Telechat Review of draft-ietf-intarea-probe-07

Request Review of draft-ietf-intarea-probe
Requested rev. no specific revision (document currently at 10)
Type Telechat Review
Team Ops Directorate (opsdir)
Deadline 2017-12-12
Requested 2017-11-28
Other Reviews Intdir Early review of -06 by Jean-Michel Combes (diff)
Genart Telechat review of -07 by Joel Halpern (diff)
Secdir Telechat review of -07 by Yaron Sheffer (diff)
Review State Completed
Reviewer Stefan Winter
Review review-ietf-intarea-probe-07-opsdir-telechat-winter-2017-12-04
Posted at
Reviewed rev. 07 (document currently at 10)
Review result Has Issues
Draft last updated 2017-12-04
Review completed: 2017-12-04



* Introduction
states "[...] if it appears in the IPv4 Address Resolution Protocol (ARP) table [RFC0826] or IPv6 Neighbor Cache [RFC4861]." "Appears" is a rather loose word, as entries in those tables can have multiple states. E.g. for IPv6, which of the states DELAY, STALE, REACHABLE do you mean? All? Or only a subset? In IPv4, do you mean the "C" flag exclusively?
Also, when the proxy operates remotely (i.e. bases the reply on ARP/Neighbor Cache rather than ifOperStatus), does it actively ping the interface in question itself? If not, how does it handle an interface address which is not in the ARP/Neighbour table simply because the entry has timed out? The interface might be up and active nontheless. In such a case, reporting "does not exist" is false.

* Request -> L-Bit. 
I don't get it. The Request part of the spec is used by the probING node. It always sends the request to a proxy node. The proxy node then is the one to figure out by local state if the interface that is to be probed is local to itself, or on a link. Now the question is of course: what purpose does setting the L-Bit on the *request* serve? The probed interface either is local to the proxy node or it's not; no amount of flipping bits changes the reality. I can see how this L-bit information could be set in a *Response* as an information element. But that's not what the document says; the document actually states two contradictory things
a) L (local) - The L-bit is set if the probed interface resides on 
   the probed node.  The L-bit is clear if the probed interface is 
   directly connected to the probed node 
   [doesn't make sense, see above]
b) If the L-bit is set, the Interface Identification Object identifies
   the probed interface by name, index or address.  It the L-bit is
   clear, the Interface Identification Object identifies the probed
   interface by address.
   [ makes more sense, but conflicts with previous statement]
The latter formulation be also begs the questions a) why would one ever clear the L-Bit; identifying an interface by address is also possible when it's set, so setting the L-bit is fit for all situations envisaged and provides a true superset of functionality that L-Bit cleared offers; b) what do you mean with "name, index **or** address". Is that an exclusive OR, or any subset of the three, or can they all three be set? 
Later text suggests that each Interface Identification Object can carry only one of the three (XOR), but previous text suggests that two such Objects might be required for unique idenficiation. So in the end either one or two can be used to identify an object, but not all three? That's totally fine, but could be made more obvious. 
I also suggest to ditch the L-Bit and operate in a mode as if the L-Bit was always set. It adds no value. I also contemplate later in the text that L-Bit set is default-on while L-Bit clear is default off already.

* Response (chapter 3)
The choice of flag names is not very intuitive. Why is IPv4 "F" and IPv6 "S"? I understand that those are the first letters of the words FOUR and SIX in English. But maybe the flags could actually be named "4" and "6". Those are ASCII characters like any other, and have a more direct recognition by humans (e.g. when the flags are displayed in protocol decoders).

Chapter 4, authorisation:
"not explicitly authorized for the incoming ICMP Extended Echo Request L-bit setting"
I don't understand why the L-bit is a major decision point for authorisation checks. It is in principle superfluous anyway as above, and then one is expecting that policy decisions of sorts "this probing address is allowed ask for interfaces based on properties different from the address, but this other node is only allowed to operate on address"? The use case for that escapes me; and also, it can be achieved with "define enabled query types" as per Security Considerations.

* Security Considerations
"For example, a malicious party can use PROBE to discover interface names." This would be discovery by brute forcing the interface name space? Because the reply doesn't give away the name when the request was via address - right? It would be good to make clear that this discovery has to happen as a hit-and-miss of guessed names rather than getting an enumeration on the silver platter. OTOH, there are many well-known naming conventions for interfaces and it's more like a dictionary attack rather than simple brute-force.

* Chapter 2, Page 4, first bullet of the "ICMP fields" enumeration. The value is TTTT0 (four T's) but you then ask IANA to register things with only TTTx (three T's). The fourth T is superfluous.