Small Internet Protocol Stack using a standard serial port.

Dependencies:   mbed

PPP-Blinky - TCP/IP Networking Over a Serial Port

Note: The source code is at the bottom of this page.

/media/uploads/nixnax/blinky-connected.gif
A Windows desktop showing PPP-Blinky in the network connections list.

Describe PPP-Blinky in Three Sentences

PPP-Blinky is a tiny library that enables Internet protocols (IPv4) to any mbed target hardware by using only a serial port.

The code runs on processors with as little as 8k RAM, for example the Nucleo-L053R8 board.

PPP-Blinky uses the industry-standard PPP (Point-to-Point) Protocol and a tiny "stateless" TCP/IP stack.

No Ethernet Port Required

No ethernet port is required - PPP-Blinky uses a serial port to send IP packets to your PC.

PPP-Blinky emulates a standard dial-up modem and therefore connects to Windows, Linux or Adroid machines.

The code runs on most ARM mbed platforms such as the LPC11U24 shown in the picture below:

/media/uploads/nixnax/blinky-to-laptop1.jpg mbed LPC11u24 acting as a webserver to a Windows laptop.

Webserver

The Webserver and WebSocket functions are ideal for building browser-based GUIs on mbed-enabled hardware.

PPP-Blinky's HTTP webserver works with most web clients such as Internet Explorer, Mozilla Firefox, Google Chrome, Safari, Curl, wget and Lynx as well as Microsoft Powershell Invoke-Webrequest command.

In the image below Firefox web browser displays the main web page embedded into PPP-Blinky's code:

/media/uploads/nixnax/ppp-blinky-firefox.jpg Firefox web browser displays a web page embedded into PPP-Blinky's code

WebSocket Service

WebSocket is the most popular protocol standard for real-time bidirectional TCP/IP communication between clients and servers.
In the image below a small Internet Explorer script has connected to PPP-Blinky's WebSocket Service.
A websocket message was then sent by the browser and was echoed back by the WebSocket, triggering the onmessage event in the script.
The WebSocket service enables bidirectional real-time interaction between PPP-Blinky and any element in the browser DOM via JavaScript.
If you already have PPP-Blinky up and running you can test your WebSocket service using this: http://jsfiddle.net/d26cyuh2/112/embedded/result
Websockets are ideal for building browser-based GUIs for mbed hardware.

/media/uploads/nixnax/ppp-blinky-websocke-2.gif

Trying PPP-Blinky on your mbed board

You will need an mbed-enabled hardware board: https://developer.mbed.org/platforms/

Establish a serial port connection between your host PC and your mbed board. The easiest way is to use mbed hardware with a USB serial debug port. I've tried the ST-Micro Nucleo-L476RG, Nucleo-L152RE, Nucleo-F401RE, Nucleo-L432KC, Nucleo-L053R8, mbed-LPC11U24 and mbed-LPC1768 boards and they all work out of the box. Use the mbed online compiler to compile the software for your target board. Save the compiled binary to your hardware.

Before establishing a network connection, you can verify the operation of the code by opening a terminal program such as Tera Term, and setting the baud rate of the COM port on your mbed board to 115200 baud. LED1 should toggle for every two 0x7E (~) (i.e. tilde) characters you type, as 0x7E is the PPP frame start/end marker. Don't forget to close the port when your'e done testing, or else Windows Dial-up Networking will report that the COM port is in use by another program when you try to connect.

Once you are certain that the serial port and firmware is working, proceed to creating a new network connection on your PC -see below.

Creating a Dial-up Connection in Windows

/media/uploads/nixnax/modem.jpg

Setting up Dial-Up Networking (DUN) on your Windows 7 or 8 PC is essentially a two-step process: First, you create a new modem device, because PPP-blinky partially emulates a standard Windows serial port modem device. Second, you create a new Internet connection (in practice, a new network adapter) which is associated with your new "modem".

Step-by-step description of how to configure Windows for PPP-Blinky here:

/users/nixnax/code/PPP-Blinky/wiki/Configuring-Windows-Dial-Up-Networking

There is also a screen on how to set up Linux dial-up networking near the bottom of this page.

Connecting to PPP-Blinky from your PC

Once Windows networking is configured you can establish a dial-up connection to your mbed board over the USB virtual com port.

