Chapter 6 Protocols and Architecture

Characteristics zDirect or indirect zMonolithic or structured zSymmetric or asymmetric zStandard or nonstandard

Direct or Indirect zDirect ySystems share a point to point link or ySystems share a multi-point link yData can pass without intervening active agent zIndirect ySwitched networks or yInternetworks or internets yData transfer depend on other entities

Monolithic or Structured zCommunications is a complex task zTo complex for single unit zStructured design breaks down problem into smaller units zLayered structure

Symmetric or Asymmetric zSymmetric yCommunication between peer entities zAsymmetric yClient/server

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

Use of Standard Protocols

Functions zEncapsulation zSegmentation and reassmebly zConnection control zOrdered delivery zFlow control zError control zAddressing zMultiplexing zTransmission services

Encapsulation zAddition of control information to data yAddress information yError-detecting code yProtocol control

Segmentation (Fragmentation) zData blocks are of bounded size zApplication layer messages may be large zNetwork packets may be smaller zSplitting larger blocks into smaller ones is segmentation (or fragmentation in TCP/IP) yATM blocks (cells) are 53 octets long yEthernet blocks (frames) are up to 1526 octets long zCheckpoints and restart/recovery

Why Fragment? zAdvantages yMore efficient error control yMore equitable access to network facilities yShorter delays ySmaller buffers needed zDisadvantages yOverheads yIncreased interrupts at receiver yMore processing time

Connection Control zConnection Establishment zData transfer zConnection termination zMay be connection interruption and recovery zSequence numbers used for yOrdered delivery yFlow control yError control

Connection Oriented Data Transfer

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

Flow Control zDone by receiving entity zLimit amount or rate of data zStop and wait zCredit systems ySliding window zNeeded at application as well as network layers

Error Control zGuard against loss or damage zError detection ySender inserts error detecting bits yReceiver checks these bits yIf OK, acknowledge yIf error, discard packet zRetransmission yIf no acknowledge in given time, re-transmit zPerformed at various levels

Addressing zAddressing level zAddressing scope zConnection identifiers zAddressing mode

Addressing level zLevel in architecture at which entity is named zUnique address for each end system (computer) and router zNetwork level address yIP or internet address (TCP/IP) yNetwork service access point or NSAP (OSI) zProcess within the system yPort number (TCP/IP) yService access point or SAP (OSI)

Address Concepts

Addressing Scope zGlobal nonambiguity yGlobal address identifies unique system yThere is only one system with address X zGlobal applicability yIt is possible at any system (any address) to identify any other system (address) by the global address of the other system yAddress X identifies that system from anywhere on the network ze.g. MAC address on IEEE 802 networks

Connection Identifiers zConnection oriented data transfer (virtual circuits) zAllocate a connection name during the transfer phase yReduced overhead as connection identifiers are shorter than global addresses yRouting may be fixed and identified by connection name yEntities may want multiple connections - multiplexing yState information

Addressing Mode zUsually an address refers to a single system yUnicast address ySent to one machine or person zMay address all entities within a domain yBroadcast ySent to all machines or users zMay address a subset of the entities in a domain yMulticast ySent to some machines or a group of users

Multiplexing zSupporting multiple connections on one machine zMapping of multiple connections at one level to a single connection at another yCarrying a number of connections on one fiber optic cable yAggregating or bonding ISDN lines to gain bandwidth

Transmission Services zPriority ye.g. control messages zQuality of service yMinimum acceptable throughput yMaximum acceptable delay zSecurity yAccess restrictions

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

The OSI Environment

OSI as Framework for Standardization

Layer Specific Standards

Elements of Standardization zProtocol specification yOperates between the same layer on two systems yMay involve different operating system yProtocol specification must be precise xFormat of data units xSemantics of all fields xallowable sequence of PCUs zService definition yFunctional description of what is provided zAddressing yReferenced by SAPs

OSI Layers (1) zPhysical yPhysical interface between devices xMechanical xElectrical xFunctional xProcedural zData Link yMeans of activating, maintaining and deactivating a reliable link yError detection and control yHigher layers may assume error free transmission

OSI Layers (2) zNetwork yTransport of information yHigher layers do not need to know about underlying technology yNot needed on direct links zTransport yExchange of data between end systems yError free yIn sequence yNo losses yNo duplicates yQuality of service

OSI Layers (3) zSession yControl of dialogues between applications yDialogue discipline yGrouping yRecovery zPresentation yData formats and coding yData compression yEncryption zApplication yMeans for applications to access OSI environment

Use of a Relay

TCP/IP Protocol Suite zDominant commercial protocol architecture zSpecified and extensively used before OSI zDeveloped by research funded US Department of Defense zUsed by the Internet

TCP/IP Protocol Architecture(1) zApplication Layer yCommunication between processes or applications  End to end or transport layer (TCP/UDP/ … ) yEnd to end transfer of data yMay include reliability mechanism (TCP) yHides detail of underlying network zInternet Layer (IP) yRouting of data

TCP/IP Protocol Architecture(2) zNetwork Layer yLogical interface between end system and network zPhysical Layer yTransmission medium ySignal rate and encoding

PDUs in TCP/IP

Some Protocols in TCP/IP Suite

Required Reading zStallings chapter 2 zComer,D. Internetworking with TCP/IP volume I zComer,D. and Stevens,D. Internetworking with TCP/IP volume II and volume III, Prentice Hall zHalsall, F> Data Communications, Computer Networks and Open Systems, Addison Wesley zRFCs