patch, 10 partsAdvertise trace2 SID in protocol capabilities

Documentation/technical/protocol-capabilities.txt, Documentation/technical/protocol-v2.txt

diff --git a/Documentation/technical/protocol-capabilities.txt b/Documentation/technical/protocol-capabilities.txt
index ba869a7d36..73d4ee7f27 100644
--- a/Documentation/technical/protocol-capabilities.txt
+++ b/Documentation/technical/protocol-capabilities.txt
@@ -27,8 +27,8 @@ and 'push-cert' capabilities are sent and recognized by the receive-pack
 (push to server) process.
 
 The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
-by both upload-pack and receive-pack protocols.  The 'agent' capability
-may optionally be sent in both protocols.
+by both upload-pack and receive-pack protocols.  The 'agent' and 'trace2-sid'
+capabilities may optionally be sent in both protocols.
 
 All other capabilities are only recognized by the upload-pack (fetch
 from server) process.
@@ -365,3 +365,12 @@ If the upload-pack server advertises the 'filter' capability,
 fetch-pack may send "filter" commands to request a partial clone
 or partial fetch and request that the server omit various objects
 from the packfile.
+
+trace2-sid=<session-id>
+-----------------------
+
+If trace2 tracing is enabled on the server, it may advertise its session ID via
+this capability. The client may choose to log the server's session ID in its
+trace logs, and advertise its own session ID back to the server for it to log
+as well. This allows for easier debugging of remote sessions when both client
+and server logs are available.
diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index e597b74da3..a5b9ef04f6 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -492,3 +492,12 @@ form `object-format=X`) to notify the client that the server is able to deal
 with objects using hash algorithm X.  If not specified, the server is assumed to
 only handle SHA-1.  If the client would like to use a hash algorithm other than
 SHA-1, it should specify its object-format string.
+
+trace2-sid=<session-id>
+~~~~~~~~~~~~~~~~~~~~~~~
+
+If trace2 tracing is enabled on the server, it may advertise its session ID via
+this capability. The client may choose to log the server's session ID in its
+trace logs, and advertise its own session ID back to the server for it to log
+as well. This allows for easier debugging of remote sessions when both client
+and server logs are available.

diff --git a/Documentation/config/trace2.txt b/Documentation/config/trace2.txt
index 01d3afd8a8..3f2e3b4425 100644
--- a/Documentation/config/trace2.txt
+++ b/Documentation/config/trace2.txt
@@ -69,3 +69,7 @@ trace2.maxFiles::
 	write additional traces if we would exceed this many files. Instead,
 	write a sentinel file that will block further tracing to this
 	directory. Defaults to 0, which disables this check.
+
+trace2.advertiseSID::
+	Boolean. When true, client and server processes will advertise their
+	trace2 session IDs to their remote counterpart. Defaults to false.

diff --git a/upload-pack.c b/upload-pack.c
index 3b858eb457..862656010c 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -27,6 +27,7 @@
 #include "commit-graph.h"
 #include "commit-reach.h"
 #include "shallow.h"
+#include "trace2/tr2_sid.h"
 
 /* Remember to update object flag allocation in object.h */
 #define THEY_HAVE	(1u << 11)
@@ -110,6 +111,7 @@ struct upload_pack_data {
 	unsigned done : 1;					/* v2 only */
 	unsigned allow_ref_in_want : 1;				/* v2 only */
 	unsigned allow_sideband_all : 1;			/* v2 only */
+	unsigned advertise_trace2_sid : 1;
 };
 
 static void upload_pack_data_init(struct upload_pack_data *data)
@@ -141,6 +143,7 @@ static void upload_pack_data_init(struct upload_pack_data *data)
 	packet_writer_init(&data->writer, 1);
 
 	data->keepalive = 5;
+	data->advertise_trace2_sid = 0;
 }
 
 static void upload_pack_data_clear(struct upload_pack_data *data)
@@ -1178,6 +1181,11 @@ static void format_symref_info(struct strbuf *buf, struct string_list *symref)
 		strbuf_addf(buf, " symref=%s:%s", item->string, (char *)item->util);
 }
 
+static void format_trace2_info(struct strbuf *buf, struct upload_pack_data *d) {
+	if (d->advertise_trace2_sid && trace2_is_enabled())
+		strbuf_addf(buf, " trace2-sid=%s", tr2_sid_get());
+}
+
 static int send_ref(const char *refname, const struct object_id *oid,
 		    int flag, void *cb_data)
 {
@@ -1193,9 +1201,11 @@ static int send_ref(const char *refname, const struct object_id *oid,
 
 	if (capabilities) {
 		struct strbuf symref_info = STRBUF_INIT;
+		struct strbuf trace2_info = STRBUF_INIT;
 
 		format_symref_info(&symref_info, &data->symref);
-		packet_write_fmt(1, "%s %s%c%s%s%s%s%s%s object-format=%s agent=%s\n",
+		format_trace2_info(&trace2_info, data);
+		packet_write_fmt(1, "%s %s%c%s%s%s%s%s%s%s object-format=%s agent=%s\n",
 			     oid_to_hex(oid), refname_nons,
 			     0, capabilities,
 			     (data->allow_uor & ALLOW_TIP_SHA1) ?
@@ -1205,9 +1215,11 @@ static int send_ref(const char *refname, const struct object_id *oid,
 			     data->stateless_rpc ? " no-done" : "",
 			     symref_info.buf,
 			     data->allow_filter ? " filter" : "",
+			     trace2_info.buf,
 			     the_hash_algo->name,
 			     git_user_agent_sanitized());
 		strbuf_release(&symref_info);
+		strbuf_release(&trace2_info);
 	} else {
 		packet_write_fmt(1, "%s %s\n", oid_to_hex(oid), refname_nons);
 	}
@@ -1299,6 +1311,8 @@ static int upload_pack_config(const char *var, const char *value, void *cb_data)
 		data->allow_sideband_all = git_config_bool(var, value);
 	} else if (!strcmp("core.precomposeunicode", var)) {
 		precomposed_unicode = git_config_bool(var, value);
+	} else if (!strcmp("trace2.advertisesid", var)) {
+		data->advertise_trace2_sid = git_config_bool(var, value);
 	}
 
 	if (current_config_scope() != CONFIG_SCOPE_LOCAL &&

diff --git a/serve.c b/serve.c
index f6341206c4..6ad73d69ab 100644
--- a/serve.c
+++ b/serve.c
@@ -7,6 +7,9 @@
 #include "ls-refs.h"
 #include "serve.h"
 #include "upload-pack.h"
+#include "trace2/tr2_sid.h"
+
+static int advertise_trace2_sid;
 
 static int always_advertise(struct repository *r,
 			    struct strbuf *value)
@@ -30,6 +33,15 @@ static int object_format_advertise(struct repository *r,
 	return 1;
 }
 
+static int trace2_advertise(struct repository *r, struct strbuf *value)
+{
+	if (!advertise_trace2_sid || !trace2_is_enabled())
+		return 0;
+	if (value)
+		strbuf_addstr(value, tr2_sid_get());
+	return 1;
+}
+
 struct protocol_capability {
 	/*
 	 * The name of the capability.  The server uses this name when
@@ -66,6 +78,7 @@ static struct protocol_capability capabilities[] = {
 	{ "fetch", upload_pack_advertise, upload_pack_v2 },
 	{ "server-option", always_advertise, NULL },
 	{ "object-format", object_format_advertise, NULL },
+	{ "trace2-sid", trace2_advertise, NULL },
 };
 
 static void advertise_capabilities(void)
@@ -261,6 +274,8 @@ static int process_request(void)
 /* Main serve loop for protocol version 2 */
 void serve(struct serve_options *options)
 {
+	git_config_get_bool("trace2.advertisesid", &advertise_trace2_sid);
+
 	if (options->advertise_capabilities || !options->stateless_rpc) {
 		/* serve by default supports v2 */
 		packet_write_fmt(1, "version 2\n");

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index bb9909c52e..1ff83c874b 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -29,6 +29,7 @@
 #include "commit-reach.h"
 #include "worktree.h"
 #include "shallow.h"
+#include "trace2/tr2_sid.h"
 
 static const char * const receive_pack_usage[] = {
 	N_("git receive-pack <git-dir>"),
@@ -54,6 +55,7 @@ static int receive_unpack_limit = -1;
 static int transfer_unpack_limit = -1;
 static int advertise_atomic_push = 1;
 static int advertise_push_options;
+static int advertise_trace2_sid;
 static int unpack_limit = 100;
 static off_t max_input_size;
 static int report_status;
@@ -248,6 +250,11 @@ static int receive_pack_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (strcmp(var, "trace2.advertisesid") == 0) {
+		advertise_trace2_sid = git_config_bool(var, value);
+		return 0;
+	}
+
 	return git_default_config(var, value, cb);
 }
 
