File content as of revision 145:098b6ed2f7f2:

// PPP-Blinky - "The Most Basic Internet Of a Thing"
// A Tiny HTTP Webserver Using Windows XP/7/8/10/Linux Dial-Up Networking Over A Serial Port.
// Receives UDP packets and responds to ping (ICMP Echo requests)
// WebSocket Service - see https://en.wikipedia.org/wiki/WebSocket

// Copyright 2016/2017 Nicolas Nackel aka Nixnax. Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

// Notes and Instructions
// http://bit.ly/PPP-Blinky-Instructions
// http://bit.ly/win-rasdial-config

// Handy reading material
// https://technet.microsoft.com/en-us/library/cc957992.aspx
// https://en.wikibooks.org/wiki/Serial_Programming/IP_Over_Serial_Connections
// http://atari.kensclassics.org/wcomlog.htm

// Handy tools
// https://ttssh2.osdn.jp/index.html.en - Tera Term, a good terminal program to monitor the debug output from the second serial port with!
// https://www.microsoft.com/en-us/download/details.aspx?id=4865 - Microsoft network monitor - real-time monitoring of PPP packets
// http://pingtester.net/ - nice tool for high rate ping testing
// http://www.sunshine2k.de/coding/javascript/crc/crc_js.html - Correctly calculates the 16-bit FCS (crc) on our frames (Choose CRC16_CCITT_FALSE), then custom relected-in=1, reflected-out=1
// https://technet.microsoft.com/en-us/sysinternals/pstools.aspx - psping for fast testing of ICMP ping function
// https://eternallybored.org/misc/netcat/ - use netcat -u 172.10.10.1 80 to send/receive UDP packets from PPP-Blinky
// Windows Powershell invoke-webrequest command - use it to stress test the webserver like this:  while (1){ invoke-webrequest -uri 172.10.10.1/x }

// Connecting PPP-Blinky to Linux
// PPP-Blinky can be made to talk to Linux - tested on Fedora - the following command, which uses pppd, works:
// pppd /dev/ttyACM0 115200 debug dump local passive noccp novj nodetach nocrtscts 172.10.10.1:172.10.10.2
// in the above command 172.10.10.1 is the adapter IP, and 172.10.10.2 is the IP of PPP-Blinky.
// See also https://en.wikipedia.org/wiki/Point-to-Point_Protocol_daemon

// Special pages when PPP-Blinky is running
// 172.10.10.2  root page
// 172.10.10.2/x  returns a number that increments every time you request a page - this is handy for testing
// 172.10.10.2/xb  also returns a number, but issues a fast refresh command. This allows you to use your browser to benchmark page load speed
// 172.10.10.2/ws  a simple WebSocket demo
// http://jsfiddle.net/d26cyuh2/  more complete WebSocket demo in JSFiddle

// Ok, enough talking, time to check out some code!!

#include "ppp-blinky.h"

// The #define below enables/disables a second (OPTIONAL) serial port that prints out interesting diagnostic messages.
// Change to SERIAL_PORT_MONITOR_YES to enable diagnostics messages. You need to wire a second serial port to your mbed hardware to monitor this.
// Using the second serial port will slow down packet response time
// Note - the LPC11U24 does NOT have a second serial port
#define SERIAL_PORT_MONITOR_NO /* change to SERIAL_PORT_MONITOR_YES for debug messages */

// here we define the OPTIONAL, second debug serial port for various mbed target boards
#ifdef SERIAL_PORT_MONITOR_YES
#if defined(TARGET_LPC1768)
Serial xx(p9, p10); // Second serial port on LPC1768 - not required to run, if you get compile error here, change #define SERIAL_PORT_MONITOR_YES to #define SERIAL_PORT_MONITOR_NO
#elif defined(TARGET_NUCLEO_F446RE) || defined(TARGET_NUCLEO_L152RE) || defined(TARGET_NUCLEO_L053R8) || defined(TARGET_NUCLEO_L476RG) || defined(TARGET_NUCLEO_F401RE)
Serial xx(PC_10, PC_11); // Second serial port on NUCLEO boards - not required to run, if you get compile error here, change #define SERIAL_PORT_MONITOR_YES to #define SERIAL_PORT_MONITOR_NO
#elif defined(TARGET_LPC11U24)
#error The LPC11U24 does not have a second serial port to use for debugging - change SERIAL_PORT_MONITOR_YES back to SERIAL_PORT_MONITOR_NO
#elif defined (TARGET_KL46Z) || (TARGET_KL25Z)
Serial xx(PTE0,PTE1); // Second serial port on FRDM-KL46Z board
#elif defined(YOUR_TARGET_BOARD_NAME_HERE)
// change the next line to YOUR target board's second serial port pin definition if it's not present - and if it works, please send it to me - thanks!!!
Serial xx(p9, p10); // change this to YOUR board second serial port pin definition - and please send it to me if it works!!!
#else
#error Add your target board's second serial port here if you want to use debugging - or simply change SERIAL_PORT_MONITOR_YES to SERIAL_PORT_MONITOR_NO
#endif
#define debugPrintf(x...) xx.printf (x) /* if we have a serial port we print debug messages */
#define debugPutc(x...) xx.putc(x)
#define debugBaudRate(x...) xx.baud(x)
#else
// if we don't have a debug port the debug print functions do nothing
#define debugPrintf(x...) {}
#define debugPutc(x...) {}
#define debugBaudRate(x...) {}
#endif

// verbosity flags used in debug printouts - change to 1 to see increasingly more detailed debug info.
#define v0 1
#define v1 0
#define v2 0
#define IP_HEADER_DUMP_YES /* YES for ip header dump */
#define TCP_HEADER_DUMP_YES /* YES for tcp header dump */

// this is the webpage we serve when we get an HTTP request to root (/)
// keep size under ~900 bytes to fit into a single PPP packet

const static char rootWebPage[] = "\
<!DOCTYPE html>\
<html>\
<head>\
<title>mbed PPP-Blinky</title>\
<script>\
window.onload=function(){\
setInterval(function(){function x(){return document.getElementById('w');};\
x().textContent=parseInt(x().textContent)+1;},100);};\
</script>\
</head>\
<body style=\"font-family: sans-serif; font-size:20px; text-align:center; color:#807070\">\
<h1>mbed PPP-Blinky Up and Running</h1>\
<h1 id=\"w\">0</h1>\
<h1><a href=\"http://bit.ly/pppBlink2\">Source on mbed</a></h1>\
<h1><a href=\"/ws\">WebSocket Demo</a></h1>\
<h1><a href=\"/x\">Benchmark 1</a></h1>\
<h1><a href=\"/xb\">Benchmark 2</a></h1>\
<h1><a href=\"http://jsfiddle.net/d26cyuh2/\">JSFiddle Demo</a></h1>\
</body>\
</html>"; // size = 634 bytes plus 1 null byte = 635 bytes

const static char webSocketPage[] = "\
<!DOCTYPE html>\
<html>\
<head>\
<title>mbed PPP-Blinky</title>\
<script>\
window.onload=function(){\
 var url=\"ws://172.10.10.2\";\
 var sts=document.getElementById(\"sts\");\
 var btn=document.getElementById(\"btn\");\
 var ctr=0;\
 function show(text){sts.textContent=text;}\
 btn.onclick=function(){\
  if(btn.textContent==\"Connect\"){\
   x=new WebSocket(url);\
    x.onopen=function(){\
    show(\"Connected to : \"+url);\
    btn.textContent=\"Send \\\"\"+ctr+\"\\\"\";\
   };\
  x.onclose=function(){show(\"closed\");};\
  x.onmessage=function(msg){show(\"PPP-Blinky Sent: \\\"\"+msg.data+\"\\\"\");};\
  } else {\
   x.send(ctr);\
   ctr=ctr+1;\
   btn.textContent=\"Send \\\"\"+ctr+\"\\\"\";\
  }\
 };\
};\
</script>\
<body style=\"font-family: sans-serif; font-size:25px; color:#807070\">\
<h1>PPP-Blinky WebSocket Test</h1>\
<div id=\"sts\">Idle</div>\
<button id=\"btn\" style=\"font-size: 100%; margin-top: 55px; margin-bottom: 55px;\">Connect</button>\
<h4><a href=\"/\">PPP-Blinky home</a></h4>\
</body>\
</html>"; // size = 916 bytes + 1 null byte = 917 bytes

