src/net/addrselect.go - go - Git at Google

 // Copyright 2015 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // +build aix darwin dragonfly freebsd linux netbsd openbsd solaris

 // Minimal RFC 6724 address selection.

 package net

 import "sort"

 func sortByRFC6724(addrs []IPAddr) {
 	if len(addrs) < 2 {
 		return
 	}
 	sortByRFC6724withSrcs(addrs, srcAddrs(addrs))
 }

 func sortByRFC6724withSrcs(addrs []IPAddr, srcs []IP) {
 	if len(addrs) != len(srcs) {
 		panic("internal error")
 	}
 	addrAttr := make([]ipAttr, len(addrs))
 	srcAttr := make([]ipAttr, len(srcs))
 	for i, v := range addrs {
 		addrAttr[i] = ipAttrOf(v.IP)
 		srcAttr[i] = ipAttrOf(srcs[i])
 	}
 	sort.Stable(&byRFC6724{
 		addrs:    addrs,
 		addrAttr: addrAttr,
 		srcs:     srcs,
 		srcAttr:  srcAttr,
 	})
 }

 // srcsAddrs tries to UDP-connect to each address to see if it has a
 // route. (This doesn't send any packets). The destination port
 // number is irrelevant.
 func srcAddrs(addrs []IPAddr) []IP {
 	srcs := make([]IP, len(addrs))
 	dst := UDPAddr{Port: 9}
 	for i := range addrs {
 		dst.IP = addrs[i].IP
 		dst.Zone = addrs[i].Zone
 		c, err := DialUDP("udp", nil, &dst)
 		if err == nil {
 			if src, ok := c.LocalAddr().(*UDPAddr); ok {
 				srcs[i] = src.IP
 			}
 			c.Close()
 		}
 	}
 	return srcs
 }

 type ipAttr struct {
 	Scope      scope
 	Precedence uint8
 	Label      uint8
 }

 func ipAttrOf(ip IP) ipAttr {
 	if ip == nil {
 		return ipAttr{}
 	}
 	match := rfc6724policyTable.Classify(ip)
 	return ipAttr{
 		Scope:      classifyScope(ip),
 		Precedence: match.Precedence,
 		Label:      match.Label,
 	}
 }

 type byRFC6724 struct {
 	addrs    []IPAddr // addrs to sort
 	addrAttr []ipAttr
 	srcs     []IP // or nil if unreachable
 	srcAttr  []ipAttr
 }

 func (s *byRFC6724) Len() int { return len(s.addrs) }

 func (s *byRFC6724) Swap(i, j int) {
 	s.addrs[i], s.addrs[j] = s.addrs[j], s.addrs[i]
 	s.srcs[i], s.srcs[j] = s.srcs[j], s.srcs[i]
 	s.addrAttr[i], s.addrAttr[j] = s.addrAttr[j], s.addrAttr[i]
 	s.srcAttr[i], s.srcAttr[j] = s.srcAttr[j], s.srcAttr[i]
 }

 // Less reports whether i is a better destination address for this
 // host than j.
 //
 // The algorithm and variable names comes from RFC 6724 section 6.
 func (s *byRFC6724) Less(i, j int) bool {
 	DA := s.addrs[i].IP
 	DB := s.addrs[j].IP
 	SourceDA := s.srcs[i]
 	SourceDB := s.srcs[j]
 	attrDA := &s.addrAttr[i]
 	attrDB := &s.addrAttr[j]
 	attrSourceDA := &s.srcAttr[i]
 	attrSourceDB := &s.srcAttr[j]

 	const preferDA = true
 	const preferDB = false

 	// Rule 1: Avoid unusable destinations.
 	// If DB is known to be unreachable or if Source(DB) is undefined, then
 	// prefer DA.  Similarly, if DA is known to be unreachable or if
 	// Source(DA) is undefined, then prefer DB.
 	if SourceDA == nil && SourceDB == nil {
 		return false // "equal"
 	}
 	if SourceDB == nil {
 		return preferDA
 	}
 	if SourceDA == nil {
 		return preferDB
 	}

 	// Rule 2: Prefer matching scope.
 	// If Scope(DA) = Scope(Source(DA)) and Scope(DB) <> Scope(Source(DB)),
 	// then prefer DA.  Similarly, if Scope(DA) <> Scope(Source(DA)) and
 	// Scope(DB) = Scope(Source(DB)), then prefer DB.
 	if attrDA.Scope == attrSourceDA.Scope && attrDB.Scope != attrSourceDB.Scope {
 		return preferDA
 	}
 	if attrDA.Scope != attrSourceDA.Scope && attrDB.Scope == attrSourceDB.Scope {
 		return preferDB
 	}

 	// Rule 3: Avoid deprecated addresses.
 	// If Source(DA) is deprecated and Source(DB) is not, then prefer DB.
 	// Similarly, if Source(DA) is not deprecated and Source(DB) is
 	// deprecated, then prefer DA.

