sr_fd / sr_fd_table[] / sr_add_minor()

One of the most important data arrays in the network service is sr_fd_table[], an array of 64 struct sr_fd's. Each sr_fd element in sr_fd_table[] corresponds to either a device or an opened file descriptor to a device (i.e., a "channel"):

typedef struct sr_fd

{

          int srf_flags;

          int srf_fd;

          int srf_port;

          sr_open_t srf_open;

          sr_close_t srf_close;

          sr_write_t srf_write;

          sr_read_t srf_read;

          sr_ioctl_t srf_ioctl;

          sr_cancel_t srf_cancel;

          mq_t *srf_ioctl_q, *srf_ioctl_q_tail;

          mq_t *srf_read_q, *srf_read_q_tail;

          mq_t *srf_write_q, *srf_write_q_tail;

} sr_fd_t;

For each device (e.g., /dev/udp0), an element in sr_fd_table[] is configured by sr_add_minor(). For example, for the following inet.conf file:

eth0 DP8390 0 { default; };
psip1;

an element (i.e., a struct sr_fd) is configured for each of the following devices:

/dev/eth0 sr_fd_table[1]
/dev/ip0 sr_fd_table[2]
/dev/tcp0 sr_fd_table[3]
/dev/udp0 sr_fd_table[4]

/dev/psip1 sr_fd_table[17]
/dev/ip1 sr_fd_table[18]
/dev/tcp1 sr_fd_table[19]
/dev/udp1 sr_fd_table[20]




sr_add_minor() is called in the initialization routines for the various protocols: mnx_eth.c (osdep_eth_init()), psip.c (psip_enable()), ip.c (ip_init()), tcp.c (tcp_init()), and udp.c (udp_init()).



When a device file (e.g., /dev/udp0) is opened by a process, the element that corresponds to the device is copied to an element that is currently unoccupied (see sr_open()). In this way, a "channel" is opened. Using this technique, a channel can be opened, closed, and manipulated without affecting the elements of the descriptors initially set by sr_add_minor().


int srf_flags:

srf_flags is a combination of the following:

#define SFF_FREE 0x00
#define SFF_MINOR 0x01
#define SFF_INUSE 0x02
#define SFF_BUSY 0x3C
#define SFF_IOCTL_IP 0x04
#define SFF_READ_IP 0x08
#define SFF_WRITE_IP 0x10
#define SFF_PENDING_REQ 0x30
#define SFF_SUSPENDED 0x1C0
#define SFF_IOCTL_SUSP 0x40
#define SFF_READ_SUSP 0x80
#define SFF_WRITE_SUSP

srf_flags is initialized to SFF_FREE for each element in sr_fd_table[]. If the channel corresponds to a device file, srf_flags is set to SFF_INUSE | SFF_MINOR. If the channel does not correspond to a device file, srf_flags is set simply to SFF_INUSE.

When a request comes in for a read, write, or ioctl operation and the network service is not already processing another request for the same operation, srf_flags is set to SFF_READ_IP, SFF_WRITE_IP, or SFF_IOCTL_IP. However, if an operation is attempted but the underlying protocol is still processing a previous request of the same nature (e.g., udp_write()), the appropriate flag (SFF_IOCTL_SUSP, SFF_READ_SUSP, or SFF_WRITE_SUSP) in srf_flags is set.


int srf_fd, srf_port:

srf_fd and srf_port are both set by sr_add_minor(). For the channels in srf_fd_table[] that correspond to the device files (e.g., /dev/udp0), srf_fd is set to the minor device number of the device. For example, if /dev/udp0 is added to sr_fd_table[] and the interface number of the device file is 0 (see comments for ip_conf[]), then the minor device number is:

if2minor(ifno, dev) = ((0)*16 + UDP_DEV = 0 + 4 = 4

For the channels in srf_fd_table[] that do not correspond to a device file, srf_fd is the file descriptor for the appropriate protocol. For example, if the file system requests that a udp channel be opened, srf_open is dereferenced and udp_open() is called. udp_open() opens a udp file descriptor and returns the index of the corresponding element in udp_fd_table[]. srf_fd is set to the index of this element.

