Overview
• Last Lecture
– I/O multiplexing
– Source: Chapter 6 of Stevens’ book
• This Lecture
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Socket options
Source: Chapter 7 of Stevens’ book
Elementary UDP sockets
Source: Chapter 8 of Stevens’ book
• Next Lecture
– Name and address conversions
– Source: Chapter 11 of Stevens’ book
TELE 402 Lecture 5: Socket options
1
Socket options 1
• There are many socket options for
programmers to set for fine control of the
underlying system and protocols
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–
–
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Generic socket options
IPv4 socket options
IPv6 socket options (to be discussed later)
ICMPv6 socket options (to be discussed later)
TCP socket options
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Socket options 2
• Options can be manipulated with the following
functions
– getsockopt and setsockopt
– fcntl (recommended by Posix)
– ioctl
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getsockopt and setsockopt 1
• These two functions apply only to sockets
int getsockopt(int sockfd,
int level,
int optname,
void *optval,
socklen_t *optlen);
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getsockopt and setsockopt 2
int setsockopt(int sockfd,
int level,
int optname,
const void *optval,
socklen_t optlen);
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getsockopt and setsockopt 3
– Both return: 0 if OK, -1 on error
– sockfd must refer to an open socket descriptor
– level specifies the code in the system to interpret the
option (the general code or protocol-specific code). See
Figure 7.1 & 7.2
– optname is an integer representing the specific option.
Use the macro definitions in Figure 7.1 & 7.2
– optval is a pointer to a variable storing the option value.
(new value for setsockopt; current value for getsockopt)
– Optlen is a result-value parameter referring to the size
of optval
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Option values 1
• There are two types of option values
– Binary options that enable or disable a certain
feature (flags with • in Figure 7.1 & 7.2)
• 0 for disable
• nonzero for enable.
– Options that fetch and return specific values
that we can either set or examine (values). The
actual values are passed between the kernel and
the user spaces.
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Option values 2
• Data types for values
– Most option values are integer
– Some are structures, such as timeval, linger, and
character array
– Refer to Datatype column in Figure 7.1 & 7.2 for
details
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Socket states 1
• Some socket options have timing considerations
about when to set or fetch the option due to the
state of the socket
• The following options are inherited by a
connected TCP socket from the listening socket
– SO_DEBUG, SO_DONTROUTE, SO_KEEPALIVE,
SO_LINGER, SO_OOBINLINE, SO_RCVBUF,
SO_SNDBUF, SO_RCVLOWAT, SO_SNDLOWAT,
TCP_MAXSEG, AND TCP_NODELAY
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Socket states 2
– For TCP, the connected socket is not returned to a
server by accept until the three-way handshake is
completed by the TCP layer.
– To ensure that one of the above options is set for the
connected socket when the three-way handshake
completes, we must set that option for the listening
socket.
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Generic socket options 1
• SO_BROADCAST
– Enable or disable the ability of the socket to send
broadcast messages
• SO_DEBUG
– Supported only by TCP. When enabled, the kernel
keeps track of detailed info about all the packets sent or
received by the socket
• SO_DONTROUTE
– Bypass the normal routing mechanism of the
underlying protocol
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Generic socket options 2
• SO_ERROR
– Get pending error and clear
• so_error is set to a Exxxx value
• Called a pending error
• Two ways for process to be immediately notified
•
•
– If blocked in a call to select, select returns
– If using signal-driven I/O, SIGIO signal is generated
Process can obtain so_error by fetching SO_ERROR option.
If so_error is nonzero
– If read is called and no data to return, -1 is returned and errno=so_error
– If read is called and data is queued, data is returned instead of the error
condition
– If write is called, -1 is returned and errno=so_error
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Generic socket options 3
• SO_KEEPALIVE
– If there is no data exchanged in either direction for 2 hours, a probe
is sent to the peer if this option is set. One of three scenarios
exists:
• Peer responds with an ACK
• Peer responds with a RST
• No response
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Ways to detect TCP conditions
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Linger 1
• SO_LINGER
– Specifies how close operates for a connection-oriented
protocol
– The following structure is used:
struct linger {
int l_onoff;
int l_linger;
}
// l_onoff - 0=off; nonzero=on
// l_linger specifies seconds
– Three scenarios:
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Linger 2
• SO_LINGER (cont)
– If l_onoff is 0, close returns immediately. If there is any data still
remaining in the socket send buffer, the system will try to deliver
the data to the peer. The value of l_linger is ignored.
