How is time synchronization implemented in Sniffer10G timestamping?

Here are the components involved in time synchronization and Myri-10G "8B" and "8C" network adapters.

Host Timestamping

1. The network adapter timestamps packets using its own free-running clock with an accuracy of ~1.5us on 8B and 8C network adapters. These timestamps are "raw" in that they are subject only to one clock and have not undergone any transformation.

2. The myri_snf driver runs a background process every second to synchronize the host and network adapter clock. The host clock is what applications would use when calling gettimeofday() whereas the network adapter clock refers to the adapter's free-running clock. This process involves sampling both clocks over PCI express and is accurate to ~200ns. The sampling runs asynchronously with respect to all other running applications and produces functional parameters that can be supplied to userspace applications to transform "raw" network adapter timestamps into host timestamps.

   Note: This entry only applies to host timestamp mode and has nothing to do with the SYNC adapter when connected to a timesource. It describes how the adapter converts adapter time to host time for the purpose of presenting a timestamp on the packet in the host time domain. The host's time is never adjusted by any of our software at any time, even when connected to a timesource.

3. Users of the Sniffer10G API always return absolute timestamps. If the underlying adapter is an 8B network adapter, the raw timestamps will be converted to host timestamps and as such will be accurate to gettimeofday() within ~200ns (because of the limitations explained in #2).

Timesource (Hardware) Timestamping

1. The network adapter timestamps packets using its own free-running clock with an accuracy of ~1.5us on 8B and 8C network adapters. These timestamps are "raw" in that they are subject only to one clock and have not undergone any transformation. If you have a 10G-PCIE2-8C2-2S-SYNC network adapter that is connected to a IRIG-B signal, the free-running clock has the additional property that it is synchronized with an external timesource accurate to ~12ns. In this case, the "raw" timestamps are also absolute timestamps because of the additional hardware on the board.

2. Users of the Sniffer10G API always return absolute timestamps. With a IRIG-B connected SYNC network adapter, the raw timestamps are absolute and referenced to the IRIG-B signal and will not undergo transformations with respect to the host system time.

3. Note (for Linux only):

   Since the host time is never adjusted by the Sniffer10G software, if you would like to sync the host's clock to the timesource, the software exports a PPS output to the system. This PPS output can be used as an input into ntp. For more details, please read: How do I configure NTP to use the PPS source?