Internet DRAFT - draft-evans-discardclass
draft-evans-discardclass
Independent Stream J. Evans
Internet-Draft O. Pylypenko
Intended status: Informational Amazon
Expires: 16 February 2024 15 August 2023
Experience from implementing a new packet discard classification scheme
draft-evans-discardclass-03
Abstract
Router reported packet loss is the primary signal of when a network
is not doing its job. Some packet loss is normal or intended in TCP/
IP networks, however. To minimise network packet loss through
automated network operations we need clear and accurate signals of
all packets which are dropped and why. This document describes our
experience from implementing a packet loss classification scheme to
provide these signals and 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
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Internet-Drafts are draft documents valid for a maximum of six months
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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 16 February 2024.
Copyright Notice
Copyright (c) 2023 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. Problem statement . . . . . . . . . . . . . . . . . . . . . . 3
3. Traffic and Discard Classification Scheme . . . . . . . . . . 4
3.1. Discard Class Descriptions . . . . . . . . . . . . . . . 8
3.2. Discard Accounting Requirements . . . . . . . . . . . . . 9
3.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . 10
4. A Possible Signal-Cause-Mitigation Mapping . . . . . . . . . 11
5. Implementation Experience . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Where do packets get dropped? . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
The job of a network is to transport packets. Understanding both
where and why packet loss occurs is essential for effective network
operation. 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 customer
impacting packet loss. Precise classification of packet loss signals
is important to ensure the right action is taken as taking the wrong
action can make problems worse.
The existing metrics for packet loss as defined in [RFC1213] - namely
ifInDiscards, ifOutDiscards, ifInErrors, ifOutErrors - do not provide
sufficient precision to be able to automatically identify the cause
of the loss and mitigate the impact. From a network operators'
perspective, ifindiscards can represent both intended packet loss
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(i.e. packets discarded due to policy) and unintended packet loss
(e.g. packets dropped in error). Further, these definitions are
ambiguous, in that vendors can and have implemented them differently.
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 a clearly defined classification
scheme and semantics for packet loss reporting.
This document describes our experience from implementing a packet
loss classification scheme across multiple hardware platforms, which
aims 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.
The terms 'packet drop' and 'discard' are considered equivalent and
are used interchangeably.
2. Problem statement
Working backwards from the goal of auto-mitigation of unintended
packet loss, there are only a relative small number of potential
auto-mitigation actions, e.g.:
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 another device
4. Roll-back a recent change to a device that might have caused the
problem
5. Escalate to a human (e.g. network operators) as a last resort
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 by moving the traffic
to other already congested links and/or devices.
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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/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, e.g. obtained with SNMP [RFC1157] or NETCONF [RFC6241].
Feature 1, however, 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.
3. Traffic and Discard Classification Scheme
We define the classification scheme as a tree which follows the
structure <component><direction><type><layer><sub-type><sub-sub-
type><metric>, where:
a. component can be interface|device
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
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| | | | `-- 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/
| | | | `-- rx/
| | | | |-- frames
| | | | |-- crc_error/
| | | | | `-- frames
| | | | |-- invalid_mac/
| | | | | `-- frames
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| | | | |-- invalid_vlan/
| | | | | `-- frames
| | | | `-- invalid_frame/
| | | | `-- frames
| | | |-- l3/
| | | | |-- rx/
| | | | | |-- packets
| | | | | |-- checksum_error/
| | | | | | `-- packets
| | | | | |-- mtu_exceeded/
| | | | | | `-- packets
| | | | | |-- invalid_packet/
| | | | | | `-- packets
| | | | | `-- ttl_expired/
| | | | | `-- packets
| | | | `-- no_route/
| | | | `-- packets
| | | `-- local/
| | | |-- packets
| | | `-- hw/
| | | |-- packets
| | | `-- parity_error/
| | | `-- packets
| | |-- policy/
| | | `-- l3/
| | | |-- packets
| | | |-- acl/
| | | | `-- packets
| | | |-- policer/
| | | | |-- packets
| | | | `-- bytes
| | | |-- null_route/
| | | | `-- packets
| | | `-- urpf/
| | | `-- packets
| | `-- no_buffer/
| | |-- class_0/
| | | |-- packets
| | | `-- bytes
| | |-- ...
| | `-- class_n/
| | |-- packets
| | `-- bytes
| `-- egress/
| |-- traffic/
| | |-- l2/
| | | |-- frames
| | | `-- bytes
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| | |-- 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/
| | |-- packets
| | `-- bytes
| `-- discards/
| |-- l2/
| | |-- frames
| | `-- bytes
| |-- l3/
| | |-- v4/
| | | |-- packets
| | | |-- bytes
| | | |-- unicast/
| | | | |-- packets
| | | | `-- bytes
| | | `-- multicast/
| | | |-- packets
| | | `-- bytes
| | `-- v6/
| | |-- packets
| | |-- bytes
| | |-- unicast/
| | | |-- packets
| | | `-- bytes
| | `-- multicast/
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| | |-- packets
| | `-- bytes
| |-- errors/
| | |-- l2/
| | | `-- tx/
| | | `-- frames
| | `-- l3/
| | `-- tx/
| | `-- packets
| |-- policy/
| | `-- l3/
| | |-- acl/
| | | `-- packets
| | `-- policer/
| | |-- packets
| | `-- bytes
| `-- no_buffer/
| |-- class_0/
| | |-- packets
| | `-- bytes
| |-- ...
| `-- class_n/
| |-- packets
| `-- bytes
`-- control_plane/
|-- packets
|-- bytes
`-- policy/
|-- acl/
| `-- packets
`-- policer/
`-- packets
For additional context, Appendix A provides an example of where
packets may be dropped in a device.
