Internet Mobility
Presented by: Nitin Bahadur
Nitin Bahadur
Distributed Systems
Spring 2000
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References
• Mary Baker, Xinhua Zhao, Stuart Cheshire, Jonathan
Stone, Supporting mobility in Mosquitonet,
Proceedings of USENIX, Technical Conference, 1996.
• Stuart Cheshire and Mary Baker, Internet Mobility 4x4,
SIGCOMM ‘96.
• Kevin Lai, Mema R., Diane Tang, Xinhua Zhao, Mary
Baker, Experiences with a Mobile Testbed,
Proceedings of WWCA ‘98.
Nitin Bahadur
Distributed Systems
Spring 2000
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References
• C. Perkins, IP Mobility Support for IPv4, Internet Draft,
draft-ietf-mobileip-rfc2002-bis-01.txt, Jan 2000.
• C. Perkins, Route optimization in Mobile IP, Internet
Draft, deaft-ietf-mobileip-opim-09.txt, Feb 2000.
• David Maltz, Pravin Bhagwat, MSOCKS: An
Architecture for transport layer mobility, IEEE
INFOCOM ‘98.
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Distributed Systems
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Outline
• Motivation and Challenges
• Some proposed solutions
• IP Mobility
• Routing Optimizations
• Security Issues
• Recent Extensions
• Conclusions
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Distributed Systems
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Motivation
• Ubiquitous connectivity, continuous connectivity.
• Ability to maintain current
conversations/connections during movement.
• Move from one kind of network to another.
• Move to networks that do NOT provide support
for mobility.
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Distributed Systems
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Challenges
• Movement causes change in IP address
• Problems with TCP connections
• Maintaining transparency
• Efficient routing to new location of mobile host
• Security issues
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Distributed Systems
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Some Solutions
• Use Host specific routes - possible with IPv6.
• Link Layer solutions
– Limited to a single medium
– A new solution for every medium
– Compatibility issues with other mediums
• Use extended DNS to register COA with DNS
– Information propagation time
– Global change in DNS
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Distributed Systems
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Basic Terminology
•
•
•
•
•
Mobile Host (MH)
Correspondent Host (CH)
Home Agent (HA)
Foreign Agent (FA)
Care of Address (COA)
Internet
CH
HA
Nitin Bahadur
FA
Distributed
Systems
MH
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Split TCP
• Two TCP connections, CH<=>HA<=>MH
• Transport Layer mechanism
TCP - I
Internet
HA
CH
TCP - II
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Distributed Systems
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Split TCP
• Two TCP connections instead of one
• Changed end points
• HA acks. packets to CH even before MH has
received them.
• Home agent is responsible for final packet delivery
• Multiple traversal through the TCP protocol stack
• HA needs to maintain a TCP connection for every TCP
connection of all its mobile hosts
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Distributed Systems
Spring 2000
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TCP Splicing
• Two TCP connections, CH<=>HA<=>MH
• Transport Layer mechanism
• Acks are sent on TCP-I to CH only when MH sends
them on TCP-II
TCP - I
Internet
HA
CH
TCP - II
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Distributed Systems
Spring 2000
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TCP Splicing
+ The scheme works like 1 TCP connection
+ Location transparency is maintained
- Multiple traversal through the TCP protocol stack
- HA still needs to maintain a TCP connection for
every TCP connection of all its mobile hosts
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Distributed Systems
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Mobility using Foreign Agent
• MH obtains COA from FA
• FA discovery using Agent Advertisement or Agent
Solicitation messages
• MH host registers COA with HA through FA
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•
•
•
HA intercepts packets for MH
HA encapsulates and sends packets to FA
FA decapsulates packets and sends it to MH
Everything done at IP level……no TCP
Same mechanism in the reverse direction
This is called Bidirectional tunneling
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Distributed Systems
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Mobility using Foreign Agent
Internet
HA
CH
FA
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Distributed Systems
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Encapsulation: IP in IP
Source = HA
Destn = COA
Source = CH
Destn = MH
HA
DATA
Source = CH
Destn = MH
Source = CH
Destn = MH
FA
DATA
DATA
• Encapsulation maintains consistency in Source and
Destination address fields.
