Subject: [Pals] Alvaro Retana's No Objection on draft-ietf-pals-mpls-tp-mac-wd-02: (with COMMENT)

From: "Alvaro Retana" <[email-masked]>
Date: Mon, 19 Oct 2015 13:54:39 -0700

Alvaro Retana has entered the following ballot position for
draft-ietf-pals-mpls-tp-mac-wd-02: No Objection

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COMMENT:
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I have some comments that, while non-blocking, I would like to see
addressed before publication.

1. Section 3. (MAC Withdraw OAM Message)  I don't understand why "when
sequence number wraps, all MAC addresses are flushed".  Presumably, the
wrap in the sequence number would be the result of a new MAC List TLV, so
why would all the addresses be flushed and not just the ones on the list?
 What am I missing?

2. Section 4.1. (Operation of Sender)  "…if a need to send a new MAC
withdraw message with updated sequence number arises then retransmission
of the older unacknowledged withdraw message MUST be suspended and
retransmit time for the new sequence number MUST be initiated."  That
sounds fine to me, but I have a related question.  Should the contents of
the withdraw message include both the un-ack'ed list as well as the new
addresses?  It is not clear from the text above.  I ask this in light of
the text in Section 3 that reads: "The receipt of same or lower sequence
number message is responded with ACK but does not cause removal of MAC
addresses."  IOW, if the un-ack'ed list is not included in the new
message then those addresses may not be withdrawn.

3. Also in 4.1. "The 'R' reset bit is set in the first MAC withdraw…" 
I'm assuming the Sequence Number with the R-bit set will always be 1, is
that true?  Does the text mean that there will in fact be a MAC List TLV
if the R-bit is set?  Later in Section 4.2 it says that a message "…with
'R' bit set…MAC withdraw message processing is performed as described
above."   All this seems problematic due to the text ("above") that says:
"If the sequence number in the received message is smaller than or equal
to the value in the register, the MAC TLV(s) is/are not processed."  IOW,
of the R-bit is set (assuming Sew Number = 1), then the Seq Number will
be smaller than whatever was received before and the MAC TLV will not be
processed.  The logic doesn't seem to work for me..

4. Security Considerations.  I traced all the way back to RFC4385, where
it hints at the ability of an attacker to disrupt the PW by misusing the
associated channel..but couldn't find an authoritative reference to
whether spoofing or changing the messages in flight is an issue.  I'm
worried about the ability of someone to, for example, inject/modify the
MAC List, or simply change the R-bit setting.  I may just be paranoid, so
please point me in the right direction.