IB Computer Science Topic A2.1.4: Network Protocols Used for Transport and Application Notes

Transport and Application Protocols: Two Layers, Five Protocols

1. A protocol is an agreed set of rules for formatting and exchanging data, and the syllabus names five of them across two layers of the TCP/IP model.
2. TCP and UDP are transport layer protocols: they get data between the correct programs on two hosts and decide how much is guaranteed on the way.
3. HTTP, HTTPS and DHCP are application layer protocols: they define what the data means and what each side should do with it.
4. Every application protocol runs on top of a transport protocol, so the two layers are always working together rather than as alternatives.

TCP: Reliability Bought With Overhead

How TCP Guarantees Delivery

1. Connection establishment happens before any data moves, using a three way handshake.
   1. SYN: the sender asks to open a connection and states its starting sequence number.

1. SYN-ACK: the receiver acknowledges that request and states its own starting sequence number.
2. ACK: the sender acknowledges the receiver's number, and the connection is open in both directions.

1. Reliable data transfer is enforced after every segment, not just at the end.
   1. The receiver returns an acknowledgement for data it received intact.
   2. If an acknowledgement does not arrive within a timeout, the sender retransmits that segment.
   3. Sequence numbers let the receiver put segments back in the right order even when they arrive out of order.
2. Flow control uses a sliding window so the sender never sends faster than the receiver can accept, which prevents data being dropped at the far end.
3. Connection termination closes the session in an orderly way with a four way exchange of FIN and ACK, so neither side stops while data is still in flight.

UDP: Speed Bought By Giving Up Guarantees

What UDP Leaves Out

1. No connection is established, so the first datagram carries real data instead of a handshake.
2. No acknowledgements and no retransmission, so a lost datagram is simply gone and the sender never learns about it.
3. No ordering, so datagrams may arrive in a different order from the one they were sent in.
4. A much smaller header than TCP, which leaves more of each transmission available for the data itself.

Where the Trade Suits the Application

1. Live audio and video cannot use a frame that arrives late, so retransmitting it would waste time and still produce a stutter.
2. Online gaming needs the newest position of a player, and an old position resent is worse than no position at all.
3. Multicasting sends one datagram to many recipients at once, which TCP cannot do because it manages one connection per pair of hosts.

HTTP: The Request and Response Protocol of the Web

How an HTTP Exchange Works

1. The client, usually a browser, sends an HTTP request naming the resource it wants and the method to apply to it.
2. The server returns an HTTP response containing either the resource or a status code explaining why it cannot be supplied.
3. HTTP is stateless: the server keeps no memory of previous requests, so every request must carry everything needed to serve it.
4. Statelessness is what lets one server handle enormous numbers of clients, and it is also why sites need cookies or tokens to remember that a user is logged in.

The Common HTTP Methods

1. GET retrieves a resource from the server and should not change anything.
2. POST submits new data to the server, such as the contents of a form.
3. PUT replaces or updates an existing resource.
4. DELETE removes a resource from the server.

HTTPS: The Same Protocol Inside an Encrypted Channel

1. HTTPS is HTTP carried inside a channel secured by Transport Layer Security (TLS), the successor to Secure Sockets Layer (SSL).
2. The requests, responses and methods are identical, so what changes is the protection around them rather than the protocol itself.
3. Encryption makes the traffic unreadable to anyone intercepting it between client and server.
4. Authentication uses a digital certificate to prove the server is who it claims to be, which is what stops a fake site impersonating a real one.
5. Integrity checking detects any alteration of the data in transit, so a modified response is rejected rather than displayed.

DHCP: Addresses Handed Out Automatically

The Four Step DHCP Exchange

1. Discover: a device joining the network broadcasts a request, because it has no address yet and does not know where the server is.
2. Offer: a DHCP server replies offering an address from its pool.
3. Request: the device broadcasts that it accepts that particular offer, which also tells any other servers their offers were declined.
4. Acknowledge: the server confirms the assignment and starts the lease.

What DHCP Supplies Beyond the Address

1. A lease duration sets how long the address is held, after which it is renewed or returned to the pool for another device.
2. A subnet mask tells the device which part of the address identifies its own network.
3. A default gateway address tells it where to send traffic bound for other networks.
4. One or more DNS server addresses let it turn domain names into IP addresses.

Comparing TCP and UDP

1. Both protocols sit at the transport layer and both use port numbers to deliver data to the right program on a host.
2. Everything else about them follows from one decision: whether the protocol takes responsibility for delivery or leaves it to the application.