rfc925.txt

RFC 相关的技术文档

RFC 925                                                     October 1984
Multi-LAN Address Resolution


         method could result in an out-of-date entry persisting in a
         cache for a very long time if ARP requests for that address
         mapping were received at just less than the time out period.

      When handling regular datagrams, the BOXes must decrement the IP
      datagram Time-To-Live field (TTL) and update the IP header check
      sum.  If the TTL becomes zero the datagram is discarded (not
      forwarded).

      ARP, as currently defined, will take the most recent information
      as the best and most up-to-date.  In a complicated multi-LAN
      environment where there are loops in the connectivity it is likely
      that one will get two (or more) responses to an ARP request for a
      host on some other LAN.  It is probable that the first response
      will be from the BOX that is the most efficient path.

      The one change to the host implementation of ARP that is suggested
      here is to prevent later responses from replacing the mapping
      recorded from the first response.

   Potential Problems

      Bad Cache Entries

         If some wrong information get into a cache entry, it will stay
         there for time T3.  The persistence of old information could
         prevent communication (for a time) if a host changed its IA:HA
         mapping.

         One way to replace bad or out-of-date entries in a cache would
         be to have the BOXes explicitly interpret a broadcast ARP reply
         to require an entry with either this IA or HA to be replaced
         with this new IA:HA mapping.  One could have important servers
         send a broadcast ARP reply when they come up.

      Non-ARP Hosts

         It seems unrealistic to expect to use both ARP hosts and
         non-ARP hosts on the same LAN and expect them to communicate.
         If all the non-ARP hosts are on the same LAN the situation is
         considered with under the next heading (Non-Broadcast LANs).

         Hosts that do not implement ARP must use some other means of
         address mapping.  Either they hold a complete table of all
         hosts, or they access some such table in a server via some
         protocol; or they expect to make all routing decisions based on
         analysis of address fields.


Postel                                                          [Page 6]



RFC 925                                                     October 1984
Multi-LAN Address Resolution


      Non-Broadcast LANs

         BOXes that are connected to LANs that do not have broadcast
         capability and/or LANs where the hosts do not respond to ARP
         may have a static or dynamic table of the IA:HA mappings for
         that LAN (or the addresses may be computed from one another).
         All the hosts on that LAN must be in the table.

         When a BOX must find the address mapping and would otherwise
         send an ARP request into a non-broadcast LAN (this can only
         happen when the sought host is not the non-broadcast LAN since
         all the hosts are in the table), it must instead send an ARP
         type request specifically to each of the other BOXes on that
         LAN.

      Size of Tables

         The worst case of the size of the tables in the BOXes is the
         number of hosts in the set of LANs for each table.  That is,
         the table kept for each LAN interface may (in the worst case)
         grow to have an entry for each host in the entire set of LANs.
         However, these tables are really caches of the entries needed
         for current communication activity and the typical case will be
         far from the worst case.  Most hosts will communicate mostly
         with other hosts on their own LAN and with a few hosts on other
         LANs.  Most communication on LANs is between work station hosts
         and server hosts.  It can be expected that there will be
         frequent communication involving the main server hosts and that
         these server hosts will be entered in the tables of most of the
         BOXes most of the time.

      Infinite Transmission Loops

         The possibility of infinite transmission loops through an
         interconnected set of LANs is prevented by keeping search lists
         in the BOXes and terminating the search when a request is
         received for an address already on the list.

         Transmission loops of regular datagrams can not persist because
         them the BOXes must decrement the TTL, and discard the datagram
         if the TTL is reduced to zero.  For debugging purposes it would
         be useful for a BOX to report to the implementer any datagrams
         discarded for this reason.






Postel                                                          [Page 7]



RFC 925                                                     October 1984
Multi-LAN Address Resolution


      Broadcast

         Note that broadcast does not really have anything to do with
         either transparent subnets or explicit subnets.  Since it was
         discussed in [1], it will be discussed here, too.  Two of the
         three broadcast functions suggested in [1] work just the same
         and have the same effects, the third can be supported, too.

         It is also argued that the support for a broadcast
         interpretation of IAs is a bigger issue that the question of
         explicit subnets versus transparent subnets and it should be
         decided separately.

         It is also suggested that broadcast is not really what is
         desired, but rather multicast is the better function.  It may
         make sense to understand how to do an Internet multicast before
         adopting a broadcast scheme.

