[j-nsp] Accuracy of interface stats

[Stefan Fouant]

Thanks Richard for the additional info.  I don't think I said 64 bytes
was the minimum IP packet size, rather I just chose that number
arbitrarily so I could do some rough comparisons, but I could see how
someone might surmise that I was referring to that as the minimum IP
size.  I totally forgot about the IFG however, but attribute that to
not working in a qualification lab since the late 90's... I'm bound to
be a little rusty.  Regardless, the addition of the IFG does little to
help the situation, and actually again means that PoS throughput is
actually better with smaller size packets than Ethernet at comparable
speeds.  Of course, as you mention, the exact opposite is true as the
frame size increases... Now of course if we are talking about 10GbE
over SONET using 10 Gig WAN PHY, that's a totally different story...



On 9/27/09, Richard A Steenbergen <ras at e-gerbil.net> wrote:
> On Sun, Sep 27, 2009 at 01:17:54PM -0400, Stefan Fouant wrote:
>> 64 Byte IP packet + 8 Byte Ethernet Preamble + 18 Byte Ethernet header =
>> 90
>> Bytes * 8 bits per byte = 720 bits per frame
>>
>> 10,000,000,000 bps GigE / 720 bits per frame = 13,888,888 fps
>>
>> Have my calculations been incorrect all these years?  What am I missing?
>
> You're a little off, but not much. The minimum IP payload size is
> actually 46 not 64, anything smaller (such as a 40 byte TCP ACK, a 28
> byte UDP packet, etc) will be padded to 46. The 64 byte minimum you're
> thinking of comes from the 46 byte payload + 14 byte ethernet header + 4
> byte FCS = 64 bytes. You also forgot the Inter-Frame Gap, which is
> another 12 bytes of layer 1 overhead.
>
> Basically every packet on Ethernet contributes at least 38 bytes in
> L1+L2 overhead (plus padding up to 46, for IP payloads less than 46
> bytes), regardless of the size. This is really really bad when the frame
> size is small (worse than ATM infact), a 46 byte IP packet burns 84
> bytes * 8 = 672 bits giving you a max rate of 14,880,952 fps. It's been
> a while since I did the math on SONET overhead but I seem to recall the
> number for OC192 being something more like 26-28Mpps for small packets.
> Of course, the exact opposite is true once the packet sizes get bigger,
> SONET overhead is worse than Ethernet at 1500 bytes.
>
> --
> Richard A Steenbergen <ras at e-gerbil.net>       http://www.e-gerbil.net/ras
> GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)
>


-- 
Stefan Fouant