Network Working Group K. Kompella
Internet-Draft J. Drake
Updates: 3031 (if approved) Juniper Networks
Intended status: Standards Track S. Amante
Expires: November 6, 2011 Level 3 Communications, LLC
W. Henderickx
Alcatel-Lucent
L. Yong
Huawei USA
May 5, 2011
The Use of Entropy Labels in MPLS Forwarding
draft-ietf-mpls-entropy-label-00
Abstract
Load 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.
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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and may be updated, replaced, or obsoleted by other documents at any
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 November 6, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used . . . . . . . . . . . . . . . . . . . . . 4
1.2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Entropy Labels . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Data Plane Processing of Entropy Labels . . . . . . . . . . . 8
4.1. Ingress LSR . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Transit LSR . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Egress LSR . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Signaling for Entropy Labels . . . . . . . . . . . . . . . . . 9
5.1. LDP Signaling . . . . . . . . . . . . . . . . . . . . . . 10
5.2. BGP Signaling . . . . . . . . . . . . . . . . . . . . . . 11
5.3. RSVP-TE Signaling . . . . . . . . . . . . . . . . . . . . 12
6. Operations, Administration, and Maintenance (OAM) and
Entropy Labels . . . . . . . . . . . . . . . . . . . . . . . . 13
7. MPLS-TP and Entropy Labels . . . . . . . . . . . . . . . . . . 14
8. Point-to-Multipoint LSPs and Entropy Labels . . . . . . . . . 15
9. Entropy Labels and Applications . . . . . . . . . . . . . . . 15
9.1. Tunnels . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.2. LDP Pseudowires . . . . . . . . . . . . . . . . . . . . . 17
9.3. BGP Applications . . . . . . . . . . . . . . . . . . . . . 18
9.3.1. Inter-AS BGP VPNs . . . . . . . . . . . . . . . . . . 19
9.4. Multiple Applications . . . . . . . . . . . . . . . . . . 20
10. Security Considerations . . . . . . . . . . . . . . . . . . . 21
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
11.1. LDP Entropy Label TLV . . . . . . . . . . . . . . . . . . 22
11.2. BGP Entropy Label Attribute . . . . . . . . . . . . . . . 22
11.3. Attribute Flags for LSP_Attributes Object . . . . . . . . 22
11.4. Attributes TLV for LSP_Attributes Object . . . . . . . . . 22
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
13.1. Normative References . . . . . . . . . . . . . . . . . . . 23
13.2. Informative References . . . . . . . . . . . . . . . . . . 23
Appendix A. Applicability of LDP Entropy Label sub-TLV . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
Load balancing, or multi-pathing, is an attempt to balance traffic
across a network by allowing the traffic to use multiple paths. Load
balancing has several benefits: it eases capacity planning; it can
help absorb traffic surges by spreading them across multiple paths;
it allows better resilience by offering alternate paths in the event
of a link or node failure.
As providers scale their networks, they use several techniques to
achieve greater bandwidth between nodes. Two widely used techniques
are: Link Aggregation Group (LAG) and Equal-Cost Multi-Path (ECMP).
LAG is used to bond together several physical circuits between two
adjacent nodes so they appear to higher-layer protocols as a single,
higher bandwidth 'virtual' pipe. ECMP is used between two nodes
separated by one or more hops, to allow load balancing over several
shortest paths in the network. This is typically obtained by
arranging IGP metrics such that there are several equal cost paths
between source-destination pairs. Both of these techniques may, and
often do, co-exist in various parts of a given provider's network,
depending on various choices made by the provider.
A very important requirement when load balancing is that packets
belonging to a given 'flow' must be mapped to the same path, i.e.,
the same exact sequence of links across the network. This is to
avoid jitter, latency and re-ordering issues for the flow. What
constitutes a flow varies considerably. A common example of a flow
is a TCP session. Other examples are an L2TP session corresponding
to a given broadband user, or traffic within an ATM virtual circuit.
To meet this requirement, a node uses certain fields, termed 'keys',
within a packet's header as input to a load balancing function
(typically a hash function) that selects the path for all packets in
a given flow. The keys chosen for the load balancing function depend
on the packet type; a typical set (for IP packets) is the IP source
and destination addresses, the protocol type, and (for TCP and UDP
traffic) the source and destination port numbers. An overly
conservative choice of fields may lead to many flows mapping to the
same hash value (and consequently poorer load balancing); an overly
aggressive choice may map a flow to multiple values, potentially
violating the above requirement.
For MPLS networks, most of the same principles (and benefits) apply.
However, finding useful keys in a packet for the purpose of load
balancing can be more of a challenge. In many cases, MPLS
encapsulation may require fairly deep inspection of packets to find
these keys at transit LSRs.
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One way to eliminate the need for this deep inspection is to have the
ingress LSR of an MPLS Label Switched Path extract the appropriate
keys from a given packet, input them to its load balancing function,
and place the result in an additional label, termed the 'entropy
label', as part of the MPLS label stack it pushes onto that packet.
The packet's MPLS entire label stack can then be used by transit LSRs
to perform load balancing, as the entropy label introduces the right
level of "entropy" into the label stack.
There are four key reasons why this is beneficial:
1. at the ingress LSR, MPLS encapsulation hasn't yet occurred, so
deep inspection is not necessary;
2. the ingress LSR has more context and information about incoming
packets than transit LSRs;
3. ingress LSRs usually operate at lower bandwidths than transit
LSRs, allowing them to do more work per packet, and
4. transit LSRs do not need to perform deep packet inspection and
can load balance effectively using only a packet's MPLS label
stack.
