diff --git a/src/core/channel/call_op_string.c b/src/core/channel/call_op_string.c
index 9a7838ce2fc..789913901ad 100644
--- a/src/core/channel/call_op_string.c
+++ b/src/core/channel/call_op_string.c
@@ -37,8 +37,8 @@
 #include <stdio.h>
 #include <string.h>
 
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
-#include <grpc/support/string.h>
 #include <grpc/support/useful.h>
 
 #define MAX_APPEND 1024
diff --git a/src/core/channel/channel_args.c b/src/core/channel/channel_args.c
index 36312e54de8..04ce519ff37 100644
--- a/src/core/channel/channel_args.c
+++ b/src/core/channel/channel_args.c
@@ -33,9 +33,9 @@
 
 #include <grpc/grpc.h>
 #include "src/core/channel/channel_args.h"
+#include "src/core/support/string.h"
 
 #include <grpc/support/alloc.h>
-#include <grpc/support/string.h>
 
 #include <string.h>
 
diff --git a/src/core/channel/client_channel.c b/src/core/channel/client_channel.c
index fa75561c78b..f6078aa8080 100644
--- a/src/core/channel/client_channel.c
+++ b/src/core/channel/client_channel.c
@@ -40,9 +40,9 @@
 #include "src/core/channel/connected_channel.h"
 #include "src/core/channel/metadata_buffer.h"
 #include "src/core/iomgr/iomgr.h"
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include <grpc/support/sync.h>
 #include <grpc/support/useful.h>
 
diff --git a/src/core/channel/connected_channel.c b/src/core/channel/connected_channel.c
index e01cb81a890..cfa76f9e488 100644
--- a/src/core/channel/connected_channel.c
+++ b/src/core/channel/connected_channel.c
@@ -37,12 +37,12 @@
 #include <stdio.h>
 #include <string.h>
 
+#include "src/core/support/string.h"
 #include "src/core/transport/transport.h"
 #include <grpc/byte_buffer.h>
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
 #include <grpc/support/slice_buffer.h>
-#include <grpc/support/string.h>
 
 #define MAX_BUFFER_LENGTH 8192
 /* the protobuf library will (by default) start warning at 100megs */
diff --git a/src/core/httpcli/httpcli.c b/src/core/httpcli/httpcli.c
index 2143eeb63d0..d6fd8ca8658 100644
--- a/src/core/httpcli/httpcli.c
+++ b/src/core/httpcli/httpcli.c
@@ -44,9 +44,9 @@
 #include "src/core/security/security_context.h"
 #include "src/core/security/google_root_certs.h"
 #include "src/core/security/secure_transport_setup.h"
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 
 typedef struct {
   gpr_slice request_text;
diff --git a/src/core/httpcli/httpcli_security_context.c b/src/core/httpcli/httpcli_security_context.c
index c7b9b330f0d..d074e163f16 100644
--- a/src/core/httpcli/httpcli_security_context.c
+++ b/src/core/httpcli/httpcli_security_context.c
@@ -36,9 +36,9 @@
 #include <string.h>
 
 #include "src/core/security/secure_transport_setup.h"
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include "src/core/tsi/ssl_transport_security.h"
 
 typedef struct {
diff --git a/src/core/iomgr/resolve_address_posix.c b/src/core/iomgr/resolve_address_posix.c
index c9c2c5378a5..f80eea7f46e 100644
--- a/src/core/iomgr/resolve_address_posix.c
+++ b/src/core/iomgr/resolve_address_posix.c
@@ -44,9 +44,9 @@
 #include "src/core/iomgr/iomgr_internal.h"
 #include "src/core/iomgr/sockaddr_utils.h"
 #include "src/core/iomgr/socket_utils_posix.h"
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include <grpc/support/thd.h>
 #include <grpc/support/time.h>
 
diff --git a/src/core/iomgr/sockaddr_utils.c b/src/core/iomgr/sockaddr_utils.c
index eca14a4f398..5bb11242843 100644
--- a/src/core/iomgr/sockaddr_utils.c
+++ b/src/core/iomgr/sockaddr_utils.c
@@ -37,8 +37,8 @@
 #include <errno.h>
 #include <string.h>
 
