tcpip/stack/registration.go - third_party/netstack - Git at Google

 // Copyright 2018 Google Inc.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 package stack

 import (
 	"sync"

 	"github.com/google/netstack/sleep"
 	"github.com/google/netstack/tcpip"
 	"github.com/google/netstack/tcpip/buffer"
 	"github.com/google/netstack/waiter"
 )

 // NetworkEndpointID is the identifier of a network layer protocol endpoint.
 // Currently the local address is sufficient because all supported protocols
 // (i.e., IPv4 and IPv6) have different sizes for their addresses.
 type NetworkEndpointID struct {
 	LocalAddress tcpip.Address
 }

 // TransportEndpointID is the identifier of a transport layer protocol endpoint.
 //
 // +stateify savable
 type TransportEndpointID struct {
 	// LocalPort is the local port associated with the endpoint.
 	LocalPort uint16

 	// LocalAddress is the local [network layer] address associated with
 	// the endpoint.
 	LocalAddress tcpip.Address

 	// RemotePort is the remote port associated with the endpoint.
 	RemotePort uint16

 	// RemoteAddress it the remote [network layer] address associated with
 	// the endpoint.
 	RemoteAddress tcpip.Address
 }

 // ControlType is the type of network control message.
 type ControlType int

 // The following are the allowed values for ControlType values.
 const (
 	ControlPacketTooBig ControlType = iota
 	ControlPortUnreachable
 	ControlUnknown
 )

 // TransportEndpoint is the interface that needs to be implemented by transport
 // protocol (e.g., tcp, udp) endpoints that can handle packets.
 type TransportEndpoint interface {
 	// HandlePacket is called by the stack when new packets arrive to
 	// this transport endpoint.
 	HandlePacket(r *Route, id TransportEndpointID, vv buffer.VectorisedView)

 	// HandleControlPacket is called by the stack when new control (e.g.,
 	// ICMP) packets arrive to this transport endpoint.
 	HandleControlPacket(id TransportEndpointID, typ ControlType, extra uint32, vv buffer.VectorisedView)
 }

 // TransportProtocol is the interface that needs to be implemented by transport
 // protocols (e.g., tcp, udp) that want to be part of the networking stack.
 type TransportProtocol interface {
 	// Number returns the transport protocol number.
 	Number() tcpip.TransportProtocolNumber

 	// NewEndpoint creates a new endpoint of the transport protocol.
 	NewEndpoint(stack *Stack, netProto tcpip.NetworkProtocolNumber, waitQueue *waiter.Queue) (tcpip.Endpoint, *tcpip.Error)

 	// MinimumPacketSize returns the minimum valid packet size of this
 	// transport protocol. The stack automatically drops any packets smaller
 	// than this targeted at this protocol.
 	MinimumPacketSize() int

 	// ParsePorts returns the source and destination ports stored in a
 	// packet of this protocol.
 	ParsePorts(v buffer.View) (src, dst uint16, err *tcpip.Error)

 	// HandleUnknownDestinationPacket handles packets targeted at this
 	// protocol but that don't match any existing endpoint. For example,
 	// it is targeted at a port that have no listeners.
 	//
 	// The return value indicates whether the packet was well-formed (for
 	// stats purposes only).
 	HandleUnknownDestinationPacket(r *Route, id TransportEndpointID, vv buffer.VectorisedView) bool

 	// SetOption allows enabling/disabling protocol specific features.
 	// SetOption returns an error if the option is not supported or the
 	// provided option value is invalid.
 	SetOption(option interface{}) *tcpip.Error

 	// Option allows retrieving protocol specific option values.
 	// Option returns an error if the option is not supported or the
 	// provided option value is invalid.
 	Option(option interface{}) *tcpip.Error
 }

 // TransportDispatcher contains the methods used by the network stack to deliver
 // packets to the appropriate transport endpoint after it has been handled by
 // the network layer.
 type TransportDispatcher interface {
 	// DeliverTransportPacket delivers packets to the appropriate
 	// transport protocol endpoint.
 	DeliverTransportPacket(r *Route, protocol tcpip.TransportProtocolNumber, vv buffer.VectorisedView)

