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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-13) exists of draft-ietf-manet-dlep-credit-flow-control-04 == Outdated reference: A later version (-16) exists of draft-ietf-manet-dlep-da-credit-extension-07 Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group B. Cheng 3 Internet-Draft D. Wiggins 4 Intended status: Standards Track MIT Lincoln Laboratory 5 Expires: November 7, 2019 L. Berger, Ed. 6 LabN Consulting, L.L.C. 7 May 6, 2019 9 DLEP Control Plane Based Pause Extension 10 draft-ietf-manet-dlep-pause-extension-07 12 Abstract 14 This document defines an extension to the Dynamic Link Exchange 15 Protocol (DLEP) that enables a modem to use DLEP messages to pause 16 and resume data traffic coming from its peer router. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at https://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on November 7, 2019. 35 Copyright Notice 37 Copyright (c) 2019 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (https://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 53 1.1. Key Words . . . . . . . . . . . . . . . . . . . . . . . . 3 54 2. Extension Usage and Identification . . . . . . . . . . . . . 3 55 3. Extension Data Items . . . . . . . . . . . . . . . . . . . . 3 56 3.1. Queue Parameters . . . . . . . . . . . . . . . . . . . . 3 57 3.1.1. Queue Parameter Sub Data Item . . . . . . . . . . . . 5 58 3.2. Pause . . . . . . . . . . . . . . . . . . . . . . . . . . 7 59 3.3. Restart . . . . . . . . . . . . . . . . . . . . . . . . . 8 60 4. Security Considerations . . . . . . . . . . . . . . . . . . . 9 61 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 62 5.1. Extension Type Value . . . . . . . . . . . . . . . . . . 9 63 5.2. Data Item Values . . . . . . . . . . . . . . . . . . . . 10 64 5.3. Queue Parameters Sub Data Item Values . . . . . . . . . . 10 65 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 66 6.1. Normative References . . . . . . . . . . . . . . . . . . 11 67 6.2. Informative References . . . . . . . . . . . . . . . . . 11 68 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 12 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 71 1. Introduction 73 The Dynamic Link Exchange Protocol (DLEP) is defined in [RFC8175]. 74 It provides the exchange of link related control information between 75 a modem and a router. DLEP defines a base set of mechanisms as well 76 as support for possible extensions. This document defines one such 77 extension. 79 The base DLEP specification does not include any data plane flow 80 control capability. The extension defined in this document supports 81 flow control of data traffic based on explicit messages sent via DLEP 82 by a modem to indicate when a router should hold off sending traffic, 83 and when it should resume. This functionality parallels the flow 84 control mechanism found in PPP over Ethernet (PPPoE) per [RFC5578]. 85 The extension also optionally supports DSCP (differentiated services 86 codepoint) aware flow control for use by DiffServ-aware modems. (For 87 general background on Differentiated Services see [RFC2475].) This 88 functionality is very similar to that provided by Ethernet Priority 89 flow control, see [IEEE.802.1Q_2014]. The extension defined in this 90 document is referred to as "Control Plane Based Pause". Other flow 91 control methods are possible with DLEP, e.g., see 92 [I-D.ietf-manet-dlep-da-credit-extension] and 93 [I-D.ietf-manet-dlep-credit-flow-control]. 95 Note that this mechanism only applies to traffic that is to be 96 transmitted on the modem's attached data channel and not to DLEP 97 control messages themselves. Furthermore it applies only to the 98 single sub-network that is used to connect a modem and a router, and 99 for traffic sent from a router to a modem. 101 This document defines a new DLEP Extension Type Value in Section 2 102 which is used to indicate the use of the extension, and three new 103 DLEP Data Items in Section 3. 105 1.1. Key Words 107 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 108 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 109 "OPTIONAL" in this document are to be interpreted as described in BCP 110 14 [RFC2119] [RFC8174] when, and only when, they appear in all 111 capitals, as shown here. 113 2. Extension Usage and Identification 115 The use of the Control Plane Based Pause Extension SHOULD be 116 configurable. To indicate that the implementation supports use of 117 the Control Plane Based Pause Extension, an implementation MUST 118 include the Control Plane Based Pause Extension Type Value in the 119 Extensions Supported Data Item. The Extensions Supported Data Item 120 is sent and processed according to [RFC8175]. 122 The Control Plane Based Pause Extension Type Value is TBA1, see 123 Section 5. 125 3. Extension Data Items 127 Three data items are defined by this extension. The Queue Parameters 128 Data Item is used by a modem to provide information about the DSCPs 129 it uses in forwarding. The Pause Data Item is used by a modem to 130 indicate when a router should cease sending packets and the Restart 131 Data Item is used by a modem to indicate when a router can resume 132 sending packets. 