Книга: Linux Network Administrator Guide, Second Edition

Protocol Overview

Protocol Overview

UUCP is used over different types of connections, such as serial lines, TCP, or sometimes even X.25; it is advantageous to transport UUCP within protocols designed specifically for the underlying network protocol. In addition, several implementations of UUCP have introduced different protocols that do roughly the same thing.

Protocols can be divided into two categories: streaming and packet protocols. Protocols of the streaming variety transfer a file as a whole, possibly computing a checksum over it. This is nearly free of overhead, but requires a reliable connection because any error will cause the whole file to be retransmitted. These protocols are commonly used over TCP connections but are not suitable for use over telephone lines. Although modern modems do quite a good job at error correction, they are not perfect, nor is there any error detection between your computer and the modem.

On the other hand, packet-oriented protocols split up the file into several chunks of equal size. Each packet is sent and received separately, a checksum is computed, and an acknowledgment is returned to the sender. To make this more efficient, sliding-window protocols have been invented, which allow for a limited number (a window) of outstanding acknowledgments at any time. This greatly reduces the amount of time uucico has to wait during a transmission. Still, the relatively large overhead compared to a streaming protocol makes packet protocols inefficient for TCP use, but ideal for telephone lines.

The width of the data path also makes a difference. Sometimes sending 8-bit characters over a serial connection is impossible; for instance, the connection could go through a stupid terminal server that strips off the eighth bit. When you transmit 8-bit characters over a 7-bit connection, they have to be quoted on transmission. In the worst-case scenerio, quoting doubles the amount of data to be transmitted, although compression done by the hardware may compensate. Lines that can transmit arbitrary 8-bit characters are usually called 8-bit clean. This is the case for all TCP connections, as well as for most modem connections.

Taylor UUCP 1.06 supports a wide variety of UUCP protocols. The most common of these are:

g

This is the most common protocol and should be understood by virtually all uucico s. It does thorough error checking and is therefore well suited for noisy telephone links. g requires an 8-bit clean connection. It is a packet-oriented protocol that uses a sliding-window technique.

i

This is a bidirectional packet protocol, which can send and receive files at the same time. It requires a full-duplex connection and an 8-bit clean data path. It is currently understood by Taylor UUCP only.

t

This protocol is intended for use over a TCP connection or other truly error-free networks. It uses packets of 1,024 bytes and requires an 8-bit clean connection.

e

This should basically do the same as t. The main difference is that e is a streaming protocol and is thus suited only to reliable network connections.

f

This is intended for use with reliable X.25 connections. It is a streaming protocol and expects a 7-bit data path. 8-bit characters are quoted, which can make it very inefficient.

G

This is the System V Release 4 version of the g protocol. It is also understood by some other versions of UUCP.

a

This protocol is similiar to ZMODEM. It requires an 8-bit connection, but quotes certain control characters like XON and XOFF.

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