 	// DeliverTransportControlPacket delivers control packets to the
 	// appropriate transport protocol endpoint.
 	DeliverTransportControlPacket(local, remote tcpip.Address, net tcpip.NetworkProtocolNumber, trans tcpip.TransportProtocolNumber, typ ControlType, extra uint32, vv buffer.VectorisedView)
 }

 // NetworkEndpoint is the interface that needs to be implemented by endpoints
 // of network layer protocols (e.g., ipv4, ipv6).
 type NetworkEndpoint interface {
 	// DefaultTTL is the default time-to-live value (or hop limit, in ipv6)
 	// for this endpoint.
 	DefaultTTL() uint8

 	// MTU is the maximum transmission unit for this endpoint. This is
 	// generally calculated as the MTU of the underlying data link endpoint
 	// minus the network endpoint max header length.
 	MTU() uint32

 	// Capabilities returns the set of capabilities supported by the
 	// underlying link-layer endpoint.
 	Capabilities() LinkEndpointCapabilities

 	// MaxHeaderLength returns the maximum size the network (and lower
 	// level layers combined) headers can have. Higher levels use this
 	// information to reserve space in the front of the packets they're
 	// building.
 	MaxHeaderLength() uint16

 	// WritePacket writes a packet to the given destination address and
 	// protocol.
 	WritePacket(r *Route, hdr buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber, ttl uint8) *tcpip.Error

 	// ID returns the network protocol endpoint ID.
 	ID() *NetworkEndpointID

 	// NICID returns the id of the NIC this endpoint belongs to.
 	NICID() tcpip.NICID

 	// HandlePacket is called by the link layer when new packets arrive to
 	// this network endpoint.
 	HandlePacket(r *Route, vv buffer.VectorisedView)

 	// Close is called when the endpoint is reomved from a stack.
 	Close()
 }

 // NetworkProtocol is the interface that needs to be implemented by network
 // protocols (e.g., ipv4, ipv6) that want to be part of the networking stack.
 type NetworkProtocol interface {
 	// Number returns the network protocol number.
 	Number() tcpip.NetworkProtocolNumber

 	// MinimumPacketSize returns the minimum valid packet size of this
 	// network protocol. The stack automatically drops any packets smaller
 	// than this targeted at this protocol.
 	MinimumPacketSize() int

 	// ParsePorts returns the source and destination addresses stored in a
 	// packet of this protocol.
 	ParseAddresses(v buffer.View) (src, dst tcpip.Address)

 	// NewEndpoint creates a new endpoint of this protocol.
 	NewEndpoint(nicid tcpip.NICID, addr tcpip.Address, linkAddrCache LinkAddressCache, dispatcher TransportDispatcher, sender LinkEndpoint) (NetworkEndpoint, *tcpip.Error)

 	// SetOption allows enabling/disabling protocol specific features.
 	// SetOption returns an error if the option is not supported or the
 	// provided option value is invalid.
 	SetOption(option interface{}) *tcpip.Error

 	// Option allows retrieving protocol specific option values.
 	// Option returns an error if the option is not supported or the
 	// provided option value is invalid.
 	Option(option interface{}) *tcpip.Error
 }

 // NetworkDispatcher contains the methods used by the network stack to deliver
 // packets to the appropriate network endpoint after it has been handled by
 // the data link layer.
 type NetworkDispatcher interface {
 	// DeliverNetworkPacket finds the appropriate network protocol
 	// endpoint and hands the packet over for further processing.
 	DeliverNetworkPacket(linkEP LinkEndpoint, dstLinkAddr, srcLinkAddr tcpip.LinkAddress, protocol tcpip.NetworkProtocolNumber, vv buffer.VectorisedView)
 }

 // LinkEndpointCapabilities is the type associated with the capabilities
 // supported by a link-layer endpoint. It is a set of bitfields.
 type LinkEndpointCapabilities uint

