>-----Original Message-----
>From:
craigdo@... [mailto:
craigdo@...]
>Sent: Wednesday, July 23, 2008 12:37 PM
>To: ''Mathieu Lacage''
>Cc: ''ns-developers''
>Subject: Re: [Ns-developers] Csma, ethernet length field, NSC and the linkmtu...
>
>[ ... ]
>
>> SetMtu is expected to fail and return false if the value given is not
>> compatible with the configuration of the underlying device.
>> Now, if you
>> make the Mtu an attribute, you will need to make sure that
>> setting that
>> value through the attribute system conforms to the device's
>> bounds and,
>> if these bounds are not static (as is the case for the csma netdevice
>> which can be dynamically configured to behave in different ways), you
>> will have to provide your own implementation of AttributeChecker which
>> performs bounds checking based on the device's configuration.
>
>I think this means we agree. Let me reiterate:
>
>o No notion of separate PHY and MAC MTU numbers exist in either p2p or CSMA
>devices;
>o There should be an internal PHY MTU which corresponds to the 1500 byte
>enet number;
I suggest to avoid "MTU" term for anything but the (Mac) MTU, or else we have another round of ifIndex problems. Perhaps "FrameSize"? Ethernet frame size is 1518 or 1522 for VLAN tagging.
>o There should be a mac level MTU attribute checked against the Phy MTU;
Yes. User should be able to set this downward (e.g., an Ethernet MTU of 1300 bytes).
>o SetMtu should attempt to set the Mac MTU and return error if it can't;
Yes, through bounds checking that considers the current encapsulation mode defined and the underlying frame size.
>o GetMtu should return the MAC MTU which is the payload;
GetMtu should return min(Mac MTU attribute, (frame size - header size)), which if the bounds checking on setting works, effectively degenerates to returning (Mac MTU)
