PPPoE and MTUs? XOFF, Please.

NETWORK WORLD NEWSLETTER: STEVE TAYLOR AND JIM METZLER ON WIDE
AREA NETWORKING
06/16/05
Today's focus: PPPoE and MTUs? XOFF, Please.

Dear networking.world@gmail.com,

In this issue:

* More reader feedback
* Links related to Wide Area Networking
* Featured reader resource
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Today's focus: PPPoE and MTUs? XOFF, Please.

By Steve Taylor and Jim Metzler

The recent discussion of Point-to-Point Protocol over Ethernet
(PPPoE) and associated issues related to Maximum Transmission
Unit (MTU) size, led to a lot of reader feedback, mostly related
to the wrong acronym translation being inserted. So far as
that's concerned, it's been addressed, and we ask for an XOFF
(which is a decimal 19, hex 13, or "control S" in the ASCII code
set).

In the midst of the responses, though there were a couple of
e-mails that were particularly useful. First of all a couple of
readers pointed out that it's the addition of PPP in addition to
Ethernet framing that caused the problem. And in many ways, the
addition of PPP adds very little. You need some lower-layer
protocol to provide framing around the IP PDUs (protocol data
units, otherwise generically called "packets"), but either PPP -
which is historically used on dial-up links - or Ethernet is
sufficient. PPPoE is a bit redundant, but adding additional
protocol layers that may or may not be needed is not a big issue
for most folks now that we have such an abundance of bandwidth.

One reader did point out "RFC 2516 defines PPPoE and is dated
February 1999. It states: 'Since Ethernet has a maximum payload
size of 1500 octets, the PPPoE header is 6 octets and the PPP
Protocol ID is 2 octets, the PPP MTU MUST NOT be greater than
1492.' So this problem was identified when PPPoE was born.
It's fairly inexcusable that Tier 1 support couldn't provide the
answer and a fix on your first call."

But, as it turns out, the answer is - for a number of reasons -
a bit more complex than simply looking for an MTU size of less
than 1492. And, depending on the number of intermediate protocol
layers that get heaped on your data, as mandated by the
Department of Redundancy Department, your MTU may need to be
considerably smaller than 1492 bytes.

Another reader, Mike Iversen of Martin Marietta Materials, wrote
that " <http://www.speedguide.net/> has been invaluable to me
for helping with MTU issues. They have a utility called
TCPOptomize that will figure out Max MTU and then update you
network card setting with the appropriate value, with no need
for manually editing the registry." He continued "MTU is
important when using a site to site VPN over PPPoE you have to
reduce the MTU again because of the overhead of IPsec etc."

Steve tried the utility and, while he has not tested it
extensively, the test for the recommended MTU size was 1300
bytes - exactly as recommended by the DSL service provider.

(If you're either old enough or you've studied
telecommunications history to have recognized the XOFF
reference, XON (decimal 17, hex 11, or "control Q") for all
other issues.)

RELATED EDITORIAL LINKS

TCP Optimizer
http://www.speedguide.net/downloads.php

Ex-Cisco executives flee Nortel
Network World, 06/13/05
http://www.networkworld.com/news/2005/061305-nortel.html?rl
_______________________________________________________________
To contact: Steve Taylor and Jim Metzler

Steve Taylor is president of Distributed Networking Associates
and publisher/editor-in-chief of Webtorials. For more detailed
information on most of the topics discussed in this newsletter,
connect to Webtorials <http://www.webtorials.com/>, the premier
site for Web-based educational presentations, white papers, and
market research. Taylor can be reached at
<mailto:taylor@webtorials.com>

Jim Metzler is the Vice President of Ashton, Metzler &
Associates, a consulting organization that focuses on leveraging
technology for business success. Jim assists vendors to refine
product strategies, service providers to deploy technologies and
services, and enterprises evolve their network infrastructure.
He can be reached at <mailto:jim@ashtonmetzler.com>
_______________________________________________________________
This newsletter is sponsored by Tacit Networks
Consolidate storage enterprise-wide and save
Slash multi-office storage costs today.

Tacit Networks' WAFS solutions can:
*Consolidate remote storage to the datacenter.
*End server proliferation at branch offices.
*Generate ROI in 9 months or less.
Get the FREE White Paper "Storage Consolidation with WAFS: The
Fast Track to ROI" and Improve Your Net Work(tm).
Click here: http://www.fattail.com/redir/redirect.asp?CID=102286
_______________________________________________________________
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