File content as of revision 103:4f5512dd11cf:

// PPP-Blinky - "The Most Basic Internet Of Things"

// A Tiny Webserver Using Windows XP/7/8/10/Linux Dial-Up Networking Over A Serial Port.
// Also receives UDP packets and responds to ping (ICMP Echo requests)

// 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

// Handy reading material
// https://technet.microsoft.com/en-us/library/cc957992.aspx
// https://en.wikibooks.org/wiki/Serial_Programming/IP_Over_Serial_Connections
// http://bit.ly/dialup777error - how to solve Dial Up Error 777 in Windows 7/8/10
// 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

// Ok, enough talking, time to check out some code!!
#include "mbed.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.
#define SERIAL_PORT_MONITOR_NO /* change to SERIAL_PORT_MONITOR_YES for debug messages */

#ifndef SERIAL_PORT_MONITOR_NO

// here we define the OPTIONAL, second debug serial port for the various target boards
// insert your target board's port here if it's not in yet - if it works, please send it to me - thanks!!!
#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)
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
#else
#error Add your target board's second serial port here if you want to use debugging - or choose SERIAL_PORT_MONITOR_NO
#endif

#define debug(x...) xx.printf (x)

#else
#define debug(x...) {}
#endif

// verbosity flag used in debug printouts - change to 1 to see more debug info. Lots of interesting info.
#define v0 0
// verbosity flag used in debug printouts - change to 1 to see more debug info. Lots of interesting info.
#define v1 0
// verbosity flag used in debug printouts - change to 1 to see more debug info. Lots of interesting info.
#define v2 0

// 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:30px; color:#807070\">\
<h1>mbed PPP-Blinky Up and Running</h1>\
<h1 id=\"w\" style=\"text-align:center;\">0</h1>\
<h1><a href=\"http://bit.ly/pppBlink2\">Source on mbed</a></h1>\
</body>\
</html>"; // around 464 bytes long

// 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.
Serial pc(USBTX, USBRX); // 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
    unsigned int ident; // our IP ident value
    int crc; // for calculating IP and TCP CRCs
    int ledState; // state of LED1
    int httpPageCount;
    struct {
#define RXBUFLEN (1<<14)
        // the serial port receive buffer and packet buffer, size is RXBUFLEN (currently 8192 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 TCP_max_size 3300
        char buf[TCP_max_size]; // send and receive buffer large enough for unstuffed (decoded) hdlc frames
    } pkt; // ppp buffer objects
    struct {
        int frameStartIndex; // frame start marker
        int frameEndIndex; // frame end marker
        int frameFound; // we have found at least one start of frame already
    } hdlc; // hdlc frame objects
};

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

// Initialize our global structure, clear the buffer, etc.
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.ident=1000;
    ppp.ledState=0;
    ppp.hdlc.frameFound=0;
    ppp.hdlc.frameStartIndex=0;
    ppp.httpPageCount=0;
}

void led1Toggle()
{
    ppp.ledState = ppp.ledState? 0 : 1;
    led1 = ppp.ledState; // toggle led
}

void crcReset()
{
    ppp.crc=0xffff;   // crc restart
}

void crcDo(int x) // cumulative crc
{
    for (int i=0; i<8; i++) {
        ppp.crc=((ppp.crc&1)^(x&1))?(ppp.crc>>1)^0x8408:ppp.crc>>1; // crc calculator
        x>>=1;
    }
}

int crcBuf(char * buf, int size) // crc on an entire block of memory
{
    crcReset();
    for(int i=0; i<size; i++)crcDo(*buf++);
    return ppp.crc;
}

// fill our own receive buffer with characters from the PPP serial port
void fillbuf()
{
    if ( pc.readable() ) {
        int hd = (ppp.rx.head+1)&(RXBUFLEN-1); // increment/wrap head index
        if ( hd == ppp.rx.rtail ) {
            debug("\nReceive buffer full\n");
            return;
        }
        ppp.rx.buf[ppp.rx.head]=pc.getc(); // insert in rx buffer
        ppp.rx.head = hd; // update head pointer
        ppp.rx.buflevel++;
    }
}

int rxbufNotEmpty()   // check if rx buffer has data
{
    int emptyStatus = (ppp.rx.head==ppp.rx.tail) ? 0 : 1 ;
    return emptyStatus;
}

