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Author Topic: New 4G USB ‘dongle’ i/f  (Read 1386 times)

Weaver

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New 4G USB ‘dongle’ i/f
« on: November 24, 2020, 11:20:25 PM »

AA have sent me a new 4G dongle free and a new SIM for my Firebrick. I was moaning about occasional failures of the 3G one that I currently have, which AA supplied. Both are Huawei but I can’t remember the model numbers; the new one might be a 3372. See also :
    https://support.aa.net.uk/Category:FireBrick_USB_Dongles
Superb service, that they just swapped it out for free when they couldn’t reproduce the rare ‘disappearances’ of the 3G dongle that I had seen every few months. And after all, they might never see the badness happen, because their network is different: signal strength different and god knows what. So how long do you wait.

I haven’t installed the 4G one yet. I will need to add something to the FB2900 XML config because the new dongle has an ethernet i/f with DHCP, as opposed to the 3G one which speaks PPP. See above URL.

I don’t really understand the remarks about NAT and the 4G dongle. I understand the the Firebrick will get some IPv4 address from the dongle by DHCP. Still doesn’t speak IPv6  >:(  :( ??? I’m assuming this is AQL or Three’s fault. What is the NAT thing all about? Will the 4G dongle mangle all the source IPv4 addresses of the hosts on my LAN when they go out onto the internet? All of my LAN IPv4 range is routed through the dongle (secondarily, on a fallback basis) to my LAN so I want the real global routable existing IPv4 addresses of the hosts to go out to the internet unmolested.
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burakkucat

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Re: New 4G USB ‘dongle’ i/f
« Reply #1 on: November 25, 2020, 12:26:55 AM »

My interpretation, having looked at the link you provided, is that the 4G dongle is a DHCP server and it will hand out an IPv4 address to the connected Firebrick.

But . . . I'm confused.  ???
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Weaver

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Re: New 4G USB ‘dongle’ i/f
« Reply #2 on: November 25, 2020, 01:14:25 AM »

Do we think it is a NAT translator too? Or is it just the FB being a NAT translator, for some reason ?

This because it shows nat="true" in the FB XML example on the website (URL given above). I have emailed AA tech support to ask for confirmation about the ‘nat’ attribute as if I can I would obviously want to avoid it as I don’t want NATing. Perhaps that example just presumed that the AA user only had one IPv4 address for their whole LAN and that was the only reason for it, because one IPv4 is the AA default unless you ask for more addresses, which are free of charge. I have explicitly put nat="false" on the ‘dongle’ element currently as I believe the default is ‘true’ iirc.

One IPv4 address is allocated to the new SIM by AA, and the existing /26 is already secondarily (backup) routed to the new dongle, ie when all DSL lines go down as detected by AA’s PPP LCP pinging then they re-route traffic to the secondary route which is through the two dongles (new and old) whichever is up. I set that up by ticking the appropriate flag check-boxes in clueless.aa.net.uk.
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Alex Atkin UK

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Re: New 4G USB ‘dongle’ i/f
« Reply #3 on: November 25, 2020, 02:53:13 AM »

I believe the dongles may indeed do NAT in certain modes, though not sure why as it seems completely unnecessary as they could simply bridge.

There does seem to be a few different protocols they can use though RNDIS is probably default, thus the virtual NIC and DHCP.  I think ACM mode requires a completely different firmware on the dongle, although you'd think AA might apply that considering?
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Weaver

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Re: New 4G USB ‘dongle’ i/f
« Reply #4 on: November 25, 2020, 03:48:38 AM »

I’m getting AA to check out my proposed FB XML config before I try the device.

One of the superb things about having readable XML or similar in config files, as I’ve said before - you can diff it, edit it and email it to someone for perusal. And you can process it. I have written a tool to prep it too, with #ifdef-like commands within significant comments.

MTU: I’m changing MTU for IPv6 compared to that with the 3G one, which was 1408 bytes: have added 48 to that to bring it up to 1408+48=1456 which is (n * 48 - 32) as before, for ATM full efficiency, the 32 because 32 bytes is the total ADSL protocol stack overhead, as before. This figure is less than the 4G MTU which is 1500 and less than 1500 - 20 = 1480 which is the MTU with the IPv6-over-IPv4 tunnel overhead subtracted because since before there’s no IPv6 support, we have to use AA’s tunnel for IPv6 traffic during failover-to-4G mode.

I can’t use the next multiple of n, because n * 48 - 32 is too large, since with n=32 : (n=32) * 48 bytes - (ovh = 32 ) = 1504 bytes which is > (1500 - 20)
« Last Edit: November 25, 2020, 04:40:44 AM by Weaver »
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Weaver

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Re: New 4G USB ‘dongle’ i/f
« Reply #5 on: November 25, 2020, 05:12:44 AM »

Pros and cons:

Cons of 3G: The 3G dongle is very slow, only about 2Mbps downstream, I can’t remember the upstream, and the low MTU of 1440 bytes is an annoyance. The rare failures where it just disappears are bad.

I don’t notice these because there are no notification messages from AA; clueless.aa.net.uk assumes that all mobile network connections are indeed mobile so loss of signal is assumed to be normal and it is assumed, wrongly, that users do not want to be pestered with useless notifications. This is completely wrong for stationary users and Firebrick dongle users in particular so I have suggested that AA make this a per-link flag/option in clueless. There are already options to control the behaviour of notifications but these are not line-specific. Because of this lack of notifications, my current 3G dongle can fail and I don’t know about it for weeks, until I just happen to inspect things.

