Internet Engineering Task Force (IETF) S. Bryant Request for Comments: 8372 Huawei Category: Informational C. Pignataro ISSN: 2070-1721 Cisco M. Chen Z. Li Huawei G. Mirsky ZTE Corp. May 2018
MPLS Flow Identification Considerations
This document discusses aspects to consider when developing a solution for MPLS flow identification. The key application that needs this solution is in-band performance monitoring of MPLS flows when MPLS is used to encapsulate user data packets.
Status of This Memo
This document is not an Internet Standards Track specification; it is published for informational purposes.
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This document discusses the aspects that need to be considered when developing a solution for MPLS flow identification. The key application that needs this is in-band performance monitoring of MPLS flows when MPLS is used to encapsulate user data packets.
There is a need to identify flows in MPLS networks for various applications such as determining packet loss and packet delay measurement. A method of loss and delay measurement in MPLS networks was defined in [RFC6374]. When used to measure packet loss, [RFC6374] depends on the use of injected Operations, Administration, and Maintenance (OAM) packets to designate the beginning and the end of the packet group over which packet loss is being measured. If the misordering of packets from one group relative to the following group
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or the misordering of any of the packets being counted relative to the Loss Measurement packet [RFC6374] occurs, then an error will occur in the packet loss measurement.
In addition, [RFC6374] did not support different granularities of flow or address a number of multipoint cases in which two or more ingress Label Switching Routers (LSRs) could send packets to one or more destinations.
Due to the very low loss rate in normal operation, improvements in link and transmission technologies have made it more difficult to assess packet loss using active performance measurement methods with synthetic traffic. That, together with more demanding service-level requirements, means that network operators now need to be able to measure the loss of the actual user data traffic using passive performance measurement methods. Any technique deployed needs to be transparent to the end user, and it needs to be assumed that they will not take any active part in the measurement process. Indeed, it is important that any flow identification technique be invisible to them and that no remnant of the measurement process leaks into their network.
Additionally, when there are multiple traffic sources, such as in multipoint-to-point and multipoint-to-multipoint network environments, there needs to be a method whereby the sink can distinguish between packets from the various sources; that is to say, a multipoint measurement model needs to be developed.
Modern networks, if not oversubscribed, generally drop relatively few packets; thus, packet loss measurement is highly sensitive to the common demarcation of the exact set of packets to be measured for loss. Without some form of coloring or batch marking such as that proposed in [RFC8321], it may not be possible to achieve the required accuracy in the loss measurement of customer data traffic. Thus, when accurate measurement of packet loss is required, it may be economically advantageous, or even be a technical requirement, to include some form of marking in the packets to assign each packet to a particular counter for loss measurement purposes.
When this level of accuracy is required and the traffic between a source-destination pair is subject to Equal-Cost Multipath (ECMP), a demarcation mechanism is needed to group the packets into batches. Once a batch is correlated at both ingress and egress, the packet accounting mechanism is then able to operate on the batch of packets that can be accounted for at both the packet ingress and the packet
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egress. Errors in the accounting are particularly acute in Label Switched Paths (LSPs) subjected to ECMP because the network transit time will be different for the various ECMP paths since:
1. the packets may traverse different sets of LSRs;
2. the packets may depart from different interfaces on different line cards on LSRs; and
3. the packets may arrive at different interfaces on different line cards on LSRs.
A consideration with this solution on modifying the identity label (the MPLS label ordinarily used to identify the LSP, Virtual Private Network, Pseudowire, etc.) to indicate the batch is the impact that this has on the path chosen by the ECMP mechanism. When the member of the ECMP path set is chosen by deep packet inspection, a change of batch represented by a change of identity label will have no impact on the ECMP path. If the path member is chosen by reference to an entropy label [RFC6790], then changing the batch identifier will not result in a change to the chosen ECMP path. ECMP is so pervasive in multipoint-to-(multi)point networks that some method of avoiding accounting errors introduced by ECMP needs to be supported.
Most of the existing delay measurement methods are active methods that depend on the extra injected test packet to evaluate the delay of a path. With the active measurement method, the rate, numbers, and interval between the injected packets may affect the accuracy of the results. Due to ECMP (or link aggregation techniques), injected test packets may traverse different links from the ones used by the data traffic. Thus, measuring the delay of the real traffic is required.
For combined loss and delay measurements, both the loss and the delay considerations apply.