Later, when the file system requests a read or a write on the open channel, srf_fd is passed into the protocol-specific read or write function (e.g., udp_read()), allowing the protocol-specific function to locate the appropriate file descriptor (e.g., udp file descriptor).

srf_port is more straight-forward. srf_port is the index in the protocol's port table. For example, if a system has two udp device files (/dev/udp0 and /dev/udp1), udp_port_table[] will have two entries, 0 and 1. Therefore, srf_port for the entry in sr_fd_table[] that corresponds to /dev/udp0 will be 0 and srf_port for the entry that corresponds to /dev/udp1 will be 1.


sr_open_t srf_open:
sr_close_t srf_close:
sr_write_t srf_write:
sr_read_t srf_read:
sr_ioctl_t srf_ioctl:
sr_cancel_t srf_cancel:

The fields above are all protocol-specific functions and and are all set by sr_add_minor(). For example, when sr_add_minor() is called by udp_init(), srf_open, srf_close, srf_write, srf_read, srf_ioctl, and srf_cancel are set to the pointers of the functions udp_open(), udp_close(), udp_write(), udp_read(), udp_ioctl(), and udp_cancel(). Later, when the file system makes a request to the network service, these functions will be called. For example, if the file system requests that data is written to a channel, srf_write is dereferenced and, if the channel is a udp channel, udp_write() is called.

mq_t *srf_ioctl_q, *srf_ioctl_q_tail:
mq_t *srf_read_q, *srf_read_q_tail:
mq_t *srf_write_q, *srf_write_q_tail:

The fields above are linked lists of ioctl, read, and write messages waiting to be processed. When a message requesting an ioctl, read, or write operation is received, the message is placed at the end of the linked list (unless there are no previous messages of this type that have not already been processed).


After the initialization of the network service, sr_rec() is called upon receipt of messages from the file system in the endless loop within main(). sr_rec() then calls a function to handle the specific request. For open requests, sr_rec() calls sr_open(); for read, write, and io requests, sr_rec() calls sr_rwio(); for close requests, sr_rec() calls sr_close(); for cancel requests, sr_rec() calls sr_cancel().

ip_open()

ip_open() finds an available ip file descriptor in ip_fd_table[], sets a few of the ip file descriptor's fields, and then returns the index of the ip file descriptor within ip_fd_table[]. ip_open() is called by higher-level code (e.g., udp_main()) and the returned ip file descriptor is then associated with a higher-level port (e.g., udp port).



Note that there will only be a few ip file descriptors open at any given time. There will be an ip file descriptor opened for each interface for each client (udp, tcp, and icmp) and there will be one ip file descriptor opened each time the /dev/ip file is opened directly (as opposed to when, for example, the /dev/udp file is opened).

ip_ioctl()

ip_ioctl(fd, req) performs one of several tasks on the ip file descriptor whose index within ip_fd_table[] is fd, the first parameter. The task performed depends on req, the second parameter.

NWIOSIPOPT: Set the options (the if_ipopt field of the ip file descriptor) on the ip file descriptor. For example, during the initialization of a physical udp port, ip_ioctl() is called with req equal to NWIOSIPOPT.

An example of an ip iption (i.e., ip flag) is the NWIO_EN_BROAD flag. This flag is set if the ip file descriptor accepts broadcast packets. The options desired are obtained from the user process. For example, if a udp port opened up the ip file descriptor, udp_get_data() is (indirectly) called to obtain the configuration data.

NWIOGIPOPT: Send the ip file descriptor's options to the user process requesting the information. The information is sent in a struct of type nwio_ipopt_t.

NWIOSIPCONF: Configure the ip port (for example, the ip address can be configured) that corresponds to the ip file descriptor fd. The fields are obtained from the user process. For a detailed description of the different settings, click here.

NWIOGIPCONF: Send the ip address/subnet information (i.e., send a nwio_ipconf_t struct) to the next higher layer. For example, if the next higher layer is udp, ip_ioctl() calls (indirectly) udp_put_data(), which sets the ip address for the udp port (i.e., sets the up_ipaddr field of the corresponding element in udp_port_table[]).

NWIOGIPIROUTE, NWIOSIPIROUTE, NWIOGIPOROUTE, NWIODIPIROUTE, NWIOSIPOROUTE: It is possible to influence the route taken by a packet. These ioctl requests alter the input and output routing tables.

bf_memreq()

After the buffers have been initialized, accessors[] looks like the following:



bf_memreq() allocates accessors to the caller. For example, if 1514 bytes of buffer space are requested immediately after the network process starts and each buffer is 512 bytes (the default), then accessors[] will look like the following:



Note that three elements of accessors[] have been removed from buf512_freelist and that the head of the chain of the 3 accessors is returned by bf_memreq(). Also note that the acc_linkC and buf_linkC fields have been set to one and acc_length and acc_offset have been set to their appropriate values.

