GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1932

Network Working Group R. Cole Request for Comments: 1932 D. Shur Category: Informational AT&T Bell Laboratories

                                                         C. Villamizar
                                                                   ANS
                                                            April 1996
                 IP over ATM: A Framework Document

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
 The discussions of the IP over ATM working group over the last
 several years have produced a diverse set of proposals, some of which
 are no longer under active consideration.  A categorization is
 provided for the purpose of focusing discussion on the various
 proposals for IP over ATM deemed of primary interest by the IP over
 ATM working group.  The intent of this framework is to help clarify
 the differences between proposals and identify common features in
 order to promote convergence to a smaller and more mutually
 compatible set of standards.  In summary, it is hoped that this
 document, in classifying ATM approaches and issues will help to focus
 the IP over ATM working group's direction.

1. Introduction

 The IP over ATM Working Group of the Internet Engineering Task Force
 (IETF) is chartered to develop standards for routing and forwarding
 IP packets over ATM sub-networks.  This document provides a
 classification/taxonomy of IP over ATM options and issues and then
 describes various proposals in these terms.
 The remainder of this memorandum is organized as follows:
 o Section 2 defines several terms relating to networking and
   internetworking.
 o Section 3 discusses the parameters for a taxonomy of the
   different ATM models under discussion.
 o Section 4 discusses the options for low level encapsulation.

Cole, Shur & Villamizar Informational [Page 1] RFC 1932 IP over ATM: A Framework Document April 1996

 o Section 5 discusses tradeoffs between connection oriented and
   connectionless approaches.
 o Section 6 discusses the various means of providing direct
   connections across IP subnet boundaries.
 o Section 7 discusses the proposal to extend IP routing to better
   accommodate direct connections across IP subnet boundaries.
 o Section 8 identifies several prominent IP over ATM proposals that
   have been discussed within the IP over ATM Working Group and
   their relationship to the framework described in this document.
 o Section 9 addresses the relationship between the documents
   developed in the IP over ATM and related working groups and the
   various models discussed.

2. Definitions and Terminology

 We define several terms:
 A Host or End System: A host delivers/receives IP packets to/from
   other systems, but does not relay IP packets.
 A Router or Intermediate System: A router delivers/receives IP
   packets to/from other systems, and relays IP packets among
   systems.
 IP Subnet: In an IP subnet, all members of the subnet are able to
    transmit packets to all other members of the subnet directly,
    without forwarding by intermediate entities.  No two subnet
    members are considered closer in the IP topology than any other.
    From an IP routing and IP forwarding standpoint a subnet is
    atomic, though there may be repeaters, hubs, bridges, or switches
    between the physical interfaces of subnet members.
 Bridged IP Subnet: A bridged IP subnet is one in which two or
    more physically disjoint media are made to appear as a single IP
    subnet.  There are two basic types of bridging, media access
    control (MAC) level, and proxy ARP (see section 6).
 A Broadcast Subnet: A broadcast network supports an arbitrary
    number of hosts and routers and additionally is capable of
    transmitting a single IP packet to all of these systems.
 A Multicast Capable Subnet: A multicast capable subnet supports
   a facility to send a packet which reaches a subset of the
   destinations on the subnet.  Multicast setup may be sender

Cole, Shur & Villamizar Informational [Page 2] RFC 1932 IP over ATM: A Framework Document April 1996

   initiated, or leaf initiated.  ATM UNI 3.0 [4] and UNI 3.1
   support only sender initiated while IP supports leaf initiated
   join.  UNI 4.0 will support leaf initiated join.
 A Non-Broadcast Multiple Access (NBMA) Subnet: An NBMA supports
   an arbitrary number of hosts and routers but does not
   natively support a convenient multi-destination connectionless
   transmission facility, as does a broadcast or multicast capable
   subnetwork.
 An End-to-End path: An end-to-end path consists of two hosts which
    can communicate with one another over an arbitrary number of
    routers and subnets.
 An internetwork: An internetwork (small "i") is the concatenation
    of networks, often of various different media and lower level
    encapsulations, to form an integrated larger network supporting
    communication between any of the hosts on any of the component
    networks.  The Internet (big "I") is a specific well known
    global concatenation of (over 40,000 at the time of writing)
    component networks.
 IP forwarding: IP forwarding is the process of receiving a packet
    and using a very low overhead decision process determining how
    to handle the packet.  The packet may be delivered locally
    (for example, management traffic) or forwarded externally.  For
    traffic that is forwarded externally, the IP forwarding process
    also determines which interface the packet should be sent out on,
    and if necessary, either removes one media layer encapsulation
    and replaces it with another, or modifies certain fields in the
    media layer encapsulation.
 IP routing: IP routing is the exchange of information that takes
    place in order to have available the information necessary to
    make a correct IP forwarding decision.
 IP address resolution: A quasi-static mapping exists between IP
    address on the local IP subnet and media address on the local
    subnet.  This mapping is known as IP address resolution.
    An address resolution protocol (ARP) is a protocol supporting
    address resolution.
 In order to support end-to-end connectivity, two techniques are used.
 One involves allowing direct connectivity across classic IP subnet
 boundaries supported by certain NBMA media, which includes ATM.  The
 other involves IP routing and IP forwarding.  In essence, the former
 technique is extending IP address resolution beyond the boundaries of
 the IP subnet, while the latter is interconnecting IP subnets.

Cole, Shur & Villamizar Informational [Page 3] RFC 1932 IP over ATM: A Framework Document April 1996

 Large internetworks, and in particular the Internet, are unlikely to
 be composed of a single media, or a star topology, with a single
 media at the center.  Within a large network supporting a common
 media, typically any large NBMA such as ATM, IP routing and IP
 forwarding must always be accommodated if the internetwork is larger
 than the NBMA, particularly if there are multiple points of
 interconnection with the NBMA and/or redundant, diverse
 interconnections.
 Routing information exchange in a very large internetwork can be
 quite dynamic due to the high probability that some network elements
 are changing state.  The address resolution space consumption and
 resource consumption due to state change, or maintenance of state
 information is rarely a problem in classic IP subnets.  It can become
 a problem in large bridged networks or in proposals that attempt to
 extend address resolution beyond the IP subnet.  Scaling properties
 of address resolution and routing proposals, with respect to state
 information and state change, must be considered.

3. Parameters Common to IP Over ATM Proposals

 In some discussion of IP over ATM distinctions have made between
 local area networks (LANs), and wide area networks (WANs) that do not
 necessarily hold.  The distinction between a LAN, MAN and WAN is a
 matter of geographic dispersion.  Geographic dispersion affects
 performance due to increased propagation delay.
 LANs are used for network interconnections at the the major Internet
 traffic interconnect sites.  Such LANs have multiple administrative
 authorities, currently exclusively support routers providing transit
 to multihomed internets, currently rely on PVCs and static address
 resolution, and rely heavily on IP routing.  Such a configuration
 differs from the typical LANs used to interconnect computers in
 corporate or campus environments, and emphasizes the point that prior
 characterization of LANs do not necessarily hold.  Similarly, WANs
 such as those under consideration by numerous large IP providers, do
 not conform to prior characterizations of ATM WANs in that they have
 a single administrative authority and a small number of nodes
 aggregating large flows of traffic onto single PVCs and rely on IP
 routers to avoid forming congestion bottlenecks within ATM.
 The following characteristics of the IP over ATM internetwork may be
 independent of geographic dispersion (LAN, MAN, or WAN).
 o The size of the IP over ATM internetwork (number of nodes).
 o The size of ATM IP subnets (LIS) in the ATM Internetwork.

