ZeroTierOne/node/Packet.hpp

/*
 * ZeroTier One - Network Virtualization Everywhere
 * Copyright (C) 2011-2016  ZeroTier, Inc.  https://www.zerotier.com/
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#ifndef ZT_N_PACKET_HPP
#define ZT_N_PACKET_HPP

#include <stdint.h>
#include <string.h>
#include <stdio.h>

#include <string>
#include <iostream>

#include "Constants.hpp"

#include "Address.hpp"
#include "Poly1305.hpp"
#include "Salsa20.hpp"
#include "Utils.hpp"
#include "Buffer.hpp"

#ifdef ZT_USE_SYSTEM_LZ4
#include <lz4.h>
#else
#include "../ext/lz4/lz4.h"
#endif

/**
 * Protocol version -- incremented only for major changes
 *
 * 1 - 0.2.0 ... 0.2.5
 * 2 - 0.3.0 ... 0.4.5
 *   + Added signature and originating peer to multicast frame
 *   + Double size of multicast frame bloom filter
 * 3 - 0.5.0 ... 0.6.0
 *   + Yet another multicast redesign
 *   + New crypto completely changes key agreement cipher
 * 4 - 0.6.0 ... 1.0.6
 *   + BREAKING CHANGE: New identity format based on hashcash design
 * 5 - 1.1.0 ... 1.1.5
 *   + Supports circuit test, proof of work, and echo
 *   + Supports in-band world (root server definition) updates
 *   + Clustering! (Though this will work with protocol v4 clients.)
 *   + Otherwise backward compatible with protocol v4
 * 6 - 1.1.5 ... 1.1.10
 *   + Network configuration format revisions including binary values
 * 7 - 1.1.10 -- 1.2.0
 *   + Introduce trusted paths for local SDN use
 * 8 - 1.2.0  -- CURRENT
 *   + Multipart network configurations for large network configs
 *   + Tags and Capabilities
 *   + Inline push of CertificateOfMembership deprecated
 *   + Certificates of representation for federation and mesh
 */
#define ZT_PROTO_VERSION 8

/**
 * Minimum supported protocol version
 */
#define ZT_PROTO_VERSION_MIN 4

/**
 * Maximum hop count allowed by packet structure (3 bits, 0-7)
 *
 * This is a protocol constant. It's the maximum allowed by the length
 * of the hop counter -- three bits. See node/Constants.hpp for the
 * pragmatic forwarding limit, which is typically lower.
 */
#define ZT_PROTO_MAX_HOPS 7

/**
 * Cipher suite: Curve25519/Poly1305/Salsa20/12/NOCRYPT
 *
 * This specifies Poly1305 MAC using a 32-bit key derived from the first
 * 32 bytes of a Salsa20/12 keystream as in the Salsa20/12 cipher suite,
 * but the payload is not encrypted. This is currently only used to send
 * HELLO since that's the public key specification packet and must be
 * sent in the clear. Key agreement is performed using Curve25519 elliptic
 * curve Diffie-Hellman.
 */
#define ZT_PROTO_CIPHER_SUITE__C25519_POLY1305_NONE 0

/**
 * Cipher suite: Curve25519/Poly1305/Salsa20/12
 *
 * This specifies Poly1305 using the first 32 bytes of a Salsa20/12 key
 * stream as its one-time-use key followed by payload encryption with
 * the remaining Salsa20/12 key stream. Key agreement is performed using
 * Curve25519 elliptic curve Diffie-Hellman.
 */
#define ZT_PROTO_CIPHER_SUITE__C25519_POLY1305_SALSA2012 1

/**
 * Cipher suite: NONE
 *
 * This differs from POLY1305/NONE in that *no* crypto is done, not even
 * authentication. This is for trusted local LAN interconnects for internal
 * SDN use within a data center.
 *
 * For this mode the MAC field becomes a trusted path ID and must match the
 * configured ID of a trusted path or the packet is discarded.
 */
#define ZT_PROTO_CIPHER_SUITE__NO_CRYPTO_TRUSTED_PATH 2

/**
 * DEPRECATED payload encrypted flag, may be re-used in the future.
 *
 * This has been replaced by the three-bit cipher suite selection field.
 */
#define ZT_PROTO_FLAG_ENCRYPTED 0x80

/**
 * Header flag indicating that a packet is fragmented
 *
 * If this flag is set, the receiver knows to expect more than one fragment.
 * See Packet::Fragment for details.
 */
#define ZT_PROTO_FLAG_FRAGMENTED 0x40

/**
 * Verb flag indicating payload is compressed with LZ4
 */
#define ZT_PROTO_VERB_FLAG_COMPRESSED 0x80

/**
 * Rounds used for Salsa20 encryption in ZT
 *
 * Discussion:
 *
 * DJB (Salsa20's designer) designed Salsa20 with a significant margin of 20
 * rounds, but has said repeatedly that 12 is likely sufficient. So far (as of
 * July 2015) there are no published attacks against 12 rounds, let alone 20.
 *
 * In cryptography, a "break" means something different from what it means in
 * common discussion. If a cipher is 256 bits strong and someone finds a way
 * to reduce key search to 254 bits, this constitues a "break" in the academic
 * literature. 254 bits is still far beyond what can be leveraged to accomplish
 * a "break" as most people would understand it -- the actual decryption and
 * reading of traffic.
 *
 * Nevertheless, "attacks only get better" as cryptographers like to say. As
 * a result, they recommend not using anything that's shown any weakness even
 * if that weakness is so far only meaningful to academics. It may be a sign
 * of a deeper problem.
 *
 * So why choose a lower round count?
 *
 * Turns out the speed difference is nontrivial. On a Macbook Pro (Core i3) 20
 * rounds of SSE-optimized Salsa20 achieves ~508mb/sec/core, while 12 rounds
 * hits ~832mb/sec/core. ZeroTier is designed for multiple objectives:
 * security, simplicity, and performance. In this case a deference was made
 * for performance.
 *
 * Meta discussion:
 *
 * The cipher is not the thing you should be paranoid about.
 *
 * I'll qualify that. If the cipher is known to be weak, like RC4, or has a
 * key size that is too small, like DES, then yes you should worry about
 * the cipher.
 *
 * But if the cipher is strong and your adversary is anyone other than the
 * intelligence apparatus of a major superpower, you are fine in that
 * department.
 *
 * Go ahead. Search for the last ten vulnerabilities discovered in SSL. Not
 * a single one involved the breaking of a cipher. Now broaden your search.
 * Look for issues with SSH, IPSec, etc. The only cipher-related issues you
 * will find might involve the use of RC4 or MD5, algorithms with known
 * issues or small key/digest sizes. But even weak ciphers are difficult to
 * exploit in the real world -- you usually need a lot of data and a lot of
 * compute time. No, virtually EVERY security vulnerability you will find
 * involves a problem with the IMPLEMENTATION not with the cipher.
 *
 * A flaw in ZeroTier's protocol or code is incredibly, unbelievably
 * more likely than a flaw in Salsa20 or any other cipher or cryptographic
 * primitive it uses. We're talking odds of dying in a car wreck vs. odds of
 * being personally impacted on the head by a meteorite. Nobody without a
 * billion dollar budget is going to break into your network by actually
 * cracking Salsa20/12 (or even /8) in the field.
 *
 * So stop worrying about the cipher unless you are, say, the Kremlin and your
 * adversary is the NSA and the GCHQ. In that case... well that's above my
 * pay grade. I'll just say defense in depth.
 */
#define ZT_PROTO_SALSA20_ROUNDS 12

/**
 * PUSH_DIRECT_PATHS flag: forget path
 */
#define ZT_PUSH_DIRECT_PATHS_FLAG_FORGET_PATH 0x01

/**
 * PUSH_DIRECT_PATHS flag: cluster redirect
 */
#define ZT_PUSH_DIRECT_PATHS_FLAG_CLUSTER_REDIRECT 0x02

