Ethernet packet size and QinQ standard?


A normative question:
AS we all know, ethernet has a max packet size of 1514 bytes:
14 bytes header (dst + src + len/type), plus 1500 bytes data.
For 802.1Q tagging, the spec was changed to allow a max packet size
of 1518 bytes: 1514 bytes, plus 4 bytes for the .1Q type/tag/prio fields.
For QinQ tagging, has the base spec been changed again,
now to a size of 1522 bytes, to allow for the double .1Q fields?
Are there any plans to go beyond this?
I'm kind of surprized because some hardware does have the packet size
hard-coded in, and while a careful study was done when the first
packet size change was made, changing this again and again may cause
trouble and I'd like to understand the rationale behind this.
(and no, jumbograms isn't the answer - suppose you have hardware which
is capable of 9kB packets, the same hardware may not be capable to carry
9kB+2bytes needed for .1Q, or 9kB+4bytes for QinQ protocols)
Thanks,
Geert Jan
Reply to
Geert Jan de Groot
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The relevant standard is 802.1ad, aka "Amendment to IEEE 802.1Q-2005. IEEE Standard for Local and Metropolitan Area Networks?Virtual Bridged Local Area Networks?Revision?Amendment 4: Provider Bridges"
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However, the maximum ethernet packet size was not changed from 1518 so it is currently out of spec to provide an end-user MTU of 1500 using Q-in-Q (as I understand it).
Work was started on 802.3as to expand the maximum frame size to 2000 bytes (support for 802.1ad being one of the objectives) but it seems to have stalled. Given the vast install-base of ethernet equipment it seems unlikely such a change would be worthwhile.
Reply to
Chris Hills
there is probably a difference between the minimum constraint the standards require and the stuff deployed in some networks.
However adding more 4 byte "wrappers" in an Ethernet frame is becoming common - at least in a carrier environment.
every MPLS label adds 4 bytes, so for a typical carrier MPLS VPN service you need 2 levels, with more for various add ons such as fast reroute.
At work the switches are set to allow 3 or 4 levels of MPLS label - and they flow through the L2 Ethernet switches that glue everything together OK (Cisco with non default max size set).
Also the Ethernet access tails deployed for MPLS use Q in Q. 801.q customer traffic is sent across a "multi service" bearer, where an extra layer of 802.1q tags are used to segregate different services from each other + management.
then there is all the malarky about large packet - where the actual maximum sze depends on the vendor + equipment + blade type....
Reply to
Stephen
no it hasnt stalled, 802.3as was ratified. I am pretty sure you'll find it included in 802.3-2008.
The frame length was increased to 2000 but the payload remains the same at 1500. The extra is to allow for every sort of encapsulation that may be needed in the future
Reply to
Marris
The more things change, the more they stay the same.
The original 1500 byte Ethernet frame was *intended* to carry 1K bytes of payload. The additional 476 bytes was for "every sort of encapsulation that may be needed in the future." Of course, folks took the entire field for the payload, and so 28 years later we had to expand the frame for the encapsulation.
Let's see if we have to do it again in 2036.
-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX
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Reply to
Rich Seifert

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