Computer Communication: An example

1
Computer Communication An example

• What happens when I click on
• http//www.cs.fsu.edu/xyuan/cen5515?

2

• Step 1 Netscape figures out what to do with the
  URL http//www.cs.fsu.edu/xyuan/cen5515
• Three components in the URL
• Where? www.cs.fsu.edu
• What? /xyuan/cen5515
• How? Through the HTTP protocol.
• Talk to the http daemon using the HTTP protocol
  on www.cs.fsu.edu to get file /xyuan/cen5515.
• Netscape finds out what is the network address of
  www.cs.fsu.edu.

3

• finding out the network address of
  www.cs.fsu.edu
• netscape goes to a domain name server (DNS) to
  find out the IP address of www.cs.fsu.edu
• host gethostbyname(www.cs.fsu.edu)
• DNS server in our department
• 128.186.121.178
• How does netscape know the address of the DNS
  server?
• Hard-coded at system (network) configuration.
  E.g. /etc/resolv.conf in Linux systems.

4

• Step 1 Get the IP address from the DNS server.
• Call its UDP protocol entity to talk to
  128.186.120.178 port 53
• UDP protocol calls IP to send a datagram to
  128.186.120.178.
• Turns out that 128.186.120.178 and 128.186.120.2
  (diablo) are on the same Ethernet domain, can
  send directly via the Ethernet.
• Needs to find out the Ethernet address of
  128.186.120.178.
• uses ARP protocol, sends an ARP packet over the
  network
• What is the address of 128.186.121.10?
• result 080020FCEF6D.
• IP asks Ethernet to send an Ethernet frame to
  080020FCEF6D.
• Ethernet on 128.186.120.178 receives an Ethernet
  frame, turns out to be an IP packet, pass it to
  the IP module.
• IP module finds out that it is a UDP packet and
  passes it to the UDP module.
• UDP realizes that a process is listening to port
  53, notifies the process.

5

• Step 1 How to get the IP address from the DNS
  server (continue)?
• Netscape calls the UDP protocol entity to send a
  message to 128.186.121.10/53
• The UDP message to the DNS server is What is the
  IP address of www.cs.fsu.edu?
• The DNS server sends a message back
  128.186.120.244

6

• Step 2 Set up a TCP connection to the remote
  (128.186.120.244) http daemon.
• Calls TCP entity to set up a connection to
  128.186.120.244/80
• TCP protocol calls IP to send a datagram to
  128.186.120.244
• turns out that 128.186.120.244 (www.cs.fsu.edu)
  and this machine are directly connected.
• Find the Ethernet address of 128.186.120.244
  using arp
• ...
• www.cs.fsu.edu receives a packet.

7

• Step 3 Talk to the http daemon on www.cs.fsu.edu
  using the http protocol.
• Use TCP to send strings (following the HTTP
  protocol) get /xyuan/cen5515 HTTP/1.1\nHost
  diablo.cs.fsu.edu\n\n
• TCP entity calls IP to send a datagram
• ..
• www.cs.fsu.edu responses with the content of
  /xyuan/cen5515
• Step 4 Netscape displays the file in the window.

8

• The example was simplified.

DNS
Netscape (http client)
http server
TCP
TCP/UDP
UDP
IP
IP
IP
Ethernet
Ethernet
Ethernet
9

• What are the problems we need to solve?
• Naming, addressing
• fragmentation/reassembly
• multiplexing/demultiplexing
• routing
• resolve contention
• Speed mismatch between sender/receiver
• error control
• ...