Computer Networking: Application Layer

Chapter 2: Application Layer

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• Book name: Computer Networking: A Top-Down Approach
• Authors: Jim Kurose, Keith Ross
• Edition : 8th
• Published: Pearson, 2020

Application Layer Overview

• Key aspects of network applications
• Focus on web and HTTP protocols
• Includes mail protocols including SMTP and IMAP
• Includes DNS protocol
• Peer-to-peer application architectures
• Video streaming and content distribution are analyzed
• Focus on Socket programming with UDP and TCP

Goals

• Focus on conceptual and implementation aspects of application layer
• Look at transport layer service models
• Including client-server and peer-to-peer models
• Learn how protocols work by looking at popular app-layer protocols and infrastructure
• Focus on HTTP, SMTP, IMAP, and DNS
• Also video streaming systems and CDNs
• Programming network applications are analysed including socket API

Network Applications

• Examples of key network applications
• Social networking, web, and text messaging.
• Email, multiplayer network games, and streaming video
• VoIP (Skype), video conferencing (Zoom), internet search, and remote login

Creating a Network App

• Focus on creating programs for end systems
• Enabling communication over a network
• Web server software interacting with browser software is required
• No need to write the core network devices
• Network-core devices are therefore free to avoid running user applications
• Facilitates rapid application development and propagation

Client-Server Paradigm

• Server characteristics
• Always-on host, with a permanent IP address
• Often in data centers for scaling purposes
• Client characteristics
• Clients contact and communicate with the server
• May connect intermittently
• Can have dynamic IP addresses
• Generally clients do NOT communicate with each other
• Examples include HTTP, IMAP, and FTP

Peer-to-Peer Architecture

• There is no always-on server
• End systems can arbitratily communicate directly with each other
• Peers request services from other peers
• Provide services in return
• Self-scaling property where each new peer brings new service capacity
• Peers connect in intermittent fashion and the IP addresses change
• Complex to manage such a network
• P2P file sharing using BitTorrent is an example

Processes Communicating

• In terms of networking a process is a program running on a host
• Within the same host, two processes can communicate using inter-process communication as defined by the OS
• Processes in different hosts communicate by exchanging messages
• Terminology of clients and servers
• The client process initiates communication
• The server process waits to be contacted

Sockets

• A process sends and receives messages through a socket
• Analogy, a socket behaves like a door
• The sending process sends message out the door, relying on the transport infrastructure
• The receiving process also has a socket
• Two sockets are used, one at each end

Addressing, Identifiers and Port Numbers

• To receive a message a process must have an identifier
• This identifier is made of
• The unique 32-bit IP address of the host machine that the process is running on
• The port number associated with the process
• Example port numbers
• HTTP server: 80
• Mail server: 25
• To connect to any web server at gaia.cs.umass.edu:
• IP address: 128.119.245.12
• port number: 80

Application-Layer Protocols

• Includes what message types are exchanged e.g. request, and response
• Message syntax defining the fields and how they are delineated
• Message semantics define information meaning
• Rules for when and how messages are sent and processed
• Open Protocols are defined in RFCs, accessible to everyone and interoperable
• HTTP and SMTP are examples of open protocols
• Proprietary Protocols are not open e.g. Skype and Zoom

App Transport Service Requirements

• Data integrity requirements
• Web browsing, file transfer require 100% reliable data transfer
• Other applications like audio can tolerate some loss
• Timing
• Internet telephony and interactive games require low delay
• Throughput
• Some apps require minimum throughput e.g. multimedia file streaming
• Other apps do not require a minimum like elastic apps
• Security
• All apps should encrypt data and ensure data integrity

Common App Transport Requirements

• File transfer and e-mail are not time-sensitive and allow no data loss
• Web documents not time-sensitive and allow no loss
• Audio and video are loss-tolerant and time-sensitive
• Interactive games are loss-tolerant and also time-sensitive
• Text messaging allows no loss and is time-sensitive

Internet Transport Protocols

• TCP service
• Provides reliable transport between sending and receiving process
• Employs flow and congestion controls
• Provides connection-oriented setup
• Does not provide timing, minimum throughput or security
• UDP service
• Provides unreliable data transfer
• Lacks reliability, flow and congestion control
• Does not guarantee timing, security, or connection setup
• One may ask why use UDP if TCP is superior in most aspects

