Lecture (2)

TCP/IP Protocol Architecture Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET) Used by the global Internet No official model but a working one. Application layer Host to host or transport layer Internet layer Network access layer Physical layer

Physical Layer Physical interface between data transmission device (e.g. workstation/computer) and transmission medium or network Characteristics of transmission medium Signal levels Data rates etc.

Network Access Layer Exchange of data between end system (e.g. workstation) and network Providing the network with Destination address Concerned with access to and routing data across a network for two end systems attached to the same network. Invoking services (like priority). Different standards have been developed e.g. frame relay  circuit switching, packet switching e.g. Ethernet  LANs

Internet Layer (IP) Concerned with cases where two devises (end systems) are attached to different networks Routing functions across multiple networks Implemented in end systems and routers Router: a processor that connects two networks and whose primary function is to relay data from one network to the other on its route from the source to the destination end system.

Transport Layer (TCP) Application Layer Reliable delivery of data Data arrive at the destination application Data id delivered, in the same order in which they sent, to a certain SAP (port) Application Layer Support for user applications e.g. http, SMTP

TCP/IP Protocol Architecture Model

Operation of TCP/IP

OSI Model Open Systems Interconnection (OSI) Developed by the International Organization for Standardization (ISO) Seven layers A theoretical system delivered too late! TCP/IP is the real standard

OSI Layers Application Presentation Session Transport Network Data Link Physical

OSI v TCP/IP

Standards Required to allow for interoperability between equipment Advantages Ensures a large market for equipment and software Allows products from different vendors to communicate Disadvantages Freeze technology May be multiple standards for the same thing

William Stallings Data and Computer Communications Chapter 2 Protocols and Architecture

Protocols Characteristics Direct or indirect Monolithic or structured Symmetric or asymmetric Standard or nonstandard

Direct or Indirect Direct Indirect Systems share a point to point link or Systems share a multi-point link Data can pass without intervening active agent Indirect Switched networks or Internetworks or internets Data transfer depend on other entities

Monolithic or Structured Communications is a complex task Too complex for single unit Structured design breaks down problem into smaller units Layered structure

Symmetric or Asymmetric Communication between peer entities Asymmetric Client/server

Standard or Nonstandard Nonstandard protocols built for specific computers and tasks K sources and L receivers leads to K*L protocols and 2*K*L implementations If common protocol used, K + L implementations needed

Use of Standard Protocols

Functions A. Encapsulation B. Segmentation and reassembly C. Connection control D. Ordered delivery E. Flow control F. Error control G. Addressing H. Multiplexing I. Transmission services

A.Encapsulation Addition of control information to data Address information (sender/receiver) Error-detecting code Protocol control information The queries and replies among communications equipment to determine the respective capabilities of each end of the communications link. Information exchanged between entities of a given layer, via the service provided by the next lower layer, to coordinate their joint operation.

A.Encapsulation (cont’d)

B. Segmentation (Fragmentation) Data blocks (PDUs) are of bounded size Application layer messages (continuous stream) may be large Network packets may be smaller Splitting larger blocks into smaller ones is segmentation (or fragmentation in TCP/IP) ATM blocks (cells) are 53 octets long Ethernet blocks (frames) are up to 1526 octets long Smaller PDU blocks result in more interrupts Checkpoints and restart/recovery

Why Fragment? Advantages Disadvantages More efficient error control More equitable access to network facilities Shorter delays Smaller buffers needed Disadvantages Overheads Increased interrupts at receiver More processing time

Reassembly The segmented data must be reassembled into messages appropriate to the application level If PDU arrive out of order, the task is complicated

C. Connection Control Connection Establishment Data transfer Connection termination May be connection interruption and recovery Conectionless data transfer (datagram): an entity may transfer data to another entity in a way that each PDU is treated independently of all prior PDUs.

Connection Oriented Data Transfer

C. Connection Control (cont’d) Sequence numbers used for Ordered delivery Flow control (managing the rate of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver)  congestion avoidance Error control Sequencing is not found in all connection oriented protocols (e.g. Frame relay and ATM)

D. Ordered Delivery PDUs may traverse different paths through network PDUs may arrive out of order Sequentially number PDUs to allow for ordering

E. Flow Control Done by receiving entity Limit amount or rate of data The simplest procedure is (Stop and wait) Each PDU must be acknowledged before the next can be sent Credit systems for transmitter Sliding window (determines the amount of data that can be sent without an ACK) Needed at application as well as transport and network access layers

F. Error Control Guard against loss or damage Implemented as two separate functions: Error detection Sender inserts error detecting bits Receiver checks these bits If OK, acknowledge If error, discard packet Retransmission If no acknowledge in given time, re-transmit Some protocols employ an error correction code Performed at various layers

End of Lecture (2)