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.

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.


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:
Websockets are ideal for building browser-based GUIs for mbed hardware.


Trying PPP-Blinky on your mbed board

You will need an mbed-enabled hardware board:

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


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:


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 ( 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, ( 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 or, if your PPP-Blinky is running, simply click on this link:


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 -
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!


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 (the Dial-Up Adapter IP) requests a TCP connection by sending an S (SYN) flag.
Frame 3 - PPP-Blinky at IP 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.


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.


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 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.



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.


RevisionDateWhoCommit message
275:bb2d582c20d6 5 weeks ago nixnax Formatting default tip
274:b1bc40deb637 2 months ago nixnax 404 code added to not found message
273:3b3de186ea2f 3 months ago nixnax Typo fix
272:9ea94556c6f6 4 months ago nixnax Favicon mirrors LED state, remove a space in the response.
271:06c2c2a4e0bf 5 months ago nixnax Changed HTTP header response code
270:98099fbc46fb 5 months ago nixnax Added LED toggle option to web page.
269:ad9e99728fdf 6 months ago nixnax Initialize PPP-Ident as 11111 so packets can be recognized.
268:a81648a9c7bf 8 months ago nixnax Remove LED toggle on every frame.
267:40ea40e040d2 9 months ago nixnax Updated Copyright Date
266:47eea4b910ad 10 months ago nixnax Comment in waitForPppFrame()
265:b05d78e226a9 11 months ago nixnax Flat file containing ShA1
264:71c191a24d30 12 months ago nixnax Stream vs RawSerial comment
263:83a6d767c349 14 months ago nixnax Toggle LED for every received frame
262:c044fed611aa 15 months ago nixnax Change ping counter to ppp packet counter
261:272ec8e2a7d4 15 months ago nixnax Make mbed-code field in websocket response optional.
260:2c9a01c9d03e 15 months ago nixnax Don't count empty PPP frames.
259:f694046fc06f 16 months ago nixnax Simplified TCP responder.
258:1eb9abd66922 17 months ago nixnax small changes to PPP handler
257:1990a41a1e31 18 months ago nixnax Fixed comments, new jsfiddle URL
256:16543c8dd55c 19 months ago nixnax License update
255:14e83e1e2b40 21 months ago nixnax new mbed library revision 172
254:7345c9a6f4c2 21 months ago nixnax Display SystemCoreClock on main HTTP page
253:ee1044976b2c 22 months ago nixnax PPP frames were double-counted.
252:b3c8c5699153 22 months ago nixnax Count all PPP frames in ping count.
251:ae27cf9ec58d 23 months ago nixnax Added LCP packets to ping counter
250:35a3a19dc184 23 months ago nixnax Remove Connection: Close from http header
249:9ff0e20325f7 23 months ago nixnax More favicon work
248:5aca1c98fc24 23 months ago nixnax Use Data URI for fake favicon image
247:7b0a8e6fbb86 23 months ago nixnax Restore the escaping of characters >127
246:68a38a3bfdbe 23 months ago nixnax Revert the escaping (using 0x7d) of characters >127
245:544cea761769 23 months ago nixnax HTML comment example
244:f062ca9b6e06 23 months ago nixnax Increase RXBUFLEN to 4000
243:24f899297799 23 months ago nixnax Trap buffer full event with LED flashing
242:1e9215c09da5 24 months ago nixnax Escape characters > 0x7f
241:dece92cb7296 24 months ago nixnax Display ICMP ping count in webpage using strstr()
240:d7a01533397d 24 months ago nixnax Show number of pings in webpage.
239:426e50519204 24 months ago nixnax Added a counter for ICMP pings
238:f39289ed5a2e 24 months ago nixnax Remove unneeded define
237:796851817c26 2018-11-23 nixnax Toggle LED1 for each HTTP GET
236:b931c4909ee1 2018-11-23 nixnax new mbed library revision 171
235:43004d18b130 2018-11-22 nixnax mbed version 171 has issues, reverting
234:c581abc0feeb 2018-11-22 nixnax New mbed library rev 171
233:1b590d78ec9c 2018-11-14 nixnax restoring RXBUFLEN to 1500
232:db31d91f71d2 2018-11-14 nixnax Reduce rxbuflen to 500
231:6bd4523833a2 2018-10-24 nixnax Remove more debugPrintf
230:ee097c486e45 2018-10-21 nixnax Change debugPrintf to debugPuts where possible
229:a81c37376187 2018-09-30 nixnax Consistency in server header
228:7fb878500862 2018-09-17 nixnax removed LCPConfigurationRequest()
227:f87156f9ef11 2018-09-17 nixnax consolidate LCP handler
226:4898247048c7 2018-09-09 nixnax Removed delay in sendPPPbuf; Edits in webpages
225:4c9994e3becb 2018-09-08 nixnax mbed library revision 170 and updated jsfiddle revision 112
224:28a12be8d52d 2018-09-01 nixnax Updated jsfiddle
223:917b554cb8a5 2018-08-28 nixnax make ppp.rx.buf volatile
222:0ec93b1ff979 2018-08-26 nixnax Updated jsfiddle version
221:1d7c32d8256b 2018-08-25 nixnax Removed putcwhilechecking and putswhilechecking.; Added ppp.xx.bufferfull
220:54f45f924707 2018-08-20 nixnax Unrolled the CRC calculation.
219:cbf6a235d1f2 2018-08-01 nixnax New jsfiddle for WebSocket demo
218:651241ad5928 2018-07-28 nixnax Reduce RXBUFLEN to 1800
217:86498f9ab967 2018-07-22 nixnax Enable MAXBUFLEVEL in the TCP/IP headers for testing
216:e3c5a5b47295 2018-07-22 nixnax RXBUFLEN can now be any value