Wireless

1
Wireless Mobile Communications Chapter 6
Network Protocols/Mobile IP

• Motivation
• Data transfer
• Encapsulation
• Security
• IPv6

• Problems
• DHCP
• Ad-hoc networks
• Routing protocols

2
Why Mobile IP?

• What do cellular networks and wireless LANs
  provide?
• Wireless connectivity
• Mobility at the data link layer
• What is Dynamic Host Configuration Protocol
  (DHCP)?
• It provides local IP addresses for mobile hosts
• Is not secure
• Does not maintain network connectivity when
  moving around
• What they do not provide
• Transparent connectivity at the network layer
• Mobility with local access
• The difference between mobility and nomadicity!

3
What is Mobile IP?

• Mobile IP provides network layer mobility
• Provides seamless roaming
• Extends the home network over the entire
  Internet

4
IP Overview 1/3

• IP Addressing
• Dotted Decimal Notation 32 bits (4x8) used to
  represent IPv4 addresses - 192.19.241.18
• Network Prefix and Host Portions p - prefix, h -
  host, p h 32. If p 24 then h 32 - 24 8.
  Using above address the network prefix will be
  192.19.241 and host will be 18. For those of you
  familiar with subnet masks, p represents the
  number of 1s in the subnet mask. If p 24,
  subnet mask is 255.255.255.0, if p 26, subnet
  mask is 255.255.255.192.

5
IP Overview 2/3

• IP Routing
• Network prefix is used for routing. Routing
  tables are used to look up next hop and the
  interface on the router that is to be used.
• In the routing tables we use the following
  notation target/prefix length, e.g.,
  192.19.241.0/24, or 192.19.241.192/26.
• If two subnet masks/prefixes fit the address, the
  one with the largest prefix is chosen for
  routing. E.g., a router with the following 3
  entries in its table 7.7.7.99/32 (p32 host
  specific) and 7.7.7.0/24 (0ltplt32 network prefix)
  and 0.0.0.0/0 (p0 default) will use entry 2 for
  an IP packet with destination 7.7.7.1 and entry 3
  for destination 192.33.14.12.

6
IP Overview 3/3

• Domain Name System (DNS) used to translate a
  host name to an IP address. A host sends a query
  to a server to obtain the IP address of a
  destination of which it only has the host name.
• Link Layer Addresses - Address Resolution
  Protocol (ARP)
• Once a host has the IP address of a destination
  it then needs to finds its layer 2 address or the
  layer 2 address of the next hop on the path. A
  broadcast message is sent and the targeted host
  responds with its layer 2 address.
• A proxy ARP is a response by a node for another
  node that cannot respond at the time the request
  is made (e.g. the node is a mobiel node and not
  on its host net at the time, its home agent will
  respond in its stead).
• A gratuitous ARP, is a reply to no ARP request,
  used by a node that just joins the network and
  wants to make its address known. Can be used by a
  mobile node upon its return to its home net.

7
Motivation for Mobile IP

• IP Routing
• based on IP destination address, network prefix
  (e.g. 129.13.42) determines physical subnet
• change of physical subnet implies change of IP
  address to have a topologically correct address
  (standard IP) or needs special entries in the
  routing tables
• Specific routes to end-systems?
• requires changing all routing table entries to
  forward packets to the right destination
• does not scale with the number of mobile hosts
  and frequent changes in the location, security
  problems
• Changing the IP-address?
• adjust the host IP address depending on the
  current location
• almost impossible to find a mobile system, DNS
  updates take long time
• TCP connections break, security problems

8
What Mobile IP does

• Mobile IP solves the following problems
• if a node moves without changing its IP address
  it will be unable to receive its packets,
• if a node changes its IP address it will have to
  terminate and restart its ongoing connections
  everytime it moves to a new network area (new
  network prefix).
• Mobile IP is a routing protocol with a very
  specific purpose.
• Mobile IP is a network layer solution to node
  mobility in the Internet.
• Mobile IP is not a complete solution to mobility,
  changes to the transport protocols need to be
  made for a better solution (i.e., the transport
  layers are unaware of the mobile nodes point of
  attachment and it might be useful if, e.g., TCP
  knew that a wireless link was being used!).