@@ -268,6 +275,8 @@ static void show_ref(const char *path, const struct object_id *oid)
 			strbuf_addf(&cap, " push-cert=%s", push_cert_nonce);
 		if (advertise_push_options)
 			strbuf_addstr(&cap, " push-options");
+		if (advertise_trace2_sid && trace2_is_enabled())
+			strbuf_addf(&cap, " trace2-sid=%s", tr2_sid_get());
 		strbuf_addf(&cap, " object-format=%s", the_hash_algo->name);
 		strbuf_addf(&cap, " agent=%s", git_user_agent_sanitized());
 		packet_write_fmt(1, "%s %s%c%s\n",

t/t5705-trace2-sid-in-capabilities.sh, transport.c

diff --git a/t/t5705-trace2-sid-in-capabilities.sh b/t/t5705-trace2-sid-in-capabilities.sh
new file mode 100755
index 0000000000..0870e78f7c
--- /dev/null
+++ b/t/t5705-trace2-sid-in-capabilities.sh
@@ -0,0 +1,64 @@
+#!/bin/sh
+
+test_description='trace2 SID in capabilities'
+
+. ./test-lib.sh
+
+REPO="$(pwd)/repo"
+LOCAL_PRISTINE="$(pwd)/local_pristine"
+
+test_expect_success 'setup repos for trace2 SID capability tests' '
+	git init "$REPO" &&
+	test_commit -C "$REPO" a &&
+	git clone "file://$REPO" "$LOCAL_PRISTINE" &&
+	test_commit -C "$REPO" b
+'
+
+for PROTO in 0 1 2
+do
+	test_expect_success "trace2 session IDs not advertised by default (fetch v${PROTO})" '
+		test_when_finished "rm -rf local tr2-client-events" &&
+		cp -r "$LOCAL_PRISTINE" local &&
+		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
+		git -c protocol.version=$PROTO -C local fetch origin &&
+		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)"
+	'
+
+	test_expect_success "trace2 session IDs not advertised by default (push v${PROTO})" '
+		test_when_finished "rm -rf local tr2-client-events" &&
+		cp -r "$LOCAL_PRISTINE" local &&
+		git -C local pull --no-rebase origin &&
+		GIT_TRACE2_EVENT_NESTING=5 \
+		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
+		git -c protocol.version=$PROTO -C local push origin &&
+		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)"
+	'
+done
+
+test_expect_success 'enable SID advertisement' '
+	git -C "$REPO" config trace2.advertiseSID true &&
+	git -C "$LOCAL_PRISTINE" config trace2.advertiseSID true
+'
+
+for PROTO in 0 1 2
+do
+	test_expect_success "trace2 session IDs advertised (fetch v${PROTO})" '
+		test_when_finished "rm -rf local tr2-client-events" &&
+		cp -r "$LOCAL_PRISTINE" local &&
+		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
+		git -c protocol.version=$PROTO -C local fetch origin &&
+		grep \"key\":\"server-sid\" tr2-client-events
+	'
+
+	test_expect_success "trace2 session IDs advertised (push v${PROTO})" '
+		test_when_finished "rm -rf local tr2-client-events" &&
+		cp -r "$LOCAL_PRISTINE" local &&
+		git -C local pull --no-rebase origin &&
+		GIT_TRACE2_EVENT_NESTING=5 \
+		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
+		git -c protocol.version=$PROTO -C local push origin &&
+		grep \"key\":\"server-sid\" tr2-client-events
+	'
+done
+
+test_done
diff --git a/transport.c b/transport.c
index 47da955e4f..d16be597bd 100644
--- a/transport.c
+++ b/transport.c
@@ -286,6 +286,8 @@ static struct ref *handshake(struct transport *transport, int for_push,
 	struct git_transport_data *data = transport->data;
 	struct ref *refs = NULL;
 	struct packet_reader reader;
+	int sid_len;
+	const char *server_trace2_sid;
 
 	connect_setup(transport, for_push);
 
@@ -297,6 +299,8 @@ static struct ref *handshake(struct transport *transport, int for_push,
 	data->version = discover_version(&reader);
 	switch (data->version) {
 	case protocol_v2:
+		if (server_feature_v2("trace2-sid", &server_trace2_sid))
+			trace2_data_string("trace2", NULL, "server-sid", server_trace2_sid);
 		if (must_list_refs)
 			get_remote_refs(data->fd[1], &reader, &refs, for_push,
 					ref_prefixes,
@@ -310,6 +314,12 @@ static struct ref *handshake(struct transport *transport, int for_push,
 				 for_push ? REF_NORMAL : 0,
 				 &data->extra_have,
 				 &data->shallow);
+		server_trace2_sid = server_feature_value("trace2-sid", &sid_len);
+		if (server_trace2_sid) {
+			char *server_sid = xstrndup(server_trace2_sid, sid_len);
+			trace2_data_string("trace2", NULL, "server-sid", server_sid);
+			free(server_sid);
+		}
 		break;
 	case protocol_unknown_version:
 		BUG("unknown protocol version");

diff --git a/fetch-pack.c b/fetch-pack.c
index b10c432315..40065fc7dd 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -23,6 +23,7 @@
 #include "fetch-negotiator.h"
 #include "fsck.h"
 #include "shallow.h"
+#include "trace2/tr2_sid.h"
 
 static int transfer_unpack_limit = -1;
 static int fetch_unpack_limit = -1;
@@ -35,6 +36,8 @@ static int fetch_fsck_objects = -1;
 static int transfer_fsck_objects = -1;
 static int agent_supported;
 static int server_supports_filtering;
+static int server_sent_trace2_sid;
+static int advertise_trace2_sid;
 static struct shallow_lock shallow_lock;
 static const char *alternate_shallow_file;
 static struct strbuf fsck_msg_types = STRBUF_INIT;
@@ -326,6 +329,8 @@ static int find_common(struct fetch_negotiator *negotiator,
 			if (deepen_not_ok)      strbuf_addstr(&c, " deepen-not");
 			if (agent_supported)    strbuf_addf(&c, " agent=%s",
 							    git_user_agent_sanitized());
+			if (advertise_trace2_sid && server_sent_trace2_sid && trace2_is_enabled())
+				strbuf_addf(&c, " trace2-sid=%s", tr2_sid_get());
 			if (args->filter_options.choice)
 				strbuf_addstr(&c, " filter");
 			packet_buf_write(&req_buf, "want %s%s\n", remote_hex, c.buf);
@@ -979,6 +984,9 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 				      agent_len, agent_feature);
 	}
 
+	if (server_supports("trace2-sid"))
+		server_sent_trace2_sid = 1;
+
 	if (server_supports("shallow"))
 		print_verbose(args, _("Server supports %s"), "shallow");
 	else if (args->depth > 0 || is_repository_shallow(r))
@@ -1191,6 +1199,8 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 		packet_buf_write(&req_buf, "command=fetch");
 	if (server_supports_v2("agent", 0))
 		packet_buf_write(&req_buf, "agent=%s", git_user_agent_sanitized());
+	if (advertise_trace2_sid && server_supports_v2("trace2-sid", 0) && trace2_is_enabled())
+		packet_buf_write(&req_buf, "trace2-sid=%s", tr2_sid_get());
 	if (args->server_options && args->server_options->nr &&
 	    server_supports_v2("server-option", 1)) {
 		int i;
@@ -1711,6 +1721,7 @@ static void fetch_pack_config(void)
 	git_config_get_bool("repack.usedeltabaseoffset", &prefer_ofs_delta);
 	git_config_get_bool("fetch.fsckobjects", &fetch_fsck_objects);
 	git_config_get_bool("transfer.fsckobjects", &transfer_fsck_objects);
+	git_config_get_bool("trace2.advertisesid", &advertise_trace2_sid);
 	if (!uri_protocols.nr) {
 		char *str;
 

serve.c, t/t5705-trace2-sid-in-capabilities.sh, upload-pack.c

diff --git a/serve.c b/serve.c
index 6ad73d69ab..6a4f0c64a9 100644
--- a/serve.c
+++ b/serve.c
@@ -202,6 +202,7 @@ static int process_request(void)
 	struct packet_reader reader;
 	struct strvec keys = STRVEC_INIT;
 	struct protocol_capability *command = NULL;
+	const char *client_sid;
 
 	packet_reader_init(&reader, 0, NULL, 0,
 			   PACKET_READ_CHOMP_NEWLINE |
@@ -265,6 +266,9 @@ static int process_request(void)
 
 	check_algorithm(the_repository, &keys);
 
+	if (has_capability(&keys, "trace2-sid", &client_sid))
+		trace2_data_string("trace2", NULL, "client-sid", client_sid);
+
 	command->command(the_repository, &keys, &reader);
 
 	strvec_clear(&keys);
diff --git a/t/t5705-trace2-sid-in-capabilities.sh b/t/t5705-trace2-sid-in-capabilities.sh
index 0870e78f7c..3fad9462d3 100755
--- a/t/t5705-trace2-sid-in-capabilities.sh
+++ b/t/t5705-trace2-sid-in-capabilities.sh
@@ -17,11 +17,14 @@ test_expect_success 'setup repos for trace2 SID capability tests' '
 for PROTO in 0 1 2
 do
 	test_expect_success "trace2 session IDs not advertised by default (fetch v${PROTO})" '
-		test_when_finished "rm -rf local tr2-client-events" &&
+		test_when_finished "rm -rf local tr2-client-events tr2-server-events" &&
 		cp -r "$LOCAL_PRISTINE" local &&
 		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
-		git -c protocol.version=$PROTO -C local fetch origin &&
-		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)"
+		git -c protocol.version=$PROTO -C local fetch \
+			--upload-pack "GIT_TRACE2_EVENT=\"$(pwd)/tr2-server-events\" git-upload-pack" \
+			origin &&
+		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)" &&
+		test -z "$(grep \"key\":\"client-sid\" tr2-server-events)"
 	'
 
 	test_expect_success "trace2 session IDs not advertised by default (push v${PROTO})" '
@@ -43,11 +46,15 @@ test_expect_success 'enable SID advertisement' '
 for PROTO in 0 1 2
 do
 	test_expect_success "trace2 session IDs advertised (fetch v${PROTO})" '
-		test_when_finished "rm -rf local tr2-client-events" &&
+		test_when_finished "rm -rf local tr2-client-events tr2-server-events" &&
 		cp -r "$LOCAL_PRISTINE" local &&
+		git -C local config trace2.advertiseSID true &&
 		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
-		git -c protocol.version=$PROTO -C local fetch origin &&
-		grep \"key\":\"server-sid\" tr2-client-events
+		git -c protocol.version=$PROTO -C local fetch \
+			--upload-pack "GIT_TRACE2_EVENT=\"$(pwd)/tr2-server-events\" git-upload-pack" \
+			origin &&
+		grep \"key\":\"server-sid\" tr2-client-events &&
+		grep \"key\":\"client-sid\" tr2-server-events
 	'
 
