netdev - Re: 802.3ad bonding brain damaged?

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Date:	Mon, 08 Aug 2011 09:57:14 +0200
From:	David Lamparter <equinox@...c24.net>
To:	Phillip Susi <psusi@....rr.com>
Cc:	netdev@...r.kernel.org
Subject: Re: 802.3ad bonding brain damaged?

Am Sonntag, den 07.08.2011, 15:52 -0400 schrieb Phillip Susi:
> - From Documentation/networking/bonding.txt:
> 
> 	Additionally, the linux bonding 802.3ad implementation
> 	distributes traffic by peer (using an XOR of MAC addresses),
> 
> This is counter to the entire point of 802.3ad. Distributing traffic by
> hash of the destination address is poor mans load balancing for
> systems not supporting 802.3ad. 

No, it isn't. 802.3ad/.1AX explicitly requires that no packet
re-ordering may ever occur, which can only be guaranteed by enqueueing
packets for one host on one TX interface. This behaviour is mandated by
802.1AX-2008 page 15 which reads:

  This standard does not mandate any particular distribution
  algorithm(s); however, any distribution algorithm shall ensure that,
  when frames are received by a Frame Collector as specified in 5.2.3,
  the algorithm shall not cause
  a) Misordering of frames that are part of any given conversation, or
  b) Duplication of frames.
| The above requirement to maintain frame ordering is met by ensuring
| that all frames that compose a given conversation are transmitted on a
| single link in the order that they are generated by the MAC Client;
  hence, this requirement does not involve the addition (or
  modification) of any information to the MAC frame, nor any buffering
  or processing on the part of the corresponding Frame Collector in
  order to reorder frames. This approach to the operation of the
  distribution function permits a wide variety of distribution and load
  balancing algorithms to be used, while also ensuring interoperability
  between devices that adopt differing algorithms.

(IMHO it is 802.3ad/.1AX that is brain damaged, I would've made this a
weak requirement with an optional mode switch for networks that require
a strong one.)

It is true that it might be possible to fulfill this requirement with a
more sophisticated approach or just ignore edge cases, but in that case
you can just use a different bonding algorithm.

> When in 802.3ad mode, packets are supposed to
> be queued to whichever interface has the shortest tx length so a single
> stream to a single host can be balanced across all links instead of
> being restricted to one, while the other is idle.

Please make sure to tag your assumptions as such and don't make
assertions without reading the specification.

-David

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