[CUWiN] HSLS Behavior Question
dyoung at pobox.com
Fri May 25 22:53:37 CDT 2007
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
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
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.
David Young OJC Technologies
dyoung at ojctech.com Urbana, IL * (217) 278-3933 ext 24
More information about the CU-Wireless