1. Onward with Chat!
Networking
CS 3470, Section 1

2. Chat v1 Handing back grading sheets Average score = A- Common issues
Makefile
Argument checking
Hardcoded port number
Even though taking it in as a parameter!

3. Makefile
# This is a comment. Don’t use C-style comments CC=gcc CFLAGS=-Wall all: chat chat: chat.c $(CC) -o chat chat.c $(CFLAGS) clean: rm chat

4. Argument Checking Part #1
int main(int argc, char **argv) {
/* Are we invoking the server or client? Let's figure it out based
* on the number of arguments. */
if(argc > 1)
client(argc, argv);
else
server();
return 0;

5. Argument Checking Part #2
int client(int argc, char **argv) {
struct hostent *hp; …
/* Checking parameters */
/* There had better be exactly 5, else we are outa here. */
if (argc!=5) {
print_usage();
exit(1); }

6. Argument Checking Part #3
/* There should be a flag in the first and third positions. */ if(strcmp(argv[1], "-p")==0) { port=atoi(argv[2]); host=argv[4]; } else if(strcmp(argv[3], "-p")==0) { port=atoi(argv[4]); host=argv[2]; else { /* Something is off */ print_usage(); exit(1);

7. Helpful Functions atoi – converts a string to an int
strcmp or strncmp – compares two strings

8. Any other questions?

9. Onward Next program: full chat! Back and forth chatting
Pretty formatting like on the original project specification
Packet formatting
Version header
Src IP header
Dst IP header
Checksum field
Length of message (always 140)
Bad chat
For testing!

10. Back and Forth Chatting
Do this first!
Both send() and rcv() must be in the client and server function’s while loops

11. Packet Formatting Your packet should look like this
(Field sizes coming soon)
Version
Src IP
Dest IP
Data
Checksum

12. Packet Formatting Version = 2 Src IP = IP of sender
Dest IP = IP of receiver
Data = chat message, always 140 bytes/characters long
Checksum
Version
Src IP
Dest IP
Data
Checksum

13. Packet Formatting How do I do this?
Create a struct packet that holds all those fields
Send the struct packet over the socket

14. Creating a Struct Packet
Assign 2 to version field
Get source and destination IP from socket, assign to src and dest fields
Get user-typed chat message from fgets(), use strcpy (“string copy”) to copy it into data field
Use checksum function on page 95 of your book to compute checksum of all previous fields

15. Checksum
Sender creates checksum, copies it to end of packet, and sends packet through socket
Receiver receives packet, creates checksum, and checks to see that computed checksum matches checksum found in packet
If matches, it shouldn’t do anything
If it doesn’t match, it should display an error to the user.

16. Bad Chat How can we test our checksum routines?
We can invoke a bad checksum routine on chat by using a “–b” flag with either the client or the server
This will invoke a second “bad checksum” function that appends an incorrect checksum to our packet
Computing checksum on receiver side should always use “good” checksum function

17. Other Things
Cannot have any pointers in struct packet before sending it over the socket
Why?

18. Other Things
Must be aware of byte ordering!

19. Byte Ordering High-order byte Low-order byte MSB 16-bit value LSB
Increasing memory address
Address A+1
Address A
Little-endian
byte order
High-order byte
Low-order byte
MSB bit value LSB
Consider a 16-bit integer that is made up of 2 bytes: there are two ways to store the 2 bytes in memory, with low order byte at the starting address, known as little endian byte order, or with the high order byte at the starting address, known as big endian byte order
Big-endian
byte order
Low-order byte
High-order byte
Address A+1
Address A
Increasing memory address

20. Implications of Byte Order
Unfortunately there is no standard between these two byte orderings and we encounter systems that use both formats
We refer to the byte ordering used by a given system as host byte order
The sender and the receiver must agree on the order in which the bytes of these multi-byte field transmitted: specify network byte order, which is big-endian byte ordering

21. Byte Order Functions
#include <netinet.h> /* Host to network */ uint16_t htons(uint16_t host16bitvalue) Converts a 16-bit integer from host to network byte order uint32_t htonl(uint32_t host32bitvalue) Converts a 32-bit integer from host to network byte order Both return: value in network byte order /* Network to host */ uint16_t ntohs(uint16_t net16bitvalue) uint32_t ntohl(uint32_t net32bitvalue) Both return: value in host byte order

22. When do we use hton/ntoh functions?
Use hton the port number in struct sockaddr_in
If we create a custom struct to hold our headers and data
Sending our data through send() and recv() functions
E.g., if our first struct member is a 2-byte header, and sender/receiver have different memory orderings, number would look very different to each machine

23. Address Conversion Functions
#include <arpa/inet.h> in_addr_t inet_addr(const char *strptr); /* return 32-bit binary network byte ordered IPv4 address; INADDR_NONE if error, deprecated and replaced by inet_aton() */ char *inet_ntoa(struct in_addr inaddr); /* returns: pointer to dotted-decimal string */