[Bluez-users] data fragment

[Bluez-users] data fragment

[Uichin (Eugene) Lee]

Hi All,

I have an urgent question. I'm sending a 2300B packet from one machine
to another machine through L2CAP. I used hcidump to see the
fragmentation in L2CAP.

I looked at the kernel code, and realized that L2CAP uses "ACL_MTU" to
do the fragmentation. hcidump at the sender side shows that

< ACL data: handle 1 flags 0x01 dlen: 120

However, hcidump at the receiver side shows like this:

> ACL data: handle 42 flags 0x01 dlen: 224
=> 10 times +
> ACL data: handle 42 flags 0x01 dlen: 64

Total 2304 (2300B payload + 4 bytes - CID/Length)

What's happening here? At the lower layer (i.e., LMP), pack those
packets and then create another ACL packet with length 224 (the size
of  DM5 packet). It seems like LMP packs uppper level data packets
into current baseband packet format. Is that true?

Thank you.
-- 
Uichin (Eugene) Lee
Ph.D. Student UCLA CS Dept.

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Re: [Bluez-users] data fragment

[Marcel Holtmann]

Hi Uichin,

> I have an urgent question. I'm sending a 2300B packet from one machine
> to another machine through L2CAP. I used hcidump to see the
> fragmentation in L2CAP.
> 
> I looked at the kernel code, and realized that L2CAP uses "ACL_MTU" to
> do the fragmentation. hcidump at the sender side shows that
> 
> < ACL data: handle 1 flags 0x01 dlen: 120
> 
> However, hcidump at the receiver side shows like this:
> 
> > ACL data: handle 42 flags 0x01 dlen: 224
> => 10 times +
> > ACL data: handle 42 flags 0x01 dlen: 64
> 
> Total 2304 (2300B payload + 4 bytes - CID/Length)
> 
> What's happening here? At the lower layer (i.e., LMP), pack those
> packets and then create another ACL packet with length 224 (the size
> of  DM5 packet). It seems like LMP packs uppper level data packets
> into current baseband packet format. Is that true?

I can't give you a clear answer to that. This depends on how the
firmware on the Bluetooth chip implements the HCI. Sometimes HCI and LMP
packet size match, but that is not a must.

Regards

Marcel



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