// The serial port on your mbed hardware. Your PC should be configured to view this port as a standard dial-up networking modem.
// On Windows the model type of the modem should be selected as "Communications cable between two computers"
// The modem baud rate should be set to 115200 baud
// See instructions at the top.
// On a typical mbed hardware platform this serial port is a USB virtual com port (VCP) and the USB serial driver is supplied by the board vendor.
BufferedSerial pc(USBTX, USBRX, 100); // usb virtual com port for mbed hardware

DigitalOut led1(LED1); // this led toggles when a packet is received

// the standard hdlc frame start/end character. It's the tilde character "~"
#define FRAME_7E (0x7e)

/// a structure to keep all our ppp globals in
struct pppType {
    int online; // we hunt for a PPP connection if this is zero
    int hostIP; // ip address of host
    int fcs; // PPP "frame check sequence" - a 16-bit HDLC-like checksum used in all PPP frames
    int ledState; // state of LED1
    int httpPageCount;
    int firstFrame; // cleared after first frame
    struct {
#define RXBUFLEN (1<<11)
        // the serial port receive buffer and packet buffer, size is RXBUFLEN (currently 2048 bytes)
        char buf[RXBUFLEN]; // RXBUFLEN MUST be a power of two because we use & operator for fast wrap-around in ring buffer
        int head;
        int tail;
        int rtail;
        int buflevel;
    } rx; // serial port objects
    struct {
        int len; // number of bytes in buffer
        int crc; // PPP CRC (frame check)
#define PPP_max_size 2500
        // we are assuming 1000 bytes more than MTU size of 1500 - due to the PPP encoding of special bytes
        char buf[PPP_max_size]; // send and receive buffer large enough for raw, encoded PPP/HDLC frames
    } pkt; // ppp buffer objects
    struct {
        int frameStartIndex; // frame start marker
        int frameEndIndex; // frame end marker
    } hdlc; // hdlc frame objects
    struct {
        unsigned int ident; // our IP ident value (outgoing frame count)
    } ip; // ip related object
};

pppType ppp; // our global - definitely not thread safe

/// Initialize the ppp structure and clear the receive buffer
void pppInitStruct()
{
    memset( ppp.rx.buf, 0, RXBUFLEN);
    ppp.online=0;
    ppp.rx.tail=0;
    ppp.rx.rtail=0;
    ppp.rx.head=0;
    ppp.rx.buflevel=0;
    ppp.pkt.len=0;
    ppp.ip.ident=10000; // easy to recognize in ip packet dumps
    ppp.ledState=0;
    ppp.hdlc.frameStartIndex=0;
    ppp.httpPageCount=0;
    ppp.firstFrame=1;
}

/// Toggle the LED on every second PPP packet received
void led1Toggle()
{
    led1 = (ppp.ledState >> 1) & 1; // use second bit, in other words toggle LED only every second packet
    ppp.ledState++;

}

/// returns 1 after a connect messasge, 0 at startup or after a disconnect message
int connected()
{
    return ppp.online;
}

/// Check for available characters from the PC and read them into our own circular serial receive buffer at ppp.rx.buf.
/// Also, if we are offline and a 0x7e frame start character is seen, we go online immediately
void checkPc()
{
    char ch;
    if ( pc.readable() ) {
        int hd = (ppp.rx.head+1)&(RXBUFLEN-1); // increment/wrap head index
        if ( hd == ppp.rx.rtail ) {
            debugPrintf("\nReceive buffer full\n");
            return;
        }
        ch = pc.getc(); // read new character
        ppp.rx.buf[ppp.rx.head] = ch; // insert in our receive buffer
        if ( ppp.online == 0 ) {
            if (ch == 0x7E) {
                ppp.online = 1;
            }
        }
        ppp.rx.head = hd; // update head pointer
        ppp.rx.buflevel++;
    }
}

/// print to debug port while checking for incoming characters
void putcWhileCheckingInput( char outByte )
{
#ifdef SERIAL_PORT_MONITOR_YES
    checkPc();
    debugPutc( outByte );
    checkPc();
#endif
}

/// puts to debug port while checking the PPP input stream
void putsWhileCheckingInput( char * data )
{
#ifdef SERIAL_PORT_MONITOR_YES
    char * nextChar = data;
    while( *nextChar != 0 ) {
        putcWhileCheckingInput( *nextChar ); // write one character to debug port while checking input
        nextChar++;
    }
#endif
}

/// a sniffer tool to assist in figuring out where in the code we are having characters in the input buffer
void qq()
{
    if ( pc.readable() ) putsWhileCheckingInput( "Character available!\n" );
}

/// Initialize the PPP FCS (frame check sequence) total
void fcsReset()
{
    ppp.fcs=0xffff;   // crc restart
}

/// update the cumulative PPP FCS (frame check sequence)
void fcsDo(int x)
{
    for (int i=0; i<8; i++) {
        ppp.fcs=((ppp.fcs&1)^(x&1))?(ppp.fcs>>1)^0x8408:ppp.fcs>>1; // crc calculator
        x>>=1;
    }
    checkPc(); // handle input
}

/// calculate the PPP FCS (frame check sequence) on an entire block of memory
int fcsBuf(char * buf, int size) // crc on an entire block of memory
{
    fcsReset();
    for(int i=0; i<size; i++)fcsDo(*buf++);
    return ppp.fcs;
}

/// Get one character from our received PPP buffer
int pc_getBuf()
{
    int x = ppp.rx.buf[ ppp.rx.tail ];
    ppp.rx.tail=(ppp.rx.tail+1)&(RXBUFLEN-1);
    ppp.rx.buflevel--;
    return x;
}

/// Dump a PPP frame to the debug serial port
/// Note - the hex output of dumpPPPFrame() can be imported into WireShark
/// Capture the frame's hex output in your terminal program and save as a text file
/// In WireShark, use "Import Hex File". Options are: Offset=None, Protocol=PPP.
void dumpPPPFrame()
{
    char pbuf[30];
    for(int i=0; i<ppp.pkt.len; i++) {
        checkPc();
        sprintf(pbuf, "%02x ", ppp.pkt.buf[i]);
        checkPc();
        putsWhileCheckingInput(pbuf);
    }
    checkPc();
    sprintf(pbuf, " CRC=%04x Len=%d\n", ppp.pkt.crc, ppp.pkt.len);
    checkPc();
    putsWhileCheckingInput(pbuf);
}

