[网络协议] Computer Network Questions

TCP is a bi-directional communication protocol, which means either end ought to be able to send data reliably.

Both parties need to establish an Initial Sequence Number (ISN), and both parties need to acknowledge the

other's ISN. Thus a two-way handshake is required in each direction as follows:

1) Alice picks an ISN and SYNchronises it with Bob.

2) Bob ACKnowledges the ISN.

3) Bob picks an ISN and SYNchronises it with Alice.

4) Alice ACKnowledges the ISN.

Steps 2 and 3 can be combined in an SYN-ACK segment, which reduces this to a 3-way handshake.

A 2-way handshake would only allow one party to establish an ISN, and the other party to acknowledge it.

Which means only one party can send data.

The process to browser a webpage by your laptop. (Already configured with IP address of local DNS server and IP address of the first-hop router)

1. Since the ARP cache on your computer is initially empty, your computer will use ARP protocol to get the MAC address of the local DNS server and first-hop router.

2. Then query the local DNS server to get the IP address of the Web page you would like to visit.

3. Once your computer has the IP address of the Web page, it will establish a TCP connection with the Web server by a 3-way handshake and send out an HTTP request via the first-hop router (assume the Web server does not reside inside the local network). The HTTP request message will be encapsulated in a TCP segment, and then encapsulated in an IP datagram and then an Ethernet frame.

4. Your computer sends the Ethernet frame destined to the first-hop router. Once the router receives the frame, it passes the encapsulated IP datagram up to the network layer, checks its routing table, and then sends the datagram encapsulated in an Ethernet frame to the next-hop router corresponding to?the routing algorithm. Then the IP datagram will be routed through the Internet until it reach the Web server.

5. The server of the Web page will send back the Web page to your computer via HTTP response messages. Those messages will be encapsulated in TCP segments and then further into IP datagrams and link-layer frames. Those IP datagrams follow the routes made by the routing algorithm and finally reach your first-hop router, and then the router will forward them to your computer by encapsulating them into Ethernet frames.

[网络协议]Computer Network Questions