[lwip-users] Questions about LwIP KEEPALIVE...

[Frédéric BERNON]

Hi group,

Can you give me some informations about he LwIP KEEPALIVE features? I use 
Socket Layer API, and I would like to know if it's possible to enable it "per 
socket" and to fix for each one a different timeout? I found in several files 
these informations :

In sockets.h/.c, I see :

#define  SO_KEEPALIVE   0x0008 /* keep connections alive */

For this one, except seeing it is set/unset in the 
sock->conn->pcb.tcp->so_options.

In ip.h "These are the same like SO_XXX." : 

#define SOF_KEEPALIVE (u16_t)0x0008U    /* keep connections alive */


In tcp.h, I see :

#define TCP_KEEPALIVE  0x02    /* send KEEPALIVE probes when idle for 
pcb->keepalive miliseconds */

/* Keepalive values */
#define  TCP_KEEPDEFAULT   7200000                       /* KEEPALIVE timer in 
miliseconds */
#define  TCP_KEEPINTVL     75000                         /* Time between 
KEEPALIVE probes in miliseconds */
#define  TCP_KEEPCNT       9                             /* Counter for 
KEEPALIVE probes */
#define  TCP_MAXIDLE       TCP_KEEPCNT * TCP_KEEPINTVL   /* Maximum KEEPALIVE 
probe time */

So, TCP_MAXIDLE is 675 seconds (11.25min), and TCP_KEEPDEFAULT is 7200 seconds 
(~120min : 2 hours) !?!?!!


In my lwipopts.h, I have set :

/* The TCP timer interval in milliseconds. */
#define TCP_TMR_INTERVAL                100

/* the fine grained timeout in milliseconds */
#define TCP_FAST_INTERVAL               TCP_TMR_INTERVAL 

/* the coarse grained timeout in milliseconds */
#define TCP_SLOW_INTERVAL              (2*TCP_TMR_INTERVAL)  


In tcp.c, I see :
 
In tcp.c : 

   /* Check if KEEPALIVE should be sent */
   if((pcb->so_options & SOF_KEEPALIVE) && ((pcb->state == ESTABLISHED) || 
(pcb->state == CLOSE_WAIT))) {
      if((u32_t)(tcp_ticks - pcb->tmr) > (pcb->keepalive + TCP_MAXIDLE) / 
TCP_SLOW_INTERVAL)  {
         LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: KEEPALIVE timeout. Aborting 
connection to %"U16_F".%"U16_F".%"U16_F".%"U16_F".\n",
                                 ip4_addr1(&pcb->remote_ip), 
ip4_addr2(&pcb->remote_ip),
                                 ip4_addr3(&pcb->remote_ip), 
ip4_addr4(&pcb->remote_ip)));
    
         tcp_abort(pcb);
      }
      else if((u32_t)(tcp_ticks - pcb->tmr) > (pcb->keepalive + pcb->keep_cnt * 
TCP_KEEPINTVL) / TCP_SLOW_INTERVAL) {
         tcp_keepalive(pcb);
         pcb->keep_cnt++;
      }
   }

If I unserstand all these informations, at socket layer, I can set with 
setsockopt on a TCP socket the TCP_KEEPALIVE with my own timeout (in the 
setsockopt value), and set the SO_KEEPALIVE option to 1 to enable KEEPALIVE 
checking.

(pcb->keepalive + TCP_MAXIDLE) / TCP_SLOW_INTERVAL) give with default values 
(TCP_KEEPDEFAULT + TCP_MAXIDLE) / TCP_SLOW_INTERVAL) : (7200000+675000)/(2*100) 
= 7875000/200 = 39375 ticks.

After that, if my connection is in ESTABLISHED or CLOSE_WAIT state, if 
(tcp_ticks - pcb->tmr) > 39375 , I suppose the tcp connection will be abort. 
Right? 

So, if I dont do any mistake, if a router between my lwip devices crashs, the 
connection will detect it in... 39375*200 ms = 7875 sec = 131.25 min !?!?!

I suppose I do a mistake somewhere, or I don't understand (because more than 2 
hours - by default - to detect a connection failure seems to be very very 
long)...

Can you help me ?

Thank you

  
====================================
Frédéric BERNON 
HYMATOM SA 
Chef de projet informatique 
Microsoft Certified Professional 
Tél. : +33 (0)4-67-87-61-10 
Fax. : +33 (0)4-67-70-85-44 
Email : address@hidden 
Web Site : http://www.hymatom.fr 
====================================
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