// Field indexes in packet header
#define ZT_PACKET_IDX_IV 0
#define ZT_PACKET_IDX_DEST 8
#define ZT_PACKET_IDX_SOURCE 13
#define ZT_PACKET_IDX_FLAGS 18
#define ZT_PACKET_IDX_MAC 19
#define ZT_PACKET_IDX_VERB 27
#define ZT_PACKET_IDX_PAYLOAD 28

/**
 * Packet buffer size (can be changed)
 *
 * The current value is big enough for ZT_MAX_PACKET_FRAGMENTS, the pragmatic
 * packet fragment limit, times the default UDP MTU. Most packets won't be
 * this big.
 */
#define ZT_PROTO_MAX_PACKET_LENGTH (ZT_MAX_PACKET_FRAGMENTS * ZT_UDP_DEFAULT_PAYLOAD_MTU)

/**
 * Minimum viable packet length (a.k.a. header length)
 */
#define ZT_PROTO_MIN_PACKET_LENGTH ZT_PACKET_IDX_PAYLOAD

// Indexes of fields in fragment header
#define ZT_PACKET_FRAGMENT_IDX_PACKET_ID 0
#define ZT_PACKET_FRAGMENT_IDX_DEST 8
#define ZT_PACKET_FRAGMENT_IDX_FRAGMENT_INDICATOR 13
#define ZT_PACKET_FRAGMENT_IDX_FRAGMENT_NO 14
#define ZT_PACKET_FRAGMENT_IDX_HOPS 15
#define ZT_PACKET_FRAGMENT_IDX_PAYLOAD 16

/**
 * Magic number found at ZT_PACKET_FRAGMENT_IDX_FRAGMENT_INDICATOR
 */
#define ZT_PACKET_FRAGMENT_INDICATOR ZT_ADDRESS_RESERVED_PREFIX

/**
 * Minimum viable fragment length
 */
#define ZT_PROTO_MIN_FRAGMENT_LENGTH ZT_PACKET_FRAGMENT_IDX_PAYLOAD

// Field incides for parsing verbs -------------------------------------------

// Some verbs have variable-length fields. Those aren't fully defined here
// yet-- instead they are parsed using relative indexes in IncomingPacket.
// See their respective handler functions.

#define ZT_PROTO_VERB_HELLO_IDX_PROTOCOL_VERSION (ZT_PACKET_IDX_PAYLOAD)
#define ZT_PROTO_VERB_HELLO_IDX_MAJOR_VERSION (ZT_PROTO_VERB_HELLO_IDX_PROTOCOL_VERSION + 1)
#define ZT_PROTO_VERB_HELLO_IDX_MINOR_VERSION (ZT_PROTO_VERB_HELLO_IDX_MAJOR_VERSION + 1)
#define ZT_PROTO_VERB_HELLO_IDX_REVISION (ZT_PROTO_VERB_HELLO_IDX_MINOR_VERSION + 1)
#define ZT_PROTO_VERB_HELLO_IDX_TIMESTAMP (ZT_PROTO_VERB_HELLO_IDX_REVISION + 2)
#define ZT_PROTO_VERB_HELLO_IDX_IDENTITY (ZT_PROTO_VERB_HELLO_IDX_TIMESTAMP + 8)

#define ZT_PROTO_VERB_ERROR_IDX_IN_RE_VERB (ZT_PACKET_IDX_PAYLOAD)
#define ZT_PROTO_VERB_ERROR_IDX_IN_RE_PACKET_ID (ZT_PROTO_VERB_ERROR_IDX_IN_RE_VERB + 1)
#define ZT_PROTO_VERB_ERROR_IDX_ERROR_CODE (ZT_PROTO_VERB_ERROR_IDX_IN_RE_PACKET_ID + 8)
#define ZT_PROTO_VERB_ERROR_IDX_PAYLOAD (ZT_PROTO_VERB_ERROR_IDX_ERROR_CODE + 1)

#define ZT_PROTO_VERB_OK_IDX_IN_RE_VERB (ZT_PACKET_IDX_PAYLOAD)
#define ZT_PROTO_VERB_OK_IDX_IN_RE_PACKET_ID (ZT_PROTO_VERB_OK_IDX_IN_RE_VERB + 1)
#define ZT_PROTO_VERB_OK_IDX_PAYLOAD (ZT_PROTO_VERB_OK_IDX_IN_RE_PACKET_ID + 8)

#define ZT_PROTO_VERB_WHOIS_IDX_ZTADDRESS (ZT_PACKET_IDX_PAYLOAD)

#define ZT_PROTO_VERB_RENDEZVOUS_IDX_FLAGS (ZT_PACKET_IDX_PAYLOAD)
#define ZT_PROTO_VERB_RENDEZVOUS_IDX_ZTADDRESS (ZT_PROTO_VERB_RENDEZVOUS_IDX_FLAGS + 1)
#define ZT_PROTO_VERB_RENDEZVOUS_IDX_PORT (ZT_PROTO_VERB_RENDEZVOUS_IDX_ZTADDRESS + 5)
#define ZT_PROTO_VERB_RENDEZVOUS_IDX_ADDRLEN (ZT_PROTO_VERB_RENDEZVOUS_IDX_PORT + 2)
#define ZT_PROTO_VERB_RENDEZVOUS_IDX_ADDRESS (ZT_PROTO_VERB_RENDEZVOUS_IDX_ADDRLEN + 1)

#define ZT_PROTO_VERB_FRAME_IDX_NETWORK_ID (ZT_PACKET_IDX_PAYLOAD)
#define ZT_PROTO_VERB_FRAME_IDX_ETHERTYPE (ZT_PROTO_VERB_FRAME_IDX_NETWORK_ID + 8)
#define ZT_PROTO_VERB_FRAME_IDX_PAYLOAD (ZT_PROTO_VERB_FRAME_IDX_ETHERTYPE + 2)

#define ZT_PROTO_VERB_EXT_FRAME_IDX_NETWORK_ID (ZT_PACKET_IDX_PAYLOAD)
#define ZT_PROTO_VERB_EXT_FRAME_LEN_NETWORK_ID 8
#define ZT_PROTO_VERB_EXT_FRAME_IDX_FLAGS (ZT_PROTO_VERB_EXT_FRAME_IDX_NETWORK_ID + ZT_PROTO_VERB_EXT_FRAME_LEN_NETWORK_ID)
#define ZT_PROTO_VERB_EXT_FRAME_LEN_FLAGS 1
#define ZT_PROTO_VERB_EXT_FRAME_IDX_COM (ZT_PROTO_VERB_EXT_FRAME_IDX_FLAGS + ZT_PROTO_VERB_EXT_FRAME_LEN_FLAGS)
#define ZT_PROTO_VERB_EXT_FRAME_IDX_TO (ZT_PROTO_VERB_EXT_FRAME_IDX_FLAGS + ZT_PROTO_VERB_EXT_FRAME_LEN_FLAGS)
#define ZT_PROTO_VERB_EXT_FRAME_LEN_TO 6
#define ZT_PROTO_VERB_EXT_FRAME_IDX_FROM (ZT_PROTO_VERB_EXT_FRAME_IDX_TO + ZT_PROTO_VERB_EXT_FRAME_LEN_TO)
#define ZT_PROTO_VERB_EXT_FRAME_LEN_FROM 6
#define ZT_PROTO_VERB_EXT_FRAME_IDX_ETHERTYPE (ZT_PROTO_VERB_EXT_FRAME_IDX_FROM + ZT_PROTO_VERB_EXT_FRAME_LEN_FROM)
#define ZT_PROTO_VERB_EXT_FRAME_LEN_ETHERTYPE 2
#define ZT_PROTO_VERB_EXT_FRAME_IDX_PAYLOAD (ZT_PROTO_VERB_EXT_FRAME_IDX_ETHERTYPE + ZT_PROTO_VERB_EXT_FRAME_LEN_ETHERTYPE)