The low MTU of the 3G dongle means that during failover, 1500-byte long IPv4 packets will get fragmented or worse, and that is not good. That’s for existing flows; new flows started up after the failover point will presumably be set off with the correct MTU, which will be a mere 1440 bytes, but that should be fine. I have I think asked about this before, but I wonder how bad in practice IPv4 fragmentation can be?

Pros of 3G: it routes my existing IPv4 /26 straight out to the internet as it should. IPv6 works through a tunnel.

Cons of 4G:

All things considered, I am thinking about which way to go should it turn out that the 4G dongle does not handle straight routing of my current global routable IPv4 /26 directly to the internet, and I find that it’s mangling source addresses of transmitted packets.

Pros of 4G: speed, possibly; full IPv4 MTU 1500, so I’m told. Hopefully 100% reliability?

If the 4G dongle misbehaves with fiddling about with tx packets, then I might even consider sticking with the 3G one despite the slowness and the low MTU as long as its reliability remains fairly reasonable.

This is all just me fretting in advance; there might be absolutely no problem.

Suggestions?
« Last Edit: November 25, 2020, 05:26:57 AM by Weaver »
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meritez

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Re: New 4G USB ‘dongle’ i/f
« Reply #6 on: November 25, 2020, 09:32:37 AM »

Looks like it has two modes, ethernet where it behaves as a gateway and hands out dhcp, or serial modem: https://firebrick.co.uk/about/news/2018-06-01-4g-dongles/

Whether serial modem mode will allow ipv6 routing is unknown
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Weaver

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Re: New 4G USB ‘dongle’ i/f
« Reply #7 on: November 25, 2020, 03:11:06 PM »

Thank you for that link. Since the mobile network doesn’t support IPv6, I’m using AA’s 6-over-4 tunnel to make it work.
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Alex Atkin UK

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Re: New 4G USB ‘dongle’ i/f
« Reply #8 on: November 25, 2020, 08:26:52 PM »

Looks like it has two modes, ethernet where it behaves as a gateway and hands out dhcp, or serial modem: https://firebrick.co.uk/about/news/2018-06-01-4g-dongles/

Whether serial modem mode will allow ipv6 routing is unknown

Like I said, the Huawei devices need to flash a different firmware to enable this mode and I'm surprised AA wouldn't have done this knowing the customer has a routable public IP range.

They default to the RDIS mode because its faster and it allows their router software to interact with it, but even in serial mode its going to be WAY faster than the 3G dongle was.
« Last Edit: November 26, 2020, 01:00:41 AM by Alex Atkin UK »
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Weaver

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Re: New 4G USB ‘dongle’ i/f
« Reply #9 on: November 26, 2020, 05:48:49 AM »

AA tell me they haven’t tested the alternative firmware (yet).
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Alex Atkin UK

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Re: New 4G USB ‘dongle’ i/f
« Reply #10 on: November 27, 2020, 01:39:34 AM »

Strange, seeing as the bonding + mobile backup is a business package you'd think they would be focusing on the business case of routable IPs at all times.  Still, at least they acknowledged its existence.
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Weaver

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Re: New 4G USB ‘dongle’ i/f
« Reply #11 on: November 27, 2020, 01:43:26 AM »

It’s annoying. With my current 3G dongle, I get fully routed IPv4 and IPv6 blocks over 3G. So if the 3G device decides to behave, then I shall probably stick with that.

The support website mentions the option of using L2TP over 4G to get around the IPv4 NATing problem, but I don’t know enough about it. I’m already using a tunnel for IPv6, but using 6 over 4.
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Alex Atkin UK

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Re: New 4G USB ‘dongle’ i/f
« Reply #12 on: November 27, 2020, 02:58:24 AM »

I'd just keep tormenting AA until they figure it out as its supposed to be part of your package.  They specifically mention supporting 3G or 4G on the product page and an IP range, so it should work correctly for both IMO.
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Weaver

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Re: New 4G USB ‘dongle’ i/f
« Reply #13 on: November 27, 2020, 09:18:56 AM »

That’s what I have been doing, sort-of. Politely.  :) The current 3G dongle, which is like a straight modem, routes the whole IPv4 range correctly, but it has been slightly unreliable in the past, ‘disappearing’ every few months. The problem for AA is that the available 4G dongles are rubbish, the stupid, unwanted NAT feature should be selectable in them. L2TP should sort that out; I’ve asked them about it.

I’m not sure when or if the 3G one is going to fail again. It’s slow and its MTU is low; those are only minor disadvantages with it.
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Alex Atkin UK

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Re: New 4G USB ‘dongle’ i/f
« Reply #14 on: November 27, 2020, 09:15:36 PM »

I can see their point, bypassing the issue entirely by using L2TP and they may be right, it might be the best option.  But its still additional overhead that the alternative firmware wouldn't have.  Unless they actually test it using both firmwares, I don't see how they can claim one is better than the other.

I agree about the NAT feature though, you'd expect a dongle to work in bridge mode seeing as there will only ever be a single client.  It seems this was just a lazy thing for Huawei so they didn't have to do something more complex for their router app to work.

On the other hand if you're going to have to use NAT anyway, wouldn't it be better to use a 4G router?  They support newer standards and get far superior reception.  I get they need an additional mains socket, but might be worth it for reliability and flexibility?  For example I like that as it broadcasts its own WiFi I can always use it directly to check my firewall rules are working correctly from the WAN side (as my phone is PAYG so don't want to waste credit doing it from my SIM there).
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