The IP address you manually assigned to the new dial-up network adapter (172.10.10.1) functions as a gateway to any valid IP address on that subnet. In the screen capture below, I'm sending pings from the Windows 8 command line to my ST-Micro Nucleo-L476RG board over the USB virtual serial Port. I'm also using a second serial port and Tera Term to capture the debug output from a second serial port on the hardware. The optional debug output from the board prints out the IP source and destination address and the first few bytes of the data payload. Note that the source is the adapter IP address, (172.10.10.1 in this case) and the destination is some other address on that subnet - all packets to the subnet are sent to our mbed hardware. For example, you could also ping 172.10.10.123 or, if your PPP-Blinky is running, simply click on this link: http://172.10.10.123

/media/uploads/nixnax/ping-cap-3.gif

One Million Pings!

In the image below the ICMP ("ping") echo reply service was tested by sending one million pings to ppp-Blinky. This took over two hours.
The ping tool used on the Windows 8 PC was psping.exe from PsTools by Mark Russinovich - http://bit.ly/PingFast
The average reply time for a short ping (1 byte of payload data) was 11 milliseconds at 115200 baud on the $10 Nucleo-L053R8 board - barely enough time for 130 bytes to be sent over the port!

/media/uploads/nixnax/ppp-blinky-ping-results.jpg

Monitoring PPP-Blinky Packets

The image below is from a Microsoft Network Monitor 3.4 capture session.

Responses from PPP-Blinky are shown in blue.

Frame 2 - Internet Explorer at IP 172.10.10.1 (the Dial-Up Adapter IP) requests a TCP connection by sending an S (SYN) flag.
Frame 3 - PPP-Blinky at IP 172.10.10.2 responds with an ACK in frame 3. One direction of the link is now established.
Frame 4 - The PC acknowledges the SYN sent by PPP-Blinky in frame 3. The TCP link is now fully established.
Frame 5 - The browser "pushes" (P flag is set) an HTTP GET request to PPP-Blinky.
Frame 6 - PPP-Blinky responds with a standard HTTP response "pushes" (P flag set) back a small web page. It also sets the A (ACK) flag to acknowledge the message sent in frame 6.
Frame 7 - The PC acknowledges reception of the HTTP payload.
Frame 8 - The PC starts to shut down the TCP connection by sending a FIN flag.
Frame 9 - PPP-Blinky acknowledges the FIN request - the connection is now closed in one direction. It also sets a FIN flag in the response to request closure of the opposite direction of the connection.
Frame 10 - The PC acknowledges the FIN request. The closing of the TCP connection is now confirmed in both directions.

/media/uploads/nixnax/ms-network-monitor-http-get-1.gif

Debug Output

PPP-Blinky can output handy debug information to an optional second serial port.
The image below shows the debug output (Ident, Source, Destination, TCP Flags) for a complete HTTP conversation.
The PC messages are displayed in black. PPP-Blinky messages are blue.
Notice how PPP-blinky automatically inserts a blank line after each full HTTP conversation.

/media/uploads/nixnax/tcp-data-3.gif

Creating a Dial-Up Connection in Linux

The screen below shows the required pppd command to connect to PPP-Blinky from a Linux machine. This was much simpler than Windows! The USB serial port of the mbed LPC1768 board registered as /dev/ttyACM0 on my Linux box. Do a websearch on pppd if you want to learn more about pppd, the Linux PPP handler. Near the bottom of the screen below, two webpages are fetched (/ and /y) by using the curl command on the command line. Gnome Webkit and Firefox work fine, too. Also try echo GET / HTTP/1.1 | nc 172.10.10.2 which uses netcat, the "Swiss army knife" of networking tools. PPP-Blinky was also tested with ApacheBench, the Apache server benchmark software. After 100000 fetches, the mean page fetch rate was reported as 6 page fetches per second for a small page.

/media/uploads/nixnax/pppd-screen.png

Caveats

PPP Blinky is an extremely sparse implementation (1.5k lines) of HTTP,WebSocket,TCP, UDP, ICMP, IPCP and LCP over PPP, requiring around 8kB of RAM. The minimum functionality required to establish connectivity is implemented. These are often acceptable tradeoffs for embedded projects as well as a handy tool to learn the practical details of everyday networking implementations.

main.cpp

Committer:
nixnax
Date:
2017-07-12
Revision:
80:753f5dd2e84e
Parent:
79:f0fc1c19a550
Child:
81:9ede60e9a2c8

File content as of revision 80:753f5dd2e84e:

#include "mbed.h"

// Copyright 2016 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.

