messages
 can be passed between agents.  However, some minimal additional
 bandwidth must be allocated in the Q.2931 SETUP to allow for the
 signaling messages themselves.
 Note that the primary disadvantage to In-Band Reservation Signaling
 is the fact that it does not make use of  the multipoint capabilities
 of ATM and will thus overreserve ATM network bandwidth and create a
 larger than necessary number of virtual circuits.

4.3 Dedicated Reservation Signaling Virtual Circuits

 One mechanism that can be used to take advantage of the full data
 transmission capabilities of ATM networks is to use Dedicated Virtual
 Circuits for reservation signaling.  This guarantees a two-way
 signaling pipe between the endpoints in a connection while enabling
 the data transmission to take advantage of the multipoint
 capabilities of ATM.  Data and Signaling are done over separate
 virtual circuits.
 When an application requests a reservation, the reservation agent
 reviews the list of targets in the CONNECT request.  For any targets
 which have no current signaling virtual circuits established, the
 agent establishes UBR (unspecified bit rate) virtual circuits and
 forwards the CONNECT message to the targets over these virtual
 circuits. ATMARP is used to resolve any endpoint addresses.  For any
 targets for which there already exist signaling virtual circuits, the
 agent simply forwards the CONNECT message over the existing virtual
 circuit.
 Once an ACCEPT message is received, the agent issues a Q.2931 SETUP
 to the associated target.  Upon receipt of a CONNECTACK, data can
 begin to flow.  As additional ACCEPTs are received, the Q.2931
 ADDPARTY message is used to add a target to the multicast and
 multipoint connection.  Depending on the cause of any ADDPARTY
 failure, the agent may attempt to establish a dedicated point-to-
 point virtual circuit to complete the multicast group.
 DISCONNECT requests result in  Q.2931 DROPPARTY messages and will
 cause a member to be dropped from a multicast and multipoint
 connection.  When all targets are dropped from a multipoint
 connection, a RELEASE can be issued to take down the virtual circuit.
 Signaling virtual circuits are shared among reservations while data
 circuits are dedicated to a particular  reservation.   Once all

Jackowski Informational [Page 12] RFC 1946 Native ATM Support for ST2+ May 1996

 reservations to a given endpoint are terminated, the signaling
 virtual circuit to that endpoint can be RELEASEd.
 Note that this approach  would allow the NSAP address to be passed as
 user data in the ACCEPT message to enable a kernel-based reservation
 protocol to establish the dedicated data circuit.  In addition,
 because the connectivity to the endpoint is identical to that of the
 data circuit, this approach assures the fact that accumulated
 information in the flowspecs retains it validity.

4.4 Reservation Signaling via IP over ATM or LAN Emulation

 As described in the previous section, it would be possible to set up
 unique SVCs for SCMP signaling, however, since the streaming,
 connection-oriented data transport offered by ST-2 is intended to be
 complementary to IP and other connectionless protocol
 implementations, it would be simpler and more elegant to simply use
 classical IP over ATM (RFC 1577) mechanisms, or to use LAN Emulation.
 The widespread deployment of IP over ATM and LAN emulation in host-
 based ATM drivers, and the assumption that most host systems will be
 running applications that do not need specific QoS and bandwidth
 provisioning, makes this the most straightforward (if not performance
 optimal) solution for signaling.  Once an end-to-end acceptance of a
 reservation request is completed via normal LAN or IP transmission,
 then a unique direct virtual circuit can be established for each data
 flow.
 If LAN Emulation is used, as long as the ST-2 implementation allows
 for different paths for SCMP and data, there would be no changes to
 the signaling mechanisms employed by the reservation agent.
 For IP over ATM, all SCMP messages would be encapsulated in IP as
 described in both RFC 1190 and RFC 1819.  This is required because
 current ATM drivers will not accept Ipv5 packets, and most drivers do
 not provide direct access to the shared signaling virtual circuits
 used for IP.
 In either case, LAN Emulation or IP over ATM, the reservation agent
 would handle SCMP messages as it normally does.  However, once the
 first ACCEPT is received for  a reservation request, a dedicated
 virtual circuit is established for the data flow.  Subsequent ACCEPTs
 will result in the use of ADDPARTY to add multicast targets to the
 multipoint virtual circuit.  In fact, processing of
 multipoint/multicast is identical to that described in section 4.3.
 Once again, the use of an out-of-band signaling mechanism makes it
 possible to carry the NSAP address of the target in the ACCEPT
 message.

