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QNEthernet, an lwIP-Based Ethernet Library For Teensy 4.1

Version: 0.22.0-snapshot

The QNEthernet library provides Ethernet functionality for the Teensy 4.1. It's compatible with the Arduino-style API.

The low-level code is based on code by Paul Stoffregen, here:
https://github.com/PaulStoffregen/teensy41_ethernet

All the Teensy stuff is under the MIT license, so I'm making this library under the MIT license as well. Please see the lwip-info/ directory for the info files provided with the lwIP release.

Table of contents

  1. Differences, assumptions, and notes
    1. Two core differences
    2. Other differences and notes
  2. Additional functions and features not in the Arduino API
    1. Ethernet
    2. EthernetClient
      1. TCP socket options
    3. EthernetServer
    4. EthernetUDP
      1. parsePacket() return values
    5. EthernetFrame
    6. MDNS
    7. DNSClient
    8. Print utilities
    9. IPAddress operators
    10. operator bool() and explicit
  3. How to run
    1. Asynchronous use is not supported
  4. How to write data to connections
    1. Write immediacy
  5. A note on the examples
  6. A survey of how connections (aka EthernetClient) work
    1. Connections and link/interface detection
    2. connect() behaviour and its return values
    3. Non-blocking connection functions, connectNoWait()
  7. How to use multicast
  8. How to use listeners
  9. How to change the number of sockets
  10. UDP receive buffering
  11. mDNS services
  12. DNS
  13. stdio
    1. Adapt stdio files to the Print interface
  14. Raw Ethernet frames
    1. Promiscuous mode
    2. Raw frame receive buffering
  15. How to implement VLAN tagging
  16. Application layered TCP: TLS, proxies, etc.
    1. About the allocator functions
  17. On connections that hang around after cable disconnect
  18. Notes on ordering and timing
  19. Notes on RAM1 usage
  20. Entropy collection
  21. Configuration macros
    1. Configuring macros using the Arduino IDE
    2. Configuring macros using PlatformIO
    3. Redefining macros in lwipopts.h
  22. Complete list of features
  23. Other notes
  24. To do
  25. Code style
  26. References

Differences, assumptions, and notes

This section describes differences from the Arduino Ethernet API.

Note: Please read the function docs in the relevant header files for more information.

Two core differences

There are two core differences that make QNEthernet not a 100% drop-in replacement for code using the Arduino Ethernet API. These are as follows:

  • The DHCP versions of Ethernet.begin(...) don't block. The blocking behaviour can be emulated by following a call to begin() with a loop that checks Ethernet.localIP() for a valid IP. See also the new Ethernet.waitForLocalIP(timeout) or Ethernet.onAddressChanged(cb).
  • Everything is inside the qindesign::network namespace. In many cases, adding the following at the top of your program will obviate the need to qualify any object uses or make any other changes: 
    using namespace qindesign::network;

For API additions beyond what the Arduino-style API provides, see:
Additional functions and features not in the Arduino API

Other differences and notes

  • Include the QNEthernet.h header instead of Ethernet.h.
  • Ethernet loop() is called from yield(). The functions that wait for timeouts rely on this. This also means that you must use delay(ms), yield(), or Ethernet.loop() when waiting on conditions; waiting without calling these functions will cause the TCP/IP stack to never refresh. Note that many of the I/O functions call loop() so that there's less burden on the calling code.
  • The Ethernet.begin(...) functions don't block.
  • EthernetServer::write(...) functions always return the write size requested. This is because different clients may behave differently.
  • The examples at https://www.arduino.cc/reference/en/libraries/ethernet/server.accept/ and https://www.arduino.cc/reference/en/libraries/ethernet/if-ethernetclient/ directly contradict each other with regard to what operator bool() means in EthernetClient. The first example uses it as "already connected", while the second uses it as "available to connect". "Connected" is the chosen concept, but different from connected() in that it doesn't check for unread data.
  • All the Arduino-defined Ethernet.begin(...) functions that use the MAC address are deprecated.
  • Ethernet.hardwareStatus(): Returns either EthernetHardwareStatus::EthernetOtherHardware or EthernetHardwareStatus::EthernetNoHardware.
  • The following Ethernet functions are deprecated and do nothing or return some default value:
    • init(uint8_t sspin): Does nothing.
    • maintain(): Returns zero.
    • setRetransmissionCount(uint8_t number): Does nothing.
    • setRetransmissionTimeout(uint16_t milliseconds): Does nothing.
  • The following EthernetUDP functions do nothing:
    • flush() because it is ill-defined.
  • The system starts with the Teensy's actual MAC address. If you want to use that address with the deprecated API, you can collect it with Ethernet.macAddress(mac) and then pass it to one of the deprecated begin(...) functions.
  • All classes and objects are in the qindesign::network namespace. This means you'll need to fully qualify any types. To avoid this, you could utilize a using directive: 
    using namespace qindesign::network;

EthernetUDP udp;

void setup() {
  Ethernet.begin();
}
    However, this pollutes the current namespace. An alternative is to choose something shorter. For example: 
    namespace qn = qindesign::network;

qn::EthernetUDP udp;

void setup() {
  qn::Ethernet.begin();
}
  • Files that configure lwIP for our system:
    • src/sys_arch.cpp
    • src/lwipopts.h ← use this one for tuning (see src/lwip/opt.h for more details)
    • src/arch/cc.h
  • The main include file, QNEthernet.h, in addition to including the Ethernet, EthernetFrame, and MDNS instances, also includes the headers for EthernetClient, EthernetServer, and EthernetUDP.
  • Most of the Ethernet functions do nothing or return some form of empty/nothing/false unless the system has been initialized.

Additional functions and features not in the Arduino API

QNEthernet defines functions that don't exist in the Arduino API as it's currently defined. (See: Arduino Ethernet library) This section documents those functions.

Features:

  • The read(buf, len) functions allow a NULL buffer so that the caller can skip data without having to read into a buffer.

Ethernet

The Ethernet object is the main Ethernet interface.

  • begin(): Initializes the library, uses the Teensy's internal MAC address, and starts the DHCP client. This returns whether startup was successful.

