Cellular IP: Introduction

Reference: “Design, implementation, and evaluation of cellular IP”; Campbell, A.T.; Gomez, J.; Kim, S.; Valko, A.G.; Chieh-Yih Wan; Turanyi, Z.R.; IEEE Personal Communications, Volume: 7 Issue: 4, Aug. 2000; Page(s): 42 –49 (CellularIPIntro-3.pdf)

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Introduction• Trend

– Large numbers of mobile users equipped with wireless IP-enabled communicators will have access to a wide array of Web-based mobile multimedia services

– High-speed access can be achieved by using smaller and smaller cell sizesResulting in coverage area with a larger number of b

ase stations

Picocellular environments: simple, low-cost wireless infrastructure

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Introduction (cont)• Seamless mobility

– The ability of the network to support fast handoff between base stations with low delay and minimum or zero packet loss

• Paging

– In essence, Mobile hosts will be in an idle state but passively connected to the network infrastructure

– It will be sufficient for the wireless Internet only to know the approximate location of its population of idle users

– The exact location of idle mobile hosts only becomes important when data needs to be forwarded to them, in which case the network needs to be able to efficiently search and find these users in a scalable and timely manner

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Introduction (cont)• Passive connectivity

– Efficient location tracking in support of idle users and paging in support of active communications

– Handle the location tracking of active and idle mobile hosts independently

– E.g. keeping the approximate location information of idle users requires less signaling and thus reduced the load of the network (air and wired)

• Mobile IP doest not support the notion of seamless mobility, passive connectivity, or paging Cellular IP (micro mobility)

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Related Work• Hawaii

– Support seamless mobility, passive connectivity, and paging

– Hawaii nodes are IP routersAssume intra-domain routing protocol is operating in the

access network, allowing each node to have routes to other nodes

The routing information is used to exchange explicit signaling messages, and forward packets between old and new access points during handoff

For large number of mobile hosts (e.g. tens of thousands), the use of an all-IP-based router solution for picocellular networks may become prohibitively expensive

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Related Work (cont)• About Cellular IP

– Layer 3 routing protocolReplaces IP routing but without modifying the IP packet

format and forwarding mechanism

Location management is integrated with routing

Per-host location information stored in Cellular IP nodes is the next hop route for a given mobile host

– Adopting the learning feature of Ethernet switchesLayer 2 solution (low cost to support large number of MH)

Cellular IP uses data packets to refresh location management state and can operate at layer two or three

The use of explicit signaling messages is limited by using the IP data packets to convey location and paging information

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Protocol Overview• Network model

Mapping (X, BS3)

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Protocol Overview (cont)• Base station

– Wireless access point and router of IP packets while performing all mobility-related functions

– Built on a regular IP forwarding engine with the exception that IP routing is replaced by Cellular IP routing

• Gateway router– Connects Cellular IP networks to the Internet

– Mobile hosts attached to an access network use the IP address of the gateway as their Mobile IP care-of-address

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Protocol Overview (cont)• General principles

– To minimize control messaging, regular data packets transmitted by mobile hosts are used to refresh host location information

– Uplink packets are routed from a mobile host to the gateway on a hop-by-hop basis

– The path taken by these packets is cached by all intermediate base stations

– To route downlink packets addressed to a mobile host, the path used by recently transmitted packets from the mobile host is reversed

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Protocol Overview (cont)– When the mobile host has no data to transmit,

it sends a small, special IP packets toward the gateway to maintain its downlink routing state

– Passive connectivity: mobile hosts that have not received packets for some period of time allow their downlink routes to be cleared from the cache (soft state timer)

– Paging is used to route packets to idle mobile hosts

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CIP Routing• Cellular IP gateway

– Periodically broadcasts a beacon packet that is flooded in the access network

• Base stations– Record the neighbor they last received this

beacon from and use it to route packets toward the gateway

• Packet transmission– All packets transmitted by mobile hosts,

regardless their destination address, are routed toward the gateway using these routes

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CIP Routing (cont)– As these packets pass each node en route to the

gateway, their route information is recorded in the routing cache of each base station en route

– For a mobile host that is not regularly transmitting data packets, to keep its routing cache mappings valid, the host transmits route update packets in the uplink at regular intervals called route-update time

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CIP Handoff• Hard handoff

– Mobile hosts listen to beacons transmitted by base stations and initiate handoff based on signal strength measurement

– To perform a handoff, a MH tunes its radio to a new BS and sends a route-update packet

– The route update message creates routing cache mappings en route to the gateway configuring the downlink route cache to point toward the new base station

– Define handoff latency = the time between handoff initiation and the arrival of the first packet along the new route

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CIP Handoff (cont)– For hard handoff, handoff latency = round-trip

time between the MH and the crossover base station. In the worst case, the crossover point is the gateway

– During the interval of handoff latency, downlink packets may be lost

– Although packets may get lost during a hard handoff, the time taken to redirect packets to the new point of attachment is shorter than that in Mobile IP

– Method to reduced packet loss during handoff: relying on interaction between the old and new base stations (the idea of HAWAII)

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Hard Handoff

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Semi-soft handoff– Reduce packet loss during handoff

– 1. In order to reduce handoff latency, the routing cache mappings associated with the new base station must be created before the actual handoff takes place

– Before a mobile host hands off to a new base station, it sends a semi-soft packet to the new base station and immediately returns to listening to the old base station

– The semi-soft packet is used to establish new routing cache mappings between the crossover and new base stations

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Semi-soft handoff (cont)– 2. After a semi-soft delay, the mobile host

performs a regular handoff

– The semi-soft delay can be an arbitrary value that is proportional to the mobile-to-gateway round-trip time

– The delay ensures that by the time the MH finally tunes its radio to the new BS, its downlink packets are being delivery through both the old and the new BS

– i.e. the downlink packets consumes twice the amount of resources during the period of semi-soft delay

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Semi-soft handoff (cont)– Q: does semi-soft handoff ensures smooth han

doff (no packet loss during handoff) ?