Cole, Shur & Villamizar Informational [Page 4] RFC 1932 IP over ATM: A Framework Document April 1996

 o Single IP subnet vs multiple IP subnet ATM internetworks.
 o Single or multiple administrative authority.
 o Presence of routers providing transit to multihomed internets.
 o The presence or absence of dynamic address resolution.
 o The presence or absence of an IP routing protocol.

IP over ATM should therefore be characterized by:

 o Encapsulations below the IP level.
 o Degree to which a connection oriented lower level is available
   and utilized.
 o Type of address resolution at the IP subnet level (static or
   dynamic).
 o Degree to which address resolution is extended beyond the IP
   subnet boundary.
 o The type of routing (if any) supported above the IP level.

ATM-specific attributes of particular importance include:

 o The different types of services provided by the ATM Adaptation
   Layers (AAL).  These specify the Quality-of-Service, the
   connection-mode, etc.  The models discussed within this document
   assume an underlying connection-oriented service.
 o The type of virtual circuits used, i.e., PVCs versus SVCs.  The
   PVC environment requires the use of either static tables for
   ATM-to-IP address mapping or the use of inverse ARP, while the
   SVC environment requires ARP functionality to be provided.
 o The type of support for multicast services.  If point-to-point
   services only are available, then a server for IP multicast is
   required.  If point-to-multipoint services are available, then
   IP multicast can be supported via meshes of point-to-multipoint
   connections (although use of a server may be necessary due to
   limits on the number of multipoint VCs able to be supported or to
   maintain the leaf initiated join semantics).
 o The presence of logical link identifiers (VPI/VCIs) and the
   various information element (IE) encodings within the ATM SVC
   signaling specification, i.e., the ATM Forum UNI version 3.1.

Cole, Shur & Villamizar Informational [Page 5] RFC 1932 IP over ATM: A Framework Document April 1996

   This allows a VC originator to specify a range of "layer"
   entities as the destination "AAL User".  The AAL specifications
   do not prohibit any particular "layer X" from attaching
   directly to a local AAL service.  Taken together these points
   imply a range of methods for encapsulation of upper layer
   protocols over ATM. For example, while LLC/SNAP encapsulation is
   one approach (the default), it is also possible to bind virtual
   circuits to higher level entities in the TCP/IP protocol stack.
   Some examples of the latter are single VC per protocol binding,
   TULIP, and TUNIC, discussed further in Section 4.
 o The number and type of ATM administrative domains/networks, and
   type of addressing used within an administrative domain/network.
   In particular, in the single domain/network case, all attached
   systems may be safely assumed to be using a single common
   addressing format, while in the multiple domain case, attached
   stations may not all be using the same common format,
   with corresponding implications on address resolution.  (See
   Appendix A for a discussion of some of the issues that arise
   when multiple ATM address formats are used in the same logical
   IP subnet (LIS).) Also security/authentication is much more of a
   concern in the multiple domain case.
 IP over ATM proposals do not universally accept that IP routing over
 an ATM network is required.  Certain proposals rely on the following
 assumptions:
 o The widespread deployment of ATM within premises-based networks,
   private wide-area networks and public networks, and
 o The definition of interfaces, signaling and routing protocols
   among private ATM networks.
 The above assumptions amount to ubiquitous deployment of a seamless
 ATM fabric which serves as the hub of a star topology around which
 all other media is attached.  There has been a great deal of
 discussion over when, if ever, this will be a realistic assumption
 for very large internetworks, such as the Internet.  Advocates of
 such approaches point out that even if these are not relevant to very
 large internetworks such as the Internet, there may be a place for
 such models in smaller internetworks, such as corporate networks.
 The NHRP protocol (Section 8.2), not necessarily specific to ATM,
 would be particularly appropriate for the case of ubiquitous ATM
 deployment.  NHRP supports the establishment of direct connections
 across IP subnets in the ATM domain.  The use of NHRP does not
 require ubiquitous ATM deployment, but currently imposes topology
 constraints to avoid routing loops (see Section 7).  Section 8.2

Cole, Shur & Villamizar Informational [Page 6] RFC 1932 IP over ATM: A Framework Document April 1996

 describes NHRP in greater detail.
 The Peer Model assumes that internetwork layer addresses can be
 mapped onto ATM addresses and vice versa, and that reachability
 information between ATM routing and internetwork layer routing can be
 exchanged.  This approach has limited applicability unless ubiquitous
 deployment of ATM holds.  The peer model is described in Section 8.4.
 The Integrated Model proposes a routing solution supporting an
 exchange of routing information between ATM routing and higher level
 routing.  This provides timely external routing information within
 the ATM routing and provides transit of external routing information
 through the ATM routing between external routing domains.  Such
 proposals may better support a possibly lengthy transition during
 which assumptions of ubiquitous ATM access do not hold.  The
 Integrated Model is described in Section 8.5.
 The Multiprotocol over ATM (MPOA) Sub-Working Group was formed by the
 ATM Forum to provide multiprotocol support over ATM. The MPOA effort
 is at an early stage at the time of this writing.  An MPOA baseline
 document has been drafted, which provides terminology for further
 discussion of the architecture.  This document is available from the
 FTP server ftp.atmforum.com in pub/contributions as the file atm95-
 0824.ps or atm95-0824.txt.

4. Encapsulations and Lower Layer Identification

 Data encapsulation, and the identification of VC endpoints,
 constitute two important issues that are somewhat orthogonal to the
 issues of network topology and routing.  The relationship between
 these two issues is also a potential sources of confusion.  In
 conventional LAN technologies the 'encapsulation' wrapped around a
 packet of data typically defines the (de)multiplexing path within
 source and destination nodes (e.g.  the Ethertype field of an
 Ethernet packet).  Choice of the protocol endpoint within the
 packet's destination node is essentially carried 'in-band'.
 As the multiplexing is pushed towards ATM and away from LLC/SNAP
 mechanism, a greater burden will be placed upon the call setup and
 teardown capacity of the ATM network.  This may result in some
 questions being raised regarding the scalability of these lower level
 multiplexing options.
 With the ATM Forum UNI version 3.1 service the choice of endpoint
 within a destination node is made 'out of band' - during the Call
 Setup phase.  This is quite independent of any in-band encapsulation
 mechanisms that may be in use.  The B-LLI Information Element allows
 Layer 2 or Layer 3 entities to be specified as a VC's endpoint.  When

Cole, Shur & Villamizar Informational [Page 7] RFC 1932 IP over ATM: A Framework Document April 1996

 faced with an incoming SETUP message the Called Party will search
 locally for an AAL User that claims to provide the service of the
 layer specified in the B-LLI.  If one is found then the VC will be
 accepted (assuming other conditions such as QoS requirements are also
 met).
 An obvious approach for IP environments is to simply specify the
 Internet Protocol layer as the VCs endpoint, and place IP packets
 into AAL--SDUs for transmission.  This is termed 'VC multiplexing' or
 'Null Encapsulation', because it involves terminating a VC (through
 an AAL instance) directly on a layer 3 endpoint.  However, this
 approach has limitations in environments that need to support
 multiple layer 3 protocols between the same two ATM level endpoints.
 Each pair of layer 3 protocol entities that wish to exchange packets
 require their own VC.

