Overview
• Socket Programming: how applications use the network

Socket Programming
EE 122: Intro to Communication Networks Vern Paxson
TAs: Lisa Fowler, Daniel Killebrew, Jorge Ortiz

– Sockets are a C-language programming interface between Layer 7 (applications) and Layer 4 (transport) – Interface is quite general and fairly abstract – Use of interface differs somewhat between clients & servers

Materials with thanks to Sukun Kim, Artur Rivilis, Jennifer Rexford, Ion Stoica, and colleagues at Princeton and UC Berkeley
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Socket: End Point of Communication
• Sending message from one process to another
– Message must traverse the underlying network

Using Ports to Identify Services
Server host 128.2.194.242 Client host
Client

• Process sends and receives through a “socket”
– In essence, the doorway leading in/out of the house

Service request for 128.2.194.242:80 (i.e., the Web server)
OS

Web server (port 80)

Echo server (port 7)

• Socket as an Application Programming Interface
– Supports the creation of network applications
User process User process Service request for 128.2.194.242:7 (i.e., the echo server)
Client

Web server (port 80) OS Echo server (port 7)

socket
Operating System

socket
Operating System
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Knowing What Port Number To Use
• Popular applications have “well-known ports”
– E.g., port 80 for Web and port 25 for e-mail – Well-known ports listed at http://www.iana.org
Or see /etc/services on Unix systems

UNIX Socket API
• In UNIX, everything is like a file
– All input is like reading a file – All output is like writing a file – File is represented by an integer file descriptor

• Well-known vs. ephemeral ports
– Server has a well-known port (e.g., port 80)
By convention, between 0 and 1023; privileged

– Client gets an unused “ephemeral” (i.e., temporary) port
By convention, between 1024 and 65535

• System calls for sockets
– Client: create, connect, write/send, read, close – Server: create, bind, listen, accept, read/recv, write/send, close
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• Uniquely identifying the traffic between the hosts
– The two IP addresses plus the two port numbers
Sometimes called the “four-tuple”

– And: underlying transport protocol (e.g., TCP or UDP)
With the above, called the “five-tuple”

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Types of Sockets
• Different types of sockets implement different service models
– Stream: SOCK_STREAM – Datagram: SOCK_DGRAM

Using Stream Sockets
• No need to packetize data • Data arrives in the form of a “byte stream” • Receiver needs to separate messages in stream
TCP may send the messages joined together, “Hi there!Hope you are well” or may send them separately, or might even split them like “Hi there!Ho” and “pe you are well”
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• Stream socket (TCP)
– – – – Connection-oriented (includes establishment + termination) Reliable (lossless) and in-order delivery guaranteed. At-most-once delivery, no duplicates E.g., SSH, HTTP (Web), SMTP (email)

application transport network data-link physical
User application sends messages “Hi there!” and “Hope you are well” separately
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• Datagram socket (UDP)
– Connectionless (just data-transfer of packets) – “Best-effort” delivery, possibly lower variance in delay – E.g., IP Telephony (Skype), simple request/reply protocols (hostname → address lookups via DNS; time synchronization via NTP)

Recovering message boundaries
• Stream socket data separation:
– Use records (data structures) to partition data stream – How do we implement variable length records? size of record A fixed length record B fixed length record C 4 variable length record

Datagram Sockets
• User packetizes data before sending • Maximum size of 64 KB • Using previous example, “Hi there!” and “Hope you are well” definitely sent in separate packets at network layer
– Message boundaries preserved – But note: your message had better fit within 64 KB or else you’ll have to layer your own boundary mechanism on top of the datagram delivery anyway

– What if field containing record size gets corrupted?
Not possible! Why?

• Structuring the byte stream so you can recover the original data boundaries is termed framing
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Creating a Socket: socket()
• Operation to create a socket
– int socket(int domain, int type, int protocol) – Returns a descriptor (or handle) for the socket – Originally designed to support any protocol suite

Sending and Receiving Data
• Sending data
– ssize_t write(int sockfd, void *buf, size_t len) – Arguments: socket descriptor, pointer to buffer of data to send, and length of the buffer – Returns the number of characters written, and -1 on error – send(): same as write() with extra flags parameter

• Domain: protocol family
– Use PF_INET for the Internet

• Receiving data
– ssize_t read(int sockfd, void *buf, size_t len) – Arguments: socket descriptor, pointer to buffer to place the data, size of the buffer – Returns the number of characters read (where 0 implies “end of file”), and -1 on error – recv(): same as read() with extra flags parameter

• Type: semantics of the communication
– SOCK_STREAM: reliable byte stream – SOCK_DGRAM: message-oriented service

• Protocol: specific protocol
– UNSPEC: unspecified. No need for us to specify, since PF_INET plus SOCK_STREAM already implies TCP, or 11 SOCK_DGRAM implies UDP.

• Closing the socket
– int close(int sockfd)
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Byte Ordering
• We talk about two numeric presentations:
– Big Endian
Architectures: Sun SPARC, PowerPC 970, IBM System/360 The most significant byte stored in memory at the lowest address. Example: 4A3B2C1D hexadecimal will be stored as:
Lowest address here 4A 3B 2C 1D

Byte Ordering Solution
• The networking API provides us the following functions: uint16_t htons(uint16_t host16bitvalue); uint32_t htonl(uint32_t host32bitvalue); uint16_t ntohs(uint16_t net16bitvalue); uint32_t ntohl(uint32_t net32bitvalue);

This is network-byte order. Architectures: Intel x86, AMD64 The least significant byte stored in memory at the lowest address. Example: 4A3B2C1D hexadecimal will be stored as:
1D 2C 3B 4A

– Little Endian

• Use for all 16-bit and 32-bit binary numbers (short, int) to be sent across network • ‘h’ stands for “host order” • These routines do nothing on big-endian hosts • Note: common mistake is to forget to use these
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• What problems can arise because of this? • What can we do to solve them?

