Re: Non-Last Small IPv6 Fragments

[Tom Herbert <[e-mail]>]

On Mon, Jan 14, 2019 at 2:04 PM Erik Kline <[e-mail]> wrote:
>
> Clearly I'm way behind in my document reading!!
>
Erik,

Please look at draft-herbert-fast-03 also. This protocol agnostic and
secure way for applications to signal the network. In order fo this to
work, as well as other similar proposals in MTU reporting and in-situ
OAM, we really need Hop-by-Hop extension header to at least pass
though intermediate nodes.

Tom

> I'd just add that it seems reasonable to see about broader Internet
> usage at some point.
>
> On Mon, 14 Jan 2019 at 13:34, Ole Troan <[e-mail]> wrote:
> >
> >
> >
> > On 14 Jan 2019, at 21:40, Erik Kline <[e-mail]> wrote:
> >
> > On Sun, 13 Jan 2019 at 22:24, Fernando Gont <[e-mail]> wrote:
> >
> >
> > Hi, Erik,
> >
> >
> > On 14/1/19 02:19, Erik Kline wrote:
> >
> >
> > Ole Troan wrote on 13/01/2019 20:03:
> >
> > Let’s fix path MTU discovery. There’s no fix for fragmentation.
> >
> >
> > pmtu discovery is hard because it needs a way for an intermediate
> >
> >    node to be able to signal a transmitting node to dynamically drop
> >
> >    the MTU if network conditions change.  The only way to reliably work
> >
> >    around this is to transmit at 1280 and claim implementation /
> >
> >    operational breakage if this cannot get through.  This, however,
> >
> >    robs the host devices of the ability to use higher MTUs.
> >
> >
> >    “Fix” as in something different than rfc8201.
> >
> >
> >
> > How about a version of the fragment header that:
> >
> >
> >    (a) *always* has the L4 header (or a configurable number of bytes)
> >
> > in every non-first fragment
> >
> >
> > This could be documented on a per-L4+ basis how much needs to be
> >
> > included, i.e. we could have one doc for UDP and (say) a separate doc
> >
> > for information QUIC would need on a per-fragment basis.
> >
> >
> > If you are requiring that the first frag always contains the L4 ehader,
> >
> > that's already achieved by RFC7112.
> >
> >
> > Not what I suggested.  I said every fragment header would include the
> > L4 header (or other specifiable bytes).
> >
> > If you really mean that the FH contains the L4 header, then:
> >
> >
> > * There's the issue of EHs in the fragmentable part
> >
> >
> > That would be addressed in an associated L4 doc, and it would probably
> > say to ignore non-transport headers.
> >
> > * It may be hard to enforce in the presence of tunnels, unless the
> >
> > tunnel reassembles and refragments where necesary
> >
> >
> >
> > Still, that doesn't solve the biggest issues with fragmention:
> >
> >
> > * a stateful operation for anotherwise stateless protocol
> >
> >
> > * i'd argue that for the IPv6 case, one of the base reasons for which
> >
> > fragments are dropped is because they employ EHs
> >
> >
> >
> >
> > This increases the overhead in a given fragment, but also helps to
> >
> > ensure that (eventually) intermediate systems can examine this field and
> >
> > preemptively make a drop/no-drop decision.
> >
> >
> > Then we're back on draft-gont-v6ops-ipv6-ehs-packet-drops
> >
> >
> > Understood, but any L3 solution is going to necessitate running into
> > this.  UDP trailers is certainly cute (and +1 from me), but every
> > transport will have to have some similar fix (or applications will
> > have to consider rfc8085#section-3.2 type guidelines).  If there's to
> > be an L3 solution to this L3 problem it will face some deployment
> > issues but if its usefulness is sufficiently attractive it could find
> > reasonably wide-spread support in due time (in the absence of ipng-ng,
> > IPv6 is it for the long haul).
> >
> > [Silly Idea #1]
> >
> > (/me thinks back to spud bof)
> >
> > Here's another random suggestion that maybe belongs in panrg or (more
> > likely) the dumpster:
> >
> > A new hop-by-hop header under "00 1", i.e.
> >
> >    00 - skip over this option and continue processing the header
> >
> >    1 - Option Data may change en route
> >
> > of the form, for example, Option Type 00 1 00001, Opt Data Len 2,
> > Option Data == MTU.  Here, the observation is that the main thing we
> > have experience with "working" (for the most generous definition of
> > "works") in this problemspace in the internet today is TCP MSS
> > Clamping.  So let's have an MTU (or MSS) HbH option that can be
> > revised monotonically downwards but never lower than 1280.  If it
> > helps pad to 8 byte alignment we can have a 2nd 2-byte field that is
> > untouched representing the sender's original MTU/MSS.
> >
> > In this way, a sender could include an HbH with either:
> >
> >    [HbH | 0 | 0x0202 | 2 | (1500=0x5dc) | Pad1 | Pad1]
> >
> > or
> >
> >    [HbH | 0 | 0x0202 | 4 | (1500=0x5dc) | (1500=0x5dc) ]
> >
> > with one of the MTU/MSS fields to be munged along the path.
> >
> > If this were the only HbH option included it adds only 8 bytes, fits
> > (along with most L4 headers) within a 128 byte hardware classifier,
> > and end systems could explore its usefulness via ping.  (Separately,
> > there could of course be sockopt/cmsg things to set/get the values.)
> >
> >
> > https://tools.ietf.org/html/draft-hinden-6man-mtu-option-00
> >
> >
> >
> > [Silly Idea #2]
> > Separate, even sillier idea: everyone converges on using 16 bits of
> > the flow ID to encode the lowest MTU encountered along the path.  Here
> > again, TCP MSS clamping style behaviour would be applied to this
> > field.
>
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