Re: Hardware timestamp ?

[Guy Harris <guy () alum mit edu>]

On Feb 26, 2009, at 5:22 PM, Guy Harris wrote:

> The *accuracy* is limited by the fact that most network adapters  
> aren't designed primarily for use when capturing traffic, so they  
> don't do their own packet timestamping, and libpcap normally just  
> plugs into the OS's built-in facilities for capturing packets, and  
> those either use the OS's networking stack, which is designed  
> primarily for regular network traffic rather than traffic capture  
> and might sacrifice low latency for higher throughput (thus adding  
> delays that get in the way of time stamping), and which plugs into  
> device drivers *also* designed primarily for regular network traffic  
> rather than traffic capture, or use something such as BPF that might  
> bypass the networking stack but *doesn't* bypass the driver.

Although it *appears* that, with current versions of the Linux kernel,  
the driver *could* timestamp the packet (setting the time in the  
skbuff) and netif_rx() won't timestamp it itself:

        int netif_rx(struct sk_buff *skb)
        {
                struct softnet_data *queue;
                unsigned long flags;

                /* if netpoll wants it, pretend we never saw it */
                if (netpoll_rx(skb))
                        return NET_RX_DROP;

                if (!skb->tstamp.tv64)
                        net_timestamp(skb);

                        ...

so that if the network adapter provided a time stamp along with the  
packet data, the driver would set the time stamp for the skbuff before  
handing it to the networking stack.

Similarly, with variants of the bpf_tap() and bpf_mtap() kernel APIs  
that take a time stamp as an argument, the BPF mechanisms in *BSD  
could let the driver supply a time stamp when it hands a packet to BPF.

That would let you have a regular network adapter that time-stamps  
packets itself, so you could get accurate time stamps on incoming  
packets and still have the adapter usable as a regular network adapter  
as well as an adapter for capturing traffic.  You still wouldn't get  
accurate time stamps on *outgoing* packets, if by "accurate" you mean  
"gives the time at which the packet was put onto the network".
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