Computer Networking Lecture Notes

11 September 2012 • The Application Layer

Outline

Principles, protocols, and architecture.
Communication
Addressing
Application Services.
- Reliability, bandwidth, and delay.
FTP
Media Services.

Principles

The business end of networks.
Software oriented.
What is communicating?
Simple and familiar introduction to network concepts.

Protocols

Applications vs application protocols.
Consider the Web: HTTP vs the rest.
Syntax and semantics.
- Syntax: message structure.
- Semantics: message meaning, message exchange sequences.

Architecture

Network applications are usually asymmetric: clients and servers.
For example
- Web browsers vs web servers.
- E-mail clients (MTA)
But most hosts support both both application clients and servers.

Communication

Application ends communicate by sending and receiving messages.
The application layer provides the mechanisms by which applications communicate.
- The application programmer’s interface (API).

Addressing

An application endpoint is located via
1. a host on the network and
2. a process in the host.
A host has a network unique address.
A process on a host has a OS-specific identifier.
- Abstracted by the network as a port.

Application Services

Application layers provide a number of services to applications.
- And perhaps several versions of each service.
The API lets the application make (and possibly change) the choices.
- That is, what services to use, and which service version if used.

Reliability

Reliable communication reproduces the sender’s output at the receiver.
- Unreliable communication doesn’t.
Reliable communication is more expensive than unreliable communication.
Some applications are loss-tolerant, and may not choose to pay for reliability.
- Media unreliability can be tolerated.
- File transfer requires reliability.

Bandwidth

Bandwidth is the data rate (bytes/sec, for example) available.
Some applications are bandwidth insensitive (e-mail, for example).
Some are bandwidth sensitive (telephony, for example).
Some are bandwidth adaptive (jpeg progressive compression).
Cost is directly proportional to bandwidth.

Delay

Delay is inversely proportional to transmission time.
Delay-sensitive applications require low, unvarying delay.
- Telephony, for example.
Delay-insensitive applications can tolerate high delay.
- E-mail, or file transfer, for example.
Cost is inversely proportional to delay.

FTP

The file transfer protocol (FTP).
- Request for comment (RFC) 959, 1971.
Moves files from system to system.

Example

$ ls README.TXT
ls: cannot access README.TXT: No such file or directory

$ ftp ftp1.FreeBSD.org
Connected to moss.cse.buffalo.edu.
220 FTP server (V. wu-2.6.2(1) Mon Aug 2 14:04:46 EDT 2010) ready.
Name (ftp1.FreeBSD.org:rclayton): anonymous
331 Guest login ok, send your complete e-mail address as password.
Password:
230 Guest login ok, access restrictions apply.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls
200 PORT command successful.
150 Opening ASCII mode data connection for /bin/ls.
total 8
drwxrwxr-x  2 0      5    512 Jun  5  2009 .snap
drwxr-xr-x  2 0      0    512 Aug  1  2008 bin
drwxrwxr-x  4 89987  546  512 Jun 13  2009 mirror
drwxrwxr-x  2 89987  546  512 Jun 14  2009 pub
226 Transfer complete.
ftp> cd pub/FreeBSD
250-If you're looking for one of the FreeBSD releases, please look in the
250-releases/${ARCH}/${RELNAME} directory, where ARCH = "alpha", "amd64",
250-"i386", "ia64", "pc98", or "sparc64" and RELNAME = the release
250-you're interested in, e.g. "7.1-RELEASE" or "8.0-RELEASE".
250-
250-Please read the file README.TXT
250-  it was last modified on Fri Jun 18 21:54:21 2004 - 3005 days ago
250 CWD command successful.
ftp> ls
200 PORT command successful.
150 Opening ASCII mode data connection for /bin/ls.
total 90
-rw-rw-r--   1 89987  546    262 Jan 28  2010 .message
-r--rw-r--   1 89987  546      0 Nov  7  1996 .notar
drwxrwxr-x   6 89987  546    512 Oct 23  2006 CERT
lrwxrwxrwx   1 89987  546     15 Apr 19  2007 CTM -> development/CTM
lrwxrwxrwx   1 89987  546     17 Apr 19  2007 CVSup -> development/CVSup
drwxrwxr-x   4 89987  546    512 Oct 23  2006 ERRATA
lrwxrwxrwx   1 89987  546     17 Apr 19  2007 FreeBSD-current -> branches/-current
lrwxrwxrwx   1 89987  546     19 Apr 19  2007 FreeBSD-stable -> branches/4.0-stable
lrwxrwxrwx   1 89987  546     25 Apr 19  2007 ISO-IMAGES-alpha -> releases/alpha/ISO-IMAGES
lrwxrwxrwx   1 89987  546     25 Apr 19  2007 ISO-IMAGES-amd64 -> releases/amd64/ISO-IMAGES
lrwxrwxrwx   1 89987  546     24 Apr 19  2007 ISO-IMAGES-i386 -> releases/i386/ISO-IMAGES
lrwxrwxrwx   1 89987  546     24 Apr 19  2007 ISO-IMAGES-ia64 -> releases/ia64/ISO-IMAGES
lrwxrwxrwx   1 89987  546     24 Apr 19  2007 ISO-IMAGES-pc98 -> releases/pc98/ISO-IMAGES
lrwxrwxrwx   1 89987  546     27 Apr 20  2007 ISO-IMAGES-powerpc -> releases/powerpc/ISO-IMAGES
lrwxrwxrwx   1 89987  546     27 Apr 19  2007 ISO-IMAGES-sparc64 -> releases/sparc64/ISO-IMAGES
-rw-rw-r--   1 89987  546   6430 Jun 19  2004 README.TXT
drwxrwxr-x   3 89987  546    512 Oct 23  2006 SOC2005
-rw-rw-r--   1 89987  546     11 Sep 11 04:55 TIMESTAMP
drwxrwxr-x   2 89987  546    512 Oct 23  2006 TrustedBSD
drwxrwxr-x   6 89987  546    512 Oct 23  2006 branches
drwxrwxr-x  10 89987  546    512 Jun  3  2008 development
-rw-r--r--   1 89987  546  48291 Sep 11 05:22 dir.sizes
lrwxrwxrwx   1 89987  546     15 Apr 19  2007 distfiles -> ports/distfiles
drwxrwxr-x  24 89987  546   1024 Jun  1 09:47 doc
drwxrwxr-x   3 89987  546    512 Oct 23  2006 misc
drwxrwxr-x  12 89987  546    512 Jun 14 15:43 ports
drwxrwxr-x  10 89987  546    512 Dec  8  2011 releases
drwxrwxr-x   2 89987  546    512 Aug 30 15:51 snapshots
drwxrwxr-x   5 89987  546   1024 Oct 24  2006 tools
drwxrwxr-x   2 89987  546    512 Oct 24  2006 tools.NEW
drwxrwxr-x   3 89987  546    512 Oct 24  2006 torrents
drwxrwxr-x   2 89987  546    512 Oct 24  2006 updates
226 Transfer complete.
ftp> get README.TXT 
local: README.TXT remote: README.TXT
200 PORT command successful.
150 Opening BINARY mode data connection for README.TXT (6430 bytes).
226 Transfer complete.
6430 bytes received in 0.02 secs (256.4 kB/s)
ftp> quit
221-You have transferred 6430 bytes in 1 files.
221-Total traffic for this session was 10019 bytes in 3 transfers.
221-Thank you for using the FTP service on moss.cse.buffalo.edu.
221 Goodbye.

