Ryota Hirose wrote: > Hi IPv6 hackers, > > Recent GbE NICs or GbE switches support Jumbo Frames, but RFC2464 > provides that the maximum MTU of an Ethernet is 1500. So we cannot > use the Jumbo Frames for IPv6.
As I read it, RFC2464 says that the maximum *default* MTU is 1500, and, anything larger than that must be manually configured. It doesn't say that it is prohibited. > 2. Maximum Transmission Unit > > The default MTU size for IPv6 [IPV6] packets on an Ethernet is 1500 > octets. This size may be reduced by a Router Advertisement [DISC] > containing an MTU option which specifies a smaller MTU, or by manual > configuration of each node. If a Router Advertisement received on an > Ethernet interface has an MTU option specifying an MTU larger than > 1500, or larger than a manually configured value, that MTU option may > be logged to system management but must be otherwise ignored. > > For example, on FreeBSD 5.4R, to send a Jumbo Frames of IPv4, I just > only set the link MTU with ifconfig command. To send a Jumbo Frames > of IPv6, I had to hack a kernel code to not compare link MTU with > ETHERMTU(1500). > > BTW, Jinmei san suggested that KAME, NetBSD and OpenBSD are already > hacked as above and this behavior may be only FreeBSD problem. > But, according to RFC2464, FreeBSD behavior is right, and other > BSDs is wrong. I haven't tried this myself on FreeBSD 5.4R, but, if so, it is a bug, because manual configuration of MTU larger than 1500 is not prohibited by RFC2464. Thanks for pointing the FreeBSD bug out. Please report it to the FreeBSD mailing lists (-current, -net). > I know that this Jumbo Frames of GbE violates a specification of IEEE > 802.3. But it is deploied already, and is used for a high performance > application like the file servers. Does anyone have a plan to change > the rule of RFC2464? Jumbo frames are essential, and, do not violate RFC2464. Unfortunately, they are effectively deprecated by requiring manual configuration of each node, since RFC2464 effectively disallows configuration of jumbo frames through router advertisements. Probably OK, though, since most jumbo frame subnets today are manually configured anyway. However, the RFC should not discourage jumbo frames. -------------------------------------------------------------------- IETF IPv6 working group mailing list ipv6@ietf.org Administrative Requests: https://www1.ietf.org/mailman/listinfo/ipv6 --------------------------------------------------------------------
