class Packet : public Event {
private:
unsigned char* bits_; // header bits
AppData* data_; // variable size buffer for 'data'
....
};
不得不说，上面两个字段域是Packet最重要的动动，其中bits_存储包头结构，而data_存储用户自定义的数据。但是，NS2其实是一个大而全的仿真平台，它在仿真时其实是将所有或者大多数数据包放在一个包里面，即使当前用不到。比如我们建立一个仿真环境，用来测试两个节点的通过TCP建立连接，然后发送数据，理论上这个仿真中只涉及到TCP/IP的基本存储功能，即应该包括hdr_mac, hdr_ll, hdr_ip, hdr_cmn, hdr_tcp，但是实际得到的包应该如下：
| hdr_ip | hdr_ll | hdr_cmn | hdr_tcp | hdr_XCP | ........ | data_ |
那如何访问数据包中相应的子包呢，可以查看packet.h开始，发现定义了许多宏，如：
#define HDR_CMN(p) (hdr_cmn::access(p))
#define HDR_ARP(p) (hdr_arp::access(p))
#define HDR_MAC(p) (hdr_mac::access(p))
#define HDR_MAC802_11(p) ((hdr_mac802_11 *)hdr_mac::access(p))
#define HDR_MAC_TDMA(p) ((hdr_mac_tdma *)hdr_mac::access(p))
#define HDR_SMAC(p) ((hdr_smac *)hdr_mac::access(p))
#define HDR_LL(p) (hdr_ll::access(p))
#define HDR_HDLC(p) ((hdr_hdlc *)hdr_ll::access(p))
#define HDR_IP(p) (hdr_ip::access(p))
#define HDR_RTP(p) (hdr_rtp::access(p))
#define HDR_TCP(p) (hdr_tcp::access(p))
#define HDR_SCTP(p) (hdr_sctp::access(p))
#define HDR_SR(p) (hdr_sr::access(p))
#define HDR_TFRC(p) (hdr_tfrc::access(p))
#define HDR_TORA(p) (hdr_tora::access(p))
就是用来访问相应的字段，比如HDR_MAC(p)，即可以取得对应的域。而具体如何取则由hdr_***::access来实现。access是每一种包头都包含的，比如hdr_cmn:
inline static hdr_cmn* access(const Packet* p) {
return (hdr_cmn*) p->access(offset_);
}
再看看包Packet的access函数，如下：
inline unsigned char* access(int off) const {
if (off < 0) abort();
return (&bits_[off]);
}
到此，基本明白了，offset其实是每个包相对于packet开始的偏移值，通过这个偏移值即可确定其在整个包中的地址。
现在问题是，在哪个地方设置这个offset值，搜索了所有的C++代码也没看到对每个包的offset的初始化。经过仔细比较，发现在tcl/lib/ns-lib.tcl中的Simulator初始化init过程中有一个调用：
$self create_packetformat
接着在ns-packet.tcl中找到了这个函数：
Simulator instproc create_packetformat { } {
PacketHeaderManager instvar tab_
set pm [new PacketHeaderManager]
foreach cl [PacketHeader info subclass] {
if [info exists tab_($cl)] {
set off [$pm allochdr $cl]
$cl offset $off
}
}
$self set packetManager_ $pm
}
应该是在这个地方实现了offset的初始化，换句话说，是在这个地方指定每个包的offset。设置断点并打印，证实了我的猜想：
PacketHeader/PBC has offset 0
PacketHeader/LRWPAN has offset 8
PacketHeader/XCP has offset 216
PacketHeader/PGM has offset 272
PacketHeader/PGM_SPM has offset 288
PacketHeader/PGM_NAK has offset 296
PacketHeader/Pushback has offset 312
PacketHeader/NV has offset 320
PacketHeader/LDP has offset 328
PacketHeader/MPLS has offset 368
PacketHeader/rtProtoLS has offset 392
PacketHeader/Ping has offset 400
PacketHeader/TFRC has offset 424
PacketHeader/TFRC_ACK has offset 480
PacketHeader/Diffusion has offset 544
PacketHeader/RAP has offset 736
PacketHeader/AOMDV has offset 760
PacketHeader/AODV has offset 1568
PacketHeader/SR has offset 2376
PacketHeader/TORA has offset 3088
PacketHeader/IMEP has offset 3120
PacketHeader/ARP has offset 3632
PacketHeader/MIP has offset 3664
PacketHeader/IPinIP has offset 3696
PacketHeader/LL has offset 3704
PacketHeader/Mac has offset 3736
PacketHeader/Encap has offset 3776
PacketHeader/HttpInval has offset 3784
PacketHeader/SRMEXT has offset 3792
PacketHeader/SRM has offset 3800
PacketHeader/aSRM has offset 3816
PacketHeader/mcastCtrl has offset 3824
PacketHeader/CtrMcast has offset 3848
PacketHeader/rtProtoDV has offset 3864
PacketHeader/GAF has offset 3872
PacketHeader/Snoop has offset 3880
PacketHeader/SCTP has offset 3904
PacketHeader/TCPA has offset 3912
PacketHeader/TCP has offset 3928
PacketHeader/IVS has offset 4008
PacketHeader/RTP has offset 4040
PacketHeader/Message has offset 4056
PacketHeader/Resv has offset 4120
PacketHeader/QS has offset 4136
PacketHeader/UMP has offset 4152
PacketHeader/Src_rt has offset 4168
PacketHeader/IP has offset 4248
PacketHeader/Common has offset 4280
PacketHeader/Flags has offset 4384
实际上，每个包的offset应该为sizeof(hdr_***)。为了验证，写了一个程序测试。比如hdr_ip,
typedef int32_t nsaddr_t;
typedef int32_t nsmask_t;
/* 32-bit addressing support */
struct ns_addr_t {
int32_t addr_;
int32_t port_;
};
struct hdr_ip {
/* common to IPv{4,6} */
ns_addr_t src_;
ns_addr_t dst_;
int ttl_;
/* Monarch extn */
u_int16_t sport_;
u_int16_t dport_;
/* IPv6 */
int fid_; /* flow id */
int prio_;
static int offset_;
inline static int& offset() { return offset_; }
/* per-field member acces functions */
ns_addr_t& src() { return (src_); }
nsaddr_t& saddr() { return (src_.addr_); }
int32_t& sport() { return src_.port_;}
ns_addr_t& dst() { return (dst_); }
nsaddr_t& daddr() { return (dst_.addr_); }
int32_t& dport() { return dst_.port_;}
int& ttl() { return (ttl_); }
/* ipv6 fields */
int& flowid() { return (fid_); }
int& prio() { return (prio_); }
};
int main(int argc, char* argv[])
{
printf("ip: %d
", sizeof(hdr_ip));
printf("tora: %d
", sizeof(hdr_tora));
getch();
return 0;
}
程序的输出验证我的猜想。
packet.h的下面定义则实现了两者间的绑定：
class PacketHeaderClass : public TclClass {
protected:
PacketHeaderClass(const char* classname, int hdrsize);
virtual int method(int argc, const char*const* argv);
void field_offset(const char* fieldname, int offset);
inline void bind_offset(int* off) { offset_ = off; }
inline void offset(int* off) {offset_= off;}
int hdrlen_; // # of bytes for this header
int* offset_; // offset for this header
public:
virtual void bind();
virtual void export_offsets();
TclObject* create(int argc, const char*const* argv);
};