// PPP-Blinky - "My Internet Of Thing"
// A Tiny Webserver Using Windows XP/7/8/10 Dial-Up Networking Over A Serial Port.

// Also receives UDP packets and responds to ping (ICMP Echo requests)

// 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!
// Wireshark - can't monitor Dial-Up network packets on windows, but useful - can import our dumpFrame routine's hex output
// Microsoft network monitor - real-time monitoring of all our 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)
// The curl.exe program in Windows Powershell - use it like this to stress test the webserver: while (1) { curl 172.10.10.1 }
// 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


// 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 /* or change to SERIAL_PORT_MONITOR_YES */

#ifndef SERIAL_PORT_MONITOR_NO
Serial xx(PC_10, PC_11); // Not required to run, if you get compile error here, change #define SERIAL_PORT_MONITOR_YES to #define SERIAL_PORT_MONITOR_NO
#define debug(x...) xx.printf (x)
#else
#define debug(x...) {}
#endif

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

// this is the webpage we serve when we get an HTTP request
// keep size under 900 bytes to fit into a single frame
const static char ourWebPage[] = "\
<!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)

// the serial port receive buffer and packet buffer

// a structure to keep all our ppp globals in
struct pppType {
    int online; // we hunt for a PPP connection if this is zero
    int ident; // our IP ident value
    unsigned int seq; // our TCP sequence number
    int crc; // for calculating IP and TCP CRCs
    int ledState; // state of LED1
    struct {
#define BUFLEN (1<<13)
        char buf[BUFLEN]; // BUFLEN MUST be a power of two because we use & operator for fast wrap-around in rxHandler
        //char * buf;
        volatile int head;
        volatile int tail;
    } rx; // serial port objects
    struct {
        int len; // number of bytes in buffer
        int crc; // PPP CRC (frame check)
        //char * buf; // the actual buffer
#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 frameBusy; // busy capturing a frame
    } hdlc; // hdlc frame objects
};

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

// Initialize our globals
void pppInitStruct()
{
    ppp.online=0;
    ppp.rx.tail=0;
    ppp.rx.head=0;
    ppp.pkt.len=0;
    ppp.ident=0;
    ppp.seq=1000;
    ppp.ledState=0;
    ppp.hdlc.frameBusy=0;
}

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

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;
}

void rxHandler() // serial port receive interrupt handler
{
    while ( pc.readable() ) {
        int hd = (ppp.rx.head+1)&(BUFLEN-1); // increment/wrap
        if ( hd == ppp.rx.tail ) break; // watch for buffer full
        ppp.rx.buf[ppp.rx.head]=pc.getc(); // insert in rx buffer
        ppp.rx.head = hd; // update head pointer
    }
}

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

int pc_getBuf() // get one character from the buffer
{
    __disable_irq(); // critical section start
    int x = ppp.rx.buf[ ppp.rx.tail ];
    ppp.rx.tail=(ppp.rx.tail+1)&(BUFLEN-1);
    __enable_irq(); // critical section end
    return x;
}

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) & (BUFLEN-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 (v0) {
        debug("PPP FCS(crc) Error CRC=%x Length = %d\n",ppp.pkt.crc,ppp.pkt.len); // ignore packets with CRC errors but print a debug line
    }
}

// Note - the hex output of dumpFrame() 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 dumpFrame()
{
    for(int i=0; i<ppp.pkt.len; i++) debug("%02x ", ppp.pkt.buf[i]);
    debug(" C=%02x %02x L=%d\n", ppp.pkt.crc&0xff, (ppp.pkt.crc>>8)&0xff, ppp.pkt.len);
}

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 sendFrame() // 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] );
    pc.putc(0x7e); // hdlc end-of-frame "flag"
}

void ipConfigRequestHandler()
{
    debug("IPCP Conf ");
    if ( ppp.pkt.buf[7] != 4 ) {
        debug("Rej\n"); // reject any options that are requested
        ppp.pkt.buf[4]=4;
        sendFrame();
    } else  {
        debug("Ack\n");
        ppp.pkt.buf[4]=2; // ack the minimum
        sendFrame(); // acknowledge
        debug("IPCP Ask\n");
        // send our own request now
        ppp.pkt.buf[4]=1; // request no options
        ppp.pkt.buf[5]++; // next sequence
        sendFrame(); // this is our request
    }
}

