The following headers are used by pml-0.02 and up. All values are in network byte order. Note that these packet formats differ from the packet formats used in pml-0.01. This backwards incompatibility should not be an issue because there is vary little actual usage of pml-0.01.

The P2P Multicast Session Description (PMSD) header is used when describing a resource using a multicast group.

	0	4	8	12	16	20	24	28
0	Version		CodingID			Payload size
1	Start		(Res'd)		SessionID
2	Source IP address
3	Multicast group IP address
4	UDP port				Bitrate (8Kbit/s)
5	Filesize (bytes)
6
7	File TigerTree Hash
8
9
10
11
12
13	Segment offset
14	Segment length
15	Segment TigerTree Hash
16
17
18
19
20
21	Filename (Max 128 bytes)

PMSD header

• The first two bytes of each header are used to multiplex both PMT and PMSD packets on the same (S,G) session. Currently, version number 0 indicates that a PMT packet is being sent and version number 1 indicates that a PMSD version 1 packet is being sent.
• For the possible CodingID values please consult the table below with the defined PMT codings
• The Start field provides the number of seconds before the actual transmission of the resource starts
• The SessionID provides an  identifier unique to the (S,G) multicast session that will be used for the transport of the resource described by the PMSD packet
• Source IP address & multicast group IP address, UDP port
• The bitrate field in the PMSP header is provided in multiples of 8Kbit/s. So a bitrate value of 8 means that the session will be transmitted at 64Kbit/s
• File size is the size of the file of which a segment is being described
• File TigerTree hash
• The segment offset plus the segment length may not be greater then the filesize.
• The filename may not exceed 128 bytes and is encoded using ASCII (UTF-8 support may be added in the future)

 

The P2P Multicast Transport  (PMT) header is used when transporting a resource from the sender to the receivers.

	0	4	8	12	16	20	24	28
0	Version		PartID		SessionID
1	PacketID
2	Payload (...)

PMT header

• The Version must be 0 for all PMT headers to be able to distinguish between PMSD and PMT headers in the same multicast session.
• The PartID is only used when Reed-Solomon coding is used for the multicast session. The PartID must be 0 when the multicast session does not use Reed-Solomon coding.
• The SessionID matches the SessionID announced in the PMSD header.
• The PacketID provides:
  • the sequence number of the packet in the part in the case of Reed-Solomon coding
  • the sequence number of the packet in the session, with the other codings
• The payload of the packet may not exceed the payload size indicated in the corresponding PMSD header

 

The following P2P Multicast Transport encodings are defined:

ID	Codec	Parameters
0	Reed-Solomon	SYMSIZE=8, Genpoly = 0x187, FCS=1, Prime=1, NROOTS=8
1	LDGM-Stairs	Left degree = 7, Data packets = int(Segment length / Payload size) + 1, FEC packets = int (0.33 * data packets), Seed = 1

PMT coding