9
Requirements to Mobile IP (RFC 2002)

• Transparency
• mobile end-systems keep their IP address
• continuation of communication after interruption
  of link possible
• point of connection to the fixed network can be
  changed
• Compatibility
• support of the same layer 2 protocols as IP
• no changes to current end-systems and routers
  required
• mobile end-systems can communicate with fixed
  systems
• Security
• authentication of all registration messages
• Efficiency and scalability
• only little additional messages to the mobile
  system required (connection typically via a low
  bandwidth radio link)
• world-wide support of a large number of mobile
  systems in the whole Internet

10
Mobile IP Terminology

• Mobile Node (MN)
• system (node) that can change the point of
  connection to the network without changing its
  IP address
• Home Agent (HA)
• system in the home network of the MN, typically a
  router
• registers the location of the MN, tunnels IP
  datagrams to the COA
• Foreign Agent (FA)
• system in the current foreign network of the MN,
  typically a router
• forwards the tunneled datagrams to the MN,
  typically also the default router for the MN
• Care-of Address (COA)
• address of the current tunnel end-point for the
  MN (at FA or MN)
• actual location of the MN from an IP point of
  view
• can be chosen, e.g., via DHCP
• Correspondent Node (CN)
• communication partner

11
Mobile IP Operation Summary

• Consists of 3 steps
• Agent discovery,
• Registration, and
• Routing/Tunneling

12
Operation Summary 1/3

• Agent Advertisement/Discovery consists of
  broadcast messages used by mobiles to detect that
  they have moved and are required to register with
  a new FA.
• FAs send agent advertisements
• MNs can solicit for agents if they have not heard
  an agent advertisement in awhile or use some
  other mechanism to obtain a COA or temp. IP
  address (e.g. DHCP).
• MNs know they are home when they recognize their
  HA.

13
Operation Summary 2/3

• Registration used by a MN to inform the FA that
  it is visiting.
• The new care of address of the MN is sent to the
  HA.
• Registration expires, duration is negotiated
  during registration
• Mobile must re-register before it expires
• All registrations are authenticated
• The MN sends a regristration request in to the FA
  which passes it along to the home agent. The HA
  responds to the FA which then informs the MN that
  all is in order and registration is complete.

14
Operation Summary 3/3

• Routing/Encapsulation/Tunneling consists of the
  delivery of the packets to the mobile node at its
  current care of address.
• Sender does not need to know that the destination
  is a MN.
• HA intercepts all packets for the MN and passes
  them along to MN using a tunnel.
• MN communicates directly with the CN.
• Referred to as Triangle Routing

15
Example network
HA
MN
Internet
router
mobile end-system
home network
(physical home network for the MN)
FA
foreign network
router
(current physical network for the MN)
CN
router
end-system
16
Data transfer to the mobile system
HA
2
MN
Internet
home network
receiver
3
FA
foreign network
1. Sender sends to the IP address of MN, HA
intercepts packet (proxy ARP) 2. HA tunnels
packet to COA, here FA, by encapsulation 3.
FA forwards the packet to the MN
1
CN
sender
17
Data transfer from the mobile system
HA
1
MN
Internet
home network
sender
FA
foreignnetwork
1. Sender sends to the IP address of the
receiver as usual, FA works as default router
CN
receiver
18
Overview
COA
foreign network
router FA
MN
router HA
home network
Internet
CN
router
foreign network
3.
router FA
MN
router HA
home network
2.
4.
Internet
1.
CN
router
19
Network integration

• Agent Advertisement Discovery
• HA and FA periodically send advertisement
  messages into their physical subnets
• MN listens to these messages and detects, if it
  is in the home or a foreign network (standard
  case for home network)
• MN reads a COA from the FA advertisement messages
• Registration (always limited lifetime!)
• MN signals COA to the HA via the FA, HA
  acknowledges via FA to MN
• these actions have to be secured by
  authentication
• Routing/Encapsulation/Tunneling
• HA advertises the IP address of the MN (as for
  fixed systems), i.e. standard routing information
• packets to the MN are sent to the HA,
• independent of changes in COA/FA

20
Agent advertisement
0
7
8
15
16
31
24
23
type
checksum
code
addresses
addr. size
lifetime
router address 1
preference level 1
router address 2
preference level 2
. . .
type
sequence number
length
registration lifetime
reserved
COA 1
COA 2
. . .
21
Registration
MN
FA
HA
MN
HA
registration request
registration request
registration request
registration reply
registration reply
t
registration reply
t
22
Mobile IP registration request
0
7
8
15
16
31
24
23
type
lifetime
rsv
home address
home agent
COA
identification
extensions . . .
23
Processing Registration Messages 1/3

