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

["Sri Gundavelli (sgundave)" <[e-mail]>]

Hi Warren,

Slight rewrite of this paragraph without changing the original intent.  Removed unnecessary text; identified the key issues.

Please let me know if this addresses your concerns.

Regards
Sri


————————————-

NEW:

3.2.  Traffic distribution schemes

When the MAG has registered multipath binding with the LMA, there will be multiple established overlay tunnels between them. The MAG and the LMA can use any one, or more of the available tunnels paths for routing the mobile node’s IP traffic.  This specification does not recommend, or define any specific traffic distribution scheme, however it identifies two well-known approaches that implementations can potentially use. These approaches are, Per-flow and Per-packet Traffic distribution schemes.

Per-Flow Traffic Distribution:

In this approach the MAG and the LMA associate each of the IP flows (upstream and downstream) to a specific tunnel path. The packets in a given IP flow are always routed on the same overlay tunnel path; they are never split and routed concurrently on more than one tunnel path.  It is possible a given flow may be moved from one tunnel path to another, but the flow is never split. The decision to bind a given IP flow to a specific tunnel path is based on traffic distribution policy. This traffic distribution policy is either statically configured on both the MAG and the LMA, or dynamically negotiated  over Proxy Mobile IPv6 signaling.   The Flow Binding extension [6089] defines the mechanism and the semantics for exchanging the traffic policy between two tunnel peers and the same mechanism and the mobility options are used here.

Per-Packet Traffic Distribution:

In this approach, packets belonging a given IP flow will be split and routed across more than one tunnel paths. The exact approach for traffic distribution, or the distribution weights is outside the scope of this specification. In a very simplistic approach, assuming the established tunnel paths have symmetric characteristics, the packets can be equally distributed on all the available tunnel paths. In a different scenario when the links have different speeds, the chosen approach can be based on weighted distribution (Ex: n:m ratio). However, in any of these chosen approaches, implementations have to be sensitive to issues related to asymmetric link characteristics and the resulting issues such as re-ordering, buffering and the impact to the application performance. Care must be taken to ensure there is no negative impact to the application performance due to the use of this approach.

—————————————————————-

OD:

3.2.  Traffic distribution schemes

   When receiving packets from the MN, the MAG distributes packets over
   tunnels that have been established.  Traffic distribution can be
   managed either on a per-flow or on a per-packet basis:


   o  Per-flow traffic management: each IP flow (both upstream and
      downstream) is mapped to a given tunnel, corresponding to a given
      WAN interface.  Flow binding extension [RFC6089] is used to
      exchange, and synchronize, IP flow management policies (i.e. rules
      associating traffic selectors [RFC6088] to a tunnel).

   o  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).  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.  For example, the algorithm may give
      precedence to one given access: the MAG overflows traffic from the
      primary access, e.g.  WLAN, to the second one, only when load on
      primary access reaches a given threshold.  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.  Sequence number can be can be used for
      that purpose, for example using GRE with sequence number option
      [RFC5845].  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.

   Because latency introduced by per-packet can cause injury to some
   application, per-flow and per-packet distribution schemes could be
   used in conjunction.  For example, high throughput services (e.g.
   video streaming) may benefit from per-packet distribution scheme,
   while latency sensitive applications (e.g.  VoIP) are not be spread
   over different WAN paths.  IP flow mobility extensions, [RFC6089] and
   [RFC6088], can be used to provision the MAG with such flow policies.




On 8/15/17, 10:46 AM, "Warren Kumari" <[e-mail]<mailto:[e-mail]>> wrote:

This document is on Thursday's telechat - I have not seen proposed
text to address my concerns, and so I'm continuing to hold my DISCUSS
position.

W

On Wed, Aug 2, 2017 at 5:18 PM,  <[e-mail]<mailto:[e-mail]>> wrote:
Hello

Please see inline

Pierrick



Sent from my cell phone, mind the typos.

-------- Message d'origine --------
De : Warren Kumari <[e-mail]<mailto:[e-mail]>>
Date : 02/08/2017 22:23 (GMT+01:00)
À : The IESG <[e-mail]<mailto:[e-mail]>>
Cc : [e-mail]<mailto:[e-mail]>, Jouni Korhonen
<[e-mail]<mailto:[e-mail]>>, [e-mail]<mailto:[e-mail]>, [e-mail]<mailto:[e-mail]>,
[e-mail]<mailto:[e-mail]>
Objet : Warren Kumari's Discuss on draft-ietf-dmm-mag-multihoming-04: (with
DISCUSS)

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

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----------------------------------------------------------------------
DISCUSS:
----------------------------------------------------------------------

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.

ok

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?

some use-cases implement per-packet distribution. However this document
does not aim to make recommendation on the way to distribute packets


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--
I don't think the execution is relevant when it was obviously a bad
idea in the first place.
This is like putting rabid weasels in your pants, and later expressing
regret at having chosen those particular rabid weasels and that pair
of pants.
   ---maf