Internetworking??Internetworking??

What is Internetwork?What is Internetwork?A collection of individual networks, connected by intermediate networking devices, that functions as a single large network

Internetworking challenges:- Support of communication between disparate technologies (different types of media, speed, etc.)-Reliable and consistent access to network resources-Centralized network mana-gement and troubleshooting-Flexibility for network expansion and new applications and services

Need of a general reference model describing how information is moved from software in one to another computer

Open Systems Interconnection Open Systems Interconnection (OSI) Reference Model(OSI) Reference Model

Developed by the International Organization for Standardization (1984)

Tasks involved with moving information are divided into 7 smaller, more manageable task groups (OSI layers) with specific particular network functions

Each layer is reasonably self-contained:◦ Layer tasks can be implemented

independently◦ Any layer can be updated without

adversely affecting the other layers

OSI LayersOSI LayersUpper layers:

◦ Deal with applications containing a communication component

◦ Implemented by software only◦ Closest to the end user

• Lower layers:– Handle data transport– Physical and Data Link

are implemented in HW and SW, the other by SW

– Closest to the transfer medium

Protocols and OSI ModelProtocols and OSI ModelOSI model = conceptual framework for

communication but not a method of communication

Protocol = a formal set of rules and conventions that governs how computers exchange information over a network medium

Actual communications is based on communication protocols:◦ LAN protocols operate on Data Link and Network

layers (communication over LAN)◦ WAN protocols – on 3 bottom layers (communication

over WAN)◦ Routing protocols – on Network layer (path

determination and traffic switching)◦ Network protocols – various upper-layer protocols

(exist in a given protocol suite)

OSI Model and Communication OSI Model and Communication between Systems and Layersbetween Systems and Layers

SW passes its information to be sent to the Application layer. Then it must pass trough all other layers as far as the Physical layer Ţ medium Ţ the Physical layer in computer B, …

A layer communicates with:◦ Directly adjacent layers in

the same system◦ Peer layer in other system

OSI-layer services:◦ User (requests services)◦ Service provider (adjacent

layer)◦ Service access point

OSI Layers and Information OSI Layers and Information ExchangeExchange

Control information (specific request and instruction) must be exchanged between peer layers◦ Forms: header and trailers◦ Encapsulation: data coming from upper layer contains

upper layer control information Ţ a new additional control info of the layer will be added

Physical LayerPhysical Layer defines the electrical, mechanical, procedural, and

functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems (such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors)

Data Link LayerData Link LayerProvides reliable transit of data across a physical

layer.Different data link layer specifications define different network and protocol specification, including:◦ physical addressing at the data link layer (opposed to

network addressing), ◦ network topology consisting of the data link layer

specifications (e.g. topology – a bus or a ring, etc.)◦ error notification – alerts upper-layers protocols at a

transmission error occurrence,◦ sequencing of frames – reorder frames transmitted out

of sequence◦ flow control – moderates transmission of data (receiving

device must not be overwhelmed with more traffic than it can handle at one time)

IEEE subdivides the data link layer into Logical Link Control (LLC) and Media Access Control (MAC)

Network LayerNetwork LayerProvides routing and related functions that enable

multiple data links to be combined into an internetwork and accomplished by the logical addressing

Supports both higher level protocols: connection-oriented and connectionless services

Transport Layer• Implements reliable internetwork data transport services that

are transparent to upper layers

• Functions:– Flow control – manage data transmission between devices (sending no

more data than can be processed in receiver) – Multiplexing data from several applications to be transmitted onto a

single physical link– Error checking – creating various mechanism for detecting errors and

taking an action to error recovery to resolve occurred error

Session LayerSession Layerestablishes, manages, and terminates communication

sessions between presentation layer entitiesSession consists of: services requests and service

responses between applications in different network devices

This communication is coordinated by protocols implemented in session layer

Presentation Layer• Provides a variety of coding and conversion functions that are

applied to application data layer to ensure readability information sent between applications in different systems

• Common activities:– Conversion of common data representation formats (e.g. ASCII and

EBCDIC)– Data compression and decompression– Standard data encryption and deciphering (MPEG, GIF, JPEG, …)

Application LayerApplication Layer

Closest to the user – interacts directly with communicating components of application software (out of OSI model scope)

Typical activities:◦ Identifying communication partners – identity and

availability◦ Determining sufficient resource availability for

requested communication◦ Synchronizing communication between application

Information FormatsInformation FormatsFrame is an information unit whose source

and destination are data link layer entities.

• Packet is an information unit whose source and destination are network-layer entities.

Information FormatsInformation FormatsDatagram usually refers to an information unit

whose source and destination are network-layer entities that use connectionless network service.

