diff --git a/proto/snpb/replicator.go b/proto/snpb/replicator.go
index 6d9800cf8..f146355f0 100644
--- a/proto/snpb/replicator.go
+++ b/proto/snpb/replicator.go
@@ -6,26 +6,42 @@ import (
 	"github.com/kakao/varlog/pkg/types"
 )
 
+// InvalidSyncPosition returns a SyncPosition with both LLSN and GLSN set to
+// types.InvalidGLSN.
 func InvalidSyncPosition() SyncPosition {
 	return SyncPosition{LLSN: types.InvalidLLSN, GLSN: types.InvalidGLSN}
 }
 
+// Invalid checks if the SyncPosition is invalid. A SyncPosition is considered
+// invalid if either LLSN or GLSN is invalid.
 func (sp SyncPosition) Invalid() bool {
 	return sp.LLSN.Invalid() || sp.GLSN.Invalid()
 }
 
+// LessThan checks if the current SyncPosition "sp" is less than another
+// SyncPosition "other". It returns true if both LLSN and GLSN of the current
+// SyncPosition are less than those of the other SyncPosition.
 func (sp SyncPosition) LessThan(other SyncPosition) bool {
 	return sp.LLSN < other.LLSN && sp.GLSN < other.GLSN
 }
 
+// InvalidSyncRange returns a SyncRange with both FirstLLSN and LastLLSN set to
+// types.InvalidLLSN.
 func InvalidSyncRange() SyncRange {
 	return SyncRange{FirstLLSN: types.InvalidLLSN, LastLLSN: types.InvalidLLSN}
 }
 
+// Invalid determines if the SyncRange is invalid. A SyncRange is considered
+// invalid if either FirstLLSN or LastLLSN is invalid, or if FirstLLSN is
+// greater than LastLLSN.
 func (sr SyncRange) Invalid() bool {
 	return sr.FirstLLSN.Invalid() || sr.LastLLSN.Invalid() || sr.FirstLLSN > sr.LastLLSN
 }
 
+// Validate checks the validity of the SyncRange. It returns an error if
+// FirstLLSN is greater than LastLLSN, or if FirstLLSN is invalid while
+// LastLLSN is valid. If both FirstLLSN and LastLLSN are invalid, it returns
+// nil, indicating that the entire log range is considered trimmed.
 func (sr SyncRange) Validate() error {
 	if sr.FirstLLSN > sr.LastLLSN {
 		return fmt.Errorf("invalid sync range: first %d, last %d", sr.FirstLLSN, sr.LastLLSN)
diff --git a/proto/snpb/replicator_test.go b/proto/snpb/replicator_test.go
index 437c06d5d..5915e3876 100644
--- a/proto/snpb/replicator_test.go
+++ b/proto/snpb/replicator_test.go
@@ -4,6 +4,8 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/require"
+
+	"github.com/kakao/varlog/pkg/types"
 )
 
 func TestSyncPositionInvalid(t *testing.T) {
@@ -80,3 +82,73 @@ func TestSyncPositionLessThan(t *testing.T) {
 		})
 	}
 }
+
+func TestInvalidSyncRange(t *testing.T) {
+	sr := InvalidSyncRange()
+	require.True(t, sr.Invalid())
+}
+
+func TestSyncRangeInvalid(t *testing.T) {
+	tcs := []struct {
+		sr       SyncRange
+		expected bool
+	}{
+		{
+			sr:       SyncRange{FirstLLSN: types.InvalidLLSN, LastLLSN: types.InvalidLLSN},
+			expected: true,
+		},
+		{
+			sr:       SyncRange{FirstLLSN: types.LLSN(1), LastLLSN: types.InvalidLLSN},
+			expected: true,
+		},
+		{
+			sr:       SyncRange{FirstLLSN: types.InvalidLLSN, LastLLSN: types.LLSN(1)},
+			expected: true,
+		},
+		{
+			sr:       SyncRange{FirstLLSN: types.LLSN(2), LastLLSN: types.LLSN(1)},
+			expected: true,
+		},
+		{
+			sr:       SyncRange{FirstLLSN: types.LLSN(1), LastLLSN: types.LLSN(2)},
+			expected: false,
+		},
+	}
+
+	for _, tc := range tcs {
+		t.Run(tc.sr.String(), func(t *testing.T) {
+			require.Equal(t, tc.expected, tc.sr.Invalid())
+		})
+	}
+}
+
+func TestSyncRangeValidate(t *testing.T) {
+	tcs := []struct {
+		sr      SyncRange
+		wantErr bool
+	}{
+		{
+			sr:      SyncRange{FirstLLSN: types.LLSN(2), LastLLSN: types.LLSN(1)},
+			wantErr: true,
+		},
+		{
+			sr:      SyncRange{FirstLLSN: types.InvalidLLSN, LastLLSN: types.LLSN(1)},
+			wantErr: true,
+		},
+		{
+			sr:      SyncRange{FirstLLSN: types.LLSN(1), LastLLSN: types.LLSN(2)},
+			wantErr: false,
+		},
+	}
+
+	for _, tc := range tcs {
+		t.Run(tc.sr.String(), func(t *testing.T) {
+			err := tc.sr.Validate()
+			if tc.wantErr {
+				require.Error(t, err)
+				return
+			}
+			require.NoError(t, err)
+		})
+	}
+}