// Copyright 2016 The Netstack Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package ipv6 contains the implementation of the ipv6 network protocol. To use
// it in the networking stack, this package must be added to the project, and
// activated on the stack by passing ipv6.ProtocolName (or "ipv6") as one of the
// network protocols when calling stack.New(). Then endpoints can be created
// by passing ipv6.ProtocolNumber as the network protocol number when calling
// Stack.NewEndpoint().
package ipv6

import (
	"github.com/google/netstack/tcpip"
	"github.com/google/netstack/tcpip/buffer"
	"github.com/google/netstack/tcpip/header"
	"github.com/google/netstack/tcpip/stack"
)

const (
	// ProtocolName is the string representation of the ipv6 protocol name.
	ProtocolName = "ipv6"

	// ProtocolNumber is the ipv6 protocol number.
	ProtocolNumber = header.IPv6ProtocolNumber

	// maxTotalSize is maximum size that can be encoded in the 16-bit
	// PayloadLength field of the ipv6 header.
	maxPayloadSize = 0xffff
)

type address [header.IPv6AddressSize]byte

type endpoint struct {
	nicid         tcpip.NICID
	id            stack.NetworkEndpointID
	address       address
	linkEP        stack.LinkEndpoint
	linkAddrCache stack.LinkAddressCache
	dispatcher    stack.TransportDispatcher
}

func (e *endpoint) DefaultTTL() uint8 {
	return header.IPv6DefaultHopLimit
}

// MTU implements stack.NetworkEndpoint.MTU. It returns the link-layer MTU minus
// the network layer max header length.
func (e *endpoint) MTU() uint32 {
	mtu := e.linkEP.MTU() - uint32(e.MaxHeaderLength())
	if mtu <= maxPayloadSize {
		return mtu
	}
	return maxPayloadSize
}

// NICID returns the ID of the NIC this endpoint belongs to.
func (e *endpoint) NICID() tcpip.NICID {
	return e.nicid
}

// ID returns the ipv6 endpoint ID.
func (e *endpoint) ID() *stack.NetworkEndpointID {
	return &e.id
}

// MaxHeaderLength returns the maximum length needed by ipv6 headers (and
// underlying protocols).
func (e *endpoint) MaxHeaderLength() uint16 {
	return e.linkEP.MaxHeaderLength() + header.IPv6MinimumSize
}

// WritePacket writes a packet to the given destination address and protocol.
func (e *endpoint) WritePacket(r *stack.Route, hdr *buffer.Prependable, payload buffer.View, protocol tcpip.TransportProtocolNumber, ttl uint8) *tcpip.Error {
	length := uint16(hdr.UsedLength())
	if payload != nil {
		length += uint16(len(payload))
	}
	ip := header.IPv6(hdr.Prepend(header.IPv6MinimumSize))
	ip.Encode(&header.IPv6Fields{
		PayloadLength: length,
		NextHeader:    uint8(protocol),
		HopLimit:      ttl,
		SrcAddr:       r.LocalAddress,
		DstAddr:       r.RemoteAddress,
	})

	return e.linkEP.WritePacket(r, hdr, payload, ProtocolNumber)
}

// HandlePacket is called by the link layer when new ipv6 packets arrive for
// this endpoint.
func (e *endpoint) HandlePacket(r *stack.Route, vv *buffer.VectorisedView) {
	h := header.IPv6(vv.First())
	if !h.IsValid(vv.Size()) {
		return
	}

	vv.TrimFront(header.IPv6MinimumSize)
	vv.CapLength(int(h.PayloadLength()))
	p := tcpip.TransportProtocolNumber(h.NextHeader())
	if p == header.ICMPv6ProtocolNumber {
		e.handleICMP(r, vv)
		return
	}
	e.dispatcher.DeliverTransportPacket(r, p, vv)
}

// Close cleans up resources associated with the endpoint.
func (*endpoint) Close() {}

type protocol struct{}

// NewProtocol creates a new protocol ipv6 protocol descriptor. This is exported
// only for tests that short-circuit the stack. Regular use of the protocol is
// done via the stack, which gets a protocol descriptor from the init() function
// below.
func NewProtocol() stack.NetworkProtocol {
	return &protocol{}
}

// Number returns the ipv6 protocol number.
func (p *protocol) Number() tcpip.NetworkProtocolNumber {
	return ProtocolNumber
}

// MinimumPacketSize returns the minimum valid ipv6 packet size.
func (p *protocol) MinimumPacketSize() int {
	return header.IPv6MinimumSize
}

// ParseAddresses implements NetworkProtocol.ParseAddresses.
func (*protocol) ParseAddresses(v buffer.View) (src, dst tcpip.Address) {
	h := header.IPv6(v)
	return h.SourceAddress(), h.DestinationAddress()
}

// NewEndpoint creates a new ipv6 endpoint.
func (p *protocol) NewEndpoint(nicid tcpip.NICID, addr tcpip.Address, linkAddrCache stack.LinkAddressCache, dispatcher stack.TransportDispatcher, linkEP stack.LinkEndpoint) (stack.NetworkEndpoint, *tcpip.Error) {
	e := &endpoint{
		nicid:         nicid,
		linkEP:        linkEP,
		linkAddrCache: linkAddrCache,
		dispatcher:    dispatcher,
	}
	copy(e.address[:], addr)
	e.id = stack.NetworkEndpointID{tcpip.Address(e.address[:])}
	return e, nil
}

// SetOption implements NetworkProtocol.SetOption.
func (p *protocol) SetOption(option interface{}) *tcpip.Error {
	return tcpip.ErrUnknownProtocolOption
}

func init() {
	stack.RegisterNetworkProtocolFactory(ProtocolName, func() stack.NetworkProtocol {
		return &protocol{}
	})
}

// LinkLocalAddr computes the default IPv6 link-local address from
// a link-layer (MAC) address.
func LinkLocalAddr(linkAddr tcpip.LinkAddress) tcpip.Address {
	// Convert a 48-bit MAC to an EUI-64 and then prepend the
	// link-local header, FE80::.
	//
	// The conversion is very nearly:
	//	aa:bb:cc:dd:ee:ff => FE80::Aabb:ccFF:FEdd:eeff
	// Note the capital A. The conversion aa->Aa involves a bit flip.
	lladdrb := [16]byte{
		0:  0xFE,
		1:  0x80,
		8:  linkAddr[0] ^ 2,
		9:  linkAddr[1],
		10: linkAddr[2],
		11: 0xFF,
		12: 0xFE,
		13: linkAddr[3],
		14: linkAddr[4],
		15: linkAddr[5],
	}
	return tcpip.Address(lladdrb[:])
}

// SolicitedNodeAddr computes the solicited-node multicast address.
// This is used for NDP. Described in RFC 4291.
func SolicitedNodeAddr(addr tcpip.Address) tcpip.Address {
	return "\xff\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\xff" + addr[len(addr)-3:]
}