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What is MTU (maximum transmission unit)?

The largest packet a link can carry in one piece; 1500 bytes is standard for Ethernet. An IP packet larger than this is fragmented along the way or dropped in IPv4, while in IPv6 only the source fragments. The mechanism for finding the smallest MTU along a path is called path MTU discovery. When extra headers are added, as in VPN tunnels, the space left for the payload shrinks accordingly.

Japanese name: MTU(最大転送単位)

MTU (maximum transmission unit) appears in 1 practice question across 1 topic, including Computer Networking Basics.

Field: Computer Networking Basics

Try it: questions about MTU (maximum transmission unit)

On 1 Gbps Ethernet, TCP data is sent continuously with IP packets filling the 1500-byte MTU. Taking the IPv4 header as 20 bytes, the TCP header as 20 bytes, the Ethernet header and FCS together as 18 bytes, and the preamble and inter-frame gap together as 20 bytes, which is closest to the upper limit on application-data throughput?

  1. AAbout 949 Mbps
  2. BAbout 973 Mbps
  3. CAbout 1000 Mbps
Show answer and explanation

Answer: A. About 949 Mbps

Each frame carries 1500 − 20 − 20 = 1460 bytes of application data. On the wire, however, each frame takes the time of 1500 + 18 + 20 = 1538 bytes. The efficiency is therefore 1460 ÷ 1538 ≈ 0.949, and the throughput limit is 1000 × 0.949 ≈ 949 Mbps. Using the TCP timestamp option adds 12 bytes to the header and lowers efficiency slightly further. The smaller the packets, the larger the share of headers and other overhead, and the lower the efficiency.

Questions that mention MTU (maximum transmission unit) (1)

  1. On 1 Gbps Ethernet, TCP data is sent continuously with IP packets filling the 1500-byte MTU. Taking the IPv4 header as 20 bytes, the TCP header as 20 bytes, the Ethernet header and FCS together as 18 bytes, and the preamble and inter-frame gap together as 20 bytes, which is closest to the upper limit on application-data throughput?

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Related terms

Ethernet TCP (Transmission Control Protocol) IP (Internet Protocol) Packet IPv4 Throughput ARP ARQ (automatic repeat request) CIDR Congestion control

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