An overview of mac802_11.cc mac802_11.h

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An overview of mac-802_11.ccmac-802_11.h
Packet Format

• Sarthak Grover

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Introduction

• In ns, two MAC layer protocols are implemented
  for mobile networks
• 802.11
• TDMA

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Overview

• The 802.11 is implemented using
  ns/mac-802_11.cc, ns/mac-802_11.h
• The MAC 802.11 doesn't try to retransmit a
  broadcast packet incase there is a collision and
  the packet is dropped

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Overview

• Coupled to this was the fact that mac-802.11
  didn't do random selection of slots in the
  contention window before it transmitted a packet.
  
• As a result there were a high number of
  collisions at the MAC layer and a lot of packets
  were lost. This was fixed by adding random
  selection of slots before MAC tx'ed a broadcast
  packet.

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mac-802_11.h

• Header file to assist the mac-802_11.cc
• Includes the following

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Other header files included

• ns_mac_80211_h
• address.h
• ip.h
• mac-timers.h
• marshall.h

To support even tracing
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Frame formats(mac-802_11.h)

• define MAC_Type_Management
• define MAC_Type_Data
• //Packet type
• define MAC_Subtype_RTS
• define MAC_Subtype_CTS
• define MAC_Subtype_ACK
• define MAC_Subtype_Data
• // BSS type
• define BSS_Infrastructure
• define BSS_Adhoc

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Frame formats(mac-802_11.h)

• For each of the subtypes, there are structures
  defined
• struct rts_frame
• struct frame_control rf_fc
• u_int16_t cf_duration
• u_char rf_raETHER_ADDR_LEN
• u_char rf_taETHER_ADDR_LEN
• u_char rf_fcsETHER_FCS_LEN

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Header Lengths(mac-802_11.h)

• In this, the lengths of headers for PLCP,
  Ethernet, RTS, CTS and ACK are defined
• define PLCP_HRD_LEN
• ((phymib_-gtPreambleLength gtgt 3)
• (phymib_-gtPLCPHeaderLength gtgt 3)
• define ETHER_RTS_LEN
• (PLCP_HDR_LEN
• sizeof(struct rts_frame))

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DSSS Parameters(mac-802_11.h)

• Parameters specific to DSSS are specified here
• define DSSS_CWmin 31
• define DSSS_CWMax 1023
• define DSSS_SlotTime 0.000020
• define DSSS_PreambleLength 144
• define DSSS_PLCPHeaderLength 48
• define DSSS_PLCPDataRate 1.0e6

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Default Values for MAC Attributes(mac-802_11.h)

• define MAC_FragmentationThreshold 2346
• define MAC_MaxTransmitMSDULifetime 512
• define MAC_MaxReceiveLifetime 512
• class MAC_MIB
• public
• MAC_MIB(Mac802_11parent)
• private
• u_int32_t RTSThreshold
• u_int32_t ShortRetryLimit
• u_int32_t LongRetryLimit
• protected
• u_int32_t Failedcount
• u_int32_t RTSSuccessCount
• u_int32_t RTSFailureCount

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802.11 MAC class(mac-802_11.h)

• public methods and definitions
• void recv(Packet p, Handler h)
• void trace_event(char , Packet )
• EventTrace et_
• inline int hdr_dst(char hdr, int dst -2)
• inline int hdr_src(char hdr, int src -2)
• inline int hdr_type(char hdr, u_int16_t type
  0)
• //To identify the BSS
• inline int bss_id() return bss_id

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802.11 MAC class(mac-802_11.h)

• Protected methods
• void backoffHandler(void)
• void deferHandler(void)
• void beaconHandler(void)
• void navHandler(void)
• void recvHandler(void)
• void sendHandler(void)
• void txHandler(void)

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802.11 MAC class(mac-802_11.h)

• Private methods
• //Called by the timers
• void recv_timer(void)
• void send_timer(void)
• int check_pktCTRL()
• int check_pktRTS()
• int check_pktTx()

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802.11 MAC class(mac-802_11.h)

