c_generic.h File Reference

ROHC generic compression context for IP-only, UDP and UDP Lite profiles. More...

#include "rohc_comp_internals.h"
#include "rohc_packets.h"
#include "comp_list.h"
#include "ip.h"
#include <stdlib.h>
#include <netinet/ip.h>

Include dependency graph for c_generic.h:

This graph shows which files directly or indirectly include this file:

Go to the source code of this file.

Data Structures
struct	ipv4_header_info
	Store information about an IPv4 header between the different compressions of IP packets. More...
struct	ipv6_header_info
	Store information about an IPv6 header between the different compressions of IP packets. More...
struct	ip_header_info
	Store information about an IP (IPv4 or IPv6) header between the different compressions of IP packets. More...
struct	generic_tmp_vars
	Structure that contains variables that are used during one single compression of packet. More...
struct	c_generic_context
	The generic compression context. More...
struct	list_comp
	The list compressor. More...
Defines
#define	IPID_MAX_DELTA   20
	The maximal delta accepted between two consecutive IPv4 ID so that it can be considered as coded in Network Byte Order (NBO)
#define	MAX_ITEM   15
	The number of compression list items.
#define	L   5
Functions
int	c_generic_create (struct c_context *const context, const struct ip_packet *ip)
	Create a new context and initialize it thanks to the given IP packet.
void	c_generic_destroy (struct c_context *const context)
	Destroy the context.
void	change_mode (struct c_context *const context, const rohc_mode new_mode)
	Change the mode of the context.
void	change_state (struct c_context *const context, const rohc_c_state new_state)
	Change the state of the context.
void	ip6_c_init_table (struct list_comp *const comp)
	Initialize the tables IPv6 extension in compressor.
int	rohc_list_decide_ipv6_compression (struct list_comp *const comp, const struct ip_packet *const ip)
	Decide whether list of IPv6 extension headers shall be sent compressed.
int	c_create_current_list (const int index, struct list_comp *const comp, const unsigned char *ext, const int index_table)
	Create the current list.
int	rohc_list_decide_type (struct list_comp *const comp)
	Decide the encoding type for compression list.
int	rohc_list_encode (struct list_comp *const comp, unsigned char *const dest, int counter, const int ps, const int size)
	Generic encoding of compressed list.
int	rohc_list_encode_type_0 (struct list_comp *const comp, unsigned char *const dest, int counter, const int ps)
	Build encoding type 0 for list compression.
int	rohc_list_encode_type_1 (struct list_comp *const comp, unsigned char *const dest, int counter, const int ps)
	Build encoding type 1 for list compression.
int	rohc_list_encode_type_2 (struct list_comp *const comp, unsigned char *const dest, int counter, const int ps)
	Build encoding type 2 for list compression.
int	rohc_list_encode_type_3 (struct list_comp *const comp, unsigned char *const dest, int counter, const int ps)
	Build encoding type 3 for list compression.
int	c_generic_encode (struct c_context *const context, const struct ip_packet *ip, const int packet_size, unsigned char *const dest, const int dest_size, rohc_packet_t *const packet_type, int *const payload_offset)
	Encode an IP packet according to a pattern decided by several different factors.
void	c_generic_feedback (struct c_context *const context, const struct c_feedback *feedback)
	Update the profile when feedback arrives.
void	decide_state (struct c_context *const context)
	Decide the state that should be used for the next packet.

---

Detailed Description

ROHC generic compression context for IP-only, UDP and UDP Lite profiles.

Author:

Didier Barvaux <didier.barvaux@toulouse.viveris.com>

The hackers from ROHC for Linux

---

Define Documentation

#define IPID_MAX_DELTA   20

The maximal delta accepted between two consecutive IPv4 ID so that it can be considered as coded in Network Byte Order (NBO)

Referenced by check_ip_identification().

#define L   5

The number of compressed list to send to make the reference list L is the name specified in the RFC

Referenced by rohc_list_decide_ipv6_compression().

