should TCPs do MTU black hole detection?

Vern Paxson vern at
Thu Nov 18 22:40:01 UTC 1999

The IETF's tcp-impl (TCP implementation) working group has a draft document
discussing problems with path MTU discovery:

The main issue we're trying to decide is whether the draft should advocate
"black hole detection".  That is, when a TCP is doing PMTU discovery, but
somewhere the necessary ICMPs are either not being generated or are being
filtered out before the TCP receives them, the TCP notices that it's losing
multiple packets of the same size, so it then tries sending smaller segments,
even though it hasn't received a "Datagram Too Big" ICMP.

The plus of black hole detection is that it can work around a sometimes very
hard to debug problem.  The minus is that it masks problems that should
instead be fixed.

To help resolve this issue, I'm wondering whether the ISP community has a
clear preference for either yes-do-detection or no-we-want-the-problems-fixed.
Comments appreciated.



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