Subject: [manet] Routing protocol (DSR) guarentees of packet delivery

From: [email-masked] (Noah H. Miller)
Date: 13 Sep 2002 10:24:33 EDT

Hello,

What requirements must the routing protocol of a MANET fulfill with regard to packet delivery?  It seems there are no guarentees of 100% packet delivery or sequential packet delivery.  But my question pertains specifically to packet duplication: must a ro
uting protocol guarentee that no packets will be duplicated?

The question is also specific to a problem in DSR, draft 7.  DSR stores in a Maintenance Buffer all the packets that it has pushed down onto the MAC layer for delivery.  If the MAC layer notifies the Maintenance Buffer that a packet with next-hop address 
X failed, all the packets in the Maintenance Buffer with next-hop address X will be removed (section 4.5).  Now, consider the case of two packets pushed to address X.  Packet 1 fails, and both Packets 1 and 2 are removed from the Maintenance Buffer, salva
ged, and retransmitted on a different route with next-hop address Z.  However, Packet 2 with next-hop address X is still in the MAC layer waiting for delivery, now alongside Packet 2 with next-hop address Z.  Packet 2:X may actually be delivered, if node 
X moves back into contact, and so the destination of Packet 2 may receive two copies, one through X and one through Z.
Is this a problem with the draft?  Is there a correction to avoid this duplication?  Or does it not matter?

Thank you,
Noah Miller
ISTS, Dartmouth College
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From: Yih-Chun Hu <[email-masked]>
Date: Fri, 13 Sep 2002 12:34:12 -0400 (EDT)

>What requirements must the routing protocol of a MANET fulfill with
>regard to packet delivery?  It seems there are no guarentees of 100%
>packet delivery or sequential packet delivery.  But my question pertains
>specifically to packet duplication: must a ro uting protocol guarentee
>that no packets will be duplicated? 

No. In the IP model, packets may be lost, duplicated, or reordered.

As for your DSR draft question, Section 3.4.2 says: 
    -  Remove the packet from the node's Network Interface Queue and
       Maintenance Buffer.
with regards to packet 2, so 2:X leaves the MAC layer.

In general, duplication is possible in manets. For example, consider a
node A using any routing protocol forwarding packet 1 to its next-hop
destination B. A sends an RTS to B, B replies with a CTS, A sends the
DATA, B sends the ACK, but A doesn't receive it. B forwards the packet,
but A does not hear the forward, due to hidden-terminal interference.
Enough traffic passes near B to flush its duplicate suppression mechanism,
then A decides to send another RTS for packet 1. B sends a CTS, receives
DATA, sends ACK, and unless B has an infinite-space duplicate suppression
mechanism, or unless there exists a _hard_ time bound on the amount of
time packet 1 will be retried at the MAC.

Hope this helps,
Yih-Chun

>The question is also specific to a problem in DSR, draft 7.  DSR stores
>in a Maintenance Buffer all the packets that it has pushed down onto the
>MAC layer for delivery.  If the MAC layer notifies the Maintenance Buffer
>that a packet with next-hop address X failed, all the packets in the
>Maintenance Buffer with next-hop address X will be removed (section 4.5). 
>Now, consider the case of two packets pushed to address X.  Packet 1
>fails, and both Packets 1 and 2 are removed from the Maintenance Buffer,
>salva ged, and retransmitted on a different route with next-hop address
>Z.  However, Packet 2 with next-hop address X is still in the MAC layer
>waiting for delivery, now alongside Packet 2 with next-hop address Z. 
>Packet 2:X may actually be delivered, if node X moves back into contact,
>and so the destination of Packet 2 may receive two copies, one through X
>and one through Z.  Is this a problem with the draft?  Is there a
>correction to avoid this duplication?  Or does it not matter? 
>
>Thank you,
>Noah Miller
>ISTS, Dartmouth College

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From: Yih-Chun Hu <[email-masked]>
Date: Fri, 13 Sep 2002 12:34:12 -0400 (EDT)

Hope this helps,
Yih-Chun

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From: Fred Baker <[email-masked]>
Date: Fri, 13 Sep 2002 14:31:07 -0700

At 10:24 AM 9/13/2002 -0400, Noah H. Miller wrote:
>What requirements must the routing protocol of a MANET fulfill with regard 
>to packet delivery?  It seems there are no guarentees of 100% packet 
>delivery or sequential packet delivery.  But my question pertains 
>specifically to packet duplication: must a routing protocol guarentee that 
>no packets will be duplicated?

For unicast traffic, the routing protocol is generally not relevant to 
packet duplication. A packet will be multiply delivered if a link layer 
event happens that causes the same packet to be handed to two different 
routing next hops (which might be the final system or any router along the 
way) or the same one twice. The obvious examples here are link or transport 
layer retransmissions - when an acknowledgement is not received by a 
sender, he cannot tell whether the reason is that the data got lost or 
whether the acknowledgement got lost, and in the latter case his 
retransmission will result in a duplicate receipt.

With multicast traffic, multicast routing generally attempts to deliver the 
packet exactly once, but there are a variety of scenarios in which that can 
fail, often related to changing routes.

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From: Fred Baker <[email-masked]>
Date: Fri, 13 Sep 2002 14:31:07 -0700