#define MAX_ITEM   15

The number of compression list items.

Referenced by rohc_list_decide_ipv6_compression().

---

Function Documentation

int c_create_current_list	(	const int	index,
		struct list_comp *const	comp,
		const unsigned char *	ext,
		const int	index_table
	)

Create the current list.

Parameters:

comp	The list compressor which is specific to the extension type
ext	The extension
index	The number of the extension
index_table	The index of the item in the based table

Returns:

1 if successful, 0 otherwise

References list_comp::based_table, list_comp::compare, c_translation::counter, list_comp::counter, list_comp::create_item, list_comp::curr_list, delete_elt(), elt_index(), get_elt(), list_comp::get_size, list_elt::index_table, insert_elt(), list_elt::item, c_translation::known, rohc_list_item::length, push_back(), rohc_debugf, list_comp::trans_table, and rohc_list_item::type.

Referenced by rohc_list_decide_ipv6_compression().

int c_generic_create	(	struct c_context *const	context,
		const struct ip_packet *	ip
	)

Create a new context and initialize it thanks to the given IP packet.

This function is one of the functions that must exist in one profile for the framework to work.

Parameters:

context	The compression context
ip	The IP packet given to initialize the new context

Returns:

1 if successful, 0 otherwise

References c_create_wlsb(), c_generic_destroy(), c_init_header_info(), c_init_tmp_variables(), C_WINDOW_WIDTH, c_generic_context::code_dynamic_part, c_generic_context::code_static_part, c_generic_context::code_UO_packet_head, c_generic_context::code_UO_packet_tail, c_generic_context::compute_crc_dynamic, compute_crc_dynamic(), compute_crc_static(), c_generic_context::compute_crc_static, c_generic_context::decide_state, decide_state(), c_generic_context::fo_count, c_generic_context::go_back_fo_count, c_generic_context::go_back_ir_count, c_profile::id, c_generic_context::init_at_IR, c_generic_context::ip_flags, ip_get_inner_packet(), ip_get_protocol(), c_generic_context::ir_count, c_generic_context::ir_dyn_count, c_generic_context::is_ip2_initialized, c_generic_context::next_header_len, c_generic_context::next_header_proto, c_context::profile, rohc_debugf, ROHC_PROFILE_IP, ROHC_PROFILE_RTP, ROHC_PROFILE_UDP, ROHC_PROFILE_UDPLITE, ROHC_PROFILE_UNCOMPRESSED, c_generic_context::sn, c_generic_context::sn_window, c_generic_context::so_count, c_context::specific, c_generic_context::specific, and c_generic_context::tmp.

Referenced by c_rtp_create(), c_udp_create(), and c_udp_lite_create().

void c_generic_destroy

(

struct c_context *const

context

)

Destroy the context.

This function is one of the functions that must exist in one profile for the framework to work.

Parameters:

context

The compression context

References c_destroy_wlsb(), list_comp::curr_list, destroy_list(), ipv6_header_info::ext_comp, ip_header_info::info, c_generic_context::ip2_flags, c_generic_context::ip_flags, ipv4_header_info::ip_id_window, IPV4, IPV6, c_generic_context::is_ip2_initialized, list_comp::ref_list, c_generic_context::sn_window, c_context::specific, c_generic_context::specific, ip_header_info::v4, ip_header_info::v6, ip_header_info::version, and zfree.

Referenced by c_generic_create(), c_rtp_create(), c_rtp_destroy(), c_udp_create(), and c_udp_lite_create().

int c_generic_encode	(	struct c_context *const	context,
		const struct ip_packet *	ip,
		const int	packet_size,
		unsigned char *const	dest,
		const int	dest_size,
		rohc_packet_t *const	packet_type,
		int *const	payload_offset
	)

Encode an IP packet according to a pattern decided by several different factors.

1. Check if we have double IP headers.
2. Check if the IP-ID fields are random and if they are in NBO.
3. Decide in which state to go (IR, FO or SO).
4. Decide how many bits are needed to send the IP-ID and SN fields and more important update the sliding windows.
5. Decide which packet type to send.
6. Code the packet.