 	// TODO(bradfitz): implement? low priority for now.

 	// Rule 4: Prefer home addresses.
 	// If Source(DA) is simultaneously a home address and care-of address
 	// and Source(DB) is not, then prefer DA.  Similarly, if Source(DB) is
 	// simultaneously a home address and care-of address and Source(DA) is
 	// not, then prefer DB.

 	// TODO(bradfitz): implement? low priority for now.

 	// Rule 5: Prefer matching label.
 	// If Label(Source(DA)) = Label(DA) and Label(Source(DB)) <> Label(DB),
 	// then prefer DA.  Similarly, if Label(Source(DA)) <> Label(DA) and
 	// Label(Source(DB)) = Label(DB), then prefer DB.
 	if attrSourceDA.Label == attrDA.Label &&
 		attrSourceDB.Label != attrDB.Label {
 		return preferDA
 	}
 	if attrSourceDA.Label != attrDA.Label &&
 		attrSourceDB.Label == attrDB.Label {
 		return preferDB
 	}

 	// Rule 6: Prefer higher precedence.
 	// If Precedence(DA) > Precedence(DB), then prefer DA.  Similarly, if
 	// Precedence(DA) < Precedence(DB), then prefer DB.
 	if attrDA.Precedence > attrDB.Precedence {
 		return preferDA
 	}
 	if attrDA.Precedence < attrDB.Precedence {
 		return preferDB
 	}

 	// Rule 7: Prefer native transport.
 	// If DA is reached via an encapsulating transition mechanism (e.g.,
 	// IPv6 in IPv4) and DB is not, then prefer DB.  Similarly, if DB is
 	// reached via encapsulation and DA is not, then prefer DA.

 	// TODO(bradfitz): implement? low priority for now.

 	// Rule 8: Prefer smaller scope.
 	// If Scope(DA) < Scope(DB), then prefer DA.  Similarly, if Scope(DA) >
 	// Scope(DB), then prefer DB.
 	if attrDA.Scope < attrDB.Scope {
 		return preferDA
 	}
 	if attrDA.Scope > attrDB.Scope {
 		return preferDB
 	}

 	// Rule 9: Use longest matching prefix.
 	// When DA and DB belong to the same address family (both are IPv6 or
 	// both are IPv4 [but see below]): If CommonPrefixLen(Source(DA), DA) >
 	// CommonPrefixLen(Source(DB), DB), then prefer DA.  Similarly, if
 	// CommonPrefixLen(Source(DA), DA) < CommonPrefixLen(Source(DB), DB),
 	// then prefer DB.
 	//
 	// However, applying this rule to IPv4 addresses causes
 	// problems (see issues 13283 and 18518), so limit to IPv6.
 	if DA.To4() == nil && DB.To4() == nil {
 		commonA := commonPrefixLen(SourceDA, DA)
 		commonB := commonPrefixLen(SourceDB, DB)

 		if commonA > commonB {
 			return preferDA
 		}
 		if commonA < commonB {
 			return preferDB
 		}
 	}

 	// Rule 10: Otherwise, leave the order unchanged.
 	// If DA preceded DB in the original list, prefer DA.
 	// Otherwise, prefer DB.
 	return false // "equal"
 }

 type policyTableEntry struct {
 	Prefix     *IPNet
 	Precedence uint8
 	Label      uint8
 }

