Eugen Leitl wrote:

In future photonic networks (which will do relativistic cut-through directly in a photonic crossbar without converting photons to electrons and back) the fiber is not just a transport channel but also a photonic buffer

Yes.

(e.g. at 10 GBit/s Ethernet a short reach fiber already buffers a standard 1500 MTU).

Wrong. 10Gbps is too slow for optical buffering. At 1Tbps, you can use 100 times less lengthy fiber than at 10Gbps to buffer packets. A 1Tbps packet can be constructed by simultaneously encoding 100 wavelengths at 10Gbps.

Of course photonic gates are expensive, individual delays do add up so even with slow light buffers

Don't try to make light slower. Slow light buffers have resonators, which means they have very very very narrow bandwidth. Instead, make communication speed faster, which shortens fiber length of fiber delay line buffers.

or optical delay loops taken into consideration current TCP/IP header layout has not been optimized for leading edge containing most significant switching/routing information, or even local-knowledge routing (with no global routes). It's too bad IPv6 was not radical enough, so today's legacy protocols have to be tunneled through the networks of the future.

Considering that, in practice, packet headers must be processed electrically, IPv4 at the photonic backbone is just fine, if most routing table entries are aggregated at /24 or better, which is the current practice. You only have to read a 16M entry SRAM. A problem of IPv6 with 128bit addresses is that route look up can not be performed within a constant time of a few nano seconds, which means packets have overrun fiber delay lines.

I presume this future is some 20-30 years away still.

Not so much. Moore's law requires much rapid bandwidth increase. My slides presented at IEEE photonics society 2009 summer topical ftp://chacha.hpcl.titech.ac.jp/IEEE-ST.ppt might be interesting for you. Masataka Ohta