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Basics of networking, TCP & UDP

  • In distributed/network applications, you can have reliable vs. unreliable
    • Reliable is slower, unreliable faster (think TCP vs UDP)
  • Protocols manage sending/receiving of messages
    • Human
      • Permission hierarchies
      • Access cards
      • Standardization
    • Network
      • Non-human, automated
      • Consistent
  • Protocols are standardized
    • IETF: Internet Engineering Task Force (also: W3C, IEEE, ITU, etc.)
      • Start with an internet draft
      • RFC: request for comments
        • Can be used in your network

Communication models:

  • Client-Server
    • Servers provide, clients requests
    • Centralized
  • P2P
    • No dedicated servers, each host can request/provide
    • Decentralized
  • Overlay networks:
    • Virtual networks running atop physical infrastructure

Data Transfer types

  • Reliable: TCP (RFC 793)
    • Byte stream between the two, checks each message successful
    • Initiates a handshake in the beginning standardizing communications
    • HTTP, FTP, SSH, etc.
  • Unreliable: UDP (RFC 768)
    • Packet transfer
    • Essentially throws the message off, regardless of validity/success
    • Packets can be in wrong order, not arrive, invalid, etc.
    • Media transfer, audio/video streaming, etc.

TCP Traffic issue remediation

  • Flow control (prevention)
    • Checking for buffer space consistently (capacity)
  • Congestion control (reaction)
    • Mechanism to react to current happenings
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"Be conservative in what you do, be liberal in what you accept from others"
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UDP

  • Best effort, connectionless (no handshake), and unreliable
  • Doesn't implement flow control, congestion control
  • Low overhead/small (no header, handshake, etc.)
  • For time-sensitive applications

Multi-client data transfer

  • Circuit switching: send off an entire message at once, then the next
  • Packet-switching: Split a message into packets, weave together messages

Packet switching

  • Data is divided into packets, which share network resources
    • Each packet uses full link bandwidth
    • Resources used as needed
  • Can create waiting queues if sending rate > bandwidth
  • Can create congestion, resource demand can be greater than available resources
  • Good for bursty data
  • Simpler
  • Excessive congestion: packet delay/loss
    • Protocols needed for reliability and congestion control
  • Relies upon statistical multiplexing & store and forward

Statistical Multiplexing

  • Multiple data streams share network resources
  • BW shared on demand in ordered to increase utilization (avoid silent periods)

Store and Forward

  • The whole packet must arrive at router before transmitted to next link
Example:
R is link txm rate, link capacity, or bandwidth
L is length of packet

R = 1.5 Mb
L = 7.5 Mb

Txm delay:
one-hop pkt txm delay L/R = 7.5/1.5 = 5 sec

Essentially, it takes L/R seconds to transmit L-bit packet into link at R bps