CS 498 Lecture 6 Packet Management

1. CS 498 Lecture 6Packet Management Jennifer Hou Department of Computer Science University of Illinois at Urbana-Champaign Reading: Chapters 3-4, The Linux Networking Architecture: Design and Implementation of Network Protocols in the Linux Kernel

2. Outline • Socket buffer sk_buff structure • APIs for creating, releasing, and duplicating socket buffers. • APIs for manipulating parameters within the sk_buff structure • APIs for managing the socket buffer queue.

3. Structure of sk_buff sk_buff_head sk_buff next sk_buff prev list stamp net_device dev h nh mac Paketdatenbereich dst len MAC-Header ... IP-Header head UDP-Header data UDP-Data tail end datarefp: 1

4. Socket Buffer • The structure used to manage a packet during its lifetime. • Created when an application passes data to a socket or when a packet arrives at the network adaptor (dev_alloc_skb() is invoked). • Packet headers of each layer are inserted in front of the payload. • The packet is copied only twice: once from the user address space to the kernel address space, and once when the packet is passed onto the network adaptor.

5. Change of sk_buff as a Packet Traverses Across the Stack sk_buff sk_buff sk_buff next next next prev prev prev ... ... ... head head head data data data tail tail tail end end end Paketdatenbereich Paketdatenbereich Paketdatenbereich IP-Header UDP-Header UDP-Header UDP-Daten UDP-Daten UDP-Daten datarefp: 1 datarefp: 1 datarefp: 1

6. Parameters of sk_buff Structure • list: points to the queue where the socket buffer is currently located. • sk: points to the socket that created the packet. • stamp: specifies the time when the packet arrived in the Linux (in jiffies) • dev: states the current network device on which the socket buffer operates. Once the routing decision is made, dev points to the network adapter on which the packet leaves.

7. Parameters of sk_buff Structure • input_dev: always points to the network device that received the packet. • h, nh, mac: are pointers to packet headers in the transport layer (h), the network layer (nh), and the MAC layer (mac). • dst: a reference to the adapter on which the packet leaves the computer, or a reference to a MAC header stored in the hard header cache. • cloned: indicates if a packet was cloned.

8. Parameters of sk_buff Structure • pkt_type: specifies the type of a packet • PACKET_HOST: a packet sent to the local host • PACKET_BROADCAST: a broadcast packet • PACKET_MULTICAST: a multicast packet • PACKET_OTHERHOST:a packet not destined for the local host, but received in the promiscuous mode. • PACKET_OTGOING: a packet leaving the host • PACKET_LOOKBACK: a packet sent by the local host to itself.

9. Parameters of sk_buff Structure • len: specifies the length of a packet. • data, tail: point to currently valid packet data. • head, end: point to the total location that can be used for packet data. • The space between head and data is called headroom, and the space between tail and end is called tailroom • datarefp: a referece counter that indicates how many times the packet has been referenced. Not part of the sk_buff structure

10. Operations on Socket Buffers Create, release, and duplicate socket buffers. Manipulate parameters within the sk_buffer structure Manage socket buffer queues.

11. 1. Creating and Releasing Socket Buffers • alloc_skb(size,gfp_mask) • No immediate attempt is made to allocate the memory with kmalloc() for the sk_buff structure. • Tries to reuse a sk_buff in the skb_head_cache queue; if not successful, tries to obtain a packet from the central socket-buffer cache (skbuff_head_cache) with kmem_cache_alloc(). • If neither is successful, the invoke kmalloc() to reserve memory. • The packet data space is reserved by use of kmalloc().

12. 1. Creating and Releasing Socket Buffers • skb_copy(skb,gfp_mask): creates a copy of the socket buffer skb, copying both the sk_buff structure and the packet data. • skb_copy_expand(skb,newheadroom, newtailroom, gfp_mask): creates a new copy of the socket buffer and packet data, and in addition, reserves a larger space before and after the packet data.

