Download Play

The 3DS dlplay title has two dlplay modes: 3DS and DS. DS dlplay is just regular dsmode dlplay, same interface and protocol as before. Like DS gamecards, holding down start+select while starting the dsmode dlplay client will disable stretching the screens.

Download Play protocol

The Download Play protocol for 3DS is completely different from the DS Wireless Multiboot (WMB) protocol. While the DS WMB protocol used to send program code in plaintext over wireless, the 3DS Download Play protocol uses UDS which uses CCMP encryption etc. See also here.

Download Play UDS protocol

This section describes the data transferred using the UDS service. All data is stored as big-endian.

UDS data_channel 0x1 is used for spectator data, while all non-spectator data uses data_channel 0x2. The spectator data is received by connecting to the network as a spectator then receiving data-frames, this is handled when scanning for DLP networks.

The spectator data includes the 48x48 icon, this has the same format as SMDH.

This is the data starting at offset 0x0 for UDS PullPacket/SendTo:

Offset	Size	Description
0x0	0x1	Must be 0x1 for spectator data.
0x3	0x3	?
0x4	0x2	Size of the entire frame. The actual_size from PullPacket is the same size as this value. The spectator handling code doesn't validate this value, but the non-spectator data-recv code does verify that frame_size is <= actual_size(from PullPacket).
0x6	0x2	u16, unknown. For spectator data this is only used for the metadata frame.
0x8	0x4	Checksum
0xC	0x1	Spectator data: frameid, must be less than total_frames. Normal data: unknown.
0xD	0x1	Spectator data: total_frames. Normal data: unknown.
0xE	0x1	Unknown. This must match a state value. When this frame value is non-zero, 0x1 is used for the frame value when doing the compare instead.
0xF	0x1	?
0x10		The frame-specific payload starts here.

Total_frames is at least 0x4 normally. When a sysupdate is included, total_frames is 0x4+<total frames required for the titlelist(normally 0x1)>. Total_frames should be <=0x20, but the code doesn't check for this specific value.

Checksum

The checksum seed is the 4-byte output from encrypting zeros with AES-CTR using keytype5. The CTR is the output from this: "for(i=0; i<0x10; i++)ctr[i] = ctrseed[i] ^ host_macaddress[i % 6];" This ctrseed is a fixed 0x10-byte random-data block stored in DLP-sysmodule .rodata. This seed is initialized after connecting to/creating the DLP network.

The checksum stored in the above data frame header is then calculated using this checksum seed.

First, the calc_checksum is initialized to 0. Then calc_checksum is added with all words in the data frame loaded as big-endian, with the data-frame checksum cleared to zero here. If the frame_size isn't word-aligned, the remaining <4-bytes are loaded as big-endian for adding as well.

Then this is run:

 //During init before the above adding, shift and count are initialized:
 //shift = (((u8*)&checksum_seed)[2] & 0xf) + 0x4;
 //count = (((u8*)&checksum_seed)[3] & 0x7) + 0x2;
 for(pos=0; pos<count; pos++)checksum = (checksum<<shift | checksum>>shift) ^ checksum_seed;//The u32 checksum_seed is byte-swapped on 3DS for this.

Lastly the calculated checksum is written to output as big-endian(hence on 3DS it's byte-swapped before writing to output).

Frames

Frameid	frame_size	Description
0x0	0x300	Metadata + start of the icon gfx, see below.
0x1-0x3	0x5A8	The remaining icon gfx.
0x4-<total frames-1>	0x5B8	Sysupdate titlelist, if any.

The structure of each spectator frame relative to "frame-specific payload" is described below.

Metadata frame

Offset	Size	Description
0x0	0x4	u32, unknown.
0x4	0x4	u32, unknown.
0x8	0x2	u16, unknown.
0xA	0x1	u8, unknown.
0xB	0x1	u8, unknown.
0xC	0x4	u32, chunk_size. This is the chunk_size used for transferring the CIA. This is the exact size used for the FS .cia file-reads on the host, and the exact size used for AM CIA file-writes on the client(s). Normally this is 0x0003FFC0. This appears to be validated by the client at some point, using a large value for this triggers an "connection interrupted" error when trying to connect.
0x10	0x2	Unused by the DLP-client(sysmodule).
0x12	0x2	u16, unknown.
0x14	0x4	Unused by the DLP-client(sysmodule).
0x18	0x10	Unknown 0x10-byte structure.
0x28	0x4	u32, unknown.
0x2C	0x4	u32, unknown. Must be <=0x02000000.
0x30	0x80 (0x3F characters are copied)	UTF-16 Application name string. The last u16 is ignored, value 0x0 is always written to output for it instead.
0xB0	0x100 (0x7F characters are copied)	UTF-16 Description string. The last u16 is ignored, value 0x0 is always written to output for it instead.
0x1B0	0x138	Array of 0x9C u16s for the start of the icon gfx.
0x2E8	0x1	u8, unknown. Written to outptr+2.
0x2E9	0x1	u8, unknown. Written to outptr+1.
0x2EA	0x1	u8, unknown. Written to outptr+0.
0x2EB	0x1	u8, unknown. Written to outptr+3.
0x2EC	0x1	u8, unknown. Written to outptr+4.
0x2ED	0x1	u8, unknown. Written to outptr+7.
0x2EE	0x1	u8, unknown. Written to outptr+6.
0x2EF	0x1	u8, unknown. Written to outptr+5.

If the u16 at frameheader+0x6 is less than 0x101, the 8-bytes at outptr are cleared to all-zero, instead of copying the data from offset 0x2E8.

Icon gfx frame

Offset	Size	Description
0x0	0x598	Array of 0x2CC u16s for the gfx data.

Sysupdate titlelist frame

Offset	Size	Description
0x0	0x2	u16 unk_x0, not used after the below check.
0x2	0x2	u16 total_entries, under this frame.
0x4	0x2	u16 out_entryindex, starts at 0x0. This is the output entryindex where this titlelist will be written, this is only non-zero starting with frameid 0x6, if it exists.
0x6	0x2	u16 unk_x6
0x8	0x5A0 (0x5A entries)	The actual titlelist, see below for the structure of each 0x10-byte entry. Unused entries are set to all-zero.

The value from (unk_x0 + unk_x6) must be <=0x100. Normally unk_x0 and total_entries are set to the same value, with the two u16s after that set to 0x0(with frameid 0x4).

Entry structure:

Offset	Size	Description
0x0	0x8	u64 titleID
0x8	0x2	u16 title_version
0xA	0x2	Padding, not used.
0xC	0x4	Title size

When writing the entry to output, the titleID is ORRed with the data stored there. The rest of the data is stored with normal writes(padding is not written in the output 0x10-byte entry).

Broadcasted application data

The Download Play protocol broadcasts 3DS application data in the CIA format. The title is installed to NAND, and is kept there until new CIA data from a different game is received through the Download Play protocol.

Remote Distribution of System-Updates

As part of the child distribution process, a 3DS acting as the server in a local Download Play session, can send system updates to another 3DS unit acting as the client, through first sending the system update package then instructing the client to install reboot and reinstantiate a connection (which it caches information about temporarily) remotely, if it finds system updates are necessary before distributing the child-application. ( eg. multiplayer game or a demo. ) Like "update" partitions on CTR Cards, this is not an "automatic feature" and not implemented for all Download Play titles. This system update data is from the application's system update CFA, prior to beginning the data transfer the host broadcasts data-frames which contain a title-list from the system update CFA.