Maximum transmission unit

Last updated

In computer networking, the maximum transmission unit (MTU) is the size of the largest protocol data unit (PDU) that can be communicated in a single network layer transaction. [1] The MTU relates to, but is not identical to the maximum frame size that can be transported on the data link layer, e.g., Ethernet frame.

Contents

Larger MTU is associated with reduced overhead. Smaller MTU values can reduce network delay. 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 protocols and network layers. The MTU is specified in terms of bytes or octets of the largest PDU that the layer can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). Standards (Ethernet, 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 and physical layers 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), resulting in an MTU of 1500 bytes.

Tradeoffs

A larger MTU brings greater efficiency because each network packet 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 link for more time than a smaller packet, causing greater delays to subsequent packets, and increasing network delay 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 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, 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. [2]

Internet protocol

The Internet protocol suite 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 into pieces, each small enough to accommodate a specified MTU limitation. This fragmentation process takes place at the internet layer. 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, 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 Ethernet frame this adds a protocol overhead of 18 bytes, or 22 bytes with an IEEE 802.1Q tag for VLAN tagging or class of service.

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 [3] and 1280 bytes for IPv6. [4]

Media for IP transportMaximum transmission unit (bytes)Notes
Internet IPv4 path MTUAt least 68, [5] max of 64 KiB [6] Systems may use Path MTU Discovery [7] to find the actual path MTU. Routing from larger MTU to smaller MTU causes IP fragmentation.
Internet IPv6 path MTUAt least 1280, [8] max of 64 KiB, but up to 4 GiB with optional jumbogram [9] Systems should use Path MTU Discovery [10] to find the actual path MTU, unless the minimum MTU (1280 bytes) is not exceeded.
X.25 Minimal 576 (sending) or 1600 (receiving) [11]
Ethernet v2 1500 [12] Nearly all IP over Ethernet implementations use the Ethernet II frame format.
Ethernet with LLC and SNAP 1492 [13]
Ethernet jumbo frames 1501–9202 [14] or more [15] The limit varies by vendor. For correct interoperation, frames should be no larger than the maximum frame size supported by any device on the network segment. [16]
PPPoE v2 1492 [17] Ethernet II MTU (1500) less PPPoE header (8)
DS-Lite over PPPoE1452Ethernet II MTU (1500) less PPPoE header (8) and IPv6 header (40)
PPPoE jumbo frames1493–9190 or more [18] Ethernet Jumbo Frame MTU (1501–9198) less PPPoE header (8)
IEEE 802.11 Wi-Fi (WLAN)2304 [19] The maximum MSDU size is 2304 before encryption. WEP will add 8 bytes, WPA-TKIP 20 bytes, and WPA2-CCMP 16 bytes. See also Frame aggregation mechanisms in 802.11n.
Token Ring (802.5) 4464
FDDI 4352 [7]

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 and the 1500-byte payload, 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 bytes 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 IPv4 [20] and IPv6 [21] . 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 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 attacks), which prevents path MTU discovery from working. Packetization Layer Path MTU Discovery [22] [23] 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 firewalls 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 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) 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.

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. [29] Network switches and some repeater hubs have a built-in capability to detect when a device is jabbering. [30] [31]

Related Research Articles

The Internet Control Message Protocol (ICMP) is a supporting protocol in the Internet protocol suite. It is used by network devices, including routers, to send error messages and operational information indicating success or failure when communicating with another IP address. For example, an error is indicated when a requested service is not available or that a host or router could not be reached. ICMP differs from transport protocols such as TCP and UDP in that it is not typically used to exchange data between systems, nor is it regularly employed by end-user network applications.

<span class="mw-page-title-main">Internet Protocol version 4</span> Fourth version of the Internet Protocol

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 deployed for production on SATNET in 1982 and on the ARPANET in January 1983. It is still used to route most Internet traffic today, even with the ongoing deployment of Internet Protocol version 6 (IPv6), its successor.

<span class="mw-page-title-main">IPv6</span> Version 6 of the Internet Protocol

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. IPv6 was developed by the Internet Engineering Task Force (IETF) to deal with the long-anticipated problem of IPv4 address exhaustion, and was intended to replace IPv4. In December 1998, IPv6 became a Draft Standard for the IETF, which subsequently ratified it as an Internet Standard on 14 July 2017.

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.

In computer networking, the User Datagram Protocol (UDP) is one of the core communication protocols of the Internet protocol suite used to send messages to other hosts on an Internet Protocol (IP) network. Within an IP network, UDP does not require prior communication to set up communication channels or data paths.

A frame is a digital data transmission unit in computer networking and telecommunication. In packet switched systems, a frame is a simple container for a single network packet. In other telecommunications systems, a frame is a repeating structure supporting time-division multiplexing.