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

// 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()
{
    for(int i=0; i<ppp.pkt.len; i++) debug("%02x ", ppp.pkt.buf[i]);
    debug(" CRC=%04x Len=%d\n", ppp.pkt.crc, ppp.pkt.len);
}

void processHDLCFrame(int start, int end)   // process received frame
{
    led1Toggle(); // change led1 state on every frame we receive
    if(start==end) {
        return;
    }
    crcReset();
    char * dest = ppp.pkt.buf;
    ppp.pkt.len=0;
    int unstuff=0;
    int idx = start;
    while(1) {
        if (unstuff==0) {
            if (ppp.rx.buf[idx]==0x7d) unstuff=1;
            else {
                *dest = ppp.rx.buf[idx];
                ppp.pkt.len++;
                dest++;
                crcDo(ppp.rx.buf[idx]);
            }
        } else { // unstuff characters prefixed with 0x7d
            *dest = ppp.rx.buf[idx]^0x20;
            ppp.pkt.len++;
            dest++;
            crcDo(ppp.rx.buf[idx]^0x20);
            unstuff=0;
        }
        idx = (idx+1) & (RXBUFLEN-1);
        if (idx == end) break;
    }
    ppp.pkt.crc = ppp.crc & 0xffff;
    if (ppp.pkt.crc == 0xf0b8) { // check for good CRC
        void determinePacketType(); // declaration only
        determinePacketType();
    } else {
        if (0) { // don't normally report FCS because windows early-terminates packets, and early-terminated packets all have FCS errors.
            debug("\nPPP FCS(crc) Error CRC=%x Length = %d\n",ppp.pkt.crc,ppp.pkt.len); // ignore packets with CRC errors but print a debug line
        }
    }
}

void hdlcPut(int ch)   // do hdlc handling of special (flag) characters
{
    if ( (ch<0x20) || (ch==0x7d) || (ch==0x7e) ) {
        pc.putc(0x7d);
        pc.putc(ch^0x20);  // these characters need special handling
    } else {
        pc.putc(ch);
    }
}

void send_pppFrame()   // send a PPP frame in HDLC format
{
    int crc = crcBuf(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)
    pc.putc(0x7e); // hdlc start-of-frame "flag"
    for(int i=0; i<ppp.pkt.len; i++) {
        hdlcPut( ppp.pkt.buf[i] ); // send a character

        if((i&0x7f)==0) fillbuf(); // handle received characters very every 128 sent
    }
    pc.putc(0x7e); // hdlc end-of-frame "flag"
}

void ipcpConfigRequestHandler()
{
    debug("Their IPCP Config Req, Our Ack\n");
    ppp.pkt.buf[4]=2; // change code to ack
    send_pppFrame(); // simply acknowledge everything they ask for
    debug("Our IPCP Ask\n");
    ppp.pkt.buf[4]=1; // change code to request
    if (ppp.pkt.buf[8]==3) { // check if this is an ip address request
        ppp.pkt.buf[13]++; // choose next ip address
    } else { // not an IP request, respond with empty request
        ppp.pkt.len=10; // shortest ipcp packet possible (4 ppp + 4 ipcp + 2 crc)
        ppp.pkt.buf[7]=4; // no options in this request
    }
    send_pppFrame(); // send our request
}

