README-Olsr-Extensions

=====================================================
      OLSRd (version 0.6.0) protocol extensions
=====================================================

1.) Credits
2.) Link quality algorithms
3.) Fisheye
4.) NIIT (ipv4 over ipv6 traffic)
5.) Smart gateways (asymmetric gateway tunnels)
6.) NatThreshold

NIIT and Smart gateways are only supported for linux at the moment.

    1.) Credits:
********************

The concept of ETX (expected transmission count) has been developed by
Douglas S. J. De Couto at the Massachusetts Institute of Technology
(see http://en.wikipedia.org/wiki/Expected_Transmission_Count).

The original ETX design has been done by the Berlin Freifunk Network
(see www.freifunk.net and www.c-base.org), the code and message format
was coded by Thomas Lopatic.

Fisheye was implemented by Thomas Lopatic in 2005.

The LQ-Plugin rewrite was done by Henning Rogge in 2008.

The NIIT kernel module was written by lynxis in 2009.

The asymmetric gateway tunnels was written by Markus Kittenberger
and Henning Rogge, but the concept was used by B.A.T.M.A.N before OLSRd.



    2.) Link quality algorithm
**********************************

Concept:
--------

OLSRd (since version 0.5.6) use a dimensionless integer value for a
representation of the 'cost' of each link. This is often called Link quality
(LQ for short). There are multiple LQ-Plugins, each of them calculating a cost
for the links of the router. At the moment (version 0.6.0) all lq_plugins are
using an ETX-metric (expected transmission count) but others would be possible
and imaginable, such as MIC [0], etc.


Each link is described by a LQ (link quality) and a NLQ (neighbor link quality)
value, which describe the quality towards the router (LQ) and towards the
neighbor (neighbor link quality, NLQ). Both LQ and NLQ can be a value between 0
and 1.  The total cost of the link is calculated as ETX = 1.0/(LQ * NLQ). The
ETX value of a link can be seen as the number of retransmissions necessary to
deliver the packet to the target. ETX 1.0 mean a perfect link without packet
loss.

     +---+              +---+
     | A |  <--- LQ --- | B |
     +---+  ---- NLQ -->+---+

Note that the LQ and NLQ are always seen as from one nodes' perspective: the LQ
of node A towards B is the percentage of packets that A can transmit to B.
Hence, in the OLSR ETX implementation, B has to tell A it's LQ.

OLSRd chooses the path towards a target by selecting the path segments with the
smallest sum of link costs. In other words:

   best_path(A,B) = minimum_sum({set of all paths between A and B})


Configuration:
--------------

The link quality system is activated by setting the config variable
"LinkQualityLevel" to 2.

You can use the "LinkQualityAlgorithm" parameter to choose the current
link quality algorithm in the config file. Some embedded OLSRd versions
are only compiled with one plugin (mostly etx_ff), so don't use the
configuration option with these agents.

There are four different link quality algorithms in OLSRd 0.6.0, two
current Funkfeuer/Freifunk ETX implementations and two legacy implementations.

LinkQuality-Algorithm "etx_ff":
-------------------------------

"Etx_ff" (ETX Funkfeuer/Freifunk) is the current default LQ algorithm for OLSRd.
It uses the sequence number of the OLSR packages (which are link specific)
to determine the current packet loss rate. Etx_ff includes a hysteresis
mechanism to suppress small fluctuations of the LQ and NLQ value. If
no packages are received from a certain neighbor at all, a timer begins
to lower the calculated LQ value until the next package is received or
the link is dropped.
Etx_ff only uses integer arithmetic, so it performs well on embedded
hardware having no FPU.

The message format of etx_ff is compatible with etx_fpm and etx_float.


LinkQuality-Algorithm "etx_ffeth"
--------------------------------

"Etx_ffeth" is an experimental and INCOMPATIBLE extension of etx_ff (meaning it
will not interoperate with etx_ff nodes).  The problem with etx_ff, etx_float
and etx_fpm is that they calculate ethernet links with the same cost as a
wireless link without packet loss (ETX 1.0) because the encoding of etx_ff
cannot encode link costs lower than 1.0. This means OLSRd prefers a single
wireless link with some loss (e.g. ETX 1.5) over a two hop route with one
ethernet link (ETX 1.0) and one perfect wireless link (ETX 1.0) *even though*
the latter path would be better!

"Etx_ffeth" tries to work around this problem by introducing a special
LQ encoding for the value ETX 0.1, which is only used for ethernet
links without packet loss. Because of the different encoding etx_ffeth
is not compatible with etx_ff, etx_fpm or etx_float. These three
implementations detect etx_ffeth nodes with LQ 0 (ETX infinite).

etx_ffeth only use integer arithmetic, so it performs well on embedded
hardware.

All ethernet interfaces must be marked with "mode ether" 
(see olsrd.conf.default.full) in their interface configuration to get any
useful advantage of etxff_eth.

At the time of this writing, etx_ffeth is the prefered metric for building new
mesh networks which include links over LAN cables (such as daisy chained
linksys routers).


