lsquic_parse_iquic_common.c revision a0e1aeee 
1/* Copyright (c) 2017 - 2019 LiteSpeed Technologies Inc.  See LICENSE. */
2/*
3 * Parsing routines shared by all IETF QUIC versions.
4 */
5
6#include <assert.h>
7#include <stddef.h>
8#include <stdint.h>
9#include <string.h>
10#include <sys/queue.h>
11#include <sys/types.h>
12
13#include <openssl/rand.h>
14
15#include "lsquic_types.h"
16#include "lsquic_int_types.h"
17#include "lsquic_packet_common.h"
18#include "lsquic_packet_in.h"
19#include "lsquic_parse_common.h"
20#include "lsquic_parse.h"
21#include "lsquic_version.h"
22#include "lsquic.h"
23#include "lsquic_logger.h"
24#include "lsquic_byteswap.h"
25#include "lsquic_varint.h"
26#include "lsquic_enc_sess.h"
27#include "lsquic_tokgen.h"
28#include "lsquic.h"
29#include "lsquic_mm.h"
30#include "lsquic_engine_public.h"
31
32
33/* [draft-ietf-quic-transport-17] Section-17.2 */
34static const enum header_type bits2ht[4] =
35{
36    [0] = HETY_INITIAL,
37    [1] = HETY_0RTT,
38    [2] = HETY_HANDSHAKE,
39    [3] = HETY_RETRY,
40};
41
42
43int
44lsquic_Q046_parse_packet_in_long_begin (struct lsquic_packet_in *packet_in,
45                size_t length, int is_server, unsigned cid_len,
46                struct packin_parse_state *state)
47{
48    const unsigned char *p = packet_in->pi_data;
49    const unsigned char *const end = p + length;
50    lsquic_ver_tag_t tag;
51    enum header_type header_type;
52    unsigned dcil, scil, packet_len;
53    int verneg;
54    unsigned char first_byte;
55    lsquic_packno_t packno;
56
57    if (length < 6)
58        return -1;
59    first_byte = *p++;
60
61    memcpy(&tag, p, 4);
62    p += 4;
63    verneg = 0 == tag;
64    if (!verneg)
65        header_type = bits2ht[ (first_byte >> 4) & 3 ];
66    else
67        header_type = HETY_VERNEG;
68
69    packet_in->pi_header_type = header_type;
70
71    dcil = p[0] >> 4;
72    if (dcil)
73        dcil += 3;
74    scil = p[0] & 0xF;
75    if (scil)
76        scil += 3;
77    ++p;
78
79    /* Chromium comments state that the client sends packets with destination
80     * CID of 8 bytes and source CID of 0 bytes and the server does it the
81     * other way around.
82     */
83    if (is_server)
84    {
85        if (!(dcil == cid_len && scil == 0))
86            return -1;
87    }
88    else
89    if (!(dcil == 0 && scil == cid_len))
90        return -1;
91
92    if (!verneg)
93    {
94        packet_in->pi_flags |= (first_byte & 3) << PIBIT_BITS_SHIFT;
95        packet_len = 1 + (first_byte & 3);
96        if (end - p < (ptrdiff_t) (dcil + scil + packet_len))
97            return -1;
98    }
99    else
100    {
101        /* Need at least one version in the version array: add 4 */
102        if (end - p < (ptrdiff_t) (dcil + scil + 4))
103            return -1;
104#ifdef WIN32
105        /* Useless initialization: */
106        packet_len = 0;
107#endif
108    }
109
110    memcpy(&packet_in->pi_dcid.idbuf, p, cid_len);
111    packet_in->pi_dcid.len = cid_len;
112    p += cid_len;
113    packet_in->pi_flags |= PI_CONN_ID;