#define ZT_PROTO_VERB_NETWORK_CONFIG_REQUEST_IDX_NETWORK_ID (ZT_PACKET_IDX_PAYLOAD)
#define ZT_PROTO_VERB_NETWORK_CONFIG_REQUEST_IDX_DICT_LEN (ZT_PROTO_VERB_NETWORK_CONFIG_REQUEST_IDX_NETWORK_ID + 8)
#define ZT_PROTO_VERB_NETWORK_CONFIG_REQUEST_IDX_DICT (ZT_PROTO_VERB_NETWORK_CONFIG_REQUEST_IDX_DICT_LEN + 2)

#define ZT_PROTO_VERB_MULTICAST_GATHER_IDX_NETWORK_ID (ZT_PACKET_IDX_PAYLOAD)
#define ZT_PROTO_VERB_MULTICAST_GATHER_IDX_FLAGS (ZT_PROTO_VERB_MULTICAST_GATHER_IDX_NETWORK_ID + 8)
#define ZT_PROTO_VERB_MULTICAST_GATHER_IDX_MAC (ZT_PROTO_VERB_MULTICAST_GATHER_IDX_FLAGS + 1)
#define ZT_PROTO_VERB_MULTICAST_GATHER_IDX_ADI (ZT_PROTO_VERB_MULTICAST_GATHER_IDX_MAC + 6)
#define ZT_PROTO_VERB_MULTICAST_GATHER_IDX_GATHER_LIMIT (ZT_PROTO_VERB_MULTICAST_GATHER_IDX_ADI + 4)

// Note: COM, GATHER_LIMIT, and SOURCE_MAC are optional, and so are specified without size
#define ZT_PROTO_VERB_MULTICAST_FRAME_IDX_NETWORK_ID (ZT_PACKET_IDX_PAYLOAD)
#define ZT_PROTO_VERB_MULTICAST_FRAME_IDX_FLAGS (ZT_PROTO_VERB_MULTICAST_FRAME_IDX_NETWORK_ID + 8)
#define ZT_PROTO_VERB_MULTICAST_FRAME_IDX_COM (ZT_PROTO_VERB_MULTICAST_FRAME_IDX_FLAGS + 1)
#define ZT_PROTO_VERB_MULTICAST_FRAME_IDX_GATHER_LIMIT (ZT_PROTO_VERB_MULTICAST_FRAME_IDX_FLAGS + 1)
#define ZT_PROTO_VERB_MULTICAST_FRAME_IDX_SOURCE_MAC (ZT_PROTO_VERB_MULTICAST_FRAME_IDX_FLAGS + 1)
#define ZT_PROTO_VERB_MULTICAST_FRAME_IDX_DEST_MAC (ZT_PROTO_VERB_MULTICAST_FRAME_IDX_FLAGS + 1)
#define ZT_PROTO_VERB_MULTICAST_FRAME_IDX_DEST_ADI (ZT_PROTO_VERB_MULTICAST_FRAME_IDX_DEST_MAC + 6)
#define ZT_PROTO_VERB_MULTICAST_FRAME_IDX_ETHERTYPE (ZT_PROTO_VERB_MULTICAST_FRAME_IDX_DEST_ADI + 4)
#define ZT_PROTO_VERB_MULTICAST_FRAME_IDX_FRAME (ZT_PROTO_VERB_MULTICAST_FRAME_IDX_ETHERTYPE + 2)

#define ZT_PROTO_VERB_HELLO__OK__IDX_TIMESTAMP (ZT_PROTO_VERB_OK_IDX_PAYLOAD)
#define ZT_PROTO_VERB_HELLO__OK__IDX_PROTOCOL_VERSION (ZT_PROTO_VERB_HELLO__OK__IDX_TIMESTAMP + 8)
#define ZT_PROTO_VERB_HELLO__OK__IDX_MAJOR_VERSION (ZT_PROTO_VERB_HELLO__OK__IDX_PROTOCOL_VERSION + 1)
#define ZT_PROTO_VERB_HELLO__OK__IDX_MINOR_VERSION (ZT_PROTO_VERB_HELLO__OK__IDX_MAJOR_VERSION + 1)
#define ZT_PROTO_VERB_HELLO__OK__IDX_REVISION (ZT_PROTO_VERB_HELLO__OK__IDX_MINOR_VERSION + 1)

#define ZT_PROTO_VERB_WHOIS__OK__IDX_IDENTITY (ZT_PROTO_VERB_OK_IDX_PAYLOAD)

#define ZT_PROTO_VERB_NETWORK_CONFIG_REQUEST__OK__IDX_NETWORK_ID (ZT_PROTO_VERB_OK_IDX_PAYLOAD)
#define ZT_PROTO_VERB_NETWORK_CONFIG_REQUEST__OK__IDX_DICT_LEN (ZT_PROTO_VERB_NETWORK_CONFIG_REQUEST__OK__IDX_NETWORK_ID + 8)
#define ZT_PROTO_VERB_NETWORK_CONFIG_REQUEST__OK__IDX_DICT (ZT_PROTO_VERB_NETWORK_CONFIG_REQUEST__OK__IDX_DICT_LEN + 2)

#define ZT_PROTO_VERB_MULTICAST_GATHER__OK__IDX_NETWORK_ID (ZT_PROTO_VERB_OK_IDX_PAYLOAD)
#define ZT_PROTO_VERB_MULTICAST_GATHER__OK__IDX_MAC (ZT_PROTO_VERB_MULTICAST_GATHER__OK__IDX_NETWORK_ID + 8)
#define ZT_PROTO_VERB_MULTICAST_GATHER__OK__IDX_ADI (ZT_PROTO_VERB_MULTICAST_GATHER__OK__IDX_MAC + 6)
#define ZT_PROTO_VERB_MULTICAST_GATHER__OK__IDX_GATHER_RESULTS (ZT_PROTO_VERB_MULTICAST_GATHER__OK__IDX_ADI + 4)

#define ZT_PROTO_VERB_MULTICAST_FRAME__OK__IDX_NETWORK_ID (ZT_PROTO_VERB_OK_IDX_PAYLOAD)
#define ZT_PROTO_VERB_MULTICAST_FRAME__OK__IDX_MAC (ZT_PROTO_VERB_MULTICAST_FRAME__OK__IDX_NETWORK_ID + 8)
#define ZT_PROTO_VERB_MULTICAST_FRAME__OK__IDX_ADI (ZT_PROTO_VERB_MULTICAST_FRAME__OK__IDX_MAC + 6)
#define ZT_PROTO_VERB_MULTICAST_FRAME__OK__IDX_FLAGS (ZT_PROTO_VERB_MULTICAST_FRAME__OK__IDX_ADI + 4)
#define ZT_PROTO_VERB_MULTICAST_FRAME__OK__IDX_COM_AND_GATHER_RESULTS (ZT_PROTO_VERB_MULTICAST_FRAME__OK__IDX_FLAGS + 1)