void ipcpAckHandler()
{
    debug("Their IPCP Grant\n");
}

void ipcpNackHandler()
{
    debug("Their IPCP Nack\n");
}

void ipcpDefaultHandler()
{
    debug("Their IPCP Other\n");
}

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();
    }
}

void UDPpacket()
{
    char * udpPkt = ppp.pkt.buf+4; // udp packet start
    int headerSizeIP = (( udpPkt[0]&0xf)*4);
    char * udpBlock = udpPkt + headerSizeIP; // udp info start
#ifndef SERIAL_PORT_MONITOR_NO
    char * udpSrc = udpBlock; // source port
    char * udpDst = udpBlock+2; // destination port
#endif
    char * udpLen = udpBlock+4; // udp data length
    char * udpInf = udpBlock+8; // actual start of info
#ifndef SERIAL_PORT_MONITOR_NO
    int srcPort = (udpSrc[0]<<8) | udpSrc[1];
    int dstPort = (udpDst[0]<<8) | udpDst[1];
    char * srcIP = udpPkt+12; // udp src addr
    char * dstIP = udpPkt+16; // udp dst addr
#endif
#define UDP_HEADER_SIZE 8
    int udpLength = ((udpLen[0]<<8) | udpLen[1]) - UDP_HEADER_SIZE; // size of the actual udp data
    if(v0) debug("UDP %d.%d.%d.%d:%d ", srcIP[0],srcIP[1],srcIP[2],srcIP[3],srcPort);
    if(v0) debug("%d.%d.%d.%d:%d ",     dstIP[0],dstIP[1],dstIP[2],dstIP[3],dstPort);
    if(v0) debug("Len %03d", udpLength);
    int printSize = udpLength;
    if (printSize > 20) printSize = 20; // print only first 20 characters
    if (v1) {
        for (int i=0; i<printSize; i++) {
            char ch = udpInf[i];
            if (ch>31 && ch<127) {
                debug("%c", ch);
            } else {
                debug("_");
            }
        }
    }
    if (v0) debug("\n");
}

unsigned int dataCheckSum(unsigned char * ptr, int len)
{

    unsigned int sum=0;
    unsigned char placeHolder;
    if (len&1) {
        placeHolder = ptr[len];    // when length is odd stuff in a zero byte
        ptr[len]=0;
    }
    for (int i=0; i<len/2; i++) {
        unsigned int hi = *ptr;
        ptr++;
        unsigned int lo = *ptr;
        ptr++;
        unsigned int val = ( (hi<<8) | lo );
        sum = sum + val;
    }
    if (len&1) {
        ptr[len] = placeHolder;    // restore the last byte for odd lengths
    }
    sum = (sum & 0xffff) + (sum>>16);
    sum = (sum & 0xffff) + (sum>>16); // sum one more time to catch any carry from the carry
    return ~sum;
}

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;
    }
    sum = sum + (sum>>16);
    sum = ~sum;
    ppp.pkt.buf[14]= (sum>>8);
    ppp.pkt.buf[15]= (sum   );
}

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;
#ifndef SERIAL_PORT_MONITOR_NO
        int icmpIdent = (icmpType[4]<<8)|icmpType[5];
        int icmpSequence = (icmpType[6]<<8)|icmpType[7];
#endif
        if(v0) debug("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]);
        if(v0) debug("Ident %04x Sequence %04d ",icmpIdent,icmpSequence);
        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 (v0) {
            for (int i=0; i<printSize; i++) {
                char ch = icmpData[i];
                if (ch>31 && ch<127) {
                    debug("%c",ch);
                } else {
                    debug("_");
                }
            }
            debug("\n");
        }
        send_pppFrame(); // reply to the ping

    } else {
        if (v0) {
            debug("ICMP type=%d \n", icmpType[0]);
        }
    }
}

void IGMPpacket()   // internet group management protocol
{
    if (v0) {
        debug("IGMP type=%d \n", ppp.pkt.buf[28]);
    }
}

void dumpHeaderIP ()
{
    if (v0) {
        char * ipPkt = ppp.pkt.buf+4; // ip packet start
#ifndef SERIAL_PORT_MONITOR_NO
        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 * ident =      ipPkt+4; // 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
#endif
        char * srcAdr =     ipPkt+12; // 4 bytes
        char * dstAdr =     ipPkt+16; // 4 bytes = total of 20 bytes

