| Network Working Group | F. Brockners |
| Internet-Draft | S. Bhandari |
| Intended status: Informational | S. Dara |
| Expires: January 9, 2017 | C. Pignataro |
| Cisco | |
| H. Gredler | |
| RtBrick Inc. | |
| July 8, 2016 |
Requirements for In-band OAM
draft-brockners-inband-oam-requirements-00
This document discusses the motivation and requirements for including specific operational and telemetry information into data packets while the data packet traverses a path between two points in the network. This method is referred to as "in-band" Operations, Administration, and Maintenance (OAM), given that the OAM information is carried with the data packets as opposed to in "out-of-band" packets dedicated to OAM. In-band OAM complements other OAM mechanisms which use dedicated probe packets to convey OAM information.
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This document discusses requirements for "in-band" Operations, Administration, and Maintenance (OAM) mechanisms. "In-band" OAM means to record OAM and telemetry information within the data packet while the data packet traverses a network or a particular network domain. The term "in-band" refers to the fact that the OAM and telemetry data is carried within data packets rather than being sent within packets specifically dedicated to OAM. In-band OAM mechanisms, which are sometimes also referred to as embedded network telemetry are a current topic of discussion. In-band network telemetry has been defined for P4 [P4]. The SPUD prototype [I-D.hildebrand-spud-prototype] uses a similar logic that allows network devices on the path between endpoints to participate explicitly in the tube outside the end-to-end context. Even the IPv4 route-record option defined in [RFC0791] can be considered an in-band OAM mechanism. In-band OAM complements "out-of-band" mechanisms such as ping or traceroute, or more recent active probing mechanisms, as described in [I-D.lapukhov-dataplane-probe]. In-band OAM mechanisms can be leveraged where current out-of-band mechanisms do not apply or do not offer the desired characteristics or requirements, such as proving that a certain set of traffic takes a pre-defined path, strict congruency is desired, checking service level agreements for the live data traffic, detailed statistics on traffic distribution paths in networks that distribute traffic across multiple paths, or scenarios where probe traffic is potentially handled differently from regular data traffic by the network devices. [RFC7276] presents an overview of OAM tools.
Compared to probably the most basic example of "in-band OAM" which is IPv4 route recording [RFC0791], an in-band OAM approach has the following capabilities:
Abbreviations used in this document:
This document defines in-band Operations, Administration, and Maintenance (in-band OAM), as the subset in which OAM information is carried along with data packets. This is as opposed to "out-of-band OAM", where specific packets are dedicated to carrying OAM information.
In several scenarios it is beneficial to make information about which path a packet took through the network available to the operator. This includes not only tasks like debugging, troubleshooting, as well as network planning and network optimization but also policy or service level agreement compliance checks. This section discusses the motivation to introduce new methods for enhanced in-band network diagnostics.
Mechanisms which add tracing information to the regular data traffic, sometimes also referred to as "in-band" or "passive OAM" can complement active, probe-based mechanisms such as ping or traceroute, which are sometimes considered as "out-of-band", because the messages are transported independently from regular data traffic. "In-band" mechanisms do not require extra packets to be sent and hence don't change the packet traffic mix within the network. Traceroute and ping for example use ICMP messages: New packets are injected to get tracing information. Those add to the number of messages in a network, which already might be highly loaded or suffering performance issues for a particular path or traffic type.
Packet scheduling algorithms, especially for balancing traffic across equal cost paths or links, often leverage information contained within the packet, such as protocol number, IP-address or MAC-address. Probe packets would thus either need to be sent from the exact same endpoints with the exact same parameters, or probe packets would need to be artificially constructed as "fake" packets and inserted along the path. Both approaches are often not feasible from an operational perspective, be it that access to the end-system is not feasible, or that the diversity of parameters and associated probe packets to be created is simply too large. An in-band mechanism is an alternative in those cases.
In-band mechanisms also don't suffer from implementations, where probe traffic is handled differently (and potentially forwarded differently) by a router than regular data traffic.
Traditional ping and traceroute tools return the OAM results to the sender of the probe. Even when the ICMP messages that are used with these tools are enhanced, and additional telemetry is collected (e.g., ICMP Multi-Part [RFC4884] supporting MPLS information [RFC4950], Interface and Next-Hop Identification [RFC5837], etc.), it would be advantageous to separate the sending of an OAM probe from the receiving of the telemetry data. In this context, it is desired to not assume there is a bidirectional working path.
