An Information Model for Packet Discard Reporting
draft-ietf-opsawg-discardmodel-00
| Document | Type | Active Internet-Draft (opsawg WG) | |
|---|---|---|---|
| Authors | John Evans , Oleksandr Pylypenko , Jeffrey Haas , Aviran Kadosh | ||
| Last updated | 2024-02-05 | ||
| Replaces | draft-opsawg-evans-discardmodel | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | Diego Lopez | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | diego.r.lopez@telefonica.com |
draft-ietf-opsawg-discardmodel-00
OPSAWG J. Evans
Internet-Draft O. Pylypenko
Intended status: Informational Amazon
Expires: 8 August 2024 J. Haas
Juniper Networks
A. Kadosh
Cisco Systems, Inc.
5 February 2024
An Information Model for Packet Discard Reporting
draft-ietf-opsawg-discardmodel-00
Abstract
The primary function of a network is to transport packets and deliver
them according to a service level objective. Understanding both
where and why packet loss occurs within a network is essential for
effective network operation. Router-reported packet loss is the most
direct signal for network operations to identify customer impact
resulting from unintended packet loss. This document defines an
information model for packet loss reporting, which classifies these
signals to enable automated network mitigation of unintended packet
loss.
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 8 August 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
4. Information Model . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Discard Class Descriptions . . . . . . . . . . . . . . . 9
4.2. Semantics . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Example Signal-Cause-Mitigation Mapping . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 12
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Where do packets get dropped? . . . . . . . . . . . 14
Appendix B. Implementation Experience . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
In automating network operations, a network operator needs to be able
to detect anomalous packet loss, diagnose or root cause the loss, and
then apply one of a set of possible actions to mitigate customer-
impacting packet loss. Some packet loss is normal or intended in IP
networks, however. Hence, precise classification of packet loss
signals is crucial both to ensure that anomalous packet loss is
easily detected and that the right action or sequence of actions are
taken to mitigate the impact, as taking the wrong action can make
problems worse.
The existing metrics for reporting packet loss, as defined in
[RFC1213] - namely ifInDiscards, ifOutDiscards, ifInErrors,
ifOutErrors - do not provide sufficient precision to automatically
identify the cause of the loss and mitigate the impact. From a
network operator's perspective, ifInDiscards can represent both
intended packet loss (e.g., packets discarded due to policy) and
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unintended packet loss (e.g., packets dropped in error).
Furthermore, these definitions are ambiguous, as vendors can and have
implemented them differently. In some implementations, ifInErrors
accounts only for errored packets that are dropped, while in others,
it accounts for all errored packets, whether they are dropped or not.
Many implementations support more discard metrics than these; where
they do, they have been inconsistently implemented due to the lack of
a standardised classification scheme and clear semantics for packet
loss reporting. [RFC7270] provides support for reporting discards
per flow in IPFIX using forwardingStatus, however, the defined drop
reason codes also lack sufficient clarity to support automated root
cause analysis and mitigation of impact.
Hence, this document defines an information model for packet loss
reporting, aiming to address these issues by presenting a packet loss
classification scheme that can enable automated mitigation of
unintended packet loss. Consistent with [RFC3444], this information
model is independent of any specific implementations or protocols
used to transport the data. There are multiple ways that this
information model could be implemented (i.e., data models), including
SNMP [RFC1157], NETCONF [RFC6241] / YANG [RFC7950], and IPFIX
[RFC5153], but they are outside of the scope of this document. We
further limit the scope of this document to reporting packet loss at
layer 3 and frames discarded at layer 2, although the information
model could be extended in future to cover segments dropped at layer
4.
Section 3 describes the problem. Section 4 defines the information
model and semantics with examples. Section 5 provides examples of
discard signal-to-cause-to-auto-mitigation action mapping.
Appendix B details the authors' experience from implementing this
model.
The terms 'packet drop' and 'discard' are considered equivalent and
are used interchangeably in this document.
This document considers only the signals that may trigger automated
mitigation plans and not how they are defined or executed.
2. 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.
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3. Problem Statement
At the highest-level, unintended packet loss is the discarding of
packets that the network operator otherwise intends to deliver, i.e.
which indicates an error state. There are many possible reasons for
unintended packet loss, including: erroring links may corrupt packets
in transit; incorrect routing tables may result in packets being
dropped because they do not match a valid route; configuration errors
may result in a valid packet incorrectly matching an access control
list (ACL) and being dropped. Whilst the specific definition of
unintended packet loss is network dependent, for any network there
are a small set of potential actions that can be taken to minimise
customer impact by auto-mitigating unintended packet loss:
1. Take a device, link, or set of devices and/or links out of
service.