134 3.1. Queue Parameters 136 The Queue Parameters Data Item is sent by a modem to to a router to 137 indicate DSCP values that may be independently paused. This data 138 item MUST be included in a Session Initialization Response Message 139 that also contains the Control Plane Based Pause Extension Type Value 140 in the Extensions Supported Data Item. Updates to these parameters 141 MAY be sent by a modem by including the data item in Session Update 142 Messages. 144 The Queue Parameters Data Item groups DSCPs into logical queues, each 145 of which is identified by a "Queue Index". The number of logical 146 queues, or queue indexes, is variable as is the number of DSCPs 147 associated with each queue. A queue size (in bytes) is provided for 148 informational purposes. Queue Indexes are numbered sequentially from 149 zero, where queue index zero is a special case covering DSCPs which 150 are not otherwise associated with a Queue Index. 152 An implementation that does not support DSCPs would indicate 1 queue 153 with 0 DSCPs, and the number of bytes that may be in its associated 154 link transmit queue. Additional logical queues are represented in a 155 variable series of Queue Parameter sub data items. 157 The format of the Queue Parameters Data Item is: 159 0 1 2 3 160 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 1 161 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 162 | Data Item Type | Length | 163 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 164 | Num Queues | Scale | Reserved | 165 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 166 | Queue Parameter Sub Data Item 1 | 167 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 168 : ... : 169 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 170 | Queue Parameter Sub Data Item n | 171 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 173 Data Item Type: TBA2 175 Length: Variable 177 Per [RFC8175] Length is the number of octets in the data item, 178 excluding the Type and Length fields. 180 Num Queues: 182 An 8-bit unsigned integer indicating the number of queues 183 represented in the data item. This field MUST contain a value of 184 at least one (1), and is equal to one greater than the number of 185 included Queue Parameter Sub Data Items. 187 Scale: 189 A 4-bit unsigned integer indicating the scale used in the Queue 190 Size fields. The valid values are: 192 Value Scale 193 ------------ 194 0 B - Bytes (Octets) 195 1 KiB - Kilobytes (1024 B) 196 2 MiB - Megabytes (1024 KiB) 197 3 GiB - Gigabytes (1024 MiB) 199 Reserved: 201 A 20-bit field that MUST be set to zero by the sender (a modem) 202 and ignored by the receiver (a router). 204 3.1.1. Queue Parameter Sub Data Item 206 Queue Parameter Sub Data Items are an unordered list composed of sub 207 data items with a common format. The format of the Queue Parameter 208 Sub Data Item is patterned after the standard DLEP data item format, 209 see [RFC8175] Section 11.3. Any errors or inconsistencies 210 encountered in parsing Sub Data Items are handled in the same fashion 211 as any other Data Item parsing error encountered in DLEP. In 212 particular, the receiving implementation MUST issue a Session 213 Termination Message containing a Status Data Item with status code 214 set to 130 'Invalid Data' and transition to the Session Termination 215 state. 217 The format of the Queue Parameter Sub Data Item is: 219 0 1 2 3 220 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 1 221 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 222 | Sub Data Item Type (1) | Length | 223 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 224 | Value... | 225 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 227 and Value has the format: 229 0 1 2 3 230 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 1 231 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 232 | Queue Index | Queue Size Qn | 233 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 234 | Num DSCPs Qn | DS Field Qn | ... : 235 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 236 : ... | DS Field Qn | 237 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 239 Sub Data Item Type: 241 A 16-bit unsigned integer that indicates the type and 242 corresponding format of the Sub Data Item's Value field. Sub Data 243 Item Types are scoped within the Data Item in which they are 244 carried, i.e., the Sub Data Item Type field MUST be used together 245 with the Queue Parameters Data Item Type to identify the format of 246 the Sub Data Item. This field MUST be set to one (1) for the 247 Queue Parameter Sub Data Item. 249 Length: Variable 251 Length is the number of octets in the sub data item, excluding the 252 Type and Length fields. 254 Queue Index: 256 An 8-bit field indicating the queue index of the queue parameter 257 represented in the sub data item. Only the first instance of a 258 particular Queue Index value is meaningful. Subsequent sub data 259 items containing the same Queue Index values, if present, MAY be 260 logged via a management interface and MUST otherwise be ignored. 261 Note that the value 255 is reserved and MUST NOT be used in this 262 field. 264 Queue Size Qn: 266 A 24-bit unsigned integer representing the size, in the octet 267 scale indicated by the Scale field, of the queue supporting 268 traffic with the DSCPs associated with the queue index. 