EEL 5718 Computer Communications Chapter 2 Layering Architecture

EEL 5718 Computer Communications Chapter 2: Layering Architecture

Outline • Examples of layering • OSI Reference Model • TCP/IP suite

Reiteration of Design Philosophy • Overall design: must be considered using system theory—top-down approach – – Goal or objective Functionalities or features or break-down Tools/budget Systems diagram/flow chart • Implementation: bottom-up – Design all tasks – Interactions among different tasks – Fine-tuning phases

Example 1 • How two philosophers communicates

Observations • Vertical interfaces – Interpreter and philosopher must understand each other – Philosopher submits task while interpreter provides services • Horizontal interfaces – Portability: two interpreters are identical in terms of their functionality – Peer-to-peer protocol: it seems that a peer talks to the same peer directly

Example 2 • Web services: client-server interaction Request HTTP client Response HTTP server

Layering Design • HTTP server HTTP client Port # Port 80 GET 80, # TCP #, 80 STATUS

Simplified File Transfer

Protocol Architecture

Addressing: Entity ID • Every device must have a unique ID: an address • Two levels of addressing required • Each computer needs a unique network address – allows network to deliver data to the right computer • Each application on a computer needs a unique address within the computer – known as service access point or SAP – allows each application to access the transport layer individually

Clarification: Network ID • Two ID system: logical ID and physical ID – The same network interface can be used by multiple devices – The same device can be used in other networks • Identify yourself: globally unique IP address – IP address identifies the network interface rather than the host/entity itself, the same “host” may have more than one IP, e. g. , router – IP address=(network ID, host ID) • Physical ID: MAC address – Each network card has a unique number, e. g. , 48 bits

Protocol Data Units (PDU) • Message unit: the format • Protocols are used to communicate at each layer • Control information is added to user data at each layer to tell the peer entity how to handle the data unit • Data from next higher layer + control information = PDU of the current layer

Transport PDU • Transport layer may fragment user data • Each fragment has a transport header added – Destination SAP (port number) – Sequence number – Error detection code • This gives a transport protocol data unit which is sent down to network layer

Network PDU • Adds network header to data received from transport layer – network addresses for source and destination computers – PDU contains information how to be handled • This gives a network protocol data unit

Protocol Data Units

Operation Overview

OSI Reference Model • Open Systems Interconnection (OSI) • Developed by the International Organization for Standardization (ISO) • A theoretical framework for developing protocol standards • Seven layers • Standardization too slow to be useful in practice! • Elegant protocol structure for better understanding!

OSI Model • • Application Presentation Session Transport Network Data Link Physical

Application Layer • provides means for applications to access OSI environment • provides management functions and useful mechanisms to support distributed applications • hosts general-purpose applications such as telnet, ftp, email, http, dns etc

Presentation Layer • defines data formats between applications • provides data transformation services, e. g. data compression and encryption

Session Layer • provides mechanism for controlling dialogues between applications in end systems • key services – dialogue discipline: duplexing – grouping: flow marking – failure recovering: checkpointing

Transport Layer • provides reliable, transparent data transfer between end systems • provides end-to-end error recovery and flow control • provides security or end-to-end Qo. S etc

Network Layer • provides a mechanism for transferring data across a subnet (routing algorithms and routing protocols) • relieves higher layers of the need to know anything about the underlying network technologies • provides congestion control and load balancing

Data link Layer • provides reliable data transfer from point to point • provides error recovery mechanism (FEC and ARQ)

Physical Layer • provides physical interface between devices • sets up rules by which bits are passed from one to another • deals about mechanical, electrical, functional, procedural characteristics of interface and devices – – Mechanical: specification of pluggable connector Electrical: representation of bits (e. g. , voltages) Functional: spec. of functions of all circuits Procedural: spec. of events for a bit transmission • REAL COMMUNICATION!

Modular Design Philosophy • Tasks of communication broken up into modules, each of which will have a protocol (peer-to-peer protocol) • 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 changes in other layers

Peer Communications

Internet • Any gateway/router must have at least the lowest three layers G net 1 G = gateway/router net 3 G G G net 2 net 5 G net 4 G

TCP/IP Suite • • • Developed by the US DARPA for ARPANET Used by the Internet, the de facto standard Not official model but a working one Defined by Vint Cerf and Robert Kahn (1974) First tested in UCLA and BBN First message sent over Internet: from UCLA to BBN by Leonard Kleinrock (first incomplete transmission from UCLA to SRI)

Five Layers Model of TCP/IP • Application layer • Host-to-host or transport layer (transmission control protocol (TCP)) • Internet layer (Internet Protocol (IP)) • Network access layer • Physical layer

Physical Layer • Physical interface between data transmission device (e. g. computer) and transmission medium or network • Specifies: – – characteristics of transmission medium nature of signals data rates etc.

Network Access Layer • Exchange of packets between end-system (computer) and network (via X. 25, Ethernet etc) • Deals with parts of OSI network layer and data link layer • Encapsulation of packets via network frame structure may be used • Software depends on the physical network

Internet Layer (IP) • • Systems may be attached to different networks Routing functions across multiple networks Implemented in end-systems and routers Router: – processor that connects two networks – primary function is to relay data from one network to the other

Transport Layer • Ensures – reliable delivery of data (to the satisfactory of applications) – right ordering of delivery • Independent of the nature of application • End-to-end protocol supporting interaction between two end-systems • Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)

Application Layer • Supports user applications, e. g. http, telnet, ftp, SNMP, . . . • Depends on applications

Two Machines Talk • Machine B Machine A Application Transport Router/Gateway Transport Internet Network Interface Network 1 Network Interface Network 2

Example • Physical layout (1, 1) (2, 1) router s Ethernet (1, 3) r w (1, 2) PPP

Example (cont) HTTP TCP Router IP IP IP Net Interface Ethernet TCP PPP PC

TCP/IP Suite

TCP/IP Upper Layers

OSI vs TCP/IP

New Trend: Paradigm Shift • A word of caution: strict layering design may NOT work well (Microsoft clumsy system) • Qo. S forces us to use cross layer information (Internet real-time service support, diffserv) • Wireless network design philosophy tends to break down the layer design tradition (active networks) (IEEE/ACM Mobi. Com’ 99) • Buzzword: cross-layer design, still hot particularly for wireless networks

Further Reading • Textbook: chapter 2 • Stallings, Data and Computer Communications (6 th ed. ), Prentice Hall, 2000, Chapter 1 & 2 • Web sites for IETF, IEEE, ITU-T, ISO • Internet Requests for Comment (RFCs)