Network-1-Application Layer

Application architectures

server
- always-on host
- permanent IP address
- data centers for scaling
clients
- communicate with server
- do not communicate directly with each other
- may be intermittently(on and off) connect
- may have dynamic IP addresses

no always-on server
arbitrary end systems directly communicate
peers request service from other peers, provide service in return to ohter peers
- self scalability - new peers bring new service capacity, as well as new service demands
peers are intermittently connected and change IP address
- complex management

process: program running within a host
- within same host, tow processes communicate using inter-process communication (defined by OS)
- processes in different hosts communicate by exchanging messages
clients, servers
- client process: process that initiates communication
- server process: process that waits to be contacted
  
  aside: applications with P2P architectures have client processes & server processes

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 to deliver message to socket at receiving process

to receive messages, process must have identifier
host device has unique 32-bit IP address
Q: does IP address of host on which process runs suffice for identifying th process?
- A: no, many processes can be running on same host
identifier include both IP adderss and port numbers
example port numbers:
- HTTP server: 80
- mail server: 25
to send HTTP message to www.naver.com web server
- IP address: 121.33.223.11
- port number: 80

data integrity
- some apps (e.g., file transfer, web transactions) require 100% reliable data transfer
- other apps (e.g., audio) can tolerate some loss
timing
- some apps (e.g., Internet telephony, interactive games) require lo delay to be “effective”
throughput
- some apps (e.g., multimedia) require minimum amount of throughtput to be “effective”
- other apps (“elastic apps”) make use of whatever throughtput they get

TCP service
- reliable transport between sending and receiving process
- flow control: sender won’t overwhelm receiver
- connection-oriented: setup required between client and server processes
- congestion control: throttle sender when network overloaded
- does not provide: timing, minimum throughput guarantee, security
UDP service
- unreliable data transfer between sending and reveiving process
- does not provide: reliability, flow control, congestiong control, timing, throughput guarantee, security, or connection setup
  Internet apps: application, transport protocols