T H E S E C U R E T R A N S M I S S I O N P R O T O C O L O F S E N S O R A D H O C N E T W O R K

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1 Z E S Z Y T Y N A U K O W E A K A D E M I I M A R Y N A R K I W O J E N N E J S C I E N T I F I C J O U R N A L O F P O L I S H N A V A L A C A D E M Y 2015 (LVI) 4 (203) A n d r z e j M r c z k DOI: / X T H E S E C U R E T R A N S M I S S I O N P R O T O C O L O F S E N S O R A D H O C N E T W O R K ABSTRACT The pper presents secure protocol of rdio Ad Hoc sensor network. This network opertes sed on TDMA multiple ccess method. Trnsmission rte on the rdio chnnel is 57.6 kps. The pper presents the construction of frmes, types of pckets nd procedures for the uthentiction, ssignment of time slots ville to the node, relesing ssigned slots nd slots ssignment conflict detection. Key words: Ad Hoc, ASAP, OFB Mode, TDMA. INTRODUCTION The Time Division Multiple Access (TDMA) method is often used in Ad Hoc sensor networks ecuse of its ility to ensure collision-free trnsmission of pckets regrdless of the mount of trffic on the network. Mny types of trnsmission scheduling protocols re used in TDMA Ad Hoc networks. Some of them do not support utonomous ehviors of moile nodes. They cnnot updte the slot ssignment of ech node due to rrivl or exit of moile node [2]. Unifying the Slot Assignment Protocol (USAP) [4] nd USAP-MA [5] Protocol, llows the opertion of networks whose topology dynmiclly chnges. However, they re chrcterized y poor chnnel utiliztion ecuse of the existence of mny conflicting or unssigned slots. Gdńsk University of Technology, Fculty of Electronics, Telecommunictions nd Informtics, G. Nrutowicz 11/12 Str., Gdńsk, Polnd; e-mil: mrczk@eti.pg.gd.pl 27

2 Andrzej Mrczk The sensor Ad Hoc network protocol, presented in this pper, uses the TDMA method nd the Adptive Slot Assignment Protocol (ASAP) protocol [1]. The ASAP protocol ws chosen ecuse of its ese of implementtion in hrdwre nd good properties [1]. This protocol is enhnced with uthentiction nd encryption procedures. TDMA FRAMES STRUCTURE A sensor network consists of Server, Personl Identifiction Module nodes (PIM) nd Reference Node nodes (RN) (fig. 1). The hierrchy level of RN indictes the numer of rdio hops to the server. Hierrchy level 0 mens tht the RN is connected vi wired connection to the server. Network nodes trnsmit seven types of pckets: dt pcket DATA; request pcket REQ; informtion pcket INF; hierrchy level pcket LEVEL; suggestion pcket SUG; reply pcket REP; uthentiction pcket AUTH. Wired connection Wireless connections Wireless connections PIM Server Reference Node Level 0 PIM Reference Node Level1 PIM PIM Fig. 1. Secure sensor network Pckets re trnsmitted in the frmes. The frmes re orgnized into superfrmes. The primry (shortest length) superfrme hs 4 time slots. The durtion of the superfrme is 20 ms. The ASAP protocol llows the use of long superfrmes. The lengths 28 Zeszyty Nukowe AMW Scientific Journl of PNA

3 The secure trnsmission protocol of sensor Ad Hoc network of such the superfrmes re multiples of the primry superfrme. The length of the long superfrme is set s power of two. The superfrmes cn e composed of 8, 16, 32 nd 64 time slots, respectively. Such superfrme durtions will e then of 40 ms, 80 ms, 160 ms nd 320 ms. Trnsmission rte t the rdio interfce is 57.6 kps, so the durtion of the 1 it is t = s, nd the durtion of 1 yte is t B = 8 x t = s. The durtion of the 33-yte dt pcket (DATA) t r = ms. The durtion of the time slot t s = 5 ms, so gurd intervls hve 2 x ms ( s), or durtion of 3 ytes. In the cse of pcket types with fewer ytes, we used the ddition of pproprite numer of zero ytes (0 x 00) to lign the pcket length. Figure 2 shows the frme nd superfrme in sensor network. The first slot in the superfrme (ZPS) hs een reserved for the new node to trnsmit control Request pckets (REQ) or uthentiction pcket (AUTH). Superfrme ZPS ts tsr Frme Premle 4 ytes Synchron. 4 ytes Kind of frme 1 yte Sender ddr. 2 ytes Receiv. Addr. 2 ytes Type of frme 4 its Frme length 1 yte Mx. frme length 1 yte Slot numer 1 yte Numer of slots 4 its Timestmp 3 ytes Dt 12 ytes CRC 2 ytes tr Fig. 2. Frme nd superfrme in secure sensor network This wy, no dt pckets (DATA) re trnsmitted in this time slot. The dt pckets cn e trnsmitted over the remining three time slots (for the superfrme with length L = 4). The sme is true in the cse of superfrmes with greter numer of time slots. The first time slot is lwys ZPS, used for the REQ or AUTH pckets to e trnsmitted, nd the remining slots re used for dt trnsmission. Ech pcket type hs fixed prt depending on the structure of the pckets sent y used the rdio modems. This fixed prt hs 4 yte premle, 4-yte synchroniztion nd 1 yte the kind of frme informtion. The PIM node ddresses will hve vlues etween 0 (0 x 00) to 127 (0 x 7F), while RN will hve ddress vlues from 128 (0 x 80) to 254 (0 x FE). Address with vlue 255 (0 x FF) is for rodcst trnsmission. In the cse of trnsmission towrds the server in pcket ddress 4 (203)