 	test_expect_success "trace2 session IDs advertised (push v${PROTO})" '
diff --git a/upload-pack.c b/upload-pack.c
index 862656010c..4238e06510 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -1059,6 +1059,7 @@ static void receive_needs(struct upload_pack_data *data,
 		const char *features;
 		struct object_id oid_buf;
 		const char *arg;
+		int feature_len;
 
 		reset_timeout(data->timeout);
 		if (packet_reader_read(reader) != PACKET_READ_NORMAL)
@@ -1110,6 +1111,12 @@ static void receive_needs(struct upload_pack_data *data,
 		if (data->allow_filter &&
 		    parse_feature_request(features, "filter"))
 			data->filter_capability_requested = 1;
+		if ((arg = parse_feature_value(features, "trace2-sid", &feature_len, 0)))
+		{
+			char *client_sid = xstrndup(arg, feature_len);
+			trace2_data_string("trace2", NULL, "client-sid", client_sid);
+			free(client_sid);
+		}
 
 		o = parse_object(the_repository, &oid_buf);
 		if (!o) {

diff --git a/send-pack.c b/send-pack.c
index eb4a44270b..56c6740e65 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -16,6 +16,7 @@
 #include "gpg-interface.h"
 #include "cache.h"
 #include "shallow.h"
+#include "trace2/tr2_sid.h"
 
 int option_parse_push_signed(const struct option *opt,
 			     const char *arg, int unset)
@@ -424,6 +425,8 @@ int send_pack(struct send_pack_args *args,
 	int use_sideband = 0;
 	int quiet_supported = 0;
 	int agent_supported = 0;
+	int server_sent_trace2_sid = 0;
+	int advertise_trace2_sid = 0;
 	int use_atomic = 0;
 	int atomic_supported = 0;
 	int use_push_options = 0;
@@ -435,6 +438,8 @@ int send_pack(struct send_pack_args *args,
 	const char *push_cert_nonce = NULL;
 	struct packet_reader reader;
 
+	git_config_get_bool("trace2.advertisesid", &advertise_trace2_sid);
+
 	/* Does the other end support the reporting? */
 	if (server_supports("report-status-v2"))
 		status_report = 2;
@@ -450,6 +455,8 @@ int send_pack(struct send_pack_args *args,
 		quiet_supported = 1;
 	if (server_supports("agent"))
 		agent_supported = 1;
+	if (server_supports("trace2-sid"))
+		server_sent_trace2_sid = 1;
 	if (server_supports("no-thin"))
 		args->use_thin_pack = 0;
 	if (server_supports("atomic"))
@@ -506,6 +513,8 @@ int send_pack(struct send_pack_args *args,
 		strbuf_addf(&cap_buf, " object-format=%s", the_hash_algo->name);
 	if (agent_supported)
 		strbuf_addf(&cap_buf, " agent=%s", git_user_agent_sanitized());
+	if (advertise_trace2_sid && server_sent_trace2_sid && trace2_is_enabled())
+		strbuf_addf(&cap_buf, " trace2-sid=%s", tr2_sid_get());
 
 	/*
 	 * NEEDSWORK: why does delete-refs have to be so specific to

builtin/receive-pack.c, t/t5705-trace2-sid-in-capabilities.sh

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 1ff83c874b..34199e4933 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -2040,6 +2040,7 @@ static struct command *read_head_info(struct packet_reader *reader,
 		if (linelen < reader->pktlen) {
 			const char *feature_list = reader->line + linelen + 1;
 			const char *hash = NULL;
+			const char *client_trace2_sid;
 			int len = 0;
 			if (parse_feature_request(feature_list, "report-status"))
 				report_status = 1;
@@ -2062,6 +2063,12 @@ static struct command *read_head_info(struct packet_reader *reader,
 			}
 			if (xstrncmpz(the_hash_algo->name, hash, len))
 				die("error: unsupported object format '%s'", hash);
+			client_trace2_sid = parse_feature_value(feature_list, "trace2-sid", &len, NULL);
+			if (client_trace2_sid) {
+				char *client_sid = xstrndup(client_trace2_sid, len);
+				trace2_data_string("trace2", NULL, "client-sid", client_sid);
+				free(client_sid);
+			}
 		}
 
 		if (!strcmp(reader->line, "push-cert")) {
diff --git a/t/t5705-trace2-sid-in-capabilities.sh b/t/t5705-trace2-sid-in-capabilities.sh
index 3fad9462d3..15f78cb7d2 100755
--- a/t/t5705-trace2-sid-in-capabilities.sh
+++ b/t/t5705-trace2-sid-in-capabilities.sh
@@ -28,13 +28,17 @@ do
 	'
 
 	test_expect_success "trace2 session IDs not advertised by default (push v${PROTO})" '
-		test_when_finished "rm -rf local tr2-client-events" &&
+		test_when_finished "rm -rf local tr2-client-events tr2-server-events" &&
+		test_when_finished "git -C local push --delete origin new-branch" &&
 		cp -r "$LOCAL_PRISTINE" local &&
 		git -C local pull --no-rebase origin &&
 		GIT_TRACE2_EVENT_NESTING=5 \
 		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
-		git -c protocol.version=$PROTO -C local push origin &&
-		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)"
+		git -c protocol.version=$PROTO -C local push \
+			--receive-pack "GIT_TRACE2_EVENT=\"$(pwd)/tr2-server-events\" git-receive-pack" \
+			origin HEAD:new-branch &&
+		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)" &&
+		test -z "$(grep \"key\":\"client-sid\" tr2-server-events)"
 	'
 done
 
@@ -58,13 +62,17 @@ do
 	'
 
 	test_expect_success "trace2 session IDs advertised (push v${PROTO})" '
-		test_when_finished "rm -rf local tr2-client-events" &&
+		test_when_finished "rm -rf local tr2-client-events tr2-server-events" &&
+		test_when_finished "git -C local push --delete origin new-branch" &&
 		cp -r "$LOCAL_PRISTINE" local &&
 		git -C local pull --no-rebase origin &&
 		GIT_TRACE2_EVENT_NESTING=5 \
 		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
-		git -c protocol.version=$PROTO -C local push origin &&
-		grep \"key\":\"server-sid\" tr2-client-events
+		git -c protocol.version=$PROTO -C local push \
+			--receive-pack "GIT_TRACE2_EVENT=\"$(pwd)/tr2-server-events\" git-receive-pack" \
+			origin HEAD:new-branch &&
+		grep \"key\":\"server-sid\" tr2-client-events &&
+		grep \"key\":\"client-sid\" tr2-server-events
 	'
 done
 

diff --git a/Documentation/config/trace2.txt b/Documentation/config/trace2.txt
index 01d3afd8a8..3f2e3b4425 100644
--- a/Documentation/config/trace2.txt
+++ b/Documentation/config/trace2.txt
@@ -69,3 +69,7 @@ trace2.maxFiles::
 	write additional traces if we would exceed this many files. Instead,
 	write a sentinel file that will block further tracing to this
 	directory. Defaults to 0, which disables this check.
+
+trace2.advertiseSID::
+	Boolean. When true, client and server processes will advertise their
+	trace2 session IDs to their remote counterpart. Defaults to false.

Documentation/technical/protocol-capabilities.txt, Documentation/technical/protocol-v2.txt

diff --git a/Documentation/technical/protocol-capabilities.txt b/Documentation/technical/protocol-capabilities.txt
index ba869a7d36..73d4ee7f27 100644
--- a/Documentation/technical/protocol-capabilities.txt
+++ b/Documentation/technical/protocol-capabilities.txt
@@ -27,8 +27,8 @@ and 'push-cert' capabilities are sent and recognized by the receive-pack
 (push to server) process.
 
 The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
-by both upload-pack and receive-pack protocols.  The 'agent' capability
-may optionally be sent in both protocols.
+by both upload-pack and receive-pack protocols.  The 'agent' and 'trace2-sid'
+capabilities may optionally be sent in both protocols.
 
