Lecture 15

mobile communication

1. 1
What if we use a Mobile computer in an internet straight away?
Host sends the IP packet with the destination address and other fields.
The destination address determines the sub-net that the host belongs to
and also the host address.

Ex., 129.13.42.99 specifies that the physical subnet with prefix
129.13.42 is to be connected to.
If the receiver has moved outside the subnet area, the packet will not
reach.
Thus, a host needs a topologically correct address. But if the host keeps
changing the physical address, it is impossible to keep track of it.

**** Mobile IP is the answer to these issues******
Mobile Network Layer

2. 2
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
topological correct address (standard IP) or needs special entries in
the routing tables
Specific routes to end-systems?
change of 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 to long
time
TCP connections break, security problems
Mobile Network Layer

3. 3
Requirements to Mobile IP
Transparency
mobile end-systems keep their IP address, Mobility to be transparent to
higher layers.
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

4. 4
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
The tunnel for the packets for the MN starts at the HA.
HA maintains a location registry. The current COA informs the HA about the current
location of the MN.
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, CN can be a fixed or a mobile node.

5. 5
Two Possibilities for COA Location
Foreign Agent COA : COA located at the FA.
COA is the IP address of the FA. FA is the tunnel end point
and forwards packets to the MN.
Many MNs using the FA can share the COA.
Co-located COA
If the MN temporarily acquires an additional IP address, this
IP address can act as COA. One problem with co-located
MNs is that it uses additional IP address which is scarce
generally.

6. 6
Example network
Internet
router
router
router
end-system
FA
HA
MN
home network
foreign
network
(physical home network
for the MN)
(current physical network
for the MN)
CN

7. 7
Data transfer to the mobile system
Internet
sender
FA
HA
MN
home network
foreign
network
receiver
1
2
3
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
CN

8. 8
Data transfer from the mobile system

Internet
receiver
FA
HA
MN
home network
foreign
network
sender
1
1. Sender sends to the IP address
of the receiver as usual,
FA works as default router
CN

9. 9
Overview

CN
router
HA
router
FA
Internet
router
1.
2.
3.
home
network
MN
foreign
network
4.
CN
router
HA
router
FA
Internet
router
home
network
MN
foreign
network
COA

10. 10
Further definitions…
Mobile Node(MN):
-It is a mobile device or Router that can change its point of attachment to internet
by the use of Mobile IP.
Correspondent Node(CN) : This is a partner needed for communication. CN can be
fixed or mobile.
Home Network : Subnet that the MN belongs to. No mobile IP support is needed
within the home network.
Foreign Network : Current network that the mobile is visiting.
- Can have a Care of Address (COA) that can act as a tunnel for sending the
packets to an MN.
- Can be a default router for the MN
- Can provide security
- Typically, it is implemented on the router for the subnet that the MN attaches to.
COA : Marks the tunnel end point; IT is the IP address of the FA.

11. 11
Further definitions…
IP Packet Delivery
-CN wants to send an IP packet to MN
-CN does not need to know the IP address of MN. MN is the destination
address.
-The internet routes the packet to the router of the home network that the
packet belongs to.
-HA intercepts the packet, being aware that MN is not in its home
network.
-The packet is encapsulated and tunneled to the COA. A new header is
put in front of the old IP header showing COA as the new destination and
HA as the source of the encapsulated packet.
-The Foreign Agent(FA) now decapsulates the packet, I.e, removes the
additional header and forwards the original packet with CN as source and
MN as destination to MN.

12. 12
Network Integration
Agent Advertisement
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
Advertisement
HA advertises the IP address of the MN (as for fixed systems), i.e. standard
routing information
routers adjust their entries, these are stable for a longer time (HA responsible
for a MN over a longer period of time)
packets to the MN are sent to the HA,
independent of changes in COA/FA

13. 13
Agent Advertisement
Foreign Agents(FSs) and Home Agents(HAs) Advertise their presents
periodically via the Agent Advt Messages. - beacon broadcast to the
subnet.
Upper part of the packet represents the ICMP (Advt Intenet Control
Message Protocol)
Lower part represents the mobility.