/// Process a received PPP frame
void processPPPFrame(int start, int end)
{
    led1Toggle(); // change led1 state on every frame we receive
    if(start==end) {
        return; // empty frame
    }
    fcsReset();
    char * dest = ppp.pkt.buf;
    ppp.pkt.len=0;
    int unstuff=0;
    int idx = start;
    while(1) {
        checkPc();
        if (unstuff==0) {
            if (ppp.rx.buf[idx]==0x7d) unstuff=1;
            else {
                *dest = ppp.rx.buf[idx];
                ppp.pkt.len++;
                dest++;
                fcsDo(ppp.rx.buf[idx]);
            }
        } else { // unstuff characters prefixed with 0x7d
            *dest = ppp.rx.buf[idx]^0x20;
            ppp.pkt.len++;
            dest++;
            fcsDo(ppp.rx.buf[idx]^0x20);
            unstuff=0;
        }
        idx = (idx+1) & (RXBUFLEN-1);
        if (idx == end) break;
    }
    ppp.pkt.crc = ppp.fcs & 0xffff;
    if(0) dumpPPPFrame(); // set to 1 to dump ALL ppp frames
    if (ppp.pkt.crc == 0xf0b8) { // check for good CRC
        determinePacketType();
    } else {
#define REPORT_FCS_ERROR_YES
#ifdef REPORT_FCS_ERROR_YES
        char pbuf[50]; // local print buffer
        checkPc();
        sprintf(pbuf, "\nPPP FCS(crc) Error CRC=%x Length = %d\n",ppp.pkt.crc,ppp.pkt.len); // print a debug line
        checkPc();
        putsWhileCheckingInput( pbuf );
        if(0) dumpPPPFrame(); // set to 1 to dump frames with errors in them
#endif
    }
}

/// output a character to the PPP port while checking for incoming characters
void pcPutcWhileCheckingInput(int ch)
{
    checkPc(); // check input
    pc.putc(ch);
    checkPc();
}

/// do PPP HDLC-like handling of special (flag) characters
void hdlcPut(int ch)
{
    if ( (ch<0x20) || (ch==0x7d) || (ch==0x7e) ) {
        pcPutcWhileCheckingInput(0x7d);
        pcPutcWhileCheckingInput(ch^0x20);  // these characters need special handling
    } else {
        pcPutcWhileCheckingInput(ch);
    }
}

/// send a PPP frame in HDLC format
void sendPppFrame()
{
    int crc = fcsBuf(ppp.pkt.buf, ppp.pkt.len-2); // update crc
    ppp.pkt.buf[ ppp.pkt.len-2 ] = (~crc>>0); // fcs lo (crc)
    ppp.pkt.buf[ ppp.pkt.len-1 ] = (~crc>>8); // fcs hi (crc)
    pcPutcWhileCheckingInput(0x7e); // hdlc start-of-frame "flag"
    for(int i=0; i<ppp.pkt.len; i++) {
        wait_us(86); // wait one character time
        checkPc();
        hdlcPut( ppp.pkt.buf[i] ); // send a character
    }
    pcPutcWhileCheckingInput(0x7e); // hdlc end-of-frame "flag"
}

/// convert a network ip address in the buffer to an integer (IP adresses are big-endian, i.e most significant byte first)
int bufferToIP(char * buffer)
{
    int result=0;
    for(int i=0; i<4; i++) result = (result<<8)|(*buffer++ & 0xff);
    return result;
}

/// handle IPCP configuration requests
void ipcpConfigRequestHandler()
{
    debugPrintf("Their IPCP Config Req, Our Ack\n");
    if(ppp.pkt.buf[8]==3) {
        ppp.hostIP = bufferToIP(ppp.pkt.buf+10);
        debugPrintf("Host IP = %d.%d.%d.%d (%08x)\n", ppp.pkt.buf[10],ppp.pkt.buf[11],ppp.pkt.buf[12],ppp.pkt.buf[13],ppp.hostIP);
    }

    ppp.pkt.buf[4]=2; // change code to ack
    sendPppFrame(); // acknowledge everything they ask for - assume it's IP addresses

    debugPrintf("Our IPCP Ask (no options)\n");
    ppp.pkt.buf[4]=1; // change code to request
    ppp.pkt.buf[7]=4; // no options in this request
    ppp.pkt.len=10; // no options in this request shortest ipcp packet possible (4 ppp + 4 ipcp + 2 crc)
    sendPppFrame(); // send our request
}

/// handle IPCP acknowledge (do nothing)
void ipcpAckHandler()
{
    debugPrintf("Their IPCP Grant\n");
}

/// Handle IPCP NACK by sending our suggested IP address if there is an IP involved.
/// This is how Linux responds to an IPCP request with no options - Windows assumes any IP address on the submnet is OK.
void ipcpNackHandler()
{
    debugPrintf("Their IPCP Nack");
    if (ppp.pkt.buf[8]==3) { // check if the NACK contains an IP address parameter
        ppp.pkt.buf[4]=1; // assume the NACK contains our "suggested" IP address
        sendPppFrame(); // let's request this IP address as ours
        debugPrintf("Our IPCP ACK (received an IP)\n");
    } // if it's not an IP nack we ignore it
}

/// handle all other IPCP requests (by ignoring them)
void ipcpDefaultHandler()
{
    debugPrintf("Their IPCP Other\n");
}

/// process an incoming IPCP packet
void IPCPframe()
{
    int code = ppp.pkt.buf[4]; // packet type is here
    switch (code) {
        case 1:
            ipcpConfigRequestHandler();
            break;
        case 2:
            ipcpAckHandler();
            break;
        case 3:
            ipcpNackHandler();
            break;
        default:
            ipcpDefaultHandler();
    }
}

/// process an incoming UDP packet
void UDPpacket()
{
    char * udpPkt = ppp.pkt.buf+4; // udp packet start
    int headerSizeIP = (( udpPkt[0]&0xf)*4);
    char * udpBlock = udpPkt + headerSizeIP; // udp info start
    char * udpInf = udpBlock+8; // actual start of info
    char * udpSrc = udpBlock; // source port
    char * udpDst = udpBlock+2; // destination port
    char * udpLen = udpBlock+4; // udp data length
    char * udpCks = udpBlock+6; // udp checksum
    char * srcIP = udpPkt+12; // udp src addr
    char * dstIP = udpPkt+16; // udp dst addr
    if (udpLen); // shut up compiler warning for unused udpLen
#ifdef SERIAL_PORT_MONITOR_YES
    int srcPort = (udpSrc[0]<<8) | udpSrc[1];
    int dstPort = (udpDst[0]<<8) | udpDst[1];
    int udpLength = ((udpLen[0]<<8) | udpLen[1]) - 8; // size of the actual udp data
    if(v0) debugPrintf("UDP %d.%d.%d.%d:%d ", srcIP[0],srcIP[1],srcIP[2],srcIP[3],srcPort);
    if(v0) debugPrintf("%d.%d.%d.%d:%d ",     dstIP[0],dstIP[1],dstIP[2],dstIP[3],dstPort);
    if(v0) debugPrintf("Len %03d", udpLength);
    if (v1) {
        int printSize = udpLength;
        if (printSize > 20) printSize = 20; // print only first 20 characters
        for (int i=0; i<printSize; i++) {
            char ch = udpInf[i];
            if (ch>31 && ch<127) {
                debugPrintf("%c", ch);
            } else {
                debugPrintf("_");
            }
        }
    }
    if (v0) debugPrintf("\n");
#endif
    if (strncmp(udpInf,"echo", 4)==0) { // if the UDP string starts with "echo" we echo it back
        char tempHold[12]; // it's 12 long because we later reuse it when building the TCP pseudo-header
        memcpy(tempHold, srcIP,4);
        memcpy(srcIP, dstIP,4);
        memcpy(dstIP, tempHold,4); // swap ip address source/dest
        memcpy(tempHold, udpSrc,2);
        memcpy(udpSrc, udpDst,2);
        memcpy(udpDst, tempHold,2); // swap udp port source/dest
        memcpy(udpInf,"Got:",4);
        memset(udpCks,0,2); // we don't compute the checksum (it's optional), so have to zero it
        sendPppFrame();
    }
}