 All other capabilities are only recognized by the upload-pack (fetch
 from server) process.
@@ -365,3 +365,12 @@ If the upload-pack server advertises the 'filter' capability,
 fetch-pack may send "filter" commands to request a partial clone
 or partial fetch and request that the server omit various objects
 from the packfile.
+
+trace2-sid=<session-id>
+-----------------------
+
+If trace2 tracing is enabled on the server, it may advertise its session ID via
+this capability. The client may choose to log the server's session ID in its
+trace logs, and advertise its own session ID back to the server for it to log
+as well. This allows for easier debugging of remote sessions when both client
+and server logs are available.
diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index e597b74da3..a5b9ef04f6 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -492,3 +492,12 @@ form `object-format=X`) to notify the client that the server is able to deal
 with objects using hash algorithm X.  If not specified, the server is assumed to
 only handle SHA-1.  If the client would like to use a hash algorithm other than
 SHA-1, it should specify its object-format string.
+
+trace2-sid=<session-id>
+~~~~~~~~~~~~~~~~~~~~~~~
+
+If trace2 tracing is enabled on the server, it may advertise its session ID via
+this capability. The client may choose to log the server's session ID in its
+trace logs, and advertise its own session ID back to the server for it to log
+as well. This allows for easier debugging of remote sessions when both client
+and server logs are available.

trace2.c, trace2.h

diff --git a/trace2.c b/trace2.c
index 2c6b570077..256120c7fd 100644
--- a/trace2.c
+++ b/trace2.c
@@ -792,3 +792,8 @@ void trace2_printf(const char *fmt, ...)
 	va_end(ap);
 }
 #endif
+
+const char *trace2_session_id(void)
+{
+	return tr2_sid_get();
+}
diff --git a/trace2.h b/trace2.h
index b18bc5529c..ede18c2e06 100644
--- a/trace2.h
+++ b/trace2.h
@@ -500,4 +500,6 @@ void trace2_collect_process_info(enum trace2_process_info_reason reason);
 	} while (0)
 #endif
 
+const char *trace2_session_id(void);
+
 #endif /* TRACE2_H */

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index bb9909c52e..2fe5a8b4fd 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -54,6 +54,7 @@ static int receive_unpack_limit = -1;
 static int transfer_unpack_limit = -1;
 static int advertise_atomic_push = 1;
 static int advertise_push_options;
+static int advertise_trace2_sid;
 static int unpack_limit = 100;
 static off_t max_input_size;
 static int report_status;
@@ -248,6 +249,11 @@ static int receive_pack_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (strcmp(var, "trace2.advertisesid") == 0) {
+		advertise_trace2_sid = git_config_bool(var, value);
+		return 0;
+	}
+
 	return git_default_config(var, value, cb);
 }
 
@@ -268,6 +274,8 @@ static void show_ref(const char *path, const struct object_id *oid)
 			strbuf_addf(&cap, " push-cert=%s", push_cert_nonce);
 		if (advertise_push_options)
 			strbuf_addstr(&cap, " push-options");
+		if (advertise_trace2_sid && trace2_is_enabled())
+			strbuf_addf(&cap, " trace2-sid=%s", trace2_session_id());
 		strbuf_addf(&cap, " object-format=%s", the_hash_algo->name);
 		strbuf_addf(&cap, " agent=%s", git_user_agent_sanitized());
 		packet_write_fmt(1, "%s %s%c%s\n",

diff --git a/upload-pack.c b/upload-pack.c
index 3b858eb457..3bb01c5427 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -110,6 +110,7 @@ struct upload_pack_data {
 	unsigned done : 1;					/* v2 only */
 	unsigned allow_ref_in_want : 1;				/* v2 only */
 	unsigned allow_sideband_all : 1;			/* v2 only */
+	unsigned advertise_trace2_sid : 1;
 };
 
 static void upload_pack_data_init(struct upload_pack_data *data)
@@ -141,6 +142,7 @@ static void upload_pack_data_init(struct upload_pack_data *data)
 	packet_writer_init(&data->writer, 1);
 
 	data->keepalive = 5;
+	data->advertise_trace2_sid = 0;
 }
 
 static void upload_pack_data_clear(struct upload_pack_data *data)
@@ -1178,6 +1180,11 @@ static void format_symref_info(struct strbuf *buf, struct string_list *symref)
 		strbuf_addf(buf, " symref=%s:%s", item->string, (char *)item->util);
 }
 
+static void format_trace2_info(struct strbuf *buf, struct upload_pack_data *d) {
+	if (d->advertise_trace2_sid && trace2_is_enabled())
+		strbuf_addf(buf, " trace2-sid=%s", trace2_session_id());
+}
+
 static int send_ref(const char *refname, const struct object_id *oid,
 		    int flag, void *cb_data)
 {
@@ -1193,9 +1200,11 @@ static int send_ref(const char *refname, const struct object_id *oid,
 
 	if (capabilities) {
 		struct strbuf symref_info = STRBUF_INIT;
+		struct strbuf trace2_info = STRBUF_INIT;
 
 		format_symref_info(&symref_info, &data->symref);
-		packet_write_fmt(1, "%s %s%c%s%s%s%s%s%s object-format=%s agent=%s\n",
+		format_trace2_info(&trace2_info, data);
+		packet_write_fmt(1, "%s %s%c%s%s%s%s%s%s%s object-format=%s agent=%s\n",
 			     oid_to_hex(oid), refname_nons,
 			     0, capabilities,
 			     (data->allow_uor & ALLOW_TIP_SHA1) ?
@@ -1205,9 +1214,11 @@ static int send_ref(const char *refname, const struct object_id *oid,
 			     data->stateless_rpc ? " no-done" : "",
 			     symref_info.buf,
 			     data->allow_filter ? " filter" : "",
+			     trace2_info.buf,
 			     the_hash_algo->name,
 			     git_user_agent_sanitized());
 		strbuf_release(&symref_info);
+		strbuf_release(&trace2_info);
 	} else {
 		packet_write_fmt(1, "%s %s\n", oid_to_hex(oid), refname_nons);
 	}
@@ -1299,6 +1310,8 @@ static int upload_pack_config(const char *var, const char *value, void *cb_data)
 		data->allow_sideband_all = git_config_bool(var, value);
 	} else if (!strcmp("core.precomposeunicode", var)) {
 		precomposed_unicode = git_config_bool(var, value);
+	} else if (!strcmp("trace2.advertisesid", var)) {
+		data->advertise_trace2_sid = git_config_bool(var, value);
 	}
 
 	if (current_config_scope() != CONFIG_SCOPE_LOCAL &&

diff --git a/serve.c b/serve.c
index f6341206c4..b40d7aad34 100644
--- a/serve.c
+++ b/serve.c
@@ -8,6 +8,8 @@
 #include "serve.h"
 #include "upload-pack.h"
 
+static int advertise_trace2_sid;
+
 static int always_advertise(struct repository *r,
 			    struct strbuf *value)
 {
@@ -30,6 +32,15 @@ static int object_format_advertise(struct repository *r,
 	return 1;
 }
 
+static int trace2_advertise(struct repository *r, struct strbuf *value)
+{
+	if (!advertise_trace2_sid || !trace2_is_enabled())
+		return 0;
+	if (value)
+		strbuf_addstr(value, trace2_session_id());
+	return 1;
+}
+
 struct protocol_capability {
 	/*
 	 * The name of the capability.  The server uses this name when
@@ -66,6 +77,7 @@ static struct protocol_capability capabilities[] = {
 	{ "fetch", upload_pack_advertise, upload_pack_v2 },
 	{ "server-option", always_advertise, NULL },
 	{ "object-format", object_format_advertise, NULL },
+	{ "trace2-sid", trace2_advertise, NULL },
 };
 
 static void advertise_capabilities(void)
@@ -261,6 +273,8 @@ static int process_request(void)
 /* Main serve loop for protocol version 2 */
 void serve(struct serve_options *options)
 {
+	git_config_get_bool("trace2.advertisesid", &advertise_trace2_sid);
+
 	if (options->advertise_capabilities || !options->stateless_rpc) {
 		/* serve by default supports v2 */
 		packet_write_fmt(1, "version 2\n");

t/t5705-trace2-sid-in-capabilities.sh, transport.c

diff --git a/t/t5705-trace2-sid-in-capabilities.sh b/t/t5705-trace2-sid-in-capabilities.sh
new file mode 100755
index 0000000000..0870e78f7c
--- /dev/null
+++ b/t/t5705-trace2-sid-in-capabilities.sh
@@ -0,0 +1,64 @@
+#!/bin/sh
+
+test_description='trace2 SID in capabilities'
+
+. ./test-lib.sh
+
+REPO="$(pwd)/repo"
+LOCAL_PRISTINE="$(pwd)/local_pristine"
+
+test_expect_success 'setup repos for trace2 SID capability tests' '
+	git init "$REPO" &&
+	test_commit -C "$REPO" a &&
+	git clone "file://$REPO" "$LOCAL_PRISTINE" &&
+	test_commit -C "$REPO" b
+'
+
+for PROTO in 0 1 2
+do
+	test_expect_success "trace2 session IDs not advertised by default (fetch v${PROTO})" '
+		test_when_finished "rm -rf local tr2-client-events" &&
+		cp -r "$LOCAL_PRISTINE" local &&
+		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
+		git -c protocol.version=$PROTO -C local fetch origin &&
+		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)"
+	'
+
+	test_expect_success "trace2 session IDs not advertised by default (push v${PROTO})" '
+		test_when_finished "rm -rf local tr2-client-events" &&
+		cp -r "$LOCAL_PRISTINE" local &&
+		git -C local pull --no-rebase origin &&
+		GIT_TRACE2_EVENT_NESTING=5 \
+		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
+		git -c protocol.version=$PROTO -C local push origin &&
+		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)"
+	'
+done
+
+test_expect_success 'enable SID advertisement' '
+	git -C "$REPO" config trace2.advertiseSID true &&
+	git -C "$LOCAL_PRISTINE" config trace2.advertiseSID true
+'
+
+for PROTO in 0 1 2
+do
+	test_expect_success "trace2 session IDs advertised (fetch v${PROTO})" '
+		test_when_finished "rm -rf local tr2-client-events" &&
+		cp -r "$LOCAL_PRISTINE" local &&
+		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
+		git -c protocol.version=$PROTO -C local fetch origin &&
+		grep \"key\":\"server-sid\" tr2-client-events
+	'
+
+	test_expect_success "trace2 session IDs advertised (push v${PROTO})" '
+		test_when_finished "rm -rf local tr2-client-events" &&
+		cp -r "$LOCAL_PRISTINE" local &&
+		git -C local pull --no-rebase origin &&
+		GIT_TRACE2_EVENT_NESTING=5 \
+		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
+		git -c protocol.version=$PROTO -C local push origin &&
+		grep \"key\":\"server-sid\" tr2-client-events
+	'
+done
+
+test_done
diff --git a/transport.c b/transport.c
index 47da955e4f..d16be597bd 100644
--- a/transport.c
+++ b/transport.c
@@ -286,6 +286,8 @@ static struct ref *handshake(struct transport *transport, int for_push,
 	struct git_transport_data *data = transport->data;
 	struct ref *refs = NULL;
 	struct packet_reader reader;
+	int sid_len;
+	const char *server_trace2_sid;
 
 	connect_setup(transport, for_push);
 
