RFC 6958
RTP Control Protocol (RTCP) Extended Report (XR) Block for Burst/Gap Loss Metric Reporting
Internet Engineering Task Force (IETF) A. Clark Request for Comments: 6958 Telchemy Category: Standards Track S. Zhang ISSN: 2070-1721 J. Zhao STTRI Q. Wu, Ed. Huawei May 2013 RTP Control Protocol (RTCP) Extended Report (XR) Block for Burst/Gap Loss Metric ReportingAbstract
This document defines an RTP Control Protocol (RTCP) Extended Report (XR) Block that allows the reporting of burst and gap loss metrics for use in a range of RTP applications. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6958. 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 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Burst/Gap Loss Metrics Block . . . . . . . . . . . . . . 2 1.2. RTCP and RTCP XR Reports . . . . . . . . . . . . . . . . 3 1.3. Performance Metrics Framework . . . . . . . . . . . . . . 3 1.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Burst/Gap Loss Metrics Block . . . . . . . . . . . . . . . . 4 3.1. Report Block Structure . . . . . . . . . . . . . . . . . 5 3.2. Definition of Fields in Burst/Gap Loss Metrics Block . . 5 3.3. Derived Metrics Based on Reported Metrics . . . . . . . . 7 4. Considerations for Voice-over-IP Applications . . . . . . . . 8 5. SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 9 5.1. SDP rtcp-xr-attrib Attribute Extension . . . . . . . . . 9 5.2. Offer/Answer Usage . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6.1. New RTCP XR Block Type Value . . . . . . . . . . . . . . 9 6.2. New RTCP XR SDP Parameter . . . . . . . . . . . . . . . . 9 6.3. Contact Information for Registrations . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 10.1. Normative References . . . . . . . . . . . . . . . . . . 11 10.2. Informative References . . . . . . . . . . . . . . . . . 11 Appendix A. Metrics Represented Using the Template from RFC 6390 131. Introduction
1.1. Burst/Gap Loss Metrics Block
This document defines a new block type to augment those defined in [RFC3611] for use in a range of RTP applications. The new block type supports the reporting of the proportion of packets lost by the network. The losses during loss bursts are reported, together with the number of bursts and additional data, allowing the calculation of statistical parameters (mean and variance) of the distribution of burst lengths. Some uses of these metrics depend on the availability of the metric "cumulative number of packets lost" from RTCP [RFC3550]. This block provides information on transient IP problems. Burst/gap metrics are typically used in Cumulative reports; however, they also may be used in Interval reports. The burstiness of packet loss affects user experience, may influence any sender strategies to mitigate the problem, and may also have diagnostic value.
The metric belongs to the class of transport-related end system metrics defined in [RFC6792]. The definitions of "burst", "gap", "loss", and "discard" are consistent with definitions in [RFC3611]. To accommodate the range of jitter buffer algorithms and packet discard logic that may be used by implementors, the method used to distinguish between bursts and gaps may be an equivalent method to that defined in [RFC3611]. The method used should produce the same result as that defined in [RFC3611] for conditions of burst packet loss but may produce different results for conditions of time-varying jitter.1.2. RTCP and RTCP XR Reports
The use of RTCP for reporting is defined in [RFC3550]. [RFC3611] defines an extensible structure for reporting by using an RTCP Extended Report (XR). This document defines a new Extended Report block for use with [RFC3550] and [RFC3611].1.3. Performance Metrics Framework
The Performance Metrics Framework [RFC6390] provides guidance on the definition and specification of performance metrics. The "Guidelines for Use of the RTP Monitoring Framework" [RFC6792] provides guidelines for reporting block format using RTCP XR. The Metrics Block described in this document is in accordance with the guidelines in [RFC6390] and [RFC6792].1.4. Applicability
These metrics are applicable to a range of RTP applications that contain jitter buffers and don't use stream repair means, e.g., Forward Error Correction (FEC) [RFC5109] and/or retransmission [RFC4588].2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
In addition, the following terms are defined:
Received, Lost, and Discarded
A packet shall be regarded as lost if it fails to arrive within an
implementation-specific time window. A packet that arrives within
this time window but is too early or late to be played out or
thrown away before playout due to packet duplication or redundancy
shall be regarded as discarded. A packet shall be classified as
one of received (or OK), discarded, or lost. The metric
"cumulative number of packets lost" defined in [RFC3550] reports a
count of packets lost from the media stream (single
Synchronization Source (SSRC) within a single RTP session).
