popular examples of layer 4 protocols are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). The primary difference between these two is that despite the fact that both protocols operate at the transport layer, TCP provides a guarantee that any traffic sent across the network will be delivered to its destination complete and in the order that it was sent. Comparably, UDP provides no such guarantee; data sent using UDP does not create any layer 4 acknowledgement from its destination that it was or was not received correctly. If a piece of data were lost in transit across the network and the application were using TCP, the receiver of the data would notify the sender and a retransmission would be arranged, whereas with UDP, it would just be lost. Although this may seem like a large drawback, whether it is or not depends on the use case; for real‐time applications such as video conferencing or Voice over Internet Protocol (VoIP) calls, it is beneficial to the user experience to allow a certain amount of lost data compared to incurring the delay of the sender having to retransmit any lost data, which can result in odd sound or video to an end user.
Although still very commonly used, the TCP and UDP protocols are not always the optimal choice. These protocols emerged in 1974 and 1980, respectively, and as such predate the applications that today generate the vast majority of traffic on the internet and its constituent networks, sometimes by several decades. Modern alternatives (such as the Stream Control Transmission Protocol [SCTP], which is designed to incorporate many of the desirable features of both TCP and UDP) are emerging and will see increasing use in the near future for use cases that require their additional capabilities such as native support for multihoming in the case of SCTP, where two endpoints may each have multiple globally unique addresses, allowing for the use of redundant network paths for added resiliency.
3.3.5 Layers 5, 6, and 7
These three layers are referred to as the session, presentation, and application layers, respectively. In practice their details are not encountered as much from the network perspective as they are by the inner workings of a specific application, which is why they are not going to be described in the same level of detail as the lower levels of the stack are in this chapter. As the functions at these layers are often handled within a single application at a single endpoint, the protocols at each of these upper layers as well as the interfaces between them tend to be less distinct than those at the lower layers where interoperation between endpoints from many sources is needed for the network to function.
As further degrees of interoperation between the network and the processing resources of the data centre both at the infrastructure edge and at the regional or national scale are used to support next generation use cases, the functions of these three upper layers of the OSI model will be increasingly used to provide that underlying network and data centre infrastructure with intelligence about the specific characteristics of the application in use, which will then be used to make nuanced decisions about how infrastructure resources can be allocated and used for optimal cost and user experience.
3.4 Ethernet
Ethernet is an example of a layer 2 protocol and is the most commonly used layer 2 protocol today. A basic understanding of some of the characteristics of Ethernet is useful in the context of infrastructure edge computing, as the protocol is so widely used both within the infrastructure edge data centre, as well as between them and between other facilities of both similar and larger scale.
Ethernet uses broadcast communication to perform network endpoint discovery. This means that when an Ethernet endpoint receives a frame with a destination MAC address and the endpoint does not have an existing entry in its switching table for that destination MAC address, a request is sent to all other Ethernet endpoints on that segment of the network, asking for the location of the endpoint which has that destination MAC address assigned to one of its interfaces. The protocol was designed in this way for implementation simplicity and low cost, both of which have helped Ethernet become established as the dominant layer 2 protocol today; but there is an equal drawback in regard to the inefficiency of this behaviour in a larger network, where the volume of broadcast traffic can be substantial enough to impact the performance of network endpoints and ultimately of the network.
The protocol is capable of supporting frame sizes between 64 and 1518 bytes as standard, and some equipment can be configured to support so‐called jumbo frames of up to 9600 bytes. The latter are useful for some use cases which rely on these jumbo frames in order to lower the overhead of large numbers of Ethernet frame headers involved when carrying data, or for protocols such as those of storage area networks (SANs), which natively use data segmentation sizes closer to a jumbo frame.
The most common type of traffic encountered on a modern network today is an Ethernet frame that encapsulates an Internet Protocol version 4 (IPv4) or Internet Protocol version 6 (IPv6) packet, using TCP or UDP as its transport layer protocol, carrying some application data from a source endpoint to its destination endpoint. This combination of protocols is used for a wide range of use cases and across almost every scale of network in common use today.
3.5 IPv4 and IPv6
Both IPv4 and IPv6 are examples of layer 3 protocols. They are the most commonly encountered layer 3 protocols, and as such, they provide a method for the end‐to‐end addressing of endpoints across the network using a globally unique address space. When each endpoint has a globally unique identifier, data can be addressed to a specific endpoint without ambiguity; this function allows data to be transmitted between endpoints which reside on different networks, even at a worldwide scale.
In the context of both the internet and infrastructure edge computing, both of the Internet Protocol (IP) versions, IPv4 and, to a growing extent, IPv6 are ubiquitous. Any application, endpoint, or piece of infrastructure must support these protocols; no real competitor currently exists and is unlikely to do so for some time due to the ubiquity of both IPv4 and IPv6, driving their integration into billions of devices and applications across the world. In addition, many of the issues with these protocols have been tempered by the industry using various means, so few see a pressing need to replace them.
IPv6 adoption, although behind its earlier cousin IPv4 as of today, is growing across the world and is expected to reach parity with and then exceed the amount of global internet traffic transmitted atop IPv6 compared to IPv4 as measured on a daily basis. One of the growth areas for IPv6 is expected to be the widespread deployment of city‐scale IoT, where potentially millions of devices must be able to connect with remote applications operating in other networks, requiring these devices to have a globally unique IP address. This need combined with the global exhaustion of the IPv4 address space looks set to drive the future adoption of IPv6, although IPv4 address conservation mechanisms such as network address translation (NAT) remain in use and will continue to be for many years ahead.
3.6 Routing and Switching
Both routing and switching are vast topics, each with significant history and many unique intricacies. The focus of this book is not on either of these fields, but they are closely related to any discussion of network design and operation, and so this section will describe some of the key points related to routing and switching that are relevant to network design and operation for modern networks so that it can be referred to during later chapters as many of the same core principles apply to the new networks being designed, deployed, and operated to support infrastructure edge computing as well.
3.6.1 Routing
On the subject of routing, which is the process where a series of network endpoints use layer 3 information as well as other characteristics of the data in transit and of the network itself to deliver data from its source to its destination, there are two primary approaches to performing this process.