@@ -297,6 +299,8 @@ static struct ref *handshake(struct transport *transport, int for_push,
 	data->version = discover_version(&reader);
 	switch (data->version) {
 	case protocol_v2:
+		if (server_feature_v2("trace2-sid", &server_trace2_sid))
+			trace2_data_string("trace2", NULL, "server-sid", server_trace2_sid);
 		if (must_list_refs)
 			get_remote_refs(data->fd[1], &reader, &refs, for_push,
 					ref_prefixes,
@@ -310,6 +314,12 @@ static struct ref *handshake(struct transport *transport, int for_push,
 				 for_push ? REF_NORMAL : 0,
 				 &data->extra_have,
 				 &data->shallow);
+		server_trace2_sid = server_feature_value("trace2-sid", &sid_len);
+		if (server_trace2_sid) {
+			char *server_sid = xstrndup(server_trace2_sid, sid_len);
+			trace2_data_string("trace2", NULL, "server-sid", server_sid);
+			free(server_sid);
+		}
 		break;
 	case protocol_unknown_version:
 		BUG("unknown protocol version");

diff --git a/fetch-pack.c b/fetch-pack.c
index b10c432315..7fbefa7b8e 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -35,6 +35,8 @@ static int fetch_fsck_objects = -1;
 static int transfer_fsck_objects = -1;
 static int agent_supported;
 static int server_supports_filtering;
+static int server_sent_trace2_sid;
+static int advertise_trace2_sid;
 static struct shallow_lock shallow_lock;
 static const char *alternate_shallow_file;
 static struct strbuf fsck_msg_types = STRBUF_INIT;
@@ -326,6 +328,8 @@ static int find_common(struct fetch_negotiator *negotiator,
 			if (deepen_not_ok)      strbuf_addstr(&c, " deepen-not");
 			if (agent_supported)    strbuf_addf(&c, " agent=%s",
 							    git_user_agent_sanitized());
+			if (advertise_trace2_sid && server_sent_trace2_sid && trace2_is_enabled())
+				strbuf_addf(&c, " trace2-sid=%s", trace2_session_id());
 			if (args->filter_options.choice)
 				strbuf_addstr(&c, " filter");
 			packet_buf_write(&req_buf, "want %s%s\n", remote_hex, c.buf);
@@ -979,6 +983,9 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 				      agent_len, agent_feature);
 	}
 
+	if (server_supports("trace2-sid"))
+		server_sent_trace2_sid = 1;
+
 	if (server_supports("shallow"))
 		print_verbose(args, _("Server supports %s"), "shallow");
 	else if (args->depth > 0 || is_repository_shallow(r))
@@ -1191,6 +1198,8 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 		packet_buf_write(&req_buf, "command=fetch");
 	if (server_supports_v2("agent", 0))
 		packet_buf_write(&req_buf, "agent=%s", git_user_agent_sanitized());
+	if (advertise_trace2_sid && server_supports_v2("trace2-sid", 0) && trace2_is_enabled())
+		packet_buf_write(&req_buf, "trace2-sid=%s", trace2_session_id());
 	if (args->server_options && args->server_options->nr &&
 	    server_supports_v2("server-option", 1)) {
 		int i;
@@ -1711,6 +1720,7 @@ static void fetch_pack_config(void)
 	git_config_get_bool("repack.usedeltabaseoffset", &prefer_ofs_delta);
 	git_config_get_bool("fetch.fsckobjects", &fetch_fsck_objects);
 	git_config_get_bool("transfer.fsckobjects", &transfer_fsck_objects);
+	git_config_get_bool("trace2.advertisesid", &advertise_trace2_sid);
 	if (!uri_protocols.nr) {
 		char *str;
 

serve.c, t/t5705-trace2-sid-in-capabilities.sh, upload-pack.c

diff --git a/serve.c b/serve.c
index b40d7aad34..ffe5ba3f8a 100644
--- a/serve.c
+++ b/serve.c
@@ -201,6 +201,7 @@ static int process_request(void)
 	struct packet_reader reader;
 	struct strvec keys = STRVEC_INIT;
 	struct protocol_capability *command = NULL;
+	const char *client_sid;
 
 	packet_reader_init(&reader, 0, NULL, 0,
 			   PACKET_READ_CHOMP_NEWLINE |
@@ -264,6 +265,9 @@ static int process_request(void)
 
 	check_algorithm(the_repository, &keys);
 
+	if (has_capability(&keys, "trace2-sid", &client_sid))
+		trace2_data_string("trace2", NULL, "client-sid", client_sid);
+
 	command->command(the_repository, &keys, &reader);
 
 	strvec_clear(&keys);
diff --git a/t/t5705-trace2-sid-in-capabilities.sh b/t/t5705-trace2-sid-in-capabilities.sh
index 0870e78f7c..3fad9462d3 100755
--- a/t/t5705-trace2-sid-in-capabilities.sh
+++ b/t/t5705-trace2-sid-in-capabilities.sh
@@ -17,11 +17,14 @@ test_expect_success 'setup repos for trace2 SID capability tests' '
 for PROTO in 0 1 2
 do
 	test_expect_success "trace2 session IDs not advertised by default (fetch v${PROTO})" '
-		test_when_finished "rm -rf local tr2-client-events" &&
+		test_when_finished "rm -rf local tr2-client-events tr2-server-events" &&
 		cp -r "$LOCAL_PRISTINE" local &&
 		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
-		git -c protocol.version=$PROTO -C local fetch origin &&
-		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)"
+		git -c protocol.version=$PROTO -C local fetch \
+			--upload-pack "GIT_TRACE2_EVENT=\"$(pwd)/tr2-server-events\" git-upload-pack" \
+			origin &&
+		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)" &&
+		test -z "$(grep \"key\":\"client-sid\" tr2-server-events)"
 	'
 
 	test_expect_success "trace2 session IDs not advertised by default (push v${PROTO})" '
@@ -43,11 +46,15 @@ test_expect_success 'enable SID advertisement' '
 for PROTO in 0 1 2
 do
 	test_expect_success "trace2 session IDs advertised (fetch v${PROTO})" '
-		test_when_finished "rm -rf local tr2-client-events" &&
+		test_when_finished "rm -rf local tr2-client-events tr2-server-events" &&
 		cp -r "$LOCAL_PRISTINE" local &&
+		git -C local config trace2.advertiseSID true &&
 		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
-		git -c protocol.version=$PROTO -C local fetch origin &&
-		grep \"key\":\"server-sid\" tr2-client-events
+		git -c protocol.version=$PROTO -C local fetch \
+			--upload-pack "GIT_TRACE2_EVENT=\"$(pwd)/tr2-server-events\" git-upload-pack" \
+			origin &&
+		grep \"key\":\"server-sid\" tr2-client-events &&
+		grep \"key\":\"client-sid\" tr2-server-events
 	'
 
 	test_expect_success "trace2 session IDs advertised (push v${PROTO})" '
diff --git a/upload-pack.c b/upload-pack.c
index 3bb01c5427..d938603c97 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -1058,6 +1058,7 @@ static void receive_needs(struct upload_pack_data *data,
 		const char *features;
 		struct object_id oid_buf;
 		const char *arg;
+		int feature_len;
 
 		reset_timeout(data->timeout);
 		if (packet_reader_read(reader) != PACKET_READ_NORMAL)
@@ -1109,6 +1110,12 @@ static void receive_needs(struct upload_pack_data *data,
 		if (data->allow_filter &&
 		    parse_feature_request(features, "filter"))
 			data->filter_capability_requested = 1;
+		if ((arg = parse_feature_value(features, "trace2-sid", &feature_len, NULL)))
+		{
+			char *client_sid = xstrndup(arg, feature_len);
+			trace2_data_string("trace2", NULL, "client-sid", client_sid);
+			free(client_sid);
+		}
 
 		o = parse_object(the_repository, &oid_buf);
 		if (!o) {

diff --git a/send-pack.c b/send-pack.c
index eb4a44270b..7478a3f7cc 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -424,6 +424,8 @@ int send_pack(struct send_pack_args *args,
 	int use_sideband = 0;
 	int quiet_supported = 0;
 	int agent_supported = 0;
+	int server_sent_trace2_sid = 0;
+	int advertise_trace2_sid = 0;
 	int use_atomic = 0;
 	int atomic_supported = 0;
 	int use_push_options = 0;
@@ -435,6 +437,8 @@ int send_pack(struct send_pack_args *args,
 	const char *push_cert_nonce = NULL;
 	struct packet_reader reader;
 
+	git_config_get_bool("trace2.advertisesid", &advertise_trace2_sid);
+
 	/* Does the other end support the reporting? */
 	if (server_supports("report-status-v2"))
 		status_report = 2;
@@ -450,6 +454,8 @@ int send_pack(struct send_pack_args *args,
 		quiet_supported = 1;
 	if (server_supports("agent"))
 		agent_supported = 1;
+	if (server_supports("trace2-sid"))
+		server_sent_trace2_sid = 1;
 	if (server_supports("no-thin"))
 		args->use_thin_pack = 0;
 	if (server_supports("atomic"))
@@ -506,6 +512,8 @@ int send_pack(struct send_pack_args *args,
 		strbuf_addf(&cap_buf, " object-format=%s", the_hash_algo->name);
 	if (agent_supported)
 		strbuf_addf(&cap_buf, " agent=%s", git_user_agent_sanitized());
+	if (advertise_trace2_sid && server_sent_trace2_sid && trace2_is_enabled())
+		strbuf_addf(&cap_buf, " trace2-sid=%s", trace2_session_id());
 
