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Upper OSI Layers. Lecture 10, May 7, 2003 Mr. Greg Vogl Data Communications and Networks Uganda Martyrs University. OSI Layers and Protocols. Functions of Transport Layer. End-to-end reliability (time to live counter) Packet sequencing (numbering, resend)

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upper osi layers

Upper OSI Layers

Lecture 10, May 7, 2003

Mr. Greg Vogl

Data Communications and Networks

Uganda Martyrs University

osi layers and protocols
OSI Layers and Protocols

Data Communications and Networks: Lecture 10: Upper OSI Layers

functions of transport layer
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)

Data Communications and Networks: Lecture 10: Upper OSI Layers

reliability and packet sequencing
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

Data Communications and Networks: Lecture 10: Upper OSI Layers

flow and error control
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

Data Communications and Networks: Lecture 10: Upper OSI Layers

transport control protocol
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

Data Communications and Networks: Lecture 10: Upper OSI Layers

tcp segment
TCP Segment

Data Communications and Networks: Lecture 10: Upper OSI Layers

tcp ports
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

Data Communications and Networks: Lecture 10: Upper OSI Layers

user datagram protocol
User Datagram Protocol
  • Connectionless, no connections like IP
  • Uses ports much like TCP
  • Lower overhead, higher speed
  • No 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

Data Communications and Networks: Lecture 10: Upper OSI Layers

udp datagram
UDP Datagram

Data Communications and Networks: Lecture 10: Upper OSI Layers

functions of session layer
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)

Data Communications and Networks: Lecture 10: Upper OSI Layers

dynamic host configuration protocol
Dynamic Host Configuration Protocol
  • Server assigns IP number to a computer
  • Client obtains IP number automatically
    • User can move PC without involving admin
    • Good for portable users
  • Can also provide IP of local router, DNS
  • Microsoft helped define DHCP
    • Based on BOOTP (for diskless workstations)

Data Communications and Networks: Lecture 10: Upper OSI Layers

domain name service
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 services
    • Local 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

Data Communications and Networks: Lecture 10: Upper OSI Layers

top level domains
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

Data Communications and Networks: Lecture 10: Upper OSI Layers

dns database structure
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

Data Communications and Networks: Lecture 10: Upper OSI Layers

network basic input output system
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

Data Communications and Networks: Lecture 10: Upper OSI Layers

remote procedure call
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

Data Communications and Networks: Lecture 10: Upper OSI Layers

functions of presentation layer
Functions of Presentation Layer
  • Network characters  specific platforms
  • Data compression, encoding, encryption
  • Network shell
    • Direct local and network requests appropriately
    • Local and network resources treated the same
    • Application development is easier
    • e.g. Microsoft Redirector
    • e.g. Sun eXternal Data Representation for RPC

Data Communications and Networks: Lecture 10: Upper OSI Layers

important application protocols
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)

Data Communications and Networks: Lecture 10: Upper OSI Layers