layer4.c File Reference

Layer 4 adds simple error detection to layer 4 services. More...

#include "layer.h"

Macros
#define	LAYER4_H
#define	CHKFRSZ   4

Functions
int	layer4_write (char *msg, int len)
	Computes a checksum of the message and sends it to layer4_read. More...
int	layer4_read (char *msg, int max)
	Reads a message into memory starting at the address specified by msg. More...

Detailed Description

Layer 4 adds simple error detection to layer 4 services.

The errors to look for here involve transmission errors; we want to somehow make sure that the message received is the same as the message that was sent. The simplest approach to error detection is to use a checksum. The checksum is the sum % 65536. The resulting two-byte checksum should be sent along with the message data; the receiving end goes through the same steps to compute the checksum, then compares the received checksum to the computed checksum.

Peer-to-peer and Interface Protocols Using C

Author

Nicholas Guthrie guthr.nosp@m.n@rp.nosp@m.i.edu http//nickguthrie.com January 30, 2014

Compile with: gcc -c layer4.c -o ../build/layer4.o

Function Documentation

int layer4_read	(	char *	msg,
		int	max
	)

Reads a message into memory starting at the address specified by msg.

Parameters

[in]	msg	No more than max bytes will be put into memory, so max must limit the size of the message read.
[in]	max	The maximum size of the message.

Returns

-1 if max size is incorrect or if there is a transmission error. The size of the message in bytes is returned otherwise.

int layer4_write	(	char *	msg,
		int	len
	)

Computes a checksum of the message and sends it to layer4_read.

Parameters

[in]	msg	The message to send.
[in]	len	The length of the message.

Returns

The number of Bytes sent.