[132602] in North American Network Operators' Group
Re: Jumbo frame Question
daemon@ATHENA.MIT.EDU (Douglas Otis)
Mon Nov 29 18:24:51 2010
Date: Mon, 29 Nov 2010 15:21:33 -0800
From: Douglas Otis <dotis@mail-abuse.org>
To: nanog@nanog.org
In-Reply-To: <4CF418A0.8030602@brightok.net>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
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
