HP ProCurve 6400cl Series Management And Configuration Manual page 279

Note on Testing
Rate-Limiting
rate limit. In this case, the inbound traffic flow does not reach the
configured rate and lower priority traffic is not forwarded into the switch
fabric from the rate-limited port. (This behavior is termed "head-of-line
blocking" and is a well-known problem with flow-control.) In another type
of situation, an outbound port can become oversubscribed by traffic
received from multiple rate-limited ports. In this case, the actual rate for
traffic on the rate-limited ports may be lower than configured because the
total traffic load requested to the outbound port exceeds the port's
bandwidth, and thus some requested traffic may be held off on inbound.
Rate-Limiting is byte-based and is applied to the available bandwidth on a port,
and not to any specific applications running through the port. If the total
bandwidth requested by all applications together is less than the available,
configured maximum rate, then no rate-limit can be applied. This situation
occurs with a number of popular throughput-testing software applications, as
well as most regular network applications. Consider the following example,
which uses the minimum packet size:
The total available bandwidth on a 100 Mbps port "X" (allowing for Inter-
packet Gap—IPG), with no rate-limiting restrictions, is:
(((100,000,000 bits) / 8 ) / 84) x 64 = 9,523,809 bytes per second
where:
– The divisor (84) includes the 12-byte IPG, 8-byte preamble, and
64-bytes of data required to transfer a 64-byte packet on a 100
Mbps link.
– Calculated "bytes-per-second" includes packet headers and data.
This value is the maximum "bytes-per-second" that 100 Mbps
can support for minimum-sized packets.
Suppose port "X" is configured with a rate limit of 50% (4,761,904 Mbytes). If
a throughput-testing application is the only application using the port, and
transmits 1 Mbyte of data through the port, it uses only 10.5% of the port's
available bandwidth, and the rate-limit of 50% has no effect. This is because
the maximum rate permitted (50%) exceeds the test application's bandwidth
usage (126,642-164,062 bytes, depending upon packet size, which is only 1.3-
1.7% of the available total). Before rate-limiting can occur, the test applica­
tion's bandwidth usage must exceed the configured rate-limit. In this example,
the bandwidth usage must exceed 50% of the port's total available bandwidth.
That is, in this example, to test the rate-limit setting, the following must be true:
bandwidth usage > (0.50 x 9,523,809)
Port Traffic Controls
Rate-Limiting
13-7