On Wed, Jun 29, 2016 at 1:17 PM, Dan Drown <[email protected]> wrote:
> Quoting Charles Swiger <[email protected]>: > >> If you'd like to consider things at layer 3, note that IPv4 normally has >> a 20-byte header size, and IPv6 has a 40-byte header. For large packets, >> the difference in protocol overhead is not very significant-- about 1%-- >> but for a 56-byte NTPv4 packet, using IPv6 means sending about 125% as many >> bits over the wire as sending the same payload via IPv4. >> >> If other factors are held equal, IPv4 is always going to perform better >> than IPv6 for NTP because smaller packets mean shorter transmit/receive >> times and thus reduced latency for NTP polls. >> >> (There's nothing magical about protocol overhead, except perhaps >> pretending that there isn't a difference. :-) >> > > But is it a difference that matters? > > Typical NTP packet sizes in bytes: > > v4: 14(ethernet) + 20(IPv4) + 8(UDP) + 48(NTP) = 90 bytes > v6: 14(ethernet) + 40(IPv6) + 8(UDP) + 48(NTP) = 110 bytes > > At 100Mbit, these take 7.2us(v4) and 8.8us(v6) to transmit (I'm ignoring > preamble and inter-packet gap) > > At 1G, these take 720ns and 880ns to transmit. > > Compare that 3.2us round-trip difference at 100M to the other sources of > measurement error: > > And each router on IPv4 has to do an IP packet header checksum on a variable length header, on IPv6 there is no packet header checksum. I'm not sure how long does it take to perform on standard hardware: https://en.wikipedia.org/wiki/IPv4_header_checksum _______________________________________________ pool mailing list [email protected] http://lists.ntp.org/listinfo/pool
