Re: PATCH v3 [1/1]: MPTCP support for Git on Linux

> Hi,
> 
> This patch is about Multi-Path TCP.
> 
> Multi-Path TCP (MPTCP) had been in development for the past 15 years
> which started with MPTCP v0 (version 0) which initially had issues
> for middleboxes and NAT Gateways.
> 
> The current iteration is MPTCP v1 which has a fallback mechanism to
> regular
> TCP to avoid issues with middleboxes and NAT Gateways.
> 
> Started to add this code change as a need as i have large git 
> codebases
> with around 50 gigabytes and i have multiple WAN links which i can
> aggregate
> bandwidth across and even when network one path (even in between my
> CPE router
> to internet) is down, i will not get interrupted.
> 
> Also i am using a Linux laptop that has WiFi and 5G module. So this 
> kind
> of adds my reason of adding support for git (on Linux)
> 
> To get MPTCP to be fully working, both ends of client and server must
> implement
> MPTCP.
> 
> My implementation adds support for the basic git protocol.
> 
> MPTCP helps in situations when one of my WAN links have a high latency
> and
> automatically choose a link with a path with less latency.
> 
> Also, MPTCP aggregates the MPTCP connection by using subflows where 
> two
> or more
> links can be utilised with subflows. A single flow of data can have
> multiple
> subflows across different IP interfaces and thus increases network
> throughput.
> 
> Apple for example had been using MPTCP for their cloud services since
> MPTCP v0
> which had issues with middleboxes (not MPTCP v1) since 2013.
> 
> The downside, even though i had never experienced it for other
> applications
> on Linux like Google Chromium[1], is that the fallback might induce
> delays
> in connectivity, if i've read it somewhere which i cannot recall 
> where.
> 
> How this patch works:
> 
> This patch enables MPTCP protocol option only when it's built on Linux
> with
> IPPROTO_MPTCP support in netinet/in.h.
> 
> On Linux, if IPPROTO_MPTCP is not defined in netinet/in.h, it will
> skipped.
> 
> IPPROTO_MPTCP should and never be enabled when it detects being built 
> on
> an OS other than Linux with defined(__linux__) check.
> 
> Another challenge is that although "getaddrinfo()" is a POSIX 
> function,
> not all glibc "getaddrinfo()" implementation is written with
> IPPROTO_MPTCP support out of the box, especially on older glibc
> versions.
> 
> getaddrinfo() IPPROTO_MPTCP support had only been added to recent 
> glibc
> in 2025 eventhough IPPROTO_MPTCP definition had been around for
> much longer in netinet/in.h.
> 
> So we run getaddrinfo() which is a code in glibc and check for errors,
> specifically "EAI_SOCKTYPE" return value which tells us that the 
> socket
> type
> is not supported and fallback to regular TCP (IPPROTO_TCP)
> 
> Also we will also check that we are building on Linux and depending on
> version number of Linux we will initialize the socket() accordingly 
> and
> if
> there is an error return value (like
> EINVAL/EPROTONOSUPPORT/ENOPROTOOPT),
> we will fall back to regular TCP.
> 
> Enabling and disabling MPTCP:
> 
> By default on the client side, MPTCP will not be enabled in git 
> client,
> however MPTCP
> can be enabled by setting an environment variable "GIT_ENABLE_MPTCP" 
> to
> any value.
> 
> Persisting the configuration can be done in your shell.
> 
> Also for server side git server (daemon.c), there is a flag to
> optionally
> enable mptcp with "--mptcp", example:
> 
> git-daemon --base-path=/all/my/repos --export-all --mptcp
> 
> This will tell the git server daemon to accept mptcp connections but
> fallback to regular tcp when mptcp connection is not available.
> 
> PS: Can someone point me about having a "knob" in Makefile or is this
> already sufficient?
> 
> [1] https://chromium-review.googlesource.com/c/chromium/src/+/6355767
> 
> Signed-off-by: Muhammad Nuzaihan Bin Kamal Luddin
> <zaihan@unrealasia.net>
>