Cole, Shur & Villamizar Informational [Page 8] RFC 1932 IP over ATM: A Framework Document April 1996

 RFC-1483 [6] notes that VC multiplexing is possible, but focuses on
 describing an alternative termed 'LLC/SNAP Encapsulation'.  This
 allows any set of protocols that may be uniquely identified by an
 LLC/SNAP header to be multiplexed onto a single VC. Figure 1 shows
 how this works for IP packets - the first 3 bytes indicate that the
 payload is a Routed Non-ISO PDU, and the Organizationally Unique
 Identifier (OUI) of 0x00-00-00 indicates that the Protocol Identifier
 (PID) is derived from the EtherType associated with IP packets
 (0x800).  ARP packets are multiplexed onto a VC by using a PID of
 0x806 instead of 0x800.
                                             .---------------.
                                             :               :
                                             :   IP Packet   :
                                             :               :
                                              ---------------
                                               :           :
                                               :           :
               8 byte header                   V           V
    .-------------.-------------.------------.---------------.
    :             :             :            :               :
    :             :             :            : Encapsulated  :
    : 0xAA-AA-03  :  0x00-00-00 :   0x08-00  :    Payload    :
    :             :             :            :               :
     -------------^-------------^------------^---------------
     :                                     :   :           :
     :   (LLC)         (OUI)         (PID) :   :           :
     V                                     V   V           V
   .----------------------------------------------------------.
   :                                                          :
   :                          AAL SDU                         :
   :                                                          :
    ----------------------------------------------------------
          Figure 1:  IP packet encapsulated in an AAL5 SDU

Cole, Shur & Villamizar Informational [Page 9] RFC 1932 IP over ATM: A Framework Document April 1996

    .----------.     .----------.    .---------.     .----------.
    :          :     :          :    :         :     :          :
    :    IP    :     :   ARP    :    :AppleTalk:     :   etc... :
    :          :     :          :    :         :     :          :
     ----------       ----------      ---------       ----------
       ^    :           ^    :         ^     :          ^     :
       :    :           :    :         :     :          :     :
       :    V           :    V         :     V          :     V
    .-----------------------------------------------------------.
    :                                                           :
    :  0x800             0x806          0x809            other  :
    :                                                           :
    :         Instance of layer using LLC/SNAP header to        :
    :            perform multiplexing/demultiplexing            :
    :                                                           :
     -----------------------------------------------------------
                             ^  :
                             :  :
                             :  V
                      .------------------.
                      :                  :
                      : Instance of AAL5 :
                      :    terminating   :
                      :      one VCC     :
                      :                  :
                       ------------------
      Figure 2: LLC/SNAP encapsulation allows more than just
                         IP or ARP per VC.
 Whatever layer terminates a VC carrying LLC/SNAP encapsulated traffic
 must know how to parse the AAL--SDUs in order to retrieve the
 packets.  The recently approved signalling standards for IP over ATM
 are more explicit, noting that the default SETUP message used to
 establish IP over ATM VCs must carry a B-LLI specifying an ISO 8802/2
 Layer 2 (LLC) entity as each VCs endpoint.  More significantly, there
 is no information carried within the SETUP message about the identity
 of the layer 3 protocol that originated the request - until the
 packets begin arriving the terminating LLC entity cannot know which
 one or more higher layers are packet destinations.
 Taken together, this means that hosts require a protocol entity to
 register with the host's local UNI 3.1 management layer as being an
 LLC entity, and this same entity must know how to handle and generate
 LLC/SNAP encapsulated packets.  The LLC entity will also require
 mechanisms for attaching to higher layer protocols such as IP and
 ARP.  Figure 2 attempts to show this, and also highlights the fact
 that such an LLC entity might support many more than just IP and ARP.

Cole, Shur & Villamizar Informational [Page 10] RFC 1932 IP over ATM: A Framework Document April 1996

 In fact the combination of RFC 1483 LLC/SNAP encapsulation, LLC
 entities terminating VCs, and suitable choice of LLC/SNAP values, can
 go a long way towards providing an integrated approach to building
 multiprotocol networks over ATM.
 The processes of actually establishing AAL Users, and identifying
 them to the local UNI 3.1 management layers, are still undefined and
 are likely to be very dependent on operating system environments.
 Two encapsulations have been discussed within the IP over ATM working
 group which differ from those given in RFC-1483 [6].  These have the
 characteristic of largely or totally eliminating IP header overhead.
 These models were discussed in the July 1993 IETF meeting in
 Amsterdam, but have not been fully defined by the working group.
 TULIP and TUNIC assume single hop reachability between IP entities.
 Following name resolution, address resolution, and SVC signaling, an
 implicit binding is established between entities in the two hosts.
 In this case full IP headers (and in particular source and
 destination addresses) are not required in each data packet.
 o The first model is "TCP and UDP over Lightweight IP" (TULIP)
   in which only the IP protocol field is carried in each packet,
   everything else being bound at call set-up time.  In this
   case the implicit binding is between the IP entities in each
   host.  Since there is no further routing problem once the binding
   is established, since AAL5 can indicate packet size, since
   fragmentation cannot occur, and since ATM signaling will handle
   exception conditions, the absence of all other IP header fields
   and of ICMP should not be an issue.  Entry to TULIP mode would
   occur as the last stage in SVC signaling, by a simple extension
   to the encapsulation negotiation described in RFC-1755 [10].
   TULIP changes nothing in the abstract architecture of the IP
   model, since each host or router still has an IP address which is
   resolved to an ATM address.  It simply uses the point-to-point
   property of VCs to allow the elimination of some per-packet
   overhead.  The use of TULIP could in principle be negotiated on a
   per-SVC basis or configured on a per-PVC basis.
 o The second model is "TCP and UDP over a Nonexistent IP
   Connection" (TUNIC). In this case no network-layer information
   is carried in each packet, everything being bound at virtual
   circuit set-up time.  The implicit binding is between two
   applications using either TCP or UDP directly over AAL5 on a
   dedicated VC.  If this can be achieved, the IP protocol field has
   no useful dynamic function.  However, in order to achieve binding
   between two applications, the use of a well-known port number

Cole, Shur & Villamizar Informational [Page 11] RFC 1932 IP over ATM: A Framework Document April 1996

   in classical IP or in TULIP mode may be necessary during call
   set-up.  This is a subject for further study and would require
   significant extensions to the use of SVC signaling described in
   RFC-1755 [10].
  Encapsulation   In setup message            Demultiplexing
  -------------+--------------------------+------------------------
  SNAP/LLC     _ nothing                  _ source and destination
               _                          _ address, protocol
               _                          _ family, protocol, ports
               _                          _
  NULL encaps  _ protocol family          _ source and destination
               _                          _ address, protocol, ports
               _                          _
  TULIP        _ source and destination   _ protocol, ports
               _ address, protocol family _
               _                          _
  TUNIC - A    _ source and destination   _ ports
               _ address, protocol family _
               _ protocol                 _
               _                          _
  TUNIC - B    _ source and destination   _ nothing
               _ address, protocol family _
               _ protocol, ports          _
              Table 1:  Summary of Encapsulation Types

TULIP/TUNIC can be presented as being on one end of a continuum opposite the SNAP/LLC encapsulation, with various forms of null encapsulation somewhere in the middle. The continuum is simply a matter of how much is moved from in-stream demultiplexing to call setup demultiplexing. The various encapsulation types are presented in Table 1.

Encapsulations such as TULIP and TUNIC make assumptions with regard to the desirability to support connection oriented flow. The tradeoffs between connection oriented and connectionless are discussed in Section 5.

Cole, Shur & Villamizar Informational [Page 12] RFC 1932 IP over ATM: A Framework Document April 1996

5. Connection Oriented and Connectionless Tradeoffs

The connection oriented and connectionless approaches each offer advantages and disadvantages. In the past, strong advocates of pure connection oriented and pure connectionless architectures have argued intensely. IP over ATM does not need to be purely connectionless or purely connection oriented.