So what happens if there are not enough buffers on the buf512_freelist to satisfy a request? On lines 2280-2290 of buf.c, functions that free buffers for the specific clients (e.g., eth_buffree()) are called until there are enough buffers on buf512_freelist.

For a complete description of the network service's buffer management, click here.

nwio_ipopt

The nwio_ipopt struct is used to pass ip option values from a higher-layer level (e.g., icmp, udp) to the ip layer during the configuration of an ip file descriptor. Note that an ip file descriptor cannot be used until it is configured.

typedef struct nwio_ipopt

{

          u32_t nwio_flags;

          ipaddr_t nwio_rem;

          ip_hdropt_t nwio_hdropt;

          u8_t nwio_tos;

          u8_t nwio_ttl;

          u8_t nwio_df;

          ipproto_t nwio_proto;

} nwio_ipopt_t;

u32_t nwio_flags:

nwio_flags will be a combination of the flags below. Most of the flags within a set are exclusionary. For example, both NWIO_REMSPEC and NWIO_REMANY can't both be set.

Note that "EN" stands for "ENable" and "DI" stands for "DIsable".

#define NWIO_EXCL 0x00000001l
#define NWIO_SHARED 0x00000002l
#define NWIO_COPY 0x00000003l

From ip(4):

"The options covered by NWIO_ACC_MASK control the number of channels that can use one IP protocol. If NWIO_EXCL is specified then only that channel can use a certain IP protocol. If NWIO_SHARED then multiple channels that all have to specify NWIO_SHARED can use the same IP protocol, but incoming packets will be delivered to a most one channel. NWIO_COPY does not impose any restrictions. Every channel gets a copy of an incoming packet."

Note that, for whatever reason, NWIO_EXCL behaves exactly as NWIO_COPY. Every channel receives a copy of an incoming packet.

The access flags are important during the read of an ip file descriptor.

#define NWIO_EN_LOC 0x00000010l
#define NWIO_DI_LOC 0x00100000l
#define NWIO_EN_BROAD 0x00000020l
#define NWIO_DI_BROAD 0x00200000l

NWIO_EN_LOC specifies that this file descriptor can receive packets destined for this machine and NWIO_EN_BROAD specifies that this file descriptor can receive broadcast packets.

#define NWIO_REMSPEC 0x00000100l
#define NWIO_REMANY 0x01000000l

If the NWIO_REMANY flag is set, this file descriptor can send packets to any destination. If, on the other hand, the NWIO_REMSPEC flag is set, this file descriptor can only communicate with a single host. This host is specified by nwio_rem (see below).

#define NWIO_PROTOSPEC 0x00000200l
#define NWIO_PROTOANY 0x02000000l

If NWIO_PROTOANY is set, the ip file descriptor will accept packets of any protocol type. However, if NWIO_PROTOSPEC is set, only packets with a protocol type of nwio_proto (see below) are accepted.

#define NWIO_HDR_O_SPEC 0x00000400l
#define NWIO_HDR_O_ANY 0x04000000l

If the NWIO_HDR_O_SPEC flag in nwio_flags is set, nwio_hdropt (see below) must be set. If this is the case, the extra header information for all outgoing packets will be taken from nwio_hdropt, nwio_tos, nwio_ttl, and nwio_df (see below).

#define NWIO_RWDATONLY 0x00001000l
#define NWIO_RWDATALL 0x10000000l

NWIO_RWDATALL is a little tricky. If the NWIO_RWDATALL flag is set, the header was omitted when passing the packet down to ip code and the NWIO_EN_LOC, NWIO_DI_BROAD, NWIO_REMSPEC, NWIO_PROTOSPEC and NWIO_HDR_O_SPEC flags must all be set (and NWIO_REMANY and NWIO_PROTOANY cannot be set). In other words, this file descriptor can only send the data to one destination using one protocol.

