Architecture of Network Implementation

1
Architecture of Network Implementation

• Reading Chapters 3 and 4

2
Layer-Based Communication Models
Intermediate system
End systems
End systems
Anwendung
Anwendung
Application
Application
(N1)-Protocols
Instance (N1)
Instance (N1)
Instance (N1)
Instance (N1)
Layer (N1)
(N)-protocol
Instance (N)
Instance (N)
Layer (N)
(N)-protocol
Instance (N-1)
Instance (N-1)
Layer (N-1)
Transmission medium
Transmission medium
3
Two Important Concepts in the Layered Model

• Protocols
• Rules that two parties talk and understand each
  other
• Horizontal
• Services
• Functions provided by a lower layer to the
  neighboring upper layer
• Vertical
• Service interface (function calls)

4
ISO/OSI Reference Model
ISO/OSI Reference model
Application
Presentation
Session
Transport
Network
Data link
Physical
5
ISO/OSI Model

• Physical layer
• Media types, coding methods, bit sequences vs.
  physical signals
• Data link layer
• Moving data between two directly connected
  stations. Flow control, error detection, etc.
  Shared medium access control. LLC/MAC
• Network layer
• Connectivity of networks. Packet
  routing/forwarding
• Transport layer
• Concerning end-to-end functions between
  applications. Flow control, packet ordering, etc.
• Session layer
• Handling structured message exchange
• Presentation layer
• Regulating data presentation (format)
• Application layer
• Protocols for different applications, HTTP, SMTP,
  FTP, etc

6
TCP/IP Reference Model
Internet reference mode
ISO/OSI reference model
Application
Application (HTTP, SMTP, SSH)
7
6
Presentation
5
Session
4
Transport (TCP/UDP)
Transport
3
Internet (IPv4/v6)
Network
Data link(802.x, PPP, SLIP)
Data link
2
Physical
1
7
Interplay of Layers, Instances, and Protocols
(N1)-Layer
(N)-IDU
(N)-Layer
(N)-ICI
(N)-SDU
(N)-PCI
(N)-PDU
(N-1)-ICI
(N)-PCI
(N)-SDU
(N-1)-IDU
(N-1)-Layer
(N-1)-SDU
(N-1)-ICI
8
PDU/PCI/SDU/ICI/IDU

• PDU protocol data unit
• Elements exchanged between two protocol instances
• PCI (protocol control information or packet
  header) SDU (service data unit)
• ICI interface control information
• Passed from layer N to layer N-1 together with
  layer N PDU
• IDU interface data unit
• ICI PDU

9
Managing Network Packets in Kernel

• Socket buffers
• Where packets are stored, manipulated.
• Socket buffer queues
• Linked list of socket buffers

10
Socket Buffers (skb)
11
Some important fields of skb

• prev, next, list
• Queues linked list
• sk
• Socket owning the packet
• dev
• Device receiving/sending packet
• h, nh, mac
• Pointers to the transport/network/MAC layer
  headers
• len
• Size of the packet
• data, head, tail, end
• Pointers associated with data packet field
• datarefp
• A counter indicating how many skb_buff points to
  the packet
• Not really a field of sbk_buff

12
Updates of skb_buff as packet passing different
layers
13
Operations on socket buffers

• Three types of operations
• Creating and releasing socket buffers
• alloc_skb(), dev_alloc_skb(), skb_clone(),
  kfree_skb(), dev_kfree_skb()
• Manipulating fields of socket buffers
• skb_put(), skb_push(), skb_pull(), skb_reserve()
• Managing socket buffer queues
• skb_queue_head(),etc

14
Creating/releasing skb

• alloc_skb(size, gpf_mask)
• dev_alloc_skb(length)
• kfree_skb(skb)
• dev_kfree_skb(skb)

• skb_clone(skb,gpf_mask)
• skb_copy(skb,gpf_mask)

15
Skb_clone()
16
skb_copy()
17
Manipulating skb fields

• skb_put(skb,len)
• skb_push(skb,len)
• skb_pull(skb,len)
• skb_reserve(skb,len)

After alloc_skb()
After skb_reserve()
An skb_buff containing data
After skb_put()
After skb_push()
18
Socket buffer queue
19
Operations on socket buffer queues

• Managing queue structures
• skb_queue_head_init(list)
• skb_queue_empty(list)
• skb_queue_len(list)
• Managing socket buffers in queues
• skb_queue_head(), skb_queue_tail(),etc

20
Managing skb in queues

• skb_queue_head(list, skb)
• skb_queue_tail(list, skb)
• skb_dequeue(list)
• skb_dequeue_tail(list)
• skb_insert(oldskb,newskb)
• skb_append(oldskb,newskb)
• skb_peek(list)
• skb_peek_tail(list)

21
Next time

• Chapter 5 network devices