Chapter 4 : TCP/IP and OSI Business Data Communications, 4e

What is a Protocol? 8Allows entities (i.e. application programs) from different systems to communicate 8Shared conventions for communicating information are called protocols 8Includes syntax, semantics, and timing

Why Use Protocol Architecture? 8Data communications requires complex procedures 8Sender identifies data path/receiver 8Systems negotiate preparedness 8Applications negotiate preparedness 8Translation of file formats 8For all tasks to occur, high level of cooperation is required

Modular Approach 8Breaks complex tasks into subtasks 8Each module handles specific subset of tasks 8Communication occurs 8between different modules on the same system 8between similar modules on different systems

Advantages of Modularity 8Easier application development 8Network can change without all programs being modified

Three-Layer Model 8Distributed data communications involves three primary components: 8Networks 8Computers 8Applications 8Three corresponding layers 8Network access layer 8Transport layer 8Application layer

Network Access Layer 8Concerned with exchange of data between computer and network 8Includes addressing, routing, prioritizing, etc 8Different networks require different software at this layer 8Example: X.25 standard for network access procedures on packet-switching networks

Transport Layer 8Concerned with reliable transfer of information between applications 8Independent of the nature of the application 8Includes aspects like flow control and error checking

Application Layer 8Logic needed to support various applications 8Each type of application (file transfer, remote access) requires different software on this layer

Addressing 8Each computer on a network requires a unique address on that network 8Each application requires a unique address within the computer to allow support for multiple applications (service access points, or SAP)

Data Transmission 8Application layer creates data block 8Transport layer appends header to create PDU (protocol data unit) 8Destination SAP, Sequence #, Error-Detection Code 8Network layer appends another header 8Destination computer, facilities (e.g. “priority”) 8See figure 4.5 in the book

Standardized Protocol Architectures 8Vendors like standards because they make their products more marketable 8Customers like standards because they enable products from different vendors to interoperate 8Two protocol standards are well-known: 8TCP/IP: widely implemented 8OSI: well-known, less used, still useful for modeling/conceptualizing

TCP/IP 8Transmission Control Protocol/Internet Protocol 8Developed by DARPA 8No official protocol standard 8Can identify five layers 8Application 8Host-to-Host (transport) 8Internet 8Network Access 8Physical

TCP/IP Physical Layer 8Physical interface between a DTE (e.g. computer or terminal) and a transmission medium 8Specifies: 8Characteristics of medium 8Nature of signals 8Data rate

TCP/IP Network Access 8Exchange of data between end system and network 8Address of host and destination 8Prioritization of transmission 8Software at this layer depends on network (e.g. X.25 vs. Ethernet) 8Segregation means that no other software needs to be concerned about net specifics

TCP/IP Internet Layer 8An Internet is an interconnection of two or more networks 8Internet layer handles tasks similar to network access layer, but between networks rather than between nodes on a network 8Uses IP for addressing and routing across networks 8Implemented in workstations and routers

TCP/IP Transport Layer 8Also called host-to-host layer 8Reliable exchange of data between applications 8Uses TCP protocols for transmission

TCP/IP Application Layer 8Logic needed to support variety of applications 8Separate module supports each type of application (e.g. file transfer)

TCP & UDP 8Most TCP/IP applications use TCP for transport layer 8TCP provides a connection (logical association) between two entities to regulate flow check errors 8UDP (User Datagram Protocol) does not maintain a connection, and therefore does not guarantee delivery, preserve sequences, or protect against duplication

IP and IPv6 8IP provides for 32-bit source and destination addresses 8IPv6 (1996 standard) provides for 128-bit addresses 8Migraqtion to IPv6 will be a very slow process

TCP/IP Applications 8SMTP (Simple Mail Transfer Protocol) 8Basic facility, transferring messages among hosts 8FTP (File Transfer Protocol) 8Sends files from one system to another on user command 8Telnet 8Remote login capability, allowing a user to emulate a terminal on the remote system

Internetworking 8Interconnected networks, usually implies TCP/IP 8Can appear to users as a single large network 8The global Internet is the largest example, but intranets and extranets are also examples

Routers 8Equipment used to interconnect independent networks 8Several essential functions 8Provide a link between networks 8Provide routing and delivery of data between processes on systems from different networks 8Provide the above functions without requiring modification of the attached networks

Router Issues 8Addressing schemes 8Maximum packet size 8Interfaces 8Reliability

TCP Segment (TCP PDU) 8Source port (16 bits) 8Destination port (16 bits) 8Sequence number (32 bits) 8Acknowledgment number (32 bits) 8Data Offset (4 bits) 8Reserved (6 bits) 8Flags (6 bits) : URG, ACK, PSH, RST, SYN, FIN 8Window (16 bits) 8Checksum (16 bits) 8Urgent Pointer (16 bits) 8Options (variable)

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

Why Study OSI? 8Still an excellent model for conceptualizing and understanding protocol architectures 8Key points: 8Modular 8Hierarchical 8Boundaries between layers=interfaces

OSI 8Open Systems Interconnection 8Developed by ISO 8Contains seven layers (see page 358) 8Application 8Presentation 8Session 8Transport 8Network 8Data Link 8Physical

OSI Lower Layers 8Physical 8Data Link 8Network

OSI Physical Layer 8Responsible for transmission of bits 8Always implemented through hardware 8Encompasses mechanical, electrical, and functional interfaces 8e.g. RS-232

OSI Data Link Layer 8Responsible for error-free, reliable transmission of data 8Flow control, error correction 8e.g. HDLC

OSI Network Layer 8Responsible for routing of messages through network 8Concerned with type of switching used (circuit v. packet) 8Handles routing between networks, as well as through packet-switching networks

OSI Upper Layers 8Transport 8Session 8Presentation 8Application

OSI Transport Layer 8Isolates messages from lower and upper layers 8Breaks down message size 8Monitors quality of communications channel 8Selects most efficient communication service necessary for a given transmission

OSI Session Layer 8Establishes logical connections between systems 8Manages log-ons, password exchange, log- offs 8Terminates connection at end of session

OSI Presentation Layer 8Provides format and code conversion services 8Examples 8File conversion from ASCII to EBDIC 8Invoking character sequences to generate bold, italics, etc on a printer

OSI Application Layer 8Provides access to network for end-user 8User’s capabilities are determined by what items are available on this layer

OSI in Action: Outgoing File Transfer 8Program issues command to Application Layer 8Application passes it to Presentation, which may reformat, passes to Session 8Session requests a connection, passes to Transport 8Transport breaks file into chunks, passes to Network 8Network selects the data’s route, passes to Data Link 8Data Link adds error- checking info, passes to Physical 8Physical transmits data, which includes information added by each layer

OSI in Action: Incoming File Transfer 8Physical receives bits, passes to Data Link 8Data Link checks for errors, passes to Network 8Network verifies routing, passes to Transport 8Transport reassembles data, passes to Session 8Session determines if transfer is complete, may end session, passes to Presentation 8Presentation may reformat, perform conversions, pass to Application layer 8Application presents results to user (e.g. updates FTP program display)