Subject: mboned: Source discovery draft for SSM

From: <[email-masked]>
Date: Thu, 12 Aug 2004 11:10:10 +0300

Hi folks,

I'd like to get your comments and discussion related to the draft published yesterday in the draft directory. 

Intention was to find a solution for multicast that utilizes SSM security/simplicity properties, works on the top of IPv4 and IPv6 and makes use well-known protocols (in this case MSDP). Solution can be implemented gradually (those hosts that want source discovery functionality can co-exist with other hosts) on application layer in end-hosts and doesn't require RP functionality and related signalling from the network. It uses always the optimal path for actual multicast data traffic and provides good scalability properties by keeping required source discovery signalling within the interest group (logical multicast group).

Please send your comments to the list and/or directly to me.

- Rami

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A New Internet-Draft is available from the on-line Internet-Drafts directories.

	Title		: Dynamic Multi-Source Discovery for SSM using MSDP
	Author(s)	: R. Lehtonen
	Filename	: draft-lehtonen-mboned-multissm-00.txt
	Pages		: 10
	Date		: 2004-8-11
	
This draft specifies dynamic multi-source discovery for
   source-specific multicast (SSM).  The source discovery is handled by
   the end-hosts and multicast routing has to be only SSM aware.  The
   source discovery is achieved by sending MSDP Source Active messages
   through the original SSM channel.  The support can be implemented on
   the application level.

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From: Beau Williamson <[email-masked]>
Date: Thu, 12 Aug 2004 10:13:31 -0500

Rami,

A couple of comments on your draft.

3.1  Source functionality

When new source, S1 wants to send to the same logical multicast
group, G, it has to send MSDP SA message to the group controller, S.
The message is sent to the unicast address of the group controller
with TCP/IP using UDP port number 639.

The above is a bit unclear as to whether the source uses UDP or TCP to send 
(and periodically refresh) these messages to the controller. I would assume 
UDP so that the controller isn't burdened with maintaining numerous TCP 
connections.

MSDP SA message MUST contain the following information:

    Address of the group controller = S
    Address of the logical multicast group = G
    Address of the new active source = S1

This brings up an issue regarding security and/or proxy MSDP source 
announcements.  It is possible that some host, S2, could send an MSDP-lite 
SA message for source S1, thereby functioning as a proxy for S1.  Should 
this be allowed?

My current thought is no, as this opens the door to a DDoS attack on the 
group by flooding the controller with "bogus" SA addresses. If the other 
members of the Logical Group respond by sending IGMPv3/MLDv2 Joins for 
these bogus sources, we would generate state for all these bogus sources in 
the network.

Of course, to some extent this is one of the weakness of SSM.  Any host can 
crank off bogus IGMPv3/MLDv2 joins and create state throughout the network. 
As a result, the idea of providing IGMP/MLD state limits per host is under 
consideration by equipment vendors. (Note that these sort of limits would 
effectively put an upper limit on the number of hosts that could 
participate in an SSM-based many-to-many multicast.

Possibly you want to add some text to Section 3.2, "Group Controller 
Functionality" regarding the policy decisions that clearly state that if 
the SA message [Source Address] field contains address S1, then it must be 
received from MSDP-Lite peer S1.  Unfortunately, there is no guarantee here 
either since source spoofing could be used by the attacker. Possibly some 
form of IPSec could be used here to authenticate the sending MSDP-lite peer.

I would think that the draft would at least have text stating that the 
Controller "SHOULD" make use of IPSec for MSDP peer authentication to 
prevent this sort of DDoS attack.

Beau Williamson