Re: [IPFIX] New Version Notification for draft-trammell-ipfix-tcpcontrolbits-revision-00.txt

[Lothar Braun <[e-mail]>]

Hi,

I have some clarification questions/comments inline (I'm commenting on version 02)


> 
> 
> IPFIX Working Group                                          B. Trammell
> Internet-Draft                                                ETH Zurich
> Intended status: Informational                                 P. Aitken
> Expires: March 13, 2014                               Cisco Systems, Inc
>                                                       September 09, 2013
> 
> 
>         Revision of the tcpControlBits IPFIX Information Element
>           draft-trammell-ipfix-tcpcontrolbits-revision-02.txt
> 
> Abstract
> 
>    This document revises the tcpControlBits IPFIX Information Element
>    defined in [RFC5102] to reflect changes to the TCP Flags header field
>    since [RFC0793].
> 
> Status of This Memo
> 
>    This Internet-Draft is submitted in full conformance with the
>    provisions of BCP 78 and BCP 79.
> 
>    Internet-Drafts are working documents of the Internet Engineering
>    Task Force (IETF).  Note that other groups may also distribute
>    working documents as Internet-Drafts.  The list of current Internet-
>    Drafts is at http://datatracker.ietf.org/drafts/current/.
> 
>    Internet-Drafts are draft documents valid for a maximum of six months
>    and may be updated, replaced, or obsoleted by other documents at any
>    time.  It is inappropriate to use Internet-Drafts as reference
>    material or to cite them other than as "work in progress."
> 
>    This Internet-Draft will expire on March 13, 2014.
> 
> Copyright Notice
> 
>    Copyright (c) 2013 IETF Trust and the persons identified as the
>    document authors.  All rights reserved.
> 
>    This document is subject to BCP 78 and the IETF Trust's Legal
>    Provisions Relating to IETF Documents
>    (http://trustee.ietf.org/license-info) in effect on the date of
>    publication of this document.  Please review these documents
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>    include Simplified BSD License text as described in Section 4.e of
>    the Trust Legal Provisions and are provided without warranty as
>    described in the Simplified BSD License.
> 
> 
> 
> 
> Trammell & Aitken        Expires March 13, 2014                 [Page 1]
> Internet-Draft            IPFIX tcpControlBits            September 2013
> 
> 
> 1.  Introduction
> 
>    Octets 12 and 13 of the TCP header encode the data offset (header
>    length) in four bits, as well as 12 bits of flags.  The least
>    significant 6 bits of these were defined in [RFC0793] as URG, ACK,
>    PSH, RST, SYN, and FIN for TCP control.  Subsequently, [RFC3168]
>    defined the CWR and ECE flags for Explicit Congestion Notification
>    (ECN) negotiation and signaling; [RFC3540] additionally defined the
>    NS flag for the ECN Nonce Sum.
> 
>    As defined in the IANA IPFIX Information Element Registry
>    [IANA-IPFIX], taken from [RFC5102], the tcpControlBits Information
>    Element for IPFIX [I-D.ietf-ipfix-protocol-rfc5101bis] only covers
>    the original six bits from [RFC0793].  To allow IPFIX to be used to
>    measure the use of ECN, and to bring the IPFIX Information Element
>    definition in line with the current definition of the TCP Flags
>    header field, it is necessary to revise this definition.
> 
>    The revised definition of the Information Element in Section 2 was
>    developed and approved through the IE-DOCTORS process
>    [I-D.ietf-ipfix-ie-doctors] in August 2013.  Section 5.1 of
>    [I-D.ietf-ipfix-ie-doctors] states "This process should not in any
>    way be construed as allowing the IE-DOCTORS to overrule IETF
>    consensus.  Specifically, Information Elements in the IANA IE
>    registry which were added with IETF consensus require IETF consensus
>    for revision or deprecation".  Since the tcpControlBits Information
>    Element was defined in [RFC5102], an IETF Proposed Standard, any
>    revision of this Information Element definition requires IETF
>    Consensus.  The publication of this document fulfills that
>    requirement.
> 
>    The following section defines the revised tcpControlBits Information
>    Element as in Section 9.1 of [I-D.ietf-ipfix-ie-doctors].
> 
> 2.  The tcpControlBits Information Element
> 
>    ElementId:   6
>    Data Type:   unsigned16
>    Data Type Semantics:   flags
>    Description:   TCP control bits observed for the packets of this
>       Flow.  This information is encoded as a bit field; for each TCP
>       control bit, there is a bit in this set.  The bit is set to 1 if
>       any observed packet of this Flow has the corresponding TCP control
>       bit set to 1.  The bit is cleared to 0 otherwise.
> 
>       The values of each bit are shown below, per the definition of the
>       bits in the TCP header [RFC0793]:
> 
> 
> 
> 
> Trammell & Aitken        Expires March 13, 2014                 [Page 2]
> Internet-Draft            IPFIX tcpControlBits            September 2013
> 
> 
>     MSb                                                         LSb
>      0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
>    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
>    |               |           | N | C | E | U | A | P | R | S | F |
>    |     Zero      |   Future  | S | W | C | R | C | S | S | Y | I |
>    | (Data Offset) |    Use    |   | R | E | G | K | H | T | N | N |
>    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
> 
>    bit    flag
>    value  name  description
>    ------+-----+-------------------------------------
>    0x8000       Zero (see tcpHeaderLength)
>    0x4000       Zero (see tcpHeaderLength)
>    0x2000       Zero (see tcpHeaderLength)
>    0x1000       Zero (see tcpHeaderLength)
>    0x0800       Future Use
>    0x0400       Future Use
>    0x0200       Future Use
>    0x0100   NS  ECN Nonce Sum
>    0x0080  CWR  Congestion Window Reduced
>    0x0040  ECE  ECN Echo
>    0x0020  URG  Urgent Pointer field significant
>    0x0010  ACK  Acknowledgment field significant
>    0x0008  PSH  Push Function
>    0x0004  RST  Reset the connection
>    0x0002  SYN  Synchronize sequence numbers
>    0x0001  FIN  No more data from sender
> 
> 
>       As the most significant four bits of octets 12 and 13 of the TCP
>       header [RFC0793] are used to encode the TCP data offset (header
>       length), the corresponding bits in this IE must be exported as
>       zero and must be ignored by the collector; use the tcpHeaderLength
>       Information Element to encode this value.

