Introduction
This document describes the format of the pseudo-header generated by the Excentis XRA-31 during the storage of captured packets in pcap/pcapng format. This header is prepended to DOCSIS® packets and encapsulates several physical layer (e.g. MER) and MAC layer (e.g. SID, IUC) parameters relevant to the packet, that would otherwise be invisible to the user. This metadata is stored in a flexible type-length-value format, defined below.
The header is officially supported by Wireshark®.
Excentis XRA Header Definition
Format
| XRA Version |
XRA Packet Type |
XRA Metadata Length |
XRA Metadata |
Payload |
- XRA Version (1 byte): current version is 1
- XRA Packet Type (1 byte): The XRA Packet Type defines what XRA Metadata is present and how the payload is decoded. By convention, the first two bits indicate direction
- 00: Downstream
- 01: Upstream
- 10, 11: Reserved
- Downstream Packet Types
- 1: Downstream SC-QAM DOCSIS MAC Frame
- 8: OFDM DOCSIS MAC Frame
- 9: OFDM NCP
- 10: OFDM PLC
- 11: OFDM PLC MMM
- Upstream Packet Types:
- 65: A-TDMA Burst
- 72: OFDMA Data Burst
- 73: OFDMA Initial Ranging
- 74: OFDMA Fine Ranging
- 75: OFDMA REQ
- 76: OFDMA Probing Sequence
- 80: Upstream DOCSIS MAC Frame
- XRA Metadata Length (2 bytes): total length of all XRA Metadata TLVs
- XRA Metadata: A list of Type (1 byte) - Length (1 byte) - Value encoded Metadata. For a definition of the distinct types, see below.
- Payload: dissection dependent on the XRA Packet Type
- Downstream SC-QAM DOCSIS MAC Frame (1): MAC Frame as described by section 6.2 "MAC Frame Formats" of the DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification (1)
- OFDM DOCSIS Mac Frame (8): MAC Frame as described by section 6.2 "MAC Frame Formats" of the DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification (1)
- OFDM NCP (9): NCP field (collection of NCP message blocks within a symbol) as described by section 8.3.4 "Next Codeword Pointer" of the DOCSIS 3.1 Physical Layer Specification (2). This means a sequence of (N-1) 3-byte NCP Data Message Blocks followed by a 3-byte NCP CRC Message Block
- OFDM PLC (10): PLC structure as described by section 6.5.1 "PLC Structure" of the DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification (1), starting with the TimeStamp Message Block (without preamble). So, the PLC structure is a sequence of message blocks.
- OFDM PLC MMM (11): An MMM Message, embedded in the Message Channel Message Block (MCMB) of the PLC. Described in section 6.5.4 "Message Channel Message Block" of the DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification (1)
- A-TDMA Burst (65): data bytes belonging to upstream burst. This can be a segment as described by section 6.3 "Segment Header Format" or a MAC Frame as described by section 6.2 "MAC Frame Formats" of the DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification (1). Distinguishing Segment Headers and MAC Frames is done with the "Segment Header Present" TLV.
- OFDMA Data Burst (72): data bytes belonging to upstream burst. This can be a segment as described by section 6.3 "Segment Header Format" or a MAC Frame as described by section 6.2 "MAC Frame Formats" of the DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification (1). Distinguishing Segment Headers and MAC Frames is done with the "Segment Header Present" TLV.
- OFDMA Initial Ranging (73): as described by section 6.4.5.7 "OFDMA Initial Ranging Request (O-INIT-RNG-REQ)" of the DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification (1)
- OFDMA Fine Ranging (74): as described by section 6.4.5.4 "Ranging Request (RNG-REQ)" of the DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification (1)
- OFDMA REQ (75): as described by section 6.2.4.5 "Queue-depth Based Request Frame" of the DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification (1)
- OFDMA Probing Sequence (76): as described in section 7.4.15.3 "Probing" of the DOCSIS 3.1 Physical Layer Specification (2). Under development – exact format to be determined in the future.
- Upstream DOCSIS MAC Frame (80): MAC Frame as described by section 6.2 "MAC Frame Formats" of the DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification (1) . E.g. MAC Frames extracted from DOCSIS Segments.
