diff --git a/cmake/FindGDRCopy.cmake b/cmake/FindGDRCopy.cmake
index 54e0ba1c..c1f786ae 100644
--- a/cmake/FindGDRCopy.cmake
+++ b/cmake/FindGDRCopy.cmake
@@ -30,15 +30,19 @@ find_library(GDRCOPY_LIBRARIES
   ${GDRCOPY_ROOT_DIR}/lib
   /usr/local/lib
   /usr/lib
-  /usr/lib/x86_64-linux-gnu)
+  /usr/lib/x86_64-linux-gnu
+  /usr/lib/aarch64-linux-gnu)
 
 if(GDRCOPY_INCLUDE_DIRS)
-    include(CheckSymbolExists)
+    include(CheckCXXSourceCompiles)
     set(CMAKE_REQUIRED_INCLUDES ${GDRCOPY_INCLUDE_DIRS})
     set(CMAKE_REQUIRED_LIBRARIES ${GDRCOPY_LIBRARIES})
-    check_symbol_exists(gdr_pin_buffer_v2 "gdrapi.h" GDRCOPY_HAS_PIN_BUFFER_V2)
-    unset(CMAKE_REQUIRED_LIBRARIES)
+    check_cxx_source_compiles("
+      #include <gdrapi.h>
+      int main() { gdr_pin_buffer_v2(0, 0, 0, 0, 0); return 0; }
+    " GDRCOPY_HAS_PIN_BUFFER_V2)
     unset(CMAKE_REQUIRED_INCLUDES)
+    unset(CMAKE_REQUIRED_LIBRARIES)
     if(NOT GDRCOPY_HAS_PIN_BUFFER_V2)
         message(STATUS "GDRCopy found but too old (gdr_pin_buffer_v2 not available). Requires >= 2.5.")
         set(GDRCOPY_INCLUDE_DIRS GDRCOPY_INCLUDE_DIRS-NOTFOUND)
diff --git a/include/mscclpp/core.hpp b/include/mscclpp/core.hpp
index 5b184f0a..4aeab654 100644
--- a/include/mscclpp/core.hpp
+++ b/include/mscclpp/core.hpp
@@ -390,7 +390,7 @@ struct EndpointConfig {
     };
 
     static constexpr int DefaultPort = -1;
-    static constexpr int DefaultGidIndex = 0;
+    static constexpr int DefaultGidIndex = 3;
     static constexpr int DefaultMaxCqSize = 1024;
     static constexpr int DefaultMaxCqPollNum = 1;
     static constexpr int DefaultMaxSendWr = 8192;
diff --git a/python/mscclpp/language/rank.py b/python/mscclpp/language/rank.py
index e5b7aab8..0c38cb06 100644
--- a/python/mscclpp/language/rank.py
+++ b/python/mscclpp/language/rank.py
@@ -304,11 +304,16 @@ class BaseBuffer:
         self.size = offset + size
 
     def __getitem__(self, key):
-        if self.offset + key.stop > self.size:
-            raise RuntimeError(
-                f"Index range from {self.offset + key.start} - {self.offset + key.stop} is out of bounds for buffer {self.buffer_type}. Buffer size: {self.size}"
-            )
-        return Chunk(self.rank, self.buffer_type, self.offset + key.start, key.stop - key.start)
+        if isinstance(key, slice):
+            start = key.start if key.start is not None else 0
+            stop = key.stop if key.stop is not None else (self.size - self.offset)
+            if self.offset + stop > self.size:
+                raise RuntimeError(
+                    f"Index range from {self.offset + start} - {self.offset + stop} is out of bounds for buffer {self.buffer_type}. Buffer size: {self.size}"
+                )
+            return Chunk(self.rank, self.buffer_type, self.offset + start, stop - start)
+        else:
+            raise TypeError(f"Buffer indices must be slices, not {type(key).__name__}")
 
