DCCP Working Group G. Renker Internet-Draft G. Fairhurst Updates: 4342, 5622 University of Aberdeen (if approved) October 17, 2010 Intended status: Standards Track Expires: April 20, 2011 Sender RTT Estimate Option for DCCP draft-ietf-dccp-tfrc-rtt-option-00.txt Abstract This document describes an update to CCID-3/4 that addresses parameter-estimation problems occurring with TFRC-based DCCP congestion control. The fix uses a recommendation made in the original TFRC specification. It avoids the inherent problems of receiver-based RTT sampling, by utilising higher-accuracy RTT samples already available at the sender. It is integrated into the feature set of DCCP as an end-to-end negotiable extension, upward and downward compatible. 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 April 20, 2011. Copyright Notice Copyright (c) 2010 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 Renker & Fairhurst Expires April 20, 2011 [Page 1]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Problems caused by sampling the RTT at the receiver . . . . . 4 2.1. List of problems encountered with a real implementation . 4 2.2. Other areas affected by the RTT sampling problems . . . . 6 2.2.1. Measured Receive Rate X_recv . . . . . . . . . . . . . 6 2.2.2. Disambiguation and Accuracy of Loss Intervals . . . . 6 2.2.3. Determining Quiescence . . . . . . . . . . . . . . . . 7 2.2.4. Practical Considerations . . . . . . . . . . . . . . . 7 3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Conventions . . . . . . . . . . . . . . . . . . . . . . . 8 3.2. Options and Features . . . . . . . . . . . . . . . . . . . 8 3.2.1. RTT Estimate Option . . . . . . . . . . . . . . . . . 8 3.2.2. Send RTT Estimate Feature . . . . . . . . . . . . . . 10 3.3. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4. Security Considerations . . . . . . . . . . . . . . . . . . . 13 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 5.1. Option Types . . . . . . . . . . . . . . . . . . . . . . . 14 5.2. Feature Numbers . . . . . . . . . . . . . . . . . . . . . 15 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 6.1. Normative References . . . . . . . . . . . . . . . . . . . 17 6.2. Informative References . . . . . . . . . . . . . . . . . . 17 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20 Renker & Fairhurst Expires April 20, 2011 [Page 2]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 1. Introduction This document lists and analyses problems observed with receiver- based RTT sampling in an actual implementation of TFRC congestion control [RFC4342], [RFC5622]. To fix these problems, this document presents a solution based on a concept first recommended in [RFC5348], 3.2.1; i.e. to measure the RTT at the sender. This results in a higher reliability and frequency of samples, and avoids the inherent problems of receiver- based RTT sampling discussed below. We begin by listing the encountered problems in the next section. The proposed solution is presented in in Section 3. We then discuss security considerations in Section 4 and list the resulting IANA considerations in Section 5. Renker & Fairhurst Expires April 20, 2011 [Page 3]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 2. Problems caused by sampling the RTT at the receiver There are at least six areas that make a TFRC receiver vulnerable to inaccuracies or absence of (receiver-based) RTT samples: o the measured sending rate, X_recv ([RFC5348], 6.2); o synthesis of the first loss interval ([RFC5348], 6.3.1); o disambiguation of loss events ([RFC4342], 10.2); o validation of loss intervals ([RFC4342], 6.1); o ensuring that at least one feedback packet is sent per RTT ([RFC4342], 10.3); o determining quiescence periods ([RFC4342], 6.4). 2.1. List of problems encountered with a real implementation This section summarizes several years of experience using the Linux implementation of CCID-3 and CCID-4. It lists the problems encountered with receiver-based RTT sampling over real networks, in a variety of wired and wireless environments and under different link- layer conditions. The Linux DCCP/TFRC implementation is based on the RTT-sampling algorithm specified in [RFC4342], 8.1. This algorithm relies on a coarse-grained window-counter (units of RTT/4), and uses packet inter-arrival times to estimate the current RTT of the network. The algorithm is effective only for packets with modulo-16 CCVal differences less than 5, due to limitations noted in sections 8.1 and 10.3 of [RFC4342]. A CCVal difference less than 4 means sampling at sub-RTT scale; RFC [RFC4342], 8.1 thus suggests differences between 2 and 4, the latter being preferable (equivalent to a full RTT). The same section limits the maximum CCVal difference between data- carrying packets to 5, to avoid wrap-around. As a consequence, the timing interval for adjacent packets with a CCVal difference greater than 4 can not be determined: such samples have to be discarded. A second problem arises when there are holes in the sequence