Requirements
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Requirements. Connectivity Resource Sharing Support for Common Services Performance. Performance Metrics. Bandwidth (throughput) data transmitted per time unit link versus end-to-end notation KB = 2 10 bytes Mbps = 10 6 bits per second Latency (delay)

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Requirements

Requirements

  • Connectivity

  • Resource Sharing

  • Support for Common Services

  • Performance


Performance metrics

Performance Metrics

  • Bandwidth (throughput)

    • data transmitted per time unit

    • link versus end-to-end

    • notation

      • KB = 210 bytes

      • Mbps = 106 bits per second

  • Latency (delay)

    • time to send message from point A to point B

    • one-way versus round-trip time (RTT)

    • components

      Latency = Propagation + Transmit + Queue

      Propagation = Distance / c

      Transmit = Size / Bandwidth

    • Examples of RTT: LAN, Cross-country link, Satellite


Delay x bandwidth product

Delay x Bandwidth Product

  • Amount of data “in flight” or “in the pipe”

  • Example: 100ms x 45Mbps = 560KB

  • Why is it important to know Delay x Bandwidth product?


Bandwidth versus latency

Bandwidth versus Latency

  • Relative importance

    • 1-byte: 1ms vs 100ms dominates 1Mbps vs 100Mbps

    • 25MB: 1Mbps vs 100Mbps dominates 1ms vs 100ms

  • Infinite bandwidth

    • RTT dominates

      • Throughput = TransferSize / TransferTime

      • TransferTime = RTT + TransferSize /Bandwidth

  • 1-MB file to 1-Gbps link as 1-KB packet to 1-Mbps link


Network architecture

Network Architecture

  • Layering and Protocols

  • ISO Architecture

  • Internet architecture


Layering

Layering

  • Use abstractions to hide complexity

  • Abstraction naturally lead to layering

  • Alternative abstractions at each layer

  • Advantages:

  • Solve small problems vs. monolithic software

  • Modularity: easily add new services

  • Drawback:

  • May hide important information

Application programs

Request/reply

Message stream

channel

channel

Host-to-host connectivity

Hardware


Protocols

Protocols

  • Building blocks of a network architecture

  • Each protocol object has two different interfaces

    • service interface: operations on this protocol (SAP)

    • peer-to-peer interface: messages exchanged with peer

  • Term “protocol” is overloaded

    • specification of peer-to-peer interface (rules)

    • module that implements this interface

  • Protocol stack: set of consecutive layers

  • Interoperability problems


Interfaces

Interfaces

Host1

Host2

Service

High-level

High-level

interface

object

object

Protocol

Protocol

Peer-to-peer

interface


Protocol machinery

Protocol Machinery

  • Protocol Graph

    • most peer-to-peer communication is indirect

    • peer-to-peer is direct only at hardware level

Host 2

Host 1

Digital

Digital

Video

Video

File

File

library

library

application

application

application

application

application

application

RRP

MSP

RRP

MSP

HHP

HHP


Machinery cont

Machinery (cont)

  • Multiplexing and Demultiplexing (demux key)

  • Encapsulation (header/body)

Host 1

Host 2

Application

Application

program

program

Data

Data

RRP

RRP

RRP

Data

RRP

Data

HHP

HHP

HHP

RRP

Data


Osi architecture reference model

OSI Architecture:Reference Model

End host

End host

Application

Application

Presentation

Presentation

Session

Session

Transport

Transport

Network

Network

Network

Network

Data link

Data link

Data link

Data link

Physical

Physical

Physical

Physical

One or more nodes

within the network


Physical layer

Physical Layer

  • Function: provides a “virtual bit pipe”

  • How: maps bits into electrical/electromagnetic signals appropriate for the channel

  • The physical layer module is called a modem (modulator/demodulator)

  • Important issues:

    • Timing: synchronous, intermittent synchronous, asynchronous (characters)

    • Interfacing the physical layer and DLC (e.g., RS-232, X.21)


Data link control layer dlc

Data Link Control Layer (DLC)

  • Receives packets from the network layer and transforms then into bits transmitted by the physical layer. Generally guarantees order and correctness.

