Subject: [pim] Query regarding the fragmentation for Register message

From: [email-masked]
Date: Fri, 7 Nov 2003 18:09:25 +0900

Hello, 

I have a question regarding the fragmentation for PIM register 
messages defined in the draft draft-ietf-pim-sm-v2-new-08.txt 
Section 4.4.1. 

    If a multicast packet is fragmented on the way into the 
    Register Tunnel, each fragment is encapsulated individually so it 
    contains IP, PIM, and inner IP headers. 

    In IPv6, an ICMP Fragmentation Required message may be sent by the 
    encapsulating DR. 

My understanding is that inner multicast data would be fragmented 
before it is encapsulated with outer IP header and PIM header. 

I want to know the reason to go through the procedure mentioned above. 
Is it wrong procedure that fragmentation is done after a multicast 
packet is encapsulated with outer IP header and PIM header ? 

In IPv6, when an original multicast packet is smaller than 
IPv6 minimum MTU(1280bytes), DR cannot send PACKET_TOO_BIG to sender.  
And DR would have to fragment the packet when it is larger than the 
Path-MTU toward the RP. 
At that time, If original packet was fragmented by the sender, 
DR cannot fragment it anymore, I think. 

Please let me know if I am making any mistake.  

Thanks, 

Shiroko 

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From: Bill Fenner <[email-masked]>
Date: Tue, 11 Nov 2003 16:01:02 -0800

Shiroko,

  You are right; if the MTU of the DR->RP path is 1280 then the DR
may not send an ICMP fragmentation required error.  The idea behind
having the fragmentation occur inside the tunnel is to prevent
the RP from being overloaded with reassembly load.

  One possible way to handle this is that if the DR->RP path MTU is
1288 or less then use fragmentation on the outer packet.  This leaves
the overhead on the end systems if possible and only moves it to the
RP when necessary.  However, it's a little obscure and possibly
confusing.  Anyone have any thoughts on this?

Thanks,
  Bill

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From: Bill Fenner <[email-masked]>
Date: Tue, 11 Nov 2003 16:01:02 -0800

Shiroko,

Thanks,
  Bill