Improve size calculation for our packets.

We were hardcoding kMaxAppMessageLen and kMaxSecureSduLengthBytes, but those
defines were not even consistent with each other, and both were overly
conservative.  Specific changes:

1. Fix computation of CHIP_SYSTEM_HEADER_RESERVE_SIZE: we don't have any "crypto
   headers".
2. Fix computation of kMaxAppMessageLen to use our actual packet buffer sizes
   and defined header sizes.
3. Fix kMaxSecureSduLengthBytes to be consistent with kMaxAppMessageLen.

src/app/StatusResponse.h

@@ -22,10 +22,11 @@
 #include <messaging/ExchangeContext.h>
 #include <protocols/interaction_model/Constants.h>
 #include <system/TLVPacketBufferBackingStore.h>
+#include <transport/raw/MessageHeader.h>
 
 namespace chip {
 namespace app {
-static constexpr size_t kMaxSecureSduLengthBytes = 1024;
+static constexpr size_t kMaxSecureSduLengthBytes = kMaxAppMessageLen + kMaxTagLen;
 
 class StatusResponse
 {

src/system/SystemConfig.h

@@ -293,55 +293,57 @@
 #endif // CHIP_SYSTEM_CONFIG_ZEPHYR_LOCKING && CHIP_SYSTEM_CONFIG_NO_LOCKING
 
 /**
- *  @def CHIP_SYSTEM_HEADER_RESERVE_SIZE
+ *  @def CHIP_SYSTEM_CRYPTO_HEADER_RESERVE_SIZE
  *
  *  @brief
  *      The number of bytes to reserve in a network packet buffer to contain
- *      the CHIP message and exchange headers.
- *
- *      This number was calculated as follows:
+ *      the Matter crypto headers.
  *
- *      CHIP Crypto Header:
- *
- *          4 -- Length of encrypted block
- *          4 -- Reserve
- *          8 -- Initialization Vector
- *          8 -- Encryption Tag
+ *      This is 0, because Matter does not have any crypto headers.  This define
+ *      is still here only for backwards compatibility.
  */
 #ifndef CHIP_SYSTEM_CRYPTO_HEADER_RESERVE_SIZE
-#define CHIP_SYSTEM_CRYPTO_HEADER_RESERVE_SIZE 24
+#define CHIP_SYSTEM_CRYPTO_HEADER_RESERVE_SIZE 0
 #endif
 
 /**
  *  @def CHIP_SYSTEM_HEADER_RESERVE_SIZE
  *
  *  @brief
  *      The number of bytes to reserve in a network packet buffer to contain
- *      the CHIP message and exchange headers.
+ *      the CHIP message and payload headers.
  *
  *      This number was calculated as follows:
  *
- *      CHIP Message Header:
+ *      Matter Message Header:
  *
  *          2 -- Frame Length
- *          2 -- Message Header
- *          4 -- Message Id
- *          8 -- Source Node Id
- *          8 -- Destination Node Id
- *          2 -- Key Id
+ *          1 -- Message Flags
+ *          2 -- Session ID
+ *          1 -- Security Flags
+ *          4 -- Message Counter
+ *          8 -- Source Node ID
+ *          8 -- Destination Node ID
+ *
+ *      Total: 26 bytes.
+ *
+ *      Matter Payload Header:
  *
- *      CHIP Exchange Header:
+ *          1 -- Exhange Flags
+ *          1 -- Protocol Opcode
+ *          2 -- Exchange ID
+ *          2 -- Protocol Vendor ID
+ *          2 -- Protocol ID
+ *          4 -- Acknowledged MEssage Counter
  *
- *          1 -- Application Version
- *          1 -- Message Type
- *          2 -- Exchange Id
- *          4 -- Profile Id
- *          4 -- Acknowledged Message Id
+ *      Total: 12 bytes.
  *
- *    @note A number of these fields are optional or not presently used. So most headers will be considerably smaller than this.
+ *    @note A number of these fields are optional or not presently used. So most
+ *          headers will be considerably smaller than this.
+ *    @note This calculation assumes ther are no Message Extensions or Secured Extensions.
  */
 #ifndef CHIP_SYSTEM_HEADER_RESERVE_SIZE
-#define CHIP_SYSTEM_HEADER_RESERVE_SIZE (38 + CHIP_SYSTEM_CRYPTO_HEADER_RESERVE_SIZE)
+#define CHIP_SYSTEM_HEADER_RESERVE_SIZE (26 + 12 + CHIP_SYSTEM_CRYPTO_HEADER_RESERVE_SIZE)
 #endif /* CHIP_SYSTEM_HEADER_RESERVE_SIZE */
 
 /**

src/transport/raw/MessageHeader.h

@@ -35,15 +35,41 @@
 #include <lib/core/PeerId.h>
 #include <lib/support/BitFlags.h>
 #include <lib/support/BufferReader.h>
+#include <lib/support/CodeUtils.h>
 #include <lib/support/TypeTraits.h>
 #include <protocols/Protocols.h>
 #include <system/SystemPacketBuffer.h>
 
 namespace chip {
 
+namespace detail {
+// Figure out the max size of a packet we can allocate, including all headers.
+static constexpr size_t kMaxIPPacketSizeBytes     = 1280;
+static constexpr size_t kMaxPacketBufferSizeBytes = System::PacketBuffer::kMaxSizeWithoutReserve;
+static constexpr size_t kMaxPacketSizeBytes       = min(kMaxIPPacketSizeBytes, kMaxPacketBufferSizeBytes);
+
+// Figure out the max size of our headers.
+// System::PacketBuffer::kDefaultHeaderReserve may or may not include space for
+// UDP headers, depending on the situation.  Make sure we always have enough
+// space for UDP headers plus Matter headers.
+static constexpr size_t kMaxUDPHeaderSizeBytes          = 48;
+static constexpr size_t kMaxPacketbufferHeaderSizeBytes = System::PacketBuffer::kDefaultHeaderReserve;
+static constexpr size_t kMaxHeaderSizeBytes =
+    max(kMaxUDPHeaderSizeBytes + CHIP_SYSTEM_HEADER_RESERVE_SIZE, kMaxPacketbufferHeaderSizeBytes);
+
+static_assert(kMaxPacketSizeBytes > kMaxHeaderSizeBytes, "Need to be able to fit our headers in a packet.");
+
+static constexpr size_t kMaxMessageSizeBytes = detail::kMaxPacketSizeBytes - detail::kMaxHeaderSizeBytes;
+} // namespace detail
+
 static constexpr size_t kMaxTagLen = 16;
 
-static constexpr size_t kMaxAppMessageLen = 1200;
+static_assert(detail::kMaxMessageSizeBytes > kMaxTagLen, "Need to be able to fit our tag in a message");
+
+// This is somewhat of an under-estimate, because in practice any time we have a
+// tag we will not have source/destination node IDs, but above we are including
+// those in the header size.
+static constexpr size_t kMaxAppMessageLen = detail::kMaxMessageSizeBytes - kMaxTagLen;
 
 static constexpr uint16_t kMsgUnicastSessionIdUnsecured = 0x0000;