Dutch Transport Card Broken

[Anne & Lynn Wheeler]

Victor Duchovni wrote:
>
> SMTP does not need TCP to provide reliability for the tail of the session,
> the application-level "." (end-of-data) and server "250" response complete
> a transaction, everything after that is optional, so for example Postfix
> will send (when PIPELINING).
>
> 	DATA<CRLF>		354 Go ahead
> 	Message-Content		Lots of acks
> 	.<CRLF>QUIT<CRLF>	250 Ok
>
> and will disconnect after reading the "250 response" without waiting
> for the 221 response. The TCP 3-way shutdown (FIN, FIN-ACK, ACK) happens
> in the kernel in the background, the SMTP server and client are by that
> point handling different connections. So the reliable shutdown latency
> is of no consequence for application throughput.
>
> A pipelined SMTP delivery can be completed over TCP in 5 RTTs not 7.
>
> 	0. SYN			SYN-ACK
> 	1. ACK			220
> 	2. EHLO			250
> 	3. MAIL RCPT DATA	250 250 354
> 	4. MSG . QUIT		250 221
> 	5. close socket
>
> TCP is fine, latency is primarily the result of application protocol
> details, not TCP overhead. The only TCP overhead above is 1 extra RTT
> for the connection setup. Everything else is SMTP not TCP, and running
> SMTP over UDP (with ideal conditions and no lost packets, ...) would
> save just 1 RTT.
>
>    
re:
http://www.garlic.com/~lynn/aadsm28.htm#21 Dutch Transport Card Broken

sorry, I didn't say that TCP required seven round-trips for reliable 
exchange;
the statement was that minimum TCP operation was seven packet
exchange (for reliable operation) .... sort of 3.5 round-trips. That
VMTP (rfc 1045) reduced that to minimum of five packet exchange
(sort of 2.5 round-tips) ... and that XTP got it to a minimum of three
packet exchange (sort of 1.5 round-trips) for reliable operation.

from my RFC index
http://www.garlic.com/~lynn/rfcietff.htm

rfc 1045 summary
http://www.garlic.com/~lynn/rfcidx3.htm#1045

1045 E
  VMTP: Versatile Message Transaction Protocol: Protocol specification,
  Cheriton D., 1988/02/01 (123pp) (.txt=264928) (Refs 955, 966, 969)
  (Ref'ed By 1050, 1072, 1105, 1106, 1190, 1263, 1323, 1453, 1458,
  1700, 2018, 2375, 2757) (VMTP)

as always, clicking on the ".txt=nnn" field (in rfc summary) retrieves 
the actual RFC.


If there is more than minimum amount of data ... TCP might involve more
than seven packet exchange ... but the minimum packet exchange is
seven packets (not round-trips).

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