Subject: [manet] Interactions between TCP and Routing

From: Pietro Michiardi <[email-masked]>
Date: Thu, 10 Mar 2005 17:32:41 +0100

Dear MANET people,
I have a question for you.

I've run a test on GlomoSim for an ad hoc network formed by 4 nodes 
placed in the following scenario:

            n2

n1                         n3

            n4

n1 has 2 neighbors: n2,n4
n3 has 2 neighbors: n2,n4
n2 has 2 neighbors: n1,n3
n4 has 2 neighbors: n1,n3

There is NO MOBILITY.
Nodes use the DSR routing protocol (implemented in a pretty old version, 
v.3 to be precise)

Now suppose n1 wants to send 10 items to n3 using a FTP session (over TCP).

Running my simulations, I have noticed the following:
 
1- it happens that n1 selects route <n1,n2,n3> to send FTP items, while 
n3 performs a new route discovery and selects route <n3,n4,n1> to send 
back acks.
Actually, TCP SYN is sent on the former route and TCP SYN ACK is sent on 
the latter: the three-way TCP handshake is performed on two different 
routes.

2- in some other cases (it depends on the simulation SEED parameter) the 
3-way handshake is carried out on the same route.

3- it might happen that TCP DATA ACK are sent on a different route than 
the one used by actual data.

Reading some DSR documents, I found that route request piggybacking is 
not implemented for such short messages (section 4.2 of the original 
paper by D. Johnson).

I have run few experiments to see if there are any variations on 
classical performance metrics such as trhoughput and delay. Apparently 
there are very small differences, probably due to the simplicity of the 
simulation scenario.

Ok, here are my questions:
- Do you guys see any problems with that?
- Did any of you make any similar experiments?
- Does this apply to any other ad hoc routing protocol? OLSR? (i didn't 
run tests since I have no olsr implementation in glomosim)
- Could performance be an issue in a more complex scenario?
- What about node mobility? Wouldn't it be better to take the same route 
for data and acks?

Thanks for your help!
Pietro

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