Find the smallest link limit
01A path’s MTU is the smallest link MTU along the path. The sender initially knows its first-hop MTU, not every later link’s limit.
The router sends ICMP Type 3 Code 4 with Next-Hop MTU 1280 back to the source. The dropped datagram never reached the destination.
Source for this drawing: RFC 1191 · Read RFC section 2 ↗
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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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A large first hop does not guarantee that a large datagram reaches the destination.
- Classic IPv4 PMTU Discovery starts with the known first-hop MTU and sends datagrams with DF (Don’t Fragment) set. A router unable to forward without fragmentation discards the datagram and returns ICMP “fragmentation needed and DF set” to its source.
- After an applicable Datagram Too Big message, the host MUST reduce the relevant path estimate and force discovery to converge. It MUST try to avoid eliciting more such messages soon.
- A host can stop setting DF in some circumstances, allowing fragmentation instead; the usual discovery path keeps DF set so lower MTUs on a changed route can be detected. RFC 1191 is IPv4, not ICMPv 6.
Chosen link limits are 1500,1280 and 1400 octets; all datagram sizes include IPv4 headers and exclude link-layer headers. The 1280 value here is simply an IPv4 example, not the IPv6 minimum rule. Source/destination and router icons are separate roles; the error is not sent by the destination. Assume the receiver can accept the example’s datagrams; TCP’s peer-MSS limit is covered in chapter three.
Read the RFC section ↗