• Allows MH to receive packets as it moves from
network to network.
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Distributed Systems
Spring 2000
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Encapsulation Issues
• IP encapsulation and automatic decapsulation is
dangerous
• How can one verify if the inner packet has a source
address if claims to be !
• Encapsulation can cause packet fragmentation
– TCP breaks data in chunks of 1460 bytes and gives it to IP
– Encapsulation will automatically lead to packet
fragmentation in such cases. So extra overhead.
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Distributed Systems
Spring 2000
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Mobility without Foreign Agent
• MH obtains a COA using DHCP
• MH registers COA with HA directly
• MH performs encapsulation and decapsulation
Internet
HA
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CH
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Implementation in MosquitoNet
Altered the route lookup function ip_rt_route
Mobile Policy Table helps in combination with ip_rt_route is
used for making routing decisions
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Distributed Systems
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Home Agent Functionality
• Maintaining information about MH’s current location
• Acting as an ARP proxy for MH
– ARP…. to get link-layer address for an IP address
– Proxy ARP…..done to answer a new ARP request on behalf
on MH
– Gratuitous ARP….done to update ARP information of MH
in all nodes
• Forwarding packets to CH and MH
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Distributed Systems
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Movement of MH away from home network
• Detection by MH
– received a different agent advertisement message
– stopped receiving agent advertisement messages
• Disable ARP
• Register with FA or HA
• HA performs Gratuitous ARP on behalf of MH
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Distributed Systems
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Movement of MH to home network
• Re-enable ARP
• De-register itself with HA
• HA performs Gratuitous ARP on behalf of MH
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Distributed Systems
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Advantages of using Foreign Agent
• No need for a temporary COA for every MH
• If MH leaves foreign network, then Inflight packets
can de directed by FA to new location of MH
– Less packet loss
• Less complexity in MH
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Distributed Systems
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Advantages of NOT using Foreign Agent
• MH can visit networks without a foreign agent
• FA is not a bottleneck or single point of failure
• No need for a FA on each network
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Distributed Systems
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Triangular Routing
• Proposed by Mobile IP working group
• CH sends packets to HA which forwards it to MH
• MH sends directly packets to CH
CH
MH
Internet
HA
CH
HA
COA
CH
MH
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MH
CH
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Problems with Routing Techniques
• Bidirectional tunneling and Triangular routing
– Inefficient, increase in RTT, increase in path length
– HA is a bottleneck and a single point of failure
– Source address filtering problem with triangular routing
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Distributed Systems
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Source Address Filtering Problem
• Foreign network might not allow transit traffic
Source = MH != foreign network
Destination = CH != foreign network
=> TRANSIT TRAFFIC => DROP IT !
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Distributed Systems
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Source Address Filtering Problem
• Filtering at CH network
Source = MH = CH network
But packet has come from a different network
…..hmmm….drop it !
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Distributed Systems
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Routing Optimizations - I
• Similar to triangular routing
• MH encapsulates packet to avoid source-address
filtering problem
• Better than triangular routing and bidirectional
tunneling !
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Distributed Systems
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Routing Optimizations - I
CH
MH
Internet
CH
HA
MH
CH
HA
COA
CH
MH
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COA
CH
MH
CH
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Routing Optimizations - I
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•
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Direct delivery to CH
Valid source address, so no source address problem
Location transparency is maintained
Will work in all situations
• CH needs to have decapsulation capability
• Indirect delivery for CH
• Will fail if CH border router does not admit packets
from current (foreign) network of MH
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Distributed Systems
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Routing Optimizations - II
• Direct delivery mechanism
• Both MH and CH encapsulate packets and send
directly to each other
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Distributed Systems
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Routing Optimizations - II
Internet
CH
HA
CH
MH
CH
MH
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Distributed Systems
CH
COA
CH
MH
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Routing Optimizations - II
Internet
CH
HA
MH
CH
MH
CH
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Distributed Systems
COA
CH
MH
CH
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Routing Optimizations - II
• Direct delivery between CH and MH
• Valid source address, so no source address problem
• CH needs to have en (de)capsulation capability
• CH needs to be aware of current location of MH
• Will fail if CH border router does not admit packets
from current (foreign) network of MH
• Validity of encapsulated packet ?