// ---------------------------------------------------------------------------

namespace ZeroTier {

/**
 * ZeroTier packet
 *
 * Packet format:
 *   <[8] 64-bit random packet ID and crypto initialization vector>
 *   <[5] destination ZT address>
 *   <[5] source ZT address>
 *   <[1] flags/cipher/hops>
 *   <[8] 64-bit MAC (or trusted path ID in trusted path mode)>
 *   [... -- begin encryption envelope -- ...]
 *   <[1] encrypted flags (MS 3 bits) and verb (LS 5 bits)>
 *   [... verb-specific payload ...]
 *
 * Packets smaller than 28 bytes are invalid and silently discarded.
 *
 * The flags/cipher/hops bit field is: FFCCCHHH where C is a 3-bit cipher
 * selection allowing up to 7 cipher suites, F is outside-envelope flags,
 * and H is hop count.
 *
 * The three-bit hop count is the only part of a packet that is mutable in
 * transit without invalidating the MAC. All other bits in the packet are
 * immutable. This is because intermediate nodes can increment the hop
 * count up to 7 (protocol max).
 *
 * For unencrypted packets, MAC is computed on plaintext. Only HELLO is ever
 * sent in the clear, as it's the "here is my public key" message.
 */
class Packet : public Buffer<ZT_PROTO_MAX_PACKET_LENGTH>
{
public:
	/**
	 * A packet fragment
	 *
	 * Fragments are sent if a packet is larger than UDP MTU. The first fragment
	 * is sent with its normal header with the fragmented flag set. Remaining
	 * fragments are sent this way.
	 *
	 * The fragmented bit indicates that there is at least one fragment. Fragments
	 * themselves contain the total, so the receiver must "learn" this from the
	 * first fragment it receives.
	 *
	 * Fragments are sent with the following format:
	 *   <[8] packet ID of packet whose fragment this belongs to>
	 *   <[5] destination ZT address>
	 *   <[1] 0xff, a reserved address, signals that this isn't a normal packet>
	 *   <[1] total fragments (most significant 4 bits), fragment no (LS 4 bits)>
	 *   <[1] ZT hop count (top 5 bits unused and must be zero)>
	 *   <[...] fragment data>
	 *
	 * The protocol supports a maximum of 16 fragments. If a fragment is received
	 * before its main packet header, it should be cached for a brief period of
	 * time to see if its parent arrives. Loss of any fragment constitutes packet
	 * loss; there is no retransmission mechanism. The receiver must wait for full
	 * receipt to authenticate and decrypt; there is no per-fragment MAC. (But if
	 * fragments are corrupt, the MAC will fail for the whole assembled packet.)
	 */
	class Fragment : public Buffer<ZT_PROTO_MAX_PACKET_LENGTH>
	{
	public:
		Fragment() :
			Buffer<ZT_PROTO_MAX_PACKET_LENGTH>()
		{
		}

		template<unsigned int C2>
		Fragment(const Buffer<C2> &b)
	 		throw(std::out_of_range) :
	 		Buffer<ZT_PROTO_MAX_PACKET_LENGTH>(b)
		{
		}

		Fragment(const void *data,unsigned int len) :
			Buffer<ZT_PROTO_MAX_PACKET_LENGTH>(data,len)
		{
		}

		/**
		 * Initialize from a packet
		 *
		 * @param p Original assembled packet
		 * @param fragStart Start of fragment (raw index in packet data)
		 * @param fragLen Length of fragment in bytes
		 * @param fragNo Which fragment (>= 1, since 0 is Packet with end chopped off)
		 * @param fragTotal Total number of fragments (including 0)
		 * @throws std::out_of_range Packet size would exceed buffer
		 */
		Fragment(const Packet &p,unsigned int fragStart,unsigned int fragLen,unsigned int fragNo,unsigned int fragTotal)
			throw(std::out_of_range)
		{
			init(p,fragStart,fragLen,fragNo,fragTotal);
		}

		/**
		 * Initialize from a packet
		 *
		 * @param p Original assembled packet
		 * @param fragStart Start of fragment (raw index in packet data)
		 * @param fragLen Length of fragment in bytes
		 * @param fragNo Which fragment (>= 1, since 0 is Packet with end chopped off)
		 * @param fragTotal Total number of fragments (including 0)
		 * @throws std::out_of_range Packet size would exceed buffer
		 */
		inline void init(const Packet &p,unsigned int fragStart,unsigned int fragLen,unsigned int fragNo,unsigned int fragTotal)
			throw(std::out_of_range)
		{
			if ((fragStart + fragLen) > p.size())
				throw std::out_of_range("Packet::Fragment: tried to construct fragment of packet past its length");
			setSize(fragLen + ZT_PROTO_MIN_FRAGMENT_LENGTH);

			// NOTE: this copies both the IV/packet ID and the destination address.
			memcpy(field(ZT_PACKET_FRAGMENT_IDX_PACKET_ID,13),p.field(ZT_PACKET_IDX_IV,13),13);

			(*this)[ZT_PACKET_FRAGMENT_IDX_FRAGMENT_INDICATOR] = ZT_PACKET_FRAGMENT_INDICATOR;
			(*this)[ZT_PACKET_FRAGMENT_IDX_FRAGMENT_NO] = (char)(((fragTotal & 0xf) << 4) | (fragNo & 0xf));
			(*this)[ZT_PACKET_FRAGMENT_IDX_HOPS] = 0;

			memcpy(field(ZT_PACKET_FRAGMENT_IDX_PAYLOAD,fragLen),p.field(fragStart,fragLen),fragLen);
		}

		/**
		 * Get this fragment's destination
		 *
		 * @return Destination ZT address
		 */
		inline Address destination() const { return Address(field(ZT_PACKET_FRAGMENT_IDX_DEST,ZT_ADDRESS_LENGTH),ZT_ADDRESS_LENGTH); }

		/**
		 * @return True if fragment is of a valid length
		 */
		inline bool lengthValid() const { return (size() >= ZT_PACKET_FRAGMENT_IDX_PAYLOAD); }

		/**
		 * @return ID of packet this is a fragment of
		 */
		inline uint64_t packetId() const { return at<uint64_t>(ZT_PACKET_FRAGMENT_IDX_PACKET_ID); }

		/**
		 * @return Total number of fragments in packet
		 */
		inline unsigned int totalFragments() const { return (((unsigned int)((*this)[ZT_PACKET_FRAGMENT_IDX_FRAGMENT_NO]) >> 4) & 0xf); }

		/**
		 * @return Fragment number of this fragment
		 */
		inline unsigned int fragmentNumber() const { return ((unsigned int)((*this)[ZT_PACKET_FRAGMENT_IDX_FRAGMENT_NO]) & 0xf); }

		/**
		 * @return Fragment ZT hop count
		 */
		inline unsigned int hops() const { return (unsigned int)((*this)[ZT_PACKET_FRAGMENT_IDX_HOPS]); }

		/**
		 * Increment this packet's hop count
		 */
		inline void incrementHops()
		{
			(*this)[ZT_PACKET_FRAGMENT_IDX_HOPS] = (((*this)[ZT_PACKET_FRAGMENT_IDX_HOPS]) + 1) & ZT_PROTO_MAX_HOPS;
		}

		/**
		 * @return Length of payload in bytes
		 */
		inline unsigned int payloadLength() const { return ((size() > ZT_PACKET_FRAGMENT_IDX_PAYLOAD) ? (size() - ZT_PACKET_FRAGMENT_IDX_PAYLOAD) : 0); }

		/**
		 * @return Raw packet payload
		 */
		inline const unsigned char *payload() const
		{
			return field(ZT_PACKET_FRAGMENT_IDX_PAYLOAD,size() - ZT_PACKET_FRAGMENT_IDX_PAYLOAD);
		}
	};

	/**
	 * ZeroTier protocol verbs
	 */
	enum Verb /* Max value: 32 (5 bits) */
	{
		/**
		 * No operation (ignored, no reply)
		 */
		VERB_NOP = 0x00,

		/**
		 * Announcement of a node's existence:
		 *   <[1] protocol version>
		 *   <[1] software major version>
		 *   <[1] software minor version>
		 *   <[2] software revision>
		 *   <[8] timestamp (ms since epoch)>
		 *   <[...] binary serialized identity (see Identity)>
		 *   <[1] destination address type>
		 *   [<[...] destination address>]
		 *   <[8] 64-bit world ID of current world>
		 *   <[8] 64-bit timestamp of current world>
		 *
		 * This is the only message that ever must be sent in the clear, since it
		 * is used to push an identity to a new peer.
		 *
		 * The destination address is the wire address to which this packet is
		 * being sent, and in OK is *also* the destination address of the OK
		 * packet. This can be used by the receiver to detect NAT, learn its real
		 * external address if behind NAT, and detect changes to its external
		 * address that require re-establishing connectivity.
		 *
		 * Destination address types and formats (not all of these are used now):
		 *   0x00 - None -- no destination address data present
		 *   0x01 - Ethernet address -- format: <[6] Ethernet MAC>
		 *   0x04 - 6-byte IPv4 UDP address/port -- format: <[4] IP>, <[2] port>
		 *   0x06 - 18-byte IPv6 UDP address/port -- format: <[16] IP>, <[2] port>
		 *
		 * OK payload:
		 *   <[8] timestamp (echoed from original HELLO)>
		 *   <[1] protocol version (of responder)>
		 *   <[1] software major version (of responder)>
		 *   <[1] software minor version (of responder)>
		 *   <[2] software revision (of responder)>
		 *   <[1] destination address type (for this OK, not copied from HELLO)>
		 *   [<[...] destination address>]
		 *   <[2] 16-bit length of world update or 0 if none>
		 *   [[...] world update]
		 *
		 * ERROR has no payload.
		 */
		VERB_HELLO = 0x01,