This memo describes why entropy labels are needed and defines the
properties of entropy labels; in particular how they are generated
and received, and the expected behavior of transit LSRs. Finally, it
describes in general how signaling works and what needs to be
signaled, as well as specifics for the signaling of entropy labels
for LDP ([RFC5036]), BGP ([RFC3107], [RFC4364]), and RSVP-TE
([RFC3209]).
1.1. Conventions used
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The following acronyms are used:
LSR: Label Switching Router;
LER: Label Edge Router;
PE: Provider Edge router;
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CE: Customer Edge device; and
FEC: Forwarding Equivalence Class.
The term ingress (or egress) LSR is used interchangeably with ingress
(or egress) LER. The term application throughout the text refers to
an MPLS application (such as a VPN or VPLS).
A label stack (say of three labels) is denoted by <L1, L2, L3>, where
L1 is the "outermost" label and L3 the innermost (closest to the
payload). Packet flows are depicted left to right, and signaling is
shown right to left (unless otherwise indicated).
The term 'label' is used both for the entire 32-bit label and the 20-
bit label field within a label. It should be clear from the context
which is meant.
1.2. Motivation
MPLS is very successful generic forwarding substrate that transports
several dozen types of protocols, most notably: IP, PWE3, VPLS and IP
VPNs. Within each type of protocol, there typically exist several
variants, each with a different set of load balancing keys, e.g., for
IP: IPv4, IPv6, IPv6 in IPv4, etc.; for PWE3: Ethernet, ATM, Frame-
Relay, etc. There are also several different types of Ethernet over
PW encapsulation, ATM over PW encapsulation, etc. as well. Finally,
given the popularity of MPLS, it is likely that it will continue to
be extended to transport new protocols.
Currently, each transit LSR along the path of a given LSP has to try
to infer the underlying protocol within an MPLS packet in order to
extract appropriate keys for load balancing. Unfortunately, if the
transit LSR is unable to infer the MPLS packet's protocol (as is
often the case), it will typically use the topmost (or all) MPLS
labels in the label stack as keys for the load balancing function.
The result may be an extremely inequitable distribution of traffic
across equal-cost paths exiting that LSR. This is because MPLS
labels are generally fairly coarse-grained forwarding labels that
typically describe a next-hop, or provide some of demultiplexing
and/or forwarding function, and do not describe the packet's
underlying protocol.
On the other hand, an ingress LSR (e.g., a PE router) has detailed
knowledge of an packet's contents, typically through a priori
configuration of the encapsulation(s) that are expected at a given
PE-CE interface, (e.g., IPv4, IPv6, VPLS, etc.). They also have more
flexible forwarding hardware. PE routers need this information and
these capabilities to:
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a) apply the required services for the CE;
b) discern the packet's CoS forwarding treatment;
c) apply filters to forward or block traffic to/from the CE;
d) to forward routing/control traffic to an onboard management
processor; and,
e) load-balance the traffic on its uplinks to transit LSRs (e.g.,
P routers).
By knowing the expected encapsulation types, an ingress LSR router
can apply a more specific set of payload parsing routines to extract
the keys appropriate for a given protocol. This allows for
significantly improved accuracy in determining the appropriate load
balancing behavior for each protocol.
If the ingress LSR were to capture the flow information so gathered
in a convenient form for downstream transit LSRs, transit LSRs could
remain completely oblivious to the contents of each MPLS packet, and
use only the captured flow information to perform load balancing. In
particular, there will be no reason to duplicate an ingress LSR's
complex packet/payload parsing functionality in a transit LSR. This
will result in less complex transit LSRs, enabling them to more
easily scale to higher forwarding rates, larger port density, lower
power consumption, etc. The idea in this memo is to capture this
flow information as a label, the so-called entropy label.
Ingress LSRs can also adapt more readily to new protocols and extract
the appropriate keys to use for load balancing packets of those
protocols. This means that deploying new protocols or services in
edge devices requires fewer concommitant changes in the core,
resulting in higher edge service velocity and at the same time more
stable core networks.
2. Approaches
There are two main approaches to encoding load balancing information
in the label stack. The first allocates multiple labels for a
particular Forwarding Equivalance Class (FEC). These labels are
equivalent in terms of forwarding semantics, but having multiple
labels allows flexibility in assigning labels to flows belonging to
the same FEC. This approach has the advantage that the label stack
has the same depth whether or not one uses label-based load
balancing; and so, consequently, there is no change to forwarding
operations on transit and egress LSRs. However, it has a major
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drawback in that there is a significant increase in both signaling
and forwarding state.
The other approach encodes the load balancing information as an
additional label in the label stack, thus increasing the depth of the
label stack by one. With this approach, there is minimal change to
signaling state for a FEC; also, there is no change in forwarding
operations in transit LSRs, and no increase of forwarding state in
any LSR. The only purpose of the additional label is to increase the
entropy in the label stack, so this is called an "entropy label".
This memo focuses solely on this approach.
3. Entropy Labels
An entropy label (as used here) is a label:
1. that is not used for forwarding;
2. that is not signaled; and
3. whose only purpose in the label stack is to provide 'entropy' to
improve load balancing.
