RE: Logical Class A Addressing

Vincent (cochise!
Sun Sep 18 13:58:34 1988


    I don't profess to be an expert and may get flamed but here goes:


    As you know, the third field of an Arpanet/Milnet style Class A IP
    address is called the logical field. The use of this field is very
    controversial. For the Milnet at least, the NIC has stated that they
    are not authorized to issue logical addresses. The reason for the
    restriction will become clear later. In theory and practice, one can
    use this style of address to place several Class A hosts behind a
    single PSN port. Figure 1 shows three hosts connected to a
    concentrator, an IP router. In DDN terms, this device is referred to
    as a transparent gateway. This transparent gateway is in turn
    connected to a single port of a PSN.

                                TO OTHER PSN'S
                             . ON THE DDN .
                            /|\ /|\
                             :..... .....:
                                  : :
                               [MODEM] [MODEM]
                                  : :
                         | |
                         | PSN 122 |
                         | |
                                     | | ^------------PSN PORT
                                     | |
                                     | |
                         | |
                         | TRANSPARENT GATEWAY |
                         | |
                                : : :
                                : [MODEM] :
                     ...........: : :..........
                     : : :
                     : [MODEM] :
                     : : :
               [-----:----] [-----+----] [-----+----]
               | HOST A | | HOST B | | HOST C |
               || || ||
               [----------] [----------] [----------]


                                  Figure 1

    In Figure 1, each host address uses port 3 of PSN 122. The only
    difference is the logical field. In this case the three hosts are
    following the standard naming convention starting at number 2 as
    address 0 and 1 are reserved. The transparent gateway gets its name
    by the fact that it does not disassemble the X.25 packet. By the use
    of header length information, the gateway looks into the IP header
    and finds the destination IP address. The gateway examines the
    logical field of the IP address and passes the packet on to the
    proper host. This is an excellent way to share the bandwidth of a
    single port. Theoretically the IP address will allow 253 (with
    logical addresses 0, 1, and 255 reserved) hosts behind the router.
    In reality it depends upon the degree of bandwidth required by each

    There are several problems with this arrangement. First and
    foremost, RFC 1005 (May 1987) contains a suggestions for rearranging
    the 32 bit IP address. This suggestion, if adopted, would render the
    transparent gateway router useless. This is why the Milnet manager
    is discouraging the use of this addressing scheme. Users are not
    restricted from using this field, they are only restricted from being
    published in the NIC's host table. (for a good horor story, ask Mary
    Stahl at the NIC about a site she worked at where this type of
    address caused many problems.) Without being published in the NIC's
    host table, it is difficult to receive electronic mail without the IP
    address as the distant end will have no way of knowing which PSN and
    port the host is on. This is not a problem if the DOMAIN server
    system is employed. The domain server system allows hosts to receive
    mail without being on the NIC's host table (see RFC 1032 through RFC
    1035). However there are many domains (like mine) which do not
    employ this wonderful and intelligent design.

> First, is anyone using this currently? I know of the definition
> of how it is to be done on the X25 connections to the net.

    Currently, the HOST.TXT file shows only 3 of the 1727 Milnet hosts
    using this addressing format. These three machines are special
    purpose Sun work stations with only a single user per machine.
    There are many others, they just are not published.

    There is another concern with using a transparent gateway. Some
    gateways may or may not fully participate with the Internet Control
    Messaging Protocol (ICMP) [1]. The ICMP is concerned with network
    flow control. A transparent gateway which does not fully participate
    with ICMP may choke the PSN, or it may not inform the host that the
    distant end is dead [2].

    Why anyone would want to use this type of gateway is beyond me. I
    really would like to hear some good reason to use it. I may not be
    aware of some special feature about remaining a Class A host. My
    understanding is folks like to use them as they believe this is the
    only way to do X.25 type multiplexing. However, there are gateway
    devices (like Cisco Systems) which allow the same type of PSN port
    sharing without the use of the logical field. By changing the hosts
    from Class A to Class B or C, a gateway device can route X.25 packets
    from a single port on the PSN to as many X.25 hosts as the gateway
    can support. This type of gateway box also gives you EGP and other
    handy services. All transparent gateway

> Furthermore, If a single host has two physical connections to the
> net and both use the same logical address, how does the PSN route
> traffic between the two links? Is it load balanced?

    PSN's route X.25 packets and don't care about IP addresses. Gateways
    and hosts put on the X.25 address for a specific PSN and port.
    Therefore it depends on the IP to X.25 mapping in a given distant end
    gateway and host. All transparent gateway boxes that I have come in
    contact with do allow for load balanced dual-homing configurations.
    However the load balancing is done for outbound traffic.


    [1] Comer, Douglas; TCP/IP pg 195
    [2] RFC 1009 "Requirements for Internet Gateways" pg 9.

    Hope this helps Barry,

    Curt Vincent
    Army Computer Engineering Center
    Army Information Systems Engineering Command
    Fort Huachuca, AZ 85613-7300


    Disclaimer: The comments are my own and do not reflect the opinions
    and philosophies of the United States Army Computer Engineering
    Center, or this Command. (Regardless if they are right, or wrong!)

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