Current Meeting Report

2.8.12 Reliable Multicast Transport (rmt)

In addition to this official charter maintained by the IETF Secretariat, there is additional information about this working group on the Web at: -- Additional RMT Web Page
NOTE: This charter is a snapshot of the 54th IETF Meeting in Yokohama, Japan. It may now be out-of-date.

Last Modifield: 05/30/2002

Roger Kermode <>
L Vicisano <>
Transport Area Director(s):
Scott Bradner <>
A. Mankin <>
Transport Area Advisor:
A. Mankin <>
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Description of Working Group:
Additional RMT web page:

The purpose of this WG is to standardize reliable multicast transport.

Initial efforts will focus solely on the standardization of the one-to-many transport of large amounts of data. Due to the large number of applications that fall into this category, and the sometimes orthogonal requirements these applications exhibit, it is believed that a "one size fits all" protocol will be unable to meet the requirements of all applications. In recognition of this observation, this working group expects to initially standardize three protocol instantiations, one each from the the following three families:

1) A NACK-based protocol 2) A Tree-based ACK protocol 3) An "Asynchronous Layered Coding protocol that uses Forward Error Correction

The WG will carry out protocol standardization by composing a set of RFCs that specify

- building blocks: A set of easily-separable coarse-grained modular components that are common to multiple protocols along with abstract APIs that define a building block's access methods and their arguments.

- protocol instantiations: Specifications that define the necessary gluing logic and minimal additional functionality required to realize a working protocol from one or more building blocks. These specifications will also include an abstract API that defines the interface between the protocol implementation and an application.

To assist in this standardization process, the WG will also complete work on three documents. The first describes the design-space in which the three one-to-many transport protocols will be developed. The second explains the concepts of building-blocks and protocol instantiations. The third provides guidelines to authors of drafts that specify building-blocks and protocol instantiations. These three documents will be initially submitted as WG drafts and subsequently proposed as Informational RFC.

The WG will generate and submit for standardization drafts of the following building-blocks for use in the construction of the three protocols: congestion control, negative acknowledgments, forward error correction, generic mechanisms for router assist, and to address the RFC 2357 security requirements.

The WG will also standardize and generate RFCs for the following three protocol instantiations: A NACK-based protocol, A Tree-based ACK protocol and an Open Loop protocol that uses Forward Error Correction.

If new requirements are identified that cannot be satisfied with the building-blocks and protocol instantiations described above, the Area Directors in consultation with the IESG may add additional building-blocks and protocol instantiations to the working group deliverables.

This working group will work closely with the Internet Research Task Force (IRTF) groups on Reliable Multicast (RMRG) and Secure Multicast (SMUG), especially for meeting the congestion control and security requirements mandated by RFC 2357. This working group may work with the Area Directors to recharter to standardize reliable multicast for additional scenarios beyond the one-to-many transport of bulk data once they are sufficiently well understood.

Goals and Milestones:
Done  Submit design-space, building-blocks, and guidelines drafts for publication as Informational RFCs
Done  Initial Drafts for the following building blocks: negative acknowledgments, forward error correction, a generic signaling mechanism for router assist, and transport protection
Done  Submit Initial Drafts for the three protocol instantiations.
Done  Review drafts at the Adelaide IETF
Done  Submit Initial Draft for Congestion Control
Done  Complete building-block drafts WG Last-Call and submit for publication as Proposed Standard
JUN 00  Protocol instantiations drafts submitted for publication as Proposed Standard.
DEC 01  Congestion control draft submitted for publication as Proposed Standard.
  • - draft-ietf-rmt-bb-fec-06.txt
  • - draft-ietf-rmt-pi-alc-08.txt
  • - draft-ietf-rmt-bb-norm-04.txt
  • - draft-ietf-rmt-bb-lct-04.txt
  • - draft-ietf-rmt-info-fec-02.txt
  • - draft-ietf-rmt-pi-norm-04.txt
  • - draft-ietf-rmt-bb-pgmcc-01.txt
  • - draft-ietf-rmt-bb-webrc-02.txt
  • Request For Comments:
    RFC2887 I The Reliable Multicast Design Space for Bulk Data Transfer
    RFC3048 I Reliable Multicast Transport Building Blocks for One-to-Many Bulk-Data Transfer
    RFC3269 I Author Guidelines for RMT Building Blocks and Protocol Instantiation documents

    Current Meeting Report

    The RMT Working Group met in Yokohama on July 16th, 2002.