Jackowski Informational [Page 13] RFC 1946 Native ATM Support for ST2+ May 1996

 One potential drawback to using LAN Emulation or SCMP messages
 encapsulated in IP over ATM, is the fact that there is no guarantee
 that the connectivity achieved to reach the target via signaling has
 any relationship to the data path.  This means that accumulated
 values in the flowspec may be rendered useless.
 In addition, it is possible that the targets will actually  reside
 outside the ATM network.  That is, there may be no direct ATM access
 to the Targets and it may be difficult to identify ATM addresses of
 the associated ATM connected routers.  This approach will involve
 some additional complexity in routing to the targets.  However, since
 ST-2 is intended to run with IP, if ATM vendors would accept IPv5
 packets or would allow direct access to the IP over ATM signaling
 virtual circuits, this approach would be optimal in minimizing the
 number of virtual circuits required.

4.5 Summary of Reservation Signaling Approaches

 Embedded Reservation Signaling (section 4.1) is ideal for homogeneous
 ATM connections, but  requires extensions to existing ATM signaling
 to support multipoint connections.  In-Band Reservation Signaling
 (section 4.2) is the easiest to implement, but cannot employ
 multipoint connections either.
 Perhaps the simplest way to do this is similar to what is suggested
 in [6]: separate the reservation signaling from the actual data
 flows, mapping the data flows directly to ATM circuits while doing
 the signaling separately.
 While there is significant complexity in doing this for IP traffic
 and RSVP, the ST2 protocols lend themselves to this quite well.  In
 fact, because SCMP reservation signaling results in streaming,
 multicast connections, the 'Shortcut' mechanism described in [6],
 which can bypass routers where direct ATM connections are possible,
 is automatically available to ST2 streams.
 Using Reservation Signaling over LAN Emulation or IP over ATM
 (section 4.4) is one multipoint-capable approach  to implement in
 hosts since most ATM drivers shipping today provide both IP over ATM
 and LAN Emulation, as well as associated address resolution
 mechanisms. However, it is not complete in its ability to accurately
 depict flowspec parameters or to resolve host ATM addresses. In
 addition, to be optimal, ATM vendors would either have to support
 IPv5 in their drivers or allow direct access to the IP signaling
 virtual circuits.  Thus the current ideal approach to implementation
 of the ST2 protocols over ATM is to use shared Dedicated Reservation
 Signaling Virtual Circuits (section 4.3) for signaling of
 reservations, and then to establish appropriate multipoint ATM

Jackowski Informational [Page 14] RFC 1946 Native ATM Support for ST2+ May 1996

 virtual circuits for the data flows.

5.0 Mapping of Reservation QoS to ATM QoS

 QoS negotiation in ST-2 (and ST-2+) is done via a two-way
 negotiation.
 The origin proposes a QoS for the connection in a Flow Specification
 (Flowspec) associated with the CONNECT message.  Most of the
 network-significant QoS parameters in the Flowspec include both a
 minimum and a desired value.  Each ST agent along the path to the
 Target validates its ability to provide the specified QoS (at least
 the minimum value for each), updates certain values in the Flowspec,
 and propagates the CONNECT until it reaches the Target.  The Target
 can either ACCEPT the Flowspec or REFUSE it if it cannot meet at
 least the minimum QoS requirements.  Negotiation takes place as part
 of the process in that the Target can specify changes to the desired
 QoS values as long as the new value meets at least the minimum
 requirements specified by the Origin system.  In addition, both the
 Target and the Origin can assess actual network performance by
 reviewing the values that are accumulated along the path.
 The primary Reservation QoS parameters that impact an ATM network
 are:

ST-2 (RFC 1190) ST-2+ (RFC 1819)

Desired PDU Bytes, Desired Message Size, Limit on PDU Bytes (minimum). Limit on Message Size.

Desired PDU Rate, Desired Rate, Limit on PDU Rate (minimum). Limit on Rate. Minimum Transmission Rate in Bytes.

Limit on Delay (maximum). Desired Delay,

                                              Limit on Delay.

Maximum Bit Error Rate.

Accumulated Delay. Accumulated Delay Variance (Jitter).

Q.2931 ATM signaling offers the following QoS parameters:

- Cumulative Transit Delay, - Maximum End to End Transit Delay.

- Forward Peak Cell Rate (PCR), - Backward Peak Cell Rate (PCR).

Jackowski Informational [Page 15] RFC 1946 Native ATM Support for ST2+ May 1996

- Forward Maximum CPCS-SDU size, - Backward Maximum CPCS-SDU size.

- Forward QoS Class, - Backward QoS Class.

- B-LLI (one byte user protocol information).

