image stream routers

Sat Sep 17 06:18:28 UTC 2005

Christopher J. Wolff wrote:
> I'd be interested to know the relative pros and cons of switching packets in
> software (Imagestream) versus handing them off to a dedicated ASIC (Cisco,
> Juniper)

[without having looked at Imagestream in any way, shape or form..]

it would be _unlikely_ that any router vendor that wants to support >OC3 
could do so with the 'standard' (non-modified) linux IP stack.  if they 
are modifying the 'standard' linux IP stack then its very unlikely that 
one could do so without having to publish the source-code to it.  (i.e. 
as per GPL).

'standard' linux on standard hardware isn't capable of much more than 
100K PPS.  sure - some folks have a few hundred packets/sec - but these 
are minimalist versus the demonstrated performance of ASIC-based 
forwarding, typically 30M-50M PPS.

one advantage of software is programmability.  if there is a bug you can 
fix it.
if there is a bug in an ASIC, it may or may not be possible to fix it - 
it depends on awful lot on how the ASIC is built (whether its 100% fixed 
functionality or supports limited programmability in various stages of 
the forwarding pipeline).
it may be that its not fixable but that the ASIC allows 
software-workarounds - in essence, 'fixing' something by diverting it to 
a (slower) software-path.

note that there is a correction to make here: not all routers _ARE_ 
ASIC-based for forwarding.  in fact, most of the Cisco /router/ product 
portfolio isn't hardware-forwarding based.  generally speaking it isn't 
necessary - UNTIL you get to the point of having interface speeds & 
number-of-interfaces which exceed the capabilities of general-purpose 
processors.  that is, typically somewhere between 100K PPS and 1M PPS.

cheers,

lincoln.