1 TCP/IP Network and OSI Model

2 What is a Protocol? n Allows entities (i.e. application programs) from different systems to communicate n Shared conventions for communicating information are called protocols n Includes syntax, semantics, and timing

3 Why Use Protocol Architecture? n Data communications requires complex procedures –Sender identifies data path/receiver –Systems negotiate preparedness –Applications negotiate preparedness –Translation of file formats n For all tasks to occur, high level of cooperation is required

4 Three-Layer Model n Distributed data communications involves three primary components: –Networks –Computers –Applications n Three corresponding layers –Network access layer –Transport layer –Application layer

5 Protocol “Layers” Networks are complex! n many “pieces”: –hosts –routers –links of various media –applications –protocols –hardware, software Question: Is there any hope of organizing structure of network? Or at least our discussion of networks?0

6 Organization of air travel n a series of steps ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing

7 Organization of air travel : a different view Layers: each layer implements a service –via its own internal-layer actions –relying on services provided by layer below ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing

8 Layered air travel: services Counter-to-counter delivery of person+bags baggage-claim-to-baggage-claim delivery people transfer: loading gate to arrival gate runway-to-runway delivery of plane airplane routing from source to destination

9 Distributed implementation of layer functionality ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing Departing airport arriving airport intermediate air traffic sites airplane routing

10 Why layering? Dealing with complex systems: n explicit structure allows identification, relationship of complex system’s pieces –layered reference model for discussion n modularization eases maintenance, updating of system –change of implementation of layer’s service transparent to rest of system –e.g., change in gate procedure doesn’t affect rest of system n layering considered harmful?

11 Internet protocol stack n application: supporting network applications –ftp, smtp, http n transport: host-host data transfer –tcp, udp n network: routing of datagrams from source to destination –ip, routing protocols n link: data transfer between neighboring network elements –ppp, ethernet n physical: bits “on the wire” application transport network link physical

12 TCP/IP n Transmission Control Protocol/Internet Protocol n Developed by DARPA n No official protocol standard n Can identify five layers n Application n Host-to-Host (transport) n Internet n Network Access n Physical

13 TCP Overview n Connection-oriented n Byte-stream –app writes bytes –TCP sends segments –app reads bytes Application process Write bytes TCP Send buffer Segment Transmit segments Application process Read bytes TCP Receive buffer … …… n Full duplex n Flow control: keep sender from overrunning receiver n Congestion control: keep sender from overrunning network

14 TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 n full duplex data: –bi-directional data flow in same connection –MSS: maximum segment size n connection-oriented: –handshaking (exchange of control msgs) init’s sender, receiver state before data exchange n flow controlled: –sender will not overwhelm receiver n point-to-point: –one sender, one receiver n reliable, in-order byte steam: –no “message boundaries” n pipelined: –TCP congestion and flow control set window size n send & receive buffers

15 TCP segment structure source port # dest port # 32 bits application data (variable length) sequence number acknowledgement number rcvr window size ptr urgent data checksum F SR PAU head len not used Options (variable length) URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) # bytes rcvr willing to accept counting by bytes of data (not segments!) Internet checksum (as in UDP)

16 TCP Segment (TCP PDU) n Source port (16 bits) n Destination port (16 bits) n Sequence number (32 bits) n Acknowledgment number (32 bits) n Data Offset (4 bits) n Reserved (6 bits) n Flags (6 bits) : URG, ACK, PSH, RST, SYN, FIN n Window (16 bits) n Checksum (16 bits) n Urgent Pointer (16 bits) n Options (variable)

17 TCP/IP Physical Layer n Physical interface between a DTE (e.g. computer or terminal) and a transmission medium n Specifies: –Characteristics of medium –Nature of signals –Data rate

18 TCP/IP Network Access n Exchange of data between end system and network n Address of host and destination n Prioritization of transmission n Software at this layer depends on network (e.g. X.25 vs. Ethernet) n Segregation means that no other software needs to be concerned about net specifics

19 Network Access Layer n Concerned with exchange of data between computer and network n Includes addressing, routing, prioritizing, etc n Different networks require different software at this layer n Example: X.25 standard for network access procedures on packet-switching networks

