Synonymous Flow Label FrameworkFururewei Technologies Incstewart.bryant@gmail.comHuaweimach.chen@huawei.comHuaweilizhenbin@huawei.comSouthend Technical Centerswallow.ietf@gmail.comCisco Systemsmsiva@cisco.comZTE Corp.gregimirsky@gmail.comMPLS Working GroupRFC 8372 describes the requirement for introducing
flow identities within the MPLS architecture. This document
describes a method of accomplishing this by using a technique called
Synonymous Flow Labels in which labels which mimic the behaviour of
other labels provide the identification service. These identifiers
can be used to trigger per-flow operations on the packet at
the receiving label switching router. describes the requirement for introducing
flow identities within the MPLS architecture.This document describes a method of accomplishing this by using a
technique called Synonymous Flow Labels (SFL) (see ) in
which labels which mimic the behaviour of other labels provide the
identification service. These identifiers can be used to trigger
per-flow operations on the packet at the receiving label switching
router.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 when, and only when, they appear in all
capitals, as shown here.An SFL is defined to be a label that causes exactly the same
behaviour at the egress Label Switching Router (LSR) as the label it
replaces, but in addition also causes an agreed action to take place
on the packet. There are many possible additional actions such as
the measurement of the number of received packets in a flow,
triggering IPFIX inspection, triggering other types of Deep Packet
Inspection, or identification of the packet source. In, for example,
a Performance Monitoring (PM) application, the agreed action could be
the recording of the receipt of the packet by incrementing a packet
counter. This is a natural action in many MPLS implementations, and
where supported this permits the implementation of high quality
packet loss measurement without any change to the packet forwarding
system.Consider an MPLS application such as a pseudowire (PW), and consider
that it is desired to use the approach specified in this document to
make a packet loss measurement. By some method outside the scope of
this text, two labels, synonymous with the PW labels are obtained
from the egress terminating provider edge (T-PE). By alternating
between these SFLs and using them in place of the PW label, the PW
packets may be batched for counting without any impact on the PW
forwarding behaviour (note that strictly only one SFL is needed in
this application, but that is an optimization that is a matter for
the implementor).Now consider an MPLS application that is multi-point to point such as
a VPN. Here it is necessary to identify a packet batch from a
specific source. This is achieved by making the SFLs source
specific, so that batches from one source are marked differently from
batches from another source. The sources all operate independently
and asynchronously from each other, independently co-ordinating with
the destination. Each ingress is thus able to establish its own SFL
to identify the sub-flow and thus enable PM per flow.Finally we need to consider the case where there is no MPLS
application label such as occurs when sending IP over an LSP. In
this case introducing an SFL that was synonymous with the LSP label
would introduce network wide forwarding state. This would not be
acceptable for scaling reasons. We therefore have no choice but to
introduce an additional label. Where penultimate hop popping (PHP)
is in use, the semantics of this additional label can be similar to
the LSP label. Where PHP is not in use, the semantics are similar to
an MPLS explicit NULL . In both of these cases the label has the
additional semantics of the SFL.Note that to achieve the goals set out in Section 1 SFLs need to be
allocated from the platform label table.As noted in it is necessary to consider two cases:Applications label presentSingle label stack shows the case in which both an LSP label and an application
label are present in the MPLS label stack. Traffic with no SFL
function present runs over the “normal” stack, and SFL enabled flows
run over the SFL stack with the SFL used to indicate the packet
batch.At the egress LSR the LSP label is popped (if present). Then the SFL
is processed in exactly the same way as the corresponding application
label would have been processed.The TTL and the Traffic Class bits in the SFL LSE would
normally be set to the same value as would have been set in the label
that the SFL is synonymous with. However it is recognised that there
may be an applications need to set the SFL to some other value. An
example would be where it was desired to cause the SFL to trigger an
action in the TTL expiry exception path as part of the label action. shows the case in which only an LSP label is present in the
MPLS label stack. Traffic with no SFL function present runs over the
“normal” stack and SFL enabled flows run over the SFL stack with the
SFL used to indicate the packet batch. However in this case it is
necessary for the ingress LSR to first push the SFL and then to push
the LSP label.At the receiving LSR it is necessary to consider two cases:Where the LSP label is still presentWhere the LSP label is penultimate hop poppedIf the LSP label is present, it processed exactly as it would
normally processed and then it is popped. This reveals the SFL which
in the case of measurements is simply counted and then
discarded. In this respect the processing of the SFL is synonymous
with an MPLS Explicit NULL. As the SFL is the bottom of stack, the IP
packet that follows is processed as normal.If the LSP label is not present due to PHP action in the upstream
LSR, two almost equivalent processing actions can take place. Either
the SFL can be treated as an LSP label that was not PHPed and the
additional associated SFL action is taken when the label is
processed. Alternatively, it can be treated as an MPLS Explicit NULL with
associated SFL actions. From the perspective of the measurement
system described in this document the behaviour of two approaches are
indistinguishable and thus either may be implemented.The TTL and the Traffic Class considerations described in
apply.There are cases where it is desirable to aggregate an SFL action
against a number of labels. For example where it is desirable to
have one counter record the number of packets received over a group
of application labels, or where the number of labels used by a single
application is large, and consequently the increase in the number of
allocated labels needed to support the SFL actions consequently
becomes too large to be viable. In these circumstances it would be
necessary to introduce an additional label in the stack to act as an
aggregate instruction. This is not strictly a synonymous action in
that the SFL is not replacing a existing label, but is somewhat
similar to the single label case shown in , and the same
signalling, management and configuration tools would be applicable.The Aggregate SFL is shown in the label stack depicted in as
preceding the application label, however the choice of position
before, or after, the application label will be application specific.