14. 14
Agent Advertisement

type = 16
length = 6 + 4 * #COAs
R: registration required
B: busy, no more registrations
H: home agent
F: foreign agent
M: minimal encapsulation
G: GRE encapsulation
r: =0, ignored (former Van Jacobson compression)
T: FA supports reverse tunneling
reserved: =0, ignored
preference level 1
router address 1
#addresses
type
addr. size lifetime
checksum
COA 1
COA 2
type = 16 sequence numberlength
0 78 1516 312423
code
preference level 2
router address 2
. . .
registration lifetime
. . .
RBHFMGr reservedT
ICMP : Internet Control Message Protocol
Mobility Extension

15. 15
Agent Advt Packet
Type : set to 9
Code =0 if the agent also routes packets from non-mobile nodes
=16 if it does not route anything other than mobiles nodes
#addresses: no. of addresses advertised with this packet
Addresses follow.
Life time: length of time that these advts are valid.
Mobile extensions :
- type : set to 16
- length = 6 + 4*no. of addresses.
- sequence no. : total no. of advts sent since initialisation.
- COA 1, COA 2… : COAs advertised.

16. 16
Agent Solicitation
If no advertisements are received in a specified interval, and if an MN
has not received a COA by any other means, MN must send Agent
Solicitation messages.
MN searches for a FN through these messages. When MNs are moving
at a good pace, <1 sec interval solicitations are required so that packets
are not lost.
If a MN does not receive reply to its solicitations, it must reduce the
solicitation interval to avoid flooding of network traffic.
At the End of Advts and Solicitation:
MN receives a COA, either for a FA or for a co-located COA.

17. 17
Registration
The main purpose of Registration is to inform HA of the current
location for correct forwarding of packets.
If the COA is at the FA
- MN sends a registration request via contained in COA to FA.
-FA forwards this request to HA.
-HA sets up a mobility binding that contains MN’s home IP
address and the current COA.
-Registration expires after the life time.
-After setting up mobility, the FA sends a reply back to FA that
forwards the same to MN

18. 18
Registration
If the COA is collocated…
MN sends the request directly to HA and vice versa.

19. 19
Registration

t
MN HA
registration
request
registration
reply
t
MN FA HA
registration
request
registration
request
registration
reply
registration
reply
Registration of the MN via FA
(Case 1)
Registration Directly (If COA is
collocated) (Case 2)

20. 20
Registration
Case-1:
MN sends the Reg request to FA. FA forwards this to HA
HA sets up a mobility binding that contains MN’s home IP
address and the current COA.
After setting up mobility binding, the HA sends a reply back to FA
and FA forwards the same to MN.
Case –2:
MN sends a Reg. Request to HA and HA replies to this request.

21. 21
Mobile IP Registration Request (UDP packet format)

home agent
home address
type = 1 lifetime
0 78 1516 312423
T x
identification
COA
extensions . . .
SBDMGr
S: simultaneous bindings
B: broadcast datagrams
D: decapsulation by MN
M mininal encapsulation
G: GRE encapsulation
r: =0, ignored
T: reverse tunneling requested
x: =0, ignored
Having received a COA, MN has to register with HA. The main purpose of registration is to inform the HA of the
current location for for correct forwarding of packets.

22. 22
Mobile IP Registration Request (UDP packet format)
Type set to 1 : Registration required
S bit enabled : MN can specify if it wants the HA to retain the prior
mobility bindings, It allows simultaneous bindings
B set to 1 : indicates that MN also wants to receive the broadcast
packets which have been received by the HA in the home network.
D Bit : indicates that the MN uses a collocated COA and it carries out
decapsulation at tunel end point.
M & G : Whether minimal encapsulation or general encapsulation is
required.
T indicates reverse tunneling. R & x are set to zero in this case.