In computing, Internet Protocol Security (IPsec) is a secure network protocol suite that authenticates and encrypts packets of data to provide secure encrypted communication between two computers over an Internet Protocol network. It is used in virtual private networks (VPNs).

EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the payload of the frame and is used at the receiving end by the data link layer to determine how the payload is processed. The same field is also used to indicate the size of some Ethernet frames.

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 packets over the DSL connection to the ISP's IP network, and from there to the rest of the Internet. A 2005 networking book noted that "Most DSL providers use PPPoE, which provides authentication, encryption, and compression." Typical use of PPPoE involves leveraging the PPP facilities for authenticating the user with a username and password, predominately via the PAP protocol and less often via CHAP. Around 2000, PPPoE was also starting to become a replacement method for talking to a modem connected to a computer or router over an Ethernet LAN displacing the older method, which had been USB. This use-case, connecting routers to modems over Ethernet is still extremely common today.

<span class="mw-page-title-main">IP fragmentation</span> Process that breaks IP packets into smaller pieces

IP fragmentation is an Internet Protocol (IP) process that breaks packets into smaller pieces (fragments), so that the resulting pieces can pass through a link with a smaller maximum transmission unit (MTU) than the original packet size. The fragments are reassembled by the receiving host.

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 the TCP header or the IP header. The IP datagram containing a TCP segment may be self-contained within a single packet, or it may be reconstructed from several fragmented pieces; either way, the MSS limit applies to the total amount of data contained in the final, reconstructed TCP segment.

A ping of death is a type of attack on a computer system that involves sending a malformed or otherwise malicious ping to a computer. In this attack, a host sends hundreds of ping requests with a packet size that is large or illegal to another host to try to take it offline or to keep it preoccupied responding with ICMP Echo replies.

In computer networking, jumbo frames are Ethernet frames with more than 1500 bytes of payload, the limit set by the IEEE 802.3 standard. The payload limit for jumbo frames is variable: while 9000 bytes is the most commonly used limit, smaller and larger limits exist. Many Gigabit Ethernet switches and Gigabit Ethernet network interface controllers and some Fast Ethernet switches and Fast Ethernet network interface cards can support jumbo frames.

In packet-switched computer networks, a jumbogram is an internet-layer packet exceeding the standard maximum transmission unit (MTU) of the underlying network technology. In contrast, large packets for link-layer technologies are referred to as jumbo frames.

6LoWPAN was a working group of the Internet Engineering Task Force (IETF). It was created with the intention of applying the Internet Protocol (IP) even to the smallest devices, enabling low-power devices with limited processing capabilities to participate in the Internet of Things.

Path MTU Discovery (PMTUD) is a standardized technique in computer networking for determining the maximum transmission unit (MTU) size on the network path between two Internet Protocol (IP) hosts, usually with the goal of avoiding IP fragmentation. PMTUD was originally intended for routers in Internet Protocol Version 4 (IPv4). However, all modern operating systems use it on endpoints. In IPv6, this function has been explicitly delegated to the end points of a communications session. As an extension to the standard path MTU discovery, a technique called Packetization Layer Path MTU Discovery works without support from ICMP.

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.

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 destination host specified by an IP address. The internet layer derives its name from its function facilitating internetworking, which is the concept of connecting multiple networks with each other through gateways.

An IPv6 packet is the smallest message entity exchanged using Internet Protocol version 6 (IPv6). Packets consist of control information for addressing and routing and a payload of user data. The control information in IPv6 packets is subdivided into a mandatory fixed header and optional extension headers. The payload of an IPv6 packet is typically a datagram or segment of the higher-level transport layer protocol, but may be data for an internet layer or link layer instead.

Generic Stream Encapsulation, or GSE for short, is a Data link layer protocol defined by DVB. GSE provides means to carry packet oriented protocols such as IP on top of uni-directional physical layers such as DVB-S2, DVB-T2 and DVB-C2.

