From f08abcb8dda6ff488fa1b52314a67d97aee059d7 Mon Sep 17 00:00:00 2001
From: Toke Høiland-Jørgensen <toke@toke.dk>
Date: Fri, 2 Jun 2023 00:31:53 +0200
Subject: Babel: Add support for the RTT extension

This adds support to the Babel protocol for the RTT extension specified
in draft-ietf-babel-rtt-extension. While this extension is not yet at the
RFC stage, it is one of the more useful extensions to Babel[0], so it
seems worth having in Bird as well.

The extension adds timestamps to Hello and IHU TLVs and uses these to
compute an RTT to each neighbour. An extra per-neighbour cost is then
computed from the RTT based on a minimum and maximum interval and cost
value specified in the configuration. The primary use case for this is
improving routing in a geographically distributed tunnel-based overlay
network.

The implementation follows the babeld implementation when picking
constants and default configuration values. It also uses the same RTT
smoothing algorithm as babeld, and follows it in adding a new 'tunnel'
interface type which enables RTT by default.

[0] https://alioth-lists.debian.net/pipermail/babel-users/2022-April/003932.html
---
 doc/bird.sgml | 51 ++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 44 insertions(+), 7 deletions(-)

(limited to 'doc/bird.sgml')

diff --git a/doc/bird.sgml b/doc/bird.sgml
index 39eae4cd..8041faa9 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -1918,7 +1918,7 @@ protocol babel [<name>] {
 	ipv6 [sadr] { <channel config> };
         randomize router id <switch>;
 	interface <interface pattern> {
-		type <wired|wireless>;
+		type <wired|wireless|tunnel>;
 		rxcost <number>;
 		limit <number>;
 		hello interval <time>;
@@ -1932,6 +1932,11 @@ protocol babel [<name>] {
 		next hop ipv4 <address>;
 		next hop ipv6 <address>;
 		extended next hop <switch>;
+		rtt cost <number>;
+		rtt min <time>;
+		rtt max <time>;
+		rtt decay <number>;
+		send timestamps <switch>;
 		authentication none|mac [permissive];
 		password "&lt;text&gt;";
 		password "&lt;text&gt;" {
@@ -1962,15 +1967,16 @@ protocol babel [<name>] {
       router ID every time it starts up, which avoids this problem at the cost
       of not having stable router IDs in the network. Default: no.
 
-      <tag><label id="babel-type">type wired|wireless </tag>
-      This option specifies the interface type: Wired or wireless. On wired
-      interfaces a neighbor is considered unreachable after a small number of
-      Hello packets are lost, as described by <cf/limit/ option. On wireless
+      <tag><label id="babel-type">type wired|wireless|tunnel </tag>
+      This option specifies the interface type: Wired, wireless or tunnel. On
+      wired interfaces a neighbor is considered unreachable after a small number
+      of Hello packets are lost, as described by <cf/limit/ option. On wireless
       interfaces the ETX link quality estimation technique is used to compute
       the metrics of routes discovered over this interface. This technique will
       gradually degrade the metric of routes when packets are lost rather than
-      the more binary up/down mechanism of wired type links. Default:
-      <cf/wired/.
+      the more binary up/down mechanism of wired type links. A tunnel is like a
+      wired interface, but turns on RTT-based metrics with a default cost of 96.
+      Default: <cf/wired/.
 
       <tag><label id="babel-rxcost">rxcost <m/num/</tag>
       This option specifies the nominal RX cost of the interface. The effective
@@ -2041,6 +2047,37 @@ protocol babel [<name>] {
       hop when IPv4 addresses are absent from the interface as described in
       <rfc id="9229">. Default: yes.
 
+      <tag><label id="babel-rtt-cost">rtt cost <m/number/</tag>
+      The RTT-based cost that will be applied to all routes from each neighbour
+      based on the measured RTT to that neighbour. If this value is set,
+      timestamps will be included in generated Babel Hello and IHU messages, and
+      (if the neighbours also have timestamps enabled), the RTT to each
+      neighbour will be computed. An additional cost is added to a neighbour if
+      its RTT is above the <ref id="babel-rtt-min" name="rtt min"> value
+      configured on the interface. The added cost scales linearly from 0 up to
+      the RTT cost configured in this option; the full cost is applied if the
+      neighbour RTT reaches the RTT configured in the <ref id="babel-rtt-max"
+      name="rtt max"> option (and for all RTTs above this value). Default: 0
+      (disabled), except for tunnel interfaces, where it is 96.
+
+      <tag><label id="babel-rtt-min">rtt min <m/time/ s|ms</tag>
+      The minimum RTT above which the RTT cost will start to be applied (scaling
+      linearly from zero up to the full cost). Default: 10 ms
+
+      <tag><label id="babel-rtt-max">rtt max <m/time/ s|ms</tag>
+      The maximum RTT above which the full RTT cost will start be applied.
+      Default: 120 ms
+
+      <tag><label id="babel-rtt-decay">rtt decay <m/number/</tag>
+      The decay factor used for the exponentional moving average of the RTT
+      samples from each neighbour, in units of 1/256. Higher values discards old
+      RTT samples faster. Must be between 1 and 256. Default: 42
+
+      <tag><label id="babel-send-timestamps">send timestamps <m/switch/</tag>
+      Whether to send the timestamps used for RTT calculation on this interface.
+      Sending the timestamps enables peers to calculate an RTT to this node,
+      even if no RTT cost is applied to the route metrics. Default: yes.
+
       <tag><label id="babel-authentication">authentication none|mac [permissive]</tag>
       Selects authentication method to be used. <cf/none/ means that packets
       are not authenticated at all, <cf/mac/ means MAC authentication is
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