13 Sep
2019
13 Sep
'19
10:01 p.m.
Peter Jeremy:
NFS ran much faster when you turned off the UDP
checksums as well.
(Though there was still the Ethernet CRC32).
Dave Horsfall:
Blerk... That just tells you that the packet came across the wire more or
less OK.
=====
UDP (and TCP) checksums are nearly useless against
the sort of corruption Larry described. Since they
are a simple addition with carry wraparound, you
can insert or remove any number of word-aligned pairs
of zero octets without the checksum changing.
I discovered this the hard way, while tracking down
a kernel bug that caused exactly that sort of corruption.
Does IPv6 require a meaningful checksum, or just the
useless old ritual one?
Norman Wilson
Toronto ON
13 Sep
13 Sep
10:30 p.m.
New subject: [SPAM] Re: SCCS
On Fri, 13 Sep 2019, Norman Wilson wrote:
UDP (and TCP) checksums are nearly useless against the
sort of
corruption Larry described. Since they are a simple addition with carry
wraparound, you can insert or remove any number of word-aligned pairs of
zero octets without the checksum changing.
I was thinking of an intermediate router (probably one that you never knew
about) corrupting the checksum-less UDP packet, recalculating the Ethernet
checksum, and your kernel happily accepting it; you now have an
application that fails for some unknown reason.
Never seen it in practice, but I've heard of it happening.
-- Dave
16 Sep
16 Sep
10:59 a.m.
New subject: [SPAM] Re: SCCS
Dave Horsfall <dave(a)horsfall.org> wrote:
I was thinking of an intermediate router (probably one that you never knew
about) corrupting the checksum-less UDP packet, recalculating the Ethernet
checksum, and your kernel happily accepting it; you now have an application
that fails for some unknown reason.
Never seen it in practice, but I've heard of it happening.
This paper has some examples:
http://conferences.sigcomm.org/sigcomm/2000/conf/paper/sigcomm2000-9-1.pdf
Tony.
--
f.anthony.n.finch <dot(a)dotat.at> http://dotat.at/
Forth, Tyne, West Dogger: Westerly 3 to 5 veering northwesterly 4 to 6. Slight
or moderate, becoming moderate or rough except in west Forth. Mainly fair.
Good.
12:11 p.m.
New subject: [SPAM] Re: SCCS
Norman Wilson <norman(a)oclsc.org> writes:
Does IPv6 require a meaningful checksum, or just the
useless old ritual one?
IPv6 has no checksum at all (for the header), and TCPv6 uses the same
checksum algorithm.
--
Leah Neukirchen <leah(a)vuxu.org> https://leahneukirchen.org/
9:45 p.m.
New subject: [SPAM] Re: SCCS
On Mon, 16 Sep 2019, Leah Neukirchen wrote:
IPv6 has no checksum at all (for the header), and
TCPv6 uses the same
checksum algorithm.
Every time I've tried to read the IPv6 spec my eyes glazed over; it was
plainly designed by a committee i.e. everybody wanted their mark on it.
-- Dave
10:21 p.m.
New subject: [SPAM] Re: SCCS
Third mistake ever was host-only fragmentation and re-assembly. The
lack of pragmatism tells here, because it means intermediate systems
have no choice but to drop.
Second biggest mistake was keeping the order SRC, DST when if the DST
is first you can latch it into a buffer and do forwarding decisions
while the rest of the packet is in processing.
First biggest mistake was ignoring TUBA.
-G
On Tue, Sep 17, 2019 at 7:46 AM Dave Horsfall <dave(a)horsfall.org> wrote:
On Mon, 16 Sep 2019, Leah Neukirchen wrote:
IPv6 has no checksum at all (for the header), and
TCPv6 uses the same
checksum algorithm.
Every time I've tried to read the IPv6 spec my eyes glazed over; it was
plainly designed by a committee i.e. everybody wanted their mark on it.
-- Dave
2068
days inactive
2071
days old
5 comments
5 participants
participants (5)
-
Dave Horsfall -
George Michaelson -
Leah Neukirchen -
Norman Wilson -
Tony Finch