On the Implementation of the TCP Urgent Mechanism
draft-ietf-tcpm-urgent-data-07

[Ballot discuss]
This is a discuss-discuss. This sounds like a feature that never really worked. Applications have been discouraged from using it for years. Many deployed middle boxes defeat it.

Maybe a better approach would be to deprecate the feature and talk about alternative methods of delivering urgent data (e.g. a separate TCP session for urgent data only).

[Ballot comment]
> This means that if this
> sysctl is set, an application might be unable to interoperate with
> itself if both the TCP sender and the TCP receiver are running on the
> same host.

Heh. Amusing, or perhaps a proof of the lack of real use.

Some questions from a review by Ari Keranen:

3.2. Semantics of the Urgent Pointer

    [...] For example, Linux provides the sysctl
    "tcp_stdurg" (i.e., net.ivp4.tcp_stdurg) that, when set, supposedly
    changes the system behavior to interpret the semantics of the TCP
    Urgent Pointer as specified in RFC 1122.

Since the Linux behavior may change later on, it would be a good idea to
note the version of the Linux with this behavior (same probably applies
to section 3.4 with Cisco PIX too). Maybe you could add a reference to
the related appendix with this info.


5. Advice to new applications employing TCP

    As a result of the issues discussed in Section 3.2 and Section 3.4,
    new applications SHOULD NOT employ the TCP urgent mechanism.

What about applications that supposedly do not use the urgent mechanism
but an attacker might use it against them? Should it be recommended for
all applications to always set the SO_OOBINLINE socket option to prevent
the attack described in [phrack]? Or perhaps recommend this to be made
the default in the kernels of the OSs -- even if they got it wrong the
first time.

[Ballot discuss]
This is GREAT document, thank you for writing it. I am reading
it very carefully due to the importance of the Updates clause.
With this in mind, please consider the following:

  As discussed in Section 1, the TCP urgent mechanism simply permits a
  point in the data stream to be designated as the end of urgent
  information, but does NOT provide a mechanism for sending out of band
  data.

As far as I can tell, Section 1 does not discuss this. The intended
semantics of the urgent mechanism are also a crucial point for the
changes suggested in this document, so I hope to see a clear reference
and a rock solid explanation why this the case. That being said,
I think the matter is pretty clear, given that there is no start
pointer for the urgent data. However, RFC 793 confuses this a bit by
saying:

  If the sending user also indicates a push, timely delivery of
  the urgent information to the destination process is enhanced.

whereas in reality all data up to and including the urgent information
gets a higher priority treatment.

I have no comments on the rest of the contents of the document. However,
I think the document is lacking in two respects. First, as far as I can
tell, it does not describe the problems (if any) that result from a
different interpretation of the pointer and other semantics on the two
endpoints. Second, if there are significant problems, perhaps all the
evidence from this document should be taken as an indication that urgent
data is simply not used in the Internet (and might be deprecated). If
there are no problems, what's the fuss?

[Ballot comment]
> This means that if this
> sysctl is set, an application might be unable to interoperate with
> itself if both the TCP sender and the TCP receiver are running on the
> same host.

Heh. Amusing, or perhaps a proof of the lack of real use.

Some questions from a review by Ari Keranen:

3.2. Semantics of the Urgent Pointer

    [...] For example, Linux provides the sysctl
    "tcp_stdurg" (i.e., net.ivp4.tcp_stdurg) that, when set, supposedly
    changes the system behavior to interpret the semantics of the TCP
    Urgent Pointer as specified in RFC 1122.

Since the Linux behavior may change later on, it would be a good idea to
note the version of the Linux with this behavior (same probably applies
to section 3.4 with Cisco PIX too). Maybe you could add a reference to
the related appendix with this info.


5. Advice to new applications employing TCP

    As a result of the issues discussed in Section 3.2 and Section 3.4,
    new applications SHOULD NOT employ the TCP urgent mechanism.

What about applications that supposedly do not use the urgent mechanism
but an attacker might use it against them? Should it be recommended for
all applications to always set the SO_OOBINLINE socket option to prevent
the attack described in [phrack]? Or perhaps recommend this to be made
the default in the kernels of the OSs -- even if they got it wrong the
first time.

