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Make NTP request more robust. #7

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Make NTP request more robust. #7

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45 changes: 25 additions & 20 deletions src/EasyNTPClient.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -41,9 +41,17 @@ void EasyNTPClient::setTimeOffset (int offset) {

unsigned long EasyNTPClient::getServerTime () {
static int udpInited = this->mUdp->begin(123); // open socket on arbitrary port
// Only the first four bytes of an outgoing NTP packet need to be set
// appropriately, the rest can be whatever.
const long ntpFirstFourBytes = 0xEC0600E3; // NTP request header
// Only the first four bytes of an NTP request have to be set. The rest
// of the packet has to be 0, to avoid the server getting confused with the
// timestamps.
// see https://labs.apnic.net/?p=462 for details about the NTP packet
// structure
byte packetBuffer[NTP_PACKET_SIZE] = {0};

packetBuffer[0] = NTP_HEADER_LI|NTP_HEADER_VN|NTP_HEADER_MODE;
// packetBuffer[1] (stratum) skipped because it is already 0
packetBuffer[2] = 6; // polling interval in log2 seconds
packetBuffer[3] = 0xEC; // precision

// Fail if WiFiUdp.begin() could not init a socket
if (! udpInited)
Expand All @@ -54,7 +62,7 @@ unsigned long EasyNTPClient::getServerTime () {

// Send an NTP request
if (! (this->mUdp->beginPacket(this->mServerPool, 123) // 123 is the NTP port
&& this->mUdp->write((byte *)&ntpFirstFourBytes, 48) == 48
&& this->mUdp->write(packetBuffer, NTP_PACKET_SIZE) == NTP_PACKET_SIZE
&& this->mUdp->endPacket()))
return 0; // sending request failed

Expand All @@ -63,31 +71,28 @@ unsigned long EasyNTPClient::getServerTime () {
const byte maxPoll = 15; // poll up to this many times
int pktLen; // received packet length
for (byte i=0; i<maxPoll; i++) {
if ((pktLen = this->mUdp->parsePacket()) == 48)
break;
delay(pollIntv);
if ((pktLen = this->mUdp->parsePacket()) == NTP_PACKET_SIZE)
break;
delay(pollIntv);
}
if (pktLen != 48)
return 0; // no correct packet received
if (pktLen != NTP_PACKET_SIZE)
return 0; // no correct packet received

// Read and discard the first useless bytes
// Set useless to 32 for speed; set to 40 for accuracy.
const byte useless = 40;
for (byte i = 0; i < useless; ++i)
this->mUdp->read();
this->mUdp->read(packetBuffer, NTP_PACKET_SIZE);

// Read the integer part of sending time
unsigned long time = this->mUdp->read(); // NTP time
for (byte i = 1; i < 4; i++)
time = time << 8 | this->mUdp->read();
// Read the integer part (32 bits) of the Transmit Timestamp (64 bits total)
unsigned long time = packetBuffer[NTP_TX_TIMESTAMP_OFFSET];
time = time << 8 | packetBuffer[NTP_TX_TIMESTAMP_OFFSET + 1];
time = time << 8 | packetBuffer[NTP_TX_TIMESTAMP_OFFSET + 2];
time = time << 8 | packetBuffer[NTP_TX_TIMESTAMP_OFFSET + 3];

// Round to the nearest second if we want accuracy
// The fractionary part is the next byte divided by 256: if it is
// The fractionary part is the last byte divided by 256: if it is
// greater than 500ms we round to the next second; we also account
// for an assumed network delay of 50ms, and (0.5-0.05)*256=115;
// additionally, we account for how much we delayed reading the packet
// since its arrival, which we assume on average to be pollIntv/2.
time += (this->mUdp->read() > 115 - pollIntv/8);
time += (packetBuffer[NTP_TX_TIMESTAMP_OFFSET + 4] > 115 - pollIntv/8);

// Discard the rest of the packet
this->mUdp->flush();
Expand Down
12 changes: 11 additions & 1 deletion src/EasyNTPClient.h
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,16 @@
#include "Arduino.h"
#include <Udp.h>

/*
* NTP packet-related macros.
* see https://labs.apnic.net/?p=462 for details about the NTP packet structure.
*/
#define NTP_PACKET_SIZE 48 // Size of an NTP packet
#define NTP_HEADER_LI 0b11000000 // leap indicator = 3 (unsynchronized)
#define NTP_HEADER_VN 0b00100000 // version number = 4
#define NTP_HEADER_MODE 0b00000011 // mode = 3 (client)
#define NTP_TX_TIMESTAMP_OFFSET 40 // offset within the packet for the TX time

class EasyNTPClient
{
public:
Expand All @@ -37,4 +47,4 @@ class EasyNTPClient
unsigned long getServerTime();
};

#endif
#endif