rfc1932.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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.




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   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



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     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.



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   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.




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   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.



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     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.