[HACKERS] client socket TIME_WAIT state after PQfinish
Dear all, Environement: - OS : Ubuntu 10.04 LTS. - DB: postgresql 8.4. - Connection to postgresql using sslmode = disable Scenario : 1. I use pgadmin to connect/disconnect to the postgresql server on port 5432 or 2. I use a progam using libpq and make PQconnectdb and PQfinish. Bug: In both cases, the client socket (pgadmin or my program) remains in TIME_WAIT state. I have used wireshark to sniff the TCP protocol. We have at the end of a connection: Client Server ---FIN,ACK --- ---FIN,ACK --- --- ACK--- This ends up in a TIME_WAIT state. The TCP protocol should be Client Server ---FIN,ACK --- --- ACK--- ---FIN,ACK --- --- ACK--- I suppose there is a bug in the postgresql server that do not send an ack to the client. Could you please clarify this situation ? I am a bit lost. Thank you, Franck Lefort
Re: [HACKERS] client socket TIME_WAIT state after PQfinish
2010/9/27 Guillaume Du Pasquier guillaume.dupasqu...@sensometrix.ch: In both cases, the client socket (pgadmin or my program) remains in TIME_WAIT state. I have used wireshark to sniff the TCP protocol. We have at the end of a connection: Client Server --- FIN,ACK --- --- FIN,ACK --- --- ACK --- This ends up in a TIME_WAIT state. The TCP protocol should be According to the Two Generals' Problem [1], one of the sides necessarily doesn't know whether the other side has received its last packet. Therefore, TCP lets one of the sides sit in TIME_WAIT status for as long as any packets could in principle survive on the network (typically defined as 2 minutes on IP networks IIRC), and potentially disturb a new connection between the same (dst IP, dst port, src IP, src port) combination. [1] URL:http://en.wikipedia.org/wiki/Two_Generals'_Problem Client Server --- FIN,ACK --- --- ACK --- --- FIN,ACK --- AFAIK, this last ACK (in above packet) is not needed: the server can ACK the client's FIN _while_ it sends its own FIN (by using an appropriate sequence number, as FIN uses one byte in the sequence). --- ACK --- I suppose there is a bug in the postgresql server that do not send an ack to the client. I don't think so. Nicolas -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] client socket TIME_WAIT state after PQfinish
On Mon, Sep 27, 2010 at 12:22 PM, Nicolas Barbier nicolas.barb...@gmail.com wrote: According to the Two Generals' Problem [1], one of the sides necessarily doesn't know whether the other side has received its last packet. Therefore, TCP lets one of the sides sit in TIME_WAIT status for as long as any packets could in principle survive on the network (typically defined as 2 minutes on IP networks IIRC), and potentially disturb a new connection between the same (dst IP, dst port, src IP, src port) combination. In other words, this is the way TCP is designed to work, not something specific to PostgreSQL. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise Postgres Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] client socket TIME_WAIT state after PQfinish
Dear Nicolas, Dear Robert, Thank you for your quick answers. We do not have such behavior using SSL, how do you explain it ? I suppose that openssl is using the setsockopt SO_LINGER that removes this behavior. Therefore, there is a RST sent to close the socket. We work with an environment that uses a lot of socket connections. Therefore, many file descriptors are opened. If after each Sql requests a TIME_WAIT arises we will end up with many file descriptors opened. By default the maximum number of file descriptors is set to 1024 and we reach that number quite fast. Do you have any advices to get rid of this TIME_WAIT problem ? Our client runs on the same machine as the postgresql server. Would it be possible to use PF_UNIX sockets ? Thank you, Franck -Original Message- From: Nicolas Barbier [mailto:nicolas.barb...@gmail.com] Sent: lundi 27 septembre 2010 18:23 To: Guillaume Du Pasquier Cc: pgsql-hackers@postgresql.org Subject: Re: [HACKERS] client socket TIME_WAIT state after PQfinish 2010/9/27 Guillaume Du Pasquier guillaume.dupasqu...@sensometrix.ch: In both cases, the client socket (pgadmin or my program) remains in TIME_WAIT state. I have used wireshark to sniff the TCP protocol. We have at the end of a connection: Client Server --- FIN,ACK --- --- FIN,ACK --- --- ACK --- This ends up in a TIME_WAIT state. The TCP protocol should be According to the Two Generals' Problem [1], one of the sides necessarily doesn't know whether the other side has received its last packet. Therefore, TCP lets one of the sides sit in TIME_WAIT status for as long as any packets could in principle survive on the network (typically defined as 2 minutes on IP networks IIRC), and potentially disturb a new connection between the same (dst IP, dst port, src IP, src port) combination. [1] URL:http://en.wikipedia.org/wiki/Two_Generals'_Problem Client Server --- FIN,ACK --- --- ACK --- --- FIN,ACK --- AFAIK, this last ACK (in above packet) is not needed: the server can ACK the client's FIN _while_ it sends its own FIN (by using an appropriate sequence number, as FIN uses one byte in the sequence). --- ACK --- I suppose there is a bug in the postgresql server that do not send an ack to the client. I don't think so. Nicolas -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] client socket TIME_WAIT state after PQfinish
On Mon, Sep 27, 2010 at 12:56 PM, Guillaume Du Pasquier guillaume.dupasqu...@sensometrix.ch wrote: Dear Nicolas, Dear Robert, Thank you for your quick answers. We do not have such behavior using SSL, how do you explain it ? I suppose that openssl is using the setsockopt SO_LINGER that removes this behavior. Therefore, there is a RST sent to close the socket. We work with an environment that uses a lot of socket connections. Therefore, many file descriptors are opened. If after each Sql requests a TIME_WAIT arises we will end up with many file descriptors opened. By default the maximum number of file descriptors is set to 1024 and we reach that number quite fast. Hrm. Does a socket in the TIME_WAIT state count against the number of open file descriptors? Certainly, it shouldn't count against the per-process limit, as the process has already closed it. Do you have any advices to get rid of this TIME_WAIT problem ? Our client runs on the same machine as the postgresql server. Would it be possible to use PF_UNIX sockets ? Yeah, actually that's the default, if you just run psql with no parameters. It looks for a socket in /tmp/.s.PGSQL.5432. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise Postgres Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] client socket TIME_WAIT state after PQfinish
2010/9/27 Robert Haas robertmh...@gmail.com: On Mon, Sep 27, 2010 at 12:56 PM, Guillaume Du Pasquier guillaume.dupasqu...@sensometrix.ch wrote: Our client runs on the same machine as the postgresql server. Would it be possible to use PF_UNIX sockets ? Yeah, actually that's the default, if you just run psql with no parameters. It looks for a socket in /tmp/.s.PGSQL.5432. It depends on the interface; e.g., the JDBC driver doesn't support unix sockets, AFAIR. Nicolas -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
