[pmtud] PLPMTUD (draft-07) comments

Ivan Beschastnikh <ivan@cs.washington.edu> Tue, 11 July 2006 23:02 UTC

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Date: Tue, 11 Jul 2006 16:02:56 -0700
From: Ivan Beschastnikh <ivan@cs.washington.edu>
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Subject: [pmtud] PLPMTUD (draft-07) comments
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Hello,

I just read the draft-ietf-pmtud-method-07 document and have a few 
comments and questions.

pg1, 2nd paragraph: "If can be configured to perform" should read "*It* 
can be.."

pg3, 5th paragraph (last paragraph in section 1): "..and draw heavily 
RFC1191 and .." should read "...and draws heavily *on* RFC..."

pg16, section 7.1: When you define search_low you say its equal to 
useful probe size minus one. Useful probe size is somewhat ambiguous- do 
you mean a probe size that hasn't been verified yet ie. the link might 
or might not support an MTU size of this value?

pg17, last paragraph: "..and rely ICMP PTB.." should read "and *..to* 
rely *on* ICMP PTB.."

pg 18, fourth paragraph: "..and per per-route configuration" should read 
"and *per route* configuration"

In section 8 you say that if an application sends a datagram larger than 
the "known path MTU," the datagram should be fragmented in the host's IP 
layer. In section 9, you say that a third mode is needed which allows 
the application to send datagrams that are larger than the current 
"estimate of the path MTU". Are "known path MTU" and "estimate of the 
path MTU" the same thing and if not, then is "estimate of path MTU" an 
application level estimate rather than a system level "known" value 
(since you're talking about application level PMTUD in this section). 
Also, I don't see how the recommendation for IPv4 implementations from 
section 8 differs from mode #1 from section 9, paragraph 2.

Along the previous comment, I see an unmentioned security repercussion 
of the scheme described in section 9, second to last paragraph where 
application level PLPMTUD results can be cached at IP layer by the OS or 
the system level vars for the method can be directly updated by the application. 
Wouldn't this mechanism effect other applications on the host that are 
doing PLPMTUD of their own or sending over the same path? Particularly 
if a rogue application decides to set the MTU for the first hop to a 
very low value to starve the bandwidth of other applications on 
the same host, how can this scenario be avoided, is this beyond the scope of 
this document?


many thanks,

ivan.




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