Similarly, the metric "number of packets discarded" defined in
[DISCARD] reports a count of packets discarded from the media
stream (single SSRC within single RTP session) arriving at the
receiver. The post-repair Loss RLE metric, which is defined in
[RFC5725], can be used to report the number of packets that are
not recovered by any repair techniques that are in use.
Bursts and Gaps
The terms "burst" and "gap" are used in a manner consistent with
that of RTCP XR [RFC3611]. RTCP XR views an RTP stream as being
divided into bursts, which are periods when the loss rate is high
enough to cause noticeable quality degradation (generally over 5
percent loss rate) and gaps, which are periods when lost packets
are infrequent and hence quality is generally acceptable.
3. Burst/Gap Loss Metrics Block
Metrics in this block report on burst/gap loss in the stream arriving
at the RTP system. The measurement of these metrics is made at the
receiving end of the RTP stream. Each instance of this Metrics Block
refers by SSRC to a separate auxiliary Measurement Information Block
[RFC6776], which describes the measurement periods in use (see RFC
6776, Section 4.2).
This Metrics Block relies on the measurement period in the
Measurement Information Block indicating the span of the report.
Senders MUST send this block in the same compound RTCP packet as the
Measurement Information Block. Receivers MUST verify that the
measurement period is received in the same compound RTCP packet as
this Metrics Block. If not, this Metrics Block MUST be discarded.
3.1. Report Block Structure
The structure of the Burst/Gap Loss Metrics Block is as follows. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | BT=20 | I |C| resv. | Block Length = 5 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of Source | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+-+-+-+-+-+-+-+ | Threshold | Sum of Burst Durations (ms) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Packets Lost in Bursts | Total... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ...Packets Expected in Bursts | Number of Bursts | Sum of| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ...Squares of Burst Durations (ms-squared) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Report Block Structure3.2. Definition of Fields in Burst/Gap Loss Metrics Block
Block Type (BT): 8 bits A Burst/Gap Loss Metrics Block is identified by the constant 20. Interval Metric flag (I): 2 bits This field is used to indicate whether the burst/gap loss metrics are Sampled, Interval, or Cumulative metrics: I=10: Interval Duration - the reported value applies to the most recent measurement interval duration between successive metrics reports. I=11: Cumulative Duration - the reported value applies to the accumulation period characteristic of cumulative measurements. I=01: Sampled Value - the reported value is a sampled instantaneous value. In this document, burst/gap loss metrics can only be measured over definite intervals and cannot be sampled. Also, the value I=00 is reserved for future use. Senders MUST NOT use the values I=00 or I=01. If a block is received with I=00 or I=01, the receiver MUST discard the block.
Loss and Discard Combination flag (C): 1 bit
The 'C' flag is used to indicate whether the loss/discard report
is combined with the burst/gap loss report in the same compound
RTCP packet. The value is set to '1' if the loss/discard report
and the burst gap loss report are combined. Otherwise, the value
is set to '0'. If the 'C' flag is set to '1' but the burst/gap
discard was not sent, the receiver MUST discard the burst/gap
loss.
Reserved (resv.): 5 bits
These bits are reserved. They MUST be set to zero by senders and
ignored by receivers (see [RFC6709], Section 4.2).
Block Length: 16 bits
The length of this report block in 32-bit words, minus one. For
the Burst/Gap Loss Metrics Block, the block length is equal to 5.
The block MUST be discarded if the block length is set to a
different value.
SSRC of Source: 32 bits
As defined in Section 4.1 of [RFC3611].
Threshold: 8 bits
The Threshold is equivalent to Gmin in [RFC3611], i.e., the number
of successive packets that must be received prior to and following
a lost packet in order for this lost packet to be regarded as part
of a gap. Note that the threshold is calculated in accordance
with the Gmin Calculation defined in Section 4.7.2 of RFC 3611.
Sum of Burst Durations (ms): 24 bits
The total duration of bursts of lost packets in the period of the
report (Interval or Cumulative).
The measured value is an unsigned value. If the measured value
exceeds 0xFFFFFD, the value 0xFFFFFE MUST be reported to indicate
an over-range measurement. If the measurement is unavailable, the
value 0xFFFFFF MUST be reported.
