src/decoding.cpp

/* decoding
 *
 * Copyright (C) 2012 Ivan Klyuchnikov
 * Copyright (C) 2012 Andreas Eversberg <jolly@eversberg.eu>
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 */
#include <decoding.h>
#include <rlc.h>
#include <gprs_debug.h>
#include <egprs_rlc_compression.h>

extern "C" {
#include <osmocom/core/utils.h>
#include <osmocom/core/bitcomp.h>
#include <osmocom/gprs/protocol/gsm_04_60.h>
}

#include <arpa/inet.h>

#include <errno.h>
#include <string.h>

#define LENGTH_TO_END 255
/*!
 * \returns num extensions fields (num frames == offset) on success,
 *          -errno otherwise.
 */
static int parse_extensions_egprs(const uint8_t *data, unsigned int data_len,
	unsigned int *offs,
	bool is_last_block,
	Decoding::RlcData *chunks, unsigned int chunks_size)
{
	const struct rlc_li_field_egprs *li;
	uint8_t e;
	unsigned int num_chunks = 0;

	e = 0;
	while (!e) {
		if (*offs > data_len) {
			LOGP(DRLCMACUL, LOGL_NOTICE, "UL DATA LI extended, "
				"but no more data\n");
			return -EINVAL;
		}

		/* get new E */
		li = (struct rlc_li_field_egprs *)&data[*offs];
		e = li->e;
		*offs += 1;

		if (!chunks)
			continue;

		if (num_chunks == chunks_size) {
			LOGP(DRLCMACUL, LOGL_NOTICE, "UL DATA LI extended, "
				"but no more chunks possible\n");
			return -ENOSPC;
		}
		if (li->li == 0 && num_chunks == 0) {
			/* TS 44.060, table 10.4.14a.1, row 2a */
			/* TS 44.060, table 10.4.14a.1, row 4 */
			chunks[num_chunks].length = 0;
			chunks[num_chunks].is_complete = true;
		} else if (li->li == 127 && li->e == 1) {
			/* TS 44.060, table 10.4.14a.1, row 3 & 5 */
			/* only filling bytes left */
			LOGP(DRLCMACUL, LOGL_DEBUG, "UL DATA LI contains "
				"only filling bytes with extention octet: LI=%d, E=%d, count=%d\n",
				li->li, li->e, num_chunks);
			break;
		} else if (li->li > 0) {
			/* TS 44.060, table 10.4.14a.1, row 1 & 2b */
			chunks[num_chunks].length = li->li;
			chunks[num_chunks].is_complete = true;
		} else {
			LOGP(DRLCMACUL, LOGL_NOTICE, "UL DATA LI contains "
				"invalid extension octet: LI=%d, E=%d, count=%d\n",
				li->li, li->e, num_chunks);
			return -EINVAL;
		}

		LOGP(DRLCMACUL, LOGL_DEBUG, "UL DATA LI contains "
			"extention octet: LI=%d, E=%d, count=%d\n",
			li->li, li->e, num_chunks);
		num_chunks += 1;

		if (e == 1) {
			/* There is space after the last chunk, add a final one */
			if (num_chunks == chunks_size) {
				LOGP(DRLCMACUL, LOGL_NOTICE,
					"UL DATA LI possibly extended, "
					"but no more chunks possible\n");
				return -ENOSPC;
			}

			chunks[num_chunks].length = LENGTH_TO_END;
			chunks[num_chunks].is_complete = is_last_block;
			num_chunks += 1;
		}
	}

	return num_chunks;
}

static int parse_extensions_gprs(const uint8_t *data, unsigned int data_len,
	unsigned int *offs,
	bool is_last_block,
	Decoding::RlcData *chunks, unsigned int chunks_size)
{
	const struct rlc_li_field *li;
	uint8_t m, e;
	unsigned int num_chunks = 0;

	e = 0;
	while (!e) {
		if (*offs > data_len) {
			LOGP(DRLCMACUL, LOGL_NOTICE, "UL DATA LI extended, "
				"but no more data\n");
			return -EINVAL;
		}