4 Andrzej Mrczk fields will e the source ddress (PIM or RN) nd the ddress of the next RN node (the destintion node). The RN node receiving the pcket contining its ddress s the trget, it checks its routing tle to the next RN node ddress in the direction of the server nd forwrds the received pcket in different time slot. In the cse of trnsmission from the server, the source ddress is the ddress of RN node sending the pcket nd the destintion ddress is the ddress PIM or RN node (if the pcket is sent to the RN node). PACKET FORMATS T r n s m i t m o d e The dt pcket (DATA) (0 x 0D) is the first type of pckets. It contins informtion on the frme length nd time slots ssigned to the sender, nd the mximum frme length of the sender nd its neighors [1]. This pcket lso contins the encrypted dt sent y node. C o n t r o l m o d e 1. Request pcket (REQ) (0 x 0C) is trnsmitted y only new node. By sending this pcket to neighors, new node requests the informtion on the frme length nd ssigned time slots of ll nodes in contention re [1]. 2. Informtion pcket (INF) (0 x 0B) contins the informtion on the frme length of the sender nd time slot ssigned to the sender nd its neighors [1]. 3. Hierrchy level pcket (LEVEL) (0 x 6) is trnsmitted periodiclly y RN node in the ZPS time slot. The RN node is sending the pcket to its neighors, informing ll nodes out its network hierrchy level. 4. Suggestion pcket (SUG) (0 x 0A) is trnsmitted y only new node. By sending this pcket to the neighors, the new node nnounces the frme length nd its ssigned slot [1]. 5. Reply pcket (REP) (0 x 05) is trnsmitted for the confirmtion of receiving SUG pcket [1]. 6. Authentiction pcket (AUTH) (0 x 01) is used in the uthentiction procedure. This pcket sends n encrypted node ddress nd the encrypted session key (in 2 consecutive pckets). 30 Zeszyty Nukowe AMW Scientific Journl of PNA

5 The secure trnsmission protocol of sensor Ad Hoc network DATA SECURITY The security of the trnsmission is ensured y the use of the lock cipher lgorithm (e.g. AES, 3DES) working in Output Feedck (OFB) mode (fig. 3). The OFB mode uses the initiliztion vector IV. In this solution IV vector is timestmp. Its uniqueness is criticl. The ciphertext is otined y the modulo 2 ddition of the plintext its (P) nd lock cipher output its (O). Block cipher output its only depend on the cipher lgorithm, session key K nd initiliztion vector IV (timestmp) [3]. One dvntge of the OFB mode is low sensitivity to trnsmission errors, nd more specificlly the lck of error propgtion [3]. Using the OFB mode we cn encrypt dt locks of ny length, even shorter thn the length of the dt lock used in encryption lgorithm (e.g. 128 its in AES lgorithm). Ciphering OFB Deciphering OFB Session key K Plintext Timestmp IV Block cipher O Session key K Ciphertext Timestmp Block cipher O IV P C C P Ciphertext Plintext Fig. 3. Ciphering nd deciphering in sensor network AUTHENTICATION PROCEDURE The OFB mode is used in the procedure of uthentiction nodes (PIM, RN), nd to encrypt dt trnsmitted in the DATA frmes. The uthentiction procedure tkes plce fter the new node determines the first time slot (ZPS) in the superfrme. All network nodes keep in the memory pir of numers (8-it ddress) nd the mster key (128 its)). The sme pir of numers is stored in the server, which cts s Key Distriution Center (KDC). Only node whose dt (ddress nd mster key) re stored in the memory server cn connect to the network. Authentiction is performed fter the connection to the KDC server. The trnsmission ssocited with the uthentiction is performed in the ZPS time slot. The s slot ssignment procedure to the new node occurs only fter successful uthentiction. The result of 4 (203)

