Socket Options Jari Kellokoski

Introduction So far we have discovered some basic usage of socket With socket options we control sockets more suitable way for our purposes (e.g. closing the socket) Covers: –Getsocketopt & setsocketopt –Socket Options like: Generic Options IPv4 IPv6 TCP SCTP

getsockopt & setsockopt int getsocketopt( int sockfd, int level, int optname, void *optval, socketlnet_t *oplen) int setsocketopt( int sockfd, int level, int optname, const void *optval, socklen_t oplen) where –sckfd – open socket descriptor –level – code in the system that interprets the option –optval – pointer to a variable into which the current value of the option is stored by getsocketopt or which the new value of the option is fetched by setsocketopt –oplen – size of optval as a value-result for getsocketopt or as a value for setsockopt

getsockopt & setsockopt Idea with options: –Some features can be enabled or disabled –Values can be set or read

Level, Optname, Flag Level: SOL_SOCKET, IPPROTO_IP, IPPROTO_ICMPv6… optname: SO_BROADCAST, IP_OPTIONS, MCAST_JOIN_GROUP… get: value can be read set: value can be set Flag: on/off Datatype: int, struct…

Generic Socket Options SO_BROADCAST SO_ERROR SO_KEEPALIVE SO_LINGER SO_RECVBUF & SO_NDBUF SO_REUSEADDR & REUSEPORT

Generic Socket Options SO_BROADCAST –enable/disable to send broadcast messages –of course this is only possible on networks supporting broadcast – like Ethernet – but not with point-to-point links –Applications must set this – wise since failed use can be problematic - flooding

Generic Socket Options SO_ERROR –Pending error for socket – standard Exxx values filled on failures –If blocked call – socket returns condition on readability or write ability –If signal-driven – SIGIO signal is generated

Generic Socket Options SO_KEEPALIVE –TCP will keep the connection alive even no actual communication data is not transferred –Scenarios to keepalive: Response with ACK – everything is ok Response with RST – crash or reboot in peer host No response – retransmissions – eventually ETIMEOUT or EHOSTUNREACH

Generic Socket Options SO_LINGER –How the close function is operating –struct linger with int l_onoff and l_linger –Where: l-onoff is 0 – close returns immediately l_onoff is nonzero and l_linger is 0 – TCP aborts the connection when it is closed – data is discarded from buffer – RST to the peer l_onoff is nonzero and l_linger is nonzero – kernel will linger when socket is closed. Remaining data is tried to be send or linger time expires

Generic Socket Options SO_RECVBUF & SO_NDBUF –Send and receive buffers –SO_RCVBUF must be set before calling connect –Buffer sizes should be at least four times the MSS –File transfer in one direction – send buffer in sending side and receiving buffer in receiving side –Bi-directional transfer – both buffer on both ends

Generic Socket Options SO_REUSEADDR & SO_REUSEPORT –Allows a listening server to bind well-know port even previously established connections exist (child was spawned) –Allows a new server to be stared on the same port as an existing server (different local IP address – IP aliasing) –Single process to bind same port to multiple sockets (UDP based server) –Completely duplicate binding – same port and IP – UDP multicasting

IPv4 Socket Options IP_HDRINCL – for building own IP header IP_TOS – (legendary) type of service field IP_TTL – setting/getting(default) time-to- live

IPv6 Socket Options IPv6_CHECKSUM – tells where to look checksum field IPv6_DONTFRAG – disables the automatic fragment header (for UDP and raw sockets) IPv6_UNICAST_HOPS – similar to IPv4 TTL – specifies the default hop limit for outgoing datagrams sent on the socket (or default hop limit that kernel uses) IPv6_PATHMTU – retrieves current MTU returned by path-MTU discovery

TCP Socket Options TCP_MAXSEG – fetching/setting MSS for connection – however we can always send less then announced MSS TCP_NODELAY – disables Nagle algorithm –Reasons for that: small packets used in communications (telnet) –small packet – packet smaller then MSS –Nagle algorithm reduces these ”small” packets however in some cases this is creating outstanding delays or making communication almost impossible –delayed ACK – the TCP ACK is delayed for piggyback reasons

SCTP Socket Options SCTP_ASSOCINFO – get information about existing association, change parameters of existing association, set defaults for future associations SCTP_AUTOCLOSE – autoclose time for SCTP endpoint SCTP_EVENTS – fetch, enable, disable various SCTP notifications SCTP_INITMSG – get or set initial parameters with INIT message SCTP_MAXBUST – max burst size – no more than this can be sent at once – avoids flooding SCTP_STATUS – current status of SCTP association (data amounts, number of streams, primary addresses and state)

fcntl File control – meaning control operations for descriptor –set to nonblocking I/O –set to signal-driven I/O –SIGIO and SIGURG signal based control for socket owner socket owner – only from listening socket – the socket owner is inherited from the listening socket by the connected socket

Summary Good to understand – SO_ERROR, SO_KEEPALIVE, SO_REUSEADDR, SO_LINGER SO_ERROR - Pending error for socket – standard Exxx values filled on failures SO_KEEPALIVE – TCP (SCTP) will keep the connection alive even no actual communication data is not transferred SO_REUSEADDR – different scenarios of address/port reusage SO_LINGER - How the close function is operating