re: [RRG] thoughts on the design space 3: caching

[Xu Xiaohu <xuxh@huawei.com>]

Hi Jari,

> First, I am not at all convinced that we actually HAVE to employ a cache
> for the forwarding lookup. The designs such as LISP are based on the
> idea that a subset of routers has to do more work than most routers in
> the Internet. This may be all that is needed; build those routers with
> the capability to have a full table. The rest of the routers will only
> need minimal amount of memory. If you believe this, then something like
> LISP + NERD would be far simpler and efficient than the other
alternatives.
> 
> Hasn't the industry built caching routers before, and what happened to
> them? Think we need a second try? I have not seen any ideas on how to
> remove the problems inherent with the caches, such as vulnerability to
> being hit by random destination addresses.

I believe the cache mechanism is still useful. However, it depends on what
you indeed expect it to do. If you just expect it to optimize the forwarding
path for the traffic, it would be OK. If you expect it to forward almost all
traffic, it would be doubtable. In other word, is it acceptable for the
overlay to forward most traffic once the cache becomes unavailable, e.g, the
cache is filled up due to some attacks.

> Second, even if you believe that we DO need a cache, there's really no
> reason why the cache design has to be cast into the rest of the
> architecture. Even if your forwarding ASIC can't employ enough memory
> for all the entries, I have a hard time convincing myself that we can't
> have a general purpose computer sitting next to it that holds the full
> table. I'm writing this on a laptop that has a 160G disk. That would be
> enough for several mapping tables containing EVERY IPV4 ADDRESS.

There is a similar idea with VA, that is, the full DFZ route is hold by
every router in its RIB, while only parts of the RIB will be installed into
its FIB. What's your opinion on this?

> Third, if you believe we must use an alternate topology for routing the
> initial packets while we are fetching for the cache entry, is that
> alternate topology a global infrastructure or a local device? Again, I
> have hard time getting myself convinced that we need to build a global
> infrastructure. The cost of the infrastructure is immense, and if you
> use the current BGP infrastructure you end up keeping there precisely
> the information that you wanted to get rid of. My take is that if you
> cannot build an ITR/ETR that holds the entire mapping table in memory,
> maybe it would be better to attach it to a general purpose computer that
> can (slowly) handle the cache misses and hand the packets back to the
> router.

In VA, the FIB can be distributed just within a single AS. So there is no
need for a global infrastructure in fact. What's your opinion on this?

Xiaohu Xu



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