2. Return a device, link, or set of devices and/or links back into
service.
3. Move traffic to other links or devices.
4. Roll back a recent change to a device that might have caused the
problem.
5. Escalate to a human (e.g., network operator) as a last resort.
A precise signal of impact is crucial, 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 by moving the traffic
to other links or devices, which are already congested.
To detect whether router-reported packet loss is a problem and to
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/or degree of the loss.
3. The duration of the loss.
4. The location of the loss.
Features 2, 3, and 4 are already addressed with passive monitoring
statistics, for example, obtained with SNMP [RFC1157] / MIB-II
[RFC1213] or NETCONF [RFC6241] / YANG [RFC7950]. Feature 1, however,
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is dependent on the classification scheme used for packet loss
reporting. In the next section, we define a new classification
scheme to address this problem.
4. Information Model
The classification scheme is defined as a tree which follows the
structure component/direction/type/layer/sub-type/sub-sub-
type/.../metric, where:
a. component can be interface|device|control_plane|flow
b. direction can be ingress|egress
c. type can be traffic|discards, where traffic accounts for packets
successfully received or transmitted, and discards account for packet
drops
d. layer can be l2|l3
.
|-- interface/
| |-- ingress/
| | |-- traffic/
| | | |-- l2/
| | | | |-- frames
| | | | `-- bytes
| | | |-- l3/
| | | | |-- v4/
| | | | | |-- packets
| | | | | |-- bytes
| | | | | |-- unicast/
| | | | | | |-- packets
| | | | | | `-- bytes
| | | | | `-- multicast/
| | | | | |-- packets
| | | | | `-- bytes
| | | | `-- v6/
| | | | |-- packets
| | | | |-- bytes
| | | | |-- unicast/
| | | | | |-- packets
| | | | | `-- bytes
| | | | `-- multicast/
| | | | |-- packets
| | | | `-- bytes
| | | `-- qos/
| | | |-- class_0/
| | | | |-- packets
| | | | `-- bytes
| | | |-- ...
| | | `-- class_n/
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| | | |-- packets
| | | `-- bytes
| | `-- discards/
| | |-- l2/
| | | |-- frames
| | | `-- bytes
| | |-- l3/
| | | |-- v4/
| | | | |-- packets
| | | | |-- bytes
| | | | |-- unicast/
| | | | | |-- packets
| | | | | `-- bytes
| | | | `-- multicast/
| | | | |-- packets
| | | | `-- bytes
| | | `-- v6/
| | | |-- packets
| | | |-- bytes
| | | |-- unicast/
| | | | |-- packets
| | | | `-- bytes
| | | `-- multicast/
| | | |-- packets
| | | `-- bytes
| | |-- errors/
| | | |-- l2/
| | | | `-- rx/
| | | | |-- frames
| | | | |-- crc_error/
| | | | | `-- frames
| | | | |-- invalid_mac/
| | | | | `-- frames
| | | | |-- invalid_vlan/
| | | | | `-- frames
| | | | `-- invalid_frame/
| | | | `-- frames
| | | |-- l3/
| | | | |-- rx/
| | | | | |-- packets
| | | | | |-- checksum_error/
| | | | | | `-- packets
| | | | | |-- mtu_exceeded/
| | | | | | `-- packets
| | | | | |-- invalid_packet/
| | | | | | `-- packets
| | | | | `-- ttl_expired/
| | | | | `-- packets
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| | | | `-- no_route/
| | | | `-- packets
| | | `-- local/
| | | |-- packets
| | | `-- hw/
| | | |-- packets
| | | `-- parity_error/
| | | `-- packets
| | |-- policy/
| | | |-- l2/
| | | | |-- frames
| | | | `-- acl/
| | | | `-- frames
| | | `-- l3/
| | | |-- packets
| | | |-- acl/
| | | | `-- packets
| | | |-- policer/
| | | | |-- packets
| | | | `-- bytes
| | | |-- null_route/
| | | | `-- packets
| | | |-- rpf/
| | | | `-- packets
| | | `-- ddos/
| | | `-- packets
| | `-- no_buffer/
| | |-- class_0/
| | | |-- packets
| | | `-- bytes
| | |-- ...
