leong@ANDREW.CMU.EDU (John Leong) (06/27/86)
For running IP on the IBM token ring, one has to handle ARP quite carefully (besides boring thing like SAP number as applicable to all LAN's using the 802.2 layer). The IBM token ring is sort of defined in the official 802.5 spec. What is missing in that definition is the bridge. Unlike Ethernet, inter-ring bridge is very much part of the IBM token ring since a basic ring will only support 260 stations due to potential jitter problem. (Bridge differs from a repeater in that it will store and forward if necessary). While IBM has contributed the Bridge design to IEEE, it was not included in the final spec. The IBM ring bridge works very different from the DEC LANbridge which is totally transparant to station software. The IBM bridge use source routing. In a simplified form, when a station does any broadcast, the bridge will forward the broadcast packet. it will also tag its ID in the Route Information Field of the packet. The Route Information Field, RIF, is considered as part of the MAC layer and below the LLC layer. When the broadcast packet gets to the other end, the receiving station will know the path. (The same thing applies to a special MAC frame called "resolve"). In the case of ARP, the recieving station must store the RIF in its ARP table since subsequent (non-broadcast) communication will have to be done with source routing : i.e. including the RIF field in the MAC header. The ARP reply must also incude the RIF as part of the hardware address so the ARP sender can source route its packet. This has at least two implications : (a) The hardware address of the ARP cache has to be a variable length (subject to a maximum : there is a maximum length for the RIF field). (b) There needs to be closer interfacing between the driver and the ARP layer since the driver must pass up the whole MAC header to the ARP handler so that the RIF field can be extracted. Personally, I think DEC's approach makes life much more easier. However, I think future revision of ARP specification for the 802 LAN's should take the above into consideration. John Leong