  • Mechanisms of the DLC:

    • Framing: header, trailer to separate packets, detect errors…

    • Multiple access schemes: when the link is shared by several nodes there is a need for addressing and controlling the access (this entity is called MAC sublayer)

    • Error detection and retransmission (LLC sublayer)


Network layer

Network Layer

  • Provides naming/addressing, routing, flow control, and scheduling/queuing in a multi-hop network

  • Makes decisions based on packet header (e.g., destination address) and module stored information (e.g., routing tables)

  • General comment: each layer looks only at its corresponding header (here packet header)

  • Routing is different on virtual circuit networks than on datagram networks


Transport layer

Transport Layer

  • Provides a reliable mechanism to transmit messages between two end-nodes through:

    • Message fragmentation into packets

    • Packets reassembly in original order

    • Retransmission of lost packets

    • End-to-end flow control

    • Congestion control


Session layer

Session Layer

  • Was intended to handle the interaction between two end points in setting up a session:

    • multiple connections

    • Service location (e.g., would achieve load sharing)

    • Control of access rights

  • For example, managing an audio and video stream in a teleconferencing application

  • In many networks these functionalities are inexistent or spread over other layers


Presentation layer

Presentation Layer

  • Concerned with the format of the data exchanged

  • Provides data encryption, data compression, and code conversion.


Application layer

Application Layer

  • What’s left over…

  • Examples:WWW, Email, Telnet, …


Internet architecture

FTP

HTTP

NV

TFTP

UDP

TCP

IP

NET

NET

NET

2

1

n

Internet Architecture

  • Defined by Internet Engineering Task Force (IETF)

  • IETF requires working implementations for standard adoption

  • Application vs. Application Protocol (FTP, HTTP)


Internet architecture1

Internet Architecture

  • Not quite layered

Application

TCP

UDP

IP

Network


Implementing network software

Implementing Network Software

  • Success of the Internet is partially due to:

    • Minimal functionality within the network

    • Most of the functionality running is software over general-purpose computers

  • Simple Application Programming Interface

  • Efficient protocol implementation


Application programming interface

Application Programming Interface

  • Each OS can have a special interface exported by the network to the applications developer

  • Most widely used network API is: socket interface

    • Initially developed by the Berkeley Distribution of Unix and today ported to almost all OS


Creating a socket

Creating a Socket

  • int socket(int domain, /* PF_INET, PF_UNIX */

    int type, /* SOCK_STREAM, SOCK_DGRAM */

    int protocol)


Active open

Active Open

  • Clients perform an active open operation using the connect routine

  • int connect (int socket, struct sockaddr *address, int addr_len)


Passive open

Passive Open

  • Servers perform a passive open

    • bind, listen, accept

  • int bind (int socket, struct sockaddr *address, int addr_len)

  • int listen (int socket, int backlog)

  • int accept (int socket, struct sockaddr *address, int *addr_len)


Sending and receiving data

Sending and Receiving Data

  • Once the connection is established, use send/recv functions to exchange data

  • int send (int socket, char *message, int msg_len, int flags)

  • int recv (int socket, char *buffer, int buf_len, int flags)


Closing a socket

Closing a Socket

  • void close (int socket)


Client server sockets

Client/Server Sockets

  • Client:

    • socket, connect, (send, recv)*, close

  • Server:

    • socket, bind, listen, (accept, (recv, send)*, close)*


Exercise

Exercise

  • Write the simplex-talk client and server programs given in Section 1.3.2 of text

    • Given in C

  • Run a client and server on same machine and test

  • Run client and server on different machines

  • Do Exercises 28, 29, and 30 of text


Protocol implementation issues

Protocol Implementation Issues

  • The socket API is an interface between the application and the network subsystem (TCP, UDP, IP, etc.)

  • Could potentially be used as a protocol-to-protocol interface

  • Turns out that the socket interface, while clean, introduces a number of inefficiencies

  • Different protocol-to-protocol interfaces designed


Process models

Process Models

Process-per-protocol

Process-per-message


Context switches

Context Switches

  • Inefficiency of process-per-protocol model:

    • Too many context switches, as the message is handed from one protocol to the next

  • Context switches become procedure calls in the process-per-message model

    • Sending a message: high-level protocol calls send on the low-level protocol

    • Receiving a message: high-level protocol calls receive on the low-level protocol


Protocol to protocol interface

Protocol-to-Protocol Interface

  • The receive operation replaced by deliver


Message buffer handling

Message Buffer Handling

  • Copying messages between buffers expensive

  • Shared message abstraction

    • Adding and stripping headers

    • Fragmenting and reassembling messages


Other common library routines

Other Common Library Routines

  • Event library: commonly need to schedule an event in the future

    • Examples: timeout, garbage collection

    • A common event & event manager abstraction

  • Map library: For maintaining bindings between identifiers

    • Example: Mapping incoming messages to the connection that will process the message


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