Linux Kernel MultiPath TCP Project

by kristianpaul • 4 years ago

95 54 4 years ago

“MultiPath TCP (MPTCP) is an effort towards enabling the simultaneous use of several IP-addresses/interfaces by a modification of TCP that presents a regular TCP interface to applications, while in fact spreading data across several subflows.”

datastoat3• 4 years ago

One other point to make about MPTCP: it has a congestion controller that takes pains to be fair with respect to regular TCP. It's a subtle problem...

MPTCP sender -> [link1 | link2] -> shared link -> MPTCP receiver

There might be a TCP flow using the shared link, in which case fairness means that the MPTCP sender should only send the same total traffic as would a single TCP flow. Or there might be a TCP flow using link1, and a second TCP flow using link2, and the shared link might be uncongested, in which case MPTCP could reasonably behave like two TCP flows. Or maybe there's a single path TCP flow using (link1 -> shared), or one using (link1 -> link2), or ...

The MPTCP congestion controller was designed so that in all of these cases it's no greedier than if it were a single TCP flow. And it has to do this without knowing the network topology, nor what other flows are using the network.

Reference: 2011 paper, https://www.usenix.org/conference/nsdi11/design-implementati...

j16sdiz• 4 years ago

Is there any path to "upgrade" TCP to MPTCP? Last time I checked, it is a separate protocol, need separate stack and no software support

xmichael9991113• 4 years ago

VPN

nynx• 4 years ago

I wonder whether this would be better spent integrating into QUIC

topspin2• 4 years ago

https://multipath-quic.org/

tracnar3• 4 years ago

The work on MPTCP started before QUIC. One big difference is that MPTCP puts a lot of effort to look like TCP to avoid being affected by middleboxes (firewall, NAT, proxies, ...). QUIC uses UDP which is more often blocked or throttled, though that will quickly change as it gets more widely deployed!

londons_explore2• 4 years ago

Programs that were built to use TCP will still use the TCP connections when running on an MPTCP-enabled kernel unless the programmer/user/admin opts-in to using Multipath TCP.

So, if MPTCP is a drop-in replacement for TCP, why do programs need to opt into it?

Without being enabled by default, this project will go nowhere.

bheadmaster• 4 years ago

I didn't see the phrase "drop-in replacement" anywhere in the linked text, so I assume it's not supposed to be a drop-in replacement.

tracnar3• 4 years ago

MPTCP is an extension of TCP and is "drop-in". But AFAIK making it opt-in was a concession to get it upstreamed in Linux, understandably as this is a risky change.

There are different ways to enable it on unmodified applications, e.g. via a wrapper or using eBPF.

kevincox• 4 years ago

Hopefully there will eventually be a sysctl to do it by default and maybe with enough testing enable tbat be default.

simoncion• 4 years ago

Without being enabled by default, this project will go nowhere.

The maintainers of Linux distributions will enable it, if they see doesn't cause trouble with regular network traffic.

After all, there are instructions for the "old" implementation: <https://multipath-tcp.org/pmwiki.php/Users/ConfigureMPTCP>, <https://multipath-tcp.org/pmwiki.php/Users/ConfigureRouting> and the "new" implementation: <https://github.com/multipath-tcp/mptcp_net-next/wiki/#how-to...>.

datastoat3• 4 years ago

MPTCP is a drop-in replacement -- but its original designers were paranoid about e.g. packet-sniffing middleboxes, so they made it opt-in.

Suppose for example there's a packet-sniffing security appliance along the path, and it only sees packets on one path of the MPTCP flow, and it says "This stream of packets doesn't look like a legitimate TCP flow, so it's probably an attack, so I'll block it."

pid-1• 4 years ago

Tangencial, but that's also the reason QUIC (used by HTTP/3) was built upon UDP.

londons_explore2• 4 years ago

I think the real answer is to test a lot of middleboxes, come up with some fallback behaviour that will at least work with 99% of them, and then just break the last 1%.

The same happened with Max Segment Size detection - and that mostly works now.

Shalle135• 4 years ago

How is this superior to bonded / teamed interfaces?

m_eiman• 4 years ago

Bonded interfaces have the same IP (iirc), MPTCP can send traffic over multiple IPs, eg over both wifi and 4g, for better roaming. Especially useful when moving around and entering/leaving different wifi networks, all without dropping the TCP connection.

miken123• 4 years ago

A bonded/teamed interface usually load balanced based on a flow. Thus, over a single TCP flow you will only use one of the interfaces.

On top of that MPTCP also offers advantages when it comes to handover in mobile networks: if your IP address changes for some reason (e.g., switching from Wifi to mobile), existing connections won't be interrupted.

zokier2• 4 years ago

It's more related to multihoming than link aggregation.

datastoat3• 4 years ago

Suppose the sender has a single interface, and it's the receiver that has multiple interfaces. With MPTCP, the sender will learn that there are multiple paths available, and it'll balance its load adaptively over those paths.

