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The maximum size is 1500 Bytes. Is there a lower limit? 1? 64? Must it be a multiple of two? Is there somewhere I can look to learn more?

3 Answers 3

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Yes, IPv4 implementations require a minimum MTU of 68 bytes per RFC 791.

See Peter's answer for more detail.

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  • 2
    You contradict yourself. The quote says "up to 576", not "at least 576".
    – Nobody
    Commented Jun 5, 2015 at 21:22
  • 1
    Edited answer for clarity. Devices can receive packets with smaller MTU's, but per the RFC, 576 bytes was the right size to use. Most OSs enforce a number around 576, it varies slightly - so there's really no reason to send smaller than necessary. Commented Jun 7, 2015 at 13:22
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    This answer is wrong, it's confusing datagram size with MTU. Commented Mar 7, 2016 at 13:11
  • Yup, you're right - edited my answer. Thanks for catching that @PeterGreen. Commented Mar 7, 2016 at 21:18
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The minimum MTU for IPv4 is 68 bytes. Specifically from https://www.rfc-editor.org/rfc/rfc791

Every internet module must be able to forward a datagram of 68 octets without further fragmentation. This is because an internet header may be up to 60 octets, and the minimum fragment is 8 octets.

However such a low MTU would be extremely inefficient.

IPv6 sets a much higher minimum of 1280 bytes and requires links that can't support that MTU to provide a link-specific fragmentation and reassembly layer. From https://www.rfc-editor.org/rfc/rfc2460

IPv6 requires that every link in the internet have an MTU of 1280 octets or greater. On any link that cannot convey a 1280-octet packet in one piece, link-specific fragmentation and reassembly must be provided at a layer below IPv6.

Presumablly the motivation is to stop low MTU links from driving inefficiency across the network. I assume 1280 bytes was chosen to allow for a packet to be wrapped in some encapsulation/tunneling crap and still fit in a standard ethernet frame.

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  • So are there some reasons in a real-world IPv4 networks to set MTU to 68 or e.g. 200 or something very small relative to the usual 1500?
    – red0ct
    Commented Sep 22, 2020 at 15:15
  • IPv4 was designed in an era when connections were much slower. If you are running over a 1200 baud modem then a 1500 byte packet will block the link for over a second, this is generally not desirable. Commented Sep 22, 2020 at 15:21
  • Thanks! In other words MTU value is determined by the physical transmission medium (which in turn is determined by the devices). So if there are some slow devices with corresponding link-layer protocol between them - there could be some small MTU value. Isn't it? But I'm not sure that this is the case in modern IP-networks.
    – red0ct
    Commented Sep 22, 2020 at 16:04
  • The physical transmission medium does not directly determine the MTU, but it does have an impact on what MTU values are likely to be considered sensible by a designer. Commented Jun 1, 2022 at 22:25
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I am not sure if it is the lowest but can has a MTU of 8 bytes. It can't be used directly for IP packets.

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    "MTU" is an IETF term for IP networks, so your answer doesn't make sense. 8 bytes cannot work as it can't even carry an IPv4 header, let alone any payload.
    – Zac67
    Commented May 31, 2022 at 19:54
  • @Zac67 Linux gives also a MTU for socketCAN devices. Of course it makes sense. From Wikipedia: "Maximum transmission unit, the size of the largest packet that a network protocol can transmit" There is nothing about IP. The OP didn't say anything about IP. Commented Jun 1, 2022 at 8:53
  • I'm afraid, SocketCAN and hosts are nothing that is on-topic here, see the help center. And the WP article about MTU talks at length about the Internet Protocol which I would see as implied unless the OP indicates otherwise.
    – Zac67
    Commented Jun 1, 2022 at 8:59
  • @Zac67 Read it. Don't see any indication why it shouldn't be on topic. It is about networks and CAN is a Network. And again, the OP didn't say anything about IP. I wouldn't imply that, maybe you are in a bubble where you almost only use IP and related stacks? Commented Jun 1, 2022 at 9:31
  • While I wouldn't strictly say CAN is off-topic here, it's a fringe topic here. If you check the help center, it very clearly states Network Engineering Stack Exchange is for asking questions about professionally managed networks in a business environment and elaborates on the various scenarios. So, if someone mentions MTU without any more context, IP is implied. If you need to discuss further, I'd suggest putting that on Network Engineering Meta.
    – Zac67
    Commented Jun 1, 2022 at 9:53

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