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MTU settings

SuperZoom
Grafter
Posts: 353
Registered: ‎17-05-2013

Re: MTU settings

Interesting stuff. Thanks, Andy. It's definitely a positive development if PlusNet are going to be offering RFC 4638 support as a matter of course.
Unless, of course, the modified PPPoE implementation causes router instability!!!...    Wink
Hopefully the "waiting for..." issues are down to something else anyway - maybe IPv6 specific, as you say.
I take it PPP activity doesn't appear in the system log by default, then?
AndyH
Grafter
Posts: 6,824
Thanks: 1
Registered: ‎27-10-2012

Re: MTU settings

I get a very limited amount of info:
Quote
Jan  1 00:00:24 pppd[372]: pppd 2.4.5 started by xxxx, uid 0
Jan  1 00:00:59 pppd[372]: Timeout waiting for PADO packets
Jan  1 00:02:14 pppd[372]: Timeout waiting for PADO packets
Jan  1 00:02:14 pppd[372]: Connected to 00:30:88:xx:xx:xx via interface eth0
Jan  1 00:02:14 pppd[372]: Connect: ppp0 <--> eth0
Jan  1 00:02:16 pppd[372]: CHAP authentication succeeded
Jan  1 00:02:16 pppd[372]: peer from calling number 00:30:88:xx:xx:xx authorized
Jan  1 00:02:16 pppd[372]: local  LL address fe80::1918:xxxx:xxxx:xxxx
Jan  1 00:02:16 pppd[372]: remote LL address fe80::0090:xxxx:xxxx:xxxx
Jan  1 00:02:17 pppd[372]: local  IP address 81.174.16x.xxx
Jan  1 00:02:17 pppd[372]: remote IP address 195.166.1xx.xxx

I need to play around to see what the problem could be on the "waiting for ...". A quick look at some of the addresses which weren't loading show they have no IPv6 address in the DNS records, which is strange because there shouldn't be issues with packets too big on IPv4.
SuperZoom
Grafter
Posts: 353
Registered: ‎17-05-2013

Re: MTU settings

I see...
I've got the latest RP-PPPOE on a Linux laptop. I may have a play with that first of all when I get some time and see if I can get a 1500 byte connection.
Isn't the BT Home Hub 4 RFC 4638 compliant out of the box as well? I've got one of those lying about so could swap it in for a bit of low-hassle testing at some point...
AndyH
Grafter
Posts: 6,824
Thanks: 1
Registered: ‎27-10-2012

Re: MTU settings

I would play with it and see how you get on. I'm not sure about the HomeHub 4 being RFC 4638 compliant - I read mixed things on their forums.
I found my "waiting for ..." problem is definitely linked to running a 1500 MTU with IPv4 (I disabled IPv6 to check this). What I haven't figured out is if it's an issue with my router.
buseng12
Grafter
Posts: 374
Thanks: 1
Registered: ‎14-06-2013

Re: MTU settings

I've telnetted my fibre FTTC router & it shows a WAN (internet)  MTU of 1492 already.
LAN (local network) shows 1500.
I have set my PC Ethernet adapter to 1492 as well.
I assume this is correct for fibre.




                 Technicolor TG582n FTTC
                   10.2.5.2.FO
                     Copyright (c) 1999-2012, Technicolor
------------------------------------------------------------------------
{admin}=>ip iflist
Interface                            Group  MTU   RX         TX         Admin  Oper
1   loop. . . . . . . . . . . . . .  local  4096  354 MB     467 MB     UP     [UP]
2   Internet. . . . . . . . . . . .  wan    1492  10183 MB   1710 MB    UP     UP
3   Mobile. . . . . . . . . . . . .  wan    1500  0          0          DOWN   DOWN
4   Mobile_trigger. . . . . . . . .  wan    1500  0          0          DOWN   [DOWN]
5   LocalNetwork. . . . . . . . . .  lan    1500  2148 MB    10 GB      UP     [UP]
6   Multicast . . . . . . . . . . .  wan    1500  266 KB     289 KB     UP     UP
{admin}=>


AndyH
Grafter
Posts: 6,824
Thanks: 1
Registered: ‎27-10-2012

Re: MTU settings

@ buseng12 - It's fine if your PC ethernet adapter is set to 1492.
@ SuperZoom - What do you make of this:
MTU 1492
Quote
root@ubuntu:/home/andrew# wget http://platform.twitter.com/widgets/tweet_button.1400006231.html
--2014-05-14 22:27:49--  http://platform.twitter.com/widgets/tweet_button.1400006231.html
Resolving platform.twitter.com (platform.twitter.com)... 199.96.57.6
Connecting to platform.twitter.com (platform.twitter.com)|199.96.57.6|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 69347 (68K) [text/html]
Saving to: `tweet_button.1400006231.html.3'
100%[==============================================================================================================>] 69,347      --.-K/s   in 0.02s  
2014-05-14 22:27:49 (4.41 MB/s) - `tweet_button.1400006231.html.3' saved [69347/69347]

