MTU breakage in f20
Wolfgang S. Rupprecht
wolfgang.rupprecht at gmail.com
Mon Nov 3 01:20:05 UTC 2014
Ed Greshko <ed.greshko at greshko.com> writes:
> [egreshko at meimei ~]$ ping -s 1200 wifi (my gw)
> PING wifi.greshko.com (192.168.1.1) 1200(1228) bytes of data.
> 1208 bytes from wifi.greshko.com (192.168.1.1): icmp_seq=1 ttl=64 time=0.487 ms
> 1208 bytes from wifi.greshko.com (192.168.1.1): icmp_seq=2 ttl=64 time=0.501 ms
> So, no trouble here. Fully updated F20 system.
Hmm. I didn't really believe it could be an across the board problem
without anyone else noticing, but that leaves me with the question as to
what is going on here. I've got a similar claimed mtu of 1500.
[wolfgang at arbol ~]$ ip link show p34p1
3: p34p1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
link/ether 08:60:6e:74:6f:e2 brd ff:ff:ff:ff:ff:ff
Using a bit of binary search it turns out my largest working ping is 512
bytes. That is a very suspicious number because it is power of two and
the actual packet still has a handful of bytes slapped onto the front
making it a non power of two.
[wolfgang at arbol ~]$ ping -s 512 gw
PING gw.wsrcc.com (192.168.35.1) 512(540) bytes of data.
520 bytes from gw.wsrcc.com (192.168.35.1): icmp_seq=1 ttl=64 time=0.538 ms
520 bytes from gw.wsrcc.com (192.168.35.1): icmp_seq=2 ttl=64 time=0.521 ms
520 bytes from gw.wsrcc.com (192.168.35.1): icmp_seq=3 ttl=64 time=0.453 ms
^C
--- gw.wsrcc.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2001ms
rtt min/avg/max/mdev = 0.453/0.504/0.538/0.036 ms
[wolfgang at arbol ~]$ ping -s 513 gw
PING gw.wsrcc.com (192.168.35.1) 513(541) bytes of data.
^C
--- gw.wsrcc.com ping statistics ---
9 packets transmitted, 0 received, 100% packet loss, time 7999ms
[wolfgang at arbol ~]$
I've turned off all the hardware accelerators that ethtool knows about.
No change.
[wolfgang at arbol ~]$ ethtool -k p34p1
Features for p34p1:
rx-checksumming: off
tx-checksumming: off
tx-checksum-ipv4: off
tx-checksum-ip-generic: off [fixed]
tx-checksum-ipv6: off [fixed]
tx-checksum-fcoe-crc: off [fixed]
tx-checksum-sctp: off [fixed]
scatter-gather: off
tx-scatter-gather: off
tx-scatter-gather-fraglist: off [fixed]
tcp-segmentation-offload: off
tx-tcp-segmentation: off
tx-tcp-ecn-segmentation: off [fixed]
tx-tcp6-segmentation: off [fixed]
udp-fragmentation-offload: off [fixed]
generic-segmentation-offload: off
generic-receive-offload: off
large-receive-offload: off [fixed]
rx-vlan-offload: off
tx-vlan-offload: off
ntuple-filters: off [fixed]
receive-hashing: off [fixed]
highdma: off [fixed]
rx-vlan-filter: off [fixed]
vlan-challenged: off [fixed]
tx-lockless: off [fixed]
netns-local: off [fixed]
tx-gso-robust: off [fixed]
tx-fcoe-segmentation: off [fixed]
tx-gre-segmentation: off [fixed]
tx-ipip-segmentation: off [fixed]
tx-sit-segmentation: off [fixed]
tx-udp_tnl-segmentation: off [fixed]
tx-mpls-segmentation: off [fixed]
fcoe-mtu: off [fixed]
tx-nocache-copy: off
loopback: off [fixed]
rx-fcs: off
rx-all: off
tx-vlan-stag-hw-insert: off [fixed]
rx-vlan-stag-hw-parse: off [fixed]
rx-vlan-stag-filter: off [fixed]
l2-fwd-offload: off [fixed]
busy-poll: off [fixed]
[wolfgang at arbol ~]$
What is left? Some weirdness caused by my router announcing a low MTU
but Fedora not reporting it? I'm grasping at straws here.
-wolfgang
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