Are there good reasons for the two "must" in this sentence? (And shouldn't this be a MUST if this really should be zero?). 

We could, at some point in the future, decide to use those four bits for something else. This happened for example with the CWR and ECE bits of the previous definition that had a requirement to be zero. Later on, it turned out that those bits had to be used for encoding something that the original definition hadn't thought about. 

I understand that these four bits are not likely to be used in the TCP header length to encode any useful information for flow data, but we might decide to use those four bits for encoding other information, e.g. some more information about the TCP connection state (or whatever else we might think about). 

What I would like to have would be some text that sets the bits to reserved and says something like:

"Bits 0x1000 through 0x.8000 are reserved for future use. A collector must not assume to receive information about the TCP header length; the field tcpHeaderLength Information Element should be used to encode this value."

What do you think?


>       Each of the three future use bits (0x800, 0x400, and 0x200) should
>       be exported as one if the corresponding bit is observed in the TCP
>       headers of the packets of this Flow, as they may be subsequent to
>       a future update of [RFC0793].

What does "as one" in "exported AS ONE if" mean? Does this mean that these bits must be exported as seen in the packet headers?

>       If exported as a single octet with reduced length encoding,

Nit: RFC 5101 calls it reduced size encoding (at least most of the time ;))

> this
>       Information Element covers the low-order octet of this field (i.e,
>       bits 0x80 to 0x01), omitting the ECN Nonce Sum and the three
>       Future Use bits.  A collector receiving this Information Element
>       with reduced length encoding must not assume anything about the
>       content of these four bits.

I just wanted to raise this point (without objecting). This would clearly be an exception to what I would expect from reduced size encoding in RFC 5101bis:

   The reduction in size can be to any number of
   octets smaller than the original type if the data value still fits,
   i.e., so that only leading zeroes are dropped.