Entropy labels are generated by an ingress LSR, based entirely on
load balancing information. However, they MUST NOT have values in
the reserved label space (0-15). Entropy labels MUST be at the
bottom of the label stack, and thus the 'Bottom of Stack' (S) bit
([RFC3032]) in the label should be set. To ensure that they are not
used inadvertently for forwarding, entropy labels SHOULD have a TTL
of 0.
Since entropy labels are generated by an ingress LSR, an egress LSR
MUST be able to tell unambiguously that a given label is an entropy
label. If any ambiguity is possible, the label above the entropy
label MUST be an 'entropy label indicator' (ELI), which indicates
that the following Label is an entropy label. An ELI is typically
signaled by an egress LSR and is added to the MPLS label stack along
with an entropy label by an ingress LSR. For many applications, the
use of entropy labels is unambiguous, and an ELI is not needed. If
used, an ELI MUST have S = 0 and SHOULD have a TTL of 0.
Applications for MPLS entropy labels include pseudowires ([RFC4447]),
Layer 3 VPNs ([RFC4364]), VPLS ([RFC4761], [RFC4762]) and Tunnel LSPs
carrying, say, IP traffic. [I-D.ietf-pwe3-fat-pw] explains how
entropy labels can be used for RFC 4447-style pseudowires, and thus
is complementary to this memo, which focuses on several other
applications of entropy labels.
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4. Data Plane Processing of Entropy Labels
4.1. Ingress LSR
Suppose that for a particular application (or service or FEC), an
ingress LSR X is to push label stack <TL, AL>, where TL is the
'tunnel label' and AL is the 'application label'. (Note the use of
the convention for label stacks described in Section 1.1. The use of
a two-label stack is just for illustrative purposes.) Suppose
furthermore that the egress LSR Y has told X that it is capable of
processing entropy labels for this application. If X can insert
entropy labels, it may use a label stack of <TL, AL, EL> for this
application, where EL is the entropy label.
When a packet for this application arrives at X, X does the
following:
1. X identifies the application to which the packet belongs,
identifies the egress LSR as Y, and thereby picks the outgoing
label stack <TL, AL> to push onto the packet to send to Y;
2. X determines which keys that it will use for load balancing;
3. X, having kept state that Y can process entropy labels for this
application, generates an entropy label EL (based on the output
of the load balancing function), and
4. X pushes <TL, AL, EL> on to the packet before forwarding it to
the next LSR on its way to Y.
EL is a 'regular' 32-bit label whose S bit MUST be 1 and whose TTL
field SHOULD be 0. The load balancing information is encoded in the
20-bit label field. If X is told (via signaling) that it must use an
entropy label indicator with label value E, then X instead pushes
<TL, AL, ELI, EL> onto the packet, where ELI is a label whose S bit
MUST be 0, whose TTL SHOULD be 0, and whose 20-bit label field MUST
be E. The CoS fields for EL and ELI can be set to any values.
Note that ingress LSR X MUST NOT include an entropy label unless the
egress LSR Y for this application has indicated that it is ready to
receive entropy labels. Furthermore, if Y has signaled that an ELI
is needed, then X MUST include the ELI before the entropy label.
Note that the signaling and use of entropy labels in one direction
(signaling from Y to X, and data path from X to Y) has no bearing on
the behavior in the opposite direction (signaling from X to Y, and
data path from Y to X).
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4.2. Transit LSR
Transit LSRs have virtually no change in forwarding behavior. For
load balancing, transit LSRs SHOULD use the whole label stack as keys
for the load balancing function. Transit LSRs MAY choose to look
beyond the label stack for further keys; however, if entropy labels
are being used, this may not be very useful. Looking beyond the
label stack may be the simplest approach in an environment where some
ingress LSRs use entropy labels and others don't, or for backward
compatibility. Thus, other than using the full label stack as input
to the load balancing function, transit LSRs are almost unaffected by
the use of entropy labels.
4.3. Egress LSR
If egresss LSR Y signals that it is capable of processing entropy
labels without an ELI for an application, then when Y receives a
packet with the application label, then Y looks to see if the S bit
is set. If so, Y applies its usual processing rules to the packet,
including popping the application label. If the S bit is not set, Y
assumes that the label below the application label is an entropy
label and pops both the application label and the entropy label. Y
SHOULD ensure that the entropy label has its S bit set. Y then
processes the packet as usual. Implementations may choose the order
in which they apply these operations, but the net result should be as
specified.
If Y signals that it is capable of processing entropy labels but that
an ELI is necessary for a given application, then when Y receives a
packet with the application label, Y processes the application label
as usual, then pops it. Y then checks whether the S bit on the
application label is set. If not, Y looks to see if the label below
the application label is the ELI. If so, Y further pops both the ELI
and the label below (which should be the entropy label). Y SHOULD
ensure that the ELI has its S bit unset, and that the entropy label
has its S bit set. If the S bit of the application label is set, or
the label below is not the ELI, Y processes the packet as usual
(there is no entropy label).
5. Signaling for Entropy Labels
An egress LSR Y may signal to ingress LSR(s) its ability to process
entropy labels on a per-application (or per-FEC) basis. As part of
this signaling, Y also signals the ELI to use, if any.
In cases where an application label is used and must be the
bottommost label in the label stack, Y MAY signal that no ELI is
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needed for that application.