23. 23
Mobile IP Registration Reply

home agent(addr of HA)
home address(addr of MN)
type = 3 lifetime
0 78 1516 31
code
Identification(64 bit - to match reg. Req with replies)
extensions . . . Example codes:
registration successful
0 registration accepted
1 registration accepted, but simultaneous mobility bindings unsupported
registration denied by FA
65 administratively prohibited
66 insufficient resources
67 mobile node failed authentication
68 home agent failed authentication
69 requested Lifetime too long
registration denied by HA
129 administratively prohibited
131 mobile node failed authentication
133 registration Identification mismatch
135 too many simultaneous mobility bindings

24. 24
Encapsulation

original IP headeroriginal data
new datanew IP header
outer header inner header original data
Original Header + Original Data
COA

25. 25
Encapsulation-1 – IP-in-IP Encapsulation

Care-of address COA
IP address of HA
TTL
IP identification
IP-in-IP IP checksum
flagsfragment offset
lengthDS (TOS)ver.IHL
IP address of MN
IP address of CN
TTL
IP identification
lay. 4 prot. IP checksum
flagsfragment offset
lengthDS (TOS)ver.IHL
TCP/UDP/ ... payload
Encapsulation of one packet into another as payload
e.g. IPv6 in IPv4 (6Bone), Multicast in Unicast (Mbone)
here: e.g. IP-in-IP-encapsulation, minimal encapsulation or GRE (Generic Record
Encapsulation)
IP-in-IP-encapsulation (mandatory, RFC 2003)
tunnel between HA and COA
New
Header
Original
Header

26. 26
Encapsulation-1 – IP-in-IP Encapsulation
Ver : Ipv Ver 4
IHL : Internet Header Length (length of the outer header in 32 bits)
DS(TOS-Type of Service) : Copied from the inner header
Length : length of completed encapsulated packet
IP Id, Flags, Fragmented Offset : No special meaning for Mobile IP
TTL (Time to Live) : Must be high enough so that packet can reach the tunnel end point.
IP-In-IP : Type of the protocol : 4 for IPv4
IP Address of HA : Source Address
Care of Addr of COA : Tunnel exit point as destination address.

27. 27
Encapsulation-II

care-of address COA
IP address of HA
TTL
IP identification
min. encap. IP checksum
flagsfragment offset
lengthDS (TOS)ver.IHL
IP address of MN
original sender IP address (if S=1)
Slay. 4 protoc. IP checksum
TCP/UDP/ ... payload
reserved
Minimal encapsulation (optional)
avoids repetition of identical fields
e.g. TTL, IHL, version, DS (RFC 2474, old: TOS)
only applicable for unfragmented packets, no space left for fragment
identification

28. 28
Generic Routing Encapsulation (GRE)

original
header
original data
new datanew header
outer header
GRE
header
original data
original
header
Care-of address COA
IP address of HA
TTL
IP identification
GRE IP checksum
flagsfragment offset
lengthDS (TOS)ver.IHL
IP address of MN
IP address of CN
TTL
IP identification
lay. 4 prot. IP checksum
flagsfragment offset
lengthDS (TOS)ver.IHL
TCP/UDP/ ... payload
routing (optional)
sequence number (optional)
key (optional)
offset (optional)checksum (optional)
protocolrec.rsv.ver.CRKSs
RFC 1701
RFC 2784
reserved1 (=0)checksum (optional)
protocolreserved0 ver.C
•Supports other other network layer
protocols in addition to IP.
•Allows encapsulation of the packets of
one protocol suite into the payload
portion of another protocol suite.

29. 29
Issue with Routing – Need for Optimization

Japanese
German
Meeting at a
conference in
Hawaii

30. 30
Issue with Routing – Need for Optimization
Japanese sends a data to German
Japanese computer sends a data to HA of German
HA in Germany encapsulates the packet and tunnels to the COA of the German Lap Top in Hawaii.
This is very inefficient since the lap tops are just meters apart.
The scheme is called Triangular Routing (CN->HA, HA->COA/MN, MN->back to CN).