 	/*
 	 * NEEDSWORK: why does delete-refs have to be so specific to

builtin/receive-pack.c, t/t5705-trace2-sid-in-capabilities.sh

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 2fe5a8b4fd..f780a66a52 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -2039,6 +2039,7 @@ static struct command *read_head_info(struct packet_reader *reader,
 		if (linelen < reader->pktlen) {
 			const char *feature_list = reader->line + linelen + 1;
 			const char *hash = NULL;
+			const char *client_trace2_sid;
 			int len = 0;
 			if (parse_feature_request(feature_list, "report-status"))
 				report_status = 1;
@@ -2061,6 +2062,12 @@ static struct command *read_head_info(struct packet_reader *reader,
 			}
 			if (xstrncmpz(the_hash_algo->name, hash, len))
 				die("error: unsupported object format '%s'", hash);
+			client_trace2_sid = parse_feature_value(feature_list, "trace2-sid", &len, NULL);
+			if (client_trace2_sid) {
+				char *client_sid = xstrndup(client_trace2_sid, len);
+				trace2_data_string("trace2", NULL, "client-sid", client_sid);
+				free(client_sid);
+			}
 		}
 
 		if (!strcmp(reader->line, "push-cert")) {
diff --git a/t/t5705-trace2-sid-in-capabilities.sh b/t/t5705-trace2-sid-in-capabilities.sh
index 3fad9462d3..15f78cb7d2 100755
--- a/t/t5705-trace2-sid-in-capabilities.sh
+++ b/t/t5705-trace2-sid-in-capabilities.sh
@@ -28,13 +28,17 @@ do
 	'
 
 	test_expect_success "trace2 session IDs not advertised by default (push v${PROTO})" '
-		test_when_finished "rm -rf local tr2-client-events" &&
+		test_when_finished "rm -rf local tr2-client-events tr2-server-events" &&
+		test_when_finished "git -C local push --delete origin new-branch" &&
 		cp -r "$LOCAL_PRISTINE" local &&
 		git -C local pull --no-rebase origin &&
 		GIT_TRACE2_EVENT_NESTING=5 \
 		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
-		git -c protocol.version=$PROTO -C local push origin &&
-		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)"
+		git -c protocol.version=$PROTO -C local push \
+			--receive-pack "GIT_TRACE2_EVENT=\"$(pwd)/tr2-server-events\" git-receive-pack" \
+			origin HEAD:new-branch &&
+		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)" &&
+		test -z "$(grep \"key\":\"client-sid\" tr2-server-events)"
 	'
 done
 
@@ -58,13 +62,17 @@ do
 	'
 
 	test_expect_success "trace2 session IDs advertised (push v${PROTO})" '
-		test_when_finished "rm -rf local tr2-client-events" &&
+		test_when_finished "rm -rf local tr2-client-events tr2-server-events" &&
+		test_when_finished "git -C local push --delete origin new-branch" &&
 		cp -r "$LOCAL_PRISTINE" local &&
 		git -C local pull --no-rebase origin &&
 		GIT_TRACE2_EVENT_NESTING=5 \
 		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
-		git -c protocol.version=$PROTO -C local push origin &&
-		grep \"key\":\"server-sid\" tr2-client-events
+		git -c protocol.version=$PROTO -C local push \
+			--receive-pack "GIT_TRACE2_EVENT=\"$(pwd)/tr2-server-events\" git-receive-pack" \
+			origin HEAD:new-branch &&
+		grep \"key\":\"server-sid\" tr2-client-events &&
+		grep \"key\":\"client-sid\" tr2-server-events
 	'
 done
 

diff --git a/serve.c b/serve.c
index f6341206c4..8c0bb84f37 100644
--- a/serve.c
+++ b/serve.c
@@ -8,6 +8,8 @@
 #include "serve.h"
 #include "upload-pack.h"
 
+static int advertise_sid;
+
 static int always_advertise(struct repository *r,
 			    struct strbuf *value)
 {
@@ -30,6 +32,15 @@ static int object_format_advertise(struct repository *r,
 	return 1;
 }
 
+static int session_id_advertise(struct repository *r, struct strbuf *value)
+{
+	if (!advertise_sid)
+		return 0;
+	if (value)
+		strbuf_addstr(value, trace2_session_id());
+	return 1;
+}
+
 struct protocol_capability {
 	/*
 	 * The name of the capability.  The server uses this name when
@@ -66,6 +77,7 @@ static struct protocol_capability capabilities[] = {
 	{ "fetch", upload_pack_advertise, upload_pack_v2 },
 	{ "server-option", always_advertise, NULL },
 	{ "object-format", object_format_advertise, NULL },
+	{ "session-id", session_id_advertise, NULL },
 };
 
 static void advertise_capabilities(void)
@@ -261,6 +273,8 @@ static int process_request(void)
 /* Main serve loop for protocol version 2 */
 void serve(struct serve_options *options)
 {
+	git_config_get_bool("transfer.advertisesid", &advertise_sid);
+
 	if (options->advertise_capabilities || !options->stateless_rpc) {
 		/* serve by default supports v2 */
 		packet_write_fmt(1, "version 2\n");

diff --git a/fetch-pack.c b/fetch-pack.c
index b10c432315..23179b8dd0 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -35,6 +35,7 @@ static int fetch_fsck_objects = -1;
 static int transfer_fsck_objects = -1;
 static int agent_supported;
 static int server_supports_filtering;
+static int advertise_sid;
 static struct shallow_lock shallow_lock;
 static const char *alternate_shallow_file;
 static struct strbuf fsck_msg_types = STRBUF_INIT;
@@ -326,6 +327,8 @@ static int find_common(struct fetch_negotiator *negotiator,
 			if (deepen_not_ok)      strbuf_addstr(&c, " deepen-not");
 			if (agent_supported)    strbuf_addf(&c, " agent=%s",
 							    git_user_agent_sanitized());
+			if (advertise_sid)
+				strbuf_addf(&c, " session-id=%s", trace2_session_id());
 			if (args->filter_options.choice)
 				strbuf_addstr(&c, " filter");
 			packet_buf_write(&req_buf, "want %s%s\n", remote_hex, c.buf);
@@ -979,6 +982,9 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 				      agent_len, agent_feature);
 	}
 
+	if (!server_supports("session-id"))
+		advertise_sid = 0;
+
 	if (server_supports("shallow"))
 		print_verbose(args, _("Server supports %s"), "shallow");
 	else if (args->depth > 0 || is_repository_shallow(r))
@@ -1191,6 +1197,8 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 		packet_buf_write(&req_buf, "command=fetch");
 	if (server_supports_v2("agent", 0))
 		packet_buf_write(&req_buf, "agent=%s", git_user_agent_sanitized());
+	if (advertise_sid && server_supports_v2("session-id", 0))
+		packet_buf_write(&req_buf, "session-id=%s", trace2_session_id());
 	if (args->server_options && args->server_options->nr &&
 	    server_supports_v2("server-option", 1)) {
 		int i;
@@ -1711,6 +1719,7 @@ static void fetch_pack_config(void)
 	git_config_get_bool("repack.usedeltabaseoffset", &prefer_ofs_delta);
 	git_config_get_bool("fetch.fsckobjects", &fetch_fsck_objects);
 	git_config_get_bool("transfer.fsckobjects", &transfer_fsck_objects);
+	git_config_get_bool("transfer.advertisesid", &advertise_sid);
 	if (!uri_protocols.nr) {
 		char *str;
 

t/t5705-session-id-in-capabilities.sh, transport.c

diff --git a/t/t5705-session-id-in-capabilities.sh b/t/t5705-session-id-in-capabilities.sh
new file mode 100755
index 0000000000..9e782f4413
--- /dev/null
+++ b/t/t5705-session-id-in-capabilities.sh
@@ -0,0 +1,64 @@
+#!/bin/sh
+
+test_description='session ID in capabilities'
+
+. ./test-lib.sh
+
+REPO="$(pwd)/repo"
+LOCAL_PRISTINE="$(pwd)/local_pristine"
+
+test_expect_success 'setup repos for session ID capability tests' '
+	git init "$REPO" &&
+	test_commit -C "$REPO" a &&
+	git clone "file://$REPO" "$LOCAL_PRISTINE" &&
+	test_commit -C "$REPO" b
+'
+
+for PROTO in 0 1 2
+do
+	test_expect_success "session IDs not advertised by default (fetch v${PROTO})" '
+		test_when_finished "rm -rf local tr2-client-events" &&
+		cp -r "$LOCAL_PRISTINE" local &&
+		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
+		git -c protocol.version=$PROTO -C local fetch origin &&
+		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)"
+	'
+
+	test_expect_success "session IDs not advertised by default (push v${PROTO})" '
+		test_when_finished "rm -rf local tr2-client-events" &&
+		cp -r "$LOCAL_PRISTINE" local &&
+		git -C local pull --no-rebase origin &&
+		GIT_TRACE2_EVENT_NESTING=5 \
+		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
+		git -c protocol.version=$PROTO -C local push origin &&
+		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)"
+	'
+done
+
+test_expect_success 'enable SID advertisement' '
+	git -C "$REPO" config transfer.advertiseSID true &&
+	git -C "$LOCAL_PRISTINE" config transfer.advertiseSID true
+'
+
+for PROTO in 0 1 2
+do
+	test_expect_success "session IDs advertised (fetch v${PROTO})" '
+		test_when_finished "rm -rf local tr2-client-events" &&
+		cp -r "$LOCAL_PRISTINE" local &&
+		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
+		git -c protocol.version=$PROTO -C local fetch origin &&
+		grep \"key\":\"server-sid\" tr2-client-events
+	'
+
+	test_expect_success "session IDs advertised (push v${PROTO})" '
+		test_when_finished "rm -rf local tr2-client-events" &&
+		cp -r "$LOCAL_PRISTINE" local &&
+		git -C local pull --no-rebase origin &&
+		GIT_TRACE2_EVENT_NESTING=5 \
+		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
+		git -c protocol.version=$PROTO -C local push origin &&
+		grep \"key\":\"server-sid\" tr2-client-events
+	'
+done
+
+test_done
diff --git a/transport.c b/transport.c
index 47da955e4f..679a35e7c1 100644
--- a/transport.c
+++ b/transport.c
@@ -286,6 +286,8 @@ static struct ref *handshake(struct transport *transport, int for_push,
 	struct git_transport_data *data = transport->data;
 	struct ref *refs = NULL;
 	struct packet_reader reader;
+	int sid_len;
+	const char *server_sid;
 
 	connect_setup(transport, for_push);
 
