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Chapter 2
Application Layer
Computer Networking: A Top Down Approach , 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April
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Chapter 2: Application layer
(^) 2.1 Principles of network applications (^) 2.2 Web and HTTP (^) 2.3 FTP (^) 2.4 Electronic Mail (^) SMTP, POP3, IMAP (^) 2.5 DNS (^) 2.6 P2P applications (^) 2.7 Socket programming with TCP (^) 2.8 Socket programming with UDP
Chapter 2: Application layer
(^) 2.1 Principles of network applications (^) 2.2 Web and HTTP (^) 2.3 FTP (^) 2.4 Electronic Mail (^) SMTP, POP3, IMAP (^) 2.5 DNS (^) 2.6 P2P applications (^) 2.7 Socket programming with TCP (^) 2.8 Socket programming with UDP
Some network apps
(^) e-mail (^) web (^) instant messaging (^) remote login (^) P2P file sharing (^) multi-user network games (^) streaming stored video clips (^) voice over IP (^) real-time video conferencing (^) grid/cluster computing (^) Java RMI (^) CORBA-Based App (^) Web Services (^) J2EE (^) ESB (Mule) (^) Social network (^) Community
Application architectures
(^) Client-server (^) Peer-to-peer (P2P) (^) Hybrid of client-server and P2P (^) Message passing (Socket) (^) Message system: Point-to-point; Publish/Subscribe (^) Distributed objects: (^) Remote method invocation (RMI) (^) Object request broker (CORBA) (^) Object space (JavaSpaces) (^) Component-based/Web Service-based (J2EE/SOA) (^) Mobile agents (IBM’s Aglets) (^) Network services (Jini)
Client-server architecture
server: (^) always-on host (^) permanent IP address (^) server farms for scaling thru VIP clients: (^) communicate with server (^) may be intermittently connected (^) may have dynamic IP addresses (^) do not communicate directly with each other client/server
Hybrid of client-server and
P2P
Skype voice-over-IP P2P application centralized server: finding address of remote party: client-client connection: direct (not through server) Instant messaging chatting between two users is P2P centralized service: client presence detection/location
- (^) user registers its IP address with central server when it comes online
- (^) user contacts central server to find IP addresses of buddies
Processes communicating
Process: program running within a host. (^) within same host, two processes communicate using inter-process communication (defined by OS). (^) processes in different hosts communicate by exchanging messages Client process: process that initiates communication Server process: process that waits to be contacted (^) Note: applications with P2P architectures have client processes & server processes
Sockets
(^) The Berkeley Unix mechanism for creating a virtual connection between processes. Sockets interface Unix's standard I/O with its network communication facilities. They can be of two types, stream (bi-directional) or datagram (fixed length destination-addressed messages). The socket library function socket() creates a communications end-point or socket and returns a file descriptor with which to access that socket. The socket has associated with it a socket address, consisting of a port number and the local host's network address. (^) From http://wombat.doc.ic.ac.uk/foldoc/foldoc.cgi? query=socket
Addressing Processes
(^) to receive messages, process must have identifier (^) host device has unique 32-bit IP address (^) Q: does IP address of host suffice for identifying the process?
App-layer protocol defines
(^) Types of messages exchanged, (^) e.g., request, response (^) Message syntax: (^) what fields in messages & how fields are delineated (^) Message semantics (^) meaning of information in fields (^) Rules for when and how processes send & respond to messages Public-domain protocols: (^) defined in RFCs (^) allows for interoperability (^) e.g., HTTP, SMTP Proprietary protocols: (^) e.g., Skype
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, interactive games) require low delay to be “effective” Throughput (^) some apps (e.g., multimedia) require minimum amount of throughput to be “effective” (^) other apps (“elastic apps”) make use of whatever throughput they get Security (^) Encryption, data integrity, …
Internet transport protocols services
TCP service: (^) connection-oriented: setup required between client and server processes (^) reliable transport between sending and receiving process (^) flow control: sender won’t overwhelm receiver (^) congestion control: throttle sender when network overloaded (^) does not provide: timing, minimum throughput guarantees, security UDP service: (^) unreliable data transfer between sending and receiving process (^) does not provide: connection setup, reliability, flow control, congestion control, timing, throughput guarantee, or security Q: why bother? Why is there a UDP?
Internet apps: application, transport protocols
Application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony Application layer 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) Underlying transport protocol TCP TCP TCP TCP TCP or UDP typically UDP
