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

2. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 2 OSI Layers and Protocols

3. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 3 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)

4. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 4 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

5. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 5 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

6. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 6 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

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

8. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 8 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

9. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 9 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

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

11. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 11 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)

12. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 12 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)

13. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 13 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

14. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 14 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

15. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 15 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

16. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 16 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

17. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 17 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

18. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 18 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

19. May 7, 2003Data Communications and Networks: Lecture 10: Upper OSI Layers 19 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)