10 bits        54 bits             64 bits
        +----------+-----------------+----------------------+
        |1111111010|       zeros     | Interface Identifier |
        +----------+-----------------+----------------------+

        

Figure 4: IPv6 link-local address in Bluetooth LE

At network interface initialization, both 6LN and 6LBR SHALL generate and assign to the Bluetooth LE network interface IPv6 link-local addresses [RFC4862] based on the 48-bit Bluetooth device addresses (see Section 2.3) that were used for establishing underlying Bluetooth LE connection. A 64-bit Interface Identifier (IID) is formed from 48-bit device address as defined in RFC 2464 [RFC2464]. The IID is then appended with prefix fe80::/64, as described in RFC 4291 [RFC4291] and as depicted in Figure 4. The same link-local address SHALL be used for the lifetime of the Bluetooth LE L2CAP channel. (After Bluetooth LE logical link has been established, it is referenced with a Connection Handle in HCI. Thus possibly changing device addresses do not impact data flows within existing L2CAP channel. Hence there is no need to change IPv6 link-local addresses even if devices change their random device addresses during L2CAP channel lifetime).

A 6LN MUST join the all-nodes multicast address. There is no need for 6LN to join the solicited-node multicast address, since 6LBR will know device addresses and hence link-local addresses of all connected 6LNs. The 6LBR will ensure no two devices with the same Bluetooth LE device address are connected at the same time. Effectively duplicate address detection for link-local addresses is performed by the 6LBR's software responsible of discovery of IP-enabled Bluetooth LE nodes and of starting Bluetooth LE connection establishment procedures. This approach increases complexity of 6LBR, but reduces power consumption on both 6LN and 6LBR at link establishment phase by reducing number of mandatory packet transmissions.

After link-local address configuration, 6LN sends Router Solicitation messages as described in [RFC4861] Section 6.3.7.

For non-link-local addresses a 64-bit IID MAY be formed by utilizing the 48-bit Bluetooth device address. Alternatively, a randomly generated IID (see Section 3.2.2) can be used instead, for example, as discussed in [I-D.ietf-6man-default-iids]. The non-link-local addresses 6LN generates must be registered with 6LBR as described in Section 3.2.2.

Only if the Bluetooth device address is known to be a public address the "Universal/Local" bit can be set to 1 [RFC4291].

The tool for a 6LBR to obtain an IPv6 prefix for numbering the Bluetooth LE network is out of scope of this document, but can be, for example, accomplished via DHCPv6 Prefix Delegation [RFC3633] or by using Unique Local IPv6 Unicast Addresses (ULA) [RFC4193]. Due to the link model of the Bluetooth LE (see Section 2.2) the 6LBR MUST set the "on-link" flag (L) to zero in the Prefix Information Option [RFC4861]. This will cause 6LNs to always send packets to the 6LBR, including the case when the destination is another 6LN using the same prefix.

3.2.2. Neighbor discovery

'Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)' [RFC6775] describes the neighbor discovery approach as adapted for use in several 6LoWPAN topologies, including the mesh topology. Bluetooth LE does not support mesh networks and hence only those aspects that apply to a star topology are considered.

The following aspects of the Neighbor Discovery optimizations [RFC6775] are applicable to Bluetooth LE 6LNs:

1. A Bluetooth LE 6LN MUST register its non-link-local addresses with the 6LBR by sending a Neighbor Solicitation (NS) message with the Address Registration Option (ARO) and process the Neighbor Advertisement (NA) accordingly. The NS with the ARO option MUST be sent irrespective of the method used to generate the IID. The 6LN MUST register only one IPv6 address per IPv6 prefix available on a link.

2. For sending Router Solicitations and processing Router Advertisements the Bluetooth LE 6LNs MUST, respectively, follow Sections 5.3 and 5.4 of the [RFC6775].

3.2.3. Header compression

Header compression as defined in RFC 6282 [RFC6282], which specifies the compression format for IPv6 datagrams on top of IEEE 802.15.4, is REQUIRED in this document as the basis for IPv6 header compression on top of Bluetooth LE. All headers MUST be compressed according to RFC 6282 [RFC6282] encoding formats.

The Bluetooth LE's star topology structure and ARO can be exploited in order to provide a mechanism for IID compression. The following text describes the principles of IPv6 address compression on top of Bluetooth LE.

The ARO option requires use of EUI-64 identifier [RFC6775]. In the case of Bluetooth LE, the field SHALL be filled with the 48-bit device address used by the Bluetooth LE node converted into 64-bit Modified EUI-64 format [RFC4291].

To enable efficient header compression, the 6LBR MUST include 6LoWPAN Context Option (6CO) [RFC6775] for all prefixes the 6LBR advertises in Router Advertisements for use in stateless address autoconfiguration.

