protocol/packp: capabilities new Capability entity and List struct, test improvements (#144)

* protocol/pakp: capabilities new Capability entity and List struct, test improvements

* etc: example cloud-config file

* removing sorting from List.String

22 files changed, 819 insertions, 474 deletions

diff --git a/common_test.go b/common_test.go
index be86b4e..1e90409 100644
--- a/common_test.go
+++ b/common_test.go
@@ -10,6 +10,7 @@ import (
 	"gopkg.in/src-d/go-git.v4/plumbing"
 	"gopkg.in/src-d/go-git.v4/plumbing/format/packfile"
 	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 	"gopkg.in/src-d/go-git.v4/plumbing/transport"
 	"gopkg.in/src-d/go-git.v4/plumbing/transport/client"
 	"gopkg.in/src-d/go-git.v4/storage/filesystem"
@@ -97,8 +98,8 @@ func (c *MockFetchPackSession) AdvertisedReferences() (*packp.AdvRefs, error) {
 
 	h := fixtures.ByURL(c.endpoint.String()).One().Head
 
-	cap := packp.NewCapabilities()
-	cap.Decode("6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEADmulti_ack thin-pack side-band side-band-64k ofs-delta shallow no-progress include-tag multi_ack_detailed no-done symref=HEAD:refs/heads/master agent=git/2:2.4.8~dbussink-fix-enterprise-tokens-compilation-1167-gc7006cf")
+	cap := capability.NewList()
+	cap.Set(capability.Agent, "go-git/tests")
 
 	ref := plumbing.ReferenceName("refs/heads/master")
 	branch := plumbing.ReferenceName("refs/heads/branch")
diff --git a/plumbing/protocol/packp/advrefs.go b/plumbing/protocol/packp/advrefs.go
index b36b180..a0587ab 100644
--- a/plumbing/protocol/packp/advrefs.go
+++ b/plumbing/protocol/packp/advrefs.go
@@ -5,14 +5,11 @@ import (
 	"strings"
 
 	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 	"gopkg.in/src-d/go-git.v4/plumbing/storer"
 	"gopkg.in/src-d/go-git.v4/storage/memory"
 )
 
-const (
-	symref = "symref"
-)
-
 // AdvRefs values represent the information transmitted on an
 // advertised-refs message.  Values from this type are not zero-value
 // safe, use the New function instead.
@@ -31,7 +28,7 @@ const (
 type AdvRefs struct {
 	Prefix       [][]byte // payloads of the prefix
 	Head         *plumbing.Hash
-	Capabilities *Capabilities
+	Capabilities *capability.List
 	References   map[string]plumbing.Hash
 	Peeled       map[string]plumbing.Hash
 	Shallows     []plumbing.Hash
@@ -41,7 +38,7 @@ type AdvRefs struct {
 func NewAdvRefs() *AdvRefs {
 	return &AdvRefs{
 		Prefix:       [][]byte{},
-		Capabilities: NewCapabilities(),
+		Capabilities: capability.NewList(),
 		References:   make(map[string]plumbing.Hash),
 		Peeled:       make(map[string]plumbing.Hash),
 		Shallows:     []plumbing.Hash{},
@@ -52,7 +49,7 @@ func (a *AdvRefs) AddReference(r *plumbing.Reference) error {
 	switch r.Type() {
 	case plumbing.SymbolicReference:
 		v := fmt.Sprintf("%s:%s", r.Name().String(), r.Target().String())
-		a.Capabilities.Add(symref, v)
+		a.Capabilities.Add(capability.SymRef, v)
 	case plumbing.HashReference:
 		a.References[r.Name().String()] = r.Hash()
 	default:
@@ -87,7 +84,7 @@ func addSymbolicRefs(s storer.ReferenceStorer, ar *AdvRefs) error {
 		return nil
 	}
 
-	for _, symref := range ar.Capabilities.Get(symref).Values {
+	for _, symref := range ar.Capabilities.Get(capability.SymRef) {
 		chunks := strings.Split(symref, ":")
 		if len(chunks) != 2 {
 			err := fmt.Errorf("bad number of `:` in symref value (%q)", symref)
@@ -105,5 +102,5 @@ func addSymbolicRefs(s storer.ReferenceStorer, ar *AdvRefs) error {
 }
 
 func hasSymrefs(ar *AdvRefs) bool {
-	return ar.Capabilities.Supports(symref)
+	return ar.Capabilities.Supports(capability.SymRef)
 }
diff --git a/plumbing/protocol/packp/advrefs_decode.go b/plumbing/protocol/packp/advrefs_decode.go
index 8d37066..696bbae 100644
--- a/plumbing/protocol/packp/advrefs_decode.go
+++ b/plumbing/protocol/packp/advrefs_decode.go
@@ -201,9 +201,8 @@ func decodeCaps(p *advRefsDecoder) decoderStateFn {
 		return decodeOtherRefs
 	}
 
-	for _, c := range bytes.Split(p.line, sp) {
-		name, values := readCapability(c)
-		p.data.Capabilities.Add(name, values...)
+	if err := p.data.Capabilities.Decode(p.line); err != nil {
+		p.error("invalid capabilities: %s", err)
 	}
 
 	return decodeOtherRefs
diff --git a/plumbing/protocol/packp/advrefs_decode_test.go b/plumbing/protocol/packp/advrefs_decode_test.go
index 2b3da72..82a05a4 100644
--- a/plumbing/protocol/packp/advrefs_decode_test.go
+++ b/plumbing/protocol/packp/advrefs_decode_test.go
@@ -7,6 +7,7 @@ import (
 
 	"gopkg.in/src-d/go-git.v4/plumbing"
 	"gopkg.in/src-d/go-git.v4/plumbing/format/pktline"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 
 	. "gopkg.in/check.v1"
 )
@@ -176,82 +177,70 @@ func (s *AdvRefsDecodeSuite) TestNoCaps(c *C) {
 }
 
