Distributed Service Location and Session Management for Ad-hoc Networks
Juhamatti Kettunen
Nokia Research Center
Computing Architectures Laboratory
P.O. Box 407, FIN-00045 NOKIA GROUP, Finland
juhamatti.kettunengnokia.com
Abstract
This paper presents distributed and authenticated service discovery and session management schemes. The SLP
and SIP protocols have been enhancedfor use in ad-hoc
networks. We have coupled the protocols with the AODV
ad-hoc routing protocol to make the signaling as efficient
as possible. The design has been implemented and tried
out in a real ad-hoc network testbed. Our results show that
the system is feasible in practice, and very efficient both in
terms of bandwidth consumption and computational load.
1. Introduction
Ad-hoc networks are dynamic networks, wherethe number of nodes and the network topology may change sporadically. Routing iS typically multi-hop, yet, this can be
performed on the MAC layer or the IP layer. Due to their
dynamic nature, ad-hoc networks must follow distributed
computing because relyng on fixed centralized servers may
not be possible. Distributed computing in ad-hoc networks
can be based on electing certain nodes to perform centralized tasks, and re-elections in case of failure, or on distributing the same functionality evenly into all nodes.
Primary design criteria for service discovery and session
management in ad-hoc networks are distributed and standalone operation without support from the Internet, ability
to handle node arrival and departure gracefully, support for
pull and push information dissemination, support for any
types of services and applications, mutual authentication,
and small message sizes for energy efficiency.
The SESSI project investigated services for ad-hoc networks, focusing on extensions to the Service Location Protocol v2 (SLP) [3] and the Session Initiation Protocol (SIP)
[4] [8]. The key extensions to SLP include better authentication of the service announcement, and proactive dissemination of service availability without prior request. Fundamentally SLP fits quite well into an ad-hoc network [5].
1-4244-0992-6/07/$25.OO © 2007 IEEE
Jukka Manner, Antti Yld-Jaaski
Helsinki University of Technology
TML Laboratory
P.O.Box 5400, 02015 TKK, Finland
j manner, anttiyj gtml. hut. fi
The basic SIP architecture includes an overlay network
of centralized servers, e.g, registrars and proxies. A new
extension to SIP, called distributed SIP (dSIP) [8, 9], removes the need for centralized servers, and enables initiating sessions in ad-hoc networks. The basic concept is
that all SIP clients broadcast their bindings (SIP Addressof-Record and IP address) to other nodes in the network, or
alternatively, request other users' bindings. This latter functionality can be implemented using SLP: users look for the
service "SIP", and get in return the bindings of users.
The SESSI project targeted W/LAN ad-hoc networks,
e.g., IEEE 802.11 in ad-hoc mode, where all nodes communicate on the same IP link. In this paper, we extend the basic SESSI framework with the Ad-hoc On-demand Distance
Vector (AODV) [14] routing protocol. With careful coupling of SLP and dSIP schemes and AODV, we are able to
send these higher layer protocol messages in single AODV
routing packets, saving a tremendous amount of bandwidth
and energy. Without coupling, a node would send route requests, service requests, and its own proactive service announcements in separate messages, with additional replies
coming back in individual messages. We show an implementation and proof-of-concept demonstration of a multihop ad-hoc network where users can look for services, including the service "SIP", and are able to initiate sessions,
e.g., instant messaging or chat, with each other. Our system
is fully distributed, thus, no single points of failure exist.
There exists research and solutions for service discovery
in ad-hoc networks (e.g., [6, 7, 10, 13]), but the solutions
usually address only a specific type of ad-hoc network environment, solve only a subset ofthe design criteria discussed
above, are based on mediators or other proxy functionality and do not consider the effect of the underlying ad-hoc
routing protocols. An old and expired Internet Draft [5]
discusses SLP and AODV integration, and a short paper [2]
presents similar ideas using ns2. Our scheme follows those
concepts, but our design goes much further in the integration, and security, and introduces the use of passive SLP.
