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breaking up of TCP/IP packets

Posted on 2003-02-25
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I am hoping for some clarification on when TCP/IP packets are broken up.  That is which machines decide to break up a transmission, and then send the frames.

For example lets say I am sending a command from terminal A(on a LAN) to server A over the internet.  Are these the only two machines that decide the packet size (whether it be the default MSS or a specified one)?  Or does the server that terminal A is connected to (on the LAN) decide this..ect?

When the packets are sent out...are they already broken up by terminal A...and kept intact throughout the internet until they reach server A?  Or do routers on the internet break up the transmission into packets as needed?

I realize that an answer to these questions may depend on the software/hardware of terminal A and server A...but this seems like a pretty straightforward question.

thanks,

b
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Question by:Brian1
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by:lrmoore
ID: 8021732
generically speaking, it is the routers' job to fragment/reassemble packets. Never the server.
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by:Brian1
ID: 8021768
So does the first router the message runs into perform the first and final separation of packets...or can the pieces(packets) of the message be further fragmented as they travel along to other routers?
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lrmoore earned 60 total points
ID: 8021866
They can get further fragmented as they move along. If you want a good explanation, read up on MTU.
There is a mechanism within the OS to detect the MTU for end-to-end and the capability to adjust accordingly, but that is also very dependent on many things in between (ie.routers and icmp)
Generally, the default MTU for Ethernet is 1500 bytes, and the MTU for dailup connections is 576.

background:
http://voip.netlab.uky.edu/~fei/teaching/cs471/slides/14.fragmentation.pdf

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by:SunBow
ID: 8022182
ditto above. The client does the first hack. After that everything should run smooth for we are all in agreement, right? Wrong. Old routers (Bay?) won't do the max, and since this is configuration option, there's always the misconfiguration opportunity. MTU differs by protocol, change one and you should change other, token ring not same as eNet. 1500 ethernet 'should' be same everywhere however, so the question should be more theoretical than it is.
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