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Key Concepts

Interoperability

It only requires devices connected to the internet to have connections at the lower levels.

Layering

Each layer builds on the last so that any type of protocol used at the lower levels doesn't dictate the higher level protocols.

Security

There are many risks to the internet that were identified by Cerf and Cain, particularly for military users of the internet. They suggest that the previous models were not as secure as the DoD model

DoD Model

·

The DoD model was first advocated by Vinton Cerf and Edward Cain in their 1983 paper The DoD Internet Architecture Model.

1

It is a framework to understand how each protocol relies on a series of other protocols. For example, wifi protocols will form the basis for anything on top of them

2

Built on the heterogeneity of the lower level systems, focussing on the widely varying specifications of things connecting to a network

HTTP

Application Layer

When looking at a website such as www.google.com, the application layer would be the browser used to view the website. The browser would be able to interpret the HyperText Markup Language (HTML). It is the layer from which the actual communication (whatever it may be) is initiated.

Utility Layer

The utility layer contains protocols for specific applications. These include: Domain Name System (DNS), Internet Relay Chat (IRC), Secure Shell (SSH) and Voice over IP (VOIP). This differs from the layers below because almost all of these will then use the same transport and internet protocols.

For looking at websites the main protocol is HyperText Transfer Protocol (HTTP). Its format is shown on the right.

Utility Layer

GET / HTTP/1.1\r\n
Host: {www.soton.ac.uk}\r\n
Connection: keep-alive\r\n
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*; q=0.8\r\n
Upgrade-Insecure-Requests: 1\r\n
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/47.0.2526.106 Safari/537.36\r\n
Accept-Encoding: gzip, deflate, sdch\r\n
Accept-Language: en-US,en;q=0.8,en-GB;q=0.6\r\n
Cookie: fos.web.server=uos-web00020-si; _ga=GA1.3.1002123292.1443177497; runId=-2242960999957318882; usid=6nhnokZcf0e++vtdI/Ce+A__\r\n \r\n.

Transport Layer

The transport layer contains protocols that deal with the moving of packets. It splits the message into packets takes each of the packets and puts a "header" with metadata to then send through the network. Below shows the process. Particularly with TCP the headers contain data to then ensure the reliable delivery of packets. Other protocols such as UDP are used when reliable delivery is not so key.

If this data was split into three packets then the data structure would change, shown in on right

Transport Layer

Header	Data
63531	GET / HTTP/1.1\r\nHost: www.soton.ac.uk\r\nConnection: keep-alive\r\n Accept: text/html, application/xhtml+xml,application/xml; q=0.9, image/webp,/; q=0.8\r\nUpgrade-Insecure-Requests:
80
2781093198
2781093376

63531	1\r\nUser-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/47.0.2526. 106 Safari/537.36\r\nAccept-Encoding: gzip, deflate, sdch\r\n
80
2781093376
2781093554

63531	Accept-Language: en-US,en; q=0.8,en-GB; q=0.6\r\nCookie: fos.web.server= uos-web00020-si; _ga=GA1.3.1002123292. 1443177497; runId=-2242960999957318882; usid=6nhnokZcf0e++vtdI/ Ce+A__\r\n\r\n
80
2781093554
2781093732

Internetwork Layer

The internetwork contains protocols by which the packets made in the transport layer are sent to other locations. It also adds its own header to sends some of its own metadata.

Version	Header Length	Total Length	Source	Destination	Time to live	Protocol
4	20	581	192.168.0.7	152.78.118.51	64	6

The internet forms a path through the computers connected, this is demonstrated below this to the right The most common protocol in this layer is IP which is used here.