In the case described in , by definition the SFL has the
full application context. In this case the positioning will depend
on whether the SFL action needs the full context of the application
to perform its action and whether the complexity of the application
will be increased by finding an SFL following the application label.The introduction to an SFL to an existing flow may cause that flow to take
a different path through the network under conditions of Equal Cost
Multipath (ECMP). This in turn may invalidate the certain uses of
the SFL such as performance measurement applications. Where this is
a problem there are two solutions worthy of consideration:The operator can elect to always run with the SFL in place in the
MPLS label stack.The operator can elect to use Entropy Labels in
a network that fully supports this type of ECMP. If this
approach is adopted, the intervening MPLS network MUST NOT
load balance on any packet field other than the entropy label.
Note that this is stricter than the text in Section 4.2 of
. In networks in which the ECMP decision is independent
of both the value of any other label in the label stack, and the MPLS
payload, the path of the flow with the SFL will be congruent with
the path without the SFL.IETF concerns on pervasive monitoring are described in
. The inclusion of originating and/or flow information in a
packet provides more identity information and hence potentially
degrades the privacy of the communication. Whilst the inclusion of
the additional granularity does allow greater insight into the flow
characteristics it does not specifically identify which node
originated the packet other than by inspection of the network at the
point of ingress, or inspection of the control protocol packets.
This privacy threat may be mitigated by encrypting the control
protocol packets, regularly changing the synonymous labels and by
concurrently using a number of such labels. Minimizing the scope
of the identity indication can be useful in minimizing the
observability of the flow characteristics.The issue noted in is a security consideration. There are
no other new security issues associated with the MPLS dataplane. Any
control protocol used to request SFLs will need to ensure the
legitimacy of the request.This draft makes no IANA requests.Multiprotocol Label Switching (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic Class" FieldThe early Multiprotocol Label Switching (MPLS) documents defined the form of the MPLS label stack entry. This includes a three-bit field called the "EXP field". The exact use of this field was not defined by these documents, except to state that it was to be "reserved for experimental use".Although the intended use of the EXP field was as a "Class of Service" (CoS) field, it was not named a CoS field by these early documents because the use of such a CoS field was not considered to be sufficiently defined. Today a number of standards documents define its usage as a CoS field.To avoid misunderstanding about how this field may be used, it has become increasingly necessary to rename this field. This document changes the name of the field to the "Traffic Class field" ("TC field"). In doing so, it also updates documents that define the current use of the EXP field. [STANDARDS-TRACK]Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.MPLS Label Stack EncodingThis document specifies the encoding to be used by an LSR in order to transmit labeled packets on Point-to-Point Protocol (PPP) data links, on LAN data links, and possibly on other data links as well. This document also specifies rules and procedures for processing the various fields of the label stack encoding. [STANDARDS-TRACK]MPLS Flow Identification ConsiderationsThis 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.Packet Loss and Delay Measurement for MPLS NetworksMany service provider service level agreements (SLAs) depend on the ability to measure and monitor performance metrics for packet loss and one-way and two-way delay, as well as related metrics such as delay variation and channel throughput. This measurement capability also provides operators with greater visibility into the performance characteristics of their networks, thereby facilitating planning, troubleshooting, and network performance evaluation. This document specifies protocol mechanisms to enable the efficient and accurate measurement of these performance metrics in MPLS networks. [STANDARDS-TRACK]The Use of Entropy Labels in MPLS ForwardingLoad balancing is a powerful tool for engineering traffic across a network. This memo suggests ways of improving load balancing across MPLS networks using the concept of "entropy labels". It defines the concept, describes why entropy labels are useful, enumerates properties of entropy labels that allow maximal benefit, and shows how they can be signaled and used for various applications. This document updates RFCs 3031, 3107, 3209, and 5036. [STANDARDS-TRACK]Pervasive Monitoring Is an AttackPervasive monitoring is a technical attack that should be mitigated in the design of IETF protocols, where possible.