		/**
		 * Error response:
		 *   <[1] in-re verb>
		 *   <[8] in-re packet ID>
		 *   <[1] error code>
		 *   <[...] error-dependent payload>
		 */
		VERB_ERROR = 0x02,

		/**
		 * Success response:
		 *   <[1] in-re verb>
		 *   <[8] in-re packet ID>
		 *   <[...] request-specific payload>
		 */
		VERB_OK = 0x03,

		/**
		 * Query an identity by address:
		 *   <[5] address to look up>
		 *
		 * OK response payload:
		 *   <[...] binary serialized identity>
		 *
		 * If querying a cluster, duplicate OK responses may occasionally occur.
		 * These should be discarded.
		 *
		 * If the address is not found, no response is generated. WHOIS requests
		 * will time out much like ARP requests and similar do in L2.
		 */
		VERB_WHOIS = 0x04,

		/**
		 * Meet another node at a given protocol address:
		 *   <[1] flags (unused, currently 0)>
		 *   <[5] ZeroTier address of peer that might be found at this address>
		 *   <[2] 16-bit protocol address port>
		 *   <[1] protocol address length (4 for IPv4, 16 for IPv6)>
		 *   <[...] protocol address (network byte order)>
		 *
		 * This is sent by a relaying node to initiate NAT traversal between two
		 * peers that are communicating by way of indirect relay. The relay will
		 * send this to both peers at the same time on a periodic basis, telling
		 * each where it might find the other on the network.
		 *
		 * Upon receipt a peer sends HELLO to establish a direct link.
		 *
		 * Nodes should implement rate control, limiting the rate at which they
		 * respond to these packets to prevent their use in DDOS attacks. Nodes
		 * may also ignore these messages if a peer is not known or is not being
		 * actively communicated with.
		 *
		 * Unfortunately the physical address format in this message pre-dates
		 * InetAddress's serialization format. :( ZeroTier is four years old and
		 * yes we've accumulated a tiny bit of cruft here and there.
		 *
		 * No OK or ERROR is generated.
		 */
		VERB_RENDEZVOUS = 0x05,

		/**
		 * ZT-to-ZT unicast ethernet frame (shortened EXT_FRAME):
		 *   <[8] 64-bit network ID>
		 *   <[2] 16-bit ethertype>
		 *   <[...] ethernet payload>
		 *
		 * MAC addresses are derived from the packet's source and destination
		 * ZeroTier addresses. This is a shortened EXT_FRAME that elides full
		 * Ethernet framing and other optional flags and features when they
		 * are not necessary.
		 *
		 * ERROR may be generated if a membership certificate is needed for a
		 * closed network. Payload will be network ID.
		 */
		VERB_FRAME = 0x06,

		/**
		 * Full Ethernet frame with MAC addressing and optional fields:
		 *   <[8] 64-bit network ID>
		 *   <[1] flags>
		 *  [<[...] certificate of network membership (DEPRECATED)>]
		 *   <[6] destination MAC or all zero for destination node>
		 *   <[6] source MAC or all zero for node of origin>
		 *   <[2] 16-bit ethertype>
		 *   <[...] ethernet payload>
		 *
		 * Flags:
		 *   0x01 - Certificate of network membership attached (DEPRECATED)
		 *   0x02 - Packet is a TEE'd packet
		 *   0x04 - Packet is a REDIRECT'ed packet
		 *
		 * An extended frame carries full MAC addressing, making them a
		 * superset of VERB_FRAME. They're used for bridging or when we
		 * want to attach a certificate since FRAME does not support that.
		 *
		 * ERROR may be generated if a membership certificate is needed for a
		 * closed network. Payload will be network ID.
		 */
		VERB_EXT_FRAME = 0x07,

		/**
		 * ECHO request (a.k.a. ping):
		 *   <[...] arbitrary payload>
		 *
		 * This generates OK with a copy of the transmitted payload. No ERROR
		 * is generated. Response to ECHO requests is optional and ECHO may be
		 * ignored if a node detects a possible flood.
		 */
		VERB_ECHO = 0x08,

		/**
		 * Announce interest in multicast group(s):
		 *   <[8] 64-bit network ID>
		 *   <[6] multicast Ethernet address>
		 *   <[4] multicast additional distinguishing information (ADI)>
		 *   [... additional tuples of network/address/adi ...]
		 *
		 * LIKEs may be sent to any peer, though a good implementation should
		 * restrict them to peers on the same network they're for and to network
		 * controllers and root servers. In the current network, root servers
		 * will provide the service of final multicast cache.
		 *
		 * VERB_NETWORK_CREDENTIALS should be pushed along with this, especially
		 * if using upstream (e.g. root) nodes as multicast databases. This allows
		 * GATHERs to be authenticated.
		 *
		 * OK/ERROR are not generated.
		 */
		VERB_MULTICAST_LIKE = 0x09,

		/**
		 * Network membership credential push:
		 *   <[...] serialized certificate of membership>
		 *   [<[...] additional certificates of membership>]
		 *   <[1] 0x00, null byte marking end of COM array>
		 *   <[2] 16-bit number of capabilities>
		 *   <[...] one or more serialized Capability>
		 *   <[2] 16-bit number of tags>
		 *   <[...] one or more serialized Tags>
		 *
		 * This is sent in response to ERROR_NEED_MEMBERSHIP_CERTIFICATE and may
		 * be pushed at any other time to keep exchanged certificates up to date.
		 *
		 * OK/ERROR are not generated.
		 */
		VERB_NETWORK_CREDENTIALS = 0x0a,

		/**
		 * Network configuration request:
		 *   <[8] 64-bit network ID>
		 *   <[2] 16-bit length of request meta-data dictionary>
		 *   <[...] string-serialized request meta-data>
		 *  [<[8] 64-bit timestamp of netconf we currently have>]
		 *
		 * This message requests network configuration from a node capable of
		 * providing it. If the optional revision is included, a response is
		 * only generated if there is a newer network configuration available.
		 *
		 * OK response payload:
		 *   <[8] 64-bit network ID>
		 *   <[2] 16-bit length of network configuration dictionary field>
		 *   <[...] network configuration dictionary (or fragment)>
		 *   [<[4] 32-bit total length of assembled dictionary>]
		 *   [<[4] 32-bit index of fragment in this reply>]
		 *
		 * Fields after the dictionary are extensions to support multipart
		 * sending of large network configs. If they are not present the
		 * sent config must be assumed to be whole.
		 *
		 * ERROR response payload:
		 *   <[8] 64-bit network ID>
		 */
		VERB_NETWORK_CONFIG_REQUEST = 0x0b,

		/**
		 * Network configuration refresh request:
		 *   <[...] array of 64-bit network IDs>
		 *
		 * This can be sent by the network controller to inform a node that it
		 * should now make a NETWORK_CONFIG_REQUEST.
		 *
		 * It does not generate an OK or ERROR message, and is treated only as
		 * a hint to refresh now.
		 */
		VERB_NETWORK_CONFIG_REFRESH = 0x0c,