#ifndef SERIAL_PORT_MONITOR_NO
        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 identIP = (ident[0]<<8)|ident[1];
        int flagsIP = flags[0]>>14&3;
        int ttlIP = ttl[0];
        int protocolIP = protocol[0];
        unsigned int checksumIP = (headercheck[0]<<8)|headercheck[1];
#endif

        char srcIP [16];
        snprintf(srcIP,16, "%d.%d.%d.%d", srcAdr[0],srcAdr[1],srcAdr[2],srcAdr[3]);
        char dstIP [16];
        snprintf(dstIP,16, "%d.%d.%d.%d", dstAdr[0],dstAdr[1],dstAdr[2],dstAdr[3]);
        debug("IP %s %s v%d h%d d%d e%d L%03d ",srcIP,dstIP,versionIP,headerSizeIP,dscpIP,ecnIP,packetLength);
        debug("i%04x f%d t%d p%d C%04x\n",identIP,flagsIP,ttlIP,protocolIP,checksumIP);
    }
}

void dumpHeaderTCP()
{
    if( v0 ) {
        int headerSizeIP     = (ppp.pkt.buf[4]&0xf)*4; // header size of ip portion
        char * tcpStart      =  ppp.pkt.buf+4+headerSizeIP; // start of tcp packet
#ifndef SERIAL_PORT_MONITOR_NO
        char * seqtcp        = tcpStart + 4;  // 4 bytes
        char * acktcp        = tcpStart + 8;  // 4 bytes
#endif
        char * flagbitstcp   = tcpStart + 12; // 9 bits
#ifndef SERIAL_PORT_MONITOR_NO
        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]);
#endif
        int flags = ((flagbitstcp[0]&1)<<8)|flagbitstcp[1];

        char flagInfo[10]; // text string presentating the TCP flags
        memset(flagInfo,'.', 9); // fill string with "........."
        memset(flagInfo+9,0,1); // null terminate string

        if (flags & (1<<0)) flagInfo[0]='F';
        if (flags & (1<<1)) flagInfo[1]='S';
        if (flags & (1<<2)) flagInfo[2]='R';
        if (flags & (1<<3)) flagInfo[3]='P';
        if (flags & (1<<4)) flagInfo[4]='A';
        if (flags & (1<<5)) flagInfo[5]='U';
        if (flags & (1<<6)) flagInfo[6]='E';
        if (flags & (1<<7)) flagInfo[7]='C';
        if (flags & (1<<8)) flagInfo[8]='N';
        debug("TCP Flags %s Seq %u Ack %u\n", flagInfo, seq, ack); // show the flags in debug
    }
}

int httpResponse(char * dataStart)
{
    int n=0; // number of bytes we have printed so far
    int nHeader; // byte size of HTTP header
    int contentLengthStart; // index where HTML starts
    int xFetch, httpGetRoot; // temporary storage of strncmp results

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

    httpGetRoot = strncmp(dataStart, "GET / HTTP/1.", 13); // found GET, respond to both HTTP/1.<anything>
    xFetch = strncmp(dataStart, "GET /x", 6); // found GET /x , respond to both HTTP/1.<anything>
    // for example, in linux, xFetch can be used as:   echo GET /x | nc 172.10.10.2
    if( (httpGetRoot==0) || (xFetch==0) ) {
        n=n+sprintf(n+dataStart,"HTTP/1.1 200 OK\r\nServer: mbed-PPP-Blinky\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
        //n=n+sprintf(n+dataStart,"%s", ourWebPage);
        memcpy(n+dataStart,rootWebPage,sizeof(rootWebPage));
        n = n + sizeof(rootWebPage);
    } else {
        if (xFetch == 0) {

#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 element)
            n=n+sprintf(n+dataStart,"<body>%d</body>",ppp.httpPageCount); // body = the http frame count
#else
#define BENCHMARK_USING_BROWSER_NO /* set to _YES if you want to use your browser as a benchmark tool of http fetches */
#ifndef BENCHMARK_USING_BROWSER_NO
            // a small script that reloads the page after 10 ms - handy for benchmarking using your web browser
            n=n+sprintf(n+dataStart, "<script>setTimeout(function(){location.reload();},10);</script><body>%d</body>",ppp.httpPageCount);
#else
            n=n+sprintf(n+dataStart,"%d",ppp.httpPageCount); // not valid html but fast, most browsers and curl are ok with it
#endif
#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>ppp-blinky-mbed</title>"); // html title (required element)
            n=n+sprintf(n+dataStart,"<body>Not Found</body>"); // not found message
        }
    }
    while( (n%4)!= 2) n=n+sprintf(n+dataStart," "); // insert spaces until n is exactly two away from a multiple of four
    n=n+sprintf(n+dataStart,"\r\n");  // add the last two characters (\r\n) - n is now an exact multiple of four

