• Socket options: a summary– Tcp options, ip options and general socket

options can be examined and sometimes modified.

• getsockopt, setsockopt routines• Some examples:

– Socket (TCP/UDP) send/receive buffer size– Socket (TCP/UDP) send/receive low/high watermark– UDP broadcasting– TCP keep alive timer– IP options: e.g. source routing

» IP Time to live (TTL)» IP quality of service (TOS)

– IP multicast options

– #include <sys/socket.h>int getsockopt (int sockfd, int level, int optname, void

*optval, socklen_t *optlen);

Int setsockopt(int sockfd, int level, int optname, const void *optval, socklen_t optlen);

• Level includes general socket option (SOL_SOCKET) and protocol-specific option (IP, TCP, etc).

• Option value can be of different types : int, in_addr, timeval, … ---- that is why we use the void pointer.

• General socket options (level = SOL_SOCKET)– Optnames:

• SO_BROADCAST(int): permit sending broadcast datagram

• SO_DEBUG: for debugging purpose

• SO_DONROUTE: used by routing daemon to make sure a packet goes through a certain interface.

• SO_ERROR: can only be got (not set), reset the error

• SO_KEEPALIVE: for TCP only, automatic send keepalive message when inactive for 2 hours (can be modified).

• SO_LINGER: for TCP only, determines the behavior when a close is called.

• SO_OOBINLINE: put out of band data in the normal input queue.

• SO_RCVBUF, SO_SNDBUF: send and receive buffer size.

• General socket options (level = SOL_SOCKET)– Optnames:

• SO_RCVLOWAT, SO_SENDLOWAT: low watermark – affect the select system call

• SO_RCVTIMEO, SO_SNDTIMEO: inherent timeout time (disable by default)

• SO_REUSEADDR: Allows the port to be reused – all server should do this to restart immediately

• SO_TYPE: socket type? SOCK_STREAM and SOCK_DGRAM.

• SO_USELOOPBACK: sending socket gets a copy?

• IP options:– Allows packets sent through a socket to have certain

behavior, e.g source routing

– Level = IPPROTO_IP• Example optnames: (manipulating IP header fields)

– IP_HDRINCL: who builds the IP header (raw socket).

– IP_OPTIONS: setting IP optional header fields

– IP_TOS

– IP_TTL

• IP options:– Level = IPPROTO_IP

• Optnames: (IP MULTICAST related)

– IP_MULTICAST_IF

– IP_MULTICAST_TTL

– IP_MULTICAST_LOOP

– IP_ADD_MEMBERSHIP

– IP_DROP_MEMBERSHIP

• TCP options:– Level = IPPROTO_TCP

• Optnames:

– TCP_KEEPALIVE: set the time

– TCP_MAXRT: maximum retransmission time

– TCP_MAXSEG: maximum segment size

– TCP_NODELAY: wait to ack or not (enable)

– See example5.c