Transcript Document

Telecommunications and Multimedia Unit
BSD TCP/IP Protocol Suite
An Overview of the Implementation
Agostinho L S Castro
[email protected]
BSD TCP/IP Protocol Suite – The Implementation
•Network Implementation
4.4 BSD supports 4 distinct communication protocol
families
1.
2.
3.
4.
TCP/IP
XNS (Xerox Network Systems)
OSI protocols
Unix domain protocols
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
•The TCP/IP protocol suite allows computers of all sizes,
from different computer vendor, running totally different
operating systems, to communicate with each other.
•TCP/IP is normally considered to be a 4 layer system
Application
Transport
Network
Link
Telecommunications and Multimedia Unit
Telnet, FTP, e-mail, etc.
TCP, UDP
IP, ICMP, IGMP
Device driver and Interface card
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
•General organization of networking code in 4.4 BSD
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Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
•Mbufs (Memory Buffers)
A fundamental concept in the design of the Berkeley
Networking is the memory buffer, called an mbuf, used
throughout the networking code to hold various pieces
of information.
sys/mbuf.h
mbuf structure, mbuff macros and definitions
linux/include/linux/skbuff.h
kern/uipc_mbuf.c
mbuf.h functions
Telecommunications and Multimedia Unit
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BSD TCP/IP Protocol Suite – The Implementation
mbuf.h
/* header at beginning of each mbuf */
struct m_hdr {
struct mbuf *mh_next ;
/* next buffer in chain */
struct mbuf *mh_nextpkt; /* next chain in queue record */
……………………………………………………………………………………
short mh_type;
/* type of data */
short mh_flags;
/* flags */
………………………………………..…………………………………………..
#define m_next m.hdr.mh_next
#define m_len m.hdr.mh_len
……………………………………………………………………..……………..
};
mbuf structures
m_flags
Description
M_BROADCAST
sent/receive as link level broadcast
m_type
Description
MT_data
Dynamic data allocation
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
•Remarks on mbuf
1. The size of the mbuf structure is always 128 bytes
2. In each of the mbuf we show the m_data member
pointing to the beginning of the corresponding buffer.
(this pointer can point anywhere in the corresponding
buff – not necessarily in the front)
3. The m_next pointer links together the mbuf forming a
single packet into an mbuf chain
4. The m_next pointer links multiple packets together to
form a queue of mbuf
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
Linked list of mbuf chains with head pointer
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
•Interface layer
4.4 BSD interface layer attempts to provide a hardwareindependent programming interface between the network
protocols and the drivers for the network devices
connected to a system
The interface layer supports provides for all devices
i. A well-defined set of interface functions
ii. A standard set of statistics and control flags
iii. A device-independent method of storing protocol
address
i. A standard queueing method for outgoing packets
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
sys/socket.h
net/if.h
 address structure definitions
 interface structure definitions
net/if_dl.h
 link-level structure definitions
net/init_main.c
 system and interface initialisations
linux/init/main.c  system and interface initialisations
net/if.c
 generic interface code
drivers/net/skeleton.c  generic interface code
net/if_loop.c
 loopback device driver
drivers/net/loopback.c  loopback device driver
Telecommunications and Multimedia Unit
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BSD TCP/IP Protocol Suite – The Implementation
•Interface layer
•Ifnet struture
•Ifaddr struture
•sockaddr struture
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BSD TCP/IP Protocol Suite – The Implementation
•ifnet structure
It contains information common to all interfaces.
During system initialization, a separate ifnet structure is
allocated for each network device.
Every ifnet structure has a list of one or more protocol
address associated with it.
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
ifnet description
•Implementation information
List construction
Common interface information (name, such as: eth0, sl0,...)
