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Network Working Group                                            R. ColeRequest for Comments: 1932                                       D. ShurCategory: Informational                           AT&T Bell Laboratories                                                           C. Villamizar                                                                     ANS                                                              April 1996IP over ATM: A Framework DocumentStatus 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:   oSection 2 defines several terms relating to networking and     internetworking.   oSection 3 discusses the parameters for a taxonomy of the     different ATM models under discussion.   oSection 4 discusses the options for low level encapsulation.Cole, Shur & Villamizar      Informational                      [Page 1]

RFC 1932           IP over ATM: A Framework Document          April 1996   oSection 5 discusses tradeoffs between connection oriented and     connectionless approaches.   oSection 6 discusses the various means of providing direct     connections across IP subnet boundaries.   oSection 7 discusses the proposal to extend IP routing to better     accommodate direct connections across IP subnet boundaries.   oSection 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.   oSection 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 (seesection 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 senderCole, 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 inSection 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.  (SeeAppendix 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 (seeSection 7).Section 8.2Cole, 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 inSection 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 inSection 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.  WhenCole, 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 1996RFC-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 SDUCole, 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 ofRFC 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 inRFC-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 inRFC-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 numberCole, 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 inRFC-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 TypesTULIP/TUNIC can be presented as being on one end of a continuum oppositethe SNAP/LLC encapsulation, with various forms of null encapsulationsomewhere in the middle.  The continuum is simply a matter of how muchis 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 tothe desirability to support connection oriented flow.  The tradeoffsbetween connection oriented and connectionless are discussed inSection5.Cole, Shur & Villamizar      Informational                     [Page 12]

RFC 1932           IP over ATM: A Framework Document          April 19965.  Connection Oriented and Connectionless TradeoffsThe connection oriented and connectionless approaches each offeradvantages and disadvantages.  In the past, strong advocates of pureconnection oriented and pure connectionless architectures have arguedintensely.  IP over ATM does not need to be purely connectionless orpurely 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 approachATM with basic AAL 5 service is connection oriented.  The IP layerabove ATM is connectionless.  On top of IP much of the traffic issupported by TCP, a reliable end-to-end connection oriented protocol.A fundamental question is to what degree is it beneficial to mapdifferent flows above IP into separate connections below IP.  There isa broad spectrum of opinion on this.As stated insection 4, at one end of the spectrum, IP would remainhighly connectionless and set up single VCs between routers which areadjacent on an IP subnet and for which there was active traffic flow.All traffic between the such routers would be multiplexed on a singleATM VC. At the other end of the spectrum, a separate ATM VC would becreated for each identifiable flow.  For every unique TCP or UDPaddress and port pair encountered a new VC would be required.  Part ofthe intensity of early arguments has been over failure to recognizethat there is a middle ground.Cole, Shur & Villamizar      Informational                     [Page 13]

RFC 1932           IP over ATM: A Framework Document          April 1996ATM offers QoS and traffic management capabilities that are wellsuited for certain types of services.  It may be advantageous to useseparate ATM VC for such services.  Other IP services such as DNS, areill suited for connection oriented delivery, due to their normal veryshort duration (typically one packet in each direction).  Shortduration transactions, even many using TCP, may also be poorly suitedfor a connection oriented model due to setup and state overhead.  ATMQoS and traffic management capabilities may be poorly suited forelastic 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                                approachCole, 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 19966.  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 (seeSection 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 ARPRFC-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 (seeRFC-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 inAppendix A on the   possible scenarios to be supported by ARP).7.  Extensions to IP RoutingRFC-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 subnetCole, 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 whichCole, 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   historicRFC-904 [11].  This turned out to be unmanageable, with   routing problems occurring when topology was changed elsewhere.8.  IP Over ATM Proposals8.1  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 inRFC-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 inRFC-1577).   o Defining the default MTU size.  This issue is addressed inRFC-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 inRFC-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 seeRFC-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 supplementRFC-1577 ARP. There has   been recent discussion of replacingRFC-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 onlyCole, 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 19968.5 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 inSection 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 19969.  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 ofAppendix 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                    _ AAL5Cole, 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 DocumentsReferences   [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 publicCole, 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 theCole, 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.netCole, Shur & Villamizar      Informational                     [Page 31]