CERF@A.ISI.EDU (02/06/89)
Art, Now I am confused. Two cases: 1. no gateways intervene 2. one or more gateways intervene between source and destination. In the first case, the source needs to initiate an X.25 VC to the destination. Given the IP destination address, known to the source, the source must translate from IP destination to comparable X.121 address. This is going to have to be either straight table look-up or algorithmic (mapping of a 32 IP address into an X.121 address - non-trivial in most cases). The receiving host will get a call set-up packet at the X.25 level which contains at least the source X.121 address. For this special case, one might be able to map from the source X.121 address in the X.25 call set-up packet to the source IP address, but this seems unnecessary since the arriving IP packet coming on the set-up link will contain source and destination IP addresses. Are you trying to bind the call set-up to a particular service process before finding out the originating IP source address? In the second case, as you point out, the calling address of the final X.25 call set-up may bear no relationship to the source IP address of the caller since the last VC is between a gateway andthe target host, not between the source host and thetarget host. That being the case, it doesn't seem possible to try to map the calling X.121 address into a source IP address at all. I have the continuing feeling that I have not understood the problem originally posed since I'm not able to make sense of your most recent answer. Vint
satz@CISCO.COM (02/07/89)
Vint, >> Now I am confused. Two cases: >> >> 1. no gateways intervene >> 2. one or more gateways intervene between source and destination. I would relabel these two cases as 1. packet sourced by host on connected subnet 2. packet sourced by host behind connected subnet (behind a router) 3. packet destined for host on connected subnet 4. packet destined for host behind connected subnet (behind a router). And, I am not sure the distinction is necessary. Since the X.25 virtual circuit source may not be from the originator but from the previous hop, you can't necessarily believe the IP source address. Also security and configuration reasons may require you to enumerate the complete list of connected hosts you will speak with. For example if you want to verify that an X.25 VC can Reverse Charge, some neighbor information will need to be preconfigured. This information is needed before the first X.25 DATA packet arrives to determine whether the VC can be accepted. The cisco implementation requires preconfigured table entries about other entities sharing the same connected subnet unless a mapping function exists (DDN or BFE). Preconfigured entries can always be used to provide additional information (such as subnet multicast). Greg