Hi Fred
I had something quite simple in mind:
Fragment the IP packet just as you do today and send each fragment as
opaque data in a simple 8 byte basic UDP payload with port set to IP.
Set the source port based on a hash of the 5 tuple. Then resemble the IP
just like you always would.
- Stewart
On 30/01/2019 16:55, Templin (US), Fred L wrote:
Hi Stewart,
>> It we really need to fragment a packet, it would be better to stick
the fragments inside a common UDP/IP(no frag) shim.
I agree. Two different approaches for UDP fragmentation that avoid IP
fragmentation
are currently under consideration:
https://datatracker.ietf.org/doc/draft-ietf-intarea-gue-extensions/
https://datatracker.ietf.org/doc/draft-ietf-tsvwg-udp-options/
Thanks - Fred
*From:*Int-area [mailto:[email protected]] *On Behalf Of
*Stewart Bryant
*Sent:* Wednesday, January 30, 2019 6:14 AM
*To:* Fred Baker <[email protected]>; Tom Herbert
<[email protected]>
*Cc:* int-area <[email protected]>
*Subject:* Re: [Int-area] Comments on draft-ietf-intarea-frag-fragile-06
On 29/01/2019 23:37, Fred Baker wrote:
Section 4.5:
"IP fragmentation causes problems for some routers that support Equal
Cost Multipath (ECMP). Many routers that support ECMP execute the
algorithm described in Section 4.4 in order to perform flow based
forwarding;
As far as I know, routers that hash fields in the IP header to select a en
ECMP next hop do so because all packets in a flow will hash the same way
(modulo the issues with the transport port number), not because they are doing
per-flow forwarding. The do so explicitly to avoid having to maintain per-flow
state and yet make all fragments of a message follow the same path.
I agree with Fred. ECMP is normally done to distribute the load over
the available next hops on a best effort basis. Originally it was done
per packet, but that gave problems with out of order packet delivery,
so the routers moved to doing it based on the five tuple described in
this draft. It is a stateless best effort ECMP process with no regard
to specific flows and the path for any five tuple may move arbitrarily
if routing changes its mind on the ECMP set.
Fragmented packets are really bad news in networks that need ECMP.
There is not enough entropy in the SA/DA/Protocol triplet and anything
else results in misorder. But if ECMP is not done this overloads the
default path.
MPLS is also stateless but there are more options, although the most
common is to look past BoS to the five tuple, however some "features"
make mistakes and look at a non-existent five tuple despite hints in
the packet that thus is a bad idea.
therefore, the exhibit they same problematic behaviors
described in Section 4.4. In IPv6, the flow label may alternatively
used as input to the algorithm as opposed to parsing the transport
layer of packets to discern port numbers. The flow label should be
consistently set for a packets of flow including fragments, such that
a device does not need to parse packets beyond the IP header for the
purposes of ECMP."
Add to section 7.3:
"Routers SHOULD use IPv6 flow label for ECMP routing as described in
[RFC6438]."
If we want to migrate to the FL then we really need to state that the
FL MUST be set by the sender. Without, that we are never going to wean
routers off looking at the five tuple, if indeed we ever succeed in
doing that.
It we really need to fragment a packet, it would be better to stick
the fragments inside a common UDP/IP(no frag) shim. Then the
forwarders could carry on just as they are. We would never get
misorder and we would not be faced with the impossible problem of
changing the Internet core forwarding behaviour to a single consistent
model.
- Stewart
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