How to tune OPENTHREAD_CONFIG to 32 child per parent

[rretanubun]

Seeking advice on our approach

We have a customer request to have a TBR or TME that can buffer 8 frames (one full MTU) per child plus 100 shared frames. Which works out to 32 +13 = 45 MTUs (full 1280-byte IP packets)

The code offers these CONFIG options:

1. #define OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE (sizeof(void *) * 32) // which is 128 bytes in a 32-bit system
2. #define OPENTHREAD_CONFIG_NUM_MESSAGE_BUFFERS 256 // which we bump to 512
3. #define OPENTHREAD_CONFIG_DEFAULT_SED_BUFFER_SIZE 1280 // which we kept as is
4. #define OPENTHREAD_CONFIG_DEFAULT_SED_DATAGRAM_COUNT 1 // which we change to 13

Rationale for the math

Why OPENTHREAD_CONFIG_NUM_MESSAGE_BUFFERS changed?

1. Each #define OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE (sizeof(void *) * 32) means the message buffer is 4*32 = 128 bytes
2. to support full 1280-byte IP packets we need 10 OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE
3. we need 45 MTUs (1280-bytes each) so we need 45*10 = 450 OPENTHREAD_CONFIG_NUM_MESSAGE_BUFFERS which is why we are changing to 512 (rounding up for wiggle room and to account for the 100 shared frames)

Why OPENTHREAD_CONFIG_DEFAULT_SED_DATAGRAM_COUNT changed?

1. The request is to buffer 8 frames (one full MTU) per child each MTU is 1280-byte and each SED datagram is 106-octet (see the info on OPENTHREAD_CONFIG_DEFAULT_SED_DATAGRAM_COUNT, to contain 1280 we need 1280/106 = 12.07, so we round up to 13.

Questions

1. Does the math checks out or am I missing something?
2. Any other CONFIG option that we should consider/tune for the goal?
3. Once a message is moved to the SED Buffer does it not longer consume space in the Message Buffer?

Thanks for everyone's time.

[abtink]

to support full 1280-byte IP packets we need 10 OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE (each 128 byte)

• Need to also consider the Metadata saved in Message
• And the reserved space for headers.
• Would recommend to account 11 or more (instead of 10) data buffer (of 128 bytes length) to contain an IPv6 message with 1280 payload.

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Once a message is moved to the SED Buffer does it not longer consume space in the Message Buffer?

No. Message Buffers are shared/used for all.

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that can buffer 8 frames (one full MTU) per child plus 100 shared frames. Which works out to 32 +13 = 45 MTUs (full 1280-byte IP packets)

• You want 11 MESSAGE_BUFFER per 1280 byte IPv6 message.
• If you want to support 8 IPv6 msg (max 1280 bytes) per child, it becomes 8 * 11 = 88 buffers per child.

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You can mostly ignore OPENTHREAD_CONFIG_DEFAULT_SED_BUFFER_SIZE, OPENTHREAD_CONFIG_DEFAULT_SED_DATAGRAM_COUNT configs.
These are only included in the emitted Connectivity TLV from device (they wont really adjust/allocate the buffers).

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Note that IPv6 Message instances are placed in the message queue for the SED child. 6LoWPAN (fragment) frames are constructed when the stack transmits to the child. Additionally, a single IPv6 Message instance is used for multicast transmissions destined for multiple children.

[rretanubun]

@abtink : Thanks for the thoughtful reply. Double checking my understanding on some of the replies:

You want 11 MESSAGE_BUFFER per 1280 byte IPv6 message.
If you want to support 8 IPv6 msg (max 1280 bytes) per child, it becomes 8 * 11 = 88 buffers per child

This mean (88 buffers per child * 32 child) + (13 MTU (for 100 shared frames) * 11 buffers-per-MTU) = 2959 for OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE if we want 8 IPv6 msg (max 1280 bytes) per child.

If we want 1 IPv6 msg (max 1280 bytes) per child, then it is 11 * (32 child +13 for-shared-frames) = 495 for OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE
Correct?

Just curious, is there an advantage to supporting 8 (vs 1) IPv6 msg per child?

You can mostly ignore OPENTHREAD_CONFIG_DEFAULT_SED_BUFFER_SIZE, OPENTHREAD_CONFIG_DEFAULT_SED_DATAGRAM_COUNT configs.
These are only included in the emitted Connectivity TLV from device (they wont really adjust/allocate the buffers).

Just making sure I follow, setting these higher does not increase the FTD/parent's capacity for processing/buffering SED specific messages for the children until they woke up?

[abtink]

f we want 1 IPv6 msg (max 1280 bytes) per child, then it is 11 * (32 child +13 for-shared-frames) = 495 for OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE
Correct?

Sounds correct to me.

Just making sure I follow, setting these higher does not increase the FTD/parent's capacity for processing/buffering SED specific messages for the children until they woke up?

Correct.