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AOS Lab 12: Network Communication
1. Lab 12: Network Communication
Advanced Operating Systems
Zubair Nabi
zubair.nabi@itu.edu.pk
April 24, 2013
2. Introduction
• In *nix systems, the networking infrastructure abstracts away
many network architectures
3. Introduction
• In *nix systems, the networking infrastructure abstracts away
many network architectures
• Each network architecture consists of
• Network-communication protocols
4. Introduction
• In *nix systems, the networking infrastructure abstracts away
many network architectures
• Each network architecture consists of
• Network-communication protocols
• The protocol family
5. Introduction
• In *nix systems, the networking infrastructure abstracts away
many network architectures
• Each network architecture consists of
• Network-communication protocols
• The protocol family
• Conventions of naming end-points
6. Introduction
• In *nix systems, the networking infrastructure abstracts away
many network architectures
• Each network architecture consists of
•
•
•
•
Network-communication protocols
The protocol family
Conventions of naming end-points
The address family or address format
7. Introduction
• In *nix systems, the networking infrastructure abstracts away
many network architectures
• Each network architecture consists of
•
•
•
•
•
Network-communication protocols
The protocol family
Conventions of naming end-points
The address family or address format
Additional facilities
8. Introduction
• In *nix systems, the networking infrastructure abstracts away
many network architectures
• Each network architecture consists of
•
•
•
•
•
Network-communication protocols
The protocol family
Conventions of naming end-points
The address family or address format
Additional facilities
• Network facilities are accessed through the socket abstraction
9. Network Subsystem
Consists of three layers:
1
Transport layer: In charge of sockets-amenable addressing
structure and protocol mechanisms, such as ordering, reliability,
etc.
10. Network Subsystem
Consists of three layers:
1
Transport layer: In charge of sockets-amenable addressing
structure and protocol mechanisms, such as ordering, reliability,
etc.
2
Network layer: Responsible for delivery of data across the
network (must maintain a routing database)
11. Network Subsystem
Consists of three layers:
1
Transport layer: In charge of sockets-amenable addressing
structure and protocol mechanisms, such as ordering, reliability,
etc.
2
Network layer: Responsible for delivery of data across the
network (must maintain a routing database)
3
Link layer: Responsible for shipping messages between hosts
connected to a common transmission medium
13. Network Subsystem (2)
• The layering is just logical layering
• The network service itself might choose to use more or fewer
layers based on its requirements
14. Network Subsystem (2)
• The layering is just logical layering
• The network service itself might choose to use more or fewer
layers based on its requirements
• For instance, raw sockets use a null implementation at one or
more layers
15. Network Subsystem (2)
• The layering is just logical layering
• The network service itself might choose to use more or fewer
layers based on its requirements
• For instance, raw sockets use a null implementation at one or
more layers
• Similarly, tunneling of one protocol through another requires
additional implementations of multiple layers
16. Memory Management
• Memory management for communication protocols is different
than regular entities as memory is required in widely varying sizes
17. Memory Management
• Memory management for communication protocols is different
than regular entities as memory is required in widely varying sizes
• A special-purpose memory-management facility exists for IPC
and networking systems
18. Memory Management
• Memory management for communication protocols is different
than regular entities as memory is required in widely varying sizes
• A special-purpose memory-management facility exists for IPC
and networking systems
• The unit of allocation is an mbuf (skbuff in Linux), which is
128 bytes long with 100 or 108 bytes reserved for data
19. Memory Management
• Memory management for communication protocols is different
than regular entities as memory is required in widely varying sizes
• A special-purpose memory-management facility exists for IPC
and networking systems
• The unit of allocation is an mbuf (skbuff in Linux), which is
128 bytes long with 100 or 108 bytes reserved for data
• A chain of mbufs can be linked together (m_next) to hold an
arbitrary quantity of data
