sorry, but ...
zenadsl6186 at zen.co.uk
Tue Aug 14 02:44:40 BST 2012
On 13/08/12 21:57, Derek Fawcus wrote:
> On Wed, Aug 01, 2012 at 10:05:57AM +0100, Andrew Bangs wrote:
>> Intercepting at switches (vs literally 'on the wire') always reminds me of Zeno's arrow paradox.
>> We somehow have to pretend that the thing to be intercepted is stationary if this is really different from doing it on the wire,
>> or at least that it's received and then has stuff done to it before being sent on its way.
> No pretense is necessary, that is usually how they operate; with store and forward on the routers/switches.
>> (for anyone thirsty for further reading, "cut-through switching" is a phrase your favourite search engine could investigate)
> I think there were some early switches which did cut through, as a way of reducing
> latency, but the down side is that damaged packets can then propogate.
> I don't know of any IP routers which do cut through forwarding when operating at the IP level.
> Likewise for most MPLS switches. The packet is fully received, and checksums verified before
> I think I may have read of some new optical switches (with an MPLS varient) which do something
> like cut-through, but it is more a case of lambda switching; where certain MPLS tags get
> associated with specific colours. This would generally be in the core of the network,
> and one is then in to having to use optical splices to intercept.
> I would suggest that 90% (or more) of current internet routers/switches are store and forward,
> even when the have hardware forwarding engines.
IIRC that's the way the internet was designed; it is built on packets
rather than connections.
Cut through switching is considerably more complicated and expensive
than store-and-forward; the expense is only really justifiable where
very low latency is critical, eg in supercomputer networks.
It's not needed at all for the everyday internet.
[and I expect even cut through switches store-and-forward at least one
bit, for a gazillionth of a second - a bit is not a communication, but
then a packet is not usually the whole of a communication either]
[not even I would expect judges to fall for that... but what do I kno'?]
-- Peter F.
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