During the configuration of an ip file descriptor being opened by the udp code, ip_ioctl() calls udp_get_data() (indirectly) to get configuration information. udp_get_data() returns the following configuration information:

NWIO_COPY | NWIO_EN_LOC | NWIO_EN_BROAD | NWIO_REMANY | NWIO_PROTOSPEC | NWIO_HDR_O_ANY | NWIO_RWDATALL

ipaddr_t nwio_rem:

If the NWIO_REMSPEC flag in nwio_flags is set (see above), nwio_rem is the ip address of the destination host.


ip_hdropt_t nwio_hdropt:

The ip header length is flexible to allow for extra options. For example, in addition to the normal fields (e.g., destination ip address), an ip header may specify the route it wishes to take or request that the route be recorded.

that it wishes to record a route or to ip_chk_hdropt().


u8_t nwio_tos:

"tos" stands for "Type Of Service". nwio_tos is initialized to 0 but can be changed by ip_ioctl().


u8_t nwio_ttl:

"ttl" stands for "Time To Live", which is the number of hops that a packet can take before being dropped by a router. nwio_ttl is initialized to 255 but can be changed by ip_ioctl().


u8_t nwio_df:

nwio_df specifies whether fragmentation is allowed or not. nwio_df is initialized to FALSE but, again, can be changed by ip_ioctl().


ipproto_t nwio_proto:

nwio_proto can take one of the values below. Obviously, if the udp code opens up an ip file descriptor, nwio_proto will be IPPROTO_UDP. The same is true for icmp and tcp.

#define IPPROTO_ICMP 1
#define IPPROTO_TCP 6
#define IPPROTO_UDP 17

This field is used in conjunction with the NWIO_PROTOSPEC flag in nwio_flags. If this flag is set, nwio_proto must be set.

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_cut()

If a section of a linked list needs to be duplicated, bf_cut(data, offset, length) is called. For example, if a section of length 50 starting at an offset of 75 of the linked list below needs to be duplicated, bf_cut(data, 75, 50) is called:



Note that the original linked list remains unchanged and that acc_linkC for all the accessors in the new linked list is one.

If length (the second parameter) is zero, simply duplicate the first accessor in the linked list but set acc_length=0 and acc_next=null. In other words, create a linked list of length one accessor whose acc_length is 0.

bf_cut() is used in a number of scenarios, including cutting a received ethernet packet to size.

For a full description of the network service's buffer management, click here.

nwio_ipconf_t

Type nwio_ipconf_t contains the ip address information of an ip port (i.e., an element in ip_port_table[]:

typedef struct nwio_ipconf

{

          u32_t nwic_flags;

          ipaddr_t nwic_ipaddr;

          ipaddr_t nwic_netmask;

} nwio_ipconf_t;

nwic_flags: reflects whether the ip address and the subnet mask have been set. The different flags are as follows:

#define NWIC_NOFLAGS 0x0
#define NWIC_FLAGS 0x3
#define NWIC_IPADDR_SET 0x1
#define NWIC_NETMASK_SET 0x2

nwic_ipaddr: the ip address (e.g., 192.168.5.5)

nwic_netmask: the subnet mask (e.g., 255.255.255.0)

bf_packIffLess()

If the data in a linked list of accessors is less than min_len (the second parameter), bf_packIffLess(pack, min_len) packs the data by calling bf_pack().

bf_packIffLess() is often called to ensure that a packet's header is in a single contiguous buffer so that the individual fields of the header can be easily accessed.

For a detailed description of the network service's buffer management, click here.

ptr2acc_data()

The macro ptr2acc_data is #define'd in inet/generic/buf.h as:

#define ptr2acc_data(/* acc_t * */ a) (bf_temporary_acc=(a), \
(&bf_temporary_acc->acc_buffer->buf_data_p[bf_temporary_acc-> \
acc_offset]))

ptr2acc_data() simply returns a pointer to the actual data within an accessor.

ptr2acc_data() is usually called so that the fields of a header (e.g., ip header) can be analyzed.

ptr2acc_data()

The macro ptr2acc_data is #define'd in inet/generic/buf.h as:

#define ptr2acc_data(/* acc_t * */ a) (bf_temporary_acc=(a), \
(&bf_temporary_acc->acc_buffer->buf_data_p[bf_temporary_acc-> \
acc_offset]))

ptr2acc_data() simply returns a pointer to the actual data within an accessor.

ptr2acc_data() is usually called so that the fields of a header (e.g., ip header) can be analyzed.

bf_align()

If data is not already packed and aligned, bf_align(acc, size, alignment) packs size (bf_align's second parameter) bytes from acc, bf_align()'s first parameter and a linked list of accessors (i.e., a packet), by calling bf_pack(). This packing is necessary to ensure that all of the fields from a header are easily accessed. For example, the ip code aligns a packet's header contained in the accessors before accessing the various ip header fields.

