Subject: PIM-SM

From: Shahzad Ali <[email-masked]>
Date: Mon, 30 Nov 1998 15:52:01 -0500 (EST)

Hello,

I had a few questions about register and BSR messages in PIM-SM.

1)  Does a NULL register packet consist of three headers?
 I mean is it outer-IP + PIM-header + Reserved Word + Inner-IP?
 The inner-IP will be an IP packet with zero data len and source and
 destination equal to the (S,G) entry for which the NULL register is being
 sent for. 

2) In register packets (NULL or Regular), is the pim checksome done on the 
 PIM-header + Reserved word OR PIM-header alone 
 OR PIM-Header + reserved word + Inner IP-header?
The spec states that the checksum is done "only on the PIM-header,
excluding the data packet portion".  This could be interpretted as either
just the PIM-header OR PIM-header + Reserved word.

3) If a router is configured as a Cand-BSR but not as a Cand-RP (Note: the
spec says " we prefer that each router configured as a C-Rp also be
configured as a C-BSR" and not "it should be"), does the C-BSR send out
empty BSR messages initially.  It seems as if it doesnot there is a
deadlock.  Consider R1 and R2 both configured as C-BSR's but only R2 is
configured to be a C-RP as well.  Also assume the R1 will be the preferred
BSR if there were to be any election betweem R1 and R2.  Now R2 will
consider itself to be the BSR until it hears from a better BSR (R1), but
the better BSR (R1) is not sending any messages.  So R2 thinks that it is
the preffered BSR (incorrect!).  The BSR messages that it sends to R1 are
not accepted as R1 figures out that R2 is an inferior BSR.  Therefore R1
never finds out about the RP advertised by R2 and there is an
inconsistency. 
Although the state machine specified in the spec probably handles this
case properly, i still wanted to make sure that it be explicitly stated in
the spec that empty BSR messages should be sent in this case.

Cheers.

-Shahzad
Torrent Networking Tech.

---

From: [email-masked]
Date: Mon, 30 Nov 1998 19:14:24 -0800 (PST)

>  destination equal to the (S,G) entry for which the NULL register is being

yes, that is correct.

>  OR PIM-Header + reserved word + Inner IP-header?

well, that's been a topic of disgussion lately.  according to the spec 
pim, the checksum should only be done over the ip header, however,
cisco and gated do a checksum over the entire packet, inner-ip
included.  

> just the PIM-header OR PIM-header + Reserved word.

rfc2362 states:

        Checksum
             The checksum is the 16-bit one's complement of the one's
             complement sum of the entire PIM message, (excluding the
                                   ^^^^^^^^^^^^^^^^^^
             data portion in the Register message).  

so that would mean the pim header and the reserved word, but not the
inner ip packet.

> empty BSR messages initially.

rfc2362 says:

   The BSR also uses its Bootstrap-timer to periodically send Bootstrap
   messages.  In particular, when the Bootstrap-timer expires, the BSR
   originates a Bootstrap message on each of its PIM interfaces. 

this makes no mention of wether there are c-rp's or not.  so the
implicitly provides for null advertisements.

- rusty