RE: secdir review for draft-ietf-ntp-checksum-trailer-04.txt

[Tal Mizrahi <[e-mail]>]

Hi Sandra,

Thanks for the comments.

In the current draft (07) I believe that all your comments have been addressed.
https://tools.ietf.org/html/draft-ietf-ntp-checksum-trailer-07

Detailed responses below.

>(1)
>
>It seems there's an impossibility of protecting the NTP packet with this
>extension.  This extension header MUST NOT be used with an NTP MAC, and
>so is usable only in those situations where NTP is used in an "unauthenticated
>mode".  One might imagine that NTP running in an unauthenticated mode
>could be run in an ipsec tunnel for protection.
>However, the description of the timestamping engine makes it seem that the
>engine modifies the packet directly before transmission.  I.e., the packet
>would be modified after the ipsec protections were applied.
>Certainly the "intermediate nodes" mentioned in 3.2.2 would be adding the
>extension header after the source applied the ipsec protection.
>If my suppositions are correct, then using this extension header means neither
>an authenticated NTP nor a protected tunnel could be used.
>All the intruder attacks mentioned in RFC5905 security considerations section
>would be possible.

Agreed. 
This issue is discussed in the security considerations section. Based on your comment the text has been slightly revised to clarify this point further. Here is the updated text:

   The concept described in this document is intended to be used only in
   unauthenticated mode. As discussed in Section 3.4. , if a
   cryptographic security mechanism is used, then the Checksum
   Complement does not simplify the implementation compared to using the
   conventional Checksum, and therefore the Checksum Complement is not
   used.


>(2)
>
>RFC5905 and draft-ietf-ntp-extension-field-07.txt both define the general
>extension header format with the padding following the value.
>This draft defines this extension header as the value following the padding.
>That conflict should be addressed.


Agreed. 
Based on this comment the field name has been changed from "Padding" to "MBZ".


>(3)
>
>Section 1 describes intermediate entities (like the timestamping engine), and
>section 3.2.2 says:
>
>  An intermediate node that receives and alters an NTP packet
>  containing a Checksum Complement extension MAY use the Checksum
>  Complement to maintain a correct UDP checksum value.
>
>However, section 4 says:
>
>                                                              The
>  extension is intended to be used internally in an NTP client or
>  server, and not intended to be used by intermediate switches and
>  routers that reside between the client and the server.
>
>That seems to contradict the earlier statements, particularly section 3.2.2.
>Are the intermediate nodes of section 3.2.2 not "switches and routers"?  This
>is probably just a wording issue, but it should be more clear.


The point is well taken.
The use of the word "intermediate" was a bit confusing in a few locations in the document. The text has been rephrased in these cases in order to clarify the meaning.
Specifically, the sentence from Section 4 that you mentioned above was rephrased to the following:

   The
   extension is intended to be used internally in an NTP client or
   server. This extension is not intended to be used by switches and
   routers that reside between the client and the server.


Best regards,
Tal.







>-----Original Message-----
>From: ietf [mailto:[e-mail]] On Behalf Of Sandra Murphy
>Sent: Thursday, March 03, 2016 6:47 PM
>To: [e-mail]; [e-mail]; The IETF
>Subject: secdir review for draft-ietf-ntp-checksum-trailer-04.txt
>
>I have reviewed this document as part of the security directorate's ongoing
>effort to review all IETF documents being processed by the IESG.  These
>comments were written primarily for the benefit of the security area
>directors.  Document editors and WG chairs should treat these comments just
>like any other last call comments.
>
>The document draft-ietf-ntp-checksum-trailer-04.txt defines a new extension
>header for NTP that will allow a hardware based timestamping engine to
>modify the timestamp in the NTP packet and then add a correction to the
>packet's UDP checksum to account for the modified timestamp.  Putting the
>correction in an extension header allows for serial processing of the packet -
>no need to backtrack from the timestamp to the UDP checksum preceding in
>the packet and correct that field.
>
>I found the document adequate for the most part.
>
>(1)
>
>It seems there's an impossibility of protecting the NTP packet with this
>extension.  This extension header MUST NOT be used with an NTP MAC, and
>so is usable only in those situations where NTP is used in an "unauthenticated
>mode".  One might imagine that NTP running in an unauthenticated mode
>could be run in an ipsec tunnel for protection.
>However, the description of the timestamping engine makes it seem that the
>engine modifies the packet directly before transmission.  I.e., the packet
>would be modified after the ipsec protections were applied.
>Certainly the "intermediate nodes" mentioned in 3.2.2 would be adding the
>extension header after the source applied the ipsec protection.
>If my suppositions are correct, then using this extension header means neither
>an authenticated NTP nor a protected tunnel could be used.
>All the intruder attacks mentioned in RFC5905 security considerations section
>would be possible.
>
>(2)
>
>RFC5905 and draft-ietf-ntp-extension-field-07.txt both define the general
>extension header format with the padding following the value.
>This draft defines this extension header as the value following the padding.
>That conflict should be addressed.
>
>(3)
>
>Section 1 describes intermediate entities (like the timestamping engine), and
>section 3.2.2 says:
>
>  An intermediate node that receives and alters an NTP packet
>  containing a Checksum Complement extension MAY use the Checksum
>  Complement to maintain a correct UDP checksum value.
>
>However, section 4 says:
>
>                                                              The
>  extension is intended to be used internally in an NTP client or
>  server, and not intended to be used by intermediate switches and
>  routers that reside between the client and the server.
>
>That seems to contradict the earlier statements, particularly section 3.2.2.
>Are the intermediate nodes of section 3.2.2 not "switches and routers"?  This
>is probably just a wording issue, but it should be more clear.
>
>--Sandy