diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackFetchConnection.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackFetchConnection.java
index 2aecf63ad..be36d2b83 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackFetchConnection.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackFetchConnection.java
@@ -12,6 +12,18 @@
 
 package org.eclipse.jgit.transport;
 
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DELIM;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DEEPEN;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DEEPEN_NOT;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DEEPEN_SINCE;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DONE;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_END;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_ERR;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_HAVE;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_SHALLOW;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_UNSHALLOW;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_WANT;
+
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
@@ -483,7 +495,7 @@ private void doFetchV2(ProgressMonitor monitor, Collection<Ref> want,
 		clearState();
 		String line = pckIn.readString();
 		// If we sent a done, we may have an error reply here.
-		if (sentDone && line.startsWith("ERR ")) { //$NON-NLS-1$
+		if (sentDone && line.startsWith(PACKET_ERR)) {
 			throw new RemoteRepositoryException(uri, line.substring(4));
 		}
 
@@ -491,7 +503,8 @@ private void doFetchV2(ProgressMonitor monitor, Collection<Ref> want,
 			line = handleShallowUnshallow(shallowCommits, pckIn);
 			if (!PacketLineIn.isDelimiter(line)) {
 				throw new PackProtocolException(MessageFormat
-						.format(JGitText.get().expectedGot, "0001", line)); //$NON-NLS-1$
+						.format(JGitText.get().expectedGot, PACKET_DELIM,
+								line));
 			}
 			line = pckIn.readString();
 		}
@@ -532,7 +545,7 @@ private boolean sendNextHaveBatch(FetchStateV2 fetchState,
 			if (c == null) {
 				break;
 			}
-			output.writeString("have " + c.getId().name() + '\n'); //$NON-NLS-1$
+			output.writeString(PACKET_HAVE + c.getId().name() + '\n');
 			n++;
 			if (n % 10 == 0 && monitor.isCancelled()) {
 				throw new CancelledException();
@@ -543,7 +556,7 @@ private boolean sendNextHaveBatch(FetchStateV2 fetchState,
 				|| (fetchState.hadAcks
 						&& fetchState.havesWithoutAck > MAX_HAVES)
 				|| fetchState.havesTotal > maxHaves) {
-			output.writeString("done\n"); //$NON-NLS-1$
+			output.writeString(PACKET_DONE + '\n');
 			output.end();
 			return true;
 		}
@@ -610,11 +623,12 @@ private boolean readAcknowledgments(FetchStateV2 fetchState,
 		if (gotReady) {
 			if (!PacketLineIn.isDelimiter(line)) {
 				throw new PackProtocolException(MessageFormat
-						.format(JGitText.get().expectedGot, "0001", line)); //$NON-NLS-1$
+						.format(JGitText.get().expectedGot, PACKET_DELIM,
+								line));
 			}
 		} else if (!PacketLineIn.isEnd(line)) {
 			throw new PackProtocolException(MessageFormat
-					.format(JGitText.get().expectedGot, "0000", line)); //$NON-NLS-1$
+					.format(JGitText.get().expectedGot, PACKET_END, line));
 		}
 		return gotReady;
 	}
@@ -726,8 +740,7 @@ private boolean sendWants(Collection<Ref> want, PacketLineOut p)
 			}
 
 			final StringBuilder line = new StringBuilder(46);
-			line.append("want "); //$NON-NLS-1$
-			line.append(objectId.name());
+			line.append(PACKET_WANT).append(objectId.name());
 			if (first && TransferConfig.ProtocolVersion.V0
 					.equals(getProtocolVersion())) {
 				line.append(enableCapabilities());
@@ -836,7 +849,7 @@ private void negotiate(ProgressMonitor monitor, boolean mayHaveShallow, Set<Obje
 			}
 
 			ObjectId o = c.getId();
-			pckOut.writeString("have " + o.name() + "\n"); //$NON-NLS-1$ //$NON-NLS-2$
+			pckOut.writeString(PACKET_HAVE + o.name() + '\n');
 			havesSent++;
 			havesSinceLastContinue++;
 
@@ -939,7 +952,7 @@ private void negotiate(ProgressMonitor monitor, boolean mayHaveShallow, Set<Obje
 			// loop above while in the middle of a request. This allows us
 			// to just write done immediately.
 			//
-			pckOut.writeString("done\n"); //$NON-NLS-1$
+			pckOut.writeString(PACKET_DONE + '\n');
 			pckOut.flush();
 		}
 
