BBS:      TELESC.NET.BR
Assunto:  Re: New offline QWK mail
De:       deon
Data:     Sun, 12 Apr 2026 10:38:42 +1000
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  hbRenb: hcRe: New offline QWK mail
  bBynb: hcRetroswim bto cBoraxman bon cSat Apr 11 2026 04:53 pmn

Howdy,

 > I thought it wasn't so much TCP, but the fact it's quite often ZModem over
 > Telnet over TCP.

There are two issues with zmodem over networks (that I'm aware of), so it takes some thought to use them:

* Over telent - due to IAC control codes
* TCP window sizes, and one side being slower than the other.

IE: I've sometimes used telnet over a TCPser line, that is serial one side and TCP the other. When the sender (TCP) sends their last byte to a receive (slow link), they start a timer and explode when it hits a timeout (if they havent received a response from the remote).

The receiver however is slowly draining the TCP buffer and when they receive the last byte, they send their ACK, however the other end has long gone. Naturally this isnt always a problem the other way around (slow sending to fast).

As DM said, zmodem can be configured to ACK or wait to send the next block, and use block sizes can be limited to small, so that the ACK timer doesnt explode before the TCP buffer is drained.


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