RE: Memory allocation

["JIM LANGSTON" <JLANGSTON@xxxxxxxxxxx>]

Buck is right.  Here is a little more detail if you need it.

You can not count on the packet size arriving via TCP/IP.  A packet may
get sent as a certain size, and then get broken up along the way if it
is too big for the routers packet size (called MTU in WinXX, Maximum
Transmission Unit).  Also, the TCP/IP stack may buffer some before it
send it, not sure.

Your 32K per read is not what you think it is, you are setting the MTU
for your socket program to 32K, which means the biggest packet you will
receive is 32K in size.  Anything smaller than that would have to be
broken up (not sure if it a 65k packet would get broken in two or cause
an error, and not sure if the program would break it up, or the sockets
driver).

Just remember that TCP/IP does not guarantee that packets won't get
broken up, it only guarantees they will arrive in order.  You have to
allow for this in any sockets programs you write.

Regards,

Jim Langston

-----Original Message-----
From: owner-rpg400-l@midrange.com [mailto:owner-rpg400-l@midrange.com]On
Behalf Of Buck Calabro
Sent: Tuesday, July 24, 2001 1:21 PM
To: RPG400-L@midrange.com
Subject: RE: Memory allocation


>I have my read set to recieve 32K per read. However, the first
>read receives 108 bytes and most of the time the next reads
>will receive 16,832 bytes. My sockets program is communicating
>with a web-server from a Windows box (if that helps). Also, the
>16,832 thing is not consistent. For an example: I received a
>document that took four reads that arrived in the following
>sequence. First read: 108; Second read: 16,832;
>Third read: 16,832; Fourth read: 8,586. I then received the
>exact same document again and it arrived as 108; 21,152;
>16,832; 4,266. It seems to me that the iSeries is buffering
>the input somewhere.

It isn't the iSeries; it's the network.  Depending on the route and routers,
TCP/IP can break a packet into smaller ones as it sees fit.  That's just the
way it works.

Buck Calabro
Commsoft; Albany, NY
Visit the Midrange archives and FAQ at http://www.midrange.com
"Where so many hours have been spent convincing myself that I am right,
 is there not some reason to fear that I may be wrong?" -- Jane Austen
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