@@ -956,7 +969,7 @@ private void negotiate(ProgressMonitor monitor, boolean mayHaveShallow, Set<Obje
 			String line = handleShallowUnshallow(shallowCommits, pckIn);
 			if (!PacketLineIn.isEnd(line)) {
 				throw new PackProtocolException(MessageFormat
-						.format(JGitText.get().expectedGot, "0000", line)); //$NON-NLS-1$
+						.format(JGitText.get().expectedGot, PACKET_END, line));
 			}
 		}
 
@@ -1041,7 +1054,7 @@ private void markAdvertised(AnyObjectId id) {
 	private void markCommon(RevObject obj, AckNackResult anr, boolean useState)
 			throws IOException {
 		if (useState && anr == AckNackResult.ACK_COMMON && !obj.has(STATE)) {
-			pckState.writeString("have " + obj.name() + '\n'); //$NON-NLS-1$
+			pckState.writeString(PACKET_HAVE + obj.name() + '\n');
 			obj.add(STATE);
 		}
 		obj.add(COMMON);
@@ -1074,41 +1087,46 @@ private void receivePack(final ProgressMonitor monitor,
 		}
 	}
 
-	private void sendShallow(Set<ObjectId> shallowCommits, PacketLineOut output) throws IOException {
+	private void sendShallow(Set<ObjectId> shallowCommits, PacketLineOut output)
+			throws IOException {
 		for (ObjectId shallowCommit : shallowCommits) {
-			output.writeString("shallow " + shallowCommit.name()); //$NON-NLS-1$
+			output.writeString(PACKET_SHALLOW + shallowCommit.name());
 		}
 
 		if (depth != null) {
-			output.writeString("deepen " + depth); //$NON-NLS-1$
+			output.writeString(PACKET_DEEPEN + depth);
 		}
 
 		if (deepenSince != null) {
-			output.writeString("deepen-since " + deepenSince.getEpochSecond()); //$NON-NLS-1$
+			output.writeString(
+					PACKET_DEEPEN_SINCE + deepenSince.getEpochSecond());
 		}
 
 		if (deepenNots != null) {
 			for (String deepenNotRef : deepenNots) {
-				output.writeString("deepen-not " + deepenNotRef); //$NON-NLS-1$
+				output.writeString(PACKET_DEEPEN_NOT + deepenNotRef);
 			}
 		}
 	}
 
-	private String handleShallowUnshallow(Set<ObjectId> advertisedShallowCommits, PacketLineIn input)
+	private String handleShallowUnshallow(
+			Set<ObjectId> advertisedShallowCommits, PacketLineIn input)
 			throws IOException {
 		String line = input.readString();
 		ObjectDatabase objectDatabase = local.getObjectDatabase();
-		HashSet<ObjectId> newShallowCommits = new HashSet<>(advertisedShallowCommits);
+		HashSet<ObjectId> newShallowCommits = new HashSet<>(
+				advertisedShallowCommits);
 		while (!PacketLineIn.isDelimiter(line) && !PacketLineIn.isEnd(line)) {
-			if (line.startsWith("shallow ")) { //$NON-NLS-1$
+			if (line.startsWith(PACKET_SHALLOW)) {
 				newShallowCommits.add(ObjectId
-						.fromString(line.substring("shallow ".length()))); //$NON-NLS-1$
-			} else if (line.startsWith("unshallow ")) { //$NON-NLS-1$
+						.fromString(line.substring(PACKET_SHALLOW.length())));
+			} else if (line.startsWith(PACKET_UNSHALLOW)) {
 				ObjectId unshallow = ObjectId
-						.fromString(line.substring("unshallow ".length())); //$NON-NLS-1$
+						.fromString(line.substring(PACKET_UNSHALLOW.length()));
 				if (!advertisedShallowCommits.contains(unshallow)) {
-					throw new PackProtocolException(MessageFormat.format(JGitText.get()
-							.notShallowedUnshallow, unshallow.name()));
+					throw new PackProtocolException(MessageFormat.format(
+							JGitText.get().notShallowedUnshallow,
+							unshallow.name()));
 				}
 				newShallowCommits.remove(unshallow);
 			}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/GitProtocolConstants.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/GitProtocolConstants.java
index 24ea552ba..be14e92d0 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/GitProtocolConstants.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/GitProtocolConstants.java
@@ -343,6 +343,106 @@ public final class GitProtocolConstants {
 	 */
 	public static final String VERSION_2_REQUEST = "version=2"; //$NON-NLS-1$
 
