Isn’t that pretty much what Geoff Huston has done with the weekly reports William quoted earlier in this thread?
Sure, that’s from a limited set of perspectives, but it probably represents the minimum achievable compression in most circumstances.
Owen
On Oct 2, 2023, at 06:41, Joshua Miller <conte...@gmail.com> wrote:
Seems like we've reached the limits of apriori speculation. At this point I'd like to see data demonstrating that it's at least viable from a statistical perspective.
If someone is motivated to demonstrate this, a "backtest" against historical data would be the next step. Later, one could design the study to reveal "transient degradation" (loops, drops, etc.) and their probability, though the duration would be more a function of the implementation. It would be best to "backtest" the status quo as a control because it too has transient degradation, for a more apples-to-apples comparison.
I'm not sufficiently motivated (nor knowledgeable in statistics) to take this on. I see this more in the domain of vendors to determine the best approach for their implementation. On Monday, 2 October, 2023 09:39, "William Herrin" <b...@herrin.us> said:
> That depends. When the FIB gets too big, routers don't immediately
> die. Instead, their performance degrades. Just like what happens with
> oversubscription elsewhere in the system.
>
> With a TCAM-based router, the least specific routes get pushed off the
> TCAM (out of the fast path) up to the main CPU. As a result, the PPS
> (packets per second) degrades really fast.
>
> With a DRAM+SRAM cache system, the least used routes fall out of the
> cache. They haven't actually been pushed out of the fast path, but the
> fast path gets a little bit slower. The PPS degrades, but not as
> sharply as with a TCAM-based router.
Spit-balling here, is there a possible design for not-Tier-1 providers where routing optimality (which is probably not a word) degrades rather than packet-shifting performance?
If the FIB is full, can we start making controlled and/or smart decisions about what to install, rather than either of the simple overflow conditions?
For starters, as long as you have *somewhere* you can point a default at in the worst case, even if it's far from the *best* route, you make damn sure you always install a default.
Then you could have knobs for what other routes you discard when you run out of space. Receiving a covering /16? Maybe you can drop the /24s, even if they have a different next hop - routing will be sub-optimal, but it will work. (I know, previous discussions around traffic engineering and whether the originating network must / does do that in practice...)
Understand which routes your customers care about / where most of your traffic goes? Set the "FIB-preference" on those routes as you receive them, to give them the greatest chance of getting installed.
Not a hardware designer, I have little idea as to how feasible this is - I suspect it depends on the rate of churn, complexity of FIB updates, etc. But it feels like there could be a way to build something other than "shortest -> punt to CPU" or "LRU -> punt to CPU".
Or is everyone who could make use of this already doing the same filtering at the RIB level, and not trying to fit a quart RIB into a pint FIB in the first place?
Thanks,
Tim.
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