space. Because there may be wrap-around of the 4-bit CCVal window counter, it is not possible to determine window-counter wrap-around whenever sequence numbers of subsequent packets are not immediately adjacent. This problem occurs when packets are delayed, reordered, or lost in the network. Renker & Fairhurst Expires April 20, 2011 [Page 4]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 As a consequence, RTT sampling has to be paused during times of loss. This however aggravates the problem, since the sender now requires new feedback from the receiver, but the receiver is unable to provide accurate and up-to-date information: the receiver is unable to sample the RTT, accordingly also not able to estimate X_recv correctly, which then in turn affects X_Bps at the sender. The third limitation arises from using inter-arrival times as representatives of network inter-packet gaps. It is well known that the inter-packet gap of packets is not constant along a network path. Furthermore, modern network interface cards do not necessarily deliver each packet at the time it is received, but rather in a bunch, to avoid overly frequent interrupts [MR97]. As a result, inter-packet arrival times may converge to zero, when subsequent packets are delivered at virtually the same time. The fourth problem is that of under-sampling and thus related to the first limitation. If loss occurs while the receiver has not yet had a chance to sample the RTT, it needs to fall back to some fixed RTT constant to plug into the equation of [RFC5348], 6.3.1. (The sender, for example, uses a fixed value of 1 second when it can not obtain an initial RTT sample, compare [RFC5348], 4.2). In particular, if the loss is caused by a transient condition, this fourth problem causes a subsequent deterioration of the connection (rate reduction), further aggravated by the fact that TFRC takes longer than common window-based protocols to recover from a reduction of its allowed sending rate. The fifth and last problem is starvation under burst loss, caused for instance by a sudden interference in a wireless transmission. The resulting burst loss sets off a vicious circle, where link-layer retransmissions and transmitter-backoff procedures and/or reverse- path loss eventually cause the nofeedback timer to be triggered at the sender. This in turn halves the sending rate, thereby doubling the inter-packet gap. Which in turn decreases X_recv sampled via RTT at the receiver. These factors contribute to an accelerated reduction of the sending rate towards zero, or rather 1 packet per 64 seconds (t_mbi). Under these conditions the connection is no longer in a usable state, unless buffering of more than 64 seconds (more is required because the sending rate is low) can be applied; which is impossible for interactive applications, and unacceptable for many audio/video applications. Trying to smooth over these effects by imposing heavy filtering on the RTT samples did not substantially improve the situation, nor does it solve the problem of under-sampling. Renker & Fairhurst Expires April 20, 2011 [Page 5]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 We are not aware of an alternative (published) algorithm to better estimate the RTT at the receiver. The TFRC sender, on the other hand, is much better equipped to estimate the RTT and can do this more accurately. This is in particular due to the use of timestamps and elapsed time information ([RFC5348], 3.2.2), which are mandatory in CCID-3 (sections 6 and 8.2 of [RFC4342]). 2.2. Other areas affected by the RTT sampling problems We here analyse the impact that unreliability of receiver-based RTT sampling has on the areas listed at the begin of this section. In addition, benefits of sender-based RTT sampling have already been pointed out in [RFC5348], and in the specification of CCID-3 [RFC4342], at the end of section 10.2. 2.2.1. Measured Receive Rate X_recv A key problem is that the reliability of X_recv [RFC4342] depends directly upon the reliability and accuracy of RTT samples. This means that failures propagate from one parameter to another. Errata IDs 610 and 611 update [RFC4342] to use the definition of the receive rate as specified in [RFC5348]. Having an explicit (rather than a coarse-grained) RTT estimate allows measurement of X_recv with greater accuracy, and isolates failure. An explicit RTT estimate also enables the receiver to more accurately perform the test in step (2) of [RFC4342], 6.2, i.e. to check whether less or more than one RTT has passed since the last feedback. 