> According to RFC 2516, PPPoE, the Maximum-Receive-Unit (MRU) option MUST
> NOT be negotiated to a larger size than 1492. If PPPoE implementations
> actually negotiated an MRU size of 1492 bytes, we would not have an
> issue. However, many implementations of PPPoE servers only support an
> MTU of about 1100 bytes, i.e. less than the minimal MTU allowed by IPv6.
>
> What should we do?
Require RFC1990 multilink capability. End of story.
1.2. Functional Description
The method discussed here is similar to the multilink protocol
described in ISO 7776 [4], but offers the additional ability to split
and recombine packets, thereby reducing latency, and potentially
increase the effective maximum receive unit (MRU). Furthermore,
there is no requirement here for acknowledged-mode operation on the
link layer, although that is optionally permitted.
[...]
The effective MRU for the logical-link entity is negotiated via an
LCP option. It is irrelevant whether Network Control Protocol
packets are encapsulated in multilink headers or not, or even over
which link they are sent, once that link identifies itself as
belonging to a multilink arrangement.
[...]
5.1.1. Multilink MRRU LCP option
Figure 4: Multilink MRRU LCP option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 17 | Length = 4 | Max-Receive-Reconstructed-Unit|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The presence of this LCP option indicates that the system sending it
implements the PPP Multilink Protocol. If not rejected, the system
will construe all packets received on this link as being able to be
processed by a common protocol machine with any other packets
received from the same peer on any other link on which this option
has been accepted.
The Max-Receive-Reconstructed unit field is two octets, and specifies
the maximum number of octets in the Information fields of reassembled
packets. A system MUST be able to receive the full 1500 octet
Information field of any reassembled PPP packet although it MAY
attempt to negotiate a smaller, or larger value. The number 1500
here comes from the specification for the MRU LCP option in PPP; if
this requirement is changed in a future version of RFC 1661, the same
rules will apply here.
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