References

  1. ↑ RFC 791. p. 25. doi: 10.17487/RFC0791 .
  2. ↑ Murray, David; Terry Koziniec; Kevin Lee; Michael Dixon (2012). "Large MTUs and internet performance". 2012 IEEE 13th International Conference on High Performance Switching and Routing. pp. 82–87. doi:10.1109/HPSR.2012.6260832. ISBN   978-1-4577-0833-6. S2CID   232321.
  3. ↑ RFC 791. p. 24. doi: 10.17487/RFC0791 . Every internet destination must be able to receive a datagram of 576 octets either in one piece or in fragments to be reassembled.
  4. ↑ RFC 2460. p. 13. doi: 10.17487/RFC2460 .
  5. ↑ RFC 791. p. 24. doi: 10.17487/RFC0791 . Every internet module must be able to forward a datagram of 68 octets without further fragmentation.
  6. ↑ RFC 791. p. 12. doi: 10.17487/RFC0791 . Total Length is the length of the datagram, measured in octets, including internet header and data. This field allows the length of a datagram to be up to 65,535 octets.
  7. 1 2 RFC 1191. doi: 10.17487/RFC1191 .
  8. ↑ RFC   2460
  9. ↑ RFC   2675, p. 1, "The IPv6 header [IPv6] has a 16-bit Payload Length field and, therefore, supports payloads up to 65,535 octets long. This document specifies an IPv6 hop-by-hop option, called the Jumbo Payload option, that carries a 32-bit length field in order to allow transmission of IPv6 packets with payloads between 65,536 and 4,294,967,295 octets in length. Packets with such long payloads are referred to as 'jumbograms'."
  10. ↑ RFC   2460
  11. ↑ RFC   1356
  12. ↑ Network Working Group of the IETF, RFC   894: A Standard for the Transmission of IP Datagrams over Ethernet Networks, Page 1, "The maximum length of the data field of a packet sent over an Ethernet is 1500 octets, thus the maximum length of an IP datagram sent over an Ethernet is 1500 octets.", ERRATA
  13. ↑ IEEE 802.3 [ page needed ]
  14. ↑ Scott Hogg (2013-03-06), Jumbo Frames, Network World , retrieved 2013-08-05, Most network devices support a jumbo frame size of 9216 bytes.
  15. ↑ Juniper Networks (2020-03-23), Physical Interface Properties , retrieved 2020-05-01
  16. ↑ Joe St Sauver (2003-02-04). "Practical Issues Associated With 9K MTUs" (PDF). uoregon.edu. p. 67. Retrieved 2016-12-15. you still need to insure that ALL upstream Ethernet switches, including any switches in your campus core, are ALSO jumbo frame capable
  17. ↑ RFC   2516 with the standard Ethernet MTU of 1500 bytes; extensions exist
  18. ↑ RFC   4638
  19. ↑ 802.11-2012, page 413, section 8.3.2.1; page 381 "The Frame Body field is of variable size. The maximum frame body size is determined by the maximum MSDU size (2304 octets), plus the length of the Mesh Control field (6, 12, or 18 octets) if present, the maximum unencrypted MMPDU size excluding the MAC header and FCS (2304 octets) or the maximum A-MSDU size (3839 or 7935 octets, depending upon the STA’s capability), plus any overhead from security encapsulation."
  20. ↑ J. Mogul; S. Deering (November 1990). Path MTU Discovery. Network Working Group. doi: 10.17487/RFC1191 . RFC 1191.Draft Standard. Obsoletes RFC  1063.
  21. ↑ J. McCann; S. Deering; J. Mogul (July 2017). R. Hinden (ed.). Path MTU Discovery for IP version 6. IETF. doi: 10.17487/RFC8201 . STD 87. RFC 8201.Internet Standard 87. Obsoletes RFC  1981.
  22. ↑ M. Mathis; J. Heffner (March 2007). Packetization Layer Path MTU Discovery. Network Working Group. doi: 10.17487/RFC4821 . RFC 4821.Proposed Standard. Updated by RFC  8899.
  23. ↑ G. Fairhurst; T. Jones; M. Tüxen; I. Rüngeler; T. Völker (September 2020). Packetization Layer Path MTU Discovery for Datagram Transports. IETF. doi: 10.17487/RFC8899 . ISSN   2070-1721. RFC 8899.Proposed Standard. Updates RFC  4821, 4960, 6951, 8085 and 8261.
  24. ↑ "Configure and Verify Maximum Transmission Unit on Cisco Nexus Platforms". Cisco. 2016-11-29. Document ID:118994. Retrieved 2017-01-04.
  25. ↑ "MTU in RouterOS". MikroTik. 2022-07-08. Retrieved 2022-09-02.
  26. ↑ "How to configure MTU (Maximum Transmission Unit) for Jumbo Frames on Dell Networking Force10 switches". Dell. 2016-06-02. Article ID: HOW10713. Retrieved 2017-01-06.
  27. ↑ "Jumbo Frames". HP Networking 2910al Switches Management and Configuration Guide. Hewlett-Packard. November 2011. P/N 5998-2874.
  28. ↑ "SRX Series Services Gateways for the Branch Physical Interface Modules Reference: MTU Default and Maximum Values for Physical Interface Modules". Juniper. 2014-01-03. Retrieved 2017-01-04.
  29. ↑ jabber, The Network Encyclopedia, retrieved 2016-07-28
  30. ↑ show interfaces, Juniper Networks , retrieved 2016-07-28
  31. ↑ IEEE 802.3 27.3.1.7 Receive jabber functional requirements
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.