Application Layer

This focuses on understanding the principles, protocols, and implementation of network applications, like web (HTTP), email (SMTP, IMAP), DNS, P2P systems, video streaming, content distribution networks (CDNs), and socket programming with UDP and TCP, while exploring client-server and peer-to-peer paradigms and the application-layer infrastructure.

Client-Server Paradigm

Server

Clients

Peer-Peer Architecture

Process

Client Process:

Server Process:

Identifier

▪ to receive messages, process must have identifier ▪ host device has unique 32-bit IP address …. includes both IP address and port numbers associated with process on host.

Processes Communicating within Same Host

Processes Communicating within Different Hosts

Sockets

Application-Layer Protocol

▪ 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 open protocols: ▪ defined in RFCs, everyone has access to protocol definition ▪ allows for interoperability ▪ e.g., HTTP, SMTP proprietary protocols

What transport service does an app need?

Internet Transport Protocols Services

TCP Service

UDP Service

Transport Layer Security (TLS)

Web Page

HyperText Transfer Protocol (HTTP)

Persistent HTTP

Non-Persistent HTTP

RTT

Non-Persistent HTTP Issues

HTTP Request Message

POST Method

GET Method

HEAD Method

PUT Method

HTTP Response Message

HTTP Response Status Codes

200 OK: Request succeeded, requested object later in this message

301 Moved Permanently: Requested object moved, new location specified later in this message (in Location: field)

400 Bad Request: Request msg not understood by server

404 Not Found: Requested document not found on this server

505 HTTP Version Not Supported

Cookies

Purpose of Cookies

Uses of Cookies

How to keep state?

Cookies and Privacy

Web Caches

Purpose of Web Caches

Conditional GET

HTTP/2

HTTP/3

E-mail

User Agent

Mail Servers

mailbox contains incoming messages for user

message queue of outgoing (to be sent) mail messages

SMTP Protocol

Comparison of SMTP with HTTP

Internet Mail Access Protocol (IMAP)

Domain Name System (DNS)

DNS Services

Root Name Servers

Top-Level Domain (TLD) Servers

Authoritative DNS Servers

Local DNS Name Servers

Iterated Query

Recursive Query

Caching DNS Information

DNS Records

type=A

type=NS

type=CNAME

type=MX

DNS Protocol Messages

DDOS Attacks

Spoofing