		/* get new E */
		li = (const struct rlc_li_field *)&data[*offs];
		e = li->e;
		m = li->m;
		*offs += 1;

		if (li->li == 0) {
			/* TS 44.060, 10.4.14, par 6 */
			e = 1;
			m = 0;
		}

		/* TS 44.060, table 10.4.13.1 */
		if (m == 0 && e == 0) {
			LOGP(DRLCMACUL, LOGL_NOTICE, "UL DATA "
				"ignored, because M='0' and E='0'.\n");
			return 0;
		}

		if (!chunks)
			continue;

		if (num_chunks == chunks_size) {
			LOGP(DRLCMACUL, LOGL_NOTICE, "UL DATA LI extended, "
				"but no more chunks possible\n");
			return -ENOSPC;
		}

		if (li->li == 0)
			/* e is 1 here */
			chunks[num_chunks].length = LENGTH_TO_END;
		else
			chunks[num_chunks].length = li->li;

		chunks[num_chunks].is_complete = li->li || is_last_block;

		LOGP(DRLCMACUL, LOGL_DEBUG, "UL DATA LI contains "
			"extention octet: LI=%d, M=%d, E=%d, count=%d\n",
			li->li, li->m, li->e, num_chunks);
		num_chunks += 1;

		if (e == 1 && m == 1) {
			if (num_chunks == chunks_size) {
				LOGP(DRLCMACUL, LOGL_NOTICE, "UL DATA LI extended, "
					"but no more chunks possible\n");
				return -ENOSPC;
			}
			/* TS 44.060, 10.4.13.1, row 4 */
			chunks[num_chunks].length = LENGTH_TO_END;
			chunks[num_chunks].is_complete = is_last_block;
			num_chunks += 1;
		}
	}

	return num_chunks;
}

int Decoding::rlc_data_from_ul_data(
	const struct gprs_rlc_data_block_info *rdbi, GprsCodingScheme cs,
	const uint8_t *data, RlcData *chunks, unsigned int chunks_size,
	uint32_t *tlli)
{
	uint8_t e;
	unsigned int data_len = rdbi->data_len;
	int num_chunks = 0, i;
	unsigned int offs = 0;
	bool is_last_block = (rdbi->cv == 0);

	if (!chunks)
		chunks_size = 0;

	e = rdbi->e;
	if (e) {
		if (chunks_size > 0) {
			/* Block without LI means it only contains data of one LLC PDU */
			chunks[num_chunks].offset = offs;
			chunks[num_chunks].length = LENGTH_TO_END;
			chunks[num_chunks].is_complete = is_last_block;
			num_chunks += 1;
		} else if (chunks) {
			LOGP(DRLCMACUL, LOGL_NOTICE, "No extension, "
				"but no more chunks possible\n");
			return -ENOSPC;
		}
	} else if (cs.isEgprs()) {
		/* if E is not set (LI follows), EGPRS */
		num_chunks = parse_extensions_egprs(data, data_len, &offs,
			is_last_block,
			chunks, chunks_size);
	} else {
		/* if E is not set (LI follows), GPRS */
		num_chunks = parse_extensions_gprs(data, data_len, &offs,
			is_last_block,
			chunks, chunks_size);
	}

	if (num_chunks < 0)
		return num_chunks;

	/* TLLI */
	if (rdbi->ti) {
		uint32_t tlli_enc;
		if (offs + 4 > data_len) {
			LOGP(DRLCMACUL, LOGL_NOTICE, "UL DATA TLLI out of block "
				"border\n");
			return -EINVAL;
		}

		memcpy(&tlli_enc, data + offs, sizeof(tlli_enc));
		if (cs.isGprs())
			/* The TLLI is encoded in big endian for GPRS (see
			 * TS 44.060, figure 10.2.2.1, note) */
			*tlli = be32toh(tlli_enc);
		else
			/* The TLLI is encoded in little endian for EGPRS (see
			 * TS 44.060, figure 10.3a.2.1, note 2) */
			*tlli = le32toh(tlli_enc);

		offs += sizeof(tlli_enc);
	} else {
		*tlli = 0;
	}

	/* PFI */
	if (rdbi->pi) {
		LOGP(DRLCMACUL, LOGL_ERROR, "ERROR: PFI not supported, "
			"please disable in SYSTEM INFORMATION\n");
		return -ENOTSUP;

		/* TODO: Skip all extensions with E=0 (see TS 44.060, 10.4.11 */
	}

	if (chunks_size == 0)
		return num_chunks;

