Chapter 11

User Protocol (UDP)

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o Process-to-process communication o User datagram o Checksum o UDP operation o Use of UDP o UDP packet

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The McGraw-Hill Companies, Inc., 2000 PDF created with FinePrint pdfFactory Pro trial version www.pdffactory.com Introduction

o A protocol usually has two responsibilities n Create a process-to-process communications

n Provide control mechanism at the transport layer o UDP does this task at a very minimal level o It only provides error control to some extent

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o Thus, UDP is a connectionless, unreliable transport protocol n Only add process-to-process to the IP n And perform very limited error checking

o Advantages n UDP is very simple protocol using a minimum of overhead

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The McGraw-Hill Companies, Inc., 2000 PDF created with FinePrint pdfFactory Pro trial version www.pdffactory.com Process-to-Process Communication

o Host-to-host communication n protocol: IP n Deliver the message only to the destination computer o Process-to-process communication n Transport layer protocol: UDP and TCP n Deliver the message to the appropriate process

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UDP Versus IP

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o Used to define the process o 0~65535 o Well-known port numbers n Some serves needs to be assign a universal port number o However, client’s port number can be defined randomly n number

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Port Numbers

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IP Addresses Versus Port Numbers

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o The IANA divides the port numbers into three ranges n Well-known: assigned and controlled by IANA o 0~1,023 n Registered: not assigned or controlled by IANA o 1,024~49,151 o Can only be registered with IANA to prevent duplication n Dynamic (or private): neither controlled nor registered o 49152~65535 o Ephemeral ports and can by used by any process

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IANA Ranges

The McGraw-Hill Companies, Inc., 2000 PDF created with FinePrint pdfFactory Pro trial version www.pdffactory.com Well-Known Ports for UDP

Port Protocol Description 7 Echo Echoes a received datagram back to the sender 9 Discard Discards any datagram that is received 11 Users Active users 13 Daytime Return the data and the time 17 Quote Returns a quote of a day 19 Chargen Return a string of characters 53 Nameserver Domain Name Service 67 Bootps Server port to download bootstrap information 68 Bootpc Client port to download bootstrap information

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Port Protocol Description 69 TFTP Trivial 111 RPC Remote Procedure Call 123 NTP Network 161 SNMP Simple Network Management Protocol 162 SNMP Simple Network Management Protocol (trap)

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o UDP needs two identifiers n The IP address n The port number

o Socket address n The combination of an IP address and a port number

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Socket Addresses

The McGraw-Hill Companies, Inc., 2000 PDF created with FinePrint pdfFactory Pro trial version www.pdffactory.com 1111.2.2 USER DATAGRAM

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o UDP packets: called user n Have a fixed-size header of 8 o The fields are n Source port number: 16-bits: 0~65535 o Usually chosen by the UDP software n Destination port number: 16-bits: 0~65535 n Length: 16 bits: o Total length (header plus data) of the user datagram o Minimum length must be 8 bytes (contains only header) n Checksum: 16 bits o Detect errors over the entire user datagram (header plus data)

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User Datagram Format

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o However, the UDP length can also be derived from the IP header n UDP length = IP length – IP header’s length o Thus, theoretically, the length field in a UDP datagram is not necessary n However, it is more efficient to derive the length of a UDP packet by the UDP package itself o Ask the IP layer is time consuming

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CHECKSUM

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o UDP checksum calculation is different from the one for IP and ICMP o The checksum includes three sections n The Pseudoheader o However, it will not appear in the UDP datagram n The UDP header n The data coming from the

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o Some part of the header of the IP packet n With some fields are filled with 0s o Fields n Source and destination IP address o To prevent that UDP datagram is correct but IP header is corrupted and be delivered to the wrong host n Protocol o To prevent the packet to deliver to the TCP o UDP has the value of 17

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o Add the pseudoheaderto the UDP datagram o Fill the checksum field with zeros o Divide the total bits into 16-bits words o If the total number of bytes is not even n Add 1 of padding (all 0s) o Add all 16-bit section using one’s complement arithmetic o Complement the result and insert it in the checksum field o Drop the pesudoheaderand any added padding o Deliver the UDP datagram to the IP software

