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packet switching is a protocol, where a message received from tcp layer is divided into packets only at sender machine ip layer and each packet is sent individually on different routes with an identification field set in ip header to help use re-assemble at destination machine.

Where as

fragmentation at ip layer is done on sender machine or any of the, on the way layer 3 device ip layer and fragmentation field is set in ip header to help use re-assemble at destination machine only.

My question:

  1. is my understanding correct?

  2. In packet switching, If message could not be re-assembled due to missing packet at destination, based on identification field, that message is discarded at ip layer of destination machine and tcp layer of sender machine will take care of retransmission of that message, am i correct?

overexchange
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1 Answers1

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packet switching is a protocol

No. Packet switching is an alternative to circuit switching at the physical layer.

where a message received from tcp layer is divided into packets only at sender machine ip layer and each packet is sent individually on different routes with an identification field set in ip header to help use re-assemble at destination machine.

None of this is correct as a description of packet switching. Packet switching implies the existence of packets, period. It doesn't impose any of these constraints.

Whereas fragmentation at ip layer is done on sender machine or any of the

... intermediate nodes

on the way layer 3 device ip layer

Layer 2

and fragmentation field is set in ip header to help use re-assemble at destination machine only.

My question:

is my understanding correct?

No. You seem to think that packet switching and fragmentation are in some kind of opposition. They aren't. Fragmentation is an extension of packet switching if anything, not an alternative to it.

In packet switching, If message could not be re-assembled due to missing packet at destination, based on identification field, that message is discarded at ip layer of destination machine and tcp layer of sender machine will take care of retransmission of that message, am i correct?

No. Again you're confused about what packet switching is. Your remark applies pretty well to TCP, but that's because of the semantics of TCP, not because of packet switching.

user207421
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  • "None of this incorrect as a description of packet switching. " Are you saying, None of this is correct in this statement? – overexchange May 26 '14 at 11:06
  • As you said, Packet switching is alternative to circuit switching, which is correct, But when you say at physical layer, i did not get you, Because i feel packet switching is possible only when you run ip protocol at network layer, Because ip layer will break data(coming from tcp) into packets and each packet is sent in multiple routes based on availability. – overexchange May 26 '14 at 11:59
  • In packet switched network, How ip layer in destination machine re-assemble all the dis-ordered packets? – overexchange May 26 '14 at 13:16
  • 1. 'This' refers to the paragraph I had just quoted, and 'as a description of packet switching' seems clear to me. 2. Packet switching is carried out by routers and switches and NICs at the Ethernet layer. Nothing to do with IP whatsoever. 3. It doesn't. It usually throws away an out of order packet. It is up to a higher layer, e.g. TCP, to re-request any missing packets. All that IP does is coalesce ordered fragments into packets. – user207421 May 26 '14 at 23:08
  • then what is the purpose of identification field in ip header? – overexchange May 27 '14 at 03:35
  • so if packet switching is about existence of packets, what about circuit switching? does that not have packets? – overexchange May 27 '14 at 03:40
  • 1. It identifies groups of fragments. 2. Circuit-switching has complete circuits, like in the old days of the telephone system. You can have packets on a switched circuit, but you don't have packet *switching.* – user207421 May 27 '14 at 22:22
  • i think identification filed is used for re-ordering of packets at destination ip layer and fragmentation field is used for grouping fragments. – overexchange May 31 '14 at 07:04