sorry, but ...

[Derek Fawcus]

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
retransmission.

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.

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