The AH itself looks like this:

+---------------+---------------+---------------+---------------+
| Next Header   |    Length     |            RESERVED           |
+---------------+---------------+---------------+---------------+
|                    Security Parameters Index                  |
+---------------+---------------+---------------+---------------+
|     Authentication Data (variable number of 32-bit words)     |
+---------------+---------------+---------------+---------------+
|1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8|