[Flent-users] DSCP / ToS

Toke Høiland-Jørgensen toke at toke.dk
Mon Feb 19 12:50:50 EET 2018

Sebastian Moeller <moeller0 at gmx.de> writes:

> Hi Toke,
>> On Feb 19, 2018, at 11:22, Toke Høiland-Jørgensen <toke at toke.dk> wrote:
>> Pete Heist <peteheist at gmail.com> writes:
>>>> On Feb 18, 2018, at 7:48 PM, Pete Heist <peteheist at gmail.com> wrote:
>>>>>> This would break —dscp’s current behavior, so I would increment the
>>>>>> version number when I do it, and save it for v1.0 in case there are
>>>>>> other breaking changes.
>>>>> Well, that means Flent would need to deal with this case and detect
>>>>> which version of irtt is available; which IMO goes into the 'too much
>>>>> effort' category ;)
>>>> I hear you. :)
>>> So my latest idea is to not break the current behavior, and rather:
>>> 1) Add the text values as described before.
>>> 2) Return an error if a value is passed in with either of the two LSBs set to 1. That will save people in many cases who make the same mistake I did of passing in the DSCP value rather than the whole DS field.
>>> 3) If ever people really need to set what is currently the ECN bits, add a new flag at that time, but they really shouldn’t be doing that.
>>> Flent should be fine since it doesn’t pass in values with the ECN bits
>>> set.
>> Yup, this sounds like an excellent plan!
>>> Incidentally, I found that yes, Go lets you set whatever you want in
>>> the DS/ToS field, but somewhere along the Internet route from the
>>> office to my house, all the bits except the ECN bits are set to 0, so
>>> something, somewhere is washing the field in this direction only. So
>>> apparently, “DS field washing" happens sometimes as well…
>> Yeah, there's a reason DiffServ failed as an end-to-end measure. This
>> kind of shenanigans is happens way too often. For some networks I guess
>> it's necessary; if, e.g., you are doing strict priority on DiffServ
>> fields you kinda need to make sure someone doesn't DOS you by flooding
>> you network with EF traffic; and just clearing the fields is easier than
>> doing proper admission control... :)
> Well, it might be even easier for an ISP to completely ignore the
> tos/dscp fileds and simply use VLAN tags for priretisation, which a)
> can be interpreted by L2-Hardware (at least that is my understanding)
> and b) require active configuration so they are orthogonal to whateber
> gunk upstream passed in the DSCP fields (CS7?).

Yeah, I'm pretty sure some networks do that. Or something equivalent
using SDN. But using VLANs basically means you're now running your whole
network on layer 2; which means you'll need to handle routing at layer 2
as well. If you're not using SDN that basically means spanning tree or
something proprietary...


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