NS3000/iX NMMGR Screens Reference Manual - Edition 8 (36922-90038)

62 Chapter4

Network Transport Conﬁguration Screens

Transmission Control Protocol (TCP) Conﬁguration

The algorithm used for congestion and ﬂow control is based on the Van

Jacobson algorithm which uses the initial retransmission

timeout for most calculations. The initial retransmission timeout is

the greater of the following two values:

• The value you conﬁgure in the Retransmission interval lower bound

ﬁeld

• The calculated smooth round trip (SRT) time which is automatically

calculated based on the average round trip time from the last few

send and acknowledgement packet timestamps.

The TCP retransmission algorithm works as follows:

• Once a packet is sent, the local node waits for a response from the

remote node. If a response is not received before the initial

retransmission timeout, the packet is transmitted again.

• The algorithm uses a multiplier of 2. For each time that a packet is

retransmitted, the multiplier is doubled, thus exponentially

increasing the wait time. Given an initial retransmission timeout of

X seconds, the sequence of timeout values would be X, 2X, 4X, 8X

seconds and so on.

• Retransmission is stopped when either one of the following happens:

• The time conﬁgured in maximum time to wait for remote

response is reached. This happens when the total time of all

retransmission intervals would exceed the maximum time to wait

for remote response.

• The number of retransmissions conﬁgured in maximum

retransmissions per packet is reached.

Let us take an example using the default values. Assume that the

calculated smooth round trip is 1 second. Since the retransmission

interval lower bound (4 secs) is the greater of the two, its value is used

as the initial retransmission timeout. If the node does not get an

acknowledgement after 4 seconds, the packet is retransmitted. If there

is still no acknowledgement, the retransmission of the outstanding

packet would be at intervals of 4, 8, 16, and 30 seconds. The connection

would be broken after 30 seconds because a maximum of 4 retries is

allowed.

Needless retransmissions may occur if you set retransmission interval

lower bound low and you set maximum timeto wait for remote response

and maximum retransmissions per packet high. If the retransmission

interval lower bound is set too high, an unnecessarily long delay occurs

when a packet is lost, and a retransmission will be necessary.