computer networking A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...

, the maximum transmission unit (MTU) is the size of the largest

protocol data unit In telecommunications, a protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network. It is composed of protocol-specific control information and user data. In the layered architectures of ...

(PDU) that can be communicated in a single

network layer In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers. Functions The network layer provides the means of tran ...

transaction. The MTU relates to, but is not identical to the maximum frame size that can be transported on the

data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...

, e.g.

Ethernet frame In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a data unit on an Ethernet link transports an Ethernet frame as its payload ...

. Larger MTU is associated with reduced overhead. Smaller MTU values can reduce

network delay Network delay is a design and performance characteristic of a telecommunications network. It specifies the latency for a bit of data to travel across the network from one communication endpoint to another. It is typically measured in multiples ...

. In many cases, MTU is dependent on underlying network capabilities and must be adjusted manually or automatically so as to not exceed these capabilities. MTU parameters may appear in association with a communications interface or standard. Some systems may decide MTU at connect time, e.g. using Path MTU Discovery.

Applicability

MTUs apply to

communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synch ...

s and

s. The MTU is specified in terms of

byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...

s or

octets Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 compo ...

of the largest PDU that the layer can pass onwards. MTU parameters usually appear in association with a communications interface (

NIC NIC may refer to: Banking and insurance companies * National Insurance Corporation, Uganda * NIC Bank, a commercial bank in Kenya Politics, government and economics * National Ice Center, an agency that provides worldwide navigational ice a ...

serial port In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel. ...

, etc.). Standards (

Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1 ...

, for example) can fix the size of an MTU; or systems (such as point-to-point serial links) may decide MTU at connect time. Underlying

data link A data link is the means of connecting one location to another for the purpose of transmitting and receiving digital information (data communication). It can also refer to a set of electronics assemblies, consisting of a transmitter and a recei ...

and

physical layer In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip. The ...

s usually add overhead to the network layer data to be transported, so for a given maximum frame size of a medium, one needs to subtract the amount of overhead to calculate that medium's MTU. For example, with Ethernet, the maximum frame size is 1518 bytes, 18 bytes of which are overhead ( header and

frame check sequence A frame check sequence (FCS) is an error-detecting code added to a frame in a communication protocol. Frames are used to send payload data from a source to a destination. Purpose All frames and the bits, bytes, and fields contained within ...

), resulting in an MTU of 1500 bytes.

Tradeoffs

A larger MTU brings greater efficiency because each

network packet In telecommunications and computer networking, a network packet is a formatted unit of data carried by a packet-switched network. A packet consists of control information and user data; the latter is also known as the ''payload''. Control inform ...

carries more user data while protocol overheads, such as headers or underlying per-packet delays, remain fixed; the resulting higher efficiency means an improvement in bulk protocol throughput. A larger MTU also requires processing of fewer packets for the same amount of data. In some systems, per-packet-processing can be a critical performance limitation. However, this gain is not without a downside. Large packets occupy a slow link for more time than a smaller packet, causing greater delays to subsequent packets, and increasing

and delay variation. For example, a 1500-byte packet, the largest allowed by Ethernet at the network layer, ties up a 14.4k modem for about one second. Large packets are also problematic in the presence of communications errors. If no

forward error correction In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for controlling errors in data over unreliable or noisy communication channels. The central idea i ...

is used, corruption of a single bit in a packet requires that the entire packet be retransmitted, which can be costly. At a given

bit error rate In digital transmission, the number of bit errors is the number of received bits of a data stream over a communication channel that have been altered due to noise, interference, distortion or bit synchronization errors. The bit error rate (BER) ...

, larger packets are more susceptible to corruption. Their greater payload makes retransmissions of larger packets take longer. Despite the negative effects on retransmission duration, large packets can still have a net positive effect on end-to-end TCP performance.