+	/**
+	 * The flush packet.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_FLUSH = "0000"; //$NON-NLS-1$
+
+	/**
+	 * An alias for {@link #PACKET_FLUSH}. "Flush" is the name used in the C git
+	 * documentation; the Java implementation calls this "end" in several
+	 * places.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_END = PACKET_FLUSH;
+
+	/**
+	 * The delimiter packet in protocol V2.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_DELIM = "0001"; //$NON-NLS-1$
+
+	/**
+	 * A "deepen" packet beginning.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_DEEPEN = "deepen "; //$NON-NLS-1$
+
+	/**
+	 * A "deepen-not" packet beginning.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_DEEPEN_NOT = "deepen-not "; //$NON-NLS-1$
+
+	/**
+	 * A "deepen-since" packet beginning.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_DEEPEN_SINCE = "deepen-since "; //$NON-NLS-1$
+
+	/**
+	 * An "ACK" packet beginning.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_ACK = "ACK "; //$NON-NLS-1$
+
+	/**
+	 * A "done" packet beginning.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_DONE = "done"; //$NON-NLS-1$
+
+	/**
+	 * A "ERR" packet beginning.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_ERR = "ERR "; //$NON-NLS-1$
+
+	/**
+	 * A "have" packet beginning.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_HAVE = "have "; //$NON-NLS-1$
+
+	/**
+	 * A "shallow" packet beginning.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_SHALLOW = OPTION_SHALLOW + ' ';
+
+	/**
+	 * A "shallow" packet beginning.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_UNSHALLOW = "unshallow "; //$NON-NLS-1$
+
+	/**
+	 * A "want" packet beginning.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_WANT = "want "; //$NON-NLS-1$
+
+	/**
+	 * A "want-ref" packet beginning.
+	 *
+	 * @since 6.3
+	 */
+	public static final String PACKET_WANT_REF = OPTION_WANT_REF + ' ';
+
 	enum MultiAck {
 		OFF, CONTINUE, DETAILED;
 	}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/ProtocolV0Parser.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/ProtocolV0Parser.java
index 4ddcb9941..21a492577 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/ProtocolV0Parser.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/ProtocolV0Parser.java
@@ -10,6 +10,11 @@
 package org.eclipse.jgit.transport;
 
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_FILTER;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DEEPEN;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DEEPEN_NOT;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DEEPEN_SINCE;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_SHALLOW;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_WANT;
 
 import java.io.EOFException;
 import java.io.IOException;
@@ -70,8 +75,9 @@ FetchV0Request recvWants(PacketLineIn pckIn)
 				break;
 			}
 
-			if (line.startsWith("deepen ")) { //$NON-NLS-1$
-				int depth = Integer.parseInt(line.substring(7));
+			if (line.startsWith(PACKET_DEEPEN)) {
+				int depth = Integer
+						.parseInt(line.substring(PACKET_DEEPEN.length()));
 				if (depth <= 0) {
 					throw new PackProtocolException(
 							MessageFormat.format(JGitText.get().invalidDepth,
@@ -89,8 +95,9 @@ FetchV0Request recvWants(PacketLineIn pckIn)
 				continue;
 			}
 
-			if (line.startsWith("deepen-not ")) { //$NON-NLS-1$
-				reqBuilder.addDeepenNot(line.substring(11));
+			if (line.startsWith(PACKET_DEEPEN_NOT)) {
+				reqBuilder.addDeepenNot(
+						line.substring(PACKET_DEEPEN_NOT.length()));
 				if (reqBuilder.getDepth() != 0) {
 					throw new PackProtocolException(
 							JGitText.get().deepenNotWithDeepen);
@@ -98,9 +105,10 @@ FetchV0Request recvWants(PacketLineIn pckIn)
 				continue;
 			}
 
-			if (line.startsWith("deepen-since ")) { //$NON-NLS-1$
+			if (line.startsWith(PACKET_DEEPEN_SINCE)) {
 				// TODO: timestamps should be long
-				int ts = Integer.parseInt(line.substring(13));
+				int ts = Integer
+						.parseInt(line.substring(PACKET_DEEPEN_SINCE.length()));
 				if (ts <= 0) {
 					throw new PackProtocolException(MessageFormat
 							.format(JGitText.get().invalidTimestamp, line));
@@ -113,9 +121,10 @@ FetchV0Request recvWants(PacketLineIn pckIn)
 				continue;
 			}
 
