Re: [6lowpan] Slight problem with format document

[Mario Mao <[e-mail]>]

I agree with gabriel, there is no interference between header compression and ICMP. However, I believe two points should be noted which may cause information loss.

1. If the processing node is lowpan gateway/router and the sent packet comes from Internet but not originates by the gateway/router itself, the IID of IP source address shouldn't be compressed.
2. If the IP destination isn't an "on-link" node (that means the destination is more than one hop away from source node), the IID of IP destination address shouldn't be compressed.

In a word, 6lowpan node could only compress the IID that derives from IEEE 802.15.4 MAC address. I think that point should be emphasized in draft, isn't it?

Regards,

Mario

  ----- Original Message ----- 
  From: gabriel montenegro 
  To: Geoff Mulligan ; 6lowpan 
  Sent: Thursday, January 18, 2007 3:07 PM
  Subject: Re: [6lowpan] Slight problem with format document


  Is this even necessary to be said? ICMP msgs will never be sent by intermediate nodes
  in a lowpan mesh, but only by the IPv6 end nodes as part of normal IPv6 processing.
  This processing takes place after the lowpan layer delivers packets to the
  IPv6 stack. So by the time the IPv6 stack gets the packet, it will have been
  decompressed. This is the same for lowpan header compression or any other type
  of header compression.

  So I wonder if the text below is needed. It would be needed if the entity sending
  back ICMP messages was the lowpan layer. Yes, it would see compressed headers.
  But the entity is the IPv6 stack, which does not see those compressed headers.

  Comments? Am I missing something?

  -gabriel


  ----- Original Message ----
  From: Geoff Mulligan <[e-mail]>
  To: 6lowpan <[e-mail]>
  Sent: Wednesday, January 17, 2007 10:36:52 PM
  Subject: [6lowpan] Slight problem with format document


  While reviewing the format document I realized that we didn't describe
  handling of ICMPv6 error messages.  Since the 6lowpan headers may be
  compressed it is necessary to uncompress the original IP and transport
  headers before sending the ICMP error message.

  Here is my proposed text for a new section 12...

  12. Handling ICMPv6 messages

  ICMPv6 (RFC2463) is used to report errors and carry IP layer information
  and functions such as diagnostics.  There are two groups of ICMPv6
  messages: error messages and informational messages. Each message
  consists of a type field (if the high order bit is 0 it is an error
  message), a code and checksum field.  These fields are followed by the
  ICMPv6 message body.  For ICMPv6 error messages (Type <128) the message
  body shall contain as much of the original (offending) IP message
  without exceeded the minimum IPv6 MTU.  

  As described in the preceding section (Header Compression), the original
  IPv6 and Transport (TCP or UDP) headers may be compressed via HC1 and
  HC2 encoding.  So that the destination node of an ICMPv6 error message
  can properly process the message the source node must decompress the
  IPv6 and transport headers before sending the ICMPv6 message.  This is
  necessary because the original MAC addresses and the HC1 and HC2
  encoding headers will be lost and the recipient would have no ability to
  reconstruct the original message nor compute the proper checksum.  

  The ICMPv6 message itself is carried inside and IPv6 packet and this
  packet may be transmitted over the 6LoWPAN network utilizing header
  compression.  ICMPv6 messages that are larger than the available payload
  of the 6LoWPAN network will need to be fragmented. Assuming the maximum
  frame overhead, maximum link layer security and including a 6LoWPAN Mesh
  Header, Fragmentation Header and Dispatch Header, sending the
  uncompressed IPv6 and transport headers should still allow for 25 octets
  of the original packet payload.  Packets using short addresses, no
  security, and just a 6LoWPAN Dispatch header will be able to carry 61
  octets of the original packet.

    


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