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RE: secdir review of draft-ietf-mpls-ldp-typed-wildcard-05

2010-02-09 10:53:22
Hi Richard,

Thank you so much for your review and critical feedback. This really
helps to improve this specification. Please see inline,

specific constraints.  Because this change is relatively minor, the
security considerations are mostly the same as the base protocol, as
noted by the Security Considerations section; however, I would prefer
if the authors explained a little better why this is the case.

Please see my response below (to the last comment).

From an editorial perspective, this document is unclear on several
important points, especially with regard to the type-specific
constraints and how they are defined and managed.  I think the
document
would would benefit from another revision, focused on making the
meaning and management of all parameters clear to ensure
interoperability.     

I am assuming that the unclarity you refer to is based on the specific
comments you have provided below.  Right? 


Specific comments:

Section 1, Para "As specified..."
With respect to the phrase "relative to an optional constraint": I
don't see anything in RFC 5036 that allows for such a constraint.  The
Wildcard FEC type "is to be applied to all FECs associated with the
label within the following label TLV."

Per RFC5036, the Label TLV is an optional parameter in Label release
(section 3.5.10) and Label withdraw (section 3.5.10) messages. Hence,
the text is section 1 is correct.


Section 1, Para "1. The Wildcard FEC Element is untyped"
It's not quite accurate to say that the element is untyped; it has
type
0x01.  Suggest something like "The Wildcard FEC element only allows
very coarse selection of FECs by label."


Type-length-value in TLV (encoding) has nothing to do with the 'typed'
(vs. 'untyped') data, which is what that statement and the whole
document refer to.

Typed data is supplemented with the value, whereas the untyped data is
not. The latter is what RFC5036 prescribed (note that there is no value
in it). The former is what the current document is prescribing.

Hence, the existing text in the para (and the whole document).

I don't see any need for changing that para. Do you?



Section 1, General
Clearly you're really after here isn't to change the Wildcard FEC
Element (as the last sentence of the section says), but to have a new
element that is a typed Wildcard.  It would be clearer and more
accurate to say this, e.g., in bullet (2), "There are situations where
it would be useful to have a wildcard-like FEC Element, with type
constraints, in Label Request messages."


Agreed. Fixed.

 
Section 2, TLV
s/Lenth/Length/

Agreed. Fixed.

 
Section 3, Para "The Typed Wildcard FEC Element..."
The language about constraints here seems vague.  (In what sense is
the
constraint "optional"?)  Suggest the following:
"
A Typed Wildcard FEC Element specifies a FEC Type and, optionally, a
constraint.  An element of this type refers to all FECs of the
specified FEC Type that meet the constraint.  The format of the
constraint field depends on the FEC Type specified.
"


Agreed. Pls allow me to suggest a slightly changed replacement instead -

~~~~~~~~~
The Typed Wildcard FEC Element refers to all FECs of the specified type
that meet the constraint. It specifies a 'FEC Element Type' and an
optional constraint, which is intended to provide additional
information.  
~~~~~~~~~

I have also put "Optional" in the figure 1, btw.

 
Section 3, Para "Additional FEC Type-specific Information ..." et seq
It is unclear whose responsibility it is to define the structure of
this field (i.e., who is the "designer"?).  Do you mean to say this:
"Additional constraints that the FEC must satisfy in order to be
selected. The format of the Additional FEC Type-specific Information
depends on the FEC type in question.  This document defines the format
of this field for the Prefix FEC type."


Agreed. Pls allow me to suggest a slightly changed replacement instead -

~~~~~~
It is important that any document specifying Typed wildcarding for any
FEC Element Type also specifies the length and format of Additional FEC
Type Specific Information, if included.
~~~~~~~~~~

The text here and in the document suggest that there should perhaps be
a procedure for defining and registering formats for this field.
However, you may want to specify that any FEC Type may be specified
with a zero-length Additional FEC Type-specific Information field to
simply match all FECs of that FEC Type (in order to make it easy to
use
without a whole lot of new RFCs).

This is already implicit in the description of the 'Leng FEC Type Info'.
I have made it explicit in the below update - 

~~~~~~~~~`
Additional FEC Type-specific Information:  (Optional) Additional
information specific to the FEC Element Type required to fully specify
the Typed Wildcard. If this field is absent, then all FECs of the
specified FEC Type would be matched.
~~~~~~~~~~~~~
        
 
Section 4, Para "It is the responsibility..." et seq
The authors of this document are the designers of the Typed Wildcard
FEC Element Type; who do you mean?  It is meaningless to have a MUST
that is conditional on "making sense".


What we mean is that the (future) specifications defining any new FEC
Element Types should prescribe whether typed wildcarding is needed for
that FEC Element Type.  

Nonetheless, that para should be in the section 3, not section 4. I have
moved it in the section 3. The 2nd para (When Typed Wildcarding....) has
been removed, since it is redundant with the existing text.

 
Section 4, Para "When a FEC TLV..."
This constraint made sense for the generic Wildcard type, since that
would overwhelm any other FEC Elements, but it's not clear why it's
necessary here.  Couldn't I have a Label Withdraw message that
withdraws all CR-LSP FECs and a single Prefix FEC?


Good question. The answer is - No. There must be two different messages.

RFC5036 (section 3.4.1) does NOT allow multiple FEC elements in FEC TLV
in any message except label mapping message.

Frankly, it would bring whole set of complexity, if we removed this
restriction, for minimal benefit. 



 
Section 6, General
You need to specify the semantics of this field.  Does it match all
FECs that are of the given address family?  Also, this doesn't allow
any constraints on prefix length or the prefix itself; is that
intentional?

