In article <[log in to unmask]>, Steve Dirickson
<[log in to unmask]> writes
>> Or, is your 3000 replying, yes, I got message #123,456,
>> here's your checksum
>> (or whatever) to confirm that I really, truly got it? If the
>> latter, it
>> sounds like a poorly defined process, since TCP/IP pretty
>> much already does all of that for you,
>
>TCP doesn't do *any* of that for you.
>
>What TCP promises is that, *at the protocol level*,
> 1) a stream of bytes accepted at one end of a connection for transfer
>will be delivered complete and in sequence to the other end, or
> 2) the error will be reported to the protocol, resulting (usually) in
>a connection reset
>
>TCP knows *nothing* about application-level return-receipt, integrity,
>or other concerns. And there's no such thing as a TCP "message".
>
>Specifically, a successful send()/IPCSEND()/whateversend() call means
>that at least some of the data provided has been accepted by the
>protocol for transport. And that's all it means. In particular, it
>does NOT mean that
> 1) Any of the data has been--or ever will be--received at the other
>end
> 2) Any of the data has been--or ever will be--placed "on the wire" at
>the sender's end
>
>Proper management of connection termination via the "linger" option
>and socket shutdown and closure functions can maximize the chance that
>the data at least gets to the hardware on the sending machine, but
>nothing is available at the socket level to check progress, receipt,
>or integrity; those must be handled by the application.

TCP is a high level (Layer 4) protocol (layered on top of IP) for the
reliable communication of data between two endpoints which are uniquely
defined by a combination of port number and IP address. Because TCP is
connection oriented, it also contains state information which can be
viewed via the NETSTAT command. TCP also contains flow control
mechanisms allowing for transparent management of network through put
variations. AFAIK TCP uses a sliding window protocol which provides for
the correct sequencing of packets (each of which can feasibly travel via
a different route) and the retransmission of lost packets.
Contrast this with UDP which is a connectionless unreliable protocol
that also layers onto IP - (lost packets are never found).
Higher layer protocols using TCP include FTP, HTTP, NNTP, POP, SMTP,
whilst RPC and DNS are examples that use UDP, which is why DNS lookups
sometimes fail to resolve first time around.

Regards

Chris
--
Chris Thompson
The Internet Agency, UK
http://www.the-internet-agency.com
European Distributors for Advanced Networks Systems Inc.
Distributors of CCS TRAX and CCS C-iX 'C' compiler for MPE
MPE and RTE migration tools and services
IBM Development Partner
Voice:  +44 7836 364575
Fax:    +44 1202 418209
Email   [log in to unmask]

ANSI -
Advanced Network Systems Inc., USA
MPE migration tools and services
Specialist in JAVA and J2EE for MPE
IBM Development Partner
http://www.advnetsys.com
Voice:  +1 908-638-3330
Fax:    +1 908-638-3331
Email   [log in to unmask]

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