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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?

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jt26
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  • The link you've given doesn't seem to link to a very helpful part of the standard. Could you edit it to point to the part of the standard that talks directly about fragmentation and reassembly? – David Richerby Jan 25 '18 at 17:46
  • Even with IPv4, reassembly is only supposed to happen at the receiver. In any case, many, many firewalls and routers disallow fragments because they can be used in attacks. _[RFC 791, Internet Protocol, Section 3.2 Discussion](https://tools.ietf.org/html/rfc791)_: "_The basic internet service is datagram oriented and provides for the fragmentation of datagrams at gateways, with reassembly taking place at the destination internet protocol module in the destination host._" – Ron Maupin Jan 26 '18 at 21:35

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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.

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Michael Hampton
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