Application Layer Flashcards
Client-Server Architecture
SERVER: Always-on host, permanent IP address
CLIENT: Communicate with server, not directly with each other, intermittently connected, dynamic IP address
Peer-Peer Architecture
- No always-on server
- End systems communicate directly with each other
- Intermittently connected and change IP address
- self scalability
Process
Program running within a host
Client, Server Process Communication
CLIENT: process that initiates communication
SERVER: Process that waits to be contacted
Socket
Software structure that serves as endpoint for sending and receiving data across the network (like a door). Two sockets, one on each side.
Addressing Processes
Includes IP ADDRESS and PORT NUMBER. To receive messages, process must have identifier.
HTTP Overview
Hypertext Transfer Protocol. Uses TCP connection, is “stateless”
CLIENT: Browser that requests, receives and displays Web objects using HTTP Protocol
SERVER: Web server sends objects in response to request using HTTP protocol
TCP
Transmission Control Protocol. Organizes data in a way that ensures secure transmission
TCP Procedure
- Client initiates TCP connection to server, port 80
- Server accepts TCP connection from client
- HTTP messages exchanged between browser (HTTP client) and Web server (HTTP server)
- Connection is closed
Another name for HTTP messages
Application-layer protocol messages
“Stateless”
Server maintains no information about past client requests to this server. Can use info from requests to other servers
Non-persistent HTTP
Can only send one object per connection. Downloading multiple objects requires multiple connections.
Non-Persistent HTTP Procedure
- TCP connection opened
- Send one object over connection
- TCP connection closed
Persistent HTTP
Can use one connection to send multiple objects.
Persistent HTTP Procedure
- TCP connection opened
- Multiple objects send between server and client
- TCP connection closed
Non-persistent HTTP Response Time (per object)
- One RTT for initiate TCP
- One RTT for HTTP request and first few bytes of HTTP response to return
- object/file transmission time
time = 2RTT + file transmission time
RTT
Round Trip Time (non-persistent HTTP). Time for a small packet to travel from client to server and back
Example Non-persistent: Client requests for web page having 4 parallel connections?
4RTT + 2 file transmission time
Example Non-persistent: Client requests for web page having 8 parallel connections?
8RTT + 4 file transmission time
Example Persistent: Client requests for web page having 4 or 8 parallel connections?
3RTT + 2 file transmission time
Types of HTTP Messages
- Request
- Response
HTTP Return Line Format
[HTTP method] space [URL] space [HTTP version]
HTTP Message Format
- Request line
- Header lines
- Body
HTTP Methods
- POST
- GET
- HEAD
- PUT
HTTP POST Method
- web page includes form input
- user input sent in body
HTTP GET Method
For sending data to server. Include user data in URL field of HTTP
HTTP HEAD Method
Request headers that would be returned if specified URL were requested with HTTP GET method
HTTP PUT Method
- Uploads new file/object to server
- Completely replaces file that exists at specified URL
- Uses body to send replacement file
Cookies
Used by web sites and client browsers to maintain some state between transactions. Unique ID maps to entry in backend database
Cookie Components
- Cookie header line of HTTP response message
- Cookie header line in next HTTP request message
- Cookie file kept on user host, managed by user’s browser
- Back-end database at web site
DNS Services
- host name to IP address translation
- host aliasing (canonical names)
- mail server aliasing
- load distribution
DNS
Domain Name System
- Distributed database implemented in hierarchy of many name servers
- implemented as application-layer protocol
- hosts and name servers communicate to resolve names
DNS Hierarchy
- Root
- Top level domain (.edu)
- Authoritative (gmu.edu)
Local DNS Servers
- each ISP has one DNS server
- host DNS queries get sent to local DNS server
- has cache
DNS Iterated Query
Contacted sever replies with name of next server to contact (“idk but ask this server”)
DNS Recursive Query
- puts burden of name resolution on contacted server
- heavy load on upper levels of hierarchy
DNS Resource Record Types
- A
- NS
- CNAME
- MX
A Resource Record Type
NAME is hostname, VALUE ip address
NS Resource Record Type
NAME is domain, VALUE is hostname of authoritative name server
CNAME Resource Record Type
NAME is alias for “canonical name”, VALUE is canonical name
MX Resource Record Type
VALUE is mail server name associated with NAME
Web Cache
- Goal is to satisfy request without origin server
- User configures browser to point to a web cache
- browser sends all HTTP requests to cache
- installed by ISP
Web Cache Hit
Object is in cache, return object to client
Web Cache Miss
Object not in cache, request object from origin server, cache returned object and return to client
Review Web Cache Example
Web Cache Conditional GET
Goal is to not send object of cache has up-to-date version
CACHE: specify date of date limit in HTTP request
SERVER: response contains no object of cached copy is up-to-date
HTTP 1.1
Key Goal: decreased delay in multi-object HTTP requests
Introduced multiple, pipelined GETs over single TCP connection. Server responds in-order to GET requests.
HTTP 2
- Objects divided into frames
- Frame transmission interleaved/synchronous
- Reduces waiting time for all objects and recovery from packet loss
- Stalls object transmissions
HTTP 3
- adds security
- per object error and congestion control over UDP
- uses QUIC protocol
SMTP
Simple Mail Transfer Protocol,
uses user agents and mail servers
SMTP User Agent
- mail reader
- composing, editing, reading mail messages
- outgoing, incoming messages stored on server
SMTP Mail Server
- MAILBOX containing incoming messages
- MESSAGE QUEUE of outgoing messages
SMTP Procedure
- User agent sends message to its mail server using SMTP
- Mail server sends message to next mail server using SMTP or puts the message in the queue
- Next mail server sends the message to the other user agent using IMAP or HTTP
File Distribution Example
BitTorrent
- type of P2P file distribution
- peers in torrent send/receive file chunks
- has TRACKERS and TORRENT
BitTorrent Tracker
Tracks peers that are participating in the torrent
BitTorrent Torrent
Group of peers exchanging chunks of a file