 // The following are the supported link endpoint capabilities.
 const (
 	CapabilityChecksumOffload LinkEndpointCapabilities = 1 << iota
 	CapabilityResolutionRequired
 	CapabilitySaveRestore
 	CapabilityDisconnectOk
 	CapabilityLoopback
 )

 // LinkEndpoint is the interface implemented by data link layer protocols (e.g.,
 // ethernet, loopback, raw) and used by network layer protocols to send packets
 // out through the implementer's data link endpoint.
 type LinkEndpoint interface {
 	// MTU is the maximum transmission unit for this endpoint. This is
 	// usually dictated by the backing physical network; when such a
 	// physical network doesn't exist, the limit is generally 64k, which
 	// includes the maximum size of an IP packet.
 	MTU() uint32

 	// Capabilities returns the set of capabilities supported by the
 	// endpoint.
 	Capabilities() LinkEndpointCapabilities

 	// MaxHeaderLength returns the maximum size the data link (and
 	// lower level layers combined) headers can have. Higher levels use this
 	// information to reserve space in the front of the packets they're
 	// building.
 	MaxHeaderLength() uint16

 	// LinkAddress returns the link address (typically a MAC) of the
 	// link endpoint.
 	LinkAddress() tcpip.LinkAddress

 	// WritePacket writes a packet with the given protocol through the given
 	// route.
 	//
 	// To participate in transparent bridging, a LinkEndpoint implementation
 	// should call eth.Encode with header.EthernetFields.SrcAddr set to
 	// r.LocalLinkAddress if it is provided.
 	WritePacket(r *Route, hdr buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.NetworkProtocolNumber) *tcpip.Error

 	// Attach attaches the data link layer endpoint to the network-layer
 	// dispatcher of the stack.
 	Attach(dispatcher NetworkDispatcher)

 	// IsAttached returns whether a NetworkDispatcher is attached to the
 	// endpoint.
 	IsAttached() bool
 }

 // A LinkAddressResolver is an extension to a NetworkProtocol that
 // can resolve link addresses.
 type LinkAddressResolver interface {
 	// LinkAddressRequest sends a request for the LinkAddress of addr.
 	// The request is sent on linkEP with localAddr as the source.
 	//
 	// A valid response will cause the discovery protocol's network
 	// endpoint to call AddLinkAddress.
 	LinkAddressRequest(addr, localAddr tcpip.Address, linkEP LinkEndpoint) *tcpip.Error

 	// ResolveStaticAddress attempts to resolve address without sending
 	// requests. It either resolves the name immediately or returns the
 	// empty LinkAddress.
 	//
 	// It can be used to resolve broadcast addresses for example.
 	ResolveStaticAddress(addr tcpip.Address) (tcpip.LinkAddress, bool)

 	// LinkAddressProtocol returns the network protocol of the
 	// addresses this this resolver can resolve.
 	LinkAddressProtocol() tcpip.NetworkProtocolNumber
 }

 // A LinkAddressCache caches link addresses.
 type LinkAddressCache interface {
 	// CheckLocalAddress determines if the given local address exists, and if it
 	// does not exist.
 	CheckLocalAddress(nicid tcpip.NICID, protocol tcpip.NetworkProtocolNumber, addr tcpip.Address) tcpip.NICID

 	// AddLinkAddress adds a link address to the cache.
 	AddLinkAddress(nicid tcpip.NICID, addr tcpip.Address, linkAddr tcpip.LinkAddress)

 	// GetLinkAddress looks up the cache to translate address to link address (e.g. IP -> MAC).
 	// If the LinkEndpoint requests address resolution and there is a LinkAddressResolver
 	// registered with the network protocol, the cache attempts to resolve the address
 	// and returns ErrWouldBlock. Waker is notified when address resolution is
 	// complete (success or not).
 	GetLinkAddress(nicid tcpip.NICID, addr, localAddr tcpip.Address, protocol tcpip.NetworkProtocolNumber, w *sleep.Waker) (tcpip.LinkAddress, *tcpip.Error)