+#include "src/core/support/string.h"
 #include <grpc/support/host_port.h>
-#include <grpc/support/string.h>
 #include <grpc/support/log.h>
 #include <grpc/support/port_platform.h>
 
diff --git a/src/core/iomgr/socket_utils_common_posix.c b/src/core/iomgr/socket_utils_common_posix.c
index 3a0639f3567..1854285b5a4 100644
--- a/src/core/iomgr/socket_utils_common_posix.c
+++ b/src/core/iomgr/socket_utils_common_posix.c
@@ -50,8 +50,8 @@
 #include <errno.h>
 
 #include "src/core/iomgr/sockaddr_utils.h"
+#include "src/core/support/string.h"
 #include <grpc/support/host_port.h>
-#include <grpc/support/string.h>
 #include <grpc/support/log.h>
 #include <grpc/support/port_platform.h>
 #include <grpc/support/sync.h>
diff --git a/src/core/iomgr/tcp_posix.c b/src/core/iomgr/tcp_posix.c
index 64996bd07d1..a9b59df8854 100644
--- a/src/core/iomgr/tcp_posix.c
+++ b/src/core/iomgr/tcp_posix.c
@@ -44,10 +44,10 @@
 #include <sys/socket.h>
 #include <unistd.h>
 
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
 #include <grpc/support/slice.h>
-#include <grpc/support/string.h>
 #include <grpc/support/sync.h>
 #include <grpc/support/time.h>
 
diff --git a/src/core/security/credentials.c b/src/core/security/credentials.c
index 628963e46cd..db73c01c1de 100644
--- a/src/core/security/credentials.c
+++ b/src/core/security/credentials.c
@@ -36,9 +36,9 @@
 #include "src/core/httpcli/httpcli.h"
 #include "src/core/iomgr/iomgr.h"
 #include "src/core/security/json_token.h"
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include <grpc/support/sync.h>
 #include <grpc/support/time.h>
 
diff --git a/src/core/security/json_token.c b/src/core/security/json_token.c
index 14ee758e8bf..82bd9b505a2 100644
--- a/src/core/security/json_token.c
+++ b/src/core/security/json_token.c
@@ -37,9 +37,9 @@
 
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 
 #include "src/core/security/base64.h"
+#include "src/core/support/string.h"
 
 #include <openssl/bio.h>
 #include <openssl/evp.h>
diff --git a/src/core/security/secure_endpoint.c b/src/core/security/secure_endpoint.c
index e73767c1aad..9f12cf5d60f 100644
--- a/src/core/security/secure_endpoint.c
+++ b/src/core/security/secure_endpoint.c
@@ -32,11 +32,11 @@
  */
 
 #include "src/core/security/secure_endpoint.h"
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
 #include <grpc/support/slice_buffer.h>
 #include <grpc/support/slice.h>
-#include <grpc/support/string.h>
 #include <grpc/support/sync.h>
 #include "src/core/tsi/transport_security_interface.h"
 
diff --git a/src/core/security/security_context.c b/src/core/security/security_context.c
index cce3c7fe04f..58cd458415d 100644
--- a/src/core/security/security_context.c
+++ b/src/core/security/security_context.c
@@ -39,12 +39,12 @@
 #include "src/core/channel/http_client_filter.h"
 #include "src/core/security/credentials.h"
 #include "src/core/security/secure_endpoint.h"
+#include "src/core/support/string.h"
 #include "src/core/surface/lame_client.h"
 #include "src/core/transport/chttp2/alpn.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
 #include <grpc/support/slice_buffer.h>
-#include <grpc/support/string.h>
 #include "src/core/tsi/fake_transport_security.h"
 #include "src/core/tsi/ssl_transport_security.h"
 
diff --git a/src/core/statistics/census_rpc_stats.c b/src/core/statistics/census_rpc_stats.c
index 39094b5f65a..dd3c07e80b3 100644
--- a/src/core/statistics/census_rpc_stats.c
+++ b/src/core/statistics/census_rpc_stats.c
@@ -39,9 +39,9 @@
 #include "src/core/statistics/census_tracing.h"
 #include "src/core/statistics/window_stats.h"
 #include "src/core/support/murmur_hash.h"
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include <grpc/support/sync.h>
 
