Experience from implementing a new packet discard classification scheme

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 23 November 2023.¶

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

Table of Contents

1. Introduction

The job of a network is to transport packets. Understanding both where and why packet loss occurs is essential to the effective operation of networks. Router-reported packet loss is the most direct signal for network operations to identify customer impact from unintended packet loss. Accurate accounting of packet loss is not enough, however, as some level of packet loss is normal in TCP/IP networks. In automating network operations, there are only a relatively small number of automated actions that can be taken to mitigate impacting packet loss. Precise signal is important to ensure the right action is taken, however, as taking the wrong action can make problems worse.¶

The existing definitions of ifindiscards, ifoutdiscards, ifinerrors, ifouterrors [RFC1213], do not provide the precision needed to be able to identify the cause of packet loss sufficient to mitigate the impact. From a network operators' perspective, ifindiscards can represent both intended packet loss (i.e. packets discarded due to policy) and unintended packet loss. Further, these definitions are ambiguous, in that vendors can and have implemented them differently with different outcomes. In some implementations, ifinerrors accounts only for errored packets which are dropped, whilst in others it accounts for all errored packets whether they are dropped or not. Many vendors support more discard metrics than these; where they do they are inconsistently implemented due to an absence of clearly defined classification and semantics for packet loss reporting.¶

This document describes our experience from implementing a packet loss classification scheme across multiple hardware platforms, which aimed to address these issues and enable automated mitigation of unintended packet loss. Section 2 describes the problem. Section 3 defines the classification scheme and the accounting requirements with examples. Section 4 gives examples of discard signal to cause to auto-mitigation action mapping. Section 5 details our experience from implementing this scheme.¶

2. Problem statement

Working backwards from auto-mitigation of unintended packet loss, there are only a relative small number of potential auto-mitigation actions, e.g.:¶

1. Take device, link or set of devices and/or links out of service¶
2. Put device, link or set of devices and/or links back into service¶
3. Move traffic¶
4. Roll-back a change¶
5. Escalate to network operators¶

Precise signal of impact is important as taking the wrong action can be worse than taking no action. For example, taking a congested device out of service can make congestion worse.¶

To be able to detect whether router reported packet loss is a problem and determine what actions should be taken to mitigate the impact and remediate the cause, depends on four primary features of the packet loss signal:¶

1. the cause of the loss¶
2. the rate and magnitude of the loss¶
3. the duration of the loss¶
4. the location of the loss¶

Features 2, 3 and 4 are already addressed with device level passive monitoring statistics, e.g. obtained with SNMP [RFC1157] or NETCONF [RFC6241]. Feature 1 is dependent on the classification scheme used for packet loss reporting. We define a new scheme to address this problem in the next section.¶

3. Classification Scheme

We define the classification scheme as a tree. At the top level we differentiate the direction: ingress or egress. At the second level we differentiate between traffic and discards. At the third level we differentiate between intended discards and unintended discards by discard class.¶

