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Forum Discussion
MR2100 - Need to Manually set MTU (due to PMTUD failure)
I've been trying out the 3 big LTE networks in the UK for comparison (EE, Three, Vodafone), I'm sticking with Vodafone which has the best reception in my area, and this is where the MTU problem starts.
The MR2100 appears to do some kind of PMTUD for the LTE connection (Path MTU Discovery) and then relays this for new wifi connections to ensure connected devices fragment packets at the appropriate size for the network, which is great, and seemingly necessary as each LTE network appear to have fairly arbitrary and low MTUs.
It correctly determines the MTU for EE and Three, e.g it uses 1380 bytes for Three which is correct, however It uses 1500 bytes for Vodafone which is incorrect. Now I'm pretty sure I know why this is - because the initial connection on Vodafone _is_ in fact 1500, the problem is with Vodafone the effective MTU _always_ lowers over time, eventually settling on 1464 bytes - I can only speculate as to why, perhaps cell aggregation causes it to settle on a different path - Anyway, the MR2100 does not detect this, it only appears to attempt PMTUD upon the initial connection, and so of course all the connected devices will continue attempting to send 1500 for large enough packet... and you can probably guess what happens, things get flakey, website loading gets a bit annoying, HTTP requests that had a particularly big header will just vanish... uploading becomes almost unworkable.
It's possible to manually work around this by first using "PMTU black hole discovery", i.e bisect the MTU size by manually pinging different sizes starting with `ping -M do -s 1472 1.1.1.1` (-28 byte header), this is how I found the correct Vodafone MTU. Now I can set this for every - single - device... which works, but is incredibly annoying.
So my question is: How can I manually set the MTU on the MR2100 to centralise this, this used to be an ordinary setting for the modem on old routers, but there doesn't appear to be any setting for this in the MR2100 user interface (192.168.1.1). I'm happy to fish around in the telnet console but don't know where to start or if it's even possible.
5 Replies
So i'm guessing this would require an AT command if it's possible, and I've now spent longer than I care to say trying to find an AT command reference for the "Qualcomm SDX24 LTE Modem" in this thing, these baseband people seem to keep this info under lock and key... Any hints for the AT command setting IP MTU?
tom- wrote:
Anyway, the MR2100 does not detect this, it only appears to attempt PMTUD upon the initial connection, and so of course all the connected devices will continue attempting to send 1500 for large enough packet... and you can probably guess what happens, things get flakey, website loading gets a bit annoying, HTTP requests that had a particularly big header will just vanish... uploading becomes almost unworkable.
This is clearly a probem with the (always ongoing) PMTUD: The last system, resp the IP stack on the Web server or the proxy/load balancer in the front of the Web server must realize that the packet is incomplete, ... Also http headers can be fragmented.
> This is clearly a probem with the (always ongoing) PMTUD: The last system, resp the IP stack on the Web server or the proxy/load balancer in the front of the Web server must realize that the packet is incomplete, ... Also http headers can be fragmented.
No, this dissapears at the first few hops irrespective of the address, i.e inside of Vodafone's network (I've verified this with multiple addresses including one of my own servers to double check, where I can see the MTU for the public interface). As I said it's pretty easy to bisect the size with `ping` (using any well behaved server that will bounce a sizable ICMP), and then also see where the packet dissapears with MTU +1 byte using something like MTR.
I'm using "PMTUD" here losely and perhaps incorrectly because I don't know what actual mechanism the modem uses to determine the appropriate IP MTU, it's more likely it's determined based on something explicitly negotiated in the LTE layer... but these modems aren't exactly very open and I'm not an LTE expert so I'm saying "PMTUD" to mean "auto IP MTU based on whatever the LTE modem says"... from what i've read on other better documented modems there tends to be an AT packet size command to either set an explicit IP MTU or "auto" mode which I presume is determined by the LTE modem.Nothing magically disappears. If your server does operate on the industry standard MTU, the IP stack must recognize that an incomplete packet arrived, and it has to inform the sender by returning an ICMP Fragmentation Needed (Type 3, Code 4). The same must happen by any L3-aware device in the data path. If these are any kind of L2 bridges with some reduced frame size, probably because your data is flowing over different data paths wtjh potentially different MTU, the next device in the data path has to do it, worst case it's the server at the end.
To answer your original question why there is no max MTU config on the MR2100 and the likes? Simply not required these days*.Or have you spotted an ability to configure the max MTU on your mobile phone, and then probably for the WiFi and the WWAN interface?
*The problem existed e.g. on connections like xDSL on two wire, where instead of transparent IP tunneling like PPPoE or PPTP, IP-stacks where substandard, crazy ideas of blocking any ICMP were common.
There are still some IP stacks in the field (Netgear introduced a problem as part of some security fixes on some Plus switch models recently) which have issues on connections with MTU below of the standard MTU, e.g. over VPNs - thus not fully standards compliant.
Last but not least, the mentioned PMTUD is not a "one-off" process, it's an ongoing thing for each session.
