Remove reliable delivery (#73)

Reliability has been moved one layer higher. See https://github.com/OpenCyphal-Garage/cy for usage.

Author: pavel-kirienko

README.md

@@ -25,16 +25,13 @@ next-generation intelligent vehicles: manned and unmanned aircraft, spacecraft,
 - Support for redundant network interfaces with seamless interface aggregation and zero fail-over delay.
 - Robust message reassembler supporting highly distorted datagram streams:
   out-of-order fragments, fragment/message deduplication, interleaving, variable MTU, ...
-- Packet loss mitigation via:
-  - reliable topics (retransmit until acknowledged; callback notifications for successful/failed deliveries).
-  - redundant interfaces (packet lost on one interface may be received on another, transparent to the application);
 - Heap not required (but supported); the library can be used with fixed-size block pool allocators.
 - Detailed time complexity and memory requirement models for the benefit of real-time high-integrity applications.
-- Scalable: designed to handle thousands of topics and hundreds of concurrent transfers with minimal resources.
+- Scalable: designed to handle thousands of subjects and hundreds of concurrent transfers with minimal resources.
 - Runs anywhere out of the box, including extremely resource-constrained baremetal environments with ~100K ROM/RAM.
   No porting required.
 - Partial MISRA C compliance (reach out to <https://forum.opencyphal.org>).
-- Full implementation in a single C file with only 2k lines of straightforward C99!
+- Full implementation in a single C file with less than 2k lines of straightforward C99!
 - Extensive verification suite.
 
 ## Usage
@@ -71,7 +68,7 @@ standards-compliant C99 compiler is available.
 
 ### v3.0 -- WORK IN PROGRESS
 
-The library has been redesigned from scratch to support Cyphal v1.1, named topics, and reliable transfers.
+The library has been redesigned from scratch to support Cyphal v1.1 and named topics.
 No porting guide is provided since the changes are too significant;
 please refer to the new API docs in `libudpard/udpard.h`.
 

cyphal_udp_header.dsdl

@@ -1,40 +1,31 @@
+# Cyphal/UDP provides UNRELIABLE UNORDERED DEDUPLICATED (at most one) delivery of UNICAST or MULTICAST datagrams
+# with GUARANTEED INTEGRITY (messages either delivered correct or not delivered).
+#
 # All Cyphal/UDP traffic is sent to port 9382.
 # The subject multicast group address is composed as 239.0.0.0 (=0xEF000000) + subject_id (23 bits).
+#
 # All frames of a transfer must share the same field values unless otherwise noted.
 # Frames may arrive out-of-order, possibly interleaved with neighboring transfers; implementations must cope.
+#
+# The origin UID is a 64-bit globally unique identifier (e.g., EUI-64). It allows nodes to use redundant interfaces
+# without source address ambiguity, and also allows live interface migration.
+#
+# Unicast traffic is sent directly to the source endpoint of the destination node, which is discovered dynamically.
+# The destination UID is not included explicitly since the IP endpoint is considered adequate for node identification.
 
 uint5 version                   #=2 in this version.
 uint3 priority                  # 0=highest, 7=lowest.
 
-uint2 KIND_MSG_BEST_EFFORT = 0  # No ack must be sent.
-uint2 KIND_MSG_RELIABLE    = 1  # Remote must acknowledge reception by sending an ACK frame back.
-uint2 KIND_ACK             = 2  # Sent P2P; the transfer_id is of the acknowledged frame. Payload empty/ignored.
-uint2 kind
-uint6 reserved_incompat         # Discard frame if any incompatibility flags are set that are not understood.
+void5                           # Send zero, ignore on reception.
+uint3 incompatibility           # Send zero, drop frame if nonzero.
 
-void16                          # Reserved for compatibility flags and fields (transmit zero, ignore on reception).
-
-# Payload reassembly information.
-# We provide both the frame index and the frame payload offset to allow various reassembly strategies depending on the
-# preferences of the implementation. The provided information is sufficient for zero-copy out-of-order reassembly.
-# Offset 4 bytes.
+uint48 transfer_id              # For multi-frame reassembly and dedup.
+uint64 sender_uid               # Origin identifier ensures invariance to the source IP address for reassembly.
 
-uint24 frame_index              # Zero-based index of the payload fragment carried by this frame.
-void8
 uint32 frame_payload_offset     # The offset of the frame payload relative to the start of the transfer payload.
 uint32 transfer_payload_size    # Total for all frames.
 
-# Transfer identification information.
-# The transfer-ID is a single field that segregates transfers by topic hash and epoch (publisher sequence restarts).
-# Offset 16 bytes.
-
-uint64 transfer_id              # For multi-frame reassembly and dedup. ACK specifies the acked tfer here.
-uint64 sender_uid               # Origin identifier ensures invariance to the source IP address for reassembly.
-
-# Integrity checking information.
-# Offset 32 bytes.
-
 uint32 prefix_crc32c            # crc32c(payload[0:(frame_payload_offset+payload_size)])
 uint32 header_crc32c            # Covers all fields above. Same as the transfer payload CRC.
 
-# End of header at 40 bytes. Payload follows.
+# Header size 32 bytes; payload follows.