.
|-- interface/
|   |-- ingress/
|   |   |-- traffic_rx/
|   |   |   |-- v4/
|   |   |   |   |-- packets
|   |   |   |   `-- bytes
|   |   |   |-- v6/
|   |   |   |   |-- packets
|   |   |   |   `-- bytes
|   |   |   |-- l2/
|   |   |   |   |-- packets
|   |   |   |   `-- bytes
|   |   |   `-- qos/
|   |   |       |-- class_0/
|   |   |       |   |-- packets
|   |   |       |   `-- bytes
|   |   |       |-- ...
|   |   |       `-- class_n/
|   |   |           |-- packets
|   |   |           `-- bytes
|   |   `-- discards/
|   |       |-- v4/
|   |       |   |-- packets
|   |       |   `-- bytes
|   |       |-- v6/
|   |       |   |-- packets
|   |       |   `-- bytes
|   |       |-- l2/
|   |       |   |-- packets
|   |       |   `-- bytes
|   |       |-- policy/
|   |       |   |-- acl/
|   |       |   |   `-- packets
|   |       |   |-- policer/
|   |       |   |   |-- packets
|   |       |   |   `-- bytes
|   |       |   `-- null_route/
|   |       |       `-- packets
|   |       |-- errors/
|   |       |   |-- rx/
|   |       |   |   |-- l2/
|   |       |   |   |   |-- frames
|   |       |   |   |   |-- fec_error/
|   |       |   |   |   |   `-- frames
|   |       |   |   |   `-- invalid_frame/
|   |       |   |   |       `-- frames
|   |       |   |   `-- l3/
|   |       |   |       |-- packets
|   |       |   |       |-- checksum_error/
|   |       |   |       |   `-- packets
|   |       |   |       `-- invalid_packet/
|   |       |   |           `-- packets
|   |       |   |-- local/
|   |       |   |   |-- packets
|   |       |   |   `-- parity_error/
|   |       |   |       `-- packets
|   |       |   `-- no_route/
|   |       |       `-- packets
|   |       |-- ttl_expired/
|   |       |   `-- packets
|   |       `-- no_buffer/
|   |           |-- class_0/
|   |           |   |-- packets
|   |           |   `-- bytes
|   |           |-- ...
|   |           `-- class_n/
|   |               |-- packets
|   |               `-- bytes
|   `-- egress/
|       |-- traffic_tx/
|       |   |-- v4/
|       |   |   |-- packets
|       |   |   `-- bytes
|       |   |-- v6/
|       |   |   |-- packets
|       |   |   `-- bytes
|       |   |-- l2/
|       |   |   |-- packets
|       |   |   `-- bytes
|       |   `-- qos/
|       |       |-- class_0/
|       |       |   |-- packets
|       |       |   `-- bytes
|       |       |-- ...
|       |       `-- class_n/
|       |           |-- packets
|       |           `-- bytes
|       `-- discards/
|           |-- v4/
|           |   |-- packets
|           |   `-- bytes
|           |-- v6/
|           |   |-- packets
|           |   `-- bytes
|           |-- l2/
|           |   |-- packets
|           |   `-- bytes
|           |-- policy/
|           |   |-- acl/
|           |   |   `-- packets
|           |   `-- policer/
|           |       `-- packets
|           |-- errors/
|           |   `-- tx/
|           |       |-- l2/
|           |       |   `-- frames
|           |       `-- l3/
|           |           `-- packets
|           `-- no_buffer/
|               |-- class_0/
|               |   |-- packets
|               |   `-- bytes
|               |-- ...
|               `-- class_n/
|                   |-- packets
|                   `-- bytes
`-- to_cpu/
    |-- packets
    |-- bytes
    `-- policy/
        |-- acl/
        |   `-- packets
        `-- policer/
            `-- packets

Notes:¶

1. The tree follows the structure <component><direction><type><sub-type><sub-sub-type><metric>, where:
   a. component can be interface|device|tunnel|to_cpu
   b. direction can be ingress|egress
   c. type can be traffic|discards¶
2. If Diffserv [RFC2475] quality of service (QOS) is not configured, class0 is considered the default for no_buffer discards¶
3. from_cpu traffic is assumed to be accounted for as transit traffic¶

See Appendix A for an example of where packets may be dropped in a device.¶

4. Signal to Cause to Mitigation Mapping

Example discard signal to cause to mitigation mappings are shown in the table below:¶

+------------------+---------+-------------------+-----------+--------+-----------+----------------------+
|Discard class     |Direction|Cause              |Rate       |Duration|Unintended?|Possible actions      |
+------------------+---------+-------------------+-----------+--------+-----------+----------------------+
|ErrorRxL2Discards |Ingress  |Upstream device    |>0(Anomaly)|O(1min) |Y          |Take upstream link or |
|                  |         |or link errror     |           |        |           |device out-of-service |
|TTLDiscards       |Ingress  |Tracert            |<=Baseline |        |           |no action             |
|TTLDiscards       |Ingress  |Convergence        |>Baseline  |O(1s)   |Y          |no action             |
|TTLDiscards       |Ingress  |Routing loop       |>Baseline  |O(1min) |Y          |Roll-back             |
|AclDiscards       |Ingress  |ACL                |           |        |N          |no action             |
|NullRouteDiscards |Ingress  |Null route         |           |        |N          |no action             |
|NoRouteDiscards   |Ingress  |Convergence        |>0(Anomaly)|O(1s)   |Y          |no action             |
|NoRouteDiscards   |Ingress  |Config error       |>0(Anomaly)|O(1min) |Y          |Roll-back             |
|NoRouteDiscards   |Ingress  |Invalid destination|>0(Anomaly)|O(10min)|N          |Escalate              |
|ErrorLocalDiscards|Ingress  |Device errors      |>0(Anomaly)|O(1min) |Y          |Take device           |
|                  |         |                   |           |        |           |out-of-service        |
|NoBufferDiscards  |Egress   |Congestion         |<=Baseline |        |N          |no action             |
|NoBufferDiscards  |Egress   |Congestion         |>Baseline  |O(1min) |Y          |Bring capacity back   |
|                  |         |                   |           |        |           |into service or move  |
|                  |         |                   |           |        |           |traffic               |
+------------------+---------+-------------------+-----------+--------+-----------+----------------------+