Internet Apps and Protocols

• FTP uses TCP
• Email uses SMTP and TCP
• Web documents uses HTTP and TCP
• Internet telephony uses SIP and RTP with either TCP or UDP

Securing TCP

• Vanilla TCP and UDP sockets
• Do not use encryption
• Send passwords without encryption
• Transport Layer Security (TLS) provides encrypted TCP connections with data integrity and end-point authentication
• Apps can use TLS libraries, build on TCP
• With the result of sending cleartext messages across the Internet in the encrypted form

Web And Http Overview

• Web pages consist of objects
• Objects can be on different web servers
• Objects are HTML file, JPEG image, Java applet, audio files
• A web page consists of a base HTML file which further references objects, and each of which is addressable by a URL

HTTP

• HTTP is hypertext transfer protocol
• Used by the web applications
• It is client-server type i.e. browser that requests, gets and finally displays web objects
• The web server sends with HTTP the requested objects in response
• Hypertext Transfer Protocol (HTTP) is the web’s application-layer protocol

HTTP Uses TCP

• The client always initiates a TCP connection
• Server accepts TCP connection from client
• The HTTP messages are exchanged between browser (HTTP client) and Web server (HTTP server)
• After a certain time, the TCP connection gets closed

HTTP Is Stateless

• Statless means the server maintains no information about past client requests protocols that maintain
• “state” are complex
• past history (state) must be maintained
• Server/client crashes, their views of “state” may be inconsistent, must be reconciled

HTTP Connections

• There are two types of connections
• Non-persistent HTTP
• Persistent HTTP (HTTP 1.1)

Non-Persistent HTTP

1. The TCP connection opened for each object file
2. At most one object gets sent over TCP connection
3. The TCP connection gets closed

• Therefore, downloading multiple objects requires multiple connections

Persistent HTTP (HTTP 1.1)

• The TCP connection opened to get a server
• The client can transfer multiple objects through the same TCP connection
• And finally the TCP connection gets closed

Non-Persistent HTTP Example

1a. HTTP client initiates TCP connection to HTTP server (process) on port 80 2. HTTP client sends HTTP request message (containing URL) into TCP connection socket 3. HTTP server receives request message and forms response message containing requested object, and sends message into its socket 4. HTTP server closes TCP 5. HTTP client receives response message containing html documents, and displays html file of requested directory etc

• Then the steps 1. to 5. are executed for another objects

HTTP Response Time

• RTT is time for delivery of packet to server and back
• For object
• One RTT for client
• One RTT for HTTP
• Total response time equals 2RTT+ file transmission time

Persistent HTTP

• Requres 2 RTTs per object
• TCP overhead for all conections
• Browsers often open TCP connections to get the requested objects

Persistent HTTP

• HTTP leaves server connection open
• Subesquent HTTP messages sent over connection open
• Client requests are available at refernced objects
• HTTP 1.1 can give the entire requested objects

HTTP Message Types

• Uses requests like request and response messages
• Uses ASCII for coding text (human readable formatting)
• Uses carriage return character
• Request uses commands (GET, POST, HEAD)

HTTP Request Message

• Uses method, URL and version
• The first header line defines
• Value, cr and If
• Can use entity bodies to describe a body

Other Request Types

• POST:
• includes forms
• user input
• entity body
• GET
• includes urls and data input www.somesite.com/animalsearch?monkeys&banana
• HEAD
• Retrurns headers if specified URL used by HTTP GET
• PUT
• Uploads object to server
• replaces content
• includes entity body and content

HTTP Response Message:

• Status line (http - phrase)
• Date of reply
• Apache response
• last modified
• content message
• data in HTML request form

HTTP Response Static Codes

• Status code appears in 1st line in server-to-client message.
• Some example 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

NetCat Server

• To test the HTTP request can send request with:
• % nc -c -v gaia.cs.umass.edu 80 (for Mac) ▪ opens TCP connection to port 80 (default HTTP - server port) at gaia.cs.umass.edu.

