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

traceroute to brown

traceroute to MIT web server

• 20 hops, less than 90 ms