Content networking is a technology that is here to stay. It will slowly become ingrained into networking products just like spanning tree and routing protocols have in Layer 2 and Layer 3 devices. While skills are not as widespread as needed, education and a
Chapter 2. Understanding Layer 2, 3, and 4 Protocols
While many of the concepts well known to traditional Layer 2 and Layer 3 networking still hold true in content switching applications, the area introduces new and more complex themes that need to be well
The OSI Seven Layer Model”What Is a Layer?
Established in 1947, the International Organization for Standardization (ISO) was
Figure 2-1. The OSI Seven Layer Model.
When we talk about Layer 2 and Layer 3 networking, it is these
Each layer plays its part in moving data from one device to another across a network infrastructure by providing a standard interface to the
The Application Layer (Layer 7)
The top layer in the stack, the Application layer is where the end-user application resides. Think of the Application layer as the browser application or email client for a
In content switching terms, Layer 7 refers to the ability to parse information directly generated by the user or application in decision making, such as the URL typed by the user in the Web browser. For example, http://www.foocorp.com is an example of Application layer data.
The Presentation Layer (Layer 6)
The Presentation layer is used to provide a common way for applications (residing at the Application layer) to translate between data formats or perform encryption and decryption. Mechanisms to convert between text formats such as ASCII and Unicode may be
The Session Layer (Layer 5)
The Session layer coordinates multiple Presentation layer processes communicating between end devices. The Session layer is used by applications at either end of the communication between end devices to tie together multiple Transport layer sessions and provide synchronization between them.
The HTTP protocol can use multiple TCP connections to retrieve objects that make up a single Web page. The Session layer provides application coordination between these separate TCP connections.
The Transport Layer (Layer 4)
The Transport layer is responsible for providing an identifiable and sometimes reliable transport mechanism between two communicating devices. User or application data, having passed through the Presentation and Session layers, will typically be
The Transport layer is the first at which we see the concept of packets or datagrams of information that will be transported across the network. TCP, UDP, and ICMP are examples of Layer 4 protocols used to provide a delivery mechanism between end
The Network Layer (Layer 3)
Whereas Layer 4 is
Traditional Internet routers
The Data Link Layer (Layer 2)
The Data Link layer also defines a lower level addressing structure to be used between end systems as well as the lower level framing and checksums being used to transmit onto the physical medium. Ethernet, Token Ring, and Frame Relay are all examples of Data Link layer or Layer 2 protocols.
Traditional Ethernet switches operate at the Data Link layer and are concerned with forwarding packets based on the Layer 2 addressing scheme. Layer 2 Ethernet switches are not concerned with whether the packet contains IP, IPX, or AppleTalk, but only with where the MAC address of the recipient end system resides.
The Physical Layer (Layer 1)
As with all computer systems, networking is ultimately about making, moving, and storing 1s and 0s. In networking terms, the Physical layer defines how the user's browser application data is turned into 1s and 0s to be transmitted onto the physical medium. The Physical layer defines the physical medium such as cabling and interface specifications. AUI, 10Base-T, and RJ45 are all examples of Layer 1 specifications.
Putting All the Layers Together
Let's take an example of a Web user visiting the Web site of Foocorp, Inc. Within the browser application, at the Application layer, the user will type in the URL, typically something like http://www.foocorp.com/ . While this is the only input the user will provide the application, there is much more information generated by the browser application itself, including:
In terms of HTTP-based Web browser traffic, these pieces of information can be thought of as the Application, Presentation, and Session layers of the OSI model. They provide not only the raw data input by the user in the application, but also information needed by the application to ensure successful communication with the end system; in this case, a Web server at Foocorp. HTTP information for the Web user would look something like:
Hypertext Transfer Protocol GET / HTTP/1.0\r\n Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg\r\n Accept-Language: en-gb\r\n User-Agent: Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)\r\n Host: www.foocorp.com\r\n Connection: Keep-Alive\r\n \r\n
Once this application information has been generated, it can be packaged and passed on to the
At the Transport layer, information about the port numbers, sequence
Transmission Control Protocol Source port: 3347 (3347) Destination port: http (80) Sequence number: 52818332 Next sequence number: 52818709 Acknowledgement number: 3364222344 Header length: 20 bytes Flags: 0x0018 (PSH, ACK) 0... .... = Congestion Window Reduced (CWR): Not set .0.. .... = ECN-Echo: Not set ..0. .... = Urgent: Not set ...1 .... = Acknowledgment: Set .... 1... = Push: Set .... .0.. = Reset: Not set .... ..0. = Syn: Not set .... ...0 = Fin: Not set Window size: 17520 Checksum: 0xb043 (correct)
Once the Transport layer information has been added to the head of the packet, it is passed to the Network layer for the Layer 3 headers to be appended. The Network layer will include information on the IP addresses of both the client and the end system, and a reference to which Transport layer protocol has been used. The Network layer information is used to ensure the correct delivery
Internet Protocol Version: 4 Header length: 20 bytes Time to live: 128 Protocol: TCP Header checksum: 0x2df9 (correct) Source: 192.168.254.201 (192.168.254.201) Destination: 184.108.40.206 (220.127.116.11)
For transmission across the local, physical network, the frame is then passed to the Data Link layer for the addition of the local physical addresses. In terms of Ethernet, this would be the Ethernet Media Access Control (MAC) address of the user machine and the MAC address of the default gateway router on the Ethernet network. The Layer 2 protocol, such as Ethernet, will also include a reference to which Layer 3 protocol has been used and a checksum to ensure data integrity. For our example, the Layer 2 information might look something like:
Ethernet II Destination: 00:20:6f:14:58:2f (00:20:6f:14:58:2f) Source: 00:30:ab:17:0d:1a (00:30:ab:17:0d:1a) Type: IP (0x0800)
Figure 2-2 depicts this process of repackaging each layer with new header information at the layer below.
Figure 2-2. Passing data through the seven OSI layers.