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In computer networking, the term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest packet 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.). The MTU may be fixed by standards (as is the case with Ethernet) or decided at connect time (as is usually the case with point-to-point serial links). A higher MTU brings higher bandwidth efficiency. However large packets can block up a slow interface for some time, increasing the lag on other packets. For example a 1500 byte packet, the largest allowed by Ethernet (and hence most of the Internet), would block up a 14.4k modem for about one second. This article or section is in need of attention from an expert on the subject. ...
In computer science a byte is a ubiquitous unit of storage measurement. ...
A packet is a formatted block of information carried by a computer network. ...
It has been suggested that this article or section be merged with protocol (computing). ...
A transitional network card with both BNC Thinnet (left) and Twisted pair (right) connectors. ...
A male DE-9 connector used for a serial port on a PC style computer. ...
Ethernet is a large and diverse family of frame-based computer networking technologies for local area networks (LANs). ...
This article or section is in need of attention from an expert on the subject. ...
IP (Internet Protocol)
The Internet protocol suite was designed to work over many networking technologies. While a host will know the MTU of its own interface and possibly that of its peer (from initial handshakes) it will not initially know what the lowest MTU on the link is and in any case a higher protocol design may require packets of a size larger than the network links support. It has been suggested that Internet Protocols be merged into this article or section. ...
To get around this issue IP performs fragmentation, sub-dividing the datagram 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. This method however has a number of drawbacks. 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 a formatted block of information carried by a computer network. ...
- When the link MTU is just below the packet size (as is common in tunneling situations), a small fragment will be removed from the end of the packet. So any reduction in MTU size means a doubling in the overhead from IP headers and any lower-layer headers, trailers or inter-packet delays. The number of routing decisions that routers must make is also doubled.
- As it is normal to make all fragments except the last as big as possible, further fragmentation can easily occur later on in the transmission, increasing the overhead even more.
- All fragments of a packet must arrive for the higher level protocol to get the packet. Therefore if any fragment is dropped, the entire packet must be resent.
Path MTU discovery 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 suffering further fragmentation. In telecommunication, the term hop has the following meanings: 1. ...
RFC 1191 describes "Path MTU discovery", a technique for determining the path MTU between two IP hosts with a view to avoiding IP fragmentation. Path MTU discovery works by setting the DF (Don't Fragment) option in the IP headers of outgoing packets—any device along the path whose MTU is smaller than the packet will drop it, and send back an ICMP "Destination Unreachable (Datagram Too Big)" message containing its MTU, allowing the source host to reduce its assumed path MTU appropriately. The process repeats until the MTU is small enough to traverse the entire path without fragmentation. The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet protocol suite. ...
The Destination Unreachable message is an ICMP message which is generated by the router to inform the client that the destination host is unreachable. ...
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. In networking, black holes refer to places in the network where incoming traffic is silently discarded (or dropped), without informing the source that the data did not reach its intended recipient. ...
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 real-time Internet chat or synchronous conferencing. ...
In computing, a client is a system that accesses a (remote) service on another computer by some kind of network. ...
Load balancing refers to the general practice of balancing a load. ...
In data networking and queueing theory, network congestion occurs when incremental increases in offered load lead either only to small increases in network throughput, or to an actual reduction in network throughput. ...
Definition An interruption in availability of the system due to the communication failure of the network. ...
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. Ethernet is a large and diverse family of frame-based computer networking technologies for local area networks (LANs). ...
Local area network scheme A local area network (LAN) is a computer network covering a local area, like a home, office, or group of buildings[1]. Current LANs are most likely to be based on switched IEEE 802. ...
PPPoE, point-to-point protocol over Ethernet, is a network protocol for encapsulating PPP frames in Ethernet frames. ...
It has been suggested that network layer firewall be merged into this article or section. ...
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 a virtual circuit protocol that is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data. ...
ATM Backbones Some providers, particularly those with a telephony background, use the ATM (Asynchronous Transfer Mode) protocol on their internal backbone network. That protocol uses 53 byte packets, 5 of which constitute a header, leaving 48 bytes payload for each packet. There are some recommendations to lower the MTU to 1488 or fewer bytes to accommodate ATM backbones better. The rationale behind this is that 1488 = 48 * 31, to avoid having a 32nd ATM packet only carrying a 4 byte payload and therefore you save some 3% bandwidth on the backbone and get 3% more throughput when the backbone is congested. Asynchronous Transfer Mode (ATM) is a cell relay, network and data link layer protocol which encodes data traffic into small (53 bytes; 48 bytes of data and 5 bytes of header information) fixed-sized cells. ...
However, this is a myth. The calculation ignores several additional overhead components:
yielding a total overhead of 40 or 44 bytes. With an MTU of 1492 bytes, the grand total is 1536 bytes, and this is exactly 32 ATM cells of 48 byte payload each. If the FCS is not present, the last of these 32 ATM cells is 4 bytes short of being completely filled, which is practically insignificant. A frame check sequence (FCS) refers to the extra checksum characters added to a Frame in a communication protocol for error detection and correction. ...
Ethernet is a large and diverse family of frame-based computer networking technologies for local area networks (LANs). ...
Segmentation and Reassembly refers to the process used to divide and reformat data so as to allow it to be transported as the 48 byte payload in multiple ATM cells, followed by its reconstitution at the destination of the ATM Permanent virtual circuit through which it has been carried. ...
// ATM Adaptation Layer 5 Computers use ATM Adaptation Layer 5 (AAL5) to send variable-length packets up to 65,535 octets in size across an ATM network. ...
PPPoE, point-to-point protocol over Ethernet, is a network protocol for encapsulating PPP frames in Ethernet frames. ...
According to the IEEE 802 family of standards, Logical Link Control (LLC) is the upper sublayer of the OSI data link layer. ...
The SubNetwork Access Protocol (SNAP) is a standard for the transmission of IP datagrams over IEEE 802 networks. ...
Therefore the 1488 byte MTU recommendation is usually wrong.
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