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Some of these packets are delivered via unicast, and some via multicast, and EIGRP runs directly on top of IP, so some method of ensuring delivery is required; to solve this Cisco has borrowed from TCP the concept of packet sequencing.
Reliable Multicast
This is Cisco's name for the method that it uses in RTP to ensure reliable, ordered delivery. As mentioned above, it is very similar to TCP in that a non-zero Sequence number is used in the packet header for all packets (multicast or unicast) it wishes delivered reliably. The sequence number is incremented by the EIGRP process on a router whenever a reliable RTP packet is sent.
For other packets, a zero is used in the Sequence field.
For other packets, a zero is used in the Sequence field.
ACKnowledgement
These packets are all required to be responded to by the neighboring router with an Acknowledgement (ACK), either:
- in the ACK field of a returning reliable RTP / EIGRP packet, or
- as a stand-alone ACK, which is essentially a Hello with the ACKed sequence number in the ACK field.
Note, however, that in RTP the ACK contains the last received sequence number, and NOT the next expected sequence number as in TCP.
Conditional Receive
EIGRP RTP "Conditional Receive" allows a list of 'lagging' routers on a multi-access interface, i.e. routers that have not ACKed (within a timeout window) reliable multicast messages one or more times, to be tracked so that they are sent unicast messages instead. This timeout is determined by the "multicast flow timer", and the interval between unicast transmissions by the "retransmission timout (RTO)"; both of these are calculated based on the smooth round-trip time (SRTT) which is a measurement of the time between the sending of a reliable packet and the receipt of its ACK.
In order to ensure that these routers do not try to process BOTH the unicast and multicast messages, the 'lagging' router list is communicated in two specific TLVs: the Sequence TLV and the Next Multicast Sequence TLV.
These are included in a Hello packet (known as a "Sequenced Hello"). All routers receiving the packet examine the Sequence TLV to see if they are on the list: any non-lagging routers will place themselves in 'Conditional Receive mode' (CR-mode); any router receiving the Hello and finding itself in the TLV, or those not receiving the Hello, will not place themselves in CR-mode.
The sending router will then send the next multicast packet with the CR flag set and those in CR-mode will pick up this packet and use it as usual, then exit CR-mode; others not in CR-mode will ignore it.
