Added first draft of a part15-tcpip write-up

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Writing a "bare metal" operating system for Raspberry Pi 4 (Part 15)
====================================================================
Adding a TCP/IP stack
---------------------
Having achieved "proof of life" from our Ethernet module in _part14-spi-ethernet_, you're doubtless wondering how to go from there to serving web pages, posting tweets on Twitter or perhaps even just simply responding to a ping!
This is where you'll need a fully-fledged TCP/IP stack that goes way beyond handcrafted ARPs, implementing many more protocols to achieve efficient bi-directional communication.
In this part we make use of some code from Guido Socher of [tuxgraphics.org](http://tuxgraphics.org/), designed to be a lightweight TCP/IP stack for embedded devices. I chose this because it was super simple to get working (or "port"), but you might want to look at [LwIP](https://en.wikipedia.org/wiki/LwIP) if you need something more advanced.
The code
--------
Most of the new code is in the _tcpip/_ subdirectory. I actually came across it in [this Github repository](https://github.com/ussserrr/maglev-ti-rtos) and, again, made only a very few cosmetic changes (`diff` is your friend!).
It did require me to expose the `strlen()` function we implemented in _lib/fb.c_, so that's added to _include/fb.h_. Similarly, we expose the `memcpy()` function we implemented in _kernel/kernel.c_, so that's added to _kernel/kernel.h_.
I also needed a single function that tells the ENC to send a packet. Nothing new here, just different packaging:
```c
void enc28j60PacketSend(unsigned short buflen, void *buffer) {
if (ENC_RestoreTXBuffer(&handle, buflen) == 0) {
ENC_WriteBuffer((unsigned char *) buffer, buflen);
handle.transmitLength = buflen;
ENC_Transmit(&handle);
}
}
```
This was also added to _kernel/kernel.h_. Finally, _kernel/kernel.c_ now calls a function called `net_test()` instead of our original `arp_test()`.
The changes to _arp.c_
----------------------
We initialise the network card in exactly the same way but, when we're done, we call this function in Guido's code:
```c
init_udp_or_www_server(myMAC, deviceIP);
```
This tells the TCP/IP library who we are, so we're all on the same page!
Finally, and aside from a little cleanup, the major change is the new `net_test()` function:
```c
void net_test(void)
{
while (1) {
while (!ENC_GetReceivedFrame(&handle));
uint16_t len = handle.RxFrameInfos.length;
uint8_t *buffer = (uint8_t *)handle.RxFrameInfos.buffer;
packetloop_arp_icmp_tcp(buffer, len);
}
}
```
This is an infinite loop which waits for an incoming packet and then simply passes it to Guido's `packetloop_arp_icmp_tcp()` function. This function implements some useful things, like responding to pings. I modified the routine to print a message to the screen when it sends a "pong" (look from line 1381), so we can see when it's in action!
_Imagine my excitement when I built, ran and could ping my RPi4 at 192.168.0.66 and get a response to both my laptop and my iPhone!_
I recommend reading [this page](http://tuxgraphics.org/electronics/200905/embedded-tcp-ip-stack.shtml) to give you some ideas about what else you might achieve with Guido's library...
![Pinging from my iPhone](images/15-tcpip-pinging.jpg)

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* packet. The client "web browser" as implemented here can also receive * packet. The client "web browser" as implemented here can also receive
* large pages. * large pages.
* *
* Chip type : ATMEGA88/168/328/644 with ENC28J60
*********************************************/ *********************************************/
#include "ip_config.h" #include "ip_config.h"
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// //
static uint8_t macaddr[6]; static uint8_t macaddr[6];
static uint8_t ipaddr[4]={0,0,0,0}; static uint8_t ipaddr[4]={0,0,0,0};
static uint8_t seqnum=0xa; // my initial tcp sequence number //static uint8_t seqnum=0xa; // my initial tcp sequence number
static void (*icmp_callback)(uint8_t *ip); static void (*icmp_callback)(uint8_t *ip);
// //
@ -1319,7 +1318,7 @@ uint8_t packetloop_icmp_checkreply(uint8_t *buf,uint8_t *ip_monitoredhost)
// of the tcp data if there is tcp data part // of the tcp data if there is tcp data part
uint16_t packetloop_arp_icmp_tcp(uint8_t *buf,uint16_t plen) uint16_t packetloop_arp_icmp_tcp(uint8_t *buf,uint16_t plen)
{ {
uint16_t len; // uint16_t len;
#if defined (TCP_client) #if defined (TCP_client)
uint8_t send_fin=0; uint8_t send_fin=0;
uint16_t tcpstart; uint16_t tcpstart;

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* *
* IP/ARP/UDP/TCP functions * IP/ARP/UDP/TCP functions
* *
* Chip type : ATMEGA88/168/328/644 with ENC28J60
*********************************************/ *********************************************/
//@{ //@{
#ifndef IP_ARP_UDP_TCP_H #ifndef IP_ARP_UDP_TCP_H

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// //
#endif #endif
//@} //@}