2.3 Computer Networking

A computer network is a telecommunications network that allows computers to exchange data. The physical connection between networked computing devices is established using either cable media or wireless media. The best-known computer network is the Internet.

Network devices that originate, route and terminate the data are called network nodes. Nodes can include hosts such as servers and personal computers, as well as networking hardware. Two devices are said to be networked when a process in one device is able to exchange information with a process in another device.

Computer networks support applications such as access to the World Wide Web, shared use of application and storage servers, printers, and fax machines, and use of email and instantmessaging applications. The remainder of this article discusses local area network technologies and classifies them according to the following characteristics: the physical media used to transmit signals, the communications protocols used to organize network traffic, along with the network’s size, its topology and its organizational intent.

2.3.1 Network Hardware

Apart from the physical communications media described above, networks comprise additional basic hardware building blocks, such as network interface controller cards, repeaters, hubs, bridges, switches, routers, and firewalls.

1．Network Interfaces

A network interface controller (NIC) is a hardware accessory that provides a computer with both a physical interface for accepting a network cable connector and the ability to process low-level network information.

In Ethernet networks, each network interface controller has a unique media access control (MAC) address which is usually stored in the card’s permanent memory. MAC address uniqueness is maintained and administered by the Institute of Electrical and Electronics Engineers (IEEE) in order to avoid address conflicts between devices on a network. The size of an Ethernet MAC address is six octets. The 3 most significant octets are reserved to identify card manufacturers. The card manufacturers, using only their assigned prefixes, uniquely assign the 3 least-significant octets of every Ethernet card they produce.

2．Repeaters and Hubs

A repeater is an electronic device that receives a network signal, cleans it of unnecessary noise, and regenerates it. The signal is retransmitted at a higher power level, or to the other side of an obstruction, so that the signal can cover longer distances without degradation. In most twisted pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. A repeater with multiple ports is known as a hub. Repeaters work on the physical layer of the OSI model. Repeaters require a small amount of time to regenerate the signal. This can cause a propagation delay which can affect network performance. As a result, many network architectures limit the number of repeaters that can be used in a row, e.g., the Ethernet 5-4-3 rule.

Repeaters and hubs have been mostly obsoleted by modern switches.

3．Bridges

A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model to form a single network. Bridges broadcast to all ports except the port on which the broadcast was received. However, bridges do not promiscuously copy traffic to all ports, as hubs do. Instead, bridges learn which MAC addresses are reachable through specific ports. Once the bridge associates a port with an address, it will send traffic for that address to that port only.

Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, the bridge assumes that the MAC address is associated with that port and stores its source address. The first time a bridge sees a previously unknown destination address, the bridge willforward the frame to all ports other than the one on which the frame arrived. Bridges come in three basic types:

Local bridges: Directly connect LANs.

Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, largely have been replaced with routers.

Wireless bridges: Can be used to join LANs or connect remote devices to LANs.

4．Switches

A network switch is a device that forwards and filters OSI layer 2datagrams between ports based on the MAC addresses in the packets. A switch is distinct from a hub in that it only forwards the frames to the ports involved in the communication rather than all ports connected. A switch breaks the collision domain but represents itself as a broadcast domain. Switches make decisions about where to forward frames based on MAC addresses. A switch normally has numerous ports, facilitating a star topology for devices, and cascading additional switches. Multi-layer switches are capable of routing based on layer 3 addressing or additional logical levels. The term switch is often used loosely to include devices such as routers and bridges, as well as devices that may distribute traffic based on load or based on application content (e.g., a Web URL identifier).

5．Routers

A router is an internetworking device that forwards packets between networks by processing the routing information included in the packet or datagram (Internet protocol information from layer 3). The routing information is often processed in conjunction with the routing table (or forwarding table). A router uses its routing table to determine where to forward packets. (A destination in a routing table can include a “null” interface, also known as the “black hole” interface because data can go into it, however, no further processing is done for said data.)

6．Firewalls

A firewall is a network device for controlling network security and access rules. Firewalls are typically configured to reject access requests from unrecognized sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase in cyber attacks.

2.3.2 Network Protocols

In the context of data communication, a network protocol is a formal set of rules, conventions and data structure that governs how computers and other network devices exchange information over a network. In other words, protocol is a standard procedure and format that two data communication devices must understand, accept and use to be able to talk to each other.

(2-4)In modern protocol design, protocols are "layered" according to the OSI 7 layer model or a similar layered model. Layering is a design principle which divides the protocol design into a number of smaller parts, each part accomplishing a particular sub-task and interacting with the other parts of the protocol only in a small number of well-defined ways. Layering allows the parts of a protocol to be designed and tested without a combinatorial explosion of cases, keeping each design relatively simple. Layering also permits familiar protocols to be adapted to unusual circumstances.