•Flags (if_flags specifies the operational state and
properties of the interface)
If_flags
Kernel Only
Descriptions
IFF_BROADCAST
x
The interface is for a broadcast network
IFF_MULTICAST
x
The interface supports multicasting
IFF_PROMISC
Telecommunications and Multimedia Unit
The interface receives all network packets
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BSD TCP/IP Protocol Suite – The Implementation
ifnet description
•Interface timer
•BSD packet filter
Through BPF, a process can receive copies of
packets transmitted or received by interface
•Hardware information
Interface characteristics (hardware address type
supported by the interface – Ethernet , SLIP,
loopback interface)
mtu, metric, baudrate
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BSD TCP/IP Protocol Suite – The Implementation
ifnet description
•Interfaces statistics
•Functions pointers (interface procedures)
It contains pointers to the standard interface-layer
functions which isolate device-specific details from
network layer
Function
if_init
if_output
if_start
Description
Initialize the inteface
Queue outgoing packets for transmission
Initiate transmission of packets
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
ifnet description
•Output queue
Each interface has its own ifnet structure and
therefore it own output queue
ifaddr structure
•Each interface maintains a linked list of ifaddr structures
because some data links, such as Ethernet, support
more then one protocol
•A separate ifaddr structure describes each address assigned to the interface, usually one address
per protocol
Telecommunications and Multimedia Unit
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BSD TCP/IP Protocol Suite – The Implementation
if.h
struct ifaddr {
struct ifaddr *ifa_next; /*next address for interface */
struct ifnet *ifa_ifp;
/*back-pointer to interface */
……………………………………………………………..…..
};
Telecommunications and Multimedia Unit
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BSD TCP/IP Protocol Suite – The Implementation
sockaddr structure
•Addressing information for an interface consists of
more then single host address. 4.4 BSD maintains
host, broadcast and network mask in structures
derived from generic sockaddr structure
socket.h
struct sockaddr {
u_char sa_len;
/* total length */
u_char sa_family; /* address family*/
.......................................................................
};
Telecommunications and Multimedia Unit
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BSD TCP/IP Protocol Suite – The Implementation
sa_family
Protocol
AF_INET
AF_ISO, AF_OSI
AF_UNIX
AF_ROUTE
Internet
OSI
Unix
Rpouting table
AF_LINK
Data link
sa_family constants
Telecommunications and Multimedia Unit
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BSD TCP/IP Protocol Suite – The Implementation
Ifnet and ifaddr specialization
•The ifnet and ifaddr structures contain general information
applicable to all network interface and protocol address.
•To accommodate additional device and protocol-specific information, each driver defines and each
protocol
allocates a specialized version of the ifnet and ifaddr structures.
Arpcom structure is common to all Ethernet drivers and contains information for the Address
Resolution Protocol (ARP)
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BSD TCP/IP Protocol Suite – The Implementation
Arragement of ifnet structure within
device-dependent structures
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BSD TCP/IP Protocol Suite – The Implementation
Ethernet Initialization
•cpu_start (init_main.c)
the kernel locates any attached network devices
le,sl,loop xxxattach – initialization function
•each device driver for a network interface initialize a
specialized ifnet structure
•if_attach function completes the initialization and
inserts the structure into linked list the interface (ifnet structure specialized, arpcom structure)
After this initialization, the interfaces are configured only
with link-level address
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
•Interface: Ethernet
File
Description
net/if_ether.h
Ethernet structures
net/if.h
net/if_ethersubr.c
dev/if_le.c
Ioctl command definitions
Generic Ethernet frames
Ethernet driver (LANCE)
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
if_le.c
leintr Function
•leintr Function examines the hardware and if a frame
has arrived. Calls lread function to transfer the frame
from the interface to a chain of mbufs
leread Function
•ether_header construction
•chain of mbufs construction
•it passes the incoming frames to BPF
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
ether_input Function
•ether_input function examines the ether_header
structure to determine the type of data has been
received and then queues the received packet for processing
•broadcast and multicast recognition
•link-level demultiplexing
•queue the packet
Telecommunications and Multimedia Unit
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BSD TCP/IP Protocol Suite – The Implementation
ether_output Function
•output frames of Ethernet frames
•network-level protocol (IP) call if_output function
(ifnet structure)
•if_output ~ ether_output (accept and queue frame for begin transmission of frame
- verification
- protocol-specific processing
- frame construction
- interface queueing
Telecommunications and Multimedia Unit
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BSD TCP/IP Protocol Suite – The Implementation
lestart Function
•interface must be initialized
•dequeue frame from output queue
•transmit frame and pass to BPF
•repeat if device is ready for more frames
•mark device as busy
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
•IP: Internet Protocol
File
Description
net/route.h
netinet/ip.h
Route entries
IP Header
netinet/ip_input.c
netinet/ip_output.c
Netinet/in_cksum.c
IP Processing
IP output Processing
Internet Checksum algorithm
Telecommunications and Multimedia Unit
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BSD TCP/IP Protocol Suite – The Implementation
IP datagram, including ip structure names
ip structure - ip.h file
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BSD TCP/IP Protocol Suite – The Implementation
input processing
ipintr Function – ip_input.c
•verification of incoming packets
dequeueing packets from ipintrq
verifies their contents
damaged or erroneous packets are silently discarded
IP version (ip_v = 4)
IP checksum
Byte ordering
Packet length
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
ipintr Function – ip_input.c
•Determines whether or not the packet has reached its final destination
NO – forward (if systems is configured as a router)
YES – transport-level protocol
•Reassembly and demultipexing
•The protocol specified in the datagram is mapped by ip_p.
