Subject: [DMM] Warren Kumari's Discuss on draft-ietf-dmm-mag-multihoming-04: (with DISCUSS)

From: Warren Kumari <[email-masked]>
Date: Wed, 02 Aug 2017 13:23:08 -0700

Warren Kumari has entered the following ballot position for
draft-ietf-dmm-mag-multihoming-04: Discuss

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DISCUSS:
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Section 3.2.  Traffic distribution schemes
"Per-packet management: the LMA and the MAG distribute packets, belonging to a
same IP flow, over more than one bindings (i.e. more than one WAN interface)."
This immediately made my out-of-order-packets antenna pop up, so I read the
section looking for mitigations. The very next sentence reads: "Packet
distribution can be done either at the transport level, e.g. using MPTCP or at
When operating at the IP packet level, different packets distribution
algorithms are possible. " -- the fact that this sentence is a: malformed and
b: hand-wavy did nothing to allay my concerns, so I read further: "The
distribution algorithm is left to implementer but whatever the algorithm is,
packets distribution likely introduces packet latency and out-of-order
delivery.  LMA and MAG shall thus be able to make reordering before packets
delivery." - I agree with the first sentence (although it is poorly worded),
but the second sentence doesn't follow from the first; "shall thus be able to"
implies that the prior text somehow provides a solution, not points out a
problem (the sentence is also malformed)-- I think you mean something like "The
LMA and MAG thus need to be able reorder packets to their original order before
delivery."

This then continues with "Sequence number can be can be used for that purpose,
for example using GRE with sequence number option [RFC5845]." - I think that
the actual reference should be RFC2890, but regardless of this, I don't think
that what you are proposing works - "The Sequence Number field is used to
maintain sequence of packets **within** the GRE Tunnel." (from RFC2890,
emphasis added). This means that sequence numbers are local to the tunnel, and
(as I understand it) your solution involves diverse tunnels. Further, Section
2.2. Sequence Number says: "The receiver may perform a small amount of
buffering in an attempt to recover the original sequence of transmitted
packets. In this case, the packet may be placed in a buffer sorted by sequence
number." - if you are proposing using a single sequence number space for
multiple tunnels, you will end up with sequence number space gabs, and lots of
buffering, etc. The section ends with: "However, more detailed considerations
on reordering  and IP packet distribution scheme (e.g. definition of packets
distribution algorithm) are out the scope of this document." - I think that,
unless the prior paragraph is significantly reworked, it should not try and
suggest any mitigations.

The whole idea of striping packets of a flow across (notably) different
transports seems like a really bad idea to me -- is it actually needed?