2 Overall Architecture
2.3
The SESSI project had three goals: service discovery,
session management, and security. One of the key design
guidelines was flexibility in order to enable different applications and usage models, independency of routing protocols, and power efficiency. Service discovery is achieved
with extensions to SLP, session management is handled
with an extended version of SIP, and the security mechanisms allow various security levels depending on the application and user requirements. The project also designed
functionality for interworking of applications between adhoc networks and the Internet, in addition to extensions to
SIP-based instant messaging and presence services [8, 9].
AODV [14] is a reactive ad-hoc routing protocol. The
route finding is based on broadcast network search and unicast replies of discovered paths. Packet duplicates and routing loops are identified with sequence numbering and routing table entries have a maximum lifetime. After route expiration, route discovery procedure is started. Network nodes
maintain established routes and in case a link breaks on an
active route, an error message is sent to the route users.
A Route request (RREQ) is sent when the destination is
not found from the route table or the route has expired. A
node receiving a new RREQ searches its local routing table
for a route to the indicated destination. If none is found,
the RREQ is broadcast further. A route reply (RREP) is
generated if the node is the destination of the RREQ or it
has an active route to the destination. A RREP is unicast to
the next hop towards the node which originated the RREQ.
AODV supports global connectivity through gateways.
Modified Router Solicitations and Router Advertisements
advertise global IPv6 prefixes to the ad-hoc network.
Service Discovery extensions [5] can be used for the services which are discovered in ad-hoc networks. Service Request (SREQ) and Service Reply (SREP) messages are used
to extend AODV. To use Service Discovery extensions, the
nodes must have a service binding table which includes service selector and IP address pairs with valid lifetime.
When a node requires access to a service, a SREQ is included in an AODV RREQ message and broadcast through
the network. The destination address is set to zero. When a
node receives SREQ, it searches for a service binding. If it
is found and a route to the source of the SREQ is available,
a RREP with Service Reply extension is generated and the
resulting SREP message is sent to the source of SREQ. If
the route is not available, SREQ is re-broadcast.
2.1
Distributed Session Management
In Distributed SIP (dSIP) all ad-hoc nodes incorporate
a local SIP proxy and registrar [8, 9]. dSIP users register
to the local SIP registrar which broadcasts a REGISTER
message to the ad-hoc network. All listening SESSI nodes
gain registration information into their local proxy. A SIP
user list can be obtained with a local query to the local
SIP registrar. The ad-hoc network registrations are separated from global registrations with an automatically added
. local string in the contact address of REGISTER messages. When a user is invited to a session, local ad-hoc
sessions use the local proxy and global sessions use preconfigured outbound SIP proxy/registrar.
2.2
Service Location
In SLP aided SIP (sSIP), the local user registers to the
local SIP registrar as with dSIP. However, instead of broadcasting REGISTER messages, the local SIP proxy registers
the SIP service and SIP user as Address of Record (AOR)
attribute to the local SLP daemon. SLP Service Requests
for SIP service and SLP Attribute Requests for the AOR attributes are used to acquire local ad-hoc network SIP users.
SLP requests can also be used to find network services like
default gateway, DNS services and SIP proxies. Active
mode service discovery is used as in standard SLP.
Instead of active request-reply mode, a new passive
mode can be used. A Service Advertisement is broadcast
to the network on local service registration [8]. A new SLP
Service Advertisement message is used to advertise services
in the network. When lifetime is set to zero, the message
can also be used to de-advertise services.
Service Advertisements can be cached in forwarding
nodes which cannot be done in request-reply architecture.
As Engelstad et al. state [1], this seems to be very useful
especially in distributed query-based environments; other
results also support this functionality [6, 7, 13].
2.4
AODV and Service Discovery
Security Framework
The security of SIP sessions relies on a new Authentication & Authorization (AA) module which uses asymmetric
X.509 certificates for mutual authentication and confidentiality [8]. Two types of identities are used: user and group
identities. The standard SLP authentication block does not
allow mutual authentication and uses normal timestamps in
authentication blocks. The SESSI SLP uses modified authentication blocks which include sender and group identifications and logical timestamps. Instead of calculating
signatures over service specific values and timestamp, the
modified SLP authentication block signature is calculated
over the whole message. The authentication and confidentiality levels have their own user and group asymmetric
keys. The modified authentication block is extended with
plain text receiver ID and ID length fields (see Figure 1).
