<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On Jan 7, 2014, at 12:56 PM, Emmanuel Thierry wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div>Hello,<br><br>Le 7 janv. 2014 à 12:37, Tore Anderson a écrit :<br><br><blockquote type="cite">Hi list,<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Does anyone know what tricks, if any, the major 6RD deployments (AT&T,<br></blockquote><blockquote type="cite">Free, Swisscom, others?) are using to alleviate any problems stemming<br></blockquote><blockquote type="cite">from the reduced IPv6 MTU? Some possibilities that come to mind are:<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">* Having the 6RD CPE lower the TCP MSS value of SYN packets as they<br></blockquote><blockquote type="cite">enter/exit the tunnel device<br></blockquote><blockquote type="cite">* Having the 6RD BR lower the TCP MSS value in the same way as above<br></blockquote><blockquote type="cite">* Having the 6RD CPE advertise a lowered MTU to the LAN in RA Options<br></blockquote><br>For your information, i see an advertised mtu of 1480 on my WiFi interface with the Free CPE.<br></div></blockquote><div><br></div><div>Section 9.1 of RFC 5969:</div><div><br></div><div><pre class="newpage" style="font-size: 1em; margin-top: 0px; margin-bottom: 0px; page-break-before: always; color: rgb(0, 0, 0); font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"> If the MTU is well-managed such that the IPv4 MTU on the CE WAN side
interface is set so that no fragmentation occurs within the boundary
of the SP, then the 6rd Tunnel MTU should be set to the known IPv4
MTU minus the size of the encapsulating IPv4 header (20 bytes). For
example, if the IPv4 MTU is known to be 1500 bytes, the 6rd Tunnel
MTU might be set to 1480 bytes. Absent more specific information,
the 6rd Tunnel MTU SHOULD default to 1280 bytes.</pre><div><br></div><div>Note I've heard some ISPs consider running Jumbo Frames under the covers so that IPv4 could carry 1520 and 1500 would be possible for IPv6, but have not yet seen that confirmed to me in practice. </div><div><br></div><div>- Mark</div></div><br><blockquote type="cite"><div><br><blockquote type="cite">* Several (or all) of the above in combination<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Also, given that some ISPs offer [only] Layer-2 service and expect/allow<br></blockquote><blockquote type="cite">their customers to bring their own Layer-3 home gateway if they want<br></blockquote><blockquote type="cite">one, I would find it interesting to learn if any of the most common<br></blockquote><blockquote type="cite">off-the-shelf home gateway products (that enable 6RD by default) also<br></blockquote><blockquote type="cite">implement any such tricks by default or not.<br></blockquote><blockquote type="cite"><br></blockquote><br>Best regards<br>Emmanuel Thierry<br><br></div></blockquote></div><br></body></html>