java-topology/tools/tickets/defects/frrouting-0001.md

id	repo	severity	status	created
frrouting-0001	FRRouting/frr	HIGH	PATCHED	2026-03-23

Defect

File: ospfd/ospf_ti_lfa.c:72,114,227,278,285 Pattern: listnode_lookup(q_space->pc_path, vertex) — linear scan through post-convergence path list Complexity: O(path_length²) per protected element during TI-LFA backup computation Language: C

Description

FRRouting's TI-LFA (Topology Independent Loop-Free Alternates) implementation uses listnode_lookup() — a linear scan through a doubly-linked list — to locate vertices in the post-convergence path (pc_path) during backup tunnel construction. The function is called 5 times in the same file. For each protected link or node, TI-LFA must find the P-node and Q-node positions within the post-convergence path, performing O(path_length) work per lookup. On a network with N routers and P protected elements, total TI-LFA computation is O(P × path_length), where both P and path_length grow with N.

Note: The main OSPF SPF implementation (ospf_spf.c) is clean — it uses the LSA_SPF_IN_SPFTREE pointer flag (O(1) membership) and a skip-list priority queue. Only the TI-LFA extension has the defect.

Fix

Replace: listnode_lookup(pc_path, vertex) — O(n) linked-list scan With: vertex index stored in the vertex struct, or a companion hash map vertex → listnode* Data structure change: add struct listnode *pc_path_node to struct vertex so lookup is O(1) via vertex->pc_path_node — set when path is built, nulled on cleanup.

Work required

• Patch in defects/frrouting/patch/frrouting-0001-pc-path-index.patch
• Unit test — asserts exact operation counts before/after (in defects/frrouting/unit/)
• Integration test (in defects/frrouting/integration/)
• Benchmark — before/after on network with V=50,100,200,400 routers with full mesh protection (in defects/frrouting/bench/)
• White paper section — whitepaper/vectors/networking/frrouting.rst