@@ -297,6 +299,8 @@ static struct ref *handshake(struct transport *transport, int for_push,
 	data->version = discover_version(&reader);
 	switch (data->version) {
 	case protocol_v2:
+		if (server_feature_v2("session-id", &server_sid))
+			trace2_data_string("transfer", NULL, "server-sid", server_sid);
 		if (must_list_refs)
 			get_remote_refs(data->fd[1], &reader, &refs, for_push,
 					ref_prefixes,
@@ -310,6 +314,12 @@ static struct ref *handshake(struct transport *transport, int for_push,
 				 for_push ? REF_NORMAL : 0,
 				 &data->extra_have,
 				 &data->shallow);
+		server_sid = server_feature_value("session-id", &sid_len);
+		if (server_sid) {
+			char *sid = xstrndup(server_sid, sid_len);
+			trace2_data_string("transfer", NULL, "server-sid", sid);
+			free(sid);
+		}
 		break;
 	case protocol_unknown_version:
 		BUG("unknown protocol version");

diff --git a/send-pack.c b/send-pack.c
index eb4a44270b..bda65c98f9 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -424,6 +424,7 @@ int send_pack(struct send_pack_args *args,
 	int use_sideband = 0;
 	int quiet_supported = 0;
 	int agent_supported = 0;
+	int advertise_sid = 0;
 	int use_atomic = 0;
 	int atomic_supported = 0;
 	int use_push_options = 0;
@@ -435,6 +436,8 @@ int send_pack(struct send_pack_args *args,
 	const char *push_cert_nonce = NULL;
 	struct packet_reader reader;
 
+	git_config_get_bool("transfer.advertisesid", &advertise_sid);
+
 	/* Does the other end support the reporting? */
 	if (server_supports("report-status-v2"))
 		status_report = 2;
@@ -450,6 +453,8 @@ int send_pack(struct send_pack_args *args,
 		quiet_supported = 1;
 	if (server_supports("agent"))
 		agent_supported = 1;
+	if (!server_supports("session-id"))
+		advertise_sid = 0;
 	if (server_supports("no-thin"))
 		args->use_thin_pack = 0;
 	if (server_supports("atomic"))
@@ -506,6 +511,8 @@ int send_pack(struct send_pack_args *args,
 		strbuf_addf(&cap_buf, " object-format=%s", the_hash_algo->name);
 	if (agent_supported)
 		strbuf_addf(&cap_buf, " agent=%s", git_user_agent_sanitized());
+	if (advertise_sid)
+		strbuf_addf(&cap_buf, " session-id=%s", trace2_session_id());
 
 	/*
 	 * NEEDSWORK: why does delete-refs have to be so specific to

serve.c, t/t5705-session-id-in-capabilities.sh, upload-pack.c

diff --git a/serve.c b/serve.c
index 8c0bb84f37..eec2fe6f29 100644
--- a/serve.c
+++ b/serve.c
@@ -201,6 +201,7 @@ static int process_request(void)
 	struct packet_reader reader;
 	struct strvec keys = STRVEC_INIT;
 	struct protocol_capability *command = NULL;
+	const char *client_sid;
 
 	packet_reader_init(&reader, 0, NULL, 0,
 			   PACKET_READ_CHOMP_NEWLINE |
@@ -264,6 +265,9 @@ static int process_request(void)
 
 	check_algorithm(the_repository, &keys);
 
+	if (has_capability(&keys, "session-id", &client_sid))
+		trace2_data_string("transfer", NULL, "client-sid", client_sid);
+
 	command->command(the_repository, &keys, &reader);
 
 	strvec_clear(&keys);
diff --git a/t/t5705-session-id-in-capabilities.sh b/t/t5705-session-id-in-capabilities.sh
index 9e782f4413..afa2159657 100755
--- a/t/t5705-session-id-in-capabilities.sh
+++ b/t/t5705-session-id-in-capabilities.sh
@@ -17,11 +17,14 @@ test_expect_success 'setup repos for session ID capability tests' '
 for PROTO in 0 1 2
 do
 	test_expect_success "session IDs not advertised by default (fetch v${PROTO})" '
-		test_when_finished "rm -rf local tr2-client-events" &&
+		test_when_finished "rm -rf local tr2-client-events tr2-server-events" &&
 		cp -r "$LOCAL_PRISTINE" local &&
 		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
-		git -c protocol.version=$PROTO -C local fetch origin &&
-		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)"
+		git -c protocol.version=$PROTO -C local fetch \
+			--upload-pack "GIT_TRACE2_EVENT=\"$(pwd)/tr2-server-events\" git-upload-pack" \
+			origin &&
+		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)" &&
+		test -z "$(grep \"key\":\"client-sid\" tr2-server-events)"
 	'
 
 	test_expect_success "session IDs not advertised by default (push v${PROTO})" '
@@ -43,11 +46,14 @@ test_expect_success 'enable SID advertisement' '
 for PROTO in 0 1 2
 do
 	test_expect_success "session IDs advertised (fetch v${PROTO})" '
-		test_when_finished "rm -rf local tr2-client-events" &&
+		test_when_finished "rm -rf local tr2-client-events tr2-server-events" &&
 		cp -r "$LOCAL_PRISTINE" local &&
 		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
-		git -c protocol.version=$PROTO -C local fetch origin &&
-		grep \"key\":\"server-sid\" tr2-client-events
+		git -c protocol.version=$PROTO -C local fetch \
+			--upload-pack "GIT_TRACE2_EVENT=\"$(pwd)/tr2-server-events\" git-upload-pack" \
+			origin &&
+		grep \"key\":\"server-sid\" tr2-client-events &&
+		grep \"key\":\"client-sid\" tr2-server-events
 	'
 
 	test_expect_success "session IDs advertised (push v${PROTO})" '
diff --git a/upload-pack.c b/upload-pack.c
index ebb4099268..dcd429dc01 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -1058,6 +1058,7 @@ static void receive_needs(struct upload_pack_data *data,
 		const char *features;
 		struct object_id oid_buf;
 		const char *arg;
+		int feature_len;
 
 		reset_timeout(data->timeout);
 		if (packet_reader_read(reader) != PACKET_READ_NORMAL)
@@ -1110,6 +1111,13 @@ static void receive_needs(struct upload_pack_data *data,
 		    parse_feature_request(features, "filter"))
 			data->filter_capability_requested = 1;
 
+		arg = parse_feature_value(features, "session-id", &feature_len, NULL);
+		if (arg) {
+			char *client_sid = xstrndup(arg, feature_len);
+			trace2_data_string("transfer", NULL, "client-sid", client_sid);
+			free(client_sid);
+		}
+
 		o = parse_object(the_repository, &oid_buf);
 		if (!o) {
 			packet_writer_error(&data->writer,

builtin/receive-pack.c, t/t5705-session-id-in-capabilities.sh

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 6ed498b6c7..deb5f859a9 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -2039,6 +2039,7 @@ static struct command *read_head_info(struct packet_reader *reader,
 		if (linelen < reader->pktlen) {
 			const char *feature_list = reader->line + linelen + 1;
 			const char *hash = NULL;
+			const char *client_sid;
 			int len = 0;
 			if (parse_feature_request(feature_list, "report-status"))
 				report_status = 1;
@@ -2061,6 +2062,12 @@ static struct command *read_head_info(struct packet_reader *reader,
 			}
 			if (xstrncmpz(the_hash_algo->name, hash, len))
 				die("error: unsupported object format '%s'", hash);
+			client_sid = parse_feature_value(feature_list, "session-id", &len, NULL);
+			if (client_sid) {
+				char *sid = xstrndup(client_sid, len);
+				trace2_data_string("transfer", NULL, "client-sid", client_sid);
+				free(sid);
+			}
 		}
 
 		if (!strcmp(reader->line, "push-cert")) {
diff --git a/t/t5705-session-id-in-capabilities.sh b/t/t5705-session-id-in-capabilities.sh
index afa2159657..f1d189d5bc 100755
--- a/t/t5705-session-id-in-capabilities.sh
+++ b/t/t5705-session-id-in-capabilities.sh
@@ -28,13 +28,17 @@ do
 	'
 
 	test_expect_success "session IDs not advertised by default (push v${PROTO})" '
-		test_when_finished "rm -rf local tr2-client-events" &&
+		test_when_finished "rm -rf local tr2-client-events tr2-server-events" &&
+		test_when_finished "git -C local push --delete origin new-branch" &&
 		cp -r "$LOCAL_PRISTINE" local &&
 		git -C local pull --no-rebase origin &&
 		GIT_TRACE2_EVENT_NESTING=5 \
 		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
-		git -c protocol.version=$PROTO -C local push origin &&
-		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)"
+		git -c protocol.version=$PROTO -C local push \
+			--receive-pack "GIT_TRACE2_EVENT=\"$(pwd)/tr2-server-events\" git-receive-pack" \
+			origin HEAD:new-branch &&
+		test -z "$(grep \"key\":\"server-sid\" tr2-client-events)" &&
+		test -z "$(grep \"key\":\"client-sid\" tr2-server-events)"
 	'
 done
 