114
115    if (!verneg)
116    {
117        READ_UINT(packno, 64, p, packet_len);
118        packet_in->pi_packno = packno;
119        p += packet_len;
120        packet_in->pi_quic_ver = 1;
121        if (is_server || HETY_0RTT != header_type)
122            packet_in->pi_nonce = 0;
123        else
124        {
125            packet_in->pi_nonce = p - packet_in->pi_data;
126            p += 32;
127        }
128    }
129    else
130    {
131        if (p >= end || (3 & (uintptr_t) (end - p)))
132            return -1;
133        packet_in->pi_quic_ver = p - packet_in->pi_data;
134        p = end;
135    }
136
137    packet_in->pi_header_sz    = p - packet_in->pi_data;
138    packet_in->pi_frame_types  = 0;
139    packet_in->pi_data_sz      = length;
140    packet_in->pi_refcnt       = 0;
141    packet_in->pi_received     = 0;
142
143    return 0;
144}
145
146
147int
148lsquic_Q046_parse_packet_in_short_begin (lsquic_packet_in_t *packet_in,
149            size_t length, int is_server, unsigned cid_len,
150            struct packin_parse_state *state)
151{
152    const unsigned char *p = packet_in->pi_data;
153    const unsigned char *const pend = packet_in->pi_data + length;
154    unsigned packet_len, header_len;
155    lsquic_packno_t packno;
156
157    if (*p & 0x40)  /* Q046 and higher */
158        packet_len = 1 + (*p & 3);
159    else
160        return -1;
161
162    if (is_server)
163        header_len = 1 + cid_len + packet_len;
164    else
165        header_len = 1 + packet_len;
166
167    if (pend - p < (ptrdiff_t) header_len)
168        return -1;
169
170    packet_in->pi_flags |= (*p & 3) << PIBIT_BITS_SHIFT;
171    ++p;
172    if (is_server)
173    {
174        memcpy(packet_in->pi_dcid.idbuf, packet_in->pi_data + 1, cid_len);
175        packet_in->pi_dcid.len = cid_len;
176        packet_in->pi_flags |= PI_CONN_ID;
177        p += cid_len;
178    }
179
180    READ_UINT(packno, 64, p, packet_len);
181    packet_in->pi_packno = packno;
182    p += packet_len;
183
184    packet_in->pi_header_type  = HETY_NOT_SET;
185    packet_in->pi_quic_ver     = 0;
186    packet_in->pi_nonce        = 0;
187    packet_in->pi_header_sz    = p - packet_in->pi_data;
188    packet_in->pi_frame_types  = 0;
189    packet_in->pi_data_sz      = length;
190    packet_in->pi_refcnt       = 0;
191    packet_in->pi_received     = 0;
192
193    return 0;
194}
195
196
197/* TODO: this only works Q044? XXX */
198ssize_t
199lsquic_generate_iquic_reset (const lsquic_cid_t *cidp, unsigned char *buf,
200                                                            size_t buf_sz)
201{
202    size_t need;
203    uint64_t id;
204
205    need = 1 /* Type */ + 20 /* Random bytes */ + 16 /* Reset token */;
206    if (buf_sz < need)
207        return -1;
208
209    *buf = 0x30;
210    (void) RAND_pseudo_bytes(buf + 1, 20);
211    /* XXX code duplication here and lsquic_generate_reset_token().  Which
212     * should call which: parse function the crypto functions or the other
213     * way around?