6 Andrzej Mrczk uthentiction procedure is to provide the RN or PIM the session key K, which is necessry for the exchnge of informtion with the server. The session key is the sme for ll nodes in the network. Its vlidity cn rnge from few to severl hours. The length of the session key depends on the encryption lgorithm. For the AES lgorithm key length is equl 128 its. The uthentiction procedure consists of four steps: 1. The node tht wnts to connect to the network trnsmits in ZPS time slot, uthentiction pcket (AUTH) contining his encrypted ddress. The ddress is encrypted using mster key. 2. The server, sed on the node ddress, serches in its memory the mster key nd decrypts the encrypted ddress. Then, it compres the two ddresses (the decrypted ddress nd the ddress sent without encryption in the ddress field of the frme). 3. If the comprison result is positive, the server encrypts 128-it session key K using the mster key nd sends it to the uthenticted node lso in the ZPS time slot. Negtive comprison result ends the uthentiction procedure. The server sends one AUTH pcket contining zero. 4. The node receives the encrypted session key K nd decrypts it. Since then, ll trnsmitted dt is secured. From tht moment, the entire trnsmission is secure. In the cse of negtive uthentiction, fter receiving the AUTH pcket with the content zero, the node, fter few superfrmes, my initite re-uthentiction procedure. TIME SLOT ASSIGNMENT PROCEDURE The time slot ssignment procedure is performed fter successful uthentiction of the new node. All RN nodes, pssing AUTH pcket with messge out the negtive uthentiction (1 pcket contining zero 8 ytes of 0 x 00) know tht the node is unuthenticted nd cnnot compete for ccess to the chnnel. After successful uthentiction the new node selects time slot ssigned to itself in four steps. R e q u e s t i n g t h e i n f o r m t i o n o n t i m e s l o t s s i g n m e n t i n t h e c o n t e n t i o n r e When new node joins the network, it does not know the informtion on network topology or time slots ssigned to other nodes in its contention re. To get this informtion, the new node listens to the chnnel nd checks pckets trnsmitted from the neighors. DATA pckets from neighors contin the informtion on their 32 Zeszyty Nukowe AMW Scientific Journl of PNA

7 The secure trnsmission protocol of sensor Ad Hoc network ssigned slots, superfrme length, nd mximum superfrme length. From these pieces of informtion, the new node knows the position of the first time slot in superfrme nd mximum superfrme length mong ll nodes in its contention re. Then the new node sends REQ pcket (0 x 0 C) in the first time slot of the next superfrme. Neighors tht hve received the REQ pcket trnsmitted from the new node, trnsit to the control mode. Ech neighor of the new node gives informtion in its superfrme length nd time slot ssigned to itself nd its neighors y trnsmitting n INF pcket (0 x 0 B) in its ssigned time slot. After ll neighors of the new node hve trnsmitted INF pckets, ll nodes in the contention re of the new node cn know its structure [1]. S e t t i n g t h e s u p e r f r m e l e n g t h n d t i m e s l o t s s i g n m e n t After receiving INF pckets from ll neighors, the new node sets its superfrme length. If ll nodes in its contention re hve the sme superfrme length, the new node sets its own superfrme length to this length. Otherwise, the new node uses the mximum superfrme length mong ll nodes in the contention re. Then, from received INF pckets, the new node knows the informtion on slot ssignment in this contention re. The new node cretes its own time slot ssignment informtion of superfrme length, S 0 where S 0 denotes the frme length tht is set to the new node. If the superfrme length of neighor is sme s S 0, the time slot ssignment informtion of the neighor is copied to tht of the new node. Otherwise, if S 0 = S i, the time slot ssignment informtion of the neighor is copied repetedly to every S 0/ slots. S i is the superfrme length of the neighor nd is n integer of power of two. The new node merges the informtion from ll neighors nd cretes its own time slot ssignment informtion [1]. For exmple, when the new node sets its superfrme length s 8, the time slot ssignment informtion in the INF pcket received from node whose superfrme length is 4 nd ssigned slot is 2 is copied repetedly to every 4 time slots in tht of the new node (fig. 4) [1]. (0) (1) (2) (3) (0) (1) (2) (3) (4) (5) (6) (7) 4 time slots 4 time slots Fig. 4. Copying informtion out node [1] 4 (203)