• A MN, depending on which registration scenario it
  is in, will figure what addresses to use in the
  various fields of the Registration request
  message.
• Link layer addresses are tricky
• A MN may not use ARP if it is using a FA COA. It
  needs to use the address of the FA as the
  destination address.
• If it is using a collocated COA, then it uses ARP
  to locate the default router using its COA as
  source. Note that if the R bit is set is uses
  the FA address as the destination address.
• For de-registration is uses ARP to locate the HA
  link address and it uses its own home address for
  the ARP message.
• For network layer addresses (i.e., IP addresses)
• It uses the FA address as destination address
  when using the FA COA and its own home address as
  the source address.
• If using a collocated COA it uses its COA as
  source address and the HA address as destination
  address. Note that if the R bit is set then is
  must use the same addresses as for the FA COA
  scenario.
• For de-registration it uses its own home address
  as source and the HA address as destination.

24
Processing Registration Messages 2/3

• For the FA
• A FA may refuse a Registration request for a
  number of reasons lifetime too long,
  authentication failed, requested tunneling not
  supported, cannot handle another MN (current load
  too high).
• If an FA does not refuse the request it relays it
  to the HA. Relaying is different from forwading
  as the FA is required to process the packet and
  create new headers.
• Some important fields of the request message are
  recorded for use later on MN link layer address,
  MN IP address, UDP source port, HA IP address,
  identification number and requested lifetime.
• Regarding a Registration reply message, the FA
  can refuse it and send a decline to the MN is it
  finds the reply from the HA to be invalid.
  Otherwise it updates its list of visiting MNs and
  begins acting on behalf of the MN.

25
Processing Registration Messages 3/3

• For a HA
• The HA will determine, as the FA did, whether it
  will accept the request. If it does not it
  returns a code in the reply message indicating
  the cause of the failed request.
• If the request is accepted, the reply is sent
  back by reversing all the IP addresses and UDP
  port numbers.
• The HA updates the binding table corresponding to
  that MN dependent upon the nature of the request.

26
Routing/Tunneling 1/5

• Routing a packet to a MN involves the following
• A router on the home link, possibly the HA,
  advertises reachability to the network prefix of
  the MNs home address.
• All packets are therefore routed to the MNs home
  link.
• A HA intercepts the packets for the MN and
  tunnels a copy to each COA in the binding table.
• At the foreign link either the MN extracts the
  packet (collocated COA) or the FA extracts the
  packet and forwards it to the MN.

27
Routing/Tunneling 2/5

• A HA can use one of two methods to intercept a
  MNs packets
• The HA is a router with multiple network
  interfaces. In that case it advertises
  reachability to the MNs home network prefix.
• The HA is not a router with multiple intefaces.
  It must use ARP to receive the MNs packets. It
  either responds to ARP requests on behalf of the
  MN (proxy ARP) or uses gratuitous ARPs to inform
  the home network that it is receiving the MNs IP
  packets. This is to update any ARP caches that
  hosts and other devices might have.

28
Routing/Tunneling 3/5

• How to fool the routing table into handling
  tunneled packets at the HA?
• A virtual interface is used to do the
  encapsulation.
• A packet destined for the MN is handled by the
  routing routine as all received IP packets are.
• The routing table has a host specific entry for
  the MN. This host specific entry is used to route
  the packet to a virtual interface that basically
  consists of a process that does encapsulation.
• Once encapsulation has been performed the packet
  is sent to be processed by the routing routine
  again. This time the destination address is the
  COA and it is routed normally.

29
Routing/Tunneling 4/5

• How to fool the routing table into handling
  tunneled packets at the FA?
• The same procedure is used as above.
• A packet coming in with a COA that is one of the
  FA addresses is handled by the routing routine.
• A host specific address (its own address) in the
  routing table points to the higher layers and the
  packet is passed on to a virtual interface.
• The virtual interface consists of a process that
  decapsulates the packet and re-routes it to the
  routing routine.
• The routing routine routes the packet normally
  based upon a host specific entry that is the
  MNs home address (for which it has the link
  layer address!).