Several network deployments leverage tunneling mechanisms to create overlay or service-layer networks. Examples include VXLAN-GPE, GRE, or LISP. One often observed attribute of overlay networks is that they do not offer the user of the overlay any insight into the underlay network. This means that the path that a particular tunneled packet takes, nor other operational details such as the per-hop delay/jitter in the underlay are visible to the user of the overlay network, giving rise to diagnosis and debugging challenges in case of connectivity or performance issues. The scope of OAM tools like ping or traceroute is limited to either the overlay or the underlay which means that the user of the overlay has typically no access to OAM in the underlay, unless specific operational procedures are put in place. With in-band OAM the operator of the underlay can offer details of the connectivity in the underlay to the user of the overlay. The operator of the egress tunnel router could choose to share the recorded information about the path with the user of the overlay.
Coupled with mechanisms such as Segment Routing (SR) [I-D.ietf-spring-segment-routing], overlay network and underlay network can be more tightly coupled: The user of the overlay has detailed diagnostic information available in case of failure conditions. The user of the overlay can also use the path recording information as input to traffic steering or traffic engineering mechanisms, to for example achieve path symmetry for the traffic between two endpoints. [I-D.brockners-lisp-sr] is an example for how these methods can be applied to LISP.
In-band OAM can help users of an overlay-service to verify that negotiated SLAs for the real traffic are met by the underlay network provider. Different from solutions which rely on active probes to test an SLA, in-band OAM based mechanisms avoid wrong interpretations and "cheating", which can happen if the probe traffic that is used to perform SLA-check is prioritized by the network provider of the underlay.
Network planners and operators benefit from knowledge of the actual traffic distribution in the network. When deriving an overall network connectivity traffic matrix one typically needs to correlate data gathered from each individual devices in the network. If the path of a packet is recorded while the packet is forwarded, the entire path that a packet took through the network is available to the egress system. This obviates the need to retrieve individual traffic statistics from every device in the network and correlate those statistics, or employ other mechanisms such as leveraging traffic engineering with null-bandwidth tunnels just to retrieve the appropriate statistics to generate the traffic matrix.
In addition, with individual path tracing, information is available at packet level granularity, rather than only at aggregate level - as is usually the case with IPFIX-style methods which employ flow-filters at the network elements. Data-center networks which use equal-cost multipath (ECMP) forwarding are one example where detailed statistics on flow distribution in the network are highly desired. If a network supports ECMP, one can create detailed statistics for the different paths packets take through the network at the egress system, without a need to correlate/aggregate statistics from every router in the system. Transit devices are off-loaded from the task of gathering packet statistics.
Bandwidth- and power-constrained, time-sensitive, or loss-intolerant networks (e.g., networks for industry automation/control, health care) require efficient OAM methods to decide when to replicate packets to a secondary path in order to keep the loss/error-rate for the receiver at a tolerable level - and also when to stop replication and eliminate the redundant flow. Many IoT networks are time sensitive and cannot leverage automatic retransmission requests (ARQ) to cope with transmission errors or lost packets. Transmitting the data over multiple disparate paths (often called bi-casting or live-live) is a method used to reduce the error rate observed by the receiver. TSN receive a lot of attention from the manufacturing industry as shown by a various standardization activities and industry forums being formed (see e.g., IETF 6TiSCH, IEEE P802.1CB, AVnu).
Several deployments use traffic engineering, policy routing, segment routing or Service Function Chaining (SFC) [RFC7665] to steer packets through a specific set of nodes. In certain cases regulatory obligations or a compliance policy require to prove that all packets that are supposed to follow a specific path are indeed being forwarded across the exact set of nodes specified. If a packet flow is supposed to go through a series of service functions or network nodes, it has to be proven that all packets of the flow actually went through the service chain or collection of nodes specified by the policy. In case the packets of a flow weren't appropriately processed, a verification device would be required to identify the policy violation and take corresponding actions (e.g., drop or redirect the packet, send an alert etc.) corresponding to the policy. In today's deployments, the proof that a packet traversed a particular service chain is typically delivered in an indirect way: Service appliances and network forwarding are in different trust domains. Physical hand-off-points are defined between these trust domains (i.e., physical interfaces). Or in other terms, in the "network forwarding domain" things are wired up in a way that traffic is delivered to the ingress interface of a service appliance and received back from an egress interface of a service appliance. This "wiring" is verified and trusted. The evolution to Network Function Virtualization (NFV) and modern service chaining concepts (using technologies such as LISP, NSH, Segment Routing, etc.) blurs the line between the different trust domains, because the hand-off-points are no longer clearly defined physical interfaces, but are virtual interfaces. Because of that very reason, networks operators require that different trust layers not to be mixed in the same device. For an NFV scenario a different proof is required. Offering a proof that a packet traversed a specific set of service functions would allow network operators to move away from the above described indirect methods of proving that a service chain is in place for a particular application.