– Consider the time to transmit packets from the crossover point to the new base station

– Case 1. New BS is behind the old oneDuplicate packets (which does not disrupt many AP

s)

– Case 2. New BS is aheadPackets will be missing

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Semi-soft handoff (cont)

Crossover

Old BS

New BS

Pkt i

Pkt i

MH

MH

Case 1

Crossover

Old BS

New BSPkt i

Pkt i

MH

MH

Case 2

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Semi-soft handoff (cont)– Solution for Case 2: by temporarily introducing

a constant delay along the new path between the crossover and new base stations using a simple delay device mechanism (delay buffers)

– Optimally, the device delay should be located at the crossover base station

– After handoff is complete, the mobile host sends a data or route-update packet along the new path, which disables the delaying mechanism.

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CIP Paging

• Base stations are geographically grouped into paging areas

• When there is no call ongoing, mobile hosts only need to report their position to the network if they move between paging areas

• This makes location update and handoff support for idle hosts unnecessary

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Paging Areas

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CIP Paging (cont)• Idle mobile host

– That has not transmitted packets for a system-specific active-state-timeout

– Transmits paging-update packets at regular intervals defined by a paging-update-time

• Base stations– May optionally maintain paging cache

– Paging cache mappings have a longer timeout period called paging-timeout, hence a longer interval exists between consecutive paging-update packets

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CIP Paging (cont)• Paging action

– When a packet is addressed to an idle mobile host, and the gateway or base stations find no valid routing cache mapping for the destination

– If the base station has no paging cache, it will forward the packet to all of its interfaces except the one the packet came through

– Base stations that have paging cache will only forward a paging packet (the first data packet) to the mapped interface if the destination has a valid paging cache mapping

– If there is no paging cache in an access network, the first packet addressed to an idle mobile will be broadcast

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CIP Security• Only authenticated (control) packets can

establish or change cache mappings

• Data packets can only refresh existing mappings

• Session keys used by mobile hosts to perform authentication must be promptly available at the new base station during handoff

• The session key is calculated using an MD5 hashing function

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CIP Security (cont)• A special session key is used in CIP access netwo

rks, and each BS can independently calculate session keys

• The session key is a secure hash, which combines:– The IP address of a MH (IPMH)

– A random # (RMH) assigned to a MH when it first registers with an access network

– A network secret (Knetwork) known by all BS within in an access network

• Session key Ksession = MD5 (IPMH, RMH, Knetwork)

• A session key is first calculated and transmitted to a MH when it first contacts the CIP network during global mobility authentication and authorization

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CIP Security (cont)AAA

Ksession ,RMH MH BS

Route-Update

Encryption with Ksession

1. BS can quickly calculate the session key by combining the IP address and the random value found in the control packet with the network secret.

2. BS can validate authentication easily with the session key.

Authentication Info.RMHIPMH

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Performance Evaluation• Testbed

100Mbps full duplex

2Mbps WaveLAN

MH can dynamically change frequency to perform handoff

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UDP Performance

MH Handoff every 5sec

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UDP Performance (cont)• Input

– The mobile host receives 100 byte UDP packets at rates of 25 and 50 packets/s while making periodic handoffs between B2 and B3 every 5s

• Discussion– Hard handoff causes packet losses

proportional to the round-trip time and to the downlink packet rate

– Semi-soft handoff eliminates packet lossBuffering a single packet in the delay device is

sufficient to eliminate loss even in the case of a large round-trip time where hard handoff results in the loss of up to 4 packets

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TCP Performance

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TCP Performance (cont)• Input

– TCP Reno, downloading 16 Mbytes of data from a correspondent host to a mobile host

• Discussion– Hard handoff

The performance of TCP degrades as the handoff frequency increases due to packet loss

– Semi-soft handoffReduced packet loss and significantly improved the

transport throughputFor 8-packet delay buffer, the packet loss is

eliminated at the higher handoff ratesThe results look promising

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TCP Performance (cont)

• The throughput measured at zero handoff rate is marginally lower than 1.6Mbps achieved using standard IP routing

– 1. IP is implemented in the kernel and CIP in user space

– 2. CIP uses PCAP (Berkeley Packet Filter’s Packet Capture Library) to forward packets which is not optimized for IP forwarding.

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Scalability• High throughput with large # of mobile hosts

– Efficient routing cache search

– Routing cache misses search paging cache

• Input– Permanent, random cache mappings

– Multi-homed 300MHz PC base station

– Radio interface 100Mbps Ethernet

• Discussion– Operation of CIP routing cache is very similar to the

self-learning operation of Ethernet switches

– CIP software base stations are capable of supporting large # of mobile hosts and high aggregate throughput

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Scalability (cont)

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Future Work

• Quality of service provisioning

• Multiple gateways in Cellular IP Networks

– Mobile host should be capable of changing gateways (care-of-address) during normal operations

• Specification, source code for Cellular IP

– Web page: comet.columbia.edu/cellularip