[HLD] DSCP/TC remapping for tunnel traffic

doc/qos/tunnel_dscp_remapping.md

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+# DSCP remapping for tunnel traffic
+
+## 1 Table of Content ###
+
+- [Revision](#11-revision)
+- [Scope](#2-scope)
+- [Definitions/Abbreviations](#3-definitionsabbreviations)
+- [Overview](#4-overview)
+- [Design](#5-design)
+    - [SWSS Schema](#51-swss-schema)
+        - [Define new table for mapping](#511-define-new-table-for-mapping)
+        - [Update existing TUNNEL table](#512-update-existing-tunnel-table)
+        - [Define new field for extra lossless queues](#513-define-new-field-for-extra-lossless-queues)
+    - [SAI attribute](#52-sai-attribute)
+    - [orchagent](#53-orchagent)
+- [Test requirement](#6-test-requirement)
+- [Open Questions](#7-open-questions)
+
+### 1.1 Revision ###
+| Rev |     Date    |       Author       | Change Description                |
+|:---:|:-----------:|:------------------:|-----------------------------------|
+| 0.1 |             | Bing Wang   | Initial version                   |
+
+
+## 2 Scope ##
+
+This document covers high level design of DSCP and TC remapping for tunnel traffic in SONiC.
+
+## 3 Definitions/Abbreviations ##
+
+
+| Term | Meaning |
+|:--------:|:---------------------------------------------:|
+| PFC | Priority-based Flow Control  |
+| TC | Traffic class|
+| DSCP| Differentiated Services Code Point |
+
+## 4 Overview
+
+In Dual-ToR scenario, PFC deadlock can happen if two servers are congested at same time and start sending PFC pause frames to both upper ToR and lower ToR. It is because the south bound traffic from T1 to standby ToR is bounced back to T1 via the same queue. Even when the block condition is resolved, the pause condition among T1 and both ToRs can stay forever and no traffic will go through.
+
+To avoid this scenario, the bounced-back traffic from standby ToR to T1 will be remapped into another queue. For example, the traffic flow is as below if we are going to remap traffic in queue 3 to queue 2.
+
+<p align=center>
+<img src="dscp-remapping-images/Bounced-back-traffic-flow.png" alt="Figure 1. Bounced back traffic flow">
+</p>
+
+When congestion happens on server, the traffic flow of PFC pause frames is as below diagram.
+
+<p align=center>
+<img src="dscp-remapping-images/Bounced-back-traffic-flow-PFC.png" alt="Figure 2. Bounced back traffic flow with PFC pause">
+</p>
+The current QoS map architecture allows for port-based selection of each QoS map. However, we are not able to override the port-based QoS map for tunnel traffic. 
+This design proposes a method to remapping DSCP and TC for tunnel traffic. 
+
+
+## 5 Design ##
+
+### 5.1 SWSS Schema
+#### 5.1.1 Define new table for mapping
+Update [qos_config.j2](https://github.com/Azure/sonic-buildimage/blob/master/files/build_templates/qos_config.j2) to generate 4 tables for remapping. Currently, the remapping is required in `dual-tor` scenario. So the tables are rendered into `config_db` only when `DEVICE_METADATA['localhost']['subtype'] = 'DualToR`. 
+
+Please be noted that below config is to remap traffic in queue 3 to queue 2, and traffic in queue 4 to queue 6.
+Before remapping to queue 4 and 6, both queues are required to be cleared. Hence the current `DSCP_TO_TC_MAP|AZURE` in [qos_config.j2](https://github.com/Azure/sonic-buildimage/blob/master/files/build_templates/qos_config.j2) is required to be updated to map DSCP `5` and `48` into other queues.
+* Table for decap
+
+    DSCP_TO_TC_MAP for mapping DSCP to TC
+
+    ```json
+    "DSCP_TO_TC_MAP": {
+        "AZURE_TUNNEL": {
+            "0": "1",
+            "1": "1",
+            "2": "2",
+            "3": "3",
+            "4": "4",
+            "5": "1",
+            "6": "6",
+            "7": "1",
+            "8": "0",
+            "9": "1"
+    }
+    ```
+
+    TC_TO_PRIORITY_GROUP_MAP for mappping TC to PG
+
+    ```json
+    "TC_TO_PRIORITY_GROUP_MAP": {
+        "AZURE_TUNNEL": {
+            "0": "0",
+            "1": "0",
+            "2": "0",
+            "3": "2",
+            "4": "6",
+            "5": "0",
+            "6": "0",
+            "7": "7"
+    }
+    ```
+
+* Table for encap
+
+    TC_TO_QUEUE_MAP for remapping queue
+
+    ```json
+    "TC_TO_QUEUE_MAP": {
+        "AZURE_TUNNEL": {
+            "0": "0",
+            "1": "1",
+            "2": "1",
+            "3": "2",
+            "4": "6",
+            "5": "5",
+            "6": "1",
+            "7": "7"
+    }
+    ``` 
+
+    TC_TO_DSCP_MAP for rewriting DSCP
+    ```json
+    "TC_TO_DSCP_MAP": {
+        "AZURE_TUNNEL": {
+            "0": "8",
+            "1": "0",
+            "2": "0",
+            "3": "2",
+            "4": "6",
+            "5": "46",
+            "6": "0",
+            "7": "48"
+    }
+    ```
+
+    To support the new table, a new YANG model `sonic-tc-dscp.yang` is required 
+#### 5.1.2 Update existing TUNNEL table
+1. Change `dscp_mode` from `uniform` to `pipe` for TC remapping
+2. Add TC remapping config if TC remapping is enabled
+
+```json
+    "TUNNEL": {
+        "MuxTunnel0": {
+            "dscp_mode": "pipe",
+            "dst_ip": "10.1.0.32",
+            "ecn_mode": "copy_from_outer",
+            "encap_ecn_mode": "standard",
+            "ttl_mode": "pipe",
+            "tunnel_type": "IPINIP",
+            "decap_dscp_to_tc_map": "[DSCP_TO_TC_MAP|AZURE_TUNNEL]",
+            "decap_tc_to_pg_map": "[TC_TO_PRIORITY_GROUP_MAP|AZURE_TUNNEL]",
+            "encap_tc_to_queue_map": "[TC_TO_QUEUE_MAP|AZURE_TUNNEL]",
+            "encap_tc_color_to_dscp_map": "[TC_TO_DSCP_MAP|AZURE_TUNNEL]"
+        }
+    }
+```
+
+#### 5.1.3 Define new field for extra lossless queues
+Two new fields are added to specify software or hardward PFC watchdog.
