Subject: [manet] Hello messages in AODV

From: Alicia Triviño <[email-masked]>
Date: Thu, 29 Sep 2005 12:38:45 +0200

In AODV protocol, nodes may use hello messages. When a node receives a 
Hello message, should it update the routing table for the entry related to 
the source that sent the hello message?

Thanks in advance,

Alicia Triviño                               / Tl. +34-95.213.71.91
Dpto. Tecnologia Electronica
E.T.S. Ing. Telecomunicacion
Universidad de Malaga
Campus Universitario de Teatinos
29071 Malaga, Spain

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From: "Gabi Kliot" <[email-masked]>
Date: Thu, 29 Sep 2005 16:22:40 +0200

Hello Alicia

Hello messages in AODV are used in the route maintenance part of the AODV protocol.
When hello message from node B is received by node A, A refreshes all entries in its routing table in which node B appears to be the next hop.

If A has not heard from B for some amount of time (no hello messages and no regular message was received by A from B), A assumes that B is no longer its neighbor and invalidates all routes through B (routes to all destinations in which B is the next hop). A may also send an RERR message now to all nodes that used to route through him in order to get to this "now unreachable" destination.

If, when receiving hello message from B,  A does not have any route in which B is the next hop - as far as I know, A does not establish this one hop route to B (but I might be wrong).

Hope it helps,

Best Regards,

Gabi Kliot

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]] 
> On Behalf Of Alicia Triviסo
> Sent: Thursday, September 29, 2005 12:39 PM
> To: [email-masked]
> Subject: [manet] Hello messages in AODV
> 
>

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From: "Charles E. Perkins" <[email-masked]>
Date: Thu, 29 Sep 2005 17:16:10 +0300

Hello folks,

>If, when receiving hello message from B,  A does not have any route in which B is the next hop - as far as I know, A does not establish this one hop route to B (but I might be wrong).
It might be a good idea anyway for A to keep track of the fact that B is
reachable, since it potentially useful information and fits naturally 
into the
route table.

Other methods besides "Hello" messages can be used to keep track of
links to next hops.  For instance, if a packet is received from a neighbor,
then that neighbor is known to be within range even if no Hello messages
have been received from it recently.  Or, perhaps there are some layer-2
indications about whether the link is still up and running.

Regards,
Charlie P.

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From: Carles Gómez <[email-masked]>
Date: Thu, 29 Sep 2005 20:48:02 +0200

Hola Alicia, 

The lack of a number of Hello messages from a neighbor in an active route may 
be used as a way to detect a link failure. 

A study on the impact of the hello message mechanism on a lab AODV ad-hoc 
network has recently been published in PIMRC'05. The paper is 
entitled "Evaluating performance of real ad-hoc networks using AODV with hello 
message mechanism for maintaining local connectivity". Some tradeoffs 
(reactivity to topology changes Vs. bandwidth and power consumption) are 
evaluated as a function of HELLO_INTERVAL. If you are interested in 
it, please e-mail me.

Best regards, 

Carles Gomez i Montenegro 
Technical University of Catalonia 

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From: "Gabi Kliot" <[email-masked]>
Date: Fri, 30 Sep 2005 10:09:58 +0200

Hi

The lack of a number of Hello messages from a neighbor is sure one of
the ways to detect link failures.
I would say that it is a proactive way to detect link failures.

The reactive way is for routing protocol to receive MAC layer
notifications after MAC fails in all its retransmission for a single
message.
Here the number of MAC layer retransmissions would play an important
role on the performance of routing algorithm and on the bandwidth and
power consumption. The smaller the number of MAC retransmissions - the
faster we will detect the link failure, the less power is wasted and
etc.. But on the bad side: the smaller the number of MAC retransmissions
- the higher is the chance for "false alarms", false detection of link
failure, which could actually be caused by temporal contention. Such
false alarm will destroy perfectly good route, can cause useless RERR
messages, etc...
As far as I know, the usually common number of MAC retransmissions is
6-7.

MAC layer failure notifications are also simulated in NS-2 and in
Glomosim.

One of the interesting ideas here could be to evaluate the tradeoffs
(the ones Carles mentioned) as a function of both HELLO_INTERVAL and MAC
layer retransmissions.

Best regards,

Gabi Kliot
Technion - Israel Institute of Technology

> On Behalf Of Carles Gףmez
> Sent: Thursday, September 29, 2005 8:48 PM
> To: [email-masked]
> Subject: Re: [manet] Hello messages in AODV
> 
> 
> Hola Alicia, 
>