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authorSven Eckelmann <sven@narfation.org>2017-02-22 17:25:42 +0100
committerSimon Wunderlich <sw@simonwunderlich.de>2017-03-04 17:31:57 +0100
commit1c2bcc766be44467809f1798cd4ceacafe20a852 (patch)
tree49ac8871263ee7a5327bf6fcbb667bf6b8704196 /MAINTAINERS
parent51c6b429c0c95e67edd1cb0b548c5cf6a6604763 (diff)
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batman-adv: Keep fragments equally sized
The batman-adv fragmentation packets have the design problem that they cannot be refragmented and cannot handle padding by the underlying link. The latter often leads to problems when networks are incorrectly configured and don't use a common MTU. The sender could for example fragment a 1271 byte frame (plus external ethernet header (14) and batadv unicast header (10)) to fit in a 1280 bytes large MTU of the underlying link (max. 1294 byte frames). This would create a 1294 bytes large frame (fragment 2) and a 55 bytes large frame (fragment 1). The extra 54 bytes are the fragment header (20) added to each fragment and the external ethernet header (14) for the second fragment. Let us assume that the next hop is then not able to transport 1294 bytes to its next hop. The 1294 byte large frame will be dropped but the 55 bytes large fragment will still be forwarded to its destination. Or let us assume that the underlying hardware requires that each frame has a minimum size (e.g. 60 bytes). Then it will pad the 55 bytes frame to 60 bytes. The receiver of the 60 bytes frame will no longer be able to correctly assemble the two frames together because it is not aware that 5 bytes of the 60 bytes frame are padding and don't belong to the reassembled frame. This can partly be avoided by splitting frames more equally. In this example, the 675 and 674 bytes large fragment frames could both potentially reach its destination without being too large or too small. Reported-by: Martin Weinelt <martin@darmstadt.freifunk.net> Fixes: ee75ed88879a ("batman-adv: Fragment and send skbs larger than mtu") Signed-off-by: Sven Eckelmann <sven@narfation.org> Acked-by: Linus Lüssing <linus.luessing@c0d3.blue> Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
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