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Intro ifnet Strucure ifaddr Structure sockaddr Structure Ifnet and ifaddr Specialization Network initialization overview Ethernet initialization SLIP initialization Lookback Initialization if_attach Function Ifinit Function
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Intro
Interface Layer provides,.. A well-defined ser of interface functions A standard set of statistics and control flags A device-independent method of string protocol addresses A standard queueing method for outgoing packets
Only Best-effort service Higher layers must compensate for lacks of reliability
Refer to three particular network interfaces from Net/3 AMD 7990 LANCE Ethernet interface Serial Line IP(SLIP) interface Lookback interface
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Code Introduction
Three Hearders and Two C files for structure and init.
Global variables
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Ifnet Steucture
Contains information common to all interfaces A separate ifnet is allocated for each network device
Every ifnet has a list of one ore more protocol addresses
Protocol address
1. SLIP support only a single protocol, but
Ethernet do multiple protocols
- Eth could have Internet and OSI protocols
2. Possiblefor one network interface to offer
multiple addresses within a single protocol
※
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ifnet structure_con’t
If_name : Short string that identifies interface type
If_bpf, if_pcount : For BSD Packet filter
- A process can receive copies of packets transmitted or received by an interface
If_unit : Identifies multiple instances of the same type
If_timer
- time in senconds until the kernel calls if_watchdog function
- if_watchdog may used by device driver to collect interface statistics at regular intervals or reset
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Ifnet structure_variables(II)
If System has two SLIP interfaces, If_name : “sl” If_unit : 1 for first, 2 is second
if_flags
Device_specific flags may or may not be modifiable depending on the device
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If_data in ifnet structure
/* other ifnet members */
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If_data in ifnet structure
If_type specifies hardware address type supported by the interface
If_addrlen is the length of the data_link address If_hdrlen is the length of the header attached to outgoing packet by HW
Ethernet header length is 14bytes If_mtu is maximum transmission unit of interface
Control size of packet created by network and transport protocols Ethernet is 1500
If_metric is usually 0 if_baudrate means transmission speed of the interface
Only set by SLIP
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If_data in ifnet structure
If_collision : incremented when trans interrupted on shared media such as ethernet
If_noproto : count the packets when the protocol is not supproted SLIP can support only IP
If_lastchange : the last time any of the statistics were changed
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Interface functions
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ifqueue in ifnet structure
ifq_len : current length of queue
ifq_maxlen : maximum length of queue
ifq_drops : packets dropped because of full queue
Each if has its own ifnet and therefore its own output queue Maximum is set to 50
From the global integer ifqmaxlen, which is initialized at compile from IFQ_MAXLEN
Queue is made up as a linked list of mbuf chains Ifq_drops counts packets discarded because of FULL
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ifqueue routines
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Ifaddr structure
Each interface maintains a linked-list of ifaddr structures Data_link, such as ethernet, offer more than one protocols
ifaddr structure describes each address assigned to the interface, usually one address per protocol
To support multiple addresses Many protocols(TCP/IP) supprot multiple addresses assigned to a
single physical interface
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ifaddr strucuture
Counts references to the ifaddr structure
- shared by the interface and routing data
structures
Support routing lookups for the interface
※ MACRO : IFAFREE
1. It decrements counter and return if there are other references
2. Avoids a function call overhead
3. If last reference, IFAFREE call ifafree func, which release the structure
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ifnet and ifaddr structures
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sockaddr structure
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Every protocol has its own address format Net/3 handles generic addresses in a sockaddr structure
sa_len : length of address sa_family
sa_data is defined to be an array of 14 bytes,.. But, when sockaddr overlays a larger are of memory, sa_data may be
up to 253 bytes long sa_len is only a single byte, so the size(total) must be less than 256
sockaddr structure
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ifnet and ifaddr Specialization
To accommodate additional device and protocol-specific info Each driver defines and each protocol allocates a
specialized version of ifnet and ifaddr These structures contain original ifnet or ifaddr as their first member So, can be accessed without consideration for additional special info
Each device structure begins with ifnet Lookback declares only ifnet
No need to require any device_dependent data
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Arrangement of ifnet within device_dependent Structure
Common to all ethernet drivers and contains info for ARP
and Ethernet multicasting
Contains additional info unique to the LANCE ethernet driver
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Specialized ifaddr structure
Each protocol has its own specialized ifaddr structure IP use in_ifaddr, OSI use iso_ifaddr
Kernel assigns each interface a link-level address when init Kernel construct link-level address : one ifaddr, two sockaddr_dl
One sockaddr for link-level address itself The other for link-level address mask
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Ethernet interface with link-level, Internet, OSI address
