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============================================================================================
Route Table Guide
============================================================================================
--------------------------------------------------------------------------------------------
RIC Message Router -- RMR
--------------------------------------------------------------------------------------------


Overview
========

Messages sent via the RIC Message Router (RMR) are routed to
an endpoint (another application) based on a combination of
the *message type* (MT) and *subscription ID* (SID) supplied
in the message. RMR determines the endpoint by matching the
MT and SID combination to an entry in a route table which has
been supplied dynamically by a *Route Manager* service, or as
a static table loaded during RMR initialisation. It is also
possible to route messages directly to an endpoint which is
the *managed entity* "owner," using the *managed entity ID*
(MEID).

For most xAPP developers the format of the RMR route table is
not important beyond understanding how to create a static
table for local testing. For developers of a *Route Manager*
service, the need is certainly a requirement. This document
describes the overall syntax of a route table and the
interface between the *Route Manager* service and RMR.


Contents of a Route Table
=========================

The table consists of a start record, one or more entry
records, and an end record. Each entry record defines one
message type, with an optional sender application, and the
endpoint(s) which accept the indicated message type. All
table records contain fields separated with vertical bars
(|), and allow for trailing comments with the standard shell
comment symbol (hash, #) provided that the start of the
comment is separated from the last token on the record by one
or more spaces. Leading and trailing white space in each
field is ignored. Figure 1 illustrates a very basic route
table with two message types, 1000 and 2000, and two
subscription IDs for message type 1000.


::

    newrt | start | rt-0928
    rte   | 2000  | logger:30311
    mse   | 1000  | 10 | forwarder:43086
    mse   | 1000  | 21 | app0:43086,app1:43086
    newrt | end   | 3

Figure 1: A basic route table.


Entry record syntax
-------------------

Two types of table entries are supported for compatibility
with the original RMR implementation, but only the *mse*
entry type is needed and that should be the entry used when
creating new tables. The following shows the syntax for both
entry types:


::

   rte | <msg-type>[,<sender-endpoint>] | <endpoint-group>[;<endpoint-group>;...]
   mse | <msg-type>[,<sender-endpoint>] | <sub-id> | <endpoint-group>[;<endpoint-group>;...]


Where:


    .. list-table::
      :widths: 25,70
      :header-rows: 0
      :class: borderless

      * - **mse, rte**
        -
          is the table entry type

      * - **<msg-type>**
        -
          is the integer message type

      * - **<sender-endpoint>**
        -
          is the endpoint description of the message sender; only that
          sender will read the entry from the table, so a single table
          may be used for all senders when a common message type is
          delivered to varying endpoints based on senders. If the
          sender endpoint is omitted from the entry, then the entry
          will be used by all applications.

      * - **<sub-id>**
        -
          is the subscription id (integer) for subscription-based
          messages, or -1 if the message type is not
          subscription-based. An *mse* entry with a sub-id of -1 is the
          **same** as an *rte* entry with the same message type.

      * - **<endpoint-group>**
        -
          is one or more, comma separated, endpoint descriptions.



When an application sends a message with the indicated type,
the message will be sent to one endpoint in the group in a
round-robin ordering. If multiple endpoint groups are given,
then the message is sent to a member selected from each
group; 3 groups, then three messages will be sent. The first
group is required.


Line separation
---------------

Table entries **must** end with a record termination sequence
which may be one of the following three sequences:


* a single newline (\\n)
* a DOS style CRLF pair (\\r\\n)
* a single carriage return (\\r)


Care must be taken when manually editing a static table; some
editors do **not** add a final record termination sequence to
the last line of a file. RMR expects the final record to have
a termination sequence to ensure that the record was not
truncated (especially important when receiving dynamic
tables).


Table framing
-------------

The route table parser within RMR assumes that route table
entries are sent via RMR messages as a stream. To ensure
synchronisation and prevent malformed tables because of
broken sessions or lost packets, each table must begin and
end with an *newrt* record. Each *newrt* record has one of
two possible syntax layouts as described below.


::

    newrt | begin [| table-id-string]
    newrt | end  [| record-count]

Figure 2: Illustration of the newrt records in the table.

The *table-id-string* is an optional string which is used by
RMR when sending an acknowledgement back to the *Route
Manager* service (see the *Route Manager Interface* section
for more details). If a *record-count* is given as the final
field on the *end* record, RMR will verify that the number of
*mse* and *rte* entries in the table matches the count; if
there is a mismatch in values the table is not used.


Comments, spaces, and blank lines
---------------------------------

Comments may be placed to the right of any table entry line
using the standard shell comment symbol (#). The start of a
comment must be separated from any previous record content by
at least one space or tab. Complete lines are treated as
comments when the first non-whitespace character of a line is
a comment symbol. Blank lines are also ignored.