 #define NUM_INTERVALS 3
diff --git a/src/core/statistics/census_tracing.c b/src/core/statistics/census_tracing.c
index 1e616020715..3c4ba66f5f4 100644
--- a/src/core/statistics/census_tracing.c
+++ b/src/core/statistics/census_tracing.c
@@ -38,10 +38,10 @@
 
 #include "src/core/statistics/census_rpc_stats.h"
 #include "src/core/statistics/hash_table.h"
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
 #include <grpc/support/port_platform.h>
-#include <grpc/support/string.h>
 #include <grpc/support/sync.h>
 #include <grpc/support/time.h>
 
diff --git a/src/core/support/cmdline.c b/src/core/support/cmdline.c
index ff163a1f6cc..a55da9dd188 100644
--- a/src/core/support/cmdline.c
+++ b/src/core/support/cmdline.c
@@ -37,9 +37,9 @@
 #include <stdio.h>
 #include <string.h>
 
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 
 typedef enum { ARGTYPE_INT, ARGTYPE_BOOL, ARGTYPE_STRING } argtype;
 
diff --git a/src/core/support/host_port.c b/src/core/support/host_port.c
index 02500551fcd..446c11ebec3 100644
--- a/src/core/support/host_port.c
+++ b/src/core/support/host_port.c
@@ -35,8 +35,8 @@
 
 #include <string.h>
 
+#include "src/core/support/string.h"
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 
 int gpr_join_host_port(char **out, const char *host, int port) {
   if (host[0] != '[' && strchr(host, ':') != NULL) {
diff --git a/src/core/support/string.c b/src/core/support/string.c
index 7960547735b..9b5cac75966 100644
--- a/src/core/support/string.c
+++ b/src/core/support/string.c
@@ -31,7 +31,7 @@
  *
  */
 
-#include <grpc/support/string.h>
+#include "src/core/support/string.h"
 
 #include <ctype.h>
 #include <stddef.h>
@@ -122,3 +122,33 @@ int gpr_parse_bytes_to_uint32(const char *buf, size_t len, gpr_uint32 *result) {
   *result = out;
   return 1;
 }
+
+void gpr_reverse_bytes(char *str, int len) {
+  char *p1, *p2;
+  for (p1 = str, p2 = str + len - 1; p2 > p1; ++p1, --p2) {
+    char temp = *p1;
+    *p1 = *p2;
+    *p2 = temp;
+  }
+}
+
+int gpr_ltoa(long value, char *string) {
+  int i = 0;
+  int neg = value < 0;
+
+  if (value == 0) {
+    string[0] = '0';
+    string[1] = 0;
+    return 1;
+  }
+
+  if (neg) value = -value;
+  while (value) {
+    string[i++] = '0' + value % 10;
+    value /= 10;
+  }
+  if (neg) string[i++] = '-';
+  gpr_reverse_bytes(string, i);
+  string[i] = 0;
+  return i;
+}
diff --git a/include/grpc/support/string.h b/src/core/support/string.h
similarity index 88%
rename from include/grpc/support/string.h
rename to src/core/support/string.h
index 68e7452a7fa..28b7029ecd6 100644
--- a/include/grpc/support/string.h
+++ b/src/core/support/string.h
@@ -60,6 +60,17 @@ char *gpr_hexdump(const char *buf, size_t len, gpr_uint32 flags);
 int gpr_parse_bytes_to_uint32(const char *data, size_t length,
                               gpr_uint32 *result);
 
+/* Minimum buffer size for calling ltoa */
+#define GPR_LTOA_MIN_BUFSIZE (3 * sizeof(long))
+
+/* Convert a long to a string in base 10; returns the length of the
+   output string (or 0 on failure).
+   output must be at least GPR_LTOA_MIN_BUFSIZE bytes long. */
+int gpr_ltoa(long value, char *output);
+
+/* Reverse a run of bytes */
+void gpr_reverse_bytes(char *str, int len);
+
 /* printf to a newly-allocated string.  The set of supported formats may vary
    between platforms.
 