For a detailed description of the network service's buffer management, click here.

ptr2acc_data()

The macro ptr2acc_data is #define'd in inet/generic/buf.h as:

#define ptr2acc_data(/* acc_t * */ a) (bf_temporary_acc=(a), \
(&bf_temporary_acc->acc_buffer->buf_data_p[bf_temporary_acc-> \
acc_offset]))

ptr2acc_data() simply returns a pointer to the actual data within an accessor.

ptr2acc_data() is usually called so that the fields of a header (e.g., ip header) can be analyzed.

bf_delhead()

If only the beginning of a linked list can be freed, bf_delhead() is called. If acc_linkC and buf_linkC are one for all of the relevant accessors and their associated buffers in the linked list, the operation is straight-forward:



bf_delhead() is often called to remove the header (e.g., ip header) from a packet.

For a detailed description of the network service's buffer management, click here.

tcp_pack_oneCsum()

tcp_pack_oneCsum() computes the checksum of a tcp packet (including several fields of its ip header). It accomplishes this by computing the checksum (by calling oneC_sum()) of each of the tcp packet's buffers.

Note that a checksum is used to determine if errors occurred during the transmission of data.

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_packIffLess()

If the data in a linked list of accessors is less than min_len (the second parameter), bf_packIffLess(pack, min_len) packs the data by calling bf_pack().

bf_packIffLess() is often called to ensure that a packet's header is in a single contiguous buffer so that the individual fields of the header can be easily accessed.

For a detailed description of the network service's buffer management, click here.

ptr2acc_data()

The macro ptr2acc_data is #define'd in inet/generic/buf.h as:

#define ptr2acc_data(/* acc_t * */ a) (bf_temporary_acc=(a), \
(&bf_temporary_acc->acc_buffer->buf_data_p[bf_temporary_acc-> \
acc_offset]))

ptr2acc_data() simply returns a pointer to the actual data within an accessor.

ptr2acc_data() is usually called so that the fields of a header (e.g., ip header) can be analyzed.

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_align()

If data is not already packed and aligned, bf_align(acc, size, alignment) packs size (bf_align's second parameter) bytes from acc, bf_align()'s first parameter and a linked list of accessors (i.e., a packet), by calling bf_pack(). This packing is necessary to ensure that all of the fields from a header are easily accessed. For example, the ip code aligns a packet's header contained in the accessors before accessing the various ip header fields.

For a detailed description of the network service's buffer management, click here.

ptr2acc_data()

The macro ptr2acc_data is #define'd in inet/generic/buf.h as:

#define ptr2acc_data(/* acc_t * */ a) (bf_temporary_acc=(a), \
(&bf_temporary_acc->acc_buffer->buf_data_p[bf_temporary_acc-> \
acc_offset]))

ptr2acc_data() simply returns a pointer to the actual data within an accessor.

ptr2acc_data() is usually called so that the fields of a header (e.g., ip header) can be analyzed.

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_delhead()

If only the beginning of a linked list can be freed, bf_delhead() is called. If acc_linkC and buf_linkC are one for all of the relevant accessors and their associated buffers in the linked list, the operation is straight-forward:



bf_delhead() is often called to remove the header (e.g., ip header) from a packet.

For a detailed description of the network service's buffer management, click here.

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_memreq()

After the buffers have been initialized, accessors[] looks like the following:



bf_memreq() allocates accessors to the caller. For example, if 1514 bytes of buffer space are requested immediately after the network process starts and each buffer is 512 bytes (the default), then accessors[] will look like the following:



Note that three elements of accessors[] have been removed from buf512_freelist and that the head of the chain of the 3 accessors is returned by bf_memreq(). Also note that the acc_linkC and buf_linkC fields have been set to one and acc_length and acc_offset have been set to their appropriate values.

So what happens if there are not enough buffers on the buf512_freelist to satisfy a request? On lines 2280-2290 of buf.c, functions that free buffers for the specific clients (e.g., eth_buffree()) are called until there are enough buffers on buf512_freelist.

For a complete description of the network service's buffer management, click here.

bf_packIffLess()

If the data in a linked list of accessors is less than min_len (the second parameter), bf_packIffLess(pack, min_len) packs the data by calling bf_pack().

bf_packIffLess() is often called to ensure that a packet's header is in a single contiguous buffer so that the individual fields of the header can be easily accessed.

For a detailed description of the network service's buffer management, click here.

ptr2acc_data()

The macro ptr2acc_data is #define'd in inet/generic/buf.h as:

#define ptr2acc_data(/* acc_t * */ a) (bf_temporary_acc=(a), \
(&bf_temporary_acc->acc_buffer->buf_data_p[bf_temporary_acc-> \
acc_offset]))

ptr2acc_data() simply returns a pointer to the actual data within an accessor.

ptr2acc_data() is usually called so that the fields of a header (e.g., ip header) can be analyzed.