3.1. Discard Class Descriptions
discards/error/l2/rx/
Frames dropped due to errors in the received L2 frame, e.g. due to
failing CRC, invalid header, invalid MAC address, invalid VLAN.
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discards/error/l3/rx/
These are drops due to errors in the received packet, i.e. which
indicate an upstream problem, rather than a problem with the device
that is dropping the errored packets. There are multiple potential
errors that can cause a packet to be dropped on receipt, e.g. header
checksum errors, incorrect version, incorrect header length, bad
options.
discards/error/l3/ttl_expired
There can also be multiple causes for TTL-exceed drops: i) trace-
route; ii) TTL set too low by the end-system; iii) routing loops
discards/error/l3/no_route/
Discards due to a packet not matching any route.
discards/error/local/
A device may drop packets within its switching pipeline due to
internal errors, e.g. parity errors. Any discards not explicitly
assigned to the above classes are accounted here.
discards/policy/
These are intended discards, i.e. packets dropped due to a configured
policy. There are multiple sub-classes.
discards/policy/l3/acl/
Discards due to packet matching an access control list (ACL).
discards/policy/l3/policer/
Discards due to packet matching a configured policer.
discards/policy/l3/null_route/
Discards due to a packet matching a route with discard action.
discards/policy/l3/urpf/
Discards due to a packet failing unicast reverse path forwarding
(RPF) check.
discards/no_buffer/
Discards due to no available buffer to enqueue the packet. These can
be tail-drop discards or due to an active queue management algorithm,
e.g. RED [RED93], CODEL [RFC8289].
3.2. Discard Accounting Requirements
Requirements 1-10 apply to the packets forwarded by the device, i.e.
rather than packets destined to/from the device:
1. All packet receipt, transmission and drops MUST be reported
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2. All packet receipt, transmission and drops SHOULD be attributed
to the physical or logical interface where they occur.
3. If a frame is discarded at L2, it MUST NOT be accounted for at
L3
4. An individual packet MUST NOT account against both the L2
traffic and L2 discard classes on a single direction, i.e.
ingress or egress
5. An individual packet MUST NOT account against both the L3
traffic and L3 discard classes on a single direction, i.e.
ingress or egress
6. The aggregate L2 and L3 traffic and discard classes MUST account
for all underlying packets received, transmitted and dropped
across all other classes
7. The aggregate QOS traffic and discard (no buffer) classes MUST
account for all underlying packets received, transmitted and
dropped across all other classes
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 sub class
9. Where there may be multiple drop reasons for a packet, the
ordering of discard class reporting MUST be defined
10. If Diffserv [RFC2475] quality of service (QOS) is not used,
no_buffer discards SHOULD be reported as class0
11. Traffic from the device control plane SHOULD be accounted for
the same as other egress traffic
3.3. 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
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A received unicast IPv6 packet dropped due to TTL 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
4. A Possible Signal-Cause-Mitigation Mapping
Example discard signal-to-cause-to-mitigation mappings are shown in
the table below:
+-------------------------------------------+---------------------+------------+----------+-------------+-----------------------+
| 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 |
+-------------------------------------------+---------------------+------------+----------+-------------+-----------------------+
The 'Baseline' in the 'Discard Rate' column is network dependent.
5. Implementation Experience
1. The number and granularity of classes described in section 3 is
a compromise between: providing sufficient detail to be able to
take the appropriate automated actions whilst: a) not providing
too much detail which may require deeper understanding rather
than helping to surface the problem quickly; b) constraining the
quantity of data produced where these metrics are produced per
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interface to limit data volume and device CPU impacts. While
further granularity is possible, we found the scheme described
to be generally sufficient.
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 causal 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. Whether a NoBuffer discard is attributed
to ingress or egress can differ accordingly. For successful
auto-mitigation: where the discards are due to egress interface
congestion, they should be reported on egress; where the
discards are due to device-level congestion (exceeding the
device forwarding rate), they should be reported on ingress.
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 limit 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 of discards reported. One method to
account for all packets 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 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 or policers, and packets dropped by ACLs or
policers which limit the number of packets to the device control
packets.
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 configuration validation before deployment or
in detecting a significant change in ACL drops after a change
compared to before.
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8. Where traffic byte counters need to be 64-bit, packet and
discard counters which increase at a lower rate may be encoded
in fewer bits, e.g. 48-bit.
9. Where the reporting device is the source or destination of a
tunnel, the ingress protocol for a packet may be different to
the egress protocol, e.g. if IPv4 is tunnelled over IPv6. In
this case, some implementations may attribute egress discards to
the ingress protocol.
10. There are multiple ways that these traffic and discard metrics
can be exposed from the device, including SNMP [RFC1157], IPFIX
[RFC5153], and NETCONF [RFC6241].
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 feedback from JR
Rivers, Ronan Waide, Chris DeBruin, Marcoz Sanz, Avinash Kadosh and
Nadav Chachmon.
10. References
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>.
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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>.
[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>.
[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?
The diagram below is 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.
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+----------+
| |
| 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
Authors' Addresses
John Evans
Amazon
Email: jevanamz@amazon.co.uk
Oleksandr Pylypenko
Amazon
Email: opyl@amazon.com
Evans & Pylypenko Expires 16 February 2024 [Page 15]