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

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

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

void IPCPframe()
{
    int code = ppp.pkt.buf[4]; // packet type is here
    switch (code) {
        case 1:
            ipConfigRequestHandler();
            break;
        case 2:
            ipAckHandler();
            break;
        case 3:
            ipNackHandler();
            break;
        default:
            ipDefaultHandler();
    }
}

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(v1) debug("UDP %d.%d.%d.%d:%d ", srcIP[0],srcIP[1],srcIP[2],srcIP[3],srcPort);
    if(v1) debug("%d.%d.%d.%d:%d ",     dstIP[0],dstIP[1],dstIP[2],dstIP[3],dstPort);
    debug("Len %d ", udpLength);
    int printSize = udpLength;
    if (printSize > 20) printSize = 20; // print only first 20 characters
    if (v0) {
        for (int i=0; i<printSize; i++) {
            char ch = udpInf[i];
            if (ch>31 && ch<127) {
                debug("%c", ch);
            } else {
                debug("_");
            }
        }
        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
        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]);
        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");
        }
        sendFrame(); // 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 ()
{
    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]);
    if (v0) debug("IP %s %s v%d h%d d%d e%d L%d ",srcIP,dstIP,versionIP,headerSizeIP,dscpIP,ecnIP,packetLength);
    if (v0) debug("i%04x f%d t%d p%d C%04x\n",identIP,flagsIP,ttlIP,protocolIP,checksumIP);
}

void dumpHeaderTCP()
{
    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';
    if (v1) {
        debug("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

    if(strncmp(dataStart, "GET / HTTP/1.1", 14) == 0 ) {
        n=n+sprintf(n+dataStart,"HTTP/1.1 200 OK\r\nServer: PPP-Blinky\r\n"); // http header
        n=n+sprintf(n+dataStart,"Content-Length: "); // http header
        int 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
        // this is where we insert our web page into the buffer
        n=n+sprintf(n+dataStart,"%s\r\n", ourWebPage);

#define CONTENTLENGTHSIZE 5
        char contentLengthString[CONTENTLENGTHSIZE+1]; // temporary buffer to create Content-Length string
        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

    } else { // all remaining requests get 404 Not Found response and heap size
        n=n+sprintf(n+dataStart,"HTTP/1.1 404 Not Found\r\nServer: PPP-Blinky\r\n"); // http header
        n=n+sprintf(n+dataStart,"Content-Length: "); // http header
        int 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

        n=n+sprintf(n+dataStart,"<!DOCTYPE html><html><head></head>"); // html start
        n=n+sprintf(n+dataStart,"<body><h1>File Not Found. Stack=0x%08x</h1></body>",&nHeader);
        n=n+sprintf(n+dataStart,"</html>\r\n"); // html end

        char contentLengthString[CONTENTLENGTHSIZE+1]; // temporary buffer to create Content-Length string
        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
}


void tcpHandler()
{
    char * ipPkt = ppp.pkt.buf+4; // ip packet start
    char * headercheck= ipPkt+10; // 2 bytes
    char * ihl =        ipPkt;    // bottom 4 bits
    char * ident =      ipPkt+4;  // 2 bytes
    char * pktLen =     ipPkt+2;  // 2 bytes
    char * protocol =   ipPkt+9;  // 1 byte
    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

    ident[0] = ppp.ident>>8;
    ident[1] = ppp.ident>>0; // insert OUR ident

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

    int tcpSize = packetLength - headerSizeIP;
    int headerSizeTCP = ((offset[0]>>4)&0x0f)*4; // size of tcp header only
    int protocolIP = protocol[0];

    int flagsTCP = ((flagbitstcp[0]&1)<<8)|flagbitstcp[1];

    char * dataStart = ppp.pkt.buf + 4 + headerSizeIP + headerSizeTCP; // start of data block after TCP header
    int tcpDataSize = tcpSize - headerSizeTCP; // size of data block after TCP header
    
    unsigned int ack = (seqtcp[0]<<24)|(seqtcp[1]<<16)|(seqtcp[2]<<8)|(seqtcp[3]) + tcpDataSize;
    unsigned int seq = (acktcp[0]<<24)|(acktcp[1]<<16)|(acktcp[2]<<8)|(acktcp[3]); // use their idea of our seq
    

#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)

    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 fastResponse = 0; // normally you wait 200ms before sending a packet but this can make it faster