 As previously noted, reservation protocols (ST and RSVP) make QoS
 reservations in one direction only. Thus, depending on the type of
 signaling used (see Section 4), the 'Backward' ATM parameters may not
 be useful.  In particular, if Multipoint ATM connections are used to
 map multicast reservations, these parameters are not available.
 However, it would be possible to implement a multiplexing scheme to
 enable reservations to share bi-directional point-to-point ATM
 connections if the reservation agent creates a split/merge point at
 the ATM boundary and sets up only point-to-point VC connections to
 targets.
 The CPCS-SDU parameters are AAL Parameters which are used by the AAL
 entity to break packets into cells.  As such, these parameters are
 not modified by the network and could conceivably be used for
 additional end-to-end signaling, along with the B-LLI.
 Finally, QoS Class is somewhat limited in its use and implementation.
 While IP over ATM recommends use of Class 0 (Unspecified QoS), this
 is not sufficient for guaranteed connections.  Instead, Class 1 with
 CLP=0 will provide at least minimum QoS services for the traffic.

5.1 CPCS-SDU Size Computation

 The CPCS-SDU size computation is the easiest QoS mapping.  Since ST-2
 does not require a Service Specific Convergence Sublayer (SSCS), if
 AAL 5 is used, the ST packet size plus 8 bytes  (for the AAL 5
 Trailer) will be the CPCS-SDU size. Note that the ST-2 packet size
 also includes an 8-byte header for ST-2.  Thus the CPCS-SDU size is:
      CPCS-SDUsize = PDUbytes + 8 + 8.
 For ST-2+, the header is larger than for ST-2, so the CPCS-SDU size
 is:
      CPCS-SDUsize = PDUbytes + 12 + 8.

Jackowski Informational [Page 16] RFC 1946 Native ATM Support for ST2+ May 1996

5.2 PCR Computation

 The Peak Cell Rate (PCR) computation is only slightly more complex.
 The PCR will be the peak packet rate divided by the ATM payload size.
 Since PDU rates in ST-2 are specified in tenths of packets per
 second, AAL 5 requires an 8 byte trailer, and the ATM payload size is
 48 bytes, the computation for PCR proceeds as follows:
      The requested maximum byte transmission rate for ST-2 is:
              PDUbytes * PDUrate * 10.
      Accounting for the AAL 5 and ST headers, the maximum byte rate
      is:
              Bytes per second = (PDUbytes + 8 + 8) * PDUrate * 10.
      Translating into cells and  eliminating the possibility of a
      fractional PDU:
              PCR = ((PDUbytes + 8 + 8 + 48) / 48) * PDUrate * 10.
 For ST-2+, not only is the header size 12 bytes, but the Rate is in
 messages per second, not tenths of packets per second.  Thus, the PCR
 for ST-2+ is:
              PCR = ((PDUbytes + 12 + 8 + 48) / 48) * PDUrate.

5.3 Maximum End to End Transit Delay.

 The End to End Transit Delay is a little more complex.   The
 requested end to end delay must account for not only the PDU size as
 requested by the user, but the additional 8-byte AAL 5 header as
 well.  The translation of the user-requested LimitOn Delay is
 preserved as long as the delay computation is based on the  CPCS-SDU
 size instead of the PDU size.
 In addition to the end to end delay introduced by the ATM network,
 there is additional delay created by the fragmentation of packets.
 Reassembly of these packets can only be accomplished at the rate at
 which they are received.  The time (in milliseconds) required to
 receive  a cell (inter-cell arrival time) is:
         T = 1000 / PCR.

Jackowski Informational [Page 17] RFC 1946 Native ATM Support for ST2+ May 1996

 The number of cells in a CPCS-SDU is:
         C = (CPCS-SDUsize + 48) / 48.
 Thus the delay for a packet is:
         LimitonDelay = (C - 1) * T + MaxCellTransitDelay.
 Therefore, the requested Maximum End to End  Transit delay is:
         MaxCellTransitDelay = Limiton Delay - (C-1) * T.

5.4 Maximum Bit Error Rate

 Q.2931 signaling does not offer the ability to directly specify the
 requested bit error rate or a corresponding cell error rate.
 Instead, this service is supposed to be offered through selection of
 QoS class.
 Since these classes have few actual implementations, at this time,
 there is no effective mapping for bit error rate.

5.5 Accumulated Mean Delay

 ST allows accumulation of the Mean Delay generated by each ST agent
 node and intervening circuits.  With an ATM circuit each agent should
 factor in the overhead of the ATM connection.  The delay associated
 with the ATM circuit is reflected in the Q.2931 CONNECT message as
 the Cummulative Transit Delay.  Since this is a cell-based
 computation, the delay experienced for an ST packet, including the
 CPCS-SDU header and ST header is, as computed in Section 5.3:
      Delay = (C - 1) * T + CummulativeTransit Delay.

5.6 Accumulated Delay Variance (Jitter)

 Cell Delay Variance is not currently available as a Q.2931 parameter.
 Thus, we can assume  that the reassembly of cells into packets will
 be consistent, since the cell transmission rate should be constant
 for each packet.  As such, except as noted by the specific ATM
 service, the ST agent should use its standard mechanisms for tracking
 packet arrival times and use this for Accumulated Delay Variance.

