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Background
Session Initiation Protocol
SigComp
Group Mobility
Hierarchical State Routing
Group mobility models
Predictive Address Reservation
Simulation part
Conclusions
Final remarks & future work

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”Langattomien teknologioiden käyttömahdollisuudet puolustusvoimien tietoliikenneverkoissa” / ”Possibilities for wireless technologies in defence networks” funded by the Finnish Defence Forces
 A joint research program of HUT Networking
Laboratory, Communications Laboratory and the Finnish Defence Forces, commenced in
2003

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Contemporary disaster relief operations rely heavily on realtime wireless communications
– these systems fall into category ”It Just Must Work”
– the technology commonly used for these ends has had propensity to be expensive
The rapid development of civilian communications technology has caused their prices to decline fast, making them an attractive alternative for the military-grade equipment
– remember the price discrimination: a price charged from a governmental authority is N-fold compared to the price charged from a civilian party
Project LaTe is an attempt to find ubiquitous, affordable and easily disposable wireless solutions to complement (and even completely substitute) the aging authority communications equipment currently in use
– Commercial Off-The-Shelf (COTS)

Netlab involvement (master’s theses)
2003 Wireless LAN Security (Ahvenainen, Marko)
2004 Mobility management with Mobile IP version 6 (Merger, Mikko)
2005 An Overview of Mobile IPv6 Home Agent Redundancy (Keränen, Heikki)
2006 Mobile IPv6 performance in 802.11 networks: handover optimizations on the link and network layer (Hautala, Mikko)
2007 Analysis of Handoff Performance in Mobile WiMAX Networks (Mäkeläinen, Antti)
2007 Supporting Group Mobility in Mission-Critical Wireless Networks for SIP-based
Applications (Repo, Marko)
2008 …

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Session Initiation Protocol (SIP)
–
–
– flexible, scalable and reliable signaling protocol inadequate in terms of bandwidth & security good starting point for application-layer mobility
Seamless handoffs during mobility
– VoIP & data
–
– inter-domain mobility assumed scarce network bandwidth & resources
Group handoffs
–
”Group Mobility” is a term originally coined in the world of ad-hoc networks
– assumes that network nodes exhibit group behavior (often realistic!)
– attempt to forecast the future need of network resources and minimize the required amount of signaling during handoff procedure

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Citing RFC3261, SIP is ”an application-layer control
(signaling) protocol for creating, modifying, and terminating sessions with one or more participants”
Has undergone a lot of development during the last half a decade
–
– and still does (various interoperability forums and events held by SIP Community) and will do (3GPP NGN/IMS, IETF, Microsoft etc.)
Has gained a significant foothold as a signaling protocol both in academia and private sector companies, competing with ITU-T H.323 mainly backed by the telecommunications industry

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Provides all needed primitives for establishing a connection between 2-N end points
Transport independent
–
UDP, TCP, SCTP, …
Supporting unicast and multicast
Extremely scalable
– Intended as a subscriber signaling protocol, but functions virtually in every network core where the intelligence is located at the edges
Intercompatible when required
–
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ITU-T H.323
ISUP (SS7)
Q.931 (ISDN)

 Issues
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UTF-8 ASCII format implies bandwidth inefficiency
 SIP was not designed for low-bandwidth wireless environment
 Attempts to alleviate the bandwidth issue have spawned mechanisms such as SigComp. Many problems and issues.
”Light-weight”?
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Way no. SIP is already as complex as H.323. By the date, the SIP specifications contain thousands of pages
Irony underneath: the protocol design started from the need for a robust signaling mechanism characterized by simplicity and lightness
Many open security questions
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 signaling media
Virtually no support for seamless mobility
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Cannot be handled with MIPv4/v6, due to the triangular routing phenomenon (too high latencies involved!)
– suitable for data connections with loose temporal requirements
The real-time streams problematic (VoIP can withstand <100ms latencies without degradation)

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Attempt to address the bandwidth issue by binary compressing text-based SIP messages
May improve efficiency especially on low-bandwidth connections
However, SigComp has some severe shortcomings
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– consumes computing power for message processing requires a lot of memory for storing state information security issues (may subject to DoS attacks) problems with mobility
After all, SigComp introduces another extra layer, and thus more complexity. So, we’ll take a different approach.

The fundamental problem with SIP:
It was never intended for narrowband airlinks. The size of a single message with a payload can range anything between a few hundreds of bytes to many kilobytes.
Ergo , even a modest number of moving nodes may generate a significant amount of SIP signaling traffic during connection hand-off.

We may try to eliminate the unnecessary signaling by dealing with groups instead of individual nodes.
Introducing
.

Creating an isolated cell using a mobile relay station (MRS) , which gains the control of the moving mobile nodes.
Suitable for public transportation vehicles (buses, trains, aeroplanes) where groups guaranteed to stay compact. Not suitable for loose or scattered groups (e.g. infantry).

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HSR: A link state protocol
– a low-latency routing solution for applications requiring group mobility
Applies hierarchical addressing to keep channel utilization efficient
– conservative on routing table sizes
Unbundles the physical affinity from the logical partition representing different logical or functional levels where the nodes may reside
The amount of signaling remains low, since there is no need for flooding
– even when the location of the corresponding node is not known

 Better in terms of complexity (=fewer routing table entries) than traditional flat routing schemes
–
–
Let N : no. nodes, M : no. hierarchy levels; then
Flat routing: O(N M );
– HSR: O(N X M).
Leads to better scalability
 The flip side of the coin: constant need for updating databases
– increased complexity + update latency
– dynamic cluster re-arrangement?

