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New fragment header, was: Re: Evolution of the IP model - ICMP and MTUs

Ok, this is what the new fragment header would look like:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| next header   |res|type |S/F|T|      fragment offset    |res|M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        identification                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          source port          |      destination port         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    observed packet length     |            checksum           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
next header: protocol number of the header following the new fragment header 
res: reserved

S/F: SYN/FIN, copied/synthesized from transport

T: 1 = addresses were translated

fragment offset: as usual

type: tcp/udp/sctp/dccp/AH/ESP/other+ports/other-ports

M: more fragments
identification: groups fragments of the same packet, copied from TCP sequence if applicable 
ports: copied from transport port numbers or IPsec SA
observed packet length: maximum packet size seen in the opposite direction in the last 5 minutes, 0 = unknown / no packets larger than 1280 bytes seen 
checksum: checksum of the IP pseudo header + transport segment bytes  
upto byte 1280 of the packet, 0 = N/A

The new fragment header is always the first header following the IPv4  
or IPv6 header. This means that filtering on protocol and port numbers  
is always easy because the information is always present in the same  
place in the packet, even if additional headers are present between  
the new fragment header and the transport header.

When routers observe a packet with the new fragment header that is too  
large to be forwarded, they truncate the packet. The minimum size of  
the first fragment is 1280 bytes. Note that traditional IPv4  
fragmentation produces the same result as truncation for the first  
fragment.

Because the new fragment header has its own checksum over the relevant  
part of the first 1280 bytes of the packet, bit errors can still be  
detected so protocols that can make use of the truncated data (such as  
TCP or DNS) may do so.

NATs MAY only translate the new fragment header.


The observed packet length field allows for passive PMTUD. A full  
PMTUD implementation can be done by initializing the PMTU to 1280 and  
then use a new fragment header for packets larger than the PMTU and/or  
in response to incoming packets that updated the OPL, updating the  
PMTU from the OPL field in incoming packets, resetting the PMTU after  
a retransmission timeout.