v3.0 discards max MRU packets on PPPoE from a dual-DSL modem

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NetAlien
DD-WRT Novice


Joined: 18 Mar 2013
Posts: 21
Location: Sanford, NC

PostPosted: Sat Feb 03, 2018 20:52    Post subject: v3.0 discards max MRU packets on PPPoE from a dual-DSL modem Reply with quote
v3.0 (currently on r34777)

Routers:
- Buffalo WZR-1750DHP running v3.0 just fine since April 2017 on single-DSL link.
- LinkSys WRT3200ACM with r34777

My ISP upgraded my DSL from 6Mbps single line to 18Mbps dual line near the end of Dec/17. The initial modem was flaky; it took until Jan 23/18 to get them to replace it. Debugging the problems drove me nuts because there were multiple issues:
- flaky modem - finally corrected.
- DD-WRT v3.0 - I'm now fairly convinced there is a MRU bug.

Symptoms:
Many websites fail, anywhere from slow, spotty and/or no reponse at all. Weirdly, if a site gives no response to the browser, it usually always fails. Some sites are just flaky. Others work flawlessly.

Looking at a number of wireshark traces, the problem occurs during SSL/TLS handshaking. The server responds with a Server Hello which is often missing. When this occurs, I see:
- on Buffalo: INDISCARDS on vlan2 and INERRORS on br0
- on LinkSys: INDISCARDS on br0 and OUTERRORS on ath2**

** Curiously, traffic occurs on ath0 and ath1; but ath2 shows ZERO traffic in both directions while showing OUT errors.

I placed a mirroring switch between the modem and router. wireshark traces here show:
- MTU/MRU negotiation succeeds
- max sized SSL/TLS (Server Hello) packets are MRU+2=MTU +20=wireMax (ie, 1492+2+20=1514 or 1490+2+20=1512)***

*** the "2" bytes may be a clue -- smells like an off-by-2, given this: before receiving the LinkSys and mirroring switch yesterday, I tried changing MTU(MRU) all the way down to 1300 to no avail; still got the discards on the Buffalo.

Some sites return 3 packets in the Server Hello (1514+1514+remainder). Only the remainder packet gets through; usually BOTH max packets are discarded. Retransmissions occur and most sessions get RST. Strangely, some evertually succeed after some retries. To the end user, this looks like totally horrible access if at all.

Don't understand why this problem would have started with the installation of the bonded dual-DSL links/modem; but the Buffalo was running fine with the old modem on one DSL.

What works:
- modem in standard router/WiFi mode
- modem in bonded bridging (RFC1483) to:
--- laptop running Linux (4.14.13-desktop-1.mga6) PPPoE (rp-pppoe-3.12-2.mga6); or,
--- LinkSys WRT3200ACM (FW_WRT3200ACM_1.0.6.186168_prod.img); or,
--- Buffalo WZR-1750DHP? [factory f/w not yet tested]

I can help debug this; my setup is:
- 2 DSL links
- Actiontec T3200 modem
- TP-Link TL-SG105E v3.0 Gigabit Smart Switch
- either Buffalo WZR-1750DHP, or LinkSys WRT3200ACM (possibly WRT54Gv8 -- only v24 available though)
- laptop running 2 wireshark: wlp3s0(WiFi) and eno1(eth. from switch)
which allows me to capture both sides of the routers simultaneously.

I have downloaded toolchains and source; but have never tried building a DD-WRT image. Willing to try if there's a doc that describes how to get a current build such as r34777...

How can I help solve this?
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NetAlien
DD-WRT Novice


Joined: 18 Mar 2013
Posts: 21
Location: Sanford, NC

PostPosted: Fri Feb 09, 2018 3:23    Post subject: Reply with quote
Installed r34876. Now, PPPoE will not connect. Wireshark sees packets from modem; but not a single packet from router. Ticket opened -- will update this thread as needed.
Per Yngve Berg
DD-WRT Guru


Joined: 13 Aug 2013
Posts: 6858
Location: Romerike, Norway

PostPosted: Fri Feb 09, 2018 9:20    Post subject: Reply with quote
Did you try to set a MTU manually on the WAN Setup tab?
NetAlien
DD-WRT Novice


Joined: 18 Mar 2013
Posts: 21
Location: Sanford, NC

PostPosted: Fri Feb 09, 2018 15:04    Post subject: Reply with quote
MTU is irrelevant in establishing a PPPoE connection, except to negotiate its size; all those packets are under 100 bytes. Minor correction, the only packets emitted by the router are ICMPv6 Router Advertisements.
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