[lwip-users] A closed pcb stays for 120seconds in the tcp_tw_pcbs. Right or wrong?

[Michael Anburaj]

Hi,

A closed pcb stays for 120seconds in the tcp_tw_pcbs.
Right or wrong?

FYI: Source line references are made w.r.to the CVS
version of the files.

The ftp server program I have been working on, follows
the following loop (pseudo code):

tcp_new

{ Loop:
tcp_bind
tcp_listen
tcp_accept

bunch of reads & writes

tcp_close
}

After 8 times through the loop tcp_listen fails at
line 400 of tcp.c

lpcb = memp_malloc(MEMP_TCP_PCB_LISTEN);

fails to get a free pcb structure veritable.


Since MEMP_NUM_TCP_PCB_LISTEN is defined as 8 after 8
times there are no free pcb structure variables in the
free_list.

/* MEMP_NUM_TCP_PCB_LISTEN: the number of listening
TCP
   connections. */
#define MEMP_NUM_TCP_PCB_LISTEN 8

Each time tcp_close gets called to free a pcb, its
state gets changed to TIME_WAIT & gets put in
tcp_tw_pcbs list.

At every 500ms ,tcp_slowtmr() gets called & process
all the pcb’s in the tcp_tw_pcbs list.

Line 709 of tcp.c

if ((u32_t)(tcp_ticks - pcb->tmr) > 2 * TCP_MSL /
TCP_SLOW_INTERVAL) {
      ++pcb_remove;
    }

Questions:
1.      Is this check intended for freeing the pcb after (2
* TCP_MSL) == 120millisec time is passed?

2.      If yes, then the above check is wrong.

(2 * TCP_MSL) == 2*60000microsec == 120000microsec ==
120millisec

TCP_SLOW_INTERVAL == 500millisec


2 * TCP_MSL / TCP_SLOW_INTERVAL == 240 ticks, which is
240*500ms = 120 seconds not equal to the intended
120millisec

So the check as such, removes a pcb after 120seconds
is passed & not 120milliseconds.



Correct code:
=========

if ((u32_t)(tcp_ticks - pcb->tmr) > 2 * TCP_MSL /
(TCP_SLOW_INTERVAL*1000)) {
      ++pcb_remove;
    }


3.      Even so, the granularity of the slow timer (500ms)
is not suited for timeout of 120ms.


Let me know if I missed something or if I uncovered a
bug.

Last question:
Looking at the code the PCB_REUSE attribute should
make use of pcb’s in tcp_tw_pcbs even before they get
freed. Am I right? Then I shouldn’t have seen this
problem…

Cheers,
-Mike.



                
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