  APPLICATION       Pure Connection Oriented Approach
  ----------------+-------------------------------------------------
  General         _ Always set up a VC
                  _
  Short Duration  _ Set up a VC.  Either hold the packet during VC
  UDP (DNS)       _ setup or drop it and await a retransmission.
                  _ Teardown on a timer basis.
                  _
  Short Duration  _ Set up a VC.  Either hold packet(s) during VC
  TCP (SMTP)      _ setup or drop them and await retransmission.
                  _ Teardown on detection of FIN-ACK or on a timer
                  _ basis.
                  _
  Elastic (TCP)   _ Set up a VC same as above.  No clear method to
  Bulk Transfer   _ set QoS parameters has emerged.
                  _
  Real Time       _ Set up a VC.  QoS parameters are assumed to
  (audio, video)  _ precede traffic in RSVP or be carried in some
                  _ form within the traffic itself.
    Table 2: Connection Oriented vs. Connectionless - a) a pure
                    connection oriented approach

ATM with basic AAL 5 service is connection oriented. The IP layer above ATM is connectionless. On top of IP much of the traffic is supported by TCP, a reliable end-to-end connection oriented protocol. A fundamental question is to what degree is it beneficial to map different flows above IP into separate connections below IP. There is a broad spectrum of opinion on this.

As stated in section 4, at one end of the spectrum, IP would remain highly connectionless and set up single VCs between routers which are adjacent on an IP subnet and for which there was active traffic flow. All traffic between the such routers would be multiplexed on a single ATM VC. At the other end of the spectrum, a separate ATM VC would be created for each identifiable flow. For every unique TCP or UDP address and port pair encountered a new VC would be required. Part of the intensity of early arguments has been over failure to recognize that there is a middle ground.

Cole, Shur & Villamizar Informational [Page 13] RFC 1932 IP over ATM: A Framework Document April 1996

ATM offers QoS and traffic management capabilities that are well suited for certain types of services. It may be advantageous to use separate ATM VC for such services. Other IP services such as DNS, are ill suited for connection oriented delivery, due to their normal very short duration (typically one packet in each direction). Short duration transactions, even many using TCP, may also be poorly suited for a connection oriented model due to setup and state overhead. ATM QoS and traffic management capabilities may be poorly suited for elastic traffic.

  APPLICATION       Middle Ground
  ----------------+-------------------------------------------------
  General         _ Use RSVP or other indication which clearly
                  _ indicate a VC is needed and what QoS parameters
                  _ are appropriate.
                  _
  Short Duration  _ Forward hop by hop.  RSVP is unlikely to precede
  UDP (DNS)       _ this type of traffic.
                  _
  Short Duration  _ Forward hop by hop unless RSVP indicates
  TCP (SMTP)      _ otherwise.  RSVP is unlikely to precede this
                  _ type of traffic.
                  _
  Elastic (TCP)   _ By default hop by hop forwarding is used.
  Bulk Transfer   _ However, RSVP information, local configuration
                  _ about TCP port number usage, or a locally
                  _ implemented method for passing QoS information
                  _ from the application to the IP/ATM driver may
                  _ allow/suggest the establishment of direct VCs.
                  _
  Real Time       _ Forward hop by hop unless RSVP indicates
  (audio, video)  _ otherwise.  RSVP will indicate QoS requirements.
                  _ It is assumed RSVP will generally be used for
                  _ this case.  A local decision can be made as to
                  _ whether the QoS is better served by a separate
                  _ VC.

Table 3: Connection Oriented vs. Connectionless - b) a middle ground

                              approach

Cole, Shur & Villamizar Informational [Page 14] RFC 1932 IP over ATM: A Framework Document April 1996

  APPLICATION       Pure Connectionless Approach
  ----------------+-------------------------------------------------
  General         _ Always forward hop by hop.  Use queueing
                  _ algorithms implemented at the IP layer to
                  _ support reservations such as those specified by
                  _ RSVP.
                  _
  Short Duration  _ Forward hop by hop.
  UDP (DNS)       _
                  _
  Short Duration  _ Forward hop by hop.
  TCP (SMTP)      _
                  _
  Elastic (TCP)   _ Forward hop by hop.  Assume ability of TCP to
  Bulk Transfer   _ share bandwidth (within a VBR VC) works as well
                  _ or better than ATM traffic management.
                  _
  Real Time       _ Forward hop by hop.  Assume that queueing
  (audio, video)  _ algorithms at the IP level can be designed to
                  _ work with sufficiently good performance
                  _ (e.g., due to support for predictive
                  _ reservation).
    Table 4: Connection Oriented vs.  Connectionless - c) a pure
                      connectionless approach
 Work in progress is addressing how QoS requirements might be
 expressed and how the local decisions might be made as to whether
 those requirements are best and/or most cost effectively accomplished
 using ATM or IP capabilities.  Table 2, Table 3, and Table 4 describe
 typical treatment of various types of traffic using a pure connection
 oriented approach, middle ground approach, and pure connectionless
 approach.
 The above qualitative description of connection oriented vs
 connectionless service serve only as examples to illustrate differing
 approaches.  Work in the area of an integrated service model, QoS and
 resource reservation are related to but outside the scope of the IP
 over ATM Work Group.  This work falls under the Integrated Services
 Work Group (int-serv) and Reservation Protocol Work Group (rsvp), and
 will ultimately determine when direct connections will be
 established.  The IP over ATM Work Group can make more rapid progress
 if concentrating solely on how direct connections are established.

Cole, Shur & Villamizar Informational [Page 15] RFC 1932 IP over ATM: A Framework Document April 1996

6. Crossing IP Subnet Boundaries

 A single IP subnet will not scale well to a large size.  Techniques
 which extend the size of an IP subnet in other media include MAC
 layer bridging, and proxy ARP bridging.
 MAC layer bridging alone does not scale well.  Protocols such as ARP
 rely on the media broadcast to exchange address resolution
 information.  Most bridges improve scaling characteristics by
 capturing ARP packets and retaining the content, and distributing the
 information among bridging peers.  The ARP information gathered from
 ARP replies is broadcast only where explicit ARP requests are made.
 This technique is known as proxy ARP.
 Proxy ARP bridging improves scaling by reducing broadcast traffic,
 but still suffers scaling problems.  If the bridged IP subnet is part
 of a larger internetwork, a routing protocol is required to indicate
 what destinations are beyond the IP subnet unless a statically
 configured default route is used.  A default route is only applicable
 to a very simple topology with respect to the larger internet and
 creates a single point of failure.  Because internets of enormous
 size create scaling problems for routing protocols, the component
 networks of such large internets are often partitioned into areas,
 autonomous systems or routing domains, and routing confederacies.
 The scaling limits of the simple IP subnet require a large network to
 be partitioned into smaller IP subnets.  For NBMA media like ATM,
 there are advantages to creating direct connections across the entire
 underlying NBMA network.  This leads to the need to create direct
 connections across IP subnet boundaries.

