Public bug reported:
TCP uses an MSS of 1440 although the IPv6 MTU size is set to 1280 by auto
configuration (router advertisements). This worked well under gutsy so far, but
now TCP connections fail because PMTU discovery does not work from the other
end correctly (probably due to tunnel ingress).
TCP must not use a value larger than the MTU
During SYN-Handshake my hardy system sends:
Transmission Control Protocol, Src Port: 47070 (47070), Dst Port: http (80),
Seq: 0, Len: 0
Source port: 47070 (47070)
Destination port: http (80)
Sequence number: 0 (relative sequence number)
Header length: 40 bytes
Flags: 0x02 (SYN)
Window size: 5760
Checksum: 0xc4b5 [correct]
Options: (20 bytes)
Maximum segment size: 1440 bytes
SACK permitted
Timestamps: TSval 58913848, TSecr 0
NOP
Window scale: 7 (multiply by 128)
Although:
net.ipv6.conf.eth0.mtu = 1280
ip -6 r s default
default via fe80::207:e9ff:fe23:e706 dev eth0 proto kernel metric 1024
expires 407sec mtu 1280 advmss 1440 hoplimit 64
advmss is obviously wrong
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
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TCP uses wrong MTU/MSS size for IPv6
https://bugs.launchpad.net/bugs/254622
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