This feature implements a new mechanism to suppress the transmission of the Hello messages following the establishment of a Targeted LDP session between two LDP peers. The Hello adjacency of the targeted session does not require periodic transmission of Hello messages as in the case of a link LDP session. In link LDP, one or more peers can be discovered over a specific network IP interface and therefore, the periodic transmission of Hello messages is required to discover new peers in addition to the periodic keepalive message transmission to maintain the existing LDP sessions. A Targeted LDP session is established to a single peer. Thus, after the Hello adjacency is established and the LDP session is brought up over a TCP connection, keepalive messages are sufficient to maintain the LDP session.
When this feature is enabled, the targeted Hello adjacency is brought up by advertising the Hold-Time value the user configured in the hello timeout parameter for the targeted session. The LSR node then starts advertising an exponentially increasing Hold-Time value in the Hello message as soon as the targeted LDP session to the peer is up. Each new incremented Hold-Time value is sent in a number of Hello messages equal to the value of the hello reduction factor before the next exponential value is advertised. This provides time for the two peers to settle on the new value. When the Hold-Time reaches the maximum value of 0xffff (binary 65535), the two peers send Hello messages at a frequency of every [(65535-1)/local helloFactor] seconds for the lifetime of the targeted-LDP session (for example, if the local hello factor is three (3), then Hello messages are sent every 21844 seconds).
Both LDP peers must be configured with this feature to bring gradually their advertised Hold-Time up to the maximum value. If one of the LDP peers does not, the frequency of the Hello messages of the targeted Hello adjacency continues to be governed by the smaller of the two Hold-Time values. This feature complies to draft-pdutta-mpls-tldp-hello-reduce.