The protocols

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.


The operation of the transport protocol is based on the Delta-t protocol, 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.

Last modified September 6, 2019