 func (s *AdvRefsDecodeSuite) TestCaps(c *C) {
+	type entry struct {
+		Name   capability.Capability
+		Values []string
+	}
+
 	for _, test := range [...]struct {
 		input        []string
-		capabilities []Capability
-	}{
-		{
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00",
-				pktline.FlushString,
-			},
-			capabilities: []Capability{},
+		capabilities []entry
+	}{{
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00",
+			pktline.FlushString,
 		},
-		{
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00\n",
-				pktline.FlushString,
-			},
-			capabilities: []Capability{},
+		capabilities: []entry{},
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00\n",
+			pktline.FlushString,
 		},
-		{
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta",
-				pktline.FlushString,
-			},
-			capabilities: []Capability{
-				{
-					Name:   "ofs-delta",
-					Values: []string(nil),
-				},
-			},
+		capabilities: []entry{},
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta",
+			pktline.FlushString,
 		},
-		{
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta multi_ack",
-				pktline.FlushString,
-			},
-			capabilities: []Capability{
-				{Name: "ofs-delta", Values: []string(nil)},
-				{Name: "multi_ack", Values: []string(nil)},
+		capabilities: []entry{
+			{
+				Name:   capability.OFSDelta,
+				Values: []string(nil),
 			},
 		},
-		{
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta multi_ack\n",
-				pktline.FlushString,
-			},
-			capabilities: []Capability{
-				{Name: "ofs-delta", Values: []string(nil)},
-				{Name: "multi_ack", Values: []string(nil)},
-			},
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta multi_ack",
+			pktline.FlushString,
 		},
-		{
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:refs/heads/master agent=foo=bar\n",
-				pktline.FlushString,
-			},
-			capabilities: []Capability{
-				{Name: "symref", Values: []string{"HEAD:refs/heads/master"}},
-				{Name: "agent", Values: []string{"foo=bar"}},
-			},
+		capabilities: []entry{
+			{Name: capability.OFSDelta, Values: []string(nil)},
+			{Name: capability.MultiACK, Values: []string(nil)},
 		},
-		{
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:refs/heads/master agent=foo=bar agent=new-agent\n",
-				pktline.FlushString,
-			},
-			capabilities: []Capability{
-				{Name: "symref", Values: []string{"HEAD:refs/heads/master"}},
-				{Name: "agent", Values: []string{"foo=bar", "new-agent"}},
-			},
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta multi_ack\n",
+			pktline.FlushString,
 		},
-	} {
+		capabilities: []entry{
+			{Name: capability.OFSDelta, Values: []string(nil)},
+			{Name: capability.MultiACK, Values: []string(nil)},
+		},
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:refs/heads/master agent=foo=bar\n",
+			pktline.FlushString,
+		},
+		capabilities: []entry{
+			{Name: capability.SymRef, Values: []string{"HEAD:refs/heads/master"}},
+			{Name: capability.Agent, Values: []string{"foo=bar"}},
+		},
+	}} {
 		ar := s.testDecodeOK(c, test.input)
 		for _, fixCap := range test.capabilities {
 			c.Assert(ar.Capabilities.Supports(fixCap.Name), Equals, true,
 				Commentf("input = %q, capability = %q", test.input, fixCap.Name))
-			c.Assert(ar.Capabilities.Get(fixCap.Name).Values, DeepEquals, fixCap.Values,
+			c.Assert(ar.Capabilities.Get(fixCap.Name), DeepEquals, fixCap.Values,
 				Commentf("input = %q, capability = %q", test.input, fixCap.Name))
 		}
 	}
@@ -260,7 +249,7 @@ func (s *AdvRefsDecodeSuite) TestCaps(c *C) {
 func (s *AdvRefsDecodeSuite) TestWithPrefix(c *C) {
 	payloads := []string{
 		"# this is a prefix\n",
-		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00foo\n",
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta\n",
 		pktline.FlushString,
 	}
 	ar := s.testDecodeOK(c, payloads)
@@ -272,7 +261,7 @@ func (s *AdvRefsDecodeSuite) TestWithPrefixAndFlush(c *C) {
 	payloads := []string{
 		"# this is a prefix\n",
 		pktline.FlushString,
-		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00foo\n",
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta\n",
 		pktline.FlushString,
 	}
 	ar := s.testDecodeOK(c, payloads)
@@ -286,98 +275,96 @@ func (s *AdvRefsDecodeSuite) TestOtherRefs(c *C) {
 		input      []string
 		references map[string]plumbing.Hash
 		peeled     map[string]plumbing.Hash
-	}{
-		{
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
-				pktline.FlushString,
-			},
-			references: make(map[string]plumbing.Hash),
-			peeled:     make(map[string]plumbing.Hash),
-		}, {
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
-				"1111111111111111111111111111111111111111 ref/foo",
-				pktline.FlushString,
-			},
-			references: map[string]plumbing.Hash{
-				"ref/foo": plumbing.NewHash("1111111111111111111111111111111111111111"),
-			},
-			peeled: make(map[string]plumbing.Hash),
-		}, {
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
-				"1111111111111111111111111111111111111111 ref/foo\n",
-				pktline.FlushString,
-			},
-			references: map[string]plumbing.Hash{
-				"ref/foo": plumbing.NewHash("1111111111111111111111111111111111111111"),
-			},
-			peeled: make(map[string]plumbing.Hash),
-		}, {
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
-				"1111111111111111111111111111111111111111 ref/foo\n",
-				"2222222222222222222222222222222222222222 ref/bar",
-				pktline.FlushString,
-			},
-			references: map[string]plumbing.Hash{
-				"ref/foo": plumbing.NewHash("1111111111111111111111111111111111111111"),
-				"ref/bar": plumbing.NewHash("2222222222222222222222222222222222222222"),
-			},
-			peeled: make(map[string]plumbing.Hash),
-		}, {
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
-				"1111111111111111111111111111111111111111 ref/foo^{}\n",
-				pktline.FlushString,
-			},
-			references: make(map[string]plumbing.Hash),
-			peeled: map[string]plumbing.Hash{
-				"ref/foo": plumbing.NewHash("1111111111111111111111111111111111111111"),
-			},
-		}, {
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
-				"1111111111111111111111111111111111111111 ref/foo\n",
-				"2222222222222222222222222222222222222222 ref/bar^{}",
-				pktline.FlushString,
-			},
-			references: map[string]plumbing.Hash{
-				"ref/foo": plumbing.NewHash("1111111111111111111111111111111111111111"),
-			},
-			peeled: map[string]plumbing.Hash{
-				"ref/bar": plumbing.NewHash("2222222222222222222222222222222222222222"),
-			},
-		}, {
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
-				"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
-				"51b8b4fb32271d39fbdd760397406177b2b0fd36 refs/pull/10/head\n",
-				"02b5a6031ba7a8cbfde5d65ff9e13ecdbc4a92ca refs/pull/100/head\n",
-				"c284c212704c43659bf5913656b8b28e32da1621 refs/pull/100/merge\n",
-				"3d6537dce68c8b7874333a1720958bd8db3ae8ca refs/pull/101/merge\n",
-				"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11\n",
-				"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11^{}\n",
-				"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
-				"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
-				pktline.FlushString,
-			},
-			references: map[string]plumbing.Hash{
-				"refs/heads/master":      plumbing.NewHash("a6930aaee06755d1bdcfd943fbf614e4d92bb0c7"),
-				"refs/pull/10/head":      plumbing.NewHash("51b8b4fb32271d39fbdd760397406177b2b0fd36"),
-				"refs/pull/100/head":     plumbing.NewHash("02b5a6031ba7a8cbfde5d65ff9e13ecdbc4a92ca"),
-				"refs/pull/100/merge":    plumbing.NewHash("c284c212704c43659bf5913656b8b28e32da1621"),
-				"refs/pull/101/merge":    plumbing.NewHash("3d6537dce68c8b7874333a1720958bd8db3ae8ca"),
-				"refs/tags/v2.6.11":      plumbing.NewHash("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c"),
-				"refs/tags/v2.6.11-tree": plumbing.NewHash("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c"),
-			},
-			peeled: map[string]plumbing.Hash{
-				"refs/tags/v2.6.11":      plumbing.NewHash("c39ae07f393806ccf406ef966e9a15afc43cc36a"),
-				"refs/tags/v2.6.11-tree": plumbing.NewHash("c39ae07f393806ccf406ef966e9a15afc43cc36a"),
-			},
+	}{{
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+			pktline.FlushString,
+		},
+		references: make(map[string]plumbing.Hash),
+		peeled:     make(map[string]plumbing.Hash),
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+			"1111111111111111111111111111111111111111 ref/foo",
+			pktline.FlushString,
+		},
+		references: map[string]plumbing.Hash{
+			"ref/foo": plumbing.NewHash("1111111111111111111111111111111111111111"),
+		},
+		peeled: make(map[string]plumbing.Hash),
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+			"1111111111111111111111111111111111111111 ref/foo\n",
+			pktline.FlushString,
+		},
+		references: map[string]plumbing.Hash{
+			"ref/foo": plumbing.NewHash("1111111111111111111111111111111111111111"),
+		},
+		peeled: make(map[string]plumbing.Hash),
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+			"1111111111111111111111111111111111111111 ref/foo\n",
+			"2222222222222222222222222222222222222222 ref/bar",
+			pktline.FlushString,
+		},
+		references: map[string]plumbing.Hash{
+			"ref/foo": plumbing.NewHash("1111111111111111111111111111111111111111"),
+			"ref/bar": plumbing.NewHash("2222222222222222222222222222222222222222"),
+		},
+		peeled: make(map[string]plumbing.Hash),
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+			"1111111111111111111111111111111111111111 ref/foo^{}\n",
+			pktline.FlushString,
+		},
+		references: make(map[string]plumbing.Hash),
+		peeled: map[string]plumbing.Hash{
+			"ref/foo": plumbing.NewHash("1111111111111111111111111111111111111111"),
 		},
-	} {
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+			"1111111111111111111111111111111111111111 ref/foo\n",
+			"2222222222222222222222222222222222222222 ref/bar^{}",
+			pktline.FlushString,
+		},
+		references: map[string]plumbing.Hash{
+			"ref/foo": plumbing.NewHash("1111111111111111111111111111111111111111"),
+		},
+		peeled: map[string]plumbing.Hash{
+			"ref/bar": plumbing.NewHash("2222222222222222222222222222222222222222"),
+		},
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+			"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+			"51b8b4fb32271d39fbdd760397406177b2b0fd36 refs/pull/10/head\n",
+			"02b5a6031ba7a8cbfde5d65ff9e13ecdbc4a92ca refs/pull/100/head\n",
+			"c284c212704c43659bf5913656b8b28e32da1621 refs/pull/100/merge\n",
+			"3d6537dce68c8b7874333a1720958bd8db3ae8ca refs/pull/101/merge\n",
+			"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11\n",
+			"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11^{}\n",
+			"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+			"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+			pktline.FlushString,
+		},
+		references: map[string]plumbing.Hash{
+			"refs/heads/master":      plumbing.NewHash("a6930aaee06755d1bdcfd943fbf614e4d92bb0c7"),
+			"refs/pull/10/head":      plumbing.NewHash("51b8b4fb32271d39fbdd760397406177b2b0fd36"),
+			"refs/pull/100/head":     plumbing.NewHash("02b5a6031ba7a8cbfde5d65ff9e13ecdbc4a92ca"),
+			"refs/pull/100/merge":    plumbing.NewHash("c284c212704c43659bf5913656b8b28e32da1621"),
+			"refs/pull/101/merge":    plumbing.NewHash("3d6537dce68c8b7874333a1720958bd8db3ae8ca"),
+			"refs/tags/v2.6.11":      plumbing.NewHash("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c"),
+			"refs/tags/v2.6.11-tree": plumbing.NewHash("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c"),
+		},
+		peeled: map[string]plumbing.Hash{
+			"refs/tags/v2.6.11":      plumbing.NewHash("c39ae07f393806ccf406ef966e9a15afc43cc36a"),
+			"refs/tags/v2.6.11-tree": plumbing.NewHash("c39ae07f393806ccf406ef966e9a15afc43cc36a"),
+		},
+	}} {
 		ar := s.testDecodeOK(c, test.input)
 		comment := Commentf("input = %v\n", test.input)
 		c.Assert(ar.References, DeepEquals, test.references, comment)
@@ -409,42 +396,40 @@ func (s *AdvRefsDecodeSuite) TestShallow(c *C) {
 	for _, test := range [...]struct {
 		input    []string
 		shallows []plumbing.Hash
-	}{
-		{
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
-				"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
-				"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
-				"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
-				pktline.FlushString,
-			},
-			shallows: []plumbing.Hash{},
-		}, {
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
-				"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
-				"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
-				"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
-				"shallow 1111111111111111111111111111111111111111\n",
-				pktline.FlushString,
-			},
-			shallows: []plumbing.Hash{plumbing.NewHash("1111111111111111111111111111111111111111")},
-		}, {
-			input: []string{
-				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
-				"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
-				"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
-				"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
-				"shallow 1111111111111111111111111111111111111111\n",
-				"shallow 2222222222222222222222222222222222222222\n",
-				pktline.FlushString,
-			},
-			shallows: []plumbing.Hash{
-				plumbing.NewHash("1111111111111111111111111111111111111111"),
-				plumbing.NewHash("2222222222222222222222222222222222222222"),
-			},
+	}{{
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+			"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+			"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+			"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+			pktline.FlushString,
+		},
+		shallows: []plumbing.Hash{},
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+			"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+			"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+			"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+			"shallow 1111111111111111111111111111111111111111\n",
+			pktline.FlushString,
+		},
+		shallows: []plumbing.Hash{plumbing.NewHash("1111111111111111111111111111111111111111")},
+	}, {
+		input: []string{
+			"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+			"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+			"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+			"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+			"shallow 1111111111111111111111111111111111111111\n",
+			"shallow 2222222222222222222222222222222222222222\n",
+			pktline.FlushString,
+		},
+		shallows: []plumbing.Hash{
+			plumbing.NewHash("1111111111111111111111111111111111111111"),
+			plumbing.NewHash("2222222222222222222222222222222222222222"),
 		},
-	} {
+	}} {
 		ar := s.testDecodeOK(c, test.input)
 		comment := Commentf("input = %v\n", test.input)
 		c.Assert(ar.Shallows, DeepEquals, test.shallows, comment)
diff --git a/plumbing/protocol/packp/advrefs_encode.go b/plumbing/protocol/packp/advrefs_encode.go
index 3c5df19..05a9c8e 100644
--- a/plumbing/protocol/packp/advrefs_encode.go
+++ b/plumbing/protocol/packp/advrefs_encode.go
@@ -7,6 +7,7 @@ import (
 