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Link layer (L2) delay
– scanning, authentication and reassociation
Movement detection (L3)
– Router Solicitation / Router Advertisement
DHCP
– Duplicate Address Detection (DAD) is a major source of delay!
Re-configuration delay
SIP re-establishment delay
– RTT for re-INVITE and message processing, a major contributor
Packet transmission time
– The time for first packet to be exchanged over the restored connection
QoS + AAA (optionally)
– Quality and security reservation introduce some latency when used

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Predictive Address Reservation (PAR) is a mechanism attempting to alleviate incurred handoff latency by eliminating the most significant sources of delay: Duplicate Address Detection (DAD) during
DHCP and SIP connection re-establishment (re-
INVITE)
Allows approximate latencies of ~60 ms, allowing possibly even better performance!
Allocate L3 addresses and the session establishment proactively, so that the handoff process is almost seamless

1.
2.
MN starts searching for a new AP/BS when the Signal-to-Noise falls below the Cell Search Threshold
MN consults its internal database and chooses a suitable target BS (TBS), then sends a reservation request to its serving BS (SBS)
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SBS consults its neighboring BS table to see whether the MAC of the TBS belongs into the same (L3) domain or not
If so, the SBS initiates a normal L2 handoff (L2HO) procedure
If not, a network level (L3) handoff is needed. The SBS requests a new IP address from the TBS, which obtains it using DHCP and allocates resources proactively.
Reservation reply containing procedure acknowledgments and a new IP address is sent to the MN

4.
Subsequently, the MN sends a re-INVITE request to its corresponding node (CN), using its newly reserved IP address
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The CN opens a new session in parallel with the old session
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The packet exchange happens through both sessions (bi-casting) until the handoff procedure is completed for minimizing the amount of lost packets
7.
When the handoff is completed, the old session will be torn down. All traffic is now sent using the new session.

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Mobility models are needed for system analysis and protocol during the design phase, but also for predicting the future availability of wireless resources
Conventional models (Random Walk, Gauss-Markov) put the emphasis on individual entities
In many cases, however, it makes sense to observe the movement and interaction characteristics for groups instead
Group mobility is currently undergoing heavy research, mainly in the world of ad-hoc networks
The future need of resources can be predicted with aid of group mobility models.
– logic: when a MN belonging into a group performs handoff, it can be anticipated that that others will follow in a certain pattern
 the rest is about queuing theory and e λt :s…

The most simple group mobility model. It is a conventional model for representing e.g. field operations involving searching activity.
The group consists of MNs associated with a line of reference, which fully characterizes the group behavior.
The participants also have a reference point on the line, around which they may freely wander.
The movement of individual nodes does not have effect on the location of group center.

Another simple model representing e.g. a chasing scenario. A target node
(TN) takes now the place of the point of reference, which denotes the group ”center”.
At any time t, the scenario can be modeled mathematically:
MN i
(
 
1 )

MN i
(
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)

A i

RM i
Where MN i is place at any time t, A is an acceleration vector of form F(TN – MN i
), i.e. position of the target node TN and the Mobile
Node i . RM i is a random motion displacement vector for any node i, RM << A

Describes activity of wandering tribes, camping for night. One may imagine that the point of reference (RP) is the camp fire.
The group motion vector GM represents the movement of the campfire
(RP), and the mobile nodes are able to wander around it randomly.
The roaming distance can be set as a parameter.

RPGM is perhaps the most generally seen ad-hoc mobility model. It can be considered of generalization of all the presented. RPGM it is also maybe the most commonly studied group mobility model as it comes to the ad-hoc mobility. Has been an inspiration for several derivative models.
The location vector for each individual node i can be written now:
PN i
(
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1 )

PN i
(
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)

GM

RM i

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Carried out using network simulator ns-2
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– several contributed modules needed
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Mobility enhancements (NIST HSNTG)
A SIP module by Rui Prior
C++ coding needed
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 insufficient 802.11b model
No way to model PAR
Attempt to demonstrate the benefits obtainable by deploying
GM-enhanced PAR-SIP with four plausible scenarios
– simulating VoIP (RTP) and data (TCP) traffic
Indicators of interest: total traffic, hand-off latency and packet loss during the hand-off process
As of May 2007, work still in progress!

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The main goal of this thesis: minimizing signaling, minimizing handoff latency!
SIP is the choice of the future, currently undergoing very rapid
& active development
– However, yet a far cry from all-around protocol
There are many ways to mitigate the incurred handoff latency.
Predictive Address Reservation (PAR) is one of them.
Group mobility mechanisms aim at minimizing the unnecessary signaling during handoff, allowing better channel utilization in many scenarios, group handoffs (= group handovers) are their realization.

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802.11x not necessarily the most realistic platform for such wide-area scenarios
– as it comes to €uro$, very alluring (comparing to WiMAX!)
– still undergoing evolution
Vertical handovers? IEEE 802.21 (Media Independent
Handover) on the verge of introduction
How about voice and data taking different routes?
– hybrid MIP-SIP
The research dealt solely with the most rudimentary transport level protocols, UDP and TCP
– how about more advanced protocols? DCCP? SCTP?
Hybrid networks? The strict division into infrastructured and adhoc networks is likely to disappear in the future
– actually, this is happening already, slowly but steadily…
– look at VIRVE/TETRA for instance, but also civilian applications
(WPANs, Bluetooth, UWB, …) although the scale is different