Maximum Transmission Unit (MTU)

What Is Maximum Transmission Unit (MTU)?

MTU, or Maximum Transmission Unit, is the largest size of a data packet that can be transmitted across a network, measured in bytes. It dictates the maximum number of bytes that a sender can include in a packet for transmission. When the MTU exceeds the maximum size supported by a receiver or a transit node along the transmission path, the packet may be fragmented or discarded, adding to the network transmission burden. Conversely, if the MTU is too small, the amount of data transmitted is inefficient, impacting transmission efficiency.

The Importance of MTU

In a network, information is usually transmitted in the form of data packets. Thus, it is crucial to establish the appropriate packet size to optimize packet transmission efficiency. A larger packet size contains more valid data, leading to higher communication efficiency. However, larger packets also result in longer transmission delays and higher probability of bit errors. Moreover, if a large packet is lost, the cost of retransmission is high. Conversely, a smaller packet size requires more packets to transmit the same amount of data, leading to increased packet processing requirements for the device. This necessitates a high line-rate forwarding capability. Setting the Maximum Transmission Unit (MTU) allows adjustment of the data packet size transmitted on the network, ensuring optimal MTU for different networks and enhancing forwarding efficiency.

Why Is the Ethernet MTU Typically Set to 1500 Bytes?

RFCs define the default Ethernet MTU as 1500 bytes. Early Ethernet networks operated in shared link mode, requiring a minimum frame length of 64 bytes and a maximum of 1518 bytes to facilitate the carrier sense multiple access with collision detection (CSMA/CD) mechanism. The 64-byte minimum ensures detection of the most extreme collisions, while the 1518-byte maximum prevents long frame transmissions from blocking other services on the shared link. Thus, the Ethernet frame size remained between 64 and 1518 bytes.

The maximum Ethernet frame size of 1518 bytes includes a 14-byte frame header, 4-byte frame trailer, and a 1500-byte upper-layer IP packet. This 1500-byte size corresponds to the default Ethernet MTU value. Network layer protocols, use the MTU to determine whether to fragment data packets. If the length of a single IP packet exceeds the MTU, the packet is fragmented into smaller packets before transmission to ensure compatibility with the network interface.

Setting the Correct MTU

In networking, MTU definitions and fragmentation mechanisms can vary between vendors and even among different models from the same vendor. This variability often leads to network issues such as protocol interconnection failures, website or link access failures, and game freezing. Therefore, setting the proper MTU is crucial.

The fundamental principle is that the MTUs of Ethernet interfaces on interconnected Layer 3 devices should match. Additionally, it's important to consider the impact of encapsulation on packet size in various scenarios. For instance, when encapsulating an MPLS label, each MPLS label adds 4 bytes of overhead to a packet. This additional overhead could potentially cause the packet length to exceed the maximum length allowed by the link layer, resulting in forwarding issues.

1. MTU Configuration Recommendations for Backbone, MAN, and Access Networks

To ensure the proper functioning of the backbone network, metropolitan area network (MAN), and access network, the MTU value should be significantly larger than the default Ethernet MTU of 1500 bytes. High-end routers and switches typically support data packets larger than 9000 bytes. However, many devices from various vendors still have a default MTU of 1500 bytes. Additionally, because routing protocols like OSPF and IS-IS, which require MTU negotiation, may be in use, it's crucial for interconnected devices from different vendors to have matching MTUs. Therefore, it is recommended to set the MTU to a large value that meets network and carrier specifications and is supported by devices from all vendors.

2. MTU Configuration Recommendations for LANs, Including Data Centers

In large-scale data center networks, MTU settings are not uniformly adjusted. The adoption of new technologies gradually exposes MTU-related issues. For instance, various tunneling technologies like Virtual Private LAN Service (VPLS) and Virtual Extensible LAN (VXLAN) are utilized for extensive Layer 2 expansion. These technologies result in the generation of oversized packets after encapsulation. Without a unified MTU plan, transmission efficiency may decrease, and services could be interrupted. Therefore, when constructing data center networks, it is advisable to set the MTU to a large value supported by devices from all vendors.