 	"gopkg.in/src-d/go-git.v4/plumbing"
 	"gopkg.in/src-d/go-git.v4/plumbing/format/pktline"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 )
 
 // Encode writes the AdvRefs encoding to a writer.
@@ -89,13 +90,11 @@ func formatSeparator(h *plumbing.Hash) string {
 	return head
 }
 
-func formatCaps(c *Capabilities) string {
+func formatCaps(c *capability.List) string {
 	if c == nil {
 		return ""
 	}
 
-	c.Sort()
-
 	return c.String()
 }
 
diff --git a/plumbing/protocol/packp/advrefs_encode_test.go b/plumbing/protocol/packp/advrefs_encode_test.go
index 207ce58..f8cc815 100644
--- a/plumbing/protocol/packp/advrefs_encode_test.go
+++ b/plumbing/protocol/packp/advrefs_encode_test.go
@@ -6,6 +6,7 @@ import (
 
 	"gopkg.in/src-d/go-git.v4/plumbing"
 	"gopkg.in/src-d/go-git.v4/plumbing/format/pktline"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 
 	. "gopkg.in/check.v1"
 )
@@ -52,10 +53,10 @@ func (s *AdvRefsEncodeSuite) TestHead(c *C) {
 }
 
 func (s *AdvRefsEncodeSuite) TestCapsNoHead(c *C) {
-	capabilities := NewCapabilities()
-	capabilities.Add("symref", "HEAD:/refs/heads/master")
-	capabilities.Add("ofs-delta")
-	capabilities.Add("multi_ack")
+	capabilities := capability.NewList()
+	capabilities.Add(capability.MultiACK)
+	capabilities.Add(capability.OFSDelta)
+	capabilities.Add(capability.SymRef, "HEAD:/refs/heads/master")
 	ar := &AdvRefs{
 		Capabilities: capabilities,
 	}
@@ -70,10 +71,10 @@ func (s *AdvRefsEncodeSuite) TestCapsNoHead(c *C) {
 
 func (s *AdvRefsEncodeSuite) TestCapsWithHead(c *C) {
 	hash := plumbing.NewHash("6ecf0ef2c2dffb796033e5a02219af86ec6584e5")
-	capabilities := NewCapabilities()
-	capabilities.Add("symref", "HEAD:/refs/heads/master")
-	capabilities.Add("ofs-delta")
-	capabilities.Add("multi_ack")
+	capabilities := capability.NewList()
+	capabilities.Add(capability.MultiACK)
+	capabilities.Add(capability.OFSDelta)
+	capabilities.Add(capability.SymRef, "HEAD:/refs/heads/master")
 	ar := &AdvRefs{
 		Head:         &hash,
 		Capabilities: capabilities,
@@ -170,10 +171,10 @@ func (s *AdvRefsEncodeSuite) TestShallow(c *C) {
 func (s *AdvRefsEncodeSuite) TestAll(c *C) {
 	hash := plumbing.NewHash("6ecf0ef2c2dffb796033e5a02219af86ec6584e5")
 
-	capabilities := NewCapabilities()
-	capabilities.Add("symref", "HEAD:/refs/heads/master")
-	capabilities.Add("ofs-delta")
-	capabilities.Add("multi_ack")
+	capabilities := capability.NewList()
+	capabilities.Add(capability.MultiACK)
+	capabilities.Add(capability.OFSDelta)
+	capabilities.Add(capability.SymRef, "HEAD:/refs/heads/master")
 
 	references := map[string]plumbing.Hash{
 		"refs/heads/master":      plumbing.NewHash("a6930aaee06755d1bdcfd943fbf614e4d92bb0c7"),
diff --git a/plumbing/protocol/packp/advrefs_test.go b/plumbing/protocol/packp/advrefs_test.go
index 6d9f488..1689938 100644
--- a/plumbing/protocol/packp/advrefs_test.go
+++ b/plumbing/protocol/packp/advrefs_test.go
@@ -10,6 +10,7 @@ import (
 	"gopkg.in/src-d/go-git.v4/plumbing"
 	"gopkg.in/src-d/go-git.v4/plumbing/format/pktline"
 	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 
 	. "gopkg.in/check.v1"
 )
@@ -52,9 +53,7 @@ func (s *SuiteDecodeEncode) test(c *C, in []string, exp []string) {
 		obtained = buf.Bytes()
 	}
 
-	c.Assert(obtained, DeepEquals, expected,
-		Commentf("input = %v\nobtained = %q\nexpected = %q\n",
-			in, string(obtained), string(expected)))
+	c.Assert(string(obtained), DeepEquals, string(expected))
 }
 
 func (s *SuiteDecodeEncode) TestNoHead(c *C) {
@@ -115,7 +114,7 @@ func (s *SuiteDecodeEncode) TestRefs(c *C) {
 	}
 
 	expected := []string{
-		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n",
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:/refs/heads/master ofs-delta multi_ack\n",
 		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
 		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
 		"7777777777777777777777777777777777777777 refs/tags/v2.6.12-tree\n",
@@ -137,7 +136,7 @@ func (s *SuiteDecodeEncode) TestPeeled(c *C) {
 	}
 
 	expected := []string{
-		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n",
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:/refs/heads/master ofs-delta multi_ack\n",
 		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
 		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
 		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
@@ -163,7 +162,7 @@ func (s *SuiteDecodeEncode) TestAll(c *C) {
 	}
 
 	expected := []string{
-		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n",
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:/refs/heads/master ofs-delta multi_ack\n",
 		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
 		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
 		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
@@ -195,7 +194,7 @@ func (s *SuiteDecodeEncode) TestAllSmart(c *C) {
 	expected := []string{
 		"# service=git-upload-pack\n",
 		pktline.FlushString,
-		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n",
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:/refs/heads/master ofs-delta multi_ack\n",
 		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
 		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
 		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
@@ -227,7 +226,7 @@ func (s *SuiteDecodeEncode) TestAllSmartBug(c *C) {
 	expected := []string{
 		"# service=git-upload-pack\n",
 		pktline.FlushString,
-		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n",
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:/refs/heads/master ofs-delta multi_ack\n",
 		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
 		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
 		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
@@ -278,9 +277,9 @@ func ExampleEncoder_Encode() {
 	ar.Head = &head
 
 	// ...add some server capabilities...
-	ar.Capabilities.Add("symref", "HEAD:/refs/heads/master")
-	ar.Capabilities.Add("ofs-delta")
-	ar.Capabilities.Add("multi_ack")
+	ar.Capabilities.Add(capability.MultiACK)
+	ar.Capabilities.Add(capability.OFSDelta)
+	ar.Capabilities.Add(capability.SymRef, "HEAD:/refs/heads/master")
 