    The meeting was opened with a summary report on the WG status presented by Roger
    Kermode, followed by update-presentations on

    the GRA signaling draft (draft-ietf-rmt-bb-gra-signalling-00.txt),
    on WEBRC congestion control (draft-ietf-rmt-bb-webrc-02.txt),
    on PGMCC congestion control (draft-ietf-rmt-bb-pgmcc-01.txt)
    and on the NORM building block (draft-ietf-rmt-bb-norm-04.txt).

    The meeting ended with a discussion on the WG progress and on future plans.

    The status-report was given by general protocol areas: most of the ALC-related
    drafts are undergoing IESG review following the submission as experimental RFCs.
    The only non-ready draft in this area is the congestion control one
    (draft-ietf-rmt-bb-webrc-02.txt). Some of the ALC drafts under IESG examination
    are also part of the NORM protocol. The main drafts related to NORM are in good
    status of advancement (draft-ietf-rmt-bb-norm-04.txt and
    draft-ietf-rmt-pi-norm-04.txt). As for the congestion control for NORM, PGMCC
    (draft-ietf-rmt-bb-pgmcc-01.txt) is current and almost complete while TFMCC is
    expired. However the last submitted version of TFMCC was almost complete. All
    the drafts specifically intended for the TRACK protocol are expired and none of
    the authors was present at the meeting [more on this later]. Finally the work on
    GRA has just restarted and it is still at an early stage.

    Tony Speakman discussed the advancement on GRA signaling. The GRA signaling
    draft, that will be submitted to the IETF directory after the meeting,
    concentrates on a general framework for end-host to request network elements to
    execute GRA function. The specific GRA functions are to be defined in separate
    specification(s). The presentation was followed by a technical discussion on the
    draft content, with two main hilights: the G-bit used to detect the GRA header
    in transport packet formats needs to be considered by the other authors of RMT
    protocol specifications, and the granularity used by the draft to describe
    network-element functions might be revised (made finer). Also the future of the
    other two GRA drafts that are expired was discussed. These will not be
    resubmitted in the short term, but might be updated at a later stage, with the
    intent of providing information on the GRA framework (the intended outcome is
    informational RFCs).

    Mike Luby presented an update on the WEBRC congestion control. This congestion
    control scheme is also discussed in a technical paper to appear in the
    proceedings of Sigcomm'2002. NS code will be available by August 1, some
    simulations in real networks have already been done but there is a need to do
    real-world experiments. The goal is for this to be an RFC by March 2003 at the

    Gianluca Iannaccone gave a brief update on PGMCC, that was revised to comply
    with the 'author guidelines" document and to fix some technical details. Next
    step is to develop more on the interaction between PGMCC and feedback
    suppression mechanisms. Two additional action-items for the draft authors came
    from the discussion: add a description of a possible attack from a malicious
    receiver that causes itself to be elected as the "acker" by sending a large
    amount of NAKs, but then drives the session rate up by ACKing in an inconsistent
    way. The specification should require implementors to protect the network
    against this type of attack through heuristics that compare the loss-rate
    reported by NAKs and the one inferred from ACKs. The other action item was to
    experiment with the protocol in real networks.

    Brian Adamson gave updates on the NORM BB draft and presented some simulation
    results (the code of his simulation experiments is shared with the reference
    implementation that he has). Next steps are: collecting more feedback from the
    WG, working on congestion control support (both TFMCC and PGMCC) and GRA
    support. There was some feedback on the NORM document structure: Roger Kermode
    suggested to revise it to comply to the RMT authors' guidelines, Tony Speakman
    to move the content around between the BB and the PI document to improve
    modularity and chance of BB reuse.

    An evaluation of the WG status an discussion on future action items closed the

    Most of the work in the charter is close to completion with the exception of the
    TRACK protocol and GRA.

    As for TRACK, it was not possible to collect information on the authors'
    intentions, as none of them was present, however the feeling of the WG was that
    the status of the work is probably an indication of lack of interest in pursuing
    it further. The plan is to stop the work on TRACK and change the WG charter
    accordingly, unless there is a clear indication/commitment from the TRACK

    The work on GRA is being carried on but is substantially behind compared to the
    rest, hence the WG will proceed towards the publication of the various BBs and
    PIs as Experimental RFC. If GRA is not ready at the time of the publication, the
    final integration work will be done in a later revision of the specs.

    Finally two possible new working items for the WG were discussed. The first was
    a session control protocol, the action item was to form a team to gather
    requirements, before making any decision on whether this work should be done in
    the WG or not. Mike Luby volunteered to lead the effort with the help of Tony
    Speakman and Roger Kermode.

    The second proposal was to open the WG to non-one-to-many RMTs. The WG
    orientation was not to pursue it at this time. The rationale behind this
    decision was to have time to consolidate the results currently achieved and
    observe how deployment plays out, before starting to standardize new pieces of


    GRA Signalling Protocol