Subject: [Detnet-dp-dt] Relay vs Transit

From: Lou Berger <[email-masked]>
Date: Fri, 1 Jul 2016 00:51:17 -0400

Wednesday's discussion was very enlightening, at least to me. I had
previously been reading the definition of relay node fairly literally
based on the architecture doc:

   relay node
           A router, transit node, bridge, Label Switch Router (LSR),
           firewall, or any other system that forwards packets from one
           interface to another.

Which to me is a typical transit/intermediate node/router/LSR, or just
transit node for short. That is, it provided functions that would
normally be expected from the underlying Network layer, e.g., IP or MPLS
with appropriate QoS mechanisms.

Jouni's comment on other ietf usage of relays brought in a completely
different perspective, where relays are application aware functions that
reside in the network.  Examples of such relays include DHCP relays and
(IMO) S-PEs in multi-segment pseudo wires.  The important distinction
here is that (IETF) relay nodes include application/service aware functions.

Looking at the layering in the DP document we have

+-----------+
|  Service  | PW, RTP, UDP, GRE
~~~~~~~~~~~~~
| Transport | IPv6, IPv4, MPLS LSPs, BIER, BIER-TE
+-----------+

I don't think this maps so well into the three concepts we now have of
end to end service, relay nodes (which may provide DetNet reliability)
and, transit nodes.  Perhaps it would be better to include three layers:

+-----------+
|  Service  | Detnet Service: PW, RTP, UDP, GRE
~~~~~~~~~~~~~
| Transport | DetNet Transport: e.g., replication/elimination
~~~~~~~~~~~~~                                or network coding
|  Network  | IPv6, IPv4, MPLS LSPs, BIER, BIER-TE
+-----------+

Which in a network look like

   +---------+                                           +---------+
   | Service |                                           | Service |
   +~~~~~~~~~+                +~~~~~~~~~+                +~~~~~~~~~+
   |Transport|                |Transport|                |Transport|
   +~~~~~~~~~+   +~~~~~~~~~+  +~~~~~~~~~+  +~~~~~~~~~+   +~~~~~~~~~+
   | Network |---| Network |--| Network |--| Network |---| Network |
   +---------+   +---------+  +---------+  +---------+   +---------+
       End         Transit       Relay       Transit          End
     System         Node         Node         Node          System

Keep in mind that the links interconnecting the network layers are
composed of NICs and other subnetworking technologies, e.g., 802.1 TSN.
If we include these, the more complex figure is

 +---------+                                           +---------+
 | Service | <------- End to End DetNet Service -----> | Service |
 +~~~~~~~~~+                +~~~~~~~~~+                +~~~~~~~~~+
 |Transport|                |Transport|                |Transport|
 +~~~~~~~~~+   +~~~~~~~~~+  +~~~~~~~~~+  +~~~~~~~~~+   +~~~~~~~~~+
 | Network |   | Network |  |Net| |Net|  | Network |   | Network |
 +-------.-+   +-.--.----+  +--.+ +-.-+  +--.---.---+   +---.----+
         | Link  |  |  ,-----.  \   | Link  |   |  ,-----.  |
         .........  +-[  Sub  ]-+   .........   +-[  Sub  ]-+
                      [Network]                   [Network]
                       `-----'                     `-----'
     End         Transit       Relay       Transit          End
   System         Node         Node         Node          System

So I suggest we keep the terms:
- Detnet Service	
	Provides end to end mechanisms needed to deliver a Detnet flow

- DetNet Transport
	Provides mechanisms to support a Detnet flow across a
	particular network technology, e.g., IP or MPLS.

- Network layer (protocol)
	Encapsulation, forwarding and QoS-related mechanisms for a
	particular network technology, e.g., IP or MPLS.

- Link
	A dedicated set of resources interconnecting two nodes

- Sub-Network
	A switched infrastructure interconnecting a collection of
	nodes, e.g., 802.1 TSN or OTN.

- End station
	A device that supports a DetNet Service end point

- Relay node
	A DetNet Service aware middle box that interconnects different
	network layer protocols or networks (instances). It may provide
	service supporting functions such as  DetNet reliability.

- Transit node
	A node that provide network layer switching across multiple
	links and/or sub-networks, e.g., and MPLS LSR.

- DetNet reliability function
	A set of mechanisms to increase the probability of lossless
	DetNet flow transport across a network. Example mechanisms
	include replication/elimination and network coding.

There are still more terms, e.g., DetNet flow, but I'm out of steam.
What do people think?

Lou