I don't know enough about bonded interfaces, but I don't see how they'd help in this scenario. This scenario is natural with mobile devices, which is probably why Apple uses MPTCP.

pgorczak2• 4 years ago

While a TCP socket needs to be created with specific flags to enable multipath, there is a tool that wraps existing applications and “upgrades” the socket calls on the fly: https://manpages.ubuntu.com/manpages/jammy/man8/mptcpize.8.h...

jiggawatts• 4 years ago

For anyone wondering what the point of this is -- MPTCP is being pushed primarily by Apple. It is intended to make mobile devices like phones or tablets be able to switch between WiFi and 4G/5G seamlessly.

Theoretically it would also be useful for corporate laptops that get unplugged from a docking station every time someone needs to go to a meeting. Any long-running downloads wouldn't have to be interrupted, SSH sessions could remain connected, etc...

ggm• 4 years ago

"Pushed" is a bit objectifying. Apple implemented and owning both sides of client (iphone) and server (iOS distribution service) have made it work for them. And iMessage. It's works, it gives persisting end to end connection (I would say session in isolation 7 layer model terms) state, it allows services to continue unabated between wifi and mobile.

It's good tech. I hope more people implement.

tyingq• 4 years ago

I don't know what Android uses for the same purpose, but it got a lot better about 2-3 years ago. I see it every day, when I leave the house in my car, and it's been seamless for a while now, even for live phone calls. I suppose YMMV, since everyone seems to be on a different release of Android, different hardware, etc.

xiphias2• 4 years ago

Can it be used for ,,pausing'' TCP connections?

When I go into an elevator, the WIFI / 4G connection can drop for half minute for example, then it reconnects later.

Or for example switching from one wifi network where the connection is slow to another one (my fast tethered mobile network).

vlan02• 4 years ago

This isn’t a gab at you. But this question doesn’t make sense. I think the things you don’t know you don’t know about TCP and distributed system design make this difficult to pick apart.

Remember, your identity in a TCP connection is your IP address. If you connect from another IP and want your session to continue, identity must be handled higher in the stack.

TCP connection technically live forever by default. It’s up to either party to complete the handshake. Without that mechanism, your box would run out of ports.

pgorczak2• 4 years ago

TCP can survive those interruptions on its own, unless the application sets short timeouts or the IP address changes. MPTCP keeps working when either side of the connection changes addresses.

zokier2• 4 years ago

I had such high expectations for mptcp when I first heard about it, it just makes lot of sense. But after a decade of work I see no wider interest in it, nevermind mptcp becoming the default option.

bonomono• 4 years ago

What about the obvious privacy concerns that arise from the use of this technology, particularly if it becomes widespread? I see nothing on this website that addresses those.

BiteCode_dev• 4 years ago

Obvious ?

loa_in_• 4 years ago

I don't see how this impacts privacy.

nonrandomstring2• 4 years ago

The impact on privacy appears positive.

Most users remain unaware that ISPs track and monitor traffic, or that services like Google and Cloudflare can fingerprint and statistically aggregate connection meta-data (packet size and timing) to build data sets even where privacy preserving technologies are deployed by the user.

Many domestic users could take advantage of this TCP kernel update because of the ubiquity of available connections. In dense urban settings people might have access to one or more wired connections, perhaps a couple of 4G /5G modems lying around, a phone to tether, and perhaps some free-open wifi connections from nearby coffee shops etc.

Multi-homing not only offers greater reliability, it can allow connections to evade spying much like "spread spectrum" cryptography, by multiplexing connections - as TCP was designed/intended to accommodate - spreading them randomly over multiple links.

Of course there are ways that multi-con TCP could lessen privacy in the hands of a naive user, perhaps by unexpectedly routing through a less secure link, but this comes down to digital literacy and good craft in the end.

yewenjie• 4 years ago

Is there anything that I can use today that lets me use both data and Wi-Fi on my mobile phone for faster internet?

cypress66• 4 years ago

Depends on your definition of faster.

Bonding can give you higher thoruhput, assuming you have multiple tcp sessions.

phh• 4 years ago

Android has Speedify app that can do that (proprietary app and back end, with no self hosting)

dveeden2• 4 years ago

Doesn't SCTP serve the same purpose? Any reason why that never became mainstream for SSH etc?

detaro• 4 years ago

In theory yes, but good luck finding a firewall or consumer router that passes SCTP. Support for it across random internet paths is atrocious, this stuff is so stuck that if you don't look like TCP or UDP it's not generally usable. MP-TCP specifically does what it can to look like normal TCP connections so it doesn't get messed up by middle boxes and degrades gracefully.

drummer• 4 years ago

Similar tech already implemented in QuantumGate with a load of other capabilities https://github.com/kareldonk/QuantumGate/wiki/QuantumGate-UD...

thomas536• 4 years ago

Is anyone using this on Android?

xmichael9991113• 4 years ago

Check out openmptcprouter.com works excellent!

xmichael9991113• 4 years ago

Check out openmptcprouter.com works excellent, currently connected using it and two home connections.

Linux Kernel MultiPath TCP Project

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