/// perform a 16-bit checksum. if the byte count is odd, stuff in an extra zero byte.
unsigned int dataCheckSum(unsigned char * ptr, int len)
{
    unsigned int i,hi,lo,sum;
    unsigned char placeHolder;
    if (len&1) {
        placeHolder = ptr[len];
        ptr[len]=0;  // if the byte count is odd, insert one extra zero byte is after the last real byte because we sum byte PAIRS
    }
    sum=0;
    i=0;
    while ( i<len ) {
        checkPc();
        hi = ptr[i++];
        lo = ptr[i++];
        sum = sum + ( (hi<<8) | lo );
    }
    if (len&1) {
        ptr[len] = placeHolder;    // restore the extra byte we made zero
    }
    sum = (sum & 0xffff) + (sum>>16);
    sum = (sum & 0xffff) + (sum>>16); // sum one more time to catch any carry from the carry
    return ~sum;
}

/// perform the checksum on an IP header
void headerCheckSum()
{
    int len =(ppp.pkt.buf[4]&0xf)*4; // length of header in bytes
    char * ptr = ppp.pkt.buf+4; // start of ip packet
    int sum=0;

    for (int i=0; i<len/2; i++) {
        int hi = *ptr;
        ptr++;
        int lo = *ptr;
        ptr++;
        int val = ( lo & 0xff ) | ( (hi<<8) & 0xff00 );
        sum = sum + val;
        checkPc();
    }
    sum = sum + (sum>>16);
    sum = ~sum;
    ppp.pkt.buf[14]= (sum>>8);
    ppp.pkt.buf[15]= (sum   );
}

/// handle a PING ICMP (internet control message protocol) packet
void ICMPpacket()   // internet control message protocol
{
    char * ipPkt = ppp.pkt.buf+4; // ip packet start
    char * pktLen = ipPkt+2;
    int packetLength = (pktLen[0]<<8) | pktLen[1]; // icmp packet length
    int headerSizeIP = (( ipPkt[0]&0xf)*4);
    char * icmpType = ipPkt + headerSizeIP; // icmp data start
    char * icmpSum = icmpType+2; // icmp checksum
#define ICMP_TYPE_PING_REQUEST 8
    if ( icmpType[0] == ICMP_TYPE_PING_REQUEST ) {
        char * ipTTL = ipPkt+8; // time to live
        ipTTL[0]--; // decrement time to live
        char * srcAdr = ipPkt+12;
        char * dstAdr = ipPkt+16;
#ifdef SERIAL_PORT_MONITOR_YES
        int icmpIdent = (icmpType[4]<<8)|icmpType[5];
        int icmpSequence = (icmpType[6]<<8)|icmpType[7];
        if(1) {
            char pbuf[50];
            checkPc();
            sprintf(pbuf, "ICMP PING %d.%d.%d.%d %d.%d.%d.%d ", srcAdr[0],srcAdr[1],srcAdr[2],srcAdr[3],dstAdr[0],dstAdr[1],dstAdr[2],dstAdr[3]);
            putsWhileCheckingInput( pbuf );
            checkPc();
            sprintf(pbuf, "Ident %04x Sequence %04d \n",icmpIdent,icmpSequence);
            checkPc();
            putsWhileCheckingInput( pbuf );
        }
#endif
        char src[4];
        char dst[4];
        memcpy(src, srcAdr,4);
        memcpy(dst, dstAdr,4);
        memcpy(srcAdr, dst,4);
        memcpy(dstAdr, src,4); // swap src & dest ip
        char * chkSum = ipPkt+10;
        chkSum[0]=0;
        chkSum[1]=0;
        headerCheckSum();  // new ip header checksum
#define ICMP_TYPE_ECHO_REPLY 0
        icmpType[0]=ICMP_TYPE_ECHO_REPLY; // icmp echo reply
        icmpSum[0]=0;
        icmpSum[1]=0; // zero the checksum for recalculation
        int icmpLength = packetLength - headerSizeIP; // length of ICMP data portion
        unsigned int sum = dataCheckSum( (unsigned char *)icmpType, icmpLength); // this checksum on icmp data portion
        icmpSum[0]=(sum>>8)&0xff;
        icmpSum[1]=(sum   )&0xff; // new checksum for ICMP data portion

        int printSize = icmpLength-8; // exclude size of icmp header
        char * icmpData = icmpType+8; // the actual payload data is after the header
        if (printSize > 10) printSize = 10; // print up to 20 characters
        if (0) {
            for (int i=0; i<printSize; i++) {
                char ch = icmpData[i];
                if (ch>31 && ch<127) {
                    putcWhileCheckingInput(ch);
                } else {
                    putcWhileCheckingInput('_');
                }
            }
            putcWhileCheckingInput('\n');
        }
        sendPppFrame(); // reply to the ping
    } else {
        if (v0) {
            debugPrintf("ICMP type=%d \n", icmpType[0]);
        }
    }
}

/// handle an IGMP (internet group managment protocol) packet (by ignoring it)
void IGMPpacket()
{
    if (v0) debugPrintf("IGMP type=%d \n", ppp.pkt.buf[28]);
}

/// dump the header of an IP pakcet on the (optional) debug serial port
void dumpHeaderIP (int outGoing)
{
#if defined(IP_HEADER_DUMP_YES) && defined(SERIAL_PORT_MONITOR_YES)
    checkPc(); // we are expecting the first character of the next packet
    char * ipPkt = ppp.pkt.buf+4; // ip packet start
    char * ident =      ipPkt+4;  // 2 bytes
#ifdef UNUSED_IP_VARIABLES
    char * srcAdr =     ipPkt+12; // 4 bytes
    char * dstAdr =     ipPkt+16; // 4 bytes = total of 20 bytes
    char * version =    ipPkt;    // top 4 bits
    char * ihl =        ipPkt;    // bottom 4 bits
    char * dscp =       ipPkt+1;  // top 6 bits
    char * ecn =        ipPkt+1;  // lower 2 bits
    char * pktLen =     ipPkt+2;  // 2 bytes
    char * flags =      ipPkt+6;  // 2 bits
    char * ttl =        ipPkt+8;  // 1 byte
    char * protocol =   ipPkt+9;  // 1 byte
    char * headercheck= ipPkt+10; // 2 bytes
    int versionIP = (version[0]>>4)&0xf;
    int headerSizeIP = (ihl[0]&0xf)*4;
    int dscpIP = (dscp[0]>>2)&0x3f;
    int ecnIP = ecn[0]&3;
    int packetLength = (pktLen[0]<<8)|pktLen[1]; // ip total packet length
    int flagsIP = flags[0]>>14&3;
    int ttlIP = ttl[0];
    int protocolIP = protocol[0];
    unsigned int checksumIP = (headercheck[0]<<8)|headercheck[1];
#endif
    int IPv4Id = (ident[0]<<8)|ident[1];
    char pbuf[50]; // local print buffer
    int n=0;
    n=n+sprintf(pbuf+n, outGoing ? "\x1b[34m" : "\x1b[30m" ); // VT100 color code, print black for incoming, blue for outgoing headers
    n=n+sprintf(pbuf+n, "%05d ",IPv4Id); // IPv4Id is a good way to correlate our dumps with net monitor or wireshark traces
#define DUMP_FULL_IP_ADDRESS_YES
#ifdef DUMP_FULL_IP_ADDRESS_YES
    char * srcAdr =     ipPkt+12; // 4 bytes
    char * dstAdr =     ipPkt+16; // 4 bytes = total of 20 bytes
    n=n+sprintf(pbuf+n, " %d.%d.%d.%d %d.%d.%d.%d ",srcAdr[0],srcAdr[1],srcAdr[2],srcAdr[3], dstAdr[0],dstAdr[1],dstAdr[2],dstAdr[3]); // full ip addresses
#endif
    putsWhileCheckingInput( pbuf );
#ifndef TCP_HEADER_DUMP_YES
    putsWhileCheckingInput('\x1b[30m\n'); // there is no TCP header dump, so terminate the line with \n and VT100 code for black
#endif
#endif
}

