Reverse-path forwarding: Difference between revisions

Revision as of 17:40, 7 August 2019

Revision as of 05:01, 13 August 2019

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== Multicast RPF ==

Multicast RPF, typically denoted simply as RPF, is used in conjunction with a multicast routing protocol such as [[Multicast Source Discovery Protocol]] or [[Protocol Independent Multicast]] to ensure loop-free forwarding of multicast packets. In multicast routing, the decision to forward traffic is based upon source address and not on destination address as in unicast routing. It does this by utilizing either a dedicated multicast routing table or, alternatively, the router's unicast routing table.

When a multicast packet enters a router's interface, the router looks up the list of networks that are reachable via that interface (i.e., it checks the paths by which the packet could have arrived). If the router finds a matching routing entry for the source IP address of the multicast packet, the RPF check passes and the packet is forwarded to all other interfaces that are participating in that multicast group. If the RPF check fails, the packet is dropped. As a result, the forwarding of the packet is decided based upon the reverse path of the packet rather than the forward path. By only forwarding packets that come into the interface that also holds the routing entry for the source of the packet, loops are prevented.<!--[[User:Kvng/RTH]]-->

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The underlying assumptions of RPF check are that:

* the unicast routing table is correct and stable,

* the path used from a sender to a router and the reverse path from the router back to the sender are symmetric.

If the first assumption is not true, the RPF check will fail because it depends upon the router's unicast routing table as a fallback. If the second assumption is not true, the RPF check would reject multicast traffic on all but the shortest path from the sender to the router and would eventually lead to non-optimal multicast tree. In cases where the links are unidirectional, the reverse-path approach can fail altogether.

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==Comparison to reverse-path filtering==

Reverse path filters are typically used to disable asymmetric routing where an IP application has a different incoming and outgoing routing path. Reverse path filtering is a Linux Kernel feature. So major functionality is to prevent packet entering from one interface leaving via the other interfaces. Reverse-path Filtering is a feature of the [[Linux Kernel]],<ref name="professional Linux-1"/> but reverse-path forwarding is the protocol of the IP [[multicast]] routing. <ref name="professional Linux-1"> professional Linux- 1 May 2018:[https://www.theurbanpenguin.com/rp_filter-and-lpic-3-linux-security/ rp_filter and LPIC-3 Linux Security | professional Linux software development'']</ref> <ref name="Cisco unicast-reverse-path-forwarding-1"> CISCO Security Research & Operations- 3 June 2015:[https://www.cisco.com/c/en/us/about/security-center/unicast-reverse-path-forwarding.html Understanding Unicast Reverse Path Forwarding | Cisco -unicast-reverse-path-forwarding '']</ref>

== See also ==

* [[Flooding (computer networking)]]

==References==

{{Reflist}}

== External links ==

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* [http://www.tldp.org/HOWTO/Adv-Routing-HOWTO/lartc.kernel.rpf.html Linux - Enabling RPF in kernel]

* [http://www.juniper.net/techpubs/software/erx/erx50x/swconfig-routing-vol1/html/ip-multicast-config7.html#120398 Juniper Networks on multicast RPF]

[[Category:Routing]]