Understanding IP Networks - Routers

In the last article we looked at the problems routers and switchers solve, and built a simple office network to show the benefits of IP networks. In this article we continue the theme of looking at a network from a broadcast engineers’ point of view so they can better communicate with the IT department, and take a deeper look at exactly how routers work by sitting on the back of an IP datagram as it travels through a network.

Continued from Part 1: Understanding IP Networks - Introduction

Congestion

Routers reduce traffic congestion by sending datagrams only to the devices that require the data, and not spraying the whole network with them. As well as reducing congestion this will help improve security as administrators can limit which hosts can exchange data; IP-cameras, IP-vision mixers, and computers etc.

Diagram showing how an Ethernet datagram encapsulates the IP datagram

Datagrams

An IP datagram consists of two parts, the header and the payload. In the case of video over IP, the camera will break its video stream into smaller chunks until they fit into the payload of an IP datagram. The header consists of the source IP address, that is the address of the device sending the datagram such as a camera, and the destination address of where the camera wants to send its’ data to, for example a vision mixer.

If we assume the studio network is CAT6/Ethernet, the camera will have its’ own media access control (MAC) address and the vision mixer will also have its own MAC address. These are unique Ethernet addresses that are hard coded into the devices during manufacture and are different for every IP enabled unit that leaves the production line.

Diagram showing a simple studio configuration with cameras and a vision mixer

Subnets

In diagram two we have a simple studio network where a vision mixer is connected to three cameras via two different routers and subnets. It is possible to connect all of these devices together on one switcher but that would leave us with a single point of failure and probably over-load the router due to the high bandwidths video-over-IP demands.

Subnets are a way of abstracting the underlying physical network to form a logical one, providing more intuitive and easier configuration. Network administrators will define the IP addressing and subnet schemes. A typical example might have subnets for each section within the studio, one for camera’s, one for monitors and one for sound. 

The router uses a combination of the IP address and netmask, called the Network-ID to route datagrams to other routers. A typical network ID in the class-less system would be 10.1.1.0/24. The “24” refers to the subnet mask and the 10.1.1.0 is the network. In this instance the router would route any datagram in the address range 10.1.1.0 to 10.1.1.255, as it only looks at the first 24 bits of the IP address, and each number within the dot is 8 bits. As another example, a network ID such as 10.1.1.0/8 will contain all hosts with an IP addresses range from 10.0.0.0 to 10.255.255.255.

Next Hop

If camera-1 was configured to send its datagram to the vision mixer its source IP address would be 10.0.1.3 and its destination address would be 10.0.3.1. From the diagram we can see that the vision mixer is physically connected to port 2 on router B, and camera-1 is connected to port 1 on router A. Camera-1’s datagram is sent from router A’s port 5, and only on port 5, to port 1 on router B.

Static routers work on a next-hop system, that is each router only knows of the existence of subnets connected to its ports, this simplifies the network design and keeps the router database to a manageable size. In this example we are using static routing so the network administrator will have to manually enter an entry into router A’s database (router table) that says “all IP datagrams with destination address range 1.0.3.0 to 1.0.3.255 should be sent on port 5”.

We’re using Ethernet as the physical interface and Camera-1 will have to set the source and destination MAC addresses of the Ethernet packet leaving its IP port. A MAC address is a unique number issued by the IEEE for every piece of equipment that is manufactured with Ethernet capability. When the IP datagram leaves camera-1 it must also set the Ethernet MAC source and destination addresses, as well as those for the IP datagram.

A system called address resolution protocol (ARP) is instigated by camera-1 which broadcasts an IP message saying “who has IP address 10.0.3.1 (the vision mixer)? And what is your Ethernet address?”. If the vision mixer was directly connected to router A then it would see this broadcast message, however, as it is connected to router B, it does not. The broadcasting of ARP messages is restricted to local subnets and routers to stop the entire network being flooded with ARP query messages.

Diagram showing the IP addresses stay the same but the MAC addresses change

At this point router A answers the ARP query from camera-1 with the address of its’ own Ethernet address. Camera-1 sets its own MAC address as the source MAC address, and the destination address is the MAC address of ports 1 to 3 on router A. Port 5 on router A will have a different MAC address than ports 1 to 3 as it’s connected to a different subnet.

IP Addresses Stay the Same

Earlier, the network admin configured the router A’s table to have an entry which detects an IP destination address in the range 10.0.3.0 to 10.0.3.255 and routes it to port 5. Prior to this, router A would have sent an ARP query on port 5 asking if any routers know of a host with address 10.0.3.1 (the vision mixer).

Many routers could be connected to port 5, but in this example only router B is connected, it knows that the vision mixer is connected to port 2, so router B sends back its MAC address and router A populates the Ethernet header with this address so camera-1’s datagram can be sent to router B. Router B then changes the source and destination MAC address of camera-1’s datagram so it can be delivered to the vision mixer.

Throughout the whole process of sending a datagram from camera-1 to the vision mixer via router A and router B, the source and destination IP addresses did not change at all. However, the source and destination Ethernet addresses changed at each node.

MAC Addresses Change

It’s entirely possible that the cameras on router A were at the Superbowl stadium, and router B and the vision mixer at the studio in New York, with an IP satellite link connecting them. The source and destination IP addresses did not change, but the MAC and physical connectivity mapping automatically changed as the datagram moved between devices. From an IP point of view, we do not know or care how the datagram travelled between router A and B, the underlying complexity of the physical, electrical and optical routing was abstracted away from us.

In the part 3 we look at how resilience works and how the network automatically routes when a link fails.

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