$ ls README.TXT 
README.TXT

$

Architecture

Dual connections: one for data (files), the other for commands.
- Out of band signaling.
- Alternative: in-band signaling.
Client-server (request-response) process architecture.
FTP ports:
- Data port 20
- Control port 21

Operation

Client connects to control port 21 on server.
Commands and replies are exchanged.
File transfers open connections on data port 20.
- Data connections are closed after each transfer.
Data connections are non-persistent; control connections are persistent.

Commands

$ ftp
ftp> ?
Commands may be abbreviated.  Commands are:

!               dir             mdelete         qc              site
$               disconnect      mdir            sendport        size
account         exit            mget            put             status
append          form            mkdir           pwd             struct
ascii           get             mls             quit            system
bell            glob            mode            quote           sunique
binary          hash            modtime         recv            tenex
bye             help            mput            reget           tick
case            idle            newer           rstatus         trace
cd              image           nmap            rhelp           type
cdup            ipany           nlist           rename          user
chmod           ipv4            ntrans          reset           umask
close           ipv6            open            restart         verbose
cr              lcd             prompt          rmdir           ?
delete          ls              passive         runique
debug           macdef          proxy           send
ftp> quit

$

Replies

Replies are in the IETF (Internet Engineering Task Force) format:

ddd explanation 

200 PORT command successful.
150 Opening ASCII data connection.

Some of the message codes.

1dd positive preliminary reply.

2dd positive completion reply.

3dd positive intermediate reply.

Media Services

Audio only and video + audio.
Media sources stored and streamed.
- Examples: YouTube, Internet radio.

Simple Implementation

This service is already implemented: FTP.
Client locates and downloads media files.
Client plays the downloaded file.
Problems:
- Complicated interactions (an interface problem).
- High delay, but low latency.

Another Implementation

Download the file, but start playing immediately.
- Not necessarily download to save.
FTP is possibly to much protocol for this.
- And mismatches in other characteristics.

Streaming Implementation

Tailor download protocol for immediate playback.
Streaming media.
Streaming from a stored source.

Live Media Streaming

The source is no longer a file, but a continuous producer.
- Examples: zoo cams, Internet radio.
Use the same techniques for the most part.
- Stream control operators may be limited.
- Example: fast forwarding through the live stream.

Distribution

Popular content viewed by millions world wide.
- Point-to-point connections suggests scaling problems.
This leads to multicast services.
- A form of selected broadcast.
Broadcast and multicast services require new network services and protocols.

Summary

References

Application Layer, Chapter 2, in Computer Networking: A Top-Down Approach by James Kurose and Keith Ross, Addison Wesley, 2009.
Streaming audio and video, section 7.4, in Computer Networks by Andrew Tanenbaum and David Wetherall, Prentice Hall, 2010.
FTP: File Transfer Protocol, Chapter 27, in TCP/IP Illustrated, vol. 2: The Implementation by W. Richard Stevens and Garry Wright, Addison-Wesley, 1995.

This page last modified on 2012 September 11.

1dd	positive preliminary reply.
2dd	positive completion reply.
3dd	positive intermediate reply.