Fields on table records are separated using the vertical bar
(|) character. Any white space (tabs or spaces) which appear
immediately before or after a separator are ignored.


Endpoint Description
--------------------

The endpoint description is either the hostname or IP address
followed by a port number; the two are separated by a single
colon. The illustration below assumes that host names (e.g.
forwarder and app1) are defined; they also make the tables
easier to read. The port number given is the port number that
the user application provides to RMR when the RMR
initialisation function is invoked (and thus is the port that
RMR is listening on).


Table Mechanics
===============

Creating a table from the two entry types is fairly simple,
however there are some subtleties which should be pointed out
to avoid unexpected behaviour. For this discussion the
following complete table will be used.

.. list-table::
  :widths: 75,10
  :header-rows: 0
  :class: borderless


  * -

        ::

            newrt | start | rt-0928
            rte | 2000 | logger:30311
            mse | 1000 | 10 | forwarder:43086
            mse | 1000,forwarder:43086 | 10 | app2:43086
            mse | 1000 | -1 | app0:43086,app1:43086; logger:20311
            newrt | end | 4

    -

        ::

          (1)
          (2)
          (3)
          (4)
          (5)
          (6)


Figure 3: A complete RMR routing table (line numbers to the
right for reference).


Table Entry Ordering
--------------------

Whether a table is read from a file on disk, or is received
from a *Route Manager* service, RMR parses the records to
build an internal route table keeping only the relevant
information. Entries are read in the order they appear (from
the file or in messages received), and RMR will use only one
entry for each MT/SID pair.

For most tables, the ordering of entries is not important,
but when there are entries which duplicate the MT/SID pair
ordering becomes significant. RMR will use the **last** valid
entry for a MT/SID pair that it encounters. An entry is
considered valid if there is no sender identified with the
message type (line 3), and when the sender (host and port)
match the the applications' location and the port provided to
RMR for listening.

Using the table in figure 3 as an example, there are two
entries which match the MT/SID pair of 1000/10. When this
table is parsed on any host, RMR will recognise and add the
first entry (line 3) to the internal representation; this
entry is valid for all applications. The second 1000/10 entry
(line 4) is valid when the table is parsed on the *forwarder*
host, and only by the application which is listening on port
43086. For this application the entry will override the more
generic entry for the MT/SID combination.

As a rule, the ordering of entries for a given MT/SID pair
should be from most generic to most specific.


Route Manager Communications
============================

During initialisation RMR will use the value of the
``RMR_RTG_SVC`` environment variable to connect to the *Route
Manager* service in order to request a route table. The
connection between RMR and the *Route Manager* is also an RMR
session and thus RMR messages will be used to exchange
requests and responses.


Table Request
-------------

During initialisation, RMR establishes a wormhole connection
to the *Route Manager* and sends a message type of 21 to
request a new table. RMR will continue to send table requests
until a table is received and accepted; in other words it is
fine for the *Route Manager* to ignore the requests if it is
not ready to respond.


Sending Tables To RMR
---------------------

Table entry data is expected to arrive via RMR message with a
message type of 20. The message may contain one or more
entries provided that the entries are newline separated.
Current versions of RMR support very large messages, however
to ensure compatibility with an xAPP built using an older
version of RMR (pre 3.8), messages should be limited to 4
KiB.


Table Acceptance and Acknowledgement
------------------------------------

When RMR receives the table end entry (newrt|end), it will
send a state message back to the *Route Manager* to indicate
the state of the received table. The message type is 22 and
the payload will contain UTF-8 tokens which indicate the
state. The second token will be the *table ID* supplied on
the start record, or the string "<id-missing>." When the
state is an error state, RMR might add a final set of tokens
which contain the reason for the failure.

Upon receipt of a status message which indicates an "OK"
response, the *Route Manager* can assume that the table has
been installed and is in use. Any other response indicates
that RMR did not use the table and has dropped it; the
previous table is still in use.


Using A Static Route Table
--------------------------

A static route table can be provided to assist with testing,
or to provide a bootstrap set of route information until a
dynamic table is received from a routing manager. The
environment variable ``RMR_SEED_RT`` is checked during RMR
initialisation and if set is expected to reference a file
containing a route table. This table will be loaded and used
until overlaid by a table sent by the *Route Manager*.

To simulate dynamic reloads during testing, and for some
specialised use cases, the static table will be reloaded
periodically if the ``RMR_RTG_SVC`` environment variable is
set to -1. When set to -1 RMR will not listen for *Route
Manager* connections, nor will it attempt to request a
dynamic table.