#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

    if (v2) {
        debug("HTTP Response: HTTP-header %d HTTP-content %d HTTP-total %d\n",nHeader,n-nHeader,n);
    }
    return n; // total byte size of our response
}

// if not an http response we just report the number of bytes received
// this is handy when you for example want to use netcat (nc.exe) to talk to PPP-Blinky
int tcpResponse(char * dataStart, int len)
{
    int n=0; // number of bytes we have printed so far

    n=n+sprintf(n+dataStart,"Got %04d bytes.\n",len); // report the number of bytes received
    while( (n%4)!= 0) n=n+sprintf(n+dataStart,"*"); // insert spaces until n is exactly two away from a multiple of four
    if (v2) {
        debug("TCP response %d byes\n",n);
    }
    return n; // total byte size of our response
}

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

    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

#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)

    // 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]=2; // tco window size = 700
    windowsizetcp[1]=0xbc; // tcp windows size = 700

    int doFin = 0; // flag to see if we have to send an extra FIN message to shut down the link

    // A sparse TCP flag interpreter that implements simple TCP connections from a single source
    // Clients are allowed ONE push packet, after which the link is closed with a FIN flag in the ACK packet
    // This strategy allows web browsers, netcat and curl to work ok while keeping the state machine simple

    switch ( flagsTCP ) {
        case TCP_FLAG_SYN:
            flagsOut = TCP_FLAG_SYN | TCP_FLAG_ACK; // something wants to connect - acknowledge it
            seq_out = seq_in+0x1000U; // create a new sequence number using their sequence as a base
            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_FIN; // set the FIN flag to start closing this TCP connection
                dataLen = httpResponse(tcpDataOut); // send an http response
            } else {
                dataLen = tcpResponse(tcpDataOut,tcpDataSize); // not a web request, just report number of received bytes
            }
            break;
        case TCP_FLAG_FIN | TCP_FLAG_PSH | TCP_FLAG_ACK:
        case TCP_FLAG_FIN | TCP_FLAG_ACK:
        case TCP_FLAG_RST:
            ack_out++; // for FIN flag we always have to increase sequence by 1
            break;
        case TCP_FLAG_FIN:
            flagsOut = TCP_FLAG_ACK | TCP_FLAG_FIN; // set the FIN flag to start closing the connection
            break;
        default:
            return; // ignore remaining packets
    } // switch

    // The TCP flag handling is now done
    // first we swap sourc 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

sendTCP:
    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 the ip ident number
    ppp.ident++; // get next ident number for our packet
    ident[0] = ppp.ident>>8;
    ident[1] = ppp.ident>>0; // insert OUR ident

    // Now we recalculate all the header sizes, and do the IP and TCP checksums
    int newPacketSize = headerSizeIP + headerSizeTCP + dataLen; // calculate size of the outgoing packet
    pktLen[0] = (newPacketSize>>8);
    pktLen[1]=newPacketSize; // ip total packet size
    ppp.pkt.len = newPacketSize+6; // ppp packet length
    tcpSize = headerSizeTCP + dataLen; // tcp packet size

    // 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

    send_pppFrame(); // All preparation complete - send the TCP response

    if (doFin==1) { // they want to shut down the link, so we have to send another packet to close our side of the link
        doFin=0;
        flagsOut = TCP_FLAG_ACK | TCP_FLAG_FIN; // tell them we are also finished
        goto sendTCP; // send our final packet for this conversation.
    }
    dumpHeaderIP();
    dumpHeaderTCP();
}

