initial call for review.. initial multi-fib (routing table) support

Bruce M. Simpson bms at
Fri Dec 14 02:51:13 PST 2007


First of all, thank you very much for starting this work in a much 
needed area.

Julian Elischer wrote:
> This is a call for review for a change that is part of a
> longer term project.
> This implements multiple routing tables. Eventually the implementation 
> will be much cleaner but
> the first implementation is designed to be backported to 6.x
> and thus must be ABI compatible. It need not be particularly 'clean'
> as the version in 8.x will be..  First itis needs to be committed to 
> -current in its 6.x form so an MFC can occur, then the cleaner version 
> can be committed over the top of it to clean it up.

Few comments:
Allocating multiple radix trie heads is one way of doing this, but it 
would be nice to be able to clean up the memory management in the radix 
trie in general.

I've seen implementations which do this by assigning index numbers or 
bit sets to the radix trie entries. That way, you don't need to keep 
multiple redundant copies of the same data around -- this IS the kernel 
FIB after all, and if you're running a router in the Default Free Zone, 
or with a considerable BGP topology, this kind of redundancy in the 
forwarding plane is not an OK use of memory resources.

It's been a few months, but I believe this is how OpenBSD does it; ipfw 
also does something similar deep in its innards, the rules are tagged 
with bitsets to specify which sets they are present in.
  [I see similar memory management issues with C++ STL containers, which 
irritates me; Boost++'s multi_index_container is an analogous idiom.]

One of the big strengths of the BSD radix trie, as implemented by Keith 
Sklower, was that it could be regression tested independently of the 
kernel. I'd very much like to see this capability retained, and perhaps 
expanded upon, as this is a sensitive area of work.

I'd encourage you to take a look at the OpenBSD changes. They are much 
less invasive than this patch, and whilst they don't provide the 
setfib() syscall functionality, that could be easily grafted on top. I 
understand your folk's requirements for multiple tables, I'm sure there 
is a possible fit here given the idioms described herein.

As I say it's been months since I last had a chance to look at this, and 
I am busy finishing up the first phase of another project, so I don't 
have all of these changes to hand -- however -- here's a good date and 
starting position:

I know there is an element of Not-Invented-Here which creeps in, but, 
when all is said and done, OpenBSD's approach is viable, compact, and 
simple, and addresses folk's immediate requirements for multi-path support.

They don't address SMP, multicast, or source address selection, but 
those are future development stories.


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