20. Memory Management
• Memory management for communication protocols is different
than regular entities as memory is required in widely varying sizes
• A special-purpose memory-management facility exists for IPC
and networking systems
• The unit of allocation is an mbuf (skbuff in Linux), which is
128 bytes long with 100 or 108 bytes reserved for data
• A chain of mbufs can be linked together (m_next) to hold an
arbitrary quantity of data
• For instance, a chain of mbufs is used to represent packets by
network protocols
24. Data Flow
• Socket-to-network-subsystem
• Calls the transport-layer modules that support the socket
abstraction
• Typically started by system calls
• Network-subsystem-to-socket
• Flows up the stack and is placed in the receive queue of the
destination socket
25. Data Flow
• Socket-to-network-subsystem
• Calls the transport-layer modules that support the socket
abstraction
• Typically started by system calls
• Network-subsystem-to-socket
• Flows up the stack and is placed in the receive queue of the
destination socket
• Asynchronously received and added to the per-protocol input
message queue
27. Network Protocols
• Defined by a set of conventions, including packet formats, states,
and state transitions
28. Network Protocols
• Defined by a set of conventions, including packet formats, states,
and state transitions
• Each communication-protocol module implements a particular
protocol and is made up of a collection of procedures and private
data structures
29. Network Protocols
• Defined by a set of conventions, including packet formats, states,
and state transitions
• Each communication-protocol module implements a particular
protocol and is made up of a collection of procedures and private
data structures
• The external interface of a module is described by a
protocol-switch structure
30. Network Protocols
• Defined by a set of conventions, including packet formats, states,
and state transitions
• Each communication-protocol module implements a particular
protocol and is made up of a collection of procedures and private
data structures
• The external interface of a module is described by a
protocol-switch structure
• This interface is used by the socket layer for all interaction
31. Network Protocols
• Defined by a set of conventions, including packet formats, states,
and state transitions
• Each communication-protocol module implements a particular
protocol and is made up of a collection of procedures and private
data structures
• The external interface of a module is described by a
protocol-switch structure
• This interface is used by the socket layer for all interaction
• The address of this structure is present within the socket’s
so_proto field
32. Network Protocols
• Defined by a set of conventions, including packet formats, states,
and state transitions
• Each communication-protocol module implements a particular
protocol and is made up of a collection of procedures and private
data structures
• The external interface of a module is described by a
protocol-switch structure
• This interface is used by the socket layer for all interaction
• The address of this structure is present within the socket’s
so_proto field
• Each time a socket is created the protocol is selected based on
the type of socket (pr_type)
33. Network Protocols
• Defined by a set of conventions, including packet formats, states,
and state transitions
• Each communication-protocol module implements a particular
protocol and is made up of a collection of procedures and private
data structures
• The external interface of a module is described by a
protocol-switch structure
• This interface is used by the socket layer for all interaction
• The address of this structure is present within the socket’s
so_proto field
• Each time a socket is created the protocol is selected based on
the type of socket (pr_type)
• Also in charge of mbuf storage management
34. Network Interfaces
• Each interface defines a link-layer path through which messages
can be sent and received
35. Network Interfaces
• Each interface defines a link-layer path through which messages
can be sent and received
• Typically, a hardware device is represented by this interface
36. Network Interfaces
• Each interface defines a link-layer path through which messages
can be sent and received
• Typically, a hardware device is represented by this interface
• The loopback interface is in software which is used to route
traffic to local sockets
37. Network Interfaces
• Each interface defines a link-layer path through which messages
can be sent and received
• Typically, a hardware device is represented by this interface
