[zeromq-dev] why PGM is not a complete multicast protocol?

Steven McCoy steven.mccoy at miru.hk
Thu Nov 5 01:26:01 CET 2015

On 4 November 2015 at 18:53, Noh, Jangho <jangho.noh at wustl.edu> wrote:

> Question#1.
> According to Juniper Networks, PGM is not a complete multicast protocol.
> Why is that?
> How PGM makes distribution path???
> PGM adds reliability to multicast traffic streams. *It is not a complete
> multicast protocol* like the Distance Vector Routing Multicast Protocol
> (DVMRP) or Protocol Independent Multicast (PIM). Adding PGM to a router
> does not enable the router to perform multicast functions. Instead, a PGM
> router with multicast capabilities and a preconfigured multicast protocol
> such as PIM can offer more reliable multicast services to PGM sources and
> receivers. PGM is not an alternative to multicast routing protocols, but an
> enhancement of the multicast capabilities already present and configured on
> the router.
> http://www.juniper.net/documentation/en_US/junos13.2/topics/topic-map/mcast-pgm.html
This refers to protocols that assist a routing element do its main job of
routing.  PGM functionality in a router improves the overall performance of
the protocol.

> Question#2.
> How PGM receivers handle out of order packets?? (related to sliding window)
> Suppose a PGM receiver expects to get packet#1 but it received packet#3.
> And later on, the receiver got packet#1 and packet#2.
> Is there any specified document for this scenario?
PGM receivers have a receive window that performs packet re-ordering and
initiates delayed backchannel recovery procedures.  Generally switched
networks re-order upto two packets and the PGM protocol by default delays
sending negative acknowledgements upstream till after two packets have been

The PGM protocol document is the best source of information:

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