rfc2226.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                                           T. Smith
Request for Comments: 2226                               IBM Corporation
Category: Standards Track                                    G. Armitage
                                                     Lucent Technologies
                                                            October 1997


                     IP Broadcast over ATM Networks


Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1997).  All Rights Reserved.

Abstract

   This memo describes how the IP multicast service being developed by
   the IP over ATM working group may be used to support IP broadcast
   transmission. The solution revolves around treating the broadcast
   problem as a special case of multicast, where every host in the
   subnet or cluster is a member of the group.

   An understanding of the services provided by RFC 2022 is assumed.

1.  Introduction.


   The IETF's first step in solving the problems of running IP over
   Asynchronous Transfer Mode (ATM) technology is described in RFC 1577
   [1].  It provides for unicast communication between hosts and routers
   within Logical IP Subnets (LISs), and proposes a centralized ATM ARP
   Server which provides IP to ATM address resolution services to LIS
   members.

   Two classes of IP service were omitted - multicast and broadcast
   transmissions. Multicasting allows a single transmit operation to
   cause a packet to be received by multiple remote destinations.






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   Broadcasting typically allows a single transmit operation to cause a
   packet to be received by all IP hosts that are members of a
   particular 'subnet'.

   To address the need for multicast support (represented by
   transmission to IP addresses in the Class D space), RFC 2022
   ("Support for Multicast over UNI 3.0/3.1 based ATM Networks") [2] was
   created.  This memo creates an analog of the RFC 1577 ARP Server - a
   new entity known as the MARS (Multicast Address Resolution Server).
   The MARS operates as a centralized registry and distribution
   mechanism for mappings between IP multicast addresses and groups of
   ATM unicast addresses. Host behavior is also defined for establishing
   and managing point to multipoint VCs, based on the information
   returned by the MARS, when hosts wish to transmit packets to a
   multicast group.

   This memo aims to show how RFC 2022 may be used to emulate IP
   broadcast within Logical IP Subnets. While the broadcast technique
   does not align itself well with the underlying point-to-point nature
   of ATM, clearly, some applications will still wish to use IP
   broadcasts.  Client-server applications where the client searches for
   a server by sending out a broadcast is one scenario.  Routing
   protocols, most notably RIP, are other examples.

2.  Review of Unicast and Multicast.

   Both the unicast and multicast cases take advantage of the point-to-
   point and point-to-multipoint capabilities defined in the ATM Forum
   UNI 3.1 document [4].  A unicast IP address has a single ATM level
   destination.  Unicast transmissions occur over point to point Virtual
   Channels (VCs) between the source and destination. The ARP Server
   holds mappings between IP destination addresses and their associated
   ATM destination address. Hosts issue an ARP_REQUEST to the ARP Server
   when they wish to ascertain a particular mapping.  The ARP Server
   replies with either an ARP_REPLY containing the ATM address of the
   destination, or an ARP_NAK when the ARP Server is unable to resolve
   the address. If the request is successful the host establishes a VC
   to the destination interface. This VC is then used to forward the
   first (and subsequent) packets to that particular IP destination. RFC
   1577 describes in further detail how hosts are administratively
   grouped in to Logical IP Subnets (LISs), and how the ARP Server
   establishes the initial mappings for members of the LIS it serves.

   The basic host behavior for multicasting is similar - the sender must
   establish and manage a point to multipoint VC whose leaf nodes are
   the group's actual members. Under UNI 3.1 these VCs can only be
   established and altered by the source (root) interface.




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   The MARS is an evolution of the ARP Server model, and performs two
   key functions.  The first function is the maintenance of a list of
   ATM addresses corresponding to the members for each group.  This list
   is created by a host registration process which involves two messages
   - a MARS_JOIN which declares that a host wishes to join the specified
   group(s), and a MARS_LEAVE which indicates that a host wishes to
   leave the specified group(s).

   MARS_JOIN and MARS_LEAVE messages are also redistributed to all
   members of the group so that active senders may dynamically adjust
   their point to multipoint VCs accordingly.

   The other major function is the retrieval of group membership from
   MARS (analogous to the ARP Server providing unicast address
   mappings). When faced with the need to transmit an IP packet with a
   Class D destination address, a host issues a MARS_REQUEST to the
   MARS. If the group has members the MARS returns a MARS_MULTI
   (possibly in multiple segments) carrying a set of ATM addresses. The
   host then establishes an initial point to multipoint VC using these
   ATM addresses as the leaf nodes. If the MARS had no mapping it would
   return a MARS_NAK.