Whenever I see reduced size encoding, I assume that there is no further information in the field and that all leading bits are zero because of this text. But I would assume this is not a hard requirement from RFC 5101(bis), and it is probably ok if the data type has an explicit definition that states that the first bits are not zero.

> 
> 
> Trammell & Aitken        Expires March 13, 2014                 [Page 3]
> Internet-Draft            IPFIX tcpControlBits            September 2013
> 
> 
>       Note that previous revisions of this Information Element's
>       definition specified that the CWR and ECE bits must be exported as
>       zero, even if observed.  Collectors should therefore not assume
>       that a value of zero for these bits in this Information Element
>       indicates the bits were never set in the observed traffic,
>       especially if these bits are zero in every Flow Record sent by a
>       given exporter.
>    References:   [RFC0793][RFC3168][RFC3540]
>    Revision:  1
> 
> 3.  IANA Considerations
> 
>    IANA will update the definition of the tcpControlBits Information
>    Element in the the IANA IPFIX Information Element Registry
>    [IANA-IPFIX] to reflect the changes in Section 2 above.
> 
> 4.  Security and Privacy Considerations
> 
>    This document has no security or privacy considerations; the security
>    considerations for IPFIX [I-D.ietf-ipfix-protocol-rfc5101bis] apply.
> 
> 5.  Acknowledgments
> 
>    Thanks to Andrew Feren for comments on the revised definition.  This
>    work is partially supported by the European Commission under grant
>    agreement FP7-ICT-318627 mPlane; this does not imply endorsement by
>    the Commission.
> 
> 6.  References
> 
> 6.1.  Normative References
> 
>    [I-D.ietf-ipfix-protocol-rfc5101bis]
>               Claise, B. and B. Trammell, "Specification of the IP Flow
>               Information eXport (IPFIX) Protocol for the Exchange of
>               Flow Information", draft-ietf-ipfix-protocol-rfc5101bis-10
>               (work in progress), July 2013.
> 
>    [I-D.ietf-ipfix-ie-doctors]
>               Trammell, B. and B. Claise, "Guidelines for Authors and
>               Reviewers of IPFIX Information Elements", draft-ietf-
>               ipfix-ie-doctors-07 (work in progress), October 2012.
> 
>    [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
>               793, September 1981.
> 
> 
> 
> 
> 
> 
> Trammell & Aitken        Expires March 13, 2014                 [Page 4]
> Internet-Draft            IPFIX tcpControlBits            September 2013
> 
> 
>    [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
>               of Explicit Congestion Notification (ECN) to IP", RFC
>               3168, September 2001.
> 
>    [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
>               Congestion Notification (ECN) Signaling with Nonces", RFC
>               3540, June 2003.
> 
> 6.2.  Informative References
> 
>    [RFC5102]  Quittek, J., Bryant, S., Claise, B., Aitken, P., and J.
>               Meyer, "Information Model for IP Flow Information Export",
>               RFC 5102, January 2008.
> 
>    [IANA-IPFIX]
>               Internet Assigned Numbers Authority, ., "IP Flow
>               Information Export Information Elements
>               (http://www.iana.org/assignments/ipfix)", .
> 
> Authors' Addresses
> 
>    Brian Trammell
>    Swiss Federal Institute of Technology Zurich
>    Gloriastrasse 35
>    8092 Zurich
>    Switzerland
> 
>    Phone: [phone]
>    Email: [e-mail]
> 
> 
>    Paul Aitken
>    Cisco Systems, Inc.
>    96 Commercial Quay
>    Commercial Street, Edinburgh EH6 6LX
>    United Kingdom
> 
>    Phone: [phone]
>    Email: [e-mail]



--
Lothar Braun
Chair for Network Architectures and Services (I8)
Department of Informatics
Technische Universität München
Boltzmannstr. 3, 85748 Garching bei München, Germany
Phone:  [phone]       Fax: [phone]
E-mail: [e-mail]