Date: Wed, 16 Nov 2022 11:19:06 +0800 From: Zhenlei Huang <zlei@FreeBSD.org> To: tuexen@freebsd.org Cc: freebsd-net <freebsd-net@freebsd.org> Subject: Re: ICMPv6 over lo0 Message-ID: <D353907A-2295-46B8-9F3A-C4AC0370C8E0@FreeBSD.org> In-Reply-To: <DDDFD026-72AF-4558-93E4-60D361C671DF@freebsd.org> References: <DDDFD026-72AF-4558-93E4-60D361C671DF@freebsd.org>
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> On Nov 16, 2022, at 5:14 AM, tuexen@freebsd.org wrote: >=20 > Dear all, >=20 > when using the master branch of today (or 13.1) I get when running >=20 > tuexen@ampere128:~ % ping6 -c 1 -b 30000 -s 20000 ::1 > PING6(20048=3D40+8+20000 bytes) ::1 --> ::1 > 20008 bytes from ::1, icmp_seq=3D0 hlim=3D64 time=3D0.709 ms >=20 > --- ::1 ping6 statistics --- > 1 packets transmitted, 1 packets received, 0.0% packet loss > round-trip min/avg/max/std-dev =3D 0.709/0.709/0.709/0.000 ms >=20 > which is expected. What I don't expect is: >=20 > tuexen@ampere128:~ % tcpdump -i lo0 -n > tcpdump: verbose output suppressed, use -v or -vv for full protocol = decode > listening on lo0, link-type NULL (BSD loopback), capture size 262144 = bytes > 22:06:38.835630 IP6 ::1 > ::1: frag (0|1232) ICMP6, echo request, seq = 0, length 1232 > 22:06:38.835639 IP6 ::1 > ::1: frag (1232|1232) > 22:06:38.835641 IP6 ::1 > ::1: frag (2464|1232) > 22:06:38.835641 IP6 ::1 > ::1: frag (3696|1232) > 22:06:38.835642 IP6 ::1 > ::1: frag (4928|1232) > 22:06:38.835642 IP6 ::1 > ::1: frag (6160|1232) > 22:06:38.835643 IP6 ::1 > ::1: frag (7392|1232) > 22:06:38.835644 IP6 ::1 > ::1: frag (8624|1232) > 22:06:38.835644 IP6 ::1 > ::1: frag (9856|1232) > 22:06:38.835645 IP6 ::1 > ::1: frag (11088|1232) > 22:06:38.835645 IP6 ::1 > ::1: frag (12320|1232) > 22:06:38.835646 IP6 ::1 > ::1: frag (13552|1232) > 22:06:38.835647 IP6 ::1 > ::1: frag (14784|1232) > 22:06:38.835647 IP6 ::1 > ::1: frag (16016|1232) > 22:06:38.835648 IP6 ::1 > ::1: frag (17248|1232) > 22:06:38.835648 IP6 ::1 > ::1: frag (18480|1232) > 22:06:38.835649 IP6 ::1 > ::1: frag (19712|296) > 22:06:38.836002 IP6 ::1 > ::1: frag (0|16336) ICMP6, echo reply, seq = 0, length 16336 > 22:06:38.836006 IP6 ::1 > ::1: frag (16336|3672) > ^C > 19 packets captured > 19 packets received by filter > 0 packets dropped by kernel >=20 > Why is for the Echo Request an MTU of 1280 used, whereas for the = response an MTU of 16384 > is used. >=20 > Since > tuexen@ampere128:~ % ifconfig lo0 > lo0: flags=3D8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384 > options=3D680003<RXCSUM,TXCSUM,LINKSTATE,RXCSUM_IPV6,TXCSUM_IPV6> > inet6 ::1 prefixlen 128 > inet6 fe80::1%lo0 prefixlen 64 scopeid 0x3 > inet 127.0.0.1 netmask 0xff000000 > groups: lo > nd6 options=3D21<PERFORMNUD,AUTO_LINKLOCAL> > is used, I would expect an MTU of 16384 also be used for the Echo = Request instead of > the minimum IPv6 MTU of 1280. I believe MTU of 16384 should be used for Echo Request. >=20 > Is this intended? At least for me, it is not expected... >=20 > Best regards > Michael
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