30
Routing/Tunneling 5/5

• How does a MN route its packets?
• It needs to find a router to send all its packets
  to.
• It can select a router in one of a number of ways
  dependent upon whether it has a FA COA or a
  collocated COA.
• Having a FA COA does not imply that the MN needs
  to use it as its default router for sending
  packets. It can use any router that sends
  advertisements or that is advertised in the Agent
  Advertisement message.
• If the MN is using a collocated COA it needs to
  listen for router advertisements or is it hears
  none, use DHCP to find the default router.
• Determining the link layer address is another
  issue. Collocated COA MNs can use ARP. FA COA
  must note the link layer address when they
  receive router advertisements or agent
  advertisements.

31
Encapsulation Process
original IP header
original data
new data
new IP header
outer header
inner header
original data
32
Types of Encapsulation

• Three types of encapsulation protocols are
  specified for Mobile IP
• IP-in-IP encapsulation required to be supported.
  Full IP header added to the original IP packet.
  The new header contains HA address as source and
  Care of Address as destination.
• Minimal encapsulation optional. Requires less
  overhead but requires changes to the original
  header. Destination address is changed to Care of
  Address and Source IP address is maintained as
  is.
• Generic Routing Encapsulation (GRE) optional.
  Allows packets of a different protocol suite to
  be encapsulated by another protocol suite.
• Type of tunneling/encapsulation supported is
  indicated in registration.

33
IP in IP Encapsulation

• IP in IP encapsulation (mandatory in RFC 2003)
• tunnel between HA and COA

length
TOS
ver.
IHL
IP identification
flags
fragment offset
TTL
IP-in-IP
IP checksum
IP address of HA
Care-of address COA
length
TOS
ver.
IHL
IP identification
flags
fragment offset
TTL
lay. 4 prot.
IP checksum
IP address of CN
IP address of MN
TCP/UDP/ ... payload
34
Minimum Encapsulation

• Minimal encapsulation (optional)
• avoids repetition of identical fields
• e.g. TTL, IHL, version, TOS
• only applicable for unfragmented packets, no
  space left for fragment identification

length
TOS
ver.
IHL
IP identification
flags
fragment offset
TTL
min. encap.
IP checksum
IP address of HA
care-of address COA
S
lay. 4 protoc.
IP checksum
reserved
IP address of MN
original sender IP address (if S1)
TCP/UDP/ ... payload
35
Generic Routing Encapsulation
36
Routing techniques

• Triangle Routing tunneling in its simplest form
  has all packets go to home network (HA) and then
  sent to MN via a tunnel.
• This involves two IP routes that need to be
  set-up, one original and the second the tunnel
  route.
• Causes unnecessary network overhead and adds to
  the latency.
• Route optimization allows the correstpondent
  node to learn the current location of the MN and
  tunnel its own packets directly. Problems arise
  with
• mobility correspondent node has to
  update/maintain its cache.
• authentication HA has to communicate with the
  correspondent node to do authentication, i.e.,
  security association is with HA not with MN.

37
Optimization of packet forwarding

• Change of FA
• packets on-the-fly during the change can be lost
• new FA informs old FA to avoid packet loss, old
  FA now forwards remaining packets to new FA
• this information also enables the old FA to
  release resources for the MN

38
Change of foreign agent
CN
HA
FAold
FAnew
MN
request
update
ACK
data
data
MN changeslocation
registration
registration
update
ACK
data
data
data
warning
update
ACK
data
data
t
39
Problems with Triangle Routing

• Triangle routing has the MN correspond directly
  with the CN using its home address as the SA
• Firewalls at the foreign network may not allow
  that
• Multicasting if a MN is to participate in a
  multicast group, it needs to use a reverse tunnel
  to maintain its association with the home
  network.
• TTL a MN might have a TTL that is suitable for
  communication when it is in its HM. This TTL may
  not be sufficient when moving around (longer
  routes possibly). When using a reverse tunnel, it
  only counts as a single hop. A MN does not want
  to change the TTL everytime it moves.
• Solution reverse tunneling

40
Reverse tunneling (RFC 2344)
HA
2
MN
Internet
home network
sender
1
FA
foreignnetwork
1. MN sends to FA 2. FA tunnels packets to HA
by encapsulation 3. HA forwards the packet to
the receiver (standard case)
3
CN
receiver
41
Mobile IP with reverse tunneling

