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Appendix A Link-Layer Support for IPv6

385

Token Ring: IEEE 802.5 SNAP

When sent over a Token Ring network, IPv6 packets use the IEEE 802.5 SNAP encapsulation. IPv6 encapsulation for Token Ring links is described in RFC 2470. Figure A-4 shows IEEE 802.5 SNAP encapsulation of IPv6 packets.

Start Delimiter

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Access Control

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Frame Control

 

 

 

 

 

 

 

 

 

IEEE 802.5 Header

 

 

 

 

 

 

 

 

 

Destination Address

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Source Address

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DSAP

 

 

 

= 0xAA

 

 

 

 

 

 

 

 

 

 

SSAP

 

 

 

= 0xAA

 

 

 

IEEE 802.2 LLC Header

 

 

 

 

 

Control

 

 

 

= 0x3

 

 

 

 

 

 

 

 

 

 

Organization Code

 

 

 

 

 

 

 

 

 

 

 

 

 

 

= 0x0

 

 

 

SNAP Header

 

 

 

 

 

 

 

EtherType

 

 

 

 

 

 

 

 

 

 

 

 

= 0x86DD

 

 

 

 

 

 

 

 

 

 

IPv6 Packet

 

 

 

 

• • •

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Frame Check Sequence

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

End Delimiter

 

 

 

 

 

 

 

 

 

IEEE 802.5 Trailer

 

 

 

 

 

 

 

 

 

Frame Status

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure A-4 IEEE 802.5 SNAP encapsulation of IPv6 packets

The fields in the IEEE 802.5 SNAP encapsulation are the following:

Start Delimiter The Start Delimiter field indicates the start of the Token Ring frame. The size of this field is 8 bits.

Access Control The Access Control field indicates the frame type (token or data frame), the frame’s priority, the frame’s reservation level, and whether the frame has passed the ring monitor station. The size of this field is 8 bits.

Frame Control The Frame Control field indicates whether the frame is a data frame or a Token Ring management frame and, if it is a Token Ring management frame, the specific type. The size of this field is 8 bits.

Destination Address The Destination Address field contains the MAC address of the destination Token Ring node. The size of this field is 48 bits.

Source Address The Source Address field contains the MAC address of the sending Token Ring node. The size of this field is 48 bits.

386Understanding IPv6, Second Edition

DSAP The DSAP field indicates the upper-layer protocol of the payload for the destination. For SNAP-encapsulated payloads, the DSAP is set to the defined value of 0xAA. The size of this field is 8 bits.

SSAP The SSAP field indicates the upper-layer protocol of the payload for the sender. For SNAP-encapsulated payloads, the SSAP is set to the defined value of 0xAA. The size of this field is 8 bits.

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 Token Ring frame. The checksum value is computed by the sending Token Ring node and verified by the receiving Token Ring node. The size of this field is 32 bits.

End Delimiter The End Delimiter field indicates the end of the Token Ring frame. The size of this field is 8 bits.

Frame Status The Frame Status field indicates whether or not the destination address was recognized and whether or not the frame was copied. The size of this field is 8 bits.

IPv6 packets sent using Token Ring have a variety of maximum transmission units (MTUs), depending on the maximum amount of time that a Token Ring node can hold the token. This amount can vary based on the data rate and the number of nodes on the ring. For more information, see RFC 2470.

FDDI

FDDI also uses the SNAP encapsulation. IPv6 encapsulation for FDDI links is described in RFC 2467. Figure A-5 shows FDDI encapsulation of IPv6 packets.

 

 

 

 

 

 

Appendix A Link-Layer Support for IPv6

387

Preamble

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Start Delimiter

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Frame Control

 

 

 

 

 

 

 

 

FDDI Header

 

 

 

 

 

 

 

 

 

 

Destination Address

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Source Address

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DSAP

 

= 0xAA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SSAP

 

= 0xAA

 

 

 

 

 

IEEE 802.2 LLC Header

 

 

 

 

 

 

 

 

Control

 

= 0x3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Organization Code

 

 

 

= 0x0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SNAP Header

 

EtherType

 

 

 

= 0x86DD

 

 

 

 

 

 

 

IPv6 Packet

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Up to 4,352 bytes

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

• • •

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Frame Check Sequence

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

End Delimiter

 

 

 

 

 

 

 

 

FDDI Trailer

 

 

 

 

 

 

 

 

 

 

Frame Status

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure A-5 FDDI encapsulation of IPv6 packets

The fields in the FDDI encapsulation are the following:

Preamble The Preamble field is used to synchronize the receiver. The size of this field is 16 bits.

Start Delimiter The Start Delimiter field indicates the start of the FDDI frame. The size of this field is 8 bits.

Frame Control The Frame Control field indicates the frame class, the size of the source and destination addresses, and the frame type (token or data frame). The size of this field is 8 bits.

Destination Address The Destination Address field contains the MAC address of the destination FDDI node. The size of this field is 48 bits.

Source Address The Source Address field contains the MAC address of the sending FDDI node. The size of this field is 48 bits.

DSAP The DSAP field indicates the upper-layer protocol of the payload for the destination. For SNAP-encapsulated payloads, the DSAP is set to the defined value of 0xAA. The size of this field is 8 bits.

SSAP The SSAP field indicates the upper-layer protocol of the payload for the sender. For SNAP-encapsulated payloads, the SSAP is set to the defined value of 0xAA. The size of this field is 8 bits.

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