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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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-21) exists of draft-ietf-6lowpan-nd-18 -- Possible downref: Non-RFC (?) normative reference: ref. 'IEEE802-2001' -- Obsolete informational reference (is this intentional?): RFC 4941 (Obsoleted by RFC 8981) Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 6LoWPAN Working Group J. Nieminen, Ed. 2 Internet-Draft B. Patil 3 Intended status: Standards Track T. Savolainen 4 Expires: November 15, 2012 M. Isomaki 5 Nokia 6 Z. Shelby 7 Sensinode 8 C. Gomez 9 Universitat Politecnica de 10 Catalunya/i2CAT 11 May 14, 2012 13 Transmission of IPv6 Packets over Bluetooth Low Energy 14 draft-ietf-6lowpan-btle-07 16 Abstract 18 Bluetooth Low Energy is a low power air interface technology defined 19 by the Bluetooth Special Interest Group (BT SIG). The standard 20 Bluetooth radio has been widely implemented and available in mobile 21 phones, notebook computers, audio headsets and many other devices. 22 The low power version of Bluetooth is a new specification and enables 23 the use of this air interface with devices such as sensors, smart 24 meters, appliances, etc. The low power variant of Bluetooth is 25 commonly specified in revision 4.0 of the Bluetooth specifications 26 and commonly refered to as Bluetooth 4.0. This document describes 27 how IPv6 is transported over Bluetooth Low Energy using 6LoWPAN 28 techniques. 30 Status of this Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on November 15, 2012. 47 Copyright Notice 48 Copyright (c) 2012 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 64 2. Bluetooth Low Energy . . . . . . . . . . . . . . . . . . . . . 3 65 2.1. Bluetooth Low Energy stack . . . . . . . . . . . . . . . . 4 66 2.2. Link layer roles and topology . . . . . . . . . . . . . . 4 67 2.3. BT-LE device addressing . . . . . . . . . . . . . . . . . 5 68 2.4. BT-LE packets sizes and MTU . . . . . . . . . . . . . . . 5 69 3. Specification of IPv6 over Bluetooth Low Energy . . . . . . . 6 70 3.1. Protocol stack . . . . . . . . . . . . . . . . . . . . . . 6 71 3.2. Link model . . . . . . . . . . . . . . . . . . . . . . . . 7 72 3.2.1. Stateless address autoconfiguration . . . . . . . . . 7 73 3.2.2. Neighbor discovery . . . . . . . . . . . . . . . . . . 8 74 3.2.3. Header compression . . . . . . . . . . . . . . . . . . 9 75 3.2.4. Unicast and Multicast address mapping . . . . . . . . 9 76 3.3. Internet connectivity scenarios . . . . . . . . . . . . . 10 77 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 78 5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 79 6. Additional contributors . . . . . . . . . . . . . . . . . . . 11 80 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 81 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 82 8.1. Normative References . . . . . . . . . . . . . . . . . . . 11 83 8.2. Informative References . . . . . . . . . . . . . . . . . . 12 84 Appendix A. Bluetooth Low Energy fragmentation and L2CAP Modes . 12 85 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 87 1. Introduction 89 Bluetooth Low Energy (BT-LE) is a radio technology targeted for 90 devices that operate with coin cell batteries or minimalistic power 91 sources, which means that low power consumption is essential. BT-LE 92 is an especially attractive technology for Internet of Things 93 applications, such as health monitors, environmental sensing, 94 proximity applications and many others. 96 Considering the potential for the exponential growth in the number of 97 sensors and Internet connected devices and things, IPv6 is an ideal 98 protocol due to the large address space it provides. In addition, 99 IPv6 provides tools for autoconfiguration, which is particularly 100 suitable for sensor network applications and nodes which have very 101 limited processing power or a full-fledged operating system. 