The header and/or trailer at each layer reflect the structure of the protocol. Detailed rules and procedures of a protocol or protocol group are often defined by a lengthy document. For example, IETF uses RFCs (Request for Comments) to define protocols and updates to the protocols.

A wide variety of communication protocols exists. These protocols were defined by many different standard organizations throughout the world and by technology vendors over years of technology evolution and development. One of the most popular protocol suites is TCP/IP, which is the heart of internetworking communications. The IP, the Internet Protocol, is responsible for exchanging information between routers so that the routers can select the proper path for network traffic, while TCP is responsible for ensuring the data packets are transmitted across the network reliably and error free. LAN and WAN protocols are also critical protocols in network communications. The LAN protocols suite is for the physical and data link layers of communications over various LAN media such as Ethernet wires and wireless radio waves. The WAN protocol suite is for the lowest three layers and defines communication over various wide-area media, such as fiber optic and copper cables.

Network communication has slowly evolved. Today’s new technologies are based on the accumulation over years of technologies, which may be either still existing or obsolete. Because of this, the protocols which define the network communication are highly inter-related. Many protocols rely on others for operation. For example, many routing protocols use other network protocols to exchange information between routers.

In addition to standards for individual protocols in transmission, there are now also interface standards for different layers to talk to the ones above or below (usually operating system specific). For example: Winsock and Berkeley sockets between layers 4 and 5; NDIS and ODI between layers 2 and 3.

The protocols for data communication cover all areas as defined in the OSI model. However, the OSI model is only loosely defined. A protocol may perform the functions of one or more of the OSI layers, which introduces complexity to understanding protocols relevant to the OSI 7 layer model. In real-world protocols, there is some argument as to where the distinctions between layers are drawn; there is no one black and white answer.

To develop a complete technology that is useful for the industry, very often a group of protocols is required in the same layer or across many different layers. Different protocols often describe different aspects of a single communication; taken together, these form a protocol suite. For example, Voice over IP (VoIP), a group of protocols developed by many vendors and standard organizations, has many protocols across the 4 top layers in the OSI model.

Protocols can be implemented either in hardware or software or a mixture of both. Typically, the lower layers are implemented in hardware, with the higher layers being implemented in software.

Protocols could be grouped into suites (or families, or stacks) by their technical functions, or origin of the protocol introduction, or both. A protocol may belong to one or multiple protocol suites, depending on how you categorize it. For example, the Gigabit Ethernet protocol IEEE 802.3z is a LAN (Local Area Network) protocol and it can also be used in MAN (Metropolitan Area Network) communications.

Most recent protocols are designed by the IETF for Internetworking communications and by the IEEE for local area networking (LAN) and metropolitan area networking (MAN). The ITU-T contributes mostly to wide area networking (WAN) and telecommunications protocols. ISO has its own suite of protocols for internetworking communications, which is mainly deployed in European countries.

2.3.3 Internet and TCP/IP

The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite (TCP/IP) to serve several billion users worldwide. According to Internet World Stats, as of December 31, 2011, there was an estimated 2,267,233,742 Internet users worldwide. This represents 32.7% of the world’s population.

(2-5) No one actually owns the Internet, and no single person or organization controls the Internet in its entirety. It is a network of networks that consists of millions of private, public, academic, business, and government networks, of local to global scope, that are linked by a broad array of electronic, wireless and optical networking technologies. The Internet carries an extensive range of information resources and services, such as the inter-linked hypertext documents of the World Wide Web (WWW), the infrastructure to support email, and peer-to-peer networks.

The Internet is a globally distributed network comprising many voluntarily interconnected autonomous networks. It operates without a central governing body. However, to maintain interoperability, the principal name spaces of the Internet are administered by the Internet Corporation for Assigned Names and Numbers (ICANN), headquartered in Marina del Rey, California. ICANN is the authority that coordinates the assignment of unique identifiers for use on the Internet, including domain names, Internet Protocol (IP) addresses, application port numbers in the transport protocols, and many other parameters. Globally unified name spaces, in which names and numbers are uniquely assigned, are essential for maintaining the global reach of the Internet. ICANN is governed by an international board of directors drawn from across the Internet technical, business, academic, and other non-commercial communities.

ICANN’s role in coordinating the assignment of unique identifiers distinguishes it as perhaps the only central coordinating body for the global Internet. The government of the United States continues to have a primary role in approving changes to the DNS root zone that lies at the heart of the domain name system. On 16 November 2005, the United Nations-sponsored World Summit on the Information Society, held in Tunis, established the Internet Governance Forum (IGF) to discuss Internet-related issues.

The technical underpinning and standardization of the Internet’s core protocols (IPv4 and IPv6) is an activity of the Internet Engineering Task Force (IETF), a non-profit organization of loosely affiliated international participants that anyone may associate with by contributing technical expertise.