@@ -57,13 +61,17 @@ do
 	'
 
 	test_expect_success "session IDs advertised (push v${PROTO})" '
-		test_when_finished "rm -rf local tr2-client-events" &&
+		test_when_finished "rm -rf local tr2-client-events tr2-server-events" &&
+		test_when_finished "git -C local push --delete origin new-branch" &&
 		cp -r "$LOCAL_PRISTINE" local &&
 		git -C local pull --no-rebase origin &&
 		GIT_TRACE2_EVENT_NESTING=5 \
 		GIT_TRACE2_EVENT="$(pwd)/tr2-client-events" \
-		git -c protocol.version=$PROTO -C local push origin &&
-		grep \"key\":\"server-sid\" tr2-client-events
+		git -c protocol.version=$PROTO -C local push \
+			--receive-pack "GIT_TRACE2_EVENT=\"$(pwd)/tr2-server-events\" git-receive-pack" \
+			origin HEAD:new-branch &&
+		grep \"key\":\"server-sid\" tr2-client-events &&
+		grep \"key\":\"client-sid\" tr2-server-events
 	'
 done
 

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index bb9909c52e..6ed498b6c7 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -54,6 +54,7 @@ static int receive_unpack_limit = -1;
 static int transfer_unpack_limit = -1;
 static int advertise_atomic_push = 1;
 static int advertise_push_options;
+static int advertise_sid;
 static int unpack_limit = 100;
 static off_t max_input_size;
 static int report_status;
@@ -248,6 +249,11 @@ static int receive_pack_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (strcmp(var, "transfer.advertisesid") == 0) {
+		advertise_sid = git_config_bool(var, value);
+		return 0;
+	}
+
 	return git_default_config(var, value, cb);
 }
 
@@ -268,6 +274,8 @@ static void show_ref(const char *path, const struct object_id *oid)
 			strbuf_addf(&cap, " push-cert=%s", push_cert_nonce);
 		if (advertise_push_options)
 			strbuf_addstr(&cap, " push-options");
+		if (advertise_sid)
+			strbuf_addf(&cap, " session-id=%s", trace2_session_id());
 		strbuf_addf(&cap, " object-format=%s", the_hash_algo->name);
 		strbuf_addf(&cap, " agent=%s", git_user_agent_sanitized());
 		packet_write_fmt(1, "%s %s%c%s\n",

diff --git a/upload-pack.c b/upload-pack.c
index 3b858eb457..ebb4099268 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -110,6 +110,7 @@ struct upload_pack_data {
 	unsigned done : 1;					/* v2 only */
 	unsigned allow_ref_in_want : 1;				/* v2 only */
 	unsigned allow_sideband_all : 1;			/* v2 only */
+	unsigned advertise_sid : 1;
 };
 
 static void upload_pack_data_init(struct upload_pack_data *data)
@@ -141,6 +142,7 @@ static void upload_pack_data_init(struct upload_pack_data *data)
 	packet_writer_init(&data->writer, 1);
 
 	data->keepalive = 5;
+	data->advertise_sid = 0;
 }
 
 static void upload_pack_data_clear(struct upload_pack_data *data)
@@ -1178,6 +1180,11 @@ static void format_symref_info(struct strbuf *buf, struct string_list *symref)
 		strbuf_addf(buf, " symref=%s:%s", item->string, (char *)item->util);
 }
 
+static void format_session_id(struct strbuf *buf, struct upload_pack_data *d) {
+	if (d->advertise_sid)
+		strbuf_addf(buf, " session-id=%s", trace2_session_id());
+}
+
 static int send_ref(const char *refname, const struct object_id *oid,
 		    int flag, void *cb_data)
 {
@@ -1193,9 +1200,11 @@ static int send_ref(const char *refname, const struct object_id *oid,
 
 	if (capabilities) {
 		struct strbuf symref_info = STRBUF_INIT;
+		struct strbuf session_id = STRBUF_INIT;
 
 		format_symref_info(&symref_info, &data->symref);
-		packet_write_fmt(1, "%s %s%c%s%s%s%s%s%s object-format=%s agent=%s\n",
+		format_session_id(&session_id, data);
+		packet_write_fmt(1, "%s %s%c%s%s%s%s%s%s%s object-format=%s agent=%s\n",
 			     oid_to_hex(oid), refname_nons,
 			     0, capabilities,
 			     (data->allow_uor & ALLOW_TIP_SHA1) ?
@@ -1205,9 +1214,11 @@ static int send_ref(const char *refname, const struct object_id *oid,
 			     data->stateless_rpc ? " no-done" : "",
 			     symref_info.buf,
 			     data->allow_filter ? " filter" : "",
+			     session_id.buf,
 			     the_hash_algo->name,
 			     git_user_agent_sanitized());
 		strbuf_release(&symref_info);
+		strbuf_release(&session_id);
 	} else {
 		packet_write_fmt(1, "%s %s\n", oid_to_hex(oid), refname_nons);
 	}
@@ -1299,6 +1310,8 @@ static int upload_pack_config(const char *var, const char *value, void *cb_data)
 		data->allow_sideband_all = git_config_bool(var, value);
 	} else if (!strcmp("core.precomposeunicode", var)) {
 		precomposed_unicode = git_config_bool(var, value);
+	} else if (!strcmp("transfer.advertisesid", var)) {
+		data->advertise_sid = git_config_bool(var, value);
 	}
 
 	if (current_config_scope() != CONFIG_SCOPE_LOCAL &&

trace2.c, trace2.h

diff --git a/trace2.c b/trace2.c
index 2c6b570077..256120c7fd 100644
--- a/trace2.c
+++ b/trace2.c
@@ -792,3 +792,8 @@ void trace2_printf(const char *fmt, ...)
 	va_end(ap);
 }
 #endif
+
+const char *trace2_session_id(void)
+{
+	return tr2_sid_get();
+}
diff --git a/trace2.h b/trace2.h
index b18bc5529c..ede18c2e06 100644
--- a/trace2.h
+++ b/trace2.h
@@ -500,4 +500,6 @@ void trace2_collect_process_info(enum trace2_process_info_reason reason);
 	} while (0)
 #endif
 
+const char *trace2_session_id(void);
+
 #endif /* TRACE2_H */

diff --git a/Documentation/config/transfer.txt b/Documentation/config/transfer.txt
index f5b6245270..505126a780 100644
--- a/Documentation/config/transfer.txt
+++ b/Documentation/config/transfer.txt
@@ -69,3 +69,7 @@ transfer.unpackLimit::
 	When `fetch.unpackLimit` or `receive.unpackLimit` are
 	not set, the value of this variable is used instead.
 	The default value is 100.
+
+transfer.advertiseSID::
+	Boolean. When true, client and server processes will advertise their
+	unique session IDs to their remote counterpart. Defaults to false.

Documentation/technical/protocol-capabilities.txt, Documentation/technical/protocol-v2.txt

diff --git a/Documentation/technical/protocol-capabilities.txt b/Documentation/technical/protocol-capabilities.txt
index ba869a7d36..9dfade930d 100644
--- a/Documentation/technical/protocol-capabilities.txt
+++ b/Documentation/technical/protocol-capabilities.txt
@@ -27,8 +27,8 @@ and 'push-cert' capabilities are sent and recognized by the receive-pack
 (push to server) process.
 
 The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
-by both upload-pack and receive-pack protocols.  The 'agent' capability
-may optionally be sent in both protocols.
+by both upload-pack and receive-pack protocols.  The 'agent' and 'session-id'
+capabilities may optionally be sent in both protocols.
 
 All other capabilities are only recognized by the upload-pack (fetch
 from server) process.
@@ -365,3 +365,16 @@ If the upload-pack server advertises the 'filter' capability,
 fetch-pack may send "filter" commands to request a partial clone
 or partial fetch and request that the server omit various objects
 from the packfile.
+
+session-id=<session id>
+-----------------------
+
+The server may advertise a session ID that can be used to identify this process
+across multiple requests. The client may advertise its own session ID back to
+the server as well.
+
+Session IDs should be unique to a given process. They must fit within a
+packet-line, and must not contain non-printable or whitespace characters. The
+current implementation uses trace2 session IDs (see
+link:api-trace2.html[api-trace2] for details), but this may change and users of
+the session ID should not rely on this fact.
diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index e597b74da3..85daeb5d9e 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -492,3 +492,16 @@ form `object-format=X`) to notify the client that the server is able to deal
 with objects using hash algorithm X.  If not specified, the server is assumed to
 only handle SHA-1.  If the client would like to use a hash algorithm other than
 SHA-1, it should specify its object-format string.
+
+session-id=<session id>
+~~~~~~~~~~~~~~~~~~~~~~~
+
+The server may advertise a session ID that can be used to identify this process
+across multiple requests. The client may advertise its own session ID back to
+the server as well.
+
+Session IDs should be unique to a given process. They must fit within a
+packet-line, and must not contain non-printable or whitespace characters. The
+current implementation uses trace2 session IDs (see
+link:api-trace2.html[api-trace2] for details), but this may change and users of
+the session ID should not rely on this fact.