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Distributed Systems
Spring 2000
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Routing Optimizations - III
• Direct delivery mechanism
• Both MH and CH DO NOT encapsulate packets
and send directly to each other using MH and not
COA
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Distributed Systems
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Routing Optimizations - III
Internet
HA
CH
CH
MH
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Distributed Systems
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Routing Optimizations - III
Internet
HA
CH
MH
CH
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Distributed Systems
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Routing Optimizations - III
Applicability
When MH and CH are on same link layer segment
• Direct delivery between CH and MH
• CH needs to be aware of current location of MH
• Invalid source address - security
• Location transparency ?
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Distributed Systems
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Routing Optimizations - IV
• Direct delivery mechanism
• Both MH and CH DO NOT encapsulate packets
and send directly to each other using COA and not
MH
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Distributed Systems
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Routing Optimizations - IV
Internet
HA
CH
CH
COA
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Distributed Systems
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Routing Optimizations - IV
Internet
HA
CH
COA
CH
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Distributed Systems
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Routing Optimizations - IV
• Direct delivery between CH and MH
• No encapsulation overhead
• CH needs to be aware of current location of MH
• No Location transparency
• Packets will be lost if MH changes location
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Distributed Systems
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Routing Optimizations - IV
Short lived connections such as HTTP browsing
Situations where location transparency is not an issue
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Distributed Systems
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Making CH intelligent
• Introducing mobile awareness in CH
• Why - for efficient routing
• How ?
– Binding Warning messages are sent by MH to HA so that HA
sends binding update message to CH
– Binding update messages are sent by HA to CH whenever
HA receives a tunneled packet
– MH can also directly send binding update messages to CH
MH can specify to HA which CH should be informed of its
current location
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Distributed Systems
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Gains using optimizations
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Distributed Systems
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Practical implementation of optimizations
• Optimizations must not cause break in connection
or packet loss
• Start with the most pessimistic routing method
• do
– Send ICMP echo messages in background using a
better method
– If that succeeds switch to the better method
• while (no more methods)
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Distributed Systems
Spring 2000
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Security Issues
• For registrations and communication between HA and
MH an authenticator is used
• Authenticator is optional for communications among
HA - {CH, FA} and MH - {CH,FA}
• Authenticator default algorithm is 128-bit keyed MD5
• Since key distribution may be a problem, messages
with FA and CH may not be authenticated
• Replay protection done using timestamps and/or
nonces
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Distributed Systems
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Mobile Policy Table Performance
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Distributed Systems
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Mobile Policy Table and Flexibility
• Supports multiple packet delivery methods
simultaneously
• Adaptively selects the most appropriate method
according to characteristics of each traffic flow
• Makes use of multiple network interfaces
simultaneously
• Controls interface selection of both outgoing and
incoming packets for different packet flows
• MH can register with HA flow specification and
corresponding interface binding for that flow
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Distributed Systems
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Why support multiple pkt delivery methods
MH pays for extra cost of mobility support only when
actually required
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Distributed Systems
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Why support multiple network intefaces
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Smoother handoffs
QoS
Link asymmetry
Cost and billing
Privacy and Security
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Distributed Systems
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Conclusion
• A schema for supporting ubiquitous and continuous
connectivity
• Support for multiple packet delivery methods
• Use of multiple packet interfaces simultaneously
• Dynamic adaptation of routing optimizations
• MH can specify which CHs should be informed of
current COA
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Distributed Systems
Spring 2000
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Issues to think about
• IP encapsulation overhead problem
• IPSec and IP-in-IP interactions
• Multicast for Mobile hosts….check out Alex
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Distributed Systems
Spring 2000
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