Traffic Management for the Available Bit Rate (ABR) Service in ...

8. Larger bu ers increase TCP throughput.
9. Larger number of window-limited sources increase TCP throughput. This is
because, the sum of the windows is larger when there are more sources.
10. Even when the bu ers are small, dropping of EOM cells should be avoided.
This avoids merging of packets at the destination AAL5 and improves fairness.
When su cient bu ers are provided for ABR, the drop policy assumes only a
minor importance, unlike its role in the UBR service.
8.14 Bu ering Requirements for TCP over ABR
In this section, we analyze the bu er requirement at switches for TCP over the
ATM-ABR service. We show by a combination of emperical and analytical studies
that the bu er requirement for TCP over ABR for zero loss transmission is:
(a RTT + b Averaging Interval Length + c feedback delay) link bandwidth,
for low values of the coe cients
This requirement is heavily dependent on the switch algorithm. With the ERICA+
algorithm, typical conservative values of the coe cients are (a =3�b=1�c=1).
The formula is a linear relation on three key factors:
Round trip time (RTT): Twice the delay through the ABR network or segment
(delimited by VS/VD switch(es)).
Averaging Interval Length: Aquantity which captures the measurement aspects
of a switch congestion control algorithm. Typical measured quantities are: ABR
capacity, average queue length, ABR input rate, numberofactive sources, and
VC's rate.
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