MTU 1500 (some sites do not respond beyond the initial TCP handshake)
Quote
root@ubuntu:/home/andrew# wget http://platform.twitter.com/widgets/tweet_button.1400006231.html
--2014-05-14 22:42:39--  http://platform.twitter.com/widgets/tweet_button.1400006231.html
Resolving platform.twitter.com (platform.twitter.com)... 68.232.35.139
Connecting to platform.twitter.com (platform.twitter.com)|68.232.35.139|:80... connected.
HTTP request sent, awaiting response...

Quote
22:42:39.418029 IP ubuntu.48722 > 68.232.35.139.http: Flags [ S], seq 1575891940, win 29200, options [mss 1460,sackOK,TS val 67244109 ecr 0,nop,wscale 7], length 0
22:42:39.423226 IP ubuntu.48722 > 68.232.35.139.http: Flags [.], ack 1077886608, win 229, length 0
22:42:39.423413 IP ubuntu.48722 > 68.232.35.139.http: Flags [P.], seq 0:156, ack 1, win 229, length 156

Quote
root@ubuntu:/home/andrew# ping -c 3 -s 1472 -M do platform.twitter.com
PING platform.twitter.com.tw.map.fastly.net (199.96.57.6) 1472(1500) bytes of data.
1480 bytes from 199.96.57.6: icmp_req=1 ttl=56 time=5.75 ms
1480 bytes from 199.96.57.6: icmp_req=2 ttl=56 time=6.06 ms
1480 bytes from 199.96.57.6: icmp_req=3 ttl=56 time=5.76 ms
--- platform.twitter.com.tw.map.fastly.net ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 10033ms
rtt min/avg/max/mdev = 5.751/5.858/6.064/0.170 ms

Quote
22:47:48.754992 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto ICMP (1), length 1500)
   ubuntu > 199.96.57.6: ICMP echo request, id 14387, seq 1, length 1480
22:47:48.761268 IP (tos 0xa0, ttl 56, id 15713, offset 0, flags [none], proto ICMP (1), length 1500)
   199.96.57.6 > ubuntu: ICMP echo reply, id 14387, seq 1, length 1480
22:47:53.770108 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto ICMP (1), length 1500)
   ubuntu > 199.96.57.6: ICMP echo request, id 14387, seq 2, length 1480
22:47:53.775918 IP (tos 0xa0, ttl 56, id 15714, offset 0, flags [none], proto ICMP (1), length 1500)
   199.96.57.6 > ubuntu: ICMP echo reply, id 14387, seq 2, length 1480
22:47:58.784205 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto ICMP (1), length 1500)
   ubuntu > 199.96.57.6: ICMP echo request, id 14387, seq 3, length 1480
22:47:58.789910 IP (tos 0xa0, ttl 56, id 15715, offset 0, flags [none], proto ICMP (1), length 1500)
   199.96.57.6 > ubuntu: ICMP echo reply, id 14387, seq 3, length 1480
chrcoluk
Grafter
Posts: 1,990
Thanks: 5
Registered: ‎11-12-2013

Re: MTU settings

Quote from: SuperZoom
If your router has support for RFC 4638 and logs the PPP session setup, you could just gateway hop and post the log.
I never bothered doing anything about setting it up since I was told there was no support.
I presume PlusNet's own routers won't be able to do it because they don't have gigabit WAN ports even if the ppp on them were to be upgraded, so are unlikely to be able to handle the larger frames.
I'm not on the IPv6 trial, but of course fragmentation isn't allowed in the path in v6 and things get messy if key ICMPv6 packets are blocked anywhere.

I be surprised if plusnet tech support are even trained on rfc 4638, they will likely be advising based on what the plusnet router can do.
Plusnet's network can handle 1500byte mtu however I noticed at least 1 gw doesnt work properly (or didnt maybe they fixed now).
I am using 1500bytes right now.
AndyH
Grafter
Posts: 6,824
Thanks: 1
Registered: ‎27-10-2012

Re: MTU settings

@ dave/Paul
There does seem to be an issue with using an MTU of 1500 for PPPoE on some gateways - this is for both IPv4 and IPv6 traffic.
On the older pcl, ptn and ptw gateways, quite a few sites will not load properly or at all (the initial TCP handshake is fine though). As soon as I change the MTU on my router back to 1492, everything works again.
I am now on pcl-bng01 and this is working fine with a MTU of 1500 for all sites:
Quote
sh-3.2# wget http://platform.twitter.com/widgets/tweet_button.1400006231.html
--2014-05-15 09:39:21--  http://platform.twitter.com/widgets/tweet_button.1400006231.html
Resolving platform.twitter.com... 199.96.57.6
Connecting to platform.twitter.com|199.96.57.6|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 69347 (68K) [text/html]
Saving to: 'tweet_button.1400006231.html'
100%[=========================================================================================================================================================================================================================================>] 69,347      --.-K/s   in 0.01s  
2014-05-15 09:39:22 (5.83 MB/s) - 'tweet_button.1400006231.html' saved [69347/69347]

