UNIX System Administration Handbook - Evi Nemeth [161]
FYIs are introductory or informational documents intended for a broad audience. They are usually an excellent place to start research on an unfamiliar topic. STDs document Internet protocols that have completed the IETF’s review and testing process and have been formally adopted as standards. BCPs document recommended procedures for Internet sites; they consist of administrative suggestions and for system administrators are often the most valuable of the RFC subseries.
RFCs, FYIs, STDs, and BCPs are numbered sequentially within their own series, so a document can bear several different identifying numbers. For example, RFC1635, How to Use Anonymous FTP, is also known as FYI0024.
RFCs are available from numerous sources. There’s a list of actively maintained RFC mirrors at www.rfc-editor.org, which is dispatch central for RFC-related matters.
13.2 NETWORKING ROAD MAP
Now that we’ve provided a bit of context, let’s take a look at the TCP/IP protocols themselves. TCP/IP is a “protocol suite,” a set of network protocols designed to work smoothly together. It includes several components, each defined by a standards-track RFC or series of RFCs:
• IP, the Internet Protocol, which routes data packets from one machine to another (RFC791)
• ICMP, the Internet Control Message Protocol, which provides several kinds of low-level support for IP, including error messages, routing assistance, and debugging help (RFC792)
• ARP, the Address Resolution Protocol, which translates IP addresses to hardware addresses (RFC823)1
• UDP, the User Datagram Protocol, and TCP, the Transmission Control Protocol, which deliver data to specific applications on the destination machine. UDP provides unverified, “best effort” transport for individual messages, whereas TCP guarantees a reliable, full duplex, flow controlled, error corrected conversation between processes on two hosts. (RFCs 768 and 793)
TCP/IP is designed around the layering scheme shown in Table 13.1.
Table 13.1 TCP/IP network model
a. Optionally addressing reliability and flow control issues
After TCP/IP had been implemented and deployed, the International Organization for Standardization came up with its own seven-layer protocol suite called OSI. It was a consummate design-by-committee white elephant, and it never really caught on because of its complexity and inefficiency. Some think a financial layer and a political layer should have been added to the original seven OSI layers.2
Exhibit A shows how the various components and clients of TCP/IP fit into its general architecture and layering scheme.
Exhibit A. One big happy TCP/IP family
13.3 PACKETS AND ENCAPSULATION
UNIX can support a variety of physical networks, including Ethernet, FDDI, token ring, ATM (Asynchronous Transfer Mode), wireless Ethernet, and serial-line-based systems. Hardware is managed within the link layer of the TCP/IP architecture, and higher-level protocols do not know or care about the specific hardware being used.
Data travels on a network in the form of packets, bursts of data with a maximum length imposed by the link layer. Each packet consists of a header and a payload. The header tells where the packet came from and where it’s going. It can also include checksums, protocol-specific information, or other handling instructions. The pay-load is the data to be transferred.
The name of the primitive data unit depends on the layer of the protocol. At the link layer it is called a frame, at the IP layer a packet, and at the TCP layer a segment. Here, we use “packet” as a generic term that encompasses all these cases.
As a packet travels down the protocol stack in preparation for being sent, each protocol adds its own header information. Each protocol’s finished packet becomes the payload part of the packet generated