 	// RemoveWaker removes a waker that has been added in GetLinkAddress().
 	RemoveWaker(nicid tcpip.NICID, addr tcpip.Address, waker *sleep.Waker)
 }

 // TransportProtocolFactory functions are used by the stack to instantiate
 // transport protocols.
 type TransportProtocolFactory func() TransportProtocol

 // NetworkProtocolFactory provides methods to be used by the stack to
 // instantiate network protocols.
 type NetworkProtocolFactory func() NetworkProtocol

 var (
 	transportProtocols = make(map[string]TransportProtocolFactory)
 	networkProtocols   = make(map[string]NetworkProtocolFactory)

 	linkEPMu           sync.RWMutex
 	nextLinkEndpointID tcpip.LinkEndpointID = 1
 	linkEndpoints                           = make(map[tcpip.LinkEndpointID]LinkEndpoint)
 )

 // RegisterTransportProtocolFactory registers a new transport protocol factory
 // with the stack so that it becomes available to users of the stack. This
 // function is intended to be called by init() functions of the protocols.
 func RegisterTransportProtocolFactory(name string, p TransportProtocolFactory) {
 	transportProtocols[name] = p
 }

 // RegisterNetworkProtocolFactory registers a new network protocol factory with
 // the stack so that it becomes available to users of the stack. This function
 // is intended to be called by init() functions of the protocols.
 func RegisterNetworkProtocolFactory(name string, p NetworkProtocolFactory) {
 	networkProtocols[name] = p
 }

 // RegisterLinkEndpoint register a link-layer protocol endpoint and returns an
 // ID that can be used to refer to it.
 func RegisterLinkEndpoint(linkEP LinkEndpoint) tcpip.LinkEndpointID {
 	linkEPMu.Lock()
 	defer linkEPMu.Unlock()

 	v := nextLinkEndpointID
 	nextLinkEndpointID++

 	linkEndpoints[v] = linkEP

 	return v
 }

 // FindLinkEndpoint finds the link endpoint associated with the given ID.
 func FindLinkEndpoint(id tcpip.LinkEndpointID) LinkEndpoint {
 	linkEPMu.RLock()
 	defer linkEPMu.RUnlock()

 	return linkEndpoints[id]
 }
	// Copyright 2018 Google Inc.
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	package stack

	import (
	"sync"

	"github.com/google/netstack/sleep"
	"github.com/google/netstack/tcpip"
	"github.com/google/netstack/tcpip/buffer"
	"github.com/google/netstack/waiter"
	)

	// NetworkEndpointID is the identifier of a network layer protocol endpoint.
	// Currently the local address is sufficient because all supported protocols
	// (i.e., IPv4 and IPv6) have different sizes for their addresses.
	type NetworkEndpointID struct {
	LocalAddress tcpip.Address
	}

	// TransportEndpointID is the identifier of a transport layer protocol endpoint.
	//
	// +stateify savable
	type TransportEndpointID struct {
	// LocalPort is the local port associated with the endpoint.
	LocalPort uint16

	// LocalAddress is the local [network layer] address associated with
	// the endpoint.
	LocalAddress tcpip.Address

	// RemotePort is the remote port associated with the endpoint.
	RemotePort uint16

	// RemoteAddress it the remote [network layer] address associated with
	// the endpoint.
	RemoteAddress tcpip.Address
	}

	// ControlType is the type of network control message.
	type ControlType int

	// The following are the allowed values for ControlType values.
	const (
	ControlPacketTooBig ControlType = iota
	ControlPortUnreachable
	ControlUnknown
	)

	// TransportEndpoint is the interface that needs to be implemented by transport
	// protocol (e.g., tcp, udp) endpoints that can handle packets.
	type TransportEndpoint interface {
	// HandlePacket is called by the stack when new packets arrive to
	// this transport endpoint.
	HandlePacket(r *Route, id TransportEndpointID, vv buffer.VectorisedView)