Packets Lost in Bursts: 24 bits
The total number of packets lost during loss bursts.
The measured value is an unsigned value. If the measured value
exceeds 0xFFFFFD, the value 0xFFFFFE MUST be reported to indicate
an over-range measurement. If the measurement is unavailable, the
value 0xFFFFFF MUST be reported.
Total Packets Expected in Bursts: 24 bits
The total number of packets expected during loss bursts (that is,
the sum of received packets and lost packets).
The measured value is an unsigned value. If the measured value
exceeds 0xFFFFFD, the value 0xFFFFFE MUST be reported to indicate
an over-range measurement. If the measurement is unavailable, the
value 0xFFFFFF MUST be reported.
Number of Bursts: 16 bits
The number of bursts in the period of the report (Interval or
Cumulative).
The measured value is an unsigned value. If the measured value
exceeds 0xFFFD, the value 0xFFFE MUST be reported to indicate an
over-range measurement. If the measurement is unavailable, the
value 0xFFFF MUST be reported.
Sum of Squares of Burst Durations (ms-squared): 36 bits
The sum of the squares of burst durations (where individual burst
durations are expressed in ms) in the period of the report
(Interval or Cumulative). The units for this quantity are
milliseconds-squared.
The measured value is an unsigned value. If the measured value
exceeds 0xFFFFFFFFD, the value 0xFFFFFFFFE MUST be reported to
indicate an over-range measurement. If the measurement is
unavailable, the value 0xFFFFFFFFF MUST be reported.
3.3. Derived Metrics Based on Reported Metrics
The metrics described here are intended to be used as described in
this section, in conjunction with information from the Measurement
Information Block [RFC6776] and also with the metric "cumulative
number of packets lost" provided in standard RTCP [RFC3550].
These metrics provide information relevant to statistical parameters,
including:
o the fraction of packets lost during bursts (i.e., Burst Loss Rate
in [SUMSTAT]), which can be calculated using the metric "Packets
Loss in Bursts" and the metric "Total Packets Expected in Bursts"
provided in the Burst/Gap Loss Metrics Block.
o the fraction of packets lost during gaps (i.e., Gap Loss Rate in
[SUMSTAT]), which can be calculated using the metric "Packets Loss
in Bursts" and the metric "Total Packets Expected in Bursts"
provided in the Burst/Gap Loss Metrics Block.
o burst duration mean [SUMSTAT], which can be calculated using the
metric "Packets Loss in Bursts" and the metric "Total Packets
Expected in Bursts" provided in the Burst/Gap Loss Metrics Block.
o burst duration variance [SUMSTAT], which can be calculated using
the metric "Packets Loss in Bursts" and the metric "Total Packets
Expected in Bursts" provided in the Burst/Gap Loss Metrics Block.
The details on calculation of these parameters in the metrics are
described in [SUMSTAT].
4. Considerations for Voice-over-IP Applications
This Metrics Block is applicable to a broad range of RTP
applications. Where the metric is used with a Voice-over-IP (VoIP)
application and the stream repair means is not available, the
following considerations apply.
RTCP XR views a call as being divided into bursts, which are periods
when the loss rate is high enough to cause noticeable call quality
degradation (generally over 5 percent loss rate), and gaps, which are
periods when lost packets are infrequent and hence call quality is
generally acceptable.
If Voice Activity Detection is used, the Burst and Gap Durations
shall be determined as if silence packets had been sent, i.e., a
period of silence in excess of Gmin packets will terminate a burst
condition.
The recommended value for the threshold Gmin in [RFC3611] causes a
burst during which the call quality is degraded to a similar extent
as it would be during a typical Pulse Code Modulation (PCM) Severely
Errored Second.