2.2.2. Disambiguation and Accuracy of Loss Intervals Since a loss event is defined as one or more lost (ECN-marked) data packets in one RTT ([RFC5348], 5.2), the receiver needs accurate RTT estimates to validate and accurately separate loss events. Moreover, [RFC5348], 5.2 expressly points out the sender RTT estimate as RECOMMENDED for this purpose. Having the sender RTT Estimate available further increases the accuracy of the information reported by the receiver. The definition of Loss Intervals in [RFC4342], 6.1 needs the RTT to separate the lossy parts; in particular, lossy parts spanning a period of more than one RTT are invalid. Renker & Fairhurst Expires April 20, 2011 [Page 6]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 A similar benefit arises in the computation of the loss event rate: as discussed in section 9.2 of [RFC4342], it may happen that sender and receiver compute different loss event rates, due to differences in the available timing information. An explicit RTT estimate increases the accuracy of information available at the receiver, thus the sender may not need to recompute the (less reliable) loss event rate reported by the receiver. 2.2.3. Determining Quiescence The quiescence period is defined as max(2 * RTT, 0.2 sec) in section 6.4 of [RFC4342]. An explicit RTT estimate avoids under- and over- estimating quiescence periods. 2.2.4. Practical Considerations Using explicit RTT estimates contributes to greater robustness and can also result in simpler implementation. First, it becomes easier to separate adjacent loss events. The 4-bit counter value wraps relatively frequently, which requires complex computations to avoid aliasing effects. Second, the receiver is better able to determine when to send feedback packets. It can perform the test described in step (2) of [RFC5348], 6.2 more accurately. Moreover, unnecessary expiration of the nofeedback timer (as described in [RFC4342], 10.3) can be avoided. Lastly, a sender-based RTT estimate option can be used by middleboxes for verification [RFC4342], 10.2. Renker & Fairhurst Expires April 20, 2011 [Page 7]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 3. Specification 3.1. Conventions 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 [RFC2119]. This document uses the conventions of [RFC5348], [RFC4340], [RFC4342], and [RFC5622]. All multi-byte field descriptions presented in this documented are in network byte order (most significant byte first). 3.2. Options and Features This document defines a single TFRC-specific option, RTT Estimate, described in the next subsection. Following the guidelines in [RFC4340], section 15, the use of the RTT Estimate Option is governed by an associated feature, Send RTT Estimate Feature. This feature is described in the second subsection. 3.2.1. RTT Estimate Option The sender communicates its current RTT estimate to the receiver using a RTT Estimate Option. Renker & Fairhurst Expires April 20, 2011 [Page 8]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 ==> RFC Editor's Note: Please replace 'XX' with IANA value when published and delete this note. +------+---------------+--------------+------------+ | Type | Option Length | Meaning | DCCP Data? | +------+---------------+--------------+------------+ | XX | 3/4/5 | RTT Estimate | Y | +------+---------------+--------------+------------+ Table 1: The RTT Estimate Option defined by this document Column meanings are as per [RFC4340], section 5.8 (table 3). This option is permitted in any DCCP packet, has option number XX and a length of 3-5 bytes. A Sender RTT Estimate Option is valid if it satisfies one of the three following formats: +--------+--------+--------+ |xxxxxxxx|00000011| RTT | +--------+--------+--------+ Type=XX Length=3 Estimate +--------+--------+--------+--------+ |xxxxxxxx|00000100| RTT | +--------+--------+--------+--------+ Type=XX Length=4 Estimate +--------+--------+--------+--------+--------+ |xxxxxxxx|00000101| RTT | +--------+--------+--------+--------+--------+ Type=XX Length=5 Estimate The 1..3 value bytes of the option data carry the current RTT estimate of the sender, using a granularity of 1 microsecond. This allows values up to 16.7 seconds (corresponding to 0xFFFFFE) to be communicated. The value 0xFFFFFF is reserved to indicate significant delay spikes, larger than 16.7 seconds. This is qualitative rather than quantitative information, to alert the receiver that there is a network problem (for instance jamming on a wireless channel). The use of the RTT Estimate Option on networks with RTTs larger than 16.7 seconds is not specified by this document. Renker & Fairhurst Expires April 20, 2011 [Page 9]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 A value of 0 indicates the absence of a valid RTT sample. The sender MUST set the value to 0 if it does not yet have an RTT estimate. The sender SHOULD select the smallest format suitable to carry the RTT estimate (i.e., less than 1 byte of leading zeroes). 