上面蓝色部分是对于网路套接字缓冲区结构的解释，红色部分是对于该结构体具体成员变量的解析；
其中在程序中使用到成员是data,end,tail,head等，当该结构在各协议层传输时，变动的也是data部分。需要重点理解的也主要是这几个关键成员变量；
该结构定义的文件：/linux-3.15/include\linux/skbuff.h,
而主要对该结构成员解释的文件是：/linux-3.15/net/core/skbuff.c
/**
*struct sk_buff – socket buffer*@next: Next buffer in list*@prev: Previous buffer in list*@tstamp: Time we arrived/left*@sk: Socket we are owned by*@dev: Device we arrived on/are leaving by*@cb: Control buffer. Free for use by every layer. Put private vars here*@_skb_refdst: destination entry (with norefcount bit)*@sp: the security path, used for xfrm*@len: Length of actual data*@data_len: Data length*@mac_len: Length of link layer header*@hdr_len: writable header length of cloned skb*@csum: Checksum (must include start/offset pair)*@csum_start: Offset from skb->head where checksumming should start*@csum_offset: Offset from csum_start where checksum should be stored*@priority: Packet queueing priority*@local_df: allow local fragmentation*@cloned: Head may be cloned (check refcnt to be sure)*@ip_summed: Driver fed us an IP checksum*@nohdr: Payload reference only, must not modify header*@nfctinfo: Relationship of this skb to the connection*@pkt_type: Packet class*@fclone: skbuff clone status*@ipvs_property: skbuff is owned by ipvs*@peeked: this packet has been seen already, so stats have been*done for it, don’t do them again*@nf_trace: netfilter packet trace flag*@protocol: Packet protocol from driver*@destructor: Destruct function*@nfct: Associated connection, if any*@nf_bridge: Saved data about a bridged frame – see br_netfilter.c*@skb_iif: ifindex of device we arrived on*@tc_index: Traffic control index*@tc_verd: traffic control verdict*@hash: the packet hash*@queue_mapping: Queue mapping for multiqueue devices*@ndisc_nodetype: router type (from link layer)*@ooo_okay: allow the mapping of a socket to a queue to be changed*@l4_hash: indicate hash is a canonical 4-tuple hash over transport*ports.*@wifi_acked_valid: wifi_acked was set*@wifi_acked: whether frame was acked on wifi or not*@no_fcs: Request NIC to treat last 4 bytes as Ethernet FCS*@dma_cookie: a cookie to one of several possible DMA operations*done by skb DMA functions *@napi_id: id of the NAPI struct this skb came from*@secmark: security marking*@mark: Generic packet mark*@dropcount: total number of sk_receive_queue overflows*@vlan_proto: vlan encapsulation protocol*@vlan_tci: vlan tag control information*@inner_protocol: Protocol (encapsulation)*@inner_transport_header: Inner transport layer header (encapsulation)*@inner_network_header: Network layer header (encapsulation)*@inner_mac_header: Link layer header (encapsulation)*@transport_header: Transport layer header*@network_header: Network layer header*@mac_header: Link layer header*@tail: Tail pointer*@end: End pointer*@head: Head of buffer*@data: Data head pointer*@truesize: Buffer size*@users: User count – see {datagram,tcp}.c*/struct sk_buff {/* These two members must be first. */struct sk_buff*next;struct sk_buff*prev;union {ktime_ttstamp;struct skb_mstamp skb_mstamp;};struct sock*sk;struct net_device*dev;/** This is the control buffer. It is free to use for every* layer. Please put your private variables there. If you* want to keep them across layers you have to do a skb_clone()* first. This is owned by whoever has the skb queued ATM.*/charcb[48] __aligned(8);unsigned long_skb_refdst;#ifdef CONFIG_XFRMstructsec_path*sp;#endifunsigned intlen,data_len;__u16mac_len,hdr_len;union {__wsumcsum;struct {__u16csum_start;__u16csum_offset;};};__u32priority;kmemcheck_bitfield_begin(flags1);__u8local_df:1,cloned:1,ip_summed:2,nohdr:1,nfctinfo:3;__u8pkt_type:3,fclone:2,ipvs_property:1,peeked:1,nf_trace:1;kmemcheck_bitfield_end(flags1);__be16protocol;void(*destructor)(struct sk_buff *skb);#if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)struct nf_conntrack*nfct;#endif#ifdef CONFIG_BRIDGE_NETFILTERstruct nf_bridge_info*nf_bridge;#endifintskb_iif;__u32hash;__be16vlan_proto;__u16vlan_tci;#ifdef CONFIG_NET_SCHED__u16tc_index;/* traffic control index */#ifdef CONFIG_NET_CLS_ACT__u16tc_verd;/* traffic control verdict */#endif#endif__u16queue_mapping;kmemcheck_bitfield_begin(flags2);#ifdef CONFIG_IPV6_NDISC_NODETYPE__u8ndisc_nodetype:2;#endif__u8pfmemalloc:1;__u8ooo_okay:1;__u8l4_hash:1;__u8wifi_acked_valid:1;__u8wifi_acked:1;__u8no_fcs:1;__u8head_frag:1;/* Encapsulation protocol and NIC drivers should use* this flag to indicate to each other if the skb contains* encapsulated packet or not and maybe use the inner packet* headers if needed*/__u8encapsulation:1;/* 6/8 bit hole (depending on ndisc_nodetype presence) */kmemcheck_bitfield_end(flags2);#if defined CONFIG_NET_DMA || defined CONFIG_NET_RX_BUSY_POLLunion {unsigned intnapi_id;dma_cookie_tdma_cookie;};#endif#ifdef CONFIG_NETWORK_SECMARK__u32secmark;#endifunion {__u32mark;__u32dropcount;__u32reserved_tailroom;};__be16inner_protocol;__u16inner_transport_header;__u16inner_network_header;__u16inner_mac_header;__u16transport_header;__u16network_header;__u16mac_header;/* These elements must be at the end, see alloc_skb() for details. */sk_buff_data_ttail;sk_buff_data_tend;unsigned char*head,*data;unsigned inttruesize;atomic_tusers;};
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