PGM Various Packet type

(From the specification document)

PGM DEFINES FOUR BASIC PACKET TYPES:three that flows downstream (SPM NCF DATA) and one that flows
upstream (NAK).

PGM runs over a datagram multicast protocol such IP multicast. A source multicasts sequenced data
packets (ODATA)
Receivers unicast selective negative knowledges (NAK)for data packets detected to be missing by the
sequence number.
Network elements forward NAKs PGMhop by PGMhop to the source, and confirm each hop by a multicast
NAK confirmation (NCF) in response, on the interface where the NACK was received.
RDATA packets are send in repais of packet loss and may be provided by the source itself or by
Designated Local Repairer (DLR)in response to NAK.

NACK reliability:
To minimize loss, PGM define a network layer hop-by-hop procedure for reliable NAK forwarding
Upon detection of a missing data packet, a receiver repeatedly unicasts a NAK to the last hop PGM
network element on teh distribution tree from the source. A receiver repeats the NAK until it
receives a NAK confirmation (NCF) multicast to the group from that PGM network element.
The same network element forward the NAK to the upstream PGM network element on the reverse of data
distribution path from the source of the original data packet until it receives a NCF from the
usptream network element....

Finally the source  itself receives and confirms the NAK by multicasting a NCF to the group - BUT
THEY ARE NOT PROPAGATED by PGM network elements since they do hop-by-hop confirmation.

Source Path Messages (SPM)
To establish source path state in PGM network elements, the basic data tranfer operation is
augmented by Source Path Messages(SPMs) from a source, periodically interleaved with ODATA.
SPM funcionality: establish source path state for a given TSI in all PGM network elements on the
distribution tree from the source. PGM net elements use this information to address returning
unicast NAKs directly to the upstream PGM network element toward the source and make sure that NAKs
return from a receiver to a source in the reverse path of the distribution list for the TSI (??)

NETWORK ELEMENT FUNCTIONS

-	they forward ODATA without intervention

-	intercept SPMs and use them to establsih source path state for the corresponding TSI (TRansport
	Session Identifier) before multicast
	forwarding then in the usual way.

- 	multicas NCFs to the group in response to any NAK received.For each NAK received, network elements
	create repair state,recording the transport session idendifier,the sequence number of the NAK and
	the input interface on which the NAK was received

-	network elements repeateadly unicast forward only the first copy of any NAK they receive to the
	upstream PGM net element on the distribution path for the TSI and in addition they may optionally
	multicast this NAK usptream with TTL 1. They keep sending it till they receive a NCF in response.
	Once confirmed by a NCF - network elements DISCARDS NAK packets

-	net elements discards duplicates of any NAK for which they already have repair state or received

-	net elements use NAKs to maintain repair state consisting of a list of interfaces upon which a given
	NAK was received, and they return the corresponding RDATA only on there interfaces.	
	
-	If a net element hears an upstream NCF (for the same distribution tree) it will establish  repair
	state without outgoing interfaces in anticipation of responding to and eliminating duplicates of the
	NAK might arrive from downstream.
	

TERMINILOGY: TERMS AND CONCEPTS

TSI - Transport Session Indentifier

Every PGM packet is identified by a TSI
it must be globally unique and only one source at a time may act as the source for a transport session.
TSIs are composed of the concatenation of a globally unique source identifier (GSI) and s source
assigned data-source port.