  • begin(ipaddr, netmask, gw): Initializes the library, uses the Teensy's internal MAC address, and uses the given parameters for the network configuration. This returns whether startup was successful. The DNS server is not set.

  • begin(ipaddr, netmask, gw, dns): Initializes the library, uses the Teensy's internal MAC address, and uses the given parameters for the network configuration. This returns whether startup was successful. The DNS server is only set if dns is not INADDR_NONE; it remains the same if dns is INADDR_NONE.

    Passing dns, if not INADDR_NONE, ensures that the DNS server IP is set before the address-changed callback is called. The alternative approach to ensure that the callback has all the information is to call setDNSServerIP(ip) before the three-parameter version.

  • broadcastIP(): Returns the broadcast IP address associated with the current local IP and subnet mask.

  • end(): Shuts down the library, including the Ethernet clocks.

  • hostname(): Gets the DHCP client hostname. An empty string means that no hostname is set. The default is "teensy-lwip".

  • interfaceStatus(): Returns the network interface status, true for UP and false for DOWN.

  • isDHCPActive(): Returns whether DHCP is active.

  • isDHCPEnabled(): Returns whether the DHCP client is enabled. This is valid whether Ethernet has been started or not.

  • linkState(): Returns a bool indicating the link state.

  • linkSpeed(): Returns the link speed in Mbps.

  • linkIsCrossover(): Returns whether a crossover cable is detected.

  • linkIsFullDuplex(): Returns whether the link is full duplex (true) or half duplex (false).

  • joinGroup(ip): Joins a multicast group.

  • leaveGroup(ip): Leaves a multicast group.

  • macAddress(mac): Fills the 6-byte mac array with the current MAC address. Note that the equivalent Arduino function is MACAddress(mac).

  • setDHCPEnabled(flag): Enables or disables the DHCP client. This may be called either before or after Ethernet has started. If DHCP is desired and Ethernet is up, but DHCP is not active, an attempt will be made to start the DHCP client if the flag is true.

  • setDNSServerIP(dnsServerIP): Sets the DNS server IP address. Note that the equivalent Arduino function is setDnsServerIP(dnsServerIP).

  • setHostname(hostname): Sets the DHCP client hostname. The empty string will set the hostname to nothing. To use something other than the default at system start, call this before calling begin().

  • setMACAddressAllowed(mac, flag): Allows or disallows Ethernet frames addressed to the specified MAC address. This is useful when processing raw Ethernet frames.

  • waitForLink(timeout): Waits for the specified timeout (milliseconds) for a link to be detected. This is useful when setting a static IP and making connections as a client. Returns whether a link was detected within the given timeout.

  • waitForLocalIP(timeout): Waits for the specified timeout (milliseconds) for the system to have a local IP address. This is useful when waiting for a DHCP-assigned address. Returns whether the system obtained an address within the given timeout. Note that this also works for a static-assigned address.

  • operator bool(): Tests if Ethernet is initialized.

  • Callback functions: Note that callbacks should be registered before any other Ethernet functions are called. This ensures that all events are captured. This includes Ethernet.begin(...).

    • onLinkState(cb): The callback is called when the link changes state, for example when the Ethernet cable is unplugged.
    • onAddressChanged(cb): The callback is called when any IP settings have changed. This might be called before the link or network interface is up if a static IP is set.
    • onInterfaceStatus(cb): The callback is called when the network interface status changes. It is called after the interface is up but before the interface goes down.

  • static constexpr bool isPromiscuousMode(): Returns whether promiscuous mode is enabled.

  • static constexpr int maxMulticastGroups(): Returns the maximum number of available multicast groups, not including the "all systems" group.

  • static constexpr size_t mtu(): Returns the MTU.

EthernetClient

  • abort(): Aborts a connection without going through the TCP close process.
  • close(): Closes a connection, but without waiting. It's similar to stop().
  • closeOutput(): Shuts down the transmit side of the socket. This is a half-close operation.
  • connectionId(): Returns an ID for the connection to which the client refers. It will return non-zero if connected and zero if not connected. Note that it's possible for new connections to reuse previously-used IDs.
  • connectNoWait(ip, port): Similar to connect(ip, port), but it doesn't wait for a connection.
  • connectNoWait(host, port): Similar to connect(host, port), but it doesn't wait for a connection. Note that the DNS lookup will still wait.
  • writeFully(b): Writes a single byte.
  • writeFully(s): Writes a string (const char *).
  • writeFully(s, size): Writes characters (const char *).
  • writeFully(buf, size): Writes a data buffer (const uint8_t *).
  • static constexpr int maxSockets(): Returns the maximum number of TCP connections.

TCP socket options

  • setNoDelay(flag): Sets or clears the TCP_NODELAY flag in order to disable or enable Nagle's algorithm, respectively. This must be changed for each new connection.
  • isNoDelay(): Returns whether the TCP_NODELAY flag is set for the current connection. Returns false if not connected.

EthernetServer

  • begin(port): Starts the server on the given port, first disconnecting any existing server if it was listening on a different port. This returns whether the server was successfully started.
  • beginWithReuse(): Similar to begin(), but also sets the SO_REUSEADDR socket option. This returns whether the server was successfully started.
  • beginWithReuse(port): Similar to begin(port), but also sets the SO_REUSEADDR socket option. This returns whether the server was successfully started.
  • end(): Shuts down the server.
  • port(): Returns the server's port, a signed 32-bit value, where -1 means the port is not set and a non-negative value is a 16-bit quantity.
  • static constexpr int maxListeners(): Returns the maximum number of TCP listeners.
  • EthernetServer(): Creates a placeholder server without a port. This form is useful when you don't know the port in advance.

All the begin functions call end() first only if the server is currently listening and the port or reuse options have changed.

EthernetUDP

  • beginWithReuse(localPort): Similar to begin(localPort), but also sets the SO_REUSEADDR socket option.
  • beginMulticastWithReuse(ip, localPort): Similar to beginMulticast(ip, localPort), but also sets the SO_REUSEADDR socket option.
  • data(): Returns a pointer to the received packet data.
  • localPort(): Returns the port to which the socket is bound, or zero if it is not bound.
  • send(host, port, data, len): Sends a packet without having to use beginPacket(), write(), and endPacket(). It causes less overhead. The host can be either an IP address or a hostname.
  • size(): Returns the total size of the received packet data.
  • operator bool(): Tests if the socket is listening.
  • static constexpr int maxSockets(): Returns the maximum number of UDP sockets.
  • EthernetUDP(queueSize): Creates a new UDP socket having the specified packet queue size. The minimum possible value is 1 and the default is 1. If a value of zero is used, it will default to 1.

