Hi Vishwas,
I understand that these are the requirements stated in RFC 2460. That
is not in question.
What I wanted was a bit more explanation on why Carsten finds it
beneficial to keep the MTU at 1280 for 6lowpan hosts. 6lowpan needs
to provide link-layer fragmentation anyway, so why add scenarios that
could make IP-layer fragmentation more likely in 6lowpan networks?
--
Jonathan Hui
On Jun 4, 2010, at 8:05 AM, Vishwas Manral wrote:
The RFC is very clear:
1. A node must be able to accept a fragmented packet that, after
reassembly, is as large as 1500 octets. A node is permitted to
accept fragmented packets that reassemble to more than 1500
octets.
2. IPv6 requires that every link in the internet have an MTU of 1280
octets or greater.
3. Links that have a configurable MTU (for example, PPP links [RFC-
1661]) must be configured to have an MTU of at least 1280 octets; it
is recommended that they be configured with an MTU of 1500 octets
The first case is after fragmentation, which the second case is
without fragmentation.
The thing you have is for 15.4 you could have had an MTU greater than
1280 (because the lower layer can support it), which means that you
have to fragment a packet whenever any header is attached to the
packet. In your case because there is no tunnel you have to go in for
IP-in-IP tunnelling at the edge and also fragmentation.
Thanks,
Vishwas
On Thu, Jun 3, 2010 at 11:02 PM, Jonathan Hui <j...@archrock.com>
wrote:
On Jun 3, 2010, at 10:44 PM, Carsten Bormann wrote:
On Jun 4, 2010, at 06:30, Jonathan Hui wrote:
I personally think it would not be a major disaster to increase
the MTU
requirement for 6LRs beyond RFC 4944's 1280, as long as hosts can
stay at
1280.
(If this is what ROLL decides should be done, let's have a separate
discussion on any implications in 6lowpan.)
Since we're on this subject, how does this relate to the
requirement in RFC
2460 that all nodes must be able to receive datagrams of up to 1500
bytes
(after reassembly if IP-layer fragmentation is used)? What is the
value of
keeping the MTU requirement at 1280 for hosts if they are still
required to
receive a 1500 byte datagram? Or is is that we have relaxed the
1500 byte
requirement for hosts in a 6lowpan network?
If a 6lowpan host needs to receive a 1500 byte datagram, I'd prefer
to let
the 6lowpan layer handle the fragmentation needs rather than both
link and
IP-layer fragmentation working simultaneously to deliver a 1500 byte
datagram.
If a 6lowpan host only needs to receive a 1280 byte datagram, then
I agree
that keeping the 1280 byte MTU for 6lowpan hosts is useful.
--
Jonathan Hui
--------------------------------------------------------------------
IETF IPv6 working group mailing list
ipv6@ietf.org
Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6
--------------------------------------------------------------------
--------------------------------------------------------------------
IETF IPv6 working group mailing list
ipv6@ietf.org
Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6
--------------------------------------------------------------------
