multiple routing tables roadmap

Wed Dec 26 15:48:35 PST 2007

On thing where FreeBSD has been falling behind, and which by chance I
have some time to work on is "policy based routing", which allows different
packet streams to be routed by more than just the destination address.

Constraints:
------------

I want to make some form of this available in the 6.x tree 
(and by extension 7.x) , but FreeBSD in general needs it so I might as well
do it in -current and back port the portions I need.

One of the ways that this can be done is to have the ability to instantiate
multiple kernel routing tables (which I will now refer to as 
"Forwarding Information Bases" or "FIBs" for political correctness 
reasons. Which FIB a particular packet uses to make the next hop decision
can be decided by a number of mechanisms. The policies these mechanisms
implement are the "Policies" referred to in "Policy based routing".

One of the constraints I have if I try to back port this work to 6.x is that
it must be implemented as a EXTENSION to the existing ABIs in 6.x so that
third party applications do not need to be recompiled in timespan
of the branch.  

Implementation method, (part 1)
-------------------------------
For this reason I have implemented a "sufficient subset" of a
multiple routing table solution in Perforce, and back-ported it
to 6.x. (also in Perforce though not yet caught up with what I
have done in -current/P4). The subset allows a number of FIBs
to be defined at compile time (sufficient for my purposes in 6.x) and 
implements the changes needed to allow IPV4 to use them. I have not done
the changes for ipv6 simply because I do not need it, and I do not
have enough knowledge of ipv6 (e.g. neighbor discovery) needed to do it.

Other protocol families are left untouched and should there be 
users with proprietary protocol families, they should continue to work
and be oblivious to the existence of the extra FIBs.

To understand how this is done, one must know that the current FIB code
starts everything off with a single dimensional array of pointers 
to FIB head structures (One per protocol family), each of which in 
turn points to the trie of routes available to that family.

The basic change in the ABI compatible version of the change is to extent that
array to be a 2 dimensional array, so that instead of protocol family X
looking at rt_tables[X] for the table it needs, it looks at rt_tables[Y][X]
when for all protocol families except ipv4 Y is always 0.
Code that is unaware of the change always just sees the first row
of the table, which of course looks just like the one dimensional
array that existed before.

The entry points rtrequest(), rtalloc(), rtalloc1(), rtalloc_ign()
are all maintained, but refer only to the first row of the array,
so that existing callers in proprietary protocols can continue to 
do the "right thing".
Some new entry points are added, for the exclusive use of ipv4 code
called in_rtrequest(), in_rtalloc(), in_rtalloc1() and in_rtalloc_ign(),
which have an extra argument which refers the code to the correct row.

In addition, there are some new entry points (currently called
dom_rtalloc() and friends) that check the Address family being looked up and 
call either rtalloc() (and friends) if the protocol is not IPv4 forcing the 
action to row 0 or to the appropriate row if it IS IPv4 (and that info is 
available). These are for calling from code that is not specific to any
particular protocol. The way these are implemented would change 
in the non ABI preserving code to be added later.

One feature of the first version of the code is that for ipv4, the 
interface routes show up automatically on all the FIBs, so that 
no matter what FIB you select you always have the basic direct attached 
hosts available to you. (rtinit() does this automatically).
you CAN delete an interface route from one FIB should you want to 
but by default it's there. ARP information is also available 
in each FIB. It's assumed that the same machine would have the same 
MAC address, regardless of which FIB you are using to get to it.

This brings us as to how the correct FIB is selected for an outgoing
IPV4 packet.

Packets fall into one of a number of classes.
1/ locally generated packets, coming from a socket/PCB.
   Such packets select a FIB from a number associated with the 
   socket/PCB. This in turn is inherited from the process,
   but can be changed by a socket option.  The process in turn 
   inherits it on fork. I have written  a utility call setfib
   that acts a bit like nice..  
      setfib -n 3 ping target.example.com  # will use fib 3 for ping.

2/ packets received on an interface for forwarding.
   By default these packets would use table 0,
   (or possibly a number settable in a sysctl(not yet)).
   but prior to routing the firewall can inspect them (see below).

3/ packets inspected by a packet classifier, which can arbitrarily
   associate a fib with it on a packet by packet basis.
   A fib assigned to a packet by a packet classifier
   (such as ipfw) would over-ride a fib associated by
   a more default source. (such as cases 1 or 2).

routing messages would be associated with their
process, and thus select one FIB or another.
In addition Netstat has been edited to be able to cope with the 
fact that the array is now 2 dimensional. (It looks in system
memory using libkvm (!)).

In addition two sysctls are added to give:
a) the number of FIBs compiled in (active)
b) the default FIB of the calling process.

Early testing experience:
-------------------------

Basically our (IronPort's) appliance does this functionality already using 
ipfw fwd but that method has some drawbacks.

For example,
It can't fully simulate a routing table because it can't influence the
socket's choice of local address when a connect() is done.

Testing during the generating of these changes has been 
remarkably smooth so far. Multiple tables have co-existed 
with no notable side effects, and packets have been routes accordingly.

I have not yet added the changes to ipfw. 
pf has some similar changes already but they seem to rely on 
the various FIBs having symbolic names. Which I do not plan to support 
in the first verion of these changes.

SCTP has interestingly enough buiold in support for this, called VRFs
in cisco parlance. it will be intersting to see how that handles it when
it suddenly actually does something.

I have not redone my testing since my last edits, but will be retesting with the 
current code asap.

Where to next:
--------------------

After committing the ABI compatible version and MFCing it, I'd 
like to proceed in a forward direction in -current. this will 
result in some rototilling in the routing code.

Firstly: the current code's idea of having a separate tree per 
protocol family, all of the same format, and pointed to by the 
1 dimensional array is a bit silly. Especially when one considers that there
is code that makes assumptions about every protocol having the same
internal structures there.  Some protocols don't WANT that
sort of structure. (for example the whole idea of a netmask is foreign
to appletalk). This needs to be made opaque to the external code.

My suggested first change is to add routing method pointers to the
'domain' structure, along with information pointing the data.
instead of having an array of pointers to uniform structures,
there would be an array pointing to the 'domain' structures
for each protocol address domain (protocol family),
and the methods this reached would be called. The methods would have
an argument that gives FIB number, but the protocol would be free
to ignore it.

Interaction with the ARP layer/ LL layer would need to be 
revisited as well. Qing Li has been working on this already.

diffs
for those with p4 access:
p4 diff2 -du  //depot/vendor/freebsd/src/sys/... at 131121 //depot/user/julian/routing/src/sys/... 

for those with the makediff perl script:
perl ~/makediff.pl //depot/vendor/freebsd/src/sys/... at 131121 //depot/user/julian/routing/src/sys/... 

for those with neither:

http://people.freebsd.org/~julian/mrt2.diff

I just put the userland utility in usr.sbin/setfib/ in p4.
and changes to netstat in usr.bin/netstat/

see:
http://perforce.freebsd.org/depotTreeBrowser.cgi?FSPC=//depot/user/julian/routing/src&HIDEDEL=NO

I'd like to get comments on this (compat) version, so that I can commit it, 
get general testing under way to start the clock for MFC, and then get
moving on the fuller implementation (that breaks ABIs) and other routing issues.

Julian