103 [RFC4944] specifies the transmission of IPv6 over IEEE 802.15.4. The 104 Bluetooth Low Energy link in many respects has similar 105 characteristics to that of IEEE 802.15.4. Many of the mechanisms 106 defined in [RFC4944] can be applied to the transmission of IPv6 on 107 Bluetooth Low Energy links. This document specifies the details of 108 IPv6 transmission over Bluetooth Low Energy links. 110 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 111 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 112 document are to be interpreted as described in RFC 2119 [RFC2119]. 114 The terms 6LN, 6LR and 6LBR are defined as in [I-D.ietf-6lowpan-nd]. 116 2. Bluetooth Low Energy 118 BT-LE is designed for transferring small amounts of data infrequently 119 at modest data rates at a very low cost per bit. Bluetooth SIG has 120 introduced two trademarks, Bluetooth Smart for single-mode devices (a 121 device that only supports BT-LE) and Bluetooth Smart Ready for dual- 122 mode devices. In the rest of the draft, the term BT-LE refers to 123 both types of devices. 125 BT-LE is an integral part of the BT 4.0 specification [BTCorev4.0]. 126 Devices such as mobile phones, notebooks, tablets and other handheld 127 computing devices which include BT 4.0 chipsets also have the low- 128 energy functionality of Bluetooth. BT-LE is also included in many 129 different types of accessories that collaborate with mobile devices 130 such as phones, tablets and notebook computers. An example of a use 131 case for a BT-LE accessory is a heart rate monitor that sends data 132 via the mobile phone to a server on the Internet. 134 2.1. Bluetooth Low Energy stack 136 The lower layer of the BT-LE stack consists of the Physical (PHY) and 137 the Link Layer (LL). The Physical Layer transmits and receives the 138 actual packets. The Link Layer is responsible for providing medium 139 access, connection establishment, error control and flow control. 140 The upper layer consists of the Logical Link Control and Adaptation 141 Protocol (L2CAP), Generic Attribute protocol (GATT) and Generic 142 Access Profile (GAP) as shown in Figure 1. GATT and BT-LE profiles 143 together enable the creation of applications in a standardized way 144 without using IP. L2CAP provides multiplexing capability by 145 multiplexing the data channels from the above layers. L2CAP also 146 provides fragmentation and reassembly for large data packets. 148 +----------------------------------------+------------------+ 149 | Applications | 150 +----------------------------------------+------------------+ 151 | Generic Attribute Profile | Generic Access | 152 +----------------------------------------+ Profile | 153 | Attribute Protocol |Security Manager | | 154 +--------------------+-------------------+------------------+ 155 | Logical Link Control and Adaptation | 156 +--------------------+-------------------+------------------+ 157 | Host Controller Interface | 158 +--------------------+-------------------+------------------+ 159 | Link Layer | Direct Test Mode | 160 +--------------------+-------------------+------------------+ 161 | Physical Layer | 162 +--------------------+-------------------+------------------+ 164 Figure 1: BT-LE Protocol Stack 166 2.2. Link layer roles and topology 168 BT-LE defines two Link Layer roles: the Master Role and the Slave 169 Role. A device in the Master Role, which is called master, can 170 manage multiple simultaneous connections with a number of devices in 171 the Slave Role, called slaves. A slave can only be connected to a 172 single master. Hence, a BT-LE network (i.e. a BT-LE piconet) follows 173 a star topology. 175 [BTLE-Slave]-----\ /-----[BTLE-Slave] 176 \ / 177 [BTLE-Slave]-----/[BTLE-Master]/-----[BTLE-Slave] 178 / \ 179 [BTLE-Slave]-----/ \-----[BTLE-Slave] 181 Figure 2: BT-LE Star Topology 183 A master is assumed to be less constrained than a slave. Hence, 184 master and slave can correspond with 6LoWPAN Border Router (6LBR) and 185 host, respectively. 187 In BT-LE, communication only takes place between a master and a 188 slave. Hence, in a BT-LE network using IP, a radio hop is equivalent 189 to an IP link and vice versa. 