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Always include ACKs for the last seen control packets

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This adds an MRU cache for the last seen packets from the peer to send acks
to all recently recently  packets. This allows packets to be acknowledged
even if a single P_ACK_V1 gets lost, avoiding retransmissions. The downside
is that we add up to 28 byte to an P_ACK_V1 (7* packet_id) and up to 24
bytes to other control channel packets (4* packet_id + peer session id).
However these small increases in packet size are a small price to pay for
increased reliability.

Currently OpenVPN will only send the absolute minimum of ACK messages. A
single lost ACK message will trigger a resend from the peer and another
ACK message.

Patch v2: fix multiple typos/grammar. Change lru to mru (this is really an
          MRU cache), add more unit test cases

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20220831134140.913337-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25143.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
This commit is contained in:
Arne Schwabe 2022-08-31 15:41:39 +02:00 committed by Gert Doering
parent 5f6ea59759
commit c879609534
6 changed files with 155 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

69
src/openvpn/reliable.c
View File

@ -206,10 +206,56 @@ reliable_ack_parse(struct buffer *buf, struct reliable_ack *ack,
return true;
}
/**
* Copies the first n acks from \c ack to \c ack_mru
*/
void
copy_acks_to_mru(struct reliable_ack *ack, struct reliable_ack *ack_mru, int n)
{
ASSERT(ack->len >= n);
/* This loop is backward to ensure the same order as in ack */
for (int i = n-1; i >= 0; i--)
{
packet_id_type id = ack->packet_id[i];
/* Handle special case of ack_mru empty */
if (ack_mru->len == 0)
{
ack_mru->len = 1;
ack_mru->packet_id[0] = id;
}
bool idfound = false;
/* Move all existing entries one to the right */
packet_id_type move = id;
for (int j = 0; j < ack_mru->len; j++)
{
packet_id_type tmp = ack_mru->packet_id[j];
ack_mru->packet_id[j] = move;
move = tmp;
if (move == id)
{
idfound = true;
break;
}
}
if (!idfound && ack_mru->len < RELIABLE_ACK_SIZE)
{
ack_mru->packet_id[ack_mru->len] = move;
ack_mru->len++;
}
}
}
/* write a packet ID acknowledgement record to buf, */
/* removing all acknowledged entries from ack */
bool
reliable_ack_write(struct reliable_ack *ack,
struct reliable_ack *ack_mru,
struct buffer *buf,
const struct session_id *sid, int max, bool prepend)
{
@ -222,23 +268,36 @@ reliable_ack_write(struct reliable_ack *ack,
{
n = max;
}
sub = buf_sub(buf, ACK_SIZE(n), prepend);
copy_acks_to_mru(ack, ack_mru, n);
/* Number of acks we can resend that still fit into the packet */
uint8_t total_acks = min_int(max, ack_mru->len);
sub = buf_sub(buf, ACK_SIZE(total_acks), prepend);
if (!BDEF(&sub))
{
goto error;
}
ASSERT(buf_write(&sub, &n, sizeof(n)));
for (i = 0; i < n; ++i)
ASSERT(buf_write_u8(&sub, total_acks));
/* Write the actual acks to the packets. Since we copied the acks that
* are going out now already to the front of ack_mru we can fetch all
* acks from ack_mru */
for (i = 0; i < total_acks; ++i)
{
packet_id_type pid = ack->packet_id[i];
packet_id_type pid = ack_mru->packet_id[i];
packet_id_type net_pid = htonpid(pid);
ASSERT(buf_write(&sub, &net_pid, sizeof(net_pid)));
dmsg(D_REL_DEBUG, "ACK write ID " packet_id_format " (ack->len=%d, n=%d)", (packet_id_print_type)pid, ack->len, n);
}
if (n)
if (total_acks)
{
ASSERT(session_id_defined(sid));
ASSERT(session_id_write(sid, &sub));
}
if (n)
{
for (i = 0, j = n; j < ack->len; )
{
ack->packet_id[i++] = ack->packet_id[j++];

