Jumbo frame Question

Douglas Otis dotis at mail-abuse.org
Mon Nov 29 17:21:33 CST 2010


On 11/29/10 1:18 PM, Jack Bates wrote:
>  On 11/29/2010 1:10 PM, John Kristoff wrote:
> > In a nutshell, as I recall, one of the prime motivating factors for
> > not standardizing jumbos was interoperability issues with the
> > installed base, which penalizes other parts of the network (e.g.
> > routers having to perform fragmentation) for the benefit of a
> > select few (e.g. modern server to server comms).
>
>  Given that IPv6 doesn't support routers performing fragmentation, and
>  many packets are sent df-bit anyways, standardized jumbos would be
>  nice. Just because the Internet as a whole may not support them, and
>  ethernet cards themselves may not exceed 1500 by default, doesn't
>  mean that a standard should be written for those instances where
>  jumbo frames would be desired.
>
>  Let's be honestly, there are huge implementations of baby giants out
>  there. Verizon for one requires 1600 byte support for cell towers
>  (tested at 1600 bytes for them, so slightly larger for transport gear
>  depending on what is wrappers are placed over that). None of this
>  indicates larger than 1500 byte IP, but it does indicate larger L2
>  MTU.
>
>  There are many in-house setups which use jumbo frames, and having a
>  standard for interoperability of those devices would be welcome. I'd
>  personally love to see standards across the board for MTU from
>  logical to physical supporting even tiered MTU with future proof
>  overheads for vlans, mpls, ppp, intermixed in a large number of ways
>  and layers (IP MTU support for X sizes, overhead support for Y
>  sizes).

The level of undetected errors by TCP or UDP checksums can be high.  The 
summation scheme is remarkably vulnerable to bus related bit errors, 
where as much as 2% of parallel bus related bit errors might go 
undetected.   Use of SCTP, TLS, or IPSEC can supplant weak TCP/UDP 
summation error detection schemes.   While Jumbo frames reduce serial 
error detection rates of the IEEE CRC restored by SCTP/CRC32c for Jumbo 
frames, serial detection is less of a concern when compared to bus 
related bit error detection rates.  CRC32c solves both the bus and Jumbo 
frame error detection and is found in 10GB/s NICs and math coprocessors.

-Doug




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