Protocol Stack

Layers

• Application
• Transport
• Network
• Link
• Physical

Application

HTTP, IMAP, SMTP, DNS

Transport

TCP

• Reliable
• Has flow control
• Congestion control

UDP

• Less Reliable
• No flow control, etc

Network

IP, routing protocols

Link

Ethernet, 801.11 (WiFi), PPP

Physical

Bits on the wire

HTTP

HTTP older then v3 uses TCP

• Client initiates TCP connection on port 80
• Client accepts TCP connection
• HTTP messages are exchanged
• TCP connection is closed

HTTP is stateless in the sense that the server maintains no information about prevoius requests

Persistence

There is non-persistent and persistent HTTP, in persistent HTTP a single TCP connection is opened and multiple objects can be sent over a singel TCP connection.
Persistence was introduced in HTTP 1.1

Methods

There are 4 HTTP request messages

• GET - GET a resource from the server
• POST - Generally used to create a resource
• HEAD - Request headers of a response if it were to be requested with GET
• PUT - Often used to replace/update an object

HTTP/2

HTTP1.1 introduced multiple, pipelined GETs over a single TCP connection:

• Server uses FCFS scheduling for responses

This isn’t super-ideal, and as such HTTP/2 increased server flexibility for sending responses:

• Transmision order now based on client-specified priority (not locked to FCFS)
• push unrequested objects to clients
• objects can be divided into frames, avoid HOL blocking (head of line blocking caused by TCP retransmission of large objects)

HTTP/3

Based on QUIC, it adds security, pipelining and runs over UDP

Email

Email consists of 3 main protocols:

• SMTP handles the delivery and storage of emails on servers
• IMAP handles messages stored on the server (retrieval, deletion, etc)
• POP3 handles message downloading (often deletes messages once they are downloaded)

HTTP is often used for web-interfaces to mailservers

DNS

DNS is an incredibly large distributed database

• Handles trillions of queries/day
• Physically decentralied
• “bulletproof”

Root Name Servers

• If no other server can resolve a name, the query is forwarded to a root name server
• DNSSEC provides integrity
• There are 13 “root servers” but they are actually replicated in the hundreds across their respective countries
• ICANN manages them

Hierarchy

• When you make a DNS request…
• First a local server is queried and answers if it can, otherwise…
• It queries a root DNS server…
• Which may query a TLD DNS server…
• Which may query the authoritative DNS server for that domain…

Responses are often cached for faster subsequent queries, cached items last for the TTL specified in the record

DNS Records