• Routers accept often only topologically correct
  addresses (firewall!)
• a packet from the MN encapsulated by the FA is
  now topologically correct
• Multicast and TTL problems solved
• Reverse tunneling does not solve
• all problems with firewalls, the reverse tunnel
  can be abused to circumvent security mechanisms
  (tunnel hijacking)
• optimization of data paths, i.e. packets will be
  forwarded through the tunnel via the HA to a
  sender (longer routes)
• The new standard is backwards compatible
• the extensions can be implemented easily

42
Mobile IP and IPv6

• Mobile IP was developed for IPv4, but IPv6
  simplifies the protocols
• security is integrated and not an add-on,
  authentication of registration is included
• COA can be assigned via auto-configuration
  (DHCPv6 is one candidate), every node has address
  autoconfiguration
• no need for a separate FA, all routers perform
  router advertisement which can be used instead of
  the special agent advertisement
• MN can signal a sender directly the COA, sending
  via HA not needed in this case (automatic path
  optimization)
• soft hand-over, i.e. without packet loss,
  between two subnets is supported
• MN sends the new COA to its old router
• the old router encapsulates all incoming packets
  for the MN and forwards them to the new COA
• authentication is always granted

43
Problems with Mobile IP

• Security
• authentication with FA problematic, for the FA
  typically belongs to another organization
• no protocol for key management and key
  distribution has been standardized in the
  Internet
• patent and export restrictions
• Firewalls
• typically mobile IP cannot be used together with
  firewalls, special set-ups are needed (such as
  reverse tunneling)
• QoS
• many new reservations in case of RSVP
• tunneling makes it hard to give a flow of packets
  a special treatment needed for the QoS
• Security, firewalls, QoS etc. are topics of
  current research and discussions!

44
Security in Mobile IP

• Security requirements (Security Architecture for
  the Internet Protocol, RFC 1825)
• Integrityany changes to data between sender and
  receiver can be detected by the receiver
• Authenticationsender address is really the
  address of the sender and all data received is
  really data sent by this sender
• Confidentialityonly sender and receiver can read
  the data
• Non-Repudiationsender cannot deny sending of
  data
• Traffic Analysiscreation of traffic and user
  profiles should not be possible
• Replay Protectionreceivers can detect replay of
  messages

45
IP security architecture 1/2

• Two or more partners have to negotiate security
  mechanisms to setup a security association
• typically, all partners choose the same
  parameters and mechanisms
• Two headers have been defined for securing IP
  packets
• Authentication-Header
• guarantees integrity and authenticity of IP
  packets
• if asymmetric encryption schemes are used,
  non-repudiation can also be guaranteed
• Encapsulation Security Payload
• protects confidentiality between communication
  partners

ESP header
IP header
encrypted data
46
IP security architecture 2/2

• Mobile Security Association for registrations
• parameters for the mobile host (MH), home agent
  (HA), and foreign agent (FA)
• Extensions of the IP security architecture
• extended authentication of registration
• prevention of replays of registrations
• time stamps 32 bit time stamps 32 bit random
  number
• responses 32 bit random number (MH) 32 bit
  random number (HA)

MH-FA authentication
FA-HA authentication
MH-HA authentication
registration request
registration request
MH
FA
HA
registration reply
registration reply
47
Key distribution

• Home agent distributes session keys
• foreign agent has a security association with the
  home agent
• mobile host registers a new binding at the home
  agent
• home agent answers with a new session key for
  foreign agent and mobile node

FA
MH
response EHA-FA session key EHA-MH session
key
HA
48
DHCP Dynamic Host Configuration Protocol

• Application
• simplification of installation and maintenance of
  networked computers
• supplies systems with all necessary information,
  such as IP address, DNS server address, domain
  name, subnet mask, default router etc.
• enables automatic integration of systems into an
  Intranet or the Internet, can be used to acquire
  a COA for Mobile IP
• Client/Server-Model
• the client sends via a MAC broadcast a request to
  the DHCP server (might be via a DHCP relay)

DHCPDISCOVER
DHCPDISCOVER
client
server
client
relay
49
DHCP - protocol mechanisms
client
server (not selected)
server (selected)
initialization
DHCPDISCOVER
DHCPDISCOVER
determine the configuration
determine the configuration
DHCPOFFER
DHCPOFFER
collection of replies
selection of configuration
time
DHCPREQUEST(reject)
DHCPREQUEST(options)
confirmation of configuration
DHCPACK
initialization completed
release
delete context
DHCPRELEASE
50
DHCP characteristics

