See below.
On 12/05/2017, 13:31, Michael Welzl wrote:
Hi,
Thanks a lot for all your comments (plus the nits we authors of the other
-usage draft received offline).
I’ll try to address them all - but there are a two technical questions in this
email that made me stop, so I’ll cut all the editorial stuff away and discuss
them here - in line below:
- Why do this??? - Isn't it better to set flow labels per interface or for the
whole stack, how can any specific transport or application pick unique labels?
TEXT:
o Specify IPv6 flow label field
Protocols: SCTP
(i.e., Is this automatable by the host and a host wide
configuration?)
Somehow the question seems irrelevant in the context of this draft, which is a
list of transport features of protocols. These features are defined in the RFCs
spec’ing the protocols - for SCTP, this is defined, and that’s why it’s here.
We can discuss this for the proposed services that a system should offer, which
we try to write up in the minset draft:
I do think that an application should be allowed to assign a label to a TAPS
flow (as we call them), which could then map to this function. I mean, isn’t a
flow label supposed to identify a transport flow? Then a system-wide
configuration wouldn't seem right to me.
I think we may disagree. Flow ids identify flows to the network layer,
they have no role at the transport layer, and need to be unique (as best
they can) for a source address.
I much prefer the idea that the Flow id is generated by the IP system,
by using a hash - possibly utilising transport data as a part of this
hash, and including the protocol number. That seems to be what ECMP is
expecting and I suspect ECMP is an improtant use-case.
The alternative (if I understand) could be: I could imagine each
application could (in theory) be provided with an API to find out what
flow-ids are currently being used for each interface it cares about and
to then reserve one of the unused IDs for the specific interface(s) that
it wishes to use. Then we need to ensure all upper layer entities
coordinate on this. To me, this seems over-kill, and the approach taken
with ephemeral port assignment is much simpler - the application simply
doesn't get involved with choosing the number.
Now if what you are saying is that you want the App to somehow signal
that it can use an existing flow ID that is in use, and combine data
with that flow to get the same network treeatment, I can understand the
case. However, that's not exactly the same thing.
-------------------
Get Interface MTU is missing from pass 2 and 3:
ADD to pass 2:
GET_INTERFACE_MTU.UDP:
Pass 1 primitive: GET_INTERFACE_MTU
Returns: Maximum datagram size (bytes)
But this doesn’t exist!
I think I don't understand your comment ... and interpretting
low-numbered RFCs is never easy - I'll use RFC1122 as my basis:
RFC 1122 says:
" A host MUST implement a mechanism to allow the transport layer
to learn MMS_S, the maximum transport-layer message size that
may be sent for a given {source, destination, TOS} triplet..."
" and EMTU_S must be less than or equal to the MTU of the network
interface corresponding to the source address of the datagram."
TCP handles this for the app.
It’s strictly an IP function and I couldn’t find it described for UDP
anywhere. I think we agreed on how a TAPS system should handle this, and this
is reflected in
https://tools.ietf.org/html/draft-gjessing-taps-minset-04#section-6.4.1
… which may require a system to implement new local functionality, maybe based
on this MTU function - but to my understanding it’s just not something that UDP
offers.
It's something that a UDP App really needs to pay attention to as per
RFC8085 - we may differ on whether you call that "offers" or needs to
function. Either way, an app that plans to use any form of PMTUD needs
to use this number.
As put in RFC1122:
" A host that does not implement local fragmentation MUST ensure
that the transport layer (for TCP) or the application layer
(for UDP) obtains MMS_S from the IP layer and does not send a
datagram exceeding MMS_S in size."
Cheers,
Michael
Gorry
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