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In computer networking, the term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest datagram that a given layer of a communications protocol can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). A computer network is a system for communication among two or more computers. ...
A byte is commonly used as a unit of storage measurement in computers, regardless of the type of data being stored. ...
A packet is the fundamental unit of information carriage in all modern computer networks. ...
This article concerns communication between pairs of electronic devices. ...
A transitional network card with both BNC Thinnet (left) and Twisted pair (right) connectors. ...
A male DE-9 serial port on the rear panel of a PC. In computing, a serial port is an interface on a computer system with which information is transferred in or out one bit at a time (contrast parallel port). ...
Theory
The Internet Protocol allows IP fragmentation, allowing a datagram to sub-divide into pieces, each small enough to pass over a link with a smaller MTU than the original datagram size. This fragmentation takes place at the IP layer (OSI layer 3) and uses the MTU parameter of the network interface that will send the packet out onto the link. The fragmentation process marks the "fragments" of the original packet so that the IP layer of the destination host can reassemble the packets into the original datagram. The Internet Protocol (IP) is a data-oriented protocol used by source and destination hosts for communicating data across a packet-switched internetwork. ...
The Internet Protocol allows IP fragmentation so that datagrams can be fragmented into pieces small enough to pass over a link with a smaller MTU than the original datagram size. ...
A packet is the fundamental unit of information carriage in all modern computer networks that use packet switching. ...
The Internet Protocol defines the "path MTU" of an Internet transmission path as the smallest MTU of any of the IP hops of the "path" from the source address to the destination address. Or, looked at another way, the "path MTU" defines the largest MTU value that can traverse the "path" without the packets sufferring further fragmentation. In telecommunication, the term hop has the following meanings: 1. ...
Look up path in Wiktionary, the free dictionary. ...
RFC 1191 describes "Path MTU discovery", a technique for determining the path MTU between two IP hosts with a view to avoiding IP fragmentation. This technique makes use of feedback to the source address of the results of sending gradually increasing datagram sizes, with the DF (Don't Fragment) bit set - any device along the path that needs to fragment the packet will drop the packet instead, and send an ICMP "datagram too big" response back to the source address. Through this probing, the source host "learns" the largest MTU that can pass that path without fragmenting. The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet protocol suite. ...
Potential problems and troubleshooting Unfortunately, increasing numbers of networks block all ICMP traffic (e.g. to prevent denial-of-service attacks) - this prevents path MTU discovery from working. One often detects such blocking in the cases where a connection works for low-volume data but hangs as soon as a host sends a large block of data at a time (for example, with irc a client might see up to the nospoof ping but get no response after that, as the large set of welcome messages freezes up the connection). Also, in an Internet Protocol network, the "path" from the source address to the destination address often gets modified dynamically, in response to various events ( load-balancing, congestion, outages, etc.) - this could result in the path MTU changing (sometimes repeatedly) during a transmission, which may introduce further packet drops before the host finds the new safe MTU. A denial-of-service attack (also, DoS attack) is an attack on a computer system or network that causes a loss of service to users, typically the loss of network connectivity and services by consuming the bandwidth of the victim network or overloading the computational resources of the victim system. ...
Internet Relay Chat (IRC) is a form of instant communication over the Internet. ...
In computing, a client is a system that accesses a (remote) service on another computer by some kind of network. ...
ping in a Windows 2000 command window Ping is the name of a computer network tool used on TCP/IP networks (such as the Internet). ...
In computing, load balancing is a technique used to spread work between many processes, computers, disks or other resources. ...
Congestion is a state of excessive accumulation or overfilling or overcrowding. ...
In telecommunications, an outage is a telecommunications system service condition in which a user is completely deprived of service by the system. ...
Most modern ethernet LANs use an MTU of 1500 bytes. But systems like PPPoE will reduce this, causing path MTU discovery to come into effect with the possible effect of making some sites behind badly-configured firewalls unreachable. One can possibly work around this, depending on which part of the network one controls; for example one can change the MSS (maximum segment size) in the initial packet that sets up the TCP connection at one's firewall. It has been suggested that IEEE 802. ...
A local area network (LAN) is a computer network covering a small local area, like a home, office, or small group of buildings such as a college. ...
This article incorporates text translated from the corresponding German Wikipedia article as of December 2005 PPPoE, point-to-point protocol over Ethernet, is a network protocol for encapsulating PPP frames in Ethernet frames. ...
In computing, a firewall is a piece of hardware and/or software which functions in a networked environment to prevent some communications forbidden by the security policy, analogous to the function of firewalls in building construction. ...
The maximum segment size (MSS) is the largest amount of data, specified in bytes, that a computer or communications device can handle in a single, unfragmented piece. ...
The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite. ...
To do so in a linux router: iptables -I FORWARD -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
Some IBM systems (the XSeries, for example) which support the older ethernet protocols might require setting the MTU value to 1492 in order to operate on a typical modern LAN. International Business Machines Corporation (IBM, or colloquially, Big Blue) NYSE: IBM (incorporated June 15, 1911, in operation since 1888) is headquartered in Armonk, NY, USA. The company manufactures and sells computer hardware, software, infrastructure services and consulting services. ...
The IBM xSeries computers form a sub-brand of International Business Machines (IBM)s eServer brand servers (the other eServer sub-brands having the names iSeries, pSeries, and zSeries). ...
It has been suggested that IEEE 802. ...
A local area network (LAN) is a computer network covering a small local area, like a home, office, or small group of buildings such as a college. ...
One can set the MTU by using (for example) the ifconfig command. The Unix command ifconfig is a tool used to configure a network interface for TCP/IP. It was originally released as part of the BSD TCP/IP suite - so in effect, its part of the original internet toolkit. ...
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