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
ip_forward Function
A packet arriving at a system other than its final destination needs to be forward. ipintr calls the
function ip_forward which implements the forwarding algorithm
Telecommunications and Multimedia Unit
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BSD TCP/IP Protocol Suite – The Implementation
ip_output Function – ip_output.c
two sources: ip_forward and the transport protocols
•Header initialization
The first section of ip_output merge options into the
outgoing packets and completes the IP header for packets that passed from the transport
protocols
IP header construction
Packet already including header (for a forward packet)
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
ip_output Function – ip_output.c
•Route selection
After completing the IP header, the next task for
ip_output is to locate a route to the destination
•Source address selection and fragmentation
The final section of ip_output ensure that the IP
header has a valid source address and the interface associated with the route
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
•TCP: Transmission Control Protocol
File
netinet/tcp.h
netinet/tcp_input.c
netinet/tcp_output
Description
tcphdr structure definition
tcp_input
tcp_output
tcp_debug.h, tcp_seq.h, tcp_timer.h, tcp_var.h, tcp_tcpip.h
tcp_debug.c, tcp_subr.c, tcp_timer.c, tcp_usrreq.c
28 functions and almost 4,500 lines of C code
Telecommunications and Multimedia Unit
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BSD TCP/IP Protocol Suite – The Implementation
TCP Header
tcp.h file
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BSD TCP/IP Protocol Suite – The Implementation
TCP Output – tcp_output.c
tcp_output is called whenever a segment to be sent on
a connection
tcp_output determines whether a segment can be sent,
and if so, what values to set all the TCP header fields
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
tcp_output description
• Send a segment
•it fills in all the fields in the TCP header and passes the segment to IP for output
Calculate amount of data to send
Build MSS (maximum segment size) option
Build window scale option
Build timestamp option
Check if options have overflowed segment
Update statistics
Allocate an mbuf for IP and TCP headers
Copy data into mbuf
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
tcp_output description
Set PSH (push flag) flag
Update statistics
Get mbuf for IP and TCP headers
Copy IP and TCP header templates into mbuf
Set sequence number filed of segment
Set acknowledgment field of segment
Set header length if option present
Don´t advertise less than one full-size segment
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
tcp_output description
Observe upper limit for advertised window on this
connection
Do not shrink window
Set urgent offset
Set retransmission timer
Persist state
Add trace record for socket debugging
Set IP length, TTL, and TOS
Pass datagram to IP
Telecommunications and Multimedia Unit
Agostinho L S Castro
[email protected]
BSD TCP/IP Protocol Suite – The Implementation
TCP Input – tcp_input.c
•The function tcp_input is about 1100 lines code.
•The function tcp_input is called by ipintr when a datagram
is received with a protocol field of TCP.
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
TCP Input – tcp_input.c
Preliminary processing
Get IP and TCP in first mbuf
Verify TCP checksum
Verify TCP offset field
Get headers plus option into first mbuf
Process timestamp option quickly
Save input flags and convert fields to host byte order
Locate Internet PCB (protocol control block)
Telecommunications and Multimedia Unit
Agostinho L S Castro
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BSD TCP/IP Protocol Suite – The Implementation
TCP Input – tcp_input.c
Preliminary processing
Drop segment and generate RST
Silently drop segment
Unscale advertised window
Save connection state and TCP/IP headers if socket debug option enabled
Create new socket if segment arrives for listening
socket
Computer window scale factor
Reset idle time and keep alive timer
Process TCP options if not in LISTEN state
Telecommunications and Multimedia Unit
Agostinho L S Castro
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Telecommunications and Multimedia Unit
Bibliography
1. Wright, G. R., Stevens, W. R.,"TCP/IP Illustrated – The Implementaion",Volume
2.,Addison-Wesley, 1995 .
2. Wright, G. R., Stevens, W. R.,"TCP/IP Illustrated – The Protocol",Volume 1.,AddisonWesley, 1994 .
3. Rubini, A., “LINUX Device Drivers”, O´Reilly& Associates, Inc.,1998.
4. Rusling, David A., “The Linux Kernel”, LDP – www.linuxdoc.org