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When a packet is lost, it can be recovered through fast retransmission (e.g., Go-Back-N in RoCE) or by using timeouts. Retransmission triggered by timeouts typically incurs significant latency. Packet trimming aims to facilitate rapid packet loss notification and, consequently, eliminate slow timeout-based retransmissions. Signed-off-by: Marian Pritsak <marianp@mellanox.com>
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# Switch Abstraction Interface Change Proposal for Packet Trimming | ||
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Title | Packet Trimming | ||
------------|---------------- | ||
Authors | Nvidia | ||
Status | In review | ||
Type | Standards track | ||
Created | 8/28/2024 | ||
SAI-Version | 1.14 | ||
---------- | ||
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## Overview | ||
When the lossy queue exceeds a buffer threshold, it drops packets without any notification to the destination host. | ||
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When a packet is lost, it can be recovered through fast retransmission (e.g., Go-Back-N in RoCE) or by using timeouts. Retransmission triggered by timeouts typically incurs significant latency. Packet trimming aims to facilitate rapid packet loss notification and, consequently, eliminate slow timeout-based retransmissions. | ||
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To help the host recover data more quickly and accurately, we introduce a packet trimming feature, that upon a failed packet admission to a shared buffer, | ||
will trim a packet to a configured size, and try sending it on a different queue to deliver a packet drop notification to an end host. | ||
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``` | ||
┌───────────────┐ | ||
│ │ | ||
│Trimmed packet │ | ||
│ │ | ||
└───────────────┘ | ||
┌─┬─┬─┬─┬────────┐ | ||
│ │ │ │ │ │ | ||
│ │ │ │ │ │ | ||
┌────────────────► │ │ │ │ │ | ||
│ │ │ │ │ │ │ Queue | ||
│ │ │ │ │ │ │ | ||
│ │ │ │ │ │ │ | ||
│ └─┴─┴─┴─┴────────┘ | ||
┌──────────────┐ │ | ||
│ │ ┌──────────────────────────────────────────────────────┐ │ ┌─┬─┬─┬─┬─┬─┬─┬─┬┐ | ||
│ │ │ │ │ │ │ │ │ │ │ │ │ ││ | ||
│ │ │ │ │ \ / │ │ │ │ │ │ │ │ ││ | ||
│ │ │ │ │ \ / │ │ │ │ │ │ │ │ ││ | ||
│ Packet │ │ Pipeline ┼────┼───────\────────► │ │ │ │ │ │ │ ││ Queue | ||
│ │ │ │ / \ │ │ │ │ │ │ │ │ ││ | ||
│ │ │ │ / \ │ │ │ │ │ │ │ │ ││ | ||
│ │ └──────────────────────────────────────────────────────┘ └─┴─┴─┴─┴─┴─┴─┴─┴┘ | ||
│ │ | ||
│ │ | ||
│ │ | ||
└──────────────┘ | ||
``` | ||
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This feature assumes that forwarding tables are configured properly, and the original packet would be delivered to the destination successfully if not for the congestion. | ||
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## Spec | ||
There is a tradeoff between trying to configure a higher threshold in a queue buffer profile and trimming the packet. | ||
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If the user chooses to configure higher thresholds for queues, the probability of a drop on a particular queue is lower only if other ports are less congested at the moment. | ||
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However, if all the ports are equally utilized, it makes sense to create a different buffer profile for these queues, with a stricter threshold to have more fairness in shared buffer. | ||
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A static trimming threshold may not be effective with shared buffer switches, where the buffer resources allocated to a queue or port can vary over time. Therefore, we propose adding a new attribute to a buffer profile to allow configuring packet trimming on such stricter profiles: | ||