Cole, Shur & Villamizar Informational [Page 16] RFC 1932 IP over ATM: A Framework Document April 1996

                              .----------.
                     ---------<  Non-ATM :
        .-------.   /       /-<  Subnet  >-\
        :Sub-ES >--/        :  ----------  :
         -------            :              :
                            :              :
                         .--^---.       .--^---.
                         :Router:       :Router:
                          -v-v--         -v-v--
                           : :            : :
                .--------. : : .--------. : : .--------.
    .-------.   :        >-/ \-<        >-/ \-<        :   .-------.
    :Sub-ES :---: Subnet :-----: Subnet :-----: Subnet :---:Sub-ES :
     -------    :        :     :        :     :        :    -------
                 --------       ---v----       --------
                                   :
                                .--^----.
                                :Sub-ES :
                                 -------
  Figure 3: A configuration with both ATM-based and non-ATM based
                              subnets.
 For example, figure 3 shows an end-to-end configuration consisting of
 four components, three of which are ATM technology based, while the
 fourth is a standard IP subnet based on non-ATM technology.  End-
 systems (either hosts or routers) attached to the ATM-based networks
 may communicate either using the Classical IP model or directly via
 ATM (subject to policy constraints).  Such nodes may communicate
 directly at the IP level without necessarily needing an intermediate
 router, even if end-systems do not share a common IP-level network
 prefix.  Communication with end-systems on the non-ATM-based
 Classical IP subnet takes place via a router, following the Classical
 IP model (see Section 8.1 below).
 Many of the problems and issues associated with creating such direct
 connections across subnet boundaries were originally being addressed
 in the IETF's IPLPDN working group and the IP over ATM working group.
 This area is now being addressed in the Routing over Large Clouds
 working group.  Examples of work performed in the IPLPDN working
 group include short-cut routing (proposed by P. Tsuchiya) and
 directed ARP RFC-1433 [5] over SMDS networks.  The ROLC working group
 has produced the distributed ARP server architectures and the NBMA
 Address Resolution Protocol (NARP) [7].  The Next Hop Resolution
 Protocol (NHRP) is still work in progress, though the ROLC WG is
 considering advancing the current document.  Questions/issues
 specifically related to defining a capability to cross IP subnet
 boundaries include:

Cole, Shur & Villamizar Informational [Page 17] RFC 1932 IP over ATM: A Framework Document April 1996

 o How can routing be optimized across multiple logical IP subnets
   over both a common ATM based and a non-ATM based infrastructure.
   For example, in Figure 3, there are two gateways/routers between
   the non-ATM subnet and the ATM subnets.  The optimal path
   from end-systems on any ATM-based subnet to the non ATM-based
   subnet is a function of the routing state information of the two
   routers.
 o How to incorporate policy routing constraints.
 o What is the proper coupling between routing and address
   resolution particularly with respect to off-subnet communication.
 o What are the local procedures to be followed by hosts and
   routers.
 o Routing between hosts not sharing a common IP-level (or L3)
   network prefix, but able to be directly connected at the NBMA
   media level.
 o Defining the details for an efficient address resolution
   architecture including defining the procedures to be followed by
   clients and servers (see RFC-1433 [5], RFC-1735 [7] and NHRP).
 o How to identify the need for and accommodate special purpose SVCs
   for control or routing and high bandwidth data transfers.
 For ATM (unlike other NBMA media), an additional complexity in
 supporting IP routing over these ATM internets lies in the
 multiplicity of address formats in UNI 3.0 [4].  NSAP modeled address
 formats only are supported on "private ATM" networks, while either 1)
 E.164 only, 2) NSAP modeled formats only, or 3) both are supported on
 "public ATM" networks.  Further, while both the E.164 and NSAP
 modeled address formats are to be considered as network points of
 attachment, it seems that E.164 only networks are to be considered as
 subordinate to "private networks", in some sense.  This leads to some
 confusion in defining an ARP mechanism in supporting all combinations
 of end-to-end scenarios (refer to the discussion in Appendix A on the
 possible scenarios to be supported by ARP).

7. Extensions to IP Routing

 RFC-1620 [3] describes the problems and issues associated with direct
 connections across IP subnet boundaries in greater detail, as well as
 possible solution approaches.  The ROLC WG has identified persistent
 routing loop problems that can occur if protocols which lose
 information critical to path vector routing protocol loop suppression
 are used to accomplish direct connections across IP subnet

Cole, Shur & Villamizar Informational [Page 18] RFC 1932 IP over ATM: A Framework Document April 1996

 boundaries.
 The problems may arise when a destination network which is not on the
 NBMA network is reachable via different routers attached to the NBMA
 network.  This problem occurs with proposals that attempt to carry
 reachability information, but do not carry full path attributes (for
 path vector routing) needed for inter-AS path suppression, or full
 metrics (for distance vector or link state routing even if path
 vector routing is not used) for intra-AS routing.
 For example, the NHRP protocol may be used to support the
 establishment of direct connections across subnetwork boundaries.
 NHRP assumes that routers do run routing protocols (intra and/or
 inter domain) and/or static routing.  NHRP further assumes that
 forwarding tables constructed by these protocols result in a steady
 state loop-free forwarding.  Note that these two assumptions do not
 impose any additional requirements on routers, beyond what is
 required in the absence of NHRP.
 NHRP runs in addition to routing protocols, and provides the
 information that allows the elimination of multiple IP hops (the
 multiple IP hops result from the forwarding tables constructed by the
 routing protocols) when traversing an NBMA network.  The IPATM and
 ROLC WGs have both expended considerable effort in discussing and
 coming to understand these limitations.
 It is well-known that truncating path information in Path Vector
 protocols (e.g., BGP) or losing metric information in Distance Vector
 protocols (e.g., RIP) could result in persistent forwarding loops.
 These loops could occur without ATM and without NHRP.
 The combination of NHRP and static routing alone cannot be used in
 some topologies where some of the destinations are served by multiple
 routers on the NBMA. The combination of NHRP and an intra-AS routing
 protocol that does not carry inter-AS routing path attributes alone
 cannot be used in some topologies in which the NBMA will provide
 inter-AS transit connectivity to destinations from other AS served by
 multiple routers on the NBMA.
 Figure 4 provides an example of the routing loops that may be formed
 in these circumstances.  The example illustrates how the use of NHRP
 in the environment where forwarding loops could exist even without
 NHRP (due to either truncated path information or loss of metric
 information) would still produce forwarding loops.
 There are many potential scenarios for routing loops.  An example is
 given in Figure 4.  It is possible to produce a simpler example where
 a loop can form.  The example in Figure 4 illustrates a loop which

Cole, Shur & Villamizar Informational [Page 19] RFC 1932 IP over ATM: A Framework Document April 1996

 will persist even if the protocol on the NBMA supports redirects or
 can invalidate any route which changes in any way, but does not
 support the communication of full metrics or path attributes.
  .----.    .----.
  : H1 >----< S1 :         Notes:
   ----      vvvv        H#n == host #n
             / : \        R#n == router #n
            /  :  \        S#n == subnet #n
    /------/   :   \
    :          :    \        S2 to R3 breaks
 .--^---.   .----. .-^--.
 :      :   : R4 : : R6 :
 : NBMA :    --v-   --v-      See the text for
 :      :      :      :       details of the
  -v--v-       =      =       looping conditions
   :   \       = SLOW =       and mechanisms
   :  .-^--.   = LINK =
   :  : R2 :   =      =
   :   --v-    :      :
   :     :  .--^-. .--^-.
 .-^--.  :  : R5 : : R7 :
 : R8 :  :   --v-   --v-
  --v-    \    :      :
    :      \  /       :
     \    .-^^-.   .--^-.
      \   : S2 :   : S4 :
       \   --v-     --v-
        \     \      /
         \     \    /
          \    .^--^.
           \   : R3 :    path before the break is
            \   -v--    H1->S1->R1->NBMA->R2->S2->R3->H2
             \  /
   .----.   .-^^-.    path after the break is
   : H2 >---< S3 :    H1->S1->R1->NBMA->R2->S2->R5->R4->S1
    ----     ----         \------<--the-loop--<-------/
    Figure 4:  A Routing Loop Due to Lost PV Routing Attributes.
 In the example in Figure 4, Host 1 is sending traffic toward Host 2.
 In practice, host routes would not be used, so the destination for
 the purpose of routing would be Subnet 3.  The traffic travels by way
 of Router 1 which establishes a "cut-through" SVC to the NBMA next-
 hop, shown here as Router 2.  Router 2 forwards traffic destined for
 Subnet 3 through Subnet 2 to Router 3.  Traffic from Host 1 would
 then reach Host 2.