As soon I as I move to pcl-ag07, it stops working again:
Quote
sh-3.2# wget http://platform.twitter.com/widgets/tweet_button.1400006231.html
--2014-05-15 09:51:13--  http://platform.twitter.com/widgets/tweet_button.1400006231.html
Resolving platform.twitter.com... 68.232.35.139
Connecting to platform.twitter.com|68.232.35.139|:80... connected.
HTTP request sent, awaiting response... ^C

Another unrelated thing - do the new BNGs have different MSIL/EPs compared to the other gateways? Because every time (i.e. 20+ times) I land on a BNG (either pcl or ptw), my minimum latency is nearly twice what it is normally on the older gateways:
pcl-bng01
Quote
--- bbc.co.uk ping statistics ---
100 packets transmitted, 100 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 7.881/8.157/8.510/0.108 ms

pcl-ag07
Quote
--- bbc.co.uk ping statistics ---
100 packets transmitted, 100 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 4.458/4.673/4.914/0.097 ms
SuperZoom
Grafter
Posts: 353
Registered: ‎17-05-2013

Re: MTU settings

So it looks as if it is gateway dependent from your testing, Andy.
It would make sense that the newer gateways can do it and the older ones fall over (even if only because of configuration rather than capability).
I'm not sure I entirely follow your captures. They seem a bit mangled. The first one doesn't seem to show the other side of the handshake and the packets don't seem to be from the same session, unless I'm reading it wrongly.
This is what I get for that same URL. (NB: The packet lengths include the 14 byte Ethernet II MAC header and  @buseng12  you can see from that that I too set the PC MTU to 1492 to avoid problems. Sequence numbers are relative here.)
Delta    Length  Info
0.000    66      1392 > 80 [SYN] Seq=0 Win=62400 Len=0 MSS=1452 WS=4
0.004    66      80 > 1392 [SYN, ACK] Seq=0 Ack=1 Win=14600 Len=0 MSS=1452 WS=9
0.000    54      1392 > 80 [ACK] Seq=1 Ack=1 Win=1045440 Len=0
0.013    222    GET /widgets/tweet_button.1400006231.html HTTP/1.1
0.004    60      80 > 1392 [ACK] Seq=1 Ack=169 Win=15872 Len=0
0.000    1506    HTTP/1.1 200 OK  (text/html)
0.000    1506    Continuation or non-HTTP traffic
0.000    54      1392 > 80 [ACK] Seq=169 Ack=2905 Win=1042528 Len=0
0.000    1506    Continuation or non-HTTP traffic
0.000    1506    Continuation or non-HTTP traffic


Note that the MSS advertised by the Twitter server is 1452, interestingly.

Don't know where that anomalous 10033ms ping run time is coming from! The packet capture below is seemingly not related...
Have you had any similar problems, chrcoluk? Do you have a PPP session setup log you can post?
SuperZoom
Grafter
Posts: 353
Registered: ‎17-05-2013

Re: MTU settings

I'm not going to have time to do any proper experiments for a few weeks myself, but it occurs to me that this business of being able to use an MTU of 1500 bytes over PPPoE may have more to do with the IPv6 trial than it does with the simple introduction of an RFC 4638 capability.
Of course, none of this is relevant to the OP, but the following extract may be useful to have in mind. Specifically the recommendation of a 1500 byte MTU for IPv6 (which may be why it is able to be accommodated on the new gateways).
RFC 2460 states:
Quote
Packet Size Issues
IPv6 requires that every link in the internet have an MTU of 1280 octets or greater.  On any link that cannot convey a 1280-octet packet in one piece, link-specific fragmentation and reassembly must be provided at a layer below IPv6.
Links that have a configurable MTU (for example, PPP links [RFC-1661]) must be configured to have an MTU of at least 1280 octets; it is recommended that they be configured with an MTU of 1500 octets or greater, to accommodate possible encapsulations (i.e., tunneling) without incurring IPv6-layer fragmentation.
From each link to which a node is directly attached, the node must be able to accept packets as large as that link's MTU.
It is strongly recommended that IPv6 nodes implement Path MTU Discovery [RFC-1981], in order to discover and take advantage of path MTUs greater than 1280 octets. However, a minimal IPv6 implementation (e.g., in a boot ROM) may simply restrict itself to sending packets no larger than 1280 octets, and omit implementation of Path MTU Discovery.
In order to send a packet larger than a path's MTU, a node may use the IPv6 Fragment header to fragment the packet at the source and have it reassembled at the destination(s).  However, the use of such fragmentation is discouraged in any application that is able to adjust its packets to fit the measured path MTU (i.e., down to 1280 octets).
PitchBlack
Grafter
Posts: 301
Registered: ‎10-01-2014

