Iljitsch van Beijnum wrote:
4.1:
It's not clear what exactly the Allowed MTU and Off-link MTU come
from, and how a router selects these values. Is one of these the
router's interface MRU? Why do you need an off-link MTU? As long as
you don't send packets longer than the router's MRU, then PMTUD will
take care of this.
From earlier in the document:
Allowed MTU:
The maximum MTU allowed administratively.
Off-link MTU:
The maximum packet size that is appropriate for communicating with
off-link correspondents.
The allowed MTU is the maximum MTU the administrator will allow. So for
instance, when a switch only supports 3000-byte packets, the
administrator can set the allowed MTU to 3000 to make sure the optimum
packet size is used without unnecessary probing at larger sizes.
The idea behind the off-link MTU is that hosts can use this value to
base their TCP MSS and packet size to off-link destinations on so that
PMTUD and possible problems related to it can be avoided easily while
it's still possible to use large packets on the local subnet. Do you
think this is unnecessary?
Yes, I believe PMTUD works fine for this.
The whole link speed thing makes me a bit uncomfortable. You
definitely want links with a much slower speed to use smaller MTUs,
but it seems like the end hosts are able to make an appropriate decision.
So do you think this should be removed? If the hosts don't make the
right decision on their own and MTU size at a certain speed is a concern
(i.e., VoIP jitter), this means that it's either necessary to advertise
a conservative MTU for all speeds, or touch all host configurations.
Yes, I think it's probably unnecessary. Devices (on both hosts and
switches) should be able to automatically limit their MTUs based on link
rate. It may be worth talking about this some, and giving guidance as
to what appropriate MTUs bounds might be for a given speed.
You forbid sending MTU detection messages more often than once per 60
seconds. I don't see this as being practical (say, on a router
interface) where you may need to send/forward packets to large numbers
of hosts that may all need to be probed individually.
It's 60 seconds or until you get a reply back. What I want is to avoid
generating large numbers of oversized packets, which could possibly
trigger undesired behavior on devices that can't handle the larger
packets. Maybe make this something that can be set administratively and
suggest a default of 60 seconds?
I still don't think this will help. My opinion is that this
responsibility needs to be pushed down a level. I think any given
network type needs to have its own set of rules. For example, I'd say
that with Ethernet, anything larger than 1500 byes should be forbidden
when in half-duplex mode. Switches and hosts should have reasonable max
MTU sizes for each link speed to prevent excessive jitter and queue
occupancy by a single packet. In the event you have some buggy
equipment such as a host interface that is known to behave badly when
receiving oversized frames, it can be protected by administratively
configuring its switch port to 1500 bytes. While this will impose some
administrative burden, I believe such cases are rare.
-John
_______________________________________________
Int-area mailing list
[email protected]
https://www1.ietf.org/mailman/listinfo/int-area
