Keep Ethernet size separate from path size
03The standard Ethernet data field can hold an IP datagram of up to 1500 bytes. A later link on the route may have a smaller Maximum Transmission Unit (MTU).
Separate input: the same size meets a smaller outgoing MTU with DF set. The router cannot fragment it; the ICMP Fragmentation Needed feedback path is shown instead.
Source for this drawing: RFC 894 · Read RFC source: Frame Format (unnumbered) ↗
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A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
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Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
A local 1500-byte Ethernet limit is not a promise about the whole Internet path.
- Verified technical errata 570 and 5141 correct the original text: 1500 is the maximum data-field length, not the minimum. Full-length packet support is encouraged.
- RFC 894 says gateway implementations MUST be prepared to accept full-length packets and fragment them if necessary. Its referenced IPv4 rules still apply: Don’t Fragment (DF) prohibits fragmentation.
- The worked downstream MTU of 576 is a chosen example. With DF clear and a 20-byte IPv4 header, 1480 data bytes can be split into 552, 552 and 376 byte pieces; all non-final pieces are multiples of 8.
- With DF set, a router that cannot forward without fragmentation reports ICMP Fragmentation Needed, as explained in RFC 1191 section 4. It does not disregard DF. ICMP is Internet Control Message Protocol.
RFC 894’s 576-byte historical note is not a universal present-day path MTU or required send size. See RFC 791 sections 3.1–3.2 for fragmentation and RFC 1191 section 4 for the feedback case. Fragment offsets below count 8-byte units; no full reassembly or ICMP implementation is claimed.
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