diff --git a/libsip/include/sip-uac.h b/libsip/include/sip-uac.h
index 9b6c251b..2a5f835c 100644
--- a/libsip/include/sip-uac.h
+++ b/libsip/include/sip-uac.h
@@ -69,8 +69,9 @@ int sip_uac_send(struct sip_uac_transaction_t* t, const void* data, int bytes, s
 /// @param[in] invite uac transaction, create by sip_uac_invite
 /// @param[in] data message payload(such as SDP), maybe NULL if don't need send anything
 /// @param[in] bytes data length in byte, >=0 only
+/// @param[in] ctype content-type header
 /// @return 0-ok, other-error
-int sip_uac_ack(struct sip_uac_transaction_t* invite, const void* data, int bytes);
+int sip_uac_ack(struct sip_uac_transaction_t* invite, const void* data, int bytes, const char* ctype);
 
 int sip_uac_transaction_addref(struct sip_uac_transaction_t* t);
 int sip_uac_transaction_release(struct sip_uac_transaction_t* t);
diff --git a/libsip/src/uac/sip-uac-ack.c b/libsip/src/uac/sip-uac-ack.c
index d1e61c71..0ae82cb3 100644
--- a/libsip/src/uac/sip-uac-ack.c
+++ b/libsip/src/uac/sip-uac-ack.c
@@ -112,13 +112,14 @@ int sip_uac_ack_3456xx(struct sip_uac_transaction_t* t, const struct sip_message
 }
 
 // 17.1.1.3 Construction of the ACK Request(Section 13.) (p129)
-int sip_uac_ack(struct sip_uac_transaction_t* invite, const void* data, int bytes)
+int sip_uac_ack(struct sip_uac_transaction_t* invite, const void* data, int bytes, const char* content_type)
 {
 	int r;
 	char ptr[1024];
 	char contact[1024];
 	struct sip_message_t* ack;
-	
+	const struct cstring_t* h;
+
 	if (!invite->dialog || !cstrvalid(&invite->dialog->remote.target.host))
 		return -1;
 
@@ -133,6 +134,8 @@ int sip_uac_ack(struct sip_uac_transaction_t* invite, const void* data, int byte
 
 	assert(ack->u.c.uri.scheme.n >= 3 && 0 == strncmp("sip", ack->u.c.uri.scheme.p, 3));
 
+	sip_message_add_header(ack, "Content-Type", content_type);
+
 	// 8.1.1.7 Via (p39)
 	// The branch parameter value MUST be unique across space and time for
 	// all requests sent by the UA.The exceptions to this rule are CANCEL
diff --git a/libsip/src/uac/sip-uac.c b/libsip/src/uac/sip-uac.c
index e634ea86..fa3f20b2 100644
--- a/libsip/src/uac/sip-uac.c
+++ b/libsip/src/uac/sip-uac.c
@@ -7,6 +7,7 @@
 #include "sip-message.h"
 #include "sip-transport.h"
 #include <stdio.h>
+#include <errno.h>
 
 int sip_uac_link_transaction(struct sip_agent_t* sip, struct sip_uac_transaction_t* t)
 {
@@ -248,7 +249,7 @@ int sip_uac_send(struct sip_uac_transaction_t* t, const void* sdp, int bytes, st
 	t->req->size = bytes;
 	t->size = sip_message_write(t->req, t->data, sizeof(t->data));
 	if (t->size < 0 || t->size >= sizeof(t->data))
-		return -1;
+		return -E2BIG; // payload too long
 
 	return sip_uac_transaction_send(t);
 }
diff --git a/libsip/test/sip-agent-test.cpp b/libsip/test/sip-agent-test.cpp
index 6619a13f..9cfe2633 100644
--- a/libsip/test/sip-agent-test.cpp
+++ b/libsip/test/sip-agent-test.cpp
@@ -242,7 +242,7 @@ static int sip_uac_oninvited(void* param, const struct sip_message_t* reply, str
 		if (200 <= code && code < 300)
 		{
 			*session = NULL;
-			sip_uac_ack(t, NULL, 0);
+			sip_uac_ack(t, NULL, 0, NULL);
 		}
 		task->event.Signal();
 	}
diff --git a/libsip/test/sip-message-test.cpp b/libsip/test/sip-message-test.cpp
index 9bd394f0..0936cd8d 100644
--- a/libsip/test/sip-message-test.cpp
+++ b/libsip/test/sip-message-test.cpp
@@ -60,7 +60,7 @@ static int sip_uac_test_oninvite(void* param, const struct sip_message_t* reply,
 	if (200 <= code && code < 300)
 	{
 		*session = test;
-		sip_uac_ack(t, NULL, 0);
+		sip_uac_ack(t, NULL, 0, NULL);
 	}
 	return 0;
 }
diff --git a/libsip/test/sip-uac-message-test.cpp b/libsip/test/sip-uac-message-test.cpp
index 6c1a3f79..208ce733 100644
--- a/libsip/test/sip-uac-message-test.cpp
+++ b/libsip/test/sip-uac-message-test.cpp
@@ -96,7 +96,7 @@ static int sip_uac_message_oninvite(void* param, const struct sip_message_t* rep
 	if (200 <= code && code < 300)
 	{
 		*session = NULL;
-		sip_uac_ack(t, NULL, 0);
+		sip_uac_ack(t, NULL, 0, NULL);
 	}
 	return 0;
 }
diff --git a/libsip/test/sip-uac-test.cpp b/libsip/test/sip-uac-test.cpp
index c95963e7..593881da 100644
--- a/libsip/test/sip-uac-test.cpp
+++ b/libsip/test/sip-uac-test.cpp
@@ -222,7 +222,7 @@ static int sip_uac_oninvited(void* param, const struct sip_message_t* reply, str
 	if (200 <= code && code < 300)
 	{
 		*session = NULL;
-		sip_uac_ack(t, NULL, 0);
+		sip_uac_ack(t, NULL, 0, NULL);
 	}
 	return NULL;
 }
diff --git a/libsip/test/sip-uac-test2.cpp b/libsip/test/sip-uac-test2.cpp
index 5984a19d..60f191b4 100644
--- a/libsip/test/sip-uac-test2.cpp
+++ b/libsip/test/sip-uac-test2.cpp
@@ -900,7 +900,7 @@ static int sip_uac_oninvited(void* param, const struct sip_message_t* reply, str
 		thread_create(&s->th, sip_work_thread, s.get());
 
 		*session = s.get();
-		sip_uac_ack(t, NULL, 0);
+		sip_uac_ack(t, NULL, 0, NULL);
 		return 0;
 	}
 	else if (code == 183)