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

find_unused_port()

find_unused_port() attempts to claim an unused port number. The first port number that is requested is equal to the index number of the udp file descriptor plus 0xC000 (49152). If this port is not available, the port number is incremented until an unclaimed port number is found. So for the sixth udp file descriptor (i.e., udp_fd_table[5]), find_unused_port() first tries port 49157 (49152 + 5), then tries 49158, and so on until an available port if found.

find_unused_port() is called only a single time: while setting a udp file descriptor's options with udp_setopt() (more specifically, if a user process attempts to set the NWUO_LP_SEL (Local Port SELect) flag).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_packIffLess()

If the data in a linked list of accessors is less than min_len (the second parameter), bf_packIffLess(pack, min_len) packs the data by calling bf_pack().

bf_packIffLess() is often called to ensure that a packet's header is in a single contiguous buffer so that the individual fields of the header can be easily accessed.

For a detailed description of the network service's buffer management, click here.

ptr2acc_data()

The macro ptr2acc_data is #define'd in inet/generic/buf.h as:

#define ptr2acc_data(/* acc_t * */ a) (bf_temporary_acc=(a), \
(&bf_temporary_acc->acc_buffer->buf_data_p[bf_temporary_acc-> \
acc_offset]))

ptr2acc_data() simply returns a pointer to the actual data within an accessor.

ptr2acc_data() is usually called so that the fields of a header (e.g., ip header) can be analyzed.

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

bf_afree()

After a chain of accessors is no longer needed, the chain (and not simply the single accessor passed as the parameter) can be freed by calling bf_free(). However, if either acc_linkC or buf_linkC of one of the accessors in the linked list is not equal to one (1), the entire chain will not be freed. For example, if buf_afree(acc1) is called for the following chain:



Then the resulting chain will be:



bf_afree() returns acc1 (accessors[63]) to acc_freelist (recall that acc_freelist is the linked list of acc_t's without an associated buffer). However, buffers512[127] cannot be freed because acc2 (accessors[64]) still references it.

bf_afree() is called after an accessor's associated data is no longer needed (for example, after a packet has been sent off by the ethernet driver).

find_unused_port()

find_unused_port() attempts to claim an unused port number. The first port number that is requested is equal to the index number of the udp file descriptor plus 0xC000 (49152). If this port is not available, the port number is incremented until an unclaimed port number is found. So for the sixth udp file descriptor (i.e., udp_fd_table[5]), find_unused_port() first tries port 49157 (49152 + 5), then tries 49158, and so on until an available port if found.

find_unused_port() is called only a single time: while setting a udp file descriptor's options with udp_setopt() (more specifically, if a user process attempts to set the NWUO_LP_SEL (Local Port SELect) flag).

ip_read()

If there are unexpired packets in the ip file descriptor fd's read queue, ip_read(fd, count) passes count (ip_read()'s second parameter) bytes off to the next-higher layer by calling packet2user(). If the packets in the file descriptor's read queue have expired, ip_read() discards the packets.

ip_read() is (indirectly) called by sr_rwio() when a process reads an ip device file (e.g., /dev/ip).

In the udp code, ip_read() is called by read_ip_packets() during the initialization of the udp code. Normally, ip_read() is not called by the udp code after the initialization.

htons() / ntohs() / htonl() / ntohl()

From htons(3):

"htons() converts a 16-bit quantity from host byte order to network byte order."

Different CPU architectures group multiple bytes differently. For example, on a "little-endian" machine (an example of which is the Intel CPU), the value 0x1234 is stored in memory as 0x3412. However, on a "big-endian" machine, the value 0x1234 is stored in memory as 0x1234.

It is important that values in a header are sent across a network in a consistent manner independent of the architecture of the sending or receiving system. For this reason, a standard was chosen. The standard chosen was big-endian although it could have just as well been little-endian.

htons() is defined in /include/net/hton.h, as:
#define htons(x) (_tmp=(x), ((_tmp>>8) & 0xff) | ((_tmp<<8) & 0xff00))

ntohs() converts a 16-bit quantity from network byte order to host byte order, the reverse of htons().

htonl() and ntohl() are identical to htons() and ntohs() except that they convert 32-bit quantities instead of 16-bit quantities.

Processes generally supply header information when sending packets. The data in these fields is converted to the network format (i.e., big-endian) by the process before the process copies the data to the network service.