191 2.3. BT-LE device addressing 193 Every BT-LE device is identified by a unique 48 bit Bluetooth Device 194 Address (BD_ADDR). A Bluetooth Smart device such as a sensor may use 195 a public (obtained from IEEE Registration Authority) or a random 196 device address (generated internally). The public address is created 197 according to the IEEE 802-2001 standard [IEEE802-2001] and using a 198 valid Organizationally Unique Identifier (OUI) obtained from the IEEE 199 Registration Authority. This specification mandates that the 200 Bluetooth Device Address MUST be a public address. 202 2.4. BT-LE packets sizes and MTU 204 Maximum size of the payload in the BT-LE data channel PDU is 27 205 bytes. Depending on the L2CAP mode in use, the amount of data 206 available for transporting IP bytes in the single BT-LE data channel 207 PDU ranges between 19 and 27 octets. For power efficient 208 communication between two BT-LE devices, data and its header should 209 fit in a single BT-LE data channel PDU. MTU larger than 27 bytes can 210 be supported by the L2CAP specification. The Basic L2CAP Mode allows 211 a maximum payload size (i.e. IP datagram size) of 65535 bytes per 212 L2CAP PDU. The rest of the L2CAP modes allow a maximum payload size 213 that ranges between 65527 and 65533 bytes per L2CAP PDU. 215 The maximum payload size of a BT-LE data channel PDU is 27 bytes, 216 from which L2CAP headers may consume additional bytes. However, data 217 packets may be much larger (e.g. IPv6 requires support for an MTU of 218 1280 bytes). Fragmentation and Recombination (FAR) functionality is 219 an inherent function of the BT-LE L2CAP layer. Larger L2CAP packets 220 can be transferred with the assistance of the FAR functionality. 222 Appendix A describes FAR operation and five L2CAP Modes. This 223 specification requires that FAR functionality MUST be provided in the 224 L2CAP layer up to the IPv6 minimum MTU of 1280 bytes. The 225 corresponding L2CAP Mode MUST be Basic Mode. Since FAR in BT-LE is a 226 function of the L2CAP layer, fragmentation functionality as defined 227 in [RFC4944] MUST NOT be used in BT-LE networks. 229 3. Specification of IPv6 over Bluetooth Low Energy 231 BT-LE technology sets strict requirements for low power consumption 232 and thus limits the allowed protocol overhead. 6LoWPAN standard 233 [RFC4944], [I-D.ietf-6lowpan-nd] and [RFC6282] provides useful 234 generic functionality like header compression, link-local IPv6 235 addresses, Neighbor Discovery and stateless IP-address 236 autoconfiguration for reducing the overhead in 802.15.4 networks. 237 This functionality can be partly applied to BT-LE. 239 A significant difference between IEEE 802.15.4 and BT-LE is that the 240 former supports both star and mesh topology (and requires a routing 241 protocol), whereas BT-LE does not currently support the formation of 242 multihop networks. In consequence, the mesh header defined in 243 [RFC4944] for mesh under routing MUST NOT be used in BT-LE networks. 244 In addition, a BT-LE device MUST NOT play the role of a 6LoWPAN 245 Router (6LR). 247 3.1. Protocol stack 249 In order to enable transmission of IPv6 packets over BT-LE, a new 250 fixed L2CAP channel ID MUST be reserved for IPv6 traffic by the BT- 251 SIG. A request for allocation of a new fixed channel ID for IPv6 252 traffic by the BT-SIG should be submitted through the liaison process 253 or formal communique from the 6lowpan chairs and respective area 254 directors. Until a channel ID is allocated by BT-SIG, the channel ID 255 0x0007 is recommended for experimentation. Once the channel ID is 256 allocated, the allocated value MUST be used. Figure 3 illustrates 257 IPv6 over BT-LE stack. 