serval-dna/route_link.c

#include "serval.h"
#include "overlay_address.h"
#include "overlay_buffer.h"
#include "overlay_packet.h"
#include "str.h"
#include "conf.h"

/*
Link state routing;

- each node sends a packet on a heartbeat
- on recieving a packet, update a link cost calculation (initially up/down only)
- when the cost changes, update a version field
- every heartbeat interval, send link cost details
  - send link cost for every neighbour, they need to know we can still hear them.
- after parsing incoming link details, if anything has changed, mark routes as dirty

*/

#define INCLUDE_ANYWAY (500)
#define MAX_LINK_STATES 512

#define FLAG_HAS_INTERFACE (1<<0)
#define FLAG_NO_PATH (1<<1)
#define FLAG_BROADCAST (1<<2)
#define FLAG_UNICAST (1<<3)
#define FLAG_HAS_ACK (1<<4)
#define FLAG_HAS_DROP_RATE (1<<5)

#define ACK_WINDOW (16)

struct link{
  struct link *_left;
  struct link *_right;

  struct subscriber *transmitter;
  struct link *parent;
  struct overlay_interface *interface;
  struct subscriber *receiver;

  // neighbour path version when path scores were last updated
  char path_version;

  // link quality stats;
  char link_version;
  char drop_rate;

  // calculated path score;
  int hop_count;
  int path_drop_rate;

  // loop prevention;
  char calculating;
};

struct neighbour_link{
  struct neighbour_link *_next;

  // which of their interfaces are these stats for?
  int neighbour_interface;
  // which interface did we hear it on?
  struct overlay_interface *interface;

  // very simple time based link up/down detection;
  // when will we consider the link broken?
  time_ms_t link_timeout;

  char unicast;
  int ack_sequence;
  uint64_t ack_mask;
};

struct neighbour{
  struct neighbour *_next;

  struct subscriber *subscriber;

  // whenever we hear about a link change, update the version to mark all link path scores as dirty
  char path_version;

  // when do we assume the link is dead because they stopped hearing us or vice versa?
  time_ms_t neighbour_link_timeout;

  // next link update
  time_ms_t next_neighbour_update;
  time_ms_t last_update;
  int ack_counter;

  // un-balanced tree of known link states
  struct link *root;

  // list of incoming link stats
  struct neighbour_link *links, *best_link;

  // is this neighbour still using selfacks?
  char legacy_protocol;
};

// one struct per subscriber, where we track all routing information, allocated on first use
struct link_state{
  // what is the current best hop count? (via subscriber->next_hop)
  struct subscriber *next_hop;
  struct subscriber *transmitter;
  int hop_count;
  int route_version;
  // if a neighbour is free'd this link will point to invalid memory.
  // do not trust this pointer unless you have just called find_best_link
  struct link *link;
  char calculating;

  // when do we need to send a new link state message.
  time_ms_t next_update;
};

static void link_send(struct sched_ent *alarm);

static struct profile_total link_send_stats={
  .name="link_send",
};
static struct sched_ent link_send_alarm={
  .function = link_send,
  .stats = &link_send_stats,
};

struct neighbour *neighbours=NULL;
int route_version=0;

static int NumberOfSetBits(uint32_t i)
{
    i = i - ((i >> 1) & 0x55555555);
    i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
    return (((i + (i >> 4)) & 0x0F0F0F0F) * 0x01010101) >> 24;
}

static struct link_state *get_link_state(struct subscriber *subscriber)
{
  if (!subscriber->link_state){
    subscriber->link_state = emalloc_zero(sizeof(struct link_state));
    subscriber->link_state->route_version = route_version -1;
  }
  return subscriber->link_state;
}

static struct neighbour *get_neighbour(struct subscriber *subscriber, char create)
{
  struct neighbour *n = neighbours;
  while(n){
    if (n->subscriber==subscriber)
      return n;
    n = n->_next;
  }
  if (create){
    n = emalloc_zero(sizeof(struct neighbour));
    n->subscriber = subscriber;
    n->_next = neighbours;
    neighbours = n;
    if (config.debug.linkstate)
      DEBUGF("LINK STATE; new neighbour %s", alloca_tohex_sid(n->subscriber->sid));
  }
  return n;
}

