Re: Small patch to add support for MPTCP on Linux

[Muhammad Nuzaihan <zaihan@unrealasia.net>]

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Hi all,

I've made the git daemon to work (for the git protocol), as attached
in the wireshark screenshot.
As i had mentioned that the most basic implementation of the protocol 
should be
the basic git protocol to test the implementation and not git over ssh 
or git over http.


Unless if everyone wants to deprecate the git protocol entirely then it
might be a different issue.

But i think there need to be a discussion as i have mentioned this 
thread
about the concerns being raised by others.

Thanks,
Zaihan

On Sat, May 17 2025 at 10:21:00 AM +0300, Hridoy Ahmed 
<ariyanhridoy130@gmail.com> wrote:
> 
> 
> On Sat, 17 May 2025 at 10:20 AM Muhammad Nuzaihan 
> <zaihan@unrealasia.net> wrote:
>> Hi Brian.
>> 
>>  On Fri, May 16 2025 at 08:33:03 PM +0000, brian m. carlson
>>  <sandals@crustytoothpaste.net> wrote:
>>  > On 2025-05-16 at 17:56:07, Muhammad Nuzaihan wrote:
>>  >>
>>  >>  Patch to enable the use of MPTCP on Linux (when available)
>>  >>
>>  >>  IPPROTO_MPTCP v1 (not the old v0) has been improved to go about 
>> the
>>  >>  limitations of middleboxes.
>>  >>
>>  >>  MPTCP protocol is an extension of vanilla TCP which enables 
>> multiple
>>  >>  IP to aggregate bandwidth at layer 4 of the OSI stack across
>>  >>  as said IP(s).
>>  >>
>>  >>  Similar to link aggregation which works at layer 2. MPTCP works 
>> on
>>  >> top
>>  >>  of IP layer.
>>  >>
>>  >>  Other than aggregating bandwidth, MPTCP also allows seamless
>>  >> failover
>>  >>  when one network path (not just link) is down (or having high
>>  >> latency)
>>  >>  by reinjecting the packets to a path that is available.
>>  >>
>>  >>  This patch enables IPPROTO_MPTCP if IPPROTO_MPTCP is available 
>> and
>>  >>  uses plain TCP if the Linux system does not support it.
>>  >
>>  > What happens here if I compile this on a system that has a kernel 
>> that
>>  > supports MPTCP but then switch to one that does not?  The reason 
>> I ask
>>  > is that I have worked at places where we shipped binaries, 
>> including
>>  > Git, based on a standard CentOS or RHEL system, but then some 
>> people
>>  > used our software on a system with a very stripped down kernel (in
>>  > some
>>  > cases, where IPv6 was not even compiled in) because doing so meant
>>  > that
>>  > they could make about $5 more per server per month.
>>  >
>>  MPTCP supports *both* IPv4 and IPv6. Don't tell me people would also
>>  remove
>>  even IPv4 as well? I had written an #ifdef statement to check if
>>  IPPROTO_MPTCP
>>  exists and enables that.
>> 
>> 
>>  > Do the operating systems which support MPTCP make it a compulsory 
>> part
>>  > of the TCP stack, or could we end up with cases where we're 
>> unable to
>>  > connect here?
>>  >
>>  > In addition, Wikipedia mentions that FreeBSD has only IPv4 
>> support,
>>  > but
>>  > I don't know if that's up to date.  What happens if we run on a 
>> system
>>  > where MPTCP is used, but it doesn't work with IPv6 and the only 
>> remote
>>  > IP is IPv6?  Do we fall back properly, or do things fail?
>> 
>>  This patch *specifically* targets Linux to check if IPPROTO_MPTCP 
>> exists
>>  in the Linux system. I think you have not read my initial patch
>>  description
>>  properly nor even read about the new changes for MPTCP.
>> 
>>  MPTCP support is now officially in the mainline kernel and not
>>  out-of-tree.
>> 
>>  This *current* implementation of MPTCP is v1 and not v0 (v0 had
>>  problems and
>>  v1 already solved the issue with middleboxes. again, please read my
>>  patch
>>  description properly)
>> 
>>  Please read up on how MPTCP falls back to regular TCP if it could 
>> not
>>  connect
>>  using MPTCP.
>>  >
>>  > I ask these questions not because I'm opposed to this feature but
>>  > because I want to be sure we don't accidentally break things for
>>  > users.
>>  >
>>  I'm not sure but you have not even bothered to read the 
>> documentation
>>  about MPTCP.
>>  > I know that for instance Go 1.24 enabled MPTCP and that ended up
>>  > causing
>>  > problems in some environments, so I would recommend that we make 
>> this
>>  > a
>>  > configurable option instead.  We can definitely default to MPTCP, 
>> but
>>  > we
>>  > probably need an option to fall back.
>>  MPTCP v1 (again i am repeating myself) and not the old MPTCP v0 does
>>  the fallback
>>  more effectively.
>> 
>>  Do you know of any references that mentions that Go 1.24 with MPTCP
>>  enabled
>>  (normally this is the current MPTCP v1) is causing the issues?
>> 
>>  If you could give me evidences of such issues, maybe i can 
>> reconsider
>>  it again.
>>  >
>>  > Of course, this code path is only used by the unauthenticated Git
>>  > protocol usually run on port 9418, which practically nobody uses
>>  > anymore
>>  > (because it lacks the privacy, integrity, and authentication 
>> which are
>>  > necessary and prudent on the modern Internet), so maybe nobody 
>> cares
>>  > about edge cases there.  My guess, though, is that the people most
>>  > likely to be using something that isn't HTTPS or SSH are also the
>>  > people
>>  > most likely to be using odd or unusual configurations, so we may 
>> very
>>  > well want to add an option for them.
>> 
>>  Again, the unauthenticated Git protocol is the *most basic* setup 
>> that
>>  anyone
>>  can use to test MPTCP out. I understand from your point of view but 
>> it
>>  does
>>  not make sense to support ssh and http when the most basic git 
>> protocol
>>  is
>>  not supported.
>> 
>>  git protocol is the *most basic* protocol. For ssh and https that 
>> would
>>  fall
>>  under other project's implementing (like openssh or apache)
>> 
>>  I would consider adding an option to read from .gitconfig to enable
>>  MPTCP
>>  where i can leave MPTCP disabled by default.
>> 
>>  But what you explained about the downsides of MPTCP (without 
>> evidences)
>>  and not even implementing MPTCP for git protocol does not make 
>> sense.
>> 
>>  Regards,
>>  Zaihan
>>  > --
>>  > brian m. carlson (they/them)
>>  > Toronto, Ontario, CA
>> 
>> 
>> 


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