This function is one of the functions that must exist in one profile for the framework to work.

Parameters:

context	The compression context
ip	The IP packet to encode
packet_size	The length of the IP packet to encode
dest	The rohc-packet-under-build buffer
dest_size	The length of the rohc-packet-under-build buffer
packet_type	OUT: The type of ROHC packet that is created
payload_offset	The offset for the payload in the IP packet

Returns:

The length of the created ROHC packet or -1 in case of failure

References c_add_ts(), c_init_header_info(), changed_dynamic_both_hdr(), generic_tmp_vars::changed_fields, changed_fields(), generic_tmp_vars::changed_fields2, changed_static_both_hdr(), check_ip_identification(), code_packet(), c_generic_context::decide_state, c_profile::id, ip_header_info::info, c_generic_context::ip2_flags, c_generic_context::ip_flags, ip_get_hdrlen(), ip_get_inner_packet(), ip_get_next_layer(), ip_get_protocol(), ip_get_total_extension_size(), ip_get_version(), IPV4, ipv4_get_header(), ipv4_get_id(), ipv6_get_header(), c_generic_context::is_ip2_initialized, generic_tmp_vars::max_size, ipv4_header_info::nbo, c_generic_context::next_header_len, c_generic_context::next_header_proto, generic_tmp_vars::nr_ip_id_bits2, generic_tmp_vars::nr_of_ip_hdr, c_context::num_sent_ir, c_context::num_sent_ir_dyn, ipv4_header_info::old_ip, ipv6_header_info::old_ip, ipv4_header_info::old_nbo, ipv4_header_info::old_rnd, PACKET_IR, PACKET_IR_DYN, generic_tmp_vars::packet_type, PACKET_UNKNOWN, c_context::profile, ipv4_header_info::rnd, rohc_debugf, ROHC_OK, ROHC_PROFILE_RTP, generic_tmp_vars::send_dynamic, generic_tmp_vars::send_static, ip_packet::size, rtphdr::sn, c_generic_context::sn, c_context::specific, c_generic_context::specific, rtp_tmp_vars::timestamp, sc_rtp_context::tmp, c_generic_context::tmp, sc_rtp_context::ts_sc, ip_header_info::v4, and ip_header_info::v6.

Referenced by c_rtp_encode(), c_udp_encode(), and c_udp_lite_encode().

void c_generic_feedback	(	struct c_context *const	context,
		const struct c_feedback *	feedback
	)

Update the profile when feedback arrives.

This function is one of the functions that must exist in one profile for the framework to work.

Parameters:

context	The compression context
feedback	The feedback information including the whole feedback packet

References c_feedback::acktype, c_ack_sn_wlsb(), change_mode(), change_state(), c_context::compressor, crc_calculate(), CRC_INIT_8, rohc_comp::crc_table_8, CRC_TYPE_8, c_feedback::data, FO, ip_header_info::info, c_generic_context::ip2_flags, c_generic_context::ip_flags, ipv4_header_info::ip_id_window, IPV4, IR, c_generic_context::ir_dyn_count, c_generic_context::is_ip2_initialized, rohc_debugf, c_feedback::size, sn, c_generic_context::sn_window, SO, c_context::specific, c_feedback::specific_offset, c_feedback::specific_size, c_context::state, c_feedback::type, ip_header_info::v4, and ip_header_info::version.

void change_mode	(	struct c_context *const	context,
		const rohc_mode	new_mode
	)

Change the mode of the context.

Parameters:

context	The compression context
new_mode	The new mode the context must enter in

References change_state(), IR, c_context::mode, and rohc_debugf.

Referenced by c_generic_feedback().

void change_state	(	struct c_context *const	context,
		const rohc_c_state	new_state
	)

Change the state of the context.