• The loopback interface is in software which is used to route
traffic to local sockets
• Also in charge of encapsulation and decapsulation of link-layer
protocol headers
38. Network Interfaces
• Each interface defines a link-layer path through which messages
can be sent and received
• Typically, a hardware device is represented by this interface
• The loopback interface is in software which is used to route
traffic to local sockets
• Also in charge of encapsulation and decapsulation of link-layer
protocol headers
• Typically implemented as a separate layer that is shared by
various hardware drivers
39. Network Interfaces
• Each interface defines a link-layer path through which messages
can be sent and received
• Typically, a hardware device is represented by this interface
• The loopback interface is in software which is used to route
traffic to local sockets
• Also in charge of encapsulation and decapsulation of link-layer
protocol headers
• Typically implemented as a separate layer that is shared by
various hardware drivers
• The selection of the interface is taken care of by the network-layer
protocol
40. Network Interfaces
• Each interface defines a link-layer path through which messages
can be sent and received
• Typically, a hardware device is represented by this interface
• The loopback interface is in software which is used to route
traffic to local sockets
• Also in charge of encapsulation and decapsulation of link-layer
protocol headers
• Typically implemented as a separate layer that is shared by
various hardware drivers
• The selection of the interface is taken care of by the network-layer
protocol
• Represented by an ifnet structure
42. Socket-to-Protocol Interface
• Enabled by two routines: 1) User request, pr_usrreq() and
2) Control output, pr_ctloutput()
• These methods are present in the protocol-switch table for each
protocol
43. Socket-to-Protocol Interface
• Enabled by two routines: 1) User request, pr_usrreq() and
2) Control output, pr_ctloutput()
• These methods are present in the protocol-switch table for each
protocol
• Control-output: Implements getsockopt and setsockopt
system calls
44. Socket-to-Protocol Interface
• Enabled by two routines: 1) User request, pr_usrreq() and
2) Control output, pr_ctloutput()
• These methods are present in the protocol-switch table for each
protocol
• Control-output: Implements getsockopt and setsockopt
system calls
• User-request: Implements all other operations
46. Protocol-to-Network-Interface Interface
• Lowest layer in the protocol family must interact with one or more
interfaces to send and receive packets
• Obviously a routing decision must have already chosen the
outgoing interface
49. Packet Sending
• Packet m is transmitted to destination dst via interface ifp
• Steps:
• Validation of the destination address
50. Packet Sending
• Packet m is transmitted to destination dst via interface ifp
• Steps:
• Validation of the destination address
• Queuing of the packet on the send queue
51. Packet Sending
• Packet m is transmitted to destination dst via interface ifp
• Steps:
• Validation of the destination address
• Queuing of the packet on the send queue
• If the interface is not busy, using an interrupt-driven routine to
transmit the packet
52. Packet Sending
• Packet m is transmitted to destination dst via interface ifp
• Steps:
• Validation of the destination address
• Queuing of the packet on the send queue
• If the interface is not busy, using an interrupt-driven routine to
transmit the packet
• The link-layer address is chosen by ARP in case of Ethernet
54. Packet Receiving
• Incoming packets are queued in the corresponding protocol’s
input packet queue
• A software interrupt is posted to initiate network-layer processing
55. Code: Packet Receiving
if (IF_QFULL(&ipintrq)) {
IF_DROP(&ipintrq);
ifp->if_iqdrops++;
m_freem(m);
} else {
schednetisr(NETISR_IP);
IF_ENQUEUE(&ipintrq, m)
}
56. Routing
• The routing system has two components; one within the kernel
and one in user-space
57. Routing
• The routing system has two components; one within the kernel
and one in user-space
• The routing mechanism is present within the kernel while routing
policies are defined in user-space
58. Routing
• The routing system has two components; one within the kernel
and one in user-space
• The routing mechanism is present within the kernel while routing
policies are defined in user-space
• The routing mechanism involves a table lookup to get a first-hop
for a given destination
59. Routing
• The routing system has two components; one within the kernel
and one in user-space
• The routing mechanism is present within the kernel while routing