Legacy LinkQuality-Algorithm "etx_float"
----------------------------------------

"Etx_float" calculates the ETX value by using exponential aging (with
a configurable aging parameter) on the incoming (or lost) Hellos.
It is easier to understand than etx_ff, but the results are not as
good as in etx_ff, since it cannot use the TC messages for link
quality calculation.
Etx_float uses floating point math, so it might use more CPU on embedded
hardware.

The message format of etx_float is compatible with etx_fpm and etx_ff.


Legacy LinkQuality-Algorithm "etx_fpm"
--------------------------------------

"Etx_fpm" is a fixed point math implementation of etx_float. It
calculates the same link qualities as etx_float, but is much faster
on embedded hardware.

The message format of etx_fpm is compatible with etx_float and etx_ff.


Building your own LinkQuality Algorithm
---------------------------------------- 

With the supplied samples OLSRd can be easily extended to support different
metrics. Please take a look at src/lq_plugin*.[ch] for inspiration and get in
contact with us on the OLSR development mailing list in case you plan to
implement a new metric.



    3.) Fisheye
*******************

Normally OLSR floods all topology control (TC) messages to all
routes in the mesh, which can create a lot of overhead for large
meshs with hundreds of routers. Reducing the rate of TCs can reduce
this overhead, but delay route changes and correction of errors
in the routing tables.

The Fisheye (sometimes called Hazy Sighted Link State Routing [1])
mechanism implements a strategy to reach a compromise between
these two problems. When activated only every 8th TC is send
to all mesh nodes. Most TCs are given a reduced TTL (time to live)
and are only transmitted to the neighborhood of the router.

The current sequence of TTLs with active fisheye mechanism is
2, 8, 2, 16, 2, 8, 2 and 255 (maximum TTL).

The problem with Fisheye is that it introduces artifical borders
for flooding TCs, which can theoretically lead to inconsistent routes
and routing loops at the border of the fisheye circles. In practice
fisheye seems to work well enough that it is a mandatory feature
for most larger Funkfeuer/Freifunk meshs.


    4.) NIIT (ipv4 over ipv6 traffic)
*****************************************
(see https://dev.dd19.de/cgi-bin/gitweb.cgi?p=niit.git;a=summary)

NIIT is a special linux kernel device that allows easy transmission of IPv4
unicast traffic through an IPv6 network. Since version 0.6.0 OLSRd has
integrated support for NIIT in the routing daemon. So setting up IPv4 traffic
over IPv6 OLSR meshs is very easy. Instead of creating routes and tunnels by
hand all the administrator of a router needs to do is to, is to set up his own
IPv4 targets as "IPv4-mapped" IPv6 HNAs.

Example configurations:
- connect a local 192.168.1.0/8 net to the mesh

HNA6 {
  0::ffff:C0A8:01:00 120
}

- announce an IPv4 internet gateway

HNA6 {
  0::ffff:0:0 96
}


More information on NIIT can be found at: http://wiki.freifunk.net/Niit
(german)


    5.) Smart gateways (asymmetric gateway tunnels)
*******************************************************

The smartgateway mechanism was written by Markus Kittenberger and
Henning Rogge to allow an OLSR user to directly choose their default
internet gateway instead of relying on the hop by hop decisions on
the way to the gateway. OLSRd 0.6.0 can create an IPIP tunnel
to the gateways OLSRd address to sidestep the same nasty effects
described in the Nat-Threshold section.

The smartgateway code can be split into two sections, one is
responsible for announcing the existence of a smartgateway uplink
and one on the client nodes to choose an uplink and create the
tunnel to the gateway. It use a modified (but backward compatible)
special HNA to signal the gateways to the clients. The clients can
use a plugin (or the integrated default code) to choose one of the
available gateways and change it if necessary. 

The smartgateway system is setup by several configuration parameters,
most of them with a sane default setting. The whole system can be
switched on/off by the following parameter:

SmartGateway <yes/no>

All other parameters will be ignored if SmartGateway is set to "no"
(the default is "yes").

On the client side there is a single additional parameter which
controls if you want to allow the selection of an outgoing ipv4
gateway with NAT (network address translation).

SmartGatewayAllowNAT <yes/no>

The uplink side of the smartgateway code has four parameters to
set up the type of the uplink.

SmartGatewayUplink defines which kind of uplink is exported to the
other mesh nodes. The existence of the uplink is detected by looking
for a local HNA of 0.0.0.0/0, ::ffff:0:0/96 or 2000::/3. The default
setting is "both".
SmartGatewayUplinkNAT defines if the ipv4 part of the uplink use NAT.
The default of this setting is "yes".
SmartGatewaySpeed sets the uplink and downlink speed of the gateway,
which could be used by a plugin to choose the right gateway for a
client. The default is 128/1024 kbit/s.
The final parameter SmartGatewayPrefix can be used to signal the
external IPv6 prefix of the uplink to the clients. This might allow
a client to change it's local IPv6 address to use the IPv6 gateway
without any kind of address translation. The maximum prefix length
is 64 bits, the default is ::/0 (no prefix).

