Introduction to Socket Details

1
Introduction to Socket Details

• Unix Network Programming
• Ch 3

2
Socket Address Structure

• Most socket functions (Posix C lib) require a
  pointer to a socket address struct as an argument
• Each supported protocol suite defines its own
  socket address struct
• Names begin with sockaddr_
• End with a unique suffix for each protocol

3
IPv4 Socket Address Struct

• /usr/include/netinet/in.h include
  ltnetinet/in.hgt

/ Internet address. / typedef uint32_t
in_addr_t struct in_addr in_addr_t
s_addr / Structure describing an
Internet socket address. / struct sockaddr_in
__SOCKADDR_COMMON (sin_) in_port_t
sin_port / Port number. / struct
in_addr sin_addr / Internet address. /
/ Pad to size of struct sockaddr'. /
unsigned char sin_zerosizeof (struct sockaddr)
- __SOCKADDR_COMMON_SIZE - sizeof
(in_port_t) - sizeof (struct in_addr)

ltbits/sockaddr.hgt
4
IPv4 Socket Address Structure

• Some struct defs have a sin_len
• IPv4 address and TCP or UDP port number are
  always stored in network byte order (discussed
  later)

5
Generic Socket Address Structure

• A socket address structure is always passed by
  reference when passed as an argument
• Any socket function must deal with structs from
  any of the supported protocol families
• Therefore they require a generic struct
• Defined in ltsys/socket.hgt (actually pulled out of
  ltbits/socket.hgt on our system

/ Structure describing a generic socket address.
/ struct sockaddr __SOCKADDR_COMMON
(sa_) / Common data address family and length.
/ char sa_data14 / Address data. /

6
Generic Socket Address Struct

• Socket functions are then defined as taking a
  pointer to the generic socket address structure
• int bind(int, struct sockaddr , socklen_t)
• To call bind, for example
• struct sockaddr_in serv / IPv4 socket address
  structure /
• / fill in serv /
• bind(sockfd, (struct sockaddr ) serv,
  sizeof(serv))

7
IPv6 Socket Address Structure

• IPv6 socket address is also defined in
  ltnetinet/in.hgt

/ IPv6 address / struct in6_addr union
uint8_t u6_addr816 uint16_t
u6_addr168 uint32_t u6_addr324
in6_u define s6_addr in6_u.u6_addr8 define
s6_addr16 in6_u.u6_addr16 define
s6_addr32 in6_u.u6_addr32 / Ditto, for
IPv6. / struct sockaddr_in6
__SOCKADDR_COMMON (sin6_) in_port_t
sin6_port / Transport layer port /
uint32_t sin6_flowinfo / IPv6 flow information
/ struct in6_addr sin6_addr / IPv6 address
/ uint32_t sin6_scope_id / IPv6 scope-id
/
8
New Generic Socket Address

• IPv6 defined a new generic socket address
  structure

struct sockaddr_storage __SOCKADDR_COMMON
(ss_) / Address family, etc. /
__ss_aligntype __ss_align / Force desired
alignment. / char __ss_padding_SS_PADSIZE

9
Comparison of Socket Address Structures

• pg. 73 fig 3.6

10
Byte Ordering Functions

• Consider a 16-bit integer that is made up of two
  bytes.
• There are 2 ways to store the two bytes in
  memory
• With the low-order byte at the starting address
  (little-endian)
• With the high-order byte at the starting address
  (big-endian)
• Show figure 3.9, pg. 77

11
Program to Determine Byte Order

• intro/byteorder.c

12
Network Byte Order

• We must deal with these byte ordering differences
  as network programmers
• Networking protocols must specify a network byte
  order
• IP uses big-endian byte ordering
• We use the following 4 functions to convert
  between host and network byte ordering

13
Network Byte Order
include ltnetinet/in.hgt uint16_t htons(uint16_t
host16bitvalue) uint32_t htonl(uint32_t
host32bitvalue) uint16_t ntohs(uint16_t
net16bitvalue) uint32_t ntohl(uint32_t
net32bitvalue)

• if __BYTE_ORDER __BIG_ENDIAN
• / The host byte order is the same as network
  byte order,
• so these functions are all just identity. /
• define ntohl(x) (x)
• define ntohs(x) (x)
• define htonl(x) (x)
• define htons(x) (x)
• else
• if __BYTE_ORDER __LITTLE_ENDIAN
• define ntohl(x) __bswap_32 (x)
• define ntohs(x) __bswap_16 (x)
• define htonl(x) __bswap_32 (x)
• define htons(x) __bswap_16 (x)
• endif
• endif
• endif

14
Byte Manipulation Functions

• 2 groups of functions operate on multibyte fields
• can't rely on C convention of null-terminated
  string, since data can contain zeros (e.g IP
  addresses)

include ltstrings.hgt void bzero(void dest,
size_t nbytes) void bcopy(const void src, void
dest, size_t nbytes) int bcmp(const void ptr1,
const void ptr2, size_t nbytes) void
memset(void dest, int c, size_t len) void
memcpy(void dest, const void src, size_t
nbytes) int memcmp(const void ptr1, const void
ptr2, size_t nbytes)
15
inet_aton, inet_addr and inet_ntoa

• Convert internet addresses between ASCII strings
  and network byte ordered binary values
• inet_aton, inet_ntoa and inet_addr convert an
  Ipv4 address from a dotted-decimal string (e.g.
  206.168.112.96) to its 32-bit network byte
  ordered binary value
• Newer functions inet_pton and inet_ntop handle
  both IPv4 and IPv6 addresses.

16
Readn, writen and readline functions

• Stream sockets (e.g. TCP sockets) exhibit a
  behavior with the read and write functions that
  differs from normal file I/O
• A read or write on a stream socket might input or
  output fewer bytes than requested
• But this is not an error condition
• All that is required is for caller to invoke the
  read or write function again