259 +-------------------+ 260 | UDP/TCP | 261 +-------------------+ 262 | IPv6 | 263 +-------------------+ 264 | 6LoWPAN adapted | 265 | to BT-LE | 266 +-------------------+ 267 | BT-LE L2CAP | 268 +-------------------+ 269 | BT-LE Link Layer | 270 +-------------------+ 271 | BT-LE Physical | 272 +-------------------+ 274 Figure 3: IPv6 over BT-LE Stack 276 3.2. Link model 278 The concept of IP link (layer 3) and the physical link (combination 279 of PHY and MAC) needs to be clear and the relationship has to be well 280 understood in order to specify the addressing scheme for transmitting 281 IPv6 packets over the BT-LE link. [RFC4861] defines a link as "a 282 communication facility or medium over which nodes can communicate at 283 the link layer, i.e., the layer immediately below IP." 285 In the case of BT-LE, L2CAP is an adaptation layer that supports the 286 transmission of IPv6 packets. L2CAP also provides multiplexing 287 capability in addition to FAR functionality. This draft assumes the 288 same common IPv6 header values as those assumed in [RFC6282]. It is 289 also assumed that the IPv6 payload length can be inferred from the 290 L2CAP header length and also assumes that IPv6 addresses assigned to 291 6LoWPAN interfaces are formed with an IID derived directly from the 292 48-bit Bluetooth device addresses, as described in subsection 3.2.1. 294 The BT-LE link between two communicating nodes can be considered to 295 be a point-to-point or point-to-multipoint link. When one of the 296 communicating nodes is in the role of a master, then the link can be 297 viewed as a point-to-multipoint link. 299 When a host connects to another BT-LE device the link is up and IP 300 address configuration and transmission can occur. 302 3.2.1. Stateless address autoconfiguration 304 A BT-LE 6LN performs stateless address autoconfiguration as per 305 [RFC4862]. The 64 bit Interface Identifier (IID) for a BT-LE 306 interface is formed from the 48-bit unique public Bluetooth device 307 address [REF Sec 2.3] as per the "IPv6 over Ethernet" specification 308 [RFC2464]. A BT-LE 6LN SHOULD utilize this EUI-64 address for 309 stateless address autoconfiguration. The 48-bit public bluetooth 310 address is globally unique and provided by the IEEE registration 311 authority. Hence the "Universal/Local" (U/L) bit MUST be set to 0. 313 The IPv6 link-local address [RFC4291] for a BT-LE node is formed by 314 appending the IID, as defined above, to the prefix FE80::/64, as 315 depicted in Figure 4. 317 The tool for a gateway to obtain an IPv6 prefix for numbering the 318 BT-LE network is out of scope of this document, but can for example 319 be accomplished via DHCPv6 Prefix Delegation. The used IPv6 prefix 320 may change due to the gateway^1s movement. 322 3.2.2. Neighbor discovery 324 [I-D.ietf-6lowpan-nd] describes the neighbor discovery approach for 325 6loWPAN nodes. BT-LE does not support mesh networks and hence only 326 those aspects of the [I-D.ietf-6lowpan-nd] that apply to a star 327 topology are considered. 329 The following aspects of 6lowpan-nd are applicable to BT-LE 6LNS: 331 1. A BT-LE 6LN MUST register its address with the router by sending 332 a NS message with the ARO option and process the NA accordingly. The 333 NA with the ARO option SHOULD be sent irrespective of whether the IID 334 is derived from the unique 48 bit BT-LE device address or the IID is 335 a random value that is generated as per the privacy extensions for 336 stateless address autoconfiguration [RFC4941]. 338 2. Sending a Router solicitation (RS) and processing Router 339 advertisements by BT-LE 6LNs MUST follow Sections 5.3 and 5.4 340 respectvely of [I-D.ietf-6lowpan-nd]. 342 10 bits 54 bits 64 bits 343 +----------+-----------------+----------------------+ 344 |1111111010| zeros | Interface Identifier | 345 +----------+-----------------+----------------------+ 347 Figure 4: IPv6 link-local address in BT-LE 349 3.2.3. Header compression 351 This document assumes [RFC6282], which specifies the compression 352 format for IPv6 datagrams on top of IEEE 802.15.4, as the basis for 353 IPv6 header compression on top of BT-LE. It is required that all 354 headers MUST be compressed according to HC base encoding. In BT-LE 355 the star topology structure can be exploited in order to provide a 356 mechanism for IID compression. The following text describes the 357 principles of IPv6 address compression on top of BT-LE. 359 In a link-local communication, both the IPv6 source and destination 360 addresses MUST be elided, since the device knows that the packet is 361 destined for it even if the packet does not have destination IPv6 362 address. On the other hand, a node SHALL learn the IID of the other 363 endpoint of each L2CAP connection it participates in. By exploiting 364 this information, a node that receives a data channel PDU containing 365 an IPv6 packet (or a part of it) can infer the corresponding IPv6 366 source address. The device MUST maintain a Neighbor Cache, in which 367 the entries include both the IID of the neighbor and the Device 368 Address that identifies the neighbor. 