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+---
+title: "Packet network protocols"
+author: "Dimitri Staessens"
+description: "what"
+date:  2019-09-05
+#type:  page
+draft: false
+---
+
+Packet switched networks move data between two applications using
+_protocols_, which specify where to move the data and how to do it.
+The Internet famously uses the Internet Protocol, IP (versions 4 and
+6, which are mutually incompatible, so in essence, there is not a
+single Internet, there are two!) as the network protocol that
+specifies how to move packets from point _A_ to point _B_. On top, it
+has the Transport Control Protocol (TCP) that takes care of things
+when IP packets get lost. TCP also does some other neat things, like
+making sure that a client does not send faster than a server can
+process, usually called _flow control_; and making sure that the
+network doesn't get overwhelmed by its users, usually called
+_congestion control_.
+
+The IPv4 protocol is specified in
+[RFC791](https://tools.ietf.org/html/rfc791), and its _header_ is
+shown here:
+
+```
+  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
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |Version|  IHL  |Type of Service|          Total Length         |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |         Identification        |Flags|      Fragment Offset    |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |  Time to Live |    Protocol   |         Header Checksum       |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |                       Source Address                          |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |                    Destination Address                        |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |                    Options                    |    Padding    |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+```
+
+The IPv6 protocol is specified in
+[RFC2460](https://tools.ietf.org/html/rfc2460), and its header format
+is:
+
+```
+  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
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |Version| Traffic Class |           Flow Label                  |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |         Payload Length        |  Next Header  |   Hop Limit   |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |                                                               |
+ +                                                               +
+ |                                                               |
+ +                         Source Address                        +
+ |                                                               |
+ +                                                               +
+ |                                                               |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |                                                               |
+ +                                                               +
+ |                                                               |
+ +                      Destination Address                      +
+ |                                                               |
+ +                                                               +
+ |                                                               |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+```
+
+For a detailed description of what all the fields in this protocol
+mean, we gladly refer you to the RFCs or the wikipedia pages (
+[IPv4](https://en.wikipedia.org/wiki/IPv4) and
+[IPv6](https://en.wikipedia.org/wiki/IPv6)).  The most interesting
+fact about the jump from IPv4 to IPv6 is that the protocol got
+noticeably simpler.
+
+As Ouroboros tries to preserve privacy as much as possible, it has an
+*absolutely minimal network protocol* (it's also configurable, this is
+the 64 bit address, 16 bit EID version):
+
+```
+  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 fields 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 ensures that packets don't get forwarded forever in
+  the network, for instance due to (transient) loops in the forwarding
+  path.
+
+* 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).
+
+* ECN: This field specifies Explicit Congestion Notification (ECN),
+  with similar intent as the ECN bits in the DSCP field in IPv4 / ToS
+  field in IPv6. It has 8-bit width as a default, and gets set higher
+  as packets are deeper and deeper in a forwarding queue. 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 values of this field is chosen by the endpoints at
+  _flow allocation_. It can be thought of as an ephemeral port.
+
+---
+Changelog:
+
+2019 09 05: Initial version.
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