[Ballot discuss]
This is GREAT document, thank you for writing it. I am reading
it very carefully due to the importance of the Updates clause.
With this in mind, please consider the following:

  As discussed in Section 1, the TCP urgent mechanism simply permits a
  point in the data stream to be designated as the end of urgent
  information, but does NOT provide a mechanism for sending out of band
  data.

As far as I can tell, Section 1 does not discuss this. The intended
semantics of the urgent mechanism are also a crucial point for the
changes suggested in this document, so I hope to see a clear reference
and a rock solid explanation why this the case. That being said,
I think the matter is pretty clear, given that there is no start
pointer for the urgent data. However, RFC 793 confuses this a bit by
saying:

  If the sending user also indicates a push, timely delivery of
  the urgent information to the destination process is enhanced.

whereas in reality all data up to and including the urgent information
gets a higher priority treatment.

I have no comments on the rest of the contents of the document. However,
I think the document is lacking in two respects. First, as far as I can
tell, it does not describe the problems (if any) that result from a
different interpretation of the pointer and other semantics on the two
endpoints. Second, if there are significant problems, perhaps all the
evidence from this document should be taken as an indication that urgent
data is simply not used in the Internet (and might be deprecated). If
there are no problems, what's the fuss?

Second,

[Ballot comment]
Some questions from a review by Ari Keranen:

3.2. Semantics of the Urgent Pointer

    [...] For example, Linux provides the sysctl
    "tcp_stdurg" (i.e., net.ivp4.tcp_stdurg) that, when set, supposedly
    changes the system behavior to interpret the semantics of the TCP
    Urgent Pointer as specified in RFC 1122.

Since the Linux behavior may change later on, it would be a good idea to
note the version of the Linux with this behavior (same probably applies
to section 3.4 with Cisco PIX too). Maybe you could add a reference to
the related appendix with this info.


5. Advice to new applications employing TCP

    As a result of the issues discussed in Section 3.2 and Section 3.4,
    new applications SHOULD NOT employ the TCP urgent mechanism.

What about applications that supposedly do not use the urgent mechanism
but an attacker might use it against them? Should it be recommended for
all applications to always set the SO_OOBINLINE socket option to prevent
the attack described in [phrack]? Or perhaps recommend this to be made
the default in the kernels of the OSs -- even if they got it wrong the
first time.

[Ballot discuss]
This is GREAT document, thank you for writing it. I am reading
it very carefully due to the importance of the Updates clause.
With this in mind, please consider the following:

  As discussed in Section 1, the TCP urgent mechanism simply permits a
  point in the data stream to be designated as the end of urgent
  information, but does NOT provide a mechanism for sending out of band
  data.

As far as I can tell, Section 1 does not discuss this. The intended
semantics of the urgent mechanism are also a crucial point for the
changes suggested in this document, so I hope to see a clear reference.

[Ballot comment]
Some questions from a review by Ari Keranen:

3.2. Semantics of the Urgent Pointer

    [...] For example, Linux provides the sysctl
    "tcp_stdurg" (i.e., net.ivp4.tcp_stdurg) that, when set, supposedly
    changes the system behavior to interpret the semantics of the TCP
    Urgent Pointer as specified in RFC 1122.

Since the Linux behavior may change later on, it would be a good idea to
note the version of the Linux with this behavior (same probably applies
to section 3.4 with Cisco PIX too). Maybe you could add a reference to
the related appendix with this info.


5. Advice to new applications employing TCP

    As a result of the issues discussed in Section 3.2 and Section 3.4,
    new applications SHOULD NOT employ the TCP urgent mechanism.

What about applications that supposedly do not use the urgent mechanism
but an attacker might use it against them? Should it be recommended for
all applications to always set the SO_OOBINLINE socket option to prevent
the attack described in [phrack]? Or perhaps recommend this to be made
the default in the kernels of the OSs -- even if they got it wrong the
first time.

