| Internet-Draft | IPFIX for FWD Exceptions | August 2021 |
| Munukutla, et al. | Expires 9 February 2022 | [Page] |
This draft proposes couple of new Forwarding exceptions related Information Elements (IEs) and Templates for the IP Flow Information Export (IPFIX) protocol. These new Information Elements and Exception Template can be used to export information about any forwarding errors in a network. This essential information is adequate to correlate packet drops to any control plane entity and map it to an impacted service. Once exceptions are correlated to a particular entity, an action can be assigned to mitigate such problems essentially enabling self-driving networks.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
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This Internet-Draft will expire on 9 February 2022.¶
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All networks are susceptible to traffic drops due to a number of factors. Traffic drops can go unnoticed unless they are service impacting. In a multi-layered network architecture, it is tedious manual work to localize and root cause traffic blackholing issues. Transient drops are even harder to detect. Existing methodologies that rely on periodically monitoring interfaces on several hosts in a network does not guarantee timely detection, and are not scalable for large networks.¶
In order to eliminate this tedious monitoring work-flow, objective is to simplify localization and build correlation of dropped packets to particular entity. The network entity shall identify the dropped packets by monitoring dropped counters or doing a deep packet inspection of the packet discarded by the forwarding ASIC. The implementation of the method used to detect the drop is outside the scope of this document. Dropped packets will be sampled in the forwarding-path and sent to a host or software queue along with type of exception, in/out interface information and other relevant meta data. This will be a push model where the node encountering the error will emit the information about dropped packets and associated meta-data. Techniques for IP Packet Selection [RFC5475] describes Sampling and Filtering techniques for IP packet selection either using Systematic Sampling or Random Sampling.¶
The IPFIX Protocol Specification [RFC7011] defines a generic exchange mechanism for collecting flow information. It supports source-triggered export of information via the push model approach. The IPFIX Information Model [IANA-IPFIX] defines a list of standard Information Elements (IEs) which can be carried by the IPFIX protocol.¶
This document focuses on telemetry information for dropped packet exceptions, and proposes an extension to IPFIX message format for collecting sampled exceptions. Some of the IPFIX Information Elements (IEs) already exist, some will be defined along with corresponding formats. It is also possible to achieve sampling of the dropped packets by using sampling methods like SFLOW but details of other sampling methods are outside the scope of this document.¶
IPFIX-specific terminology (e.g. Information Element, Template, Template Record, Options Template Record, Template Set, Collector, Exporter, Data Record) used in this document is defined in Section 2 of [RFC7011]. As in [RFC7011] these IPFIX-specific terms have the first letter of a word capitalized. This document also makes use of the same terminology and definitions as Section 2 of [RFC5470].¶
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.¶
This document specifies the information model used for reporting packet-based forwarding exceptions. [RFC7011] provides guidance on the choices of the transport protocols used for IPFIX and their effects. Encoded IPFIX exception packets need to be reliably transported to the collector. The choice of the actual transport protocol is beyond the scope of this document.¶
This document assumes that all devices reporting exceptions will use existing IPFIX framework/module to send encoded packets to the collector. This would mean that the network device will specify the template that it is going to use for each of the events. The templates can be of varying length, and there could be multiple templates that a network device could use to encode the exceptions.¶
The implementation details of the collector application are beyond the scope of this document.¶
The Exception template could contain a subset of the IEs shown in Table 1, depending upon the exception reported.¶
Whenever packet drop happens inside forwarding plane, following information is key to understanding the issue: reason for packet drop, flow which encountered the drop (packet content), additional meta-data e.g. flow direction (ingress/egress), nexthop index, input interface, output interface, etc. on which this packet was flowing.¶
The following table includes all the existing IEs that a device reporting IPFIX Exceptions using various Exception Templates would typically need. The formats of IEs and IPFIX IDs are listed in the table below.¶
+-------------------------------+--------+-------+--------------+
| Field Name | Size | IANA | Description |
| | (bits) | IPFIX | |
| | | ID | |
+-------------------------------+--------+-------+--------------+
| flowDirection | 8 | 61 | The direction|
| | | | of the Flow |
| | | | observed at |
| | | | Observation |
| | | | point. |
| | | | |
| ingressInterface | 32 | 10 | Index of IP |
| | | | interface |
| | | | where packets|
| | | | of this flow |
| | | | are being |
| | | | received. |
| | | | |
| egressInterface | 32 | 14 | Index of IP |
| | | | interface |
| | | | where packets|
| | | | of this flow |
| | | | are being |
| | | | sent. |
| | | | |
| dataLinkFrameSize | 16 | 312 | Specified |
| | | | length of |
| | | | data link |
| | | | frame. |
| | | | |
| dataLinkFrameSection | 65535 | 315 | Carries n |
| | | | octets from |
| | | | data link |
| | | | frame of |
| | | | selected |
| | | | frame. |
| | | | |
| commonPropertiesID | 64 | 137 | Identifier of|
| | | | a set of |
| | | | common |