	// HandleControlPacket is called by the stack when new control (e.g.,
	// ICMP) packets arrive to this transport endpoint.
	HandleControlPacket(id TransportEndpointID, typ ControlType, extra uint32, vv buffer.VectorisedView)
	}

	// TransportProtocol is the interface that needs to be implemented by transport
	// protocols (e.g., tcp, udp) that want to be part of the networking stack.
	type TransportProtocol interface {
	// Number returns the transport protocol number.
	Number() tcpip.TransportProtocolNumber

	// NewEndpoint creates a new endpoint of the transport protocol.
	NewEndpoint(stack Stack, netProto tcpip.NetworkProtocolNumber, waitQueue waiter.Queue) (tcpip.Endpoint, *tcpip.Error)

	// MinimumPacketSize returns the minimum valid packet size of this
	// transport protocol. The stack automatically drops any packets smaller
	// than this targeted at this protocol.
	MinimumPacketSize() int

	// ParsePorts returns the source and destination ports stored in a
	// packet of this protocol.
	ParsePorts(v buffer.View) (src, dst uint16, err *tcpip.Error)

	// HandleUnknownDestinationPacket handles packets targeted at this
	// protocol but that don't match any existing endpoint. For example,
	// it is targeted at a port that have no listeners.
	//
	// The return value indicates whether the packet was well-formed (for
	// stats purposes only).
	HandleUnknownDestinationPacket(r *Route, id TransportEndpointID, vv buffer.VectorisedView) bool

	// SetOption allows enabling/disabling protocol specific features.
	// SetOption returns an error if the option is not supported or the
	// provided option value is invalid.
	SetOption(option interface{}) *tcpip.Error

	// Option allows retrieving protocol specific option values.
	// Option returns an error if the option is not supported or the
	// provided option value is invalid.
	Option(option interface{}) *tcpip.Error
	}

	// TransportDispatcher contains the methods used by the network stack to deliver
	// packets to the appropriate transport endpoint after it has been handled by
	// the network layer.
	type TransportDispatcher interface {
	// DeliverTransportPacket delivers packets to the appropriate
	// transport protocol endpoint.
	DeliverTransportPacket(r *Route, protocol tcpip.TransportProtocolNumber, vv buffer.VectorisedView)

	// DeliverTransportControlPacket delivers control packets to the
	// appropriate transport protocol endpoint.
	DeliverTransportControlPacket(local, remote tcpip.Address, net tcpip.NetworkProtocolNumber, trans tcpip.TransportProtocolNumber, typ ControlType, extra uint32, vv buffer.VectorisedView)
	}

	// NetworkEndpoint is the interface that needs to be implemented by endpoints
	// of network layer protocols (e.g., ipv4, ipv6).
	type NetworkEndpoint interface {
	// DefaultTTL is the default time-to-live value (or hop limit, in ipv6)
	// for this endpoint.
	DefaultTTL() uint8

	// MTU is the maximum transmission unit for this endpoint. This is
	// generally calculated as the MTU of the underlying data link endpoint
	// minus the network endpoint max header length.
	MTU() uint32

	// Capabilities returns the set of capabilities supported by the
	// underlying link-layer endpoint.
	Capabilities() LinkEndpointCapabilities

	// MaxHeaderLength returns the maximum size the network (and lower
	// level layers combined) headers can have. Higher levels use this
	// information to reserve space in the front of the packets they're
	// building.
	MaxHeaderLength() uint16

	// WritePacket writes a packet to the given destination address and
	// protocol.
	WritePacket(r Route, hdr buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber, ttl uint8) tcpip.Error

	// ID returns the network protocol endpoint ID.
	ID() *NetworkEndpointID

	// NICID returns the id of the NIC this endpoint belongs to.
	NICID() tcpip.NICID

	// HandlePacket is called by the link layer when new packets arrive to
	// this network endpoint.
	HandlePacket(r *Route, vv buffer.VectorisedView)

