CMPE 257: Wireless and
Mobile Networking
Spring 2005
Wireless Internetworking
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Announcements
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Homework on MAC up.
Midterm moved to 05.10.
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Wireless Internet
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Extension of Internet services to
wireless/mobile users.
Challenges?
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Wireless medium.
Node mobility.
TCP/IP stack.
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Challenges
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Network layer.
Transport layer.
Application.
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Address mobility
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IP assumes fixed nodes.
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Hierarchical addresses.
IP address = network number+host number.
IP address uniquely identifies host’s PoA.
Host must attach to network specified by its IP
address to send/receive datagrams.
But what if nodes move?
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Change address?
How about packets destined to them?
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Mobile IP
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Address redirection.
Manages mobility at the IP layer.
Hides mobility from upper layers.
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Mobile IP: Goals
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Nodes can receive datagrams no matter
where they attach to the Internet.
IMHP (Internet Mobile Host Protocol) as
Mobile IP precursor.
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“Last-hop” Mobility
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Mobile IP is the Internet standard for “lasthop” mobility support in IP networks (RFC
2290).
How do we deliver IP packets when the
endpoints move?
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Mobile host must be able to communicate after
changing its link-layer point-of-attachment.
Mobile host must be able to communicate using its
permanent (home) IP address.
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Mobile IP: Design Issues
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Issues:
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Impact on IP addressing.
Impact on routing.
Impact on higher layers.
Key design considerations:
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Scale.
Compatibility.
Transparency.
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Terminology
Home Agent (HA)
Foreign Agent (FA)
HN
CH
Mobile Host (MH)
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Terminology (Cont’d)
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Similar to cellular.
Mobile Node (MN or MH): node
changing its PoA.
Correspondent Host (CH).
Home Network (HN) and Foreign
Network (FN).
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Terminology (Cont’d)
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Mobility Agents:
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Home Agent (HA): router on MN’s HN that tunnels
datagrams to MH when away and keeps MH’s
current location info.
Foreign Agent (FA): router on foreign network;
delivers datagrmas to MH while on FN.
Home Address (HoA) and Care-of Address
(CoA):
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HoA: MH’s permanent address on HN.
CoA: MH’s temporary address on FN.
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Care of Address
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FA-based.
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FN-based.
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MN’s address is its current FA’s address.
Locally-assigned address in FN.
E.g., DHCP address.
What’s the difference?
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Mobile-IP: Basic Operation
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MH normally uses its home address HoA.
When MH visits a foreign network,
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Registration with FA.
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Registration with HA.
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Discover mobile agent and CoA.
Binding update (HoA -> CoA).
Communicating with MN: use HoA.
HA forwards packet from HoA to CoA.
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Discovering Agents
Agents periodically beacon
advertisements
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Agent Discovery
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Agent advertisement (beaconing):
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Mobile agent broadcast agent
advertisement at regular intervals (“I am
here”).
Agent solicitation:
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MH can poll (“anyone here?”).
Mobile agent responds to poll.
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Discovering Agents
MH polls; agent responds.
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Agent Advertisement
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Follows ICMP router advertisement
message.
List one or more available care-of
addresses.
Inform the MN about special features
provided by FA.
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Example: Alternative encapsulation
techniques, header compression.
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Registration
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Registering
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When away, MH registers its CoA with
HA (binding update).
Binding: (HoA->CoA)
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Binding has a lifetime.
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Registration Process
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MH sends a registration request with
CoA.
HA authenticates request.
HA approves or disapproves the
request.
HA adds necessary information to its
routing table.
HA sends registration reply back to MH.
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Registration Process (cont’d…)
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In the case of FA-based CoA:
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FA is involved in registration.
FA is also involved in packet forwarding.
Encapsulation.
Tunneling.
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Tunneling
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HA tunnels datagrams destined to MN
when MN is away.
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Datagrams sent to MH directly.
Or sent to FA which forwards to MN’s CoA.
Tunnel terminates at MH’s CoA (either
the MH or the FA).
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Tunneling
SRC
Tunneled Data Packet
HA keeps binding
between MH and CoA
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Encapsulation
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Tunneling requires encapsulation.
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Sending the original packet (CH->MH) in
another packet (HA->CoA).
Default encapsulation mechanism:
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IP-within-IP (tunnel).
Tunnel header: new IP header inserted by
the tunnel source (home agent).
Destination IP: CoA
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Tunneling in Mobile IP
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The Triangle Routing Problem
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Aka, “dogleg” routing.
MH->CH: direct.
CH->MH: CH->HA->MH
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Inefficient
Solution: route optimization.
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Deliver binding updates directly to CH.
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Route Optimization
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Binding caches:
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Nodes can keep caches with CoA for MHs.
If node has entry for MH, sends data
directly.
Otherwise, “triangulates” with HA.
Binding cache entries have TTL.
HA, FA, or MH can send binding cache
updates to CH.
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Simultaneous Bindings
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MN can register multiple CoA’swith HA.
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Why?
De-registration.
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Explicit.
Implicit.
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Handoffs
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MH moving among FN.
New CoA registered with HA.
Previous FA not necessarily notified.
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Old registration will expire.
New data delivered to new CoA.
In-flight data?
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Dropped and retransmitted by upper layers, or
FA notified of new CoA; FA forwards data to new
CoA.
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Types of Handoffs
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MN-initiated:
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Handoff managed by MN.
MN measures signal strength to AP.
Decides target AP and switchs over.
Network-initiated:
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APs decide when to hand over and to
whom.
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Hard versus Soft Handoff
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Hard handoff: only a single active
connection between MN and AP.
Soft handoff: two active connections
during handoff.
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Handoff Signaling
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Forward handoff:
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Target AP contacts current AP to initiate
handoff.
Backward handoff:
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Current AP contacts the target one.
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Handoff Delay
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3 components:
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Detect need of handoff.
Link establishment between MN and new AP.
Registration with HA.
Pre- and post-registration handoffs:
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Pre-registration registers MN with HA before
handoff.
Post-registration: HA registration happens after
handoff.
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Authentication
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Malicious nodes can infiltrate FNs.
Mobile IP registration includes
authentication info exchange.
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MH-HA.
MH-FA.
HA-FA.
Protection against replay attacks.
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Timestamp and nonces.
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Mobility Support in IPv6
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Route optimization is default.
Fields for specifying both CoA and
permanent IP address.
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No need for encapsulation.
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TCP Performance in Mobile-IP
(Choong)
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Source of overhead: triangle routing.
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Additional processing at HA and FA.
Additional delay due to “triangulation”.
Additional delay due to fragmentation
(extra IP header).
Handoffs.
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Goal
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Determine the impact on TCP
performance of
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Combined overhead sources.
Individual overhead sources.
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Methodology
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Several scenarios that compound or
isolate overhead sources.
Compare performance of between
scenario pairs.
FTP transfer btween MH and CH.
Metric: TCP throughput.
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Summary of Results
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Dogleg routing as main cause of TCP
throughput degradation.
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Solution: route optimization.
Handoff is second.
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Mobile-IP’s inherent delay in re-establish
connectivity with new FA.
Solutions:
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Increase frequency of router advertisements.
Use link-layer information to trigger handoff.
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