31. 31
Optimization of Packet Forwarding
Triangular Routing
sender sends all packets via HA to MN
higher latency and network load
“Solutions”
sender learns the current location of MN
direct tunneling to this location
HA informs a sender about the location of MN
big security problems!
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

32. 32
Change of Foreign Agent

CN HA FA
old
FA
new
MN
MN changes
location
t
Data
Data
Data
Update
ACK
Data
Data
Registration
Update
ACK
Data
Data
Data
Warning
Request
Update
ACK
Data
Data

33. 33
Change of Foreign Agent
Binding Request : CN sends a request to HA for the current location of HA
Binding Update : If MN has allowed dissemination of the current location, it can send
an update.
Binding ACK : CN acknowledges this update message and stores the mobility
binding. CN can directly send the data to the current foreign
agent(Faold). Faold forwards the packets to MN.
Binding Warning : If a node decapsulates a packet for MN but it is not the correct FA
for the MN, this node sends a binding warning. The recipient knows
that it has to get a fresh binding.
Tunneling and Encapsulation is now done by CN and not HA.

34. 34
Change of Foreign Agent
MN changes the Location :
MN changes location and registers with a new foreign agent FAnew.
This registration is forwarded to HA for Location Database Update. FAnew informs
FAold about the current registration of MN. This is done via an update message.
Faold acknowledges it.
CN acknowledges this update and stores the mobility binding.
Now, CN can send the Data directly to FAold.

35. 35
Reverse Tunneling
Internet
receiver
FA
HA
MN
home network
foreign
network
sender
3
2
1
1. MN sends to FA
2. FA tunnels packets to HA
by encapsulation
3. HA forwards the packet to the
receiver (standard case)
CN

36. 36
Mobile IP with Reverse Tunneling
Router accept often only “topological correct“ addresses (firewall!)
a packet from the MN encapsulated by the FA is now topological
correct
furthermore multicast and TTL problems solved (TTL in the home
network correct, but MN is to far away from the receiver)
Reverse tunneling does not solve
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 (double triangular routing)
The standard is backwards compatible
the extensions can be implemented easily and cooperate with current
implementations without these extensions
Agent Advertisements can carry requests for reverse tunneling

37. 37
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;
addresses are always co-located
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

38. 38
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!

39. 39
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

40. 40
IP Security Architecture-I
not encrypted encrypted
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
Authentification-HeaderIP-Header UDP/TCP-Paketauthentication headerIP header UDP/TCP data
ESP headerIP header encrypted data

41. 41
IP Security Architecture-II

registration reply
registration request
registration request
MH FA HA
registration reply
MH-HA authentication
MH-FA authentication FA-HA authentication
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
nonces: 32 bit random number (MH) + 32 bit random number (HA)

42. 42
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
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
HA
response:
E
HA-FA
{session key}
E
HA-MH
{session key}

43. 43
IP Micro Mobility Support
Micro-mobility support:
Efficient local handover inside a foreign domain
without involving a home agent
Reduces control traffic on backbone
Especially needed in case of route optimization
Example approaches:
Cellular IP
HAWAII
Hierarchical Mobile IP (HMIP)
Important criteria:
Security Efficiency, Scalability, Transparency, Manageability

44. 44
Cellular IP

CIP Gateway
Internet
BS
MN1
data/control
packets
from MN 1
Mobile IP
BSBS
MN2
packets from
MN2 to MN 1
Operation:
„CIP Nodes“ maintain routing
entries (soft state) for MNs
Multiple entries possible
Routing entries updated based on
packets sent by MN
CIP Gateway:
Mobile IP tunnel endpoint
Initial registration processing
Security provisions:
all CIP Nodes share
„network key“
MN key: MD5(net key, IP addr)
MN gets key upon registration

45. 45
Cellular IP : Security
Advantages:
Initial registration involves authentication of MNs
and is processed centrally by CIP Gateway
All control messages by MNs are authenticated
Replay-protection (using timestamps)
Potential problems:
MNs can directly influence routing entries
Network key known to many entities
(increases risk of compromise)
No re-keying mechanisms for network key
No choice of algorithm (always MD5, prefix+suffix mode)
Proprietary mechanisms (not, e.g., IPSec AH)