 	// ...add a couple of references...
 	ar.References["refs/heads/master"] = plumbing.NewHash("2222222222222222222222222222222222222222")
diff --git a/plumbing/protocol/packp/capabilities.go b/plumbing/protocol/packp/capabilities.go
deleted file mode 100644
index d77c2fa..0000000
--- a/plumbing/protocol/packp/capabilities.go
+++ /dev/null
@@ -1,136 +0,0 @@
-package packp
-
-import (
-	"fmt"
-	"sort"
-	"strings"
-)
-
-// Capabilities contains all the server capabilities
-// https://github.com/git/git/blob/master/Documentation/technical/protocol-capabilities.txt
-type Capabilities struct {
-	m map[string]*Capability
-	o []string
-}
-
-// Capability represents a server capability
-type Capability struct {
-	Name   string
-	Values []string
-}
-
-// NewCapabilities returns a new Capabilities struct
-func NewCapabilities() *Capabilities {
-	return &Capabilities{
-		m: make(map[string]*Capability),
-	}
-}
-
-func (c *Capabilities) IsEmpty() bool {
-	return len(c.o) == 0
-}
-
-// Decode decodes a string
-func (c *Capabilities) Decode(raw string) {
-	params := strings.Split(raw, " ")
-	for _, p := range params {
-		s := strings.SplitN(p, "=", 2)
-
-		var value string
-		if len(s) == 2 {
-			value = s[1]
-		}
-
-		c.Add(s[0], value)
-	}
-}
-
-// Get returns the values for a capability
-func (c *Capabilities) Get(capability string) *Capability {
-	return c.m[capability]
-}
-
-// Set sets a capability removing the values
-func (c *Capabilities) Set(capability string, values ...string) {
-	if _, ok := c.m[capability]; ok {
-		delete(c.m, capability)
-	}
-
-	c.Add(capability, values...)
-}
-
-// Add adds a capability, values are optional
-func (c *Capabilities) Add(capability string, values ...string) {
-	if !c.Supports(capability) {
-		c.m[capability] = &Capability{Name: capability}
-		c.o = append(c.o, capability)
-	}
-
-	if len(values) == 0 {
-		return
-	}
-
-	c.m[capability].Values = append(c.m[capability].Values, values...)
-}
-
-// Supports returns true if capability is present
-func (c *Capabilities) Supports(capability string) bool {
-	_, ok := c.m[capability]
-	return ok
-}
-
-// SymbolicReference returns the reference for a given symbolic reference
-func (c *Capabilities) SymbolicReference(sym string) string {
-	if !c.Supports("symref") {
-		return ""
-	}
-
-	for _, symref := range c.Get("symref").Values {
-		parts := strings.Split(symref, ":")
-		if len(parts) != 2 {
-			continue
-		}
-
-		if parts[0] == sym {
-			return parts[1]
-		}
-	}
-
-	return ""
-}
-
-// Sorts capabilities in increasing order of their name
-func (c *Capabilities) Sort() {
-	sort.Strings(c.o)
-}
-
-func (c *Capabilities) String() string {
-	if len(c.o) == 0 {
-		return ""
-	}
-
-	var o string
-	for _, key := range c.o {
-		cap := c.m[key]
-
-		added := false
-		for _, value := range cap.Values {
-			if value == "" {
-				continue
-			}
-
-			added = true
-			o += fmt.Sprintf("%s=%s ", key, value)
-		}
-
-		if len(cap.Values) == 0 || !added {
-			o += key + " "
-		}
-	}
-
-	if len(o) == 0 {
-		return o
-	}
-
-	return o[:len(o)-1]
-}
diff --git a/plumbing/protocol/packp/capabilities_test.go b/plumbing/protocol/packp/capabilities_test.go
deleted file mode 100644
index a909e4c..0000000
--- a/plumbing/protocol/packp/capabilities_test.go
+++ /dev/null
@@ -1,42 +0,0 @@
-package packp
-
-import (
-	. "gopkg.in/check.v1"
-)
-
-type SuiteCapabilities struct{}
-
-var _ = Suite(&SuiteCapabilities{})
-
-func (s *SuiteCapabilities) TestDecode(c *C) {
-	cap := NewCapabilities()
-	cap.Decode("symref=foo symref=qux thin-pack")
-
-	c.Assert(cap.m, HasLen, 2)
-	c.Assert(cap.Get("symref").Values, DeepEquals, []string{"foo", "qux"})
-	c.Assert(cap.Get("thin-pack").Values, DeepEquals, []string{""})
-}
-
-func (s *SuiteCapabilities) TestSet(c *C) {
-	cap := NewCapabilities()
-	cap.Add("symref", "foo", "qux")
-	cap.Set("symref", "bar")
-
-	c.Assert(cap.m, HasLen, 1)
-	c.Assert(cap.Get("symref").Values, DeepEquals, []string{"bar"})
-}
-
-func (s *SuiteCapabilities) TestSetEmpty(c *C) {
-	cap := NewCapabilities()
-	cap.Set("foo", "bar")
-
-	c.Assert(cap.Get("foo").Values, HasLen, 1)
-}
-
-func (s *SuiteCapabilities) TestAdd(c *C) {
-	cap := NewCapabilities()
-	cap.Add("symref", "foo", "qux")
-	cap.Add("thin-pack")
-
-	c.Assert(cap.String(), Equals, "symref=foo symref=qux thin-pack")
-}
diff --git a/plumbing/protocol/packp/capability/capability.go b/plumbing/protocol/packp/capability/capability.go
new file mode 100644
index 0000000..06fbfca
--- /dev/null
+++ b/plumbing/protocol/packp/capability/capability.go
@@ -0,0 +1,249 @@
+package capability
+
+// Capability describes a server or client capability
+type Capability string
+
+func (n Capability) String() string {
+	return string(n)
+}
+
+const (
+	// MultiACK capability allows the server to return "ACK obj-id continue" as
+	// soon as it finds a commit that it can use as a common base, between the
+	// client's wants and the client's have set.
+	//
+	// By sending this early, the server can potentially head off the client
+	// from walking any further down that particular branch of the client's
+	// repository history.  The client may still need to walk down other
+	// branches, sending have lines for those, until the server has a
+	// complete cut across the DAG, or the client has said "done".
+	//
+	// Without multi_ack, a client sends have lines in --date-order until
+	// the server has found a common base.  That means the client will send
+	// have lines that are already known by the server to be common, because
+	// they overlap in time with another branch that the server hasn't found
+	// a common base on yet.
+	//
+	// For example suppose the client has commits in caps that the server
+	// doesn't and the server has commits in lower case that the client
+	// doesn't, as in the following diagram:
+	//
+	//        +---- u ---------------------- x
+	//       /              +----- y
+	//      /              /
+	//     a -- b -- c -- d -- E -- F
+	//        \
+	// 	+--- Q -- R -- S
+	//
+	// If the client wants x,y and starts out by saying have F,S, the server
+	// doesn't know what F,S is.  Eventually the client says "have d" and
+	// the server sends "ACK d continue" to let the client know to stop
+	// walking down that line (so don't send c-b-a), but it's not done yet,
+	// it needs a base for x. The client keeps going with S-R-Q, until a
+	// gets reached, at which point the server has a clear base and it all
+	// ends.
+	//
+	// Without multi_ack the client would have sent that c-b-a chain anyway,
+	// interleaved with S-R-Q.
+	MultiACK Capability = "multi_ack"
+	// MultiACKDetailed is an extension of multi_ack that permits client to
+	// better understand the server's in-memory state.
+	MultiACKDetailed Capability = "multi_ack_detailed"
+	// NoDone should only be used with the smart HTTP protocol. If
+	// multi_ack_detailed and no-done are both present, then the sender is
+	// free to immediately send a pack following its first "ACK obj-id ready"
+	// message.
+	//
+	// Without no-done in the smart HTTP protocol, the server session would
+	// end and the client has to make another trip to send "done" before
+	// the server can send the pack. no-done removes the last round and
+	// thus slightly reduces latency.
+	NoDone Capability = "no-done"
+	// ThinPack is one with deltas which reference base objects not
+	// contained within the pack (but are known to exist at the receiving
+	// end). This can reduce the network traffic significantly, but it
+	// requires the receiving end to know how to "thicken" these packs by
+	// adding the missing bases to the pack.
+	//
+	// The upload-pack server advertises 'thin-pack' when it can generate
+	// and send a thin pack. A client requests the 'thin-pack' capability
+	// when it understands how to "thicken" it, notifying the server that
+	// it can receive such a pack. A client MUST NOT request the
+	// 'thin-pack' capability if it cannot turn a thin pack into a
+	// self-contained pack.
+	//
+	// Receive-pack, on the other hand, is assumed by default to be able to
+	// handle thin packs, but can ask the client not to use the feature by
+	// advertising the 'no-thin' capability. A client MUST NOT send a thin
+	// pack if the server advertises the 'no-thin' capability.
+	//
+	// The reasons for this asymmetry are historical. The receive-pack
+	// program did not exist until after the invention of thin packs, so
+	// historically the reference implementation of receive-pack always
+	// understood thin packs. Adding 'no-thin' later allowed receive-pack
+	// to disable the feature in a backwards-compatible manner.
+	ThinPack Capability = "thin-pack"
+	// Sideband means that server can send, and client understand multiplexed
+	// progress reports and error info interleaved with the packfile itself.
+	//
+	// These two options are mutually exclusive. A modern client always
+	// favors Sideband64k.
+	//
+	// Either mode indicates that the packfile data will be streamed broken
+	// up into packets of up to either 1000 bytes in the case of 'side_band',
+	// or 65520 bytes in the case of 'side_band_64k'. Each packet is made up
+	// of a leading 4-byte pkt-line length of how much data is in the packet,
+	// followed by a 1-byte stream code, followed by the actual data.
+	//
+	// The stream code can be one of:
+	//
+	//  1 - pack data
+	//  2 - progress messages
+	//  3 - fatal error message just before stream aborts
+	//
+	// The "side-band-64k" capability came about as a way for newer clients
+	// that can handle much larger packets to request packets that are
+	// actually crammed nearly full, while maintaining backward compatibility
+	// for the older clients.
+	//
+	// Further, with side-band and its up to 1000-byte messages, it's actually
+	// 999 bytes of payload and 1 byte for the stream code. With side-band-64k,
+	// same deal, you have up to 65519 bytes of data and 1 byte for the stream
+	// code.
+	//
+	// The client MUST send only maximum of one of "side-band" and "side-
+	// band-64k".  Server MUST diagnose it as an error if client requests
+	// both.
+	Sideband    Capability = "side-band"
+	Sideband64k Capability = "side-band-64k"
+	// OFSDelta server can send, and client understand PACKv2 with delta
+	// referring to its base by position in pack rather than by an obj-id. That
+	// is, they can send/read OBJ_OFS_DELTA (aka type 6) in a packfile.
+	OFSDelta Capability = "ofs-delta"
+	// Agent the server may optionally send this capability to notify the client
+	// that the server is running version `X`. The client may optionally return
+	// its own agent string by responding with an `agent=Y` capability (but it
+	// MUST NOT do so if the server did not mention the agent capability). The
+	// `X` and `Y` strings may contain any printable ASCII characters except
+	// space (i.e., the byte range 32 < x < 127), and are typically of the form
+	// "package/version" (e.g., "git/1.8.3.1"). The agent strings are purely
+	// informative for statistics and debugging purposes, and MUST NOT be used
+	// to programmatically assume the presence or absence of particular features.
+	Agent Capability = "agent"
+	// Shallow capability adds "deepen", "shallow" and "unshallow" commands to
+	// the  fetch-pack/upload-pack protocol so clients can request shallow
+	// clones.
+	Shallow Capability = "shallow"
+	// DeepenSince adds "deepen-since" command to fetch-pack/upload-pack
+	// protocol so the client can request shallow clones that are cut at a
+	// specific time, instead of depth. Internally it's equivalent of doing
+	// "rev-list --max-age=<timestamp>" on the server side. "deepen-since"
+	// cannot be used with "deepen".
+	DeepenSince Capability = "deepen-since"
+	// DeepenNot adds "deepen-not" command to fetch-pack/upload-pack
+	// protocol so the client can request shallow clones that are cut at a
+	// specific revision, instead of depth. Internally it's equivalent of
+	// doing "rev-list --not <rev>" on the server side. "deepen-not"
+	// cannot be used with "deepen", but can be used with "deepen-since".
+	DeepenNot Capability = "deepen-not"
+	// DeepenRelative if this capability is requested by the client, the
+	// semantics of "deepen" command is changed. The "depth" argument is the
+	// depth from the current shallow boundary, instead of the depth from
+	// remote refs.
+	DeepenRelative Capability = "deepen-relative"
+	// NoProgress the client was started with "git clone -q" or something, and
+	// doesn't want that side band 2. Basically the client just says "I do not
+	// wish to receive stream 2 on sideband, so do not send it to me, and if
+	// you did, I will drop it on the floor anyway".  However, the sideband
+	// channel 3 is still used for error responses.
+	NoProgress Capability = "no-progress"
+	// IncludeTag capability is about sending annotated tags if we are