The most basic unit of identification is the identity of the node that processed the packet on its entry to the MPLS network. However, the required unit of identification may vary depending on the use case for accounting, performance measurement, or other types of packet observations. In particular, note that there may be a need to impose identity at several different layers of the MPLS label stack.
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This document considers the identification of the following traffic components:
o Per source LSR: everything from one source is aggregated
o Per group of LSPs chosen by an ingress LSR: an ingress LSP aggregates a group of LSPs (e.g., all the LSPs of a tunnel)
o Per LSP: the basic form
o Per flow [RFC6790] within an LSP: a fine-grained method
Note that a fine-grained identity resolution is needed when there is a need to perform these operations on a flow not readily identified by some other element in the label stack. Such a fine-grained resolution may be possible by deep packet inspection. However, this may not always be possible, or it may be desired to minimize processing costs by doing this only on entry to the network. Adding a suitable identifier to the packet for reference by other network elements minimizes the processing needed by other network elements. An example of such a fine-grained case might be traffic belonging to a certain service or from a specific source, particularly if matters related to service level agreement or application performance were being investigated.
We can thus characterize the identification requirement in the following broad terms:
o There needs to be some way for an egress LSR to identify the ingress LSR with an appropriate degree of scope. This concept is discussed further in Section 6.
o There needs to be a way to identify a specific LSP at the egress node. This allows for the case of instrumenting multiple LSPs operating between the same pair of nodes. In such cases, the identity of the ingress LSR is insufficient.
o In order to conserve resources such as labels, counters, and/or compute cycles, it may be desirable to identify an LSP group so that an operation can be performed on the group as an aggregate.
o There needs to be a way to identify a flow within an LSP. This is necessary when investigating a specific flow that has been aggregated into an LSP.
The unit of identification and the method of determining which packets constitute a flow will be specific to the application or use case and are out of scope of this document.
We need to consider a number of types of LSP. The two simplest types to monitor are point-to-point LSPs and point-to-multipoint LSPs. The ingress LSR for a point-to-point LSP, such as those created using the Resource Reservation Protocol - Traffic Engineering (RSVP-TE) [RFC5420] signaling protocol or those that conform to the MPLS Transport Profile (MPLS-TP) [RFC5654], may be identified by inspection of the top label in the stack because, at any provider- edge (PE) or provider (P) router on the path, the top label is unique to the ingress-egress pair at every hop at a given layer in the LSP hierarchy. Provided that Penultimate Hop Popping (PHP) is disabled, the identity of the ingress LSR of a point-to-point LSP is available at the egress LSR; thus, determining the identity of the ingress LSR must be regarded as a solved problem. Note, however, that the identity of a flow cannot to be determined without further information being carried in the packet or gleaned from some aspect of the packet payload.
In the case of a point-to-multipoint LSP, and in the absence of PHP, the identity of the ingress LSR may also be inferred from the top label. However, it may not possible to adequately identify the flow from the top label alone; thus, further information may need to be carried in the packet or gleaned from some aspect of the packet payload. In designing any solution, it is desirable that a common flow identification solution be used for both point-to-point and point-to-multipoint LSP types. Similarly, it is desirable that a common method of LSP group identification be used. In the above cases, a context label [RFC5331] needs to be used to provide the required identity information. This is a widely supported MPLS feature.
A more interesting case is the case of a multipoint-to-point LSP. In this case, the same label is normally used by multiple ingress or upstream LSRs; hence, source identification is not possible by inspection of the top label by the egress LSRs. It is therefore necessary for a packet to be able to explicitly convey any of the identity types described in Section 4.
Similarly, in the case of a multipoint-to-multipoint LSP, the same label is normally used by multiple ingress or upstream LSRs; hence, source identification is not possible by inspection of the top label by egress LSRs. The various identity types described in Section 4 are again needed. Note, however, that the scope of the identity may be constrained to be unique within the set of multipoint-to- multipoint LSPs terminating on any common node.
The scope of identification can be constrained to the set of flows that are uniquely identifiable at an ingress LSR or some aggregation thereof. There is no need for an ingress LSR to seek assistance from outside the MPLS protocol domain.
In any solution that constrains itself to carrying the required identity in the MPLS label stack rather than in some different associated data structure, constraints on the choice of label and label stack size imply that the scope of identity resides within that MPLS domain. For similar reasons, the identity scope of a component of an LSP is constrained to the scope of that LSP.
In any network, it is unlikely that all LSRs will have the same capability to support the methods of identification discussed in this document. It is therefore an important constraint on any flow identity solution that it is backwards compatible with deployed MPLS equipment to the extent that deploying the new feature will not disable anything that currently works on the legacy equipment.
