This chapter should provide enough knowledge on networking to enable a systems administrator to connect a Linux server to a network and troubleshoot basic network-related problems. First, we will go over the basics of the 7-layer Open Systems Interconnection (:term:`OSI`) model, which is a standard framework with which to implement communication systems. Next, we will delve into each layer of the OSI model in more detail as it applies to the role of systems administration.

Before any discussion of networking, however, it's important to have a working knowledge of the numbered Request for Comments (:term:`RFC`) documents and how they apply to computer networking. These documents describe the mechanisms for the OSI layer implementations (e.g. TCP, IP, HTTP, SMTP) and as such are the authoritative source for how computers communicate with one another.

Starting in 1969, the RFC document series describes standards for how computers communicate. The series gets its name from the RFC process, wherein industry experts publish documents for the community at large and solicit comments on them. If the Internet community finds errors in a document, a new, revised version is published. This new version obsoletes the prior versions. Some documents, such as the document specifying email messages, have had several revisions.

The RFC Editor manages the RFC archive, as well as associated standards. New documents go to the RFC Editor for publication [1], whether revision or new standard. These documents go through a standardization process, eventually becoming Internet-wide standards. Many of the networking protocols discussed in later chapters have RFCs governing their behavior, and each section should provide information on the relevant RFCs.

There are a number of RFCs which don't pertain to any specific technology but which are nevertheless seminal. These documents establish procedure or policy which have shaped everything after, and as such have a timeless quality. In some cases, later documents make references to them. This list is given in increasing numerical order, though is not exhaustive.

• RFC 1796: Not All RFCs are Standards

  This document describes the different kinds of documents in the RFC series.

• RFC 2026: The Internet Standards Process

  This document (and those that update it) describes in detail how RFCs are published and how they become Internet standards.

• RFC 2119: Key words for use in RFCs to Indicate Requirement Levels

  This document, referenced in many following RFCs, presents a common vocabulary for specifying the relationship between a standard and implementations of that standard. It provides keywords that specify how closely an implementation needs to follow the standard for it to be compliant.

• RFC 5000: Internet Official Protocol Standards

  This document provides an overview of the current standards documented by the RFCs and which RFC is the most recent for each standard. This document is regularly updated with the current standards document status.

The OSI model describes seven layers of abstraction that enable software programs to communicate with each other on separate systems. The seven layers are designed to allow communication to occur between systems at a given level of abstraction without concern for how the lower levels are implemented. In this way, more complex protocols can be built on top of simpler ones that can be used interchangeably without modifying the higher-level code. The job of each layer is to provide some service to the layer above by using the services provided by the layer below.

• Layer 1 - Physical layer

  The physical layer describes the physical connections between devices. Most enterprise networks today implement Ethernet at the physical layer, described in IEEE 802.3 for wired connections and IEEE 802.11 for wireless networks.

• Layer 2 - Data link layer

  The data link layer defines the basic protocol for communicating between two points on a network that may consist of many intermediate devices and cables, possibly spanning a large geographic area. Ethernet defines the data link layer in addition to the physical layer, including (Media Access Control (:term:`MAC`) addresses that allow hosts to address their data as being relevant to one or more other hosts in particular.

• Layer 3 - Network layer

  The network layer is what allows many "Layer 2" networks to be interconnected, forming much larger "Layer 3" networks. It is this layer of the OSI model that enables the Internet to exist, using Internet Protocol (IP) addressing. IP addressing allows for a logical taxonomy of systems and networks built on top of the MAC addresses provided by Ethernet, which are more closely tied to the physical hardware. Version 4 of the Internet Protocol, most commonly found in production networks, is described in RFC 791.

• Layer 4 - Transport layer

  The transport layer is where things really start to get interesting for the systems administrator. It is at the transport layer that the Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Internet Control Message Protocol (ICMP) are defined. The TCP and UDP protocols allow data to be sent from one system to another using simple "socket" APIs that make it just as easy to send text across the globe as it is to write to a file on a local disk - a technological miracle that is often taken for granted. The ICMP protocol, used by the ubiquitous ping utility, allows small test packets to be sent to a destination for troubleshooting purposes.

• Layer 5 - Session layer

  The purpose of the session layer is to provide a mechanism for ongoing conversations between devices using application-layer protocols. Notable "Layer 5" protocols include Transport Layer Security / Secure Sockets Layer (TLS/SSL) and, more recently, Google's SPDY protocol.

