RE: [nsp] a question CRC error and resent

Date: Sun Apr 22 2001 - 05:22:42 EDT

> The only time a router will retransmit a packet transiting it is in the
> case of half-duplex Ethernet link when a collision occurs. This is
> required as part of the Ethernet spec., and is the case for any device
> on the Ethernet network, not just routers.

And if you want to go into details, it is disputable if this should be
called a retransmit, since a collision will be detected during the first
64 bytes of a packet (ie. the whole packet may not have been sent in the
first place).

There is no link layer (hop by hop) retransmission for the media that
TCP normally runs on - TCP retransmit happens directly between the end
systems. You can of course run TCP on top of a link layer with hop by
hop retransmission (X.25 and LAPB comes to mind) - but this is normally
done only in very special circumstances.

(Old story: Many years ago we needed to establish communication between
Cisco routers and HP-ER routers. At the time, the only link layer these
had in common for serial lines was X.25 - so X.25 was used. Later PPP
became available, and we switched everything to PPP.)

Steinar Haug, Nethelp consulting,

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