Chap 11 udp

CChhaapptteerr 1111
UUsseerr DDaattaaggrraamm
PPrroottooccooll
Objectives
Upon completion you will be able to:
• Be able to explain process-to-process communication
• Know the format of a UDP user datagram
• Be able to calculate a UDP checksum
• Understand the operation of UDP
• Know when it is appropriate to use UDP
• Understand the modules in a UDP package
TCP/IP Protocol Suite 1

Figure 11.1 Position of UDP in the TCP/IP protocol suite

11.1 PROCESS-TO-PROCESS
COMMUNICATION
Before we examine UDP, we must first uunnddeerrssttaanndd hhoosstt--ttoo--hhoosstt
ccoommmmuunniiccaattiioonn aanndd pprroocceessss--ttoo--pprroocceessss ccoommmmuunniiccaattiioonn aanndd tthhee
ddiiffffeerreennccee bbeettwweeeenn tthheemm..
TThhee ttooppiiccss ddiissccuusssseedd iinn tthhiiss sseeccttiioonn iinncclluuddee::
PPoorrtt NNuummbbeerrss
SSoocckkeett AAddddrreesssseess

Figure 11.2 UDP versus IP

Figure 11.3 Port numbers

Figure 11.4 IP addresses versus port numbers

Figure 11.5 ICANN ranges

NNoottee::
The well-known port numbers are less
than 1024.

TTaabbllee 1111..11 WWeellll--kknnoowwnn ppoorrttss uusseedd wwiitthh UUDDPP

ExamplE 1
In UNIX, the well-known ports are stored in a file called
/etc/services. Each line in this file gives the name of the server
and the well-known port number. We can use the grep utility to
extract the line corresponding to the desired application. The
following shows the port for TFTP. Note TFTP can use port 69
on either UDP or TCP.
$ grep tftp /etc/services
tftp 69/tcp
tftp 69/udp
See Next Slide

ExamplE 1 (ContinuEd)
SNMP uses two port numbers (161 and 162), each for a
different purpose, as we will see in Chapter 21.
$ grep snmp /etc/services
snmp 161/tcp #Simple Net Mgmt Proto
snmp 161/udp #Simple Net Mgmt Proto
snmptrap 162/udp #Traps for SNMP

Figure 11.6 Socket address

11.2 USER DATAGRAM
UDP packets are called user datagrams and have aa ffiixxeedd--ssiizzee hheeaaddeerr ooff
88 bbyytteess..

Figure 11.7 User datagram format

NNoottee::
UDP length =
IP length − IP header’s length

11.3 CHECKSUM
UDP checksum calculation is different from tthhee oonnee ffoorr IIPP aanndd IICCMMPP..
HHeerree tthhee cchheecckkssuumm iinncclluuddeess tthhrreeee sseeccttiioonnss:: aa ppsseeuuddoohheeaaddeerr,, tthhee UUDDPP
hheeaaddeerr,, aanndd tthhee ddaattaa ccoommiinngg ffrroomm tthhee aapppplliiccaattiioonn llaayyeerr..
CChheecckkssuumm CCaallccuullaattiioonn aatt SSeennddeerr
CChheecckkssuumm CCaallccuullaattiioonn aatt RReecceeiivveerr
OOppttiioonnaall UUssee ooff tthhee CChheecckkssuumm

Figure 11.8 Pseudoheader for checksum calculation

Figure 11.9 Checksum calculation of a simple UDP user datagram

11.4 UDP OPERATION
UDP uses concepts common to the transport layer. TThheessee ccoonncceeppttss wwiillll
bbee ddiissccuusssseedd hheerree bbrriieeffllyy,, aanndd tthheenn eexxppaannddeedd iinn tthhee nneexxtt cchhaapptteerr oonn tthhee
TTCCPP pprroottooccooll..
CCoonnnneeccttiioonnlleessss SSeerrvviicceess
FFllooww aanndd EErrrroorr CCoonnttrrooll
EEnnccaappssuullaattiioonn aanndd DDeeccaappssuullaattiioonn
QQuueeuuiinngg
MMuullttiipplleexxiinngg aanndd DDeemmuullttiipplleexxiinngg

Figure 11.10 Encapsulation and decapsulation

Figure 11.11 Queues in UDP

Figure 11.12 Multiplexing and demultiplexing

11.5 USE OF UDP
We discuss some uses of the UDP pprroottooccooll iinn tthhiiss sseeccttiioonn..

11.6 UDP PACKAGE
To show how UDP handles the sending and receiving ooff UUDDPP ppaacckkeettss,,
wwee pprreesseenntt aa ssiimmppllee vveerrssiioonn ooff tthhee UUDDPP ppaacckkaaggee.. TThhee UUDDPP ppaacckkaaggee
iinnvvoollvveess ffiivvee ccoommppoonneennttss:: aa ccoonnttrrooll--bblloocckk ttaabbllee,, iinnppuutt qquueeuueess,, aa
ccoonnttrrooll--bblloocckk mmoodduullee,, aann iinnppuutt mmoodduullee,, aanndd aann oouuttppuutt mmoodduullee..
CCoonnttrrooll--BBlloocckk TTaabbllee
IInnppuutt QQuueeuueess
CCoonnttrrooll--BBlloocckk MMoodduullee
IInnppuutt MMoodduullee
OOuuttppuutt MMoodduullee

Figure 11.13 UDP design

TTaabbllee 1111..22 TThhee ccoonnttrrooll--bblloocckk ttaabbllee aatt tthhee bbeeggiinnnniinngg ooff eexxaammpplleess

ExamplE 2
The first activity is the arrival of a user datagram with
destination port number 52,012. The input module searches for
this port number and finds it. Queue number 38 has been
assigned to this port, which means that the port has been
previously used. The input module sends the data to queue 38.
The control-block table does not change.

ExamplE 3
After a few seconds, a process starts. It asks the operating
system for a port number and is granted port number 52,014.
Now the process sends its ID (4,978) and the port number to
the control-block module to create an entry in the table. The
module takes the first FREE entry and inserts the information
received. The module does not allocate a queue at this moment
because no user datagrams have arrived for this destination
(see Table 11.3).
See Next Slide

TTaabbllee 1111..33 CCoonnttrrooll--bblloocckk ttaabbllee aafftteerr EExxaammppllee 33

ExamplE 4
A user datagram now arrives for port 52,011. The input
module checks the table and finds that no queue has been
allocated for this destination since this is the first time a user
datagram has arrived for this destination. The module creates
a queue and gives it a number (43). See Table 11.4.
See Next Slide

TTaabbllee 1111..44 CCoonnttrrooll--bblloocckk aafftteerr EExxaammppllee 44

ExamplE 5
After a few seconds, a user datagram arrives for port 52,222.
The input module checks the table and cannot find an entry for
this destination. The user datagram is dropped and a request is
made to ICMP to send an “unreachable port” message to the
source.

Chap 11 udp

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Chap 11 udp