370 When a BT-LE slave transmits an IPv6 packet to a remote destination 371 using global IPv6 addresses, the slave MUST elide the IPv6 source 372 address, if a context is defined for the prefix of the IPv6 source 373 address. In this case, the 6LBR/master can infer the elided IPv6 374 source address since 1) the master/6LBR has previously assigned the 375 prefix to the slaves; and 2) the master/6LBR maintains a Neighbor 376 Cache that relates the Device Address and the IID of the 377 corresponding slave. If a context is defined for the IPv6 378 destination address, the slave MUST also elide the prefix of the 379 destination IPv6 address. In that case, the 6LBR/master can infer 380 the elided destination prefix by using the context. 382 When a master/6LBR receives an IPv6 packet sent by a remote node 383 outside the BT-LE network, and the destination of the packet is a 384 slave, the master/6LBR MUST elide the IPv6 destination address of the 385 packet before forwarding it to the slave. The slave can infer that 386 the IPv6 destination address of the packet is its own IPv6 address. 387 If a context is defined for the prefix of the IPv6 source address, 388 the master/6LBR MUST elide that prefix as well. 390 3.2.4. Unicast and Multicast address mapping 392 The BT-LE link layer does not support multicast. Hence traffic is 393 always unicast between two BT-LE devices. Even in the case where a 394 master is attached to multiple slave BT-LE devices, the master device 395 cannot do a multicast to all the connected slave devices. If the 396 master device needs to send a multicast packet to all its slave 397 devices, it has to replicate the packet and unicast it on each link. 398 However, this may not be energy-efficient and particular care must be 399 taken if the master is battery-powered. In the opposite direction, a 400 slave can only transmit data to a single destination (i.e. the 401 master). Hence, if a slave transmits an IPv6 multicast packet, the 402 slave can unicast the corresponding BT-LE packet to the master. It 403 is required that the master MUST provide a table for mapping 404 different types of multicast addresses (all-nodes, all-routers and 405 solicited-node multicast addresses) to the corresponding IIDs and 406 Device Addresses. 408 3.3. Internet connectivity scenarios 410 In a typical scenario, BT-LE network is connected to the Internet. 412 h ____________ 413 \ / \ 414 h ---- 6LBR --- | Internet | 415 / \____________/ 416 h 417 h: host 418 <-- BT-LE --> 6LBR: 6LoWPAN Border Router 420 Figure 5: BT-LE network connected to the Internet 422 In some scenarios, the BT-LE network may transiently or permanently 423 be an isolated network. 425 h h h: host 426 \ / 6LBR: 6LoWPAN Border Router 427 h --- 6LBR -- h 428 / \ 429 h h 431 Figure 6: Isolated BT-LE network 433 Host-to-master and master-to-host communication MUST use the same 434 mechanisms as would be used in global IPv6 communications. The 435 gateway is used to route the packets to one of its slaves. 437 4. IANA Considerations 439 While there are no actions for IANA, we do expect BT SIG to allocate 440 an L2CAP channel ID (see Section 3.1). 442 5. Security Considerations 444 The transmission of IPv6 over BT-LE links has similar requirements 445 and concerns for security as for IEEE 802.15.4. IPv6 over BT-LE 446 SHOULD be protected by using BT-LE Link Layer security. 