学习Linux-4.12内核网路协议栈（1.2）——协议栈的初始化(sk_buff)

sk_buff 是网络数据包的承载，是最关键的结构体之一

/**
 *      struct sk_buff - socket buffer
 *      @next: Next buffer in list
 *      @prev: Previous buffer in list
 *      @tstamp: Time we arrived/left
 *      @rbnode: RB tree node, alternative to next/prev for netem/tcp
 *      @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
 *      @ignore_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
 *      @pkt_type: Packet class
 *      @fclone: skbuff clone status
 *      @ipvs_property: skbuff is owned by ipvs
 *      @tc_skip_classify: do not classify packet. set by IFB device
 *      @tc_at_ingress: used within tc_classify to distinguish in/egress
 *      @tc_redirected: packet was redirected by a tc action
 *      @tc_from_ingress: if tc_redirected, tc_at_ingress at time of 
redirect
 *      @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 (with nfctinfo bits)
 *      @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
 *      @hash: the packet hash
 *      @queue_mapping: Queue mapping for multiqueue devices
 *      @xmit_more: More SKBs are pending for this queue
 *      @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.
 *      @sw_hash: indicates hash was computed in software stack
 *      @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
 *      @dst_pending_confirm: need to confirm neighbour
  *     @napi_id: id of the NAPI struct this skb came from
 *      @secmark: security marking
 *      @mark: Generic packet mark
 *      @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