Protocols involved in requesting a web page

Scenario: 

user (68.85.2.101) connects to School network’s Ethernet switch and downloads a webpage from Google;

Ethernet switch is connected to School’s router (68.85.2.1), within which DHCP server is running;

School’s router is connected to ISP (Comcast.net), which provides DNS service for the school;

DHCP – for client to obtain IP address, only the last 2 DHCP steps of the 4 are necessary.

user client (00:16:D3:23:68:8A)

  -> creates a DHCP request message

  -> puts the DHCP message within a UDP segment

  -> puts the UDP segment within an IP datagram with destination IP address 255.255.255.255 (broadcast) and source IP address 0.0.0.0

  -> put the IP datagram within an Ethernet frame with destination MAC addresses FF:FF:FF:FF:FF:FF (broadcast);

Ethernet switch

  -> receives the Ethernet frame and broadcast it;

gateway router (00:22:68:45:1F:1B)

  -> receives the Ethernet frame, extracts the IP datagram

  -> demultiplexes the datagram’s payload to UDP

  -> extracts the DHCP request message to DHCP server;

  -> DHCP server allocates IP address (68.85.2.101) to user client, creates a DHCP ACK message that contains the allocated 1) IP address, 2) IP address of DNS server, 3) IP address of default gateway router (68.85.2.1), and network mask (68.80.2.0/24);

  -> puts the DHCP message within a UDP segment;

  -> puts the UDP segment within an IP datagram;

  -> puts the IP datagram within an Ethernet frame addressed to MAC address of the user;

Ethernet switch

  -> forwards the Ethernet frame to user (it knows how to forward because it is self-learning and previously received frame from user);

user client

  -> receives the Ethernet frame, extracts the IP datagram;

  -> extracts the UDP segment;

  -> extracts the DHCP ACK message;

  -> records the allocated IP address and the IP address of DNS server; Installs the address of default gateway into its IP forwarding table (so all datagrams with destination outside of its subnet will be sent to the gateway);

DNS and ARP

user client

Web browser creates a TCP socket that will be used to send HTTP request. To create the socket, need to know IP address of the web URI:

  -> creates a DNS query message;

  -> puts the DNS message within a UDP segment;

  -> puts the UDP segment within an IP datagram, addressed to IP address of the DNS server (68.87.71.226);

  -> puts the IP datagram within an Ethernet frame;

To send the Ethernet frame to gateway, need to know MAC address of the school’s gateway router:

  -> creates an ARP query message addressed to default gateway’s IP address (68.85.2.1);

  -> puts the ARP message within an Ethernet frame, with broadcast destination address;

Ethernet switch

  -> receives the Ethernet frame, broadcasts it;

gateway router

  -> receives the Ethernet frame, finds the target IP address matches the IP address of its interface, thus creates an ARP reply indicating its MAC address 00:22:6B:45:1F:1B corresponds to IP address 68.85.2.1;

  -> puts the ARP message within an Ethernet frame, sends it to user client.

user client

  -> receives the ARP reply message, extracts the MAC address of the gateway router, thus can address the Ethernet frame that contains the DNS query;

  -> sends the Ethernet frame to switch, … .

Intra-Domain Routing

gateway router

  -> receives the Ethernet frame, extracts the IP datagram;

  -> looks up the forwarding table to determines where to forward the datagram;

  -> puts the IP datagram within a link-layer frame appropriate for the link connecting the router to the target router;

router in another (Camcast’s) network

  -> receives the frame, extracts the IP datagram;

  -> looks up the forwarding table to determine where to forward the datagram. The forwarding table has been filled by intra-domain protocols (e.g. RIP, OSPF, IS-IS) and inter-domain protocol BGP.

DNS server

  -> receives …, extracts the DNS query message;

  -> looks up the DNS database, finds the corresponding DNS resource record;

  -> creates a DNS reply message, puts the DNS reply message within a UDP segment, puts the UDP segment within an IP datagram addressed to user;

… the datagram is forwarded back to user, user then can contact the target server.

Web Client-Server Interaction

user client

  -> creates a TCP socket, perform a three-way handshake with the TCP in server side:

   --> creates a TCP SYN segment (addressed to port 80 for HTTP);

   --> puts the TCP segment within an IP datagram (addressed to server’s IP address);

   --> puts the IP datagram within a frame (addressed to MAC address of the gateway router);

… the datagram is forwarded to server.

Server (www.google.com)

   --> receives …, extracts the TCP SYN message;

   --> demultiplexes to welcome socket (associated with port 80), thus creates a connection socket;

   --> creates a TCP SYNACK segment;

   --> puts the TCP segment within a datagram (addressed to user’s IP address);

   --> puts the datagram within a link-layer frame (addressed to its first-hop router);

… The datagram is forwarded to user (ethernet card in his PC).

user client

   --> receives …, demultiplexes to TCP socket, TCP socket thus enters the connected state;

  -> browser creates HTTP GET message containing the URL to be fetched;

  -> writes the HTTP GET message into socket, thus puts the message within a TCP segment;

  -> puts the TCP segment within a datagram addressed to server.

… The datagram is forwarded to server.

Server (www.google.com)

  -> receives …, reads the HTTP GET message from TCP socket;

  -> creates an HTTP response message, puts the requested Web page content in the body of the message;

  -> sends the HTTP response message into TCP socket;

  …

… the datagram is forwarded to user;

user client

  -> receives …, Web browser reads the HTTP response from socket;

  -> Web browser extracts the html from HTTP response, the Web page is displayed.

 

 

Possible additional protocols omitted:

· NAT (running in gateway router);

· wireless access (to the network);

· security protocols (for accessing the network or encrypting segments/datagrams) 

· network management protocols;

· Web caching, DNS hierarchy (possibly encountered in public Internet).