[Ballot comment]
1. Section 2.1 states:

  TCP incorporates an "urgent mechanism" that allows the sending user
  to stimulate the receiving user to accept some "urgent data" and to
  permit the receiving TCP to indicate to the receiving user when all
  the currently known urgent data have been received by the user.

The phrase "have been received by the user" is ambiguous -- do you mean the sending user or the receiving user here? I would assume the receiving user, but it would be good to make that explicit. You could change "by the user" to "by the receiving user" or simply remove "by the user" at the end of the sentence.

[Ballot comment]
While consistent with RFC 793, I found the references to "user" in section 2.1 (paragraphs 1 and 2) a little odd.  Is
this still the accepted terminology for TCP implementations?

[Ballot discuss]
This is a discuss-discuss.  To be clear, I am not requesting any changes in the document at this time.  I would
like to discuss several issues before I either clear or make this an actionable discuss.

(1) Given the content in the preceding sections, I was somewhat disappointed with the scope of sections 4, 5 and 6.
Why did the wg choose not to update the allowed length of urgent data, given that almost every implementation
supports only one byte of urgent data?  Even if  the wg was not comfortable updating the specifications in section 4,
wouldn't it be prudent to admonish applications not to use this mechanism for more than a single byte in section 6?
Similarly, section 5 reiterates that TCP implementations MUST support the urgent mechanism.  Do these implementations need to support urgent data of a single byte or "any length"?

(2) I wonder why section 6 does not reiterate the closing text in 3.4: that applications need to be designed for correct
operation even in the case where the URG flag is cleared by middleboxes.

(3) I think the most important sentence in this document is the first sentence of section 5:
"... new applications SHOULD NOT employ the TCP urgent mechanism."  Reading the Abstract and Introduction
provides no clue that such important guidance is hidden within.  Shouldn't this be highlighted?

(4) Last, but perhaps most important of all: given the guidance cited in (3), did the wg consider making this a BCP?

[Ballot comment]
Section 2.1 (editorial): are "sending user" and "receiving user" typical
terms in specifications of urgent data; do they imply human users?  Would
"sender" and "receiver" be more general?

In section 3.2, I'm guessing "net.ivp4.tcp_stdurg" is a typo (ipv4?)

[Ballot comment]
Section 2.1 (editorial): are "sending user" and "receiving user" typical terms in specifications of urgent data; do they imply human users?  Would "sender" and "receiver" be more general?

In section 3.2, I'm guessing "net.ivp4.tcp_stdurg" is a typo (ipv4?)

[Ballot comment]
3.4.  Interaction of middle-boxes with TCP urgent indications

  This should discourage
  applications from depending on urgent indications for their correct
  operation, as urgent indications may not be not reliable in the

Did you mean "may not be *reliable*"?

  current Internet.



<>

draft-ietf-tcpm-urgent-data
 


  (1.a) Who is the Document Shepherd for this document? Has the
        Document Shepherd personally reviewed this version of the
        document and, in particular, does he or she believe this
        version is ready for forwarding to the IESG for publication?


Wesley Eddy (Wesley.M.Eddy@nasa.gov) is the document shepherd.  He
has personally reviewed this version and believes it is ready for
forwarding to the IESG for publication.



  (1.b) Has the document had adequate review both from key WG members
        and from key non-WG members? Does the Document Shepherd have
        any concerns about the depth or breadth of the reviews that
        have been performed? 


The document has had review in the TCPM working group, and underwent
several revisions based on mailing list discussion, face-to-face
presentations at IETF meetings, and interaction with the stewards of
several TCP implementations.  The shepherd has no concerns about the
depth or breadth of review.


  (1.c) Does the Document Shepherd have concerns that the document
        needs more review from a particular or broader perspective,
        e.g., security, operational complexity, someone familiar with
        AAA, internationalization or XML?


No concerns.


  (1.d) Does the Document Shepherd have any specific concerns or
        issues with this document that the Responsible Area Director
        and/or the IESG should be aware of? For example, perhaps he
        or she is uncomfortable with certain parts of the document, or
        has concerns whether there really is a need for it. In any
        event, if the WG has discussed those issues and has indicated
        that it still wishes to advance the document, detail those
        concerns here. Has an IPR disclosure related to this document
        been filed? If so, please include a reference to the
        disclosure and summarize the WG discussion and conclusion on
        this issue.