Parameters:

context	The compression context
new_state	The new state the context must enter in

References c_generic_context::fo_count, c_generic_context::ir_count, rohc_debugf, c_generic_context::so_count, c_context::specific, and c_context::state.

Referenced by c_generic_feedback(), change_mode(), decide_state(), periodic_down_transition(), rtp_decide_state(), and udp_decide_state().

void decide_state

(

struct c_context *const

context

)

Decide the state that should be used for the next packet.

The three states are:

• Initialization and Refresh (IR),
  
• First Order (FO),
  
• Second Order (SO).

Parameters:

context

The compression context

References change_state(), FO, c_generic_context::fo_count, IR, c_generic_context::ir_count, MAX_FO_COUNT, MAX_IR_COUNT, c_context::mode, periodic_down_transition(), generic_tmp_vars::send_dynamic, generic_tmp_vars::send_static, SO, c_context::specific, c_context::state, c_generic_context::tmp, and U_MODE.

Referenced by c_generic_create(), c_udp_lite_create(), rtp_decide_state(), and udp_decide_state().

void ip6_c_init_table

(

struct list_comp *const

comp

)

Initialize the tables IPv6 extension in compressor.

Parameters:

comp	The list compressor

References AH, list_comp::based_table, c_translation::counter, rohc_list_item::data, DEST, HBH, c_translation::item, c_translation::known, rohc_list_item::length, RTHDR, list_comp::trans_table, and rohc_list_item::type.

Referenced by c_init_header_info().

int rohc_list_decide_ipv6_compression	(	struct list_comp *const	comp,
		const struct ip_packet *const	ip
	)

Decide whether list of IPv6 extension headers shall be sent compressed.

Parameters:

comp	The list compressor which is specific to the extension type
ip	The IP packet to compress

Returns:

• 1 if a compressed list must be sent,
• 0 if not,
• -1 if error

References c_create_current_list(), c_translation::counter, list_comp::counter, list_comp::curr_list, delete_elt(), empty_list(), c_list::gen_id, get_elt(), list_comp::get_extension, list_comp::get_index_table, list_elt::index_table, insert_elt(), ip_get_raw_data(), list_comp::islist, list_elt::item, c_translation::known, L, MAX_ITEM, list_comp::ref_list, rohc_debugf, size_list(), list_comp::trans_table, rohc_list_item::type, and list_comp::update_done.

Referenced by code_ipv6_dynamic_part().

int rohc_list_decide_type

(

struct list_comp *const

comp

)

Decide the encoding type for compression list.

Parameters:

comp	The list compressor

Returns:

the encoding type among [0-3]

References list_comp::curr_list, c_list::first_elt, get_elt(), list_elt::index_table, list_comp::islist, list_elt::item, c_translation::known, list_comp::list_compress, list_comp::ref_list, rohc_debugf, size_list(), list_comp::trans_table, and type_is_present().

Referenced by rohc_list_encode().

int rohc_list_encode	(	struct list_comp *const	comp,
		unsigned char *const	dest,
		int	counter,
		const int	ps,
		const int	size
	)

Generic encoding of compressed list.

Parameters:

comp	The list compressor
dest	The ROHC packet under build
counter	The current position in the rohc-packet-under-build buffer
ps	The size of the index
size	The number of element in current list

Returns:

The new position in the rohc-packet-under-build buffer, -1 in case of error

References rohc_debugf, rohc_list_decide_type(), rohc_list_encode_type_0(), rohc_list_encode_type_1(), rohc_list_encode_type_2(), and rohc_list_encode_type_3().

Referenced by code_ipv6_dynamic_part().

int rohc_list_encode_type_0	(	struct list_comp *const	comp,
		unsigned char *const	dest,
		int	counter,
		const int	ps
	)

Build encoding type 0 for list compression.

