cvincent@cochise.UUCP (Vincent) (09/18/88)
Barry, I don't profess to be an expert and may get flamed but here goes: LOGICAL HOSTS ON AN ARPANET STYLE CLASS A NETWORK 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] : : |=======[16][17][18]======| | | | PSN 122 | | | |=====[1][2][3][4][5]=====| | | ^------------PSN PORT | | | | |=========[PORT]=========| | | | TRANSPARENT GATEWAY | | | |=====[2][3][4][5][6]====| : : : : [MODEM] : ...........: : :.......... : : : : [MODEM] : : : : [-----:----] [-----+----] [-----+----] | HOST A | | HOST B | | HOST C | |26.3.2.122| |26.3.4.122| |26.3.6.122| [----------] [----------] [----------] LOGICAL HOSTS CONNECTED VIA A TRANSPARENT GATEWAY 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 host. 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. REFERENCES [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!)