diff --git a/internal/martian/header/via_modifier.go b/internal/martian/header/via_modifier.go
index 633c3daf..462245cf 100644
--- a/internal/martian/header/via_modifier.go
+++ b/internal/martian/header/via_modifier.go
@@ -20,25 +20,42 @@ import (
 	"fmt"
 	"io"
 	"net/http"
-	"regexp"
 	"strings"
 
 	"github.com/saucelabs/forwarder/internal/martian"
 )
 
-var whitespace = regexp.MustCompile("[\t ]+")
+const (
+	h20Prefix = "2.0"
+	h11Prefix = "1.1"
+	h10Prefix = "1.0"
+)
 
 // ViaModifier is a header modifier that checks for proxy redirect loops.
 type ViaModifier struct {
-	requestedBy string
-	boundary    string
+	tag string
 }
 
 // NewViaModifier returns a new Via modifier.
 func NewViaModifier(requestedBy string) *ViaModifier {
+	return NewViaModifierWithBoundary(requestedBy, randomBoundary())
+}
+
+// randomBoundary generates a 10 character string to ensure that Martians that
+// are chained together with the same requestedBy value do not collide.  This func
+// panics if io.Readfull fails.
+func randomBoundary() string {
+	var buf [10]byte
+	_, err := io.ReadFull(rand.Reader, buf[:])
+	if err != nil {
+		panic(err)
+	}
+	return hex.EncodeToString(buf[:])
+}
+
+func NewViaModifierWithBoundary(requestedBy, boundary string) *ViaModifier {
 	return &ViaModifier{
-		requestedBy: requestedBy,
-		boundary:    randomBoundary(),
+		tag: requestedBy + "-" + boundary,
 	}
 }
 
@@ -49,10 +66,13 @@ func NewViaModifier(requestedBy string) *ViaModifier {
 //
 // http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-14#section-9.9
 func (m *ViaModifier) ModifyRequest(req *http.Request) error {
-	via := fmt.Sprintf("%d.%d %s-%s", req.ProtoMajor, req.ProtoMinor, m.requestedBy, m.boundary)
+	via := req.Header.Get("Via")
+
+	var sb strings.Builder
+	sb.Grow(m.nextLen(via))
 
-	if v := req.Header.Get("Via"); v != "" {
-		if m.hasLoop(v) {
+	if via != "" {
+		if strings.Contains(via, m.tag) {
 			req.Close = true
 			return martian.ErrorStatus{
 				Err:    fmt.Errorf("via: detected request loop, header contains %s", via),
@@ -60,48 +80,37 @@ func (m *ViaModifier) ModifyRequest(req *http.Request) error {
 			}
 		}
 
-		via = fmt.Sprintf("%s, %s", v, via)
+		sb.WriteString(via)
+		sb.WriteString(", ")
+	}
+
+	switch req.ProtoMajor*10 + req.ProtoMinor {
+	case 20:
+		sb.WriteString(h20Prefix)
+	case 11:
+		sb.WriteString(h11Prefix)
+	case 10:
+		sb.WriteString(h10Prefix)
+	default:
+		fmt.Fprintf(&sb, "%d.%d", req.ProtoMajor, req.ProtoMinor)
 	}
 
-	req.Header.Set("Via", via)
+	sb.WriteByte(' ')
+	sb.WriteString(m.tag)
+
+	req.Header.Set("Via", sb.String())
 
 	return nil
 }
 
-// hasLoop parses via and attempts to match requestedBy against the contained
-// pseudonyms/host:port pairs.
-func (m *ViaModifier) hasLoop(via string) bool {
-	for _, v := range strings.Split(via, ",") {
-		parts := whitespace.Split(strings.TrimSpace(v), 3)
-
-		// No pseudonym or host:port, assume there is no loop.
-		if len(parts) < 2 {
-			continue
-		}
+func (m *ViaModifier) nextLen(via string) int {
+	l := 0
 
-		if fmt.Sprintf("%s-%s", m.requestedBy, m.boundary) == parts[1] {
-			return true
-		}
+	if via != "" {
+		l += len(via) + 2
 	}
 
-	return false
-}
+	l += len(h11Prefix) + 1 + len(m.tag)
 
-// SetBoundary sets the boundary string (random 10 character by default) used to
-// disabiguate Martians that are chained together with identical requestedBy values.
-// This should only be used for testing.
-func (m *ViaModifier) SetBoundary(boundary string) {
-	m.boundary = boundary
-}
-
-// randomBoundary generates a 10 character string to ensure that Martians that
-// are chained together with the same requestedBy value do not collide.  This func
-// panics if io.Readfull fails.
-func randomBoundary() string {
-	var buf [10]byte
-	_, err := io.ReadFull(rand.Reader, buf[:])
-	if err != nil {
-		panic(err)
-	}
-	return hex.EncodeToString(buf[:])
+	return l
 }
diff --git a/internal/martian/header/via_modifier_test.go b/internal/martian/header/via_modifier_test.go
index b8e62ca2..6377030e 100644
--- a/internal/martian/header/via_modifier_test.go
+++ b/internal/martian/header/via_modifier_test.go
@@ -42,7 +42,7 @@ func TestViaModifier(t *testing.T) {
 		t.Errorf("req.Header.Get(%q): got %q, want prefixed with %q", "Via", got, want)
 	}
 
-	m.SetBoundary("boundary")
+	m = NewViaModifierWithBoundary("martian", "boundary")
 	req.Header.Set("Via", "1.0\talpha\t(martian), 1.1 martian-boundary, 1.1 beta")
 	if err := m.ModifyRequest(req); err == nil {
 		t.Fatal("ModifyRequest(): got nil, want request loop error")
@@ -51,3 +51,22 @@ func TestViaModifier(t *testing.T) {
 		t.Fatalf("req.Close: got %v, want true", req.Close)
 	}
 }
+
+func BenchmarkViaModifier(b *testing.B) {
+	const via = "1.0\talpha\t(martian), 1.1 martian-boundary, 1.1 beta"
+
+	req, err := http.NewRequest(http.MethodGet, "/", http.NoBody)
+	if err != nil {
+		b.Fatalf("http.NewRequest(): got %v, want no error", err)
+	}
+
+	m := NewViaModifier("martian")
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		req.Header.Set("Via", via)
+		if err := m.ModifyRequest(req); err != nil {
+			b.Fatalf("ModifyRequest(): got %v, want no error", err)
+		}
+	}
+}