TCP/IP Architecture and Application Layers

• The lecture covers TCP/IP basics, initial application scenarios, architecture and different layers

Driving Application

• File transfer (FTP) was the first application
• FTP transfers data from a server to a client and vice versa
• Communication occurs between two endpoints, a source, and a destination
• Communication endpoints are identified by an IP address
• The client-server model is typical

Evolution of Applications

• More applications emerged including email, messaging, and real-time applications
• Mobile applications introduced challenges, including best-effort delivery with no guarantees

TCP/IP Architecture - Why the Narrow Waist

• The TCP/IP architecture includes the Application, Transport, Network, Data link and Physical layers
• IP works over anything and anything works over IP

Application Layer

• Applications are aware of URLs, essentially names like www.facebook.com
• Application data resides in the payload of an HTTP/HTTPS message
• Applications understand URLs; the network understands IP addresses
• Translation of URLs to IP addresses occurs through the Domain Name System (DNS)

Creating a Network App

• Programs run on different end systems and communicate over the network, like web server software communicating with browser software
• Network-core devices don't run user applications
• Applications on end systems allow for rapid app development and propagation
• Possible application structures are client-server and peer-to-peer (P2P)

Client and Server Architecture

• The server is always-on, has a permanent IP address, and utilizes data centers for scaling
• Clients communicate with the server, may be intermittently connected, may have dynamic IP addresses and don't communicate directly

Peer-to-Peer Architecture

• P2P does not use an always-on server
• Arbitrary end systems communicate directly which allows new peers to bring new service capacity
• Peers request service from other peers & also provide service in return to other peers
• Peers are intermittently connected and change IP addresses, which leads to complex management

Processes Communicating

• A process is a program running within a host
• Processes within the same host communicate using inter-process communication defined by the OS
• Processes in different hosts communicate by exchanging messages
• Client processes initiate communication and server processes wait to be contacted
• Applications with P2P architectures have client processes and server processes

Sockets

• A process sends/receives messages to/from its socket
• A socket is analogous to a door: the sending process shoves a message out the door
• The process relies on the transport infrastructure on the other side of the door to deliver a message to the socket at the receiving process

Addressing Processes

• To receive messages, a process must have an identifier
• Host devices have a unique 32-bit IP address
• An identifier includes both IP address and port numbers associated with the process on the host
• HTTP server ports are 80 and mail server ports are 25
• To send an HTTP message to the umsl.edu web server, the IP address is 134.124.1.234 and the port number is 80

App-Layer Protocol Definitions

• Specify the types of messages exchanged, such as request and response
• Define message syntax, including what fields are in messages and how they are delineated
• Dictate message semantics, meaning the information in the fields

Open Protocols

• Open protocols are defined in RFCs and allow for interoperability, for example, HTTP and SMTP

Proprietary Protocols

• Skype is an example

Transport Layer

• Focuses on providing reliable data transfer

Transport Service

• Data integrity means some apps require 100% reliable data transfer
• Other apps can tolerate some loss
• Some apps require low delay, like Internet telephony or interactive games
• Some apps require a minimum throughput to be effective
• Security including encryption and data integrity are other transport service requirements

Transport requirements for applications:

• File transfer and e-mail require no data loss
• Real-time audio/video are loss-tolerant
• Interactive games need few kbps and are time sensitive
• Both file transfer and email require elastic throughput

Internet Transport Protocol Services: TCP

• TCP provides reliable transport between sending and receiving processes
• TCP uses flow control to prevent the sender overwhelming the receiver
• Congestion control throttles the sender when the network is overloaded
• TCP does not provide timing, minimum throughput guarantees, or security
• TCP is connection-oriented and requires setup between client and server processes

Internet Transport Protocol Services: UDP

• UDP provides unreliable data transfer between sending and receiving processes
• UDP does not provide reliability, flow control, congestion control, timing, throughput guarantees, or security
• HTTP/HTTPS requests/responses are encapsulated into TCP segments or UDP datagrams
• TCP delivers reliably by establishing a connection between 2 hosts and doing error checks
• UDP delivers with a best-effort and is connectionless

TCP Connection Establishment

• TCP uses a 3-way handshake

TCP Reliability

• A sequence number identifies the latest byte sent by the sender
• The receiver acknowledges the latest byte of data received

Packet Losses

• TCP uses timeout and re-sends the packets

Flow and Congestion Control

• Flow control avoids flooding the receiver
• Congestion control avoids flooding the network

TCP Congestion

• Network congestion should be avoided to promote stability

Van Jacobson Congestion Control

• Network congestion can be modeled as a control problem with an equation that helps the system reach stability through a feedback loop
• For every successful transmission increase the window by a constant number
• On loss, decrease the transmission window by a constant
• Probe for usable bandwidth until the capacity is reached

UDP

• UDP offers connection-less and unreliable data delivery
• Why use UDP? There is no delay to establish a connection, it offers finer control, and has minimal header overhead, only 8 bytes

Applications that use UDP

• Multimedia (real-time) apps use UDP as re-transmitting packets is not required
• Naive query-based protocols like DNS, NTP, and DHCP use UDP and allows those applications to retransmit if necessary

Network Layer - The Network Core

• Packet switching and circuit switching are methods of data transfer through the network

Network Core: Circuit Switching

• Has end-end resources reserved for each "call"
• The resources are dedicated
• A call setup is needed

Network Core: Packet Switching

• Each end-end data stream is divided into packets
• Users share network resources and this requires resource contention

Packet Switching vs. Circuit Switching

• Packets allows more users to use a network
• Packet switching suffers from packet delay and loss

The Internet

• Stateless, best-effort IP puts the Internet together
• The communication model is between 2 hosts. Source and destination IP addresses define them

IPV4 vs IPV6

• Specific IPV4 address blocks can cost millions, thus IPV6 was developed
• IPV6 aims to improve security, performance, multi-cast, and network configuration

Why has IPV6 not taken off then

• ISPS have invested so much money in IPV4
• There not enough good ways for the to interoperate with IPv4
• Network Address Translation (NAT) does the job for now

Data Link Layer

• There are many different protocols

Physical Layer

• Important, but not a focus.

Security

• The TCP/IP architecture does not provide security

TCP/IP security

• Accountability of resources was the last design goal of ARPANET and people trusted each other at the time
• The Internet has been successful due to so many new and innovative apps
• This is why networking people need to fulfil application requirements
• The recent trend is Ubiquitous Augmented Reality or Virtual Reality needing ultra-low user perceived latency