– If l_onoff is nonzero and linger is 0, TCP aborts the connection
when close is called. TCP discards data in the send buffer and
sends RST to the peer.
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Linger 3
• SO_LINGER (cont)
– If l_onoff is nonzero and linger is nonzero, the kernel will linger
when close is called.
• If there is any data in the send buffer, the process is put to sleep until
either:
– the data is sent and acknowledged
Or
– the linger time expires (for a nonblocking socket the process will not
wait for close to complete)
• When using this feature, the return value of close must be checked. If
the linger time expires before the remaining data is send and
acknowledged, close returns EWOULDBLOCK and any remaining
data in the buffer is ignored.
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close scenarios 1
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close scenarios 2a
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close scenarios 2b
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close scenarios 3
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close scenarios 4
• Application ACK to confirm the receipt of data
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close and shutdown
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Generic socket options 4
• SO_OOBINLINE
– Out-of-band data can be placed in the normal input
queue
• SO_RCVBUF and SO_SNDBUF
– Get/set the send buffer size and receive buffer size
– These sizes are related to capacity of the connection
• SO_RCVLOWAT and SO_SNDLOWAT
– Decide the conditions for readable and writable
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Generic socket options 5
• SO_RCVTIMEO and SO_SNDTIMEO
– Place a timeout on socket receives and sends
– They affect read and write function families
• SO_TYPE
– Returns the socket type such as SOCK_STREAM and
SOCK_DGRAM
• SO_USELOOPBACK
– Applies only to routing sockets
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Generic socket options 6
• SO_REUSEADDR
– Allows a listening server to start and bind its wellknown port even if previously established connections
exist that use this port as their local port
– Allows multiple instances of the same server to be
started on the same port, as long as each instance binds
a different local IP address.
– Allows a single process to bind the same port to
multiple sockets, as long as each bind specifies a
different local IP address
– Allows completely duplicate bindings: a bind of an IP
address and port, when that same IP address and port
are already bound to another socket (normally for
support of multicasting).
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IPv4 socket options 1
• IP_HDRINCL
– For raw IP socket to build user specified header
– The header is completely built, except the checksum,
ID field, and the source IP address if the address is set
INADDR_ANY
• IP_OPTIONS
– Allow us to set IP options in the IP header
• IP_RECVDSTADDR
– Causes the destination IP address of a received UDP
datagram to be returned as ancillary data by recvmsg
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IPv4 socket options 2
• IP_RECVIF
– Causes the index of the interface on which a UDP datagram is
received to be returned as ancillary data by recvmsg
• IP_TOS
– Get/set the type-of-service field in the IP header
• IP_TTL
– Get/set the default TTL
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TCP socket options 1
• TCP_KEEPALIVE (new with Posix)
– Specifies the idle time in seconds for the
connection before TCP starts sending keepalive
probe
– It is effective only when SO_KEEPALIVE
socket option is enabled
• TCP_MAXRT (new with Posix)
– Specifies the amount of time before a
connection is broken once TCP starts
retransmitting data
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TCP socket options 2
• TCP_MAXSEG
– Get/set the maximum segment size for a TCP
connection
• TCP_NODELAY
– Disable the delay algorithm (Nagle algorithm)
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Nagle algorithm
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fcntl function 1
• fcntl
int fcntl(int fd,
int cmd, …)
– Returns: depends on cmd if OK, -1 on error
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fcntl and ioctl
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fcntl function 2
• fcntl provides the following features related to
network programming
– Nonblocking I/O (be aware of error-handling in the
following code)
flags=fcntl(fd, F_GETFL, 0);
flags |= O_NONBLOCK;
fcntl(fd, F_SETFL, flags);
– Signal driven I/O
flags=fcntl(fd, F_GETFL, 0);
flags |= O_ASYNC;
fcntl(fd, F_SETFL, flags);
– Set socket owner to receive SIGIO signals
fcntl(fd, F_SETOWN, getpid());
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UDP client/server
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recvfrom and sendto 1
ssize_t recvfrom(int sockfd,
void *buff, size_t nbytes,
int flags, struct sockaddr *from,
socklen_t *addrlen);
ssize_t sendto(int sockfd,
const void *buff, size_t nbytes,
int flags, const struct sockaddr *to,
socklen_t addrlen);
• Both return: number of bytes read or written if OK, -1 on
error
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recvfrom and sendto 2
• sockfd, buff, and nbytes are identical to
read/write
• flags is normally set 0, but can be set for advanced
functions
• The final two arguments to recvfrom are similar to the
final two arguments to accept. They can be NULL.
