[Ns-developers] Csma, ethernet length field, NSC and the link mtu...

[Tom Henderson]

craigdo at ee.washington.edu wrote:
>>> In CsmaNetDevice::AddHeader we have this:
>>>     case LLC: {
>>>       lengthType = p->GetSize () + header.GetSerializedSize () + 
>>>         trailer.GetSerializedSize ();
>>>       LlcSnapHeader llc;
>>>       llc.SetType (protocolNumber);
>>>       p->AddHeader (llc);
>>>     } break;
>>>
>>> This looks bogus -- Why is the LLC header size ignored?
>>> And is the ethernet header really expected to be accounted 
>> for in the length field?
>>
>> no idea.
> 
> This seems wrong to me.  The LlcSnapHeader header comes after the
> dst/src/length and consumes eight bytes.  This means the maximum payload
> should be reduced to 1492 bytes when LLC/SNAP encapsulation is used.  
> 
> The EthernetHeader (header in the code above) represents the dst/src/length
> of the packet.  The LLC + SNAP header represents the LLC part
> (DSAP/SSAP/HDLC Mode) and the SNAP part (ORG/EtherType).  So my take is that
> the above code should really be,
> 
>       LlcSnapHeader llc;
>       llc.SetType (protocolNumber);
>       p->AddHeader (llc);
>       lengthType = header.GetSerializedSize () + llc.GetSerializedSize + 
>         p->GetSize () + trailer.GetSerializedSize ();
> 
> Now, there's the question of MTU.  The Ethernet MTU is 1500 bytes.  The MTU
> is 1500 bytes independent of what kind of headers and trailers are added by
> the device, right?

No, MTU is the maximum IP packet size carred by the device.  If the 
device has different headers/trailers (such as LLC/SNAP), the MTU 
changes accordingly.

> 
> Aren't we really talking about having an MTU of 1500 bytes and a maximum SDU
> (service data unit) of 1492 bytes in this case?  Is that being too pedantic?

I think we are talking about having an MTU of 1500 when IpArp 
encapsulation is used, and an MTU of 1492 bytes when LLC/SNAP 
encapsulation is used.

> 
> I can take care of getting the counts right, putting in a rational MTU and
> updating the regression traces tomorrow AM.
> 
> I'm thinking that we should add a GetSdu to net-device.h that works with the
> current MTU and mode and returns a max payload value that can be used by
> clients.  This probably also applies to the point-to-point device since I
> remember adding a ppp header but not finding any place to reduce a payload
> (it also has a 0xffff MTU where this all probably descended from).  Wifi
> too?
> 

I do not think that adding SDU really adds anything.  The MTU is the 
Ethernet SDU, but I haven't typically heard the SDU/PDU terminology when 
talking about TCP/IP.

I suggest the default for Csma should be the DIX frame variant, not the 
LLC.  We call this "IpArp" encapsulation, I believe.  Maybe this should 
be renamed to DIX or Ethernet II, or at least commented as such.

http://en.wikipedia.org/wiki/Ethernet_II_framing

Of course, ns-3 Csma is not truly Ethernet but we probably are going to 
treat it as such for now and people will put the traces through Wireshark.

Tom