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Wireless & Mobile Communications
Chapter 6: Network Protocols/Mobile IP
Motivation
 Data transfer
 Encapsulation
 Security
 IPv6

 Problems
 DHCP
 Ad-hoc networks
 Routing protocols
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!
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.2
What is Mobile IP?

Mobile IP provides network layer mobility

Provides seamless roaming

‘‘Extends’’ the home network over the entire Internet
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.3
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 1’s in the subnet mask. If p = 24, subnet mask is
255.255.255.0, if p = 26, subnet mask is 255.255.255.192.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.4
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 (p=32 host specific)
and 7.7.7.0/24 (0<p<32 network prefix) and 0.0.0.0/0 (p=0
default) will use entry 2 for an IP packet with destination
7.7.7.1 and entry 3 for destination 192.33.14.12.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.5
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.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.6
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.7
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 node’s point of
attachment and it might be useful if, e.g., TCP knew that a
wireless link was being used!).
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.8
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.9
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.10
Mobile IP Operation: Summary

Consists of 3 steps:

Agent discovery,

Registration, and

Routing/Tunneling
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.11
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.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.12
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.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.13
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.14
Example network
HA
MN
router
home network
mobile end-system
Internet
(physical home network
for the MN)
FA
foreign
network
router
(current physical network
for the MN)
CN
end-system
ICS 243E - Ch 6 Net. Protocols
router
Winter 2001
6.15
Data transfer to the mobile system
HA
2
MN
home network
receiver
3
Internet
FA
1
CN
sender
ICS 243E - Ch 6 Net. Protocols
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
Winter 2001
6.16
Data transfer from the mobile system
HA
1
home network
MN
sender
Internet
FA
foreign
network
1. Sender sends to the IP address
of the receiver as usual,
FA works as default router
CN
receiver
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.17
Overview
COA
home
network
router
FA
router
HA
MN
foreign
network
Internet
CN
router
3.
home
network
router
HA
router
FA
2.
MN
4.
Internet
foreign
network
1.
CN
router
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.18
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.19
Agent advertisement
0
7 8
type
#addresses
15 16
23 24
checksum
lifetime
31
code
addr. size
router address 1
preference level 1
router address 2
preference level 2
...
type
length
registration lifetime
sequence number
R B H F M G V reserved
COA 1
COA 2
...
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.20
Registration
MN
FA
HA
MN
HA
t
t
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.21
Mobile IP registration request
0
7 8
type
15 16
S B DMG V rsv
home address
home agent
COA
23 24
lifetime
31
identification
extensions . . .
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.22
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.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.23
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.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.24
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.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.25
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 MN’s home address.

All packets are therefore routed to the MN’s 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.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.26
Routing/Tunneling 2/5

A HA can use one of two methods to intercept a MN’s
packets:

The HA is a router with multiple network interfaces. In that
case it advertises reachability to the MN’s home network
prefix.

The HA is not a router with multiple intefaces. It must use ARP
to receive the MN’s 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 MN’s
IP packets. This is to update any ARP caches that hosts and
other devices might have.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.27
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.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.28
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 MN’s home address (for which it
has the link layer address!).
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.29
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.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.30
Encapsulation Process
original IP header
new IP header
outer header
ICS 243E - Ch 6 Net. Protocols
original data
new data
inner header
Winter 2001
original data
6.31
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.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.32
IP in IP Encapsulation

IP in IP encapsulation (mandatory in RFC 2003)