Cole, Shur & Villamizar Informational [Page 20] RFC 1932 IP over ATM: A Framework Document April 1996

 Router 1's cut-through routing implementation caches an association
 between Host 2's IP address (or more likely all of Subnet 3) and
 Router 2's NBMA address.  While the cut-through SVC is still up, Link
 1 fails.  Router 5 loses it's preferred route through Router 3 and
 must direct traffic in the other direction.  Router 2 loses a route
 through Router 3, but picks up an alternate route through Router 5.
 Router 1 is still directing traffic toward Router 2 and advertising a
 means of reaching Subnet 3 to Subnet 1.  Router 5 and Router 2 will
 see a route, creating a loop.
 This loop would not form if path information normally carried by
 interdomain routing protocols such as BGP and IDRP were retained
 across the NBMA. Router 2 would reject the initial route from Router
 5 due to the path information.  When Router 2 declares the route to
 Subnet 3 unreachable, Router 1 withdraws the route from routing at
 Subnet 1, leaving the route through Router 4, which would then reach
 Router 5, and would reach Router 2 through both Router 1 and Router
 5.  Similarly, a link state protocol would not form such a loop.
 Two proposals for breaking this form of routing loop have been
 discussed.  Redirect in this example would have no effect, since
 Router 2 still has a route, just has different path attributes.  A
 second proposal is that is that when a route changes in any way, the
 advertising NBMA cut-through router invalidates the advertisement for
 some time period.  This is similar to the notion of Poison Reverse in
 distance vector routing protocols.  In this example, Router 2 would
 eventually readvertise a route since a route through Router 6 exists.
 When Router 1 discovers this route, it will advertise it to Subnet 1
 and form the loop.  Without path information, Router 1 cannot
 distinguish between a loop and restoration of normal service through
 the link L1.
 The loop in Figure 4 can be prevented by configuring Router 4 or
 Router 5 to refuse to use the reverse path.  This would break backup
 connectivity through Router 8 if L1 and L3 failed.  The loop can also
 be broken by configuring Router 2 to refuse to use the path through
 Router 5 unless it could not reach the NBMA. Special configuration of
 Router 2 would work as long as Router 2 was not distanced from Router
 3 and Router 5 by additional subnets such that it could not determine
 which path was in use.  If Subnet 1 is in a different AS or RD than
 Subnet 2 or Subnet 4, then the decision at Router 2 could be based on
 path information.

Cole, Shur & Villamizar Informational [Page 21] RFC 1932 IP over ATM: A Framework Document April 1996

                      .--------.    .--------.
                      : Router :    : Router :
                       --v-v---      ---v-v--
                         : :            : :
 .--------.   .--------. : : .--------. : : .--------.   .--------.
 : Sub-ES :---: Subnet :-/ \-: Subnet :-/ \-: Subnet :---: Sub-ES :
  --------     --------       --------       --------     --------

Figure 5: The Classical IP model as a concatenation of three separate

                          ATM IP subnets.
 In order for loops to be prevented by special configuration at the
 NBMA border router, that router would need to know all paths that
 could lead back to the NBMA. The same argument that special
 configuration could overcome loss of path information was posed in
 favor of retaining the use of the EGP protocol defined in the now
 historic RFC-904 [11].  This turned out to be unmanageable, with
 routing problems occurring when topology was changed elsewhere.

8. IP Over ATM Proposals

8.1 The Classical IP Model

 The Classical IP Model was suggested at the Spring 1993 IETF meeting
 [8] and retains the classical IP subnet architecture.  This model
 simply consists of cascading instances of IP subnets with IP-level
 (or L3) routers at IP subnet borders.  An example realization of this
 model consists of a concatenation of three IP subnets.  This is shown
 in Figure 5.  Forwarding IP packets over this Classical IP model is
 straight forward using already well established routing techniques
 and protocols.
 SVC-based ATM IP subnets are simplified in that they:
 o limit the number of hosts which must be directly connected at any
   given time to those that may actually exchange traffic.
 o The ATM network is capable of setting up connections between
   any pair of hosts.  Consistent with the standard IP routing
   algorithm [2] connectivity to the "outside" world is achieved
   only through a router, which may provide firewall functionality
   if so desired.
 o The IP subnet supports an efficient mechanism for address
   resolution.
 Issues addressed by the IP Over ATM Working Group, and some of the
 resolutions, for this model are:

Cole, Shur & Villamizar Informational [Page 22] RFC 1932 IP over ATM: A Framework Document April 1996

 o Methods of encapsulation and multiplexing.  This issue is
   addressed in RFC-1483 [6], in which two methods of encapsulation
   are defined, an LLC/SNAP and a per-VC multiplexing option.
 o The definition of an address resolution server (defined in
   RFC-1577).
 o Defining the default MTU size.  This issue is addressed in
   RFC-1626 [1] which proposes the use of the MTU discovery
   protocol (RFC-1191 [9]).
 o Support for IP multicasting.  In the summer of 1994, work began
   on the issue of supporting IP multicasting over the SVC LATM
   model.  The proposal for IP multicasting is currently defined by
   a set of IP over ATM WG Works in Progress, referred to collectively
   as the IPMC documents.  In order to support IP multicasting the
   ATM subnet must either support point-to- multipoint SVCs, or
   multicast servers, or both.
 o Defining interim SVC parameters, such as QoS parameters and
   time-out values.
 o Signaling and negotiations of parameters such as MTU size
   and method of encapsulation.  RFC-1755 [10] describes an
   implementation agreement for routers signaling the ATM network
   to establish SVCs initially based upon the ATM Forum's UNI
   version 3.0 specification [4], and eventually to be based
   upon the ATM Forum's UNI version 3.1 and later specifications.
   Topics addressed in RFC-1755 include (but are not limited to)
   VC management procedures, e.g., when to time-out SVCs, QOS
   parameters, service classes, explicit setup message formats for
   various encapsulation methods, node (host or router) to node
   negotiations, etc.
 RFC-1577 is also applicable to PVC-based subnets.  Full mesh PVC
 connectivity is required.
 For more information see RFC-1577 [8].

8.2 The ROLC NHRP Model

 The Next Hop Resolution Protocol (NHRP), currently a work in progress
 defined by the Routing Over Large Clouds Working Group (ROLC),
 performs address resolution to accomplish direct connections across
 IP subnet boundaries.  NHRP can supplement RFC-1577 ARP. There has
 been recent discussion of replacing RFC-1577 ARP with NHRP. NHRP can
 also perform a proxy address resolution to provide the address of the
 border router serving a destination off of the NBMA which is only

Cole, Shur & Villamizar Informational [Page 23] RFC 1932 IP over ATM: A Framework Document April 1996

 served by a single router on the NBMA. NHRP as currently defined
 cannot be used in this way to support addresses learned from routers
 for which the same destinations may be heard at other routers,
 without the risk of creating persistent routing loops.