This is particularly the case when the deployment is incremental or the feature is not required for all LSRs or all LSPs. Thus, the flow identification design must support the coexistence of LSRs that can identify the traffic components described in Section 4 and those that cannot. In addition, the identification of the traffic components described in Section 4 must be an optional feature that is disabled by default. As a design simplification, a solution may require that all egress LSRs of a point-to-multipoint or a multipoint-to- multipoint LSP support the identification type in use so that a single packet can be correctly processed by all egress devices. The corollary of this last point is that either all egress LSRs are enabled to support the required identity type or none of them are.
There is a huge installed base of MPLS equipment; typically, this type of equipment remains in service for an extended period of time, and in many cases, hardware constraints mean that it is not possible to upgrade its data-plane functionality. Changes to the MPLS data plane are therefore expensive to implement, add complexity to the network, and may significantly impact the deployability of a solution that requires such changes. For these reasons, MPLS users have set a very high bar to changes to the MPLS data plane, and only a very small number have been adopted. Hence, it is important that the method of identification must minimize changes to the MPLS data
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plane. Ideally, method(s) of identification that require no changes to the MPLS data plane should be given preferential consideration. If a method of identification that makes a change to the data plane is chosen, it will need to have a significant advantage over any method that makes no change, and the advantage of the approach will need to be carefully evaluated and documented. If a change to the MPLS data plane proves necessary, it should be (a) as small a change as possible and (b) a general-purpose method, so as to maximize its use for future applications. It is imperative that, as far as can be foreseen, any necessary change made to the MPLS data plane does not impose any foreseeable future limitation on the MPLS data plane.
Stack size is an issue with many MPLS implementations both as a result of hardware limitations and due to the impact on networks and applications in which a large number of small payloads need to be transported. In particular, one MPLS payload may be carried inside another. For example, one LSP may be carried over another LSP, or a Pseudowire (PW) or similar multiplexing construct may be carried over an LSP, and identification may be required at both layers. Of particular concern is the implementation of low-cost edge LSRs that, for cost reasons, have a significant limit on the number of Label Stack Entries (LSEs) that they can impose or dispose. Therefore, any method of identity must not consume an excessive number of unique labels and must not result in an excessive increase in the size of the label stack.
The design of the MPLS data plane provides two types of special- purpose labels: the original 16 reserved labels and the much larger set of special-purpose labels defined in [RFC7274]. The original reserved labels need one LSE, and the newer special-purpose labels [RFC7274] need two LSEs. Given the tiny number of original reserved labels, it is core to the MPLS design philosophy that this scarce resource is only used when it is absolutely necessary. Using a special-purpose label to encode flow identity requires two label stack entries, one for the reserved label and one for the flow identity. Use of extended special-purpose labels [RFC7274] requires a total of three label stack entries to encode the flow identity. The larger set of [RFC7274] labels requires two label stack entries for the special-purpose label itself; hence, a total of three label stack entries is needed to encode the flow identity.
The use of special-purpose labels [RFC7274] as part of a method to encode the identity information therefore has a number of undesirable implications for the data plane. Thus, while a solution may use special-purpose labels, methods that do not require special-purpose labels need to be carefully considered.
The inclusion of originating and/or flow information in a packet provides more identity information and hence potentially degrades the privacy of the communication.
Recent IETF concerns on pervasive monitoring [RFC7258] have resulted in a preference for a solution that does not degrade the privacy of user traffic below that of an MPLS network not implementing the flow identification feature. The choice of using MPLS technology for this OAM solution has a privacy advantage, as the choice of the label identifying a flow is limited to the scope of the MPLS domain and does not have any dependency on the identification of the user data. This minimizes the observability of the flow characteristics.
Any flow identification solution must not degrade the security of the MPLS network below that of an equivalent network not deploying the specified identity solution. In order to preserve present assumptions about MPLS privacy properties, propagation of identification information outside the MPLS network imposing it must be disabled by default. Any solution should provide for the restriction of the identity information to those components of the network that need to know it. It is thus desirable to limit the knowledge of the identify of an endpoint to only those LSRs that need to participate in traffic flow. The choice of using MPLS technology for this OAM solution, with MPLS encapsulation of user traffic, provides for a key advantage over other data-plane solutions, as it provides for a controlled access and trusted domain within a service provider's network.
For a more comprehensive discussion of MPLS security and attack mitigation techniques, please see "Security Framework for MPLS and GMPLS Networks" [RFC5920].