Bandwidth Estimation on Protocol Mechanisms

A simple mechanism to measure the available bandwidth on a link is the packet-pair
method. It entails sending two packets back-to-back on a link, and measuring the
inter-arrival time of those packets at the receiver. If the packets are sent on a
point-to-point link with no other traffic, the inter-arrival time measures the raw
bandwidth of the link for that size of packets. It is the absolute minimum period at
which packets of that size can be sent. Sending packets at a smaller spacing will
only queue packets at the outbound interface, with no increase in throughput. If the
packets are sent on a multiple hop path mixed with other traffic, routers on the way
may insert other packets between the two packets that were sent back-to-back,
making them arrive farther apart. The number of packets inserted is directly
proportional to the load on the outbound port each router uses to send the packets,
and does not depend on packet size if no fragmentation occurs, as time in the
routers is normally bound by protocol processing and not packet size. If packet size
is equal to the path MTU, the inter-arrival time measured at the receiver is a snapshot
of the bandwidth of the path. The interarrival time is the minimum period at which
packets can be sent that will not create a queue in any of the routers on the path.
If the load of all routers in the path is a constant, then the inverse of the inter-arrival
time defines the optimal rate to send packets through this link. The load not being a
constant, the measurement will have to be repeated from time to time to adjust the 
rate to the current conditions.