All the begin functions call stop() first only if the socket is currently listening and the local port or reuse options have changed.

parsePacket() return values

The EthernetUDP::parsePacket() function in QNEthernet is able to detect zero-length UDP packets. This means that a zero return value indicates a valid packet.

Many Arduino examples do the following to test whether there's packet data:

int packetSize = udp.parsePacket();
if (packetSize) {  // <-- THIS IS NOT CORRECT
  // ...do something...
}

This is not correct because negative values mean that there's no packet available, and negative values are implicitly converted to a Boolean true. Instead, the code should look like this:

int packetSize = udp.parsePacket();
if (packetSize >= non_negative_value) {  // non_negative_value >= 0
  // ...do something...
}

Note that if (packetSize > 0) would also be correct, or even something like if (packetSize >= 4), just as long as the if (packetSize) form is not used.

EthernetFrame

The EthernetFrame object adds the ability to send and receive raw Ethernet frames. It provides an API that is similar in feel to EthernetUDP. Because, like EthernetUDP, it derives from Stream, the Stream API can be used to read from a frame and the Print API can be used to write to the frame.

  • beginFrame(): Starts a new frame. New data can be added using the Print API. This is similar to EthernetUDP::beginPacket().
  • beginFrame(dstAddr, srcAddr, typeOrLen): Starts a new frame and writes the given addresses and EtherType/length.
  • beginVLANFrame(dstAddr, srcAddr, vlanInfo, typeOrLen): Starts a new VLAN-tagged frame and writes the given addresses, VLAN info, and EtherType/length.
  • data(): Returns a pointer to the received frame data.
  • endFrame(): Sends the frame. This returns whether the send was successful. A frame must have been started, its data length must be in the range 60-1518, and Ethernet must have been initialized. This is similar to EthernetUDP::endPacket().
  • parseFrame(): Checks if a new frame is available. This is similar to EthernetUDP::parseFrame().
  • send(frame, len): Sends a raw Ethernet frame without the overhead of beginFrame()/write()/endFrame(). This is similar to EthernetUDP::send(data, len).
  • setReceiveQueueSize(size): Sets the receive queue size. The minimum possible value is 1 and the default is 1. If a value of zero is used, it will default to 1. If the new size is smaller than the number of items in the queue then all the oldest frames will get dropped.
  • size(): Returns the total size of the received frame data.
  • static constexpr int maxFrameLen(): Returns the maximum frame length including the FCS. Subtract 4 to get the maximum length that can be sent or received using this API.
  • static constexpr int minFrameLen(): Returns the minimum frame length including the FCS. Subtract 4 to get the minimum length that can be sent or received using this API.

MDNS

The MDNS object provides an mDNS API.

  • begin(hostname): Starts the mDNS responder and uses the given hostname as the name. This first calls end() only if the responder is currently running and the hostname is different.
  • end(): Stops the mDNS responder.
  • addService(type, protocol, port): Adds a service. The protocol will be set to "_udp" for anything other than "_tcp". The strings should have a "_" prefix. Uses the hostname as the service name.
  • addService(type, protocol, port, getTXTFunc): Adds a service and associated TXT records.
  • hostname(): Returns the hostname if the responder is running and an empty string otherwise.
  • removeService(type, protocol, port): Removes a service.
  • restart(): Restarts the responder, for use when the cable has been disconnected for a while and then reconnected. This isn't normally needed because the responder already watches for link reconnect.
  • operator bool(): Tests if the mDNS responder is operating.
  • static constexpr int maxServices(): Returns the maximum number of supported services.

DNSClient

The DNSClient class provides an interface to the DNS client.

  • setServer(index, ip): Sets a DNS server address.
  • getServer(index): Gets a DNS server address.
  • getHostByName(hostname, callback, timeout): Looks up a host by name and calls the callback when there's a result. The callback is not called once the timeout has been reached. The timeout is ignored if it's set to zero.
  • getHostByName(hostname, ip, timeout): Looks up a host by name.
  • static constexpr int maxServers(): Returns the maximum number of DNS servers.

Print utilities

The util/PrintUtils.h file declares some useful output functions. Note that this file is included when QNEthernet.h is included; there's no need to include it separately.

The functions are in the qindesign::network::util namespace, so if you've already added using namespace qindesign::network; to your code, they can be called with util::writeMagic() syntax. Otherwise, they need to be fully qualified: qindesign::network::util::writeMagic().

  1. writeFully(Print &, buf, size, breakf = nullptr): Attempts to completely write bytes to the given Print object; the optional breakf function is used as the stopping condition. It returns the number of bytes actually written. The return value will be less than the requested size only if breakf returned true before all bytes were written. Note that a NULL breakf function is assumed to return false.

    For example, the EthernetClient::writeFully(...) functions use this and pass the "am I disconnected" condition as the breakf function.

  2. writeMagic(Print &, mac, breakf = nullptr): Writes the payload for a Magic packet to the given Print object. This uses writeFully(...) under the covers and passes along the breakf function as the stopping condition.

There is also a Print decorator for stdio output files, util::StdioPrint (assuming the qindesign::network namespace is in scope). It provides a Print interface so that it is easy to print Printable objects to stdout or stderr without having to worry about buffering and the need to flush any output before printing a Printable directly to, say, Serial.

IPAddress operators

The core library version of IPAddress is missing == and != operators that can compare const IPAddress values. Provided in this library are these two operators. They are declared as follows in the usual namespace:

  1. bool operator==(const IPAddress &a, const IPAddress &b);
  2. bool operator!=(const IPAddress &a, const IPAddress &b);

operator bool() and explicit

All the operator bool() functions in the API are marked as explicit. This means that you might get compiler errors in some circumstances when trying to use a Boolean-convertible object.

You can use the object as a Boolean expression. For example in an if statement or ternary conditional.