+	// sending objects they point to.  If we pack an object to the client, and
+	// a tag object points exactly at that object, we pack the tag object too.
+	// In general this allows a client to get all new annotated tags when it
+	// fetches a branch, in a single network connection.
+	//
+	// Clients MAY always send include-tag, hardcoding it into a request when
+	// the server advertises this capability. The decision for a client to
+	// request include-tag only has to do with the client's desires for tag
+	// data, whether or not a server had advertised objects in the
+	// refs/tags/* namespace.
+	//
+	// Servers MUST pack the tags if their referrant is packed and the client
+	// has requested include-tags.
+	//
+	// Clients MUST be prepared for the case where a server has ignored
+	// include-tag and has not actually sent tags in the pack.  In such
+	// cases the client SHOULD issue a subsequent fetch to acquire the tags
+	// that include-tag would have otherwise given the client.
+	//
+	// The server SHOULD send include-tag, if it supports it, regardless
+	// of whether or not there are tags available.
+	IncludeTag Capability = "include-tag"
+	// ReportStatus the receive-pack process can receive a 'report-status'
+	// capability, which tells it that the client wants a report of what
+	// happened after a packfile upload and reference update. If the pushing
+	// client requests this capability, after unpacking and updating references
+	// the server will respond with whether the packfile unpacked successfully
+	// and if each reference was updated successfully. If any of those were not
+	// successful, it will send back an error message.  See pack-protocol.txt
+	// for example messages.
+	ReportStatus Capability = "report-status"
+	// DeleteRefs If the server sends back this capability, it means that
+	// it is capable of accepting a zero-id value as the target
+	// value of a reference update.  It is not sent back by the client, it
+	// simply informs the client that it can be sent zero-id values
+	// to delete references
+	DeleteRefs Capability = "delete-refs"
+	// Quiet If the receive-pack server advertises this capability, it is
+	// capable of silencing human-readable progress output which otherwise may
+	// be shown when processing the received pack. A send-pack client should
+	// respond with the 'quiet' capability to suppress server-side progress
+	// reporting if the local progress reporting is also being suppressed
+	// (e.g., via `push -q`, or if stderr does not go to a tty).
+	Quiet Capability = "quiet"
+	// Atomic If the server sends this capability it is capable of accepting
+	// atomic pushes. If the pushing client requests this capability, the server
+	// will update the refs in one atomic transaction. Either all refs are
+	// updated or none.
+	Atomic Capability = "atomic"
+	// PushOptions If the server sends this capability it is able to accept
+	// push options after the update commands have been sent, but before the
+	// packfile is streamed. If the pushing client requests this capability,
+	// the server will pass the options to the pre- and post- receive hooks
+	// that process this push request.
+	PushOptions Capability = "push-options"
+	// AllowTipSHA1InWant if the upload-pack server advertises this capability,
+	// fetch-pack may send "want" lines with SHA-1s that exist at the server but
+	// are not advertised by upload-pack.
+	AllowTipSHA1InWant Capability = "allow-tip-sha1-in-want"
+	// AllowReachableSHA1InWant if the upload-pack server advertises this
+	// capability, fetch-pack may send "want" lines with SHA-1s that exist at
+	// the server but are not advertised by upload-pack.
+	AllowReachableSHA1InWant Capability = "allow-reachable-sha1-in-want"
+	// PushCert the receive-pack server that advertises this capability is
+	// willing to accept a signed push certificate, and asks the <nonce> to be
+	// included in the push certificate.  A send-pack client MUST NOT
+	// send a push-cert packet unless the receive-pack server advertises
+	// this capability.
+	PushCert Capability = "push-cert"
+	// SymRef symbolic reference support for better negotiation.
+	SymRef Capability = "symref"
+)
+
+var valid = map[Capability]bool{
+	MultiACK: true, MultiACKDetailed: true, NoDone: true, ThinPack: true,
+	Sideband: true, Sideband64k: true, OFSDelta: true, Agent: true,
+	Shallow: true, DeepenSince: true, DeepenNot: true, DeepenRelative: true,
+	NoProgress: true, IncludeTag: true, ReportStatus: true, DeleteRefs: true,
+	Quiet: true, Atomic: true, PushOptions: true, AllowTipSHA1InWant: true,
+	AllowReachableSHA1InWant: true, PushCert: true, SymRef: true,
+}
+
+var requiresArgument = map[Capability]bool{
+	Agent: true, PushCert: true, SymRef: true,
+}
+
+var multipleArgument = map[Capability]bool{
+	SymRef: true,
+}
diff --git a/plumbing/protocol/packp/capability/list.go b/plumbing/protocol/packp/capability/list.go
new file mode 100644
index 0000000..73d1f25
--- /dev/null
+++ b/plumbing/protocol/packp/capability/list.go
@@ -0,0 +1,161 @@
+package capability
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"strings"
+)
+
+var (
+	// ErrUnknownCapability is returned if a unknown capability is given
+	ErrUnknownCapability = errors.New("unknown capability")
+	// ErrArgumentsRequired is returned if no arguments are giving with a
+	// capability that requires arguments
+	ErrArgumentsRequired = errors.New("arguments required")
+	// ErrArguments is returned if arguments are given with a capabilities that
+	// not supports arguments
+	ErrArguments = errors.New("arguments not allowed")
+	// ErrEmtpyArgument is returned when an empty value is given
+	ErrEmtpyArgument = errors.New("empty argument")
+	// ErrMultipleArguments multiple argument given to a capabilities that not
+	// support it
+	ErrMultipleArguments = errors.New("multiple arguments not allowed")
+)
+
+// List represents a list of capabilities
+type List struct {
+	m    map[Capability]*entry
+	sort []string
+}
+
+type entry struct {
+	Name   Capability
+	Values []string
+}
+
+// NewList returns a new List of capabilities
+func NewList() *List {
+	return &List{
+		m: make(map[Capability]*entry),
+	}
+}
+
+// IsEmpty returns true if the List is empty
+func (l *List) IsEmpty() bool {
+	return len(l.sort) == 0
+}
+
+// Decode decodes list of capabilities from raw into the list
+func (l *List) Decode(raw []byte) error {
+	for _, data := range bytes.Split(raw, []byte{' '}) {
+		pair := bytes.SplitN(data, []byte{'='}, 2)
+
+		c := Capability(pair[0])
+		if len(pair) == 1 {
+			if err := l.Add(c); err != nil {
+				return err
+			}
+
+			continue
+		}
+
+		if err := l.Add(c, string(pair[1])); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// Get returns the values for a capability
+func (l *List) Get(capability Capability) []string {
+	if _, ok := l.m[capability]; !ok {
+		return nil
+	}
+
+	return l.m[capability].Values
+}
+
+// Set sets a capability removing the previous values
+func (l *List) Set(capability Capability, values ...string) error {
+	if _, ok := l.m[capability]; ok {
+		delete(l.m, capability)
+	}
+
+	return l.Add(capability, values...)
+}
+
+// Add adds a capability, values are optional
+func (l *List) Add(c Capability, values ...string) error {
+	if err := l.validate(c, values); err != nil {
+		return err
+	}
+
+	if !l.Supports(c) {
+		l.m[c] = &entry{Name: c}
+		l.sort = append(l.sort, c.String())
+	}
+
+	if len(values) == 0 {
+		return nil
+	}
+
+	if !multipleArgument[c] && len(l.m[c].Values) > 0 {
+		return ErrMultipleArguments
+	}
+
+	l.m[c].Values = append(l.m[c].Values, values...)
+	return nil
+}
+
+func (l *List) validate(c Capability, values []string) error {
+	if _, ok := valid[c]; !ok {
+		return ErrUnknownCapability
+	}
+
+	if requiresArgument[c] && len(values) == 0 {
+		return ErrArgumentsRequired
+	}
+
+	if !requiresArgument[c] && len(values) != 0 {
+		return ErrArguments
+	}
+
+	if !multipleArgument[c] && len(values) > 1 {
+		return ErrMultipleArguments
+	}
+
+	for _, v := range values {
+		if v == "" {
+			return ErrEmtpyArgument
+		}
+	}
+
+	return nil
+}
+
+// Supports returns true if capability is present
+func (l *List) Supports(capability Capability) bool {
+	_, ok := l.m[capability]
+	return ok
+}
+
+// String generates the capabilities strings, the capabilities are sorted in
+// insertion order
+func (l *List) String() string {
+	var o []string
+	for _, key := range l.sort {
+		cap := l.m[Capability(key)]
+		if len(cap.Values) == 0 {
+			o = append(o, key)
+			continue
+		}
+
+		for _, value := range cap.Values {
+			o = append(o, fmt.Sprintf("%s=%s", key, value))
+		}
+	}
+
+	return strings.Join(o, " ")
+}
diff --git a/plumbing/protocol/packp/capability/list_test.go b/plumbing/protocol/packp/capability/list_test.go
new file mode 100644
index 0000000..6d350b0
--- /dev/null
+++ b/plumbing/protocol/packp/capability/list_test.go
@@ -0,0 +1,141 @@
+package capability
+
+import (
+	"testing"
+
+	check "gopkg.in/check.v1"
+)
+
+func Test(t *testing.T) { check.TestingT(t) }
+
+type SuiteCapabilities struct{}
+
+var _ = check.Suite(&SuiteCapabilities{})
+
+func (s *SuiteCapabilities) TestIsEmpty(c *check.C) {
+	cap := NewList()
+	c.Assert(cap.IsEmpty(), check.Equals, true)
+}
+
+func (s *SuiteCapabilities) TestDecode(c *check.C) {
+	cap := NewList()
+	err := cap.Decode([]byte("symref=foo symref=qux thin-pack"))
+	c.Assert(err, check.IsNil)
+
+	c.Assert(cap.m, check.HasLen, 2)
+	c.Assert(cap.Get(SymRef), check.DeepEquals, []string{"foo", "qux"})
+	c.Assert(cap.Get(ThinPack), check.IsNil)
+}
+
+func (s *SuiteCapabilities) TestDecodeWithErrArguments(c *check.C) {
+	cap := NewList()
+	err := cap.Decode([]byte("thin-pack=foo"))
+	c.Assert(err, check.Equals, ErrArguments)
+}
+
+func (s *SuiteCapabilities) TestDecodeWithEqual(c *check.C) {
+	cap := NewList()
+	err := cap.Decode([]byte("agent=foo=bar"))
+	c.Assert(err, check.IsNil)
+
+	c.Assert(cap.m, check.HasLen, 1)
+	c.Assert(cap.Get(Agent), check.DeepEquals, []string{"foo=bar"})
+}
+
+func (s *SuiteCapabilities) TestDecodeWithErrUnknownCapability(c *check.C) {
+	cap := NewList()
+	err := cap.Decode([]byte("foo"))
+	c.Assert(err, check.Equals, ErrUnknownCapability)
+}
+
+func (s *SuiteCapabilities) TestString(c *check.C) {
+	cap := NewList()
+	cap.Set(Agent, "bar")
+	cap.Set(SymRef, "foo:qux")
+	cap.Set(ThinPack)
+
+	c.Assert(cap.String(), check.Equals, "agent=bar symref=foo:qux thin-pack")
+}
+
+func (s *SuiteCapabilities) TestStringSort(c *check.C) {
+	cap := NewList()
+	cap.Set(Agent, "bar")
+	cap.Set(SymRef, "foo:qux")
+	cap.Set(ThinPack)
+
+	c.Assert(cap.String(), check.Equals, "agent=bar symref=foo:qux thin-pack")
+}
+
+func (s *SuiteCapabilities) TestSet(c *check.C) {
+	cap := NewList()
+	err := cap.Add(SymRef, "foo", "qux")
+	c.Assert(err, check.IsNil)
+	err = cap.Set(SymRef, "bar")
+	c.Assert(err, check.IsNil)
+
+	c.Assert(cap.m, check.HasLen, 1)
+	c.Assert(cap.Get(SymRef), check.DeepEquals, []string{"bar"})
+}
+
+func (s *SuiteCapabilities) TestSetEmpty(c *check.C) {
+	cap := NewList()
+	err := cap.Set(Agent, "bar")
+	c.Assert(err, check.IsNil)
+
+	c.Assert(cap.Get(Agent), check.HasLen, 1)
+}
+
+func (s *SuiteCapabilities) TestGetEmpty(c *check.C) {
+	cap := NewList()
+	c.Assert(cap.Get(Agent), check.HasLen, 0)
+}
+
+func (s *SuiteCapabilities) TestAdd(c *check.C) {
+	cap := NewList()
+	err := cap.Add(SymRef, "foo", "qux")
+	c.Assert(err, check.IsNil)
+
+	err = cap.Add(ThinPack)
+	c.Assert(err, check.IsNil)
+
+	c.Assert(cap.String(), check.Equals, "symref=foo symref=qux thin-pack")
+}
+
+func (s *SuiteCapabilities) TestAddErrUnknownCapability(c *check.C) {
+	cap := NewList()
+	err := cap.Add(Capability("foo"))
+	c.Assert(err, check.Equals, ErrUnknownCapability)
+}
+
+func (s *SuiteCapabilities) TestAddErrArgumentsRequired(c *check.C) {
+	cap := NewList()
+	err := cap.Add(SymRef)
+	c.Assert(err, check.Equals, ErrArgumentsRequired)
+}
+
+func (s *SuiteCapabilities) TestAddErrArgumentsNotAllowed(c *check.C) {
+	cap := NewList()
+	err := cap.Add(OFSDelta, "foo")
+	c.Assert(err, check.Equals, ErrArguments)
+}
+
+func (s *SuiteCapabilities) TestAddErrArgumendts(c *check.C) {
+	cap := NewList()
+	err := cap.Add(SymRef, "")
+	c.Assert(err, check.Equals, ErrEmtpyArgument)
+}
+
+func (s *SuiteCapabilities) TestAddErrMultipleArguments(c *check.C) {
+	cap := NewList()
+	err := cap.Add(Agent, "foo")
+	c.Assert(err, check.IsNil)
+
+	err = cap.Add(Agent, "bar")
+	c.Assert(err, check.Equals, ErrMultipleArguments)
+}
+
+func (s *SuiteCapabilities) TestAddErrMultipleArgumentsAtTheSameTime(c *check.C) {
+	cap := NewList()
+	err := cap.Add(Agent, "foo", "bar")
+	c.Assert(err, check.Equals, ErrMultipleArguments)
+}
diff --git a/plumbing/protocol/packp/common.go b/plumbing/protocol/packp/common.go
index c4b44f7..3d7786b 100644
--- a/plumbing/protocol/packp/common.go
+++ b/plumbing/protocol/packp/common.go
@@ -1,7 +1,5 @@
 package packp
 
