Upper OSI Layers Lecture 10, May 7, 2003 Mr. Greg Vogl Data Communications and Networks Uganda...

Upper OSI Layers

Lecture 10, May 7, 2003

Mr. Greg Vogl

Data Communications and Networks

Uganda Martyrs University

May 7, 2003 Data Communications and Networks: Lecture 10: Upper OSI Layers

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OSI Layers and Protocols


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Functions of Transport Layer

• End-to-end reliability (time to live counter)

• Packet sequencing (numbering, resend)

• Message segmentation/assembly (max. size)

• Flow control (ack, wait)

• Error detection/correction (checksum/CRC)

• Addressing (network, node, process)

• Security (encryption keys, user/password)


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Reliability and packet sequencing

• Time-to-live counter: no. of hops or secs.– When counter reaches 0, discard packet– Keeps a packet from moving forever

• Sequence numbers in transport header– Used to check if packets are missing– Receiver sends ack of received packets– Missing packets are resent– In connectionless sessions, restore packet order


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Flow and error control

• Flow control– Ack is needed after sending some messages– If receiver’s buffer fills, tell sender to wait– Rate control depends on source and destination

processing speed

• Error control– checksum in header– in addition to error checking in lower layers


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Transport Control Protocol

• Connection-based; full duplex– Set up, manage, close a virtual connection– Messages transferred as stream of bytes

• Sequenced delivery• Flow control

– Sliding window of acks

• Error detection and recovery; reliable• Port-level addressing


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TCP Segment


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TCP Ports

• Client and server processes communicate– network + host + port = unique process address

• Server uses well known port address 0-1023– FTP-21, Telnet-23, SMTP-25, HTTP-80, etc.– No need to include port number in URL

• http://www.w3.org:80 = http://www.w3.org

• Client uses any unused port• Handshake sets up source/dest port numbers


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User Datagram Protocol

• Connectionless, no connections like IP• Uses ports much like TCPLower overhead, higher speedNo reliability/ack• Used for non-critical messages; good for LANs

– MS Messenger Service, BOOTP, DHCP, RIP

• Higher levels manage delivery and reliability– e.g. NFS, use of timeout counters


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UDP Datagram


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Functions of Session Layer

• Establishes sessions between services• Synchronises and performs naming services• Example protocols:

– Dynamic Host Configuration Protocol (DHCP)– Bootstrap Protocol (BOOTP)– Domain Name Service (DNS)– Network Basic Input Output System (NetBIOS)– Remote Procedure Call (RPC)


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Dynamic Host Configuration Protocol

• Server assigns IP number to a computer

• Client obtains IP number automaticallyUser can move PC without involving adminGood for portable users

• Can also provide IP of local router, DNS

• Microsoft helped define DHCP– Based on BOOTP (for diskless workstations)


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Domain Name Service

• Translates IP numbers host names– E.g. 207.46.230.229 microsoft.com Easier to remember names than IP numbers– Uses: both providing and receiving servicesLocal control, available to all, fast, robust

• DNS servers keep a distributed database– Static hosts files would be too big to maintain– Web browsers and mail servers use DNS server


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Top-Level Domains

• .com: commercial/business/for profit

• .edu: educational

• .gov: US government

• .mil: US military

• .net: service providers, online organisations

• .org: nonprofits, associations, societies

• .ug, .us, .ca, etc.: country codes


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DNS Database Structure

• Tree structure with final “.” to indicate root

• Top-level domain names have 2-3 letters

• First-level name is the organisation name

• Second and third levels used by large org’s– home.umu.ac.ug is a subdomain of umu.ac.ug

• Each domain name has at least 1 IP number

• DNS servers: Win2000 Server, Unix BIND


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Network Basic Input/Output System

• The first Windows networking technology– Designed as API for small peer-peer LANs

• Windows now uses NBF and/or NetBT– NetBEUI has become a NetBIOS Frame (NBF)– NetBIOS routed in TCP/IP packets (NetBT)

• Uses Windows Internet Name Service– with DNS, resolves NetBIOS resource names


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Remote Procedure Call

• One host’s program executes another’s code

• Client requests to execute code on server

• A result message is returned to the caller

• Used to implement client-server computing

• Developed by Sun, several implementations

• Developers write communications code for:– authentication, sessions, errors, custom services


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Functions of Presentation Layer

• Network characters specific platforms• Data compression, encoding, encryption• Network shell

– Direct local and network requests appropriatelyLocal and network resources treated the sameApplication development is easier– e.g. Microsoft Redirector– e.g. Sun eXternal Data Representation for RPC


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Important Application Protocols

• File Transfer Protocol (FTP), TFTP, NTP

• Telnet; rlogin, rcp, rexec, rsh; ssh

• Simple Mail Transfer Protocol

• “Simple” Network Management Protocol

• (Secure) Hypertext Transfer Protocol

• Network File System (Sun remote mount)

• X Window System (Unix client-server GUI)

Upper OSI Layers Lecture 10, May 7, 2003 Mr. Greg Vogl Data Communications and Networks Uganda...

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