Re: [Int-area] Comments on draft-ietf-intarea-frag-fragile-06

[Stewart Bryant <[e-mail]>]

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:[e-mail]] *On Behalf Of 
> *Stewart Bryant
> *Sent:* Wednesday, January 30, 2019 6:14 AM
> *To:* Fred Baker <[e-mail]>; Tom Herbert 
> <[e-mail]>
> *Cc:* int-area <[e-mail]>
> *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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