Application Layer: Chapter 2

• Chapter 2 is about the Application Layer
• The goal includes conceptual and implementational aspects of the application layer, transport-layer service models, and client/server/peer-to-peer paradigms.

Core of Network Application Development

• It involves writing programs on different end systems to communicate with each other over networks

Example in Web Applications

• Web apps use 2 distinct programs that communicate, browser programs on user hosts, and web server programs on web server hosts.

NW Application Examples

• Examples include e-mail, the web, instant messaging, remote login, P2P file sharing, and multi-user network games.

Application Programs

• These run-in end systems (hosts), communicate over a network, and exchange messages to implement applications like e-mail, the web, P2P file sharing and instant messaging.
• No software is required for network-core devices

Network-Core devices

• These don't run user applications

Application-Layer Protocols

• These protocols define messages exchanged by apps and actions that are taken
• Communication services are provided by lower-layer protocols like TCP and UDP

Network Applications

• Network applications follow the client-server or peer-to-peer (P2P) architecture

Server

• An always-on host, has a permanent IP address and is often in data centers for scaling.

Clients

• Clients contact and communicate with the server intermittently, and may have dynamic IP addresses
• Clients do not communicate directly with each other
• Examples include HTTP, IMAP, and FTP

Peer-to-peer architecture

• This architecture has no always-on server
• Arbitrary end systems communicate directly
• Peers can request a service from other peers and provide the service in return
• New peers offer service and scalability
• Peers are intermittently connected and change IP addresses
• Management is complex
• P2p filesharing is an example

Server Architecture

• Servers are always-on hosts with permanent IP addresses
• Provides requested services to clients, such as web pages or email

Client Architecture

• Clients initiate contact with the server
• Clients may have dynamic IP Addresses
• Clients don't communicate directly with each other

Client/Server Processes

• Client processes initiate the communication
• Server processes wait to be contacted
• Browser processes initialize contact with a Web server process on the Web

Processes

• This is a program that is running within a host
• Communicate by exchanging messages

Socket

• Serves as the interface between the process and the computer network
• Sockets are also known as Service Access Points (SAP)
• Messaging process and the transport are key

API

• Sockets can also be referred to as Application Programming Interfaces through port-based transport protocols

Process Receiving Messages

• Needs an identifier
• Every host has an unique 32-bit IP address
• Multiple processes can run on the same host
• Identifier includes both IP addresses and host associated port numbers

Example Port Numbers

• Web-HTTP protocol uses an identifier port number of 80
• Mail-SMTP protocol uses an identifier port number of 25
• HTTP messages to google.com connects to IP address (216.58.216.164) on port number 80

Application Transport Needs

Data Integrity

• Some applications (file transfer, web transactions) require 100% reliability
• Some applications (audio) tolerate loss

Timing

• Some applications (Internet telephony, interactive games) require low delay to be "effective"

Throughput

• Some apps (multimedia) require minimum bandwidth to be "effective"
• Other apps ("elastic apps") make use of whatever throughput they get

Security

• Requires security encryption, data integrity, etc

Transport Services of Common Apps

File transfer

• File transfer requires no loss and elastic throughputs and timing isn't an issue

Email

• Email also requires no loss and elastic throughput and timing isn't an issue

Web documents

• Timing isn't an issue, but there's no loss, and it uses elastic throughput

Real time audio video

• Is loss-tolerant, the audio uses 5Kbps-1Mbps, video uses 10kbps-5Mbps, and the timing is 10's of msec

Stored audio video

• Uses the same configurations but the timing is in few secs

Interactive Games

• Needs very lossless tolerant conditions and timing is 10's of msec

TCP Service

• Functions with reliable transport between sending and receiving processes
• Has flow control such that the sender won't overwhelm the receiver
• Throttles sender when overloaded
• TCP is reliable, no timing, and is connection-oriented

UDP Service

• Works with unreliable transfer between sending and receiving
• Provides no reliability or security
• There's no flow or congestion control, no guaranteed timing throughput and no set up connections.

Application Layer Protocols

• FTP [RFC 959] uses TCP
• SMTP [RFC 5321] uses TCP
• HTTP 1.1 [RFC 7320] uses TCP
• SIP [RFC 3261] uses TCP or UDP
• Proprietary HTTP [RFC 7320], DASH uses TCP
• WOW,FPS (proprietary) uses either UDP or TCP

Web/HTTP Terminology

• Web pages consist of objects, such as HTML, JPEG and audio files
• Most web pages consist of a base HTML-file and includes several referenced objects
• Web objects are addressable by URLs

HTTP Protocol Overview

• HTTP stands for Hypertext Transfer Protocol
• Functions on The Web's application layer
• This protocol uses the client/server model
• Clients request, receive and display (using HTTP protocol) Web objects
• The server sends (using HTTP protocol) objects in response to requests

HTTP/TCP Use

• This protocol uses a TCP port to initiate and create connections
• In this case, HTTP has a server that approves TCP connections
• HTTP translates exchanged application-layer protocols

HTTP

• Is stateless
• The server maintains no information on the client

HTTP Connections

• Non Persistent: using TCP connections, at most one object sent over TCP connection and the connection closes Downloading multiple objects requires multiple connections
• Persistent: Use ongoing TCP connections, multiple objects can be sent over a single TCP connection between a client/server, and closes with the connection

Non-Persistent HTTP

• The client initiates a TCP connection and the server accepts
• Next, the client sends a request to retrieve the object
• Then the server sends the object, and closes the connection
• The server receives and displays the object

Response time, rtt

• RTT (definition): Time for a small packet to travel from client to server and back
• Formula: 2RTT + file transmission time

HTTP and RTT

• Persistent protocol is the preferred and faster
• HTTP persistent connection: allows to send multiple objects reducing the RTT to more than half