diff --git a/src/core/surface/call.c b/src/core/surface/call.c
index 05f66612548..bb5aa41f11b 100644
--- a/src/core/surface/call.c
+++ b/src/core/surface/call.c
@@ -35,11 +35,11 @@
 #include "src/core/channel/channel_stack.h"
 #include "src/core/channel/metadata_buffer.h"
 #include "src/core/iomgr/alarm.h"
+#include "src/core/support/string.h"
 #include "src/core/surface/channel.h"
 #include "src/core/surface/completion_queue.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 
 #include <stdio.h>
 #include <stdlib.h>
diff --git a/src/core/surface/channel_create.c b/src/core/surface/channel_create.c
index 41093d78eff..6939b92c60c 100644
--- a/src/core/surface/channel_create.c
+++ b/src/core/surface/channel_create.c
@@ -48,10 +48,10 @@
 #include "src/core/iomgr/tcp_client.h"
 #include "src/core/surface/channel.h"
 #include "src/core/surface/client.h"
+#include "src/core/support/string.h"
 #include "src/core/transport/chttp2_transport.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include <grpc/support/sync.h>
 #include <grpc/support/useful.h>
 
diff --git a/src/core/surface/client.c b/src/core/surface/client.c
index 74c79bdf9b1..fe3a81f1b9f 100644
--- a/src/core/surface/client.c
+++ b/src/core/surface/client.c
@@ -34,9 +34,9 @@
 #include "src/core/surface/client.h"
 
 #include "src/core/surface/call.h"
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 
 typedef struct { void *unused; } call_data;
 
diff --git a/src/core/surface/completion_queue.c b/src/core/surface/completion_queue.c
index 652f23e8889..e1cc114cda6 100644
--- a/src/core/surface/completion_queue.c
+++ b/src/core/surface/completion_queue.c
@@ -37,13 +37,13 @@
 #include <string.h>
 
 #include "src/core/iomgr/pollset.h"
+#include "src/core/support/string.h"
 #include "src/core/surface/call.h"
 #include "src/core/surface/event_string.h"
 #include "src/core/surface/surface_trace.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/atm.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 
 #define NUM_TAG_BUCKETS 31
 
diff --git a/src/core/surface/event_string.c b/src/core/surface/event_string.c
index 8ae2af74726..e38ef06c9ff 100644
--- a/src/core/surface/event_string.c
+++ b/src/core/surface/event_string.c
@@ -35,7 +35,7 @@
 
 #include <stdio.h>
 
-#include <grpc/support/string.h>
+#include "src/core/support/string.h"
 #include <grpc/byte_buffer.h>
 
 static size_t addhdr(char *p, grpc_event *ev) {
diff --git a/src/core/surface/lame_client.c b/src/core/surface/lame_client.c
index a5244dbe61f..056c98646b8 100644
--- a/src/core/surface/lame_client.c
+++ b/src/core/surface/lame_client.c
@@ -36,11 +36,11 @@
 #include <string.h>
 
 #include "src/core/channel/channel_stack.h"
+#include "src/core/support/string.h"
 #include "src/core/surface/channel.h"
 #include "src/core/surface/call.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 
 typedef struct { void *unused; } call_data;
 
diff --git a/src/core/surface/secure_channel_create.c b/src/core/surface/secure_channel_create.c
index 3d5727927d0..a231b2708e6 100644
--- a/src/core/surface/secure_channel_create.c
+++ b/src/core/surface/secure_channel_create.c
@@ -48,13 +48,13 @@
 #include "src/core/security/auth.h"
 #include "src/core/security/security_context.h"
 #include "src/core/security/secure_transport_setup.h"
+#include "src/core/support/string.h"
 #include "src/core/surface/channel.h"
 #include "src/core/surface/client.h"
 #include "src/core/transport/chttp2_transport.h"
 #include <grpc/grpc_security.h>
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include <grpc/support/sync.h>
 #include <grpc/support/useful.h>
 #include "src/core/tsi/transport_security_interface.h"
diff --git a/src/core/surface/server.c b/src/core/surface/server.c
index cbdd3bfa308..9585e4e8ea6 100644
--- a/src/core/surface/server.c
+++ b/src/core/surface/server.c
@@ -40,12 +40,12 @@
 #include "src/core/channel/channel_args.h"
 #include "src/core/channel/connected_channel.h"
 #include "src/core/iomgr/iomgr.h"
+#include "src/core/support/string.h"
 #include "src/core/surface/call.h"
 #include "src/core/surface/channel.h"
 #include "src/core/surface/completion_queue.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include <grpc/support/useful.h>
 
 typedef enum { PENDING_START, ALL_CALLS, CALL_LIST_COUNT } call_list;
diff --git a/src/core/transport/chttp2/frame_data.c b/src/core/transport/chttp2/frame_data.c
index 00b020b31b1..dee61cee500 100644
--- a/src/core/transport/chttp2/frame_data.c
+++ b/src/core/transport/chttp2/frame_data.c
@@ -35,9 +35,9 @@
 