Node I
Authentication Level:
Link
ReEeier'
ID
Physical
(1..N nodes)
Node 2
Link
SLPd: Create SLP
Sgaig
Message
ID
LeghSrieRqet(de)SLP:i~
sdreq
AODVd: Check route
AO DV:
_11
(found: all nodes) sdreq
802.1llg: -- ->
Confidentiality Level:
___________________________________________________________________
I
SLP Servicesde
R
sde
eceiver's Receiver'sRe quest
Original SLPv2 Original SLPV2 Extension Block &
Mesg
ID
edrMessage Body Authenticdation Blck)I
RREQ Process sdreq
H Encrypted with Sequence
Encrypted with Shared Symmetric Key or
Create SLP
Receivers Public Key
LiSymmetric Secret Key
__
~~~~~~~~~~ ~ ~~~~~~~~~~~~~~~~~~~~~~AODV Route
RREQService
Not encrypted
Request
(sdrep) [L4YReply
AODVd:
REPAODVd:
Create RREP
IBuffer sdrep
Check route
I I ~~~~~~11~RTS .
i(none: create
Original SLPv2
Extension Block & Receiver's
Authentication Block
LengthsLd
Figure 1. Security level message structures.
_RTSJ
AODV Route-
3 Efficient Service Discovery with AODV
ReplEP4
>4,
~~~~RREQ)
-
ACK
ACK[~~~J~
RREP
'
sdrep
AODVd:
Add route
Send-Our work concentrated onthe integration of AODV and
I RTS 011RSen
~~~~~~buffered
~RTS
user bindings using SLP, the sosdrep
SLP Service
I~_CTS
I
called sSIP scheme. Our aim is to minimize the amount
Reply
sde
of requests and traffic, taking into account the AODV and
sdrep
Sd:Adsrie sdrep
AC
link layer properties, thus, minimizing power consumption;
to service list
>11. ~several technical papers suggest that reactive protocols enCreate SLP Attribute
able more efficient power control than proactive protocols
ROVdCeques rattre attreq RT
(found: Node 2)
> RTS
(node transceiver does not need to be active all the time).
CLTttiutSTSAn example of service discovery without integrating
Request
AODV and SLP is shown in Figure 2. sSIP service disI
~~~~~~~~~~~aftreq
attreq
covery functions can be characterized to three functionaliiSLPd: Create
ACK
attreq
ftibteRel
AC
Attiue
el
attrep
ties: SLP Service Request, SLP Attribute Request and SLP
aftrep
~ ~ ~ ~ ~ (at rep)
E
AODVd:
Check
tRTS
Service Advertisement. Because both SLP and AODV use
I [1*
~~~~~~~~~~~~~~~~~~~~~route
_Y
fud oe1
multicast in requests and unicast in replies, these properties
SLP Attribute
can be efficiently integrated. The existing proposal [5] mustRel-CTattrep
be enhanced because SREQ and SREP messages are introIattrep
C
duced as additional routing extensions. If these service exACK
tLodAddicEAO atrp
atiuels
tensions are used as specified, the AODV daemon in nodes
must be extended to implement service discovery functions,
or even a new routing table with service-destination pair.
