1. Project by: Michelle Davies
CIS-180

2.  The Maximum Transmission Unit (also
abbreviated as MTU) is the largest physical
packet size, measured in bytes, that a network
can transmit (Webopedia).
 The Internet's Transmission Control Protocol
(TCP) uses the MTU to determine the maximum
size of each packet in any transmission.
 MTU sizes are inherent properties of physical
network interfaces, normally measured in bytes
(About.com).

3. Maximum
Media Transmission Notes
Unit (bytes)
Practical path MTUs are generally higher. IPv4 links must be
able to forward packets of size up to 68 bytes. Systems may use
Path MTU Discovery to find the actual path MTU. This should
not be mistaken with the packet size every host must be able
Internet IPv4 Path MTU At least 68 to handle, which is 576.
Practical path MTUs are generally higher. Systems must use
Internet IPv6 Path MTU At least 1280 Path MTU Discovery to find the actual path MTU.
Nearly all IP over Ethernet implementations use the Ethernet
Ethernet v2 1500 V2 frame format.
Ethernet with LLC and SNAP,
PPPoE 1492 Rarely used
The limit varies by vendor. For correct interoperation, the
whole Ethernet network must have the same MTU. Jumbo
Ethernet Jumbo Frames 1500-9000 frames are usually only seen in special purpose networks.
WLAN (802.11) 2272
Token Ring (802.5) 4464
FDDI 4352
Source: Wikipedia

4. If your MTU is set too high
or too low you will have issues,
it needs to be just right.
The next few slides will use
Ethernet V2 in the examples.

5. How do you determine the correct MTU size?
Too Low Too High
If set too low, streams of If the maximum TCP packet
network traffic will be size is set too high, it will
broken up into a relatively exceed the network's
large number of small physical MTU and also
packets that adversely degrade performance by
affects performance requiring that each packet
(latency). Xbox Live, for be subdivided into smaller
example, requires the value ones (a process known as
of MTU (packet size) by at fragmentation).
least 1365 bytes.
Source: About.com

6.  This can be tested using the ping utility within
the operating system loaded onto your
computer.
 In Windows, Go to Start -> Run -> Type in
“CMD” (w/out quotes) and hit enter. This
populates the DOS prompt.
 Type ping www.domain.com -f -l xxxx (mtu
size) and hit enter.
PING Parameter Definitions:
-f : Specifies that Echo Request messages are sent with the Don't Fragment flag in the IP header set to
1. The Echo Request message cannot be fragmented by routers in the path to the destination. This
parameter is useful for troubleshooting path Maximum Transmission Unit (PMTU) problems.
-l Size : Specifies the length, in bytes, of the Data field in the Echo Request messages sent. The
default is 32. The maximum size is 65,527.
Source: Microsoft.com
Source: Expedient

7. Wrong MTU Setting – Too High
**Note the results above indicate that the packet needs to be fragmented. Lower the size
the packet in increments of +/-10 (e.g. 1472, 1462, 1440, 1400) until you have a packet
size that does not fragment.
Correct MTU Setting
Source: Expedient

8.  Oncethe optimal MTU size has been
determined, the MTU setting will need to be
adjusted within your LAN’s router or locally
on the machine affected within the operating
system.
 Tweaking the MTU: There are a few
different ways to adjust the MTU on a
Windows machine, we will explore the two
most popular in the following slides.

9.  In Windows, Go to Start -> Run -> Type in
“REGEDIT” (w/out quotes) and hit enter.
This populates the Windows Registry Editor.
NOTE: Use extreme caution when making
changes or deleting keys. Not a good idea
for those who are not tech savvy.
 Navigate to:
HKEY_LOCAL_MACHINESYSTEMCurrentCo
ntrolSetServicesNdisWanParameters
through the tree structure (example shown
on next slide)

10. Note: MTU settings in the
Windows registry editor are
stored in Hexadecimal.

11.  Add a Protocols
 subkey (if it does not already exist). To do so:


On the Edit menu, point to New, and then click Key.
Type Protocols, and then press ENTER.
This example changes the
 Add a 0
MTU in Windows to 1500.
 (zero) subkey to the Protocols
 subkey. To do so:
 Click the Protocols
 subkey that you created step 3.
 On the Edit menu, point to New, and then click Key.

 Type 0 (zero), and then press ENTER.
Click the 0
Sound complicated? You
 subkey that you created in step 4. bet it is…but there’s an


On the Edit menu, point to New, and then click DWORD Value.
In the Value data box, type ProtocolType, and then click OK.
easier way…
 On the Edit menu, click Modify.
 In the Value data box, type 800, make sure Hexadecimal is selected under Base, and then click OK.
 On the Edit menu, point to New, and then click DWORD Value.
 Type PPPProtocolType, and then press ENTER.
 On the Edit menu, click Modify.
 In the Value data box, type 21, make sure Hexadecimal is selected under Base, and then click OK.
 On the Edit menu, point to New, and then click DWORD Value.
 Type ProtocolMTU, and then press ENTER.
 On the Edit menu, click Modify.
 Under Base, click Decimal, type the MTU size that you want in the Value data box, and then click OK.
 Quit Registry Editor.
 Restart your computer.
Source: Microsoft

12.  Dr. TCP is a program that can be used in lieu
of using the Windows Registry editor to
tweak the MTU of the local machine which is
a better option for PC novices.
 The application was created by Tolunay
Orkun.
 You can download this utility for free by
visiting either DSLreports.com or
Download.com

13. Set the Path MTU
Discovery to “No”,
more on that later.
•Make sure your ethernet adapter is selected in the drop down box, and
then take the size you determined is optimal from your ping tests and enter
it into the MaxMTU field.
•Click apply, then exit.
•The machine will need to be rebooted for the setting to be
correctly applied.