+
+* `pfc_wd_sw_enable` Specify the queue(s) to enable software PFC watchdog
+* `pfc_wd_hw_enable` Specify the queue(s) to enable hardware PFC watchdog.
+
+In current version, software PFC watchdog will read field `pfc_enable` to determine PFCWD is enabled on which queue(s). To maintain compatible with current logic, `db_migrator` script is required to be updated.
+
+```json
+"PORT_QOS_MAP": {
+        "Ethernet0": {
+            "dscp_to_tc_map": "[DSCP_TO_TC_MAP|AZURE]",
+            "pfc_enable": "3,4,2,6",
+            "pfc_wd_sw_enable": "3,4",
+            "pfc_wd_hw_enable": "2,6",
+            "pfc_to_queue_map": "[MAP_PFC_PRIORITY_TO_QUEUE|AZURE]",
+            "tc_to_pg_map": "[TC_TO_PRIORITY_GROUP_MAP|AZURE]",
+            "tc_to_queue_map": "[TC_TO_QUEUE_MAP|AZURE]"
+        }
+}
+```
+
+To support new field `pfc_wd_sw_enable` and `pfc_wd_hw_enable`, [sonic-port-qos-map.yang](https://github.com/Azure/sonic-buildimage/blob/master/src/sonic-yang-models/yang-models/sonic-port-qos-map.yang) is required to be updated.
+
+
+### 5.2 SAI attribute
+TC remapping requires below SAI attributes change.
+```cpp
+    /**
+     * @brief Enable TC AND COLOR -> DSCP MAP on tunnel at encapsulation (access-to-network) node to remark the DSCP in tunnel header
+     */
+    SAI_TUNNEL_ATTR_ENCAP_QOS_TC_AND_COLOR_TO_DSCP_MAP,
+
+    /**
+     * @brief Enable TC -> Queue MAP on tunnel encap
+     */
+    SAI_TUNNEL_ATTR_ENCAP_QOS_TC_TO_QUEUE_MAP,
+
+    /**
+     * @brief Enable DSCP -> TC MAP on tunnel at termination (Network-to-access) node. This map if configured overrides the port MAP
+     */
+    SAI_TUNNEL_ATTR_DECAP_QOS_DSCP_TO_TC_MAP,
+
+    /**
+     * @brief Enable TC -> Priority Group MAP. TC is derived from the tunnel MAP
+     */
+    SAI_TUNNEL_ATTR_DECAP_QOS_TC_TO_PRIORITY_GROUP_MAP,
+```
+### 5.3 orchagent
+
+Code change in orchagent
+
+1. Update `tunneldecaporch` to read and set new tunnel attributes when creating decap tunnel.
+
+    | Attribute |     Value    |  
+    |---|-----------|
+    | SAI_TUNNEL_ATTR_ENCAP_QOS_TC_AND_COLOR_TO_DSCP_MAP | [TC_TO_DSCP_MAP\|AZURE_TUNNEL]|
+    | SAI_TUNNEL_ATTR_ENCAP_QOS_TC_TO_QUEUE_MAP | [TC_TO_QUEUE_MAP\|AZURE_TUNNEL] |
+
+2. Update `create_tunnel` defined in `muxorch.cpp` to read and set new tunnel attributes when creating tunnel.
+    | Attribute |     Value    |  
+    |---|-----------|
+    | SAI_TUNNEL_ATTR_DECAP_QOS_DSCP_TO_TC_MAP | [DSCP_TO_TC_MAP\|AZURE_TUNNEL]|
+    | SAI_TUNNEL_ATTR_DECAP_QOS_TC_TO_PRIORITY_GROUP_MAP | [TC_TO_PRIORITY_GROUP_MAP\|AZURE_TUNNEL |
+## 6 Test requirement
+All changes are to be covered by system test.
+* Encap at standby side
+
+    * Test case 1 Verify DSCP re-writing
+    * test case 2 Verify traffic is egressed at expected queue
+    * Test case 3 Verify PFC frame generation at expected queue
+
+* Decap at active side
+
+    * Test case 1 Verify packets egressed to server at expected queue
+    * Test case 2 Verify PFC pause frame block expected queue
+    * Test case 3 Verify PFC frame generation at expected queue
+
+
+## 7 Open Questions
+
+
+