| | | | properties |
| | | | that is |
| | | | unique per |
| | | | observation |
| | | | domain. |
+-------------------------------+--------+-------+--------------+
Table 1: Forwarding Exception Information Elements
The proposed new IEs that a device reporting Exceptions using Exception template would need are listed in Table 2 below.¶
+---------------------------+---------------------+-----------------+
| Field Name | Abstract Data Type | Description |
| | | |
+---------------------------+---------------------+-----------------+
| forwardingExceptionCode | unsigned32 | Unique code for |
| | | every exception |
| | | |
| forwardingNextHopID | unsigned64 | Forwarding NH - |
| | | index associated|
| | | with packet that|
| | | encountered |
| | | this exception |
| | | |
| forwardingLookupType | unsigned8 | Last Lookup |
| | | type performed |
| | | on the packet in|
| | | in ingress path.|
| | | For instance, |
| | | IPV4, IPV6, |
| | | Bridge, MPLS, |
| | | Unknown etc. |
+---------------------------+---------------------+-----------------+
Table 2: New Information Elements
The Information Elements defined in Table 2 are proposed to be incorporated into the IANA IPFIX Information Elements registry [IANA-IPFIX]¶
Every network will encounter issues like packet loss, from time to time. Some of the causes for such a loss of traffic or a block in transmission of data packets include overloaded system conditions, misconfiguration, profiles and policies that restrict the bandwidth or priority of traffic, network outages, or disruption with physical cable faults. Packet loss could also happen because of incorrect stitching of the forwarding path or a mismatch between control plane and data plane state. Exception code entails the reason/error code due to which this packet has been dropped.¶
forwardingExceptionCode will be defined in "IPFIX Information Elements" registry. This list can be expanded in the future as necessary. The data record will have corresponding exception code value to indicate forwarding error that caused the traffic drop.¶
An implementation may choose to encode device internal exception codes as forwardingExceptionCode. In such scenarios, Enterprise Bit MUST be set to 1 and corresponding Enterprise Number MUST be present as described in [RFC7011]¶
There is an existing IE 89 - forwardingStatus [IANA-IPFIX] but it allows a very limited number of exceptions to be reported from the system (6-bit reason code). The exception codes also need to be standardized for use. Different forwarding ASICs would have different pipelines and hence discard reasons (which could be very specific to that pipeline) cannot be generalized. Hence it makes sense to have a standalone IE for reporting exception which not only provides support to report larger number of exceptions but also provides freedom for reporting application specific exceptions using the enterprise bit.¶
forwardingExceptionCode will also describe status of the flow with first two bits. An implementation may choose to export forwardingExceptionCode instead of IE89 - forwardingStatus.¶
A list of commonly used forwarding Exception codes will be identified and listed as part of Table 3 below.¶
+----------------+--------------------------------------------------+ | Forwarding | Reason | | Exception Code | | +----------------+--------------------------------------------------+ | 1 | FIREWALL_DISCARD | | 2 | TTL_EXPIRY | | 3 | DISCARD_ROUTE | | 4 | BAD_IPV4_CHECKSUM | | 5 | REJECT_ROUTE | | 6 | BAD_IPV4_HEADER (Version incorrect or IHL < 5)| | 7 | BAD_IPV6_HEADER (Version incorrect) | | 8 | BAD_IPV4_HEADER_LENGTH (V4 frame is too short) | | 9 | BAD_IPV6_HEADER_LENGTH | | 10 | BAD_IPV6_OPTIONS_PACKET(too many option headers) | | .. | .. | +----------------+--------------------------------------------------+
In terms of a network device, next hop is the gateway to which packet should be forwarded corresponding to the path to final destination. A given router doesn't need to store the entire forwarding path information for a destination. As long as it can identify the next hop to be used for forwarding to a destination, the end to end forwarding can happen. This helps reduce size of forwarding table. The nexthop index uniquely identifies the egress path a packet would take to reach the destination. This could include information about the outgoing interface, layer 2 address to be used, forwarding features configured for the packet path etc.¶
For instance, consider we have a L3VPN topology like below¶
CE1 -------- PE1 ----- MPLS Network ----- PE2 ------- CE2
Figure 1 above illustrates an example where reporting of exception can provide an insight into the error scenario. CE1 and CE2 communicate with each other over an MPLS VPN network. The labels are typically advertised using protocols like RSVP or LDP. When a packet is received from core network on PE1, a lookup on MPLS label results in packet getting forwarded towards CE1. The entries in MPLS table are populated by corresponding protocol. If label entries don't get populated in the MPLS table due to a probable glitch in the protocol configuration or some software inconsistency, the packets traversing on that LSP tunnel path shall get discarded on PE1.¶
In case of route lookups, that result in hierarchical forwarding chains, the mis-programming may manifest at different levels of the forwarding structure. The forwarding lookup may fail on any level of the hierarchy in the forwarding chain. It is expected that software at least report the nexthop where the lookup terminates. Its desirable for software to report the top level nexthop in the chain.¶
Using the mechanism described in this RFC, it will be possible to capture such packets and report them in IPFIX format with corresponding exception set (eg. DISCARD_ROUTE) along with relevant packet bytes and meta-data. This can help the operator/software to immediately understand root cause of the problem and take appropriate action.¶