		/**
		 * Request endpoints for multicast distribution:
		 *   <[8] 64-bit network ID>
		 *   <[1] flags>
		 *   <[6] MAC address of multicast group being queried>
		 *   <[4] 32-bit ADI for multicast group being queried>
		 *   <[4] 32-bit requested max number of multicast peers>
		 *  [<[...] network certificate of membership (DEPRECATED)>]
		 *
		 * Flags:
		 *   0x01 - COM is attached (DEPRECATED)
		 *
		 * This message asks a peer for additional known endpoints that have
		 * LIKEd a given multicast group. It's sent when the sender wishes
		 * to send multicast but does not have the desired number of recipient
		 * peers.
		 *
		 * More than one OK response can occur if the response is broken up across
		 * multiple packets or if querying a clustered node.
		 *
		 * Send VERB_NETWORK_CREDENTIALS prior to GATHERing if doing so from
		 * upstream nodes like root servers that are not involved in our network.
		 *
		 * OK response payload:
		 *   <[8] 64-bit network ID>
		 *   <[6] MAC address of multicast group being queried>
		 *   <[4] 32-bit ADI for multicast group being queried>
		 *   [begin gather results -- these same fields can be in OK(MULTICAST_FRAME)]
		 *   <[4] 32-bit total number of known members in this multicast group>
		 *   <[2] 16-bit number of members enumerated in this packet>
		 *   <[...] series of 5-byte ZeroTier addresses of enumerated members>
		 *
		 * ERROR is not generated; queries that return no response are dropped.
		 */
		VERB_MULTICAST_GATHER = 0x0d,

		/**
		 * Multicast frame:
		 *   <[8] 64-bit network ID>
		 *   <[1] flags>
		 *  [<[...] network certificate of membership (DEPRECATED)>]
		 *  [<[4] 32-bit implicit gather limit>]
		 *  [<[6] source MAC>]
		 *   <[6] destination MAC (multicast address)>
		 *   <[4] 32-bit multicast ADI (multicast address extension)>
		 *   <[2] 16-bit ethertype>
		 *   <[...] ethernet payload>
		 *
		 * Flags:
		 *   0x01 - Network certificate of membership attached (DEPRECATED)
		 *   0x02 - Implicit gather limit field is present
		 *   0x04 - Source MAC is specified -- otherwise it's computed from sender
		 *
		 * OK and ERROR responses are optional. OK may be generated if there are
		 * implicit gather results or if the recipient wants to send its own
		 * updated certificate of network membership to the sender. ERROR may be
		 * generated if a certificate is needed or if multicasts to this group
		 * are no longer wanted (multicast unsubscribe).
		 *
		 * OK response payload:
		 *   <[8] 64-bit network ID>
		 *   <[6] MAC address of multicast group>
		 *   <[4] 32-bit ADI for multicast group>
		 *   <[1] flags>
		 *  [<[...] network certficate of membership (DEPRECATED)>]
		 *  [<[...] implicit gather results if flag 0x01 is set>]
		 *
		 * OK flags (same bits as request flags):
		 *   0x01 - OK includes certificate of network membership (DEPRECATED)
		 *   0x02 - OK includes implicit gather results
		 *
		 * ERROR response payload:
		 *   <[8] 64-bit network ID>
		 *   <[6] multicast group MAC>
		 *   <[4] 32-bit multicast group ADI>
		 */
		VERB_MULTICAST_FRAME = 0x0e,

		// 0x0f is reserved for an old deprecated message

		/**
		 * Push of potential endpoints for direct communication:
		 *   <[2] 16-bit number of paths>
		 *   <[...] paths>
		 *
		 * Path record format:
		 *   <[1] 8-bit path flags>
		 *   <[2] length of extended path characteristics or 0 for none>
		 *   <[...] extended path characteristics>
		 *   <[1] address type>
		 *   <[1] address length in bytes>
		 *   <[...] address>
		 *
		 * Path record flags:
		 *   0x01 - Forget this path if currently known (not implemented yet)
		 *   0x02 - Cluster redirect -- use this in preference to others
		 *
		 * The receiver may, upon receiving a push, attempt to establish a
		 * direct link to one or more of the indicated addresses. It is the
		 * responsibility of the sender to limit which peers it pushes direct
		 * paths to to those with whom it has a trust relationship. The receiver
		 * must obey any restrictions provided such as exclusivity or blacklists.
		 * OK responses to this message are optional.
		 *
		 * Note that a direct path push does not imply that learned paths can't
		 * be used unless they are blacklisted explicitly or unless flag 0x01
		 * is set.
		 *
		 * Only a subset of this functionality is currently implemented: basic
		 * path pushing and learning. Blacklisting and trust are not fully
		 * implemented yet (encryption is still always used).
		 *
		 * OK and ERROR are not generated.
		 */
		VERB_PUSH_DIRECT_PATHS = 0x10,

		/**
		 * Source-routed circuit test message:
		 *   <[5] address of originator of circuit test>
		 *   <[2] 16-bit flags>
		 *   <[8] 64-bit timestamp>
		 *   <[8] 64-bit test ID (arbitrary, set by tester)>
		 *   <[2] 16-bit originator credential length (includes type)>
		 *   [[1] originator credential type (for authorizing test)]
		 *   [[...] originator credential]
		 *   <[2] 16-bit length of additional fields>
		 *   [[...] additional fields]
		 *   [ ... end of signed portion of request ... ]
		 *   <[2] 16-bit length of signature of request>
		 *   <[...] signature of request by originator>
		 *   <[2] 16-bit length of additional fields>
		 *   [[...] additional fields]
		 *   <[...] next hop(s) in path>
		 *
		 * Flags:
		 *   0x01 - Report back to originator at middle hops
		 *   0x02 - Report back to originator at last hop
		 *
		 * Originator credential types:
		 *   0x01 - 64-bit network ID for which originator is controller
		 *
		 * Path record format:
		 *   <[1] 8-bit flags (unused, must be zero)>
		 *   <[1] 8-bit breadth (number of next hops)>
		 *   <[...] one or more ZeroTier addresses of next hops>
		 *
		 * The circuit test allows a device to send a message that will traverse
		 * the network along a specified path, with each hop optionally reporting
		 * back to the tester via VERB_CIRCUIT_TEST_REPORT.
		 *
		 * Each circuit test packet includes a digital signature by the originator
		 * of the request, as well as a credential by which that originator claims
		 * authorization to perform the test. Currently this signature is ed25519,
		 * but in the future flags might be used to indicate an alternative
		 * algorithm. For example, the originator might be a network controller.
		 * In this case the test might be authorized if the recipient is a member
		 * of a network controlled by it, and if the previous hop(s) are also
		 * members. Each hop may include its certificate of network membership.
		 *
		 * Circuit test paths consist of a series of records. When a node receives
		 * an authorized circuit test, it:
		 *
		 * (1) Reports back to circuit tester as flags indicate
		 * (2) Reads and removes the next hop from the packet's path
		 * (3) Sends the packet along to next hop(s), if any.
		 *
		 * It is perfectly legal for a path to contain the same hop more than
		 * once. In fact, this can be a very useful test to determine if a hop
		 * can be reached bidirectionally and if so what that connectivity looks
		 * like.
		 *
		 * The breadth field in source-routed path records allows a hop to forward
		 * to more than one recipient, allowing the tester to specify different
		 * forms of graph traversal in a test.
		 *
		 * There is no hard limit to the number of hops in a test, but it is
		 * practically limited by the maximum size of a (possibly fragmented)
		 * ZeroTier packet.
		 *
		 * Support for circuit tests is optional. If they are not supported, the
		 * node should respond with an UNSUPPORTED_OPERATION error. If a circuit
		 * test request is not authorized, it may be ignored or reported as
		 * an INVALID_REQUEST. No OK messages are generated, but TEST_REPORT
		 * messages may be sent (see below).
		 *
		 * ERROR packet format:
		 *   <[8] 64-bit timestamp (echoed from original>
		 *   <[8] 64-bit test ID (echoed from original)>
		 */
		VERB_CIRCUIT_TEST = 0x11,

