Subject: Re: [pim] TTL value of 1 for some PIM-SM messages

From: "" <[email-masked]>
Date: Fri, 3 Sep 2004 00:09:46 -0400 (EDT)

Hi Chris,

I need some clarfication regarding "Other control<br>messages are multicast to the link local group ALL-PIM-ROUTERS".

Does this mean that a control message (JOIN message) cannot cross a

LAN boundary. Since PIM-SM can be used in  a WAN I am confused.

Please clarify.

Regards,

Arut

 --- On Thu 09/02, Christopher Thomas Brown < [email-masked] > wrote:

From: Christopher Thomas Brown [mailto: [email-masked]]

To: [email-masked]

     Cc: [email-masked]

Date: Thu, 02 Sep 2004 15:42:14 -0400

Subject: Re: [pim] TTL value of 1 for some PIM-SM messages 

On Thu, 02 Sep 2004 07:00:52 EDT<br>>Hi All,<br>>I would like to know why a TTL value of 1 is used for Register and<br>>C-RP ADV mesgs. whereas, a gretaer TTL value is used for other control mesgs.<br><br>You have this reversed. Register messages and C-RP Advertisement messages<br>(as well as Register STop) are unicasted and may travel multiple hop to<br>reach the distination so the TTL should be greater than 1. Other control<br>messages are multicast to the link local group ALL-PIM-ROUTERS. Since<br>they are not intended to be forwarded (and should not be forwarded)<br>they are given a TTL of 1.<br><br>>Also <br>><br>>- why is the upstream neighbor addr. included in JOIN-PRUNE messages ?<br><br>PIM does not require that a router keep track of all interested downstream<br>routers. So when a router sends a prune to the upstsream its other neighbors<br>need to be able to process the prune so that they may override the prune<br>by sending a join. So this message must be multica!
 sted for all neighbors<br>to process it. As such it needs a field to indicate for which neighbor the<br>message it intended.<br><br><br>Chris<br>

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From: Prashant Jhingran <[email-masked]>
Date: Fri, 03 Sep 2004 13:34:54 +0800

Hi Arut,

Check out the pim-sm draft section "4.9.  PIM Packet Formats" its quite
self explanatory...

Its enough for control mesgs to travel single hop because a PIM router
wants to convey its requests or information about the domain to there
neighbors alone, the neighbors can in turn propogate the mesgs to others
(there neighbors) and hence TTL = 1 is sufficient for multicast control
packets. 

In case of unicast contol packets, the packets MAY need to travel
multiple hops and hence TTL >=1.

regards,
Prashant
========================================================================
=====
"4.9.  PIM Packet Formats
This section describes the details of the packet formats for PIM control
messages.

All PIM control messages have IP protocol number 103.

PIM messages are either unicast (e.g.  Registers and Register-Stop), or
multicast with TTL 1 to the `ALL-PIM-ROUTERS' group (e.g. Join/Prune,
Asserts, etc.).  The source address used for unicast messages is a
domain-wide reachable address; the source address used for multicast
messages is the link-local address of the interface on which the message
is being sent.

The IPv4 `ALL-PIM-ROUTERS' group is `224.0.0.13'.  The IPv6 `ALL-PIM-
ROUTERS' group is `ff02::d'."
Type Types for specific PIM messages.  PIM Types are:

Message Type                          Destination
Mes---------------------------------------------------------------------
------
0 = Hello                             Multicast to ALL-PIM-ROUTERS
1 = Register                          Unicast to RP
2 = Register-Stop                     Unicast to source of Register
packet
3 = Join/Prune                        Multicast to ALL-PIM-ROUTERS
4 = Bootstrap                         Multicast to ALL-PIM-ROUTERS
5 = Assert                            Multicast to ALL-PIM-ROUTERS
6 = Graft (used in PIM-DM only)       Multicast to ALL-PIM-ROUTERS
7 = Graft-Ack (used in PIM-DM only)   Unicast to source of Graft packet
8 = Candidate-RP-Advertisement        Unicast to Domain's BSR
========================================================================
=====

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
[email-masked]
Sent: Friday, September 03, 2004 12:10 PM
To: [email-masked]; [email-masked]
Cc: [email-masked]
Subject: Re: [pim] TTL value of 1 for some PIM-SM messages

Hi Chris,

I need some clarfication regarding "Other control<br>messages are
multicast to the link local group ALL-PIM-ROUTERS".

Please clarify.

Regards,

Arut

 --- On Thu 09/02, Christopher Thomas Brown < [email-masked] >
wrote:

To: [email-masked]

     Cc: [email-masked]

On Thu, 02 Sep 2004 07:00:52 EDT<br>>Hi All,<br>>I would like to know
why a TTL value of 1 is used for Register and<br>>C-RP ADV mesgs.
whereas, a gretaer TTL value is used for other control mesgs.<br><br>You
have this reversed. Register messages and C-RP Advertisement
messages<br>(as well as Register STop) are unicasted and may travel
multiple hop to<br>reach the distination so the TTL should be greater
than 1. Other control<br>messages are multicast to the link local group
ALL-PIM-ROUTERS. Since<br>they are not intended to be forwarded (and
should not be forwarded)<br>they are given a TTL of 1.<br><br>>Also
<br>><br>>- why is the upstream neighbor addr. included in JOIN-PRUNE
messages ?<br><br>PIM does not require that a router keep track of all
interested downstream<br>routers. So when a router sends a prune to the
upstsream its other neighbors<br>need to be able to process the prune so
that they may override the prune<br>by sending a join. So this message
must be multica!
 sted for all neighbors<br>to process it. As such it needs a field to
indicate for which neighbor the<br>message it
intended.<br><br><br>Chris<br>

---

From: Christopher Thomas Brown <[email-masked]>
Date: Fri, 03 Sep 2004 17:49:48 -0400

On Fri, 03 Sep 2004 00:09:46 EDT
>LAN boundary. Since PIM-SM can be used in  a WAN I am confused.

PIM control messages other than the ones that I noted previously
(register, register stop, crp-adv) are not forwarded off the local network.
Using a link local group as the destination and a TTL of 1 both help to
enforce this.

Join state must be instantiated at each router along the path from the
receiver to the root of the tree. As a simplification, it appears as if
the 'join' travels hop by hop along the path. But it is not a single join
control message that is just forwarded from one router to the next. Its the
instantiation of the join state that causes the next router in the path
to send its own join control message towards its upstream.

Chris