svn commit: r191259 - head/sys/netinet

[Marko Zec]

On Monday 20 April 2009 09:01:25 Kip Macy wrote:
...
> > But it seems to me that CAM lookups are pretty resilient against DoSing
> > by throwing malicious synthetic flows on them, whereas flow caches will
> > melt down easily.
>
> Actually a CAM is a hardware implementation of a hash table. It has
> the same limitations. To claim that routers don't use flow tables
> because they are handled in hardware is a very strange thing to say.

Well I may be missing something, but TCAMs typically used for routing lookups 
are populated by the router's control plane, i.e. routing protocols, which 
means that the number of entries in the FIB / TCAM correlates to the size of 
RIB, i.e. it definitely doesn't grow / shrink dynamically in response to the 
current flow pattern.

And I may not know how CAMs are implemented internally, but I'm not aware of 
any current vendor who would use (T)CAMs indexed by a flow hash for routing 
lookups.  Wouldn't it be a more common case for a TCAM to hold a FIB table, 
sorted in a way which lets more specific prefixes having precedence?

i.e.

FIB            TCAM
10.0.1.0/24 -> 00001010 00000000 00000001 XXXXXXXX -> output port X
10.0.0.0/8  -> 00001010 XXXXXXXX XXXXXXXX XXXXXXXX -> output port Y
0.0.0.0/0   -> XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX -> output port Z

This definitely doesn't change with flows dynamics IMO.

Marko