Chapter 2 Protocols and Architecture

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William Stallings

Data and Computer

Communications

Chapter 2

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Characteristics

 _{Direct or indirect}

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Direct or Indirect

 _Direct

 _{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}

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Monolithic or Structured

 _{Communications is a complex task}  _{To complex for single unit}

 _{Structured design breaks down problem}

into smaller units

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Symmetric or Asymmetric

 _Symmetric

 _{Communication between peer entities}  _Asymmetric

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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}

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Functions

 _{Encapsulation}

 _{Segmentation and reassmebly}  _{Connection control}

 _{Ordered delivery}  _{Flow control}

 _{Error control}  _Addressing  _Multiplexing

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Encapsulation

 _{Addition of control information to data}  _{Address information}

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Segmentation (Fragmentation)

 _{Data blocks are of bounded size}

 _{Application layer messages 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

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Why Fragment?

 _Advantages

 _{More efficient error control}

 _{More equitable access to network facilities}  _{Shorter delays}

 _{Smaller buffers needed}

 _{Disadvantages}  _Overheads

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Connection Control

 _{Connection Establishment}  _{Data transfer}

 _{Connection termination}

 _{May be connection interruption and}

recovery

 _{Sequence numbers used for}  _{Ordered delivery}

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Ordered Delivery

 _{PDUs may traverse different paths through}

network

 _{PDUs may arrive out of order}

 _{Sequentially number PDUs to allow for}

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Flow Control

 _{Done by receiving entity}

 _{Limit amount or rate of data}  _{Stop and wait}

 _{Credit systems}  _{Sliding window}

 _{Needed at application as well as network}

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Error Control

 _{Guard against loss or damage}  _{Error detection}

 _{Sender inserts error detecting bits}  _{Receiver checks these bits}

 _{If OK, acknowledge}

 _{If error, discard packet}

 _{Retransmission}

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Addressing

 _{Addressing level}  _{Addressing scope}

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Addressing level

 _{Level in architecture at which entity is named}

 _{Unique address for each end system}

(computer) and router

 _{Network level address}

 _{IP or internet address (TCP/IP)}

 _{Network service access point or NSAP (OSI)}  _{Process within the system}

 _{Port number (TCP/IP)}

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Addressing Scope

 Global nonambiguity

 _{Global address identifies unique system}

 _{There is only one system with address X}

 _{Global applicability}

 _{It is possible at any system (any address)}

to identify any other system (address) by the global address of the other system

 _{Address X identifies that system from}

anywhere on the network

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Connection Identifiers

 _{Connection oriented data transfer (virtual}

circuits)

 _{Allocate a connection name during the}

transfer phase

 _{Reduced overhead as connection identifiers}

are shorter than global addresses

 _{Routing may be fixed and identified by}

connection name

 _{Entities may want multiple connections -}

multiplexing

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Addressing Mode

 _{Usually an address refers to a single system}  _{Unicast address}

 _{Sent to one machine or person}

 _{May address all entities within a domain}  _Broadcast

 _{Sent to all machines or users}

 _{May address a subset of the entities in a}

domain

 _Multicast

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Multiplexing

 _{Supporting multiple connections on one}

machine

 _{Mapping of multiple connections at one}

level to a single connection at another

 _{Carrying a number of connections on}

one fiber optic cable

 _{Aggregating or bonding ISDN lines to}

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Transmission Services

 _Priority

 _{e.g. control messages}  _{Quality of service}

 _{Minimum acceptable throughput}  _{Maximum acceptable delay}

 _Security

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OSI - The Model

 _{A layer model}

 _{Each layer performs a subset of the}

required communication functions

 _{Each layer relies on the next lower layer to}

perform more primitive functions

 _{Each layer provides services to the next}

higher layer

 _{Changes in one layer should not require}

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Elements of Standardization

 _{Protocol specification}

 _{Operates between the same layer on two systems}

 _{May involve different operating system}

 _{Protocol specification must be precise}

 _{Format of data units}

 _{Semantics of all fields}

 _{allowable sequence of PCUs}

 _{Service definition}

 _{Functional description of what is provided}

 _Addressing

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OSI Layers (1)

 _Physical

 _{Physical interface between devices}  _Mechanical

 _Electrical  _Functional  _Procedural  _{Data Link}

 _{Means of activating, maintaining and} deactivating a reliable link

 _{Error detection and control}

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OSI Layers (2)

 _Network

 _{Transport of information}

 _{Higher layers do not need to know about}

underlying technology

 _{Not needed on direct links}

 _Transport

 _{Exchange of data between end systems}

 _{Error free}

 _{In sequence}

 _{No losses}

 _{No duplicates}

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OSI Layers (3)

 _Session

 _{Control of dialogues between applications}

 _{Dialogue discipline}

 _Grouping

 _Recovery

 _Presentation

 _{Data formats and coding}

 _{Data compression}

 _Encryption

 _Application

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TCP/IP Protocol Suite

 _{Dominant commercial protocol}

architecture

 _{Specified and extensively used before OSI}  _{Developed by research funded US}

Department of Defense

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TCP/IP Protocol Architecture(1)

 _{Application Layer}

 _{Communication between processes or}

applications

 _{End to end or transport layer (TCP/UDP/…)}  _{End to end transfer of data}

 _{May include reliability mechanism (TCP)}  _{Hides detail of underlying network}

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TCP/IP Protocol Architecture(2)

 _{Network Layer}

 _{Logical interface between end system}

and network

 _{Physical Layer}

 _{Transmission medium}

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Required Reading

 _{Stallings chapter 2}

 _{Comer,D. Internetworking with TCP/IP}

volume I

 Comer,D. and Stevens,D. Internetworking

with TCP/IP volume II and volume III, Prentice Hall

 _{Halsall, F> Data Communications,}

Computer Networks and Open Systems, Addison Wesley