17
src/openvpn/reliable.h
View File

@ -197,6 +197,9 @@ reliable_ack_outstanding(struct reliable_ack *ack)
*
* @param ack The acknowledgment structure containing packet IDs to be
* acknowledged.
* @param ack_mru List of packets we have acknowledged before. Packets from
* \c ack will be moved here and if there is space in our
* ack structure we will fill it with packets from this
* @param buf The buffer into which the acknowledgment record will be
* written.
* @param sid The session ID of the VPN tunnel associated with the
@ -211,6 +214,7 @@ reliable_ack_outstanding(struct reliable_ack *ack)
* @li False, if an error occurs during processing.
*/
bool reliable_ack_write(struct reliable_ack *ack,
struct reliable_ack *ack_mru,
struct buffer *buf,
const struct session_id *sid, int max, bool prepend);
@ -370,6 +374,19 @@ bool reliable_ack_acknowledge_packet_id(struct reliable_ack *ack, packet_id_type
*/
struct reliable_entry *reliable_get_entry_sequenced(struct reliable *rel);
/**
* Copies the first n acks from \c ack to \c ack_mru
*
* @param ack The reliable structure to copy the acks from
* @param ack_mru The reliable structure to insert the acks into
* @param n The number of ACKS to copy
*/
void
copy_acks_to_mru(struct reliable_ack *ack, struct reliable_ack *ack_mru, int n);
/**
* Remove an entry from a reliable structure.
*

5
src/openvpn/ssl.c
View File

@ -357,7 +357,8 @@ calc_control_channel_frame_overhead(const struct tls_session *session)
overhead += SID_SIZE;
/* ACK array and remote SESSION ID (part of the ACK array) */
overhead += ACK_SIZE(min_int(reliable_ack_outstanding(ks->rec_ack), CONTROL_SEND_ACK_MAX));
int ackstosend = reliable_ack_outstanding(ks->rec_ack) + ks->lru_acks->len;
overhead += ACK_SIZE(min_int(ackstosend, CONTROL_SEND_ACK_MAX));
/* Message packet id */
overhead += sizeof(packet_id_type);
@ -993,6 +994,7 @@ key_state_init(struct tls_session *session, struct key_state *ks)
ALLOC_OBJ_CLEAR(ks->send_reliable, struct reliable);
ALLOC_OBJ_CLEAR(ks->rec_reliable, struct reliable);
ALLOC_OBJ_CLEAR(ks->rec_ack, struct reliable_ack);
ALLOC_OBJ_CLEAR(ks->lru_acks, struct reliable_ack);
/* allocate buffers */
ks->plaintext_read_buf = alloc_buf(TLS_CHANNEL_BUF_SIZE);
@ -1063,6 +1065,7 @@ key_state_free(struct key_state *ks, bool clear)
reliable_free(ks->rec_reliable);
free(ks->rec_ack);
free(ks->lru_acks);
free(ks->key_src);
packet_id_free(&ks->crypto_options.packet_id);

1
src/openvpn/ssl_common.h
View File

@ -233,6 +233,7 @@ struct key_state
struct reliable *send_reliable; /* holds a copy of outgoing packets until ACK received */
struct reliable *rec_reliable; /* order incoming ciphertext packets before we pass to TLS */
struct reliable_ack *rec_ack; /* buffers all packet IDs we want to ACK back to sender */
struct reliable_ack *lru_acks; /* keeps the most recently acked packages*/
/** Holds outgoing message for the control channel until ks->state reaches
* S_ACTIVE */

3
src/openvpn/ssl_pkt.c
View File

@ -179,7 +179,8 @@ write_control_auth(struct tls_session *session,
ASSERT(link_socket_actual_defined(&ks->remote_addr));
ASSERT(reliable_ack_write
(ks->rec_ack, buf, &ks->session_id_remote, max_ack, prepend_ack));
(ks->rec_ack, ks->lru_acks, buf, &ks->session_id_remote,
max_ack, prepend_ack));
msg(D_TLS_DEBUG, "%s(): %s", __func__, packet_opcode_name(opcode));