SmartGatewayUplink <none/ipv4/ipv6/both>
SmartGatewayUplinkNAT <yes/no>
SmartGatewaySpeed <uplink> <downlink>
SmartGatewayPrefix <prefix>

On the SmartGW server (the OLSR instance anncouning 'Internet here!' via
HNA0/0 or similar) the implicit tunl0 interface is used to forward  incoming 
packets originated by SmartGW clients to the internet route. This may be 
protected by the sysctl rp_filter setting. Note, that at least with RedHat kernel 
2.6.18, the net.ipv4.conf.tunl0.rp_filter sysctl file is not present after 
loading the "ipip" kernel module. Which prevents OLSRD from switching off the 
filter. As a workaround, add an "ip addr add 0.0.0.0/32 dev tunl0" after 
the "modprobe ipip" line in your OLSRD startup script. 

While the SmartGW function does a fine job on stand-alone PCs, system builders 
should keep in mind the following facts when setting up routing, firewalls 
and gateways:

a) The SmartGW tunnel communicates asymmetrically. An IP packet destinned to 
an Internet server is sent via the IPIP tunnel but returned via the standard 
OLSRD host route.

b) On the SmartGW server, you should double check your firewall rules and 
rp_filter defaults. While it's normally not possible to simply encap e.g. 
a "telnet 127.0.0.1" into IPIP and sent that to the SmartGW server, your 
specific configuration may open up other attack vectors for an intruder.

c) Do not forget to un-firewall tunl0->internet and (if required to 
NAT/MASQUERADE) this communication path.

d) While the SmartGW server does not use special routing, the SmartGW client
inserts policy routing rules for it's function. By using the default configuration,
the OLSRD standard default route is maintained in table 223 and the OLSRD SmartGW 
default route in table 224. Both tables are examined only, if you do not have
a default route in the main table (visible with "ip route ls"). Use "ip route ls
table 223" or "ip route ls table 224" for debugging/monitoring. You may also
activate the txtinfo plugin and "wget -O - http://localhost:2006/gateway".

e) For the stand-alone client (Notebook user running OLSRD in order to browse) 
the lowered IPIP tunnel MTU is no problem. If you do proxy routing, e.g. for 
attached LAN clients without OLSRD, you may want MSS-clamping for the tunnel 
interface created by OLSRD. Because OLSRD uses an arbitrary name for the tunnel
iface (e.g. tnl_7c41c668) you may want to include a wildcard iptables rule. Example:
iptables -A FORWARD -o tnl_+ -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

Furthermore (or alternatively) you might consider to clamp all traffic to your ipip mtu
on your gateway nodes that is leaving your mesh. (regardless if the traffic comes out 
of the smartgateway tunnel or not!)
iptables -A FORWARD -o [your_gateway_interface] -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1480

Especially as during olsrd startup, before an smartgateway is choosen (which is delayed), 
new connections would use a larger MSS as the smartgateway tunnel can handle. 
So the approach to clamp on the gateways should give better results.

But if you don't NAT on your gateways (but want to use SmartGateway for some some 
special reason), you would have to do this on ALL gateways (Even on gateways that 
do not provide the SmartGateway functionality!)

    6.) NatThreshold
************************

The NatThreshold option was introduced by Sven Ola to suppress a very annoying
problem with OLSRd, switching default gateways. If a router is located between
two internet gateways with similar path costs the default route (0.0.0.0/0)
will constantly switch between the two gateways due to normal fluctuations of
the link metrics. Whenever OLSRd decides that the other NAT gateway is
"better", then switching to this new gateway will result in termination of all
connected sessions (TCP and HTTP). 
The user experience will be rather painful and users will experience hanging
SSH and HTTP sessions (or anything using TCP).

NatThreshold tries to help by introducing a hysteresis factor for
choosing the route to the default gateway. Only if the new gateway has
a lower cost than the current gateways path cost multiplied by
NatThreshold the node will switch the gateway. 
In short:

  if (cost(new_gateway) < cost(current_gw)*NatThreshold)) {
	switch_gateway();
  }


Practical experience shows that this leads to much better quality of default
gateway selection, even if (in theory) a small NatThreshold together with
Fisheye can lead to  persistent routing loops.
Please note that even with NatThreshold enabled, some users will still experience
gateway switching. However, most users will not.

Smart Gateways can replace NatThreshold all together because they allow sending
traffic directly to a gateway circumventing the problems described above which
stem from a hop-by-hop routing approach 



     7.) References
************************
[0] MIC Metric: "Designing Routing Metrics for Mesh Networks", 
	Yaling Yang, Jun Wang, Robin Kravets
	http://www.cs.ucdavis.edu/~prasant/WIMESH/p6.pdf

[1] "Making link-state routing scale for ad hoc networks",
	Cesar A. Santivanez, Ram Ramanathan, Ioannis Stavrakakis
	http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.16.5940