SKB - Linux Network Buffers
HARALD WELTE LAFORGE@gnumonks.org1.3, 2000/10/14 21:27:02
Short Description About The Linux Network Buffers (SKB's)
INTRODUCTION
At the time I wanted to know more about the Linux network stack, I always wanted a document like this to exist. But unfortunately I never found one. After I gained some basic knowledge about the Linux network stack internals, I wrote one.
I'm happy if this document is of any use for other people................
Please Let Me Know of any bugs in this document. It Should Reemble Kernel Revision 2.4.0-Test4
2. SKBUFF's
skbuffs are the buffers in which the linux kernel handles network packets. The packet is received by the network card, put into a skbuff and then passed to the network stack, which uses the skbuff all the time.
2.1 Struct SK_Buff
The struct SK_Buff Is Defined in
NEXT
Next buffer in list
Prev
Previous Buffer in List
List
List we are on
SK
Socket We belong to
Stamp
TimeVal We Arrived AT
DEV
Device we are leaving by
RX_DEV
Device we arrived at
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TRANSPORT LAYER Header (TCP, UDP, ICMP, IGMP, SPX, RAW)
NH
NetWork Layer Header (IP, IPv6, ARP, IPX, RAW)
Mac
Link Layer HEADER
DST
FIXME:
CB
Control Buffer, Used INTERNALLY
Len
Length of actual data
CSum
CHECKSUM
Used
FIXME: Data Moved to User and Not MSG_PEEK
IS_clone
WE Are A Clone
cloned
Head May Be Cloned
PKT_TYPE
Packet class
IP_SUMMED
Driver Fed US IP CHECKSUM
Priority
Packet queuing priority
Users
User count
Protocol
Packet Protocol from Driver
Security
Security level of packet
TrueSize
Real size of the buffer
HEAD
Pointer to Head of buffer
Data
Data Head Pointer
TAIL
Tail pointerend
END POINTER
Destructor
Destructor function
NFMark
Netfilter Mark
NFCache
Netfilter Internal Caching Info
NFCT
Associated Connection, IF ANY
Tc_index
Traffic Control Index
2.2 SKB Support Functions
There Are A Bunch of Skb Support Functions Provided by The Sk_Buff Layer. I Briefly Describe The Most Important Ones in this section.
Allocation / Free / Copy / Clone and Expansion Functions
Struct SK_Buff * Alloc_skb (unsigned int size, int GFP_mask)
This Function Allocates A New Skb. This is provided by The Skb Layer To Initialize Some Privat Data And Do Memory Statistics. The Returned Buffer Has No Headroom and A Tailroom OF / SIZE / BYTES.
Void kfree_skb (struct SK_Buff * SKB)
Decrement the skb's usage count by one and free the skb if no references.
Struct SK_Buff * SKB_GET (STRUCT SK_BUFF * SKB)
Increments the SKB'S USAGE Count by One and Returns a Pointer To IT.
STRUCT SK_BUFF * SKB_CLONE (Struct SK_Buff * SKB, INT GFP_MASK)
This Function Clones A SKB. Both Copies Share The Packet Data But Have Their OWN STRUCT SK_BUF. The New Copy IS NOT OWNED by Any Socket, Reference Count Is 1.
STRUCT SK_BUFF * SKB_COPY (const struct SK_Buff * SKB, INT GFP_MASK)
Makes A Real Copy of the SKB, INCLUDING PACKET DATA. This is needed, if you wish to modify the packet data. Reference Count of the New SKB IS 1.
STRUCT SKB_COPY_EXPAND (Const Struct SK_Buff * SKB, INT New_HEADROOM, INT New_TAILROOM, INT GFP_MASK)
Make a Copy of the SKB, Including Packet Data. Additional The New Skb Has A Haedroom of / New_Headroom / Bytes Size and a Tailroom OF / New_TAILROOM / BYTES.
Anciliary Functions
INT SKB_CLONED (STRUCT SK_BUFF * SKB)
Is the SKB A Clone?
INT SKB_SHARED (STRUCT SK_BUFF * SKB)
Is this SKB Shared? (Is The Reference Count> 1)? Operations on Lists of Skb's
Struct SK_Buff * SKB_PEEK (STRUCT SK_BUFF_HEAD * LIST_)
Peek a SKB from Front of the List; Does Not Remove Skb from the list
Struct SK_Buff * SKB_PEEK_TAIL (Struct Sk_buff_head * list_)
Peek A SKB from Tail of The List; Does Not Remove Sk from the list
__U32 SKB_QUEUE_LEN (SK_BUFF_HEAD * LIST_)
Return the length of the given SKB LIST
Void SKB_QUEUE_HEAD (STRUCT SK_BUFF_HEAD * LIST_, STRUCT SK_BUFF * Newsk)
Enqueue a SKB At the Head of a Given List
Void SKB_QUE_TAIL (STRUCT SK_BUFF_HEAD * LIST_, STRUCT SK_BUFF * Newsk)
ENQUEUE A SKB AT The End of a Given List.
STRUCT SK_BUFF * SKB_DEQUEUE (struct SK_BUFF_HEAD * LIST_)
DEQUEUE A SKB from from the head of the given list.
Struct SK_BUFF * SBK_DEQUE_TAIL (STRUCT SK_BUFF_HEAD * LIST_)
Dequeue a SKB from the Tail of the Given List
Operations on SKB DATA
Unsigned char * SKB_PUT (Struct Sk_buff * SBK, INT LEN)
Extends The Data Area of The Skb. If The Total Size Exceeds The Size of The SKB, The Kernel Will Panic. a Pointer To The First Byte Of New Data is Returned.
Unsigned char * SKB_PUSH (Struct Sk_buff * SKB, INT LEN)
Extends The Data Area of The Skb. If The Total Size Exceeds The Size of The SKB, The Kernel Will Panic. a Pointer To The First Byte Of New Data is Returned.
Unsigned char * SKB_PULL (Struct Sk_buff * SKB, INT LEN)
Remove Data from The Start of a buffer, return the bytes to headroom. a PointR to the next data in the buffer is returned.
INT SKB_HEADROOM (Struct SK_Buff * SKB)
Return the Amount of Bytes of Free Space At the Head of SKB
INT SKB_TAILROOM (Struct Sk_buff * SKB)
Return The Amount of Bytes of Free Space At the end of skb
Struct SK_Buff * SKB_COW (Struct SK_Buff * SKB, INT Headroom If The Buffer Passed Lacks Sufficient Headroom Or Is A Clone It is Copied and Additional Headroom Made Available.