Web Cache

• The goal is to satisfy client requests without involving an origin server
• The user can configure the browser to point to a web cache
• The HTTP requests the cache
• The cache returns to the client and caches a copy from the server
• Caches act as both sides of client server relationship

Web Caching

• Can lead to the reduction of response time and overall access time
• It helps reduce traffic across institutions using the same connections
• It enables content for slow networks

Email

• Involves user agents, mail servers, and SMTP protocol
• User agents = mail readers
• Includes composing and editing features

Mail Servers

• These contain incoming messages for users
• Also contains a "message queue" of outgoing messages
• Uses SMTP to send emails

SMTP functions

• This uses TCP to reliably transfer messages from client's to the server's port 25.
• This transfers directly, which means SMTP protocol interacts directly from the sending server to the receiving.
• This contains 3 steps: Handshaking, Transferring, and Closing.
• All messages are in ASCII

Alice Sends an Email

• Alice uses a user agent to create the email message
• The user agent relays the message toward the mailbox
• The SMTP opens a TCP connection and sends the email
• Lastly, Bob the receiver uses a user agent to see the email

Protocols

• SMTP delivers messages to a designated Mailbox
• Other applications can provide access to that location depending on the type, (HTTP, IMAP)

DNS: Domain Name System

• This maps a hostname to it's IP
• Every device on every network has one Internet Hosts, Routers:
• IP addresses use 32 bits for addressing
• The "name" of each device uses a alphanumeric naming convention
• The DNS maps between these address types

DNS: Functions

• Manages names and addresses
• is a database implemented as a hierarchy containing info regarding name servers.
• Its a set of app layers for the distributed databases allowing names and addresses to be resolved by it

Centralized DNS

• Is non scalable
• Can be a single failure point
• Traffic volume could overwhelm single server
• Can't be "close" to every client (causing significant delays)
• Maintenance: DNS would need to keep a record of every Internet host

DNS and Hierarchy

• When looking for a name and IP address, first query the root, then queries for the domain, then queries for the name

Root DNS Servers

• "Official", contact of last resort to find resource records from the names that can resolve
• There are 13 logical root name "servers"
• ICANN manages all Root domains

TLD: Top-Level Domain

• Responsible for .com, .org, .net, .edu, .aero, .jobs, .museums, and all top-level country domains, e.g.: .cn, .uk, .fr, .ca, .jp etc
• Networking solutions include networking TLD etc

Local DNS Names

• These are set by the ISP
• Also called a default name server
• It will remember names for future queries as a result.
• It can act as a form of proxy server, forwarding queries

Iterated Queries

• Contacted servers reply with the name of a server to contract
• The phrase " I don't know this name, but ask this server" is generally used
• These tend to query recursively

Reclusive Queries

• These add the burden of name resolution of the contacted name server heavy query load on higher levels of hierarchy

DNS Records

• Learns mappings
• Can be not up to date (Outdated Best Efforts)
• Update/Notify: Internet Engineering Task Force proposed a system to notify a server to update it's records

DNS message header includes

• Indentification for the query
• Flags: whether it's a query or reply
• Sections for questions, answers, authority, and information for a DNS server to use.

DNS Security

• Concerns include the security, integrity, and availability of the service as a whole.
• DDoS attacks bombard the DNS servers in an attempt to negatively impact the reliability of the service.
• Redirect attacks occur from intercepted queries and DNS poisoning.

P2P Architecture

• Arbitrary system with one server acting as the main transfer point
• Peers request services and give in return
• Peers may be intermittent and dynamic addresses
• Sharing over Bittorrent and Kankan

Bittorrent

• The protocol used to fileshare
• Distributes files in a torrent with the help of peers

Bittorrent and Torrenting

• Peers are uploading and downloading to each other, sending parts of a singular file
• A program will seek out other peers and transfer file chunks to reassemble
• These are transferred while others are transferred to a given system
• Churn causes a peer to find another resource stream

Constant Bit Rate

• CBR is a video encoding rate that is fixed

Variable Bit Rate

• VBR is an encoding rate that changes depending on spacial conditions

Video Example Qualities and Speeds

• Qualities such as DVD versus Streaming VOD determines speed and bitrate

Streaming Stored Video

• Server to Client bandwidth will vary over time on bandwidth, and video qualities.
• Network conditions can ruin a video stream experience.

Dynamic Adaptive Streaming over HTTP

• Dynamic transfers stream data along with HTTP
• Files are encoded, encoded at different bit rates.
• Then clients consult these manifests which give bandwidth and sustained rates. Clients have encoding rate points through points.

Content distribution networks

• Netflix uses CDN to stream multiple copies of a video at multiple geographical sites (stores/serves)
• These are deployed to servers that are "deep" within access networks.
• Servers are deployed to countries across the planet.
• Examples include Akamai and Limelight

CDN

• Stores copies of content at CDN nodes
• Subscribers request content and can directed to it
• Can change copy it its congested

Netflix

• Relies on Amazon cloud and its own CDN to deliver content

Socket Programming

• This builds client and server applications to communicate through sockets
• Sockets acts as doors between applications

UDP Socket Programming

• Has no defined handshakes

UDP Server

• The application viewpoint is that the transfer will be delivered by UDP
• Is not reliable

Client/Server Socket Interaction: UDP transfer

• Requires a socket connection
• Must send a destination IP to send its information

TCP

• Creates a new socket for each user/ client upon contacting it and sets up a pipe to talk on
• Sets up a viewpoint for application

Client/Server Socket Interaction: TCP transfer

• TCP connections require host ID
• Connections use code commands like: connect, send, read and close

This chapter explores the conceptual and implementational aspects of the application layer. Topics covered include transport-layer service models and client/server/peer-to-peer paradigms. It also covers application programs that run in end systems.