Internet Core Protocols: The Definitive Guide

Slow start

One of the most common problems related to congestion is that senders attempt to transmit data as fast as they can, as soon as they can. When a user asks for a big file, the server gleefully tries to send it at full speed immediately.

While this might seem like it would help to complete the transfer quickly, in reality it tends to cause problems. If there are any bottlenecks between the sender and receiver, then this burst-mode form of delivery will find them very quickly, causing congestion problems immediately (most likely resulting in a dropped segment). The user may experience a sudden burst of data, followed by a sudden stop as their system attempts to recover one or more lost segments, followed by another sudden burst. Slow start is the technique used to avoid this particular scenario.

In addition, slow start is used to recover from near-fatal congestion errors, where the congestion window has been reset to one segment, due to an acknowledgment timer reaching zero, or from an ICMP Source Quench error message being received.

Slow start works by exponentially increasing the size of the congestion window. Every time a segment is sent and acknowledged, the size of the congestion window is increased by one segment’s worth of data (as determined by the discovered MTU/MRU sizes of the virtual circuit), allowing for more and more data to be sent.

For example, if the congestion window is set to one segment (with the segment size being set to whatever value was determined during ...