/// dump a TCP header on the optional debug serial port
void dumpHeaderTCP(int outGoing)
{
#if defined(TCP_HEADER_DUMP_YES) && defined(SERIAL_PORT_MONITOR_YES)
    int headerSizeIP     = (ppp.pkt.buf[4]&0xf)*4; // header size of ip portion
    char * tcpStart      =  ppp.pkt.buf+4+headerSizeIP; // start of tcp packet
    char * seqtcp        = tcpStart + 4;  // 4 bytes
    char * acktcp        = tcpStart + 8;  // 4 bytes
    char * flagbitstcp   = tcpStart + 12; // 9 bits
    unsigned int seq = (seqtcp[0]<<24)|(seqtcp[1]<<16)|(seqtcp[2]<<8)|(seqtcp[3]);
    unsigned int ack = (acktcp[0]<<24)|(acktcp[1]<<16)|(acktcp[2]<<8)|(acktcp[3]);
    if (seq && ack) {} // shut up the compiler about unused variables
    int flags = ((flagbitstcp[0]&1)<<8)|flagbitstcp[1];
    char flagInfo[9]; // text string presenting the 8 most important TCP flags
#define PRINT_ALL_TCP_FLAGS_YES
#ifdef PRINT_ALL_TCP_FLAGS_YES
    memset(flagInfo,'.', 8); // fill string with "........"
    flagInfo[8]=0; // null terminate string
    if (flags & (1<<0)) flagInfo[7]='F';
    if (flags & (1<<1)) flagInfo[6]='S';
    if (flags & (1<<2)) flagInfo[5]='R';
    if (flags & (1<<3)) flagInfo[4]='P';
    if (flags & (1<<4)) flagInfo[3]='A';
    if (flags & (1<<5)) flagInfo[2]='U';
    if (flags & (1<<6)) flagInfo[1]='E';
    if (flags & (1<<7)) flagInfo[0]='C';
#else
    if (flags & (1<<4)) flagInfo[0]='A'; // choose the most important flag to print
    if (flags & (1<<1)) flagInfo[0]='S';
    if (flags & (1<<0)) flagInfo[0]='F';
    if (flags & (1<<3)) flagInfo[0]='P';
    if (flags & (1<<2)) flagInfo[0]='R';
    flagInfo[1]=0; // ' '
    flagInfo[2]=0;
#endif
    putsWhileCheckingInput( flagInfo );
#define EVERY_PACKET_ON_A_NEW_LINE_YES
#ifdef EVERY_PACKET_ON_A_NEW_LINE_YES
    putsWhileCheckingInput("\x1b[30m\n"); // write a black color and newline after every packet
#endif
    if( outGoing && ( flags == 0x11 ) ) { // ACK/FIN - if this is an outgoing ACK/FIN its the end of a tcp conversation
        putcWhileCheckingInput('\n'); // insert an extra new line to mark the end of an HTTP the conversation
    }
#endif
}

/// Encode a buffer in base-64
void enc64(char * in, char * out, int len)
{
    const static char lut [] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
    int i,j,a,b,c;
    i=0;
    j=0;
    while(1) {
        if (i<len) {
            a = in[i++];
            out[j++] = lut[ ( (a >> 2) & 0x3f) ];
        } else break;
        if (i<len) {
            b = in[i++];
            out[j++] = lut[ ( (a << 4) & 0x30) | ( (b >> 4) & 0x0f) ];
            out[j++] = lut[ ( (b << 2) & 0x3c)  ];
        } else out[j++] = '=';
        if (i<len) {
            c = in[i++];
            j--;
            out[j++] = lut[ ( (b << 2) & 0x3c) | ( (c >> 6) & 0x03) ];
            out[j++] = lut[ ( (c >> 0) & 0x3f) ];
        } else out[j++] = '=';
    }
    out[j]=0;
}

/// handle a request for an http websocket
/// we end up here if we enter the following javascript in a web browser console: x = new WebSocket("ws://172.10.10.2");
int webSocketHandler(char * dataStart)
{
    int n=0; // byte counter
    char * key = strstr(dataStart, "Sec-WebSocket-Key: "); // search for the key in the payload
    if (key != NULL) {
        if (v0) putsWhileCheckingInput("WebSocket Request\n");
        char challenge [70];
        strncpy(challenge,key+19,70); // a local buffer
        *strchr(challenge,'\r')=0; // insert null so we can use sprintf
        strncat(challenge,"258EAFA5-E914-47DA-95CA-C5AB0DC85B11",70); // append websocket gui code
        char shaOutput [20]; // sha1 output
        sha1( shaOutput, challenge, strlen(challenge));
        char encOut[50];
        enc64( shaOutput, encOut, 20);
        char * versionstring = strstr(dataStart, "Sec-WebSocket-Version:");
        char * version = challenge;
        strncpy(version, versionstring,70); // copy version string
        *strchr(version,'\r')=0; // null terminate so we can sprintf it
        memset(dataStart,0,500); // blank out old data befor send the websocket response header
        n=n+sprintf(dataStart+n, "HTTP/1.1 101 Switching Protocols\r\n");
        n=n+sprintf(dataStart+n, "Upgrade: websocket\r\n");
        n=n+sprintf(dataStart+n, "Connection: Upgrade\r\n");
        n=n+sprintf(dataStart+n, "Sec-WebSocket-Accept: %s\r\n",encOut);
        n=n+sprintf(dataStart+n, "%s\r\n",version);
        n=n+sprintf(dataStart+n, "mbed-Code:  PPP-Blinky\r\n");
        n=n+sprintf(dataStart+n, "\r\n"); // websocket response header ending
    }
    return n; // this response should satisfy a web browser's websocket protocol request
}

#define TCP_FLAG_ACK (1<<4)
#define TCP_FLAG_SYN (1<<1)
#define TCP_FLAG_PSH (1<<3)
#define TCP_FLAG_RST (1<<2)
#define TCP_FLAG_FIN (1<<0)