214     */
215    /* TODO test this */
216    memcpy(&id, cidp->idbuf, GQUIC_CID_LEN);
217#if __BYTE_ORDER == __LITTLE_ENDIAN
218    id = bswap_64(id);
219#endif
220    memcpy(buf + 21, &id, sizeof(id));
221    memset(buf + 21 + sizeof(id), 0, SRST_LENGTH - sizeof(id));
222    return need;
223}
224
225
226/* This is a bare-bones version of lsquic_Q046_parse_packet_in_long_begin()
227 */
228int
229lsquic_is_valid_iquic_hs_packet (const unsigned char *buf, size_t length,
230                                                        lsquic_ver_tag_t *tagp)
231{
232    const unsigned char *p = buf;
233    const unsigned char *const end = p + length;
234    lsquic_ver_tag_t tag;
235    unsigned dcil, scil, packet_len;
236    unsigned char first_byte;
237    const unsigned cid_len = 8;
238
239    if (length < 6)
240        return 0;
241    first_byte = *p++;
242
243    memcpy(&tag, p, 4);
244    p += 4;
245    if (0 == tag)
246        return 0;   /* Client never sends version negotiation */
247
248    dcil = p[0] >> 4;
249    if (dcil)
250        dcil += 3;
251    scil = p[0] & 0xF;
252    if (scil)
253        scil += 3;
254    ++p;
255
256    if (!(dcil == cid_len && scil == 0))
257        return 0;
258
259    packet_len = first_byte & 3;
260
261    if (end - p >= (ptrdiff_t) (dcil + scil + packet_len))
262    {
263        *tagp = tag;
264        return 1;
265    }
266    else
267        return 0;
268}
269
270
271const enum quic_frame_type lsquic_iquic_byte2type[0x100] =
272{
273    [0x00] = QUIC_FRAME_PADDING,
274    [0x01] = QUIC_FRAME_PING,
275    [0x02] = QUIC_FRAME_ACK,
276    [0x03] = QUIC_FRAME_ACK,
277    [0x04] = QUIC_FRAME_RST_STREAM,
278    [0x05] = QUIC_FRAME_STOP_SENDING,
279    [0x06] = QUIC_FRAME_CRYPTO,
280    [0x07] = QUIC_FRAME_NEW_TOKEN,
281    [0x08] = QUIC_FRAME_STREAM,
282    [0x09] = QUIC_FRAME_STREAM,
283    [0x0A] = QUIC_FRAME_STREAM,
284    [0x0B] = QUIC_FRAME_STREAM,
285    [0x0C] = QUIC_FRAME_STREAM,
286    [0x0D] = QUIC_FRAME_STREAM,
287    [0x0E] = QUIC_FRAME_STREAM,
288    [0x0F] = QUIC_FRAME_STREAM,
289    [0x10] = QUIC_FRAME_MAX_DATA,
290    [0x11] = QUIC_FRAME_MAX_STREAM_DATA,
291    [0x12] = QUIC_FRAME_MAX_STREAMS,
292    [0x13] = QUIC_FRAME_MAX_STREAMS,
293    [0x14] = QUIC_FRAME_BLOCKED,
294    [0x15] = QUIC_FRAME_STREAM_BLOCKED,
295    [0x16] = QUIC_FRAME_STREAMS_BLOCKED,
296    [0x17] = QUIC_FRAME_STREAMS_BLOCKED,
297    [0x18] = QUIC_FRAME_NEW_CONNECTION_ID,
298    [0x19] = QUIC_FRAME_RETIRE_CONNECTION_ID,
299    [0x1A] = QUIC_FRAME_PATH_CHALLENGE,
300    [0x1B] = QUIC_FRAME_PATH_RESPONSE,