In cases where no application label exists, or where the application
label may not be the bottommost label in the label stack, Y MUST
signal a valid ELI to be used in conjunction with the entropy label
for this FEC. In this case, an ingress LSR will either not add an
entropy label, or push the ELI before the entropy label. This makes
the use or non-use of an entropy label by the ingress LSR
unambiguous. Valid ELI label values are strictly greater than 15.
It should be noted that egress LSR Y may use the same ELI value for
all applications for which an ELI is needed. The ELI MUST be a label
that does not conflict with any other labels that Y has advertised to
other LSRs for other applications. Furthermore, it should be noted
that the ability to process entropy labels (and the corresponding
ELI) may be asymmetric: an LSR X may be willing to process entropy
labels, whereas LSR Y may not be willing to process entropy labels.
The signaling extensions below allow for this asymmetry.
For an illustration of signaling and forwarding with entropy labels,
see Figure 9.
5.1. LDP Signaling
When using LDP for signaling tunnel labels ([RFC5036]), a Label
Mapping Message sub-TLV (Entropy Label sub-TLV) is used to signal an
egress LSR's ability to process entropy labels.
The presence of the Entropy Label sub-TLV in the Label Mapping
Message indicates to ingress LSRs that the egress LSR can process an
entropy label. In addition, the Entropy Label sub-TLV contains a
label value for the ELI. If the ELI is zero, this indicates the
egress doesn't need an ELI for the signaled application; if not, the
egress requires the given ELI with entropy labels. An example where
an ELI is needed is when the signaled application is an LSP that can
carry IP traffic.
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The structure of the Entropy Label sub-TLV is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type (TBD) | Length (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Entropy Label sub-TLV
where:
U: Unknown bit. This bit MUST be set to 1. If the Entropy Label
sub-TLV is not understood, then the TLV is not known to the
receiver and MUST be ignored.
F: Forward bit. This bit MUST be set be set to 1. Since this
sub-TLV is going to be propagated hop-by-hop, the sub-TLV should
be forwarded even by nodes that may not understand it.
Type: sub-TLV Type field, as specified by IANA.
Length: sub-TLV Length field. This field specifies the total
length in octets of the Entropy Label sub-TLV.
Value: value of the Entropy Label Indicator Label.
5.2. BGP Signaling
When BGP [RFC4271] is used for distributing Network Layer
Reachability Information (NLRI) as described in, for example,
[RFC3107], [RFC4364] and [RFC4761], the BGP UPDATE message may
include the Entropy Label attribute. This is an optional, transitive
BGP attribute of type TBD. The inclusion of this attribute with an
NLRI indicates that the advertising BGP router can process entropy
labels as an egress LSR for that NLRI. If the attribute length is
less than three octets, this indicates that the egress doesn't need
an ELI for the signaled application. If the attribute length is at
least three octets, the first three octets encode an ELI label value
as the high order 20 bits; the egress requires this ELI with entropy
labels. An example where an ELI is needed is when the NLRI contains
unlabeled IP prefixes.
A BGP speaker S that originates an UPDATE should only include the
Entropy Label attribute if both of the following are true:
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A1: S sets the BGP NEXT_HOP attribute to itself; AND
A2: S can process entropy labels for the given application.
If both A1 and A2 are true, and S needs an ELI to recognize entropy
labels, then S MUST include the ELI label value as part of the
Entropy Label attribute. An UPDATE SHOULD contain at most one
Entropy Label attribute.
Suppose a BGP speaker T receives an UPDATE U with the Entropy Label
attribute ELA. T has two choices. T can simply re-advertise U with
the same ELA if either of the following is true:
B1: T does not change the NEXT_HOP attribute; OR
B2: T simply swaps labels without popping the entire label stack and
processing the payload below.
An example of the use of B1 is Route Reflectors; an example of the
use of B2 is illustrated in Section 9.3.1.2.
However, if T changes the NEXT_HOP attribute for U and in the data
plane pops the entire label stack to process the payload, T MUST
remove ELA. T MAY include a new Entropy Label attribute ELA' for
UPDATE U' if both of the following are true:
C1: T sets the NEXT_HOP attribute of U' to itself; AND
C2: T can process entropy labels for the given application.
Again, if both C1 and C2 are true, and T needs an ELI to recognize
entropy labels, then T MUST include the ELI label value as part of
the Entropy Label attribute.
5.3. RSVP-TE Signaling
Entropy Label support is signaled in RSVP-TE [RFC3209] using an
Entropy Label Attribute TLV (Type TBD) of the LSP_ATTRIBUTES object
[RFC5420]. The presence of this attribute indicates that the
signaler (the egress in the downstream direction using Resv messages;
the ingress in the upstream direction using Path messages) can
process entropy labels. The Entropy Label Attribute contains a value
for the ELI. If the ELI is zero, this indicates that the signaler
doesn't need an ELI for this application; if not, then the signaler
requires the given ELI with entropy labels. An example where an ELI
is needed is when the signaled LSP can carry IP traffic.
The format of the Entropy Label Attribute is as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Entropy Label Attribute | Length (4) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ELI Label | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An egress LSR includes the Entropy Label Attribute in a Resv message
to indicate that it can process entropy labels in the downstream
direction of the signaled LSP.
An ingress LSR includes the Entropy Label Attribute in a Path message
for a bi-directional LSP to indicate that it can process entropy
labels in the upstream direction of the signaled LSP. If the
signaled LSP is not bidirectional, the Entropy Label Attribute SHOULD
NOT be included in the Path message, and egress LSR(s) SHOULD ignore
the attribute, if any.