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o Add the pseudoheaderto the UDP user datagram o Add padding if needed o Divide the total bits into 16-bit sections o Add all 16-bit sections using one’s complement arithmetic o Complement the result o If the result is all 0s n Drop the pseudoheaderand any added padding n Accept the user datagram o Otherwise n Discard the user datagram

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o The calculation of the checksum in UDP is optional

o If the checksum is not calculated n The field is filled with 0s

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o Connectionless Services

o Flow and Error Control

o Encapsulation and Decapsulation

o Queuing

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o Each user datagram is an independent datagram n Even coming from the same source process o The user datagram is not numbered

o No connection establishment and no connection termination

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o No flow control n The receiver may overflow with incoming messages

o No error control except for the checksum n The sender does not know if a message has been lost or duplicated

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Encapsulation and Decapsulation

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Queues in UDP

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Multiplexing and Demultiplexing

The McGraw-Hill Companies, Inc., 2000 PDF created with FinePrint pdfFactory Pro trial version www.pdffactory.com 1111.5.5 USE OF UDP

The McGraw-Hill Companies, Inc., 2000 PDF created with FinePrint pdfFactory Pro trial version www.pdffactory.com Uses of UDP

o For a process that require simple request-response communication with little concern for flow and error control o For a process with internal flow and error-control n TFTP (Trivial File Transfer Protocol) o For muticastingand broadcasting n Embedded in UDP but not in the TCP software o For some management processes o For some route updating protocol n RIP (Routing Information Protocol)

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o Five components n A control-block table n Input queues n A control-block module n An input module n An output module

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The McGraw-Hill Companies, Inc., 2000 PDF created with FinePrint pdfFactory Pro trial version www.pdffactory.com Control-Block Table

o Keep track of the open ports

o Eachentry has a minimum of four fields n State: FREE or IN-USE n Process ID n Port number n Corresponding queue number

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o A set of input queues n One for each process

o In the book’s design n They do not use output queues

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o Manage the control-block table o Pseudo code n Receive: a process ID and a port number n Search the control block table for a FREE entry o If (not found) n Delete an entry using a predefined strategy o Create a new entry with the state IN-USE o Enter the process ID and the port number n Return

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o Receive: a user datagram from IP o Look for the corresponding entry in the control- block table n If (found) o Check the queue field to see if a queue is allocated o If (no) n Allocate a queue o Enqueuethe data in the corresponding queue n If (not found) o Ask the ICMP module to send an “unreachable port” message o Discard the user datagram o Return

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o Receive: data and information from a process o Create a UDP data datagram o Send the user datagram o Return

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State Process ID Port Number Queue Number ------IN-USE2,34552,01034 IN-USE3,42252,011 FREE IN-USE4,65252,01238 FREE

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

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o After a few seconds, a process starts o It asks the for a port number and is granted port number 52,014 o Now the process sends its ID (4,978) and the port number to the control-block module to create an entry in the table o The module does not allocate a queue at this moment n because no user datagram have arrived for this destination

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State Process ID Port Number Queue Number ------IN-USE2,34552,01034 IN-USE3,42252,011 IN-USE4,97852,014 IN-USE4,65252,01238 FREE

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o A user datagram now arrives for port 52,011 o The input module checks the table and finds that no queue has been allocated for this destination n This is the first time a user datagram has arrived for this destination o The module creates a queue and gives it a number (43)

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State Process ID Port Number Queue Number ------IN-USE2,34552,01034 IN-USE3,42252,01143 IN-USE4,97852,014 IN-USE4,65252,01238 FREE

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o After a few seconds, a user datagram arrives for port 52,222 o The input module checks the table n Cannot find the entry for this destination o The user datagram is dropped o A request is made to ICMP to send an “unreachable port” message to the source

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o After a few seconds, a process needs to send a user datagram n It delivers the data to the output module which adds the UDP header and sends it

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