 #include <string.h>
 
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include <grpc/support/useful.h>
 #include "src/core/transport/transport.h"
 
diff --git a/src/core/transport/chttp2/hpack_parser.c b/src/core/transport/chttp2/hpack_parser.c
index 64e08ffac72..c98b90e5d11 100644
--- a/src/core/transport/chttp2/hpack_parser.c
+++ b/src/core/transport/chttp2/hpack_parser.c
@@ -38,10 +38,10 @@
 #include <assert.h>
 
 #include "src/core/transport/chttp2/bin_encoder.h"
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
 #include <grpc/support/port_platform.h>
-#include <grpc/support/string.h>
 #include <grpc/support/useful.h>
 
 typedef enum {
diff --git a/src/core/transport/chttp2/timeout_encoding.c b/src/core/transport/chttp2/timeout_encoding.c
index 2706c369a6d..23c4554cf27 100644
--- a/src/core/transport/chttp2/timeout_encoding.c
+++ b/src/core/transport/chttp2/timeout_encoding.c
@@ -36,6 +36,8 @@
 #include <stdio.h>
 #include <string.h>
 
+#include "src/core/support/string.h"
+
 static int round_up(int x, int divisor) {
   return (x / divisor + (x % divisor != 0)) * divisor;
 }
@@ -53,15 +55,21 @@ static int round_up_to_three_sig_figs(int x) {
 }
 
 /* encode our minimum viable timeout value */
-static void enc_tiny(char *buffer) { strcpy(buffer, "1n"); }
+static void enc_tiny(char *buffer) { memcpy(buffer, "1n", 3); }
+
+static void enc_ext(char *buffer, long value, char ext) {
+  int n = gpr_ltoa(value, buffer);
+  buffer[n] = ext;
+  buffer[n+1] = 0;
+}
 
 static void enc_seconds(char *buffer, long sec) {
   if (sec % 3600 == 0) {
-    sprintf(buffer, "%ldH", sec / 3600);
+    enc_ext(buffer, sec / 3600, 'H');
   } else if (sec % 60 == 0) {
-    sprintf(buffer, "%ldM", sec / 60);
+    enc_ext(buffer, sec / 60, 'M');
   } else {
-    sprintf(buffer, "%ldS", sec);
+    enc_ext(buffer, sec, 'S');
   }
 }
 
@@ -69,23 +77,23 @@ static void enc_nanos(char *buffer, int x) {
   x = round_up_to_three_sig_figs(x);
   if (x < 100000) {
     if (x % 1000 == 0) {
-      sprintf(buffer, "%du", x / 1000);
+      enc_ext(buffer, x / 1000, 'u');
     } else {
-      sprintf(buffer, "%dn", x);
+      enc_ext(buffer, x, 'n');
     }
   } else if (x < 100000000) {
     if (x % 1000000 == 0) {
-      sprintf(buffer, "%dm", x / 1000000);
+      enc_ext(buffer, x / 1000000, 'm');
     } else {
-      sprintf(buffer, "%du", x / 1000);
+      enc_ext(buffer, x / 1000, 'u');
     }
   } else if (x < 1000000000) {
-    sprintf(buffer, "%dm", x / 1000000);
+    enc_ext(buffer, x / 1000000, 'm');
   } else {
     /* note that this is only ever called with times of less than one second,
        so if we reach here the time must have been rounded up to a whole second
        (and no more) */
-    strcpy(buffer, "1S");
+    memcpy(buffer, "1S", 3);
   }
 }
 