As described in Section 8, there is also the need to distribute an
ELI from the ingress (upstream label allocation). In the case of
RSVP-TE, this is accomplished using the Upstream ELI Attribute TLV of
the LSP_ATTRIBUTES object, as shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Upstream ELI Attribute | Length (4) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ELI Label | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6. Operations, Administration, and Maintenance (OAM) and Entropy Labels
Generally OAM comprises a set of functions operating in the data
plane to allow a network operator to monitor its network
infrastructure and to implement mechanisms in order to enhance the
general behavior and the level of performance of its network, e.g.,
the efficient and automatic detection, localization, diagnosis and
handling of defects.
Currently defined OAM mechanisms for MPLS include LSP Ping/Traceroute
[RFC4379] and Bidirectional Failure Detection (BFD) for MPLS
[RFC5884]. The latter provides connectivity verification between the
endpoints of an LSP, and recommends establishing a separate BFD
session for every path between the endpoints.
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The LSP traceroute procedures of [RFC4379] allow an ingress LSR to
obtain label ranges that can be used to send packets on every path to
the egress LSR. It works by having ingress LSR sequentially ask the
transit LSRs along a particular path to a given egress LSR to return
a label range such that the inclusion of a label in that range in a
packet will cause the replying transit LSR to send that packet out
the egress interface for that path. The ingress provides the label
range returned by transit LSR N to transit LSR N + 1, which returns a
label range which is less than or equal in span to the range provided
to it. This process iterates until the penultimate transit LSR
replies to the ingress LSR with a label range that is acceptable to
it and to all LSRs along path preceding it for forwarding a packet
along the path.
However, the LSP traceroute procedures do not specify where in the
label stack the value from the label range is to be placed, whether
deep packet inspection is allowed and if so, which keys and key
values are to be used.
This memo updates LSP traceroute by specifying that the value from
the label range is to be placed in the entropy label. Deep packet
inspection is thus not necessary, although an LSR may use it,
provided it do so consistently, i.e., if the label range to go to a
given downstream LSR is computed with deep packet inspection, then
the data path should use the same approach and the same keys.
In order to have a BFD session on a given path, a value from the
label range for that path should be used as the EL value for BFD
packets sent on that path.
As part of the MPLS-TP work, an in-band OAM channel is defined in
[RFC5586]. Packets sent in this channel are identified with a
reserved label, the Generic Associated Channel Label (GAL) placed at
the bottom of the MPLS label stack. In order to use the inband OAM
channel with entropy labels, this memo relaxes the restriction that
the GAL must be at the bottom of the MPLS label stack. Rather, the
GAL is placed in the MPLS label stack above the entropy label so that
it effectively functions as an application label.
7. MPLS-TP and Entropy Labels
Since MPLS-TP does not use ECMP, entropy labels are not applicable to
an MPLS-TP deployment.
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8. Point-to-Multipoint LSPs and Entropy Labels
Point-to-Multipoint (P2MP) LSPs [RFC4875] typically do not use ECMP
for load balancing, as the combination of replication and
multipathing can lead to duplicate traffic delivery. However, P2MP
LSPs can traverse Bundled Links [RFC4201] and LAGs. In both these
cases, load balancing is useful, and hence entropy labels can be of
some value for P2MP LSPs.
There are two potential complications with the use of entropy labels
in the context of P2MP LSPs, both a consequence of the fact that the
entire label stack below the P2MP label must be the same for all
egress LSRs. First, all egress LSRs must be willing to receive
entropy labels; if even one egress LSR is not willing, then entropy
labels MUST NOT be used for this P2MP LSP. Second, if an ELI is
required, all egress LSRs must agree to the same value of ELI. This
can be achieved by upstream allocation of the ELI; in particular, for
RSVP-TE P2MP LSPs, the ingress LSR distributes the ELI value using
the Upstream ELI Attribute TLV of the LSP_ATTRIBUTES object, defined
in Section 5.3.
With regard to the first issue, the ingress LSR MUST keep track of
the ability of each egress LSR to process entropy labels, especially
since the set of egress LSRs of a given P2MP LSP may change over
time. Whenever an existing egress LSR leaves, or a new egress LSR
joins the P2MP LSP, the ingress MUST re-evaluate whether or not to
include entropy labels for the P2MP LSP.
In some cases, it may be feasible to deploy two P2MP LSPs, one to
entropy label capable egress LSRs, and the other to the remaining
egress LSRs. However, this requires more state in the network, more
bandwidth, and more operational overhead (tracking EL-capable LSRs,
and provisioning P2MP LSPs accordingly). Furthermore, this approach
may not work for some applications (such mVPNs and VPLS) which
automatically create and/or use P2MP LSPs for their multicast
requirements.
9. Entropy Labels and Applications
This section describes the usage of entropy labels in various
scenarios with different applications.
9.1. Tunnels
Tunnel LSPs, signaled with either LDP or RSVP-TE, typically carry
other MPLS applications such as VPNs or pseudowires. This being the
case, if the egress LSR of a tunnel LSP is willing to process entropy
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labels, it would signal the need for an Entropy Label Indicator to
distinguish between entropy labels and other application labels.