Todo:

this function is inefficient as it loops many times on the same list (see get_elt especially)

 Encoding type 0 (5.8.6.1):

      0   1   2   3   4   5   6   7
     --- --- --- --- --- --- --- ---
 1  | ET = 0| GP| PS|   CC = m      |
    +---+---+---+---+---+---+---+---+
 2  :            gen_id             : 1 octet, if GP = 1
    +---+---+---+---+---+---+---+---+
    |       XI 1, ..., XI m         | m octets, or m * 4 bits
 3  /               --- --- --- --- /
    |               :    Padding    : if PS = 0 and m is odd
    +---+---+---+---+---+---+---+---+
    |                               |
 4  /      item 1, ..., item n      / variable length
    |                               |
    +---+---+---+---+---+---+---+---+

 ET: Encoding type is zero.

 GP: Indicates presence of gen_id field.

 PS: Indicates size of XI fields:
     PS = 0 indicates 4-bit XI fields;
     PS = 1 indicates 8-bit XI fields.

 CC: CSRC counter from original RTP header.

 gen_id: Identifier for a sequence of identical lists.  It is
     present in U/O-mode when the compressor decides that it may use
     this list as a future reference list.

 XI 1, ..., XI m: m XI items. The format of an XI item is as
     follows:

              +---+---+---+---+
     PS = 0:  | X |   Index   |
              +---+---+---+---+

                0   1   2   3   4   5   6   7
              +---+---+---+---+---+---+---+---+
     PS = 1:  | X |           Index           |
              +---+---+---+---+---+---+---+---+

     X = 1 indicates that the item corresponding to the Index
           is sent in the item 0, ..., item n list.
     X = 0 indicates that the item corresponding to the Index is
               not sent.

     When 4-bit XI items are used and m > 1, the XI items are placed in
     octets in the following manner:

          0   1   2   3   4   5   6   7
        +---+---+---+---+---+---+---+---+
        |     XI k      |    XI k + 1   |
        +---+---+---+---+---+---+---+---+

 Padding: A 4-bit padding field is present when PS = 0 and m is
     odd.  The Padding field is set to zero when sending and ignored
     when receiving.

 Item 1, ..., item n:
     Each item corresponds to an XI with X = 1 in XI 1, ..., XI m.

Parameters:

comp	The list compressor
dest	The ROHC packet under build
counter	The current position in the rohc-packet-under-build buffer
ps	The size of the index

Returns:

The new position in the rohc-packet-under-build buffer

References list_comp::curr_list, rohc_list_item::data, c_list::gen_id, get_elt(), list_elt::index_table, list_elt::item, c_translation::known, rohc_list_item::length, m, rohc_debugf, size_list(), list_comp::trans_table, and rohc_list_item::type.

Referenced by rohc_list_encode().

int rohc_list_encode_type_1	(	struct list_comp *const	comp,
		unsigned char *const	dest,
		int	counter,
		const int	ps
	)

Build encoding type 1 for list compression.

Todo:

this function is inefficient as it loops many times in the current and reference lists (see get_elt and type_is_present especially)

 Encoding type 1 (5.8.6.2):

      0   1   2   3   4   5   6   7
     --- --- --- --- --- --- --- ---
 1  | ET = 1| GP| PS|     XI 1      |
    +---+---+---+---+---+---+---+---+
 2  :            gen_id             : 1 octet, if GP = 1
    +---+---+---+---+---+---+---+---+
 3  |            ref_id             |
    +---+---+---+---+---+---+---+---+
 4  /       insertion bit mask      / 1-2 octets
    +---+---+---+---+---+---+---+---+
    |           XI list             | k octets, or (k - 1) * 4 bits
 5  /               --- --- --- --- /
    |               :    Padding    : if PS = 0 and k is even
    +---+---+---+---+---+---+---+---+
    |                               |
 6  /      item 1, ..., item n      / variable
    |                               |
    +---+---+---+---+---+---+---+---+

 ET: Encoding type is one (1).

 GP: Indicates presence of gen_id field.

 PS: Indicates size of XI fields:
     PS = 0 indicates 4-bit XI fields;
     PS = 1 indicates 8-bit XI fields.

