cs2005 question 1 revision

Application Layer

sends the message/data - supporting network applications (FTP, SMTP, HTTP)

Transport Layer

receive the data from the application, divide it into smaller pieces called packets, add a destination address, and then pass the packets along to the next protocol layer (TCP, UDP)

Network Layer

routing of datagrams from source to destination - encloses the packet in an Internet Protocol (IP) datagram, puts in the datagram header and trailer, decides where to send the datagram (either directly to a destination or else to a gateway) (IP, routing protocols)

Link Layer

data transfer between neighbouring elements - accepts IP datagrams and transmits them as frames over a specific network hardware, such as Ethernet or Token-Ring networks (Ethernet, Wifi, PPP)

Physical Layer

the actual hardware and wire

Data

Packets/Segments (transport), Datagrams (network), Frame (link)

ISO/OSI reference model

Presentation and Session come after application

Layers of ISO/OSI reference model

Application, Presentation, Session, Transport, Network, Data Link, and Physical

Presentation Layer

Allow applications to interpret the meaning of data e.g. encryption

Session Layer

synchronisation, checkpointing, recovery of data exchange

Bandwidth

Transmission rate measured in bits per second

Packet Switches

forward packets (chunks of data). Routers and switches

Internet

network of networks - interconnected ISPs

Protocols

control the sending, and receiving of messages e.g. TCP, ICP, SMTP etc

Internet Standards

established through RFC - Request For Comments, Governed by IETF - Internet Engineering Task Force

Infrastructure

provides services to applications: Web, VoIP, email, etc

Programming Interface

hooks that allow sending and receiving app programs to connect to the internet

Service Options

analogous to postal service

Message Transmission

Protocols define format, order of messages sent and received among network entities and action taken on message transmission receipt

Client to Server

Server is always on-host, has a permanent IP and has data centres for scaling. Clients must communicate with the server, may be intermittently connected, may have dynamic IP addresses and do not communicate directly with each other

Peer to Peer

no always-on server, arbitrary end systems directly communicate, peers request service from other peers and provide service

Peers

are intermittently connected and change IP addresses

Process

a program running within a host

Client process

a process that initiates communication

Server process

a process that waits to be contacted

Socket

serve as endpoints for sending and receiving messages, analogous to a door for sending process to shove message out

Transport infrastructure

delivers the message to the socket at the receiving process

Identifier

to receive messages, the process must have an identifier

IP address

host device has a unique 32-bit IP address

Port numbers

associated with the process on host, e.g. HTTP server: 80, mail server: 25

App-layer protocol

defines types of messages exchanged, e.g. request, response

Message syntax

what fields in messages & how fields are defined

Message semantics

meaning of info in fields

Open protocols

defined in RFCs, allows for interoperability, e.g. HTTP, SMTP

Proprietary protocols

e.g., Skype

Data integrity

some apps require 100% reliable data transfer

Timing

some apps require low delay to be effective

Throughput

some apps require a minimum amount of throughput to be effective

Security

encryption, data integrity

TCP service

reliable transport between sending and receiving process

Flow control

sender won't overwhelm receiver

Congestion control

throttle sender when network overloaded

Connection-oriented

setup required between client and server processes

UDP service

unreliable data transfer between sending and receiving process

Securing TCP

TLS/SSL provides encrypted TCP connection, data integrity, end-point authentication

HTTP Overview

HTTP is a client/server protocol where the client requests, receives and displays Web objects

Non-persistent HTTP

At most one object is sent over a TCP connection and then the connection is closed. Requires multiple connections to download multiple objects.

Persistent HTTP

Multiple objects can be sent over a single TCP connection.

Example of Non-persistent HTTP

Transferring a Web page from server to client. The page consists of a base HTML file and 10 JPEG images and all 11 objects reside on the same server.

Round-trip time (RTT)

The time it takes for a small packet to travel from client to server and then back to the client including delays: packet-propagation delays, packet queuing delays in intermediate routers and switches, and packet-processing delays.

HTTP response time for TCP connection

1 RTT.

HTTP response time for file request + response

1 RTT.

Non-persistent HTTP response time

2 RTT plus file transmission time (roughly).

Persistent HTTP connection behavior

The server leaves the connection open after sending the response message. Subsequent HTTP messages between the same client/server are sent over the open connection.

User-server state: cookies

An HTTP server is stateless, but it is useful for the server to keep session state information to identify users and restrict access.

Components of an HTTP cookie

A cookie header line in the HTTP response message, a cookie header line in the HTTP request message, a cookie file kept on the user's end system and managed by the user's browser, and a back-end database on the Web site.

Web cache (proxy server)

A web cache satisfies HTTP requests on behalf of an origin Web server. It is both a client (to the original server) and a server (to the client).

Benefits of web caching

Reduces response time and network traffic (access link to the Internet).

Conditional GET in HTTP

Manifested by the If-Modified-Since: and Last-Modified: header lines to avoid stale data.

Components of Electronic Mail

User agents, mail servers, and Simple Mail Transfer Protocol (SMTP).

User agents

Compose, edit, read and save outgoing and incoming mail messages.

Mail servers

Have a mailbox for incoming messages, a message queue for outgoing messages, and an SMTP implementation.

Simple Mail Transfer Protocol (SMTP)

Protocol between mail servers to send email messages using TCP for reliable email transfer and port 25.

SMTP phases of transfer

Handshaking (greeting), transfer of messages, and closure.