Date: Tue, 11 Jan 2005 14:01:29 -0800 From: "Kevin Oberman" <oberman@es.net> To: freebsd-net@freebsd.org Cc: nesg@es.net Subject: IPv6 TCP transfers are hanging Message-ID: <20050111220129.356815D07@ptavv.es.net>
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I think I have found a problem with TCP when run over IPv6. I set my MSS for TCP to 1460 to allow a full 1500 byte MTU to be utilized on my systems. (Yes, I see that this does break some things like communicating via links where PMTUD is blocked and one or more links restrict MTU to some size less than 1500 bytes. What I am specifically seeing is a packet being sent out with a TCP length of 1460. While this is fine for IPv4, it's too back for IPv6 and, as you might expect, the far end never receives this packet. There is a sysctl for net.inet.tcp.v6mssdflt which is set to 1024. This should be fine, but it appears that it is not being honored and the V4 value is always used. Am I mis-analyzing things or is TCP at least a bit broken when running over V6? (Or am I at fault for setting the large MSS because ti is honored with v6 even though there is a separate sysctl for IPv6? -- R. Kevin Oberman, Network Engineer Energy Sciences Network (ESnet) Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab) E-mail: oberman@es.net Phone: +1 510 486-8634
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