You can't return the object as a bool from a function. For example, the following code should give a compiler error:

EthernetClient client_;

bool isConnected() {
  return client_;
}

Instead, use the following code; it fixes the problem:

EthernetClient client_;

bool isConnected() {
  return client_ ? true : false;
  // Or this:
  // if (client_) {
  //   return true;
  // } else {
  //   return false;
  // }
}

This will also work:

bool isConnected() {
  return static_cast<bool>(client_);
}

See also:

  1. The safe bool problem
  2. explicit specifier

How to run

This library works with both PlatformIO and Arduino. To use it with Arduino, here are a few steps to follow:

  1. Add #include <QNEthernet.h>. Note that this include already includes the header for EthernetUDP. Some external examples also include SPI.h. This is not needed unless you're actually using SPI in your program.

  2. Below that, add: using namespace qindesign::network;

  3. You likely don't want or need to set/choose your own MAC address, so just call Ethernet.begin() with no arguments to use DHCP, and the three- or four-argument version (IP, subnet mask, gateway[, DNS]) to set your own address. If you really want to set your own MAC address, see setMACAddress(mac) or one of the deprecated begin(...) functions that takes a MAC address parameter.

  4. There is an Ethernet.waitForLocalIP(timeout) convenience function that can be used to wait for DHCP to supply an address because Ethernet.begin() doesn't wait. Try 10 seconds (10000 ms) and see if that works for you.

    Alternatively, you can use listeners to watch for address, link, and network interface activity changes. This obviates the need for waiting and is the preferred approach.

  5. Ethernet.hardwareStatus() can detect both the Ethernet and no-Ethernet versions of the hardware.

  6. Most other things should be the same.

Please see the examples for more things you can do with the API, including but not limited to:

  • Using listeners to watch for network changes,
  • Monitoring and sending raw Ethernet frames, and
  • Setting up an mDNS service.

Asynchronous use is not supported

Asynchronous use of QNEthernet is not currently supported. This includes ISR approaches and multi-threading approaches.

First, the underlying lwIP stack must be configured and used a certain way in order to provide asynchronous support. QNEthernet does not configure lwIP for this. Second, the QNEthernet API, the layer on top of lwIP, isn't designed for asynchronous use.

Note that there is a group of libraries that claims to use QNEthernet and that claims to provide asynchronous support. Neither of these claims is true.

How to write data to connections

I'll start with these statements:

  1. Don't use the printX(...) functions when writing data to connections.
  2. Always check the write(...) and printX(...) return values, retrying if necessary.
  3. Data isn't necessarily sent immediately.

The write(...) and printX(...) functions in the Print API all return the number of bytes actually written. This means that you must always check the return value, retrying any missing bytes if you want all your data to get sent.

For example, the following code won't necessarily send all 250×102 bytes. Buffers might get full. There might be retries. Etcetera.

void sendTestData(EthernetClient& client) {
  for (int i = 0; i < 250; i++) {
    // 102-byte string (println appends CRLF)
    client.println("1234567890"
                   "1234567890"
                   "1234567890"
                   "1234567890"
                   "1234567890"
                   "1234567890"
                   "1234567890"
                   "1234567890"
                   "1234567890"
                   "1234567890");
  }
}

The following modification will print a message every time the number of bytes actually written does not match the number of bytes sent to the function. You might find that the message prints one or more times.

void sendTestData(EthernetClient& client) {
  for (int i = 0; i < 250; i++) {
    // 102-byte string (println appends CRLF)
    size_t written = client.println("1234567890"
                                    "1234567890"
                                    "1234567890"
                                    "1234567890"
                                    "1234567890"
                                    "1234567890"
                                    "1234567890"
                                    "1234567890"
                                    "1234567890"
                                    "1234567890");
    if (written != 102) {
      // This is not an error!
      Serial.println("Didn't write fully");
    }
  }
}

The solution is to utilize the raw write(...) functions and retry any bytes that aren't sent. Let's create a writeFully(...) function and use that to send the data:

// Keep writing until all the bytes are sent or the connection
// is closed.
void writeFully(EthernetClient &client, const char *data, int len) {
  // Don't use client.connected() as the "connected" check because
  // that will return true if there's data available, and this loop
  // does not check for data available or remove it if it's there.
  while (len > 0 && client) {
    size_t written = client.write(data, len);
    len -= written;
    data += written;
  }
}

void sendTestData(EthernetClient& client) {
  for (int i = 0; i < 250; i++) {
    // 102-byte string
    size_t written = writeFully(client, "1234567890"
                                        "1234567890"
                                        "1234567890"
                                        "1234567890"
                                        "1234567890"
                                        "1234567890"
                                        "1234567890"
                                        "1234567890"
                                        "1234567890"
                                        "1234567890"
                                        "\r\n");
  }
}

Note that the library implements writeFully(...); you don't have to roll your own.

Rewriting this to use the library function:

void sendTestData(EthernetClient& client) {
  for (int i = 0; i < 250; i++) {
    // 102-byte string
    size_t written = client.writeFully("1234567890"
                                       "1234567890"
                                       "1234567890"
                                       "1234567890"
                                       "1234567890"
                                       "1234567890"
                                       "1234567890"
                                       "1234567890"
                                       "1234567890"
                                       "1234567890"
                                       "\r\n");
  }
}

Let's go back to our original statement about not using the printX(...) functions. Their implementation is opaque and they sometimes make assumptions that the data will be "written fully". For example, Teensyduino's current print(const String &) implementation attempts to send all the bytes and returns the number of bytes sent, but it doesn't tell you which bytes were sent. For the string "12345", print(s) might send "12", fail to send "3", and successfully send "45", returning the value 4.

Similarly, we have no idea what print(const Printable &obj) does because the Printable implementation passed to it is beyond our control. For example, Teensyduino's IPAddress::printTo(Print &) implementation prints the address without checking the return value of the print(...) calls.

Also, most examples I've seen that use any of the printX(...) functions never check the return values. Common practice seems to stem from this style of usage. Network applications work a little differently, and there's no guarantee all the data gets sent.

The write(...) functions don't have this problem (unless, of course, there's a faulty implementation). They attempt to send bytes and return the number of bytes actually sent.

