lec4

Get a basic conceptual understanding of the application layer on the network stack.

Application Layer Intro

Things we don't care about in this layer:

Routing
Network buffer(queue) overflows

These things should be handled by the network layers beneath this one.

What the App-layer defines

types of messages

Requests and responses.

Message syntax
Message Semantics
Open protocols
Rules

Client Server Architecture

Server

Hoster of content, usually with a static ip address

Client

Requesters of content, ip addresses here aren't necessarily dynamic however if you host a server of some kind, never assume a client has a consistent ip. Also clean any requests from users to avoid random catastrophe.

Peer to Peer

Here there is no concept of a host that is always on and has a static ip. Instead we have clients communicating with each other directly.

Sockets

processes which allows applications to send/receive data.

man socket in the terminal on any *nix based system(maybe not mac)

Each socket will use some port for which to communicate through.

TCP & UDP Protocols

TCP

Provides flow and congestion control.

Drawback is that we have someover head for the safety.

UDP

Does not provide flow or congestion control.

Advantage is that we don't have to concern ourselve with any packet safety, like accounting for dropped packets.

Streaming services would be a good use case since it's more acceptable for a milisecond(colloquially speaking) of data to be lost since the user probably won't notice anyway.

More and more networking level improvements and hardware improvements however allow us to use TCP with its overhead without worry.

HTTP Protocol

Uses the client/server architecture assumptions.

Clients make GET requests and Servers usually send OK packets.

KeepAlive

Modern(ish) concept that a connection opened with a server/client should remain open for some time; that way we don't have to reopen that connection for every single request a single user makes. Yes this save us on overhead, and yes its worth it.

Stateless

Maintains no information about past client requests. Cookies add the concept of statefulness.

HTTP Structures

Now we'll go over some of the various structures which HTTP uses.

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