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Use a median filter to protect against bad timeservers.

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This commit is contained in:
JoelKatz
2012-08-05 20:07:08 -07:00
parent 65cbd319c1
commit 05e193f370
2 changed files with 24 additions and 8 deletions
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29
src/SNTPClient.cpp
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@@ -16,6 +16,9 @@ static uint8_t SNTPQueryData[48] = {
// NTP query frequency - 5 minutes
#define NTP_QUERY_FREQUENCY (5 * 60)
// NTP sample window (should be odd)
#define NTP_SAMPLE_WINDOW 9
// NTP timestamp constant
#define NTP_UNIX_OFFSET 0x83AA7E80
@@ -138,16 +141,26 @@ void SNTPClient::processReply()
timev -= now;
timev -= NTP_UNIX_OFFSET;
if ((timev == -1) || (timev == 1)) // small corrections likely do more harm than good
timev = 0;
// FIXME: We really should use the median of all recent valid offsets
if ((mLastOffsetUpdate == (time_t) -1) || (mLastOffsetUpdate < (now - 180)))
mOffset = timev;
else
mOffset = ((mOffset * 7) + timev) / 8;
// add offset to list, replacing oldest one if appropriate
mOffsetList.push_back(timev);
if (mOffsetList.size() >= NTP_SAMPLE_WINDOW)
mOffsetList.pop_front();
mLastOffsetUpdate = now;
// select median time
std::list<int> offsetList = mOffsetList;
offsetList.sort();
int j = offsetList.size();
std::list<int>::iterator it = offsetList.begin();
for (int i = 0; i < (j / 2); ++i)
++it;
mOffset = *it;
if ((j % 2) == 0)
mOffset = (mOffset + (*--it)) / 2;
if ((mOffset == -1) || (mOffset == 1)) // small corrections likely do more harm than good
mOffset = 0;
Log(lsTRACE) << "SNTP: Offset is " << timev << ", new system offset is " << mOffset;
}

3
src/SNTPClient.h
View File

@@ -4,6 +4,7 @@
#include <string>
#include <map>
#include <vector>
#include <list>
#include <boost/thread/mutex.hpp>
#include <boost/asio.hpp>
@@ -33,8 +34,10 @@ protected:
boost::asio::ip::udp::resolver mResolver;
std::vector< std::pair<std::string, time_t> > mServers;
int mOffset;
time_t mLastOffsetUpdate;
std::list<int> mOffsetList;
std::vector<uint8_t> mReceiveBuffer;
boost::asio::ip::udp::endpoint mReceiveEndpoint;