Yes. That's intentional (since it is already covered by the Prefix FEC
Element).

Wrt semantics, not sure which particular field's semantics you are
referring to, but the procedures are already specified in section 4,
hence, there wasn't any benefit in replicating the text.

        
Section 7, Para "In other words ..."
s/can not/MUST NOT/

Agreed. Fixed.
 
Section 9, General
I would like to see a little more explanation of why this extension to
LDP does not create additional security considerations.  It seems like
at the very least it increases the risk of misconfiguration by adding
much more flexible wildcard matching rules; that is, it seems more
likely that an LSR operator will accidentally match things he doesn't
intend to, or vice versa.

Strictly speaking, the security exposure is reduced by this
specification, since the wildcarding is now limited to FECs of a
particular type (AFI, say), not all the FECs. 
        
Nonetheless, I agree to your suggestion about having some text to
clarify a bit better and suggest adding the following (as 2nd para) to
the security consideration section:

~~~~~~~~~~
One could deduce that the security exposure is reduced by this document,
since an LDP speaker using Typed Wildcard FEC Element could use a single
message to request, withdraw or release all the label mappings of a
particular type (a particular AFI, for example), whereas an LDP speaker
using Wildcard FEC Element, as defined in based LDP specification
[RFC5036], could use a single message to request, withdraw or release
all the label mappings of all types (all AFIs, for example).
~~~~~~~

Would this be sufficient? 

Cheers,
Rajiv


-----Original Message-----
From: Richard L. Barnes [mailto:rbarnes(_at_)bbn(_dot_)com]
Sent: Monday, February 01, 2010 10:40 PM
To: secdir; The IESG; The IETF
Cc: draft-ietf-mpls-ldp-typed-wildcard(_at_)tools(_dot_)ietf(_dot_)org
Subject: secdir review of draft-ietf-mpls-ldp-typed-wildcard-05

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.


This document extends the matching capabilities of the LDP Wildcard
FEC
element, which matches all Forwarding Equivalence Classes bound to a
given label, by adding a second Typed Wildcard FEC element, which
matches all FECs of a given type, with optional additional type-
specific constraints.  Because this change is relatively minor, the
security considerations are mostly the same as the base protocol, as
noted by the Security Considerations section; however, I would prefer
if the authors explained a little better why this is the case.


From an editorial perspective, this document is unclear on several
important points, especially with regard to the type-specific
constraints and how they are defined and managed.  I think the
document
would would benefit from another revision, focused on making the
meaning and management of all parameters clear to ensure
interoperability.     


Detailed comments follow.


--Richard



Specific comments:

Section 1, Para "As specified..."
With respect to the phrase "relative to an optional constraint": I
don't see anything in RFC 5036 that allows for such a constraint.  The
Wildcard FEC type "is to be applied to all FECs associated with the
label within the following label TLV."

Section 1, Para "1. The Wildcard FEC Element is untyped"
It's not quite accurate to say that the element is untyped; it has
type
0x01.  Suggest something like "The Wildcard FEC element only allows
very coarse selection of FECs by label."

Section 1, General
Clearly you're really after here isn't to change the Wildcard FEC
Element (as the last sentence of the section says), but to have a new
element that is a typed Wildcard.  It would be clearer and more
accurate to say this, e.g., in bullet (2), "There are situations where
it would be useful to have a wildcard-like FEC Element, with type
constraints, in Label Request messages."

Section 2, TLV
s/Lenth/Length/

Section 3, Para "The Typed Wildcard FEC Element..."
The language about constraints here seems vague.  (In what sense is
the
constraint "optional"?)  Suggest the following:
"
A Typed Wildcard FEC Element specifies a FEC Type and, optionally, a
constraint.  An element of this type refers to all FECs of the
specified FEC Type that meet the constraint.  The format of the
constraint field depends on the FEC Type specified.
"

Section 3, Para "Additional FEC Type-specific Information ..." et seq
It is unclear whose responsibility it is to define the structure of
this field (i.e., who is the "designer"?).  Do you mean to say this:
"Additional constraints that the FEC must satisfy in order to be
selected. The format of the Additional FEC Type-specific Information
depends on the FEC type in question.  This document defines the format
of this field for the Prefix FEC type."
The text here and in the document suggest that there should perhaps be
a procedure for defining and registering formats for this field.
However, you may want to specify that any FEC Type may be specified
with a zero-length Additional FEC Type-specific Information field to
simply match all FECs of that FEC Type (in order to make it easy to
use
without a whole lot of new RFCs).

Section 4, Para "It is the responsibility..." et seq
The authors of this document are the designers of the Typed Wildcard
FEC Element Type; who do you mean?  It is meaningless to have a MUST
that is conditional on "making sense".

Section 4, Para "When a FEC TLV..."
This constraint made sense for the generic Wildcard type, since that
would overwhelm any other FEC Elements, but it's not clear why it's
necessary here.  Couldn't I have a Label Withdraw message that
withdraws all CR-LSP FECs and a single Prefix FEC?

Section 6, General
You need to specify the semantics of this field.  Does it match all
FECs that are of the given address family?  Also, this doesn't allow
any constraints on prefix length or the prefix itself; is that
intentional?

Section 7, Para "In other words ..."
s/can not/MUST NOT/

Section 9, General
I would like to see a little more explanation of why this extension to
LDP does not create additional security considerations.  It seems like
at the very least it increases the risk of misconfiguration by adding
much more flexible wildcard matching rules; that is, it seems more
likely that an LSR operator will accidentally match things he doesn't
intend to, or vice versa.
























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