Figure 2. Network phases when service and
attribute
This creates additional overhead to routing. Thus, we conrequests are sent separately.
sider plain flooding to be simple, yet, efficient, enough for
our goal. Moreover, flooding has been shown to be very
reliable and efficient compared to multicast in ad-hoc netseveral asymmetric checksums for every SLP message in
works [12]-typically multicast is also sent as a broadcast
the same AODV SLP extension of source. For efficiency,
in many common link layers.
the SESSI modified Authentication Block can be integrated
The AODV SLP extension encapsulates several SLP
to and used as AODV extension. There it can be used to
messages into one AODV extension. The encapsulated
guarantee authenticity or secrecy for all extensions and the
messages are standard SLP messages with any preferred
checksum can be calculated in one procedure. The Receiver
combination. Services can be requested, replied or adverID authentication block trailers, described in Figure 1, are
tised in the same SLP extension message. The encapsulated
not needed either - the AODV destination can be used to
SLP extension can be passed through an application interidentify the receiver. Moreover, the standard Authenticaface to a SLP daemon, which processes messages and offers
tion Block of SLP is sufficient. The AODV Authentication
replies whenever needed.
Block is used as last included extension and the signature
field is calculated over all AODV extensions.
SLP, and querying SIP
-
3.1
Authentication
SESSI uses a modified Authentication Block to identify valid SLP messages. When several messages are included in one AODV message, it is not feasible to calculate
3.2
-
-
SLP Messages
In the new coupled AODV-SLP scheme, five types
of messages can be identified: Service Request (SREQ),
Service Reply (SREP), Service Introduction (SINT), SerMultihop ad hooc
-eernetW
vice Advertisement (SADV) and Service De-advertisement
fter6bh
can
include
several
different
SLP
Service
(SDADV). SREQ
Requests and SREP includes grouped SLP Service Reply
bob@rode3
xdci0bff
ce@node4xdcidab, i
bob@nOdej3AXuClatSEa
answers. SINT is used to merge several messages together,
3612
s
X
0m
250!l128
e.g., SREQs and SADVs. The SINT group is useful when
h0de8xdcabfi
a node enters the network: all wanted services can be requested and own services advertised in a single message.
Figure 4. Multi-hop ad-hoc peer-to-peer netWhen Figures 2 and 3 are compared, the main advanwork.
tages of using SLP message grouping can be seen. In
Figure 3, a SINT is sent to network. The overall network latency and overhead is efficiently reduced. This also
tension with S I P SREQ, AOR Attribute Request and
means that in contention-based network, the network overSADV for Bob's services (see Figure 5). Node8 floods
all throughput will be much improved. Still, the same inthe SINT message.
formation is gained from SINT as with service and attribute
requests illustrated in Figure 2. Moreover, in addition to
alice@node4
.
~~bob@node3
node8
I
I
the requests, Node 1 services are advertised to all receiving.
pLPeer
network nodes.
E
Node 1
SLPd: Create SLP
Service and Attribute fl
Request and Service
Advertisement (SINT)
SINT
AODVd: Check route
(found: all nodes)
AODV Route Request
SLP ServiceAdvertisement
SLP Service Request
SLP Attribute Request
Link
Link
Physical
nodes)
Signaling:
802.1
l
L
RTS
AODV Route Reply,S
Repli/
SLP Service
Replff
SLPd: Add services
and attributes to list
AODVd: Add route
SREP
[ ACK
AODV RR
RR
RREP + SREP
AODV/SLP
r1
Clhk uCTSte
SREP
SLPd:
NT
-RTS
IXE
EP
g:L
I
_NVTE
I
SINT
T
Node 2
(1..N
RTS
SLP Attribute
+
SREP
Process SINT
Create SLP
Service andI[
Attribute Reply
200 OK
LI OK
200
I
Aq
(SREP)
AODVd:
Add route
Buffer SREP
(found: Node 1)
Figure
5.
Integrated
session initiation.
service
discovery
and
CK
CK
Figure 3. Network phases when a AODVISLP
integrated SINT message is sent.
2. Alice adds Bob's route to her route table and replies
with an AODV SLP extension SREP message which
includes Alice's Service Reply to service SIP and At-
tribute Reply to attribute AOR.
3. Bob saves Alice's information and sends a SIP INAODV supports global connectivity through gateways.
VITE to Alice at address sip:alice@node4.xdclab.