	/* LLC */
	for (i = 0; i < num_chunks; i++) {
		chunks[i].offset = offs;
		if (chunks[i].length == LENGTH_TO_END) {
			if (offs == data_len) {
				/* There is no place for an additional chunk,
				 * so drop it (this may happen with EGPRS since
				 * there is no M flag. */
				num_chunks -= 1;
				break;
			}
			chunks[i].length = data_len - offs;
		}
		offs += chunks[i].length;
		if (offs > data_len) {
			LOGP(DRLCMACUL, LOGL_NOTICE, "UL DATA out of block "
				"border, chunk idx: %d, size: %d\n",
				i, chunks[i].length);
			return -EINVAL;
		}
	}

	return num_chunks;
}

uint8_t Decoding::get_ms_class_by_capability(MS_Radio_Access_capability_t *cap)
{
	int i;

	for (i = 0; i < cap->Count_MS_RA_capability_value; i++) {
		if (!cap->MS_RA_capability_value[i].u.Content.Exist_Multislot_capability)
			continue;
		if (!cap->MS_RA_capability_value[i].u.Content.Multislot_capability.Exist_GPRS_multislot_class)
			continue;
		return cap->MS_RA_capability_value[i].u.Content.Multislot_capability.GPRS_multislot_class;
	}

	return 0;
}

uint8_t Decoding::get_egprs_ms_class_by_capability(MS_Radio_Access_capability_t *cap)
{
	int i;

	for (i = 0; i < cap->Count_MS_RA_capability_value; i++) {
		if (!cap->MS_RA_capability_value[i].u.Content.Exist_Multislot_capability)
			continue;
		if (!cap->MS_RA_capability_value[i].u.Content.Multislot_capability.Exist_EGPRS_multislot_class)
			continue;
		return cap->MS_RA_capability_value[i].u.Content.Multislot_capability.EGPRS_multislot_class;
	}

	return 0;
}

/**
 * show_rbb needs to be an array with 65 elements
 * The index of the array is the bit position in the rbb
 * (show_rbb[63] relates to BSN ssn-1)
 */
void Decoding::extract_rbb(const uint8_t *rbb, char *show_rbb)
{
	for (int i = 0; i < 64; i++) {
		uint8_t bit;

		bit = !!(rbb[i/8] & (1<<(7-i%8)));
		show_rbb[i] = bit ? 'R' : 'I';
	}

	show_rbb[64] = '\0';
}

void Decoding::extract_rbb(const struct bitvec *rbb, char *show_rbb)
{
	unsigned int i;
	for (i = 0; i < rbb->cur_bit; i++) {
		uint8_t bit;
		bit = bitvec_get_bit_pos(rbb, i);
		show_rbb[i] = bit == 1 ? 'R' : 'I';
	}

	show_rbb[i] = '\0';
}

int Decoding::rlc_parse_ul_data_header(struct gprs_rlc_data_info *rlc,
	const uint8_t *data, GprsCodingScheme cs)
{
	unsigned int cur_bit = 0;
	switch(cs.headerTypeData()) {
	case HEADER_GPRS_DATA :
		cur_bit = rlc_parse_ul_data_header_gprs(rlc, data, cs);
		break;
	case HEADER_EGPRS_DATA_TYPE_3 :
		cur_bit = rlc_parse_ul_data_header_egprs_type_3(rlc, data, cs);
		break;
	case HEADER_EGPRS_DATA_TYPE_2 :
		cur_bit = rlc_parse_ul_data_header_egprs_type_2(rlc, data, cs);
		break;
	case HEADER_EGPRS_DATA_TYPE_1 :
		cur_bit = rlc_parse_ul_data_header_egprs_type_1(rlc, data, cs);
		break;
	default:
		LOGP(DRLCMACDL, LOGL_ERROR,
			"Decoding of uplink %s data blocks not yet supported.\n",
			cs.name());
		return -ENOTSUP;
	};

	return cur_bit;
}

int Decoding::rlc_parse_ul_data_header_egprs_type_3(
	struct gprs_rlc_data_info *rlc,
	const uint8_t *data,
	const GprsCodingScheme &cs)
{
	int punct, punct2, with_padding, cps;
	unsigned int e_ti_header, offs, cur_bit = 0;
	const struct gprs_rlc_ul_header_egprs_3 *egprs3;

	egprs3 = static_cast < struct gprs_rlc_ul_header_egprs_3 * >
			((void *)data);