6.0 Data Stream Transmission

 Once virtual circuits for data transmission are established though
 one of the mechanisms described in section 4, the ST data must be

Jackowski Informational [Page 18] RFC 1946 Native ATM Support for ST2+ May 1996

 transmitted over the connection.  RFC 1483 describes mechanisms for
 encapsulating packet transmissions over AAL5.  While the LLC
 encapsulation could be used, it is not necessary.  If it is used, the
 computations in section 5 should be redone to include the LLC headers
 in addition to the AAL5 trailer currently used.  These new values
 should be substituted for the QoS values in the SETUP message.
 Instead, ST data packets can be encapsulated in standard AAL5 format
 with an 8 byte trailer and sent directly over the data virtual
 circuit.   The mechanisms for computing the QoS values in the SETUP
 message are described in section 5.

7.0 Implementation Experience and Conclusions

 All of the signaling mechanisms described in Section 4 were
 implemented and tested in a mixed ATM network/routed LAN environment.
 Initially it appeared that the best approach was to do signaling via
 IP over ATM or LANE.  However, because it required IP encapsulation
 of the SCMP packets (for IP over ATM), and because some applications
 use the accumulated values in the flowspecs (which are not guaranteed
 to be accurate in LANE and IP/ATM), using virtual circuits dedicated
 to SCMP signaling  turned out to be the best implementation for
 taking full advantage of the ATM features.
 Also, the issue of mapping ATM address to E.164 NSAP addresses was
 resolved through an external signaling mechanism (the User Data field
 of the ST-2 CONNECT and ACCEPT messages).  It appears that ATM
 vendors need to implement a consistent addressing mechanism
 throughout their interfaces.
 From a performance point of view, using ST over ATM provided more
 than triple the performance of raw IP.  The differences became
 increasingly clear as more simultaneous applications were run.  This
 resulted in dedicated virtual circuits for the ST traffic while the
 IP traffic suffered (saw inconsistent performance) over shared
 circuits.  Even more dramatic were results in mixed network
 environments where all traffic shared the same LAN/router
 connections, and, when both IP and ST traffic was sent, the ST
 traffic maintained its quality while the IP traffic saw increasing
 variation in performance.
 Clearly, using a connection-oriented, origin-oriented reservation
 protocol to provide consistent end-to-end guaranteed QoS and
 bandwidth in mixed ATM/internet environments is not only feasible, it
 results in dramatic performance and quality improvements for
 transmission of realtime traffic.

Jackowski Informational [Page 19] RFC 1946 Native ATM Support for ST2+ May 1996

8.0 Security Considerations

 This memo raises no security considerations.  However, with their
 connection-oriented and origin controlled natures, ST-2 and ST-2+
 lend themselves to better internet security.  Discussion of this is
 beyond the scope of this document.

9.0 References

 [1] Laubach, M., "Classical IP and ARP over ATM", RFC 1577, Hewlett
     Packard Laboratories, December, 1993.
 [2] Borden, M., Crawley, E., Davie, B., and S. Batsell, "Integration
     of Real-time Services in an IP-ATM network Architecture", RFC
     1821, August 1995.
 [3] Braden, R., Zhang, L., Estrin, D., Herzog, S., and S. Jamin,
     "Resource ReSerVation Protocol (RSVP Version 1 Functional
     Specification", Work in Progress, November 1995.
 [4] Topolcic, C., "Experimental Internet Stream Protocol: Version 2
     (ST-II)", RFC 1190, October 1990.
 [5] DelGrossi, L., and L. Berger, "Internet STream Protocol Version
     2+", RFC 1819, July 1995.
 [6] V. Firoiu, R. Guerin, D. Kandlur, A. Birman "Provisioning of
     RSVP-based Services over a Large ATM Network', IBM T.J. Watson
     Research Center, October 1995.
 [7] S. Damaskos, A. Anastassios Gavras, "Connection Oriented
     Protocols over ATM: A Case Study", German National Research
     Corporation for Mathematics and Data Processing (GMD) and
     Research Centre for Open Communications Systems (FOKUS), February
     1994.
 [8] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
     Layer 5", RFC 1483, July 1993.
 [9] M. Graf, T. Kober, H. Stuttgen, "ST-II over ATM Implementation
     Issues", IBM European Networking Center, October 1995.

Jackowski Informational [Page 20] RFC 1946 Native ATM Support for ST2+ May 1996

10.0 Author's Address

     Steve Jackowski
     NetManage Incorporated
     269 Mt. Hermon Road, Suite 201
     Scotts Valley, Ca 95066
     Phone:  (408) 439-6834
     Fax:    (408) 438-5115
     EMail:  Stevej@NetManage.com

Jackowski Informational [Page 21]

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