TLV List
| Description |
Type |
Length |
Notes |
| Downstream Channel ID |
1 |
1 |
Identifier of downstream channel. Unique per MAC domain. |
| Downstream Frequency (Hz) |
2 |
4 |
Central frequency of SC-QAM channel. PLC frequency of OFDM channel. |
| Modulation |
3 |
1 |
0: 64-QAM
1: 256-QAM |
| Annex |
4 |
1 |
0: annex A
1: annex B |
| Profile ID |
5 |
1 |
0: profile A
1: profile B
... |
| Codeword Info |
6 |
Total length of sub-TLV bytes |
|
| Profile Parity |
6.1 |
1 |
0: even
1: odd |
| Number of Info Bytes |
6.2 |
2 |
|
| BCH Decoding Successful |
6.3 |
1 |
0: false
1: true |
| BCH Number of Corrected Bits |
6.4 |
1 |
|
| LDPC Number of Code Bits |
6.5 |
2 |
|
| LDPC Decoding Successful |
6.6 |
1 |
0: false
1: true |
| LDPC Number of Corrected Bits |
6.7 |
1 |
|
| LDPC Number of Iterations |
6.8 |
1 |
|
| Reed-Solomon Decoding Sucessful |
6.9 |
1 |
0: false
1: true |
| Reed-Solomon Number of Corrected Symbols |
6.10 |
1 |
|
| Truncated due to Uncorrectables |
7 |
1 |
0: false
1: true |
| Symbol ID |
8 |
4 |
Identifier of first symbol of packet, assigned by hardware. Used for timing calculations. |
| MER (0.25 dB units, unsigned) |
9 |
1 |
Modulation error ratio. |
| Upstream Channel ID |
10 |
1 |
Identifier of logical upstream channel. Unique per MAC domain. |
| SID |
11 |
2 |
Service Identifier corresponding to the upstream data burst. |
| IUC |
12 |
1 |
Interval Usage Code corresponding to the upstream data burst. |
| Burst ID |
13 |
8 |
Unique burst identifier. Used in upstream data bursts*. |
| Burst Info |
14 |
Total length of sub-TLV bytes |
|
| Burst ID Reference |
14.1 |
8 |
Used to associate an upstream MAC frame with its corresponding upstream data bursts*. |
| Minislot Info |
15 |
Total length of sub-TLV bytes |
|
| Start Minislot ID |
15.1 |
4 |
Start minislot identifier. |
| Start Minislot ID (relative in OFDMA frame) |
15.2 |
1 |
See section 7.4.1 "Signal Processing Requirements" of the DOCSIS 3.1 Physical Layer Specification (2). Minislot with lowest subcarriers is 0. |
| Stop Minislot ID (relative in OFDMA frame) |
15.3 |
1 |
See section 7.4.1 "Signal Processing Requirements" of the DOCSIS 3.1 Physical Layer Specification (2). Minislot with lowest subcarriers is 0. |
| UCD CCC Parity |
16 |
1 |
0: even
1: odd |
| Grant Size (bits) |
17 |
3 |
|
| Segment Header Present |
18 |
1 |
Whether an upstream data burst belongs to a "segment header on" service flow.
0: false
1: true |
| Number of OFDMA Frames |
19 |
2 |
|
| Estimated Timing Adjust (1/204.8 μs units) |
20 |
4 |
Timing adjust of an initial or fine ranging signal, as estimated by the XRA-31. |
| Esimated Power Level (0.10 dBmV units, signed) |
21 |
2 |
|
| Subslot ID |
22 |
4 |
See DOCSIS 3.1 Physical Layer Specification (2). |
| Control Word |
23 |
1 |
Control word that specifies interleaving parameters of a downstream SC-QAM annex B channel. See Table B.2 of (3). |
| Configuration Info |
254 |
Variable |
Human readable text string. |
| Extension Type |
255 |
Variable |
|
Clarification on Burst ID
Burst ID is used to associate upstream MAC frames to their corresponding data bursts. A data burst can e.g. contain a segment (see section 7.2.4 "Continuous Concatenation and Fragmentation" of the DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification (1)). This means a segment can contain multiple MAC frames, or a MAC frame can be spread over multiple segments. The Exentis XRA-31 extracts the MAC frames from the segments. To store the information of which MAC frame belongs to which segment (or multiple segments), a Burst ID is used: each data burst gets a globally unique Burst ID. In the MAC Frame the "Burst Info"/"Burst ID reference" is used to reference these Burst IDs. Burst ID reference is a sub-TLV of Burst Info. Each Burst ID reference is contained within a Burst Info TLV.
E.g. a MAC frame is reconstructed from segments with Burst ID 7, 8 and 9. In that case the MAC Frame will contain Burst ID References with values 7, 8 and 9, each contained within a Burst Info TLV.
Burst IDs (or references) are currently represented 64-bit unsigned integers. However, they should be treated as an opaque array of bytes, with no significance to the values of the bytes, and used only for matching bursts and the MAC frames contained within them by comparing the byte arrays for equality.
References
(1) DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification:CM-SP-MULPIv3.1-I12-170906
(2) DOCSIS 3.1 Physical Layer Specification: CM-SP-PHYv3.1-I12-171026
(3) ITU-T Rec. J.83 (12/07) Digital multi-programme systems for television, sound and data services for cable distribution
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