Transmission Control Protocol and Java
The Transmission Control Protocol (TCP) is a stream-based method of network communication that is far different from any discussed previously. This chapter discusses TCP streams and how they operate under Java.
TCP provides an interface to network communications that is radically different from the User Datagram Protocol (UDP) discussed in Chapter 5. The properties of TCP make it highly attractive to network programmers, as it simplifies network communication by removing many of the obstacles of UDP, such as ordering of packets and packet loss. While UDP is concerned with the transmission of packets of data, TCP focuses instead on establishing a network connection, through which a stream of bytes may be sent and received.
In Chapter 5 we saw that packets may be sent through a network using various paths and may arrive at different times. This benefits performance and robustness, as the loss of a single packet doesn't necessarily disrupt the transmission of other packets. Nonetheless, such a system creates extra work for programmers who need to guarantee delivery of data. TCP eliminates this extra work by guaranteeing delivery and order, providing for a reliable byte communication stream between client and server that supports two-way communication. It establishes a "virtual connection" between two machines, through which streams of data may be sent (see Figure 6-1).
Figure 6-1 TCP establishes a virtual connection to transmit data.
TCP uses a lower-level communications protocol, the Internet Protocol (IP), to establish the connection between machines. This connection provides an interface that allows streams of bytes to be sent and received, and transparently converts the data into IP datagram packets. A common problem with datagrams, as we saw in Chapter 5, is that they do not guarantee that packets arrive at their destination. TCP takes care of this problem. It provides guaranteed delivery of bytes of data. Of course, it's always possible that network errors will prevent delivery, but TCP handles the implementation issues such as resending packets, and alerts the programmer only in serious cases such as if there is no route to a network host or if a connection is lost.
The virtual connection between two machines is represented by a socket. Sockets, introduced in Chapter 5, allow data to be sent and received; there are substantial differences between a UDP socket and a TCP socket, however. First, TCP sockets are connected to a single machine, whereas UDP sockets may transmit or receive data from multiple machines. Second, UDP sockets only send and receive packets of data, whereas TCP allows transmission of data through byte streams (represented as an InputStream and OutputStream). They are converted into datagram packets for transmission over the network, without requiring the programmer to intervene (as shown in Figure 6-2).
Figure 6-2 TCP deals with streams of data such as protocol commands, but converts streams into IP datagrams for transport over the network.
6.1.1 Advantages of TCP over UDP
The many advantages to using TCP over UDP are briefly summarized below.
188.8.131.52 Automatic Error Control
Data transmission over TCP streams is more dependable than transmission of packets of information via UDP. Under TCP, data packets sent through a virtual connection include a checksum to ensure that they have not been corrupted, just like UDP. However, delivery of data is guaranteed by the TCP data packets lost in transit are retransmitted.
You may be wondering just how this is achievedafter all, IP and UDP do not guarantee delivery; neither do they give any warning when datagram packets are dropped. Whenever a collection of data is sent by TCP using datagrams, a timer is started. Recall our UDP examples from Chapter 5, in which the DatagramSocket.setSoTimeout method was used to start a timer for a receive() operation. In TCP, if the recipient sends an acknowledgment, the timer is disabled. But if an acknowledgment isn't received before the time runs out, the packet is retransmitted. This means that any data written to a TCP socket will reach the other side without the need for further intervention by programmers (barring some catastrophe that causes an entire network to go down). All of the code for error control is handled by TCP.
Since the data sent between two machines participating in a TCP connection is transmitted by IP datagrams, the datagram packets will frequently arrive out of order. This would throw for a loop any program reading information from a TCP socket, as the order of the byte stream would be disrupted and frequently unreliable. Fortunately, issues such as ordering are handled by TCPeach datagram packet contains a sequence number that is used to order data. Later packets arriving before earlier packets will be held in a queue until an ordered sequence of data is available. The data will then be passed to the application through the interface of the socket.
184.108.40.206 Ease of Use
While storing information in datagram packets is certainly not beyond the reach of programmers, it doesn't lead to the most efficient way of communication between computers. There's added complexity, and it can be argued that the task of designing and creating software within a deadline provides complexity enough for programmers. Developers typically welcome anything that can reduce the complexity of software development, and the TCP does just this. TCP allows the programmer to think in a completely different way, one that is much more streamlined. Rather than being packaged into discrete units (datagram packets), the data is instead treated as a continuous stream, like the I/O streams the reader is by now familiar with. TCP sockets continue the tradition of Unix programming, in which communication is treated in the same way as file input and output. The mechanism is the same whether the developer is writing to a network socket, a communications pipe, a data structure, the user console, or a file. This also applies, of course, to reading information. This makes communicating via TCP sockets far simpler than communicating via datagram packets.
6.1.2 Communication between Applications Using Ports
It is clear that there are significant differences between TCP and UDP, but there is also an important similarity between these two protocols. Both share the concept of a communications port, which distinguishes one application from another. Many services and clients run on the same port, and it would be impossible to sort out which one was which without distributing them by port number. When a TCP socket establishes a connection to another machine, it requires two very important pieces of information to connect to the remote endthe IP address of the machine and the port number. In addition, a local IP address and port number will be bound to it, so that the remote machine can identify which application established the connection (as illustrated in Figure 63). After all, you wouldn't want your e-mail to be accessible by another user running software on the same system.
Figure 6-3 Local ports identify the application establishing a connection from other programs, allowing multiple TCP applications to run on the same machine.
Ports in TCP are just like ports in UDPthey are represented by a number in the range 165535. Ports below 1024 are restricted to use by well-known services such as HTTP, FTP, SMTP, POP3, and telnet. Table 6-1 lists a few of the well-known services and their associated port numbers.
6.1.3 Socket Operations
TCP sockets can perform a variety of operations. They can:
- Establish a connection to a remote host
- Send data to a remote host
- Receive data from a remote host
- Close a connection
In addition, there is a special type of socket that provides a service that will bind to a specific port number. This type of socket is normally used only in servers, and can perform the following operations:
- Bind to a local port
- Accept incoming connections from remote hosts
- Unbind from a local port
Table 6-1 Protocols and Their Associated Ports
Simple Mail Transfer Protocol
HyperText Transfer Protocol
Post Office Protocol 3
These two sockets are grouped into different categories, and are used by either a client or a server (since some clients may also be acting as servers, and some servers as clients). However, it is normal practice for the role of client and server to be separate.