Internetwork Layer

traceroute to 8.8.8.8 (8.8.8.8), 64 hops max, 52 byte packets routerlogin.net (192.168.0.1)
sotn-core-2a-xe-111-0.network.virginmedia.net (62.255.45.57)
popl-bb-1b-ae0-0.network.virginmedia.net (62.254.42.90)
eislou2-ic-1-ae0-0.network.virginmedia.net (62.254.42.18)
m350-mp2.cvx3-a.ltn.dial.ntli.net (213.104.85.94)
213.104.85.90 (213.104.85.90)
72.14.233.249 (72.14.233.249)
74.125.37.119 (74.125.37.119)
216.239.57.161 (216.239.57.161)
209.85.244.199 (209.85.244.199)
google-public-dns-a.google.com (8.8.8.8)

Network Layer

The network layer deals with all the other types of connections apart from the internet. The protocols within this layer deal with the "packet" net. In the past local networks would have had their own networking protocols to support different services

Link Layer

The link layer is used to move packets from one network location to another. The packets that are transferred are called frames. Each frame can be visualised as hexadecimal or bits, the example of a frame here is hexadecimal. Each paragraph can be interpreted as one line from each of the layers starting with the internetwork, followed by the transport layer and finishing with the utility. The syntax for the hardware is also a part of the frame which is the first part.

Link Layer

50 6a 03 43 69 1b c4 2c 03 24 55 c4 08 00 45 00 02 45 b1 56 40 00 40 06 00 00 c0 a8 00 07 98 4e 76 33 f8 2b 00 50 a5 c4 1d 4e a8 52 86 38 80 18 ff ff d1 68 00 00 01 01 08 0a 4b 1b ab 3b cb 41 7b 67 47 45 54 20 2f 20 48 54 54 50 2f 31 2e 31 0d 0a 48 6f 73 74 3a 20 77 77 77 2e 73 6f 74 6f 6e 2e 6e 3a 20 6b 65 65 70 2d 61 6c 69 76 65 0d 0a 41 63 63 65 70 74 3a 20 74 65 78 74 2f 68 74 6d 6c 2c 61 70 70 6c 69 63 61 74 69 6f 6e 2f 78 68 74 6d 6c 2b 78 6d 6c 2c 61 70 70 6c 69 63 61 74 69 6f 6e 2f 78 6d 6c 3b 71 3d 30 2e 39 2c 69 6d 61 67 65 2f 77 65 62 70 2c 2a 2f 2a 3b 71 3d 30 2e 38 0d 0a 55 73 65 72 2d 41 67 65 6e 74 3a 20 4d 6f 7a 69 6c 6c 61 2f 35 2e 30 20 28 4d 61 63 69 2d 73 69 3b 20 5f 67 61 3d 47 41 31 2e 33 2e 31 30 30 32 31 32 33 32 39 32 2e 31 34 34 33 31 37 37 34 39 37 3b 20 72 75 6e 49 64 3d 2d 32 32 34 32 39 36 30 39 39 39 39 35 37 33 31 38 38 38 32 3b 20 75 73 69 64 3d 36 6e 68 6e 6f 6b 5a 63 66 30 65 2b 2b 76 74 64 49 2f 43 65 2b 41 5f 5f 0d 0a

Physical Layer

The physical layer is made up of the hardware that links computers together. In your house you probably have cables that come into the house from the street. These will then be connected to a router that will send out a wireless connection or a wired connection such as ethernet. The wired connections communicate through their ports with the rest of the computer. The ports, ethernet cables and fibre optic cables are all part of the physical layer.

The computers then send electronic messages down the wires (or using radiowaves) to transmit and receive

SMTP

Application Layer

When sending or receiving an email, the application layer would be the email client such as Microsoft Outlook or Mozilla Thunderbird. It is the layer from which the actual communication (whatever it may be) is initiated.

Utility Layer

The utility layer contains protocols for specific applications. These include: Domain Name System (DNS), Internet Relay Chat (IRC), Secure Shell (SSH) and Voice over IP (VOIP). This differs from the layers below because almost all of these will then use the same transport and internet protocols.

For sending and receiving emails the main protocol is Simple Mail Transfer Protocol (SMTP) is used. It is a series of connections made to server, its format displayed to the right. The initial connection is made by opening the link but nothing is written).

Utility Layer

SERVER: 220 SMTP.example.com ESMTP CLIENT: HELO mydomain.com
SERVER: 250 Hello mydomain.com CLIENT: MAIL FROM: sender@mydomain.com
SERVER: 250 Ok CLIENT: RCPT TO: recipient@anotherdomain.com
SERVER: 250 Accepted CLIENT: DATA
SERVER: 354 Enter message, ending with "." on a line by itself CLIENT: (content) CLIENT: .
SERVER: 250 Ok CLIENT: QUIT
SERVER: 221 www.example.com closing connection

Transport Layer

The transport layer contains protocols that deal with the moving of packets. It splits the message into packets takes each of the packets and puts a "header" with metadata to then send through the network. Below shows the process. Particularly with TCP the headers contain data to then ensure the reliable delivery of packets. Other protocols such as UDP are used when reliable delivery is not so key.

