IP Fragmentation

IP 分片

IP协议允许Datagram进行分片（英语：fragmentation，又译分段），将Datagram分割成更小的单位。
这样的话，当数据包比链路最大传输单元（MTU）大时，就可以被分解为很多的足够小片段，以便能
够在其上进行传输。

skb_shared_info

skb_shared_info 的定义

/* This data is invariant across clones and lives at
 * the end of the header data, ie. at skb->end.
 */
struct skb_shared_info {
    atomic_t    dataref;
    unsigned short  nr_frags;
    unsigned short  gso_size;
    /* Warning: this field is not always filled in (UFO)! */
    unsigned short  gso_segs;
    unsigned short  gso_type;
    __be32          ip6_frag_id;
    struct sk_buff  *frag_list;
    skb_frag_t  frags[MAX_SKB_FRAGS];
};

用于skb clone的字段：
dataref : This field contains the reference count for this skb. It is incremented each time the buffer is cloned.
用于TCP 分段的字段：
nr_frags : the number of fragments in this packet. This field is used by TCP segmentation.
frags : the array of page table entries. Each entry is actually a TCP segment.
用于IP 分片的字段：
frag_list : This field points to the list of fragments for this packet if it is fragmented.

skb_shared_info 内存中的位置

This structure is placed at the end of the attached data buffer and pointed to by the end field
in the socket buffer structure(struct sk_buff), which points to the end of the data portion of the packet.

 430 #ifdef NET_SKBUFF_DATA_USES_OFFSET
 431 static inline unsigned char *skb_end_pointer(const struct sk_buff *skb)
 432 {
 433     return skb->head + skb->end;
 434 }
 435 #else
 436 static inline unsigned char *skb_end_pointer(const struct sk_buff *skb)
 437 {
 438     return skb->end;
 439 }
 440 #endif
 441 
 442 /* Internal */
 443 #define skb_shinfo(SKB) ((struct skb_shared_info *)(skb_end_pointer(SKB)))

skb_shared_info 的作用

1. IP分片
2. TCP分段
3. 记录SKB的clone次数
4. TODO GSO

TODO GRO

hello world

枯荣大师问道:“你在一阳指上的修为,已到第几品境界?
”本因额头出汗,答道:“弟子根钝,又兼未能精进,只修得
到第四品。”枯荣大师再问:“以你所见,大理段氏的一阳指
与少林拈花指、多罗叶指、无相劫指三项指法相较,孰优
孰劣?”本因道:“指法无优劣,功力有高下。”枯荣大师道:
“不错。咱们的一阳指若能练到第一品,那便如何?”本因
道:“渊深难测,弟子不敢妄说。”枯荣道:“倘若你再活一
百岁,能练到第几品?”本因额上汗水涔涔而下,颤声道:
“弟子不知。”枯荣道:“能修到第一品吗?”
本因道:“决计不能。”枯荣大师就此不再说话。
本因道:“师叔指点甚是,咱们自己的一阳指尚自修习不
得周全,要旁人的武学奇经作甚?

你好，世界

我的第一篇文章

08 Jan 2016

Basic

Neque porro quisquam est, qui dolorem ipsum, quia dolor sit, amet, consectetur, adipisci velit.

• lorem
• ipsum

1. dolor
2. sit

Basic

Neque porro quisquam est, qui dolorem ipsum, quia dolor sit, amet, consectetur, adipisci velit.

• lorem
• ipsum

1. dolor
2. sit

Blockquote

They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety.

Benjamin Franklin

Code

static void asyncEnabled(Dict* args, void* vAdmin, String* txid, struct Allocator* requestAlloc)
{
    struct Admin* admin = Identity_check((struct Admin*) vAdmin);
    int64_t enabled = admin->asyncEnabled;
    Dict d = Dict_CONST(String_CONST("asyncEnabled"), Int_OBJ(enabled), NULL);
    Admin_sendMessage(&d, txid, admin);
}