-import "bytes"
-
 type stateFn func() stateFn
 
 const (
@@ -31,15 +29,3 @@ var (
 	deepenSince     = []byte("deepen-since ")
 	deepenReference = []byte("deepen-not ")
 )
-
-// Capabilities are a single string or a name=value.
-// Even though we are only going to read at moust 1 value, we return
-// a slice of values, as Capability.Add receives that.
-func readCapability(data []byte) (name string, values []string) {
-	pair := bytes.SplitN(data, []byte{'='}, 2)
-	if len(pair) == 2 {
-		values = append(values, string(pair[1]))
-	}
-
-	return string(pair[0]), values
-}
diff --git a/plumbing/protocol/packp/ulreq.go b/plumbing/protocol/packp/ulreq.go
index 6ec5b96..be68b26 100644
--- a/plumbing/protocol/packp/ulreq.go
+++ b/plumbing/protocol/packp/ulreq.go
@@ -4,6 +4,7 @@ import (
 	"time"
 
 	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 )
 
 // UploadRequest values represent the information transmitted on a
@@ -11,7 +12,7 @@ import (
 // safe, use the New function instead.
 // This is a low level type, use UploadPackRequest instead.
 type UploadRequest struct {
-	Capabilities *Capabilities
+	Capabilities *capability.List
 	Wants        []plumbing.Hash
 	Shallows     []plumbing.Hash
 	Depth        Depth
@@ -46,7 +47,7 @@ func (d DepthReference) isDepth() {}
 // wanted hash.
 func NewUploadRequest() *UploadRequest {
 	return &UploadRequest{
-		Capabilities: NewCapabilities(),
+		Capabilities: capability.NewList(),
 		Wants:        []plumbing.Hash{},
 		Shallows:     []plumbing.Hash{},
 		Depth:        DepthCommits(0),
diff --git a/plumbing/protocol/packp/ulreq_decode.go b/plumbing/protocol/packp/ulreq_decode.go
index 0124cd0..812af5b 100644
--- a/plumbing/protocol/packp/ulreq_decode.go
+++ b/plumbing/protocol/packp/ulreq_decode.go
@@ -115,10 +115,8 @@ func (d *ulReqDecoder) decodeCaps() stateFn {
 	}
 
 	d.line = bytes.TrimPrefix(d.line, sp)
-
-	for _, c := range bytes.Split(d.line, sp) {
-		name, values := readCapability(c)
-		d.data.Capabilities.Add(name, values...)
+	if err := d.data.Capabilities.Decode(d.line); err != nil {
+		d.error("invalid capabilities: %s", err)
 	}
 
 	return d.decodeOtherWants
diff --git a/plumbing/protocol/packp/ulreq_decode_test.go b/plumbing/protocol/packp/ulreq_decode_test.go
index eb12c90..e7d9d7c6 100644
--- a/plumbing/protocol/packp/ulreq_decode_test.go
+++ b/plumbing/protocol/packp/ulreq_decode_test.go
@@ -10,6 +10,7 @@ import (
 	"gopkg.in/src-d/go-git.v4/plumbing/format/pktline"
 
