Understanding network layers and protocols
Figure 1. Network layers
A computer network is composed of a number of "network layers," each providing a different restriction and/or guarantee about the data at that layer. The protocols at each network layer generally have their own packet formats, headers, and layout.
The seven traditional layers of a network (please see the Resources section for a link to a discussion of these) are divided into two groups: upper layers and lower layers. The sockets interface provides a uniform API to the lower layers of a network, and allows you to implement upper layers within your sockets application. And application data formats may themselves constitute further layers.
While the sockets interface theoretically allows access to protocol families other than IP, in practice, every network layer you use in your sockets application will use IP. For this tutorial we only look at IPv4; in the future IPv6 will become important also, but the principles are the same. At the transport layer, sockets support two specific protocols: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).
Sockets cannot be used to access lower (or higher) network layers; for example, a socket application does not know whether it is running over ethernet, token ring, 802.11b, or a dial-up connection. Nor does the sockets pseudo-layer know anything about higher-level protocols like NFS, HTTP, FTP, and the like (except in the sense that you might yourself write a sockets application that implements those higher-level protocols).
At times, the sockets interface is not your best choice for a network programming API. Many excellent libraries exist (in various languages) to use higher-level protocols directly, without your having to worry about the details of sockets. While there is nothing wrong with writing your own SSH client, for example, there is no need to do so simply to let an application transfer data securely. Lower-level layers than those sockets address fall pretty much in the domain of device driver programming.
As mentioned, when you program a sockets application, you have a choice between using TCP and using UDP. Each has its own benefits and disadvantages.
TCP is a stream protocol, while UDP is a datagram protocol. In other words, TCP establishes a continuous open connection between a client and a server, over which bytes may be written (and correct order guaranteed) for the life of the connection. However, bytes written over TCP have no built-in structure, so higher-level protocols are required to delimit any data records and fields within the transmitted bytestream.
UDP, on the other hand, does not require that any connection be established between client and server; it simply transmits a message between addresses. A nice feature of UDP is that its packets are self-delimiting; that is, each datagram indicates exactly where it begins and ends. A possible disadvantage of UDP, however, is that it provides no guarantee that packets will arrive in order, or even at all. Higher-level protocols built on top of UDP may, of course, provide handshaking and acknowledgments.
A useful analogy for understanding the difference between TCP and UDP is the difference between a telephone call and posted letters. The telephone call is not active until the caller "rings" the receiver and the receiver picks up. On the other hand, when you send a letter, the post office starts delivery without any assurance the recipient exists, nor any strong guarantee about how long delivery will take. The recipient may receive various letters in a different order than they were sent, and the sender may receive mail interspersed in time with those she sends. Unlike with the postal service (ideally, anyway), undeliverable mail always goes to the dead letter office, and is not returned to sender.
Beyond the protocol, TCP or UDP, there are two things a peer (a
client or server) needs to know about the machine it communicates
with: an IP address and a port. An IP address is a 32-bit data
value, usually represented for humans in "dotted quad" notation,
220.127.116.11. A port is a 16-bit data value,
usually simply represented as a number less than 65536, most often
one in the tens or hundreds range. An IP address gets a packet
to a machine; a port lets the machine decide which
process or service (if any) to direct it to. That is a slight
simplification, but the idea is correct.
The above description is almost right, but it misses something.
Most of the time when humans think about an Internet host (peer),
we do not remember a number like
instead a name like
gnosis.cx. Part 1 of
this tutorial demonstrated the use of DNS and local lookups to
find IP addresses from domain names.