If the file given by the ``RMR_SEED_RT`` variable does not
exist, and the ``RMR_RTG_SVC`` variable is set to -1, RMR
will block until the table is created. This simulates a
delayed dynamic load during testing, and can be used when the
xAPP is reading the route table saved by another local
process rather than one sent directly by the *Route Manager*.


Table Stashing
--------------

To assist with debugging, and to allow an application to
share the route table received from *Route Manager*, RMR will
stash the route table updates it received. Updates are
stashed in a file named by the ``RMR_STASH_RT`` environment
variable, and if that variable is not present, the
``RR_SEED_RT`` variable will be used with an added
``.stash`` extension.

The primary use of route table stashing is to assist with
debugging of applications, and because there are risks for an
application to share its table, table sharing is **NOT**
recommended. Table sharing can be enabled by setting the
``RMR_STASH_RT`` variable for the application that will be
the source of the route table updates, and using the file
named for tha application when defining the
``RMR_SEED_RT`` variable for applications which are to read
the table information. Obviously, all applications must be
running in the same container, on the same host, or have a
common disk volum between their environments. Known risks to
using table sharing include


* An update to the table (not a complete table) may be
  received prior to one or more readers accessing the file,
  and thus the reader may not receive a valid or complete
  table.

* Any entry which has a sender:port associated with the
  message type will likely be ignored by all readers.



Routing Using MEID
==================

Starting with version 1.13.0, RMR provides the ability to
select the endpoint for a message based on the MEID (managed
entity ID) in the message, rather than selecting the endpoint
from the round-robin list for the matching route table entry.
When the MEID is used, the message is sent to the endpoint
which *owns,* or is responsible for the managed entity.
Should the *owner* change messages will be routed to the new
owner when the route table is updated. To make use of MEID
routing, there must be one or more route table entries which
list the special endpoint name ``%meid`` instead of providing
a round robin list. As an example, consider the following
route table entry:


::

    mse| 1000,forwarder:43086 | 10 | %meid

Figure 4: Sample route entry with the meid flag.

The final field of the entry doesn't specify a round-robin
group which means that when an application attempts to send a
message with type 1000, and the subscription ID of 10, the
MEID in the message will be used to select the endpoint.


MEID endpoint selection
-----------------------

To select an endpoint for the message based on the MEID in a
message, RMR must know which endpoint owns the MEID. This
information, known as an MEID map, is provided by the *Route
Manager* over the same communication path as the route table
is supplied. The following is the syntax for an MEID map.


::

    meid_map | start | <table-id>
    mme_ar | <owner-endpoint> | <meid> [<meid>...]
    mme_del | <meid> [<meid>...]
    meid_map | end | <count> [| <md5sum> ]

Figure 5: Meid map table.

The mme_ar records are add/update records and allow for the
list of MEIDs to be associated with (owned by) the indicated
endpoint. The <owner-endpoint> is the hostname:port, or IP
address and port, of the application which owns the MEID and
thus should receive any messages which are routed based on a
route table entry with %meid as the round-robin group. The
mme_del records allow for MEIDs to be deleted from RMR's
view. Finally, the <count> is the number of add/replace and
delete records which were sent; if RMR does not match the
<count> value to the number of records, then it will not add
the data to the table. Updates only need to list the
ownership changes that are necessary; in other words, the
*Route Manager* does not need to supply all of the MEID
relationships with each update.

The optional <md5sum> field on the end record should be the
MD5 hash of all of the records between the start and end
records. This allows for a precise verification that the
transmitted data was correctly received.

If a static seed file is being used for the route table, a
second section can be given which supplies the MEID map. The
following is a small example of a seed file:


::

   newrt|start | id-64306
   mse|0|-1| %meid
   mse|1|-1|172.19.0.2:4560
   mse|2|-1|172.19.0.2:4560
   mse|3|-1|172.19.0.2:4560
   mse|4|-1|172.19.0.2:4560
   mse|5|-1|172.19.0.2:4560
   newrt|end

   meid_map | start | id-028919
   mme_ar| 172.19.0.2:4560 | meid000 meid001 meid002 meid003 meid004 meid005
   mme_ar| 172.19.0.42:4560 | meid100 meid101 meid102 meid103
   mme_del | meid1000
   meid_map | end | 1

Figure 6: Illustration of both a route table and meid map in
the same file.

The tables above will route all messages with a message type
of 0 based on the MEID. There are 10 meids which are owned by
two different endpoints. The table also deletes the MEID
meid1000 from RMR's view.


Reserved Message Types
======================

RMR is currently reserving message types in the range of 0
through 99 (inclusive) for its own use. Please do not use
these types in any production or test environment as the
results may be undesired.



