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<!DOCTYPE html>
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<meta charset="utf-8">
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<title>Computer Networks (2b)</title>
<meta name="cours-n" content="2">

<meta name="author" content="Rémi Emonet">
<meta name="venue" content="DWA M1 WI/MLDM">
<meta name="date" content="2017">
<meta name="affiliation" content="Université Jean Monnet − Laboratoire Hubert Curien">
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<section class="smart">

# @chunk: chunks/title.md

# @chunk: chunks/objectives.md


<!-- pre -->
## Part 2: Application Layer{overview}
@SVG: media/stack-index.svg 100px 400px {svg floatright app unapp}

- Application layer
- high level of abstraction
- « client » of the host to host network
- interacts with the transport layer
// <div class="anim-removeclass slide" data-what=".svg" data-class="unapp"></div>
- Part 2{slide}
- application layer and protocols
- interaction with the transport layer
- design of protocols
// by studying existing protocols
- programming connected application (socket)
// API


<!-- plan -->
## Part 2: Application Layer{#plan overview}
@SVG: media/stack-index.svg 100px 400px {svg floatright app margin-left-minus-100}

- Goal{shaded}
- protocols: general principles and existing protocols
- sockets: programming and services from the transport layer
- Overview
- Principles of distributed applications {it1}
// and interactions with the transport layer
- HTTP and the web {it2}
- FTP: file transfer {it3}
- Electronic mail {it4}
- DNS: name resolution and more {it5}
- P2P Applications (peer to peer) {it6}
- Network programming: using sockets {it7}
# @copy:#plan







<!-- distributed applications -->
# @copy:#plan: %+class:inred: .it1

# Principles of Distributed Applications {no-print}

## Examples of Distributed Applications
- e-mail{col1}
- social networks{col2}
- web{col1}
- P2P file sharing{col2}
- instant messaging {col1}
- remote connections {col2}
- multiplayer games {col1}
- telephone{col2}
- video and audio streaming {col1}
- real time video-conferencing{col2}
- …{col2}

## Design of Distributed Applications
@SVG: media/part2/internet-app-to-app.svg 400px 500px {svg floatright}

- Writing programs
- executed on different hosts
- communicating through the network
- @anim: #mask-core + #mask-border | #stacks | #arrows
- Network abstraction and separation{slide}
- the application ignores the numerous details
- the network core does not execute the application
- Canonical types of architectures{slide}
- client-server
- P2P (peer to peer)

## Architectures for Distributed Applications
@SVG: media/part2/internet-client-server-p2p.svg 400px 500px {svg floatright}

- Client-Server{cs}
- Server
- always on
- fixed (IP) address
- server farms
- Clients
- intermittent comm.
- changing address
- comm. only <br/> with the server
- @anim:.svg + #mask-core + #mask-border | #arrowsclientserver + .cs | -#mask-border + -#arrowsclientserver + #mask-border2 | .p2p + #arrowsp2p
- P2P (peer to peer) {p2p}
- host = both client and server
- no central server
- complicated, dynamic management
- better scalability

## Network Abstraction: interprocess comm.
- Process
- program running on a host
- exchanging messages over the network
- server process{slide}
- waiting to be contacted
- client process {slide}
- contacting a server
- P2P: client and server at the same time{slide}
- Inter-process communications(IPC) {slide}
- alternative to the network
- works only on a single host
// api dédiées, mémoire partagée

## Network Abstraction: socket
- socket
- used by a process (application)
- interface to the rest of the network stack
- interface to another (remote) process
- @anim: .svg | #stack1 + #stack2 | #cloudconnect | #process1 + #process2 | %viewbox:#zsocket | #threed1 + #socket1 | #threed2 + #socket2 + %viewbox:#zpage | #cable

@SVG: media/part2/socket.svg 800px 300px {svg}


## Network Abstraction: <br/>process identification
- Address of an host
// "network" layer
- IP address: 32 bits
- example: 78.109.84.114
- but: there could be multiple processes on a host
- Process identifier {slide}
- address of the host
- port number
- example: 80
- &rArr; 78.109.84.114:80