No concerns.



  (1.e) How solid is the WG consensus behind this document? Does it
        represent the strong concurrence of a few individuals, with
        others being silent, or does the WG as a whole understand and
        agree with it? 


There has been no resistance to this document and reasonable support
for it at its inception.  It is felt that this resolves a long-standing
issue in specification and implementation divergence.



  (1.f) Has anyone threatened an appeal or otherwise indicated extreme
        discontent? If so, please summarise the areas of conflict in
        separate email messages to the Responsible Area Director. (It
        should be in a separate email because this questionnaire is
        entered into the ID Tracker.)


No.


  (1.g) Has the Document Shepherd personally verified that the
        document satisfies all ID nits? (See the Internet-Drafts Checklist
        and http://tools.ietf.org/tools/idnits/). Boilerplate checks are
        not enough; this check needs to be thorough. Has the document
        met all formal review criteria it needs to, such as the MIB
        Doctor, media type and URI type reviews?



There are three IDNITS warnings, two of which appear to be spurious, and
the third is merely an unused reference, which the RFC Editor can correct.



  (1.h) Has the document split its references into normative and
        informative? Are there normative references to documents that
        are not ready for advancement or are otherwise in an unclear
        state? If such normative references exist, what is the
        strategy for their completion? Are there normative references
        that are downward references, as described in [RFC3967]? If
        so, list these downward references to support the Area
        Director in the Last Call procedure for them [RFC3967].


The references are properly split.



  (1.i) Has the Document Shepherd verified that the document IANA
        consideration section exists and is consistent with the body
        of the document? If the document specifies protocol
        extensions, are reservations requested in appropriate IANA
        registries? Are the IANA registries clearly identified? If
        the document creates a new registry, does it define the
        proposed initial contents of the registry and an allocation
        procedure for future registrations? Does it suggest a
        reasonable name for the new registry? See [RFC5226]. If the
        document describes an Expert Review process has Shepherd
        conferred with the Responsible Area Director so that the IESG
        can appoint the needed Expert during the IESG Evaluation?



The IANA Considerations are present and specify no actions for IANA.



  (1.j) Has the Document Shepherd verified that sections of the
        document that are written in a formal language, such as XML
        code, BNF rules, MIB definitions, etc., validate correctly in
        an automated checker?


Not Applicable.



  (1.k) The IESG approval announcement includes a Document
        Announcement Write-Up. Please provide such a Document
        Announcement Write-Up? Recent examples can be found in the
        "Action" announcements for approved documents. The approval
        announcement contains the following sections:

    Technical Summary
        Relevant content can frequently be found in the abstract
        and/or introduction of the document. If not, this may be
        an indication that there are deficiencies in the abstract
        or introduction.


From abstract:

  This document analyzes how current TCP implementations process TCP
  urgent indications, and how the behavior of some widely-deployed
  middle-boxes affect how urgent indications are processed by end
  systems.  This document updates the relevant specifications such that
  they accommodate current practice in processing TCP urgent
  indications, provides advice to applications that make use of the
  urgent mechanism, and raises awareness about the reliability of TCP
  urgent indications in the current Internet.





    Working Group Summary
        Was there anything in WG process that is worth noting? For
        example, was there controversy about particular points or
        were there decisions where the consensus was particularly
        rough?

Nothing exceptional occurred during the working group process for this
document.


    Document Quality
        Are there existing implementations of the protocol? Have a
        significant number of vendors indicated their plan to
        implement the specification? Are there any reviewers that
        merit special mention as having done a thorough review,
        e.g., one that resulted in important changes or a
        conclusion that the document had no substantive issues? If
        there was a MIB Doctor, Media Type or other expert review,
        what was its course (briefly)? In the case of a Media Type
        review, on what date was the request posted?

This document specifically is aimed at bringing the specification and
existing implementations into line clearly with each other.  The
implementation practices in this document have been adopted in multiple
implementations and this document provides guidance to aid in future
compatibility with applications in new and old TCP implementations.