I am so sorry I was messing around with git send email and stupidly sent a random patch that is not mine by accident. Please ignore and forgive me.
On Wed, Sep 17, 2025 at 6:17 PM julian-lagattuta julian.lagattuta@gmail.com wrote:
From: Frédéric Danis frederic.danis@collabora.com
OBEX download from iPhone is currently slow due to small packet size used to transfer data which doesn't follow the MTU negotiated during L2CAP connection, i.e. 672 bytes instead of 32767:
< ACL Data TX: Handle 11 flags 0x00 dlen 12 L2CAP: Connection Request (0x02) ident 18 len 4 PSM: 4103 (0x1007) Source CID: 72
ACL Data RX: Handle 11 flags 0x02 dlen 16
L2CAP: Connection Response (0x03) ident 18 len 8 Destination CID: 14608 Source CID: 72 Result: Connection successful (0x0000) Status: No further information available (0x0000)< ACL Data TX: Handle 11 flags 0x00 dlen 27 L2CAP: Configure Request (0x04) ident 20 len 19 Destination CID: 14608 Flags: 0x0000 Option: Maximum Transmission Unit (0x01) [mandatory] MTU: 32767 Option: Retransmission and Flow Control (0x04) [mandatory] Mode: Enhanced Retransmission (0x03) TX window size: 63 Max transmit: 3 Retransmission timeout: 2000 Monitor timeout: 12000 Maximum PDU size: 1009
ACL Data RX: Handle 11 flags 0x02 dlen 26
L2CAP: Configure Request (0x04) ident 72 len 18 Destination CID: 72 Flags: 0x0000 Option: Retransmission and Flow Control (0x04) [mandatory] Mode: Enhanced Retransmission (0x03) TX window size: 32 Max transmit: 255 Retransmission timeout: 0 Monitor timeout: 0 Maximum PDU size: 65527 Option: Frame Check Sequence (0x05) [mandatory] FCS: 16-bit FCS (0x01)< ACL Data TX: Handle 11 flags 0x00 dlen 29 L2CAP: Configure Response (0x05) ident 72 len 21 Source CID: 14608 Flags: 0x0000 Result: Success (0x0000) Option: Maximum Transmission Unit (0x01) [mandatory] MTU: 672 Option: Retransmission and Flow Control (0x04) [mandatory] Mode: Enhanced Retransmission (0x03) TX window size: 32 Max transmit: 255 Retransmission timeout: 2000 Monitor timeout: 12000 Maximum PDU size: 1009
ACL Data RX: Handle 11 flags 0x02 dlen 32
L2CAP: Configure Response (0x05) ident 20 len 24 Source CID: 72 Flags: 0x0000 Result: Success (0x0000) Option: Maximum Transmission Unit (0x01) [mandatory] MTU: 32767 Option: Retransmission and Flow Control (0x04) [mandatory] Mode: Enhanced Retransmission (0x03) TX window size: 63 Max transmit: 3 Retransmission timeout: 2000 Monitor timeout: 12000 Maximum PDU size: 1009 Option: Frame Check Sequence (0x05) [mandatory] FCS: 16-bit FCS (0x01)...
ACL Data RX: Handle 11 flags 0x02 dlen 680
Channel: 72 len 676 ctrl 0x0202 [PSM 4103 mode Enhanced Retransmission (0x03)] {chan 8} I-frame: Unsegmented TxSeq 1 ReqSeq 2< ACL Data TX: Handle 11 flags 0x00 dlen 13 Channel: 14608 len 9 ctrl 0x0204 [PSM 4103 mode Enhanced Retransmission (0x03)] {chan 8} I-frame: Unsegmented TxSeq 2 ReqSeq 2
ACL Data RX: Handle 11 flags 0x02 dlen 680
Channel: 72 len 676 ctrl 0x0304 [PSM 4103 mode Enhanced Retransmission (0x03)] {chan 8} I-frame: Unsegmented TxSeq 2 ReqSeq 3The MTUs are negotiated for each direction. In this traces 32767 for iPhone->localhost and no MTU for localhost->iPhone, which based on '4.4 L2CAP_CONFIGURATION_REQ' (Core specification v5.4, Vol. 3, Part A):
The only parameters that should be included in the L2CAP_CONFIGURATION_REQ packet are those that require different values than the default or previously agreed values. ... Any missing configuration parameters are assumed to have their most recently explicitly or implicitly accepted values.
and '5.1 Maximum transmission unit (MTU)':
If the remote device sends a positive L2CAP_CONFIGURATION_RSP packet it should include the actual MTU to be used on this channel for traffic flowing into the local device. ... The default value is 672 octets.
is set by BlueZ to 672 bytes.
