[e2e] Packet dropping (Khaled Elsayed)

Agarwal, Anil Anil.Agarwal at viasat.com
Thu May 3 08:43:10 PDT 2007

David reed wrote:

> Actually, TCP typically retransmits all the "out of order" packets you
refer to Arjuna, 
> because the sender won't know the receiver has received 
> them unless SACK is working, an option that is not necessarily there.

> So the receiver can just drop all the out of order packets after a
loss due to 
> congestion without affecting throughput.

This is not quite true (perhaps this is a simplified explanation),
since the sender, after a timeout will proceed to do slow start,
starting with cwnd = 1 segment. If the receiver has buffered segments
received after 
the lost packet in question, it will ack all segments after the lost
segment retransmission
is received; the other segments will not be retransmitted.

Similarly, a fast retransmit (without SACK) will cause the lost segment
to be retransmitted. 
If the retransmission succeeds, then no other segments will be
retransmitted, if the receiver
has buffered other segments. Also, slow-start will not be triggered.

Hence, it helps to buffer out-of-sequence receive segments.


Arjuna Sathiaseelan wrote:
> My belief is as Craig said, for real-time packets - dropping the 
> oldest packet would be the best solution - so it would be better to 
> drop from the front of the queue, as most of the real-time packets 
> (VoIP,
> videoconferencing) would be carried on UDP or DCCP - which do not 
> require transport layer retransmissions. We need to note dropping 
> real-time packets such as VoIP packets (carried by UDP or DCCP) would 
> be more of a concern to the application layer rather than the
transport layer.
> But for non-real time applications running over TCP - then I would 
> prefer to see the new packet being dropped rather the oldest packet - 
> as it would be a burden to the transport layer - since the transport 
> layer has to buffer up all the out of order packets!
> Arjuna
> ------------------------------
> Message: 2
> Date: Wed, 02 May 2007 06:33:08 -0400
> From: Craig Partridge <craig at aland.bbn.com>
> Subject: Re: [e2e] Packet dropping
> To: Khaled Elsayed <kelsayed at gmail.com>
> Cc: end2end-interest at postel.org
> Message-ID: <20070502103308.0E521123842 at aland.bbn.com>
> For non-real time, the answer I believe is drop the new packet.
> Dropping the earlier packet (assuming the earlier packet has a lower 
> sequence number) is more likely to slow effective delivery of data to 
> the recipient and require a more complex set of retransmissions to 
> recover from.
> Craig
> ***************************************

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