In summary, my strong suggestion is to use the write(...) functions when sending network data, checking the return values and acting on them. Or you can use the library's writeFully(...) functions.

See the discussion at: https://forum.pjrc.com/threads/68389-NativeEthernet-stalling-with-dropped-packets

Write immediacy

Data isn't necessarily completely sent across the wire after write(...) or writeFully(...) calls. Instead, data is merely enqueued until the internal buffer is full or a timer expires. Now, if the data to send is larger than the internal TCP buffer then data will be sent and the extra data will be enqueued. In other words, data is only sent when either the buffer is full or an internal timer has expired.

To send any buffered data, call flush().

To quote lwIP's tcp_write() docs:

Write data for sending (but does not send it immediately).

It waits in the expectation of more data being sent soon (as it can send them more efficiently by combining them together). To prompt the system to send data now, call tcp_output() after calling tcp_write().

flush() is what always calls tcp_output() internally. The write(...) and writeFully(...) functions only call this when the buffer is full. The suggestion is to call flush() when done sending a "packet" of data, for some definition of "packet" specific to your application. For example, after sending a web page to a client or after a chunk of data is ready for the server to process.

A note on the examples

The examples aren't meant to be simple. They're meant to be functional. There are plenty of Arduino-style Ethernet library examples out there. This library does not aim to be just like the others, it aims to provide some more powerful tools to enable more powerful programs. The Teensy 4.1 is a serious device; it demands serious examples.

An attempt was made to use a more robust programming style and modern C++ tools. For those not as experienced with C++ or larger projects, these may have a steeper learning curve. Hopefully this brings up lots of questions and an exposure to new concepts. For example, state machines, lambdas, callbacks, data structures, vector::emplace_back, and std::move.

For those that do have more C++ and larger project experience, I invite you to improve or add to the examples so that the set of examples here becomes what you've always hoped Teensy library examples could be.

A survey of how connections (aka EthernetClient) work

Hopefully this section disambiguates some details about what each function does:

  1. connected(): Returns whether connected OR data is still available (or both).
  2. operator bool(): Returns whether connected (at least in QNEthernet).
  3. available(): Returns the amount of data available, whether the connection is closed or not.
  4. read(...): Reads data if there's data available, whether the connection is closed or not.

Connections will be closed automatically if the client shuts down a connection, and QNEthernet will properly handle the state such that the API behaves as expected. In addition, if a client closes a connection, any buffered data will still be available via the client API. If it were up to me, I'd have swapped the meaning of operator bool() and connected(), but see the above list as a guide.

Some options:

  1. Keep checking connected() (or operator bool()) and available()/read(...) to keep reading data. The data will run out when the connection is closed and after all the buffers are empty. The calls to connected() (or operator bool()) will indicate connection status (plus data available in the case of connected() or just connection state in the case of operator bool()).
  2. Same as the above, but without one of the two connection-status calls (connected() or operator bool()). The data will just run out after connection-closed and after the buffers are empty.

Connections and link/interface detection

A link and active network interface must be present before a connection can be made. Either call Ethernet.waitForLink(timeout) or check the link state or network interface status before attempting to connect. Which approach you use will depend on how your code is structured or intended to be used.

Be aware when using a listener approach to start or stop services that it's possible, when setting a static IP, for the address-changed callback to be called before the link or network interface is up.

Note that this section also applies to the DNS client.

connect() behaviour and its return values

Firstly, connect() blocks. See the next section for a non-blocking way to connect.

The Arduino API, here, defines a set of possible return values for this function. Note that the example on that page is not correct (at the time of this writing) because it assumes that only a return value of '1' will evaluate to true. The C++ spec, here, however, says that all non-zero numeric values will be converted to true. The example, therefore, assumes that a successful connection has been made even if an error code was returned.

Currently, QNEthernet's implementation of this function will return one of the following values:

Value Meaning Arduino Name
1 Success SUCCESS
0 Can't create connection (eg. no more sockets, network isn't up, etc.) None
-1 Connection timeout TIMED_OUT
-2 DNS lookup failed for the given name INVALID_SERVER

Non-blocking connection functions, connectNoWait()

The connectNoWait() functions implement non-blocking TCP connections. These functions behave similarly to connect(), however they do not wait for the connection to be established.

To check for connection establishment, simply call either connected() or the Boolean operator. If a connection can't be established then close() must be called on the object.

Note that DNS lookups for hostnames will still wait.

How to use multicast

There are a few ways in the API to utilize multicast to send or receive packets. For reception, you must join a multicast group.

The first is by using the EthernetUDP::beginMulticast(ip, port) function. This both binds to a specific port, for only receiving traffic on that port, and joins the specified group address. It's similar to EthernetUDP::begin(localPort) in that the socket will "own" the port.

Since only one socket at a time can be bound to a specific port, you will need to use the same socket if you want to receive traffic sent to multiple groups on the same port. You can accomplish this by either calling beginMulticast(ip, port) multiple times, or by using begin(localPort) once, and then calling Ethernet.joinGroup(ip) for each group you want to join.

To send multicast traffic, simply send to the appropriate IP address. There's no need to join a group.

The lwIP stack keeps track of a group "use count". This means:

  1. That joinGroup(ip) can be called multiple times, it just needs to be paired with a matching number of calls to leaveGroup(ip). Each call increments an internal count.
  2. Each call to leaveGroup(ip) decrements a count, and when that count reaches zero, the stack actually leaves the group.

How to use listeners

Instead of waiting for certain states at system start, for example link-up or address-changed, it's possible to watch for state changes using listeners, and then act on those state changes. This will make your application more robust and responsive to state changes during program operation.

The relevant functions are (see the Ethernet section for further descriptions):

  1. Ethernet.onLinkState(cb)
  2. Ethernet.onAddressChanged(cb)
  3. Ethernet.onInterfaceStatus(cb)

Link-state events occur when an Ethernet link is detected or lost.

Address-changed events occur when the IP address changes, but its effects are a little more subtle. When setting an address via DHCP, the link and network interface must already be up in order to receive the information. However, when setting a static IP address, the event may occur before the link or network interface is up. This means that if a connection or DNS lookup is attempted when it is detected that the address is valid, the attempt will fail.