-			if (line.startsWith("shallow ")) { //$NON-NLS-1$
+			if (line.startsWith(PACKET_SHALLOW)) {
 				reqBuilder.addClientShallowCommit(
-						ObjectId.fromString(line.substring(8)));
+						ObjectId.fromString(
+								line.substring(PACKET_SHALLOW.length())));
 				continue;
 			}
 
@@ -133,7 +142,7 @@ FetchV0Request recvWants(PacketLineIn pckIn)
 				continue;
 			}
 
-			if (!line.startsWith("want ") || line.length() < 45) { //$NON-NLS-1$
+			if (!line.startsWith(PACKET_WANT) || line.length() < 45) {
 				throw new PackProtocolException(MessageFormat
 						.format(JGitText.get().expectedGot, "want", line)); //$NON-NLS-1$
 			}
@@ -147,7 +156,8 @@ FetchV0Request recvWants(PacketLineIn pckIn)
 				}
 			}
 
-			reqBuilder.addWantId(ObjectId.fromString(line.substring(5)));
+			reqBuilder.addWantId(
+					ObjectId.fromString(line.substring(PACKET_WANT.length())));
 			isFirst = false;
 		}
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/ProtocolV2Parser.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/ProtocolV2Parser.java
index e502831a2..b38deb69c 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/ProtocolV2Parser.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/ProtocolV2Parser.java
@@ -20,7 +20,14 @@
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_SIDE_BAND_64K;
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_THIN_PACK;
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_WAIT_FOR_DONE;
-import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_WANT_REF;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DEEPEN;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DEEPEN_NOT;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DEEPEN_SINCE;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DONE;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_HAVE;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_SHALLOW;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_WANT;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_WANT_REF;
 
 import java.io.IOException;
 import java.text.MessageFormat;
@@ -115,15 +122,17 @@ FetchV2Request parseFetchRequest(PacketLineIn pckIn)
 