/// respond to an HTTP request
int httpResponse(char * dataStart)
{
    int n=0; // number of bytes we have printed so far
    n = webSocketHandler( dataStart ); // test for and handle WebSocket upgrade requests
    if (n>0) return n; // if it's a WebSocket we already have the response, so return

    int nHeader; // byte size of HTTP header
    int contentLengthStart; // index where HTML starts
    int httpGet5,httpGet6,httpGetx, httpGetRoot; // temporary storage of strncmp results

    ppp.httpPageCount++; // increment the number of frames we have made

    httpGetRoot = strncmp(dataStart, "GET / HTTP/1.", 13);  // found a GET to the root directory
    httpGetx    = strncmp(dataStart, "GET /x", 6);          // found a GET to /x which we will treat special (anything starting with /x, e.g. /x, /xyz, /xABC?pqr=123
    httpGet5    = dataStart[5]; // the first character in the path name, we use it for special functions later on
    httpGet6    = dataStart[6]; // the second character in the path name, we use it for special functions later on
    // for example, you could try this using netcat (nc):    echo "GET /x" | nc 172.10.10.2
    if( (httpGetRoot==0) || (httpGetx==0) ) {
        n=n+sprintf(n+dataStart,"HTTP/1.1 200 OK\r\nServer: mbed-PPP-Blinky-v1\r\n"); // 200 OK header
    } else {
        n=n+sprintf(n+dataStart,"HTTP/1.1 404 Not Found\r\nServer: mbed-PPP-Blinky\r\n"); // 404 header
    }
    n=n+sprintf(n+dataStart,"Content-Length: "); // http header
    contentLengthStart = n; // remember where Content-Length is in buffer
    n=n+sprintf(n+dataStart,"?????\r\n"); // leave five spaces for content length - will be updated later
    n=n+sprintf(n+dataStart,"Connection: close\r\n"); // close connection immediately
    n=n+sprintf(n+dataStart,"Content-Type: text/html; charset=us-ascii\r\n\r\n"); // http header must end with empty line (\r\n)
    nHeader=n; // size of HTTP header
    if( httpGetRoot == 0 ) {
        // this is where we insert our web page into the buffer
        memcpy(n+dataStart,rootWebPage,sizeof(rootWebPage));
        n = n + sizeof(rootWebPage)-1; // one less than sizeof because we don't count the null byte at the end
    } else if ( (httpGet5 == 'w') && (httpGet6 == 's') ) { // "ws" is a special page for websocket demo
        memcpy(n+dataStart,webSocketPage,sizeof(webSocketPage));
        n = n + sizeof(webSocketPage)-1; // one less than size
    } else {
        if (httpGetx == 0) { // the page request started with "GET /x" - here we treat anything starting with /x special:

#define W3C_COMPLIANT_RESPONSE_NO
// change the above to W3C_COMPLIANT_RESPONSE_YES if you want a W3C.org compliant HTTP response
#ifdef W3C_COMPLIANT_RESPONSE_YES
            n=n+sprintf(n+dataStart,"<!DOCTYPE html><title>mbed PPP-Blinky</title>"); // html title (W3C.org required elements)
            n=n+sprintf(n+dataStart,"<body>%d</body>",ppp.httpPageCount); // body = the http frame count
#else
            if( httpGet6 == 'b' ) { // if the fetched page is "xb" send a meta command to let the browser continuously reload
                n=n+sprintf(n+dataStart, "<meta http-equiv=\"refresh\" content=\"0\">"); // reload loop - handy for benchmarking
            }
            // /x is a very short page, in fact, it is only a decimal number showing the http Page count
            n=n+sprintf(n+dataStart,"%d ",ppp.httpPageCount); // not really valid html but most browsers and curl are ok with it
#endif
        } else {
            // all other requests get 404 Not Found response with a http frame count - nice for debugging
            n=n+sprintf(n+dataStart,"<!DOCTYPE html><title>mbed PPP-Blinky</title>"); // html title (required element)
            n=n+sprintf(n+dataStart,"<body>Not Found</body>"); // not found message
        }
    }
#define CONTENTLENGTHSIZE 5
    char contentLengthString[CONTENTLENGTHSIZE+1];
    snprintf(contentLengthString,CONTENTLENGTHSIZE+1,"%*d",CONTENTLENGTHSIZE,n-nHeader); // print Content-Length with leading spaces and fixed width equal to csize
    memcpy(dataStart+contentLengthStart, contentLengthString, CONTENTLENGTHSIZE); // copy Content-Length to it's place in the send buffer
    return n; // total byte size of our response
}

/// handle TCP data that is not an HTTP get
/// this is the response if we have TCP data but it's not an HTTP GET
/// this is handy when you for example want to use netcat (nc.exe) to talk to PPP-Blinky
/// this could also be a websocket receive event - especially if the first byte is 0x81 (websocket data push)
int tcpResponse(char * dataStart, int len, int * outFlags)
{
    int n=0; // number of bytes we have printed so far
    if (dataStart[0] == 0x81) { // check if this is a websocket push message
        // this is most likely a websocket push message. you get this when you enter this in your browser console: x.send("my message");
        if (0) putsWhileCheckingInput( "Got data from websocket send()\n" );

        // for now we simply echo the websocket data back to the client - the client should therefore see an onmessage event
        // to display the echoed data in your browser, enter the following into the browser console: x.onmessage = function(msg){ console.log( msg.data ); }
        if (1) {
            char mask [4];
            memcpy ( mask, dataStart+2, 4); // websocket messages are "masked", so first we obtain the 4-byte mask
            int websocketMessageSize = len - 6;  // 1 byte prefix (0x81), 1 byte, 4 bytes mask = 6 bytes
            if((dataStart[1]&0x80)==0x80) // test if the mask bit is set, which means all data is xor'ed with the mask
                for (int i=0; i<websocketMessageSize; i++) dataStart[i+6]^= mask[i%4]; // unmask each byte with one of the mask bytes
            dataStart[1] = len-2; // add four extra bytes to the message length because we don't use mask bytes for the send
            memcpy(dataStart+2, "Got:",4); // insert our own text into the four mask bytes
            n = len; // our response size remains exactly the same length as what we received
        }
    } else if ( (dataStart[0]==0x88) && (dataStart[1]==0x80) && (len == 6) ) { // test for a websocket close request
        n=2; // our close command is only two bytes long because we don't use the four mask bytes
        dataStart[1]=0; // we don't have mask bytes on
    } else if (v1) putsWhileCheckingInput("TCP data received\n");
    return n; // total byte size of our response
}

/// handle an incoming TCP packet
/// use the first few bytes to figure out if it's a websocket, an http request or just pure incoming TCP data
void tcpHandler()
{
    // IP header
    char * ipPkt = ppp.pkt.buf+4; // ip packet start
    char * ihl =        ipPkt;    // bottom 4 bits
    char * pktLen =     ipPkt+2;  // 2 bytes
    char * ident =      ipPkt+4;  // 2 bytes
    char * protocol =   ipPkt+9;  // 1 byte
    char * headercheck= ipPkt+10; // 2 bytes
    char * srcAdr =     ipPkt+12; // 4 bytes
    char * dstAdr =     ipPkt+16; // 4 bytes = total of 20 bytes
    int headerSizeIP = (ihl[0]&0xf)*4;
    int packetLength = (pktLen[0]<<8)|pktLen[1]; // ip total packet length

