HP 5300 Technické informace stáhnout pdf (Strana 256)

Port Traffic Controls

Rate-Limiting

configured rate and lower priority traffic is not forwarded into the

backplane 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.

Note on Testing Rate-Limiting is byte-based and is applied to the available bandwidth on a port,

Rate-Limiting

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)

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HP 5300 Technické informace Strana 256