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+---
+date: 2021-02-12
+title: "Ouroboros 0.18"
+linkTitle: "Ouroboros 0.18"
+description: "Major additions and changes in 0.18.0"
+author: Dimitri Staessens
+---
+
+With version 0.18 come a number of interesting updates to the prototype.
+
+### Automated Repeat-Request (ARQ) and flow control
+
+We finished the implementation of the base retransmission
+logic. Ouroboros will now send, receive and handle acknowledgments
+under packet loss conditions.  It will also send and handle window
+updates for flow control. The operation of flow control is very
+similar to the operation of window-based flow control in TCP, the main
+difference being that our sequence numbers are per-packet instead of
+per-byte.
+
+The previous version of FRCP had some partial implementation of the
+ARQ functionality, such as piggybacking ACK information on _writes_
+and handling sequence numbers on _reads_. But now, Ourobroos will also
+send (delayed) ACK packets without data if the application is not
+sending and finish sending when a flow is closed if not everything was
+acknowledged (can be turned off with the FRCTFLINGER flag).
+
+Recall that Ouroboros has this logic implemented in the application
+library, it's not a separate component (or kernel) that is managing
+transmit and receive buffers and retransmission. Furthermore, our
+implementation doesn't add a thread to the application. If a
+single-threaded application uses ARQ, it will remain single-threaded.
+
+It's not unlikely that in the future we will add the option for the
+library to start a dedicated thread to manage ARQ as this may have
+some beneficial characteristics for read/write call durations. Other
+future addditions may include fast-retransmit and selective ACK
+support.
+
+The most important characteristic of Ouroboros FRCP compared to TCP
+and derivative protocols (QUIC, SCTP, ...) is that it is 100%
+independent of congestion control, which allows for it to operate at
+real RTT timescales (i.e. microseconds in datacenters) without fear of
+RTT underestimates severely capping throughput. Another characteristic
+is that the RTT estimate is really measuring the responsiveness of the
+application, not the kernel on the machine.
+
+A detailed description of the operation of ARQ can be found
+in the [protocols](/docs/concepts/protocols/#operation-of-frcp)
+section.
+
+### Congestion Avoidance
+
+The next big addition is congestion avoidance. By default, the unicast
+layer's default configuration will now congestion-control all client
+traffic sent over them[^1]. As noted above, congestion avoidance in
+Ouroboros is completely independent of the operation of ARQ and flow
+control. For more information about how this all works, have a look at
+the developer blog
+[here](/blog/2020/12/12/congestion-avoidance-in-ouroboros/) and
+[here](/blog/2020/12/19/exploring-ouroboros-with-wireshark/).
+
+### Revision of the flow allocator
+
+We also made a change to the flow allocator, more specifically the
+Endpoint IDs to use 64-bit identifiers. The reason for this change is
+to make it harder to guess these endpoint identifiers.  In TCP,
+applications can listen to sockets that are bound to a port on a (set
+of) IP addresses. You can't imagine how many hosts are trying to brute
+force password guess SSH logins on TCP port 22. To make this at least
+a bit harder, Ouroboros has no well-known application ports, and after
+this patch they are roughtly equivalent to a 32-bit random
+number. Note that in an ideal Ouroboros deployment, sensitive
+applications such as SSH login should run on a different layer/network
+than publicly available applications.
+
+### Revision of the ipcpd-udp
+
+The ipcpd-udp has gone through some revisions during its lifetime. In
+the beginning, we wanted to emulate the operation of an Ouroboros
+layers, having the flow allocator listening on a certain UDP port, and
+mapping endpoints identifiers to random ephemeral UDP ports.  So as an
+example, the source would generate a UDP socket, e.g. on port 30927,
+and send a request for a new flow the fixed known Ouroboros UDP port
+(3531) at the receiver. This also generates a socket on an ephemeral
+UDP port, say 23705, and it sends a response back to the source on UDP
+port 3531. Traffic for the "client" flow would be on UDP port pair
+(30927, 23705). This was giving a bunch of headaches with computers
+behind NAT firewalls, rendering that scheme only useful in lab
+environments. To make it more useable, the next revision used a single
+fixed incoming UDP port for the flow allocator protocol, using an
+ephemeral UDP port from the sender side per flow and added the flow
+allocator endpoints as a "next header" inside UDP. So traffic would
+always be sent to destination UDP port 3531. Benefit was that only a
+single port was needed in the NAT forwarding rules, and that anyone
+running Ouroboros would be able to receive allocation messages, and
+this is enforcing a bit all users to participate in a mesh topology.
+However, opening a certain UDP port is still a hassle, so in this
+(most likely final) revision, we just run the flow allocator in the
+ipcpd-udp as a UDP server on a (configurable) port. No more NAT
+firewall configurations required if you want to connect (but if you
+want to accept connections, opening UDP port 3531 is still required).
+
+The full changelog can be browsed in
+[cgit](/cgit/ouroboros/log/?showmsg=1).
+
+[^1]: This is not a claim that every packet inside a layer is
+      flow-controlled: internal management traffic to the layer (flow
+      allocator protocol, etc) is not congestion-controlled.
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diff --git a/content/en/docs/Releases/_index.md b/content/en/docs/Releases/_index.md
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+
+---
+title: "Releases"
+linkTitle: "Release notes"
+weight: 120
+---