tcpip architecture design and implementation in linux by Unknown

Author:Unknown
Language: eng
Format: epub
Published: 2014-03-30T13:29:39+00:00

KEEPALIVE TIMER

355

cs 10.29a. tcp_keepalive_timer() .

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356

TCP TIMERS

cs 10.29b. tcp_keepalive_timer() ( continued ).

Get the probe interval by calling keepalive_intvl_when() . In the case where the

probe interval was not transmitted successfully, we need to send it at the earliest.

So, the expiry time for the keepalive timer is reduced to

TCP_RESOURCE_

PROBE_INTERVAL at line 631, because we are not able to transmit because of

lack of resources. Next we call tcp_mem_reclaim() to reclaim some memory. We do

this here because if our connection has consumed its quanta of memory allocated,

the next processing of the incoming segment will take it to the slow path. So, we do

this check in advance here. Next we call tcp_reset_keepalive_timer() at line 642 to

reset the keepalive probe timer to whatever expiry time we have calculated above.

We release the socket lock and leave.

10.6 SYN - ACK TIMER

There is a timer maintained by Linux to manage connection requests that are not

being accepted for a given period of time. The entire idea of having this timer is

that if we are not able to accept more connections (accept queue is full) because

the application is not able to get CPU or it is busy doing something else, we need

to manage the connection request. There are two main cases where connection

requests need to be managed:

1. Established connections are not being accepted because the accept queue is

full and the application is not accepting new connections.

2. We don ’ t get ACK for the SYN - ACK we sent; that is, the third step in the

three - way handshake is not completed.

10.6.1 When Is the SYN - ACK Timer Activated?

The timer is activated when we get a connection request and there is no pending

connection request in the listening socket ’ s SYN queue to be processed. lopt → qlen

is the counter that is incremented by 1 whenever a new connection requested arrives

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