I Am Not An Isp <patrick@ianai.net> writes:

However, there is nothing to distinguish a packet with RFC1918 space as the source address from any other "legal" packet on the 'Net other than your own administrative policies - which can break *anything* on the 'Net, not just PMTU with RFC1918 space. Sorry, but I have no control over your policy. So, if someone asks "does this break...", the answer is no.

except that it is still a violation of RFC1918 to be using them in that manner, as they clearly excluded from category 1 (by virtue of being used in transit), and are clearly in Category 3. Category 1: hosts that do not require access to hosts in other enterprises or the Internet at large; hosts within this category may use IP addresses that are unambiguous within an enterprise, but may be ambiguous between enterprises. Category 2: hosts that need access to a limited set of outside services (e.g., E-mail, FTP, netnews, remote login) which can be handled by mediating gateways (e.g., application layer gateways). For many hosts in this category an unrestricted external access (provided via IP connectivity) may be unnecessary and even undesirable for privacy/security reasons. Just like hosts within the first category, such hosts may use IP addresses that are unambiguous within an enterprise, but may be ambiguous between enterprises. Category 3: hosts that need network layer access outside the enterprise (provided via IP connectivity); hosts in the last category require IP addresses that are globally unambiguous. We will refer to the hosts in the first and second categories as "private". We will refer to the hosts in the third category as "public". It's also a violation of RFC1918 to be using them in any way which will generate packets with those source addresses, which would mean traceroutes and using PMTU discovery. Because private addresses have no global meaning, routing information about private networks shall not be propagated on inter-enterprise links, and packets with private source or destination addresses should not be forwarded across such links. Routers in networks not using private address space, especially those of Internet service providers, are expected to be configured to reject (filter out) routing information about private networks. If such a router receives such information the rejection shall not be treated as a routing protocol error. So, if the question is "does using RFC1918 address break PMTU discovery?" the answer should be "maybe it won't, break it, but it's supposed to" Darrell