3.2.2. Send RTT Estimate Feature The Send RTT Estimate feature lets endpoints negotiate whether the sender MUST provide RTT Estimate options on its data packets. Renker & Fairhurst Expires April 20, 2011 [Page 10]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 ==> RFC Editor's Note: Please replace 'YY' with IANA value when published and delete this note. Send RTT Estimate has feature number YY and is server-priority. It takes one-byte Boolean values. Values greater than 1 are invalid and MUST be ignored. +--------+-------------------+------------+---------------+-------+ | Number | Meaning | Rec'n Rule | Initial Value | Req'd | +--------+-------------------+------------+---------------+-------+ | YY | Send RTT Estimate | SP | 0 | N | +--------+-------------------+------------+---------------+-------+ Table 2: The Send RTT Estimate feature defined by this document The column meanings are described in [RFC4340], section 6.4. In particular, the feature is by default off (initial value of 0), and the extension is not required to be understood by every DCCP implementation (cf. [RFC4340], section 15). DCCP B sends a "Change R(Send RTT Estimate, 1)" to ask DCCP A to send RTT Estimate options as part of its data traffic. 3.3. Usage When the Send RTT Estimate Feature is enabled, the sender MUST provide an RTT Estimate Option on all of its Data, DataAck, Sync, and SyncAck packets. It MAY in addition provide the RTT Estimate Option on other packet types, such as DCCP-Ack. When the receiver has requested the use of the RTT Estimate Option, it MUST use the RTT value reported by that option in all places that require a RTT (listed at the begin of Section 2). For reasons described below, it SHOULD also continue to track CCVal window counter values. When the Send RTT Estimate Feature is disabled, the sender MUST NOT send RTT Estimate options on any of its packets, the receiver MUST ignore the RTT Estimate Option on all incoming packets, and SHOULD try to estimate the RTT in some other way (not specified by this document). The sender MUST implement and continue to update CCVal window counter RTT values as specified in [RFC4342], section 8.1, even when the Send RTT Estimate Feature is on. Renker & Fairhurst Expires April 20, 2011 [Page 11]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 The remainder of this section specifies robustness measures for the receiver when the Send RTT Estimate Feature is on. If the receiver encounters an invalid RTT Estimate Option (Section 3.2.1), it MUST reset the connection with Reset Code 5, "Option Error", where the Data 1..3 fields are set to the first 3 bytes of the offending RTT Estimate Option. The receiver SHOULD track the long-term RTT estimate using a moving average, such as the one specified in [RFC5348], 4.3. It SHOULD use this long-term value if the sender occasionally sends a 0-valued RTT Estimate Option, but SHOULD NOT use the long-term value otherwise. The sender is permitted to send up to MAX_INVALID_RTT=2 successive 0-valued RTT estimate options. If the receiver encounters more than MAX_INVALID_RTT RTT Estimate options in succession, it MUST behave as if the Send RTT Estimate Feature were off, and disregard all subsequent RTT Estimate options. At the begin of a connection the sender may not have a valid RTT sample yet. If the path RTT is known, e.g. from a previous connection [RFC2140], the receiver MAY reuse the previously known path RTT value to seed its long-term RTT estimate. When receiving the special RTT Estimate Option of value 0xFFFFFF, indicating a network exception, the receiver SHOULD NOT continue to sample the RTT. Instead, it SHOULD take appropriate measures to improve the quality of the connection (e.g. by attempting to associate with a different access point). To bail out from a hanging session, the receiver MAY close the connection after encountering MAX_INVALID_RTT=2 exception-indicating RTT Estimate Options. It is however RECOMMENDED to make this behaviour configurable, to support alternative recovery strategies for links with frequent transient failures. Renker & Fairhurst Expires April 20, 2011 [Page 12]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 4. Security Considerations Security considerations for CCID-3 have been discussed in section 11 of [RFC4342]; for CCID-4 these have been discussed in section 13 of [RFC5622], referring back to the same section of [RFC4342]. This document introduces an extension to communicate the current RTT estimate of the sender to the receiver of a TFRC communication. By altering the value of the RTT Estimate Option, it is possible to interfere with the behaviour of a flow using TFRC. In particular, since accuracy of the RTT estimate directly influences the accuracy of the measured sending rate X_recv, it would be possible to obtain either higher or lower sending rates than are warranted by the current network conditions. This is only possible if an attacker is on the same path as the DCCP sender and receiver, and is able to guess valid sequence numbers. Therefore the considerations of section 18 in [RFC4340] apply. Renker & Fairhurst Expires April 20, 2011 [Page 13]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 5. IANA Considerations This document requests identical allocation in the dccp-ccid3- parameters and the dccp-ccid4-parameters registries. 