Re: [core] draft-tschofenig-core-coap-tcp-tls-03 : TCP Session Establishment

[Carsten Bormann <[e-mail]>]

weigengyu wrote:
> But what the application depends on to make the decision?
> From the viewpoint of network, it should be MTU.
> The MTU limits the size of UDP and CoAP message.

(Talking about the UDP case:)
That is indeed an area where the current IPv6 architecture doesn't
provide as much help as one would like it to.  The MTU is not
necessarily a good choice, in particular when hops with adaptation layer
fragmentation are involved.

The problem is mitigated a bit by the fact that usually a constrained
network with a preference for small packets is the last (incoming) or
first (outgoing) hop to a constrained node, so this node may have more
insight into good packet size choices than the network as a whole is
able to provide.  Using -block, this node can then communicate its
choice to a peer.

A while ago, I have proposed a way to get more (and more detailed)
information out of the network:
https://tools.ietf.org/html/draft-bormann-intarea-alfi-04
Earlier drafts of that had the problem that IPv6 hop-by-hop options tend
not to traverse the general Internet intact.  So I came up with a
SPUD-like alternative solution in -04 (see its section 5:
https://tools.ietf.org/html/draft-bormann-intarea-alfi-04#section-5
).  With (the real) SPUD now being looked at for communication to
middleboxes in general, that basic approach might actually go forward later.

Grüße, Carsten