8.3 "Conventional" Model

 The "Conventional Model" assumes that a router can relay IP packets
 cell by cell, with the VPI/VCI identifying a flow between adjacent
 routers rather than a flow between a pair of nodes.  A latency
 advantage can be provided if cell interleaving from multiple IP
 packets is allowed.  Interleaving frames within the same VCI requires
 an ATM AAL such as AAL3/4 rather than AAL5.  Cell forwarding is
 accomplished through a higher level mapping, above the ATM VCI layer.
 The conventional model is not under consideration by the IP/ATM WG.
 The COLIP WG has been formed to develop protocols based on the
 conventional model.

8.4 The Peer Model

 The Peer Model places IP routers/gateways on an addressing peer basis
 with corresponding entities in an ATM cloud (where the ATM cloud may
 consist of a set of ATM networks, inter-connected via UNI or P-NNI
 interfaces).  ATM network entities and the attached IP hosts or
 routers exchange call routing information on a peer basis by
 algorithmically mapping IP addressing into the NSAP space.  Within
 the ATM cloud, ATM network level addressing (NSAP-style), call
 routing and packet formats are used.
 In the Peer Model no provision is made for selection of primary path
 and use of alternate paths in the event of primary path failure in
 reaching multihomed non-ATM destinations.  This will limit the
 topologies for which the peer model alone is applicable to only those
 topologies in which non-ATM networks are singly homed, or where loss
 of backup connectivity is not an issue.  The Peer Model may be used
 to avoid the need for an address resolution protocol and in a proxy-
 ARP mode for stub networks, in conjunction with other mechanisms
 suitable to handle multihomed destinations.
 During the discussions of the IP over ATM working group, it was felt
 that the problems with the end-to-end peer model were much harder
 than any other model, and had more unresolved technical issues.
 While encouraging interested individuals/companies to research this
 area, it was not an initial priority of the working group to address
 these issues.  The ATM Forum Network Layer Multiprotocol Working
 Group has reached a similar conclusion.

Cole, Shur & Villamizar Informational [Page 24] RFC 1932 IP over ATM: A Framework Document April 1996

8.5 The PNNI and the Integrated Models

 The Integrated model (proposed and under study within the
 Multiprotocol group of ATM Forum) considers a single routing protocol
 to be used for both IP and for ATM. A single routing information
 exchange is used to distribute topological information.  The routing
 computation used to calculate routes for IP will take into account
 the topology, including link and node characteristics, of both the IP
 and ATM networks and calculates an optimal route for IP packets over
 the combined topology.
 The PNNI is a hierarchical link state routing protocol with multiple
 link metrics providing various available QoS parameters given current
 loading.  Call route selection takes into account QoS requirements.
 Hysteresis is built into link metric readvertisements in order to
 avoid computational overload and topological hierarchy serves to
 subdivide and summarize complex topologies, helping to bound
 computational requirements.
 Integrated Routing is a proposal to use PNNI routing as an IP routing
 protocol.  There are several sets of technical issues that need to be
 addressed, including the interaction of multiple routing protocols,
 adaptation of PNNI to broadcast media, support for NHRP, and others.
 These are being investigated.  However, the ATM Forum MPOA group is
 not currently performing this investigation.  Concerned individuals
 are, with an expectation of bringing the work to the ATM Forum and
 the IETF.
 PNNI has provisions for carrying uninterpreted information.  While
 not yet defined, a compatible extension of the base PNNI could be
 used to carry external routing attributes and avoid the routing loop
 problems described in Section 7.

Cole, Shur & Villamizar Informational [Page 25] RFC 1932 IP over ATM: A Framework Document April 1996

             ++++++++++++++++++++++++++++++++++++++++++
             +   .------------.      .------------.   +
 .---------. + .-:            :-.  .-:            :-. +
 : Host or >-+-< : Single ATM : >--< : Single ATM : >-+-----\
 : Router  : + : :   Domain   : :  : :   Domain   : : +     :
  ---------  +  -:            :-    -:            :-  + .---^----.
             +    ------------        ------------    + : Router :
             +                       .------------.   +  ---v----
 .---------. +                     .-:            :-. +     :
 : Host or >-+- ...          ... --< : Single ATM : >-+-----/
 : Router  : +                     : :   Domain   : : +
  ---------  +  ATM Cloud           -:            :-  +
             +                        ------------    +
             ++++++++++++++++++++++++++++++++++++++++++
                Note: IS within ATM cloud are ATM IS
Figure 6: The ATM transition model assuming the presence of gateways
   or routers between the ATM networks and the ATM peer networks.

8.6 Transition Models

 Finally, it is useful to consider transition models, lying somewhere
 between the Classical IP Models and the Peer and Integrated Models.
 Some possible architectures for transition models have been suggested
 by Fong Liaw.  Others are possible, for example Figure 6 showing a
 Classical IP transition model which assumes the presence of gateways
 between ATM networks and ATM Peer networks.
 Some of the models described in the prior sections, most notably the
 Integrated Model, anticipate the need for mixed environment with
 complex routing topologies.  These inherently support transition
 (possibly with an indefinite transition period).  Models which
 provide no transition support are primarily of interest to new
 deployments which make exclusive, or near exclusive use of ATM or
 deployments capable of wholesale replacement of existing networks or
 willing to retain only non-ATM stub networks.
 For some models, most notably the Peer Model, the ability to attach
 to a large non-ATM or mixed internetwork is infeasible without
 routing support at a higher level, or at best may pose
 interconnection topology constraints (for example: single point of
 attachment and a static default route).  If a particular model
 requires routing support at a higher level a large deployment will
 need to be subdivided to provide scalability at the higher level,
 which for some models degenerates back to the Classical model.

Cole, Shur & Villamizar Informational [Page 26] RFC 1932 IP over ATM: A Framework Document April 1996

9. Application of the Working Group's and Related Documents

 The IP Over ATM Working Group has generated several Works in Progress
 and RFCs.  This section identifies the relationship of these and
 other related documents to the various IP Over ATM Models identified
 in this document.  The documents and RFCs produced to date are the
 following references, RFC-1483 [6], RFC-1577 [8], RFC-1626 [1], RFC-
 1755 [10] and the IPMC documents.  The ROLC WG has produced the NHRP
 document.  Table 5 gives a summary of these documents and their
 relationship to the various IP Over ATM Models.

Acknowledgments

 This memo is the direct result of the numerous discussions of the IP
 over ATM Working Group of the Internet Engineering Task Force.  The
 authors also had the benefit of several private discussions with H.
 Nguyen of AT&T Bell Laboratories.  Brian Carpenter of CERN was kind
 enough to contribute the TULIP and TUNIC sections to this memo.
 Grenville Armitage of Bellcore was kind enough to contribute the
 sections on VC binding, encapsulations and the use of B-LLI
 information elements to signal such bindings.  The text of Appendix A
 was pirated liberally from Anthony Alles' of Cisco posting on the IP
 over ATM discussion list (and modified at the authors' discretion).
 M. Ohta provided a description of the Conventional Model (again which
 the authors modified at their discretion).  This memo also has
 benefitted from numerous suggestions from John T. Amenyo of ANS, Joel
 Halpern of Newbridge, and Andy Malis of Ascom-Timplex.  Yakov Rekhter
 of Cisco provided valuable comments leading to the clarification of
 normal loop free NHRP operation and the potential for routing loop
 problems only with the improper use of NHRP.
  Documents         Summary
  ----------------+-------------------------------------------------
  RFC-1483        _ How to identify/label multiple
                  _ packet/frame-based protocols multiplexed over
                  _ ATM AAL5. Applies to any model dealing with IP
                  _ over ATM AAL5.
                  _
  RFC-1577        _ Model for transporting IP and ARP over ATM AAL5
                  _ in an IP subnet where all nodes share a common
                  _ IP network prefix.  Includes ARP server/Inv-ARP
                  _ packet formats and procedures for SVC/PVC
                  _ subnets.
                  _
  RFC-1626        _ Specifies default IP MTU size to be used with
                  _ ATM AAL5. Requires use of PATH MTU discovery.
                  _ Applies to any model dealing with IP over ATM
                  _ AAL5

