This is xnu-4903.221.2. See this file in:
/*
 * Copyright (c) 2018 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 *
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */

/*
 * RFC 2408 Internet Security Association and Key Management Protocol
 */

#ifndef _NETINET_ISAKMP_H_
#define _NETINET_ISAKMP_H_

typedef u_char cookie_t[8];
typedef u_char msgid_t[4];

/* 3.1 ISAKMP Header Format (IKEv1 and IKEv2)
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                          Initiator                            !
 !                            Cookie                             !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                          Responder                            !
 !                            Cookie                             !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !  Next Payload ! MjVer ! MnVer ! Exchange Type !     Flags     !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                          Message ID                           !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 !                            Length                             !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 */
struct isakmp {
	cookie_t i_ck;		/* Initiator Cookie */
	cookie_t r_ck;		/* Responder Cookie */
	uint8_t np;		/* Next Payload Type */
	uint8_t vers;
#define ISAKMP_VERS_MAJOR	0xf0
#define ISAKMP_VERS_MAJOR_SHIFT	4
#define ISAKMP_VERS_MINOR	0x0f
#define ISAKMP_VERS_MINOR_SHIFT	0
	uint8_t etype;		/* Exchange Type */
	uint8_t flags;		/* Flags */
	msgid_t msgid;
	uint32_t len;		/* Length */
};

/* 3.2 Payload Generic Header
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 ! Next Payload  !   RESERVED    !         Payload Length        !
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 */
struct isakmp_gen {
	uint8_t  np;       /* Next Payload */
	uint8_t  critical; /* bit 7 - critical, rest is RESERVED */
	uint16_t len;      /* Payload Length */
};

#endif /* _NETINET_ISAKMP_H_ */