Principles of Ad Hoc Networking Michel Barbeau and Evangelos Kranakis January 20, 2009

CHAPTER 4 - WIRELESS NETWORK PROGRAMMING

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Section 4.1 - Structure of information • Address representation

• ifconfig command

• iwconfig command

• Frame structure

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Address representation

• Types: unicast, multicast and broadcast

• Six hexadecimal values, between 00 and FF

• Example: 00:60:1D:1E:31:18

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Address representation, in C++ #define WLAN_ADDR_LEN 6 struct WLANAddr { unsigned char data[WLAN_ADDR_LEN]; char * wlan2asc(); Outcome str2wlan(char * s); };

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Configuration data: ifconfig # ifconfig eth0 Link encap:Ethernet HWaddr 00:60:1D:1E:31:18 inet addr:134.117.5.12 Bcast:134.117.5.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:5786 errors:0 dropped:0 overruns:0 frame:0 TX packets:352 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:100 RX bytes:870844 (850.4 Kb) TX bytes:52664 (51.4 Kb) Interrupt:10 Base address:0x100

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Configuration data: iwconfig # iwconfig eth0 IEEE 802.11-DS ESSID:"ENTRUSTW" Nickname:"localhost" Mode:Managed Frequency:2.422GHz Access Point: 00:60:1D:F1:F0:27 Bit Rate:11Mb/s Tx-Power=15 dBm Sensitivity:1/3 Retry limit:4 RTS thr:off Fragment thr:off Encryption key:off Power Management:off Link Quality:22/92 Signal level:-70 dBm Noise level:-92 dBm Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:6 Tx excessive retries:0 Invalid misc:0 Missed beacon:0

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Channels sets

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RF parameters

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Range parameters

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Signal strength, sensitivity and range

weak

-94

-91

-87

-83

strong

Receive signal strength (dBm)

1/109.4

1/109.1

1/108.7

1/108.3

Receive signal strength (milli Watt)

1

2

5.5

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Data rate (Mbps) BER 10-5 11

Distance (m) vs rate (Mbps) 600 500 400 300 200 100 0 1

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5.5

11

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Reconfiguration in the ad hoc mode iwconfig iwconfig iwconfig iwconfig

eth0 eth0 eth0 eth0

essid mynet mode Ad-hoc channel 3 rate auto

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Format of WiFi/802.11 frames

header Destination Address

Source Address

Type

Body 0 - 2312 bytes

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Header in C++ struct WLANHeader { WLANAddr destAddr; WLANAddr srcAddr; unsigned short type; }; #define WLAN_HEADER_LEN 14 #define IP_TYPE x0800

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Section 4.2 - Packet socket • Ifconfig structure

• socket() system call

• close() system call

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Parameters of packet socket struct Ifconfig ifconfig

struct sockaddr_ll sll

ifindex

sll_family

sockid

Process level Kernel level

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1

mtu

1

Extracted from interface table using ioctl()

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Created using socket()

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Promiscuous mode set using setsockopt()

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Relationship created using bind()

sll_ifindex

2

hwaddr

1

sll_protocol

Interface table ...

Data link Socket (Packet)

eth0 4

...

True

3

...

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Configuration parameters struct Ifconfig { int sockid; int ifindex; WLANAddr hwaddr; int mtu; }; Ifconfig ifconfig;

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Creation and release packet_socket = socket(PF_PACKET, int socket_type, int protocol); ifconfig.sockid = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL)) close(ifconfig.sockid);

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Section 4.3 - Parameters and control • ioctl() system call • ifreq structure • setsockopt() system call • packet mreq structure • bind() system call • sockaddr ll structure 20

Sender and receiver interaction

Sender Receiver

Data Ack Time

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Sender

Data

Receiver

Ack

Persistence

Time

non persistent

Data Data

p-persistent 1-persistent

Data Data

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Wireless interface parameters: ioctl() system call // signature int ioctl(int d, int request, ...); ... // system call char device[] = "eth0"; // name of interface // input parameter struct ifreq ifr; strcpy(ifr.ifr_name, device); ioctl(ifconfig.sockid, SIOGIFINDEX, &ifr); // fetch index of wireless interface ifconfig.ifindex = ifr.ifr_ifindex; 23

ioctl() fetches the hardware address and MTU ioctl(ifconfig.sockid, SIOCGIFHWADDR, &ifr); ... ioctl(ifconfig.sockid, SIOCGIFMTU, &ifr); ifconfig.mtu = ifr.ifr_mtu;

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Socket parameters: setsockopt() system call // set promiscuous mode struct packet_mreq mr; memset(&mr,0,sizeof(mr)); mr.mr_ifindex = ifconfig.ifindex; mr.mr_type = PACKET_MR_PROMISC; setsockopt(ifconfig.sockid, SOL_PACKET, PACKET_ADD_MEMBERSHIP, (char *)&mr, sizeof(mr)); ... // bind wireless interface to socket struct sockaddr_ll sll; memset(&sll, 0, sizeof(sll)); sll.sll_family = AF_PACKET; sll.sll_ifindex = ifconfig.ifindex; sll.sll_protocol = htons(ETH_P_ALL); bind(ifconfig.sockid, (struct sockaddr*)&sll, sizeof(sll)); 25

Section 4.4 - Receiving frames • recvfrom() system call

• sockaddr ll structure

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recvfrom() system call // signature int recvfrom(int s, void *buf, size_t len, int flags, struct sockaddr *from, socklen_t *fromlen); ... // storage for incoming data and source address unsigned char * buff = new unsigned char[ifconfig.mtu]; unsigned int i; struct sockaddr_ll from; socklen_t fromlen = sizeof(struct sockaddr_ll);

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recvfrom() in a while loop while (true) { i = recvfrom(ifconfig.sockid, buff, ifconfig.mtu, 0, (struct sockaddr *) &from, &fromlen); if (i == -1) { printf("cannot receive data: %s\n", strerror(errno)); usleep(10000); } else { break; } } 28

Section 4.5 - Sending frames • sendto() system call

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sendto() system call // signature int sendto(int s, const void *msg, size_t len, int flags, const struct sockaddr *to, socklen_t tolen); ... // send buffer variable # define BUFFSIZE 256 unsigned char buff[BUFFSIZE];

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Header construction // header variable WLANHeader hdr; // destination address in ASCII char * rp = "00:60:1D:F1:F0:27"; // address in binary form WLANAddr daddr; // construction daddr.str2wlan(rp); memmove(&hdr.destAddr, daddr.data, WLAN_ADDR_LEN); memmove(&hdr.srcAddr, ifconfig.hwaddr.data, WLAN_ADDR_LEN); hdr.type = htons(IP_TYPE); memmove(buff, &hdr, WLAN_HEADER_LEN); 31

Body of frame // data char * dp = "This is a short message!"; // load data in frame buffer memmove(buff+WLAN_HEADER_LEN, dp, strlen(dp)); // construct address structure struct sockaddr_ll to; int tolen = sizeof(to); to.sll_family = AF_PACKET; to.sll_ifindex = ifconfig.ifindex; memmove(&(to.sll_addr), daddr.data, WLAN_ADDR_LEN); to.sll_halen = WLAN_ADDR_LEN; 32

Send frame int sentlen = sendto( ifconfig.sockid, buff, WLAN_HEADER_LEN+strlen(dp), 0, (sockaddr *) &to, tolen); if (sentlen == -1 ) { printf("sendto failed\n"); return NOK; } return OK;

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