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+---
+title: "The protocols"
+author: "Dimitri Staessens"
+#description: protocols
+date: 2019-09-06
+#type: page
+draft: false
+description: >
+ A brief introduction to the main protocols.
+---
+
+# Network protocol
+
+As Ouroboros tries to preserve privacy as much as possible, it has an
+*absolutely minimal network protocol*:
+
+```
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | |
+ + Destination Address +
+ | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Time-to-Live | QoS | ECN | EID |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | EID |
+ +-+-+-+-+-+-+-+-+
+```
+
+The 5 fields in the Ouroboros network protocol are:
+
+* **Destination address**: This specifies the address to forward the
+ packet to. The width of this field is configurable based on various
+ preferences and the size of the envisioned network. The Ouroboros
+ default is 64 bits. Note that there is _no source address_, this is
+ agreed upon during _flow allocation_.
+
+* **Time-to-Live**: Similar to IPv4 and IPv6 (where this field is called
+ Hop Limit), this is decremented at each hop to ensures that packets
+ don't get forwarded forever in the network, for instance due to
+ (transient) loops in the forwarding path. The Ouroboros default for
+ the width is one octet (byte).
+
+* **QoS**: Ouroboros supports Quality of Service via a number of methods
+ (out of scope for this page), and this field is used to prioritize
+ scheduling of the packets when forwarding. For instance, if the
+ network gets congested and queues start filling up, higher priority
+ packets (e.g. a voice call) get scheduled more often than lower
+ priority packets (e.g. a file download). By default this field takes
+ one octet.
+
+* **ECN**: This field specifies Explicit Congestion Notification (ECN),
+ with similar intent as the ECN bits in the Type-of-Service field in
+ IPv4 / Traffic Class field in IPv6. The Ouroboros ECN field is by
+ default one octet wide, and its value is set to an increasing value
+ as packets are queued deeper and deeper in a congested routers'
+ forwarding queues. Ouroboros enforces Forward ECN (FECN).
+
+* **EID**: The Endpoint Identifier (EID) field specified the endpoint for
+ which to deliver the packet. The width of this field is configurable
+ (the figure shows 16 bits). The values of this field is chosen by
+ the endpoints, usually at _flow allocation_. It can be thought of as
+ similar to an ephemeral port. However, in Ouroboros there is no
+ hardcoded or standardized mapping of an EID to an application.
+
+# Transport protocol
+
+Packet switched networks use transport protocols on top of their
+network protocol in order to deal with lost or corrupted packets.
+
+The Ouroboros Transport protocol (called the _Flow and Retransmission
+Control Protocol_, FRCP) has only 4 fields:
+
+```
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Flags | Window |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Sequence Number |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Acknowledgment Number |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+```
+
+* **Flags**: There are 7 flags defined for FRCP.
+
+ - **DATA**: Indicates that the packet is carrying data (this allows
+ for 0 length data).
+
+ - **DRF** : Data Run Flag, indicates that there are no unacknowledged
+ packets in flight for this connection.
+
+ - **ACK** : Indicates that this packet carries an acknowledgment.
+ - **FC** : Indicates that this packet updates the flow control window.
+ - **RDVZ**: Rendez-vous, this is used to break a zero-window deadlock
+ that can arise when an update to the flow control window
+ gets lost. RDVZ packets must be ACK'd.
+ - **FFGM**: First Fragment, this packet contains the first fragment of
+ a fragmented payload.
+ - **MFGM**: More Fragments, this packet is not the last fragment of a
+ fragmented payload.
+
+* **Window**: This updates the flow control window.
+
+* **Sequence Number**: This is a monotonically increasing sequence number
+ used to (re)order the packets at the receiver.
+
+* **Acknowledgment Number**: This is set by the receiver to indicate the
+ highest sequence number that was received in
+ order.
+
+# Operation
+
+The operation of the transport protocol is based on the [Delta-t
+protocol]((https://www.osti.gov/biblio/5542785-delta-protocol-specification-working-draft)),
+which is a timer-based protocol that is a bit simpler in operation
+than the equivalent functionalities in TCP. In contrast with TCP/IP,
+Ouroboros does congestion control purely in the network protocol, and
+fragmentation and flow control purely in the transport protocol.
+
+
+
+
+--- Changelog:
+
+2019 09 05: Initial version.<br>
+2019 09 06: Added section on transport protocol. \ No newline at end of file