On 9/23/06, Sally Floyd <sallyfloyd@xxxxxxx> wrote:
> Why not just calculate a packet rate per second? Or am I missing
> something obvious?
No, that is a good question.
One reason for including the packet size s is discussed in
Section 5.3 of RFC 4342:
"The packet size s is used in the TCP throughput equation. A CCID 3
implementation MAY calculate s as the segment size averaged over
multiple round trip times -- for example, over the most recent four
loss intervals, for loss intervals as defined in Section 6.1.
Alternately, a CCID 3 implementation MAY use the Maximum Packet Size
to derive s. In this case, s is set to the Maximum Segment Size
(MSS), the maximum size in bytes for the data segment, not including
the default DCCP and IP packet headers. Each packet transmitted then
counts as one MSS, regardless of the actual segment size, and the TCP
throughput equation can be interpreted as specifying the sending rate
in packets per second."
Thus, an implementation MAY calculate the allowed sending rate
in bytes per second, using for s the average segment size.
Or an implementation may use the MSS for s, and in fact calculate
the allowed sending rate simply in packets per second. This would be
a purely local implementation decision.
- Sally
I'm still thinking this all through and want to discuss this a bit further.
The papers by Padhye and Floyd for the TCP throughput equation all
seem to say that given a fixed RTT and fixed loss rate that you have a
fixed throughput rate in packets per second. Have I read this
correctly?
The throughput rate is then based on the packet size for a given loss
rate and RTT. If packet sizes increase, throughput increases but the
packet transmission rate remains the same.
Said another way we don't alter packet sending rate if we alter the
size of the packet. I'm not sure that this is correct on things like
wireless links but this is the starting point for TFRC at present.
If the mss is used then t_ipi = s/X_inst according to TFRC spec. That
means that it will get less than fair share which makes sense (Richard
I had this around the wrong way when we were discussing).
I personally don't see why you would do all the calculations for
average packet size etc when you can simplify it by just counting
packets. I'm guessing that it was done in this way because people
wanted to calculate throughput rates due to the protocols being
thought of but this is far less relevant when you are datagram based
and don't normally fragment.
If I get time I'll experiment with implementations in Linux.
Regards,
Ian
--
Ian McDonald
Web: http://wand.net.nz/~iam4
Blog: http://imcdnzl.blogspot.com
WAND Network Research Group
Department of Computer Science
University of Waikato
New Zealand
