The Maximum Transmission Unit (also abbreviated as MTU) is the largest physical packet size, measured in bytes, that a network can transmit (Webopedia). The Internets 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).
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 ableInternet IPv4 Path MTU At least 68 to handle, which is 576. Practical path MTUs are generally higher. Systems must useInternet IPv6 Path MTU At least 1280 Path MTU Discovery to find the actual path MTU. Nearly all IP over Ethernet implementations use the EthernetEthernet 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. JumboEthernet Jumbo Frames 1500-9000 frames are usually only seen in special purpose networks.WLAN (802.11) 2272Token Ring (802.5) 4464FDDI 4352 Source: Wikipedia
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 useEthernet V2 in the examples.
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 networks 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
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 Dont 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
Wrong MTU Setting – Too High**Note the results above indicate that the packet needs to be fragmented. Lower the sizethe packet in increments of +/-10 (e.g. 1472, 1462, 1440, 1400) until you have a packetsize that does not fragment.Correct MTU Setting Source: Expedient
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.
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)
Note: MTU settings in theWindows registry editor arestored in Hexadecimal.
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
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
Set the Path MTU Discovery to “No”, more on that later.•Make sure your ethernet adapter is selected in the drop down box, andthen take the size you determined is optimal from your ping tests and enterit into the MaxMTU field.•Click apply, then exit.•The machine will need to be rebooted for the setting to becorrectly applied.
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 youre 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.
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 wont 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
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
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
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 Dont 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 . 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Website URL Author (if applicable) Notes http://compnetworking.about.com/od/networkprotocols/g/mtu-About.com maximum.htm Bradley Mitchell Too Low/Too High informationWebopedia http://www.webopedia.com/TERM/M/MTU.html DefinitionWikipedia http://en.wikipedia.org/wiki/Maximum_transmission_unit Only used for chart on slide 3Expedient http://help.stargate.net/broadband/mtu_ping_test.shtml MTU Testing informationMicrosoft Ping http://www.microsoft.com/resources/documentation/windows/xParameters p/all/proddocs/en-us/ping.mspx?mfr=trueMicrosoft MTUAdjustment http://support.microsoft.com/kb/826159DSL Reports http://www.dslreports.com/drtcp Dr. TCP informationDownload.com http://download.cnet.com/Dr-TCP/3000-18487_4-68761.html Tolunay Orkun Dr. TCP Author information None Listed, Netheaven Support, support@NetHeaven.coNetheaven http://www.netheaven.com/pmtu.html m PathMTU Discovery information PPPoE and MTU issueADSL CUTW http://adsl.cutw.net/mtu.html Fenn Bailey information J. Mogul & S. DeeringRFC 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