OSI and TCP/IP models — exam-style exercises with layers, PDUs, switches and routers (and the “network layer” that cost me half a mark)

Answer: A protocol: the same rules about the format, meaning and timing of the messages

A protocol is the set of rules that define how two entities communicate: the format of the messages, their meaning and their timing. As long as both sides follow the same rules, brand and operating system do not matter.

Answer: The layered organisation of several protocols: what each layer does, how they relate and what information each one adds to the message

The key is three things: layers, what each one does and what each one adds (its header). TCP/IP and OSI are two protocol architectures.

The TCP header being 20 bytes long and starting with the ports is a rule of syntax. A header code meaning ‘this chunk arrived damaged, send it again’ is a rule of semantics. The server not receiving faster than it can process, and the chunks going in order, is a rule of timing.

Syntax = data format and signal levels (what the message looks like). Semantics = control information and error handling (what it means). Timing = speed matching and sequencing (when and in what order).

Answer: The browser you upload the assignment with · The email program · A file transfer program

Entities = user applications (email, terminals, browser…). Systems = the computer, terminal or sensor they run on. Careful, this one subtracts: ticking the laptop or the sensor costs you.

• T — The transport layer is independent of the application using it and of the network the data travels over.
• F — The network access layer works the same on any kind of network, whether a LAN or a packet-switched network.
• T — The application layer lets programs use the network without knowing how it works underneath.
• F — The application layer is the one that delivers each piece of data to the right application using ports.

The network access layer depends on the type of network (that is why it changes when you switch from Ethernet to Wi-Fi). Delivering data to the right application with ports (SAP) is the job of transport, not application.

• Application → Serves programs such as email or file transfer
• Transport → End-to-end communication: takes the data to the right application and splits it up
• Network access → Data exchange between the computer and the network it is attached to

A trick to keep them apart: application = the programs; transport = which program on the other side receives it; network access = how I get out of my machine onto my network.

Answer: Network access

That is the point of layering: a change in the medium only touches the layer that talks to the network. Transport and application stay the same.

To know which service it is for inside the destination host, it carries the destination port (or SAP); to put the chunks back together, the sequence number; and to find out whether something got damaged on the way, an error detection code.

The three items in the transport header: destination port/SAP, sequence number and error detection code. The MAC and the IP belong to lower layers.

Answer: The address of the destination computer · A request for certain resources or services, such as a priority

Transport answers which service is it for? (port); network access answers which host do I deliver it to? (host address) and requests resources. This one subtracts: ticking the port costs you.

Correct order:

1. The application hands over the data
2. Transport adds its header and forms the segment
3. Internet adds the IP header and forms the datagram
4. Network access adds its header and forms the frame
5. The physical layer sends the frame as bits

When sending you go top to bottom and each layer wraps what the layer above gives it. When receiving it is the reverse: each layer strips its header and passes the rest up.

• Segment → Transport
• Datagram → Internet
• Frame → Network access
• Bits → Physical

Segment (transport, TCP), datagram or packet (Internet, IP), frame (network access) and bits (physical). Session does not exist in TCP/IP: it is an OSI layer.

Add from the inside out: 900 + 20 = 920 (segment); 920 + 20 = 940 (datagram); 940 + 18 = 958 (frame). In bits, 958 × 8 = 7664.

Headers: 958 − 900 = 58 B, and 58 / 958 × 100 = 6.05 %. The data never changes: only the envelopes grow.

Answer: 5

Five: application, transport, Internet, network access and physical. Some books merge the bottom two and talk about four, but in the IC2 exam it is five.

Answer: 7

Seven: physical, data link, network, transport, session, presentation and application.

Correct order:

1. Application
2. Transport
3. Internet
4. Network access
5. Physical

Top to bottom: application → transport (host-to-host) → Internet → network access → physical.

• Application → Serves user applications such as HTTP or SMTP
• Transport → Reliable, in-order end-to-end delivery (TCP)
• Internet → Routes across several networks; adds the IP address
• Network access → Exchange with the local network; adds the MAC address
• Physical → Defines the medium, the nature of the signals and the speed

The filler is the trap: encrypting and keeping sessions are jobs of presentation and session, which exist in OSI but not as separate layers in TCP/IP.

Answer: From ARPANET, an experimental packet-switching network funded by DARPA

TCP/IP comes from ARPANET, funded by DARPA. There is no official TCP/IP model, but there is a functional one, and it is the one that took over the Internet.

• T — TCP is connection-oriented and delivers the data in order and without duplicates.
• F — UDP guarantees that everything sent arrives, just faster than TCP.
• T — TCP uses acknowledgements (ACK) to know what has arrived.
• T — UDP has less overhead and lower latency than TCP.

UDP does not guarantee delivery or order: that is why it is lighter. TCP acknowledges with ACK and reorders; in exchange, it adds more overhead and latency.

Answer: UDP

Real time = UDP. Submitting an assignment, a web page or an email = TCP, because not a single byte can be missing.

Correct order:

1. Physical
2. Data link
3. Network
4. Transport
5. Session
6. Presentation
7. Application

1 physical, 2 data link, 3 network, 4 transport, 5 session, 6 presentation, 7 application. The first four have a direct partner in TCP/IP.

• Physical → Transmits the raw bit stream over the network hardware
• Data link → Formats the data on the network and watches for format errors and flow
• Network → Manages the addresses and the passage of data between networks
• Session → Keeps connections open and controls sessions
• Presentation → Puts the data into a usable format; encryption happens here

If you hesitate between session and presentation: session = the conversation stays open; presentation = the data is understandable (format, encryption).