Why Can’t Sockets Hide These Details?
• Dealing with endian differences is tedious
– Couldn’t the socket implementation deal with this – … by swapping the bytes as needed?

Clients vs. Server setup
• Client
– Create a Socket – Determine server address and port number – Connect to server – Write/read data to connected socket – Close connection by closing the socket

• No, swapping depends on the data type
– Two-byte short int: (byte 1, byte 0) vs. (byte 0, byte 1) – Four-byte long int: (byte 3, byte 2, byte 1, byte 0) vs. (byte 0, byte 1, byte 2, byte 3) – String of one-byte characters: (char 0, char 1, char 2, …) never requires reordering

• Server
– Create a socket – Wait for connection from client – Accept incoming client connection – Read/write data to connected socket – Close connection by closing the socket
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• Socket layer doesn’t know the data types
– Sees the data as simply a buffer pointer and a length – Doesn’t have enough information to do the swapping
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Connecting Client socket to Server
• Translate the server’s name to an address
– struct hostent *gethostbyname(char *name) – name: the name of the host (e.g., “www.cnn.com”) – Returns a structure that includes the host address
Or NULL if host doesn’t exist

Server Preparing its Socket
• Bind socket to the local address and port number
– int bind (int sockfd, struct sockaddr *my_addr, socklen_t addrlen) – Arguments: socket descriptor, server address, address length – Returns 0 on success, and -1 if an error occurs

• Identifying the service’s port number
– struct servent *getservbyname(char *name, char *proto) – E.g., getservbyname(“http”, “tcp”)

• Define how many connections can be pending
– int listen(int sockfd, int backlog) – Arguments: socket descriptor and acceptable backlog – Returns 0 on success, and -1 on error

• Establishing the connection
– int connect(int sockfd, struct sockaddr *server_address, socketlen_t addrlen) – Arguments: socket descriptor, server address, and address size 17 – Returns 0 on success, and -1 if an error occurs

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Accepting a New Client Connection
• Accept a new connection from a client
– int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen) – Arguments: socket descriptor, structure that will provide client address and port, and length of the structure – Returns descriptor for a new socket for this connection

Putting it All Together
Server socket() bind() Client listen() socket() accept() connect() block read() process request write()
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• Questions
– What happens if no clients are around?
The accept() call blocks waiting for a client

write()

– What happens if too many clients are around?
Some connection requests don’t get through … But, that’s okay, because the Internet makes no promises

read()

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Same, But Emphasizing the Client
• Stream socket: Transmission Control Protocol (TCP) • Note, does not create any state inside the network • Sequence of actions: socket() I want to check out latest Cal Football stats.

socket() bind(80) listen() accept()

Let’s Look At Some Code
(though you’ll still need to read the manual pages too …)

connect()
[connection established]

write() read() close()

read() write() close()
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Client Establishes connection struct sockaddr_in sin; struct hostent *host = gethostbyname (argv[1]); in_addr_t server_addr = *(in_addr_t *) host->h_addr_list[0]; unsigned short server_port = atoi (argv[2]);

Server Binding Port to Socket
• Want port at server end to use a particular number struct sockaddr_in sin; memset (&sin, 0, sizeof (sin));

memset (&sin, 0, sizeof (sin)); sin.sin_family = AF_INET; sin.sin_addr.s_addr = server_addr; /* already in network order */ sin.sin_port = htons (server_port); if (connect(sock, (struct sockaddr *) &sin, sizeof (sin)) 0) { if (FD_ISSET(sock, read_set)) { if (receive_packets(buffer, buffer_len, &bytes_read) != 0) { interpret break; result } if (FD_ISSET(fileno(stdin), read_set)) { if (read_user(user_buffer, user_buffer_len, &user_bytes_read) != 0) { break; } } } }

I/O Multiplexing: Select (3)
• Explanation:
– – – – FD_ZERO(fd_set *set) -- clears a file descriptor set FD_SET(int fd, fd_set *set) -- adds fd to the set FD_CLR(int fd, fd_set *set) -- removes fd from the set FD_ISSET(int fd, fd_set *set) -- tests to see if fd is in the set

• When does the call return?
– An error occurs on an fd. – Data becomes available on an fd. – (Other cases you needn’t worry about now)

• What do I check?
– You use FD_ISSET to see if a particular fd is set, and if it is then you need to handle it in some way. – All non-active fds are cleared (so you need to reset the fd_set if you want to select again on a certain fd). – More than one fd set may be set after select returns.
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Server Reusing Its Port
• After TCP connection closes, it waits several minutes before freeing up the associated port • But server port numbers are fixed ⇒ must be reused • Solution: int sock, optval = 1; if ((sock = socket (AF_INET, SOCK_STREAM, 0)) < 0) { perror (”couldn’t create TCP socket"); abort (); } /* Note, this call must come *before* call to bind() */ if (setsockopt (sock, SOL_SOCKET, SO_REUSEADDR,&optval, sizeof (optval)) < 0) { perror (“couldn’t reuse address"); abort (); }

Common Mistakes + Hints
• Common mistakes:
– – – – – Byte-ordering! Use of select() Separating records in TCP stream Not coping with a read() that returns only part of a record Server can’t bind because old connection hasn’t yet gone away – Not knowing what exactly gets transmitted on the wire
Use Wireshark/Ethereal (covered in Section next week) or tcpdump

• Hints:
– Use man pages – Check out Web (ex. Beej’s Guide), programming books – Look at sample code.
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