Thus, SIP users can register to their pre-defined registrar
fi.local for a session. Bob's local SIP proxy forwards
servers, and initiate sessions with users outside the ad-hoc
the invitation and node8 routes it further.
network. If the ad-hoc network uses IPv4 and a NAT, a
4. Alice accepts the session and answers with a 200 OK
SIP proxy must be located on the gateway, and its service
announced to the ad-hoc network [11].
message to Bob at address. Alice's local SIP proxy
forwards the 200 OK message and node8 routes it.
When a user wants to start a SIP session, the SIP stack
should first query the ad-hoc network for the target SIP ad5. Bob receives the confirmation and acknowledges it
dress. If the user is found, the SIP session can be established
with an ACK message.
directly within the ad-hoc network. If the target is not found
locally, and a gateway is available, a lookup can be estabcan initiate the actual applicalished through the gateway and outside SIP registrars.
Afterthese
ses,ithe
ers
used
over
this
session
tion
e.g. Bob starts a MSRP client in
An example of the power of our scheme in a network
and
Alice
starts
a MSRP client and connects
depicted inFigure 4 isas follows:server mode,
to Bob. When session is to be closed, a BYE message is
1. Bob broadcasts a SINT message as AODV SLP exsent by either of the parties.
4 Implementation and Experiments
Our testbed for the design discussed above was composed of Nokia 770 Linux-based devices connected together with IEEE 802.1 Ig WLAN. An extended AODV
routing daemon, SLP and SIP software were included in all
ad-hoc network nodes. We used IPv6 stacks on the ad-hoc
nodes, gateways, and external SIP servers,
4.1
Proof-of-concept Experiments
Our experiments where composed of two phases, proofof-concept and functional experiments of our designs, and
measurements for comparing decoupled AODV and SLP
with our coupled approach. The proof-of-concept experiments considered (1) connectivity of nodes through a
gateway with separated subnets, (2) connectivity of nodes
through a gateway within the same subnet, and (3) multihop service discovery and connectivity between nodes.
The environment included two laptop computers as gateway nodes which offered connectivity to outer networks SIP
registration server and advertised their gateway services to
the ad-hoc network. Gateways used different IEEE 802.1 Ig
ad-hoc SSID on non-disturbing channels without authentication. Other ad-hoc nodes were the Nokia 770 devices
with wireless networking capabilities. These nodes could
move freely between ad-hoc networks and join in either one
of these networks. After joining, a new registration was
made to the outer SIP registration server to update location
information. SIP sessions could be initiated with the other
ad-hoc nodes, or through a gateway to other networks or
back to the same inner network nodes. The AODV daemon
was embedded in all network nodes, except the SIP registration server which was assumed to be located in the outer
network. With AODV, a node did not need to be within
gateway coverage area other nodes could provide connectivity if needed. Default standard AODV settings were used
in the network without Hello messages.
As a result of integration, a rather dynamic demonstration environment could be created. With the Nokia 770
Connectivity manager program, all ad-hoc networks in a
certain area could be scanned at any time and after joining to the chosen ad-hoc network, SIP gateway and service
users could be easily requested. If local users were found,
they could be invited to a SIP-based chat session. If a gateway was found, other users could be reached with their SIP
addresses through the gateway and SIP registration server.
The AODV daemon offered multi-hop service discovery capabilities. In the measurement environment, service discovery could be done with several nodes and also over multiple
hops with very low delay.
4.2 Measurements
The measurement environment used was similar to Figure 4. The absolute performance of an ad-hoc network, particularly in our case the signaling performance of AODV
and SLP, is totally dependent on the amount of interference and node movement. Since our aim was to compare
the decoupled and coupled operation of AODV and SLP
in terms of signaling load introduced into the network, we
eliminated any outside interferences and background load
of other user data. Thus, our measurements show a comparison of the two schemes, and a best-case operation in an
unloaded network.
Service discovery queries and replies were integrated as
one AODV SLP extension which lowered overall service
discovery query data traffic and minimized the delay of
queries. The gain is less protocol headers and less individual messages. Several separated queries would add additional link, network and transport layer header overhead,
and add contention for the shared wireless medium.
