[RAM] Tunnelling & GigE jumbo frame size
RJ Atkinson <rja@extremenetworks.com> Wed, 12 September 2007 14:24 UTC
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From: RJ Atkinson <rja@extremenetworks.com>
Date: Wed, 12 Sep 2007 10:24:52 -0400
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Subject: [RAM] Tunnelling & GigE jumbo frame size
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Dino's notes all sound correct, with one exception. The exception is a detail and does not alter his main point, which is that by using Jumbo Ethernet, tunnelling of 1518 byte Ethernet frames ought not create any fragmentation issues. However, just to be crystal clear to anyone who isn't tracking Ethernet closely, here is the summary analysis. Correction: Just as "4K" MTU really means 4470 bytes, Jumbo Ethernet sizes of "9K" really mean an IP MTU of 9180 bytes. This means the link MTU for Jumbo Ethernet is really: (9180 + sizeof(Ethernet header) + sizeof(VLAN tag)) Implementers who want to be more widely deployable will have a "9K" link MTU that allows for at least 2 VLAN tags (considering the widespread use of Q-in-Q encapsulation) or 1 VLAN tag plus 2 Ethernet MAC headers (for MAC-in-MAC encapsulation). Both Q-in-Q and MAC-in-MAC encapsulations are commonly used in Metro Service Provider deployments of Ethernet (e.g. in Japan, Korea, Sweden, and in the US). History: The Jumbo Ethernet 9K frame size was demanded by Ethernet users who were transitioning away from ATM/AAL5. So they wanted Jumbo Ethernet to support existing IP packet lengths for IP/ATM AAL5 [see RFC-1626] *without* any risk of IP fragmentation. Caveats: Jumbo Ethernet is explicitly not conformant to IEEE 802.3 standards. So all implementations that I'm aware of have a default MTU (i.e. out of the box) that is IEEE 802.3 compliant. Normally, users must explicitly configure the larger MTU size on each interface before it is enabled. 1 GigE standards permit deployment with CSMA/CD, although I am not personally aware of any such deployments. A GigE CSMA/CD deployment is likely not to work properly with an Ethernet frame size above that permitted by IEEE 802.3 standards. Switched Ethernet should be fine, however, and all actual 1 GigE deployments that I am aware of are switched and do not use CSMA/CD. Yours, Ran rja@extremenetworks.com _______________________________________________ RAM mailing list RAM@iab.org https://www1.ietf.org/mailman/listinfo/ram
- [RAM] Tunnelling & GigE jumbo frame size RJ Atkinson
- Re: [RAM] Tunnelling & GigE jumbo frame size Iljitsch van Beijnum
- Re: [RAM] Tunnelling & GigE jumbo frame size RJ Atkinson
- Re: [RAM] Tunnelling & GigE jumbo frame size Iljitsch van Beijnum
- Re: [RAM] Tunnelling & GigE jumbo frame size Dino Farinacci
- Re: [RAM] Tunnelling & GigE jumbo frame size RJ Atkinson
- Re: [RAM] Tunnelling & GigE jumbo frame size Iljitsch van Beijnum
- Re: [RAM] Tunnelling & GigE jumbo frame size Dino Farinacci
- Re: [RAM] Tunnelling & GigE jumbo frame size Gert Doering
- Re: [RAM] Tunnelling & GigE jumbo frame size RJ Atkinson
- Re: [RAM] Tunnelling & GigE jumbo frame size Gert Doering