Can the length of the multicast message be optimized?

[jinpeng1989]

Currently, the datagram size of the multicast messages we are using has reached 157, 6LoWPAN will divide it into two fragments of lengths 136 and 21, and transmit two MAC frames. Sending the data in two separate MAC frames results in relatively poor communication reliability. We hope to be able to optimize it to a single MAC frame.

We will do our best to optimize the message content of the CoAP layer. I would like to inquire if there is any possibility of optimization for the other several layers? For instance, the two IPv6 layers occupy a total of 88 bytes. Can this be optimized? Have we made any incorrect usage here?

[jinpeng1989]

Below is the corresponding pcapng file. The networkkey is 984292b4b31f9d2134afefe4541e1c87
TBR_CH25_984292b4b31f9d2134afefe4541e1c87_00001_20250911164719.zip

If the other layers are not optimized, then how long should our CoAP layer be optimized to be able to be carried by a MAC frame?

[jwhui]

Note that the IPv6 headers are compressed when transmitted in an 802.15.4 frame. You should review pkt#4591 for the compressed header. For example, the outer IP header is 22 bytes (not 48) when transmitted in an 802.15.4 frame.

This PR should help reduce the outer IP header overhead even further:

• [mpl] use RLOC as IPv6 source address #11843

There is an effort to completely remove the outer IP header, but that requires a change to the Thread Specification and will take more time:

• [mpl] remove IP-in-IP header #11868