Interface-status events happen when the network interface comes up or goes down. No network operations can happen before the network interface is up. For example, when setting a static IP address, the address-changed event may occur before the network interface has come up. This means that, for example, any connection attempts or DNS lookup attempts will fail.

It is suggested, therefore, that when taking an action based on an address-changed event, the link state and network interface status are checked in addition.

Servers, on the other hand, can be brought up even when there's no link or active network interface.

In summary, no network operations can be done before all three of link-up, address_changed-to-valid, and interface-up occur.

Last, no network tasks should be performed inside the listeners. Instead, set a flag and then process that flag somewhere in the main loop.

See: Connections and link/interface detection

How to change the number of sockets

lwIP preallocates almost everything. This means that the number of sockets (UDP, TCP, etc.) the stack can handle is set at compile time. The following table shows how to change the number for each socket type.

Socket Type Macro in lwipopts.h Notes
UDP MEMP_NUM_UDP_PCB
TCP MEMP_NUM_TCP_PCB Simultaneously active
TCP (listening) MEMP_NUM_TCP_PCB_LISTEN Listening

UDP receive buffering

If UDP packets come in at a faster rate than they are consumed, some may get dropped. To help mitigate this, the EthernetUDP(queueSize) constructor can be called with a size > 1. The minimum size is 1, meaning any new packets will cause any existing packet to get dropped. If it's set to 2 then there will be space for one additional packet for a total of 2 packets, and so on. Setting a value of zero will use the default of 1.

mDNS services

It's possible to register mDNS services. Some notes:

  • Similar to Ethernet, there is a global MDNS object. It too is in the qindesign::network namespace.
  • It's possible to add TXT items when adding a service. For example, the following code adds "path=/" to the TXT of an HTTP service: 
      MDNS.begin("Device Name");
  MDNS.addService("_http", "_tcp", 80, []() {
    return std::vector<String>{"path=/"};
  });
    You can add more than one item to the TXT record by adding to the vector.
  • When adding a service, the function that returns TXT items defaults to NULL, so it's not necessary to specify that parameter. For example: 
      MDNS.begin("Device Name");
  MDNS.addService("_http", "_tcp", 80);
  • The host name is normally used as the service name, but there are also functions that let you specify the service name. For example: 
    MDNS.begin("Host Name");
MDNS.addService("my-http-service", "_http", "_tcp", 80);

DNS

The library interfaces with DNS using the DNSClient class. Note that all the functions are static.

Things you can do:

  1. Look up an IP address by name, and
  2. Set multiple DNS servers.

The Ethernet.setDNSServerIP(ip) function sets the zeroth DNS server.

stdio

Internally, lwIP uses printf for debug output and assertions. QNEthernet defines its own _write() function so that it has more control over stdout and stderr than that provided by the internal output support.

Note: As of Teensyduino 1.58-beta4, there's internal support for printf, partially obviating the need for QNEthernet to define its own. However, it always maps all of stdin, stdout, and stderr specifically to Serial.

Compared to the internal printf support, the QNEthernet version:

  1. Can map output to any Print interface, not just Serial,
  2. Can separate stdout and stderr outputs, and
  3. Disallows stdin as a valid output.

To enable the QNEthernet version, define the QNETHERNET_ENABLE_CUSTOM_WRITE macro and set the stdoutPrint or stderrPrint variables to point to a valid Print implementation. Note that if the feature is disabled, then neither stdoutPrint nor stderrPrint will be defined.

Both variables default to NULL.

For example:

// Define QNETHERNET_ENABLE_CUSTOM_WRITE somewhere

void setup() {
  Serial.begin(115200);
  while (!Serial && millis() < 4000) {
    // Wait for Serial
  }
  qindesign::network::stdoutPrint = &Serial;
}

If your application wants to define its own _write() implementation or to use the system default, then leave the QNETHERNET_ENABLE_CUSTOM_WRITE macro undefined.

Adapt stdio files to the Print interface

There is a utility class for decorating stdio FILE* objects with the Print interface. See the StdioPrint class in src/util/PrintUtils.h.

This is useful when:

  1. A FILE* object does its own buffering and you also need to write to the underlying Print object directly, Serial for example. It avoids having to remember to call fflush() on the FILE* before writing to the underlying Print object.
  2. There's a need to easily print Printable objects to the FILE*.

Raw Ethernet frames

There is support for sending and receiving raw Ethernet frames. See the EthernetFrame API, above.

This API doesn't receive any known Ethernet frame types. These include:

  1. IPv4 (0x0800)
  2. ARP (0x0806)
  3. IPv6 (0x86DD) (if enabled)

If frames are addressed to a MAC address that doesn't belong to the device and isn't a multicast group MAC address then it is necessary to tell the Ethernet stack about it. See the Ethernet.setMACAddressAllowed(mac, flag) function.

An example that uses such MAC addresses is the Precision Time Protocol (PTP) over Ethernet. It uses 01-1B-19-00-00-00 for forwardable frames and 01-80-C2-00-00-0E for non-forwardable frames. See PTP Message Transport

To enable raw frame support, define the QNETHERNET_ENABLE_RAW_FRAME_SUPPORT macro. This will use some space.

Promiscuous mode

It's possible to enable promiscuous mode so that all frames are received, even ones whose destination MAC address would normally be filtered out by the Ethernet hardware. To do this, define the QNETHERNET_ENABLE_PROMISCUOUS_MODE macro.

Raw frame receive buffering

Similar to UDP buffering, if raw frames come in at a faster rate than they are consumed, some may get dropped. To help mitigate this, the receive queue size can be adjusted with the EthernetFrame.setReceiveQueueSize(size) function. The default queue size is 1 and the minimum size is also 1 (if a zero is passed in then 1 will be used instead).

For a size of 1, any new frames will cause any existing frame to get dropped. If the size is 2 then there will be space for one additional frame for a total of 2 frames, and so on.

How to implement VLAN tagging

The lwIP stack supports VLAN tagging. Here are the steps for how to implement it. Note that all defines should go inside lwipopts.h. Documentation for these defines can be found in src/lwip/opt.h.