## Protocols from the Application Layer
// what do we have in an application protocol (as in all protocols)

- Types of messages{slide}
- initialization, request, response, …
- Syntax and format of messages{slide}
- structure of messages
- fields and their size
- encoding, separators, …
- Semantic of messages{slide}
- meaning of the different message types
- interpretation of the fields
- Processing rules {slide}
- how to answer the message?
- when to answer?
- Open protocols (HTTP, ...) vs proprietary protocols (Skype){slide}
// inseparability, standard etc

## What are the advantages of open protocols? <br/> … and of proprietary protocols?{question no-print}

## Services from the Transport Layer<br/> (from the application point of view) {libyli}
@SVG: media/stack-index.svg 100px 400px {svg floatright app margin-left-minus-100}

- Transport integrity
- guaranteed reception of all bits sent
- Latency (delay)
- reception of messages after a small time interval
- guarantee on a maximum delay
- Throughput (bandwidth)
- guarantee on the average data transfer rate
- guarantee on a (minimal) constant rate
- Security
- encryption, privacy protection
- integrity (non-corruption)

## How sensitive to these aspects are the following application?{question bottom}
- Aspects: integrity, latency, throughput, security
- Applications
- file transfer, e-mail, web browsing,
- real-time audio/video, audio/video streaming,
- multiplayer games, instant messaging

## Options for Transport with Internet <img src="media/part2/author-plug.svg" style="display: inline; height:1.5em; margin-left: .5em; margin-bottom: -.5em; margin-top: -10em;"> // not any network?
- Transport with TCP {col1 slide}
- connection oriented (stream)
- transfer integrity
- from socket to socket
- flow control{slide}
- prevent “spam”
- congestion control{slide}
- adaptation to network load
- missing services{slide}
- guaranteed latency
- guaranteed rate
- security
- UDP {col2 slide}
- packet oriented (datagram)
- transport not guaranteed
- missing services{slide}
- transfer integrity
- flow control
- congestion control
- guaranteed latency
- guaranteed rate
- security
- <img src="media/question-cube.jpg" class="floatright" width="100px"/> Question: so, why UDP?{col12 slide}


## Internet Apps and Transfer Protocols
<div>
<table class="clean1 centered">
<tr><th>Application</th><th>Application Protocols</th><th>Transfer Protocols</th></tr>
<tr><td>e-mail</td>
<td>SMTP (RFC2821)</td>
<td>TCP</td></tr>
<tr><td>Web Browsing</td>
<td>HTTP (RFC2616)</td>
<td>TCP</td></tr>
<tr><td>remote access (terminal)</td>
<td>Telnet (RFC854)</td>
<td>TCP</td></tr>
<tr><td>remote access (terminal)</td>
<td>SSH (RFC4251)<div class="comment">multiple RFC actually (the others too)</td>
<td>TCP</td></tr>
<tr><td>file transfer</td>
<td>FTP (RFC959)</td>
<td>TCP</td></tr>
<tr><td>Streaming</td>
<td>HTTP, RTP (RFC1889)</td>
<td>TCP, UDP</td></tr>
<tr><td>Voice over IP</td>
<td>SIP, RTP, prop.</td>
<td>TCP or UDP</td></tr>
</table>
</div>

## Absence of Security in TCP and UDP {libyli}
- TCP and UDP do not propose encryption
- data sent “as is”, including passwords etc
- possibility for any router to read these
- TLS (Transport Layer Security)
- evolution/renaming of SSL (Secure Sockets Layer)
- systematic encryption before sending through TCP
- authentication/identification of hosts (with “certificates”)
- Notes about TLS
- TLS is an application layer protocol
- TLS is just a software library
// with an API close to plain sockets
- the `ssh` command allows users to create secured tunnels



</section>


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