 	. "gopkg.in/check.v1"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 )
 
 type UlReqDecodeSuite struct{}
@@ -87,8 +88,8 @@ func (s *UlReqDecodeSuite) TestWantWithCapabilities(c *C) {
 	c.Assert(ur.Wants, DeepEquals, []plumbing.Hash{
 		plumbing.NewHash("1111111111111111111111111111111111111111")})
 
-	c.Assert(ur.Capabilities.Supports("ofs-delta"), Equals, true)
-	c.Assert(ur.Capabilities.Supports("multi_ack"), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.OFSDelta), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.MultiACK), Equals, true)
 }
 
 func (s *UlReqDecodeSuite) TestManyWantsNoCapabilities(c *C) {
@@ -168,8 +169,8 @@ func (s *UlReqDecodeSuite) TestManyWantsWithCapabilities(c *C) {
 	sort.Sort(byHash(expected))
 	c.Assert(ur.Wants, DeepEquals, expected)
 
-	c.Assert(ur.Capabilities.Supports("ofs-delta"), Equals, true)
-	c.Assert(ur.Capabilities.Supports("multi_ack"), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.OFSDelta), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.MultiACK), Equals, true)
 }
 
 func (s *UlReqDecodeSuite) TestSingleShallowSingleWant(c *C) {
@@ -189,8 +190,8 @@ func (s *UlReqDecodeSuite) TestSingleShallowSingleWant(c *C) {
 	}
 
 	c.Assert(ur.Wants, DeepEquals, expectedWants)
-	c.Assert(ur.Capabilities.Supports("ofs-delta"), Equals, true)
-	c.Assert(ur.Capabilities.Supports("multi_ack"), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.OFSDelta), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.MultiACK), Equals, true)
 
 	c.Assert(ur.Shallows, DeepEquals, expectedShallows)
 }
@@ -220,8 +221,8 @@ func (s *UlReqDecodeSuite) TestSingleShallowManyWants(c *C) {
 
 	sort.Sort(byHash(ur.Wants))
 	c.Assert(ur.Wants, DeepEquals, expectedWants)
-	c.Assert(ur.Capabilities.Supports("ofs-delta"), Equals, true)
-	c.Assert(ur.Capabilities.Supports("multi_ack"), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.OFSDelta), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.MultiACK), Equals, true)
 
 	c.Assert(ur.Shallows, DeepEquals, expectedShallows)
 }
@@ -250,8 +251,8 @@ func (s *UlReqDecodeSuite) TestManyShallowSingleWant(c *C) {
 	sort.Sort(byHash(expectedShallows))
 
 	c.Assert(ur.Wants, DeepEquals, expectedWants)
-	c.Assert(ur.Capabilities.Supports("ofs-delta"), Equals, true)
-	c.Assert(ur.Capabilities.Supports("multi_ack"), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.OFSDelta), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.MultiACK), Equals, true)
 
 	sort.Sort(byHash(ur.Shallows))
 	c.Assert(ur.Shallows, DeepEquals, expectedShallows)
@@ -289,8 +290,8 @@ func (s *UlReqDecodeSuite) TestManyShallowManyWants(c *C) {
 
 	sort.Sort(byHash(ur.Wants))
 	c.Assert(ur.Wants, DeepEquals, expectedWants)
-	c.Assert(ur.Capabilities.Supports("ofs-delta"), Equals, true)
-	c.Assert(ur.Capabilities.Supports("multi_ack"), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.OFSDelta), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.MultiACK), Equals, true)
 
 	sort.Sort(byHash(ur.Shallows))
 	c.Assert(ur.Shallows, DeepEquals, expectedShallows)
@@ -500,9 +501,8 @@ func (s *UlReqDecodeSuite) TestAll(c *C) {
 	sort.Sort(byHash(expectedWants))
 	sort.Sort(byHash(ur.Wants))
 	c.Assert(ur.Wants, DeepEquals, expectedWants)
-
-	c.Assert(ur.Capabilities.Supports("ofs-delta"), Equals, true)
-	c.Assert(ur.Capabilities.Supports("multi_ack"), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.OFSDelta), Equals, true)
+	c.Assert(ur.Capabilities.Supports(capability.MultiACK), Equals, true)
 
 	expectedShallows := []plumbing.Hash{
 		plumbing.NewHash("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"),
diff --git a/plumbing/protocol/packp/ulreq_encode.go b/plumbing/protocol/packp/ulreq_encode.go
index b422a5f..b2ca491 100644
--- a/plumbing/protocol/packp/ulreq_encode.go
+++ b/plumbing/protocol/packp/ulreq_encode.go
@@ -63,7 +63,6 @@ func (e *ulReqEncoder) encodeFirstWant() stateFn {
 	if e.data.Capabilities.IsEmpty() {
 		err = e.pe.Encodef("want %s\n", e.sortedWants[0])
 	} else {
-		e.data.Capabilities.Sort()
 		err = e.pe.Encodef(
 			"want %s %s\n",
 			e.sortedWants[0],
diff --git a/plumbing/protocol/packp/ulreq_encode_test.go b/plumbing/protocol/packp/ulreq_encode_test.go
index 1eb3175..3b3b6c2 100644
--- a/plumbing/protocol/packp/ulreq_encode_test.go
+++ b/plumbing/protocol/packp/ulreq_encode_test.go
@@ -8,6 +8,7 @@ import (
 	"gopkg.in/src-d/go-git.v4/plumbing/format/pktline"
 
 	. "gopkg.in/check.v1"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 )
 
 type UlReqEncodeSuite struct{}
@@ -59,14 +60,14 @@ func (s *UlReqEncodeSuite) TestOneWant(c *C) {
 func (s *UlReqEncodeSuite) TestOneWantWithCapabilities(c *C) {
 	ur := NewUploadRequest()
 	ur.Wants = append(ur.Wants, plumbing.NewHash("1111111111111111111111111111111111111111"))
-	ur.Capabilities.Add("sysref", "HEAD:/refs/heads/master")
-	ur.Capabilities.Add("multi_ack")
-	ur.Capabilities.Add("thin-pack")
-	ur.Capabilities.Add("side-band")
-	ur.Capabilities.Add("ofs-delta")
+	ur.Capabilities.Add(capability.MultiACK)
+	ur.Capabilities.Add(capability.OFSDelta)
+	ur.Capabilities.Add(capability.Sideband)
+	ur.Capabilities.Add(capability.SymRef, "HEAD:/refs/heads/master")
+	ur.Capabilities.Add(capability.ThinPack)
 
 	expected := []string{
-		"want 1111111111111111111111111111111111111111 multi_ack ofs-delta side-band sysref=HEAD:/refs/heads/master thin-pack\n",
+		"want 1111111111111111111111111111111111111111 multi_ack ofs-delta side-band symref=HEAD:/refs/heads/master thin-pack\n",
 		pktline.FlushString,
 	}
 
@@ -101,14 +102,14 @@ func (s *UlReqEncodeSuite) TestWantsWithCapabilities(c *C) {
 	ur.Wants = append(ur.Wants, plumbing.NewHash("2222222222222222222222222222222222222222"))
 	ur.Wants = append(ur.Wants, plumbing.NewHash("5555555555555555555555555555555555555555"))
 
-	ur.Capabilities.Add("sysref", "HEAD:/refs/heads/master")
-	ur.Capabilities.Add("multi_ack")
-	ur.Capabilities.Add("thin-pack")
-	ur.Capabilities.Add("side-band")
-	ur.Capabilities.Add("ofs-delta")
+	ur.Capabilities.Add(capability.MultiACK)
+	ur.Capabilities.Add(capability.OFSDelta)
+	ur.Capabilities.Add(capability.Sideband)
+	ur.Capabilities.Add(capability.SymRef, "HEAD:/refs/heads/master")
+	ur.Capabilities.Add(capability.ThinPack)
 
 	expected := []string{
-		"want 1111111111111111111111111111111111111111 multi_ack ofs-delta side-band sysref=HEAD:/refs/heads/master thin-pack\n",
+		"want 1111111111111111111111111111111111111111 multi_ack ofs-delta side-band symref=HEAD:/refs/heads/master thin-pack\n",
 		"want 2222222222222222222222222222222222222222\n",
 		"want 3333333333333333333333333333333333333333\n",
 		"want 4444444444444444444444444444444444444444\n",
@@ -122,7 +123,7 @@ func (s *UlReqEncodeSuite) TestWantsWithCapabilities(c *C) {
 func (s *UlReqEncodeSuite) TestShallow(c *C) {
 	ur := NewUploadRequest()
 	ur.Wants = append(ur.Wants, plumbing.NewHash("1111111111111111111111111111111111111111"))
-	ur.Capabilities.Add("multi_ack")
+	ur.Capabilities.Add(capability.MultiACK)
 	ur.Shallows = append(ur.Shallows, plumbing.NewHash("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"))
 
 	expected := []string{
@@ -137,7 +138,7 @@ func (s *UlReqEncodeSuite) TestShallow(c *C) {
 func (s *UlReqEncodeSuite) TestManyShallows(c *C) {
 	ur := NewUploadRequest()
 	ur.Wants = append(ur.Wants, plumbing.NewHash("1111111111111111111111111111111111111111"))
-	ur.Capabilities.Add("multi_ack")
+	ur.Capabilities.Add(capability.MultiACK)
 	ur.Shallows = append(ur.Shallows, plumbing.NewHash("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"))
 	ur.Shallows = append(ur.Shallows, plumbing.NewHash("dddddddddddddddddddddddddddddddddddddddd"))
 	ur.Shallows = append(ur.Shallows, plumbing.NewHash("cccccccccccccccccccccccccccccccccccccccc"))
@@ -225,11 +226,11 @@ func (s *UlReqEncodeSuite) TestAll(c *C) {
 	ur.Wants = append(ur.Wants, plumbing.NewHash("2222222222222222222222222222222222222222"))
 	ur.Wants = append(ur.Wants, plumbing.NewHash("5555555555555555555555555555555555555555"))
 
