rfc824.txt

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      smallest number of Ethernet multicast addresses that can be

      simultaneously attended by any host in the VLN, or 64,511,

      whichever is smaller.  A network composed of hosts with the Intel




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      and Interlan controllers mentioned above, for example, would have

      Min_Attendable equal to 7; (10) a network composed only of hosts

      with 3COM Model 3C400 controllers would have Min_Attendable equal

      to 64,511, since the controller itself does not restrict the

      number of Ethernet multicast addresses to which a host may

      attend.  (11)


           The local address field of a VLN multicast address can be

      represented in two octets, in hexadecimal:


             mm-mm


      From Table 1, mm-mm considered as a decimal integer M is in the

      range 1,024 to 65,534.  When SendVLNDatagram is invoked with a

      VLN multicast datagram, there are two cases:

        1.  (M - 1,023) <= Min_Attendable.  In this case, the datagram
            is encapsulated in a "DoD IP" Ethernet frame, and multicast
            with the Ethernet address

                    09-00-08-00-mm-mm

            A VLN component which attends VLN multicast addresses in
      _______________
      (10) Min_Attendable is 7, rather than 8,  because  one  multicast
      slot  in  the  controller must be reserved for the host's MHA, as
      described in Section 4.2.
      (11) For the Cronus Advanced Development Model, Min_Attendable is
      currently defined to be 60.




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            this range should receive Ethernet multicast addresses in
            this format, if necessary by registering the addresses with
            its Ethernet controller.

        2.  (M - 1,023) > Min_Attendable.  The datagram is encapsulated
            in a "DoD IP" Ethernet frame, and transmitted to the
            Ethernet broadcast address.  A VLN component which attends
            VLN multicast addresses in this range must receive all
            broadcast frames, and filter them on the basis of frame
            type and VLN destination address (found in the IP
            destination address field).


           There are two drawbacks to this protocol that might induce a

      more complex design:  1) because Min_Attendable is the "lowest

      common denominator" for the ability of Ethernet controllers to

      recognize multicast addresses, some controller capabilities may

      be wasted; 2) small VLN addresses (less than Max_Attendable +

      1,024) will probably be handled more efficiently than large VLN

      multicast addresses.  The second factor complicates the

      assignment of VLN multicast addresses to functions, since the

      particular assignment affects multicast performance.














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      4.4  Reliability Guarantees


           Delivered datagrams are accurate copies of transmitted

      datagrams because VLN components do not deliver incoming

      datagrams with invalid Frame Check Sequences.  The 32 bit CRC

      error detecting code applied to Ethernet frames is very powerful,

      and the probability of an undetected error occuring "on the wire"

      is very small.  The probability of an error being introduced

      before the checksum is computed or after it is checked is

      comparable to the probability of an error in a disk subsystem

      before a write operation or after a read; often, but not always,

      it can be ignored.


           Datagram duplication does not occur because the VLN layer

      does not perform datagram retransmissions, the primary source of

      duplicates in other networks.  Ethernet controllers do perform

      retransmission as a result of "collisions" on the channel, but

      the "collision enforcement" or "jam" assures that no controller

      receives a valid frame if a collision occurs.


           The sequencing guarantees hold because mutually exclusive

      access to the transmission medium defines a total ordering on




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      Ethernet transmissions, and because a VLN component buffers all

      datagrams in FIFO order, if it buffers more than one datagram.






      4.5  Use of Assigned Numbers


           On a philosophical note, protocols such as IP and TCP exist

      to provide communication services to extensible sets of clients;

      new clients and usages continue to emerge over the life of a

      protocol.  Because a protocol implementation must have some

      unambiguous knowledge of the "names" of the clients, sockets,

      hosts, networks, etc., with which it interacts, a need arises for

      the continuing administration of the 'assigned numbers' related

      to the protocol.  Typically the organization which declares a

      protocol to be a standard also becomes the administrator for its

      assigned numbers.  The organization will designate an office to

      assign numbers to the clients, sockets, hosts, networks, etc.,

      that emerge over time.  The office will also prepare lists of

      number assignments that are distributed to protocol users; the

      reference [4] is a list of this kind.





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           There are three organizations responsible for number

      assignment related to the Ethernet-based VLN implementation:

      DARPA, Xerox, and the DOS Design/Implementation Project; their

      respective roles are described below.





