Lecture 3

• Lecture 3
  • Video
  • Slides
  • Agenda
  • Background
  • Network Functionalities : Implementation
  • Secure Transmission of Data
  • NExt Lecture

Video

link

Slides

link

Agenda

• End to End Arguement in System Design paper

Background

• Network functions
  • Reliable and sequenced delivery
  • addressing and routing
  • security
  • ethernet collision detection
  • multicast
  • real-time guarantees
  • error recovery
  • duplicate message supression
  • congestion and flow control
• How to decompose the functions?
  • Monolithic approach vs
    • implement as single module
    • maintenance and mgmt will be difficult
  • Modular approach
    • Internet me different layers hai na

Network Functionalities : Implementation

• Key behind layered architecture
  • How this layered architecture works
  • 5 layers
    • Application
    • Transport
    • Network
    • Data Link
    • Physical
  • Communication happens between adjacent layers only
  • Communication happens with well defined interfaces
    • changing implementation of some functionality ( changing internals of some layer) should not result to change in interface change
    • eg: Routing algorithm might be changed but it should not affect interface of any layer
• Where to place a functionality?
  • Inside the network (in switching elements)
    • Routers
  • At the edges(i.e. hosts)
    • Hosts (higher layers of system - app and transport)
  • As a joint venture
    • some part of functionality at edges
    • some inside network
  • Redundantly at both places
• What is the right approach?
  • take the example of reliable file transfer
  • 
  • What are the requirements?
    • initially file is at A, at the end it is at B also
    • reliable => file at B is same as file at A
  • What are basic components
    • Os se system call karke file read kari = FILE System program in OS
    • then appln me File Transfer program
    • appln will ask os to transfer that contents
    • host b = client reveives it
    • os transfers to appln
    • appln stores in disk with help of os
  • Threats involved in File Transfer
    • File read may contain incorrect data due to h/w faults in the disk storage system
    • the file system, file transfer program, data communiation system might make mistake in buffering and copying the data of the file, either at A or B (writing correct program can avoid this!!)
    • Processor or its memory might have transient erorr while doing buffering and copying
    • communication system might drop/flip some bits in a packet or drop a packet
    • Either of the host may ceash at any point of time
    • When I want to make my program reliable, I need to consider all these threats
  • How to cope up with the threats
    • Solution1
      • make each step reliable, and then concatenate them
        • by duplicate copies, timeout entry, redundancy
        • when u read data from disk, check checksum’s validity
    • Solution2
      • do not verify each individual step
  • E2E : File transfer
    • Even if network guaranteed reliable delivery
      • need to provide end-to-end checks
        • buffering errors, h/w fault in disk storage system, network card may malfunction
      • Ther eceiver has to do the check anyway!
    • Reliability can only be implemented at application layer; no need for reliability from lower layers
    • Arguement
      • There are functions that can only be correctly implementerd at end hosts, and should not be completely implemented somewhere else
• Questions
  • Does FTP look like E2E file transfer
    • no
    • even though it uses TCP
    • but TCP is OS to Os communication
    • but transfer from disk to os at source, and from os to disk in destination can cause buffering errors and all
    • so, here we need to make sure ki jo A me tha and jo B me tha should be same
    • so destn me likhne ke bad it should read data again and calculate checksum, and send to A, and A checks if that checksum matches with the checksum which was already written to it, then send yes, DONE

Secure Transmission of Data

• If data transmission system performs encryption and decryption
• Responsibilities of the system
  • key management
• My data transmission happens between data communication systems
• data is in decrypted format when in appln of destn, and disk of source
• but other applications can steal the data
• if the process is happening between application to application, then it is fine
• So,
  • the lower level implementation of functionality does not give completeness, I will need to implement it end-to-end
• How to decide the end points?
  • vary from app to app
    • eg real time voice transmission vs non-real time voice transmission
      • real time me I don’t need too much reliability, person will say ki repeat if he does not understands it but non-real time me I require it ( and end point is disk )
    • RISC chips vs CISC chips
    • feature implementation at OS level vs application level

NExt Lecture

• Tussle in Cyber space : Defining tomorrow’s internet
  • paper link