301    [0x1C] = QUIC_FRAME_CONNECTION_CLOSE,
302    [0x1D] = QUIC_FRAME_CONNECTION_CLOSE,
303    [0x1E] = QUIC_FRAME_INVALID,
304    [0x1F] = QUIC_FRAME_INVALID,
305    [0x20] = QUIC_FRAME_INVALID,
306    [0x21] = QUIC_FRAME_INVALID,
307    [0x22] = QUIC_FRAME_INVALID,
308    [0x23] = QUIC_FRAME_INVALID,
309    [0x24] = QUIC_FRAME_INVALID,
310    [0x25] = QUIC_FRAME_INVALID,
311    [0x26] = QUIC_FRAME_INVALID,
312    [0x27] = QUIC_FRAME_INVALID,
313    [0x28] = QUIC_FRAME_INVALID,
314    [0x29] = QUIC_FRAME_INVALID,
315    [0x2A] = QUIC_FRAME_INVALID,
316    [0x2B] = QUIC_FRAME_INVALID,
317    [0x2C] = QUIC_FRAME_INVALID,
318    [0x2D] = QUIC_FRAME_INVALID,
319    [0x2E] = QUIC_FRAME_INVALID,
320    [0x2F] = QUIC_FRAME_INVALID,
321    [0x30] = QUIC_FRAME_INVALID,
322    [0x31] = QUIC_FRAME_INVALID,
323    [0x32] = QUIC_FRAME_INVALID,
324    [0x33] = QUIC_FRAME_INVALID,
325    [0x34] = QUIC_FRAME_INVALID,
326    [0x35] = QUIC_FRAME_INVALID,
327    [0x36] = QUIC_FRAME_INVALID,
328    [0x37] = QUIC_FRAME_INVALID,
329    [0x38] = QUIC_FRAME_INVALID,
330    [0x39] = QUIC_FRAME_INVALID,
331    [0x3A] = QUIC_FRAME_INVALID,
332    [0x3B] = QUIC_FRAME_INVALID,
333    [0x3C] = QUIC_FRAME_INVALID,
334    [0x3D] = QUIC_FRAME_INVALID,
335    [0x3E] = QUIC_FRAME_INVALID,
336    [0x3F] = QUIC_FRAME_INVALID,
337    [0x40] = QUIC_FRAME_INVALID,
338    [0x41] = QUIC_FRAME_INVALID,
339    [0x42] = QUIC_FRAME_INVALID,
340    [0x43] = QUIC_FRAME_INVALID,
341    [0x44] = QUIC_FRAME_INVALID,
342    [0x45] = QUIC_FRAME_INVALID,
343    [0x46] = QUIC_FRAME_INVALID,
344    [0x47] = QUIC_FRAME_INVALID,
345    [0x48] = QUIC_FRAME_INVALID,
346    [0x49] = QUIC_FRAME_INVALID,
347    [0x4A] = QUIC_FRAME_INVALID,
348    [0x4B] = QUIC_FRAME_INVALID,
349    [0x4C] = QUIC_FRAME_INVALID,
350    [0x4D] = QUIC_FRAME_INVALID,
351    [0x4E] = QUIC_FRAME_INVALID,
352    [0x4F] = QUIC_FRAME_INVALID,
353    [0x50] = QUIC_FRAME_INVALID,
354    [0x51] = QUIC_FRAME_INVALID,
355    [0x52] = QUIC_FRAME_INVALID,
356    [0x53] = QUIC_FRAME_INVALID,
357    [0x54] = QUIC_FRAME_INVALID,
358    [0x55] = QUIC_FRAME_INVALID,
359    [0x56] = QUIC_FRAME_INVALID,
360    [0x57] = QUIC_FRAME_INVALID,
361    [0x58] = QUIC_FRAME_INVALID,
362    [0x59] = QUIC_FRAME_INVALID,
363    [0x5A] = QUIC_FRAME_INVALID,
364    [0x5B] = QUIC_FRAME_INVALID,
365    [0x5C] = QUIC_FRAME_INVALID,
366    [0x5D] = QUIC_FRAME_INVALID,
367    [0x5E] = QUIC_FRAME_INVALID,
368    [0x5F] = QUIC_FRAME_INVALID,
369    [0x60] = QUIC_FRAME_INVALID,
370    [0x61] = QUIC_FRAME_INVALID,
371    [0x62] = QUIC_FRAME_INVALID,