| | `-- class_n/
| | |-- packets
| | `-- bytes
| `-- egress/
| |-- traffic/
| | |-- l2/
| | | |-- frames
| | | `-- bytes
| | |-- l3/
| | | |-- v4/
| | | | |-- packets
| | | | |-- bytes
| | | | |-- unicast/
| | | | | |-- packets
| | | | | `-- bytes
| | | | `-- multicast/
| | | | |-- packets
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| | | | `-- bytes
| | | `-- v6/
| | | |-- packets
| | | |-- bytes
| | | |-- unicast/
| | | | |-- packets
| | | | `-- bytes
| | | `-- multicast/
| | | |-- packets
| | | `-- bytes
| | `-- qos/
| | |-- class_0/
| | | |-- packets
| | | `-- bytes
| | |-- ...
| | `-- class_n/
| | |-- packets
| | `-- bytes
| `-- discards/
| |-- l2/
| | |-- frames
| | `-- bytes
| |-- l3/
| | |-- v4/
| | | |-- packets
| | | |-- bytes
| | | |-- unicast/
| | | | |-- packets
| | | | `-- bytes
| | | `-- multicast/
| | | |-- packets
| | | `-- bytes
| | `-- v6/
| | |-- packets
| | |-- bytes
| | |-- unicast/
| | | |-- packets
| | | `-- bytes
| | `-- multicast/
| | |-- packets
| | `-- bytes
| |-- errors/
| | |-- l2/
| | | `-- tx/
| | | `-- frames
| | `-- l3/
| | `-- tx/
| | `-- packets
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| |-- policy/
| | `-- l3/
| | |-- acl/
| | | `-- packets
| | `-- policer/
| | |-- packets
| | `-- bytes
| `-- no_buffer/
| |-- class_0/
| | |-- packets
| | `-- bytes
| |-- ...
| `-- class_n/
| |-- packets
| `-- bytes
`-- control_plane/
`-- ingress/
|-- traffic/
| |-- packets
| `-- bytes
`-- discards/
|-- packets
|-- bytes
`-- policy/
`-- packets
For additional context, Appendix A provides an example of where
packets may be dropped in a device.
4.1. Discard Class Descriptions
discards/policy/: These are intended discards, meaning packets
dropped due to a configured policy. There are multiple sub-
classes.
discards/error/l2/rx/: Frames dropped due to errors in the received
L2 frame. There are multiple sub-classes, such as those resulting
from failing CRC, invalid header, invalid MAC address, or invalid
VLAN.
discards/error/l3/rx/: These drops occur due to errors in the
received packet, indicating an upstream problem rather than an
issue with the device dropping the errored packets. There are
multiple sub-classes, including header checksum errors, MTU
exceeded, and invalid packet, i.e. due to incorrect version,
incorrect header length, or invalid options.
discards/error/l3/rx/ttl_expired: There can be multiple causes for
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TTL-exceed (or Hop limit) drops: i) trace-route; ii) TTL (Hop
limit) set too low by the end-system; iii) routing loops.
discards/error/l3/no_route/: Discards occur due to a packet not
matching any route.
discards/error/local/: A device may drop packets within its
switching pipeline due to internal errors, such as parity errors.
Any errored discards not explicitly assigned to the above classes
are also accounted for here.
discards/no_buffer/: Discards occur due to no available buffer to
enqueue the packet. These can be tail-drop discards or due to an
active queue management algorithm, such as RED [RED93] or CODEL
[RFC8289].
4.2. Semantics
Rules 1-10 relate to packets forwarded by the device; rule 11 relates
to packets destined to/from the device:
1. All instances of frame or packet receipt, transmission, and
drops MUST be reported.
2. All instances of frame or packet receipt, transmission, and
drops SHOULD be attributed to the physical or logical interface
of the device where they occur.
3. An individual frame MUST only be accounted for by either the L2
traffic class or the L2 discard classes within a single
direction, i.e., ingress or egress.
4. An individual packet MUST only be accounted for by either the L3
traffic class or the L3 discard classes within a single
direction, i.e., ingress or egress.
5. A frame accounted for at L2 SHOULD NOT be accounted for at L3
and vice versa. An implementation MUST expose which layers a
discard is counted against.
6. The aggregate L2 and L3 traffic and discard classes SHOULD
account for all underlying packets received, transmitted, and
dropped across all other classes.
7. The aggregate Quality of Service (QoS) traffic and no buffer
discard classes MUST account for all underlying packets
received, transmitted, and dropped across all other classes.
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8. In addition to the L2 and L3 aggregate classes, an individual
dropped packet MUST only account against a single error, policy,
or no_buffer discard subclass.
9. When there are multiple drop reasons for a packet, the ordering
of discard class reporting MUST be defined.
10. If Diffserv [RFC2475] is not used, no_buffer discards SHOULD be
reported as class0.
11. Traffic to the device control plane has its own class, however,
traffic from the device control plane SHOULD be accounted for in
the same way as other egress traffic.