Internet protocol

The

Internet protocol suite The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the sui ...

was designed to work over many different networking technologies, each of which may use packets of different sizes. While a host will know the MTU of its own interface and possibly that of its peers (from initial handshakes), it will not initially know the lowest MTU in a chain of links to other peers. Another potential problem is that higher-level protocols may create packets larger than even the local link supports. IPv4 allows fragmentation which divides the

datagram A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The deliv ...

into pieces, each small enough to accommodate a specified MTU limitation. This fragmentation process takes place at the

internet layer The internet layer is a group of internetworking methods, protocols, and specifications in the Internet protocol suite that are used to transport network packets from the originating host across network boundaries; if necessary, to the destinat ...

. The fragmented packets are marked so that the IP layer of the destination host knows it should reassemble the packets into the original datagram. All fragments of a packet must arrive for the packet to be considered received. If the network drops any fragment, the entire packet is lost. When the number of packets that must be fragmented or the number of fragments is great, fragmentation can cause unreasonable or unnecessary overhead. For example, various tunneling situations may exceed the MTU by very little as they add just a header's worth of data. The addition is small, but each packet now has to be sent in two fragments, the second of which carries very little payload. The same amount of payload is being moved, but every intermediate router has to forward twice as many packets. The Internet Protocol requires that hosts must be able to process IP datagrams of at least 576 bytes (for IPv4) or 1280 bytes (for IPv6). However, this does not preclude link layers with an MTU smaller than this minimum MTU from conveying IP data. For example, according to IPv6's specification, if a particular link layer cannot deliver an IP datagram of 1280 bytes in a single frame, then the link layer must provide its own fragmentation and reassembly mechanism, separate from the IP fragmentation mechanism, to ensure that a 1280-byte IP datagram can be delivered, intact, to the IP layer.

MTUs for common media

In the context of

Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. ...

, MTU refers to the maximum size of an IP packet that can be transmitted without fragmentation over a given medium. The size of an IP packet includes IP headers but excludes headers from the link layer. In the case of an

this adds an overhead of 18 bytes, or 22 bytes with an IEEE 802.1Q tag for VLAN tagging or

class of service Class of service (COS or CoS) is a parameter used in data and voice protocols to differentiate the types of payloads contained in the packet being transmitted. The objective of such differentiation is generally associated with assigning prioritie ...

. The MTU should not be confused with the minimum datagram size that all hosts must be prepared to accept. This is 576 bytes for

IPv4 Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version d ...

and of 1280 bytes for

IPv6 Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. I ...

Ethernet maximum frame size

The IP MTU and Ethernet maximum frame size are configured separately. In Ethernet switch configuration, MTU may refer to Ethernet maximum frame size. In Ethernet-based routers, MTU normally refers to the IP MTU. If jumbo frames are allowed in a network, the IP MTU should also be adjusted upwards to take advantage of this. Since the IP packet is carried by an Ethernet frame, the Ethernet frame has to be larger than the IP packet. With the normal untagged Ethernet frame overhead of 18 bytes, the Ethernet maximum frame size is 1518 bytes. If a 1500 byte IP packet is to be carried over a tagged Ethernet connection, the Ethernet frame maximum size needs to be 1522 due to the larger size of an 802.1Q tagged frame. 802.3ac increases the standard Ethernet maximum frame size to accommodate this.

Path MTU Discovery

The Internet Protocol defines the ''path MTU'' of an Internet transmission path as the smallest MTU supported by any of the hops on the path between a source and destination. Put another way, the path MTU is the largest packet size that can traverse this path without suffering fragmentation. ''Path MTU Discovery'' is a technique for determining the path MTU between two IP hosts, defined for both

and

. It works by sending packets with the DF (don't fragment) option in the IP header set. Any device along the path whose MTU is smaller than the packet will drop such packets and send back an ICMP Destination Unreachable (Datagram Too Big) message which indicates its MTU. This information allows the source host to reduce its assumed path MTU appropriately. The process repeats until the MTU becomes small enough to traverse the entire path without fragmentation. Standard Ethernet supports an MTU of 1500 bytes and Ethernet implementation supporting jumbo frames, allow for an MTU up to 9000 bytes. However, border protocols like

PPPoE The Point-to-Point Protocol over Ethernet (PPPoE) is a network protocol for encapsulating Point-to-Point Protocol (PPP) frames inside Ethernet frames. It appeared in 1999, in the context of the boom of DSL as the solution for tunneling pack ...