         This IP Network

            If the IA of this network number and an all ones host number
            (e.g., 36.255.255.255) is used, an IP level broadcast to all
            hosts on this Network (all LANs) is intended.  A BOX must
            forward this datagram.  A BOX must examine the datagram for
            potential significance to the BOX itself.

            To prevent infinite transmission loops each BOX must keep a
            list of recent broadcasts.  The entries in this list contain
            the source IA and the Identification field from the datagram
            header.  If a broadcast is received and matches an entry on
            the list it is discarded and not forwarded.  The entries on
            this list time out in time T2.

         This LAN Only

            If the IA of all ones (i.e., 255.255.255.255) is used an IP
            level broadcast to all hosts on this LAN only is intended.
            A BOX must not forward this datagram.  A BOX must examine
            the datagram for potential significance to the BOX itself.

         Another LAN Only

            Since the LANs are not individually identified in the IA
            this can not be supported in the same way. Some have also
            argued that this is a silly capability to provide.

            One way to provide it is to establish a specific IA for each


Postel                                                          [Page 8]



RFC 925                                                     October 1984
Multi-LAN Address Resolution


            LAN that means "broadcast on this LAN".  For example,
            36.255.255.128 means broadcast on LAN A, and 36.255.255.187
            means broadcast on LAN B, etc.  These addresses would be
            specially interpreted by the BOXes attached to the specific
            LAN where they had the special interpretation, other BOXes
            would treat these address as any other IAs.   Where these
            addresses are specially interpreted they are converted to
            the broadcast on this LAN only address.

DISCUSSION

   The claim for the extended ARP scheme is that the average host need
   not even know it is in a multi-LAN environment.

      If a host took the trouble to analyze its local cache of IA:AH
      address mappings it might discover that several of the IAs mapped
      to the same HA.  And if it took timing measurements it might
      discover that some hosts responded with less delay that others.
      And further, it might be able to find a correlation between these
      discoveries.  But few hosts would take the trouble.

   Address Structure

      In the explicit subnet scheme, some IA bits are devoted to
      identifying the subnet (i.e., the LAN).  The address is broken up
      into network, subnet, and host fields.  Generally, when fields are
      use the density of the assigned addresses in the address space
      goes down.  That is, there is a less efficient use of the address
      space.  Significant implementation problems may arise if more
      subnets than planned are installed and it becomes necessary to
      change the size of the subnet field.  It seems totally impractical
      to use the explicit subnet scheme with a class C IA.

      In the extended ARP scheme the address is simply the network, and
      host fields.  The extended ARP scheme may be used with any class
      of IA.

   Relocating Hosts

      In the explicit subnet scheme when a host is unplugged from one
      LAN and plugged into another its IA must change.

      In the extended ARP scheme it may keep the same IA.






Postel                                                          [Page 9]



RFC 925                                                     October 1984
Multi-LAN Address Resolution


   One view of the situation suggests that there are really two
   problems:

      1. How does the host discover if the destination is in this LAN or
      some other LAN?

         This question assumes that a host should know the difference
         and should do something different in the two cases, and further
         that once the host knows the answer it also know how to send
         the data (e.g., directly to the host, or to the box).

            The claim here is that the hosts should not know the
            difference and should always do the same thing.

      2. How do the BOXes that connect LANs know which BOXes are the
      routes to which LANs?

         This question assumes that the BOXes need some kind of
         topological knowledge, and exchange BOX-to-BOX protocol
         information about connectivity.

            The claim here is that the BOXes do not need topological
            knowledge and do not need to explicitly know about the
            existence of other BOXes.

   It has been suggested that there are two problems: first, how the
   hosts do routing; and second, how the BOXes do routing.  A claim has
   been made that the competing strategies each have an approach to each
   problems and one could select a solution made up partly from one
   approach and partly from another.

      For example: use ARP within the LAN and have the BOX send ARP
      replies and act as a agent (as in the extended ARP scheme), but
      use a BOX-to-BOX protocol to get the "which hosts are where"
      information into the BOXes (as in the explicit subnet scheme).

   There are two places where code is involved: a large number of hosts,
   and a small number of BOXes.  In considering the trade off between
   explicit subnet scheme and extended ARP scheme, the work done in the
   hosts should weigh a lot more than the work done in the BOXes.

      What do hosts do?

         Explicit Subnet Scheme

            The host must be able to decide if this IA is on this LAN or



Postel                                                         [Page 10]