3Com 3C13636 Configuration Manual page 834

3Com Router 3000 Ethernet Family
Configuration Guide
For a specific FEC, if LSR originates label assignment and distribution even without
receiving label request messages from upstream, it is in DU mode.
For a specific FEC, if LSR begins label assignment and distribution only after receiving
label request messages from upstream, it is in DoD mode.
The upstream and downstream, which have adjacency relation in label distribution,
should reach agreement on label distribution mode.
For the peers, LSR can use LDP messages to distribute labels or bear labels over other
routing protocol messages.
Note:
Upstream and downstream are just on a relative basis: For a packet forwarding
process, the transmit router serves as upstream LSR and receive router serves as
downstream LSR.
4) Label control mode
There are two types of label control modes: independent mode and ordered mode.
In independent control mode, each LSR can notify label mapping messages anytime.
In ordered control mode, a LSR can send label mapping messages to upstream only
when it receives a specific label mapping messages of the next hop of a FEC or the
LSR serves as LSP (Label distribution protocol) egress node.
5) Label retention mode
There are two label-retention modes: liberal label retention mode and conservative
label retention mode.
For a specific FEC, if LSR Ru has received the label binding from LSR Rd, in case Rd is
not the next hop of Ru and Ru saves this binding, then it is called liberal label retention
mode. If Ru discards this binding, then it is called conservative label retention mode.
In case it is required that LSR is capable of adapting route variation rapidly, liberal label
retention mode can be adopted. In case it is required that a few labels are saved in LSR,
then conservative label retention mode can be used.
1.2.3 LDP
Label distribution protocol (LDP) is the signaling control protocol in MPLS, which
controls binding of exchange labels and FECs between LSRs and coordinates a series
of procedures between LSRs.
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Chapter 1 MPLS Architecture