will reduce this. Path MTU Discovery exposes the difference between the MTU seen by Ethernet end-nodes and the Path MTU. Unfortunately, increasing numbers of networks drop ICMP traffic (for example, to prevent

denial-of-service attack In computing, a denial-of-service attack (DoS attack) is a cyber-attack in which the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host conne ...

s), which prevents path MTU discovery from working. ''Packetization Layer Path MTU Discovery'' is a Path MTU Discovery technique which responds more robustly to ICMP filtering. In an IP network, the path from the source address to the destination address may change in response to various events ( load-balancing, congestion, outages, etc.) and this could result in the path MTU changing (sometimes repeatedly) during a transmission, which may introduce further packet drops before the host finds a new reliable MTU. A failure of Path MTU Discovery carries the possible result of making some sites behind badly configured

firewall Firewall may refer to: * Firewall (computing), a technological barrier designed to prevent unauthorized or unwanted communications between computer networks or hosts * Firewall (construction), a barrier inside a building, designed to limit the spre ...

s unreachable. A connection with mismatched MTU may work for low-volume data but fail as soon as a host sends a large block of data. For example, with

Internet Relay Chat Internet Relay Chat (IRC) is a text-based chat system for instant messaging. IRC is designed for group communication in discussion forums, called '' channels'', but also allows one-on-one communication via private messages as well as chat an ...

a connecting client might see the initial messages up to and including the initial ping (sent by the server as an anti-spoofing measure), but get no response after that. This is because the large set of welcome messages sent at that point are packets that exceed the path MTU. 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 The maximum segment size (MSS) is a parameter of the ''options'' field of the TCP header that specifies the largest amount of data, specified in bytes, that a computer or communications device can receive in a single TCP segment. It does not count ...

) in the initial packet that sets up the TCP connection at one's firewall.

In other contexts

MTU is sometimes used to describe the maximum PDU sizes in communication layers other than the network layer. *

Cisco Systems Cisco Systems, Inc., commonly known as Cisco, is an American-based multinational digital communications technology conglomerate corporation headquartered in San Jose, California. Cisco develops, manufactures, and sells networking hardware, ...

and

MikroTik MikroTik (officially SIA "Mikrotīkls") is a Latvian network equipment manufacturer. The company develops and sells wired and wireless network routers, network switches, access points, as well as operating systems and auxiliary software. The com ...

use ''L2 MTU'' for the maximum frame size. *

Dell Dell is an American based technology company. It develops, sells, repairs, and supports computers and related products and services. Dell is owned by its parent company, Dell Technologies. Dell sells personal computers (PCs), servers, data ...

/ Force10 use ''MTU'' for the maximum frame size. *

Hewlett Packard The Hewlett-Packard Company, commonly shortened to Hewlett-Packard ( ) or HP, was an American multinational information technology company headquartered in Palo Alto, California. HP developed and provided a wide variety of hardware components ...

used just ''MTU'' for the maximum frame size including the optional IEEE 802.1Q tag. *

Juniper Networks Juniper Networks, Inc. is an American multinational corporation headquartered in Sunnyvale, California. The company develops and markets networking products, including routers, switches, network management software, network security product ...

use several MTU terms: ''Physical Interface MTU'' (L3 MTU plus some unspecified protocol overhead), ''Logical Interface MTU'' (consistent with IETF MTU) and ''Maximum MTU'' (maximum configurable frame size for jumbo frames). The transmission of a packet on a physical network segment that is larger than the segment's MTU is known as jabber. This is almost always caused by faulty devices.

Network switch A network switch (also called switching hub, bridging hub, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destination device. A ...

es and some repeater hubs have a built-in capability to detect when a device is jabbering.IEEE 802.3 ''27.3.1.7 Receive jabber functional requirements''

References

External links

*
Tweaking your MTU / RWin for Orange Broadband Usersmturoute
nbsp;– a console utility for debugging mtu problems {{DEFAULTSORT:Maximum Transmission Unit Packets (information technology)