388Understanding IPv6, Second Edition

■Control For SNAP-encapsulated payloads, the Control field is set to the defined value of 0x3, indicating that the 802.2 frame is an unnumbered frame. The size of this field is 8 bits.

■Organization Code The Organization Code field indicates the ID of the organization that defines the values in the 16-bit field that follows the Organization Code field. For SNAP encapsulation, the Organization Code field is set to 0, indicating the IETF, which administers the values of the EtherType field. The size of this field is 24 bits.

■EtherType The EtherType field indicates the upper-layer protocol of the payload. The size of this field is 16 bits. The EtherType field is set to 0x86DD for IPv6 packets.

■Frame Check Sequence The value of this field is a checksum that is used to check for bit-level errors in the FDDI frame. The checksum value is computed by the sending FDDI node and verified by the receiving FDDI node. The size of this field is 32 bits.

■End Delimiter The End Delimiter field indicates the end of the FDDI frame. The size of this field is 8 bits.

■Frame Status The Frame Status field indicates whether or not the destination address was recognized, whether or not the frame was copied, and whether or not the Frame Check Sequence field is valid. The size of this field is 16 bits.

FDDI allows an MTU of 4352 bytes for IPv6 packets. The IPv6 MTU is derived from the 4500-byte maximum payload for FDDI, less 22 bytes for FDDI MAC overhead, 8 bytes for the SNAP header, and 118 bytes that are reserved for future MAC header uses. All IPv6 packets are transmitted as asynchronous LLC frames using unrestricted tokens.

IEEE 802.11

The IEEE 802.11 standard for wireless LAN networks also uses the SNAP encapsulation. Figure A-6 shows IEEE 802.11 encapsulation for IPv6 packets that are sent between wireless nodes or between a wireless node and a wireless access point (AP) (rather than between wireless APs).

Figure A-6 shows an unencrypted 802.11 frame.

The fields in the IEEE 802.11 encapsulation are the following:

■Frame Control A 16-bit field that contains control information that defines the type of frame and how to process the frame. For more information, see the “Frame Control Field” section later in this appendix.

■Duration/ID Field A 16-bit field that is used to indicate the duration of time in microseconds needed to transmit the frame and the acknowledgment.

390 Understanding IPv6, Second Edition

If the payload of a data frame is encrypted with Wired Equivalent Privacy (WEP), the IPv6 packet is preceded by a plaintext 32-bit field containing an Initialization Vector (IV) field and followed with an encrypted 32-bit Integrity Check Value (ICV) field, lowering the maximum IPv6 packet size to 2304 bytes.

If the payload of a data frame is encrypted with Wi-Fi Protected Access (WPA) and the Temporal Key Integrity Protocol (TKIP), the IPv6 packet is preceded by a plaintext 64-bit field containing the IV and followed with an encrypted 64-bit Message Integrity Code (MIC) and 32-bit ICV field, lowering the maximum IPv6 packet size to 2292 bytes.

If the payload of a data frame is encrypted with Wi-Fi Protected Access 2 (WPA2) and the Advanced Encryption Standard (AES), the IPv6 packet is preceded by a plaintext 64-bit field containing the Packet Number field and followed with an encrypted 64-bit Message Integrity Code (MIC), lowering the maximum IPv6 packet size to 2296 bytes.

Frame Control Field

Figure A-7 shows the Frame Control field in the IEEE 802.11 header.

Protocol Version

Type

Subtype

To DS

From DS

More Fragments

Retry

Power Management

More Data

WEP

Order

Figure A-7 The Frame Control field in the IEEE 802.11 header

The Frame Control field contains the following subfields:

■Protocol Version A 2-bit field that indicates the version of the 802.11 protocol used to construct the frame. This field is set to 0 for the current version of IEEE 802.11. If the Protocol Version field is set to a value that is not supported by the receiving wireless node, the frame is silently discarded.

■Type A 2-bit field that indicates the type of IEEE 802.11 frame. There are three defined values: 00 for management frames, 01 for control frames, and 10 for data frames. The value of 11 is currently reserved.

■Subtype A 4-bit field that indicates the specific type of management, control, or data frame.