In the figures below, the following convention is used to depict
information signaled between X and Y:
X ---------- ... ---------- Y
app: <--- [label L, ELI value]
This means Y signals to X label L for application app. The ELI value
can be one of:
-: meaning entropy labels are NOT accepted;
0: meaning entropy labels are accepted, no ELI is needed; or
E: entropy labels are accepted, ELI label E is required.
The following illustrates a simple intra-AS tunnel LSP.
X -------- A --- ... --- B -------- Y
tunnel LSP L: [TL, E] <--- ... <--- [TL0, E]
IP pkt: push <TL, E, EL> --------------->
Figure 2: Tunnel LSPs and Entropy Labels
Tunnel LSPs may cross Autonomous System (AS) boundaries, usually
using BGP ([RFC3107]). In this case, the AS Border Routers (ASBRs)
MAY simply propagate the egress LSR's ability to process entropy
labels, or they MAY declare that entropy labels may not be used. If
an ASBR (say A2 below) chooses to propagate the egress LSR Y's
ability to process entropy labels, A2 MUST also propagate Y's choice
of ELI.
X ---- ... ---- A1 ------- A2 ---- ... ---- Y
intra-AS LSP A2-Y: <--- [TL0, E]
inter-AS LSP A1-A2: [AL, E]
intra-AS LSP X-A1: <--- [TL1, E]
IP pkt: push <TL1, E, EL>
Here, ASBR A2 chooses to propagate Y's ability to process entropy
labels, by "translating" Y's signaling of entropy label capability
(say using LDP) to BGP; and A1 translate A2's BGP signaling to (say)
RSVP-TE. The end-to-end tunnel (X to Y) will have entropy labels if
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X chooses to insert them.
Figure 3: Inter-AS Tunnel LSP with Entropy Labels
X ---- ... ---- A1 ------- A2 ---- ... ---- Y
intra-AS LSP A2-Y: <--- [TL0, E]
inter-AS LSP A1-A2: [AL, E]
intra-AS LSP X-A1: <--- [TL1, -]
IP pkt: push <TL1> -->
Here, ASBR A1 decided that entropy labels are not to be used; thus,
the end-to-end tunnel cannot have entropy labels, even though both X
and Y may be capable of inserting and processing entropy labels.
Figure 4: Inter-AS Tunnel LSP with no Entropy Labels
9.2. LDP Pseudowires
[I-D.ietf-pwe3-fat-pw] describes the signaling and use of entropy
labels in the context of RFC 4447 pseudowires, so this will not be
described further here.
[RFC4762] specifies the use of LDP for signaling VPLS pseudowires.
An egress VPLS PE that can process entropy labels can indicate this
by adding the Entropy Label sub-TLV in the LDP message it sends to
other PEs. An ELI is not required. An ingress PE must maintain
state per egress PE as to whether it can process entropy labels.
X -------- A --- ... --- B -------- Y
tunnel LSP L: [TL, E] <--- ... <--- [TL0, E]
VPLS label: <------------------------ [VL, 0]
VPLS pkt: push <TL, VL, EL> -------------->
Figure 5: Entropy Labels with LDP VPLS
Note that although the underlying tunnel LSP signaling indicated the
need for an ELI, VPLS packets don't need an ELI, and thus the label
stack pushed by X do not have one.
[RFC4762] also describes the notion of "hierarchical VPLS" (H-VPLS).
In H-VPLS, 'hub PEs' remove the label stack and process VPLS packets;
thus, they must make their own decisions on the use of entropy
labels, independent of other hub PEs or spoke PEs with which they
exchange signaling. In the example below, spoke PEs X and Y and hub
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PE B can process entropy labels, but hub PE A cannot.
X ---- ... ---- A ---- ... ---- B ---- ... ---- Y
spoke PW1: <--- [SL1, 0]
hub-hub PW: <---- [HL, 0]
spoke PW2: <--- [SL2, -]
SPW2 pkt: push <TL1, SL2>
H-H PW pkt: push <TL2,HL,EL>
SPW1 pkt: push <TL3,SL1,EL>
Figure 6: Entropy Labels with H-VPLS
9.3. BGP Applications
Section 9.1 described a BGP application for the creation of inter-AS
tunnel LSPs. This section describes two other BGP applications, IP
VPNs ([RFC4364]) and BGP VPLS ([RFC4761]). An egress PE for either
of these applications indicates its ability to process entropy labels
by adding the Entropy Label attribute to its BGP UPDATE message.
Again, ingress PEs must maintain per-egress PE state regarding its
ability to process entropy labels. In this section, both of these
applications will be referred to as VPNs.
In the intra-AS case, PEs signal application labels and entropy label
capability to each other, either directly, or via Route Reflectors
(RRs). If RRs are used, they must not change the BGP NEXT_HOP
attribute in the UPDATE messages; furthermore, they can simply pass
on the Entropy Label attribute as is.
X -------- A --- ... --- B -------- Y
tunnel LSP L: [TL, E] <--- ... <--- [TL0, E]
BGP VPN label: <------------------------ [VL, 0]
BGP VPN pkt: push <TL, VL, EL> -------------->
Figure 7: Entropy Labels with Intra-AS BGP apps
For BGP VPLS, the application label is at the bottom of stack, so no
ELI is needed. For BGP IP VPNs, the application label is usually at
the bottom of stack, so again no ELI is needed. However, in the case
of Carrier's Carrier (CsC) VPNs, the BGP VPN label may not be at the
bottom of stack. In this case, an ELI is necessary for CsC VPN
packets with entropy labels to distinguish them from nested VPN
packets. In the example below, the nested VPN signaling is not
shown; the egress PE for the nested VPN (not shown) must signal
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whether or not it can process egress labels, and the ingress nested
VPN PE may insert an entropy label if so.