static void free_links(struct link *link)
{
  if (!link)
    return;
  free_links(link->_left);
  link->_left=NULL;
  free_links(link->_right);
  link->_right=NULL;
  free(link);
}

static struct link *find_link(struct neighbour *neighbour, struct subscriber *receiver, char create)
{
  struct link **link_ptr=&neighbour->root, *link=neighbour->root;
  while(1){
    if (link==NULL){
      if (create){
        link = *link_ptr = emalloc_zero(sizeof(struct link));
        link->receiver = receiver;
        link->path_version = neighbour->path_version -1;
      }
      break;
    }
    if (receiver == link->receiver)
      break;
    if (memcmp(receiver->sid, link->receiver->sid, SID_SIZE)<0){
      link_ptr = &link->_left;
    }else{
      link_ptr = &link->_right;
    }
    link = *link_ptr;
  }
  return link;
}

static struct link *get_parent(struct neighbour *neighbour, struct link *link)
{
  // root of the routing table.
  if (link->receiver == neighbour->subscriber || link->transmitter == NULL)
    return NULL;

  if (!link->parent)
    link->parent = find_link(neighbour, link->transmitter, 0);

  return link->parent;
}

static void update_path_score(struct neighbour *neighbour, struct link *link){
  if (link->path_version == neighbour->path_version)
    return;
  if (link->calculating)
    return;

  link->calculating = 1;
  int hop_count = -1;
  int drop_rate = 0;

  if (link->transmitter == my_subscriber){
    if (link->receiver==neighbour->subscriber){
      hop_count = 1;
    }
  }else{
    struct link *parent = get_parent(neighbour, link);
    if (parent && (!parent->calculating)){
      update_path_score(neighbour, parent);
      // TODO more interesting path cost metrics...
      if (parent->hop_count>0){
        hop_count = parent->hop_count+1;
        drop_rate = parent->path_drop_rate;
      }
    }
  }

  // ignore occasional dropped packets due to collisions
  if (link->drop_rate>2)
    drop_rate += link->drop_rate;

  if (config.debug.verbose && config.debug.linkstate && hop_count != link->hop_count)
    DEBUGF("LINK STATE; path score to %s via %s version %d = %d",
	alloca_tohex_sid(link->receiver->sid),
	alloca_tohex_sid(neighbour->subscriber->sid),
	neighbour->path_version,
	hop_count);

  link->hop_count = hop_count;
  link->path_version = neighbour->path_version;
  link->path_drop_rate = drop_rate;
  link->calculating = 0;
}

static int find_best_link(struct subscriber *subscriber)
{
  if (subscriber->reachable==REACHABLE_SELF)
    return 0;

  struct link_state *state = get_link_state(subscriber);
  if (state->route_version == route_version)
    return 0;

  if (state->calculating)
    return -1;
  state->calculating = 1;

  struct neighbour *neighbour = neighbours;
  struct overlay_interface *interface = NULL;
  int best_hop_count = 99;
  int best_drop_rate = 99;
  struct link *best_link = NULL;
  struct subscriber *next_hop = NULL, *transmitter=NULL;
  time_ms_t now = gettime_ms();

  while (neighbour){
    if (neighbour->neighbour_link_timeout < now)
      goto next;

    struct link *link = find_link(neighbour, subscriber, 0);
    if (!(link && link->transmitter))
      goto next;

    if (link->transmitter != my_subscriber){
      struct link_state *parent_state = get_link_state(link->transmitter);
      find_best_link(link->transmitter);
      if (parent_state->next_hop != neighbour->subscriber)
	goto next;
    }

    update_path_score(neighbour, link);

    if (link->hop_count>0){
      if (link->path_drop_rate < best_drop_rate ||
         (link->path_drop_rate == best_drop_rate && link->hop_count < best_hop_count)){
        next_hop = neighbour->subscriber;
        best_hop_count = link->hop_count;
	best_drop_rate = link->path_drop_rate;
        transmitter = link->transmitter;
        interface = link->interface;
        best_link = link;
      }
    }

