Question about PPPoE parameters and HE IPv6...

w0w

I have a PPPoE connection with a default MTU and MRU of 1492. This configuration works fine, but when I want to use Hurricane Electric (HE), they recommend setting my MTU to 1452. I can adjust the MTU on the specific interface, and it is already done, but I still see that PPPoE negotiates the MRU to 1492. Is this a problem or not? I have seen some recommendations that MTU/MRU ideally should match.

Gertjan

@w0w

Traffic between your pfSense and the he.net tunnel endpoint is IPv6 traffic stashed into IPv4 packets.
And your IPv4 traffic is ... IPv4 into pppoe traffic.
Packets are 1500, and with pppoe you loose 8 bytes : MTU is 1492.
What I guess : the he.net tunnel traffic needs '20' bytes more, see here.

My he.net MTU is set to

= 1500-20 = 1480.
Your should be set to 1492-20=1472

w0w

@Gertjan
You may be right, but that's not what the question was about at all. By the way, the number was chosen through a different method. I used a ping with the "don't fragment" flag to determine the MTU. The question was about MRU.

Gertjan

@w0w said in Question about PPPoE parameters and HE IPv6...:

The question was about MRU.

I saw you were mentioning "RMU".
And still looking what it should be (I was hopping for a miss spell).
On the he.net side : no RMU whatsoever. On the pfSense side : didn't locate it yet - I never bothered with MSS neither. I've used he.net many years, with pppoe and without it, but don't have any souvenir of it ...

stephenw10

That's still the outer connection though. The HE tunnel runs across that whether it's transmit or receive. You should set the tunnel MTU so it doesn't create fragmented packets at the PPPoE level. So 8 bytes less than a non-pppoe connection.

w0w

@Gertjan said in Question about PPPoE parameters and HE IPv6...:

"RMU".

Maximum Transit Unit/Maximum Receive Unit (MTU/MRU)

You can find some under Interfaces - Assigments - PPPs - select your PPPoE and edit it, press Advanced Configuration, you will see all setting including MTU/MRU
But description is a bit confusing:

MTU defaults to 1492
MRU will be auto-negotiated by default

Does it mean that MRU is ignored always or what?

@stephenw10 said in Question about PPPoE parameters and HE IPv6...:

The HE tunnel runs across that whether it's transmit or receive. You should set the tunnel MTU so it doesn't create fragmented packets at the PPPoE level.

I get the reasoning, but I don't understand if the MRU size should match the MTU size. If it should, why is this requirement ignored in my case?

2024-05-01 09:22:40.175056+03:00 	ppp 	77623 	[wan_link0] MRU 1492
2024-05-01 09:22:40.175028+03:00 	ppp 	77623 	[wan_link0] PROTOCOMP
2024-05-01 09:22:40.174986+03:00 	ppp 	77623 	[wan_link0] LCP: rec'd Configure Ack #2 (Ack-Sent)
2024-05-01 09:22:40.174266+03:00 	ppp 	77623 	[wan_link0] MAGICNUM 0xe1dee174
2024-05-01 09:22:40.174240+03:00 	ppp 	77623 	[wan_link0] MRU 1492
2024-05-01 09:22:40.174213+03:00 	ppp 	77623 	[wan_link0] PROTOCOMP
2024-05-01 09:22:40.174170+03:00 	ppp 	77623 	[wan_link0] LCP: SendConfigReq #2
2024-05-01 09:22:40.174133+03:00 	ppp 	77623 	[wan_link0] MRU 1492
2024-05-01 09:22:40.174098+03:00 	ppp 	77623 	[wan_link0] LCP: rec'd Configure Nak #1 (Ack-Sent)
2024-05-01 09:22:40.174041+03:00 	ppp 	77623 	[wan_link0] LCP: state change Req-Sent --> Ack-Sent
2024-05-01 09:22:40.173965+03:00 	ppp 	77623 	[wan_link0] MAGICNUM 0xbdd28c42
2024-05-01 09:22:40.173937+03:00 	ppp 	77623 	[wan_link0] AUTHPROTO CHAP MD5
2024-05-01 09:22:40.173909+03:00 	ppp 	77623 	[wan_link0] MRU 1492

Does this mean that I have asymmetrical MRU and MTU values at the PPPoE level?
I see MTU 1452 and MRU logged in PPPoE connection as 1492. Packet capture shows for both directions maximum size:

Frame 4936: 1494 bytes on wire (11952 bits), 1494 bytes captured (11952 bits)
Ethernet II, Src: Intel_c9:77:84 (b4:96:91:c9:77:84), Dst: Cisco_94:de:c1 (64:9e:f3:94:de:c1)
    Destination: Cisco_94:de:c1 (64:9e:f3:94:de:c1)
    Source: Intel_c9:77:84 (b4:96:91:c9:77:84)
    Type: PPPoE Session (0x8864)
PPP-over-Ethernet Session
    0001 .... = Version: 1
    .... 0001 = Type: 1
    Code: Session Data (0x00)
    Session ID: 0x51f3
    Payload Length: 1474
Point-to-Point Protocol
    Protocol: Internet Protocol version 4 (0x0021)
Internet Protocol Version 4, Src: 
Transmission Control Protocol, Src Port: 28447, Dst Port: 28193, Seq: 66512, Ack: 158, Len: 1432
Data (1432 bytes)

Setting currently are
So I assume everything is OK and PPPoE log just misleading?

stephenw10

You can probably request it but the server side might just send 1492 anyway.

However I'm not sure why you'd want to change it from 1492 in either direction. It seems like you should be setting the HE gif tunnel MTU to 1452 o prevent it creating packets too large for the 1492 PPPoE WAN.

w0w

@stephenw10
Yes, you're absolutely right, and I apologize for the inaccuracy in my initial question. Actually, I set the MTU for the GIF tunnel to 1452 and for PPPoE to 1472. I remember that with the default 1492, there were many fragments and retransmissions, so I suspect the problem is actually with the provider. I mixed two questions together, which seems to have confused everyone, including myself. Thanks for your response!

Bob.Dig

@w0w said in Question about PPPoE parameters and HE IPv6...:

I set the MTU for the GIF tunnel to 1452

Don't forget to set the MTU accordingly on their website.

w0w

@Bob-Dig
Sure, thanks. I plan to re-evaluate my values and start from scratch.