270 Num DSCPs Qn: 272 An 8-bit unsigned integer indicating the number of DSCPs 273 associated with the queue index associated with the sub data item. 274 This field MUST contain a value of at least one (1). 276 DS Field Qn: 278 The data item contains a sequence of 8 bit DS Fields. The number 279 of DS Fields present MUST equal the sum of all Num DSCPs field 280 values. 282 The DS Field structure is the same as [RFC2474]. 284 0 1 2 3 4 5 6 7 285 +---+---+---+---+---+---+---+---+ 286 | DSCP | CU | 287 +---+---+---+---+---+---+---+---+ 289 DSCP: differentiated services codepoint 290 CU: currently unused, MUST be zero 292 3.2. Pause 294 The Pause Data Item is sent by a modem to a router to indicate to its 295 peer that traffic is to be suppressed, i.e., paused. The motivating 296 use case for this data item is when a modem's internal queue length 297 exceeds a particular threshold. Other use cases are possible, e.g., 298 when there are non queue related congestion points within a modem. 299 Such cases are not explicitly described in this document. 301 A modem can indicate that traffic is to be suppressed on a device- 302 wide or destination-specific basis. An example of when a modem might 303 use device wide indications is when output queues are shared across 304 all destinations, and destination specific might be used when per 305 destination queuing is used. To indicate that suppression applies to 306 all destinations, a modem MUST send the Pause Data Item in a Session 307 Update Message. To indicate that suppression applies to a particular 308 destination a modem MUST send the Pause Data Item in a Destination 309 Update Message. 311 Each Pause Data Item identifies the traffic to be suppressed by the 312 Queue Index defined by Section 3.1, which in turn indicates a set of 313 traffic identified by DSCPs. The special value of 255 is used to 314 indicate that all traffic is to be suppressed. 316 While there is no restriction on the number of Messages containing 317 Pause Data Item that may be sent by a modem, a modem SHOULD include 318 multiple queue indexes in the same message when possible. 320 A router which receives the Pause Data Item MUST cease sending the 321 identified traffic to the modem. This may of course translate into 322 the router's queues exceeding their own thresholds. If a received 323 Pause Data Item contains a Queue Index value other than 255 or a 324 queue index established by a Session Initialization or Session Update 325 Message, the router MUST terminate the session with a Status Data 326 Item indicating Invalid Data. 328 The format of the Pause Data Item is: 330 0 1 2 3 331 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 1 332 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 333 | Data Item Type | Length | 334 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 335 | Queue Index | ... : 336 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 337 : ... | Queue Index | 338 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 340 Data Item Type: TBA3 342 Length: Variable 344 Per [RFC8175] Length is the number of octets in the data item, 345 excluding the Type and Length fields. It will equal the number of 346 Queue Index fields carried in the data item. 348 Queue Index: 350 One or more 8-bit fields used to indicate a queue index defined by 351 a Queue Parameters Data Item. The special value of 255 indicates 352 all traffic is to be suppressed to the modem, when the data item 353 is carried in a Session Update Message, or is to be suppressed to 354 a destination, when the data item is carried in Destination Update 355 Message. 357 3.3. Restart 359 The Restart Data Item is sent by a modem to a router to indicate to 360 its peer that transmission of previously suppressed traffic may be 361 resumed. An example of when a modem might send this data item is 362 when an internal queue length drops below a particular threshold. 364 The sending of this data item parallels the Pause Data Item, see the 365 previous section, and follows the same rules. As above, to indicate 366 that transmission can resume to all destinations, a modem MUST send 367 the Restart Data Item in a Session Update Message. It also includes 368 that to indicate that transmission can resume to a particular 369 destination a modem MUST send the Pause Restart Item in a Destination 370 Update Message. Finally, queue indexes are interpreted in the same 371 way as in the Pause Data Item.. 373 A router which receives the Restart Data Item SHOULD resume 374 transmission of the identified traffic to the modem. 376 The format of the Restart Data Item matches the Pause Data Item and 377 is: 379 0 1 2 3 380 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 1 381 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 382 | Data Item Type | Length | 383 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 384 | Queue Index | ... : 385 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 386 : ... | Queue Index | 387 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 389 Data Item Type: TBA4 391 Length: See Section 3.2. 393 Queue Index: See Section 3.2. 395 4. Security Considerations 397 The extension introduces a new mechanism for flow control between a 398 router and modem using DLEP. The extension does not inherently 399 introduce any additional vulnerabilities above those documented in 400 [RFC8175]. The approach taken to Security in that document applies 401 equally when running the extension defined in this document. 