	// Close is called when the endpoint is reomved from a stack.
	Close()
	}

	// NetworkProtocol is the interface that needs to be implemented by network
	// protocols (e.g., ipv4, ipv6) that want to be part of the networking stack.
	type NetworkProtocol interface {
	// Number returns the network protocol number.
	Number() tcpip.NetworkProtocolNumber

	// MinimumPacketSize returns the minimum valid packet size of this
	// network protocol. The stack automatically drops any packets smaller
	// than this targeted at this protocol.
	MinimumPacketSize() int

	// ParsePorts returns the source and destination addresses stored in a
	// packet of this protocol.
	ParseAddresses(v buffer.View) (src, dst tcpip.Address)

	// NewEndpoint creates a new endpoint of this protocol.
	NewEndpoint(nicid tcpip.NICID, addr tcpip.Address, linkAddrCache LinkAddressCache, dispatcher TransportDispatcher, sender LinkEndpoint) (NetworkEndpoint, *tcpip.Error)

	// SetOption allows enabling/disabling protocol specific features.
	// SetOption returns an error if the option is not supported or the
	// provided option value is invalid.
	SetOption(option interface{}) *tcpip.Error

	// Option allows retrieving protocol specific option values.
	// Option returns an error if the option is not supported or the
	// provided option value is invalid.
	Option(option interface{}) *tcpip.Error
	}

	// NetworkDispatcher contains the methods used by the network stack to deliver
	// packets to the appropriate network endpoint after it has been handled by
	// the data link layer.
	type NetworkDispatcher interface {
	// DeliverNetworkPacket finds the appropriate network protocol
	// endpoint and hands the packet over for further processing.
	DeliverNetworkPacket(linkEP LinkEndpoint, dstLinkAddr, srcLinkAddr tcpip.LinkAddress, protocol tcpip.NetworkProtocolNumber, vv buffer.VectorisedView)
	}

	// LinkEndpointCapabilities is the type associated with the capabilities
	// supported by a link-layer endpoint. It is a set of bitfields.
	type LinkEndpointCapabilities uint

	// The following are the supported link endpoint capabilities.
	const (
	CapabilityChecksumOffload LinkEndpointCapabilities = 1 << iota
	CapabilityResolutionRequired
	CapabilitySaveRestore
	CapabilityDisconnectOk
	CapabilityLoopback
	)

	// LinkEndpoint is the interface implemented by data link layer protocols (e.g.,
	// ethernet, loopback, raw) and used by network layer protocols to send packets
	// out through the implementer's data link endpoint.
	type LinkEndpoint interface {
	// MTU is the maximum transmission unit for this endpoint. This is
	// usually dictated by the backing physical network; when such a
	// physical network doesn't exist, the limit is generally 64k, which
	// includes the maximum size of an IP packet.
	MTU() uint32

	// Capabilities returns the set of capabilities supported by the
	// endpoint.
	Capabilities() LinkEndpointCapabilities

	// MaxHeaderLength returns the maximum size the data link (and
	// lower level layers combined) headers can have. Higher levels use this
	// information to reserve space in the front of the packets they're
	// building.
	MaxHeaderLength() uint16

	// LinkAddress returns the link address (typically a MAC) of the
	// link endpoint.
	LinkAddress() tcpip.LinkAddress

	// WritePacket writes a packet with the given protocol through the given
	// route.
	//
	// To participate in transparent bridging, a LinkEndpoint implementation
	// should call eth.Encode with header.EthernetFields.SrcAddr set to
	// r.LocalLinkAddress if it is provided.
	WritePacket(r Route, hdr buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.NetworkProtocolNumber) tcpip.Error

	// Attach attaches the data link layer endpoint to the network-layer
	// dispatcher of the stack.
	Attach(dispatcher NetworkDispatcher)

	// IsAttached returns whether a NetworkDispatcher is attached to the
	// endpoint.
	IsAttached() bool
	}

