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tcp: Check if connection is local or low RTT was seen before using large MSS

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If the connection is local or the RTT was comparable to the time it
takes to queue a batch of messages, we can safely use a large MSS
regardless of the sending buffer, but otherwise not.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
This commit is contained in:
Stefano Brivio 2021-10-04 22:01:16 +02:00
parent f6bff339a9
commit e1a2e2780c
1 changed files with 59 additions and 0 deletions
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59
tcp.c
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@ -369,6 +369,9 @@
#define PORT_DETECT_INTERVAL 1000
#define LOW_RTT_TABLE_SIZE 8
#define LOW_RTT_THRESHOLD 5 /* us */
/* We need to include <linux/tcp.h> for tcpi_bytes_acked, instead of
* <netinet/tcp.h>, but that doesn't include a definition for SOL_TCP
*/
@ -440,6 +443,7 @@ struct tcp_tap_conn;
* @a.a4.a: IPv4 address
* @tap_port: Guest-facing tap port
* @sock_port: Remote, socket-facing port
* @local: Destination is local
* @state: TCP connection state
* @seq_to_tap: Next sequence for packets to tap
* @seq_ack_from_tap: Last ACK number received from tap
@ -476,6 +480,7 @@ struct tcp_tap_conn {
} a;
in_port_t tap_port;
in_port_t sock_port;
int local;
enum tcp_state state;
uint32_t seq_to_tap;
@ -536,6 +541,9 @@ static int tcp_sock_init_lo [USHRT_MAX][IP_VERSIONS];
static int tcp_sock_init_ext [USHRT_MAX][IP_VERSIONS];
static int tcp_sock_ns [USHRT_MAX][IP_VERSIONS];
/* Table of destinations with very low RTT (assumed to be local), LRU */
static struct in6_addr low_rtt_dst[LOW_RTT_TABLE_SIZE];
/**
* tcp_remap_to_tap() - Set delta for port translation toward guest/tap
* @port: Original destination port, host order
@ -680,6 +688,47 @@ static int init_sock_pool6 [TCP_SOCK_POOL_SIZE];
static int ns_sock_pool4 [TCP_SOCK_POOL_SIZE];
static int ns_sock_pool6 [TCP_SOCK_POOL_SIZE];
/**
* tcp_rtt_dst_low() - Check if low RTT was seen for connection endpoint
* @conn: Connection pointer
* Return: 1 if destination is in low RTT table, 0 otherwise
*/
static int tcp_rtt_dst_low(struct tcp_tap_conn *conn)
{
int i;
for (i = 0; i < LOW_RTT_TABLE_SIZE; i++)
if (!memcmp(&conn->a.a6, low_rtt_dst + i, sizeof(conn->a.a6)))
return 1;
return 0;
}
/**
* tcp_rtt_dst_check() - Check tcpi_min_rtt, insert endpoint in table if low
* @conn: Connection pointer
* @info: Pointer to struct tcp_info for socket
*/
static void tcp_rtt_dst_check(struct tcp_tap_conn *conn, struct tcp_info *info)
{
int i, hole = -1;
if (!info->tcpi_min_rtt || (int)info->tcpi_min_rtt > LOW_RTT_THRESHOLD)
return;
for (i = 0; i < LOW_RTT_TABLE_SIZE; i++) {
if (!memcmp(&conn->a.a6, low_rtt_dst + i, sizeof(conn->a.a6)))
return;
if (hole == -1 && IN6_IS_ADDR_UNSPECIFIED(low_rtt_dst + i))
hole = i;
}
memcpy(low_rtt_dst + hole++, &conn->a.a6, sizeof(conn->a.a6));
if (hole == LOW_RTT_TABLE_SIZE)
hole = 0;
memcpy(low_rtt_dst + hole, &in6addr_any, sizeof(conn->a.a6));
}
/**
* tcp_tap_state() - Set given TCP state for tap connection, report to stderr
* @conn: Connection pointer
@ -1258,6 +1307,11 @@ static int tcp_send_to_tap(struct ctx *c, struct tcp_tap_conn *conn, int flags,
mss -= sizeof(struct iphdr);
else
mss -= sizeof(struct ipv6hdr);
if (!conn->local && !tcp_rtt_dst_low(conn))
mss = MIN(mss, PAGE_SIZE);
else
mss = ROUND_DOWN(mss, PAGE_SIZE);
}
*(uint16_t *)data = htons(mss);
@ -1588,6 +1642,11 @@ static void tcp_conn_from_tap(struct ctx *c, int af, void *addr,
tcp_hash_insert(c, conn, af, addr);
if (!bind(s, sa, sl))
tcp_rst(c, conn); /* Nobody is listening then */
if (errno != EADDRNOTAVAIL)
conn->local = 1;
if (connect(s, sa, sl)) {
tcp_tap_state(conn, TAP_SYN_SENT);