An implementation may choose to report linecard number, linecard type, forwarding ASIC type and forwarding ASIC number on which an exception occurs, but mechanism to export these fields is out of the scope of this document.¶
A packet might undergo multiple lookups in forwarding chain. Lookup may fail on any level of the hierarchy When an exception is reported in such cases, type of last lookup performed on the packet may help in identifying the erroneous level in the hierarchy.¶
For instance, a Firewall Discard may happen for Layer2 or Layer3 packet. All such packets may be treated as FIREWALL_DISCARD for generic exception reporting purposes. However, exact place of error in the pipeline (IPV4, IPV6, MPLS etc.) may help with easily correlating the exception.¶
This section presents a list of templates for reporting exceptions using newly proposed IEs in addition to few existing Information Elements (IEs).¶
Templates listed below are sample templates to demonstrate the utility of newly introduced Information Elements in conjuction with existing Information Elements to report meaningful data to the collector. A specific implementation may add or remove Information Elements from below templates based on their reporting requirements.¶
Exception Template defined in Figure 1 demonstrates a sample set of data to export forwarding Exceptions.¶
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Set ID = 2 | Length = N octets | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Template ID = 256 | Field Count = N | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| forwardingExceptionCode | Field Length = 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| forwardingNextHopId | Field Length = 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| forwardingLookupType | Field Length = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| flowDirection | Field Length = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| ingressInterface | Field Length = 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| egressInterface | Field Length = 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| dataLinkFrameSize | Field Length = 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| dataLinkFrameSection | Field Length = 65535 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Padding (opt) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Alternatively, Exception Template defined in Figure 2 is a sample template to export forwarding exceptions. This template demonstrates the use of Information Element 137 to represent following fields: forwardingExceptionCode, forwardingNexthopId, ingressInterface, underlyingIngressInterface and egressInterface.¶
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Set ID = 2 | Length = N octets | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Template ID = 256 | Field Count = N | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| commonPropertiesId1 | Field Length = 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| flowDirection | Field Length = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| forwardingLookupType | Field Length = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| commonPropertiesId2 | Field Length = 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| commonPropertiesId3 | Field Length = 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| commonPropertiesId4 | Field Length = 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| commonPropertiesId5 | Field Length = 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| dataLinkFrameSize | Field Length = 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0| dataLinkFrameSection | Field Length = 65535 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Padding (opt) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IANA manages the IPFIX Information Elements registry at [IANA-IPFIX]. This document introduces two new IPFIX Information Elements.¶
Name: forwardingExceptionCode ElementID: TBD Description: Exception code is an identifier uniquely describing cause of irregularity or traffic drop on a device. Abstract Data Type: unsigned32 Data Type Semantics: identifier¶
Name: forwardingNexthopId ElementID: TBD Description: Nexthop ID is a unique identifier for a Nexthop on a device. Abstract Data Type: unsigned64 Data Type Semantics: identifier¶
Name: forwardingLookupType ElementID: TBD Description: Represents the last lookup performed on the packet in forwarding path. Abstract Data Type: unsigned8 Data Type Semantics: identifier¶
This document requests addition of a new registry for Forwarding Exception Codes.¶
+----------------+--------------------------------------------------+ | Forwarding | Reason | | Exception Code | | +----------------+--------------------------------------------------+ | 1 | FIREWALL_DISCARD | | 2 | TTL_EXPIRY | | 3 | DISCARD_ROUTE | | 4 | BAD_IPV4_CHECKSUM | | 5 | REJECT_ROUTE | | 6 | BAD_IPV4_HEADER (Version incorrect or IHL < 5)| | 7 | BAD_IPV6_HEADER (Version incorrect) | | 8 | BAD_IPV4_HEADER_LENGTH (V4 frame is too short) | | 9 | BAD_IPV6_HEADER_LENGTH | | 10 | BAD_IPV6_OPTIONS_PACKET(too many option headers) | | .. | .. | +----------------+--------------------------------------------------+
All assignments in this registry are to be performed via Expert Review.¶
Security Considerations listed in detail for IPFIX in [RFC7011] apply to this document as well. As described in [RFC7011], the IPFIX messages exchanged between network device and collector MUST be protected to provide confidentiality, integrity, and authenticity. Without those characteristics, the messages are subject to various kinds of attacks. These attacks are described in great detail in [RFC7011].¶
Manikandan Musuvathi Poornachary Juniper Networks, Inc. Electra Exora Business Park~Marathahalli-Sarjapur Outer Ring Road, Bangalore, KA - 560103 India Email: mpoornachary@juniper.net¶
Vishnu Pavan Beeram Juniper Networks, Inc. 1133 Innovation Way Sunnyvale, CA 94089 USA Email: vbeeram@juniper.net¶
Raveendra Torvi Juniper Networks, Inc. 10 Technology Park Dr Westford, MA 01886 USA Email: rtorvi@juniper.net¶