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TFRC for Voice: the VoIP Variant

This document discusses the TFRC variant for Voice over IP (VoIP), emphasizing fairness in bandwidth allocation and packet transmission intervals. It covers modifications, restrictions, notions of fairness, oscillations in loss event rates, and TFRC-SP and TFRC-PS variants. Additional details include calculations, simulations, and future considerations.

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TFRC for Voice: the VoIP Variant

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  1. TFRC for Voice: the VoIP Variant Sally Floyd, Eddie Kohler. November 2006 draft-ietf-dccp-tfrc-voip-06.txt DCCP Working Group, IETF Slides: http://www.icir.org/floyd/talks.html Graphics: http://www.icir.org/floyd/papers/voipimages-06.pdf

  2. VoIP: fairness in Bps. • In the TCP throughput equation, use the measured loss event rate and a packet size of 1460 bytes. • Reduce the allowed transmit rate to account for the fraction of the VoIP bandwidth that would be used by 40-byte headers: • Enforce a Min Interval between packets of 10 ms. • For short loss intervals (at most two RTTs), count the actual packet loss rate (but don’t increase the number of loss intervals).

  3. Changes from this WG Last Call: • Added a restriction that the most recent loss interval is not included in the calculation of the average loss interval if the most recent loss interval is short. • Added a discussion to Section 8 on “Fairness with different packet header sizes”. • Added Appendix C on “Exploring Possible Oscillations in the Loss Event Rate” . • Added a paragraph about “TFRC-SP” and “TFRC-PS”. • Moved simulations to the appendix. • Various editing changes, rephrasing, and bug fixes.

  4. Adding a restriction: • “Section 5.4 of RFC 3448 specifies that the calculation of the average loss interval includes the most recent loss interval only if this increases the calculated average loss interval. • TFRC-SP adds the restriction that the calculation of the average loss interval can include the most recent loss interval only if more than two round-trip times have passed since the beginning of that loss interval.”

  5. Exploring Possible Oscillations in the Loss Event Rate: • What happens when the loss interval size oscillates between short and not-short? • Are there oscillations in the estimate of the average packet drop rate? • In simulations, we didn’t see any problems. • So we decided not to change the method for estimating the loss interval size for short intervals.

  6. “TFRC-SP” and “TFRC-PS”. • TFRC-SP: the variant of TFRC specified by this internet-draft. • TFRC-PS: from RFC 3448, for TFRC-PacketSize. • Refers to a variant of TFRC for applications with a fixed rate, but that can vary their packet size in response to congestion. • The questions of how an adaptive application would use TFRC-SP, varying its packet size, are beyond the scope of this document. • This needs to be addressed in a document that is more application-specific.

  7. “TFRC-SP” and “TFRC-PS”. • “RFC 3448, the protocol specification for TFRC, stated that TFRC-PS (for TFRC-PacketSize), a variant of TFRC for applications that have a fixed sending rate but vary their packet size in response to congestion, would be specified in a later document. • This document instead specifies TFRC-SP, a variant of TFRC designed for applications that send small packets, where applications could either have a fixed or varying packet size or could adapt their packet size in response to congestion. • However, as discussed in Section 6 of this document, there are many questions about how such an adaptive application would use TFRC-SP that are beyond the scope of this document, and that would need to be addressed in documents that are more application-specific.”

  8. Still to do, from recent email: • Add pseudocode about the change of not using the current interval in estimating the loss event rate if the current interval is short. • Email from Ladan Gharai. • Say more about TFRC-SP on paths where the MTU is less than 1500 bytes. • Email from Gorry Fairhurst. • Say more about apps gaming about the packet size. • Email from Gorry.

  9. Thanks. • Thanks to Lars Eggert, Gorry Fairhurst, Ladan Gharai, and Mark Handley for feedback on this round.

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