An Investigation into Transport Protocols and Data Transport ...

5.2. TCP Tuning & Performance Improvements 90
5.2.4 Delayed Acknowledgments & Appropriate Byte
Sizing
At high throughput rates, TCP receivers would send a lot of acknowledgement
packets on the reverse path of the bulk transfer to facilitate ack-clocking
and to slide the TCP window. Under bulk one-way transport, acks are often
composed of an empty TCP packet (with only header information), and
are therefore normally quite small (56B). As demonstrated in Section A.2.3,
small packets can pose a serious processing burden on both the end-host and
the intermediate switches and routers.
As acks cover all segments prior to the sequence number within the ack,
the advancement of the TCP window is not diminished by reducing the
number of acks sent by the TCP receiver [Cla82b].
Under delayed acknowledgments [Cla82b, APS99], a TCP receiver does
not acknowledge a received segment immediately, but waits for a certain time
(typically 500msec). If a data segment is sent from the receiver during this
time (i.e. as with two way communication), the acknowledgment is piggy
backed into it. Alternatively, if another data segment arrives from the TCP
sender, then the receiver will send a single ack that confirms both segments
at once.
It is suggested by the IETF that delayed acks should be incorporated
into TCP implementations [Cla82b]. However, the consequence may be that
‘stretched’ acks result that will acknowledge more than two full-sized segments.
This would lead to potentially large line-rate bursts of traffic [Pax97]
(which can also occur with large amounts of ack loss).
Typically each arriving ack at the sender advances the sliding window
and increases the congestion window by one segment; thus, a connection with