  1. Define ETHARP_SUPPORT_VLAN as 1.
  2. To set VLAN tags, define LWIP_HOOK_VLAN_SET.
  3. To validate VLAN tags on input, define one of:
    1. LWIP_HOOK_VLAN_CHECK, (see LWIP_HOOK_VLAN_CHECK)
    2. ETHARP_VLAN_CHECK_FN, (see ETHARP_SUPPORT_VLAN)
    3. ETHARP_VLAN_CHECK. (see ETHARP_SUPPORT_VLAN)

Application layered TCP: TLS, proxies, etc.

lwIP provides a way to decorate the TCP layer. It's called "Application Layered TCP." It enables an application to add things like TLS and proxies without changing the source code.

Here are the steps to add decorated TCP:

  1. Set LWIP_ALTCP and optionally LWIP_ALTCP_TLS to 1 in lwipopts.h.
  2. Implement two functions somewhere in your code, having these names and signatures: 
    std::function<void(const ip_addr_t *ipaddr, uint16_t port,
                   altcp_allocator_t *allocator)> qnethernet_get_allocator;
std::function<void(altcp_allocator_t *allocator)> qnethernet_free_allocator;
  3. Implement all the functions necessary for the wrapping implementation. For example, for TLS, this means all the functions declared in src/lwip/altcp_tls.h.

See src/lwip/altcp.c for more information.

About the allocator functions

The functions from step 2 create and destroy any resources used by the altcp wrapper. The first function fills in the allocator function and an argument appropriate to that allocator function. For example, altcp_tcp_alloc() doesn't need an argument, but altcp_tls_alloc() needs a pointer to a struct altcp_tls_config.

The second function frees any resources that haven't already been freed. It's up to the application and TCP wrapper implementation to properly manage resources and to provide a way to determine whether a resource needs to be freed. It is only called if a socket could not be created.

The ipaddr and port parameters indicate what the calling code is trying to do:

  1. If ipaddr is NULL then the application is trying to listen.
  2. If ipaddr is not NULL then the application is trying to connect.

On connections that hang around after cable disconnect

Ref: EthernetServer accept no longer connects clients after unplugging/plugging ethernet cable ~7 times

TCP tries its best to maintain reliable communication between two endpoints, even when the physical link is unreliable. It uses techniques such as timeouts, retries, and exponential backoff. For example, if a cable is disconnected and then reconnected, there may be some packet loss during the disconnect time, so TCP will try to resend any lost packets by retrying at successively larger intervals.

The TCP close process uses some two-way communication to properly shut down a connection, and therefore is also subject to physical link reliability. If the physical link is interrupted or the other side doesn't participate in the close process then the connection may appear to become "stuck", even when told to close. The TCP stack won't consider the connection closed until all timeouts and retries have elapsed.

It turns out that some systems drop and forget a connection when the physical link is disconnected. This means that the other side may still be waiting to continue or close the connection, timing out and retrying until all attempts have failed. This can be as long as a half hour, or maybe more, depending on how the stack is configured.

The above link contains a discussion where a user of this library couldn't accept any new connections, even when all the connections had been closed, until all the existing connections timed out after about a half hour. What happened was this: connections were being made, the Ethernet cable was disconnected and reconnected, and then more connections were made. When the cable was disconnected, all connections were closed using the close() function. The Teensy side still maintained connection state for all the connections, choosing to do what TCP does: make a best effort to maintain or properly close those connections. Once all the available sockets had been exhausted, no more connections could be accepted.

Those connections couldn't be cleared and sockets made available until all the TCP retries had elapsed. The main problem was that the other side simply dropped the connections when it detected a link disconnect. If the other system had maintained those connections, it would have continued the close processes as normal when the Ethernet cable was reconnected. That's why tests on my system couldn't reproduce the issue: the IP stack on the Mac maintained state across cable disconnects/reconnects. The issue reporter was using Windows, and the IP stack there apparently drops a connection if the link disconnects. This left the Teensy side waiting for replies and retrying, and the Windows side no longer sending traffic.

To mitigate this problem, there are a few possible solutions, including:

  1. Reduce the number of retransmission attempts by changing the TCP_MAXRTX setting in lwipopts.h, or
  2. Abort connections upon link disconnect.

To accomplish #2, there's an EthernetClient::abort() function that simply drops a TCP connection without going though the normal TCP close process. This could be called on connections when the link has been disconnected. (See also Ethernet.onLinkState(cb) or Ethernet.linkState().)

Fun links:

Notes on ordering and timing

  • Link status is polled about 8 times a second.
  • For static IP addresses, the address-changed callback is called before lwIP's netif_default is set. MDNS.begin() relies on netif_default, so that function and anything else that relies on netif_default should be called after Ethernet.begin(...), and not from the listener.
  • The DNS lookup timeout is DNS_MAX_RETRIES * DNS_TMR_INTERVAL, where DNS_TMR_INTERVAL is 1000.

Notes on RAM1 usage

By default, the Ethernet RX and TX buffers will go into RAM2. If, for whatever reason, you'd prefer to put them into RAM1, define the QNETHERNET_BUFFERS_IN_RAM1 macro. [As of this writing, no speed comparison tests have been done.]

Entropy collection

This library defines functions for accessing the processor's internal "true random number generator" (TRNG) for entropy. If your project needs to use the Entropy library instead, define the QNETHERNET_USE_ENTROPY_LIB so that any internal entropy collection doesn't interfere with your project's entropy collection.

The Entropy library does the essentially same things as the internal TRNG functions, it just requires an additional dependency.

See the function declarations in src/security/entropy.h if you want to use them yourself.

Configuration macros

There are several macros that can be used to configure the system:

Macro Description Link
QNETHERNET_BUFFERS_IN_RAM1 Put the RX and TX buffers into RAM1 Notes on RAM1 usage
QNETHERNET_ENABLE_CUSTOM_WRITE Use expanded stdio output behaviour stdio
QNETHERNET_ENABLE_PROMISCUOUS_MODE Enable promiscuous mode Promiscuous mode
QNETHERNET_ENABLE_RAW_FRAME_SUPPORT Enable raw frame support Raw Ethernet Frames
QNETHERNET_USE_ENTROPY_LIB Use Entropy library instead of internal functions Entropy collection

Configuring macros using the Arduino IDE

[Current as of this writing: Arduino IDE 2.0.4, Teensyduino 1.58]

The Arduino IDE provides a facility to override the build options specified in a platform's build configuration file, platform.txt. It does this by looking for a file named platform.local.txt in the same place. Any options in that "local" file override equivalent options in the main file.