5. Implementation Experience

1. The number of classes is a compromise between: providing sufficient detail to be able to take the appropriate actions whilst: a) not providing too much detail which can require deeper understanding rather than helping to surface the problem quickly; b) constraining the quantity of data produced where these metrics are produced per interface.¶
2. There are multiple ways that we could have defined the discard classification tree, e.g. we could have used a multi-rooted tree, rooted in each protocol. We opted instead to define a tree where protocol discards and cause discards are accounted orthogonally, as this reduces the number of classes and we found it sufficient to determine mitigation actions.¶
3. NoBuffer discards can be realised differently with different memory architectures. Hence, whether a NoBuffer discard is attributed to ingress or egress can differ accordingly. A packet dropped due to NoBuffer discard should never be accounted for both on ingress and on egress.¶
4. Most platforms account for the number of packets where the TTL has expired, and the CPU has returned an ICMP Time Exceeded message. In practise, however, there is often a policer applied to traffic on the to_CPU path, which limits the number of packets to the CPU. Implicitly, this limits the rate of TTL discards processed by the CPU and hence it limits the number reported. One method to account for all TTL discards, even those that are dropped by a policer when being punted to the CPU, is to use accounting of all ingress packets received with TTL=1, i.e. before TTL processing.¶
5. Where a no_route discard is implemented with a default null route, separate accounting is needed for any explicit null routes configured, in order to differentiate between interface/ingress/discards/policy/null_route/packets and interface/ingress/discards/errors/no_route/packets.¶
6. It is useful to account separately for transit packets dropped by transit ACLs/policers, and to_cpu packets dropped by ACLs/policers which limit the number of packets to the CPU¶
7. It is not possible to identify a configuration error - i.e. when intended discards are unintended - with device packet loss metrics alone. For example, to determine if ACL drops are intended or due to a misconfigured ACL some other method is needed, e.g. with config validation before deployment or in detecting a significant change in ACL drops before/after a change.¶

6. Security Considerations

There are no new security considerations introduced by this document.¶

7. IANA Considerations

There are no new IANA considerations introduced by this document.¶

8. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

9. Acknowledgments

The content of this draft has benefitted from discussions with JR Rivers, Ronan Waide and Chris DeBruin.¶

Appendix A. Where do packets get dropped?

The diagram below is an example of where and why packets may be dropped in a typical single ASIC, shared buffered type device.¶

                                           +----------+
                                           |          |
                                           |  CPU     |
                                           |          |
                                           +--+---^---+
                                     from_cpu |   | to_cpu
                                              |   |
               +------------------------------v---+-------------------------------+
               |                                                                  |

 +----------+  +----------+  +----------+  +----------+  +----------+  +----------+  +----------+
 |          |  |          |  |          |  |          |  |          |  |          |  |          |
->  Phy     +-->  Mac     +--> Ingress  +--> Buffers  +--> Egresss  +-->  Mac     +-->  Phy     |>
 |          |  |          |  |  Pipeline|  |          |  |  Pipeline|  |          |  |          |
 +----------+  +----------+  +----------+  +----------+  +----------+  +----------+  +----------+

                error/rx/l2   error/rx/l3   no_buffer     error/tx/l3
                              error/local
                              no_route
                              ttl

                              policy/acl                  policy/acl
                              policy/policer              policy/policer
                              null_route

Authors' Addresses