	cps    = (egprs3->cps_hi << 0)  | (egprs3->cps_lo << 2);
	gprs_rlc_mcs_cps_decode(cps, cs, &punct, &punct2, &with_padding);
	gprs_rlc_data_info_init_ul(rlc, cs, with_padding);

	rlc->r      = egprs3->r;
	rlc->si     = egprs3->si;
	rlc->tfi    = (egprs3->tfi_hi << 0)  | (egprs3->tfi_lo << 2);
	rlc->cps    = cps;
	rlc->rsb    = egprs3->rsb;

	rlc->num_data_blocks = 1;
	rlc->block_info[0].cv  = egprs3->cv;
	rlc->block_info[0].pi  = egprs3->pi;
	rlc->block_info[0].spb = egprs3->spb;
	rlc->block_info[0].bsn =
			(egprs3->bsn1_hi << 0) | (egprs3->bsn1_lo << 5);

	cur_bit += rlc->data_offs_bits[0] - 2;
	offs = rlc->data_offs_bits[0] / 8;
	OSMO_ASSERT(rlc->data_offs_bits[0] % 8 == 1);
	e_ti_header = (data[offs-1] + (data[offs] << 8)) >> 7;
	rlc->block_info[0].e   = !!(e_ti_header & 0x01);
	rlc->block_info[0].ti  = !!(e_ti_header & 0x02);
	cur_bit += 2;
	/* skip data area */
	cur_bit += cs.maxDataBlockBytes() * 8;

	return cur_bit;
}

int Decoding::rlc_parse_ul_data_header_egprs_type_2(
	struct gprs_rlc_data_info *rlc,
	const uint8_t *data,
	const GprsCodingScheme &cs)
{
	const struct gprs_rlc_ul_header_egprs_2 *egprs2;
	unsigned int e_ti_header, offs, cur_bit = 0;
	int punct, punct2, with_padding, cps;

	egprs2 = static_cast < struct gprs_rlc_ul_header_egprs_2 * >
			((void *)data);

	cps    = (egprs2->cps_hi << 0)  | (egprs2->cps_lo << 2);
	gprs_rlc_mcs_cps_decode(cps, cs, &punct, &punct2, &with_padding);
	gprs_rlc_data_info_init_ul(rlc, cs, with_padding);

	rlc->r      = egprs2->r;
	rlc->si     = egprs2->si;
	rlc->tfi    = (egprs2->tfi_hi << 0)  | (egprs2->tfi_lo << 2);
	rlc->cps    = cps;
	rlc->rsb    = egprs2->rsb;

	rlc->num_data_blocks = 1;
	rlc->block_info[0].cv  = egprs2->cv;
	rlc->block_info[0].pi  = egprs2->pi;
	rlc->block_info[0].bsn =
		(egprs2->bsn1_hi << 0) | (egprs2->bsn1_lo << 5);

	cur_bit += rlc->data_offs_bits[0] - 2;

	offs = rlc->data_offs_bits[0] / 8;
	OSMO_ASSERT(rlc->data_offs_bits[0] % 8 == 7);

	e_ti_header = (data[offs] & 0x60) >> 5;
	rlc->block_info[0].e   = !!(e_ti_header & 0x01);
	rlc->block_info[0].ti  = !!(e_ti_header & 0x02);
	cur_bit += 2;

	/* skip data area */
	cur_bit += cs.maxDataBlockBytes() * 8;

	return cur_bit;
}

int Decoding::rlc_parse_ul_data_header_egprs_type_1(
	struct gprs_rlc_data_info *rlc,
	const uint8_t *data, const GprsCodingScheme &cs)
{
	struct gprs_rlc_ul_header_egprs_1 *egprs1;
	unsigned int e_ti_header, cur_bit = 0, offs;
	int punct, punct2, with_padding;

	egprs1 = static_cast < struct gprs_rlc_ul_header_egprs_1 * >
		((void *)data);
	gprs_rlc_mcs_cps_decode(egprs1->cps, cs, &punct, &punct2,
		&with_padding);
	gprs_rlc_data_info_init_ul(rlc, cs, with_padding);

