Networking 6

Question 1

Internet Control Message Protocol (ICMP)

Answer 1

It is a protocol designed to transmit error messages and operational information between hosts on a network. For the most part, ICMP is used almost exclusively by network devices such as routers.

Each ICMP packet consists of a ‘type’ and a ‘code’. Used together they describe the packet’s purpose. For example, the ‘ping’ program which is used to determine if a host on the network is available uses an ICMP type 8 code 0 packet.

Another example of an ICMP packet is the ICMP type 11 code 0 packet, or the ICMP time exceeded packet. This packet is usually sent by a gateway (router) to a system that sent a packet, where the TTL (the number of hops the packet can travel before it is considered lost and gets discarded) of that packet has expired.

Question 2

Dynamic Host Configuration Protocol (DHCP)

Answer 2

DHCP allows a DHCP server present on the network to assign network configuration settings to each host automatically when they join the network.

This makes joining a network seamless to the end user while removing the possibility of IP address conflicts

Only one DHCP server should be used on any network, otherwise there is the possibility of conflicts where one DHCP server doesn’t know the other has already assigned an IP address.

Question 3

Open Systems Interconnection (OSI) Model

Answer 3

A way of describing the way computers communicate with each other over a network.

Layer 7 - Application
Layer 6 - Presentation
Layer 5 - Session
Layer 4 - Transport
Layer 3 - Network
Layer 2 - Data Link
Layer 1 - Physical

The idea is that you start at the top and work your way down the model. Each layer transforms the data somehow until at the physical layer the data is converted into electrical signals, which are transmitted over the network.

The computer receiving that information receives it at the physical layer and then works up to the application layer again until the user sees it in its original form on the receiving computer.

Question 4

Application Layer (OSI)

Answer 4

Layer 7

An example of an application layer protocol is something like HTTP or FTP. For example, an HTTP GET request is an application layer protocol because the web browser will form the request necessary for displaying a page or submitting a form.

Question 5

Presentation Layer (OSI)

Answer 5

Layer 6

An example of a presentation layer protocol is XML or JSON. The presentation layer deals with formatting data in a way that the intended recipient can understand. Encryption also happens in the presentation layer, if the data is meant to be encrypted. An example might be a file transfer, where the file must be converted to binary data before it can be sent.

Question 6

Session Layer (OSI)

Answer 6

Layer 5

The session layer handles opening, closing and managing connections between computers. The session layer is a bit of an oddity because these days most people think TCP when they think connections. TCP is actually a transport layer protocol, which doesn’t respect the OSI model, so it does NOT fall under the session layer.

Question 7

Transport Layer (OSI)

Answer 7

Layer 4

The transport layer is responsible for end-to-end connections between computers on a network. The TCP and UDP protocols fall into the transport layer, and it is at this point that the TCP or UDP header gets added to the packet.

Question 8

Network Layer (OSI)

Answer 8

Layer 3

The network layer is responsible for routing the packet over the internet. The IP (Internet Protocol) falls into the network layer. At this point, the IP header is added to the packet.

Question 9

Data Link Layer (OSI)

Answer 9

Layer 2

The data link layer is responsible for encoding and decoding packets into bits. At this point the destination and source MAC address is added to the packet (in the form of the Ethernet frame, or wireless, or Bluetooth), indicating which network card on the network the packet is going to next.

Question 10

Physical Layer (OSI)

Answer 10

Layer 1

The physical layer is responsible for converting the packet into electrical signals, which are sent over the network.

Question 11

TCP/IP Model

Answer 11

The TCP/IP model is another theoretical model designed to show how computers communicate over the network. The TCP/IP model has only four layers, but ultimately everything the OSI model covers is also covered by the TCP/IP model, it is merely the case that several layers have been joined into one in places.

Layer 4 - Application
Layer 3 - Transport
Layer 2 - Internet
Layer 1 - Network Access

Question 12

Application Layer (TCP/IP)

Answer 12

Layer 4

The application layer is basically the same as the application layer in the OSI model, except it also includes the responsibilities of the presentation and session layers from the OSI model. Examples of protocols in the application layer are HTTP and FTP, basically any protocol that applications define and use.

OSI equivalent: Application+Presentation+Session layers

Question 13

Transport Layer (TCP/IP)

Answer 13

Layer 3

The transport layer is a direct copy of the transport layer of the OSI model, in other words, the TCP and UDP protocols are implemented here.

OSI equivalent: Transport layer

Question 14

Internet Layer (TCP/IP)

Answer 14

Layer 2

The internet layer is effectively the network layer from the OSI model, responsible for routing traffic over the network.

OSI equivalent: Network layer

Question 15

Network Access Layer (TCP/IP)

Answer 15

Layer 1

The network access layer is a combination of the data link and physical layers from the OSI model.

OSI equivalent: Data link + Physical layers

Question 16

Packet Headers

Answer 16

Starting with application-level data, we encapsulate that data into a packet ready for transmission over the network. The process of encapsulation occurs by adding headers to the data.

HTTP Process:

HTTP/1.1 GET /
To transmit this request to the site in question, the first thing that happens is a TCP header is added to the data. The TCP header consists of a source port, a destination port (80, or 443 usually`), a sequence number, an acknowledgement number and anything else the TCP protocol implements. The original data, the HTTP/1.1 GET / is still there, but the TCP header now sits in front of it.

The next stage is to add the IP header. The IP header consists of information such as the version (4 or 6), the source IP address, the destination IP address, the size of the packet and anything else the IP protocol implements.

The next stage after that is to add the data link layer protocol (let’s say we’re using Ethernet to transmit the data in this case). The Ethernet frame (it isn’t a header because it consists of a header AND a footer, so it frames the packet) consists of the source MAC address, destination MAC address and anything else the Ethernet protocol implements.

Finally, the packet is converted into a series of electrical impulses, which are transmitted over the network. On the receiving end, the packet works its way back up through the layers, first being converted from electrical impulses into data.

Question 17

Address Resolution Protocol (ARP)

Answer 17

Computer A will send a broadcast request (a request to everyone on the local network) asking for the MAC address of 192.168.0.10. Computer B will respond with its MAC address. Computer A will then store that mapping in an ARP table which caches results (so it doesn’t have to keep asking the same question over and over).

Keep in mind ARP is only for the local network. If you are sending data to the internet, then your computer should know that the IP address in question is not in your local network (because of the subnet mask). So your computer will send the data to the router and will need the router’s MAC address (it will already know the IP of the router/gateway because it will be in the network settings on that computer). If your computer doesn’t have it stored in the ARP table, then it will ask for the router’s MAC address using ARP.