If this data was split into three packets then the data structure would change, shown in on right

Transport Layer

Header	Data
25	220
587
2781093198
2781093199

25	SMTP.example.com
587
2781093199
2781093200

25	ESMTP
587
2781093200
2781093205

Internetwork Layer

The internetwork contains protocols by which the packets made in the transport layer are sent to other locations. It also adds its own header to sends some of its own metadata.

Version	Header Length	Total Length	Source	Destination	Time to live	Protocol
4	20	7	192.168.0.7	152.78.118.51	64	6

The internet forms a path through the computers connected, this is demonstrated below this to the right The most common protocol in this layer is IP which is used here.

Internetwork Layer

traceroute to 8.8.8.8 (8.8.8.8), 64 hops max, 52 byte packets routerlogin.net (192.168.0.1)
sotn-core-2a-xe-111-0.network.virginmedia.net (62.255.45.57)
popl-bb-1b-ae0-0.network.virginmedia.net (62.254.42.90)
eislou2-ic-1-ae0-0.network.virginmedia.net (62.254.42.18)
m350-mp2.cvx3-a.ltn.dial.ntli.net (213.104.85.94)
216.239.41.207 (216.239.41.207)
72.14.233.249 (72.14.233.249)
74.125.37.119 (74.125.37.119)
216.239.57.161 (216.239.57.161)
209.85.244.199 (209.85.244.199)
google-public-dns-a.google.com (8.8.8.8)

Network Layer

The network layer deals with all the other types of connections apart from the internet. The protocols within this layer deal with the "packet" net. In the past local networks would have had their own networking protocols to support different services

Link Layer

The link layer is used to move packets from one network location to another. The packets that are transferred are called frames. Each frame can be visualised as hexadecimal or bits, the example of a frame here is hexadecimal. Each paragraph can be interpreted as one line from each of the layers starting with the internetwork, followed by the transport layer and finishing with the utility. The syntax for the hardware is also a part of the frame which is the first part.

Link Layer

50 6a 03 43 69 1b c4 2c 03 24 55 c4 08 00 45 00 02 45 b1 56 40 00 40 06 00 00 c0 a8 00 07 98 4e 76 33 f8 2b 00 50 a5 c4 1d 4e a8 52 86 38 80 18 ff ff d1 68 00 00 01 01 08 0a 4b 1b ab 3b cb 41 7b 67 48 45 4c 4f 20 6d 79 64 6f 6d 61 69 6e 2e 63 6f 6d 0a

Physical Layer

The physical layer is made up of the hardware that links computers together. In your house you probably have cables that come into the house from the street. These will then be connected to a router that will send out a wireless connection or a wired connection such as ethernet. The wired connections communicate through their ports with the rest of the computer. The ports, ethernet cables and fibre optic cables are all part of the physical layer.

The computers then send electronic messages down the wires (or using radiowaves) to transmit and receive

Impacts

·

The DOD model was first advocated by Vinton Cerf and Edward Cain in their 1983 paper The DoD Internet Architecture Model.

1

The key contribution to Computer Science is that this model still exists today (albeit in a different form) today. The modern day TCP/IP model is based on DOD architecture, although the ISO and CCITT architectures are used.

2

The fact that it is built on the heterogeneity of the lower level systems, has made it more applicable across a wide array of systems, which contributes towards its longevity.

References

Information on application layer: http://www.tutorialspoint.com/data_communication_computer_network/application_layer_introduction.htm
Protocol numbers found at: http://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml
TCP http://www.freesoft.org/CIE/Course/Section4/8.htm
Other content is based on Cerf, Vinton G., and Edward Cain. "The DoD internet architecture model." Computer Networks (1976) 7.5 (1983): 307-318.
Images are either creative commons or taken from the Cerf and Cain, 1983 paper

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