      4.5.1  DARPA


           DARPA administers the internet network number and internet

      protocol number assignments.  The Ethernet-based VLN

      implementation does not involve DARPA assigned numbers, but any

      particular 'instance' of a Cronus VLN is expected to have a class

      A or B internet network number assigned by DARPA.  For example,

      the prototype Cronus system (the Advanced Development Model)

      being constructed at Bolt Beranek and Newman, Inc., has class B

      network number 128.011.xxx.xxx.


           Protocols built above the VLN will make use of other DARPA

      assigned numbers, e.g., the Cronus object-operation protocol

      requires an internet protocol number.







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      4.5.2  The Xerox Ethernet Address Administration Office


           The Ethernet Address Administration Office at Xerox Corp.

      administers Ethernet specific and multicast address assignments,

      and Ethernet frame type assignments.


           It is the intent of the Xerox internetworking model that

      every Ethernet host have a distinct specific address, and that

      the address space be large enough to accomodate a very large

      population of inexpensive hosts (e.g., personal workstations).

      They have therefore chosen to delegate the authority to assign

      specific addresses to the manufacturers of Ethernet controllers,

      by granting them large blocks of addresses on request.

      Manufacturers are expected to assign specific addresses from

      these blocks densely, e.g., sequentially, one per controller, and

      to consume all of them before requesting another block.


           The preceding paragraph explains the Xerox address

      assignment policy not because the DOS Design/Implementation

      Project intends to manufacture Ethernet controllers (!), but

      because Xerox has chosen to couple the assignment of specific and

      multicast Ethernet addresses.  An assigned block is defined by a




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      23-bit constant, which specifies the contents of the first three

      octets of an Ethernet address, except for the broadcast/multicast

      bit (the least significant bit of the first octet).  The

      possessor of an assigned block thus has in hand 2**24 specific

      addresses and 2**24 multicast addresses, to parcel out as

      necessary.


           The block assigned for use in the Cronus system is defined

      by the octets 08-00-08 (hex).  The specific addresses in this

      block range from 08-00-08-00-00-00 to 08-00-08-FF-FF-FF (hex),

      and the multicast addresses range from 09-00-08-00-00-00 to 09-

      00-08-FF-FF-FF (hex).  Only a fraction of the multicast addresses

      are actually utilized, as explained in Sections 4.2 and 4.3.


           The Ethernet Address Administration Office has designated a

      public frame type, "DoD IP", 08-00 (hex), to be used for

      encapsulated internet protocol datagrams.  The Ethernet VLN

      implementation uses this frame type exclusively for datagram

      encapsulation. In addition, the Cronus system uses two private

      Ethernet frame types, assigned by the Ethernet Address

      Administration Office:





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              NAME             TYPE

              Cronus VLN       80-03
              Cronus Direct    80-04

      (The use of the "Cronus Direct" frame type is not described in

      this note.)


           The same Ethernet address and frame type assignments will be

      used by every instance of a Cronus VLN; no further assignments

      from the Ethernet Address Administration Office are anticipated.






      4.5.3  The DOS Design/Implementation Project


           The DOS Design/Implementation Project assumes responsibility

      for the assignment of subtypes of the Ethernet frame type "Cronus

      VLN".  No assignments of subtypes for purposes unrelated to the

      Cronus system design are expected, nor are assignments to other

      organizations.  The subtypes currently assigned are:









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              NAME                 SUBTYPE

              Mapping Update       00-01







































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                                 REFERENCES


      [1]
          "On holy wars and a plea for peace," Danny Cohen, Computer,
          V 14 N 10, October 1981, pp. 48-54.

      [2]
          "48-bit absolute internet and Ethernet host numbers," Yogen
          K. Dalal and Robert S. Printis, Proc. of the 7th Data
          Communications Symposium, October 1981.

      [3]
          "The Ethernet:  a local area network, data link layer and
          physical layer specifications," Digital Equipment Corp., Intel
          Corp., and Xerox Corp., Version 1.0, September 1980.

      [4]
          "Assigned numbers," Jon Postel, RFC 790, USC/Information
          Sciences Institute, September 1981.

      [5]
          "Internet Protocol - DARPA internet program protocol
          specification," Jon Postel, ed., RFC 791, USC/Information
          Sciences Institute, September 1981.

      [6]
          "Internet protocol transition workbook," Network Information
          Center, SRI International, Menlo Park, California, March 1982.

      [7]
          "IP - Local Area Network Addressing Issues," Robert Gurwitz
          and Robert Hinden, Bolt Beranek and Newman Inc., (draft)
          August 1982.










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