• Server
• several servers can be configured for DHCP,
  coordination not yet standardized (i.e., manual
  configuration)
• Renewal of configurations
• IP addresses have to be requested periodically,
  simplified protocol
• Options
• available for routers, subnet mask, NTP (network
  time protocol) timeserver, SLP (service location
  protocol) directory, DNS (domain name system)
• Big security problems!
• no authentication of DHCP information specified

51
Ad hoc networks

• Standard Mobile IP needs an infrastructure
• Home Agent/Foreign Agent in the fixed network
• DNS, routing etc. are not designed for mobility
• Sometimes there is no infrastructure!
• remote areas, ad-hoc meetings, disaster areas
• cost can also be an argument against an
  infrastructure!
• Main topic routing
• no default router available
• every node should be able to forward

A
B
C
52
Routing examples for an ad-hoc network
N1
N1
N2
N3
N2
N3
N4
N4
N5
N5
good link weak link
time t1
time t2
53
Traditional routing algorithms

• Distance Vector
• periodic exchange of messages with all physical
  neighbors that contain information about who can
  be reached at what distance
• selection of the shortest path if several paths
  available
• Link State
• periodic notification of all routers about the
  current state of all physical links
• router get a complete picture of the network
• Example
• ARPA packet radio network (1973), DV-Routing
• every 7.5s exchange of routing tables including
  link quality
• updating of tables also by reception of packets
• routing problems solved with limited flooding

54
Problems of traditional routing algorithms

• Dynamics of the topology
• frequent changes of connections, connection
  quality, participants
• Limited performance of mobile systems
• periodic updates of routing tables need energy
  without contributing to the transmission of user
  data, sleep modes difficult to realize
• limited bandwidth of the system is reduced even
  more due to the exchange of routing information
• links can be asymmetric, i.e., they can have a
  direction dependent transmission quality
• Problem
• protocols have been designed for fixed networks
  with infrequent changes and typically assume
  symmetric links

55
DSDV (Destination Sequenced Distance Vector)

• Expansion of distance vector routing
• Sequence numbers for all routing updates
• assures in-order execution of all updates
• avoids loops and inconsistencies
• Decrease of update frequency
• store time between first and best announcement of
  a path
• inhibit update if it seems to be unstable (based
  on the stored time values)

56
Dynamic source routing I

• Split routing into discovering a path and
  maintainig a path
• Discover a path
• only if a path for sending packets to a certain
  destination is needed and no path is currently
  available
• Maintaining a path
• only while the path is in use one has to make
  sure that it can be used continuously
• No periodic updates needed!

57
Dynamic source routing II

• Path discovery
• broadcast a packet with destination address and
  unique ID
• if a station receives a broadcast packet
• if the station is the receiver (i.e., has the
  correct destination address) then return the
  packet to the sender (path was collected in the
  packet)
• if the packet has already been received earlier
  (identified via ID) then discard the packet
• otherwise, append own address and broadcast
  packet
• sender receives packet with the current path
  (address list)
• Optimizations
• limit broadcasting if maximum diameter of the
  network is known
• caching of address lists (i.e. paths) with help
  of passing packets
• stations can use the cached information for path
  discovery (own paths or paths for other hosts)

58
Dynamic Source Routing III

• Maintaining paths
• after sending a packet
• wait for a layer 2 acknowledgement (if
  applicable)
• listen into the medium to detect if other
  stations forward the packet (if possible)
• request an explicit acknowledgement
• if a station encounters problems it can inform
  the sender of a packet or look-up a new path
  locally

59
Clustering of ad-hoc networks
Internet
cluster
super cluster
60
Interference-based routing

• Routing based on assumptions about interference
  between signals

N1
N2
R1
S1
N3
N4
N5
R2
N6
S2
N9
N8
N7
61
Examples for interference based routing

• Least Interference Routing (LIR)
• calculate the cost of a path based on the number
  of stations that can receive a transmission
• Max-Min Residual Capacity Routing (MMRCR)
• calculate the cost of a path based on a
  probability function of successful transmissions
  and interference
• Least Resistance Routing (LRR)
• calculate the cost of a path based on
  interference, jamming and other transmissions
• LIR is very simple to implement, only information
  from direct neighbors is necessary