Re: MTU settings

Quote from: SuperZoom
Of course, none of this is relevant to the OP

Not a problem SuperZoom, keep it coming, I am enjoying reading what you and Andy H are supplying. I have set my MTU to 1492, 1472,1444 and now at 1500, none of them make a difference, as soon as I turn my router firewall on some web pages fail to load as I mentioned in a previous post above, none specific and say speedtest page can fail to load, then 5 minutes later it loads. I have a Belkin F7D 1401uk which I bought maybe 5 years ago, so maybe it is a bit old in the tooth for win 8 and 8.1.
chrcoluk
Grafter
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Thanks: 5
Registered: ‎11-12-2013

Re: MTU settings

and I pointed this out in another thread weeks ago Wink
currently I am on pcl-ag03 and it works.
the reason its probably not so noticeble on ipv4 is that mtu discovery mechanisms for ipv6 seem much more flaky so when its wrong on ipv6 it hurts a lot on performance.
on pcl-ag03
admin@RT-AC66U:/tmp/home/root# wget http://platform.twitter.com/widgets/tweet_button.1400006231.html
Connecting to platform.twitter.com (199.96.57.6:80)
tweet_button.1400006 100% |*******************************| 69347  0:00:00 ETA
admin@RT-AC66U:/tmp/home/root# ls
tweet_button.1400006231.html
chrcoluk
Grafter
Posts: 1,990
Thanks: 5
Registered: ‎11-12-2013

Re: MTU settings

Quote from: SuperZoom
I see...
I've got the latest RP-PPPOE on a Linux laptop. I may have a play with that first of all when I get some time and see if I can get a 1500 byte connection.
Isn't the BT Home Hub 4 RFC 4638 compliant out of the box as well? I've got one of those lying about so could swap it in for a bit of low-hassle testing at some point...

supposedbly it is but when I tested on my hh5, I could only get a 1492byte mtu working.  I be surprised if hh4 supports newer tech than hh5.
AndyH
Grafter
Posts: 6,824
Thanks: 1
Registered: ‎27-10-2012

Re: MTU settings

@ chrcoluk - Is your ppp0 definitely using a 1500 MTU?
I'm on pcl-ag05 and it's failing (testing from the router also now).
Also, is anyone else on one of the BNGs at the moment?
SuperZoom
Grafter
Posts: 353
Registered: ‎17-05-2013

Re: MTU settings

Quote from: PitchBlack
Quote from: SuperZoom
Of course, none of this is relevant to the OP

Not a problem SuperZoom, keep it coming, I am enjoying reading what you and Andy H are supplying. I have set my MTU to 1492, 1472,1444 and now at 1500, none of them make a difference, as soon as I turn my router firewall on some web pages fail to load as I mentioned in a previous post above, none specific and say speedtest page can fail to load, then 5 minutes later it loads. I have a Belkin F7D 1401uk which I bought maybe 5 years ago, so maybe it is a bit old in the tooth for win 8 and 8.1.

I don't know the router, but if the firewall makes the difference then at least that provides a good starting point for troubleshooting.
If you've been making a lot of changes it might just be worth resetting the Windows TCP/IP stack and double-checking that the MTU is indeed set to 1500 at the router, and restarting it, before doing anything else.
Then perhaps disable any IPv6 elements (assuming you don't need them) and proceed from there.
First thing, can you ping the BBC with 1472 bytes, with and without the router firewall on?
If not, can you ping the router itself?
That gets MTU out of the way for starters, then it's a question of identifying something which consistently (or regularly) fails and taking a closer look. If it doesn't fail on other OSs then you have another point of comparison...

@chrcoluk
At least we're all on the same page then  Cool
I have used the HH4 on my line before (just for testing) but the MTU was 1492 then. 1500 hasn't worked previously on PlusNet with PPPoE, as far as I'm aware. Whether the HH4 can do it or not, I don't know. I'd heard it could, but that may well be misinformation. I'll give it a try when I get chance.
When you say it 'works' on PCL-AG03, are you actually using an MTU of 1500 on your client machine as well as on the router, or is your client set to 1492?
Some PPP logs would reveal what was going on at that level, although I presume you're getting the same limited info as Andy for those, which is why you haven't posted them?...

@ dave / kelly / paul
Any chance of saving us going round in circles of guessing and probing by telling us whether or not RFC 4638 is now supported?...