Internet Communication Models

Two Communication Paradigms: Stream vs. Message StreamMessage Connection-orientedConnectionless 1-1 communication1-1, 1-many, many-many Sequence of bytesSequence of messages Arbitrary length transferLimited to 64K bytes Widely usedMultimedia applications Based on TCP (Guaranteed)Built on UDP (Not Guaranteed)

Message (Connectionless) Transport In the Internet The message service does not make any guarantees. A packet may be – Lost – Duplicated – Delayed – Delivered out-of-order A programmer who chooses the message service must ensure that the application operates correctly, even if packets are lost or reordered.

Why using message service? Fine application-level control over what data is sent, and when No connection establishment – Get straight to the point No connection state – A server can support more active clients simultaneously Small packet header overhead

Some Examples ApplicationTCP or UDP? TCP Remote terminal accessTCP WebTCP File transferTCP Remote file serverTypically UDP Streaming multimediaUDP, TCP Internet telephonyUDP, TCP Name translationTypically UDP TCP today: >95% Internet traffic ~75% on-demand and live streaming

Stream (Connection-Oriented) Communication Usually, a machine, or a server, is out there waiting for connection calls. – Port is open. Web browser Web server

Client-Server Server SoftwareClient Software Starts firstStarts second No need to know clients (can have access limits) Must know which server to visit Passively waits for clientsActively initiates contact Available most of timeTerminated after work

Why Client/Server Structure? Specialization: – Clients: user interface and data presentation. – Servers: managing data and providing services. Web servers, server, database server Type of clients does not matter. Sharing: – Many clients can be supported by few servers. Google.com – Not just one server, but multiple servers.

Server-Class Machine vs. Server Service Server-class computers – Powerful hardware Speed, bandwidth, storage capacity, etc. Server software – Usually hosted in server computers – But can also be run in less-powerful machines Windows 7 have several server software running: Remote desktop Mobile Phone – Windows PCs v.s. Unix/Linux machines Windows: clear distinction between client and server – Few server services in client PCs (Windows 7, XP, etc.) – Windows Server does not have a strong support for client applications. Unix/Linux: a machine is both server and client. – Has both client and server software.

IP-Ready Devices Clients and servers do not have to be PC-like devices. IP Cameras – Cameras have a web server built in and can have their own IPs. – Can be directly accessed through the Internet. IP-Enabled home automation – IP-enabled appliance – Smart TV

Peer-To-Peer Motivation: Client/Server model create a central bottleneck between the Internet and the server

Peer-To-Peer Idea: no dedicated server machine is necessary. – Client machines communicate with each others.

Music Swap: C/S Model server Machine A 1. I want “Grenade by Bruno Mars” 2. Here it is.

Music Swap: P2P Model I want “Grenade by Bruno Mars”. My address is … I want “Grenade by Bruno Mars”. My address is … Someone wants “Grenade by Bruno Mars”. The address is … Someone wants “Grenade by Bruno Mars”. The address is … I have it.

Distinctions Client-server – Asymmetric relationship Client make requests, and a server simply replies. – Dedicated servers with server software Peer-to-peer – Symmetric relationship A machine is a server and a client. – Server software on client machines

Social Consequences Intellectual property – Servers can be easily targeted. Napster (previous life) failed. – How about machines in P2P? MPAA and RIAA against college students Lawsuits against software developers, website that index pirated contents, … Pressures on service providers – Comcast

Ch – 3.23 Concepts related to network programming – Not a concern of midterms.

After Class Read Ch. 4 – Internet applications