In article <20000714194722.AD3EA35DC2@smb.research.att.com>, "Steven M. Bellovin" <smb@research.att.com> wrote:

No -- 1918 addresses would only break PMTU if folks did ingress or egress filtering for 1918 addresses.

It is easy to argue that using interface 1918 addresses and PMTUD are incompatible with connecting to the Internet, for PMTUD requires sending an ICMP message with the source address of the interface, and using 1918 space on the Internet is not allowed. RFC 1918 section 3: "In order to use private address space, an enterprise needs to determine which hosts do not need to have network layer connectivity outside the enterprise in the foreseeable future and thus could be classified as private. Such hosts will use the private address space defined above. [...] However, they cannot have IP connectivity to any host outside of the enterprise." "Indirect references to such addresses should be contained within the enterprise. Prominent examples of such references are DNS Resource Records and other information referring to internal private addresses. In particular, Internet service providers should take measures to prevent such leakage." -- Shields.