Three cases are shown: a plain BGP VPN packet, a CsC VPN packet
originating from X, and a transit nested VPN packet originating from
a nested VPN ingress PE (conceptually to the left of X). It is
assumed that the nested VPN packet arrives at X with label stack <ZL,
CVL> where ZL is the tunnel label (to be swapped with <TL, CL>) and
CVL is the nested VPN label. Note that Y can use the same ELI for
the tunnel LSP and the CsC VPN (and any other application that needs
an ELI).
X -------- A --- ... --- B -------- Y
tunnel LSP L: [TL, E] <--- ... <--- [TL0, E]
BGP VPN label: <------------------------ [VL, 0]
BGP CsC VPN label: <------------------------ [CL, E]
BGP VPN pkt: push <TL, VL, EL> -------------->
CsC VPN pkt: push <TL, CL, E, EL> ----------->
nested VPN pkt: swap <ZL> with <TL, CL> -------->
Figure 8: Entropy Labels with CoC VPN
9.3.1. Inter-AS BGP VPNs
There are three commonly used options for inter-AS IP VPNs and BGP
VPLS, known informally as "Option A", "Option B" and "Option C".
This section describes how entropy labels can be used in these
options.
9.3.1.1. Option A Inter-AS VPNs
In option A, an ASBR pops the full label stack of a VPN packet
exiting an AS, processes the payload header (IP or Ethernet), and
forwards the packet natively (i.e., as IP or Ethernet, but not as
MPLS) to the peer ASBR. Thus, entropy label signaling and insertion
are completely local to each AS. The inter-AS paths do not use
entropy labels, as they do not use a label stack.
9.3.1.2. Option B Inter-AS VPNs
The ASBRs in option B inter-AS VPNs have a choice (usually determined
by configuration) of whether to just swap labels (from within the AS
to the neighbor AS or vice versa), or to pop the full label stack and
process the packet natively. This choice occurs at each ASBR in each
direction. In the case of native packet processing at an ASBR,
entropy label signaling and insertion is local to each AS and to the
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inter-AS paths (which, unlike option A, do have labeled packets).
In the case of simple label swapping at an ASBR, the ASBR can
propagate received entropy label signaling onward. That is, if a PE
signals to its ASBR that it can process entropy labels (via an
Entropy Label attribute), the ASBR can propagate that attribute to
its peer ASBR; if a peer ASBR signals that it can process entropy
labels, the ASBR can propagate that to all PEs within its AS). Note
that this is the case even though ASBRs change the BGP NEXT_HOP
attribute to "self", because of clause B2 in Section 5.2.
9.3.1.3. Option C Inter-AS VPNs
In Option C inter-AS VPNs, the ASBRs are not involved in signaling;
they do not have VPN state; they simply swap labels of inter-AS
tunnels. Signaling is PE to PE, usually via Route Reflectors;
however, if RRs are used, the RRs do not change the BGP NEXT_HOP
attribute. Thus, entropy label signaling and insertion are on a PE-
pair basis, and the intermediate routers, ASBRs and RRs do not play a
role.
9.4. Multiple Applications
It has been mentioned earlier that an ingress PE must keep state per
egress PE with regard to its ability to process entropy labels. An
ingress PE must also keep state per application, as entropy label
processing must be based on the application context in which a packet
is received (and of course, the corresponding entropy label
signaling).
In the example below, an egress LSR Y signals a tunnel LSP L, and is
prepared to receive entropy labels on L, but requires an ELI.
Furthermore, Y signals two pseudowires PW1 and PW2 with labels PL1
and PL2, respectively, and indicates that it can receive entropy
labels for both pseudowires without the need of an ELI; and finally,
Y signals a L3 VPN with label VL, but Y does not indicate that it can
receive entropy labels for the L3 VPN. Ingress LSR X chooses to send
native IP packets to Y over L with entropy labels, thus X must
include the given ELI (yielding a label stack of <TL, ELI, EL>). X
chooses to add entropy labels on PW1 packets to Y, with a label stack
of <TL, PL1, EL>, but chooses not to do so for PW2 packets. X must
not send entropy labels on L3 VPN packets to Y, i.e., the label stack
must be <TL, VL>.
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X -------- A --- ... --- B -------- Y
tunnel LSP L: [TL, E] <--- ... <--- [TL0, E]
PW1 label: <----------------------- [PL1, 0]
PW2 label: <----------------------- [PL2, 0]
VPN label: <----------------------- [VL, -]
IP pkt: push <TL, ELI, EL> ------------->
PW1 pkt: push <TL, PL1, EL> ------------->
PW2 pkt: push <TL, PL2> ----------------->
VPN pkt: push <TL, VL> ------------------>
Figure 9: Entropy Labels for Multiple Applications
10. Security Considerations
This document describes advertisement of the capability to support
receipt of entropy-labels and an Entropy Label Indicator that an
ingress LSR may apply to MPLS packets in order to allow transit LSRs
to attain better load-balancing across LAG and/or ECMP paths in the
network.
This document does not introduce new security vulnerabilities to LDP.