68
tests/unit_tests/openvpn/test_packet_id.c
View File

@ -210,6 +210,70 @@ test_get_num_output_sequenced_available(void **state)
reliable_free(rel);
}
static void
test_copy_acks_to_lru(void **state)
{
struct reliable_ack ack = { .len = 4, .packet_id = {2, 1, 3, 2} };
struct reliable_ack mru_ack = {0 };
/* Test copying to empty ack structure */
copy_acks_to_mru(&ack, &mru_ack, 4);
assert_int_equal(mru_ack.len, 3);
assert_int_equal(mru_ack.packet_id[0], 2);
assert_int_equal(mru_ack.packet_id[1], 1);
assert_int_equal(mru_ack.packet_id[2], 3);
/* Copying again should not change the result */
copy_acks_to_mru(&ack, &mru_ack, 4);
assert_int_equal(mru_ack.len, 3);
assert_int_equal(mru_ack.packet_id[0], 2);
assert_int_equal(mru_ack.packet_id[1], 1);
assert_int_equal(mru_ack.packet_id[2], 3);
/* Copying just the first two element should not change the order
* as they are still the most recent*/
struct reliable_ack mru_ack2 = mru_ack;
copy_acks_to_mru(&ack, &mru_ack2, 2);
assert_int_equal(mru_ack2.packet_id[0], 2);
assert_int_equal(mru_ack2.packet_id[1], 1);
assert_int_equal(mru_ack2.packet_id[2], 3);
/* Adding just two packets shoudl ignore the 42 in array and
* reorder the order in the MRU */
struct reliable_ack ack2 = { .len = 3, .packet_id = {3, 2, 42} };
copy_acks_to_mru(&ack2, &mru_ack2, 2);
assert_int_equal(mru_ack2.packet_id[0], 3);
assert_int_equal(mru_ack2.packet_id[1], 2);
assert_int_equal(mru_ack2.packet_id[2], 1);
/* Copying a zero array into it should also change nothing */
struct reliable_ack empty_ack = { .len = 0 };
copy_acks_to_mru(&empty_ack, &mru_ack, 0);
assert_int_equal(mru_ack.len, 3);
assert_int_equal(mru_ack.packet_id[0], 2);
assert_int_equal(mru_ack.packet_id[1], 1);
assert_int_equal(mru_ack.packet_id[2], 3);
/* Or should just 0 elements of the ack */
copy_acks_to_mru(&ack, &mru_ack, 0);
assert_int_equal(mru_ack.len, 3);
assert_int_equal(mru_ack.packet_id[0], 2);
assert_int_equal(mru_ack.packet_id[1], 1);
assert_int_equal(mru_ack.packet_id[2], 3);
struct reliable_ack ack3 = { .len = 7, .packet_id = {5, 6, 7, 8, 9, 10, 11}};
/* Adding multiple acks tests if the a full array is handled correctly */
copy_acks_to_mru(&ack3, &mru_ack, 7);
struct reliable_ack expected_ack = { .len = 8, .packet_id = {5, 6, 7, 8, 9, 10, 11, 2}};
assert_int_equal(mru_ack.len, expected_ack.len);
assert_memory_equal(mru_ack.packet_id, expected_ack.packet_id, sizeof(expected_ack.packet_id));
}
int
main(void)
{
@ -232,7 +296,9 @@ main(void)
cmocka_unit_test_setup_teardown(test_packet_id_write_long_wrap,
test_packet_id_write_setup,
test_packet_id_write_teardown),
cmocka_unit_test(test_get_num_output_sequenced_available)
cmocka_unit_test(test_get_num_output_sequenced_available),
cmocka_unit_test(test_copy_acks_to_lru)
};
return cmocka_run_group_tests_name("packet_id tests", tests, NULL, NULL);