• T — It was developed by ISO (the International Organization for Standardization).
• F — Its seven-layer stack is what the Internet uses today as a real protocol.
• T — Layer 5 is the session layer.
• F — The presentation layer is the one that routes packets between networks.

OSI is a reference model: it is used to talk about layers, but as a real stack it never caught on; TCP/IP is the dominant one. Routing between networks is layer 3 (network).

Answer: Application (7) · Presentation (6) · Session (5)

The TCP/IP application layer does the job of the top three OSI layers: session, presentation and application. From transport down they match one to one.

Answer: Network access

The switch reads the frame’s MAC address: the network access layer in TCP/IP (equivalent to data link, layer 2, in OSI).

Answer: Internet

The router opens the frame, reads the datagram’s IP address and decides the next hop: the Internet layer in TCP/IP (network, layer 3, in OSI).

The switch forwards frames by looking at the MAC address and works at the data link layer. The router forwards datagrams by looking at the IP address and works at the network layer.

Switch: layer 2 (data link), frames, MAC. Router: layer 3 (network), datagrams, IP. The datagram lives in the OSI network layer (Internet in TCP/IP).

Answer: The source and destination MAC addresses

The MAC is for one hop (from one device to the next); the IP and the ports are for the whole trip. That is why the router sees the next hop and the switch only the address inside the LAN.

Answer: A switch joins the room’s computers (MAC) and a router connects the room to the campus (IP)

Switch = same network, MAC addresses, layer 2. Router = different networks, IP addresses, layer 3.

Answer: Decides where to send each packet based on the destination network · Looks for the next gateway (the next hop) · At home it usually also does NAT and DHCP

The router works up to the Internet/network layer: it routes, chooses the next hop and, at home, usually adds NAT and DHCP. This one subtracts: the ports and encryption options cost you.

Model answer:

TCP/IP is the protocol architecture the Internet runs on; it was born in ARPANET, the experimental packet-switching network funded by DARPA, and it has no official model, but it does have a functional five-layer one. The physical layer defines the medium, the signals and the speed; the network access layer handles the exchange between the host and its network and adds the MAC address; the Internet layer routes datagrams across different networks and adds the IP address, and it is implemented both in end hosts and in routers; the transport layer connects the two applications end to end through ports, with TCP, reliable and ordered, or UDP, lighter and without guarantees; and the application layer serves programs such as HTTP or SMTP.

OSI is the seven-layer reference model proposed by ISO. Both are organised in layers, and OSI physical, data link, network and transport map one to one onto TCP/IP physical, network access, Internet and transport. The main difference is at the top: OSI splits into session, presentation and application what TCP/IP handles in a single application layer. That is why the switch sits at OSI layer 2 (network access in TCP/IP) and the router at layer 3 (Internet). Finally, OSI remained a theoretical model for explaining networks, whereas TCP/IP became the real standard.

If you are short of time, prioritise the five layers with their function and the mapping onto OSI: that is what scores most.

Model answer:

A protocol is the set of rules that determine how two entities located in different systems communicate, that is, the format, the meaning and the timing of the messages they exchange. Its three key points are syntax, which defines the data format and the signal levels; semantics, which defines the control information and how errors are handled; and timing, which deals with speed matching and the sequencing of the messages.

A protocol architecture is the layered organisation of several protocols: it states what task each layer performs, how the layers interact and what control information each one adds to the message so that it can be transmitted. For example, to send a file the work can be split into an application module that handles files and commands, a communication service module that makes sure the data reaches the application on the other side correctly, and a network access module that talks to the specific network the computer is attached to.

It is the definition that comes up most in the mid-terms: learn the two definition sentences almost word for word and add the three key points.

Model answer:

At each layer a protocol adds its own control information to the data it receives from the layer above; the result is the protocol data unit, or PDU, of that layer. When sending, the transport layer may split the application data and gives each chunk a header with the destination port or SAP, the sequence number and an error detection code, forming the segment. The Internet layer adds a header with the source and destination IP addresses and forms the datagram, and the network access layer adds the source MAC address and the next-hop MAC address, together with a check code at the end, forming the frame, which the physical layer sends as bits.

At the destination the reverse path is followed: each layer removes its header and passes the rest to the layer above. Each header answers a different question: the port says which service in the host it is for, the IP which network and host, and the MAC which device on the local network receives it on this hop.

A drawing of envelopes inside envelopes (like the one in the theory article) adds a lot if you are allowed to draw.

Model answer:

The switch works with physical (MAC) addresses and joins the hosts of a single local network: it receives a frame, looks at the destination MAC and forwards it only through the port where that host is. Since it does not need to open anything beyond the frame, it sits at the network access layer of TCP/IP, equivalent to the data link layer, layer 2, of OSI.

The router, on the other hand, needs the IP address because it joins different networks: it opens the frame, reads the destination IP of the datagram, decides which is the next gateway and builds a new frame for the next hop, so the MAC addresses change at every hop while the IP stays the same from start to finish. That is why it reaches the Internet layer of TCP/IP, equivalent to the network layer, layer 3, of OSI. In a home network the router usually also does NAT and DHCP to connect the local network to the Internet.

The sentence that sums it up: the switch sees the final address inside the LAN; the router sees the next hop.