The PRP duplicate discard algorithm does not work reliably with certain
link faults. Especially with packet loss on one link, the duplicate discard
algorithm drops valid packets which leads to packet loss on the PRP
interface where the link fault should in theory be perfectly recoverable by
PRP. This happens because the algorithm opens the drop window on the lossy
link, covering received and lost sequence numbers. If the other, non-lossy
link receives the duplicate for a lost frame, it is within the drop window
of the lossy link and therefore dropped.
Since IEC 62439-3:2012, a node has one sequence number counter for frames
it sends, instead of one sequence number counter for each destination.
Therefore, a node can not expect to receive contiguous sequence numbers
from a sender. A missing sequence number can be totally normal (if the
sender intermittently communicates with another node) or mean a frame was
lost.
The algorithm, as previously implemented in commit 05fd00e5e7 ("net: hsr:
Fix PRP duplicate detection"), was part of IEC 62439-3:2010 (HSRv0/PRPv0)
but was removed with IEC 62439-3:2012 (HSRv1/PRPv1). Since that, no
algorithm is specified but up to implementers. It should be "designed such
that it never rejects a legitimate frame, while occasional acceptance of a
duplicate can be tolerated" (IEC 62439-3:2021).
For the duplicate discard algorithm, this means that 1) we need to track
the sequence numbers individually to account for non-contiguous sequence
numbers, and 2) we should always err on the side of accepting a duplicate
than dropping a valid frame.
The idea of the new algorithm is to store the seen sequence numbers in a
bitmap. To keep the size of the bitmap in control, we store it as a "sparse
bitmap" where the bitmap is split into blocks and not all blocks exist at
the same time. The sparse bitmap is implemented using an xarray that keeps
the references to the individual blocks and a backing ring buffer that
stores the actual blocks. New blocks are initialized in the buffer and
added to the xarray as needed when new frames arrive. Existing blocks are
removed in two conditions:
1. The block found for an arriving sequence number is old and therefore not
relevant to the duplicate discard algorithm anymore, i.e., it has been
added more than the entry forget time ago. In this case, the block is
removed from the xarray and marked as forgotten (by setting its
timestamp to 0).
2. Space is needed in the ring buffer for a new block. In this case, the
block is removed from the xarray, if it hasn't already been forgotten
(by 1.). Afterwards, the new block is initialized in its place.
This has the nice property that we can reliably track sequence numbers on
low traffic situations (where they expire based on their timestamp) and
more quickly forget sequence numbers in high traffic situations before they
potentially wrap over and repeat before they are expired.
When nodes are merged, the blocks are merged as well. The timestamp of a
merged block is set to the minimum of the two timestamps to never keep
around a seen sequence number for too long. The bitmaps are or'd to mark
all seen sequence numbers as seen.
All of this still happens under seq_out_lock, to prevent concurrent
access to the blocks.
The KUnit test for the algorithm is updated as well. The updates are done
in a way to match the original intends pretty closely. Currently, there is
much knowledge about the actual algorithm baked into the tests (especially
the expectations) which may need some redesign in the future.
Reported-by: Steffen Lindner <steffen.lindner@de.abb.com>
Signed-off-by: Felix Maurer <fmaurer@redhat.com>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Steffen Lindner <steffen.lindner@de.abb.com>
Link: https://patch.msgid.link/8ce15a996099df2df5b700969a39e7df400e8dbb.1770299429.git.fmaurer@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
415e636751