should TCPs do MTU black hole detection?

Alex P. Rudnev alex at
Thu Nov 18 23:25:26 UTC 1999

Unfortunately,  the MTU problem can be caused by the client's network admin as
well as by the ISP; it's very difficult to explain what's wrong, for this
admins, and MTU discovery is not the part of traditional IP approach. This means
that black-hole detection whould be implemented anyway to prevent lost of
connectivity which we have   sometimes nopw when some MS-based server   or
crlient refuse to allow ip fragmentation.

On Thu, 18 Nov 1999, Vern Paxson wrote:

> Date: Thu, 18 Nov 1999 14:40:01 PST
> From: Vern Paxson <vern at>
> To: nanog at
> Subject: should TCPs do MTU black hole detection?
> 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.
> 	Thanks,
> 		Vern

Aleksei Roudnev, Network Operations Center, Relcom, Moscow
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