• Private methods (contd.)
• //Packet Transmission Functions
• void send(Packet p, Handler h)
• void sendRTS(int dst)
• void sendCTS(int dst, double duration)
• void sendACK(int dst)
• void sendDATA(Packet p)
• void RetransmitRTS()
• void RetransmitDATA()

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802.11 MAC class(mac-802_11.h)

• Private methods (contd.)
• //Packet Transmission Functions
• void recvRTS(Packet p)
• void recvCTS(Packet p)
• void recvACK(Packet p)
• void recvDATA(Packet p)

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Internal MAC State(mac-802_11.h)

• Defines Network allocation vector, incoming
  state, outgoing state and if the transmitter is
  active or not
• double nav_ //Network Allocation Vector
• MacState rx_state //incoming state
• MacState tx_state //outgoing state
• int tx_active_ //Transmitter is active
• u_int32_t cw_ //Contention Window

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Duplicate Detection State(mac-802_11.h)

• u_int16_t sta_seqno
• int cache_node_count_
• Host cache_

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Header Files(mac-802_11.cc)

• delay.h
• connector.h
• packet.h
• random.h
• mobilenode.h
• arp.h
• 11.h
• mac.h
• mac-timers.h
• mac-802_11.h
• cmu-trace.h
• agent.h
• basetrace.h

To support even tracing
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Global Variables(mac-802_11.cc)

• DSSS parameters are defined here
• DSSS_CWMin
• DSSS_CWMax
• DSSS_SlotTime
• DSSS_CCATime
• DSSS_RxTxTurnaroundTime
• DSSS_SIFSTime
• DSSS_PreambleLength
• DSSS_PLCPHeaderLength
• DSSS_PLCPDataRate

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Outgoing packet routines(mac-802_11.cc)

• Checks if the medium is idle or not, if it is not
  idle, it makes sure it doesnt send CTS
• case MAC_Subtype_CTS
• if(!is_idle())
• discard(pktCTRL_, DROP_MAC_BUSY) pktCTRL_
  0
• return 0

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Outgoing packet routines(mac-802_11.cc)

• Timeout is then calculated by adding CTS tx time
  to the CTS duration and subtracting ACK tx time
• timeout txtime(phymib_.getCTSlen(),
  basicRate_)
• DSSS_MaxPropagationDelay
  
• sec(mh-gtdh_duration)
• DSSS_MaxPropagationDelay
  
• - phymib_.getSIFS()
• - txtime(phymib_.getACKlen
  (), basicRate_)

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Outgoing packet routines(mac-802_11.cc)

• Acknowledgements are sent after an SIFS, without
  regard to the busy/idle state of the medium
• case MAC_Subtype_ACK
• setTxState(MAC_ACK)
• timeout txtime(phymib_.getACKlen(),
  basicRate_)
• break

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Packet Format (packet.h)

• Enum defines the various packet types which
  include and are not limited to TCP, UDP, CBR,
  ACK, Audio, Video, Prune, Stop, Accept, Confirm,
  FTP, Inval etc
• enum packet_t
• PT_TCP,
• PT_UDP,
• PT_CBR,
• PT_AUDIO,
• PT_VIDEO,
• PT_ACK,
• PT_START,
• PT_STOP,

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Packet Format (Packet.h)

• Each packet contains a packet type field used by
  Traceformat to print out the type of packet
  encountered.
• The type field is defined in the TraceHeader
  class, and is considered to be part of the trace
  support it is not interpreted elsewhere in the
  simulator.

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Packet Format (Packet.h)

• Initialization of the type field in packets is
  performed by the Agentallocpkt(void) method.
• The type field is set to integer values
  associated with the definition passed to the
  Agent constructor

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Class Packet Info (Packet.h)

• This class is used as a glue to bind numeric
  packet type values with their symbolic names.
• When a new packet type is defined, its numeric
  code should be added to the enumeration packet_t
• class p_info
• public
• p_info()
• name_PT_TCP "tcp"
• name_PT_UDP "udp"
• name_PT_CBR "cbr"
• name_PT_AUDIO "audio"
• name_PT_VIDEO "video"

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Class Packet Info (Packet.h)

