On Wed, 22 Mar 2000, Fred Wright wrote:
> There's a potential downside, however, in the lack of link-level flow
> control. Suppose ports A and B both want to send to port C. They can't
> do this indefinitely at full speed, since the switch can only have finite
> buffering. With a hub there'd be collisions and/or deferrals, and the
> stations would only send data as fast as it could be delivered. But with
> a switch running the ports in full-duplex mode, there's no possibility to
> report collisions, and the only recourse is to drop packets, which must be
> recovered by much slower higher-level retransmissions.
This isn't really much of an issue with IP, because TCP uses a slow start
algorithm and a back-off algorithm that decreases the speed at which
packets are sent when packets are lost in transit. There could be problems
with UDP, but UPD applications typically do not try to use all the
available bandwidth.
However, you should never forget that in the real world packets get lost.
Although there are many vendors that claim their switches are non-blocking,
there really isn't such a thing. So once in a while (or often, depending on
traffic) switches will drop packets because they can't forward them fast
enough. Also, no medium is perfect: there are always CRC errors. And some
types of equipment are designed to drop packets under certain conditions.
For instance: a Cisco router will drop the packet if the MAC address for
the next hop isn't in the ARP table. Packets are not buffered until there
is an ARP reply, but simply discarded. Also, very few systems have both the
hardware and software infrastructure to keep up with a continuous flow of
packets at wire speed.
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