next:
    neighbour = neighbour->_next;
  }

  int changed =0;
  if (state->next_hop != next_hop || state->transmitter != transmitter || state->link != best_link)
    changed = 1;
  if (next_hop == subscriber && (interface != subscriber->interface))
    changed = 1;

  state->next_hop = next_hop;
  state->transmitter = transmitter;
  state->hop_count = best_hop_count;
  state->route_version = route_version;
  state->calculating = 0;
  state->link = best_link;

  int reachable = subscriber->reachable;
  if (next_hop == NULL){
    if (!(subscriber->reachable & REACHABLE_ASSUMED))
      reachable = REACHABLE_NONE;
  } else if (next_hop == subscriber){
    // reset the state of any unicast probe's if the interface has changed
    if (subscriber->interface != interface){
      reachable = 0;
      subscriber->last_probe=0;
      bzero(&subscriber->address, sizeof subscriber->address);
    }
    reachable = REACHABLE_BROADCAST | (subscriber->reachable & REACHABLE_UNICAST);
    next_hop = NULL;
    subscriber->interface = interface;
  } else {
    reachable = REACHABLE_INDIRECT;
  }
  subscriber->next_hop = next_hop;
  set_reachable(subscriber, reachable);

  if (changed){
    if (config.debug.linkstate){
      if (next_hop == subscriber){
	DEBUGF("LINK STATE; neighbour %s is reachable on interface %s",
	  alloca_tohex_sid(subscriber->sid), 
	  interface->name);
      } else {
        DEBUGF("LINK STATE; next hop for %s is now %d hops, %s via %s", 
	  alloca_tohex_sid(subscriber->sid), 
	  best_hop_count,
	  next_hop?alloca_tohex_sid(next_hop->sid):"UNREACHABLE", 
	  transmitter?alloca_tohex_sid(transmitter->sid):"NONE");
      }
    }
    monitor_announce_link(best_hop_count, transmitter, subscriber);
    state->next_update = now;
  }

  return 0;
}

static int monitor_announce(struct subscriber *subscriber, void *context){
  if (subscriber->reachable & REACHABLE){
    struct link_state *state = get_link_state(subscriber);
    monitor_announce_link(state->hop_count, state->transmitter, subscriber);
  }
  return 0;
}

int link_state_announce_links(){
  enum_subscribers(NULL, monitor_announce, NULL);
  return 0;
}

static int append_link_state(struct overlay_buffer *payload, char flags, 
                             struct subscriber *transmitter, struct subscriber *receiver, 
                             int interface, int version, int ack_sequence, uint32_t ack_mask, 
                             int drop_rate)
{
  if (interface!=-1)
    flags|=FLAG_HAS_INTERFACE;
  if (!transmitter)
    flags|=FLAG_NO_PATH;
  if (ack_sequence!=-1)
    flags|=FLAG_HAS_ACK;
  if (drop_rate!=-1)
    flags|=FLAG_HAS_DROP_RATE;

  int length_pos = ob_position(payload);
  if (ob_append_byte(payload, 0))
    return -1;

  if (ob_append_byte(payload, flags))
    return -1;

  if (overlay_address_append(NULL, payload, receiver))
    return -1;

  if (ob_append_byte(payload, version))
    return -1;

  if (transmitter)
    if (overlay_address_append(NULL, payload, transmitter))
      return -1;

  if (interface!=-1)
    if (ob_append_byte(payload, interface))
      return -1;

  if (ack_sequence!=-1){
    if (ob_append_byte(payload, ack_sequence))
      return -1;
    if (ob_append_ui32(payload, ack_mask))
      return -1;
  }

  if (drop_rate!=-1)
    if (ob_append_byte(payload, drop_rate))
      return -1;