   (RFC 2022 also discusses how the MARS can arrange for Class D groups
   to be supported by either multicast servers, or meshes of point to
   multipoint VCs from host to host.  However, from the host's
   perspective this is transparent, and is not central to this
   discussion of IP broadcast support.)

   This memo describes how a host may utilize the registration and group
   management functions in an existing MARS based IP/ATM network to
   emulate IP broadcasts.

3.  Broadcast as a special case of Multicast.

   Many of the problems that occur when implementing a broadcast
   solution also occur in when implementing a multicast solution.  In
   fact, broadcast may be considered a special case of multicast.  That
   is, broadcast is a multicast group whose members include all members
   in the LIS.

   There are two broadcast groups which this memo addresses:

      1) 255.255.255.255 - "All ones" broadcast

      2) x.z - CIDR-prefix (subnet) directed broadcast






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   Broadcast (1) is sometimes referred to as a limited broadcast to this
   physical network.  Broadcast (2) can be thought of as the the
   broadcast for subnets or networks in the old paradigm. As described
   in [6] and [7], the notion of subnets and networks is being replaced
   with a more efficient utilization of the routing address space known
   as Classless Inter-Domain Routing.  The CIDR-prefix (x) is the
   combination of IP address and subnet mask that denotes the subnet
   number.  The host portion of the address (z) is all ones.  One should
   note that while these broadcasts have different scopes at the IP or
   network layer, they have precisely the same scope at the link layer
   -- namely that all members of the LIS will receive a copy.

   These addresses may be used in two environments:

      o  Broadcasting to all members of a given LIS where
         a priori knowledge of a host's IP address and
         subnet mask are known (e.g. the CIDR-prefix directed
         broadcast).

      o  Broadcasting to all members of a physical network
         without knowledge of a host's IP address and
         subnet mask (e.g. the all ones broadcast).

   On a broadcast medium like Ethernet, these two environments result in
   the same physical destination.  That is, all stations on that network
   will receive the broadcast even if they are on different logical
   subnets, or are non-IP stations.  With ATM, this may not be the case.
   Because ATM is non-broadcast, a registration process must take place.
   And if there are stations that register to some broadcast groups, but
   not others, then the different broadcast groups will have different
   memberships.  The notion of broadcast becomes inconsistent.

   One case that requires the use of the all ones broadcast is that of
   the diskless boot, or bootp client, where the host boots up, and does
   not know its own IP address or subnet mask.  Clearly, the host does
   not know which subnet it belongs to.   So, to send a broadcast to its
   bootp server, the diskless workstation must use the group which
   contains no subnet information, i.e. the 255.255.255.255 broadcast
   group.  Carrying the example a little further, the bootp server,
   after receiving the broadcast, can not send either a directed frame
   nor a subnet directed broadcast to respond to the diskless
   workstation.  Instead, the bootp server must also use the
   255.255.255.255 group to communicate with the client.

   While the all ones broadcast is required at the IP layer, it also has
   relevance at the link layer when deciding which broadcast group to
   register with in MARS.  In other words, a bootp client wishing to
   register for a link layer broadcast, can only register for



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   255.255.255.255 in the MARS address space because the client's subnet
   is unknown at the time.  Given that some applications must use the
   all ones address in MARS for their broadcast group, and that we wish
   to minimize the number of broadcast groups used by LIS members, the
   all ones group in MARS MUST be used by all members of the LIS when
   registering to receive broadcast transmissions.  The VCC used for
   transmitting any broadcast packet will be based on the members
   registered in the MARS under the 255.255.255.255 address position.
   This VCC will be referred to as the "broadcast channel" through the
   remainder of this memo.

4.  The MARS role in broadcast.

   Many solutions have been proposed, some of which are listed in
   Appendix A.  This memo addresses a MARS solution which appears to do
   the best job of solving the broadcast problem.

   There are a number of characteristics of the MARS architecture that
   should be kept intact.  They include:

   o  MARS contains no knowledge of subnet prefixes and subnet masks.
      Each group address registered with MARS is managed independently.

   o  A MARS may only serve one LIS. This insures that the
      broadcast group 255.255.255.255 is joined by hosts from one
      LIS, keeping its scope bound to conventional interpretation.

   o  The Multicast Server (MCS) described in [2] may be used to service
      the broadcast groups defined in this memo without modification.
      The MCS will reduce the number of channels used by the network.