The suggested way to override compiler options is with the compiler.cpp.extra_flags and compiler.c.extra_flags properties. However, this only works if platform.txt uses those options. If it does not then there's nothing to override. The current Teensyduino installation's platform.txt file does not use these options.

Here's how to implement the behaviour:

  1. Insert this section somewhere in platform.txt: 
    # These can be overridden in platform.local.txt
compiler.c.extra_flags=
compiler.cpp.extra_flags=
  2. Insert {compiler.cpp.extra_flags} before {includes} in:
    1. recipe.preproc.includes
    2. recipe.preproc.macros
    3. recipe.cpp.o.pattern
  3. Insert {compiler.c.extra_flags} before {includes} in:
    1. recipe.c.o.pattern

Next, create a platform.local.txt file in the same directory as the platform.txt file and add the options you need. For example, to enable raw frame support:

compiler.cpp.extra_flags=-DQNETHERNET_ENABLE_RAW_FRAME_SUPPORT
compiler.c.extra_flags=-DQNETHERNET_ENABLE_RAW_FRAME_SUPPORT

Note that both properties are needed because QNEthernet contains a mixture of C and C++ sources.

The properties of most interest are probably the ones in this example. There are other ones defined in the Arduino AVR version, but aren't really needed here.

Lest you think I've forgotten to add it, here're the locations of the files for the current latest version of the IDE:

  • Mac: ~/Library/Arduino15/packages/teensy/hardware/avr
  • Linux: ~/.arduino15/packages/teensy/hardware/avr
  • Windows: %userprofile%\AppData\Local\Arduino15\packages\teensy\hardware\avr

References:

  1. Additional compiler options - Arduino Forum
  2. Arduino IDE: Where can I pass defines to the compiler? - Arduino Forum
  3. Request for Arduino IDE "extra_flags" support - Teensy Forum
  4. Platform specification - Arduino CLI
  5. This one started it all → RawFrameMonitor example seems to be missing something... · Issue #33 · QNEthernet
  6. Open the Arduino15 folder - Arduino Help Center

Configuring macros using PlatformIO

Simply add compiler flags to the build_flags build option in platformio.ini.

For example:

build_flags = -DQNETHERNET_ENABLE_RAW_FRAME_SUPPORT

Redefining macros in lwipopts.h

The lwipopts.h file defines certain macros needed by the system. It is appropriate for the user to alter some of those macros if needed, for example, to change the number of UDP sockets or IGMP groups.

These macros can either be modified directly in the file (lwipopts.h) or from the command line with a -D directive. The ones that can be modified from the command line are either wrapped in an #ifndef block or not defined at all.

Useful macro list; please see further descriptions in opt.h and in mdns_opts.h:

Macro Description
DNS_MAX_RETRIES Maximum number of DNS retries
LWIP_MDNS_RESPONDER Zero to disable
LWIP_STATS 1 to enable
MEM_SIZE Heap memory size
MEMP_NUM_IGMP_GROUP Number of multicast groups
MEMP_NUM_TCP_PCB Number of listening TCP sockets
MEMP_NUM_TCP_PCB_LISTEN Number of TCP sockets
MEMP_NUM_UDP_PCB Number of UDP sockets
MDNS_MAX_SERVICES Maximum number of mDNS services

Some extra conditions to keep in mind:

  • MEMP_NUM_IGMP_GROUP: Count must include one for the "all systems" group.
  • MEMP_NUM_UDP_PCB: Count must include one if mDNS is enabled.

Complete list of features

This section is an attempt to provide a complete list of features in the QNEthernet library.

  1. Mostly compatible with the Arduino Ethernet API
  2. Additional functions and features not in the Arduino API
  3. Automatic MAC address detection; it's not necessary to initialize the library with your own MAC address
  4. A DNS client
  5. mDNS support
  6. Raw Ethernet frame support
  7. stdio output support for stdout and stderr — implemented to support lwIP's printf() calls, but user code can use this too
  8. VLAN tagging support
  9. Zero-length UDP packets
  10. UDP and raw frame receive buffering
  11. Listeners to watch link, network interface, and address state
  12. With some additions:
    1. IPv6
    2. IEEE 1588, Precision Time Protocol (PTP)
    3. TLS and TCP proxies
    4. IEEE 802.3az, Energy-Efficient Ethernet (EEE)
    5. Secure TCP initial sequence numbers (ISNs)
  13. Client shutdown options: close (start close process without waiting), closeOutput (close just the output side, also called a "half-close"), abort (shuts down the connection without going through the TCP close process), stop (close and wait)
  14. Ability to fully write data to a client connection
  15. Multicast support
  16. Promiscuous mode
  17. SO_REUSEADDR support (see EthernetServer and EthernetUDP)
  18. TCP_NODELAY support
  19. Configuration via Configuration macros
  20. UDP and raw frame queueing for when data arrives in bursts that are faster than the ability of the program to process them
  21. Non-blocking TCP connections.

Other notes

I'm not 100% percent certain where this library will go, but I want it to be amazing. It's meant to be an alternative to the NativeEthernet/FNET library, using a different underlying TCP/IP stack.

I'm also not settled on the name.

Input is welcome.

To do

  • Tune lwIP.
  • A better API design than the Arduino-defined API.
  • Perhaps zero-copy is an option.
  • More unit tests.
  • I have seen Assertion "tcp_slowtmr: TIME-WAIT pcb->state == TIME-WAIT" failed at line 1442 in src/lwip/tcp.c when sending a large amount of data. Either it's an lwIP bug or I'm doing something wrong. See: https://lists.gnu.org/archive/html/lwip-users/2010-02/msg00013.html
  • More examples.
  • Fix reduced frame reception when Ethernet is restarted via end()/begin(...). This is a vexing one. See also: #31

Code style

Code style for this project mostly follows the Google C++ Style Guide.

Other conventions are adopted from Bjarne Stroustrup's and Herb Sutter's C++ Core Guidelines.

References

Copyright (c) 2021-2023 Shawn Silverman