448 BT-LE Link Layer supports encryption and authentication by using the 449 Counter with CBC-MAC (CCM) mechanism [RFC3610] and a 128-bit AES 450 block cipher. Upper layer security mechanisms may exploit this 451 functionality when it is available. (Note: CCM does not consume 452 bytes from the maximum per-packet L2CAP data size, since the link 453 layer data unit has a specific field for them when they are used.) 455 Key management in BT-LE is provided by the Security Manager Protocol 456 (SMP). 458 6. Additional contributors 460 Kanji Kerai, Jari Mutikainen, David Canfeng-Chen and Minjun Xi from 461 Nokia have contributed significantly to this document. 463 7. Acknowledgements 465 Samita Chakrabarti and Erik Nordmark have provided valuable feedback 466 for this draft. 468 8. References 470 8.1. Normative References 472 [BTCorev4.0] 473 "Bluetooth Core Specification v4.0, http:// 474 www.bluetooth.org/Technical/Specifications/adopted.htm". 476 [I-D.ietf-6lowpan-nd] 477 Shelby, Z., Chakrabarti, S., and E. Nordmark, "Neighbor 478 Discovery Optimization for Low Power and Lossy Networks 479 (6LoWPAN)", draft-ietf-6lowpan-nd-18 (work in progress), 480 October 2011. 482 [IEEE802-2001] 483 "IEEE 802-2001 standard, 484 http://standards.ieee.org/findstds/standard/ 485 802-2001.html". 487 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 488 Requirement Levels", BCP 14, RFC 2119, March 1997. 490 [RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet 491 Networks", RFC 2464, December 1998. 493 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 494 Architecture", RFC 4291, February 2006. 496 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 497 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 498 September 2007. 500 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 501 Address Autoconfiguration", RFC 4862, September 2007. 503 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 504 "Transmission of IPv6 Packets over IEEE 802.15.4 505 Networks", RFC 4944, September 2007. 507 [RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6 508 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, 509 September 2011. 511 8.2. Informative References 513 [RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with 514 CBC-MAC (CCM)", RFC 3610, September 2003. 516 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 517 Extensions for Stateless Address Autoconfiguration in 518 IPv6", RFC 4941, September 2007. 520 Appendix A. Bluetooth Low Energy fragmentation and L2CAP Modes 522 This section provides an overview of Fragmentation and Recombination 523 (FAR) method and L2CAP modes in Bluetooth Low Energy. FAR is an 524 L2CAP mechanism, in which an L2CAP entity can take the (large) upper 525 layer PDU, prepend the L2CAP header (4 bytes in the Basic L2CAP mode) 526 and break the resulting L2CAP PDU into fragments which can then be 527 directly encapsulated into Data channel PDUs. There are bits in the 528 Data channel PDUs which identify whether the payload is a complete 529 L2CAP PDU or the first of a set of fragments, or one of the rest of 530 the fragments. 532 There are five L2CAP modes defined in the BT 4.0 spec. These modes 533 are: Retransmission Mode (a Go-Back-N mechanism is used), Enhanced 534 Retransmission Mode (includes selective NAK among others), Flow 535 Control Mode (PDUs are numbered, but there are no retransmissions), 536 Streaming Mode (PDUs are numbered, but there are no ACKs of any kind) 537 and Basic L2CAP Mode. 539 Authors' Addresses 541 Johanna Nieminen (editor) 542 Nokia 543 Itaemerenkatu 11-13 544 FI-00180 Helsinki 545 Finland 547 Email: johanna.1.nieminen@nokia.com 549 Basavaraj Patil 550 Nokia 551 6021 Connection drive 552 Irving, TX 75039 553 USA 555 Email: basavaraj.patil@nokia.com 557 Teemu Savolainen 558 Nokia 559 Hermiankatu 12 D 560 FI-33720 Tampere 561 Finland 563 Email: teemu.savolainen@nokia.com 565 Markus Isomaki 566 Nokia 567 Keilalahdentie 2-4 568 FI-02150 Espoo 569 Finland 571 Email: markus.isomaki@nokia.com 572 Zach Shelby 573 Sensinode 574 Hallituskatu 13-17D 575 FI-90100 Oulu 576 Finland 578 Email: zach.shelby@sensinode.com 580 Carles Gomez 581 Universitat Politecnica de Catalunya/i2CAT 582 C/Esteve Terradas, 7 583 Castelldefels 08860 584 Spain 586 Email: carlesgo@entel.upc.edu