Date: Mon, 07 Aug 2000 17:32:14 -0500 From: Bob Van Valzah <Bob@WhiteBarn.Com> To: Garrett Wollman <wollman@khavrinen.lcs.mit.edu> Cc: freebsd-net@FreeBSD.ORG Subject: Re: VLAN MTU? 1500? 1504? Why? Message-ID: <398F38ED.776E84E2@WhiteBarn.Com> References: <398A3549.902A31F5@WhiteBarn.Com> <200008040434.AAA41523@whizzo.transsys.com> <398AB99C.9D5938B9@WhiteBarn.Com> <200008041446.KAA27928@khavrinen.lcs.mit.edu>
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Ok, it looks to me like some code is needed that doesn't exist. I'd be glad to help make it happen if I had a sample. Can anybody help get me started? Here's the state of the world as I understand it based on comments here and my own experimentation. Please set me straight if I'm off base. 1) A 1500-byte IPv4 MTU on an Ethernet VLAN is desirable. Things sometimes things break over VLAN interfaces that only support 1496-byte IPv4 MTUs. 2) 4.1-RELEASE contains a working vlan pseudo device driver, but only the ti driver is capable of running a 1500-byte IPv4 MTUs with it "as shipped." 3) A patch is available (http://www.euitt.upm.es/~pjlobo/fxp-mtu-patch-4.x) for the fxp driver of 4.0-RELEASE that allows a 1500-byte IPv4 MTU. However, this patch doesn't "negotiate" with the physical device--it just assumes that all physical devices can handle the larger link-layer headers. 4) It is possible (see below) for such negotiation to take place. I haven't been able to find an example of it yet. If somebody could provide me with a sample of this negotiation, I'd be happy to port it to the fxp and all the other capable cards I have around here. Bob Garrett Wollman wrote: > <<On Fri, 04 Aug 2000 07:39:56 -0500, Bob Van Valzah <Bob@WhiteBarn.Com> said: > > > Is there any existing mechanism for "negotiation" of MTU between the > > physical layer and virtual layer? > > Yes. In FreeBSD's implementation, the network interface driver > indicates its ability to handle 802.1p-enacapsulated frames (as used > in 802.1Q) by advertising a header length of 18 rather than the > standard (no encapsulation) 14 octets. (The current implementation > does not, however, have a mechanism for rejecting 1518-byte > *unencapsulated* packets, as should officially be done. I don't think > this is a serious problem.) > > -GAWollman > > -- > Garrett A. Wollman | O Siem / We are all family / O Siem / We're all the same > wollman@lcs.mit.edu | O Siem / The fires of freedom > Opinions not those of| Dance in the burning flame > MIT, LCS, CRS, or NSA| - Susan Aglukark and Chad Irschick To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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