 		boolean filterReceived = false;
 		for (String line2 : pckIn.readStrings()) {
-			if (line2.startsWith("want ")) { //$NON-NLS-1$
-				reqBuilder.addWantId(ObjectId.fromString(line2.substring(5)));
+			if (line2.startsWith(PACKET_WANT)) {
+				reqBuilder.addWantId(ObjectId
+						.fromString(line2.substring(PACKET_WANT.length())));
 			} else if (transferConfig.isAllowRefInWant()
-					&& line2.startsWith(OPTION_WANT_REF + " ")) { //$NON-NLS-1$
+					&& line2.startsWith(PACKET_WANT_REF)) {
 				reqBuilder.addWantedRef(
-						line2.substring(OPTION_WANT_REF.length() + 1));
-			} else if (line2.startsWith("have ")) { //$NON-NLS-1$
-				reqBuilder.addPeerHas(ObjectId.fromString(line2.substring(5)));
-			} else if (line2.equals("done")) { //$NON-NLS-1$
+						line2.substring(PACKET_WANT_REF.length()));
+			} else if (line2.startsWith(PACKET_HAVE)) {
+				reqBuilder.addPeerHas(ObjectId
+						.fromString(line2.substring(PACKET_HAVE.length())));
+			} else if (line2.equals(PACKET_DONE)) {
 				reqBuilder.setDoneReceived();
 			} else if (line2.equals(OPTION_WAIT_FOR_DONE)) {
 				reqBuilder.setWaitForDone();
@@ -135,11 +144,13 @@ FetchV2Request parseFetchRequest(PacketLineIn pckIn)
 				reqBuilder.addClientCapability(OPTION_INCLUDE_TAG);
 			} else if (line2.equals(OPTION_OFS_DELTA)) {
 				reqBuilder.addClientCapability(OPTION_OFS_DELTA);
-			} else if (line2.startsWith("shallow ")) { //$NON-NLS-1$
+			} else if (line2.startsWith(PACKET_SHALLOW)) {
 				reqBuilder.addClientShallowCommit(
-						ObjectId.fromString(line2.substring(8)));
-			} else if (line2.startsWith("deepen ")) { //$NON-NLS-1$
-				int parsedDepth = Integer.parseInt(line2.substring(7));
+						ObjectId.fromString(
+								line2.substring(PACKET_SHALLOW.length())));
+			} else if (line2.startsWith(PACKET_DEEPEN)) {
+				int parsedDepth = Integer
+						.parseInt(line2.substring(PACKET_DEEPEN.length()));
 				if (parsedDepth <= 0) {
 					throw new PackProtocolException(
 							MessageFormat.format(JGitText.get().invalidDepth,
@@ -154,16 +165,18 @@ FetchV2Request parseFetchRequest(PacketLineIn pckIn)
 							JGitText.get().deepenNotWithDeepen);
 				}
 				reqBuilder.setDepth(parsedDepth);
-			} else if (line2.startsWith("deepen-not ")) { //$NON-NLS-1$
-				reqBuilder.addDeepenNot(line2.substring(11));
+			} else if (line2.startsWith(PACKET_DEEPEN_NOT)) {
+				reqBuilder.addDeepenNot(
+						line2.substring(PACKET_DEEPEN_NOT.length()));
 				if (reqBuilder.getDepth() != 0) {
 					throw new PackProtocolException(
 							JGitText.get().deepenNotWithDeepen);
 				}
 			} else if (line2.equals(OPTION_DEEPEN_RELATIVE)) {
 				reqBuilder.addClientCapability(OPTION_DEEPEN_RELATIVE);
-			} else if (line2.startsWith("deepen-since ")) { //$NON-NLS-1$
-				int ts = Integer.parseInt(line2.substring(13));
+			} else if (line2.startsWith(PACKET_DEEPEN_SINCE)) {
+				int ts = Integer.parseInt(
+						line2.substring(PACKET_DEEPEN_SINCE.length()));
 				if (ts <= 0) {
 					throw new PackProtocolException(MessageFormat
 							.format(JGitText.get().invalidTimestamp, line2));
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/ReceivePack.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/ReceivePack.java
index 2542105c0..b70eedca6 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/ReceivePack.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/ReceivePack.java
@@ -20,6 +20,8 @@
 import static org.eclipse.jgit.transport.GitProtocolConstants.CAPABILITY_REPORT_STATUS;
 import static org.eclipse.jgit.transport.GitProtocolConstants.CAPABILITY_SIDE_BAND_64K;
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_AGENT;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_ERR;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_SHALLOW;
 import static org.eclipse.jgit.transport.SideBandOutputStream.CH_DATA;
 import static org.eclipse.jgit.transport.SideBandOutputStream.CH_ERROR;
 import static org.eclipse.jgit.transport.SideBandOutputStream.CH_PROGRESS;
@@ -1247,7 +1249,7 @@ private void unlockPack() throws IOException {
 	public void sendAdvertisedRefs(RefAdvertiser adv)
 			throws IOException, ServiceMayNotContinueException {
 		if (advertiseError != null) {
-			adv.writeOne("ERR " + advertiseError); //$NON-NLS-1$
+			adv.writeOne(PACKET_ERR + advertiseError);
 			return;
 		}
 
@@ -1255,7 +1257,7 @@ public void sendAdvertisedRefs(RefAdvertiser adv)
 			advertiseRefsHook.advertiseRefs(this);
 		} catch (ServiceMayNotContinueException fail) {
 			if (fail.getMessage() != null) {
-				adv.writeOne("ERR " + fail.getMessage()); //$NON-NLS-1$
+				adv.writeOne(PACKET_ERR + fail.getMessage());
 				fail.setOutput();
 			}
 			throw fail;
@@ -1339,8 +1341,9 @@ private void recvCommands() throws IOException {
 					break;
 				}
 
-				if (line.length() >= 48 && line.startsWith("shallow ")) { //$NON-NLS-1$
-					parseShallow(line.substring(8, 48));
+				int len = PACKET_SHALLOW.length() + 40;
+				if (line.length() >= len && line.startsWith(PACKET_SHALLOW)) {
+					parseShallow(line.substring(PACKET_SHALLOW.length(), len));
 					continue;
 				}
 
