Yes. But somehow I'm worried about this, particularly when the MTU size field in ND is 32 bits. Is there any danger that a false claim of a large MTU size will lead to something bad happening? Or are we relying on the sender's hardware to not accept overly large packets for transmission?
False claims can be mitigated by employing mechanisms to authenticate ND messages, e.g., SEND.
SEND can assure that RAs come from an authorized router, and that NAs come from the owner of an address. SEND may not be able to assure that the sender of an NA is trusted. That is, we know it comes from the node in question, but we may not be able to trust all the parameters it sends. In conclusion if the MTU info comes from a router, SEND is sufficient, but if its host to host, SEND may not be sufficient.
Anyway, I still think this danger is _mostly_ a non-issue -- your regular 10 Mbit/s ethernet hardware would hopefully decline to send a 2^32 byte probe packet and spend 3400 seconds sending it. However, there might be some special network technology where this might be an issue. Anyway, anything beyond 2^16 would require a jumbogram.
Also, if the IP header is 40 bytes/packet, one exchange of two 9k packets
It is not an exchange of two 9K packets; it is a 9K probe packet followed by a much smaller acknowledgement packet - on the order of the size of a NA message. The MTU/MRU probing is unidirectional.
consumes as much bandwidth as
the overhead from 225 packets -- a 337 K transmission
at 1500 byte MTU. So perhaps you wouldn't like to do this
every time.
I didn't quite catch this - can you re-phrase?
I'm assuming the reason for choosing a larger MTU is to save in header overhead and processing time. But the savings in header overhead must be balanced against the cost of negotiating a larger MTU, particularly if that negotiation involves sending large probe packets.
So, one 9K probe is about the same size as the overhead from extra 225 IPv6 headers. Using the standard 1500 byte MTU you get to send about 337 K before spending this much in overhead. That is, a 9K probe packet does not make sense bandwidth-wise if you are communicating less than 337 K with your peer.
--Jari
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