		/**
		 * Circuit test hop report:
		 *   <[8] 64-bit timestamp (from original test)>
		 *   <[8] 64-bit test ID (from original test)>
		 *   <[8] 64-bit reserved field (set to 0, currently unused)>
		 *   <[1] 8-bit vendor ID (set to 0, currently unused)>
		 *   <[1] 8-bit reporter protocol version>
		 *   <[1] 8-bit reporter major version>
		 *   <[1] 8-bit reporter minor version>
		 *   <[2] 16-bit reporter revision>
		 *   <[2] 16-bit reporter OS/platform>
		 *   <[2] 16-bit reporter architecture>
		 *   <[2] 16-bit error code (set to 0, currently unused)>
		 *   <[8] 64-bit report flags (set to 0, currently unused)>
		 *   <[8] 64-bit source packet ID>
		 *   <[5] upstream ZeroTier address from which test was received>
		 *   <[1] 8-bit source packet hop count (ZeroTier hop count)>
		 *   <[...] local wire address on which packet was received>
		 *   <[...] remote wire address from which packet was received>
		 *   <[2] 16-bit length of additional fields>
		 *   <[...] additional fields>
		 *   <[1] 8-bit number of next hops (breadth)>
		 *   <[...] next hop information>
		 *
		 * Next hop information record format:
		 *   <[5] ZeroTier address of next hop>
		 *   <[...] current best direct path address, if any, 0 if none>
		 *
		 * Circuit test reports can be sent by hops in a circuit test to report
		 * back results. They should include information about the sender as well
		 * as about the paths to which next hops are being sent.
		 *
		 * If a test report is received and no circuit test was sent, it should be
		 * ignored. This message generates no OK or ERROR response.
		 */
		VERB_CIRCUIT_TEST_REPORT = 0x12,

		/**
		 * Request proof of work:
		 *   <[1] 8-bit proof of work type>
		 *   <[1] 8-bit proof of work difficulty>
		 *   <[2] 16-bit length of proof of work challenge>
		 *   <[...] proof of work challenge>
		 *
		 * This requests that a peer perform a proof of work calucation. It can be
		 * sent by highly trusted peers (e.g. root servers, network controllers)
		 * under suspected denial of service conditions in an attempt to filter
		 * out "non-serious" peers and remain responsive to those proving their
		 * intent to actually communicate.
		 *
		 * If the peer obliges to perform the work, it does so and responds with
		 * an OK containing the result. Otherwise it may ignore the message or
		 * response with an ERROR_INVALID_REQUEST or ERROR_UNSUPPORTED_OPERATION.
		 *
		 * Proof of work type IDs:
		 *   0x01 - Salsa20/12+SHA512 hashcash function
		 *
		 * Salsa20/12+SHA512 is based on the following composite hash function:
		 *
		 * (1) Compute SHA512(candidate)
		 * (2) Use the first 256 bits of the result of #1 as a key to encrypt
		 *     131072 zero bytes with Salsa20/12 (with a zero IV).
		 * (3) Compute SHA512(the result of step #2)
		 * (4) Accept this candiate if the first [difficulty] bits of the result
		 *     from step #3 are zero. Otherwise generate a new candidate and try
		 *     again.
		 *
		 * This is performed repeatedly on candidates generated by appending the
		 * supplied challenge to an arbitrary nonce until a valid candidate
		 * is found. This chosen prepended nonce is then returned as the result
		 * in OK.
		 *
		 * OK payload:
		 *   <[2] 16-bit length of result>
		 *   <[...] computed proof of work>
		 *
		 * ERROR has no payload.
		 */
		VERB_REQUEST_PROOF_OF_WORK = 0x13
	};

	/**
	 * Error codes for VERB_ERROR
	 */
	enum ErrorCode
	{
		/* No error, not actually used in transit */
		ERROR_NONE = 0x00,

		/* Invalid request */
		ERROR_INVALID_REQUEST = 0x01,

		/* Bad/unsupported protocol version */
		ERROR_BAD_PROTOCOL_VERSION = 0x02,

		/* Unknown object queried */
		ERROR_OBJ_NOT_FOUND = 0x03,

		/* HELLO pushed an identity whose address is already claimed */
		ERROR_IDENTITY_COLLISION = 0x04,

		/* Verb or use case not supported/enabled by this node */
		ERROR_UNSUPPORTED_OPERATION = 0x05,

		/* Tried to join network, but you're not a member */
		ERROR_NETWORK_ACCESS_DENIED_ = 0x07, /* extra _ at end to avoid Windows name conflict */

		/* Multicasts to this group are not wanted */
		ERROR_UNWANTED_MULTICAST = 0x08
	};

#ifdef ZT_TRACE
	static const char *verbString(Verb v)
		throw();
	static const char *errorString(ErrorCode e)
		throw();
#endif

	template<unsigned int C2>
	Packet(const Buffer<C2> &b) :
 		Buffer<ZT_PROTO_MAX_PACKET_LENGTH>(b)
	{
	}

	Packet(const void *data,unsigned int len) :
		Buffer<ZT_PROTO_MAX_PACKET_LENGTH>(data,len)
	{
	}

	/**
	 * Construct a new empty packet with a unique random packet ID
	 *
	 * Flags and hops will be zero. Other fields and data region are undefined.
	 * Use the header access methods (setDestination() and friends) to fill out
	 * the header. Payload should be appended; initial size is header size.
	 */
	Packet() :
		Buffer<ZT_PROTO_MAX_PACKET_LENGTH>(ZT_PROTO_MIN_PACKET_LENGTH)
	{
		Utils::getSecureRandom(field(ZT_PACKET_IDX_IV,8),8);
		(*this)[ZT_PACKET_IDX_FLAGS] = 0; // zero flags, cipher ID, and hops
	}

	/**
	 * Make a copy of a packet with a new initialization vector and destination address
	 *
	 * This can be used to take one draft prototype packet and quickly make copies to
	 * encrypt for different destinations.
	 *
	 * @param prototype Prototype packet
	 * @param dest Destination ZeroTier address for new packet
	 */
	Packet(const Packet &prototype,const Address &dest) :
		Buffer<ZT_PROTO_MAX_PACKET_LENGTH>(prototype)
	{
		Utils::getSecureRandom(field(ZT_PACKET_IDX_IV,8),8);
		setDestination(dest);
	}

	/**
	 * Construct a new empty packet with a unique random packet ID
	 *
	 * @param dest Destination ZT address
	 * @param source Source ZT address
	 * @param v Verb
	 */
	Packet(const Address &dest,const Address &source,const Verb v) :
		Buffer<ZT_PROTO_MAX_PACKET_LENGTH>(ZT_PROTO_MIN_PACKET_LENGTH)
	{
		Utils::getSecureRandom(field(ZT_PACKET_IDX_IV,8),8);
		setDestination(dest);
		setSource(source);
		(*this)[ZT_PACKET_IDX_FLAGS] = 0; // zero flags and hops
		setVerb(v);
	}

	/**
	 * Reset this packet structure for reuse in place
	 *
	 * @param dest Destination ZT address
	 * @param source Source ZT address
	 * @param v Verb
	 */
	inline void reset(const Address &dest,const Address &source,const Verb v)
	{
		setSize(ZT_PROTO_MIN_PACKET_LENGTH);
		Utils::getSecureRandom(field(ZT_PACKET_IDX_IV,8),8);
		setDestination(dest);
		setSource(source);
		(*this)[ZT_PACKET_IDX_FLAGS] = 0; // zero flags, cipher ID, and hops
		setVerb(v);
	}

	/**
	 * Generate a new IV / packet ID in place
	 *
	 * This can be used to re-use a packet buffer multiple times to send
	 * technically different but otherwise identical copies of the same
	 * packet.
	 */
	inline void newInitializationVector() { Utils::getSecureRandom(field(ZT_PACKET_IDX_IV,8),8); }

	/**
	 * Set this packet's destination
	 *
	 * @param dest ZeroTier address of destination
	 */
	inline void setDestination(const Address &dest) { dest.copyTo(field(ZT_PACKET_IDX_DEST,ZT_ADDRESS_LENGTH),ZT_ADDRESS_LENGTH); }