	rlc->r      = egprs1->r;
	rlc->si     = egprs1->si;
	rlc->tfi    = (egprs1->tfi_hi << 0)  | (egprs1->tfi_lo << 2);
	rlc->cps    = egprs1->cps;
	rlc->rsb    = egprs1->rsb;
	rlc->num_data_blocks = 2;
	rlc->block_info[0].cv  = egprs1->cv;
	rlc->block_info[0].pi  = egprs1->pi;
	rlc->block_info[0].bsn =
			(egprs1->bsn1_hi << 0) | (egprs1->bsn1_lo << 5);

	cur_bit += rlc->data_offs_bits[0] - 2;
	offs = rlc->data_offs_bits[0] / 8;
	OSMO_ASSERT(rlc->data_offs_bits[0] % 8 == 0);

	e_ti_header = data[offs - 1] >> 6;
	rlc->block_info[0].e   = (e_ti_header & 0x01);
	rlc->block_info[0].ti  = !!(e_ti_header & 0x02);
	cur_bit += 2;

	rlc->block_info[1].cv  = egprs1->cv;
	rlc->block_info[1].pi  = egprs1->pi;
	rlc->block_info[1].bsn = rlc->block_info[0].bsn +
		((egprs1->bsn2_hi << 0) | (egprs1->bsn2_lo << 2));
	rlc->block_info[1].bsn = rlc->block_info[1].bsn &  (RLC_EGPRS_SNS - 1);

	if ((rlc->block_info[1].bsn != rlc->block_info[0].bsn) &&
			(rlc->block_info[0].cv == 0))
		rlc->block_info[0].cv = 1;

	cur_bit = rlc->data_offs_bits[1] - 2;

	offs = rlc->data_offs_bits[1] / 8;
	OSMO_ASSERT(rlc->data_offs_bits[1] % 8 == 2);

	e_ti_header = (data[offs] & (0x03));
	rlc->block_info[1].e   = (e_ti_header & 0x01);
	rlc->block_info[1].ti  = !!(e_ti_header & 0x02);
	cur_bit += 2;
	/* skip data area */
	cur_bit += cs.maxDataBlockBytes() * 8;

	return cur_bit;
}

int Decoding::rlc_parse_ul_data_header_gprs(struct gprs_rlc_data_info *rlc,
	const uint8_t *data, const GprsCodingScheme &cs)
{
	const struct rlc_ul_header *gprs;
	unsigned int cur_bit = 0;

	gprs = static_cast < struct rlc_ul_header * >
		((void *)data);

	gprs_rlc_data_info_init_ul(rlc, cs, false);

	rlc->r      = gprs->r;
	rlc->si     = gprs->si;
	rlc->tfi    = gprs->tfi;
	rlc->cps    = 0;
	rlc->rsb    = 0;
	rlc->num_data_blocks = 1;
	rlc->block_info[0].cv  = gprs->cv;
	rlc->block_info[0].pi  = gprs->pi;
	rlc->block_info[0].bsn = gprs->bsn;
	rlc->block_info[0].e   = gprs->e;
	rlc->block_info[0].ti  = gprs->ti;
	rlc->block_info[0].spb = 0;
	cur_bit += rlc->data_offs_bits[0];
	/* skip data area */
	cur_bit += cs.maxDataBlockBytes() * 8;

	return cur_bit;
}

/**
 * \brief Copy LSB bitstream RLC data block to byte aligned buffer.
 *
 * Note that the bitstream is encoded in LSB first order, so the two octets
 * 654321xx xxxxxx87 contain the octet 87654321 starting at bit position 3
 * (LSB has bit position 1). This is a different order than the one used by
 * CSN.1.
 *
 * \param data_block_idx  The block index, 0..1 for header type 1, 0 otherwise
 * \param src     A pointer to the start of the RLC block (incl. the header)
 * \param buffer  A data area of a least the size of the RLC block
 * \returns  the number of bytes copied
 */
unsigned int Decoding::rlc_copy_to_aligned_buffer(
	const struct gprs_rlc_data_info *rlc,
	unsigned int data_block_idx,
	const uint8_t *src, uint8_t *buffer)
{
	unsigned int hdr_bytes;
	unsigned int extra_bits;
	unsigned int i;

	uint8_t c, last_c;
	uint8_t *dst;
	const struct gprs_rlc_data_block_info *rdbi;