@@ -1795,9 +1798,9 @@ private void sendStatusReport(Throwable unpackError) throws IOException {
 			@Override
 			void sendString(String s) throws IOException {
 				if (reportStatus) {
-					pckOut.writeString(s + "\n"); //$NON-NLS-1$
+					pckOut.writeString(s + '\n');
 				} else if (msgOut != null) {
-					msgOut.write(Constants.encode(s + "\n")); //$NON-NLS-1$
+					msgOut.write(Constants.encode(s + '\n'));
 				}
 			}
 		};
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java
index 409161d58..65dbf12b2 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java
@@ -36,6 +36,12 @@
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_SIDE_BAND_64K;
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_THIN_PACK;
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_WAIT_FOR_DONE;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_ACK;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_DONE;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_ERR;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_HAVE;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_SHALLOW;
+import static org.eclipse.jgit.transport.GitProtocolConstants.PACKET_UNSHALLOW;
 import static org.eclipse.jgit.transport.GitProtocolConstants.VERSION_2_REQUEST;
 import static org.eclipse.jgit.util.RefMap.toRefMap;
 
@@ -1076,9 +1082,10 @@ else if (requestValidator instanceof AnyRequestValidator)
 			deepenNots = parseDeepenNots(req.getDeepenNots());
 			if (req.getDepth() != 0 || req.getDeepenSince() != 0 || !req.getDeepenNots().isEmpty()) {
 				computeShallowsAndUnshallows(req, shallow -> {
-					pckOut.writeString("shallow " + shallow.name() + '\n'); //$NON-NLS-1$
+					pckOut.writeString(PACKET_SHALLOW + shallow.name() + '\n');
 				}, unshallow -> {
-					pckOut.writeString("unshallow " + unshallow.name() + '\n'); //$NON-NLS-1$
+					pckOut.writeString(
+							PACKET_UNSHALLOW + unshallow.name() + '\n');
 					unshallowCommits.add(unshallow);
 				}, deepenNots);
 				pckOut.end();
@@ -1227,7 +1234,7 @@ private void fetchV2(PacketLineOut pckOut) throws IOException {
 					GitProtocolConstants.SECTION_ACKNOWLEDGMENTS + '\n');
 			for (ObjectId id : req.getPeerHas()) {
 				if (walk.getObjectReader().has(id)) {
-					pckOut.writeString("ACK " + id.getName() + "\n"); //$NON-NLS-1$ //$NON-NLS-2$
+					pckOut.writeString(PACKET_ACK + id.getName() + '\n');
 				}
 			}
 			processHaveLines(req.getPeerHas(), ObjectId.zeroId(),
@@ -1245,12 +1252,13 @@ private void fetchV2(PacketLineOut pckOut) throws IOException {
 			if (mayHaveShallow) {
 				if (sectionSent)
 					pckOut.writeDelim();
-				pckOut.writeString("shallow-info\n"); //$NON-NLS-1$
+				pckOut.writeString(
+						GitProtocolConstants.SECTION_SHALLOW_INFO + '\n');
 				for (ObjectId o : shallowCommits) {
-					pckOut.writeString("shallow " + o.getName() + '\n'); //$NON-NLS-1$
+					pckOut.writeString(PACKET_SHALLOW + o.getName() + '\n');
 				}
 				for (ObjectId o : unshallowCommits) {
-					pckOut.writeString("unshallow " + o.getName() + '\n'); //$NON-NLS-1$
+					pckOut.writeString(PACKET_UNSHALLOW + o.getName() + '\n');
 				}
 				sectionSent = true;
 			}
@@ -1314,7 +1322,7 @@ private void objectInfo(PacketLineOut pckOut) throws IOException {
 						.format(JGitText.get().missingObject, oid.name()), e);
 			}
 
-			pckOut.writeString(oid.getName() + " " + size); //$NON-NLS-1$
+			pckOut.writeString(oid.getName() + ' ' + size);
 		}
 
 		pckOut.end();
@@ -1386,7 +1394,7 @@ private void serviceV2(PacketLineOut pckOut) throws IOException {
 			protocolV2Hook
 					.onCapabilities(CapabilitiesV2Request.builder().build());
 			for (String s : getV2CapabilityAdvertisement()) {
-				pckOut.writeString(s + "\n"); //$NON-NLS-1$
+				pckOut.writeString(s + '\n');
 			}
 			pckOut.end();
 