``` | ||
/** | ||
* @brief Enum defining queue actions in case the packet fails to pass the admission control. | ||
*/ | ||
typedef enum _sai_buffer_profile_packet_admission_fail_action_t | ||
{ | ||
/** | ||
* @brief Drop the packet. | ||
* | ||
* Default action. Packet has nowhere to go | ||
* and will be dropped. | ||
*/ | ||
SAI_BUFFER_PROFILE_PACKET_ADMISSION_FAIL_ACTION_DROP, | ||
/** | ||
* @brief Trim the packet. | ||
* | ||
* Try sending a shortened packet over a different | ||
* queue. Original packet will be dropped and trimmed copy of the packet will be send. | ||
* The IP length and checksum fields will be updated in a trimmed copy. | ||
* SAI_QUEUE_STAT_DROPPED_PACKETS as well as SAI_QUEUE_STAT_DROPPED_BYTES | ||
* will count the original discarded frames even if they will be trimmed afterwards. | ||
* Interface statistics must show dropped packets. | ||
* Interface statistics may show sent trimmed packets. | ||
*/ | ||
SAI_BUFFER_PROFILE_PACKET_ADMISSION_FAIL_ACTION_DROP_AND_TRIM, | ||
} sai_buffer_profile_packet_admission_fail_action_t; | ||
``` | ||
``` | ||
/** | ||
* @brief Buffer profile discard action | ||
* | ||
* Action to be taken upon packet discard due to | ||
* buffer profile configuration. Applicable only | ||
* when attached to a queue. | ||
* | ||
* @type sai_buffer_profile_packet_admission_fail_action_t | ||
* @flags CREATE_AND_SET | ||
* @default SAI_BUFFER_PROFILE_PACKET_ADMISSION_FAIL_ACTION_DROP | ||
*/ | ||
SAI_BUFFER_PROFILE_ATTR_PACKET_ADMISSION_FAIL_ACTION, | ||
``` | ||
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Trimming engine attributes are configured globally. | ||
``` | ||
/** | ||
* @brief Trim packets to this size to reduce bandwidth | ||
* | ||
* @type sai_uint32_t | ||
* @flags CREATE_AND_SET | ||
* @default 128 | ||
*/ | ||
SAI_SWITCH_ATTR_PACKET_TRIMMING_SIZE, | ||
/** | ||
* @brief New packet trimming DSCP value | ||
* | ||
* @type sai_uint8_t | ||
* @flags CREATE_AND_SET | ||
* @default 0 | ||
*/ | ||
SAI_SWITCH_ATTR_PACKET_TRIMMING_DSCP_VALUE, | ||
/** | ||
* @brief Is the new queue index for a trimmed packet mapped from DSCP | ||
* | ||
* @type sai_bool_t | ||
* @flags READ_ONLY | ||
*/ | ||
SAI_SWITCH_ATTR_PACKET_TRIMMING_QUEUE_INDEX_MAPPED_FROM_DSCP, | ||
/** | ||
* @brief New packet trimming queue index | ||
* | ||
* @type sai_uint8_t | ||
* @flags CREATE_AND_SET | ||
* @default 0 | ||
* @validonly SAI_SWITCH_ATTR_PACKET_TRIMMING_QUEUE_INDEX_MAPPED_FROM_DSCP == false | ||
*/ | ||
SAI_SWITCH_ATTR_PACKET_TRIMMING_QUEUE_INDEX, | ||
``` | ||
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If more granularity is needed (e.g. trim a specific protocol, or packets within protocol), ACL action is added to disable trimming even if the packet is eligible due to a queue with a buffer profile attached that has trimming enabled. | ||
``` | ||
/** | ||
* @brief Disable packet trimming for a given match condition. | ||
* | ||
* This rule takes effect only when packet trimming is configured on a buffer profile of a queue to which a packet belongs. | ||
* | ||
* @type sai_acl_action_data_t bool | ||
* @flags CREATE_AND_SET | ||
* @default disabled | ||
*/ | ||
SAI_ACL_ENTRY_ATTR_ACTION_DISABLE_TRIMMING = SAI_ACL_ENTRY_ATTR_ACTION_START + 0x39, | ||
``` | ||
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Both the queue and the port have the packet counter to reflect the number of trimmed packet. | ||
``` | ||
/** Packets trimmed due to failed admission [uint64_t] */ | ||
SAI_QUEUE_STAT_TRIMMED_PACKETS = 0x00000028, | ||
``` | ||
``` | ||
/** Packets trimmed due to failed shared buffer admission [uint64_t] */ | ||
SAI_PORT_STAT_TRIMMED_PACKETS, | ||
``` |
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