-	ur.Capabilities.Add("sysref", "HEAD:/refs/heads/master")
-	ur.Capabilities.Add("multi_ack")
-	ur.Capabilities.Add("thin-pack")
-	ur.Capabilities.Add("side-band")
-	ur.Capabilities.Add("ofs-delta")
+	ur.Capabilities.Add(capability.MultiACK)
+	ur.Capabilities.Add(capability.OFSDelta)
+	ur.Capabilities.Add(capability.Sideband)
+	ur.Capabilities.Add(capability.SymRef, "HEAD:/refs/heads/master")
+	ur.Capabilities.Add(capability.ThinPack)
 
 	ur.Shallows = append(ur.Shallows, plumbing.NewHash("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"))
 	ur.Shallows = append(ur.Shallows, plumbing.NewHash("dddddddddddddddddddddddddddddddddddddddd"))
@@ -240,7 +241,7 @@ func (s *UlReqEncodeSuite) TestAll(c *C) {
 	ur.Depth = DepthSince(since)
 
 	expected := []string{
-		"want 1111111111111111111111111111111111111111 multi_ack ofs-delta side-band sysref=HEAD:/refs/heads/master thin-pack\n",
+		"want 1111111111111111111111111111111111111111 multi_ack ofs-delta side-band symref=HEAD:/refs/heads/master thin-pack\n",
 		"want 2222222222222222222222222222222222222222\n",
 		"want 3333333333333333333333333333333333333333\n",
 		"want 4444444444444444444444444444444444444444\n",
diff --git a/plumbing/protocol/packp/ulreq_test.go b/plumbing/protocol/packp/ulreq_test.go
index be02f9d..5e9e978 100644
--- a/plumbing/protocol/packp/ulreq_test.go
+++ b/plumbing/protocol/packp/ulreq_test.go
@@ -8,6 +8,7 @@ import (
 
 	"gopkg.in/src-d/go-git.v4/plumbing"
 	"gopkg.in/src-d/go-git.v4/plumbing/format/pktline"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 )
 
 func ExampleUlReqEncoder_Encode() {
@@ -20,8 +21,8 @@ func ExampleUlReqEncoder_Encode() {
 	ur.Wants = append(ur.Wants, plumbing.NewHash("2222222222222222222222222222222222222222"))
 
 	// And some capabilities you will like the server to use
-	ur.Capabilities.Add("sysref", "HEAD:/refs/heads/master")
-	ur.Capabilities.Add("ofs-delta")
+	ur.Capabilities.Add(capability.OFSDelta)
+	ur.Capabilities.Add(capability.SymRef, "HEAD:/refs/heads/master")
 
 	// Add a couple of shallows
 	ur.Shallows = append(ur.Shallows, plumbing.NewHash("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"))
@@ -36,7 +37,7 @@ func ExampleUlReqEncoder_Encode() {
 	// ...and encode the upload-request to it.
 	_ = e.Encode(ur) // ignoring errors for brevity
 	// Output:
-	// 005bwant 1111111111111111111111111111111111111111 ofs-delta sysref=HEAD:/refs/heads/master
+	// 005bwant 1111111111111111111111111111111111111111 ofs-delta symref=HEAD:/refs/heads/master
 	// 0032want 2222222222222222222222222222222222222222
 	// 0032want 3333333333333333333333333333333333333333
 	// 0035shallow aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
@@ -48,7 +49,7 @@ func ExampleUlReqEncoder_Encode() {
 func ExampleUlReqDecoder_Decode() {
 	// Here is a raw advertised-ref message.
 	raw := "" +
-		"005bwant 1111111111111111111111111111111111111111 ofs-delta sysref=HEAD:/refs/heads/master\n" +
+		"005bwant 1111111111111111111111111111111111111111 ofs-delta symref=HEAD:/refs/heads/master\n" +
 		"0032want 2222222222222222222222222222222222222222\n" +
 		"0032want 3333333333333333333333333333333333333333\n" +
 		"0035shallow aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\n" +
@@ -79,7 +80,7 @@ func ExampleUlReqDecoder_Decode() {
 		fmt.Println("depth =", string(depth))
 	}
 	// Output:
-	// capabilities = ofs-delta sysref=HEAD:/refs/heads/master
+	// capabilities = ofs-delta symref=HEAD:/refs/heads/master
 	// wants = [1111111111111111111111111111111111111111 2222222222222222222222222222222222222222 3333333333333333333333333333333333333333]
 	// shallows = [aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb]
 	// depth = 2015-01-02 03:04:05 +0000 UTC
diff --git a/plumbing/transport/test/common.go b/plumbing/transport/test/common.go
index 7818fbb..ed2e141 100644
--- a/plumbing/transport/test/common.go
+++ b/plumbing/transport/test/common.go
@@ -15,6 +15,7 @@ import (
 	"gopkg.in/src-d/go-git.v4/storage/memory"
 
 	. "gopkg.in/check.v1"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 )
 
 type FetchPackSuite struct {
@@ -64,7 +65,9 @@ func (s *FetchPackSuite) TestDefaultBranch(c *C) {
 
 	info, err := r.AdvertisedReferences()
 	c.Assert(err, IsNil)
-	c.Assert(info.Capabilities.SymbolicReference("HEAD"), Equals, "refs/heads/master")
+	symrefs := info.Capabilities.Get(capability.SymRef)
+	c.Assert(symrefs, HasLen, 1)
+	c.Assert(symrefs[0], Equals, "HEAD:refs/heads/master")
 }
 
 func (s *FetchPackSuite) TestCapabilities(c *C) {
@@ -74,7 +77,7 @@ func (s *FetchPackSuite) TestCapabilities(c *C) {
 
 	info, err := r.AdvertisedReferences()
 	c.Assert(err, IsNil)
-	c.Assert(info.Capabilities.Get("agent").Values, HasLen, 1)
+	c.Assert(info.Capabilities.Get(capability.Agent), HasLen, 1)
 }
 
 func (s *FetchPackSuite) TestFullFetchPack(c *C) {
diff --git a/remote.go b/remote.go
index feab72a..e2f9ae4 100644
--- a/remote.go
+++ b/remote.go
@@ -9,6 +9,7 @@ import (
 	"gopkg.in/src-d/go-git.v4/plumbing"
 	"gopkg.in/src-d/go-git.v4/plumbing/format/packfile"
 	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 	"gopkg.in/src-d/go-git.v4/plumbing/storer"
 	"gopkg.in/src-d/go-git.v4/plumbing/transport"
 	"gopkg.in/src-d/go-git.v4/plumbing/transport/client"
@@ -90,7 +91,7 @@ func (r *Remote) AdvertisedReferences() *packp.AdvRefs {
 }
 
 // Capabilities returns the remote capabilities
-func (r *Remote) Capabilities() *packp.Capabilities {
+func (r *Remote) Capabilities() *capability.List {
 	return r.advRefs.Capabilities
 }
 
diff --git a/remote_test.go b/remote_test.go
index ff543f2..365e118 100644
--- a/remote_test.go
+++ b/remote_test.go
@@ -19,6 +19,7 @@ import (
 	osfs "gopkg.in/src-d/go-git.v4/utils/fs/os"
 
 	. "gopkg.in/check.v1"
+	"gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability"
 )
 
 const FixRefSpec = config.RefSpec("+refs/heads/*:refs/remotes/origin/*")
@@ -57,7 +58,7 @@ func (s *RemoteSuite) TestInfo(c *C) {
 	c.Assert(r.AdvertisedReferences(), IsNil)
 	c.Assert(r.Connect(), IsNil)
 	c.Assert(r.AdvertisedReferences(), NotNil)
-	c.Assert(r.AdvertisedReferences().Capabilities.Get("ofs-delta"), NotNil)
+	c.Assert(r.AdvertisedReferences().Capabilities.Get(capability.Agent), NotNil)
 }
 
 func (s *RemoteSuite) TestDefaultBranch(c *C) {
@@ -73,7 +74,7 @@ func (s *RemoteSuite) TestCapabilities(c *C) {
 	r.client = &MockClient{}
 
 	c.Assert(r.Connect(), IsNil)
-	c.Assert(r.Capabilities().Get("agent").Values, HasLen, 1)
+	c.Assert(r.Capabilities().Get(capability.Agent), HasLen, 1)
 }
 
 func (s *RemoteSuite) TestFetch(c *C) {