372    [0x63] = QUIC_FRAME_INVALID,
373    [0x64] = QUIC_FRAME_INVALID,
374    [0x65] = QUIC_FRAME_INVALID,
375    [0x66] = QUIC_FRAME_INVALID,
376    [0x67] = QUIC_FRAME_INVALID,
377    [0x68] = QUIC_FRAME_INVALID,
378    [0x69] = QUIC_FRAME_INVALID,
379    [0x6A] = QUIC_FRAME_INVALID,
380    [0x6B] = QUIC_FRAME_INVALID,
381    [0x6C] = QUIC_FRAME_INVALID,
382    [0x6D] = QUIC_FRAME_INVALID,
383    [0x6E] = QUIC_FRAME_INVALID,
384    [0x6F] = QUIC_FRAME_INVALID,
385    [0x70] = QUIC_FRAME_INVALID,
386    [0x71] = QUIC_FRAME_INVALID,
387    [0x72] = QUIC_FRAME_INVALID,
388    [0x73] = QUIC_FRAME_INVALID,
389    [0x74] = QUIC_FRAME_INVALID,
390    [0x75] = QUIC_FRAME_INVALID,
391    [0x76] = QUIC_FRAME_INVALID,
392    [0x77] = QUIC_FRAME_INVALID,
393    [0x78] = QUIC_FRAME_INVALID,
394    [0x79] = QUIC_FRAME_INVALID,
395    [0x7A] = QUIC_FRAME_INVALID,
396    [0x7B] = QUIC_FRAME_INVALID,
397    [0x7C] = QUIC_FRAME_INVALID,
398    [0x7D] = QUIC_FRAME_INVALID,
399    [0x7E] = QUIC_FRAME_INVALID,
400    [0x7F] = QUIC_FRAME_INVALID,
401    [0x80] = QUIC_FRAME_INVALID,
402    [0x81] = QUIC_FRAME_INVALID,
403    [0x82] = QUIC_FRAME_INVALID,
404    [0x83] = QUIC_FRAME_INVALID,
405    [0x84] = QUIC_FRAME_INVALID,
406    [0x85] = QUIC_FRAME_INVALID,
407    [0x86] = QUIC_FRAME_INVALID,
408    [0x87] = QUIC_FRAME_INVALID,
409    [0x88] = QUIC_FRAME_INVALID,
410    [0x89] = QUIC_FRAME_INVALID,
411    [0x8A] = QUIC_FRAME_INVALID,
412    [0x8B] = QUIC_FRAME_INVALID,
413    [0x8C] = QUIC_FRAME_INVALID,
414    [0x8D] = QUIC_FRAME_INVALID,
415    [0x8E] = QUIC_FRAME_INVALID,
416    [0x8F] = QUIC_FRAME_INVALID,
417    [0x90] = QUIC_FRAME_INVALID,
418    [0x91] = QUIC_FRAME_INVALID,
419    [0x92] = QUIC_FRAME_INVALID,
420    [0x93] = QUIC_FRAME_INVALID,
421    [0x94] = QUIC_FRAME_INVALID,
422    [0x95] = QUIC_FRAME_INVALID,
423    [0x96] = QUIC_FRAME_INVALID,
424    [0x97] = QUIC_FRAME_INVALID,
425    [0x98] = QUIC_FRAME_INVALID,
426    [0x99] = QUIC_FRAME_INVALID,
427    [0x9A] = QUIC_FRAME_INVALID,
428    [0x9B] = QUIC_FRAME_INVALID,
429    [0x9C] = QUIC_FRAME_INVALID,
430    [0x9D] = QUIC_FRAME_INVALID,
431    [0x9E] = QUIC_FRAME_INVALID,
432    [0x9F] = QUIC_FRAME_INVALID,
433    [0xA0] = QUIC_FRAME_INVALID,
434    [0xA1] = QUIC_FRAME_INVALID,
435    [0xA2] = QUIC_FRAME_INVALID,
436    [0xA3] = QUIC_FRAME_INVALID,
437    [0xA4] = QUIC_FRAME_INVALID,
438    [0xA5] = QUIC_FRAME_INVALID,
439    [0xA6] = QUIC_FRAME_INVALID,
440    [0xA7] = QUIC_FRAME_INVALID,
441    [0xA8] = QUIC_FRAME_INVALID,
442    [0xA9] = QUIC_FRAME_INVALID,
443    [0xAA] = QUIC_FRAME_INVALID,
