On 5/29/2013 1:06 PM, Linda Dunbar wrote:
Ron,
I do have a few questions and suggestion about the practices documented in the
draft:
- Section 4.1, second paragraph:
why DF bit "MUST" set to 1 when the payload header has "0"? I would think default should
be same as the "payload" DF setting.
It depends on the goal:
The current mechanism always sets DF=1 so it (GRE) can discover the
tunnel MTU.
Setting it to match would expose the endpoint to the overall path MTU,
but comes at a risk. If a packet is too big somewhere inside the tunnel,
that router will send an ICMP that goes back to the tunnel ingress --
which might not have enough information to relay an ICMP back to the source.
This was discussed here:
http://tools.ietf.org/html/draft-ietf-intarea-tunnels
(this is still a WG doc, but we didn't get much feedback on the issues
back in 2010 - maybe it's time to revive and complete it??)
...
- There should be two options when the encapsulated data frame
exceeds MTU: a) split the data frame to two smaller frames, with each
frame being encapsulated;
FWIW, fragmentation rarely splits things into equal sizes; it's more
typical to split into "max that fits" and "leftover".
Joe
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