William Stallings Data and Computer Communications Chapter 2

William Stallings Data and Computer Communications Chapter 2 Protocols and Architecture

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

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

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

Symmetric or Asymmetric z Symmetric y. Communication between peer entities z Asymmetric y. Client/server

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

Use of Standard Protocols

Functions z Encapsulation z Segmentation and reassmebly z Connection control z Ordered delivery z Flow control z Error control z Addressing z Multiplexing z Transmission services

Encapsulation z Addition of control information to data y. Address information y. Error-detecting code y. Protocol control

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

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

Connection Control z Connection Establishment z Data transfer z Connection termination z May be connection interruption and recovery z Sequence numbers used for y. Ordered delivery y. Flow control y. Error control

Connection Oriented Data Transfer

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

Flow Control z Done by receiving entity z Limit amount or rate of data z Stop and wait z Credit systems y. Sliding window z Needed at application as well as network layers

Error Control z Guard against loss or damage z Error detection y. Sender inserts error detecting bits y. Receiver checks these bits y. If OK, acknowledge y. If error, discard packet z Retransmission y. If no acknowledge in given time, re-transmit z Performed at various levels

Addressing z Addressing level z Addressing scope z Connection identifiers z Addressing mode

Addressing level z Level in architecture at which entity is named z Unique address for each end system (computer) and router z Network level address y. IP or internet address (TCP/IP) y. Network service access point or NSAP (OSI) z Process within the system y. Port number (TCP/IP) y. Service access point or SAP (OSI)

Address Concepts

Addressing Scope z Global nonambiguity y. Global address identifies unique system y. There is only one system with address X z Global applicability y. It is possible at any system (any address) to identify any other system (address) by the global address of the other system y. Address X identifies that system from anywhere on the network z e. g. MAC address on IEEE 802 networks

Connection Identifiers z Connection oriented data transfer (virtual circuits) z Allocate a connection name during the transfer phase y. Reduced overhead as connection identifiers are shorter than global addresses y. Routing may be fixed and identified by connection name y. Entities may want multiple connections - multiplexing y. State information

Addressing Mode z Usually an address refers to a single system y. Unicast address y. Sent to one machine or person z May address all entities within a domain y. Broadcast y. Sent to all machines or users z May address a subset of the entities in a domain y. Multicast y. Sent to some machines or a group of users

Multiplexing z Supporting multiple connections on one machine z Mapping of multiple connections at one level to a single connection at another y. Carrying a number of connections on one fiber optic cable y. Aggregating or bonding ISDN lines to gain bandwidth

Transmission Services z Priority ye. g. control messages z Quality of service y. Minimum acceptable throughput y. Maximum acceptable delay z Security y. Access restrictions

OSI - The Model z A layer model z Each layer performs a subset of the required communication functions z Each layer relies on the next lower layer to perform more primitive functions z Each layer provides services to the next higher layer z Changes in one layer should not require changes in other layers

The OSI Environment

OSI as Framework for Standardization

Layer Specific Standards

Elements of Standardization z Protocol specification y. Operates between the same layer on two systems y. May involve different operating system y. Protocol specification must be precise x. Format of data units x. Semantics of all fields xallowable sequence of PCUs z Service definition y. Functional description of what is provided z Addressing y. Referenced by SAPs

OSI Layers (1) z Physical y. Physical interface between devices x. Mechanical x. Electrical x. Functional x. Procedural z Data Link y. Means of activating, maintaining and deactivating a reliable link y. Error detection and control y. Higher layers may assume error free transmission

OSI Layers (2) z Network y. Transport of information y. Higher layers do not need to know about underlying technology y. Not needed on direct links z Transport y. Exchange of data between end systems y. Error free y. In sequence y. No losses y. No duplicates y. Quality of service

OSI Layers (3) z Session y. Control of dialogues between applications y. Dialogue discipline y. Grouping y. Recovery z Presentation y. Data formats and coding y. Data compression y. Encryption z Application y. Means for applications to access OSI environment

Use of a Relay

TCP/IP Protocol Suite z Dominant commercial protocol architecture z Specified and extensively used before OSI z Developed by research funded US Department of Defense z Used by the Internet

TCP/IP Protocol Architecture(1) z Application Layer y. Communication between processes or applications z End to end or transport layer (TCP/UDP/…) y. End to end transfer of data y. May include reliability mechanism (TCP) y. Hides detail of underlying network z Internet Layer (IP) y. Routing of data

TCP/IP Protocol Architecture(2) z Network Layer y. Logical interface between end system and network z Physical Layer y. Transmission medium y. Signal rate and encoding

PDUs in TCP/IP

Some Protocols in TCP/IP Suite

Required Reading z Stallings chapter 2 z Comer, D. Internetworking with TCP/IP volume I z Comer, D. and Stevens, D. Internetworking with TCP/IP volume II and volume III, Prentice Hall z Halsall, F> Data Communications, Computer Networks and Open Systems, Addison Wesley z RFCs