444    [0xAB] = QUIC_FRAME_INVALID,
445    [0xAC] = QUIC_FRAME_INVALID,
446    [0xAD] = QUIC_FRAME_INVALID,
447    [0xAE] = QUIC_FRAME_INVALID,
448    [0xAF] = QUIC_FRAME_INVALID,
449    [0xB0] = QUIC_FRAME_INVALID,
450    [0xB1] = QUIC_FRAME_INVALID,
451    [0xB2] = QUIC_FRAME_INVALID,
452    [0xB3] = QUIC_FRAME_INVALID,
453    [0xB4] = QUIC_FRAME_INVALID,
454    [0xB5] = QUIC_FRAME_INVALID,
455    [0xB6] = QUIC_FRAME_INVALID,
456    [0xB7] = QUIC_FRAME_INVALID,
457    [0xB8] = QUIC_FRAME_INVALID,
458    [0xB9] = QUIC_FRAME_INVALID,
459    [0xBA] = QUIC_FRAME_INVALID,
460    [0xBB] = QUIC_FRAME_INVALID,
461    [0xBC] = QUIC_FRAME_INVALID,
462    [0xBD] = QUIC_FRAME_INVALID,
463    [0xBE] = QUIC_FRAME_INVALID,
464    [0xBF] = QUIC_FRAME_INVALID,
465    [0xC0] = QUIC_FRAME_INVALID,
466    [0xC1] = QUIC_FRAME_INVALID,
467    [0xC2] = QUIC_FRAME_INVALID,
468    [0xC3] = QUIC_FRAME_INVALID,
469    [0xC4] = QUIC_FRAME_INVALID,
470    [0xC5] = QUIC_FRAME_INVALID,
471    [0xC6] = QUIC_FRAME_INVALID,
472    [0xC7] = QUIC_FRAME_INVALID,
473    [0xC8] = QUIC_FRAME_INVALID,
474    [0xC9] = QUIC_FRAME_INVALID,
475    [0xCA] = QUIC_FRAME_INVALID,
476    [0xCB] = QUIC_FRAME_INVALID,
477    [0xCC] = QUIC_FRAME_INVALID,
478    [0xCD] = QUIC_FRAME_INVALID,
479    [0xCE] = QUIC_FRAME_INVALID,
480    [0xCF] = QUIC_FRAME_INVALID,
481    [0xD0] = QUIC_FRAME_INVALID,
482    [0xD1] = QUIC_FRAME_INVALID,
483    [0xD2] = QUIC_FRAME_INVALID,
484    [0xD3] = QUIC_FRAME_INVALID,
485    [0xD4] = QUIC_FRAME_INVALID,
486    [0xD5] = QUIC_FRAME_INVALID,
487    [0xD6] = QUIC_FRAME_INVALID,
488    [0xD7] = QUIC_FRAME_INVALID,
489    [0xD8] = QUIC_FRAME_INVALID,
490    [0xD9] = QUIC_FRAME_INVALID,
491    [0xDA] = QUIC_FRAME_INVALID,
492    [0xDB] = QUIC_FRAME_INVALID,
493    [0xDC] = QUIC_FRAME_INVALID,
494    [0xDD] = QUIC_FRAME_INVALID,
495    [0xDE] = QUIC_FRAME_INVALID,
496    [0xDF] = QUIC_FRAME_INVALID,
497    [0xE0] = QUIC_FRAME_INVALID,
498    [0xE1] = QUIC_FRAME_INVALID,
499    [0xE2] = QUIC_FRAME_INVALID,
500    [0xE3] = QUIC_FRAME_INVALID,
501    [0xE4] = QUIC_FRAME_INVALID,
502    [0xE5] = QUIC_FRAME_INVALID,
503    [0xE6] = QUIC_FRAME_INVALID,
504    [0xE7] = QUIC_FRAME_INVALID,
505    [0xE8] = QUIC_FRAME_INVALID,
506    [0xE9] = QUIC_FRAME_INVALID,
507    [0xEA] = QUIC_FRAME_INVALID,
508    [0xEB] = QUIC_FRAME_INVALID,
509    [0xEC] = QUIC_FRAME_INVALID,
510    [0xED] = QUIC_FRAME_INVALID,
511    [0xEE] = QUIC_FRAME_INVALID,
512    [0xEF] = QUIC_FRAME_INVALID,
513    [0xF0] = QUIC_FRAME_INVALID,
514    [0xF1] = QUIC_FRAME_INVALID,
515    [0xF2] = QUIC_FRAME_INVALID,
