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I have a question: I have a 4Mbit Connection on the one side (call it Side A) and a 1Gbit connection on the other side (side B) of a router. If now a packet of 1500 bytes is coming in on Side A. Does the router wait until it received everything of the packet (all the 1500 bytes) on Side A until it forwards it to Side B, or is it sending it in chunks?

Sorry if the question is too fundamental, but I really did not find a good answer to this question.

megaflux
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  • Protocols may split data into packets, and these packets may be truncated too. So I'd say it's protocol and implementation dependent. Please, be specific on which protocol and system you're referring to. – jweyrich Jul 04 '10 at 06:25
  • It's a TCP-IP package, containing a proprietary protocol inside, which then contain Zlib and Huffman compressed structures. So the thing is, there is a big tcp packet coming in on side A. It has 1500 bytes, which takes on a 4Meg line which takes 3ms, of course the router needs first the tcp header( with ip, mac,.. ) 48 Bytes, which is therefore 100 micros... I can read the content of the tcp message also in chunks. So if I decrease the MTU size on Side B, does it split the packages already while data is incoming or does it wait the 3ms until it receives it – megaflux Jul 04 '10 at 06:36

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if it's an IP router (so called 'layer 3 routing') it gets the whole packet (which can be less or more than 1536 bytes), verifies the checksum, and then forwards to the next hop.

'layer 3 switches' (in theory it would be an oxymoron, in practice it's not all uncommon) claim to do 'routing at the speed of switching', which might hint at starting to retransmit before receiving the whole packet; but AFAIK, even layer 2 switches (also known as 'switches') do get the whole ethernet frame before retransmitting it (at least to be able to discard bad frames). So I guess it means that it does all the table lookups as soon as the header is received, so it's ready to retransmit as soon as it's finished. when doing layer 3, the switch tables include IP address in addition to MAC addresses.

Javier
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  • thank you very much for the fast and good answer. I just start to love this side. Could have saved me hours of research if just would have asked ;-) – megaflux Jul 04 '10 at 06:38