1.4 A Day in the Life of a Packet

  • Application: stream of data

  • Transport: segments of data

  • Network: packets of data

TCP Byte Stream

  • 3-way handshake

    • Client sends a synchronize message to the server ("SYN")

    • Server responds synchronize message that also acknowledges the client's synchronize, or ("SYN/ACK")

    • Client responds by acknowledging the server's synchronize ("ACK")

  • Network layer: responsible for delivering packets to computers

  • Transport layer: responsible for delivering data to applications

  • To open a TCP stream, needs

    • IP address: address that network layer uses to deliver packets to the computer

    • TCP port: tell the computer's software which application to deliver data to

      • Web server: TCP port 80

Inside the Stream

  • Hop: link connecting two routers

  • A router can have many links connecting it. As packet arrives, a router decides which of its links to send it out on.

  • Routers have IP addresses

    • Can deliver to its own software instead of forwarding it

Inside each hop

  • "Best": most specific match (longest prefix match)

  • Default: least-specific pattern (matches every IP addresses)

    • Useful in edge networks

      • E.x. stanford example, if the destination address isn't stanford, then should send it out to larger internet

Under the Hood

  • 3-way handshakes

  • More packets for HTTP requests and response

  • tracerout:

    • Shows the hop that packet takes

    • traceroute -w 1 www.cs.brown.edu

  • 20 hops, less than 90 ms

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