5. SDP Signaling
[RFC3611] defines the use of the Session Description Protocol (SDP) [RFC4566] for signaling the use of XR blocks. XR blocks MAY be used without prior signaling.5.1. SDP rtcp-xr-attrib Attribute Extension
This section augments the SDP [RFC4566] attribute "rtcp-xr" defined in [RFC3611] by providing an additional value of "xr-format" to signal the use of the report block defined in this document. The ABNF [RFC5234] syntax is below. xr-format =/ xr-bgl-block xr-bgl-block = "burst-gap-loss"5.2. Offer/Answer Usage
When SDP is used in the offer/answer context, the SDP Offer/Answer usage defined in [RFC3611] for unilateral "rtcp-xr" attribute parameters applies. For detailed usage of offer/answer for unilateral parameters, refer to Section 5.2 of [RFC3611].6. IANA Considerations
New block types for RTCP XR are subject to IANA registration. For general guidelines on IANA considerations for RTCP XR, refer to [RFC3611].6.1. New RTCP XR Block Type Value
This document assigns the block type value 20 in the IANA "RTP Control Protocol Extended Reports (RTCP XR) Block Type Registry" to the "Burst/Gap Loss Metrics Block".6.2. New RTCP XR SDP Parameter
This document also registers a new parameter "burst-gap-loss" in the "RTP Control Protocol Extended Reports (RTCP XR) Session Description Protocol (SDP) Parameters Registry".
6.3. Contact Information for Registrations
The contact information for the registrations is: Qin Wu (sunseawq@huawei.com) 101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China7. Security Considerations
This block does not provide per-packet statistics, so the risk to confidentiality documented in Section 7, paragraph 3 of [RFC3611] does not apply. However, the gaps indicated within this block could be used to detect the timing of other events on the path between the sender and receiver. For example, a competing multimedia stream might cause a loss burst for the duration of the stream, allowing the receiver of this block to know when the competing stream was active. This risk is not a significant threat since the only information leaked is the timing of the loss, not the cause. Besides this, it is believed that this proposed RTCP XR report block introduces no other new security considerations beyond those described in [RFC3611].8. Contributors
Geoff Hunt wrote the initial draft of this document.9. Acknowledgments
The authors gratefully acknowledge reviews and feedback provided by Bruce Adams, Philip Arden, Amit Arora, Bob Biskner, Kevin Connor, Claus Dahm, Randy Ethier, Roni Even, Jim Frauenthal, Albert Higashi, Tom Hock, Shane Holthaus, Paul Jones, Rajesh Kumar, Keith Lantz, Mohamed Mostafa, Amy Pendleton, Colin Perkins, Mike Ramalho, Ravi Raviraj, Albrecht Schwarz, Tom Taylor, Hideaki Yamada, Adam Roach, Dan Romascanu, Chris Lonvick, Alfred C. Morton Jr., Pete Resnick, Ted Lemon, Stephen Farrell, Richard Barnes, and Benoit Claise.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, November 2003. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5725] Begen, A., Hsu, D., and M. Lague, "Post-Repair Loss RLE Report Block Type for RTP Control Protocol (RTCP) Extended Reports (XRs)", RFC 5725, February 2010.10.2. Informative References
[DISCARD] Clark, A., Huang, R., and Q. Wu, Ed., "RTP Control Protocol (RTCP) Extended Report (XR) Block for Discard Count metric Reporting", Work in Progress, April 2013. [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. Hakenberg, "RTP Retransmission Payload Format", RFC 4588, July 2006. [RFC5109] Li, A., "RTP Payload Format for Generic Forward Error Correction", RFC 5109, December 2007. [RFC6390] Clark, A. and B. Claise, "Guidelines for Considering New Performance Metric Development", BCP 170, RFC 6390, October 2011. [RFC6709] Carpenter, B., Aboba, B., and S. Cheshire, "Design Considerations for Protocol Extensions", RFC 6709, September 2012.
[RFC6776] Clark, A. and Q. Wu, "Measurement Identity and Information Reporting Using a Source Description (SDES) Item and an RTCP Extended Report (XR) Block", RFC 6776, October 2012. [RFC6792] Wu, Q., Hunt, G., and P. Arden, "Guidelines for Use of the RTP Monitoring Framework", RFC 6792, November 2012. [SUMSTAT] Zorn, G., Schott, R., Wu, Q., Ed., and R. Huang, "RTP Control Protocol (RTCP) Extended Report (XR) Blocks for Summary Statistics Metrics Reporting", Work in Progress, March 2013.
