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Out of order impact?

asked 2019-12-08 18:48:12 +0000

TheWizard gravatar image

Hey!

I'm looking at a pcap with alot of "out of order" packets. There are some fast retransmission occuring but fast/Retransmission are rarely occuring as it seems to catching up pretty fast to the correct sequence number.

Does out of order have any impact if you don't see retransmission or will tcp seamlessy arrange them and present to the next layer/application

What usually causes out of order packets besides multipath? Buffering? Firewalls? Capture on local machine?

Don't think it will be possible to share the pcap.

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answered 2019-12-15 07:58:43 +0000

The receiving device will put the packets back into the correct order before extracting the data and passing it up t the application.

You run into problems when one packet is overtaken by 3 or more packets because this triggers the Dup-Ack and/or SACK mechanism - which causes unnecessary retransmissions. This doesn't always cause performance problems though.

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I would like to put a nuance to that. The (unnecessary) retransmissions will trigger a reduction off the Congestion Window on the sender side. On links with a round-trip-time of a few milliseconds or more, this might limit the bandwidth of the connection as there can only be sent one congestion window worth of data per roundtrip.

SYN-bit gravatar imageSYN-bit ( 2019-12-15 10:16:51 +0000 )edit

That's a good point @SYN-bit. Yes, that could cause a performance hit.

Philst gravatar imagePhilst ( 2019-12-15 10:59:43 +0000 )edit
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answered 2019-12-11 01:48:05 +0000

Hi,

Another reason for out of order may be fragmentation. Fragment arrives out of order from rest of traffic for instance.

Out of order packets need to be put back in order at some point, usually on the receiving host.

This means memory is allocated to hold the packets in buffers while waiting for all the packets to arrive.

This also means time is "wasted" doing the reordering.

How bad this affects your traffic depends on the quantity of out of order packets, the capability of the receiving host and the type of traffic.

Cheers,

JF

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Asked: 2019-12-08 18:48:12 +0000

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Last updated: Dec 15 '19