  // TODO insert future fields here


  // patch the record length
  int end_pos = ob_position(payload);
  if (ob_set(payload, length_pos, end_pos - length_pos))
    return -1;

  ob_checkpoint(payload);
  return 0;
}

static int append_link(struct subscriber *subscriber, void *context)
{
  if (subscriber == my_subscriber)
    return 0;

  struct overlay_buffer *payload = context;
  struct link_state *state = get_link_state(subscriber);

  time_ms_t now = gettime_ms();

  if (find_best_link(subscriber))
    return 0;

  if (state->next_update - INCLUDE_ANYWAY <= now){
    if (subscriber->reachable==REACHABLE_SELF){
      // Other entries in our keyring are always one hop away from us.
      if (append_link_state(payload, 0, my_subscriber, subscriber, -1, 1, -1, 0, 0)){
        link_send_alarm.alarm = now;
        return 1;
      }
    } else {
      struct link *link = state->link;
      if (append_link_state(payload, 0, state->transmitter, subscriber, -1, link?link->link_version:-1, -1, 0, link?link->drop_rate:32)){
        link_send_alarm.alarm = now;
        return 1;
      }
    }
    // include information about this link every 5s
    state->next_update = now + 5000;
  }

  if (state->next_update < link_send_alarm.alarm)
    link_send_alarm.alarm = state->next_update;

  return 0;
}

static void free_neighbour(struct neighbour **neighbour_ptr){
  struct neighbour *n = *neighbour_ptr;
  if (config.debug.linkstate && config.debug.verbose)
    DEBUGF("LINK STATE; all links from neighbour %s have died", alloca_tohex_sid(n->subscriber->sid));

  struct neighbour_link *link = n->links;
  while(link){
    struct neighbour_link *l=link;
    link = l->_next;
    free(l);
  }

  free_links(n->root);
  n->root=NULL;
  *neighbour_ptr = n->_next;
  free(n);
  route_version++;
}

static void clean_neighbours(time_ms_t now)
{
  struct neighbour **n_ptr = &neighbours;
  while (*n_ptr){
    struct neighbour *n = *n_ptr;
    struct neighbour_link **list = &n->links;
    while(*list){
      struct neighbour_link *link = *list;
      if (link->interface->state!=INTERFACE_STATE_UP || link->link_timeout < now){
        if (config.debug.linkstate && config.debug.verbose)
          DEBUGF("LINK STATE; link expired from neighbour %s on interface %s", 
            alloca_tohex_sid(n->subscriber->sid),
            link->interface->name);
        *list=link->_next;
        free(link);
      }else{
        list = &link->_next;
      }
    }
    if (!n->links){
      free_neighbour(n_ptr);
    }else{
      n_ptr = &n->_next;
    }
  }
}

static int send_legacy_self_announce_ack(struct neighbour *neighbour, struct neighbour_link *link, time_ms_t now){
  struct overlay_frame *frame=emalloc_zero(sizeof(struct overlay_frame));
  frame->type = OF_TYPE_SELFANNOUNCE_ACK;
  frame->ttl = 6;
  frame->destination = neighbour->subscriber;
  frame->source = my_subscriber;
  frame->payload = ob_new();
  ob_append_ui32(frame->payload, neighbour->last_update);
  ob_append_ui32(frame->payload, now);
  ob_append_byte(frame->payload, link->neighbour_interface);
  frame->queue=OQ_MESH_MANAGEMENT;
  if (overlay_payload_enqueue(frame)){
    op_free(frame);
    return -1;
  }
  return 0;
}

static int link_send_neighbours(struct overlay_buffer *payload)
{
  time_ms_t now = gettime_ms();
  clean_neighbours(now);
  struct neighbour *n = neighbours;

  while (n){
    // TODO compare other link stats to find the best...
    struct neighbour_link *best_link=n->links;
    if (best_link){
      struct neighbour_link *link=best_link->_next;
      while(link){
        if (link->interface != best_link->interface &&
	  overlay_interface_compare(best_link->interface, link->interface))
	  best_link = link;
        link = link->_next;
      }
    }

    if (n->best_link != best_link){
      n->best_link = best_link;
      n->next_neighbour_update = now;
      if (config.debug.linkstate && config.debug.verbose)
        DEBUGF("LINK STATE; best link from neighbour %s is now on interface %s", 
          alloca_tohex_sid(n->subscriber->sid),
          best_link?best_link->interface->name:"NONE");
    }