516    [0xF3] = QUIC_FRAME_INVALID,
517    [0xF4] = QUIC_FRAME_INVALID,
518    [0xF5] = QUIC_FRAME_INVALID,
519    [0xF6] = QUIC_FRAME_INVALID,
520    [0xF7] = QUIC_FRAME_INVALID,
521    [0xF8] = QUIC_FRAME_INVALID,
522    [0xF9] = QUIC_FRAME_INVALID,
523    [0xFA] = QUIC_FRAME_INVALID,
524    [0xFB] = QUIC_FRAME_INVALID,
525    [0xFC] = QUIC_FRAME_INVALID,
526    [0xFD] = QUIC_FRAME_INVALID,
527    [0xFE] = QUIC_FRAME_INVALID,
528    [0xFF] = QUIC_FRAME_INVALID,
529};
530
531
532#if __GNUC__
533#   define popcount __builtin_popcount
534#else
535static int
536popcount (unsigned v)
537{
538    int count, i;
539    for (i = 0, count = 0; i < sizeof(v) * 8; ++i)
540        if (v & (1 << i))
541            ++count;
542    return count;
543}
544
545
546#endif
547
548
549int
550lsquic_Q046_gen_ver_nego_pkt (unsigned char *buf, size_t bufsz,
551         const lsquic_cid_t *scid, const lsquic_cid_t *dcid, unsigned versions,
552         uint8_t rand)
553{
554    unsigned slen, dlen;
555    size_t need;
556    int r;
557
558    need = 1 /* Type */ + 4 /* Packet number */ + 1 /* SCIL */
559                        + scid->len + dcid->len + popcount(versions) * 4;
560
561    if (need > bufsz)
562        return -1;
563
564    *buf++ = 0x80 | 0x40 | (rand & 0xF);
565    memset(buf, 0, 4);
566    buf += 4;
567
568    /* From [draft-ietf-quic-transport-11], Section 4.3:
569     *
570     *  The server MUST include the value from the Source Connection ID field
571     *  of the packet it receives in the Destination Connection ID field.
572     *  The value for Source Connection ID MUST be copied from the
573     *  Destination Connection ID of the received packet, which is initially
574     *  randomly selected by a client.  Echoing both connection IDs gives
575     *  clients some assurance that the server received the packet and that
576     *  the Version Negotiation packet was not generated by an off-path
577     *  attacker.
578     */
579
580    dlen = dcid->len;
581    if (dlen)
582        dlen -= 3;
583    slen = scid->len;
584    if (slen)
585        slen -= 3;
586    *buf++ = (dlen << 4) | slen;
587
588    memcpy(buf, dcid->idbuf, dcid->len);
589    buf += dcid->len;
590    memcpy(buf, scid->idbuf, scid->len);
591    buf += scid->len;
592
593    r = lsquic_gen_ver_tags(buf, bufsz - 1 - 4 - 1 - dcid->len - scid->len,
594                                                                    versions);
595    if (r < 0)
596        return -1;
597    assert((unsigned) r == popcount(versions) * 4u);
598
599    return need;
600}
601
602
603