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6LowPAN

i have a really simple question relating to 6LoWPAN, (i am new to this field so ignore any mistakes i make in terminology) in 6LoWPAN fragmentation there is no reliability because it uses UDP at transport layer i.e unlike TCP there is no ACK mechanism. is there any way that the source can know about the fragment loss on the way to the destination device? or any other device (intermediate or destination) can inform the source about it?

  • You can add some packet information from your application to know the integrity of sending packets.

  • thankyou for the reply but i mean any predifined mechanism in 6LoWPAN protocol which is specifically defined for fragmentation, which can tell the source about fragmentation losses
  • No, there’s no predefined mechanism in Contiki for this.
  • I'd say it depends on whether the underlying MAC protocol supports ACKs.

    For example, even if you are using UDP, if it is a unicast to a specific device, the 6lowpan layer might fragment it and tell the MAC/RDC layer to send a queue of packets to a unicast destination. If that layer in turn uses ACKs with unicasts (for every unicast packet, and it should), unicasts that are not ACKed will be retransmitted a few times. Broadcasts no, since it doesn't use ACKs on that layer. How the stack handles this can have a large impact on performance.

    And I'd say it's not a simple question, rather the opposite! Both how to handle it, but it's also a quite deep question for a 6lowpan beginner.
  • (let me clarify that, UDP unicasts are sent as unicasts on the 15.4 layer too, and hence are ACKed. UDP broadcasts are not)
  • i guess you are referring to ARQ but that just provides reliability for MAC layer frames and it is between two immediate neighbours, but is there any mechanism for reliable delivery of fragments from a source to a destination which are a few hops apart.