    if (n->next_neighbour_update - INCLUDE_ANYWAY <= now){
      if (n->legacy_protocol){
	// send a self announce ack instead.
	send_legacy_self_announce_ack(n, best_link, now);
      } else {
        char flags=0;
        if (best_link->unicast)
          flags|=FLAG_UNICAST;
        else
          flags|=FLAG_BROADCAST;

        if (append_link_state(payload, flags, n->subscriber, my_subscriber, best_link->neighbour_interface, 1, best_link->ack_sequence, best_link->ack_mask, -1)){
          link_send_alarm.alarm = now;
	  return 1;
        }
      }
      n->last_update = now;
      n->next_neighbour_update = now + best_link->interface->tick_ms;
      n->ack_counter = ACK_WINDOW;
    }

    if (n->next_neighbour_update < link_send_alarm.alarm)
      link_send_alarm.alarm = n->next_neighbour_update;

    n = n->_next;
  }
  return 0;
}

// send link details
static void link_send(struct sched_ent *alarm)
{
  time_ms_t now = gettime_ms();

  alarm->alarm=now + 10000;

  struct overlay_frame *frame=emalloc_zero(sizeof(struct overlay_frame));
  frame->type=OF_TYPE_DATA;
  frame->source=my_subscriber;
  frame->ttl=1;
  frame->queue=OQ_MESH_MANAGEMENT;
  frame->payload = ob_new();
  ob_limitsize(frame->payload, 400);

  overlay_mdp_encode_ports(frame->payload, MDP_PORT_LINKSTATE, MDP_PORT_LINKSTATE);
  ob_checkpoint(frame->payload);
  int pos = ob_position(frame->payload);

  if (link_send_neighbours(frame->payload)==0){
    enum_subscribers(NULL, append_link, frame->payload);
  }

  ob_rewind(frame->payload);

  if (ob_position(frame->payload) == pos)
    op_free(frame);
  else if (overlay_payload_enqueue(frame))
    op_free(frame);

  alarm->deadline = alarm->alarm;
  schedule(alarm);
}

static void update_alarm(time_ms_t limit){
  if (link_send_alarm.alarm>limit || link_send_alarm.alarm==0){
    unschedule(&link_send_alarm);
    link_send_alarm.alarm = limit;
    link_send_alarm.deadline = limit+10;
    schedule(&link_send_alarm);
  }
}

struct neighbour_link * get_neighbour_link(struct neighbour *neighbour, struct overlay_interface *interface, int sender_interface, char unicast)
{
  struct neighbour_link *link = neighbour->links;
  while(link){
    if (link->interface == interface && link->neighbour_interface == sender_interface && link->unicast == unicast)
      return link;
    link=link->_next;
  }
  link = emalloc_zero(sizeof(struct neighbour_link));
  link->interface = interface;
  link->neighbour_interface = sender_interface;
  link->unicast = unicast;
  link->ack_sequence = -1;
  link->ack_mask = 0;
  link->_next = neighbour->links;
  if (config.debug.linkstate && config.debug.verbose)
    DEBUGF("LINK STATE; new possible link from neighbour %s on interface %s/%d", 
      alloca_tohex_sid(neighbour->subscriber->sid),
      interface->name,
      sender_interface);
  neighbour->links = link;
  return link;
}

int link_state_interface_has_neighbour(struct overlay_interface *interface)
{
  struct neighbour *neighbour = neighbours;
  while(neighbour){
    if (neighbour->best_link && neighbour->best_link->interface == interface)
      return 1;

    neighbour = neighbour->_next;
  }
  return 0;
}

// track stats for receiving packets from this neighbour
int link_received_packet(struct subscriber *subscriber, struct overlay_interface *interface, int sender_interface, int sender_seq, int unicast)
{
  // TODO better handling of unicast routes
  if (unicast)
    return 0;

  struct neighbour *neighbour = get_neighbour(subscriber, 1);
  struct neighbour_link *link=get_neighbour_link(neighbour, interface, sender_interface, unicast);
  time_ms_t now = gettime_ms();
  time_ms_t next_update = neighbour->next_neighbour_update;

  neighbour->ack_counter --;