    // TCP header
    char * tcp             = ppp.pkt.buf+4+headerSizeIP; // start of tcp packet
    char * srctcp        = tcp + 0;  // 2 bytes
    char * dsttcp        = tcp + 2;  // 2 bytes
    char * seqtcp        = tcp + 4;  // 4 bytes
    char * acktcp        = tcp + 8;  // 4 bytes
    char * offset        = tcp + 12; // 4 bits
    char * flagbitstcp   = tcp + 12; // 9 bits
    char * windowsizetcp = tcp + 14; // 2 bytes
    char * checksumtcp   = tcp + 16; // 2 bytes

    if(ident) {}; // shut up unused variable reference warning
    int tcpSize = packetLength - headerSizeIP;
    int headerSizeTCP = ((offset[0]>>4)&0x0f)*4; // size of tcp header only
    int protocolIP = protocol[0];
    char * tcpDataIn = tcp + headerSizeTCP; // start of data block after TCP header
    int tcpDataSize = tcpSize - headerSizeTCP; // size of data block after TCP header
    char * tcpDataOut = tcp + 20; // start of outgoing data
    unsigned int seq_in = (seqtcp[0]<<24)|(seqtcp[1]<<16)|(seqtcp[2]<<8)|(seqtcp[3]);
    unsigned int ack_in = (acktcp[0]<<24)|(acktcp[1]<<16)|(acktcp[2]<<8)|(acktcp[3]);
    unsigned int ack_out = seq_in + tcpDataSize;
    unsigned int seq_out = ack_in; // use their version of our current sequence number

    // first we shorten the TCP response header to only 20 bytes. This means we ignore all TCP option requests
    tcpSize = 20; // shorten total TCP packet size to 20 bytes (no data)
    headerSizeTCP = 20; // shorten outgoing TCP header size 20 bytes
    offset[0] =  (headerSizeTCP/4)<<4; // shorten tcp header size to 20 bytes
    packetLength = 40; // shorten total packet size to 40 bytes (20 ip + 20 tcp)
    pktLen[1] = 40; // set total packet size to 40 bytes (20 ip + 20 tcp)
    pktLen[0] =  0; // set total packet size to 40 bytes (20 ip + 20 tcp)

    int dataLen = 0; // most of our responses will have zero TCP data, only a header
    int flagsOut = TCP_FLAG_ACK; // the default case is an ACK packet
    int flagsTCP = ((flagbitstcp[0]&1)<<8)|flagbitstcp[1]; // the tcp flags we received
    windowsizetcp[0] = (700 >> 8  );   // tcp window size hi byte
    windowsizetcp[1] = (700 & 0xff);   // tcp window size lo byte

    // A sparse TCP flag interpreter that implements stateless TCP connections

    switch ( flagsTCP ) {
        case TCP_FLAG_ACK:
            return;
        case TCP_FLAG_SYN:
            flagsOut = TCP_FLAG_SYN | TCP_FLAG_ACK; // something wants to connect - acknowledge it
            seq_out = seq_in+0x10000000U; // create a new sequence number using their sequence as a starting point, increase the highest digit
            ack_out++; // for SYN flag we have to increase the sequence by 1
            break;
        case TCP_FLAG_ACK | TCP_FLAG_PSH:
            if ( (strncmp(tcpDataIn, "GET /", 5) == 0) ) { // check for an http GET command
                flagsOut = TCP_FLAG_ACK | TCP_FLAG_PSH; // set outgoing FIN flag to ask them to close from their side
                dataLen = httpResponse(tcpDataOut); // send an http response
            } else {
                dataLen = tcpResponse(tcpDataOut,tcpDataSize, &flagsOut); // not a web request, send a packet reporting number of received bytes
            }
            break;
        case TCP_FLAG_FIN:
        case TCP_FLAG_FIN | TCP_FLAG_ACK:
        case TCP_FLAG_FIN | TCP_FLAG_PSH | TCP_FLAG_ACK:
            flagsOut = TCP_FLAG_ACK | TCP_FLAG_FIN; // set outgoing FIN flag to ask them to close from their side
            ack_out++; // for FIN flag we have to increase the sequence by 1
            break;
        default:
            return; // ignore remaining packets
    } // switch

    // The TCP flag handling is now done
    // first we swap source and destination TCP addresses and insert the new ack and seq numbers
    char tempHold[12]; // it's 12 long because we later reuse it when building the TCP pseudo-header
    memcpy(tempHold, srcAdr,4);
    memcpy(srcAdr, dstAdr,4);
    memcpy(dstAdr, tempHold,4); // swap ip address source/dest
    memcpy(tempHold, srctcp,2);
    memcpy(srctcp, dsttcp,2);
    memcpy(dsttcp, tempHold,2); // swap ip port source/dest

    acktcp[0]=ack_out>>24;
    acktcp[1]=ack_out>>16;
    acktcp[2]=ack_out>>8;
    acktcp[3]=ack_out>>0; // save ack 32-bit integer
    seqtcp[0]=seq_out>>24;
    seqtcp[1]=seq_out>>16;
    seqtcp[2]=seq_out>>8;
    seqtcp[3]=seq_out>>0; // save seq 32-bit integer

    flagbitstcp[1] = flagsOut; // update the TCP flags

    // increment our outgoing ip packet counter
    ppp.ip.ident++; // get next ident number for our packet

    // Now we recalculate all the header sizes
    tcpSize = headerSizeTCP + dataLen; // tcp packet size
    int newPacketSize = headerSizeIP + tcpSize; // calculate size of the outgoing packet
    pktLen[0] = (newPacketSize>>8);
    pktLen[1]=newPacketSize; // ip total packet size
    ppp.pkt.len = newPacketSize+4+2; // ip packet length + 4-byte ppp prefix (ff 03 00 21) + 2 fcs (crc) bytes bytes at the end of the packet

    // the header is all set up, now do the IP and TCP checksums
    headercheck[0]=0; // IP header checksum
    headercheck[1]=0; // IP header checksum
    headerCheckSum(); // calculate the IP header checksum

    // now we have to build the so-called 12-byte TCP "pseudo-header" in front of the TCP header (containing some IP header values) in order to correctly calculate the TCP checksum
    // this header  contains the most important parts of the IP header, i.e. source and destination address, protocol number and data length.
    char * pseudoHeader = tcp-12; // mark the start of the TCP pseudo-header
    memcpy(tempHold, pseudoHeader, 12); // preserve the 12 bytes of the IP header where the TCP pseudo-Header will be built
    memcpy( pseudoHeader+0, srcAdr, 8); // IP source and destination addresses from IP header
    memset( pseudoHeader+8, 0, 1); // reserved, set to zero
    memset( pseudoHeader+9, protocolIP, 1); // protocol from IP header
    memset( pseudoHeader+10, tcpSize>>8, 1); // size of IP data (TCP packet size)
    memset( pseudoHeader+11, tcpSize, 1); // size of IP data (TCP packet size)

    // pseudo-header built, now we can calculate TCP checksum
    checksumtcp[0]=0;
    checksumtcp[1]=0;
    unsigned int pseudoHeaderSum=dataCheckSum((unsigned char *)pseudoHeader,tcpSize+12); // calculate the TCP checksum starting at the pseudo-header
    checksumtcp[0]=pseudoHeaderSum>>8;
    checksumtcp[1]=pseudoHeaderSum;
    memcpy( tcp-12, tempHold, 12); // restore the 12 bytes that the pseudo-header overwrote
    dumpHeaderIP(1); // dump outgoing IP header
    dumpHeaderTCP(1); // dump outgoing TCP header
    for (int i=0; i<45*1000/10; i++) { // 45 ms delay before sending frame - a typical internet delay time
        checkPc(); // catch any incoming characters
        wait_us(10); // wait less than 1 character duration at 115200
    }
    sendPppFrame(); // All preparation complete - send the TCP response
    if(0) dumpPPPFrame(); // set to 1 to dump transmitted ppp frame
    memset(ppp.pkt.buf+44,0,500); // flush out traces of previous data that we may scan for
}