Appendix A -- Glossary
======================

Many terms in networking can be interpreted with multiple
meanings, and several terms used in various RMR documentation
are RMR specific. The following definitions are the meanings
of terms used within RMR documentation and should help the
reader to understand the intent of meaning.

    .. list-table::
      :widths: 25,70
      :header-rows: 0
      :class: borderless

      * - **application**
        -
          A programme which uses RMR to send and/or receive messages
          to/from another RMR based application.

      * - **Critical error**
        -
          An error that RMR has encountered which will prevent further
          successful processing by RMR. Critical errors usually
          indicate that the application should abort.

      * - **Endpoint**
        -
          An RMR based application that is defined as being capable of
          receiving one or more types of messages (as defined by a
          *routing key.*)

      * - **Environment variable**
        -
          A key/value pair which is set externally to the application,
          but which is available to the application (and referenced
          libraries) through the ``getenv`` system call. Environment
          variables are the main method of communicating information
          such as port numbers to RMR.

      * - **Error**
        -
          An abnormal condition that RMR has encountered, but will not
          affect the overall processing by RMR, but may impact certain
          aspects such as the ability to communicate with a specific
          endpoint. Errors generally indicate that something, usually
          external to RMR, must be addressed.

      * - **Host name**
        -
          The name of the host as returned by the ``gethostbyname``
          system call. In a containerised environment this might be the
          container or service name depending on how the container is
          started. From RMR's point of view, a host name can be used to
          resolve an *endpoint* definition in a *route* table.)

      * - **IP**
        -
          Internet protocol. A low level transmission protocol which
          governs the transmission of datagrams across network
          boundaries.

      * - **Listen socket**
        -
          A *TCP* socket used to await incoming connection requests.
          Listen sockets are defined by an interface and port number
          combination where the port number is unique for the
          interface.

      * - **Message**
        -
          A series of bytes transmitted from the application to another
          RMR based application. A message is comprised of RMR specific
          data (a header), and application data (a payload).

      * - **Message buffer**
        -
          A data structure used to describe a message which is to be
          sent or has been received. The message buffer includes the
          payload length, message type, message source, and other
          information.

      * - **Message type**
        -
          A signed integer (0-32000) which identifies the type of
          message being transmitted, and is one of the two components
          of a *routing key.* See *Subscription ID.*

      * - **Payload**
        -
          The portion of a message which holds the user data to be
          transmitted to the remote *endpoint.* The payload contents
          are completely application defined.

      * - **RMR context**
        -
          A set of information which defines the current state of the
          underlying transport connections that RMR is managing. The
          application will be give a context reference (pointer) that
          is supplied to most RMR functions as the first parameter.

      * - **Round robin**
        -
          The method of selecting an *endpoint* from a list such that
          all *endpoints* are selected before starting at the head of
          the list.

      * - **Route table**
        -
          A series of "rules" which define the possible *endpoints* for
          each *routing key.*

      * - **Route table manager**
        -
          An application responsible for building a *route table* and
          then distributing it to all applicable RMR based
          applications.

      * - **Routing**
        -
          The process of selecting an *endpoint* which will be the
          recipient of a message.

      * - **Routing key**
        -
          A combination of *message type* and *subscription ID* which
          RMR uses to select the destination *endpoint* when sending a
          message.

      * - **Source**
        -
          The sender of a message.

      * - **Subscription ID**
        -
          A signed integer value (0-32000) which identifies the
          subscription characteristic of a message. It is used in
          conjunction with the *message type* to determine the *routing
          key.*

      * - **Target**
        -
          The *endpoint* selected to receive a message.

      * - **TCP**
        -
          Transmission Control Protocol. A connection based internet
          protocol which provides for lossless packet transportation,
          usually over IP.

      * - **Thread**
        -
          Also called a *process thread, or pthread.* This is a
          lightweight process which executes in concurrently with the
          application and shares the same address space. RMR uses
          threads to manage asynchronous functions such as route table
          updates.

      * - **Trace information**
        -
          An optional portion of the message buffer that the
          application may populate with data that allows for tracing
          the progress of the transaction or application activity
          across components. RMR makes no use of this data.

      * - **Transaction ID**
        -
          A fixed number of bytes in the *message* buffer) which the
          application may populate with information related to the
          transaction. RMR makes use of the transaction ID for matching
          response messages with the &c function is used to send a
          message.

      * - **Transient failure**
        -
          An error state that is believed to be short lived and that
          the operation, if retried by the application, might be
          successful. C programmers will recognise this as
          ``EAGAIN.``

      * - **Warning**
        -
          A warning occurs when RMR has encountered something that it
          believes isn't correct, but has a defined work round.

      * - **Wormhole**
        -
          A direct connection managed by RMR between the user
          application and a remote, RMR based, application.