4.3. Examples
Assuming all the requirements are met, a "good" unicast IPv4 packet
received would increment:
- interface/ingress/traffic/l3/v4/unicast/packets
- interface/ingress/traffic/l3/v4/unicast/bytes
- interface/ingress/traffic/qos/class_0/packets
- interface/ingress/traffic/qos/class_0/bytes
A received unicast IPv6 packet dropped due to Hop Limit expiry would
increment:
- interface/ingress/discards/l3/v6/unicast/packets
- interface/ingress/discards/l3/v6/unicast/bytes
- interface/ingress/discards/l3/rx/ttl_expired/packets
An IPv4 packet dropped on egress due to no buffers would increment:
- interface/egress/discards/l3/v4/unicast/packets
- interface/egress/discards/l3/v4/unicast/bytes
- interface/egress/discards/no_buffer/class_0/packets
- interface/egress/discards/no_buffer/class_0/bytes
5. Example Signal-Cause-Mitigation Mapping
Figure 1 gives an example discard signal-to-cause-to-mitigation
action mapping. Mappings for a specific network will be dependent on
the definition of unintended packet loss for that network.
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+-------------------------------------------+---------------------+------------+----------+-------------+-----------------------+
| Discard class | Cause | Discard | Discard | Unintended? | Possible actions |
| | | rate | duration | | |
+-------------------------------------------+---------------------+------------+----------+-------------+-----------------------+
| ingress/discards/errors/l2/rx | Upstream device | >Baseline | O(1min) | Y | Take upstream link or |
| | or link errror | | | | device out-of-service |
| ingress/discards/errors/l3/rx/ttl_expired | Tracert | <=Baseline | | N | no action |
| ingress/discards/errors/l3/rx/ttl_expired | Convergence | >Baseline | O(1s) | Y | no action |
| ingress/discards/errors/l3/rx/ttl_expired | Routing loop | >Baseline | O(1min) | Y | Roll-back change |
| .*/policy/.* | Policy | | | N | no action |
| ingress/discards/errors/l3/no_route | Convergence | >Baseline | O(1s) | Y | no action |
| ingress/discards/errors/l3/no_route | Config error | >Baseline | O(1min) | Y | Roll-back change |
| ingress/discards/errors/l3/no_route | Invalid destination | >Baseline | O(10min) | N | Escalate to operator |
| ingress/discards/errors/local | Device errors | >Baseline | O(1min) | Y | Take device |
| | | | | | out-of-service |
| egress/discards/no_buffer | Congestion | <=Baseline | | N | no action |
| egress/discards/no_buffer | Congestion | >Baseline | O(1min) | Y | Bring capacity back |
| | | | | | into service or move |
| | | | | | traffic |
+-------------------------------------------+---------------------+------------+----------+-------------+-----------------------+
Figure 1: Example Signal-Cause-Mitigation Mapping
The 'Baseline' in the 'Discard Rate' column is network dependent.
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. Contributors
Nadav Chachmon
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
United States of America
Email: nchachmo@cisco.com
9. Acknowledgments
The content of this draft has benefitted from feedback from JR
Rivers, Ronan Waide, Chris DeBruin, and Marcoz Sanz.
10. References
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10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
10.2. Informative References
[RED93] Jacobson, V., "Random Early Detection gateways for
Congestion Avoidance", n.d..
[RFC1157] Case, J., Fedor, M., Schoffstall, M., and J. Davin,
"Simple Network Management Protocol (SNMP)", RFC 1157,
DOI 10.17487/RFC1157, May 1990,
<https://www.rfc-editor.org/rfc/rfc1157>.
[RFC1213] McCloghrie, K. and M. Rose, "Management Information Base
for Network Management of TCP/IP-based internets: MIB-II",
STD 17, RFC 1213, DOI 10.17487/RFC1213, March 1991,
<https://www.rfc-editor.org/rfc/rfc1213>.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/rfc/rfc2475>.
[RFC3444] Pras, A. and J. Schoenwaelder, "On the Difference between
Information Models and Data Models", RFC 3444,
DOI 10.17487/RFC3444, January 2003,
<https://www.rfc-editor.org/rfc/rfc3444>.
[RFC5153] Boschi, E., Mark, L., Quittek, J., Stiemerling, M., and P.
Aitken, "IP Flow Information Export (IPFIX) Implementation
Guidelines", RFC 5153, DOI 10.17487/RFC5153, April 2008,
<https://www.rfc-editor.org/rfc/rfc5153>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/rfc/rfc6241>.