 XI 1: When PS = 0, the first 4-bit XI item is placed here.
       When PS = 1, the field is set to zero when sending, and
       ignored when receiving.

 ref_id: The identifier of the reference CSRC list used when the
       list was compressed.  It is the 8 least significant bits of
       the RTP Sequence Number in R-mode and gen_id (see section
       5.8.2) in U/O-mode.

 insertion bit mask: Bit mask indicating the positions where new
           items are to be inserted.  See Insertion Only scheme in
           section 5.8.3.  The bit mask can have either of the
           following two formats:

      0   1   2   3   4   5   6   7
    +---+---+---+---+---+---+---+---+
    | 0 |        7-bit mask         |  bit 1 is the first bit
    +---+---+---+---+---+---+---+---+

    +---+---+---+---+---+---+---+---+
    | 1 |                           |  bit 1 is the first bit
    +---+      15-bit mask          +
    |                               |  bit 7 is the last bit
    +---+---+---+---+---+---+---+---+

 XI list: XI fields for items to be inserted.  When the insertion
    bit mask has k ones, the total number of XI fields is k.  When
    PS = 1, all XI fields are in the XI list.  When PS = 0, the
    first XI field is in the XI 1 field, and the remaining k - 1
    XI fields are in the XI list.

 Padding: Present when PS = 0 and k is even.

 item 1, ..., item n: One item for each XI field with the X bit set.

Parameters:

comp	The list compressor
dest	The ROHC packet under build
counter	The current position in the rohc-packet-under-build buffer
ps	The size of the index

Returns:

The new position in the rohc-packet-under-build buffer

References list_comp::curr_list, rohc_list_item::data, c_list::gen_id, get_elt(), list_elt::index_table, list_elt::item, c_translation::known, rohc_list_item::length, m, list_comp::ref_list, rohc_debugf, size_list(), list_comp::trans_table, rohc_list_item::type, and type_is_present().

Referenced by rohc_list_encode().

int rohc_list_encode_type_2	(	struct list_comp *const	comp,
		unsigned char *const	dest,
		int	counter,
		const int	ps
	)

Build encoding type 2 for list compression.

Todo:

this function is inefficient as it loops many times in the current and reference lists (see get_elt and type_is_present especially)

 Encoding type 2 (5.8.6.3):

      0   1   2   3   4   5   6   7
    +---+---+---+---+---+---+---+---+
 1  | ET = 2| GP|res|    Count      |
    +---+---+---+---+---+---+---+---+
 2  :            gen_id             : 1 octet, if GP = 1
    +---+---+---+---+---+---+---+---+
 3  |            ref_id             |
    +---+---+---+---+---+---+---+---+
 4  /        removal bit mask       / 1-2 octets
    +---+---+---+---+---+---+---+---+

 ET: Encoding type is 2.

 GP: Indicates presence of gen_id field.

 res: Reserved.  Set to zero when sending, ignored when
      received.

 Count: Number of elements in ref_list.

 removal bit mask: Indicates the elements in ref_list to be
    removed in order to obtain the current list.  See section
    5.8.3.  The bit mask can have either of the following two
    formats:

      0   1   2   3   4   5   6   7
    +---+---+---+---+---+---+---+---+
    | 0 |        7-bit mask         |  bit 1 is the first bit
    +---+---+---+---+---+---+---+---+

    +---+---+---+---+---+---+---+---+
    | 1 |                           |  bit 1 is the first bit
    +---+      15-bit mask          +
    |                               |  bit 7 is the last bit
    +---+---+---+---+---+---+---+---+

Parameters:

comp	The list compressor
dest	The ROHC packet under build
counter	The current position in the rohc-packet-under-build buffer
ps	The size of the index

Returns:

The new position in the rohc-packet-under-build buffer

References list_comp::curr_list, c_list::gen_id, get_elt(), list_elt::item, list_comp::ref_list, rohc_debugf, size_list(), and type_is_present().