•            >ncat –C gaia.cs.umass.edu 80 (for Windows) ▪ anything typed in will be sent to port 80 at
                           -   gaia.cs.umass.edu
  

• request format:
• GET /kurose_ross/interactive/index.php HTTP/1.1
• Host: gaia.cs.umass.edu
• You send the data to browser

Cookie Server State

• How do you implement it?
• Use some stateful protocol to interact
• HTTP stateless protocol without notion of message interaction
• No exchange of multi-point transaction

How Cookies Maintain User States

• the Web sites and the client browser that uses cookies to maintain information from all transaction in the state

1. use HTTP response messages
2. next HTTP request
3. file host user is using
4. back-end web data

Cookie Protocols

• You can implement data on lapto of client by tracking states in website
• Susan uses browser
• For the first time sites create
• Cookie IDs
• Add backed databases
• Sites can track info by finding and storing this cookie (such as knowing that it is really Susan sending a request)

Server Requests and Actions for Cookies

• Browser can add http to a request file
• Amazon sites store data for tracking.
• For a first time login can add a state by set-cookie to a file and to a database

HTTP Security

• How To use Authorization
• track shopping carts
• Recommends
• Allow users to do e-mail

Cookie Tracking Protocols

• Can track users Can not track by cookies:
• HTTP requests Third party cookies
• Disables data by tracking codes
• Cookies in the Google Chrome

GDPR Tracking Methods

• Informs if site uses cookies
• Users can allow cookies on site or not
• Cookies can protect info

Web Caching

• Goal to satify request with origin to server with browser cache
• Use local web client browsers - HTTP
• In cache: cache returns object to user
• No cache: object obtained from server cached and delivered to client

Cache Acting

• The cache acts as client and user
• Reduces time with a closer client
• enables poor providers to quickly deliver object code

HTTP Caching Example

• If network fast average request may be long e.g. in seconds
• High utilization causes issues queue size issue
• The network can limit bandwith use

Improving Caching

• With slow or fast speed improve the local site
• Buy and create a fast cache cheap and easy
• For hit of 0.4 reduce queue in local area, lower average overall by 154MB

Browser Caching Conditional

• Can respond without object in a new system
• It avoids transmittiom delays for code transmission
• Gets info with dates specified
• It allows response whether is changed

HTTP 2

• Goal decrease for multi objects
• Introduces multiple TCP pipelining
• Server responses order
• Lossy can stall
• It uses HOL for the protocol

HOL Blocking

• Objects devided into frames
• Multiple server
• Slow if more objects
• Quick with a bit load

HTTP3 Transfers

• New versions increase transmission speed
• New protocols include security control

eMail Overview

• It uses SMTP. IMAPS, POP3
• It use message distribution

E-mail Components

• Components
• User
• Server Three Parts
• Agents transport and simple message protocol

E-mail Servers

• Mailboxes
• Agents used SMTP

SMTP Functions

• Transmission of reliable messages with ports
• Command transfers
• Asc II status
• Handshakes hello requests

eMail Message Handling

• How to compose an E-mail
• SMTP protocol to do transfer
• Reading the text
• Server protocol to perform reads

SMTP Samples

• Messages are sent using:
• codes
• actions
• texts

Retreiving eMails

• To use and retrieve the Emails the IMAPS transport can be implemented
• The HTTP transport uses this
• Transports need to implement servers for requests

DNS Overview

• Can map between 10's of addresses
• Need server host to map
• How to translate

DNS Network Protocol

• Is a distributed Data base
• Host to serve is a layer
• Transports can not solve the problems

DNs Structures

• The services add to the network IP protocol
• Alias hosts
• Can set for many IP addresses

DNS Hierachy

• Data is managed by each one
• Many trillons access it a day with milisecods counts
• It requires easy simple managment

Names in DNS

• Root contains everything and a to data base
• top level is com or org
• Can transfer IPs

DNS Root Names

• Official last message transfers
• Names can not can access if does respond

Top Level servers

• Reponosbile for top levels like com and net
• Has servers for organization

Authorative Servers

• Organizations set domain and IP hosts
• Data to maintain service

Local Name Servers

• A host has IP for finding all servers to resolve
• Protocols to resolve
• Mac to DNS

DNS Name Servers • Request the name with host • Asks to server for the server • Asks for what to request

DNS Protocol Data

• Data includes
• Queury
• Replay
• Has a specific format
• Authentication
• Uses replay codes

Protocol Types

• Query of data
• Can access data with questions
• Authentication