jas@MONK.PROTEON.COM (John A. Shriver) (11/02/87)
> Interestingly enough, this is an area where the source routing > scheme of IBM/ 802.5 is superior to the "proxy-ARP" routers (and maybe to > the TransLan ether bridge schemes). With the latter approaches, if a > bridge/router which is not within "local broadcast" range fails, then there > is no way for the local TCP to request that the path be redetermined. > In the source route (802.5 variety) scheme, the local TCP merely causes a > new route to be determined via a new "all rings broadcast" XID. It is correct that the TCP cannot ask a DEC LANbridge (or other equivalenly implemented learnig bridges) to try again on the route. That is because the management protocols they use will detect the failure before the TCP does, activate a new (previously backup) bridge, and dynamically rethread the route. The whole point of a LEARNING bridge is that you don't have to TEACH it. You do have to TEACH an IBM Token-Ring bridge. Of course, IP routers running dynamic routing protocols offer exactly the same advantages. They too will reconfigure faster than TCP can notice when one intermediate path fails. The host does not need to ask for any help, at most it will get an ICMP redirect. The only case where this advantage of routers will fail is in the ARP proxy subnet hack. This is why ARP proxy subnetting REQUIRES ARP cache aging, which pure ARP, used for its intended purpose, does not. That's why ARP proxy subnetting is a workaround, and not a standard. Oh yes, you lose all the advantages of the frame-copied and address-recognized bits in 802.5 when you use IBM bridges. All that frame copied means to the sender is that the first bridge copied it. A later bridge may have to drop it (congestion), or be dead. You don't lose these advantages with routers, because they try again if they don't get frame-copied. This is why a ring makes a good router backbone. So far as I can see, IBM Token-Ring source routing combines the bad aspects of routing and bridging into a big layering violation. It will persist, however, due to SNA's reliance on it. (These are my opinions, and have not been certified or blessed by Proteon.)