A laptop has an active TCP connection to a server. The user moves to a different WiFi network, and the laptop gets a new IP address. Why does the TCP connection break, even though both networks are working correctly?
ATCP connections break whenever the physical link layer changes, regardless of whether the IP address changes
BThe new network uses a different MTU, causing packet fragmentation that terminates the connection
CTCP connections are bound to IP address pairs; when the laptop's IP changes, the server cannot route responses to the new address, and the connection state referencing the old IP becomes invalid
DTCP does not support user mobility and requires a new three-way handshake whenever a user roams to any new location
This is the fundamental tension that Mobile IP solves: IP addresses serve dual purposes as both host identifiers and location descriptors. TCP connections are defined by four-tuples (source IP, source port, destination IP, destination port). When the source IP changes because a host moves, the server still sends responses to the old IP, which is now on a different subnet — packets are misrouted or undeliverable, and the connection breaks. Mobile IP solves this by keeping the home address constant while routing through the care-of address.
Question 2 Multiple Choice
In Mobile IP, a remote host sends a packet to the mobile node's home address while the node is visiting a foreign network. The home agent tunnels the packet to the care-of address. What problem does this triangle routing create?
AThe packet is delivered twice — once directly and once via the tunnel — causing duplicate processing
BTriangle routing forces the packet to travel via the home network even if the remote host is geographically close to the foreign network, wasting bandwidth and adding latency
CThe home agent cannot distinguish the tunnel header from the original packet, causing header corruption
DIP-in-IP tunneling is incompatible with wireless links, so the tunnel fails on any mobile network
Triangle routing is Mobile IP's main performance drawback. If a mobile node is in Tokyo and its home network is in New York, every packet from a San Francisco host must travel to New York, get tunneled, and travel back to Tokyo — a massive detour. Route optimization addresses this by allowing the remote host to learn the care-of address directly and send packets straight to the foreign network, eliminating the home-agent detour after the first exchange.
Question 3 True / False
The main architectural advantage of Mobile IPv6 over Mobile IPv4 is that it integrates mobility support directly into the protocol, eliminating the need for separate foreign agents.
TTrue
FFalse
Answer: True
Mobile IPv6 leverages built-in IPv6 features — neighbor discovery, address autoconfiguration, and mandatory IPsec support — to implement mobility without requiring foreign agents on every visited network. The mobile node can form a care-of address directly using stateless address autoconfiguration and register it with its home agent using binding update messages. This simplifies deployment significantly compared to Mobile IPv4, which requires foreign agent infrastructure at every foreign network.
Question 4 True / False
In soft handover, the mobile device disconnects from the old access point before establishing a connection to the new one, ensuring a clean single-connection transition.
TTrue
FFalse
Answer: False
This describes hard handover, not soft. In soft handover, the device communicates with both the old and new access points simultaneously during the transition period, then drops the old connection once the new one is stable. This eliminates the gap that would otherwise cause dropped packets or interrupted calls. Cellular networks like LTE implement soft handover at the radio layer through base station coordination. WiFi typically uses hard handover because access points operate independently without coordination mechanisms.
Question 5 Short Answer
Explain why IP routing creates a fundamental problem for mobile devices, and how Mobile IP's home agent / care-of address architecture solves it.
Think about your answer, then reveal below.
Model answer: IP addresses encode both host identity and network location — routers use the address prefix to determine which subnet a packet should be delivered to. When a device moves and joins a new subnet, it must get a new address, which breaks any existing connections bound to the old address. Mobile IP solves this by letting the device keep its permanent home address (identity) while using a care-of address (location) on the foreign network. The home agent intercepts packets sent to the home address and tunnels them to the care-of address, decoupling identity from location.
The core insight is that the dual role of IP addresses — identity and location — is not a bug but a feature exploited by routing. Mobile IP adds an indirection layer: the home agent maintains the mapping from permanent identity to current location, and tunneling bridges the two. This allows the rest of the network to continue routing based on the home address as if the device never moved, while the tunneling mechanism handles the actual delivery to wherever the device currently is.