It seems that the iPhone used the lowest negotiated value to transfer data to the localhost instead of the negotiated one for the incoming direction.
This could be fixed by using the MTU negotiated for the other direction, if exists, in the L2CAP_CONFIGURATION_RSP. This allows to use segmented packets as in the following traces:
< ACL Data TX: Handle 11 flags 0x00 dlen 12 L2CAP: Connection Request (0x02) ident 22 len 4 PSM: 4103 (0x1007) Source CID: 72 < ACL Data TX: Handle 11 flags 0x00 dlen 27 L2CAP: Configure Request (0x04) ident 24 len 19 Destination CID: 2832 Flags: 0x0000 Option: Maximum Transmission Unit (0x01) [mandatory] MTU: 32767 Option: Retransmission and Flow Control (0x04) [mandatory] Mode: Enhanced Retransmission (0x03) TX window size: 63 Max transmit: 3 Retransmission timeout: 2000 Monitor timeout: 12000 Maximum PDU size: 1009
ACL Data RX: Handle 11 flags 0x02 dlen 26
L2CAP: Configure Request (0x04) ident 15 len 18 Destination CID: 72 Flags: 0x0000 Option: Retransmission and Flow Control (0x04) [mandatory] Mode: Enhanced Retransmission (0x03) TX window size: 32 Max transmit: 255 Retransmission timeout: 0 Monitor timeout: 0 Maximum PDU size: 65527 Option: Frame Check Sequence (0x05) [mandatory] FCS: 16-bit FCS (0x01)< ACL Data TX: Handle 11 flags 0x00 dlen 29 L2CAP: Configure Response (0x05) ident 15 len 21 Source CID: 2832 Flags: 0x0000 Result: Success (0x0000) Option: Maximum Transmission Unit (0x01) [mandatory] MTU: 32767 Option: Retransmission and Flow Control (0x04) [mandatory] Mode: Enhanced Retransmission (0x03) TX window size: 32 Max transmit: 255 Retransmission timeout: 2000 Monitor timeout: 12000 Maximum PDU size: 1009
ACL Data RX: Handle 11 flags 0x02 dlen 32
L2CAP: Configure Response (0x05) ident 24 len 24 Source CID: 72 Flags: 0x0000 Result: Success (0x0000) Option: Maximum Transmission Unit (0x01) [mandatory] MTU: 32767 Option: Retransmission and Flow Control (0x04) [mandatory] Mode: Enhanced Retransmission (0x03) TX window size: 63 Max transmit: 3 Retransmission timeout: 2000 Monitor timeout: 12000 Maximum PDU size: 1009 Option: Frame Check Sequence (0x05) [mandatory] FCS: 16-bit FCS (0x01)...
ACL Data RX: Handle 11 flags 0x02 dlen 1009
Channel: 72 len 1005 ctrl 0x4202 [PSM 4103 mode Enhanced Retransmission (0x03)] {chan 8} I-frame: Start (len 21884) TxSeq 1 ReqSeq 2ACL Data RX: Handle 11 flags 0x02 dlen 1009
Channel: 72 len 1005 ctrl 0xc204 [PSM 4103 mode Enhanced Retransmission (0x03)] {chan 8} I-frame: Continuation TxSeq 2 ReqSeq 2This has been tested with kernel 5.4 and BlueZ 5.77.
Cc: stable@vger.kernel.org Signed-off-by: Frédéric Danis frederic.danis@collabora.com Signed-off-by: Luiz Augusto von Dentz luiz.von.dentz@intel.com
net/bluetooth/l2cap_core.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c index a5bde5db58ef..40daa38276f3 100644 --- a/net/bluetooth/l2cap_core.c +++ b/net/bluetooth/l2cap_core.c @@ -3415,7 +3415,7 @@ static int l2cap_parse_conf_req(struct l2cap_chan *chan, void *data, size_t data struct l2cap_conf_rfc rfc = { .mode = L2CAP_MODE_BASIC }; struct l2cap_conf_efs efs; u8 remote_efs = 0;
u16 mtu = L2CAP_DEFAULT_MTU;
u16 mtu = 0; u16 result = L2CAP_CONF_SUCCESS; u16 size;@@ -3520,6 +3520,13 @@ static int l2cap_parse_conf_req(struct l2cap_chan *chan, void *data, size_t data /* Configure output options and let the other side know * which ones we don't like. */
/* If MTU is not provided in configure request, use the most recently* explicitly or implicitly accepted value for the other direction,* or the default value.*/if (mtu == 0)mtu = chan->imtu ? chan->imtu : L2CAP_DEFAULT_MTU;if (mtu < L2CAP_DEFAULT_MIN_MTU) result = L2CAP_CONF_UNACCEPT; else {-- 2.45.2