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Reordering of Packets in Multilink

Hi,

Following is the multilink config

Router#sh ppp multilink interface m3

Multilink3, bundle name is INTIN_DPH_10001

Bundle up for 1d06h, 11/255 load

Receive buffer limit 24384 bytes, frag timeout 1000 ms

0/0 fragments/bytes in reassembly list

5 lost fragments, 2864076 reordered

45/11668 discarded fragments/bytes, 0 lost received

0x187D9C received sequence, 0xE73DF5 sent sequence

Member links: 2 active, 0 inactive (max not set, min not set)

Se2/0.1/2/5/2:0, since 21:28:34

Se2/0.1/2/5/3:0, since 21:28:38

I have VOIP running over this Multilink interface ...

Can you suggest if reordered packets are within limits.

What are the reasons of the reordered packets and how to troubleshoot such problem..

Regards,

GP

1 REPLY
Anonymous
N/A

Re: Reordering of Packets in Multilink

Multilinking basically bundles a number of physical links in a logical bundle. When a packet is to be sent out this interface it is split into a number of parts equal to the number of physical links and the parts are sent down each link and received at the other end where they are reassembled.

eg for 2 physical links

LINK1 ------------------Part1-------------------- LINK1

PACKET ->

LINK2 ------------------Part2-------------------- LINK2

These physical links are generally BRI T1 or some kind of serial connection that probably goes through a telco's circuit switched cloud. The circuits chosen by the switches in the telco network will more than likely be different for each link - even to the point that one link may have more switch hops than the other. In any case the result is that the there will be a differnt delay on each link (nothing significant - we are talking fractions of a second, but a delay nonetheless).

The result of this delay is that in some instances part2 of the packet could arrive a brief time before part1.

When this is the case the router will wait for part1 and when it is recieved it will put the parts together in the correct order and pass the packet through the router to be processed and forwarded. At the same time the reorder counter is incremented.

This is absolutely normal and nothing to worry about. If the second part of the packet never arrives (there is a timer that counts down when the first packet is recieved and if the second part does not come it regards it as lost) - then a drop is counted and this is something to be concerned about.

There is nothing you can do about this - it is simple physics, even if the links provided by the telco followed the exact path there will still be some delay due to cable lenght temperature etcetera - nor is it something to be worried about.

This is very different from TCP reorders - TCP packets will be part of the traffic that goes into the multilink bundle.. I will try to explain this in a diagram - Please view with a fixed size font

eg courier

LINK1 4a 3a 2a 1a LINK1

1 2 3 4 ====================================== 1 2 3 4

LINK2 4b 3b 2b 1b LINK2

Here we see four tcp packets being sent through the multilink bundle - there is varying delay on the links so we see that the receiving router will get the packets in the following order

1a, 1b, 2a, 2b, 3b, 3a, 4b, 4a

From this we can see that it has to reorder the parts of packet 3 and packet 4 BUT the TCP pakets still arrive in the sent order (eg 1,2,3,4).

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