LDP Downstream-on-Demand in Seamless MPLS
Extracted elements (25)
Conservative label retention mode cannot be used with access LDP DoD because it prevents LSRs from requesting and maintaining label mappings for backup routes not currently used for forwarding, disabling IPFRR LFA and primary/backup static route protection. Liberal retention mode enables pre-programming of alternate forwarding paths.
Independent label distribution control is incompatible with LDP DoD in access networks because an LSR binds labels without waiting for downstream confirmation, preventing propagation of label binding failures along the access chain and making alternate-path switchover impossible for upstream LSRs.
LDP Downstream Unsolicited (DU) advertisement is inadequate for access nodes because it floods labels for all valid RIB routes, exceeding the constrained memory on devices that only need labels for a small subset of routes tied to provisioned services. LDP DoD enables an AN-driven subscription model where only required labels are requested and installed.
The Queue Request TLV addresses a gap in RFC 5036 where a 'No Route' response followed by exponential backoff can impose significant delay before re-requesting a label for a route that becomes available shortly after. By queuing the request at the downstream LSR, the label mapping is sent immediately upon route availability, eliminating the backoff penalty.
When LDP DoD is negotiated for IPv4 and IPv6 FECs on a session between adjacent access LSR/ABR nodes, PWE3 FECs on the same LDP session still use Downstream Unsolicited label advertisement as specified in RFC 4447.
All access LSRs (AN and AGN1x nodes) must use LDP DoD with ordered label distribution control and liberal label retention mode to achieve Seamless MPLS design goals of large network scale without compromising fast service restoration.
ANs must support Inter-area LDP (RFC 5283) in all static-routing access topologies in order to use the IP default route to match LDP FECs advertised by AGN1x and other ANs across area boundaries.
Downstream and upstream LSRs SHOULD implement the Queue Request TLV extension if fast-up convergence is desired.
If an access LSR only supports LDP DoD and the peer proposes Downstream Unsolicited mode, the access LSR must send a Notification with status 'Session Rejected/Parameters Advertisement Mode', close the session, and retry with exponential backoff starting at 15 seconds and growing to a maximum delay of 2 minutes.
Upon receipt of an unknown TLV with U-bit=1, the TLV MUST be silently ignored and the rest of the message processed as if the TLV did not exist. If the requested route is not available and the Queue Request TLV is unrecognized, the downstream LSR MUST ignore the TLV and send a 'No Route' notification to ensure backward compatibility.
A downstream access LSR/ABR withdraws a label via Label Withdraw in five cases: BGP route withdrawal, IGP route withdrawal or downstream LDP session loss, downstream label withdrawal, next-hop change with no label from the new next hop, or configured policy rejection. The LSR attempts local LSP convergence before withdrawing upstream labels.
A downstream access LSR responds to a Label Request with a Label Mapping only when the FEC has a route in its RIB and a valid downstream label binding exists (from LDP DoD or DU), or when acting as egress. If no downstream label is yet available but an LDP DoD session to the next hop exists, the downstream LSR propagates the request and responds only after receiving the label from the next hop.
AGN1x, acting as the access ABR, implements ordered label distribution control when redistributing FECs between the access side (LDP DoD with static routing or access IGP) and the network side (labeled BGP or core IGP with LDP DU), enabling failure propagation between access and metro/core domains.
An upstream access LSR/ABR sends a Label Release when it receives a Label Withdraw, when a /32 static route with DoD policy is deleted, when a service is decommissioned with no corresponding static route, when a next-hop change makes the label invalid, or when an unsolicited label mapping is received on a DoD session.
Fast-up convergence operation: upstream LSR sends Label Request with Queue Request TLV; supporting downstream LSR queues the request when no route is available and sends the Label Mapping as soon as the route comes up, without sending 'No Route'. Upstream does not retry while the request is queued. Upstream aborts a queued request by sending a Label Abort Request message.
For ECMP and IPFRR LFA local-repair, the access LSR/ABR proactively requests labels on both best next-hop and alternate next-hop DoD sessions. For remote LFA, it also requests labels from the alternate next hop toward the PQ node and from the PQ node toward the destination FEC.
LDP DoD session negotiation is performed by setting the 'A' bit to 1 in the Common Session Parameters TLV of the LDP Initialization message. Both access LSR/ABR peers must propose DoD mode; negotiation rules follow RFC 5036.
On receiving a 'No Route' notification, the access LSR retries via exponential backoff. Per RFC 5036, if no response is received to a Label Request, a duplicate request must not be sent; this is treated as a protocol error. The Queue Request TLV extension (Section 5) addresses this convergence limitation.
Seamless MPLS defines six reference access topology variants: [I1] single AN to single AGN, [I] daisy-chained ANs to single AGN, [V] single AN dual-homed to two AGNs, [U2] two ANs dual-homed to two AGNs, [Y] chained ANs to two AGNs, and [U] ring-chained ANs dual-homed to two AGNs. All topologies use either static routing or a dedicated access IGP, with AGN1x nodes acting as access ABRs.
To support ECMP and IPFRR LFA local-repair, the upstream access LSR/ABR sends LDP DoD Label Requests to both best next-hop and alternate next-hop neighbors and installs all received labels as primary and alternate entries in its LIB and LFIB.
IANA updated the 'TLV Type Name Space' registry (defined by RFC 5036) with a new assignment: Queue Request TLV type 0x0971.
LDP DoD deployments in access networks face the same threats as any MPLS LDP deployment. Baseline security per RFC 5920 and RFC 5036 is recommended, including message authenticity, integrity, and protection against spoofing and DoS attacks. Access nodes in physically insecure locations (e.g., VDSL street cabinets) require additional physical and monitoring controls.
LDP DoD's request-driven nature is an inherent security property: unsolicited label mappings are not accepted by upstream LSRs, limiting unauthorized label injection. ABR LSRs act as MPLS network gateways controlling FEC access. With static routing, compromised ANs cannot influence ABR routing decisions; with access IGP, FEC stealing is topologically constrained and detectable via session loss.
TCP Authentication Option (TCP-AO, RFC 5925) with strong hashing (e.g., SHA1) is recommended over MD5 for LDP session security. LDP Hello spoofing is mitigated by accepting Basic Hellos only on trusted interfaces, ignoring non-multicast-addressed Hellos, and optionally using the Cryptographic Authentication TLV during LDP discovery.
Queue Request TLV: 16-bit Type field=0x0971, 16-bit Length field=0x0000 (zero-length value). U-bit=1 causes unknown-TLV receivers to silently ignore the TLV and process the rest of the message; F-bit=0 prevents forwarding of the unrecognized TLV. Carried as an Optional Parameter in the LDP Label Request message.