policies are defined in user-space
• The routing mechanism involves a table lookup to get a first-hop
for a given destination
• Routing policies include components that help in choosing
first-hop routes
61. Kernel Routing Mechanism
• Implements a routing table for first/next hop lookup
• Two distinct portions:
1
A data structure with routing entries, one per specific route
62. Kernel Routing Mechanism
• Implements a routing table for first/next hop lookup
• Two distinct portions:
1
2
A data structure with routing entries, one per specific route
A lookup algorithm to locate the correct route for each possible
destination
63. Kernel Routing Mechanism
• Implements a routing table for first/next hop lookup
• Two distinct portions:
1
2
A data structure with routing entries, one per specific route
A lookup algorithm to locate the correct route for each possible
destination
• Each destination is represented by a sockaddr structure
64. Kernel Routing Mechanism
• Implements a routing table for first/next hop lookup
• Two distinct portions:
1
2
A data structure with routing entries, one per specific route
A lookup algorithm to locate the correct route for each possible
destination
• Each destination is represented by a sockaddr structure
• Routes are either:
1
Host or network
65. Kernel Routing Mechanism
• Implements a routing table for first/next hop lookup
• Two distinct portions:
1
2
A data structure with routing entries, one per specific route
A lookup algorithm to locate the correct route for each possible
destination
• Each destination is represented by a sockaddr structure
• Routes are either:
1
2
Host or network
Direct or indirect
67. User-space Routing Policies
• Policies add, delete, or modify kernel routing table entries
• A number of routing policies exist, including the Routing
Information Protocol (RIP)
68. Protocol Control Blocks
• For each TCP or UDP socket, an Internet protocol control block
(inpcb) is created to hold address, ports, routing information,
and pointers to any additional data structures
69. Protocol Control Blocks
• For each TCP or UDP socket, an Internet protocol control block
(inpcb) is created to hold address, ports, routing information,
and pointers to any additional data structures
• TCP in addition creates a TCP control block (tcpcb) to hold
implementation-specific information
70. Protocol Control Blocks
• For each TCP or UDP socket, an Internet protocol control block
(inpcb) is created to hold address, ports, routing information,
and pointers to any additional data structures
• TCP in addition creates a TCP control block (tcpcb) to hold
implementation-specific information
• TCP and UDP protocol modules each have a private doubly
linked list of inpcbs
71. Protocol Control Blocks
• For each TCP or UDP socket, an Internet protocol control block
(inpcb) is created to hold address, ports, routing information,
and pointers to any additional data structures
• TCP in addition creates a TCP control block (tcpcb) to hold
implementation-specific information
• TCP and UDP protocol modules each have a private doubly
linked list of inpcbs
• Common routines are used by the modules to manipulate these
lists
72. Protocol Control Blocks
• For each TCP or UDP socket, an Internet protocol control block
(inpcb) is created to hold address, ports, routing information,
and pointers to any additional data structures
• TCP in addition creates a TCP control block (tcpcb) to hold
implementation-specific information
• TCP and UDP protocol modules each have a private doubly
linked list of inpcbs
• Common routines are used by the modules to manipulate these
lists
• Traffic is multiplexed by the IP layer on the basis of the protocol
identifier in the protocol and passed on to the individual transport
protocol
73. Protocol Control Blocks
• For each TCP or UDP socket, an Internet protocol control block
(inpcb) is created to hold address, ports, routing information,
and pointers to any additional data structures
• TCP in addition creates a TCP control block (tcpcb) to hold
implementation-specific information
• TCP and UDP protocol modules each have a private doubly
linked list of inpcbs
• Common routines are used by the modules to manipulate these
lists
• Traffic is multiplexed by the IP layer on the basis of the protocol
identifier in the protocol and passed on to the individual transport
protocol
• Each protocol is then responsible for passing a direct message to
the appropriate socket
75. Reading(s)
• Chapter 12 and 13 from “The Design and Implementation of the
4.4BSD Operating System” by Marshall Kirk McKusick, Keith
Bostic, Michael J. Karels, and John S. Quarterman.
