Contents Telektronikk - Telenor

166
Throughput [Mbit/s]
Throughput [Mbit/s]
70
60
50
40
30
20
10
0
0 4096 8192 12288 16384 20480 24576 28672 32768
MSS segments to be transmitted back-toback.
There is also a throughput increase from
a 40k to a 52428 byte window size. This
increase is also caused by the size of the
socket buffers. The segment flow is still
primarily two MSSs in-between
acknowledgments for these window
sizes. Both window sizes have bytes for
two MSS segments ready in the socket
buffer when the acknowledgment arrives.
24576 byte window
32768 byte window
40960 byte window
52428 byte window
User data size [byte]
(a) TCP throughput dependent on window and user data size
60
50
40
30
20
10
0
0 16384 32768 49152 65536
Window size [byte]
(b)Throughput dependent on window size
4096 byte window
8192 byte window
16384 byte window
CPU utilization [%]
100
75
50
25
Transmit
Receive
0
0 16384 32768 49152 65536
Window size [byte]
(c) CPU utilization
Figure 15 TCP throughput, Sparc10 SBA-200
With a 52428 byte window size, the
sender has buffer space for more than
5*MSS bytes. It sometimes manages to
transmit 3 MSSs before the acknowledgment
arrives. In short, the window is better
utilized on connections with a 52428
byte window size which results in a higher
throughput.
For large window sizes and large user
data sizes there is no degradation in
throughput as with the SBA-100 adapt-
ers. With the SBA-200 adapter, the receiver
is less loaded than the sender.
Thus, the read system calls return before
the entire user receive buffer is filled.
Thereby, the socket layer more often
calls TCP to check if a window update
should be returned.
7.1 Throughput peak patterns
For small window sizes there are
throughput peaks which are even more
characteristic than peaks observed with
the SBA-100 interfaces on the Sparc10s.
For example, with an 8 kbyte window the
variation can be as high as 10 Mbit/s for
different user data sizes. This is caused
by a mismatch between user data size
and window size which directly affects
the segment flow. Figure 15 shows that
the throughput pattern to some extent
repeats for every 4096 bytes. With the
SBA-200 interface, the high byte-dependent
overhead of the segment processing
is reduced. This implies a relative
increase in the fixed overhead per segment,
and the number of segments will
have a larger impact on throughput performance.
Window and user data sizes
that result in the least number of segments
are expected to give the highest
performance. This is reflected in the
throughput peaks for user data sizes of an
integer multiple of 4096 bytes. With the
SBA-100 adapters, the byte dependent
processing was much higher, and the
throughput was therefore less dependent
on the number of transmitted segments.
For large window sizes there are no
throughput peak patterns. For these window
sizes, the number of outstanding
unacknowledged bytes seldom reaches
the window size. The segment flow is
primarily maximum sized segments of
9148 bytes with an acknowledgment
returned for every other segment.
8 Summary and
conclusions
In this paper we have presented TCP/IP
throughput measurements over a local
area ATM network from FORE Systems.
The purpose of these measurements was
to show how and why end-system hardware
and software parameters influence
achievable throughput. Both the host
architecture and the host network interface
(driver + adapter) as well as software
configurable parameters such as
window and user data size affect the
measured throughput.