 type policyTable []policyTableEntry

 // RFC 6724 section 2.1.
 var rfc6724policyTable = policyTable{
 	{
 		Prefix:     mustCIDR("::1/128"),
 		Precedence: 50,
 		Label:      0,
 	},
 	{
 		Prefix:     mustCIDR("::/0"),
 		Precedence: 40,
 		Label:      1,
 	},
 	{
 		// IPv4-compatible, etc.
 		Prefix:     mustCIDR("::ffff:0:0/96"),
 		Precedence: 35,
 		Label:      4,
 	},
 	{
 		// 6to4
 		Prefix:     mustCIDR("2002::/16"),
 		Precedence: 30,
 		Label:      2,
 	},
 	{
 		// Teredo
 		Prefix:     mustCIDR("2001::/32"),
 		Precedence: 5,
 		Label:      5,
 	},
 	{
 		Prefix:     mustCIDR("fc00::/7"),
 		Precedence: 3,
 		Label:      13,
 	},
 	{
 		Prefix:     mustCIDR("::/96"),
 		Precedence: 1,
 		Label:      3,
 	},
 	{
 		Prefix:     mustCIDR("fec0::/10"),
 		Precedence: 1,
 		Label:      11,
 	},
 	{
 		Prefix:     mustCIDR("3ffe::/16"),
 		Precedence: 1,
 		Label:      12,
 	},
 }

 func init() {
 	sort.Sort(sort.Reverse(byMaskLength(rfc6724policyTable)))
 }

 // byMaskLength sorts policyTableEntry by the size of their Prefix.Mask.Size,
 // from smallest mask, to largest.
 type byMaskLength []policyTableEntry

 func (s byMaskLength) Len() int      { return len(s) }
 func (s byMaskLength) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
 func (s byMaskLength) Less(i, j int) bool {
 	isize, _ := s[i].Prefix.Mask.Size()
 	jsize, _ := s[j].Prefix.Mask.Size()
 	return isize < jsize
 }

 // mustCIDR calls ParseCIDR and panics on any error, or if the network
 // is not IPv6.
 func mustCIDR(s string) *IPNet {
 	ip, ipNet, err := ParseCIDR(s)
 	if err != nil {
 		panic(err.Error())
 	}
 	if len(ip) != IPv6len {
 		panic("unexpected IP length")
 	}
 	return ipNet
 }

 // Classify returns the policyTableEntry of the entry with the longest
 // matching prefix that contains ip.
 // The table t must be sorted from largest mask size to smallest.
 func (t policyTable) Classify(ip IP) policyTableEntry {
 	for _, ent := range t {
 		if ent.Prefix.Contains(ip) {
 			return ent
 		}
 	}
 	return policyTableEntry{}
 }

 // RFC 6724 section 3.1.
 type scope uint8

 const (
 	scopeInterfaceLocal scope = 0x1
 	scopeLinkLocal      scope = 0x2
 	scopeAdminLocal     scope = 0x4
 	scopeSiteLocal      scope = 0x5
 	scopeOrgLocal       scope = 0x8
 	scopeGlobal         scope = 0xe
 )

 func classifyScope(ip IP) scope {
 	if ip.IsLoopback() || ip.IsLinkLocalUnicast() {
 		return scopeLinkLocal
 	}
 	ipv6 := len(ip) == IPv6len && ip.To4() == nil
 	if ipv6 && ip.IsMulticast() {
 		return scope(ip[1] & 0xf)
 	}
 	// Site-local addresses are defined in RFC 3513 section 2.5.6
 	// (and deprecated in RFC 3879).
 	if ipv6 && ip[0] == 0xfe && ip[1]&0xc0 == 0xc0 {
 		return scopeSiteLocal
 	}
 	return scopeGlobal
 }

 // commonPrefixLen reports the length of the longest prefix (looking
 // at the most significant, or leftmost, bits) that the
 // two addresses have in common, up to the length of a's prefix (i.e.,
 // the portion of the address not including the interface ID).
 //
 // If a or b is an IPv4 address as an IPv6 address, the IPv4 addresses
 // are compared (with max common prefix length of 32).
 // If a and b are different IP versions, 0 is returned.
 //
 // See https://tools.ietf.org/html/rfc6724#section-2.2
 func commonPrefixLen(a, b IP) (cpl int) {
 	if a4 := a.To4(); a4 != nil {
 		a = a4
 	}
 	if b4 := b.To4(); b4 != nil {
 		b = b4
 	}
 	if len(a) != len(b) {
 		return 0
 	}
 	// If IPv6, only up to the prefix (first 64 bits)
 	if len(a) > 8 {
 		a = a[:8]
 		b = b[:8]
 	}
 	for len(a) > 0 {
 		if a[0] == b[0] {
 			cpl += 8
 			a = a[1:]
 			b = b[1:]
 			continue
 		}
 		bits := 8
 		ab, bb := a[0], b[0]
 		for {
 			ab >>= 1
 			bb >>= 1
 			bits--
 			if ab == bb {
 				cpl += bits
 				return
 			}
 		}
 	}
 	return
 }
	// Copyright 2015 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris

	// Minimal RFC 6724 address selection.

	package net

	import "sort"

	func sortByRFC6724(addrs []IPAddr) {
	if len(addrs) < 2 {
	return
	}
	sortByRFC6724withSrcs(addrs, srcAddrs(addrs))
	}

	func sortByRFC6724withSrcs(addrs []IPAddr, srcs []IP) {
	if len(addrs) != len(srcs) {
	panic("internal error")
	}
	addrAttr := make([]ipAttr, len(addrs))
	srcAttr := make([]ipAttr, len(srcs))
	for i, v := range addrs {
	addrAttr[i] = ipAttrOf(v.IP)
	srcAttr[i] = ipAttrOf(srcs[i])
	}
	sort.Stable(&byRFC6724{
	addrs: addrs,
	addrAttr: addrAttr,
	srcs: srcs,
	srcAttr: srcAttr,
	})
	}

	// srcsAddrs tries to UDP-connect to each address to see if it has a
	// route. (This doesn't send any packets). The destination port
	// number is irrelevant.
	func srcAddrs(addrs []IPAddr) []IP {
	srcs := make([]IP, len(addrs))
	dst := UDPAddr{Port: 9}
	for i := range addrs {
	dst.IP = addrs[i].IP
	dst.Zone = addrs[i].Zone
	c, err := DialUDP("udp", nil, &dst)
	if err == nil {
	if src, ok := c.LocalAddr().(*UDPAddr); ok {
	srcs[i] = src.IP
	}
	c.Close()
	}
	}
	return srcs
	}

	type ipAttr struct {
	Scope scope
	Precedence uint8
	Label uint8
	}

	func ipAttrOf(ip IP) ipAttr {
	if ip == nil {
	return ipAttr{}
	}
	match := rfc6724policyTable.Classify(ip)
	return ipAttr{
	Scope: classifyScope(ip),
	Precedence: match.Precedence,
	Label: match.Label,
	}
	}

	type byRFC6724 struct {
	addrs []IPAddr // addrs to sort
	addrAttr []ipAttr
	srcs []IP // or nil if unreachable
	srcAttr []ipAttr
	}

	func (s *byRFC6724) Len() int { return len(s.addrs) }

	func (s *byRFC6724) Swap(i, j int) {
	s.addrs[i], s.addrs[j] = s.addrs[j], s.addrs[i]
	s.srcs[i], s.srcs[j] = s.srcs[j], s.srcs[i]
	s.addrAttr[i], s.addrAttr[j] = s.addrAttr[j], s.addrAttr[i]
	s.srcAttr[i], s.srcAttr[j] = s.srcAttr[j], s.srcAttr[i]
	}

	// Less reports whether i is a better destination address for this
	// host than j.
	//
	// The algorithm and variable names comes from RFC 6724 section 6.
	func (s *byRFC6724) Less(i, j int) bool {
	DA := s.addrs[i].IP
	DB := s.addrs[j].IP
	SourceDA := s.srcs[i]
	SourceDB := s.srcs[j]
	attrDA := &s.addrAttr[i]
	attrDB := &s.addrAttr[j]
	attrSourceDA := &s.srcAttr[i]
	attrSourceDB := &s.srcAttr[j]

	const preferDA = true
	const preferDB = false

	// Rule 1: Avoid unusable destinations.
	// If DB is known to be unreachable or if Source(DB) is undefined, then
	// prefer DA. Similarly, if DA is known to be unreachable or if
	// Source(DA) is undefined, then prefer DB.
	if SourceDA == nil && SourceDB == nil {
	return false // "equal"
	}
	if SourceDB == nil {
	return preferDA
	}
	if SourceDA == nil {
	return preferDB
	}

