MPLS Label Distribution

References:
RFC3031 - Multiprotocol Label Switching Architecture
RFC7032 - LDP Downstream-on-Demand in Seamless MPLS
Minei, I. and Lucek, J. (2011). MPLS-enabled applications. Chichester, West Sussex, U.K.: John Wiley & Sons Ltd, pp.15-16.
Monge, A. and Szarkowicz, K. (n.d.). MPLS in the SDN era. pp.61-62.

Liberal Retention Mode vs. Conservative Retention Mode
Liberal retention mode causes an LSR to store all labels learnt via a label distributing protocol, even if the advertising LSR is not the next hop for the received label. Conservative retention mode is used so that an LSR only stores the labels it requires for data forward (which align to its routing, so labels received from their next-hop LSR). Liberal label retention mode uses more label space and can be an issue on LSR with a small LFIB capacity however with conservative retention mode, if there is a topology change in the case of LDP there is no ability to request a label so the LSR has to wait to receive a new label and the interim time can mean traffic is dropped.

Downstream Unsolicited vs. Downstream on Demand
Downstream unsolicited means that an LSR is advertising labels without receiving a request for them. Downstream on demand means that an LSR advertises labels as they are requested. Both LDP and RSVP-TE support a "request" method meaning they both support downstream unsolicited and downstream on demand. LDP is typically used in downstream unsolicited mode with a SP core or edge networks. Seamless MPLS leverages downstream on demand to extend LDP to the access layer.

Ordered Distribution Control vs. Independent Distribution Control
Ordered distribution means that an LSR will only advertise a label for a FEC (for which it is not the LER/egress LSR) if it has received a valid label from an upstream LSR. This means the egress LSR needs to start the process by advertising a label for a local FEC, which is checked by its neighbour (downstream) LSR to be (1) the shortest IGP path and (2) weather it should use the label for forwarding (it might have a better label path already). If this is the preferred path and valid then the downstream LSR will advertise this label to its downstream LSR and the process repeats to the ingress LER. In the case the LSR is the LER it may originate a label advertisement for a FEC that is locally connected or that it has connectivity to which extends outside of the label switching domain; without the need to receive a valid label from another LSR.

For independent distribution control all LSRs may advertise a label for a FEC (such as an IGP shared route) without having received a valid label from an upstream LSR. If one LSR in the path from ingress to egress LER doesn't have a valid LFIB entry though, the end-to-end LSP is broken. In this case all LSR are acting independently of the egress LER compared to ordered distribution in which the egress LER is "controlling" the LSP build/tear-down process.

For LDP running on Cisco and Juniper
Both IOS and IOS-XR by default use downstream unsolicited label distribution (downstream on demand is supported too), liberal label retention modes and independent distribution control mode (ordered distribution control mode is not supported). Per-prefix label allocation mode is the default for IOS and IOS-XR. Junos uses ordered label distribution control (independent distribution mode is not supported) with liberal retention mode (conservative mode is not supported) and downstream unsolicited label distribution (although downstream on demand is supported). Junos uses "per-NH" label allocation mode by default.

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