NODELAY and NODELACK socket options - OpenVMS 7.3-2

[Magnus Josefsson]

What is the difference between the NODELAY
and NODELACK socket options - the documentation
I have found is not very clear on this subject.

[Bojan Nemec]

Magnus,

In theory NODELAY option means that TCPIP may delay transmits to coolect multiple transmits in short time in a larger packet.

NODELACK option means that TCPIP will delay the ACK packet for a short time, so multiple packets can be received and all are acknowledged with one ACK packet.

In short NODELAY is turning off transmit optimisation and NODELACK is turning off receive optimisation.

Bojan

[Wim Van den Wyngaert]

Normally TCP may delay a transmit for a short period of time so that if there are multiple rapid transmits, they can be coalesced into larger packets, placing fewer packets on the network and in general causing improved network performance.

This can be over-ridden by setting NODELAY on a connection.

Normally TCP will delay the ACK of a received segment for a short eriod of time (up to 200ms) so that if multiple segments
are received in that time and no data is being sent back, multiple received segments can all be ACKed in one ACK segment.

This can be over-ridden by setting NODELACK on a connection.


We set nodelayedack to improve access to Sybase databases.

Wim

[Ian Miller.]

setting NODELACK has been known to improve performance for some versions of pathworks and samba

[Magnus Josefsson]

It appears as if NODELAY would turn of
the Nagle algorithm as described in
RFC 896 - i.e. do not send until message
(ACK, window update) from remote host
arrives, unless the connection is idle.

I understand that this algorithm in
collaboration with delayed ACK transmission
from the remote host can be a performance
bottleneck - I guess that's why NODELACK
has been known to improve throughput in
some cases.

But then turning of Nagle (NODELAY) should
give the same result ... or am I missing
something !? The actual functionality behind
NODELAY still appears a bit unclear - what
kind of delay (timed, logic !?) is it about ?

[Wim Van den Wyngaert]

This might explain it.
http://www.thinkingsecure.com/docs/TCPIP-Illustrated-1/tcp_int.htm

Wim

[Wim Van den Wyngaert]

Summary : delay ack has more impact because 200 msec is still hardcoded.

delay ack : up to 200 msec
delay : about 1 msec (check with ping, time column)

Wim