• Boolean type of data packets are also added
  within this class
• static bool data_packet(packet_t type)
• return ( (type) PT_TCP \
• (type) PT_TELNET \
• (type) PT_CBR \
• (type) PT_AUDIO \
• (type) PT_VIDEO \
• (type) PT_ACK \
• (type) PT_SCTP \
• (type) PT_SCTP_APP1 \
• )

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Class Packet Event (Packet.h)

• This class holds a pointer to a generic array of
  unsigned characters called BOB (Bag of Bits)
  where packet header fields are stored
• Private, protected and public variables are
  defined here
• These initialize the packet header, length of
  buffer, the queues and packet free list
• Also provide backward compatibility i.e assuming
  user data is PacketData and return to its pointer

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Class Packet Event (Packet.h)

• Private
• unsigned char bits_ // header bits
• AppData data_ // variable size buffer for
  'data'
• static void init(Packet) // initialize pkt
  hdr
• bool fflag_
• Protected
• static Packet free_ // packet free list
• int ref_count_ // free the pkt until count to 0

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Class Packet Event (Packet.h)

• Public
• Packet next_ // for queues and the free list
• static int hdrlen_
• inline unsigned char const bits()return
  (bits_)
• inline Packet copy() const
• inline Packet refcopy() ref_count_ return
  this
• inline int ref_count() return (ref_count_)
• static inline Packet alloc()
• static inline Packet alloc(int)
• inline void allocdata(int)
• inline void initdata() data_ 0
• static inline void free(Packet)

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Class Packet Event (Packet.h)

• Backward Compatibility
• inline unsigned char accessdata() const
• if (data_ 0)
• return 0
• assert(data_-gttype() PACKET_DATA)
• return (((PacketData)data_)-gtdata())

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Class Packet Event (Packet.h)

• PacketStamp txinfo_
• The pkt stamp carries all info about how/where
  the pkt was sent needed for a receiver to
  determine if it correctly receives the pkt
• u_int8_t incoming
• This flag is set by the MAC layer on an incoming
  packet and is cleared by the link layer

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hdr_cmn class(Packet.h)

• Every packet in the simulator has a common
  header which is defined in ns/packet.h as
  follows
• struct hdr_cmn
• double ts_ / timestamp for q-delay measurement
  /
• packet_t ptype_ / packet type (see above) /
• int uid_ / unique id /
• int size_ / simulated packet size /
• int iface_ / receiving interface (label) /

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hdr_cmn class(Packet.h)

• / Packet header access functions /
• static int offset_
• inline static int offset() return offset_
• inline static hdr_cmn access(Packet p)
• return (hdr_cmn) p-gtaccess(offset_)
• / Per-field member functions /
• int ptype() return (ptype_)
• int uid() return (uid_)
• int size() return (size_)
• int iface() return (iface_)
• double timestamp() return (ts_)

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hdr_cmn class(Packet.h)

• This structure primarily defines fields used for
  tracing the flow of packets or measuring other
  quantities.
• The time stamp field is used to measure queuing
  delay at switch nodes.
• The ptype_ field is used to identify the type of
  packets, which makes reading traces simpler.
• The uid_ field is used by the scheduler in
  scheduling packet arrivals.
• The size_ field is of generaluse and gives the
  simulated packets size in bytes.
• The iface_ field is used by the simulator when
  performing multicast distribution tree
  computations. It is a label indicating
• (typically) on which link a packet was received.

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Allocation and Deallocation (Packets.h)

• The other methods of Packet are for creating new
  packets and storing old ones in a private free
  list.
• Such allocation and deallocation is performed by
• inline Packet Packetalloc()
• inline Packet Packetalloc(int n)
• inline void Packetfree(Packet p)
• inline void Packetallocdata(int n)

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Commands at a glance

• Following are a few of packet-header related
  procedures
• ns_ create_packetformat
• This is an internal simulator procedure and is
  called once during the simulator configuration to
  setup a packetHeaderManager object.
• pktHdrManager allochdr
• This is another internal procedure of Class
  PacketHeaderManager that keeps track of a
  variable called hdrlen_ as new packet-headers are
  enabled. It also allows 8-byte allignment for any
  newly-enabled pkt header.