8 Andrzej Mrczk S e l e c t i n g n s s i g n e d t i m e s l o t Bsed on the time slot ssignment informtion, the new node selects time slot ssigned to itself y three procedures: Getting n unssigned slot (GU) [1]. If some unssigned time slots re found in the time slot ssignment informtion, the new node ssigns one of them to itself. For exmple when unssigned time slots 3 nd 7 re found, the new node cn ssign time slot either 3 or 7 to itself (fig. 5) [1]. (0) (1) (2) (3) (4) c (5) (6) (7) Unssigned time slots Fig. 5. Getting unssigned time slots [1] Relesing multiple ssigned time slots (RMA) [1]. If no unssigned time slot is found, the new node checks whether some nodes in the contention re re ssigned multiple time slots. If such node is found, the new node releses one of these time slots nd ssigns it to itself. If there re more thn one node to which multiple time slots re ssigned, the node with the lrgest numer of ssigned time slots mong them is chosen to relese time slot [1]. For exmple, when node nd c re ssigned multiple time slots, the new node selects time slot from time slots 1, 3, 6 nd 7 which re ssigned to nodes nd c, nd ssigns the selected time slot to itself (fig. 6) [1]. (0) (1) (2) (3) c (4) d (5) e (6) (7) c Node c d e Time slot 1, 6 2 3, Douling the frme (DF) [1]. Multiple time slots Fig. 6. Relesing multiple ssigned time slots [1] If no unssigned time slot is found nd no node hs multiple ssigned slots which re le to e ssigned to the new node, the new node doules the superfrme 34 Zeszyty Nukowe AMW Scientific Journl of PNA

9 The secure trnsmission protocol of sensor Ad Hoc network length of the slot ssignment informtion nd copies the ssignment informtion to oth of the former hlf nd the ltter hlf of douled superfrme. The first time slot in the superfrme is not ssigned to ny nodes. Therefore fter douling the superfrme length, the first time slot in the ltter hlf ecomes unssigned slot. The new node ssigns this time slot to itself [1]. For exmple when the new node doules the superfrme length, slot 8 cn e ssigned to itself (fig. 7) [1]. (0) (1) (2) (3) d (4) c (5) f (6) g (7) e (0) (1) (2) (3) d (4) c (5) f (6) g (7) e (8) (9) (10) (11) d (12) c (13) f (14) g (15) e Unssigned time slot Fig. 7. Douling the superfrme [1] A n n o u n c e m e n t o f u p d t i n g t h e t i m e s l o t s s i g n m e n t i n f o r m t i o n After selecting time slot ssignment, the new node in network sends SUG pcket to its neighors. The SUG pcket contins informtion on the superfrme length nd the ssigned time slot. When the neighoring nodes receive this pcket, they updte time slot ssignment informtion. After updting the informtion sed on the received SUG pcket, ech neighoring node sends REP pcket to its neighoring nodes. Sending this pcket implies the confirmtion of the SUG pcket for the new node nd nnouncement of updting the time slot ssignment informtion nd exiting from the control mode. The sender nd receivers of the REP pcket dopt the new time slot ssignment nd cn restrt dt trnsmission from the next superfrme. The new node, fter receiving the REP pckets from ll neighoring nodes, trnsits to the trnsmit mode [1]. DETECTION OF CONFLICT In the protocol, conflict of slot ssignment occurs when new node connects to two or more nodes to which the sme time slots re ssigned. In the exmple (fig. 8), conflict occurs t new node etween node c nd node f in time slot 5. 4 (203)

10 Andrzej Mrczk 3 1 d 2 e 3 4 g c f 5 5 New node Fig. 8. Exmple of conflict n ssigned time slot [1] When new node detects the conflict, it solves this conflict in the following procedure. D i v i d i n g t h e s s i g n m e n t If multiple slots re conflicting t the new node, these slots re divided to the nodes which hve cused the conflict. In the exmple (fig. 9), conflicting slot 4 nd 12 re divided to nodes nd [1]. 4, 12 4, 12 Divide time slots 4 12 Fig. 9. Dividing the ssignment of time slots [1] D e l e t i n g c o n f l i c t i n g s l o t If in the network re some un-conflicting slots ssigned to nodes cusing the conflict, the conflicting slot is relesed from ll the nodes except for tht with the smllest numer of ssigned slots (fig. 10) [1]. 4, 8, 16 4, 12 Delete time slot 4 from node 8, 16 4, 12 Fig. 10. Deleting the conflicting time slot [1] 36 Zeszyty Nukowe AMW Scientific Journl of PNA