	OSMO_ASSERT(data_block_idx < rlc->num_data_blocks);
	rdbi = &rlc->block_info[data_block_idx];

	hdr_bytes = rlc->data_offs_bits[data_block_idx] >> 3;
	extra_bits = (rlc->data_offs_bits[data_block_idx] & 7);

	if (extra_bits == 0) {
		/* It is aligned already */
		memmove(buffer, src + hdr_bytes, rdbi->data_len);
		return rdbi->data_len;
	}

	dst = buffer;
	src = src + hdr_bytes;
	last_c = *(src++);

	for (i = 0; i < rdbi->data_len; i++) {
		c = src[i];
		*(dst++) = (last_c >> extra_bits) | (c << (8 - extra_bits));
		last_c = c;
	}

	return rdbi->data_len;
}

/**
 * \brief Get a pointer to byte aligned RLC data.
 *
 * Since the RLC data may not be byte aligned to the RLC block data such that a
 * single RLC data byte is spread over two RLC block bytes, this function
 * eventually uses the provided buffer as data storage.
 *
 * \param src     A pointer to the start of the RLC block (incl. the header)
 * \param buffer  A data area of a least the size of the RLC block
 * \returns A pointer to the RLC data start within src if it is aligned, and
 *          buffer otherwise.
 */
const uint8_t *Decoding::rlc_get_data_aligned(
	const struct gprs_rlc_data_info *rlc,
	unsigned int data_block_idx,
	const uint8_t *src, uint8_t *buffer)
{
	unsigned int hdr_bytes;
	unsigned int extra_bits;

	OSMO_ASSERT(data_block_idx < ARRAY_SIZE(rlc->data_offs_bits));

	hdr_bytes = rlc->data_offs_bits[data_block_idx] >> 3;
	extra_bits = (rlc->data_offs_bits[data_block_idx] & 7);

	if (extra_bits == 0)
		/* It is aligned already, return a pointer that refers to the
		 * original data. */
		return src + hdr_bytes;

	Decoding::rlc_copy_to_aligned_buffer(rlc, data_block_idx, src, buffer);
	return buffer;
}

static int handle_final_ack(bitvec *bits, int *bsn_begin, int *bsn_end,
	gprs_rlc_dl_window *window)
{
	int num_blocks, i;

	num_blocks = window->mod_sns(window->v_s() - window->v_a());
	for (i = 0; i < num_blocks; i++)
		bitvec_set_bit(bits, ONE);

	*bsn_begin = window->v_a();
	*bsn_end   = window->mod_sns(*bsn_begin + num_blocks);
	return num_blocks;
}

int Decoding::decode_egprs_acknack_bits(const EGPRS_AckNack_Desc_t *desc,
	bitvec *bits, int *bsn_begin, int *bsn_end, gprs_rlc_dl_window *window)
{
	int urbb_len = desc->URBB_LENGTH;
	int crbb_len = 0;
	int num_blocks = 0;
	struct bitvec urbb;
	int i;
	bool have_bitmap;
	int implicitly_acked_blocks;
	int ssn = desc->STARTING_SEQUENCE_NUMBER;
	int rc;

	if (desc->FINAL_ACK_INDICATION)
		return handle_final_ack(bits, bsn_begin, bsn_end, window);

	if (desc->Exist_CRBB)
		crbb_len = desc->CRBB_LENGTH;

	have_bitmap = (urbb_len + crbb_len) > 0;

	/*
	 * bow & bitmap present:
	 *   V(A)-> [ 11111...11111 0 SSN-> BBBBB...BBBBB ] (SSN+Nbits) .... V(S)
	 * bow & not bitmap present:
	 *   V(A)-> [ 11111...11111 ] . SSN .... V(S)
	 * not bow & bitmap present:
	 *   V(A)-> ... [ 0 SSN-> BBBBB...BBBBB ](SSN+N) .... V(S)
	 * not bow & not bitmap present:
	 *   V(A)-> ... [] . SSN .... V(S)
	 */

	if (desc->BEGINNING_OF_WINDOW) {
		implicitly_acked_blocks = window->mod_sns(ssn - 1 - window->v_a());

		for (i = 0; i < implicitly_acked_blocks; i++)
			bitvec_set_bit(bits, ONE);

		num_blocks += implicitly_acked_blocks;
	}

	if (!have_bitmap)
		goto aborted;

