[Babel-users] [Henning Rogge] Re: Ideas about Babel

[Juliusz Chroboczek]

>> Section 3.4.1 of the protocol spec, which is implemented in the function
>> update_neighbour in the Babel sample implementation.

> Hmm... on a first look it does seem to be a straight forward Hello
> mechanism with adaptive timings, which are allowed in NHDP. I think
> I even remember NHDP talk about constraints how you can change the data
> rate and tell it your neighbors correctly.

Section 5.3 of the NHDP draft.  Unless I'm missing something, it doesn't
reliably detect changes in the hello interval in the presence of packet
loss (it will mis-compute the link quality in that case).  Babel doesn't
have that issue.

>> I'd rather you explained how OLSRv2 solves the problems of mis-behaviour of
>> link-state in unstable networks.  I do understand the mechanisms that avoid
>> this misbehaviour in OSPF.  I've seen no such mechanisms in OLSRv2.

> Unstable links should be handled on a link level... just create a routing 
> metric that add a penalty to unstable link so you try to use better links.

That is not the issue.

In the presence of instabilities in the network topology, the link-state
databases of different nodes become unsynchronised.  In the presence of
packet loss, this desynchronisation may persist for relatively long periods
of time.  Given enough instability and packet loss, the link-state
databases never get a chance to synchronise.

The SPF algorithm is not a distributed algorithm, it's a replicated one.
Hence, it creates routing loops and other pathologies in the presence of
desynchronised link-state databases.

OSPF and IS-IS deal with that by using a reliable protocol for flooding of
LSAs, which ensures timely resynchronisation.

EIGRP, DSDV, AODV and Babel deal with that by using a distributed algorithm
that behaves well even when the data is desynchronised.

Please point me at the place in the OLSRv2 spec where they either deal with
the issue of desynchronised link-state databases, or ensure that the
link-state databases will resynchronise in a timely manner.

                                        Juliusz