[Bloat] Sidebar re compatibility (was: tc linklayer ADSL calc broken after commit 56b765b79)

[David Collier-Brown]

Jesper Dangaard Brouer <jbrouer at redhat.com> wrote:

> On Wed, 29 May 2013 16:18:26 -0700 Eric Dumazet
> <eric.dumazet at gmail.com> wrote:
>> On Wed, 2013-05-29 at 15:50 -0700, Stephen Hemminger wrote:
>>> On Wed, 29 May 2013 08:52:04 -0700
>>> Eric Dumazet <eric.dumazet at gmail.com> wrote:
>>>
>>>> On Wed, 2013-05-29 at 15:13 +0200, Jesper Dangaard Brouer wrote:
>>>>> I recently discovered that the (traffic control) tc linklayer
>>>>> calculations for ATM/ADSL have been broken by:
>>>>>  commit 56b765b79 (htb: improved accuracy at high rates).
>>>>>
>>>>> Thus, people shaping on ADSL links, using e.g.:
>>>>>  tc class add ... htb rate X ceil Y linklayer atm overhead 10
>>>>>
>>>>> Will no-longer get ATM cell tax/overhead adjusted.
>>>>>
>> Adding the logic on the kernel is doable, by adding some clean
>> attributes so that tc can setup the feature, and report the attributes
>> back.
> Yes, doing the logic in the kernel might be a better solution.
> But the question is how do we keep iproute2 backward compatible with
> older kernels?

If you're adding a new functionality, and distinguish it from the
existing ones in some reasonable way, the problem should not exist.

In the special case of fixing a bug that no-one depends on, the change
is strictly to one's advantage.

The only time one requires backward compatibility is when someone
depends on the previous behaviour. If someone legitimately depends on
the bug, you have the choice of visibly distinguishing the fixed version
from the buggy one, or invisibly ensuring that old users get the old
buggy behaviour and new ones get the new behaviour. The latter is
usually done with linker tricks, so that anyone who compiles and links
with the fixed version gets the fixed behaviour, while anyone who merely
re-links old software gets the old behaviour.

[I can blather on about this interminably: for more detail see
http://cacm.acm.org/magazines/2009/11/48444-you-dont-know-jack-about-software-maintenance
and send me offline email]

--dave

-- 
David Collier-Brown,         | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
davecb at spamcop.net           |                      -- Mark Twain
(416) 223-8968