• The final two arguments to sendto are similar to the
final two arguments to connect
• Send 0 bytes is ok; likewise receive 0 bytes is ok
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Simple UDP C/S
• Refer to udpcliserv/udpserv01.c, lib/dg_echo.c,
udpcliserv/udpcli01.c, and lib/dg_cli.c for details
• Sockets are created with type SOCK_DGRAM
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UDP C/S with two clients
• If a datagram is lost, the client will wait forever
• Timeout is needed, but not enough (duplicate problem)
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Comparison with TCP C/S
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Verifying Received Response
• Any process that knows the client’s
ephemeral port could send datagrams to the
client.
• Change call to recvfrom to return the IP
address and port of the replies.
• Check they are from the intended server.
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Server Not Running
• What happens and why?
• ARP request maybe
• Server host responds with ICMP ‘port
unreachable’ message
• Client blocks forever in call to recvfrom
• Called asynchronous error
• Basic rule - asynchronous errors are not
returned for a UDP socket unless it is
connected.
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Server not running
• If the server is not running, when the client send a
datagram to the server, the server host responds
with an ICMP packet (port unreachable)
• But the client just blocks in the call to recvfrom,
with no error returned
– This error is called asynchronous error, since it is
caused by a previous sendto
• The basic rule is that asynchronous errors are not
returned for UDP sockets unless the socket is
connected
– The reason is that, if a socket is not connected, it is not
clear which destination is not reachable if multiple
destinations are involved
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Summary of UDP C/S
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Summary of UDP C/S (cont.)
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Socket address information
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Connected UDP sockets 1
• Three things change with connected UDP sockets
– We can no longer specify the destination IP address and port for an
output operation.
– No point to use recvfrom, since the datagrams received come from
the same source as the socket is connected to
– Asynchronous errors are returned
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Connected UDP sockets 2
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Connected UDP sockets 3
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Connected UDP sockets 4
• A UDP client or server can call connect only if that
process uses the UDP socket to communicate with exactly
one peer
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Multiple calls to connect
• We can call connect again on a connected UDP
socket
– Specify a new IP address and port for connect
– Unconnect the socket by setting the family member of
the socket address structure to AF_UNSPEC. This
might return an error of EAFNOSUPPORT but that is
ok
• Performance
– When an application needs to send multiple datagrams
to the same peer, it is more efficient to use a connected
socket than an unconnected socket.
• Outgoing interface for connected UDP sockets
– The outgoing interface (local IP address and port) can
be determined using getsockname
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Flow control in UDP
• There is no flow control in UDP
– If a client sends UDP datagrams very fast to a slow
server, the number of datagrams received may be
smaller than the number of datagrams sent.
– The reason is that the receive buffer of the UDP socket
is limited. When it is full, datagrams are dropped.
• Increasing the socket receive buffer may help to
some extent, but can’t fix the problem
– You can try to change the size of the UDP socket
receive buffer with the socket option SO_RCVBUF
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