@@ -93,18 +101,18 @@ static void enc_micros(char *buffer, int x) {
   x = round_up_to_three_sig_figs(x);
   if (x < 100000) {
     if (x % 1000 == 0) {
-      sprintf(buffer, "%dm", x / 1000);
+      enc_ext(buffer, x / 1000, 'm');
     } else {
-      sprintf(buffer, "%du", x);
+      enc_ext(buffer, x, 'u');
     }
   } else if (x < 100000000) {
     if (x % 1000000 == 0) {
-      sprintf(buffer, "%dS", x / 1000000);
+      enc_ext(buffer, x / 1000000, 'S');
     } else {
-      sprintf(buffer, "%dm", x / 1000);
+      enc_ext(buffer, x / 1000, 'm');
     }
   } else {
-    sprintf(buffer, "%dS", x / 1000000);
+    enc_ext(buffer, x / 1000000, 'S');
   }
 }
 
diff --git a/src/core/transport/chttp2/timeout_encoding.h b/src/core/transport/chttp2/timeout_encoding.h
index a4582566adc..d1e47760324 100644
--- a/src/core/transport/chttp2/timeout_encoding.h
+++ b/src/core/transport/chttp2/timeout_encoding.h
@@ -34,8 +34,11 @@
 #ifndef __GRPC_INTERNAL_TRANSPORT_CHTTP2_TIMEOUT_ENCODING_H_
 #define __GRPC_INTERNAL_TRANSPORT_CHTTP2_TIMEOUT_ENCODING_H_
 
+#include "src/core/support/string.h"
 #include <grpc/support/time.h>
 
+#define GRPC_CHTTP2_TIMEOUT_ENCODE_MIN_BUFSIZE (GPR_LTOA_MIN_BUFSIZE + 1)
+
 /* Encode/decode timeouts to the GRPC over HTTP2 format;
    encoding may round up arbitrarily */
 void grpc_chttp2_encode_timeout(gpr_timespec timeout, char *buffer);
diff --git a/src/core/transport/chttp2_transport.c b/src/core/transport/chttp2_transport.c
index 17b37d6d4ae..6dfffa1fc50 100644
--- a/src/core/transport/chttp2_transport.c
+++ b/src/core/transport/chttp2_transport.c
@@ -37,6 +37,7 @@
 #include <stdio.h>
 #include <string.h>
 
+#include "src/core/support/string.h"
 #include "src/core/transport/chttp2/frame_data.h"
 #include "src/core/transport/chttp2/frame_goaway.h"
 #include "src/core/transport/chttp2/frame_ping.h"
@@ -53,7 +54,6 @@
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
 #include <grpc/support/slice_buffer.h>
-#include <grpc/support/string.h>
 #include <grpc/support/useful.h>
 
 #define DEFAULT_WINDOW 65535
diff --git a/test/core/echo/echo_test.c b/test/core/echo/echo_test.c
index 16d381fb654..6449b2414f3 100644
--- a/test/core/echo/echo_test.c
+++ b/test/core/echo/echo_test.c
@@ -42,10 +42,10 @@
 #include <sys/wait.h>
 
 #include "src/core/iomgr/socket_utils_posix.h"
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/host_port.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include "test/core/util/port.h"
 
 int test_client(const char *root, const char *host, int port) {
diff --git a/test/core/echo/server.c b/test/core/echo/server.c
index 35f118dc9b1..57b083779c3 100644
--- a/test/core/echo/server.c
+++ b/test/core/echo/server.c
@@ -39,11 +39,11 @@
 #include <string.h>
 #include <time.h>
 
+#include "src/core/support/string.h"
 #include "test/core/util/test_config.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/host_port.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include <grpc/support/time.h>
 #include "test/core/util/port.h"
 
diff --git a/test/core/end2end/cq_verifier.c b/test/core/end2end/cq_verifier.c
index 82c0ef1700b..49b131c2362 100644
--- a/test/core/end2end/cq_verifier.c
+++ b/test/core/end2end/cq_verifier.c
@@ -45,10 +45,10 @@
 #include <string.h>
 
 #include "src/core/surface/event_string.h"
+#include "src/core/support/string.h"
 #include <grpc/byte_buffer.h>
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include <grpc/support/time.h>
 #include <grpc/support/useful.h>
 
diff --git a/test/core/fling/fling_stream_test.c b/test/core/fling/fling_stream_test.c
index f6fe69824b3..7f52fb1bad1 100644
--- a/test/core/fling/fling_stream_test.c
+++ b/test/core/fling/fling_stream_test.c
@@ -41,9 +41,9 @@
 #include <sys/types.h>
 #include <sys/wait.h>
 