	/* next bit refers to V(Q) and thus is always zero (and not
	 * transmitted) */
	bitvec_set_bit(bits, ZERO);
	num_blocks += 1;

	if (crbb_len > 0) {
		int old_len = bits->cur_bit;

		LOGP(DRLCMACDL, LOGL_DEBUG, "Compress bitmap exists, "
			"CRBB LEN = %d and Starting color code = %d",
			desc->CRBB_LENGTH, desc->CRBB_STARTING_COLOR_CODE);
		rc = egprs_compress::decompress_crbb(desc->CRBB_LENGTH,
			desc->CRBB_STARTING_COLOR_CODE, desc->CRBB, bits);
		if (rc < 0) {
			LOGP(DRLCMACUL, LOGL_NOTICE,
				"Failed to decode CRBB: length %d, data '%s'\n",
				desc->CRBB_LENGTH, osmo_hexdump(
					desc->CRBB, (desc->CRBB_LENGTH + 7)/8));
			/* We don't know the SSN offset for the URBB,
			 * return what we have so far and assume the
			 * bitmap has stopped here */
			goto aborted;
		}

		LOGP(DRLCMACDL, LOGL_DEBUG,
			"CRBB len: %d, decoded len: %d, cc: %d, crbb: '%s'\n",
			desc->CRBB_LENGTH, bits->cur_bit - old_len,
			desc->CRBB_STARTING_COLOR_CODE,
			osmo_hexdump(
				desc->CRBB, (desc->CRBB_LENGTH + 7)/8)
		    );

		num_blocks += (bits->cur_bit - old_len);
	}

	urbb.cur_bit = 0;
	urbb.data = (uint8_t *)desc->URBB;
	urbb.data_len = sizeof(desc->URBB);

	for (i = urbb_len; i > 0; i--) {
		/*
		 * Set bit at the appropriate position (see 3GPP TS
		 * 44.060 12.3.1)
		 */
		int is_ack = bitvec_get_bit_pos(&urbb, i-1);
		bitvec_set_bit(bits, is_ack == 1 ? ONE : ZERO);
	}
	num_blocks += urbb_len;

aborted:
	*bsn_begin = window->v_a();
	*bsn_end   = window->mod_sns(*bsn_begin + num_blocks);

	return num_blocks;
}

int Decoding::decode_gprs_acknack_bits(const Ack_Nack_Description_t *desc,
	bitvec *bits, int *bsn_begin, int *bsn_end, gprs_rlc_dl_window *window)
{
	int urbb_len = RLC_GPRS_WS;
	int num_blocks;
	struct bitvec urbb;

	if (desc->FINAL_ACK_INDICATION)
		return handle_final_ack(bits, bsn_begin, bsn_end, window);

	*bsn_begin = window->v_a();
	*bsn_end   = desc->STARTING_SEQUENCE_NUMBER;

	num_blocks = window->mod_sns(*bsn_end - *bsn_begin);

	if (num_blocks < 0 || num_blocks > urbb_len) {
		*bsn_end  = *bsn_begin;
		LOGP(DRLCMACUL, LOGL_NOTICE,
			"Invalid GPRS Ack/Nack window %d:%d (length %d)\n",
			*bsn_begin, *bsn_end, num_blocks);
		return -EINVAL;
	}

	urbb.cur_bit = 0;
	urbb.data = (uint8_t *)desc->RECEIVED_BLOCK_BITMAP;
	urbb.data_len = sizeof(desc->RECEIVED_BLOCK_BITMAP);

	/*
	 * TS 44.060, 12.3:
	 * BSN = (SSN - bit_number) modulo 128, for bit_number = 1 to 64.
	 * The BSN values represented range from (SSN - 1) mod 128 to (SSN - 64) mod 128.
	 *
	 * We are only interested in the range from V(A) to SSN-1 which is
	 * num_blocks large. The RBB is laid out as
	 *   [SSN-1] [SSN-2] ... [V(A)] ... [SSN-64]
	 * so we want to start with [V(A)] and go backwards until we reach
	 * [SSN-1] to get the needed BSNs in an increasing order. Note that
	 * the bit numbers are counted from the end of the buffer.
	 */
	for (int i = num_blocks; i > 0; i--) {
		int is_ack = bitvec_get_bit_pos(&urbb, urbb_len - i);
		bitvec_set_bit(bits, is_ack == 1 ? ONE : ZERO);
	}

	return num_blocks;
}