The Internet protocol suite is the networking model and a set of communications protocols used for the Internet and similar networks. It is commonly known as TCP/IP, because its most important protocols, the Transmission Control Protocol (TCP) and the Internet Protocol (IP) were the first networking protocols defined in this standard. It is occasionally known as the DoD model due to the foundational influence of the ARPANET in the 1970s (operated by DARPA, an agency of the United States Department of Defense).

The Internet protocol suite uses encapsulation to provide abstraction of protocols and services. Encapsulation is usually aligned with the division of the protocol suite into layers of general functionality. In general, an application (the highest level of the model) uses a set of protocols to send its data down the layers, being further encapsulated at each level.

The layers of the protocol suite near the top are logically closer to the user application, while those near the bottom are logically closer to the physical transmission of the data. Viewing layers as providing or consuming a service is a method of abstraction to isolate upper layer protocols from the details of transmitting bits over, for example, Ethernet and collision detection, while the lower layers avoid having to know the details of each and every application and its protocol.

Even when the layers are examined, the assorted architectural documents—there is no single architectural model such as ISO 7498, the Open Systems Interconnection (OSI) model—have fewer and less rigidly defined layers than the OSI model, and thus provide an easier fit for real-world protocols. One frequently referenced document, RFC 1958, does not contain a stack of layers. The lack of emphasis on layering is a major difference between the IETF and OSI approaches. It only refers to the existence of the internetworking layer and generally to upper layers; this document was intended as a 1996 snapshot of the architecture: “The Internet and its architecture have grown in evolutionary fashion from modest beginnings, rather than from a Grand Plan. While this process of evolution is one of the main reasons for the technology’s success, it nevertheless seems useful to record a snapshot of the current principles of the Internet architecture.”

RFC 1122, entitled Host Requirements, is structured in paragraphs referring to layers, but the document refers to many other architectural principles not emphasizing layering. It loosely defines a four-layer model, with the layers having names, not numbers, as follows:

(1) Application layer (process-to-process): This is the scope within which applications create user data and communicate this data to other processes or applications on another or the same host. The communications partners are often called peers. This is where the higher level protocols such as SMTP, FTP, SSH, HTTP, etc. operate.

(2) Transport layer (host-to-host): The transport layer constitutes the networking regime between two network hosts, either on the local network or on remote networks separated by routers. The transport layer provides a uniform networking interface that hides the actual topology (layout) of the underlying network connections. This is where flow-control, error-correction, and connection protocols exist, such as TCP. This layer deals with opening and maintaining connections between Internet hosts.

(3) Internet layer: The internet layer has the task of exchanging datagrams across network boundaries. It is therefore also referred to as the layer that establishes internetworking, indeed, it defines and establishes the Internet. This layer defines the addressing and routing structures used for the TCP/IP protocol suite. The primary protocol in this scope is the Internet Protocol, which defines IP addresses. Its function in routing is to transport datagrams to the next IP router that has the connectivity to a network closer to the final data destination.

(4) Link layer: This layer defines the networking methods within the scope of the local network link on which hosts communicate without intervening routers. This layer describes the protocols used to describe the local network topology and the interfaces needed to effect transmission of internet layer datagrams to next-neighbor hosts.

The Internet protocol suite and the layered protocol stack design were in use before the OSI model was established. Since then, the TCP/IP model has been compared with the OSI model in books and classrooms, which often results in confusion because the two models use different assumptions and goals, including the relative importance of strict layering.

This abstraction also allows upper layers to provide services that the lower layers do not provide. While the original OSI model was extended to include connectionless services (OSIRM CL), IP is not designed to be reliable and is a best effort delivery protocol. This means that all transport layer implementations must choose whether or how to provide reliability. UDP provides data integrity via a checksum but does not guarantee delivery; TCP provides both data integrity and delivery guarantee by retransmitting until the receiver acknowledges the reception of the packet.

This model lacks the formalism of the OSI model and associated documents, but the IETF does not use a formal model and does not consider this a limitation, as illustrated in the comment by David D. Clark, “We reject: kings, presidents and voting. We believe in: rough consensus and running code.” Criticisms of this model, which have been made with respect to the OSI model, often do not consider ISO’s later extensions to that model.

For multiaccess links with their own addressing systems (e.g. Ethernet), an address mapping protocol is needed. Such protocols can be considered to be below IP but above the existing link system. While the IETF does not use the terminology, this is a subnetwork dependent convergence facility according to an extension to the OSI model, the internal organization of the network layer (IONL).

ICMP & IGMP operate on top of IP but do not transport data like UDP or TCP. Again, this functionality exists as layer management extensions to the OSI model in its Management Framework (OSIRM MF).

The library operates above the transport layer (uses TCP) but below application protocols. Again, the SSL/TLS re was no intention, on the part of the designers of these protocols, to comply with OSI architecture.

The link is treated like a black box. The IETF explicitly does not intend to discuss transmission systems, which is a less academic but practical alternative to the OSI model.