	/**
	 * Set this packet's source
	 *
	 * @param source ZeroTier address of source
	 */
	inline void setSource(const Address &source) { source.copyTo(field(ZT_PACKET_IDX_SOURCE,ZT_ADDRESS_LENGTH),ZT_ADDRESS_LENGTH); }

	/**
	 * Get this packet's destination
	 *
	 * @return Destination ZT address
	 */
	inline Address destination() const { return Address(field(ZT_PACKET_IDX_DEST,ZT_ADDRESS_LENGTH),ZT_ADDRESS_LENGTH); }

	/**
	 * Get this packet's source
	 *
	 * @return Source ZT address
	 */
	inline Address source() const { return Address(field(ZT_PACKET_IDX_SOURCE,ZT_ADDRESS_LENGTH),ZT_ADDRESS_LENGTH); }

	/**
	 * @return True if packet is of valid length
	 */
	inline bool lengthValid() const { return (size() >= ZT_PROTO_MIN_PACKET_LENGTH); }

	/**
	 * @return True if packet is fragmented (expect fragments)
	 */
	inline bool fragmented() const { return (((unsigned char)(*this)[ZT_PACKET_IDX_FLAGS] & ZT_PROTO_FLAG_FRAGMENTED) != 0); }

	/**
	 * Set this packet's fragmented flag
	 *
	 * @param f Fragmented flag value
	 */
	inline void setFragmented(bool f)
	{
		if (f)
			(*this)[ZT_PACKET_IDX_FLAGS] |= (char)ZT_PROTO_FLAG_FRAGMENTED;
		else (*this)[ZT_PACKET_IDX_FLAGS] &= (char)(~ZT_PROTO_FLAG_FRAGMENTED);
	}

	/**
	 * @return True if compressed (result only valid if unencrypted)
	 */
	inline bool compressed() const { return (((unsigned char)(*this)[ZT_PACKET_IDX_VERB] & ZT_PROTO_VERB_FLAG_COMPRESSED) != 0); }

	/**
	 * @return ZeroTier forwarding hops (0 to 7)
	 */
	inline unsigned int hops() const { return ((unsigned int)(*this)[ZT_PACKET_IDX_FLAGS] & 0x07); }

	/**
	 * Increment this packet's hop count
	 */
	inline void incrementHops()
	{
		unsigned char &b = (*this)[ZT_PACKET_IDX_FLAGS];
		b = (b & 0xf8) | ((b + 1) & 0x07);
	}

	/**
	 * @return Cipher suite selector: 0 - 7 (see #defines)
	 */
	inline unsigned int cipher() const
	{
		return (((unsigned int)(*this)[ZT_PACKET_IDX_FLAGS] & 0x38) >> 3);
	}

	/**
	 * Set this packet's cipher suite
	 */
	inline void setCipher(unsigned int c)
	{
		unsigned char &b = (*this)[ZT_PACKET_IDX_FLAGS];
		b = (b & 0xc7) | (unsigned char)((c << 3) & 0x38); // bits: FFCCCHHH
		// Set DEPRECATED "encrypted" flag -- used by pre-1.0.3 peers
		if (c == ZT_PROTO_CIPHER_SUITE__C25519_POLY1305_SALSA2012)
			b |= ZT_PROTO_FLAG_ENCRYPTED;
		else b &= (~ZT_PROTO_FLAG_ENCRYPTED);
	}

	/**
	 * Get the trusted path ID for this packet (only meaningful if cipher is trusted path)
	 *
	 * @return Trusted path ID (from MAC field)
	 */
	inline uint64_t trustedPathId() const { return at<uint64_t>(ZT_PACKET_IDX_MAC); }

	/**
	 * Set this packet's trusted path ID and set the cipher spec to trusted path
	 *
	 * @param tpid Trusted path ID
	 */
	inline void setTrusted(const uint64_t tpid)
	{
		setCipher(ZT_PROTO_CIPHER_SUITE__NO_CRYPTO_TRUSTED_PATH);
		setAt(ZT_PACKET_IDX_MAC,tpid);
	}

	/**
	 * Get this packet's unique ID (the IV field interpreted as uint64_t)
	 *
	 * @return Packet ID
	 */
	inline uint64_t packetId() const { return at<uint64_t>(ZT_PACKET_IDX_IV); }

	/**
	 * Set packet verb
	 *
	 * This also has the side-effect of clearing any verb flags, such as
	 * compressed, and so must only be done during packet composition.
	 *
	 * @param v New packet verb
	 */
	inline void setVerb(Verb v) { (*this)[ZT_PACKET_IDX_VERB] = (char)v; }

	/**
	 * @return Packet verb (not including flag bits)
	 */
	inline Verb verb() const { return (Verb)((*this)[ZT_PACKET_IDX_VERB] & 0x1f); }

	/**
	 * @return Length of packet payload
	 */
	inline unsigned int payloadLength() const { return ((size() < ZT_PROTO_MIN_PACKET_LENGTH) ? 0 : (size() - ZT_PROTO_MIN_PACKET_LENGTH)); }

	/**
	 * @return Raw packet payload
	 */
	inline const unsigned char *payload() const { return field(ZT_PACKET_IDX_PAYLOAD,size() - ZT_PACKET_IDX_PAYLOAD); }

	/**
	 * Armor packet for transport
	 *
	 * @param key 32-byte key
	 * @param encryptPayload If true, encrypt packet payload, else just MAC
	 */
	void armor(const void *key,bool encryptPayload);

	/**
	 * Verify and (if encrypted) decrypt packet
	 *
	 * This does not handle trusted path mode packets and will return false
	 * for these. These are handled in IncomingPacket if the sending physical
	 * address and MAC field match a trusted path.
	 *
	 * @param key 32-byte key
	 * @return False if packet is invalid or failed MAC authenticity check
	 */
	bool dearmor(const void *key);

	/**
	 * Attempt to compress payload if not already (must be unencrypted)
	 *
	 * This requires that the payload at least contain the verb byte already
	 * set. The compressed flag in the verb is set if compression successfully
	 * results in a size reduction. If no size reduction occurs, compression
	 * is not done and the flag is left cleared.
	 *
	 * @return True if compression occurred
	 */
	bool compress();

	/**
	 * Attempt to decompress payload if it is compressed (must be unencrypted)
	 *
	 * If payload is compressed, it is decompressed and the compressed verb
	 * flag is cleared. Otherwise nothing is done and true is returned.
	 *
	 * @return True if data is now decompressed and valid, false on error
	 */
	bool uncompress();

private:
	static const unsigned char ZERO_KEY[32];

	/**
	 * Deterministically mangle a 256-bit crypto key based on packet
	 *
	 * This uses extra data from the packet to mangle the secret, giving us an
	 * effective IV that is somewhat more than 64 bits. This is "free" for
	 * Salsa20 since it has negligible key setup time so using a different
	 * key each time is fine.
	 *
	 * @param in Input key (32 bytes)
	 * @param out Output buffer (32 bytes)
	 */
	inline void _salsa20MangleKey(const unsigned char *in,unsigned char *out) const
	{
		const unsigned char *d = (const unsigned char *)data();

		// IV and source/destination addresses. Using the addresses divides the
		// key space into two halves-- A->B and B->A (since order will change).
		for(unsigned int i=0;i<18;++i) // 8 + (ZT_ADDRESS_LENGTH * 2) == 18
			out[i] = in[i] ^ d[i];

		// Flags, but with hop count masked off. Hop count is altered by forwarding
		// nodes. It's one of the only parts of a packet modifiable by people
		// without the key.
		out[18] = in[18] ^ (d[ZT_PACKET_IDX_FLAGS] & 0xf8);

		// Raw packet size in bytes -- thus each packet size defines a new
		// key space.
		out[19] = in[19] ^ (unsigned char)(size() & 0xff);
		out[20] = in[20] ^ (unsigned char)((size() >> 8) & 0xff); // little endian

		// Rest of raw key is used unchanged
		for(unsigned int i=21;i<32;++i)
			out[i] = in[i];
	}
};

} // namespace ZeroTier

#endif