csc/ece 573 internet protocols user datagram protocol
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CSC/ECE 573Internet Protocols
User Datagram Protocol
Copyright Rudra Dutta, NCSU, Spring, 2003 2
User Datagram Protocol (RFC 768)Same service as IP connectionless, no state maintained unreliable, no notification if discarded, no reordering no flow control
Application program must assume responsibility for reliable communication by dealing with… message loss duplication excessive delay out of order delivery loss of connectivity
Copyright Rudra Dutta, NCSU, Spring, 2003 3
UDP (cont'd)UDP adds ports differentiation of services within host payload checksum (optional) data integrity
User datagram encapsulated within IP datagram overhead: 8 bytes protocol identifier (in IP header): 17 for reply: reverse source and destination port
numbers
Demultiplexing by port # identifies to what process a message should be
delivered each datagram received by one process only
Copyright Rudra Dutta, NCSU, Spring, 2003 4
Process-level CommunicationIP provides host-to-host communication
Support for communication between processes on different machines required how do we identify processes on other machines? communication frequently is same as “request for service”
Difficulties processes created and destroyed dynamically (no static
ID) a single process may provide multiple services; must be
clear which service the requestor needs Binding of “service required” and “process that handles this
service” should be determined by the server, not the client
Copyright Rudra Dutta, NCSU, Spring, 2003 5
Protocol PortsPort = abstraction of source or destination point identified by a positive integer
OS provides an interface for processes to specify/access ports
Synchronous access to ports (usually)process blocks (suspends execution) when port
unavailableBuffered (finite queue)
Messages carry destination port and source port (for replies)
Copyright Rudra Dutta, NCSU, Spring, 2003 6
UDP Ports
Copyright Rudra Dutta, NCSU, Spring, 2003 7
UDP Ports (cont'd)
Copyright Rudra Dutta, NCSU, Spring, 2003 8
UDP Port Numbers
Copyright Rudra Dutta, NCSU, Spring, 2003 9
Some Well-Known Ports
Listed in file /etc/services
PORT # SERVICE
7 ECHO
9 DISCARD
11 SYSTAT
13 DAYTIME
19 CHARGEN
37 TIME
42 NAMESERVER
43 WHOIS
53 DOMAIN
67 BOOTPS
68 BOOTPC
69 TFTP
111 SUNRPC
123 NTP
161 SNMP
162 SNMPTRAP
513 WHO
Copyright Rudra Dutta, NCSU, Spring, 2003 10
UDP Multiplexing, Demultiplexing• Enabled by the port abstraction
Copyright Rudra Dutta, NCSU, Spring, 2003 11
UDP Input QueueApplication programs negotiate with OS to use a given port
OS creates internal queue to hold arriving messages for the port application may specify/change queue size
Upon receipt of a datagram, UDP checks destination port against ports currently in use if no match, sends ICMP “port unreachable” error and
discards datagram if match is found, enqueues datagram to
corresponding port if port queue is full, discards datagram, no error
message
Copyright Rudra Dutta, NCSU, Spring, 2003 12
UDP Queues
Copyright Rudra Dutta, NCSU, Spring, 2003 13
UDP Datagram FormatSource and destination ports
Length: of UDP header and data
Checksum: across pseudo-header, UDP header, data
Copyright Rudra Dutta, NCSU, Spring, 2003 14
UDP Checksum ComputationOne's complement of one's complement sum of 16-bit words may need to pad with 1 byte (all 0) at end if an odd number of bytes If received checksum is incorrect, datagram is silently discarded
Checksum may be turned off for performance at your peril Set to zero (all 0’s)
Violates layering principle (why?)
Copyright Rudra Dutta, NCSU, Spring, 2003 15
When to Use UDPUDP is a suitable transport protocol for simple request-response communication with little concern for
error/flow control (non-critical application) e.g., time of day service
a process with application-level flow/error control mechanisms (e.g., TFTP)
multicast and broadcast communication (why preferred over reliable transport protocol?)
Barebones meta-protcols (SNMP, RIP)
Also handy for use with ICMP-based and other probes Route determination (traceroute) Path MTU discovery Investigate ARP Source Quench