IPv6 reassembling

Question

RFC8200 specifically says that fragmentation is done only by the source node and not by any intermediate node. It also says that fragments are reassembled at the receiver. Can I conclude from this that reassembly is done only at the receiver and no node other than the destination can reassemble packets?

Answer 1

Section 4.5 says, in part:

(Note: unlike IPv4, fragmentation in IPv6 is performed only by source nodes, not by routers along a packet's delivery path -- see Section 5.)

Note that "no router can reassemble packets" isn't strictly true, and isn't strictly prohibited. Some firewalls will reassemble IPv6 fragments in memory in order to process them correctly, for instance; if they didn't reassemble them, they wouldn't always be able to determine whether the traffic should be passed or blocked. But generally a router won't reassemble packets.

This is really all the answer that there is to your given question, though I suspect you have more questions or want more details about this than you've actually posted.

As a general rule, we really don't want to see fragmentation on the Internet; see RFC 8201 for an explanation of Path MTU Discovery, which lets us avoid fragmentation. See also Fragmentation Considered Harmful from 1987, Fragmentation Considered Very Harmful from 2006, and How bad is ip fragmentation from right here on SO.