UDP: User Datagram Protocol

1
UDP User Datagram Protocol
2
UDP User Datagram Protocol RFC 768

• Pros
• No connection establishment
• No delay to start sending/receiving packets
• Simple
• no connection state at sender, receiver
• Small segment header
• Just 8 bytes of header
• Cons
• best effort transport service means, UDP
  segments may be
• lost
• delivered out of order to app
• no congestion control UDP can blast away as fast
  as desired

• bare bones, best effort transport protocol
• connectionless
• no handshaking between UDP sender, receiver
  before packets start being exchanged
• each UDP segment handled independently of others
• Just provides multiplexing/demultiplexing

3
UDP more
Used for Mux/Demux

• often used for streaming multimedia apps
• loss tolerant
• rate sensitive
• other UDP uses
• DNS
• SNMP
• reliable transfer over UDP add reliability at
  application layer
• application-specific error recovery!

32 bits
source port
dest port
Length, in bytes of UDP segment, including header
checksum
length
Application data (message)
UDP segment format
4
UDP Demultiplexing

• Create sockets with port numbers
• DatagramSocket mySocket1 new DatagramSocket(6428
  )
• DatagramSocket mySocket2 new DatagramSocket(4567
  )
• UDP socket identified by two-tuple
• (dest IP address, dest port number)

• When host receives UDP segment
• checks destination port number in segment
• directs UDP segment to socket with that port
  number
• IP datagrams with different source IP addresses
  and/or source port numbers directed to same socket

5
UDP Demultiplexing Example

• DatagramSocket serverSocket new
  DatagramSocket(6428)

SP 9157
client IP A
Client IPB
server IP C
DP 6428
Source Port (SP) provides return address
Identifies the process at the other end of the
line
6
UDP checksum
Goal detect errors (e.g., flipped bits) in
transmitted segment

• Receiver
• compute checksum of received segment
• check if computed checksum equals checksum field
  value
• NO - error detected
• YES - no error detected. But maybe errors
  nonetheless?
• Reordered bytes
• Why checksum at UDP if LL provides error
  checking?
• IP is supposed to run over ANY LL, so UDP does
  its own error checking

• Sender
• treat segment contents as sequence of 16-bit
  integers
• checksum addition (1s complement sum) of
  segment contents
• sender puts checksum value into UDP checksum field

7
How to program using the UDP?

• Socket Layer
• Programmers API to the protocol stack

• Typical network app has two pieces client and
  server
• Server Passive entity. Provides service to
  clients
• e.g., Web server responds with the requested Web
  page
• Client initiates contact with server (speaks
  first)
• typically requests service from server, e.g., Web
  Browser

8
Socket Creation

• mySock socket(family, type, protocol)
• UDP/TCP/IP-specific sockets
• Socket reference
• File (socket) descriptor in UNIX
• Socket handle in WinSock

9
UDP Client/Server Interaction
Server starts by getting ready to receive
client messages

• Server
• Create a UDP socket
• Assign a port to socket
• Communicate (receive/send messages)
• When done, close the socket

• Client
• Create a UDP socket
• Communicate (send/receive messages)
• When done, close the socket

10
UDP Client/Server Interaction
/ Create socket for incoming messages /
if ((servSock socket(PF_INET, SOCK_DGRAM,
IPPROTO_UDP)) lt 0) Error("socket()
failed")

• Server
• Create a UDP socket
• Assign a port to socket
• Communicate (receive/send messages)
• When done, close the socket

• Client
• Create a UDP socket
• Communicate (send/receive messages)
• When done, close the socket

11
UDP Client/Server Interaction
ServAddr.sin_family PF_INET
/ Internet address family /
ServAddr.sin_addr.s_addr htonl(INADDR_ANY)
/ Any incoming interface /
ServAddr.sin_port htons(20000)
/ Local port 20000 / if
(bind(servSock, (struct sockaddr ) ServAddr,
sizeof(ServAddr)) lt 0) Error("bind()
failed")

• Server
• Create a UDP socket
• Assign a port to socket
• Communicate (receive/send messages)
• When done, close the socket

• Client
• Create a UDP socket
• Communicate (send/receive messages)
• When done, close the socket

12
Specifying Addresses

• struct sockaddr
• unsigned short sa_family / Address family
  (e.g., PF_INET) /
• char sa_data14 /
  Protocol-specific address information /
• struct sockaddr_in
• unsigned short sin_family / Internet
  protocol (PF_INET) /
• unsigned short sin_port / Port
  (16-bits) /
• struct in_addr sin_addr / Internet
  address (32-bits) /
• char sin_zero8 / Not used /
• struct in_addr
• unsigned long s_addr / Internet
  address (32-bits) /

Generic
IP Specific
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UDP Client/Server Interaction
struct sockaddr_in peer
int peerSize sizeof(peer) char
buffer65536 recvfrom(servSock, buffer,
65536, 0, (struct sockaddr )peer, peerSize)

• Server
• Create a UDP socket
• Assign a port to socket
• Communicate (receive/send messages)
• When done, close the socket

• Client
• Create a UDP socket
• Communicate (send/receive messages)
• When done, close the socket

14
UDP Client/Server Interaction
Server is now blocked waiting for a message from
a client

• Server
• Create a UDP socket
• Assign a port to socket
• Communicate (receive/send messages)
• When done, close the socket

• Client
• Create a UDP socket
• Communicate (send/receive messages)
• When done, close the socket

15
UDP Client/Server Interaction
Later, a client decides to talk to the server

• Server
• Create a UDP socket
• Assign a port to socket
• Communicate (receive/send messages)
• When done, close the socket

• Client
• Create a UDP socket
• Communicate (send/receive messages)
• When done, close the socket

16
UDP Client/Server Interaction
/ Create socket for outgoing messages /
if ((clientSock socket(PF_INET, SOCK_DGRAM,
IPPROTO_UDP)) lt 0) Error("socket()
failed")

• Server
• Create a UDP socket
• Assign a port to socket
• Communicate (receive/send messages)
• When done, close the socket

• Client
• Create a UDP socket
• Communicate (send/receive messages)
• When done, close the socket

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UDP Client/Server Interaction
// Initialize servers address and port
struct sockaddr_in server server.sin_family
AF_INET server.sin_addr.s_addr
inet_addr(10.10.100.37) server.sin_port
htons(20000) // Send it to the server
sendto(clientSock, buffer, msgSize, 0, (struct
sockaddr )server, sizeof(server))

• Server
• Create a UDP socket
• Assign a port to socket
• Communicate (receive/send messages)
• When done, close the socket

• Client
• Create a UDP socket
• Communicate (send/receive messages)
• When done, close the socket

18
UDP Client/Server Interaction
close(clientSock)
close(serverSock)

• Server
• Create a UDP socket
• Assign a port to socket
• Communicate (receive/send messages)
• When done, close the socket

• Client
• Create a UDP socket
• Communicate (send/receive messages)
• When done, close the socket