   The MARS needs no additional code or special algorithms to handle the
   resolution of IP broadcast addresses. It is simply a general database
   that holds {Protocol address, ATM.1, ATM.2, ... ATM.n} mappings, and
   imposes no constraints on the type and length of the 'Protocol
   address'. Whether the hosts view it as Class D or 'broadcast' (or
   even IP) is purely a host side issue.

   It is likely that end points will want to use the IP broadcast
   emulation described here in order to support boot time location of
   the end point's IP address. This leads to the observation that the
   MARS should NOT expect to see both the IP source and ATM source
   address fields of the MARS_JOIN filled in.  This is reasonable, since
   only the ATM source address is used when registering the end point as
   a group member.

   The MARS architecture is sufficient to insure the integrity of the
   broadcast group list without any modification.



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5.  Host Requirements for Broadcast.

   The following list of bullets describes additional characteristics of
   a MARS-compliant host.  These characteristics are required to take
   advantage of the broadcast function.

   o  A host must register as a MARS client.

   o  A host, soon after registration MUST issue a MARS_JOIN to the
      all ones broadcast address (i.e. 255.255.255.255) with the
      mar$flags.layer3grp reset.

   o  When transmitting packets, the host should map all IP layer
      broadcasts to the VCC (broadcast channel) created and maintained
      based on the all ones entry in MARS.

   o  A host MUST monitor the MARS_JOIN/MARS_LEAVE messages
      for 255.255.255.255 to keep the broadcast channel current.

   o  A broadcast channel should be torn down after a period of
      inactivity.  The corresponding timeout period MAY be specified
      with a minimum value of one minute, and a RECOMMENDED
      default value of 20 minutes.

   One should note that while every member participating in the
   broadcast MUST be a member of the all ones group, not all members
   will choose to transmit broadcast information.  Some members will
   only elect to receive broadcast information passively.  Therefore, in
   a LIS with n stations, there may be less than n channels terminated
   at each station for broadcast information.  Further reductions may be
   gained by adding a Multicast Server (MCS) to the broadcast
   environment which could reduce the number of VCs to two (one
   incoming, one outgoing), or one for a station that only wishes to
   listen.

   It is well understood that broadcasting in this environment may tax
   the resources of the network and of the hosts that use it.
   Therefore, an implementer MAY choose to provide a mechanism for
   retracting the host's entry in the broadcast group after it has been
   established or prior to joining the group.  The MARS_LEAVE is used to
   request withdrawal from the group if the host wishes to disable
   broadcast reception after it has joined the group.  The default
   behavior SHALL be to join the all ones broadcast group in MARS.








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6.  Implications of IP broadcast on ATM level resources.

   RFC 2022 discusses some of the implications of large multicast groups
   on the allocation of ATM level resources, both within the network and
   within end station ATM interfaces.

   The default mechanism is for IP multicasting to be achieved using
   meshes of point to multipoint VCs, direct from source host to group
   members. Under certain circumstances system administrators may, in a
   manner completely transparent to end hosts, redirect multicast
   traffic through ATM level Multicast Servers (MCSs). This may be
   performed on an individual group basis.

   It is sufficient to note here that the IP broadcast 'multicast group'
   will constitute the largest consumer of VCs within your ATM network
   when it is active. For this reason it will probably be the first
   multicast group to have one or more ATM MCSs assigned to support it.
   However, there is nothing unique about an MCS assigned to support IP
   broadcast traffic, so this will not be dealt with further in this
   memo. RFC 2022 contains further discussion on the possible
   application of multiple MCSs to provide fault-tolerant architectures.

7.  Further discussion.

   A point of discussion on the ip-atm forum revolved around "auto
   configuration" and "diskless boot".  This memo describes a broadcast
   solution that requires the use of the MARS.  Therefore, at a minimum,
   the ATM address of the MARS must be manually configured into a
   diskless workstation.  Suggestions such as universal channel numbers,
   and universal ATM addresses have been proposed, however, no agreement
   has been reached.

   Another topic for discussion is multiprotocol support.  MARS is
   designed for protocol independence.  This memo specifically addresses
   the IP broadcast case, identifying which addresses are most effective
   in the IP address space.  However, the principles apply to any layer
   3 protocol.  Further work should be performed to identify suitable
   addresses for other layer 3 protocols.

   Finally, there has been support voiced for a link layer broadcast
   that would be independent of the layer 3 protocol.  Such a solution
   may provide a simpler set of rules through which broadcast
   applications may be used.  In addition, some solutions also provide
   for more efficient use of VCCs.







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