Appendix A. Metrics Represented Using the Template from RFC 6390
The six metrics defined in this document are described below using the template from Section 5.4.4 of RFC 6390. a. Threshold Metric * Metric Name: Threshold in RTP * Metric Description: The Threshold is equivalent to Gmin in [RFC3611], i.e., the number of successive RTP packets that must be received prior to and following a lost RTP packet in order for this lost RTP packet to be regarded as part of a gap. * Method of Measurement or Calculation: See Section 3.2, Threshold definition. * Units of Measurement: See Section 3.2, Threshold definition. * Measurement Point(s) with Potential Measurement Domain: See Section 3, 1st paragraph. * Measurement Timing: See Section 3, 2nd paragraph for measurement timing and Section 3.2 for Interval Metric flag. * Use and Applications: See Section 1.4. * Reporting Model: See RFC 3611. b. Sum of Burst Durations Metric * Metric Name: Sum of Burst Durations in RTP * Metric Description: The total duration of bursts of lost RTP packets in the period of the report. * Method of Measurement or Calculation: See Section 3.2, Sum of Burst Durations definition. * Units of Measurement: See Section 3.2, Sum of Burst Durations definition. * Measurement Point(s) with Potential Measurement Domain: See Section 3, 1st paragraph. * Measurement Timing: See Section 3, 2nd paragraph for measurement timing and Section 3.2 for Interval Metric flag.
* Use and Applications: See Section 1.4.
* Reporting Model: See RFC 3611.
c. Packets Lost in Bursts Metric
* Metric Name: RTP Packets lost in bursts
* Metric Description: The total number of RTP packets lost
during loss bursts.
* Method of Measurement or Calculation: See Section 3.2, Packets
Lost in Bursts definition.
* Units of Measurement: See Section 3.2, Packets lost in bursts
definition.
* Measurement Point(s) with Potential Measurement Domain: See
Section 3, 1st paragraph.
* Measurement Timing: See Section 3, 2nd paragraph for
measurement timing and Section 3.2 for Interval Metric flag.
* Use and Applications: See Section 1.4.
* Reporting Model: See RFC 3611.
d. Total Packets Expected in Bursts Metric
* Metric Name: Total RTP packets expected in bursts
* Metric Description: The total number of RTP packets expected
during loss bursts (that is, the sum of received RTP packets
and lost RTP packets).
* Method of Measurement or Calculation: See Section 3.2, Total
packets expected in bursts definition.
* Units of Measurement: See Section 3.2, Total packets expected
in bursts definition.
* Measurement Point(s) with Potential Measurement Domain: See
Section 3, 1st paragraph.
* Measurement Timing: See Section 3, 2nd paragraph for
measurement timing and Section 3.2 for Interval Metric flag.
* Use and Applications: See Section 1.4.
* Reporting Model: See RFC 3611.
e. Number of Bursts Metric
* Metric Name: Number of bursts in RTP
* Metric Description: The total duration of bursts of lost RTP
packets in the period of the report.
* Method of Measurement or Calculation: See Section 3.2, Number
of bursts definition.
* Units of Measurement: See Section 3.2, Number of bursts
definition.
* Measurement Point(s) with Potential Measurement Domain: See
Section 3, 1st paragraph.
* Measurement Timing: See Section 3, 2nd paragraph for
measurement timing and Section 3.2 for Interval Metric flag.
* Use and Applications: See Section 1.4.
* Reporting Model: See RFC 3611.
f. Sum of Squares of Burst Durations Metric
* Metric Name: Sum of Squares of Burst Durations in RTP
* Metric Description: The sum of the squares of burst durations
(where individual burst durations are expressed in ms) over in
the period of the report.
* Method of Measurement or Calculation: See Section 3.2, Sum of
Squares of Burst Durations definition.
* Units of Measurement: See Section 3.2, Sum of Squares of Burst
Durations definition.
* Measurement Point(s) with Potential Measurement Domain: See
Section 3, 1st paragraph.
* Measurement Timing: See Section 3, 2nd paragraph for
measurement timing and Section 3.2 for Interval Metric flag.
* Use and Applications: See Section 1.4.
* Reporting Model: See RFC 3611.
Authors' Addresses
Alan Clark Telchemy Incorporated 2905 Premiere Parkway, Suite 280 Duluth, GA 30097 USA EMail: alan.d.clark@telchemy.com Sunshine Zhang Shanghai Research Institute of China Telecom Corporation Limited No. 1835, South Pudong Road Shanghai 200122 China EMail: zhangyx@sttri.com.cn Jing Zhao Shanghai Research Institute of China Telecom Corporation Limited No. 1835, South Pudong Road Shanghai 200122 China EMail: zhaojing@sttri.com.cn Qin Wu (editor) Huawei 101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China EMail: sunseawq@huawei.com