5.1. Option Types This document defines a single CCID-specific option for communicating RTT estimates from the HC-sender to the HC-receiver. Following [RFC4340], 10.3, this requires an option number for the RTT Estimate Option in the range 128...191. Renker & Fairhurst Expires April 20, 2011 [Page 14]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 Note to IANA and the RFC editor When the IANA has allocated an option number for the `RTT Estimate' option, please replace all occurrences of the placeholder `XX' in this text with that number and delete this note. (Due to [RFC4340], 19.3 and [RFC4342], 12.2, the option number would be allocated in the range 128...183/191.) 5.2. Feature Numbers This document defines a single CCID-specific feature number for the Send RTT Estimate feature which is located at the HC-sender. Following [RFC4340], 10.3, a feature number in the range 128...191 is required. Renker & Fairhurst Expires April 20, 2011 [Page 15]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 Note to IANA and the RFC editor When the IANA has allocated an option number for the `Send RTT Estimate' feature, please replace all occurrences of the placeholder `YY' in this text with that number and delete this note. (Due to [RFC4340], 19.4 and [RFC4342], 12.3, the feature number would be allocated in the range 128...183/191.) Renker & Fairhurst Expires April 20, 2011 [Page 16]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 6. References 6.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006. [RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for Datagram Congestion Control Protocol (DCCP) Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342, March 2006. [RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP Friendly Rate Control (TFRC): Protocol Specification", RFC 5348, September 2008. 6.2. Informative References [MR97] Mogul, J. and K. Ramakrishnan, "Eliminating Receive Livelock in an Interrupt-Driven Kernel", ACM Transactions on Computer Systems (TOCS), 15(3):217-252, August 1997. [RFC2140] Touch, J., "TCP Control Block Interdependence", RFC 2140, April 1997. [RFC5622] Floyd, S. and E. Kohler, "Profile for Datagram Congestion Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Rate Control for Small Packets (TFRC-SP)", RFC 5622, August 2009. Renker & Fairhurst Expires April 20, 2011 [Page 17]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 ==> NOTE TO THE RFC EDITOR: PLEASE REMOVE THIS LOG PRIOR TO PUBLICATION The following changelog lists the changes since revision 01 of the preceding individual submissions draft-renker-dccp-tfrc-rtt-option-00/01. o General: - added detailed changelog to track comments - changed document name to reflect working group - updated date to October - made spelling of RTT Estimate Option (singular) consistent - moved reference to RFC 5622 from normative to informative, since document status is Experimental o Section 2.1: - clarified problematic cases of too small CCVal differences and CCVal differences > 4, feedback by Pasi o Section 3.1: - clarified the byte ordering used by this document, feedback by Pasi o Section 3.2: - corrected naming of Send RTT Estimate Feature, feedback by Eddie - removed superfluous remark regarding scaling to microsecond granularity in 3.2.1, feedback by Pasi - removed recommendation of preferring long-term RTT stamples, since this can not be generalized (connection may be short or path RTT may change, in both cases a long-term sample would not be useful), feedback by Pasi - made option variable-length (3/4/5 bytes), feedback by Eddie - specified condition for syntactic option validity - limited the maximum option size to 3 bytes and justified decision why not to support RTTs greater than 16 seconds, in Renker & Fairhurst Expires April 20, 2011 [Page 18]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 reply to feedback by Eddie - clarified that the sender MUST use 0 to indicate absence of a valid RTT estimate - clarified the highest path RTT value supported by this document (16.7 sec) - reserved 0xFFFFFF as special value to communicate out-of- bounds exceptions, network problems resulting in disproportionately high delay spikes (> 16.7 seconds) o Section 3.3: - corrected naming of Send RTT Estimate Feature, feedback by Eddie - specified what happens if invalid RTT Estimate options are received - specified what happens if the sender persistently sends 0-valued RTT Estimate options, feedback by Eddie - specified how the exceptional value 0xFFFFFF should be handled - added reference for reusing previously known path RTT value ====> END OF NOTE TO THE RFC EDITOR <==== Renker & Fairhurst Expires April 20, 2011 [Page 19]
Internet-Draft Sender RTT Estimate Option for DCCP October 2010 Authors' Addresses Gerrit Renker University of Aberdeen School of Engineering Fraser Noble Building Aberdeen AB24 3UE Scotland Email: gerrit@erg.abdn.ac.uk URI: http://www.erg.abdn.ac.uk Godred Fairhurst University of Aberdeen School of Engineering Fraser Noble Building Aberdeen AB24 3UE Scotland Email: gorry@erg.abdn.ac.uk URI: http://www.erg.abdn.ac.uk Renker & Fairhurst Expires April 20, 2011 [Page 20]