• Layer 6 - Presentation layer

  The job of the presentation layer is to handle data encoding and decoding as required by the application. An example of this function is the Multipurpose Internet Mail Extensions (MIME) protocol, used to encode things other than unformatted ASCII text into email messages. Both the session layer and the presentation layer are often neglected when discussing TCP/IP because many application-layer protocols implement the functionality of these layers internally.

• Layer 7 - Application layer

  The application layer is where most of the interesting work gets done, standing on the shoulders of the layers below. It is at the application layer that we see protocols such as Domain Name System (DNS), HyperText Transfer Protocol (HTTP), Simple Mail Transfer Protocol (SMTP), and Secure SHell (SSH). The various application-layer protocols are at the core of a good systems administrator's knowledge base.

Internet Protocol Version 4 (IPv4) is the fourth version of the Internet protocol, the first version to be widely deployed. This is the version of the protocol you're most likely to encounter, and the default version of the IP protocol in Linux.

IPv4 uses a 32-bit address space most typically represented in 4 dotted decimal notation, each octet contains a value between 0-255, and is separated by a dot. An example address is below:

10.199.0.5

There are several other representations, like dotted hexadecimal, dotted octal, hexadecimal, decimal, and octal. These are infrequently used, and will be covered in later sections.

Both TCP RFC 793 and UDP RFC 768 provide data transfer between processes through ports. These process ports can be on the same computer or separate computers connected by a network. TCP provides the following: reliability, flow control, and connections (see Example Difference 1 below). UDP is less feature-rich, it does its work with a header that only contains a source port, destination port, a length, and a checksum. TCP provides its capabilities by sending more header data, more packets between ports and performing more processing. UDP requires less header data in the individual packets and requires fewer packets on the network to do its work. UDP does no bookkeeping about the fate of the packets sent from a source. They could be dropped because of a full buffer at a random router between the source and destination and UDP wouldn't account for it in itself (other monitoring systems can be put in place to do the accounting, however that is beyond the UDP protocol).

The choice of protocols to use is often based on whether the risk of losing packets in real-time without immediate alerting is acceptable. In some cases UDP may be acceptable, such as video or audio streaming where programs can interpolate over missing packets. However, TCP will be required due to its reliable delivery guarantee in systems that support banking or healthcare.

• Example 1

  The TCP protocol requires upfront communication and the UDP protocol does not. TCP requires an initial connection, known as the "three way handshake", in order to begin sending data. That amounts to one initial packet sent between ports from initiator of the communication to the receiver, then another packet sent back, and then a final packet sent from the initiator to the receiver again. All that happens before sending the first byte of data. In UDP the first packet sent contains the first byte of data.

• Example 2

  TCP and UDP differ in the size of their packet headers. The TCP header is 20 bytes and the UDP header is 8 bytes. For programs that send a lot of packets with very little data, the header length can be a large percentage of overhead data (e.g. games that send small packets about player position and state).

A subnet is a logical division of an IP network, and allows the host system to identify which other hosts can be reached on the local network. The host system determines this by the application of a routing prefix. There are two typical representations of this prefix: a netmask and CIDR.

Netmasks typically appear in the dotted decimal notation, with values between 0-255 in each octet. These are applied as bitmasks, and numbers at 255 mean that this host is not reachable. Netmask can also be referred to as a Subnet Mask and these terms are often used interchangeably. An example IP Address with a typical netmask is below:

CIDR notation is a two-digit representation of this routing prefix. Its value can range between 0 and 32. This representation is typically used for networking equipment. Below is the same example as above with CIDR notation:

Certain ranges of addresses were reserved for private networks. Using this address space you cannot communicate with public machines without a NAT gateway or proxy. There are three reserved blocks:

Cat5e, Cat6, and Cat6a are all coper transport mediums. They use twisted pair wiring, relying on the twist with differential signaling to prevent noise. This is the most common form of cabling for connecting computers in a network.

Fiber is a generic term that refers to optical transport mediums. It comes in several types, all of which look identical but are generally incompatible.

Multimode fiber is a less expensive fiber optic cable, that is typically useable with lower cost optical components. Depending on the application and bandwidth required, multimode fiber can have a range up to 2000 meters, but as low as 33 meters. It is very common to see it used for building backbones, and system to switch applications.

LC and SC connectors are the two most common type of fiber connectors.

LC is also known as a Lucent Connector. They are typically used for high-density applications, and are the type of connector used on SFPs or XFPs. Typically the connector is packaged in a duplex configuration with each cable side by side.

SC connectors are also know as Subscriber Connector, Square Connector, or Standard Connector. This is the type of connector typically used in the telecom industry. They have a larger form factor than the LC connectors, and are often found in single and duplex configurations.