  // for now we'll use a simple time based link up/down flag + dropped packet count

  if (sender_seq >=0){
    if (link->ack_sequence != -1){
      int offset = (link->ack_sequence - 1 - sender_seq)&0xFF;
      if (offset < 32){
        if (config.debug.verbose && config.debug.linkstate)
          DEBUGF("LINK STATE; late seq %d from %s on %s", 
	    sender_seq, alloca_tohex_sid(subscriber->sid), interface->name);
	link->ack_mask |= (1<<offset);
      }else{
        link->ack_mask = (link->ack_mask << 1) | 1;
        while(1){
          link->ack_sequence = (link->ack_sequence+1)&0xFF;
	  if (link->ack_sequence == sender_seq)
	    break;
	  // missed a packet? send a link state soon
          if (config.debug.verbose && config.debug.linkstate)
            DEBUGF("LINK STATE; missed seq %d from %s on %s", 
	      link->ack_sequence, alloca_tohex_sid(subscriber->sid), interface->name);
	  link->ack_mask = link->ack_mask << 1;
	  next_update = now+100;
        }
      }
    }else
      link->ack_sequence = sender_seq;
  }

  // force an update soon when we need to ack packets
  if (neighbour->ack_counter <=0)
    next_update = now+10;
  // force an update when we start hearing a new neighbour link
  if (link->link_timeout < now)
    next_update = now;

  if (next_update < neighbour->next_neighbour_update){
    neighbour->next_neighbour_update = next_update;
  }
  update_alarm(neighbour->next_neighbour_update);
  link->link_timeout = now + (interface->tick_ms *5);
  return 0;
}

// parse incoming link details
int link_receive(overlay_mdp_frame *mdp)
{
  struct overlay_buffer *payload = ob_static(mdp->out.payload, mdp->out.payload_length);
  ob_limitsize(payload, mdp->out.payload_length);

  struct subscriber *sender = find_subscriber(mdp->out.src.sid, SID_SIZE, 0);
  struct neighbour *neighbour = get_neighbour(sender, 1);

  struct decode_context context;
  bzero(&context, sizeof(context));
  time_ms_t now = gettime_ms();
  char changed = 0;

  while(ob_remaining(payload)>0){
    context.invalid_addresses=0;

    struct subscriber *receiver=NULL, *transmitter=NULL;
    struct overlay_interface *interface = NULL;
    int start_pos = ob_position(payload);
    int length = ob_get(payload);
    if (length <=0)
      break;

    int flags = ob_get(payload);
    if (flags<0)
      break;
    if (overlay_address_parse(&context, payload, &receiver))
      break;
    int version = ob_get(payload);
    if (version < 0)
      break;
    if (!(flags & FLAG_NO_PATH)){
      if (overlay_address_parse(&context, payload, &transmitter))
        break;
    }
    int interface_id = -1;
    if (flags & FLAG_HAS_INTERFACE){
      interface_id = ob_get(payload);
      if (interface_id < 0)
        break;
      if (interface_id >= OVERLAY_MAX_INTERFACES)
	continue;
    }

    int ack_seq = -1;
    uint32_t ack_mask = 0;
    int drop_rate = 0;

    if (flags & FLAG_HAS_ACK){
      ack_seq = ob_get(payload);
      ack_mask = ob_get_ui32(payload);

      drop_rate = 15 - NumberOfSetBits((ack_mask & 0x7FFF));
      // we can deal with low packet loss, it's not interesting if it changes, ignore it.
      if (drop_rate <=2)
	drop_rate = 0;
    }

    if (flags & FLAG_HAS_DROP_RATE){
      drop_rate = ob_get(payload);
      if (drop_rate <0)
	break;
    }

    // jump to the position of the next record, even if there's more data we don't understand
    payload->position = start_pos + length;

    if (context.invalid_addresses)
      continue;

    if (config.debug.verbose && config.debug.linkstate)
      DEBUGF("LINK STATE; record - %s, %s, %d, %d, %d, %d",
	receiver?alloca_tohex_sid(receiver->sid):"NULL",
	transmitter?alloca_tohex_sid(transmitter->sid):"NULL",
	interface_id,
	ack_seq,
	ack_mask,
	drop_rate);

    // ignore any links that our neighbour is using to route through us.
    if (receiver == my_subscriber)
      continue;

    if (receiver == sender){
      // who can our neighbour hear?

