Path Computation Algorithms
Introduction
Both RSVPTE and Segment Routing Flex Algo need to compute end to end path with other constraints as the standard IGP metric. Based on Shortest Path First (SPF) algorithms, a new class of Constrained SPF (CSPF) is provided by the FRR library.
Supported constraints are as follow:  Standard IGP metric (here, CSPF provides the same result as a normal SPF)  Traffic Engineering (TE) IGP metric  Delay from the IGP Extended Metrics  Bandwidth for a given Class of Service (CoS) for bandwidth reservation
Algorithm
The CSPF algorithm is based on a Priority Queue which store the ongoing possible path sorted by their respective weights. This weight corresponds to the cost of the cuurent path from the source up to the current node.
The algorithm is as followed:
cost = MAX_COST;
Priority_Queue.empty();
Visited_Node.empty();
Processed_Path.empty();
src = new_path(source_address);
src.cost = 0;
dst = new_destinatio(destination_address);
dst.cost = MAX_COST;
Processed_Path.add(src);
Processed_Path.add(dst);
while (Priority_Queue.count != 0) {
current_path = Priority_Queue.pop();
current_node = next_path.destination;
Visited_Node.add(current_node);
for (current_node.edges: edge) {
if (prune_edge(current_path, edge)
continue;
if (relax(current_path) && cost > current_path.cost) {
optim_path = current_path;
cost = current_path.cost;
}
}
}
prune_edge(path, edge) {
// check that path + edge meet constraints e.g.
if (current_path.cost + edge.cost > constrained_cost)
return false;
else
return true;
}
relax_edge(current_path, edge) {
next_node = edge.destination;
if (Visited_Node.get(next_node))
return false;
next_path = Processed_Path.get(edge.destination);
if (!next_path) {
next_path = new path(edge.destination);
Processed_Path.add(next_path);
}
total_cost = current_path.cost + edge.cost;
if (total_cost < next_path.cost) {
next_path = current_path;
next_path.add_edge(edge);
next_path.cost = total_cost;
Priority_Queue.add(next_path);
}
return (next_path.destination == destination);
}
Definition

struct constraints
This is the constraints structure that contains:
cost: the total cost that the path must respect
ctype: type of constraints:
CSPF_METRIC for standard metric
CSPF_TE_METRIC for TE metric
CSPF_DELAY for delay metric
bw: bandwidth that the path must respect
cos: Class of Service (COS) for the bandwidth
family: AF_INET or AF_INET6
type: RSVP_TE, SR_TE or SRV6_TE

struct c_path
This is the Constraint Path structure that contains:
edges: List of Edges that compose the path
status: FAILED, IN_PROGRESS, SUCCESS, NO_SOURCE, NO_DESTINATION, SAME_SRC_DST
weight: the cost from source to the destination of the path
dst: key of the destination vertex

struct cspf
This is the main structure for path computation. Even if it is public, you don’t need to set manually the internal field of the structure. Instead, use the following functions:
Function to create an empty cspf for future call of path computation

struct cspf *cspf_init(struct cspf *algo, const struct ls_vertex *src, const struct ls_vertex *dst, struct constraints *csts);
This function initialize the cspf with source and destination vertex and constraints and return pointer to the cspf structure. If input cspf structure is NULL, a new cspf structure is allocated and initialize.

struct cspf *cspf_init_v4(struct cspf *algo, struct ls_ted *ted, const struct in_addr src, const struct in_addr dst, struct constraints *csts);
Same as cspf_init, but here, source and destination vertex are extract from the TED data base based on respective IPv4 source and destination addresses.

struct cspf *cspf_init_v6(struct cspf *algo, struct ls_ted *ted, const struct in6_addr src, const struct in6_addr dst, struct constraints *csts);
Same as cspf_init_v4 but with IPv6 source and destination addresses.
Clean internal structure of cspf in order to reuse it for another path computation.
Delete cspf structure. A call to cspf_clean() function is perform prior to free allocated memeory.
Compute point to point path from the ted and cspf. The function always return a constraints path. The status of the path gives indication about the success or failure of the algorithm. If cspf structure has not been initialize with a call to cspf_init() or cspf_init_XX(), the algorithm returns a constraints path with status set to FAILED. Note that a call to cspf_clean() is performed at the end of this function, thus it is mandatory to initialize the cspf structure again prior to call again the path computation algorithm.
Usage
Of course, CSPF algorithm needs a network topology that contains the various metrics. Link State provides such Traffic Engineering Database.
To perform a Path Computation with given constraints, proceed as follow:
struct cspf *algo;
struct ls_ted *ted;
struct in_addr src;
struct in_addr dst;
struct constraints csts;
struct c_path *path;
// Create a new CSPF structure
algo = cspf_new();
// Initialize constraints
csts.cost = 100;
csts.ctype = CSPF_TE_METRIC;
csts.family = AF_INET;
csts.type = SR_TE;
csts.bw = 1000000;
csts.cos = 3;
// Then, initialise th CSPF with source, destination and constraints
cspf_init_v4(algo, ted, src, dst, &csts);
// Finally, got the Computed Path;
path = compute_p2p_path(ted, algo);
if (path.status == SUCCESS)
zlog_info("Got a valid constraints path");
else
zlog_info("Unable to compute constraints path. Got %d status", path>status);
If you would compute another path, you must call cspf_init() prior to compute_p2p_path() to change source, destination and/or constraints.