	// Rule 2: Prefer matching scope.
	// If Scope(DA) = Scope(Source(DA)) and Scope(DB) <> Scope(Source(DB)),
	// then prefer DA. Similarly, if Scope(DA) <> Scope(Source(DA)) and
	// Scope(DB) = Scope(Source(DB)), then prefer DB.
	if attrDA.Scope == attrSourceDA.Scope && attrDB.Scope != attrSourceDB.Scope {
	return preferDA
	}
	if attrDA.Scope != attrSourceDA.Scope && attrDB.Scope == attrSourceDB.Scope {
	return preferDB
	}

	// Rule 3: Avoid deprecated addresses.
	// If Source(DA) is deprecated and Source(DB) is not, then prefer DB.
	// Similarly, if Source(DA) is not deprecated and Source(DB) is
	// deprecated, then prefer DA.

	// TODO(bradfitz): implement? low priority for now.

	// Rule 4: Prefer home addresses.
	// If Source(DA) is simultaneously a home address and care-of address
	// and Source(DB) is not, then prefer DA. Similarly, if Source(DB) is
	// simultaneously a home address and care-of address and Source(DA) is
	// not, then prefer DB.

	// TODO(bradfitz): implement? low priority for now.

	// Rule 5: Prefer matching label.
	// If Label(Source(DA)) = Label(DA) and Label(Source(DB)) <> Label(DB),
	// then prefer DA. Similarly, if Label(Source(DA)) <> Label(DA) and
	// Label(Source(DB)) = Label(DB), then prefer DB.
	if attrSourceDA.Label == attrDA.Label &&
	attrSourceDB.Label != attrDB.Label {
	return preferDA
	}
	if attrSourceDA.Label != attrDA.Label &&
	attrSourceDB.Label == attrDB.Label {
	return preferDB
	}

	// Rule 6: Prefer higher precedence.
	// If Precedence(DA) > Precedence(DB), then prefer DA. Similarly, if
	// Precedence(DA) < Precedence(DB), then prefer DB.
	if attrDA.Precedence > attrDB.Precedence {
	return preferDA
	}
	if attrDA.Precedence < attrDB.Precedence {
	return preferDB
	}

	// Rule 7: Prefer native transport.
	// If DA is reached via an encapsulating transition mechanism (e.g.,
	// IPv6 in IPv4) and DB is not, then prefer DB. Similarly, if DB is
	// reached via encapsulation and DA is not, then prefer DA.

	// TODO(bradfitz): implement? low priority for now.

	// Rule 8: Prefer smaller scope.
	// If Scope(DA) < Scope(DB), then prefer DA. Similarly, if Scope(DA) >
	// Scope(DB), then prefer DB.
	if attrDA.Scope < attrDB.Scope {
	return preferDA
	}
	if attrDA.Scope > attrDB.Scope {
	return preferDB
	}

	// Rule 9: Use longest matching prefix.
	// When DA and DB belong to the same address family (both are IPv6 or
	// both are IPv4 [but see below]): If CommonPrefixLen(Source(DA), DA) >
	// CommonPrefixLen(Source(DB), DB), then prefer DA. Similarly, if
	// CommonPrefixLen(Source(DA), DA) < CommonPrefixLen(Source(DB), DB),
	// then prefer DB.
	//
	// However, applying this rule to IPv4 addresses causes
	// problems (see issues 13283 and 18518), so limit to IPv6.
	if DA.To4() == nil && DB.To4() == nil {
	commonA := commonPrefixLen(SourceDA, DA)
	commonB := commonPrefixLen(SourceDB, DB)

	if commonA > commonB {
	return preferDA
	}
	if commonA < commonB {
	return preferDB
	}
	}

	// Rule 10: Otherwise, leave the order unchanged.
	// If DA preceded DB in the original list, prefer DA.
	// Otherwise, prefer DB.
	return false // "equal"
	}

