Fit TCP data inside the new limit
03A new path estimate limits the complete datagram. TCP must also respect the peer’s MSS (Maximum Segment Size: its advertised data limit) and subtract the headers actually sent.
Datagrams 1280 and 260 fit the 1280 bottleneck. A repeated ICMP notification is not another reason to duplicate the same data.
Source for this drawing: RFC 1191 · Read RFC section 3.1 ↗
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Keep peer receive limits and the path’s packet limit separate.
- In the no-option IPv4/TCP example,20 bytes of IPv4 header plus 20 bytes of TCP header leave PMTU−40 bytes for data. With options, subtract their actual extra space too; this is not a universal 40-byte-overhead rule.
- TCP must respect the peer’s received MSS even if PMTU allows more. Current RFC 9293 §3.7.1 gives the effective send MSS calculation; if no IPv4 MSS option arrived, the default send MSS is 536, not 1460.
- A PMTU decrease informs packetization. RFC 1191 §6.4 suggests retransmitting known dropped data promptly or waiting for the normal timer, but warns one MUST NOT retransmit in response to every duplicate Too Big notification. PMTU discovery does not rewrite the MSS already sent to the peer.
Chosen peer MSS 1460, PMTU 1280, and 20+20 headers yield max data 1240. The missing 1460 data bytes are represented as half-open relative ranges[0,1460), not absolute TCP sequence numbers. Splitting into 1240+220 preserves data; the datagrams are 1280 and 260 octets. Sizes shown are allowed bounds, not a claim that both packets are scheduled simultaneously or that congestion control is complete.
Read the RFC section ↗