/// dump the TCP data to the debug serial port
void dumpDataTCP()
{
    int ipPktLen     = (ppp.pkt.buf[6]<<8)|ppp.pkt.buf[7]; // overall length of ip packet
    int ipHeaderLen  = (ppp.pkt.buf[4]&0xf)*4; // length of ip header
    int headerSizeTCP = ((ppp.pkt.buf[4+ipHeaderLen+12]>>4)&0xf)*4;; // length of tcp header
    int dataLen = ipPktLen - ipHeaderLen - headerSizeTCP; // data is what's left after the two headers
    if (v1) {
        char pbuf[50]; // local print buffer
        checkPc();
        sprintf(pbuf, "TCP %d ipHeader %d tcpHeader %d Data %d\n", ipPktLen, ipHeaderLen, headerSizeTCP, dataLen);    // 1 for more verbose
        checkPc();
        putsWhileCheckingInput( pbuf );
    }
    if (dataLen > 0) {
        ppp.pkt.buf[4+ipHeaderLen+headerSizeTCP+dataLen]=0; // insert a null after the data so debug printf stops printing after the data
        putsWhileCheckingInput( ppp.pkt.buf+4+ipHeaderLen+headerSizeTCP );    // print the tcp payload data
        putsWhileCheckingInput("\n");
    }
}

/// handle an incoming TCP packet
void TCPpacket()
{
    dumpHeaderIP(0);     // dump incoming packet header
    dumpHeaderTCP(0);   // dump incoming packet header
    if (v2) dumpDataTCP();
    tcpHandler();
}

/// handle the remaining IP protocols by ignoring them
void otherProtocol()
{
    debugPrintf("Other IP protocol");
}

/// process an incoming IP packet
void IPframe()
{
    int protocol = ppp.pkt.buf[13];
    switch (protocol) {
        case    1:
            ICMPpacket();
            break;
        case    2:
            IGMPpacket();
            break;
        case   17:
            UDPpacket();
            break;
        case    6:
            TCPpacket();
            break;
        default:
            otherProtocol();
    }
}

/// respond to LCP (line configuration protocol) configuration request) by allowing no options
void LCPconfReq()
{
    debugPrintf("LCP Config ");
    if (ppp.pkt.buf[7] != 4) {
        ppp.pkt.buf[4]=4; // allow only "no options" which means Maximum Receive Unit (MRU) is default 1500 bytes
        debugPrintf("Reject\n");
        sendPppFrame();
    } else {
        ppp.pkt.buf[4]=2; // ack zero conf
        debugPrintf("Ack\n");
        sendPppFrame();
        debugPrintf("LCP Ask\n");
        ppp.pkt.buf[4]=1; // request no options
        sendPppFrame();
    }
}

/// handle LCP acknowledge packets by ignoring them
void LCPconfAck()
{
    debugPrintf("LCP Ack\n");
}

/// handle LCP end (disconnect) packets by acknowledging them and by setting ppp.online to false
void LCPend()
{
    ppp.pkt.buf[4]=6;
    sendPppFrame(); // acknowledge
    ppp.online=0; // start hunting for connect string again
    pppInitStruct(); // flush the receive buffer
    debugPrintf("LCP End (Disconnect from host)\n");
}

/// respond to other LCP requests by ignoring them
void LCPother()
{
    debugPrintf("LCP Other\n");
    dumpPPPFrame();
}

/// process incoming LCP packets
void LCPframe()
{
    int code = ppp.pkt.buf[4];
    switch (code) {
        case 1:
            LCPconfReq();
            break; // config request
        case 2:
            LCPconfAck();
            break; // config ack
        case 5:
            LCPend();
            break; // end connection
        default:
            LCPother();
    }
}

/// discard packets that are not IP, IPCP, or LCP
void discardedFrame()
{
    if (v0) debugPrintf("Frame is not IP, IPCP or LCP: %02x %02x %02x %02x\n", ppp.pkt.buf[0],ppp.pkt.buf[1],ppp.pkt.buf[2],ppp.pkt.buf[3]);
}

/// determine the packet type (IP, IPCP or LCP) of incoming packets
void determinePacketType()
{
    if ( ppp.pkt.buf[0] != 0xff ) {
        debugPrintf("byte0 != ff\n");
        return;
    }
    if ( ppp.pkt.buf[1] != 3    ) {
        debugPrintf("byte1 !=  3\n");
        return;
    }
    if ( ppp.pkt.buf[3] != 0x21 ) {
        debugPrintf("byte2 != 21\n");
        return;
    }
    int packetType = ppp.pkt.buf[2];
    switch (packetType) {
        case 0xc0:
            LCPframe();
            break;  // link control
        case 0x80:
            IPCPframe();
            break;  // IP control
        case 0x00:
            IPframe();
            break;  // IP itself
        default:
            discardedFrame();
    }
}

/// scan the PPP serial input stream for frame start markers
void waitForPppFrame()
{
    while(1) {
        checkPc(); // handle received characters
        if ( ppp.rx.head != ppp.rx.tail ) {
            int oldTail = ppp.rx.tail; // remember where the character is located in the buffer
            int rx = pc_getBuf(); // get the character
            if (rx==FRAME_7E) {
                if (ppp.firstFrame) { // is this the start of the first frame start
                    ppp.firstFrame=0;
                    ppp.rx.rtail = ppp.rx.tail; // update real-time tail with the virtual tail
                    ppp.hdlc.frameStartIndex = ppp.rx.tail; // remember where first frame started
                }  else {
                    ppp.hdlc.frameEndIndex=oldTail; // mark the frame end character
                    processPPPFrame(ppp.hdlc.frameStartIndex, ppp.hdlc.frameEndIndex); // process the frame
                    ppp.rx.rtail = ppp.rx.tail; // update real-time tail with the virtual tail
                    ppp.hdlc.frameStartIndex = ppp.rx.tail; // remember where next frame started
                    break;
                }
            }
        }
    }
}

/// scan for a connect string ("CLIENT") from the host PC, if found, we set ppp.oline to true, which will the IP packet scanner
void scanForConnectString()
{
    while(ppp.online == 0) {
        checkPc(); // gather received characters
        // search for Windows Dialup Networking "Direct Connection Between Two Computers" expected connect string
        char * found1 = strstr( (char *)ppp.rx.buf, "CLIENT" );
        if (found1 != NULL) {
            // respond with Windows Dialup networking expected "Direct Connection Between Two Computers" response string
            if (v0) debugPrintf("Connected: Found connect string \"CLIENT\", sent \"CLIENTSERVER\"\n");
            pc.puts("CLIENTSERVER");
            ppp.online=1; // we are connected - set flag so we stop looking for the connect string
            checkPc();
        }
    }
}

/// Initialize PPP data structure and set serial port(s) baud rate(s)
void initializePpp()
{
    pc.baud(115200); // USB serial port to pc
    debugBaudRate(115200); // baud rate for (optional) debug serial port
    debugPrintf("\x1b[2J\x1b[H\x1b[30mmbed PPP-Blinky HTTP & WebSocket server ready :)\n"); // VT100 codes for clear_screen, home, black_text - Tera Term is a handy VT100 terminal
    pppInitStruct(); // initialize all the PPP properties
}