@@ -1613,7 +1621,7 @@ public void sendAdvertisedRefs(RefAdvertiser adv,
 	 */
 	public void sendMessage(String what) {
 		try {
-			msgOut.write(Constants.encode(what + "\n")); //$NON-NLS-1$
+			msgOut.write(Constants.encode(what + '\n'));
 		} catch (IOException e) {
 			// Ignore write failures.
 		}
@@ -1720,24 +1728,26 @@ private boolean negotiate(FetchRequest req,
 				if (commonBase.isEmpty() || multiAck != MultiAck.OFF)
 					pckOut.writeString("NAK\n"); //$NON-NLS-1$
 				if (noDone && sentReady) {
-					pckOut.writeString("ACK " + last.name() + "\n"); //$NON-NLS-1$ //$NON-NLS-2$
+					pckOut.writeString(PACKET_ACK + last.name() + '\n');
 					return true;
 				}
 				if (!biDirectionalPipe)
 					return false;
 				pckOut.flush();
 
-			} else if (line.startsWith("have ") && line.length() == 45) { //$NON-NLS-1$
-				peerHas.add(ObjectId.fromString(line.substring(5)));
+			} else if (line.startsWith(PACKET_HAVE)
+					&& line.length() == PACKET_HAVE.length() + 40) {
+				peerHas.add(ObjectId
+						.fromString(line.substring(PACKET_HAVE.length())));
 				accumulator.haves++;
-			} else if (line.equals("done")) { //$NON-NLS-1$
+			} else if (line.equals(PACKET_DONE)) {
 				last = processHaveLines(peerHas, last, pckOut, accumulator, Option.NONE);
 
 				if (commonBase.isEmpty())
 					pckOut.writeString("NAK\n"); //$NON-NLS-1$
 
 				else if (multiAck != MultiAck.OFF)
-					pckOut.writeString("ACK " + last.name() + "\n"); //$NON-NLS-1$ //$NON-NLS-2$
+					pckOut.writeString(PACKET_ACK + last.name() + '\n');
 
 				return true;
 
@@ -1798,14 +1808,15 @@ private ObjectId processHaveLines(List<ObjectId> peerHas, ObjectId last,
 				//
 				switch (multiAck) {
 				case OFF:
-					if (commonBase.size() == 1)
-						out.writeString("ACK " + obj.name() + "\n"); //$NON-NLS-1$ //$NON-NLS-2$
+					if (commonBase.size() == 1) {
+						out.writeString(PACKET_ACK + obj.name() + '\n');
+					}
 					break;
 				case CONTINUE:
-					out.writeString("ACK " + obj.name() + " continue\n"); //$NON-NLS-1$ //$NON-NLS-2$
+					out.writeString(PACKET_ACK + obj.name() + " continue\n"); //$NON-NLS-1$
 					break;
 				case DETAILED:
-					out.writeString("ACK " + obj.name() + " common\n"); //$NON-NLS-1$ //$NON-NLS-2$
+					out.writeString(PACKET_ACK + obj.name() + " common\n"); //$NON-NLS-1$
 					break;
 				}
 			}
@@ -1844,11 +1855,11 @@ private boolean shouldGiveUp(List<ObjectId> peerHas, PacketLineOut out, int miss
 							break;
 						case CONTINUE:
 							out.writeString(
-									"ACK " + id.name() + " continue\n"); //$NON-NLS-1$ //$NON-NLS-2$
+									PACKET_ACK + id.name() + " continue\n"); //$NON-NLS-1$
 							break;
 						case DETAILED:
 							out.writeString(
-									"ACK " + id.name() + " ready\n"); //$NON-NLS-1$ //$NON-NLS-2$
+									PACKET_ACK + id.name() + " ready\n"); //$NON-NLS-1$
 							readySent = true;
 							break;
 						}
@@ -1861,7 +1872,7 @@ private boolean shouldGiveUp(List<ObjectId> peerHas, PacketLineOut out, int miss
 		if (multiAck == MultiAck.DETAILED && !didOkToGiveUp
 				&& okToGiveUp()) {
 			ObjectId id = peerHas.get(peerHas.size() - 1);
-			out.writeString("ACK " + id.name() + " ready\n"); //$NON-NLS-1$ //$NON-NLS-2$
+			out.writeString(PACKET_ACK + id.name() + " ready\n"); //$NON-NLS-1$
 			readySent = true;
 		}
 
@@ -2552,7 +2563,7 @@ private class PackProtocolErrorWriter implements ErrorWriter {
 		@Override
 		public void writeError(String message) throws IOException {
 			new PacketLineOut(requireNonNull(rawOut))
-					.writeString("ERR " + message + '\n'); //$NON-NLS-1$
+					.writeString(PACKET_ERR + message + '\n');
 		}
 	}
 }