14.  Steps for Ethernet in Mac OS X 10.5.x
 From the Apple menu, choose System Preferences.
 Click Network.
 In the left sidebar, select "Built-in Ethernet 1".
 If you see a lock icon in the bottom left corner, click the lock icon and enter your
administrator password.
 Click the "Advanced..." button.
 Click the Ethernet tab.
 From the Configure: pop-up menu, choose "Manually".
 From the MTU pop-up menu, choose "Custom".
 Enter your desired setting (72 to 9000). Some ISPs may have a specific suggested
MTU value. If you're changing it without a specific recommendation, make
incremental changes (such as 1500 to 1400 to 1300, and so forth).
 Click OK, then click Apply to save your changes.

16.  RFC (Request for Comment) 1191 located at
http://tools.ietf.org/html/rfc1191 was published in 1990, but
was not widely practiced until the early 2000’s
 The basic procedure is simple - send the largest packet you can,
and if it won't fit through some link get back a notification saying
what size will fit. The notifications arrive as ICMP (Internet
Control Message Protocol) packets known as "fragmentation
needed" ICMPs (ICMP type 3, subtype 4). The notifications are
requested by setting the "do not fragment" (DF) bit in packets
that are sent out.
 Drawback: Some network and system administrators view all
ICMPs as risky and block them all, disabling path MTU discovery,
usually without even realizing it. Of the several dozen ICMP
types and subtypes, some do pose some risk, but the risk is
mostly mild and is of the "denial of service" nature. That is, an
attacker can use them to interfere with service on and from the
network.
Source: Netheaven

17.  If you have a broadband connection that uses the
PPPoE protocol (point to point protocol over
ethernet), you have an even smaller data packet
to work with – 1492 - as PPPoE has 8 bytes of
overhead.
 Websites that have PathMTU discovery turned off
(PNC Bank is an example) will have issues
loading on your machine if you have PathMTU
enabled and the MTU set too high.
 I’ve worked at an ISP that provided DSL service
for 7+ years and as an observation, MTU related
issues were among the most frequently asked
question by DSL users.
Source: Netheaven & http://adsl.cutw.net/mtu.html

18. PNC’s Network You wanted 1500, so
PathMTU
Discovery = Off
that’s what I’m giving
you. Too bad.
Sorry, we can
only accept How much
I want a
1492 ISP using 1500 sized money do I
packet, have?
PPPoE
Hmmm…www.
please.
End Result: pncbank.com
MTU: 1500
Using PPPoE
(1492)
PathMTU
Discovery = On

19.  Always, always, ALWAYS check with your Internet Service Provider to see what their
recommendations are.
 Refer to the RFC for further details:
Router specification
When a router is unable to forward a datagram because it exceeds the
MTU of the next-hop network and its Don't Fragment bit is set, the
router is required to return an ICMP Destination Unreachable message
to the source of the datagram, with the Code indicating
"fragmentation needed and DF set". To support the Path MTU Discovery
technique specified in this memo, the router MUST include the MTU of
that next-hop network in the low-order 16 bits of the ICMP header
field that is labelled "unused" in the ICMP specification [7]. The
high-order 16 bits remain unused, and MUST be set to zero. Thus, the
message has the following format:
0 1 2 3
01234567890123456789012345678901
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Code = 4 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused = 0 | Next-Hop MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + 64 bits of Original Datagram Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

20. Website URL Author (if applicable) Notes
http://compnetworking.about.com/od/networkprotocols/g/mtu-
About.com maximum.htm Bradley Mitchell Too Low/Too High information
Webopedia http://www.webopedia.com/TERM/M/MTU.html Definition
Wikipedia http://en.wikipedia.org/wiki/Maximum_transmission_unit Only used for chart on slide 3
Expedient http://help.stargate.net/broadband/mtu_ping_test.shtml MTU Testing information
Microsoft Ping http://www.microsoft.com/resources/documentation/windows/x
Parameters p/all/proddocs/en-us/ping.mspx?mfr=true
Microsoft MTU
Adjustment http://support.microsoft.com/kb/826159
DSL Reports http://www.dslreports.com/drtcp Dr. TCP information
Download.com http://download.cnet.com/Dr-TCP/3000-18487_4-68761.html Tolunay Orkun Dr. TCP Author information
None Listed, Netheaven
Support,
support@NetHeaven.co
Netheaven http://www.netheaven.com/pmtu.html m PathMTU Discovery information
PPPoE and MTU issue
ADSL CUTW http://adsl.cutw.net/mtu.html Fenn Bailey information
J. Mogul & S. Deering
RFC http://datatracker.ietf.org/doc/rfc1191/ Packet information
http://www.macgurulounge.com/manually-managing-mtu-size-in-
MacGuru Lounge mac-os-x/ "MATT" Adjusting MTU in Mac OS X