A solution approach could be based on OAM data which is added to every packet for achieving Proof Of Transit. The OAM data is updated at every hop and is used to verify whether a packet traversed all required nodes. When the verifier receives each packet, it can validate whether the packet traversed the service chain correctly. The detailed mechanisms used for path verification along with the procedures applied to the OAM data carried in the packet for path verification are beyond the scope of this document. Details are addressed in [draft-brockners-proof-of-transit]. In this document the term "proof" refers to a discrete set of bits that represents an integer or string carried as OAM data. The OAM data is used to verify whether a packet traversed the nodes it is supposed to traverse.
In-band OAM could be leveraged for several use cases, including:
The implementation of an in-band OAM mechanism needs to take several considerations into account, including administrative boundaries, how information is recorded, Maximum Transfer Unit (MTU), Path MTU discovery and packet size, etc.
The information gathered for in-band OAM can be categorized into three main categories: Information with a per-hop scope, such as path tracing; information which applies to a specific set of nodes, such as path or service chain verification; information which only applies to the edges of a domain, such as sequence numbers.
The recorded data at every hop may lead to packet size exceeding the Maximum Transmit Unit (MTU). Based on the transport protocol used MTU is discovered as a configuration parameter or Path MTU (PMTU) is discovered dynamically. Example: IPv6 recommends PMTU discovery before data packets are sent to prevent packet fragmentation. It specifies 1280 octets as the default PDU to be carried in a IPv6 datagram. A detailed discussion of the implications of oversized IPv6 header chains if found in [RFC7112].
The Path MTU restricts the amount of data that can be recorded for purpose of OAM within a data packet. The total size of data to be recorded needs to be preset to avoid packet size exceeding the MTU. It is recommended to pre-calculate and configures network devices to limit the in-band OAM data that is attached to a packet.
There are challenges in enabling in-band OAM in the public Internet across administrative domains:
The following considerations will be discussed in a future version of this document: If the packet is dropped due to the presence of the in-band OAM; If the policy failure is treated as feature disablement and any further recording is stopped but the packet itself is not dropped, it may lead to every node in the path to make this policy decision.
Deployment dependent, in-band OAM could either be used for all, or only a subset of the overall traffic. While it might be desirable to apply in-band OAM to all traffic and then selectively use the data gathered in case needed, it might not always be feasible. Depending on the forwarding infrastructure used, in-band OAM can have an impact on forwarding performance. The SPUD prototype for example uses the notion of "pipes" to describe the portion of the traffic that could be subject to in-path inspection. Mechanisms to decide which traffic would be subject to in-band OAM are outside the scope of this document.
Since packets have a finite maximum size, the data recording or carrying capacity of one packet in which the in-band OAM meta data is present is limited. In-band OAM should use its own dedicated namespace (confined to the domain in-band OAM operates in) to represent node and interface IDs to save space in the header. Generic representations of node and interface identifiers which are globally unique (such as a UUID) would consume significantly more bits of in-band OAM data.
When recorded data is required to be analyzed on a source node that issues a packet and inserts in-band OAM data, the recorded data needs to be carried back to the source node.
One way to carry the in-band OAM data back to the source is to utilize an ICMP Echo Request/Reply (ping) or ICMPv6 Echo Request/Reply (ping6) mechanism. In order to run the in-band OAM mechanism appropriately on the ping/ping6 mechanism, the following two operations should be implemented by the ping/ping6 target node:
The above discussed use cases require different types of in-band OAM data. This section details requirements for in-band OAM derived from the discussion above.
General Security considerations will be addressed ín a later version of this document. Security considerations for Proof of Transit alone are discussed below.
Threat Model: Attacks on the deployments could be due to malicious administrators or accidental misconfigurations resulting in bypassing of certain nodes. The solution approach should meet the following requirements:
[RFC Editor: please remove this section prior to publication.]
This document has no IANA actions.
The authors would like to thank Steve Youell, Eric Vyncke, Nalini Elkins, Srihari Raghavan, Ranganathan T S, Karthik Babu Harichandra Babu, Akshaya Nadahalli, and Andrew Yourtchenko for the comments and advice. This document leverages and builds on top of several concepts described in [draft-kitamura-ipv6-record-route]. The authors would like to acknowledge the work done by the author Hiroshi Kitamura and people involved in writing it.