Cole, Shur & Villamizar Informational [Page 27] RFC 1932 IP over ATM: A Framework Document April 1996

                  _
  RFC-1755        _ Defines how implementations of IP over ATM
                  _ should use ATM call control signaling
                  _ procedures, and recommends values of mandatory
                  _ and optional IEs focusing particularly on the
                  _ Classical IP model.
                  _
  IPMC            _ Defines how to support IP multicast in Classical
                  _ IP model using either (or both) meshes of
                  _ point-to-multipoint ATM VCs, or multicast
                  _ server(s).  IPMC is work in progress.
                  _
  NHRP            _ Describes a protocol that can be used by hosts
                  _ and routers to determine the NBMA next hop
                  _ address of a destination in "NBMA
                  _ connectivity"
                  _ of the sending node.  If the destination is not
                  _ connected to the NBMA fabric, the IP and NBMA
                  _ addresses of preferred egress points are
                  _ returned.  NHRP is work in progress (ROLC WG).
                 Table 5:  Summary of WG Documents

References

 [1] Atkinson, R., "Default IP MTU for use over ATM AAL5", RFC 1626,
     Naval Research Laboratory, May 1994.
 [2] Braden, R., and J. Postel, "Requirements for Internet Gateways",
     STD 4, RFC 1009, USC/Information Sciences Institute, June 1987.
 [3] Braden, R., Postel, J., and Y. Rekhter, "Internet Architecture
     Extensions for Shared Media", RFC 1620, USC/Information Sciences
     Institute, IBM Research, May 1994.
 [4] ATM Forum, "ATM User-Network Interface Specification",  Prentice
     Hall, September 1993.
 [5] Garrett, J., Hagan, J., and J. Wong, "Directed ARP", RFC 1433,
     AT&T Bell Labs, University of Pennsylvania, March 1993.
 [6] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
     Layer 5", RFC 1483, Telecom Finland, July 1993.
 [7] Heinanen, J., and R. Govindan, "NBMA Address Resolution Protocol
     (NARP)", RFC 1735, Telecom Finland, USC/Information Sciences
     Institute, December 1994.

Cole, Shur & Villamizar Informational [Page 28] RFC 1932 IP over ATM: A Framework Document April 1996

 [8] Laubach, M., "Classical IP and ARP over ATM", RFC 1577,
     Hewlett-Packard Laboratories, January 1994.
 [9] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
     DECWRL, Stanford University, November 1990.
[10] Perez, M., Liaw, F., Grossman, D., Mankin, A., and A. Hoffman,
     "ATM signalling support for IP over ATM", RFC  1755,
     USC/Information Sciences Institute, FORE Systems, Inc., Motorola
     Codex, Ascom Timeplex, Inc., January 1995.
[11] Mills, D., "Exterior Gateway Protocol Formal Specification",
     STD 18, RFC 904, BBN, April 1984.

A Potential Interworking Scenarios to be Supported by ARP

 The architectural model of the VC routing protocol, being defined by
 the Private Network-to-Network Interface (P-NNI) working group of the
 ATM Forum, categorizes ATM networks into two types:
 o Those that participate in the VC routing protocols and use NSAP
   modeled addresses UNI 3.0 [4] (referred to as private networks,
   for short), and
 o Those that do not participate in the VC routing protocol.
   Typically, but possibly not in all cases, public ATM networks
   that use native mode E.164 addresses UNI 3.0 [4] will fall into
   this later category.
 The issue for ARP, then is to know what information must be returned
 to allow such connectivity.  Consider the following scenarios:
 o Private host to Private Host, no intervening public transit
   network(s): Clearly requires that ARP return only the NSAP
   modeled address format of the end host.
 o Private host to Private host, through intervening public
   networks: In this case, the connection setup from host A to host
   B must transit the public network(s).  This requires that at
   each ingress point to the public network that a routing decision
   be made as to which is the correct egress point from that public
   network to the next hop private ATM switch, and that the native
   E.164 address of that egress point be found (finding this is a VC
   routing problem, probably requiring configuration of the public
   network links and connectivity information).  ARP should return,
   at least, the NSAP address of the endpoint in which case the
   mapping of the NSAP addresses to the E.164 address, as specified
   in [4], is the responsibility of ingress switch to the public

Cole, Shur & Villamizar Informational [Page 29] RFC 1932 IP over ATM: A Framework Document April 1996

   network.
 o Private Network Host to Public Network Host: To get connectivity
   between the public node and the private nodes requires the
   same kind of routing information discussed above - namely, the
   directly attached public network needs to know the (NSAP format)
   ATM address of the private station, and the native E.164 address
   of the egress point from the public network to that private
   network (or to that of an intervening transit private network
   etc.).  There is some argument, that the ARP mechanism could
   return this egress point native E.164 address, but this may
   be considered inconsistent for ARP to return what to some is
   clearly routing information, and to others is required signaling
   information.
 In the opposite direction, the private network node can use, and
 should only get, the E.164 address of the directly attached public
 node.  What format should this information be carried in?  This
 question is clearly answered, by Note 9 of Annex A of UNI 3.0 [4],
 vis:
    "A call originated on a Private UNI destined for an host which
    only has a native (non-NSAP) E.164 address (i.e.  a system
    directly attached to a public network supporting the native E.164
    format) will code the Called Party number information element in
    the (NSAP) E.164 private ATM Address Format, with the RD, AREA,
    and ESI fields set to zero.  The Called Party Subaddress
    information element is not used."
 Hence, in this case, ARP should return the E.164 address of the
 public ATM station in NSAP format.  This is essentially implying an
 algorithmic resolution between the native E.164 and NSAP addresses of
 directly attached public stations.
 o Public network host to Public network host, no intervening
   private network: In this case, clearly the Q.2931 requests would
   use native E.164 address formats.
 o Public network host to Public network host, intervening private
   network: same as the case immediately above, since getting
   to and through the private network is a VC routing, not an
   addressing issue.
 So several issues arise for ARP in supporting arbitrary connections
 between hosts on private and public network.  One is how to
 distinguish between E.164 address and E.164 encoded NSAP modeled
 address.  Another is what is the information to be supplied by ARP,
 e.g., in the public to private scenario should ARP return only the

Cole, Shur & Villamizar Informational [Page 30] RFC 1932 IP over ATM: A Framework Document April 1996

 private NSAP modeled address or both an E.164 address, for a point of
 attachment between the public and private networks, along with the
 private NSAP modeled address.

Authors' Addresses

 Robert G. Cole
 AT&T Bell Laboratories
 101 Crawfords Corner Road, Rm. 3L-533
 Holmdel, NJ 07733
 Phone: (908) 949-1950
 Fax: (908) 949-8887
 EMail: rgc@qsun.att.com
 David H. Shur
 AT&T Bell Laboratories
 101 Crawfords Corner Road, Rm. 1F-338
 Holmdel, NJ 07733
 Phone: (908) 949-6719
 Fax: (908) 949-5775
 EMail: d.shur@att.com
 Curtis Villamizar
 ANS
 100 Clearbrook Road
 Elmsford, NY 10523
 EMail: curtis@ans.net

Cole, Shur & Villamizar Informational [Page 31]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1932.txt · Last modified: 1996/04/05 20:50 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki