[AusNOG] "large PPPoE MTU" support

[McDonald Richards]

It works intermittently on the iiNet network in my experience, with all
backhaul conditioned correctly. I think the reason I was given at the time
was due to different models of DSLAMs having support for different MTU
sizes. By intermittently I do mean from one line to the next, not on and
off on a single service. Cannot remember how it was on Telstra or other
networks.

>From memory no special magic was needed on the BNG/LNS templates, just
leaving them at the default 1500 byte MTU was sufficient.

Macca



On Wed, Jul 23, 2014 at 8:44 PM, Mark ZZZ Smith <markzzzsmith at yahoo.com.au>
wrote:

> Hi,
>
> I've been recently looking into "large PPPoE MTU" support, meaning support
> for a PPPoE/PPP MTU greater than 1492, ideally 1500 bytes, to get rid of
> fragmentation or PMTUD having to occur over the PPPoE link (a so-called
> "dumbell MTU" scenario - LAN MTU 1500, PPPoE MTU 1492, Internet MTU of
> 1500). The 'special' thing it requires is the support of 'baby' jumbo
> Ethernet frames i.e., an ethernet payload MTU of 1508 rather than the
> traditional 1500.
>
> A PPPoE client requests support by attaching a PPP-Max-Payload option to
> it's PADI, asking for support from the PPPoE server for the requested MTU.
> If the PPPoE server supports it, then it returns the PPP-Max-Payload option
> in the PADO. The client then attaches it to the PPPoE PADR, and if the
> PPPoE server accepts PADR, returns the option again. After that, PPP can
> negotiate the supported MTU (if not the default of PPP MTU 1500, as it is
> below) e.g.,
>
> 20:14:32.176605 PPPoE PADI [Service-Name] [Host-Uniq 0x000020A7]
> ***[PPP-Max-Payload 0x05DC]***
> 20:14:32.184804 PPPoE PADO [Service-Name] [Host-Uniq 0x000020A7]
> ***[PPP-Max-Payload 0x05DC]*** [AC-Name "lns20.mel6"] [AC-Cookie
> 0xB5C51927599BA0BDA14E1203CA7693C2]
> 20:14:32.184885 PPPoE PADR [Service-Name] [Host-Uniq 0x000020A7]
> ***[PPP-Max-Payload 0x05DC]*** [AC-Cookie
> 0xB5C51927599BA0BDA14E1203CA7693C2]
> 20:14:32.191830 PPPoE PADS [ses 0xb882] [Service-Name] [Host-Uniq
> 0x000020A7] ***[PPP-Max-Payload 0x05DC]*** [AC-Cookie
> 0xB5C51927599BA0BDA14E1203CA7693C2]
> 20:14:32.205350 PPPoE  [ses 0xb882] LCP, Conf-Request (0x01), id 1, length
> 12
>  encoded length 10 (=Option(s) length 6)
>    Magic-Num Option (0x05), length 6: 0x5afba3ce
> 20:14:32.210789 PPPoE  [ses 0xb882] LCP, Conf-Request (0x01), id 1, length
> 17
>  encoded length 15 (=Option(s) length 11)
>    Auth-Prot Option (0x03), length 5: CHAP, MD5
>    Magic-Num Option (0x05), length 6: 0x15fa3801
> 20:14:32.210943 PPPoE  [ses 0xb882] LCP, Conf-Ack (0x02), id 1, length 17
>  encoded length 15 (=Option(s) length 11)
>    Auth-Prot Option (0x03), length 5: CHAP, MD5
>    Magic-Num Option (0x05), length 6: 0x15fa3801
> 20:14:32.211035 PPPoE  [ses 0xb882] LCP, Conf-Ack (0x02), id 1, length 12
>  encoded length 10 (=Option(s) length 6)
>    Magic-Num Option (0x05), length 6: 0x5afba3ce
> 20:14:32.211110 PPPoE  [ses 0xb882] LCP, Echo-Request (0x09), id 0, length
> 10
>  encoded length 8 (=Option(s) length 4)
>    Magic-Num 0x5afba3ce
> 20:14:32.217176 PPPoE  [ses 0xb882] LCP, Echo-Reply (0x0a), id 0, length 10
>  encoded length 8 (=Option(s) length 4)
>    Magic-Num 0x15fa3801
>
> RFC4638, "Accommodating a Maximum Transit Unit/Maximum Receive Unit
> (MTU/MRU) Greater Than 1492 in the Point-to-Point Protocol over Ethernet
> (PPPoE)" specifies how it works.
>
> The above is against Internode's BRASes here in Melbourne, using the 14.07
> RC1 beta release of OpenWRT. All I needed to do was to set the WAN
> interface MTU to 1508 in the 'luci web gui' under 'Interfaces - WAN,
> Advanced Settings, Override MTU, 1508'. Apparently Cisco have supported
> this for quite a while in their IOS pppoe client, with the 'pppoe-client
> ppp-max-payload' command.
>
> From what I've read, it seems both Cisco and Juniper's BRASes
> automatically support the PPP-Max-Payload option as long as the underlying
> Ethernet interface payload MTU is > 1500. Alternatively with a Cisco, it
> might be necessary to add the 'tag ppp-max-payload <min> <max>' command
> under the 'bba group, virtual template' section of the config.
>
>
> The above 1500 byte MTU isn't actually working for me (it negotiates 1500,
> but I get 100% loss for 1500 byte packets), possibly because my ADSL modem
> (a Netcomm NBPlus4W bridging) doesn't support 1508 payload Ethernet frames,
> however 1496 is, so I set the OpenWRT Override MTU to 1504 instead.
>
> I'm curious if anybody else has experimented with this and perhaps knows
> if Telstra and other carriers support it (whether they know it or not!)
>
> Thanks,
> Mark.
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