11 The secure trnsmission protocol of sensor Ad Hoc network D o u l i n g t h e s u p e r f r m e n d d i v i d i n g t h e s s i g n m e n t If the conflict occurs mong nodes to which only one time slot is ssigned, this conflict cnnot e solved with the current superfrme length. In this cse, the superfrme length of these nodes is douled nd the time slot ssignment is divided in the douled superfrme. In exmple (fig. 11) the spce for conflicting time slot is douled y douling superfrme length. The spce cn e divided to nodes nd [1]. Superfrme length Doule superfrme length 2, 6 Superfrme length 8 2, 6 Superfrme length 8 Divide time slots 2 6 Fig. 11. Douling the superfrme nd dividing the ssignment [1] After reconfiguring the time slot ssignment, the new node sends SUG pcket with the informtion on the reconfigured slot ssignment nd the selected slot. Neighoring nodes which hve received this SUG pcket lso reconfigure their slot ssignment nd send REP pckets with reconfigured informtion. The new node my fil to collect the informtion on the slot ssignment correctly due to the collisions of INF pckets. Then the new node sends the informtion on the slot in which collisions hve occurred to ll neighoring nodes insted of the SUG pcket. Neighoring nodes of the new node, which hve sent the INF pckets in the conflicting time slot, retrnsmit the INF pckets fter witing for certin superfrmes determined t rndom [1]. RELEASING TIME SLOT ASSIGNMENT When node exits from the network, it stops trnsmitting DATA pckets nd releses time slots ssigned to itself. Neighoring nodes detect the exit of the node when no pckets from exited node hve een received during the time of the superfrme length of the exited node. Then they relese the time slot ssigned to 4 (203)

12 Andrzej Mrczk the exited node from their time slot ssignment informtion. They lso relese the time slots ssigned to nodes tht hve gone out of their contention re due to exit of the node. After reconfiguring the time slot ssignment, neighoring nodes of the exited node send the updted informtion to their neighoring nodes. The nodes which hve received this informtion reconfigure the time slot ssignment y relesing the time slots ssigned to the exited node [1]. CONCLUSION The pper presents the construction of the secure Ad Hoc sensor network protocol. The opertion of this protocol is sed on the ASAP protocol, whose efficiency mesured in terms of the rdio chnnel utiliztion is much lrger thn the USAP protocol [1]. The protocol descried in this pper hs een extended with dditionl functions relted to dt security (uthentiction of new nodes nd encryption of dt trnsmission), nd the determintion of the network hierrchy level of the reference nodes. Two new types of pckets hve een proposed: the uthentiction pcket (AUTH) nd the hierrchy level pcket (LEVEL). The protocol is currently implemented on the hrdwre devices tht will form the sensor network. Acknoledgements This work ws supported in prt y the Project DOBR-BIO4/058/13045/2013. REFERENCES [1] Knzki A., Uemuki T., Hr T., Nishio S., Dynmic TDMA Slot Assignment in Ad Hoc Networks, Proceedings of the 17th Interntionl Conference on Advnced Informtion Networking nd Applictions (AINA 03), Mr. 2003, pp [2] Knzki A., Hr T., Nishio S., An Adptive TDMA Slot Assignment in Ad Hoc Sensor Networks, Proceedings of the 2005 ACM Symposium on Applied computing (SAC 05), pp [3] Stllings W., Kryptogrfi i ezpieczeństwo sieci komputerowych. Mtemtyk szyfrów i techniki kryptologii, trnsltion A. Grżyński, Ed. Helion, Gliwice 2012 [Cryptogrphy nd Network Security. Principles nd Prctice originl title]]. [4] Young C. D., USAP: unifying dynmic distriuted multichnnel TDMA slot ssignment protocol, Proc. IEEE MILCOM, 1996, Vol. 1, pp [5] Young C. D., USAP multiple ccess: dynmic resource lloction for moile multihop multichnnel wireless networking, Proc. IEEE MILCOM, 1999, Vol. 1, pp Zeszyty Nukowe AMW Scientific Journl of PNA

13 The secure trnsmission protocol of sensor Ad Hoc network B E Z P I E C Z N Y P R O T O K Ó Ł T R A N S M I S J I S E N S O R O W E J S I E C I A D H O C STRESZCZENIE W rtykule zprezentowno ezpieczny protokół rdiowej sieci sensorowej Ad Hoc. Sieć t prcuje w oprciu o metodę wielodostępu TDMA. Szykość trnsmisji w knle rdiowym wynosi 57,6 k/s. Przedstwiono udowę rmek, rodzje pkietów orz procedury uwierzytelnini, przypisni wolnych szczelin czsowych do węzł, zwlnini przypisnych szczelin i wykrywni konfliktów przyporządkowni szczelin. Słow kluczowe: Ad Hoc, TDMA, ASAP. 4 (203)