On 16 April 2014 14:42, James Harper <james.harper@bendigoit.com.au> wrote:
Hi James, I am explicitly setting mtu and mru to 1492 in the pppd config. I don't have any of the faulty modems connected right now, so can't check until tonight -- but last time I brought this up on list, I checked and said "If I'm reading the tcpdump correctly (see reply to myself with it this morning) then yes, there is a frag-required response." I will re-confirm this tonight. So outbound (masq clients to internet via linux router) appeared to be doing the right thing.
Last time this came up, in 2013, you checked the inbound packets, and reported: "I can confirm what we thought - pings <= 1492 bytes get a response, pings > 1492 bytes get no response, not even a 'fragmentation required'."
Yep. I remember now!
The issue that confuses me deeply is that half the modems work, and half don't - yet they have similar internals, and are configured as identically as they can be (given differing user interfaces). They are all setup to use LLC, an 8/35 VPI/VCI and to bridge PPPoE in full-bridge mode.
I think that fact rules out my ISP or there being a dodgy router somewhere beyond the ISP, as surely that would affect me regardless of which modem is bridging?
I was willing to write off one modem as having mysteriously broken firmware, but it seems unlikely that two modems from different vendors would be broken this way -- and also likely that I'll continue to hit the problem if I buy more modems :(
In the absence of anything else, I'm thinking that your modem is doing deep inspection on the PPPoE packets and is setting the MSS on the encapsulated TCP packets according to the current PPP parameters. In that case the "bad" modems aren't doing this (I would call them "good" modems because I don't want something to screw with my packets, but that's just me :)
I guess that's possible, but it seems so unlikely to me.. These are just consumer-grade ADSL modems, and not even particularly high-end ones at that. Reconstructing the tcp streams inside the pppoe streams inbetween the ISP and the linux server and then putting it all back together sounds beyond their means to me. But yeah.. it's possible.
To find out all you'd have to do is telnet to somewhere where you are running tcpdump and see what is captured depending on what modem you are using.
I'll investigate tonight.