isisd: Add SRv6 SID data structure

Add `struct isis_srv6_sid` data structure to represent an SRv6 SID.

The struct has the following members:
* next: pointer to the next SID, used to build linked lists of SRv6 SIDs
* flags: SID flags
* behavior: the behavior bound to the SRv6 SID (as defined in RFC 8986)
* value: the SID value (i.e., an IPv6 address)
* locator: a backpointer to the parent locator from which the SID has
been allocated.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>

isisd/isis_srv6.h

@@ -10,12 +10,34 @@
 #ifndef _FRR_ISIS_SRV6_H
 #define _FRR_ISIS_SRV6_H
 
+#include "lib/srv6.h"
+
 /* Maximum SRv6 SID Depths supported by the router */
 #define SRV6_MAX_SEG_LEFT 3
 #define SRV6_MAX_END_POP 3
 #define SRV6_MAX_H_ENCAPS 2
 #define SRV6_MAX_END_D 5
 
+/* SRv6 SID not bound to any adjacency */
+struct isis_srv6_sid {
+	struct isis_srv6_sid *next;
+
+	/* SID flags */
+	uint8_t flags;
+
+	/* SID value */
+	struct in6_addr sid;
+
+	/* Endpoint behavior bound to the SID */
+	enum srv6_endpoint_behavior_codepoint behavior;
+
+	/* Parent SRv6 locator */
+	struct srv6_locator_chunk *locator;
+
+	/* Backpointer to IS-IS area */
+	struct isis_area *area;
+};
+
 /* Per-area IS-IS SRv6 Data Base (SRv6 DB) */
 struct isis_srv6_db {