20 TCP/IP Internet Layer n An Internet is an interconnection of two or more networks n Internet layer handles tasks similar to network access layer, but between networks rather than between nodes on a network n Uses IP for addressing and routing across networks n Implemented in workstations and routers

21 TCP/IP Transport Layer n Also called host-to-host layer n Reliable exchange of data between applications n Uses TCP protocols for transmission

22 Transport Layer n Concerned with reliable transfer of information between applications n Independent of the nature of the application n Includes aspects like flow control and error checking

23 TCP/IP Application Layer n Logic needed to support variety of applications n Each type of application (file transfer, remote access) requires different software on this layer

24 TCP/IP Applications n SMTP (Simple Mail Transfer Protocol) –Basic facility, transferring messages among hosts n FTP (File Transfer Protocol) –Sends files from one system to another on user command n Telnet –Remote login capability, allowing a user to emulate a terminal on the remote system

25 TCP & UDP n Most TCP/IP applications use TCP for transport layer n TCP provides a connection (logical association) between two entities to regulate flow check errors n UDP (User Datagram Protocol) does not maintain a connection, and therefore does not guarantee delivery, preserve sequences, or protect against duplication

26 UDP: User Datagram Protocol [RFC 768] n “no frills,” “bare bones” Internet transport protocol n “best effort” service, UDP segments may be: –lost –delivered out of order to app n connectionless: –no handshaking between UDP sender, receiver –each UDP segment handled independently of others Why is there a UDP? n no connection establishment (which can add delay) n simple: no connection state at sender, receiver n small segment header n no congestion control: UDP can blast away as fast as desired

27 UDP: more n often used for streaming multimedia apps –loss tolerant –rate sensitive n other UDP uses (why?): –DNS –SNMP n reliable transfer over UDP: add reliability at application layer –application-specific error recover! source port #dest port # 32 bits Application data (message) UDP segment format length checksum Length, in bytes of UDP segment, including header

28 Addressing n Each computer on a network requires a unique address on that network n Each application requires a unique address within the computer to allow support for multiple applications (service access points, or SAP)

29 IPv4 Header n Version (4 bits) n Internet header length (4 bits) n Type of Service (8 bits) n Total Length (16 bits) n Identification (16 bits) n Flags (3 bits) n Fragment Offset (13 bits) n Time to Live (8 bits) n Protocol (8 bits n Header Checksum (16 bits) n Source Address ( 32 bits) n Destination Address (32 bits) n Options (variable) n Padding (variable)

30 IP and IPv6 n IP provides for 32-bit source and destination addresses n IPv6 (1996 standard) provides for 128-bit addresses n Migraqtion to IPv6 will be a very slow process

31 Data Transmission n Application layer creates data block n Transport layer appends header to create PDU (protocol data unit) –Destination SAP, Sequence #, Error-Detection Code n Network layer appends another header –Destination computer, facilities (e.g. “priority”)

32 Standardized Protocol Architectures n Vendors like standards because they make their products more marketable n Customers like standards because they enable products from different vendors to interoperate n Two protocol standards are well-known: –TCP/IP: widely implemented –OSI: well-known, less used, still useful for modeling/conceptualizing

33 Routers n Equipment used to interconnect independent networks n Several essential functions –Provide a link between networks –Provide routing and delivery of data between processes on systems from different networks –Provide the above functions without requiring modification of the attached networks

34 Router Issues n Addressing schemes n Maximum packet size n Interfaces n Reliability

35 Internetworking n Interconnected networks, usually implies TCP/IP n Can appear to users as a single large network n The global Internet is the largest example, but intranets and extranets are also examples

36 IP Internet n Concatenation of Networks n Protocol Stack R2 R1 H4 H5 H3 H2 H1 Network 2 (Ethernet) Network 1 (Ethernet) H6 Network 3 (FDDI) Network 4 (point-to-point) H7R3H8 R1 ETH FDDI IP ETH TCP R2 FDDI PPP IP R3 PPP ETH IP H1 IP ETH TCP H8

37 Internetworking Protocols

38 Question ???