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Computer Technology

In: Computers and Technology

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Computer Networks Applica3on Layer


Examples of network applica3ons •

e-­‐mail web instant messaging remote login P2P file sharing mul3-­‐user network games streaming stored video clips

• social networks • voice over IP • real-­‐3me video conferencing • grid compu3ng



Crea3ng a network app write programs that

application transport network data link physical – run on (different) end systems – communicate over network – e.g., web server soJware communicates with browser soJware

No need to write soJware for network-­‐core devices – Network-­‐core devices do not run user applica3ons

– applica3ons on end systems

allows for rapid app development, propaga3on

application transport network data link physical application transport network data link physical 3

Applica3on architectures • Client-­‐server – Including data centers / cloud compu3ng

• Peer-­‐to-­‐peer (P2P) • Hybrid of client-­‐server and P2P



Client-­‐server architecture server:

– always-­‐on host – permanent IP address – server farms for scaling

clients: client/server

– communicate with server – may be intermiPently connected – may have dynamic IP addresses – do not communicate directly with each other


Google Data Centers • Es3mated cost of data center: $600M • Google spent $2.4B in 2007 on new data centers • Each data center uses 50-­‐100 megawaPs of power


Pure P2P architecture • no always-­‐on server • arbitrary end systems directly communicate peer-peer
• peers are intermiPently connected and change IP addresses

Highly scalable but difficult to manage


Hybrid of client-­‐server and P2P Skype

– voice-­‐over-­‐IP P2P applica3on – centralized server: finding address of remote party:

– client-­‐client connec3on: direct (not through server)

Instant messaging

– cha]ng between two users is P2P – centralized service: client presence detec3on/ loca3on • user registers its IP address with central server when it comes online • user contacts central server to find IP addresses of buddies 8


Processes communica3ng Process: program running within a host. • within same host, two processes communicate using inter-­‐process communica3on (defined by OS). • processes in different hosts communicate by exchanging messages

Client process: process that ini3ates communica3on Server process: process that waits to be contacted

• Note: applica3ons with P2P

architectures have client processes & server processes


Sockets • process sends/receives messages to/from its socket • socket analogous to door – sending process shoves message out door – sending process relies on transport infrastructure on other side of door which brings message to socket at receiving process

host or server host or server process

controlled by app developer

process socket socket
TCP with buffers, variables


TCP with buffers, variables

controlled by OS

• API: (1) choice of transport protocol;

(2) ability to fix a few parameters



Addressing processes Q: does IP address of host on which process runs suffice for iden3fying the process? – A: No, many processes can be running on same Iden/fier includes both IP address and port numbers associated with process on host. • Example port numbers:

• to receive messages, process

must have iden/fier • host device has unique 32-­‐bit IP address • Exercise: use ipconfig from command prompt to get • your IP address (Windows)

– HTTP server: 80 – Mail server: 25 11

Applica3on-­‐layer protocol defines • Types of messages exchanged,

– e.g., request, response

• Message syntax: – what fields in messages & how fields are delineated

• Message seman3cs

– meaning of informa3on in fields

Public-­‐domain protocols: • defined in RFCs • allows for interoperability • e.g., HTTP, SMTP, BitTorrent Proprietary protocols: • e.g., Skype, ppstream

• Rules for when and how processes send & respond to messages 12


What transport service does an app need? Data loss • some apps (e.g., audio) can tolerate some loss • other apps (e.g., file transfer, telnet) require 100% reliable data transfer

Timing • some apps (e.g., Internet telephony, interac3ve games) require low delay to be “effec3ve”

Throughput • some apps (e.g., mul3media) require a certain minimum amount of throughput to be “effec3ve” • other apps (“elas3c apps”) make use of whatever throughput they get

Security • Encryp3on, data integrity, …


Transport service requirements of common apps Data loss


Time Sensitive

file transfer e-mail Web documents real-time audio/video

no loss no loss no loss loss-tolerant no no no yes, 100’s msec

stored audio/video interactive games instant messaging

loss-tolerant loss-tolerant no loss

elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic Application

yes, few secs yes, 100’s msec yes and no



Internet transport protocols services TCP service:

UDP service:

• unreliable data transfer • connec/on-­‐oriented: setup between sending and required between client and receiving process server processes • does not provide: connec3on • reliable transport between setup, reliability, flow control, sending and receiving process conges3on control, 3ming, • flow control: sender won’t throughput guarantee, or overwhelm receiver


• conges/on control: throPle sender when network overloaded Q: why bother? Why is there a • does not provide: 3ming, UDP? minimum throughput guarantees, security 15

Internet apps: applica3on, transport protocols Application e-mail remote terminal access
file transfer streaming multimedia
Internet telephony

Application layer protocol

Underlying transport protocol

SMTP [RFC 2821]
Telnet [RFC 854]
HTTP [RFC 2616]
FTP [RFC 959]
HTTP (eg Youtube),
RTP [RFC 1889]
SIP, RTP, proprietary
(e.g., Skype)

TCP or UDP typically UDP



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