Date: Fri, 5 Apr 2002 15:45:22 -0800 From: Bill Fenner <fenner@research.att.com> To: bmah@FreeBSD.ORG Cc: net@FreeBSD.ORG Subject: Re: IP fragmentation (was Re: Fatal trap 12: page fault while in kernel mode) Message-ID: <200204052345.PAA21989@windsor.research.att.com> References: <20020403181854.I42720-100000@angui.sh> <15532.29114.310072.957330@grasshopper.cs.duke.edu> <200204050504.g355493C001200@intruder.bmah.org> <15533.46222.49598.958821@grasshopper.cs.duke.edu> <3CADE0E7.ED472650@mindspring.com> <15533.57961.725030.692387@grasshopper.cs.duke.edu> <200204052120.g35LKW00034174@intruder.bmah.org>
next in thread | previous in thread | raw e-mail | index | archive | help
>The only difference I can see is that in ip_input(), if nipq > maxnipq, >all of the fragments for some other packet in the current hash bucket >get dropped (with the wonderfully descriptive comment "gak"). The best thing to do is to drop the oldest frag queue; that's obviously non-trivial with the current data structures. The problem with the ip_nfragments code is that if space becomes available in the middle of reception of an entire packet, a queue will be created to reassemble a packet that will never completely arrive (since we dropped some of the beginning of it due to no space). I guess the nipq code has a similar problem: it will gladly free a queue that contains fragments that go with the next fragment that arrives. In fact, if datagrams that hash to the same bucket arrive with interleaved fragments, the nipq code could thrash between the two packets, creating and deleting a frag queue for each fragment arrival, dropping both datagrams. To address this kind of problem, it might be worth creating a "drop" frag queue entry, which is a way to remember that we dropped fragments from a given datagram so we should drop all the other fragments too. (I'd also go for modifying the data structures to make it easy to drop the oldest frag queue.) Bill To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200204052345.PAA21989>