gateway: Implement daemon for translating babeld to BGP Metrics

[spolack]

In order to attract traffic from internet on the closest gateway (mesh perspective), thus reducing load on gre tunnels we'll have to translate the mesh metric into a BGP metric. Only AS Path is usable for traffic engineering across multiple upstream autonomous systems.

This is already prepared on the bird side in https://github.com/freifunk-berlin/bbb-configs/blob/main/roles/cfg_openwrt/templates/gateway/bird.conf.j2#L14

Basically we'd just need the script to write the export policy in /dev/shm/announcement*.bird.conf and then trigger a bird reload.

The calculations could be done at a configurable interval.

I've written a very scary PoC to validate the strategy. It's on our gateways under /root/metrix.sh

#!/bin/sh

# for every babel metric of $PREPEND_STEP we increase the as path by $PREPEND_STEP_SIZE
PREPEND_STEP=500
PREPEND_STEP_SIZE=2
MAX_PREPENDS=10

FILE="/dev/shm/announcement_babel_to_prepend.bird.conf"


get_routes_metrics() {
  prefix=$1
  # retrieve metrics where nexthop is not gre 
  result="$(echo "dump" | nc ::1 33123 | grep "installed yes" | grep "::/56"  | grep $prefix | grep -v "gre4-" | cut -d " " -f 13)"

  # put in 5k for every nexthop gre interface
  for i in $(echo "dump" | nc ::1 33123 | grep "installed yes" | grep "::/56"  | grep $prefix | grep "gre4-"); do
    result="$result 5000"
  done
  echo $result
}



echo "" > $FILE

for pfx in $(ip -6 r s | grep /44 | cut -d" " -f 2); do
  pfx_grep=$(echo $pfx | awk -F '::' '{print $1}')
  echo Calcualating average for prefix: $pfx
  metrics="$(get_routes_metrics $pfx_grep)"
  avg=$(echo "$metrics" | awk '{x+=$0}END{print x/NR}');
  
  prepends=$(echo "(($avg - $PREPEND_STEP) / $PREPEND_STEP) * $PREPEND_STEP_SIZE" | bc)
  if [[ $prepends -gt $MAX_PREPENDS ]]; then
    prepends=10
  fi
  echo "Average Metric: $avg - Caluclated AS Path Prepends: $prepends"
  
  if [[ $prepends -eq 0 ]]; then
    continue
  fi

  echo "if net = $pfx then {" >> $FILE
  for i in $(seq 1 $prepends); do 
    echo "        bgp_path.prepend(44194);" >> $FILE
  done
  echo "}" >> $FILE
done
birdc configure

Export policy produced by the PoC:

root@saarbruecker-gw:~# cat /dev/shm/announcement_babel_to_prepend.bird.conf 

if net = 2001:bf7:760::/44 then {
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
}
if net = 2001:bf7:780::/44 then {
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
}
if net = 2001:bf7:790::/44 then {
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
}
if net = 2001:bf7:800::/44 then {
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
}
if net = 2001:bf7:810::/44 then {
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
}
if net = 2001:bf7:820::/44 then {
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
}
if net = 2001:bf7:830::/44 then {
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
}
if net = 2001:bf7:840::/44 then {
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
}
if net = 2001:bf7:850::/44 then {
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
}
if net = 2001:bf7:860::/44 then {
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
        bgp_path.prepend(44194);
}