what is the meaning of lt20 when download windows version?

[BackMountainDevil]

I try download latest win version from sourceforge. But there are 4 types of v4.5.5.

1. qbittorrent_4.5.5_lt20_qt5_x64_setup.exe
2. qbittorrent_4.5.5_qt6_x64_setup.exe
3. qbittorrent_4.5.5_lt20_qt6_x64_setup.exe
4. qbittorrent_4.5.5_x64_setup.exe

I know qt6 vs qt5. A C++ UI framework. I have searched via Bing, this discussions and issues. I still not get the meaning of lt20. I have 2 question as below.

1. What is the meaning of lt20?
2. qbittorrent_4.5.5_x64_setup.exe default qt5 or qt6? with lt20 or not?

[thalieht]

What is the meaning of lt20?

libtorrent version 2.0.x. It has some problems with disk I/O which affects high speeds and maybe other problems too so the default for now is libtorrent 1.2.x. 1.2.x is stable but libtorrent development is mainly done in 2.0.x.

qbittorrent_4.5.5_x64_setup.exe default qt5 or qt6? with lt20 or not?

qt5 + libtorrent 1.2.x

[osemc3]

Please add this info to the Frequently Asked Questions wiki page. I do appreciate keeping the stable libtorrent 1.x release as the default until the 2.x issues are resolved, but it does make user curious, and I searched for an hour or so before I found this discussion thread.

[luzpaz]

@xavier2k6 can this be added to the wiki and sourceforge ?

[xavier2k6]

@xavier2k6 can this be added to the wiki and sourceforge ?

1. Don't know about updating sourceforge as don't have permission/access.

2. Wiki - don't see why not.

Maybe something like:

---

A note on libtorrent versions:

• libtorrent is the engine qBittorrent uses to handle torrents

• Different versions affect speed, features, and compatibility.
  (Development is mainly done in 2.0.x & 1.2.x is considered "Stable")

• For features like (I2P / 256 MiB Piece size / BitTorrent v2 Support etc.).
  Use builds denoted with lt20 / Qt6 and libtorrent 2.0.x series
  Installer example: qbittorrent_5.1.0_lt20_qt6_x64_setup.exe

• For better compatibility with older or private trackers.
  Use non-lt20 builds denoted with Qt6 and libtorrent 1.2.x series
  Installer example: qbittorrent_5.1.0_x64_setup.exe

[Obegg]

libtorrent version 2.0.x. It has some problems with disk I/O which affects high speeds and maybe other problems too so the default for now is libtorrent 1.2.x. 1.2.x is stable but libtorrent development is mainly done in 2.0.x.

Since this was posted years ago - Did anything change regarding issues with libtorrent 2.0.11?
Is libtorrent 2.0.11 better now than libtorrent 1.2.20?

[dftf-stu]

@Obegg: yeah, I checked the FAQ and nothing has been added to it to explain either the advantages of LibTorrent 2.0, nor what issues might remain with it that stops it becoming the only one they ship.

Would be really great if they could add into the FAQ:

• "What is different in the qBittorrent downloads that have "lt20" in the filename?"
• "What are the pros and cons of the LibTorrent 2.0 version of qBittorrent?"

As of now, I still only-ever use the 1.2 downloads, but I always wonder: "by not using the 2.0 version, does that mean there are some trackers, magnets or torrents I won't be able to connect with"?

[dftf-stu]

Also, for anyone looking at this ticket more-recently: the "qt5" builds of qBittorrent were discontinued with the release of the 5.0 version.

Qt6 on Windows requires Windows 10 or later; the Qt5 builds were only kept-around for Windows 8 and 7 support.

So if you look on the "Downloads" page now and wonder "what is this Qt5 the OP mentioned" -- you won't see it now, yes.

[xavier2k6]

my comment from may 19th this year is still relevant

[dftf-stu]

@xavier2k6: I did read your comment, but it doesn't explain what the new features of 2.0 are , just names them.

So for LibTorrent 2.0, what are the benefits or disadvantages of:

• I2P
• 256 MiB piece size
• BitTorrent v2 support

When you say "For better compatibility with older or private trackers[,] use non-lt20 builds ..." then does that mean by using the LibTorrent 2.0 version of qBittorrent that all old torrents and magnets will still work, it's just some trackers won't?

Again, this is the sort of info the FAQ should really detail, as I want to ensure by using the lt12 release, I'm not making it so certain trackers or seeds will ignore me as they only support 2.0 clients. But then, in-reverse, if I use 2.0, will some seeds on very-old torrent clients also not be able to connect to me?

[dftf-stu]

@xavier2k6: okay, so from what I've found:

BitTorrent v2 uses SHA-256 hashes for each piece (or chunk) of a file that gets downloaded, as SHA-1 generates a smaller length hash and so it's possible that someone could send chunks of different-data (possibly malicious code) that when the final-file gets reconstructed, the final SHA-1 hash matches the file you wanted to receive, but you actually get a different one. With SHA-256, this is much-harder to pull-off.

Okay, so obvious security-benefit there (though must not be happening much "in-the-wild", if LibTorrent 1.2 is still the default.)

I2P is "an anonymous, fully-encrypted network-layer that enables peer-to-peer communication with a focus on censorship-resistance and privacy, allowing users to connect and communicate without being easily tracked or monitored".

So... sounds like BitTorrent v2 connections always operate like a VPN tunnel, essentially? Though right-now, in the LibTorrent 1.2 builds of qBittorrent, you can go to Tools > Options > BitTorrent and set "Encryption Mode" to "Require Encryption". Not sure if that does the same sort-of thing, or is more-rudimentary than what v2 offers? 🤔

256 Mib piece-size I can't quite get my head-around, but essentially that piece-size was chosen for backwards-compatibility with v1.2 clients, as the truncated-hashes (or something) would break when calculated on pieces above that size. I've not been able to find much info though on why a larger-size is a benefit in the first-place though. I guess on modern Internet speeds, receiving one 256 MiB chunk, and generating a SHA-256 hash on that might be quicker than doing-so for say 256 individual 1 MiB chunks, but beyond that the only benefit would be DRAM-less SSDs, as writing larger files is much-better than many smaller ones. But are many people who use Torrent apps really saving files directly onto external USB thumb-drives regularly (i.e. setting the download location as that external-drive, versus copying files to them later from an internal-drive)? 🤷🏻‍♂️

[dftf-stu]

So, if you go to https://blog.libtorrent.org/2020/09/bittorrent-v2/ and scroll to the bottom, there are two test links there.

For the "v2 only torrent", both the .torrent file and 🧲 URL fail in qBittorrent v5.0.5 using LibTorrent 1.2, with the error:

Couldn't add torrent

The hybrid one works-fine.

So yes, there could be torrents or magnets out-there which are v2-only, and so the 1.2 builds won't support them... but I cannot say I've ever encountered any "in-the-wild" yet, so I guess moving to the lt2.0 builds isn't a priority on that-front.

Whether trackers might also support v2.0 only though, I'm not sure -- if I'm trying to download a rare or obscure torrent, am I cutting myself off from potential seeds by not using LibTorrent 2.0? I've not found a definite answer to that so-far... 🤔