On Mon, 12 Jun 2006 00:00:00 GMT, Ted MacNEIL <[EMAIL PROTECTED]> wrote:
>>TCP keeps track of it all, requesting a resend of dropped/lost/missing packets. > >NOT in this case! >The packets are dropped! >They are not re-sent and the app is blown off the air. >It works under SNA; it bellies up under TCP/IP. >Every time! Repeatable! >... I'm way behind on on IBM-Main so am probably jumping onto a cold thread, but want to add my belated 2 cents worth. Ted is probably right about the behavior of the application and is spot-on with "Repeatable!", but is a bit off on his criticizm of TCP/IP. Yes, IP drops packets. But also, yes, TCP keeps track of it all and requests retransmissions. The reason this works much better in SNA (IM not so HO) is that the error detection and correction in SNA happens at a much lower level than in TCP/IP - at what would be the IP level - at the level where timers can match "line" characteristics - very short timers. There is guaranteed delivery at a very low level; data gets through or the route is declared dead. On the other hand, IP does not guarantee delivery. It is left up to the higher (TCP) level to detect need for retransmission. And since any 2 packets can flow across different routes with very different speeds, it has to have very long timers or risk requesting unneeded retransmissions. If the retransmission timers are longer than timers in the application (something completely unneeded in an SNA app), the app may choke even though data is on the way. In other words, TCP guarrentees delivery of data, but the app may not be willing or able to wait. One aspect of this is that behavior of an SNA network is MUCH more predictable than a TCP/IP behavior. Network management is self- contradictory in TCP/IP because behavior is too random. And in regard to the DNS issues in this thread, that's sort of a red herring. IP addressing is based on numeric addresses. SNA addressing is based on <netid>.<resource> names (with numeric addresses used within a domain, and to a lesser extent, between domains in SNI configurations). What would an SNA name server do? A bigger issue is that a public network like the internet does not map well to SNA's 2-level name structure. Pat O'Keefe ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