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/host_port.h>
-#include <grpc/support/string.h>
 #include "test/core/util/port.h"
 
 int main(int argc, char **argv) {
diff --git a/test/core/fling/fling_test.c b/test/core/fling/fling_test.c
index 4607aa5f988..b2272f20c8e 100644
--- a/test/core/fling/fling_test.c
+++ b/test/core/fling/fling_test.c
@@ -43,7 +43,7 @@
 
 #include <grpc/support/alloc.h>
 #include <grpc/support/host_port.h>
-#include <grpc/support/string.h>
+#include "src/core/support/string.h"
 #include "test/core/util/port.h"
 
 int main(int argc, char **argv) {
diff --git a/test/core/security/credentials_test.c b/test/core/security/credentials_test.c
index 9c60f4c2338..ec21e0d42f7 100644
--- a/test/core/security/credentials_test.c
+++ b/test/core/security/credentials_test.c
@@ -37,9 +37,9 @@
 
 #include "src/core/httpcli/httpcli.h"
 #include "src/core/security/json_token.h"
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include <grpc/support/time.h>
 #include "test/core/util/test_config.h"
 #include <openssl/rsa.h>
diff --git a/test/core/support/string_test.c b/test/core/support/string_test.c
index e87a606aba2..a01ec6f87f3 100644
--- a/test/core/support/string_test.c
+++ b/test/core/support/string_test.c
@@ -31,7 +31,7 @@
  *
  */
 
-#include <grpc/support/string.h>
+#include "src/core/support/string.h"
 
 #include <stddef.h>
 #include <stdlib.h>
diff --git a/test/core/transport/chttp2/bin_encoder_test.c b/test/core/transport/chttp2/bin_encoder_test.c
index ea24f5cbd7a..048ed7edd3c 100644
--- a/test/core/transport/chttp2/bin_encoder_test.c
+++ b/test/core/transport/chttp2/bin_encoder_test.c
@@ -35,9 +35,9 @@
 
 #include <string.h>
 
+#include "src/core/support/string.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 
 static int all_ok = 1;
 
diff --git a/test/core/transport/chttp2/stream_encoder_test.c b/test/core/transport/chttp2/stream_encoder_test.c
index cebc2634fba..eb0f688f58e 100644
--- a/test/core/transport/chttp2/stream_encoder_test.c
+++ b/test/core/transport/chttp2/stream_encoder_test.c
@@ -35,10 +35,10 @@
 
 #include <stdio.h>
 
+#include "src/core/support/string.h"
 #include "src/core/transport/chttp2/hpack_parser.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include "test/core/util/parse_hexstring.h"
 #include "test/core/util/slice_splitter.h"
 #include "test/core/util/test_config.h"
diff --git a/test/core/transport/chttp2/timeout_encoding_test.c b/test/core/transport/chttp2/timeout_encoding_test.c
index 4bb84e3f0bf..ffa0070e341 100644
--- a/test/core/transport/chttp2/timeout_encoding_test.c
+++ b/test/core/transport/chttp2/timeout_encoding_test.c
@@ -43,7 +43,7 @@
 #define LOG_TEST() gpr_log(GPR_INFO, "%s", __FUNCTION__)
 
 static void assert_encodes_as(gpr_timespec ts, const char *s) {
-  char buffer[32];
+  char buffer[GRPC_CHTTP2_TIMEOUT_ENCODE_MIN_BUFSIZE];
   grpc_chttp2_encode_timeout(ts, buffer);
   gpr_log(GPR_INFO, "check '%s' == '%s'", buffer, s);
   GPR_ASSERT(0 == strcmp(buffer, s));
diff --git a/test/core/transport/transport_end2end_tests.c b/test/core/transport/transport_end2end_tests.c
index 5d26ef53b90..8e9b4a2cc9e 100644
--- a/test/core/transport/transport_end2end_tests.c
+++ b/test/core/transport/transport_end2end_tests.c
@@ -37,10 +37,10 @@
 #include <stdio.h>
 #include <string.h>
 
+#include "src/core/support/string.h"
 #include "src/core/transport/transport.h"
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
-#include <grpc/support/string.h>
 #include <grpc/support/thd.h>
 #include <grpc/support/useful.h>