Referenced by rohc_list_encode().

int rohc_list_encode_type_3	(	struct list_comp *const	comp,
		unsigned char *const	dest,
		int	counter,
		const int	ps
	)

Build encoding type 3 for list compression.

Todo:

this function is inefficient as it loops many times in the current and reference lists (see get_elt and type_is_present especially)

 Encoding type 3 (5.8.6.4):

      0   1   2   3   4   5   6   7
    +---+---+---+---+---+---+---+---+
 1  | ET=3  |GP |PS |     XI 1      |
    +---+---+---+---+---+---+---+---+
 2  :            gen_id             : 1 octet, if GP = 1
    +---+---+---+---+---+---+---+---+
 3  |            ref_id             |
    +---+---+---+---+---+---+---+---+
 4  /        removal bit mask       / 1-2 octets
    +---+---+---+---+---+---+---+---+
 5  /       insertion bit mask      / 1-2 octets
    +---+---+---+---+---+---+---+---+
    |           XI list             | k octets, or (k - 1) * 4 bits
 6  /               --- --- --- --- /
    |               :    Padding    : if PS = 0 and k is even
    +---+---+---+---+---+---+---+---+
    |                               |
 7  /      item 1, ..., item n      / variable
    |                               |
    +---+---+---+---+---+---+---+---+

 ET: Encoding type is 3.

 GP: Indicates presence of gen_id field.

 PS: Indicates size of XI fields:
     PS = 0 indicates 4-bit XI fields;
     PS = 1 indicates 8-bit XI fields.

 gen_id: Identifier for a sequence of identical lists.  It is
     present in U/O-mode when the compressor decides that it may use
     this list as a future reference list.

 ref_id: The identifier of the reference CSRC list used when the
       list was compressed.  It is the 8 least significant bits of
       the RTP Sequence Number in R-mode and gen_id (see section
       5.8.2) in U/O-mode.

 removal bit mask: Indicates the elements in ref_list to be
    removed in order to obtain the current list.  See section
    5.8.3.  The bit mask can have either of the following two
    formats:

      0   1   2   3   4   5   6   7
    +---+---+---+---+---+---+---+---+
    | 0 |        7-bit mask         |  bit 1 is the first bit
    +---+---+---+---+---+---+---+---+

    +---+---+---+---+---+---+---+---+
    | 1 |                           |  bit 1 is the first bit
    +---+      15-bit mask          +
    |                               |  bit 7 is the last bit
    +---+---+---+---+---+---+---+---+

 insertion bit mask: Bit mask indicating the positions where new
           items are to be inserted.  See Insertion Only scheme in
           section 5.8.3.  The bit mask can have either of the
           following two formats:

      0   1   2   3   4   5   6   7
    +---+---+---+---+---+---+---+---+
    | 0 |        7-bit mask         |  bit 1 is the first bit
    +---+---+---+---+---+---+---+---+

    +---+---+---+---+---+---+---+---+
    | 1 |                           |  bit 1 is the first bit
    +---+      15-bit mask          +
    |                               |  bit 7 is the last bit
    +---+---+---+---+---+---+---+---+

 XI list: XI fields for items to be inserted.  When the insertion
    bit mask has k ones, the total number of XI fields is k.  When
    PS = 1, all XI fields are in the XI list.  When PS = 0, the
    first XI field is in the XI 1 field, and the remaining k - 1
    XI fields are in the XI list.

 Padding: Present when PS = 0 and k is even.

 item 1, ..., item n: One item for each XI field with the X bit set.

Parameters:

comp	The list compressor
dest	The ROHC packet under build
counter	The current position in the rohc-packet-under-build buffer
ps	The size of the index

Returns:

The new position in the rohc-packet-under-build buffer

References list_comp::curr_list, rohc_list_item::data, c_list::gen_id, get_elt(), list_elt::index_table, list_elt::item, c_translation::known, rohc_list_item::length, m, list_comp::ref_list, rohc_debugf, size_list(), list_comp::trans_table, rohc_list_item::type, and type_is_present().

Referenced by rohc_list_encode().