IB Computer Science Topic A2.1.5: Function of the TCP/IP Model Notes

The TCP/IP Model: Four Layers, Each Solving One Delivery Problem

1. The TCP/IP model splits network communication into four layers, each with a job the layer above it can then take for granted.
2. The layers from the top down are application, transport, internet and network interface.
3. Each layer adds its own header as data moves down the sending host, and each removes its own header as data moves up the receiving host.
4. Splitting the work this way means a change in one layer leaves the others alone: moving from copper to fibre changes only the network interface layer, and every application above keeps working unaltered.

Application Layer: Where the Data Still Has Meaning

1. This layer holds the protocols that programs use to exchange data, and it is the only layer that understands what the data is for.
2. HTTP and HTTPS carry web pages and web application traffic.
3. SMTP carries outgoing email between mail servers.
4. DNS turns a domain name into the IP address needed by the layer below.
5. DHCP hands out addresses and configuration to devices joining a network.

Transport Layer: Getting Data to the Right Program

1. This layer splits data into segments, labels each with a port number so the receiving host knows which program it belongs to, and decides how much delivery is guaranteed.
2. TCP provides ordered, acknowledged and error checked delivery, retransmitting anything that goes missing.
3. UDP sends datagrams without acknowledgement, which is faster to start and leaves reliability to the application.
4. The internet layer below has no idea which program the data belongs to, which is why port numbers have to be added here.

Internet Layer: Getting Packets to the Right Host

1. This layer wraps each segment in a packet carrying the source and destination IP addresses.
2. It performs routing, choosing which network to forward the packet towards at each step of the journey.
3. It performs fragmentation when a packet is too large for the next link, splitting it and letting the destination reassemble it.
4. The IP header is the one part of the message that travels unchanged from the source host to the destination host.

Network Interface Layer: Getting Frames Across One Physical Hop

1. This layer wraps each packet in a frame and turns it into the signals the medium actually carries, whether that is voltage on copper, light on fibre or radio for Wi-Fi.
2. Framing adds start and end markers so the receiver can tell where one frame stops and the next begins.
3. MAC addressing identifies the specific piece of hardware the frame is going to on this one link.
4. A frame only ever survives one hop, so it is rebuilt at every device along the route.

Encapsulation: Each Layer Wraps the One Above It

1. Encapsulation is the process of adding a header at each layer on the way down, and decapsulation is removing it on the way up.
2. Application layer: the program produces the data and adds information about what it is, such as an HTTP request line.
3. Transport layer: the data is split into segments, each given port numbers, a sequence number and error checking information.
4. Internet layer: each segment is wrapped in a packet carrying source and destination IP addresses.
5. Network interface layer: each packet is wrapped in a frame carrying MAC addresses, then sent as signals.
6. The receiving host reverses the process, and each layer reads only the header its own counterpart wrote.

Tracing a File from Computer A to Computer B

On Computer A: Down Through the Stack

1. The user starts the transfer in an application, and the application layer adds a header naming the protocol in use and the type of data being sent.
2. The transport layer takes that data and breaks it into segments.
   1. Each segment receives source and destination port numbers so the correct program is reached at the far end.
   2. Each segment receives a sequence number so segments can be reassembled correctly, in the same way packets are reassembled after packet switching.
   3. Each segment receives error checking information.
   4. If TCP is being used, a handshake with Computer B opens the connection before any segment is sent.
3. The internet layer wraps each segment in a packet with the source and destination IP addresses, and fragments it if the next link cannot carry a packet that size.
4. The network interface layer wraps each packet in a frame with MAC addresses and start and end markers, then transmits it over Ethernet, Wi-Fi or fibre.

Between the Two Computers: Routers Rebuild the Frame at Every Hop

1. Routers read the destination IP address and forward the packet towards Computer B.
2. At each router the incoming frame is discarded and a new one is built for the next link, because a frame is only valid for one hop.
3. The IP header is not rewritten, which is why the packet still knows its final destination after crossing many different networks.

On Computer B: Up Through the Stack

1. The network interface layer checks the MAC address, strips the frame header and passes the packet up.
2. The internet layer checks the destination IP address, reassembles any fragments, strips the IP header and passes the segment up.
3. The transport layer puts the segments back in order using their sequence numbers, checks for errors, requests retransmission of anything missing, and uses the port number to hand the data to the right program.
4. The application layer receives the reassembled data and presents the completed file to the user.
5. The TCP connection is then closed with a four way exchange, releasing the resources both hosts had reserved for it.

What the Layers Together Deliver: Reliable Transmission

1. No single layer makes transmission reliable; reliability is the combined result of several mechanisms sitting at different layers.
2. Error checking at the transport and network interface layers uses checksums to detect data corrupted in transit.
3. Acknowledgements from the receiving transport layer tell the sender which segments arrived intact.
4. Sequencing lets the receiver rebuild the original order even when packets take different routes and arrive out of order.
5. Retransmission replaces any segment that was lost or failed its check, so a dropped packet costs delay rather than data.
6. Flow control stops a fast sender from overwhelming a slow receiver, which prevents loss before it happens.
7. Routing at the internet layer keeps traffic moving by choosing a different path when a link fails.