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) {
        debug("TCP %d ipHeader %d tcpHeader %d Data %d\n", ipPktLen, ipHeaderLen, headerSizeTCP, dataLen);    // 1 for more verbose
    }
    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
        debug("%s\n",ppp.pkt.buf+4+ipHeaderLen+headerSizeTCP);    // show the data
    }
}

void TCPpacket()
{
    dumpHeaderIP();
    dumpHeaderTCP();
    if (v2) {
        dumpDataTCP();
    }
    tcpHandler();
}

void otherProtocol()
{
    debug("Other IP protocol");
}

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();
    }
}

void LCPconfReq()
{
    debug("LCP Config ");
    if (ppp.pkt.buf[7] != 4) {
        ppp.pkt.buf[4]=4; // allow only "no options" which means Maximum Receive Unit (MRU) is 1500 bytes
        debug("Reject\n");
        send_pppFrame();
    } else {
        ppp.pkt.buf[4]=2; // ack zero conf
        debug("Ack\n");
        send_pppFrame();
        debug("LCP Ask\n");
        ppp.pkt.buf[4]=1; // request no options
        send_pppFrame();
    }
}

void LCPconfAck()
{
    debug("LCP Ack\n");
}

void LCPend()
{
    ppp.pkt.buf[4]=6;
    send_pppFrame(); // acknowledge
    ppp.online=0; // start hunting for connect string again
    pppInitStruct(); // flush the receive buffer
    debug("LCP End\n");
}

void LCPother()
{
    debug("LCP Other\n");
    dumpPPPFrame();
}

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();
    }
}

void discardedFrame()
{
    if (v0) {
        debug("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]);
    }
}

void determinePacketType()
{
    if ( ppp.pkt.buf[0] != 0xff ) {
        debug("byte0 != ff\n");
        return;
    }
    if ( ppp.pkt.buf[1] != 3    ) {
        debug("byte1 !=  3\n");
        return;
    }
    if ( ppp.pkt.buf[3] != 0x21 ) {
        debug("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();
    }
}

void wait_for_HDLC_frame()
{
    while(1) {
        fillbuf(); // handle received characters
        if ( rxbufNotEmpty() ) {
            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.hdlc.frameFound == 0) { // we are still waiting for a frame start
                    ppp.hdlc.frameFound = 1; // we found our first frame start
                    ppp.hdlc.frameStartIndex=ppp.rx.tail;  // remember where the frame character is in the buffer
                } else {
                    // we have previously found a frame start
                    ppp.hdlc.frameEndIndex=oldTail; // mark the frame end character
                    processHDLCFrame(ppp.hdlc.frameStartIndex, ppp.hdlc.frameEndIndex); // process the frame
                    ppp.rx.rtail = ppp.rx.tail;
                    ppp.hdlc.frameStartIndex = ppp.rx.tail; // where next frame will start
                    break;
                }
            }
        }
    }
}

void scanForConnectString()
{
    while(ppp.online == 0) {
        fillbuf();
        // search for Windows Dialup Networking "Direct Connection Between Two Computers" expected connect string
        char * found1 = strstr( (char *)ppp.rx.buf, "CLIENTCLIENT" );
        // also search for HDLC frame start character 0x7e
        void * found2 = memchr( (char *)ppp.rx.buf, 0x7e, RXBUFLEN );
        if( (found1 != NULL) | (found2 != NULL) ) {
            if (found1 != NULL) {
                // respond with Windows Dialup networking expected "Direct Connection Between Two Computers" response string
                pc.puts("CLIENTSERVER");
                if (v0) debug("Found connect string \"CLIENTCLIENT\"\n");
            }
            if (found2 != NULL) {
                if (v0) debug("Found HDLC frame start (7E)\n");
            }
            ppp.online=1; // we are connected, so stop looking for the string
        }
    }
}


int main()
{
    pc.baud(115200); // USB virtual serial port
#ifndef SERIAL_PORT_MONITOR_NO
    xx.baud(115200); // second serial port for debug messages
    xx.puts("\x1b[2J\x1b[HReady\n"); // VT100 code for clear screen & home
#endif
    pppInitStruct(); // initialize all the PPP properties
    while(1) {
        scanForConnectString(); // respond to connect command from windows dial up networking
        while(ppp.online) {
            wait_for_HDLC_frame();
        }
    }
}