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

Stewart Bryant <stewart.bryant@gmail.com> Wed, 30 January 2019 18:34 UTC

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To: "Templin (US), Fred L" <Fred.L.Templin@boeing.com>, Fred Baker <fredbaker.ietf@gmail.com>, Tom Herbert <tom@herbertland.com>
Cc: int-area <int-area@ietf.org>
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From: Stewart Bryant <stewart.bryant@gmail.com>
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Date: Wed, 30 Jan 2019 18:34:50 +0000
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Subject: Re: [Int-area] Comments on draft-ietf-intarea-frag-fragile-06
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I think pretty much anything would need that wouldn't it?

- Stewart

On 30/01/2019 18:29, Templin (US), Fred L wrote:
>
> Hi Stewart,
>
> Sounds like that would require some sort of encapsulation protocol and
>
> low-level code in the kernel or hardware to strip the UDP headers, right?
>
> Fred
>
> *From:*Stewart Bryant [mailto:stewart.bryant@gmail.com]
> *Sent:* Wednesday, January 30, 2019 10:15 AM
> *To:* Templin (US), Fred L <Fred.L.Templin@boeing.com>om>; Fred Baker 
> <fredbaker.ietf@gmail.com>om>; Tom Herbert <tom@herbertland.com>
> *Cc:* int-area <int-area@ietf.org>
> *Subject:* Re: [Int-area] Comments on draft-ietf-intarea-frag-fragile-06
>
> 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:int-area-bounces@ietf.org] *On Behalf Of
>     *Stewart Bryant
>     *Sent:* Wednesday, January 30, 2019 6:14 AM
>     *To:* Fred Baker <fredbaker.ietf@gmail.com>
>     <mailto:fredbaker.ietf@gmail.com>; Tom Herbert
>     <tom@herbertland.com> <mailto:tom@herbertland.com>
>     *Cc:* int-area <int-area@ietf.org> <mailto:int-area@ietf.org>
>     *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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>           
>
>         --------------------------------------------------------------------------------
>
>         Victorious warriors win first and then go to war,
>
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>
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>
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