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Network Initialization Overview
All structures described are allocated and attached during kernel initialization
Some device ,such as SLIP, lookback, are implemented entirely in software These pseudo-devices are represented by a pdevinit structure pdevinit is stored in the global pdevinit Constructed during kernel configuration
The number of devices to create
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Main Function : Network initialization
/* none network code */
/* none network code */
/* none network code */
Locate & Initialize all the Hardware devices
Finish initialization of interfaces and protocols
Scheduler begins the kernel process scheduler
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Ethernet Initialization
ac_enaddr : ethernet HW address copied by the LANCE device driver
- when? During cpu_startup()
ac_ipaddr : the las IP address assigned to the device
ac_multiaddrs : a list of ethernet multicast addresses represented by ether_multi structure
ac_multicnt : counts the entries in the list
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leattach() function for LANCE ethernet driver
Hd contains HP-specific info, since this driver
is written for an HP workstation
Points to specialized ifnet Points to first member of that structure
HW address is copied from device to sc_addr
Initialize the ifnet structure/* 1500 */
Register interface with BPF
Insert initialized ifnet into linked-list of interface
LANCE doesn’t receive its own transmissions
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SLIP Initialization
Initialized when main calls ‘slattach’ indirectly through the ‘pdev_attach’ pointer in SLIP’s pdevinit structure
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SLIP Initialization
sc_fastq : packets requesting low_delay service sc_ttyp : pointer to the associated terminal sc_flags
sc_escape : used by IP encapsulation mechanism for SLIP
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SLIP Initialization
Initialize sl_softc structure
※ Unlike leattach, which initializes only one interface at a time, the kernel calls slattach once
and slattach initializes all SLIP interfaces
296 bytes(SLMTU)
(DEFAULT)
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Lookback Initialization
Places any outgoing packets back on an appropriate input Q No hardware Initialized when main calls ‘loopattach’ indirectly through the
‘pdev_attach’ pointer in the lookback’s pdevinit
No link header or HW address
/* 1536 */
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if_attach Functioan
1. Three times if_attach has been called
- leattach => lesoftc structure
- slattach => sl_softc, loopattach => loif
2. Create link-level ifaddr(which contains two sockaddr_dl) => p21,22
3. Not yet configured about higherprotocoladdress(eg., IP)
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sockaddr_dl
Contains a name and numeric index for interface Supports a tablelookup for converting between an interface index and
the associated ‘ifaddr’ structure
Sdl_len : entire address length
Sdl_family : address family
Sdl_index : interface within kernel
- eth index is 1, SLIP 2, loopback 3
If_index : last index assigned by the kernel
Sdl_type : initialized by ifent’s if_type
/* unused */
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if_attach
Updates Two global variables if_index : hold the index of the last interface in the system ifnet_addr : pointer to an array of ifaddr pointers
Have a single argument ‘ifp’ a pointer to the ifnet While loop locates the end of the list, so saves address of null
pointer at the end of the list in ‘p’ After loop, the new ifnet is attached to the end of the list, if_index
is incremented The new index is assigned to ifp->if_index
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if_attach()
Previous explanation
Array becomes full
1. Blocks the calling process if it needs to wait for free
Memory to become available
2. What about M_DONWAIT? No bloack and retuen NULL
when not enough memory
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if_attach()
Constructs the name of the interface from if_unit and if_name
- sprint_d converts numeric value if_unit to a string in workbuf
Masklength : the number of bytes occupied by the information before sdl_data in the sockaddr_dl array plus size of text
name for interface
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if_attach : allocate and initialize link-level address
Points to the first sockaddr_dl
Second sockaddr_dl is a bit mask
1
2
1 : Initialize the sockaddr_dl address
2 : Initialize the sockaddr_dl mask
3 : ether_ifattach function is called for Ethernet devices
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/* ifp pointer type : ifnet */
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if_attach()
Structures after the first interface has been attached by if_attach
Initialized Ehternet sockaddr_dl structure
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ether_ifattach
/* HW address length is 6 bytes *//* total length : 14 bytes */
/* it performs ifnet structure initialization common to all Ethernet devices */
The Ethernet address that was copied into the arpcom structure during system Init is now copied into the link-level
address
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ifinit Function
After the interface structures are initialized and linked together, main calls ifinit()
The ‘for’ loop traverse the interface list and sets the maximum size of each interface output queue to 50 If it hasn’t already been set by the interface’s attach function
Ifqueue structure
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if_slowtimo
if_slowtimo starts the interface watchdog timers When timer expires, kernel calls watchdog function for the interface
Interface can reset timer periodically to prevent the watchdog func from call Or, set if_timer to 0 if the watchdog func is not needed
Not used
It blocks packet processing during if_slowtimo
To schedule a call to itself in hz / IFNET_SLOWHZ clock ticks
hz : the number of clock ticks in 1sec
IFNET_SLOWZ : defined to be 1,.
Kernel calls if_slowtimo once every hz clock ticks, which is once per second