46. 46
Cellular IP : Other Issues
Advantages:
Simple and elegant architecture
Mostly self-configuring (little management needed)
Integration with firewalls / private address support possible
Potential problems:
Not transparent to MNs (additional control messages)
Public-key encryption of MN keys may be a problem
for resource-constrained MNs
Multiple-path forwarding may cause inefficient use of
available bandwidth

47. 47
HAWAII (Handoff Aware wireless access internet infrastructure)
Operation:
MN obtains co-located COA
and registers with HA
Handover: MN keeps COA,
new BS answers Reg. Request
and updates routers
MN views BS as foreign agent
Security provisions:
MN-FA authentication mandatory
Challenge/Response Extensions mandatory
BS
3
Backbone
Router
Internet
BS
MN
BS
MN
Crossover
Router
DHCP
Server
HA
DHCP
Mobile IP
Mobile IP
1
2
4
3

48. 48
HAWAII (Handoff Aware wireless access internet infrastructure)
Step 1: On entering the HAWAII domain, MN obtains the colocated COA
Step 2: MN Registers with HA
Step 3: When moving to another cell within a foreign domain, MN sends a
Registration request to the new base station as to a foreign agent (mixing the
concepts of colocated COA and foreign agent COA)
Step 4: The Base station intercepts the registration request and sends out a Handoff
update message. Thus, all routers are reconfigured from the path from old and the
new Base Sattions to the so-called cross-over router.
When routing has been reconfigured successfully, the base station sends a
registration reply to the mobile node, again as if it were a foreign agent.

49. 49
HAWAII: Security
Advantages:
Mutual authentication and C/R extensions mandatory
Only infrastructure components can influence routing entries
Potential problems:
Co-located COA raises DHCP security issues
(DHCP has no strong authentication)
Decentralized security-critical functionality
(Mobile IP registration processing during handover)
in base stations
Authentication of HAWAII protocol messages unspecified
(potential attackers: stationary nodes in foreign network)
MN authentication requires PKI or AAA infrastructure

50. 50
HAWAII: Other issues
Advantages:
Mostly transparent to MNs
(MN sends/receives standard Mobile IP messages)
Explicit support for dynamically assigned home
addresses
Potential problems:
Mixture of co-located COA and FA concepts may
not be
supported by some MN implementations
No private address support possible
because of co-located COA

51. 51
Hierarchical Mobile IPv6 (HMIPv6)
Operation:
Network contains mobility anchor point (MAP)
mapping of regional COA (RCOA) to link COA (LCOA)
Upon handover, MN informs
MAP only
gets new LCOA, keeps RCOA
HA is only contacted if MAP
changes
Security provisions:
no HMIP-specific
security provisions
binding updates should be
authenticated
MAP
Internet
AR
MN
AR
MN
HA
binding
update
RCOA
LCOA
oldLCOA
new

52. 52
Hierarchical Mobile IP: Security
Advantages:
Local COAs can be hidden,
which provides some location privacy
Direct routing between CNs sharing the same link is
possible (but might be dangerous)
Potential problems:
Decentralized security-critical functionality
(handover processing) in mobility anchor points
MNs can (must!) directly influence routing entries
via binding updates (authentication necessary)

53. 53
Hierarchical Mobile IP: Other issues
Advantages:
Handover requires minimum number
of overall changes to routing tables
Integration with firewalls / private address support
possible
Potential problems:
Not transparent to MNs
Handover efficiency in wireless mobile scenarios:
Complex MN operations
All routing reconfiguration messages
sent over wireless link

54. 54
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)
client relay
clientserver
DHCPDISCOVER
DHCPDISCOVER

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DHCP - protocol mechanisms

time
server
(not selected)
client
server
(selected)
initialization
collection of replies
selection of configuration
initialization completed
release
confirmation of
configuration
delete context
determine the
configuration
DHCPDISCOVER
DHCPOFFER
DHCPREQUEST
(reject)
DHCPACK
DHCPRELEASE
DHCPDISCOVER
DHCPOFFER
DHCPREQUEST
(options)
determine the
configuration

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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

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