So you think you understand IP fragmentation?

Posted Feb 8, 2024 18:29 UTC (Thu) by vaurora (guest, #38407)
In reply to: So you think you understand IP fragmentation? by paulj
Parent article: So you think you understand IP fragmentation?

I see! No, everyone I talked to *believed* that the application was already setting the DF bit on all the packets, and I believed them. It wasn't until the PMTU discovery code failed that I did the packet dump and realized the application was not setting DF.

My takeaway from that was that multiple network experts were writing code assuming that the packets weren't fragmentable when they actually were, and no one noticed for years because things just worked anyway.

The solution on slide 9 is for fragquiz, which is a packet generator, not an implementation of DPLPMTUD (which I pronounce "dpblublubblubbbbd").

So you think you understand IP fragmentation?

Posted Feb 9, 2024 11:01 UTC (Fri) by paulj (subscriber, #341) [Link]

Ah, ok. That isn't quite clear from the article. :) E.g., the bit that says:

> "They were confident that the software already only sent packets that couldn't be fragmented, so all we had to do is send the right size of probe packets, using a built-in ping feature, and record the response. Imagine our surprise when the packet captures turned out to be full of fragmented packets."

The reader here can reasonably conclude from "all we had to is send the right size of probe" that we are talking about software under your control to do probing. And "surprise when the packet captures turned out to be full of [frags]" suggests surprise at seeing frags, which would imply the software /had/ set DF - for otherwise frags on large packets are not that surprising.

Perhaps a bit more word-smithing there? Something like "Surprise at seeing [frags], even where [condition where an expert wouldn't always predict frags]"? Or some other change there?

Great seeing detailed networking articles on LWN, and thanks for the tool! More please. :)