    // 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_ACK:
            if ( tcpDataSize != 1 ) return;
            ack++;
        case TCP_FLAG_SYN:
            flagsOut = TCP_FLAG_SYN | TCP_FLAG_ACK; // something wants to connect - ack it
            ppp.seq = ppp.seq + 12345; // create a new sequence number (normally random)
            seq = ppp.seq; // create a new sequence number (normally random)
            ack++; // for SYN flag we have to increase their sequence by 1
            break;
        case TCP_FLAG_ACK | TCP_FLAG_PSH:
            flagsOut = TCP_FLAG_ACK | TCP_FLAG_FIN; // for every push we answer once AND close the link
            fastResponse = 1; // we can respond fast to a push
            if ( strncmp(dataStart, "GET ", 4) == 0) { // do we see an http GET command
                dataLen = httpResponse(dataStart); // send an http response
                while((dataLen %4 ) !=0) { // dataLen must be a multiple of four
                    dataLen++; // must be a multiple of four
                    dataStart[dataLen-1]=0; // clear the byte in the buffer
                    ack++;
                }
            }
            break;
        case TCP_FLAG_FIN:
        case TCP_FLAG_FIN | TCP_FLAG_ACK:
        case TCP_FLAG_FIN | TCP_FLAG_ACK | TCP_FLAG_PSH:
            ack++; // for FIN flag we have to increase sequence by 1
            break;
        default:
            return; // ignore remaining packets
    }
    

    // The TCP flag handling is now done
    // Now we have to recalculate all the header sizes, swap IP address/port source and destination, and do the IP and TCP checksums

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

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

    flagbitstcp[1] = flagsOut; // set up the new flags

    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 = s-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( s-12, tempHold, 12); // restore the 12 bytes that the pseudo-header overwrote

    if (fastResponse==1) {
        fastResponse=0; // reset and skip 200 ms wait
    } else {
        // normally, you wait 200 ms before sending a TCP packet
        // remove the wait to respond faster
        // wait(0.2);
    }
    sendFrame(); // All done! Send the TCP packet
}

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()
{
    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];
    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]);
    if (v0) {
        debug("IP %s %s v%d h%d d%d e%d L%d ",srcIP,dstIP,versionIP,headerSizeIP,dscpIP,ecnIP,packetLength);
    }
    if (v0) {
        debug("i%04x f%d t%d p%d C%04x\n",identIP,flagsIP,ttlIP,protocolIP,checksumIP);
    }
    if (v1) {
        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
        debug("Reject\n");
        sendFrame();
    } else {
        ppp.pkt.buf[4]=2; // ack zero conf
        debug("Ack\n");
        sendFrame();
        debug("LCP Ask\n");
        ppp.pkt.buf[4]=1; // request no options
        sendFrame();
    }
}

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

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

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

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 ( rxbufNotEmpty() ) {
        int rx = pc_getBuf();
        if (ppp.hdlc.frameBusy) {
            if (rx==FRAME_7E) {
                if (ppp.rx.tail == 0) { // did we just wrap around?
                    ppp.hdlc.frameEndIndex=BUFLEN-1; // wrap back to end of buffer
                } else {
                    ppp.hdlc.frameEndIndex=ppp.rx.tail-1; // remember where frame ends
                }
                processHDLCFrame(ppp.hdlc.frameStartIndex, ppp.hdlc.frameEndIndex);
                ppp.hdlc.frameStartIndex = ppp.rx.tail; // where next frame will start
            }
        } else {
            if (rx==FRAME_7E) {
                ppp.hdlc.frameBusy=1; // start gathering frame
                ppp.hdlc.frameStartIndex=ppp.rx.tail; // remember where frame started
            }
        }
    }
}

void scanForConnectString()
{
    if ( ppp.online==0 ) {
        // look for Windows Dialup Networking "Direct Connection Between Two Computers" expected connect string
        char * clientFound = strstr( (char *)ppp.rx.buf, "CLIENTCLIENT" );
        if( clientFound ) {
            strcpy( clientFound, "FOUND!FOUND!" ); // overwrite so we don't find it again
            // respond with Windows Dialup networking expected "Direct Connection Between Two Computers" response string
            pc.printf("CLIENTSERVER");
            ppp.online=1; // we are connected, so stop looking for the string
            debug("Connect string found\n");
        }
    }
}

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
    pc.attach(&rxHandler,Serial::RxIrq); // start the receive handler
    while(1) {
        scanForConnectString(); // respond to connect command from windows dial up networking
        wait_for_HDLC_frame();
    }
}