 
 class Buffer(BaseBuffer):
diff --git a/test.json b/test.json
new file mode 100644
index 00000000..294c2a13
--- /dev/null
+++ b/test.json
@@ -0,0 +1,218 @@
+{
+  "name": "send_recv_test",
+  "collective": "test",
+  "protocol": "Simple",
+  "inplace": false,
+  "reuse_resources": false,
+  "gpus": [
+    {
+      "id": 0,
+      "input_chunks": 1,
+      "output_chunks": 1,
+      "scratch_chunks": 0,
+      "threadblocks": [
+        {
+          "id": 0,
+          "ops": [
+            {
+              "name": "signal",
+              "channel_ids": [
+                0
+              ],
+              "channel_type": "port"
+            },
+            {
+              "name": "wait",
+              "channel_ids": [
+                0
+              ],
+              "channel_type": "port"
+            },
+            {
+              "name": "nop"
+            },
+            {
+              "name": "put",
+              "src_buff": [
+                {
+                  "type": "i",
+                  "index": 0,
+                  "size": 1
+                }
+              ],
+              "dst_buff": [
+                {
+                  "buffer_id": 0,
+                  "index": 0,
+                  "size": 1
+                }
+              ],
+              "channel_ids": [
+                0
+              ],
+              "channel_type": "port"
+            },
+            {
+              "name": "nop"
+            },
+            {
+              "name": "signal",
+              "channel_ids": [
+                0
+              ],
+              "channel_type": "port"
+            },
+            {
+              "name": "wait",
+              "channel_ids": [
+                0
+              ],
+              "channel_type": "port"
+            }
+          ],
+          "channels": [
+            {
+              "channel_type": "port",
+              "channel_ids": [
+                0
+              ]
+            }
+          ],
+          "remote_buffer_refs": [
+            {
+              "access_channel_type": "port",
+              "remote_buffer_ids": [
+                0
+              ]
+            }
+          ]
+        }
+      ],
+      "channels": [
+        {
+          "channel_type": "port",
+          "connected_to": [
+            1
+          ]
+        }
+      ],
+      "remote_buffers": [
+        {
+          "rank": 1,
+          "type": "o",
+          "access_channel_types": [
+            "port"
+          ]
+        }
+      ],
+      "semaphores": []
+    },
+    {
+      "id": 1,
+      "input_chunks": 1,
+      "output_chunks": 1,
+      "scratch_chunks": 0,
+      "threadblocks": [
+        {
+          "id": 0,
+          "ops": [
+            {
+              "name": "signal",
+              "channel_ids": [
+                0
+              ],
+              "channel_type": "port"
+            },
+            {
+              "name": "wait",
+              "channel_ids": [
+                0
+              ],
+              "channel_type": "port"
+            },
+            {
+              "name": "nop"
+            },
+            {
+              "name": "put",
+              "src_buff": [
+                {
+                  "type": "i",
+                  "index": 0,
+                  "size": 1
+                }
+              ],
+              "dst_buff": [
+                {
+                  "buffer_id": 0,
+                  "index": 0,
+                  "size": 1
+                }
+              ],
+              "channel_ids": [
+                0
+              ],
+              "channel_type": "port"
+            },
+            {
+              "name": "nop"
+            },
+            {
+              "name": "signal",
+              "channel_ids": [
+                0
+              ],
+              "channel_type": "port"
+            },
+            {
+              "name": "wait",
+              "channel_ids": [
+                0
+              ],
+              "channel_type": "port"
+            }
+          ],
+          "channels": [
+            {
+              "channel_type": "port",
+              "channel_ids": [
+                0
+              ]
+            }
+          ],
+          "remote_buffer_refs": [
+            {
+              "access_channel_type": "port",
+              "remote_buffer_ids": [
+                0
+              ]
+            }
+          ]
+        }
+      ],
+      "channels": [
+        {
+          "channel_type": "port",
+          "connected_to": [
+            0
+          ]
+        }
+      ],
+      "remote_buffers": [
+        {
+          "rank": 0,
+          "type": "o",
+          "access_channel_types": [
+            "port"
+          ]
+        }
+      ],
+      "semaphores": []
+    }
+  ],
+  "num_threads_per_block": 1024,
+  "use_double_scratch_buffer": false,
+  "buffer_alignment": 16,
+  "min_message_size": 0,
+  "max_message_size": 18446744073709551615
+}