	// A LinkAddressResolver is an extension to a NetworkProtocol that
	// can resolve link addresses.
	type LinkAddressResolver interface {
	// LinkAddressRequest sends a request for the LinkAddress of addr.
	// The request is sent on linkEP with localAddr as the source.
	//
	// A valid response will cause the discovery protocol's network
	// endpoint to call AddLinkAddress.
	LinkAddressRequest(addr, localAddr tcpip.Address, linkEP LinkEndpoint) *tcpip.Error

	// ResolveStaticAddress attempts to resolve address without sending
	// requests. It either resolves the name immediately or returns the
	// empty LinkAddress.
	//
	// It can be used to resolve broadcast addresses for example.
	ResolveStaticAddress(addr tcpip.Address) (tcpip.LinkAddress, bool)

	// LinkAddressProtocol returns the network protocol of the
	// addresses this this resolver can resolve.
	LinkAddressProtocol() tcpip.NetworkProtocolNumber
	}

	// A LinkAddressCache caches link addresses.
	type LinkAddressCache interface {
	// CheckLocalAddress determines if the given local address exists, and if it
	// does not exist.
	CheckLocalAddress(nicid tcpip.NICID, protocol tcpip.NetworkProtocolNumber, addr tcpip.Address) tcpip.NICID

	// AddLinkAddress adds a link address to the cache.
	AddLinkAddress(nicid tcpip.NICID, addr tcpip.Address, linkAddr tcpip.LinkAddress)

	// GetLinkAddress looks up the cache to translate address to link address (e.g. IP -> MAC).
	// If the LinkEndpoint requests address resolution and there is a LinkAddressResolver
	// registered with the network protocol, the cache attempts to resolve the address
	// and returns ErrWouldBlock. Waker is notified when address resolution is
	// complete (success or not).
	GetLinkAddress(nicid tcpip.NICID, addr, localAddr tcpip.Address, protocol tcpip.NetworkProtocolNumber, w sleep.Waker) (tcpip.LinkAddress, tcpip.Error)

	// RemoveWaker removes a waker that has been added in GetLinkAddress().
	RemoveWaker(nicid tcpip.NICID, addr tcpip.Address, waker *sleep.Waker)
	}

	// TransportProtocolFactory functions are used by the stack to instantiate
	// transport protocols.
	type TransportProtocolFactory func() TransportProtocol

	// NetworkProtocolFactory provides methods to be used by the stack to
	// instantiate network protocols.
	type NetworkProtocolFactory func() NetworkProtocol

	var (
	transportProtocols = make(map[string]TransportProtocolFactory)
	networkProtocols = make(map[string]NetworkProtocolFactory)

	linkEPMu sync.RWMutex
	nextLinkEndpointID tcpip.LinkEndpointID = 1
	linkEndpoints = make(map[tcpip.LinkEndpointID]LinkEndpoint)
	)

	// RegisterTransportProtocolFactory registers a new transport protocol factory
	// with the stack so that it becomes available to users of the stack. This
	// function is intended to be called by init() functions of the protocols.
	func RegisterTransportProtocolFactory(name string, p TransportProtocolFactory) {
	transportProtocols[name] = p
	}

	// RegisterNetworkProtocolFactory registers a new network protocol factory with
	// the stack so that it becomes available to users of the stack. This function
	// is intended to be called by init() functions of the protocols.
	func RegisterNetworkProtocolFactory(name string, p NetworkProtocolFactory) {
	networkProtocols[name] = p
	}

	// RegisterLinkEndpoint register a link-layer protocol endpoint and returns an
	// ID that can be used to refer to it.
	func RegisterLinkEndpoint(linkEP LinkEndpoint) tcpip.LinkEndpointID {
	linkEPMu.Lock()
	defer linkEPMu.Unlock()

	v := nextLinkEndpointID
	nextLinkEndpointID++

	linkEndpoints[v] = linkEP

	return v
	}

	// FindLinkEndpoint finds the link endpoint associated with the given ID.
	func FindLinkEndpoint(id tcpip.LinkEndpointID) LinkEndpoint {
	linkEPMu.RLock()
	defer linkEPMu.RUnlock()

	return linkEndpoints[id]
	}