      // TODO build a map of everyone in our 2 hop neighbourhood to control broadcast flooding?

      if (transmitter == my_subscriber && interface_id != -1){
	// TODO get matching neighbour link and combine scores

	// TODO use ack_sequence && ack_mask to control (re)sending packets 

        // they can hear us? we can route through them!
	interface = &overlay_interfaces[interface_id];
	if (interface->state != INTERFACE_STATE_UP)
	  continue;

	if (neighbour->neighbour_link_timeout < now)
	  changed = 1;

	neighbour->neighbour_link_timeout = now + interface->tick_ms * 5;

      }else
        continue;
    }else if(transmitter == my_subscriber)
      transmitter = NULL;

    struct link *link = find_link(neighbour, receiver, transmitter?1:0);

    if (link && transmitter == my_subscriber){
      // TODO combine our link stats with theirs
      version = link->link_version;
      if (drop_rate != link->drop_rate || transmitter != link->transmitter)
	version++;
    }

    if (link && (link->transmitter != transmitter || link->link_version != version)){
      changed = 1;
      link->transmitter = transmitter;
      link->link_version = version;
      link->interface = interface;
      link->drop_rate = drop_rate;
      // TODO other link attributes...
    }
  }

  send_please_explain(&context, my_subscriber, sender);

  if (changed){
    route_version++;
    neighbour->path_version ++;
    if (link_send_alarm.alarm>now || link_send_alarm.alarm==0){
      unschedule(&link_send_alarm);
      link_send_alarm.alarm=now;
      // read all incoming packets first
      link_send_alarm.deadline=now+10;
      schedule(&link_send_alarm);
    }
  }

  return 0;
}

// if a neighbour asks for a subscriber explaination, make sure we repeat relevant link information immediately.
void link_explained(struct subscriber *subscriber)
{
  time_ms_t now = gettime_ms();
  struct link_state *state = get_link_state(subscriber);
  state->next_update = now;
  update_alarm(now);
}

void link_interface_down(struct overlay_interface *interface)
{
  clean_neighbours(gettime_ms());
}

/* if an ancient node on the network uses their old protocol to tell us that they can hear us;
  - send the same format back at them
  - treat the link as up.
  - but we aren't going to use this link in either routing protocol
*/
int link_state_legacy_ack(struct overlay_frame *frame, time_ms_t now)
{
  if (frame->payload->sizeLimit<9) 
    return WHY("selfannounce ack packet too short");

  ob_get_ui32(frame->payload);
  ob_get_ui32(frame->payload);
  int iface=ob_get(frame->payload);

  // record that we have a possible link to this neighbour
  struct neighbour *neighbour = get_neighbour(frame->source, 1);
  struct link *link = find_link(neighbour, frame->source, 1);
  int changed = 0;

  if (!neighbour->legacy_protocol){
    changed = 1;
    if (config.debug.linkstate)
      DEBUGF("LINK STATE; new legacy neighbour %s", alloca_tohex_sid(frame->source->sid));
  }
  if (neighbour->neighbour_link_timeout < now)
    changed = 1;
  if (link->transmitter != my_subscriber)
    changed = 1;

  link->transmitter = my_subscriber;
  link->link_version = 1;
  link->interface = &overlay_interfaces[iface];

  // give this link a high cost, we aren't going to route through it anyway...
  link->drop_rate = 32;

  neighbour->legacy_protocol = 1;
  neighbour->neighbour_link_timeout = now + link->interface->tick_ms * 5;

  if (changed){
    route_version++;
    neighbour->path_version ++;
    if (link_send_alarm.alarm>now || link_send_alarm.alarm==0){
      unschedule(&link_send_alarm);
      link_send_alarm.alarm=now;
      // read all incoming packets first
      link_send_alarm.deadline=now+10;
      schedule(&link_send_alarm);
    }
  }

  return 0;
}