2023-09-12

Introduction

Summary

keywords architecture, socket, protocols, criteria in choosing protocols, TCP, UDP

TODO

HW What is the default port number of HTTPS?

Exercise*

Next time HTTP request messages.

---

Recap

Application layer

Paradigms of network apps

Client-Server paradigm

• Server

  • Always on host

  • permanent IP address

  • need to think about scaling, security.

  • Could be hosting on standalone, or on data center

  • Data center hosting is superior in scaling and security.

• Clients

  • maybe intermittently connected.

  • could be dynamic IP address. (DHCP)

  • DO NOT communicate direct with each other.

Protocols in Client-Server paradigm : HTTP, IMAP, FTP

Peer-to-Peer paradigm

Every device is server, and a client.

• no always-on server.

• peers request service from other peers, provide service in return.

• Self scalability : new peers bring new service capacity, along with the demands.

• complex.

We'll only focus on Client-server paradigms & HTTP.

---

Process Communicating

What is a Process?

program running within a host

Client process, server process inter-process communications : process (hosted in different devices) communicating by exchanging data.

Socket

Points where two things are connected.

How do you identify the socket?

IP + Port

By IP address (device) + Port (process inside the device) ex. http servers run on port 80, https servers run on port 443

Analogy. Envelope. Family members share a same home address (IP), and we have a name (port)

UDP socket, TCP socket.

Google didn't like any of the transport layer. So they made QUIC(Quick UDP Internet Connections)

A browser (also) runs on a prot number

Protocol defines..

• Types of messages (Res, Req)

• message syntax (How fields are delineated)

• message semantics (meaning of info)

• rules (when and how processes send & respond to messages)

Open Protocols

defined in RFCs, everyone can read and use allows for interoperability ex, HTTP, SMTP

There also are (not-open) proprietary protocols.

Transport service

What criteria should we care when choosing transport service for an application?? * data integrity (making sure the files are completely transferred) * Okay if you can tolerate the data loss (Ex. audio format file) * timing (low delay) * Okay if the service is not live-streaming * throughput (bandwidth of edge..) * Okay if service is elastic.

![[../images/20230912135617.png]]

Properties of TCP & UDP

	TCP	UDP
transport reliability	Y	N
flow control	Y	N
Conjestion control	Y	N
connection-oriented	Y	N

TCP Do not provide: depends on the bandwidth. Cannot change throughput by itself. timing, minimum throughput guarantee, security

UDP Do not provide : reliability, flow control, congestion, control, timeing, throughput guarantee, security, connection setup..

throughput is not guarantee by any Protocols in this Internet. Security is not supported by TCP & UDP TCP cares about lost packets. UDP doesn't. TCP is connection-oriented. We should open and close a connection.

Q. Why use UDP?

UDP is much faster. Header is smaller. less overhead.

![[../images/20230912135815.png]]

Securing TCP

• TCP, UDP has no encryption

• cleartext. human-readable.

We use TLS (Transport Layer Security, Originally called SSL) * There are TLS libraries you can use in application layer.

---

Web, HTTP

The idea is "Sending meaningful objects through internet"

• URL : includes information about the target device & what you want

HTTP

Hypertext transfer protocol

• application layer protocol

• browser is the interpreter(of the HTTP packets) + displayer(of the object).

HTTP uses TCP.HTTP is stateless. : state information is implemented by cookies.HTTP nowadays (HTTP 1.1) are Persistent. : 1 connection can be used for multiple objects sending. before closed. You choose how long, how many.

Non-persistent HTTP has high RTT (round trip time) #todo : problems of Non-persistent HTTP