Version 1: Release Note 28 November 2014



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GBT Message processing


The GBT Initiator shall, once the Remote Party Message is fully constructed and all cryptographic protections are applied, slice the octet string produced so that:

  1. GBT Block Data with GBT Initiator Block Number of 1 is the 1149 most significant octets of the Remote Party Message, or all of the octets if the size of the Remote Party Message is less than 1149 bytes;

  2. GBT Block Data with GBT Initiator Block Number of 2 is the next 1149 most significant octets of the Remote Party Message, or all of the octets if the size of the remaining octets in Remote Party Message is less than 1149 bytes; and

  3. remaining GBT Block Data are created by repeating Step 2, each time incrementing GBT Initiator Block Number by 1, until there are no remaining octets in the Remote Party Message.

The GBT Recipient shall not undertake any processing, in the sense of Section 6, of the Remote Party Message carried in a GBT Message Series until it has received:

  • a GBT Message in this GBT Message Series where the ‘last-block’ field contains 0b1 (meaning last block); and

  • all GBT Messages in this GBT Message Series with ‘block-number’ fields less than the ‘block-number’ field in the last block. Where the GBT Recipient has not received all such GBT Messages, it shall send a GBT Message (Request Block Resend) for each missing block-number. Where the GBT Recipient is a Device, it may discard all blocks in a GBT Message Series if it has received no response to a GBT Message (Request Block Resend) after 60 minutes.

When a GBT Recipient receives a GBT Message with ‘block-number’ being an integer multiple of GBT Streaming Window for this GBT Message Series, it shall send a GBT Message (Acknowledgement).

GBT Recipient Block Number shall be set to 0x0001 in the first GBT Message sent by the GBT Recipient. It shall be incremented by 1 in each subsequent GBT Message it sends.



GBT Initiator Block Number Ack shall be the highest of:

  • 0x0000; and

  • the highest block-number in any GBT Message the GBT Initiator has received in this GBT Message Series.

GBT Recipient Block Number Ack shall:

  • in a GBT Message (Acknowledgement), be the highest block-number in any GBT Message the GBT Recipient has received in this GBT Message Series; and

  • in a GBT Message (Request Block Resend), the value of block-number up to which the GBT Recipient has received all the prior numbered GBT Messages in this GBT Message Series.

Where the GBT Initiator is a Device, the Device shall be able to resend any GBT Message within a GBT Message Series, for a minimum period from when it sends the first GBT Message in that series, to whichever is the sooner of:

  • it receiving an authenticated GBT Message (Acknowledgement) where the GBT Recipient Block Number Ack contains a value equal to the highest value of GBT Initiator Block Number Ack in this GBT Message Series; or

  • 24 hours later.

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