[CUWiN] HSLS Behavior Question

[David Young]

On Mon, May 21, 2007 at 05:12:47PM -0400, Sanjai Narain wrote:
> In our HSLS testbed with simulated mobility and losses I observed an odd 
> behavior as listed below. I would appreciate any explanation.
> 
> * There are four nodes, 6, 7, 10, 66.
> * Between times 292 and 598, 6 continues to advertise 10, 66, 7 as its 
> neighbors in Hello messages
> * Yet 10 does not advertise 6 in its Hello messages.
> 
> I assume that if 6 is advertising 10 as its neighbor, it has heard 10's 
> hello. So, 10 should advertise 6 as its neighbor. Thanks. -- Sanjai

Sanjai,

Sorry for the slow response, I was out of town for four days, and I fell
behind on email.

In a Hello packet, a node advertises only "in" links from adjacent
routers, that is, a node P advertises all of the links

P<--Q1
P<--Q2
...
P<--Qn

in its Hello packets.

When there are asymmetric links in the network, you may find that P
advertises P<--Q1 in its Hellos, however, Q1 does not advertise Q1<--P.

I do not expect asymmetry, per se, to be much of a problem in practice.
However, I believe there is more that we can do in order to keep routers
within the 2-hop "topology zone" (in the parlance of BBN Tech. Memo
#1301) of a destination up-to-date.  First, HSLS routers should compute
the bidirectional flag on linkstates, transmit and receive LSAs with
the flag.  Now and then I have called this the "bidirectional closure."
The BBN memo describes it.  Second, and more importantly, more events
should trigger a 1-hop linkstate advertisement.  For example, if the
number of hops to any "nearby" destination (0 hops or 1 hop away) has
grown following an SPF recalculation, then it is possible that the router
has just become part of a small loop that its neighbors will dissolve if
it sends a 1-hop linkstate advertisement.  Perhaps an HSLS router should
also send a 1-hop linkstate advertisement when any metric increases or
decreases by more than a certain threshold percentage.

Dave

-- 
David Young             OJC Technologies
dyoung at ojctech.com      Urbana, IL * (217) 278-3933 ext 24