tunnel between HA and COA
ver.
IHL
TOS
length
IP identification
flags
fragment offset
TTL
IP-in-IP
IP checksum
IP address of HA
Care-of address COA
ver. IHL
TOS
length
IP identification
flags
fragment offset
TTL
lay. 4 prot.
IP checksum
IP address of CN
IP address of MN
TCP/UDP/ ... payload
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.33
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
ver.
IHL
TOS
length
IP identification
flags
fragment offset
TTL
min. encap.
IP checksum
IP address of HA
care-of address COA
lay. 4 protoc. S reserved
IP checksum
IP address of MN
original sender IP address (if S=1)
TCP/UDP/ ... payload
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.34
Generic Routing Encapsulation
outer header
new header
GRE
header
original
header
original data
original
header
original data
new data
ver.
IHL
TOS
length
IP identification
flags
fragment offset
TTL
GRE
IP checksum
IP address of HA
Care-of address COA
C R K S s rec.
rsv.
ver.
protocol
checksum (optional)
offset (optional)
key (optional)
sequence number (optional)
routing (optional)
ver.
IHL
TOS
length
IP identification
flags
fragment offset
TTL
lay. 4 prot.
IP checksum
IP address of CN
IP address of MN
TCP/UDP/ ... payload
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.35
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.
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.36
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.37
Change of foreign agent
CN
HA
FAold
FAnew
MN
request
update
ACK
data
data
MN changes
location
registration
registration
update
ACK
data
data
warning
data
update
ACK
data
data
t
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.38
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.39
Reverse tunneling (RFC 2344)
HA
2
MN
home network
sender
1
Internet
FA
3
CN
receiver
ICS 243E - Ch 6 Net. Protocols
foreign
network
1. MN sends to FA
2. FA tunnels packets to HA
by encapsulation
3. HA forwards the packet to the
receiver (standard case)
Winter 2001
6.40
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.41
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.42
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!
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.43
Security in Mobile IP

Security requirements (Security Architecture for the
Internet Protocol, RFC 1825)






Integrity
any changes to data between sender and receiver can be
detected by the receiver
Authentication
sender address is really the address of the sender and all data
received is really data sent by this sender
Confidentiality
only sender and receiver can read the data
Non-Repudiation
sender cannot deny sending of data
Traffic Analysis
creation of traffic and user profiles should not be possible
Replay Protection
receivers can detect replay of messages
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.44
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
IP-Header
IP header

Authentification-Header
authentication header
UDP/TCP-Paket
UDP/TCP data
Encapsulation Security Payload
protects confidentiality between communication partners
not encrypted
IP header
ICS 243E - Ch 6 Net. Protocols
encrypted
ESP header
Winter 2001
encrypted data
6.45
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
MH-FA authentication
FA-HA authentication
MH-HA authentication
registration request
MH

registration reply
registration request
FA
registration reply
HA
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)
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.46
Key distribution

Home agent distributes session keys
FA
HA



MH
response:
EHA-FA {session key}
EHA-MH {session key}
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.47
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
server
client
ICS 243E - Ch 6 Net. Protocols
client
relay
Winter 2001
6.48
DHCP - protocol mechanisms
client
initialization
server
(not selected)
determine the
configuration
DHCPDISCOVER
DHCPDISCOVER
DHCPOFFER
DHCPOFFER
server
(selected)
determine the
configuration
collection of replies
selection of configuration
DHCPREQUEST
(reject)
DHCPREQUEST
(options)
confirmation of
configuration
DHCPACK
initialization completed
release
DHCPRELEASE
ICS 243E - Ch 6 Net. Protocols
Winter 2001
delete context
6.49
DHCP characteristics

Server


Renewal of configurations


IP addresses have to be requested periodically, simplified
protocol
Options


several servers can be configured for DHCP, coordination not
yet standardized (i.e., manual configuration)
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.50
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
ICS 243E - Ch 6 Net. Protocols
B
Winter 2001
C
6.51
Routing examples for an ad-hoc network
N1
N1
N2
N3
N4
N4
N5
time = t1
ICS 243E - Ch 6 Net. Protocols
N3
N2
good link
weak link
Winter 2001
N5
time = t2
6.52
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.53
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.54
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)
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.55
Dynamic source routing I


Split routing into discovering a path and maintainig a path
Discover a path


Maintaining a path


only if a path for sending packets to a certain destination is
needed and no path is currently available
only while the path is in use one has to make sure that it can
be used continuously
No periodic updates needed!
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.56
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)
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.57
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.58
Clustering of ad-hoc networks
Internet
cluster
super cluster
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.59
Interference-based routing

Routing based on assumptions about interference between
signals
N1
N2
R1
S1
N3
N4
N5
N6
R2
S2
neighbors
(i.e. within radio range)
N7
ICS 243E - Ch 6 Net. Protocols
N8
Winter 2001
N9
6.60
Examples for interference based routing

Least Interference Routing (LIR)


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 the number of stations
that can receive a transmission
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
ICS 243E - Ch 6 Net. Protocols
Winter 2001
6.61