Even though header information is only a small part of
overall traffic distribution in common environments, its relative share of service discovery data is significant when the
AODV SLP extension is not used. Savings of transmitted data are IEEE 802.11 (32 B), IPv6 (40 B) and UDP
(8 B) headers reduced with AODV SLP extension header
size (4 B). As an example, when a service request (SREQ)
is flooded with one SLP Service Request and two SLP Attribute Requests and several service replies (SREPs) with
one SLP Service Reply and one SLP Attribute Reply are
received, the transmitted traffic gain Tg can be viewed in
Equation (4) where n represents the number of replying
nodes in the network, rr-eq the average request count, rep
the average reply count, hreq and hrep the amount ofheader
data of upper layers and c- the average hop count. With
the non-integrated version (plain flooding of separated SLP
messages), the service request would consist of three and
the replies of two separated messages. The control traffic of
different layers is not included in the equation.
Xx2) x c
(rkeq X hrq + r x
Tintegrated (hreq + hrep x n) x c
re-integrtd
Tnon-integrated
T
Tg + Tintegrated
1) x hrq + (rra..
1) X h
X n))
x
(1)
(2)
(3)
(4)
When SREQ size assumed to be 188 bytes, SREP size 96
bytes, r q = 3 and r - = 2, differences to overall traffic
in one to five node environment can be viewed in Figure 6.
The latencies in looking for and announcing services can
be calculated with the following equations. The term i is the
node internal processing delay. With our devices, the delay
is very constant at around 2 ms.
32500-
SIP users in the networks. All this functionality is fully dis-
30000-
tributed and is therefore robust. Security frameworks are
included in the system based on two distinct security services, authentication of data, and confidentiality.
a) 20000
/
Our scheme does not break the normal operation of the
0U 17500
S
o
\ NoninteqrSLP and SIP protocols. Furthermore, SIP is only used
X
sU 15000
8 12500
as a good and practical example, naturally any other service, e.g., gateway services, can be discovered through our
1)0007500scheme. In the future, we plan to extend our implementation
to cover service location between separate networks,
XDco 2500and
instant
messaging and presence services over SIP. Ex01 Number
2
3
4
of nodes in network
periments with ad-hoc networks of tens of nodes and active users would also help us understand the scalability of
the scheme. Furthermore, our solution could be integrated
Figure 6. Data difference of service discovery
with other ad-hoc routing protocols, such as, the proactive
methods when c = 1.5.
OLSR. We are specifically enthusiastic about the numerous successful proof-of-concept experiments which give us
confidence to further develop the concepts presented.
*- 27500=
D 25000
22500-
5
(rrcq1 X RTTa,g
2
Dinegraed
+
(i +
Dnon-integrated
D9
=
((rk.q + rv,_l
References
Dnon-integrated
x n)
=
x
RTTa,gx
2
+
x
)
(i +
(5)
2
/) RTTavg /
xn
(6)
2
Dg + Dintegrated
RTTa\)
- (1 + nn)
(7)
x i
(8)
The delay difference between the non-integrated and integrated solutions grows tremendeously with the size of the
network. For example, if the average request-reply count
is only five, our integrated solution is over five times faster
than the ordinary non-integrated signaling.
The time it takes for announcing the SLP services between three nodes in an empty network is very short. In our
experiments, it took on average 13 ms to get an answer from
the closest peer, and 20 ms from the peer two-hops away.
With larger multi-hop and multi-path ad-hoc networks this
delay grows exponentially. Still, we expect the delay to remain reasonable even in large networks.
caused by the radio interference and background trafraioServiocaused
Sevieavailability depends naturally
on
the packet loss
by the radio interference and background
traf-
fic. Periodic retransmissions are used to solve the packet
loss. The retransmission procedure per node could be based
dynamically on an estimate of the link quality and background load, i.e., interface load.
5. Summary and Conclusions
This paper showed efficient service location for wireless
ad-hoc networks. AODV was extended to carry SLP service
requests, and the SIP stack made use of SLP in discovering
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