Segment usually refers to an information unit whose source and destination are transport-layer entities.

Message is an information unit whose source and destination entities exist above the network layer (often the application layer).

Cell is an information unit of a fixed size whose source and destination are data-link layer entities and it is usually used in switched environments, such as Asynchronous Transfer Mode (ATM) networks.A cell is composed of the header and payload. The header contains control information intended for the destination data-link layer entity and is typically 5 bytes long. The payload contains upper-layer data that is encapsulated in the cell header and is typically 48 bytes long.

Connection in Network ServicesConnection in Network Services

Connection-oriented service = uses a specific path established for the duration of a connection. Phases:◦ Establishment of connection = static reservation of a path

to ensure consistent grade of services (e.g. guaranteed throughput rate)

◦ Data transfer = sequential transfer – data always arrives in the order in which it was sent (disadvant.: broken line = connection interruption)

◦ Termination of connection = new communication needs establishment a new connection

Connectionless service = no specific transfer path is determined = packets go through different path Ţ each packet must be completely addressed and is handled independently Ţ ◦ No guarantee of packet sequencing at receiving and

throughput rate◦ Dynamic = more efficient using of network resources◦ Broken line recovering

Internetwork AddressingInternetwork AddressingData Link Layer Addressing = fixed physical

(hardware) addresses uniquely identifying each interface on a device

MAC addresses = subset of data link addresses used especially in LAN (e.g. Ethernet – 24 + 24bits: vendor identification + serial number)

Network Layer Addressing = unfixed virtual (logical) addresses one for each network protocol in a device interface (but only one physical address). Assignment:◦ Static = network administrator◦ Dynamic = getting a new one at each connecting to

network (usually by server)

Introduction to the Introduction to the InternetInternet

BackgroundBackground

Developed in mid-1970s by Stanford University under the Defense Advanced Research Project Agency grant

Suite of protocols known also as TCP/IP protocols

Internet Protocol (IP)Internet Protocol (IP)

Basic network-layer protocol containing addressing and some control information that enable packet to be routed

IP has two primaryresponsibilities:◦ providing connectionless,

best-effort delivery of datagramsthrough an internetwork;

◦ providing fragmentation andreassembly of datagrams tosupport data links with differentunit sizes.

IP AddressingIP Addressing Each host on TCP/IP network has assigned a unique 32-

bit logical address divide into the network number and the host

Network number must be assigned by the Network Information Center

Host number – by the network administrator

IP Address ClassesIP Address Classes

Classes D – multicast groups and E – experimental are not available for commercial use.

Masks – some part of host address may be designated for creating subnets

ARP and RARPARP and RARPARP = discovering the MAC address corresponding to

IP addressRARP = reverse ARP = mapping MAC address to IP

address (in networks with dynamic assignation of IP addresses)

• Network-layer protocol that provides message packets to report error and other information regarding IP packet processing

• ICMP messages: destination unreachable, host unreachable, protocol unreachable, port unreachable, echo request, redirect messages, time-exceeded messages

Internet Control Message Protocol (ICMP)

Transmission Control Protocol Transmission Control Protocol (TCP)(TCP)

Connection-less transport-layer protocolProvides reliable transmission with:

◦ Stream data transfer – unstructured data stream is divided into sequence identified by number and passed to IP

◦ Reliability – three-way handshaking based on forward acknowledgement technique

◦ Efficient flow control – sending back acknowledgement with highest sequence number which can be received

◦ Full-duplex operation◦ Multiplexing – many simultaneous upper-layer can be

multiplexed over a single connection

TCP Packet FormatTCP Packet Format

Ports = points for processing TCP services by various upper-layerprotocols

Sequence number = number of first data byte in sequence

Acknowledgement num. = number of expected data byte in sequence

Flags = various flag bits includingSYN and ACK bits

Window = receiver buffer capacity

User Datagram Protocol (UDP)User Datagram Protocol (UDP)

Connection-less transport-layer protocol – interface between IP and upper-layer protocols

Unlike TCP, UDP adds no reliability, flow-control, and error recovery functions to IP

Consumes less network overhead than TCPUsed for NFS, SNMP, DNS, TFTP, …

Upper-layer Internet ProtocolsUpper-layer Internet ProtocolsNumerous suite of various protocols, e.g.:

◦ File Transfer Protocol – FTP◦ Telnet – terminal emulation◦ Network File System (NFS), External Data

Representation (XDR), and Remote Procedure Call (RPC) – work together to enable transparent access to remote network resources

◦ Simple Mail Transfer Protocol (SMTP) – electronic mail services

◦ Domain Name System (DNS) – translation of network node names into network addresses

◦ Hypertext Transfer Protocol (HTTP) – transfers in WWW

◦ …