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[RFC7270] Yourtchenko, A., Aitken, P., and B. Claise, "Cisco-
Specific Information Elements Reused in IP Flow
Information Export (IPFIX)", RFC 7270,
DOI 10.17487/RFC7270, June 2014,
<https://www.rfc-editor.org/rfc/rfc7270>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/rfc/rfc7950>.
[RFC8289] Nichols, K., Jacobson, V., McGregor, A., Ed., and J.
Iyengar, Ed., "Controlled Delay Active Queue Management",
RFC 8289, DOI 10.17487/RFC8289, January 2018,
<https://www.rfc-editor.org/rfc/rfc8289>.
Appendix A. Where do packets get dropped?
Figure 2 depicts an example of where and why packets may be dropped
in a typical single ASIC, shared buffered type device, where packets
ingress on the left and egress on the right.
+----------+
| |
| CPU |
| |
+--+---^---+
from_cpu | | to_cpu
| |
+------------------------------v---+-------------------------------+
| |
+----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+
| | | | | | | | | | | | | |
Packet rx -> Phy +--> Mac +--> Ingress +--> Buffers +--> Egresss +--> Mac +--> Phy |> Packet tx
| | | | | Pipeline| | | | Pipeline| | | | |
+----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+
Intended policy/acl policy/acl
Discards: policy/policer policy/policer
policy/urpf
null_route
Unintended error/rx/l2 error/rx/l3 no_buffer error/tx/l3
Discards: error/local
no_route
ttl
Figure 2: Example of where packets get dropped
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Appendix B. Implementation Experience
This appendix captures the authors' experience gained from
implementing and applying this information model across multiple
vendors' platforms, as guidance for future implementers.
1. The number and granularity of classes described in Section 3
represent a compromise. It aims to offer sufficient detail to
enable appropriate automated actions while avoiding excessive
detail which may hinder quick problem identification.
Additionally, it helps constrain the quantity of data produced
per interface to manage data volume and device CPU impacts.
Although further granularity is possible, the scheme described
has generally proven to be sufficient for the task of auto-
mitigating unintended packet loss.
2. There are multiple possible ways to define the discard
classification tree. For example, we could have used a multi-
rooted tree, rooted in each protocol. Instead, we opted to
define a tree where protocol discards and causal discards are
accounted for orthogonally. This decision reduces the number of
combinations of classes and has proven sufficient for
determining mitigation actions.
3. NoBuffer discards can be realized differently with different
memory architectures. Hence, whether a NoBuffer discard is
attributed to ingress or egress can differ accordingly. For
successful auto-mitigation, discards due to egress interface
congestion should be reported on egress, while discards due to
device-level congestion (exceeding the device forwarding rate)
should be reported on ingress.
4. Platforms often account for the number of packets dropped where
the TTL has expired (or Hop Limit exceeded), and the CPU has
returned an ICMP Time Exceeded message. There is typically a
policer applied to limit the number of packets sent to the
device CPU, however, which implicitly limits the rate of TTL
discards that are processed. One method to account for all
packet discards due to TTL exceeded, even those that are dropped
by a policer when being forwarded to the CPU, is to use
accounting of all ingress packets received with TTL=1.
5. Where no route discards are implemented with a default null
route, separate discard accounting is required 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.
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6. It is useful to account separately for transit packets dropped
by transit ACLs or policers, and packets dropped by ACLs or
policers which limit the number of packets to the device control
plane.
7. It is not possible to identify a configuration error - e.g.,
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, i.e., with configuration validation before deployment or
by detecting a significant change in ACL drops after a change
compared to before.
8. Where traffic byte counters need to be 64-bit, packet and
discard counters that increase at a lower rate may be encoded in
fewer bits, e.g., 48-bit.
9. Aggregate counters need to be able to deal with the possibility
of discontinuities in the underlying counters.
10. In cases where the reporting device is the source or destination
of a tunnel, the ingress protocol for a packet may differ from
the egress protocol; if IPv4 is tunneled over IPv6 for example.
Some implementations may attribute egress discards to the
ingress protocol.
11. While the classification tree is seven layers deep, a minimal
implementation may only implement the top six layers.
Authors' Addresses
John Evans
Amazon
1 Principal Place, Worship Street
London
EC2A 2FA
United Kingdom
Email: jevanamz@amazon.co.uk
Oleksandr Pylypenko
Amazon
410 Terry Ave N
Seattle, WA 98109
United States of America
Email: opyl@amazon.com
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Jeffrey Haas
Juniper Networks
1133 Innovation Way
Sunnyvale, CA 94089
United States of America
Email: jhaas@juniper.net
Aviran Kadosh
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
United States of America
Email: akadosh@cisco.com
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