Set `IPV6_TCLASS` for IPv6

[fujiapple852]

The socket option IPV6_TCLASS is used to set the Traffic class field on the IPv6 header. This option is not currently exposed by socket2.

[fujiapple852]

socket2 now supports this: rust-lang/socket2#420

[fujiapple852]

Not supported by Windows as a socket option and therefore still not supported by socket2: rust-lang/socket2#364 (comment)

We would need to use the Windows QoS API:

Do not use. Type of Service (TOS) settings should only be set using the Quality of Service API. See Differentiated Services in the Quality of Service section of the Platform SDK for more information.

[fujiapple852]

The windows-sys crate does define IPV6_TCLASS but it is not listed in the winsock documentation so unclear if it is supported.

[moeller0]

side note: i have been told that windows reallly wants to clamp down on setting TOS/DSCPs directly and the supplied QoS tooling only seems to allow to set a small enumerated set of DSCPs, one work around is to use powershell and NetQosPolicy which allows to force a whole application to set a specific DSCP... this might not do for trippy's needs, but for testing that can be useful.

# windows QoS: https://docs.microsoft.com/en-us/powershell/module/netqos/?view=win10-ps
Get-NetQosPolicy
# temporary -PolicyStore ActiveStore, otherwise omit -PolicyStore
New-NetQosPolicy -Name "putty" -AppPathNameMatchCondition "putty.exe" -ThrottleRateActionBitsPerSecond 1MB -PolicyStore ActiveStore -NetworkProfile All -DSCPAction 46 -WhatIf


New-NetQosPolicy -Name "putty" -AppPathNameMatchCondition "putty.exe" -PolicyStore ActiveStore -NetworkProfile All -DSCPAction 46

Get-NetQosPolicy -Store ActiveStore

Name           : putty_ps
Owner          : PowerShell / WMI
NetworkProfile : All
Precedence     : 127
AppPathName    : putty.exe
JobObject      :
DSCPValue      : 46

Remove-NetQosPolicy -Name "putty_ps" -PolicyStore ActiveStore

Seems to work, and I confirmed via tcpdump that putty's ssh packet originating from the windows VM host actually used DSCP 46 (aka EF). I note that powershell needs to be run as administrator, and the policies do not end up visible in Local Group Policy Editor. "-PolicyStore ActiveStore" will not survive a reboot, but for novice users that might be nice so a reboot cleans the slate in good windows tradition. I assume that after initial testing one could omit the "-PolicyStore ActiveStore" directive and store tested configs more permanently.

There also is a registry hack to allow local use of the group policy editor, but that still is not solving the issue how a running program can consciously set a specific DSCP in windows...

I wonder would it be possible to allow to set TCLASS values only for OS that allow that and just document windows issue in the manual? Being able to measure DSCP path modifications with IPv6 would be really a great feature, given that more and more mobile and home links are IPv6 only.... (I guess my hobby-horse shows ;) )

[fujiapple852]

I've updated the WIP impl (#1543) to set DSCP for IPv6 on Windows. It is untested as I do not have a way to test IPv6 on my Windows VM.

It uses the follow sequence of Win32 Win32::NetworkManagement::QoS functions to set the DSCP on the socket:

• QOSCreateHandle
• QOSAddSocketToFlow
• QOSSetFlow
• QOSCloseHandle

Setting the ECN is more problematic as it needs to be set per-packet and therefore requires rewriting the code from Socket:sent_to to use WSASendMsg.