When a 6LN is sending a packet to or through a 6LBR, it MUST fully elide the source address if it is a link-local address or a non-link-local address 6LN has registered with ARO to the 6LBR for the indicated prefix. That is, if SAC=0 and SAM=11 the 6LN MUST be using the link-local IPv6 address derived from Bluetooth LE device address, and if SAC=1 and SAM=11 the 6LN MUST have registered the source IPv6 address with the prefix related to compression context identified with Context Identifier Extension. The destination IPv6 address MUST be fully elided if the destination address is the same address to which the 6LN has succesfully registered its source IPv6 address with ARO (set DAC=0, DAM=11). The destination IPv6 address MUST be fully or partially elided if context has been set up for the destination address. For example, DAC=0 and DAM=01 when destination prefix is link-local, and DAC=1 and DAM=01 with Context Identifier Extension if compression context has been configured for the used destination prefix.

When a 6LBR is transmitting packets to 6LN, it MUST fully elide the source IID if the source IPv6 address is the one 6LN has used to register its address with ARO (set SAC=0, SAM=11), and it MUST elide the source prefix or address if a compression context related to the IPv6 source address has been set up. The 6LBR also MUST elide the destination IPv6 address registered by the 6LN with ARO and thus 6LN can determine it based on indication of link-local prefix (DAC=0) or indication of other prefix (DAC=1 with Context Identifier Extension).

3.2.3.1. Remote destination example

When a 6LN transmits an IPv6 packet to a remote destination using global Unicast IPv6 addresses, if a context is defined for the 6LN's global IPv6 address, the 6LN has to indicate this context in the corresponding source fields of the compressed IPv6 header as per Section 3.1 of RFC 6282 [RFC6282], and has to elide the full IPv6 source address previously registered with ARO. For this, the 6LN MUST use the following settings in the IPv6 compressed header: CID=1, SAC=1, SAM=11. In this case, the 6LBR can infer the elided IPv6 source address since 1) the 6LBR has previously assigned the prefix to the 6LNs; and 2) the 6LBR maintains a Neighbor Cache that relates the Device Address and the IID the device has registered with ARO. If a context is defined for the IPv6 destination address, the 6LN has to also indicate this context in the corresponding destination fields of the compressed IPv6 header, and elide the prefix of or the full destination IPv6 address. For this, the 6LN MUST set the DAM field of the compressed IPv6 header as DAM=01 (if the context covers a 64-bit prefix) or as DAM=11 (if the context covers a full, 128-bit address). CID and DAC MUST be set to CID=1 and DAC=1. Note that when a context is defined for the IPv6 destination address, the 6LBR can infer the elided destination prefix by using the context.

When a 6LBR receives an IPv6 packet sent by a remote node outside the Bluetooth LE network, and the destination of the packet is a 6LN, if a context is defined for the prefix of the 6LN's global IPv6 address, the 6LBR has to indicate this context in the corresponding destination fields of the compressed IPv6 header. The 6LBR has to elide the IPv6 destination address of the packet before forwarding it, if the IPv6 destination address is inferable by the 6LN. For this, the 6LBR will set the DAM field of the IPv6 compressed header as DAM=11. CID and DAC needs to be set to CID=1 and DAC=1. If a context is defined for the IPv6 source address, the 6LBR needs to indicate this context in the source fields of the compressed IPv6 header, and elide that prefix as well. For this, the 6LBR needs to set the SAM field of the IPv6 compressed header as SAM=01 (if the context covers a 64-bit prefix) or SAM=11 (if the context covers a full, 128-bit address). CID and SAC are to be set to CID=1 and SAC=1.

3.2.4. Unicast and Multicast address mapping

The Bluetooth LE link layer does not support multicast. Hence traffic is always unicast between two Bluetooth LE nodes. Even in the case where a 6LBR is attached to multiple 6LNs, the 6LBR cannot do a multicast to all the connected 6LNs. If the 6LBR needs to send a multicast packet to all its 6LNs, it has to replicate the packet and unicast it on each link. However, this may not be energy-efficient and particular care must be taken if the master is battery-powered. In the opposite direction, a 6LN always has to send packets to or through 6LBR. Hence, when a 6LN needs to transmit an IPv6 multicast packet, the 6LN will unicast the corresponding Bluetooth LE packet to the 6LBR. The 6LBR will then forward the multicast packet to other 6LNs. To avoid excess of unwanted multicast traffic being sent to 6LNs, the 6LBR SHOULD implement MLD Snooping feature [RFC4541].

3.3. Internet connectivity scenarios

In a typical scenario, the Bluetooth LE network is connected to the Internet as shown in the Figure 5.

                      6LN 
                       \              ____________     
                        \            /            \    
                6LN ---- 6LBR ----- |  Internet   |
                        /            \____________/
                       /
                      6LN 
                                        
               <-- Bluetooth LE -->             

        

                     6LN      6LN         
                      \      /
                       \    /            
                6LN --- 6LBR --- 6LN
                       /    \ 
                      /      \    
                     6LN      6LN

               <--- Bluetooth LE --->