403 Implementations of the extension defined in this document MUST 404 support configuration of TLS usage, as describe in [RFC8175], in 405 order to protect configurations where injection attacks are possible, 406 i.e., when the link between a modem and router is not otherwise 407 protected. 409 Note that this extension does allow a compromised or impersonating 410 modem to suppress transmission by the router or a switch that 411 interconnects the modem and router. Similar attacks are generally 412 possible base DLEP, for example an impersonating modem may cause a 413 session reset or a compromised modem simply can drop all traffic 414 destined to, or sent by a router. [RFC8175] defines the use of TLS 415 to protect against the impersonating attacker. 417 5. IANA Considerations 419 This document requests the assignment of 4 values by IANA. All 420 assignments are to registries defined by [RFC8175]. 422 5.1. Extension Type Value 424 This document requests 1 new assignment to the DLEP Extensions 425 Registry named "Extension Type Values" in the range with the 426 "Specification Required" policy. The requested value is as follows: 428 +------+---------------------------+ 429 | Code | Description | 430 +------+---------------------------+ 431 | TBA1 | Control Plane Based Pause | 432 +------+---------------------------+ 434 Table 1: Requested Extension Type Value 436 5.2. Data Item Values 438 This document requests 3 new assignments to the DLEP Data Item 439 Registry named "Data Item Type Values" in the range with the 440 "Specification Required" policy. The requested values are as 441 follows: 443 +-----------+------------------+ 444 | Type Code | Description | 445 +-----------+------------------+ 446 | TBA2 | Queue Parameters | 447 | | | 448 | TBA3 | Pause | 449 | | | 450 | TBA4 | Restart | 451 +-----------+------------------+ 453 Table 2: Requested Data Item Values 455 5.3. Queue Parameters Sub Data Item Values 457 Upon approval of this document, IANA is requested to create a new 458 DLEP registry, named "Queue Parameters Sub Data Item Type Values". 460 The following table provides initial registry values and the 461 [RFC8126] defined policies that should apply to the registry: 463 +-------------+------------------------+ 464 | Type Code | Description/Policy | 465 +-------------+------------------------+ 466 | 0 | Reserved | 467 | | | 468 | 1 | Queue Parameter | 469 | | | 470 | 2-65407 | Specification Required | 471 | | | 472 | 65408-65534 | Private Use | 473 | | | 474 | 65535 | Reserved | 475 +-------------+------------------------+ 477 Table 3 479 6. References 481 6.1. Normative References 483 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 484 Requirement Levels", BCP 14, RFC 2119, 485 DOI 10.17487/RFC2119, March 1997, 486 . 488 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 489 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 490 May 2017, . 492 [RFC8175] Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B. 493 Berry, "Dynamic Link Exchange Protocol (DLEP)", RFC 8175, 494 DOI 10.17487/RFC8175, June 2017, 495 . 497 6.2. Informative References 499 [I-D.ietf-manet-dlep-credit-flow-control] 500 Cheng, B., Wiggins, D., Berger, L., and S. Ratliff, "DLEP 501 Credit-Based Flow Control Messages and Data Items", draft- 502 ietf-manet-dlep-credit-flow-control-04 (work in progress), 503 March 2019. 505 [I-D.ietf-manet-dlep-da-credit-extension] 506 Cheng, B., Wiggins, D., and L. Berger, "DLEP DiffServ 507 Aware Credit Window Extension", draft-ietf-manet-dlep-da- 508 credit-extension-07 (work in progress), March 2019. 510 [IEEE.802.1Q_2014] 511 IEEE, "IEEE Standard for Local and metropolitan area 512 networks--Bridges and Bridged Networks", IEEE 802.1Q-2014, 513 DOI 10.1109/ieeestd.2014.6991462, December 2014, 514 . 517 [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, 518 "Definition of the Differentiated Services Field (DS 519 Field) in the IPv4 and IPv6 Headers", RFC 2474, 520 DOI 10.17487/RFC2474, December 1998, 521 . 523 [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., 524 and W. Weiss, "An Architecture for Differentiated 525 Services", RFC 2475, DOI 10.17487/RFC2475, December 1998, 526 . 528 [RFC5578] Berry, B., Ed., Ratliff, S., Paradise, E., Kaiser, T., and 529 M. Adams, "PPP over Ethernet (PPPoE) Extensions for Credit 530 Flow and Link Metrics", RFC 5578, DOI 10.17487/RFC5578, 531 February 2010, . 533 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 534 Writing an IANA Considerations Section in RFCs", BCP 26, 535 RFC 8126, DOI 10.17487/RFC8126, June 2017, 536 . 538 Appendix A. Acknowledgments 540 The sub data item format was inspired by Rick Taylor's "Data Item 541 Containers" idea. 543 Authors' Addresses 545 Bow-Nan Cheng 546 MIT Lincoln Laboratory 547 Massachusetts Institute of Technology 548 244 Wood Street 549 Lexington, MA 02421-6426 551 Email: bcheng@ll.mit.edu 552 David Wiggins 553 MIT Lincoln Laboratory 554 Massachusetts Institute of Technology 555 244 Wood Street 556 Lexington, MA 02420-9108 558 Email: David.Wiggins@ll.mit.edu 560 Lou Berger (editor) 561 LabN Consulting, L.L.C. 563 Email: lberger@labn.net