How to set TCP parameter?
Patrick Mahan
mahan at mahan.org
Wed Aug 20 18:12:23 UTC 2008
EdwardKing presented these words - circa 8/19/08 9:39 PM->
> $sysctl net.inet.tcp.msl
> net.inet.tcp.msl 30000
>
> It is 30000ms or 30000s?
>
It's ms, basically it is 30*hz (see tcp_timer.h). So it is by default,
30 secs.
> I want to modify it,because I met with following question:
>
> There is a server and a client,when client send a message to server,server can send a reply to client.
The status of server and client is ESTABLISHED.Then I halt the client by press ctrl+c,I find the server status is CLOSE_WAIT
and the client status is FIN_WAIT_2. Many minutes passed,I find the the server status still is CLOSE_WAIT and the client
status still is FIN_WAIT_2. At last,the client status is disappeared,but the server status still is CLOSE_WAIT.Why the status of
server isn't CLOSED? I am puzzled with it for a very long time. Would you tell the reason?
This is going to require you digging into the TCP protocol (you can start with RFC-793 but I would also recommend
some books on the subject, I prefer the Stevens book "TCP/IP Illustrated, Vol 1"). I've attached the TCP state diagram
to this email for you.
Let's look at what happens, we start with both the client and the server in the TCP state ESTAB
Client Event Server
====== ====== ======
ESTAB ESTAB
You issue a CTRL-C, causing the client to close the connection to the server. This causes the following:
ESTAB->CLOSE ESTAB
--> <FIN>
CLOSE->FIN_WAIT_1 ESTAB
The TCP layer moves the connection to CLOSE state and sends a FIN to start connection tear down.
The server receives the FIN.
ESTAB
<ACK><--
FIN_WAIT_1 ESTAB->CLOSE_WAIT
On the server the reception of the FIN causes the TCP layer to send an ACK of the FIN and moves the
connection to the CLOSE_WAIT state.
FIN_WAIT_1->FIN_WAIT_2 CLOSE_WAIT
What should be happening is that now the server should send a FIN in response, but
either 1) it fails too or 2) it is lost in between. Eventually, the TCP layer on the client times
out the FIN_WAIT_2 state and moves the connection to the CLOSED state. However,
the server cannot move out of the CLOSE_WAIT state since it hasn't seen the ACK to it's
FIN.
There is no TCP variable in the FreeBSD implementation to change this (that I know of).
I have seen this happen at times when there is a buggy NAT firewall between you and the
server that causes the FIN from the server to get dropped. If you have control of both sides,
you can do packet traces using tcpdump to see if the FIN from the server is ever sent or
receive. It is possible that your client is dropping the packet itself.
Good luck,
Patrick
>
> Thanks.
>
> ----- Original Message -----
> From: "Patrick Mahan" <mahan at mahan.org>
> To: "EdwardKing" <zhangsc at neusoft.com>
> Sent: Wednesday, August 20, 2008 12:06 PM
> Subject: Re: How to set TCP parameter?
>
>
>>
>> EdwardKing presented these words - circa 8/19/08 7:14 PM->
>>> Yes,I want to modify spent in the "TIME_WAIT" state? How to realize it?
>>>
>>> I use sysctl to view TCP parameter,but it failed
>>> $sysctl TCP_TIME_WAIT_INTERVAL
>>> sysctl: unknown oid 'TCP_TIME_WAIT_INTERVAL'
>>>
>>> Where is wrong?
>>>
>> There is no such variable in the TCP protocol implementation on FreeBSD
>> (that I can see). Instead, you can try to modify the TCP Maximum Segment
>> Lifetime (sysctl net.inet.tcp.msl). TIME_WAIT should wait a maximum of
>> 2*msl before timing out. However, changing it can adversly affect other parts
>> of the protocol.
>>
>> Why do you want to modify it?
>>
>> Patrick
>>
>>> ----- Original Message -----
>>> From: "Patrick Mahan" <mahan at mahan.org>
>>> To: "EdwardKing" <zhangsc at neusoft.com>
>>> Cc: "FreeBSD" <freebsd-questions at freebsd.org>
>>> Sent: Wednesday, August 20, 2008 9:59 AM
>>> Subject: Re: How to set TCP parameter?
>>>
>>>
>>>> EdwardKing presented these words - circa 8/19/08 6:17 PM->
>>>>> How to set TCP parameters,such as tcp_time_wait_interval?
>>>>>
>>>> You should use 'sysctl'. See 'man 8 sysctl'.
>>>>
>>>> However, I don't see any time wait variables available via sysctl. Are you
>>>> trying to modify the time spent in the "TIME_WAIT" state?
>>>>
>>>> Patrick
>>>>
>>>>> Thanks
>>>>>
>>>>>
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-------------- next part --------------
September 1981
Transmission Control Protocol
Functional Specification
+---------+ ---------\ active OPEN
| CLOSED | \ -----------
+---------+<---------\ \ create TCB
| ^ \ \ snd SYN
passive OPEN | | CLOSE \ \
------------ | | ---------- \ \
create TCB | | delete TCB \ \
V | \ \
+---------+ CLOSE | \
| LISTEN | ---------- | |
+---------+ delete TCB | |
rcv SYN | | SEND | |
----------- | | ------- | V
+---------+ snd SYN,ACK / \ snd SYN +---------+
| |<----------------- ------------------>| |
| SYN | rcv SYN | SYN |
| RCVD |<-----------------------------------------------| SENT |
| | snd ACK | |
| |------------------ -------------------| |
+---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+
| -------------- | | -----------
| x | | snd ACK
| V V
| CLOSE +---------+
| ------- | ESTAB |
| snd FIN +---------+
| CLOSE | | rcv FIN
V ------- | | -------
+---------+ snd FIN / \ snd ACK +---------+
| FIN |<----------------- ------------------>| CLOSE |
| WAIT-1 |------------------ | WAIT |
+---------+ rcv FIN \ +---------+
| rcv ACK of FIN ------- | CLOSE |
| -------------- snd ACK | ------- |
V x V snd FIN V
+---------+ +---------+ +---------+
|FINWAIT-2| | CLOSING | | LAST-ACK|
+---------+ +---------+ +---------+
| rcv ACK of FIN | rcv ACK of FIN |
| rcv FIN -------------- | Timeout=2MSL -------------- |
| ------- x V ------------ x V
\ snd ACK +---------+delete TCB +---------+
------------------------>|TIME WAIT|------------------>| CLOSED |
+---------+ +---------+
TCP Connection State Diagram
Figure 6.
[Page 23]
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