	type policyTableEntry struct {
	Prefix *IPNet
	Precedence uint8
	Label uint8
	}

	type policyTable []policyTableEntry

	// RFC 6724 section 2.1.
	var rfc6724policyTable = policyTable{
	{
	Prefix: mustCIDR("::1/128"),
	Precedence: 50,
	Label: 0,
	},
	{
	Prefix: mustCIDR("::/0"),
	Precedence: 40,
	Label: 1,
	},
	{
	// IPv4-compatible, etc.
	Prefix: mustCIDR("::ffff:0:0/96"),
	Precedence: 35,
	Label: 4,
	},
	{
	// 6to4
	Prefix: mustCIDR("2002::/16"),
	Precedence: 30,
	Label: 2,
	},
	{
	// Teredo
	Prefix: mustCIDR("2001::/32"),
	Precedence: 5,
	Label: 5,
	},
	{
	Prefix: mustCIDR("fc00::/7"),
	Precedence: 3,
	Label: 13,
	},
	{
	Prefix: mustCIDR("::/96"),
	Precedence: 1,
	Label: 3,
	},
	{
	Prefix: mustCIDR("fec0::/10"),
	Precedence: 1,
	Label: 11,
	},
	{
	Prefix: mustCIDR("3ffe::/16"),
	Precedence: 1,
	Label: 12,
	},
	}

	func init() {
	sort.Sort(sort.Reverse(byMaskLength(rfc6724policyTable)))
	}

	// byMaskLength sorts policyTableEntry by the size of their Prefix.Mask.Size,
	// from smallest mask, to largest.
	type byMaskLength []policyTableEntry

	func (s byMaskLength) Len() int { return len(s) }
	func (s byMaskLength) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
	func (s byMaskLength) Less(i, j int) bool {
	isize, _ := s[i].Prefix.Mask.Size()
	jsize, _ := s[j].Prefix.Mask.Size()
	return isize < jsize
	}

	// mustCIDR calls ParseCIDR and panics on any error, or if the network
	// is not IPv6.
	func mustCIDR(s string) *IPNet {
	ip, ipNet, err := ParseCIDR(s)
	if err != nil {
	panic(err.Error())
	}
	if len(ip) != IPv6len {
	panic("unexpected IP length")
	}
	return ipNet
	}

	// Classify returns the policyTableEntry of the entry with the longest
	// matching prefix that contains ip.
	// The table t must be sorted from largest mask size to smallest.
	func (t policyTable) Classify(ip IP) policyTableEntry {
	for _, ent := range t {
	if ent.Prefix.Contains(ip) {
	return ent
	}
	}
	return policyTableEntry{}
	}

	// RFC 6724 section 3.1.
	type scope uint8

	const (
	scopeInterfaceLocal scope = 0x1
	scopeLinkLocal scope = 0x2
	scopeAdminLocal scope = 0x4
	scopeSiteLocal scope = 0x5
	scopeOrgLocal scope = 0x8
	scopeGlobal scope = 0xe
	)

	func classifyScope(ip IP) scope {
	if ip.IsLoopback() \|\| ip.IsLinkLocalUnicast() {
	return scopeLinkLocal
	}
	ipv6 := len(ip) == IPv6len && ip.To4() == nil
	if ipv6 && ip.IsMulticast() {
	return scope(ip[1] & 0xf)
	}
	// Site-local addresses are defined in RFC 3513 section 2.5.6
	// (and deprecated in RFC 3879).
	if ipv6 && ip[0] == 0xfe && ip[1]&0xc0 == 0xc0 {
	return scopeSiteLocal
	}
	return scopeGlobal
	}

	// commonPrefixLen reports the length of the longest prefix (looking
	// at the most significant, or leftmost, bits) that the
	// two addresses have in common, up to the length of a's prefix (i.e.,
	// the portion of the address not including the interface ID).
	//
	// If a or b is an IPv4 address as an IPv6 address, the IPv4 addresses
	// are compared (with max common prefix length of 32).
	// If a and b are different IP versions, 0 is returned.
	//
	// See https://tools.ietf.org/html/rfc6724#section-2.2
	func commonPrefixLen(a, b IP) (cpl int) {
	if a4 := a.To4(); a4 != nil {
	a = a4
	}
	if b4 := b.To4(); b4 != nil {
	b = b4
	}
	if len(a) != len(b) {
	return 0
	}
	// If IPv6, only up to the prefix (first 64 bits)
	if len(a) > 8 {
	a = a[:8]
	b = b[:8]
	}
	for len(a) > 0 {
	if a[0] == b[0] {
	cpl += 8
	a = a[1:]
	b = b[1:]
	continue
	}
	bits := 8
	ab, bb := a[0], b[0]
	for {
	ab >>= 1
	bb >>= 1
	bits--
	if ab == bb {
	cpl += bits
	return
	}
	}
	}
	return
	}