13. Copying Socket Buffers skb_copy sk_buff sk_buff sk_buff next next next prev prev prev ... ... ... head head head data data data tail tail tail end end end Paket dat storage Paket data storage Paket data storage IP-Header IP-Header IP-Header UDP-Header UDP-Header UDP-Header UDP-Daten UDP-Daten UDP-Daten datarefp: 1 datarefp: 1 datarefp: 1

14. 1. Creating and Releasing Socket Buffers • skb_clone(): creates a new socket buffer sk_buff, but not the packet data. Pointers in both sk_buffs point to the same packet data space. • Needed in multicast.

15. Cloning Socket Buffers skb_clone sk_buff sk_buff sk_buff next next next prev prev prev ... ... ... head head head data data data tail tail tail end end end Paketdatenbereich Paketdatenbereich IP-Header IP-Header UDP-Header UDP-Header UDP-Daten UDP-Daten datarefp: 1 datarefp: 2

16. 1. Creating and Releasing Socket Buffers • kfree_skb(): calls kfree_skbmem() to release the packet memory, and does all the sanity check (e.g., calls the destructor() for the socket buffer) • kfree_skbmem(): if skb_cloned() is tested for null, and datarefp is tested for one • invokes kfree() to release the packet data memory. • Invokes skb_head_to_pool() to insert the sk_buffer structure into the socket_buffer cache.

17. 2. Manipulating Packet Data Space • skb_unshare(skb): uses skb_cloned to check if the socket buffer is available for exclusive use. If not, then creates a copy of skb and returns the pointer. • skb_put(skb,len): appends data to the end of the packet; increments the pointer tail and skblen by len; need to ensure the tailroom is sufficient. • skb_push(skb,len): inserts data in front of the packet data space; decrements the pointer data by len, and increment skblen by len; need to check the headroom size.

18. 2. Manipulating Packet Data Space • skb_pull(skb,len): truncates len bytes at the beginning of a packet. • skb_tailroom(skb): returns the size of the tailroom (in bytes). • skb_headroom(skb): returns the size of the headroom (data-head) • skb_realloc_headroom(skb,newheadroom) creates a new socket buffer with a headroom of size newheadroom. • skb_reserve(skb,len): shifts the entire packet data backward by len bytes.

19. 3. Managing the Socket Buffer Queue • The socket buffers are arranged in a dual-concatenated ring structure. struct sk_buff_head { struct sk_buff *next; struct sk_buff *prev; __u32qlen; spinlock_tlock; }; Used for atomic execution of operations on the queue under critical conditions.

20. Packet Queue in the Linux Kernel sk_buff_head next prev qlen: 3 sk_buff sk_buff sk_buff next next next prev prev prev ... ... ... head head head data data data tail tail tail end end end Paketdaten Paketdaten Paketdaten

21. 3. Managing the Socket Buffer Queue • skb_queue_head_init(list): initializes an skb_queue_head structure • skb_queue_empty(list): checks whether the queue list is empty; listqlen is returned. • skb_queue_len(list): returns the actual length of the queue. • skb_queue_head(list,skb): inserts the socket buffer skb at the head of the queue and increment listqlen by one. • skb_queue_tail(list,skb): appends the socket buffer skb to the end of the queue and increment listqlen by one.

22. 3. Managing the Socket Buffer Queue • skb_dequeue(list): removes the top packet from the queue and returns the pointer to the packet. • skb_dequene_tail(list): removes the last packet from the queue and returns the pointer to the packet. • skb_queue_purge(): empties the queue list; all packets are removed via kfree_skb(). • skb_insert(oldskb,newskb): inserts newskbin front ofoldskb in the queue of oldskb. • skb_append(oldskb,newskb): inserts newskbbehindoldskb in the queue of oldskb.

23. 3. Managing the Socket Buffer Queue • skb_unlink(skb): removes the socket buffer skb from its queue and decrement the queue length. • skb_peek(list): returns a pointer to the first element of a list, if this list is not empty; otherwise, returns NULL. • skb_peek_tail(list): returns a pointer to the last element of a queue; if the list is empty, returns NULL.