Re: Last Call: <draft-ietf-6man-rfc1981bis-04.txt> (Path MTU Discovery for IP version 6) to Internet Standard

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I have some late transport comments on this ID. The update seems to retain a lot of thinking that is really historical and I'd really encourage people to look again to making the document uptodate.

Detailed comments follow.

Best wishes,

Gorry

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The following text strikes me as a little odd in an update:
 " Moreover, TCP implementations that follow the "slow-
   start" congestion-avoidance algorithm [CONG] typically calculate and
   cache several other values derived from the PMTU.  It may be simpler
   to receive asynchronous notification when the PMTU changes, so that
   these variables may be updated.”
- A modern TCP caches at least some path information in the TCB, why start with this clause at all: "Moreover, TCP implementations that follow the "slow start" congestion-avoidance algorithm [CONG] typically calculate and”
and simply replace this with something like:
"TCP implementations”?
—---

The following text also seems to not reflect a modern TCP stack:
" It is sufficient
   to treat this as any other dropped segment, and wait until the
   retransmission timer expires to cause retransmission of the segment.”
(and following 3 paras).
Could this be replaced by text that does not exclude modern retransmission methods:
" It is sufficient
   to treat this in the same way as any other dropped segment, and
   will be recovered by normal retransmission methods."
—
There is a block of text that describes retransmission triggered by ICMPv6.
Has this code been implemented in modern releases of TCP?:
"   Alternatively, the retransmission could be done in immediate response
   to a notification that the Path MTU has changed, but only for the
   specific connection specified by the Packet Too Big message.”
- It seems to expose a number of attack vectors that really should not be exposed!!
---
The discussion of NFS may still be a reasonable historic example, but to be current it should really refer also to NFSv4/TCP as utlising the MTU discovery provided by TCP, since UDP-based NFS is no longer a key application.
---
There is no mention that paths including tunnels can eat ICMPv6 PTB messages on the tunnel segment, blackholing them, which prevents reaching the destination.
---
I think the security consideration is naive!

This statement in particular seems to open DOS vulnerability:
"
  When a node receives a Packet Too Big message, it MUST reduce its
   estimate of the PMTU for the relevant path, based on the value of the
   MTU field in the message."
- Introdueces a significant vulnerability. A rogue PTB message that reduces the PMTU to a minimum, can result in a path too small to carry an encapsulated packet. (Recently noted by Fernando Gont).

Moreover, other layers view ICMP messages with suspicion and have long noted the need to check ICMP payload and match only packets that relate to actual 5-tuples in use (effectively reducing vulnerability to off-path attacks). For example, the Guidelines for UDP, rfc5405bis, state:

" Applications SHOULD appropriately validate the payload of ICMP
   messages to ensure these are received in response to transmitted
   traffic (i.e., a reported error condition that corresponds to a UDP
   datagram actually sent by the application). …“
- clearly handling this in IP-layer tunnels can be troublesome, but that's a problem that should be described, not obscured.

——

I’d finally like to add my concerns about the understatement of the value of PLPMTUD, which seems to not reflect the recommendations to use this method:
“  It defines a method for Packetization Layer Path
   MTU Discovery (PLPMTUD) designed for use over paths where delivery of
   ICMP messages to a host is not assured.”
This seems under-stating the value and recommendations to deploy PLMTUD, compared with current transport-area recommendations, for instance, the UDP Guidelines provide much more on this important design consideration:

"   Packetization Layer Path MTU Discovery (PLPMTUD) [RFC4821] does not
   rely upon network support for ICMP messages and is therefore
   considered more robust than standard PMTUD.  It is not susceptible to
   "black holing" of ICMP message.  To operate, PLPMTUD requires changes
   to the way the transport is used, both to transmit probe packets, and
   to account for the loss or success of these probes.  This updates not
   only the PMTU algorithm, it also impacts loss recovery, congestion
   control, etc.  These updated mechanisms can be implemented within a
   connection-oriented transport (e.g., TCP, SCTP, DCCP), but are not a
   part of UDP, but this type of feedback is not typically present for
   unidirectional applications."

----

The examples used in the definition of "upper layer" and "link" also makes this document appear as historic, rather than a new RFC!




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