Please refer to the Security Considerations section of LDP
([RFC5036]) for security mechanisms applicable to LDP.
Given that there is no end-user control over the values used for
entropy labels, there is little risk of Entropy Label forgery which
could cause uneven load-balancing in the network.
If Entropy Label Capability is not signaled from an egress PE to an
ingress PE, due to, for example, malicious configuration activity on
the egress PE, then the PE's will fall back to not using entropy
labels for load-balancing traffic over LAG or ECMP paths which, in
some cases, in no worse than the behavior observed in current
production networks. That said, operators are recommended to monitor
changes to PE configurations and, more importantly, the fairness of
load distribution over equal-cost LAG or ECMP paths. If the fairness
of load distribution over a set of paths changes that could indicate
a misconfiguration, bug or other non-optimal behavior on their PE's
and they should take corrective action.
Given that most applications already signal an Application Label,
e.g.: IPVPNs, LDP VPLS, BGP VPLS, whose Bottom of Stack bit is being
re-used to signal entropy label capability, there is little to no
additional risk that traffic could be misdirected into an
inappropriate IPVPN VRF or VPLS VSI at the egress PE.
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In the context of downstream-signaled entropy labels that require the
use of an Entropy Label Indicator (ELI), there should be little to no
additional risk because the egress PE is solely responsible for
allocating an ELI value and ensuring that ELI label value DOES NOT
conflict with other MPLS labels it has previously allocated. On the
other hand, for upstream-signaled entropy labels, e.g.: RSVP-TE
point-to-point or point-to-multipoint LSP's or Multicast LDP (mLDP)
point-to-multipoint or multipoint-to-multipoint LSP's, there is a
risk that the head-end MPLS LER may choose an ELI value that is
already in use by a downstream LSR or LER. In this case, it is the
responsibility of the downstream LSR or LER to ensure that it MUST
NOT accept signaling for an ELI value that conflicts with MPLS
label(s) that are already in use.
11. IANA Considerations
11.1. LDP Entropy Label TLV
IANA is requested to allocate the next available value from the IETF
Consensus range in the LDP TLV Type Name Space Registry as the
"Entropy Label TLV".
11.2. BGP Entropy Label Attribute
IANA is requested to allocate the next available Path Attribute Type
Code from the "BGP Path Attributes" registry as the "BGP Entropy
Label Attribute".
11.3. Attribute Flags for LSP_Attributes Object
IANA is requested to allocate a new bit from the "Attribute Flags"
sub-registry of the "RSVP TE Parameters" registry.
Bit | Name | Attribute | Attribute | RRO
No | | Flags Path | Flags Resv |
----+----------------------+------------+------------+-----
TBD Entropy Label LSP Yes Yes No
11.4. Attributes TLV for LSP_Attributes Object
IANA is requested to allocate the next available value from the
"Attributes TLV" sub-registry of the "RSVP TE Parameters" registry.
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12. Acknowledgments
We wish to thank Ulrich Drafz for his contributions, as well as the
entire 'hash label' team for their valuable comments and discussion.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC3107] Rekhter, Y. and E. Rosen, "Carrying Label Information in
BGP-4", RFC 3107, May 2001.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC5420] Farrel, A., Papadimitriou, D., Vasseur, JP., and A.
Ayyangarps, "Encoding of Attributes for MPLS LSP
Establishment Using Resource Reservation Protocol Traffic
Engineering (RSVP-TE)", RFC 5420, February 2009.
13.2. Informative References
[I-D.ietf-pwe3-fat-pw]
Bryant, S., Filsfils, C., Drafz, U., Kompella, V., Regan,
J., and S. Amante, "Flow Aware Transport of Pseudowires
over an MPLS PSN", draft-ietf-pwe3-fat-pw-05 (work in
progress), October 2010.
[RFC4201] Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling
in MPLS Traffic Engineering (TE)", RFC 4201, October 2005.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
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February 2006.
[RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
Heron, "Pseudowire Setup and Maintenance Using the Label
Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
(VPLS) Using BGP for Auto-Discovery and Signaling",
RFC 4761, January 2007.
[RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[RFC4875] Aggarwal, R., Papadimitriou, D., and S. Yasukawa,
"Extensions to Resource Reservation Protocol - Traffic
Engineering (RSVP-TE) for Point-to-Multipoint TE Label
Switched Paths (LSPs)", RFC 4875, May 2007.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC5586] Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
Associated Channel", RFC 5586, June 2009.
[RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, June 2010.
Appendix A. Applicability of LDP Entropy Label sub-TLV
In the case of unlabeled IPv4 (Internet) traffic, the Best Current
Practice is for an egress LSR to propagate eBGP learned routes within
a SP's Autonomous System after resetting the BGP next-hop attribute
to one of its Loopback IP addresses. That Loopback IP address is
injected into the Service Provider's IGP and, concurrently, a label
assigned to it via LDP. Thus, when an ingress LSR is performing a
forwarding lookup for a BGP destination it recursively resolves the
associated next-hop to a Loopback IP address and associated LDP label
of the egress LSR.
Thus, in the context of unlabeled IPv4 traffic, the LDP Entropy Label
sub-TLV will typically be applied only to the FEC for the Loopback IP
address of the egress LSR and the egress LSR will not announce an
entropy label capability for the eBGP learned route.
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Authors' Addresses
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: kireeti@juniper.net
John Drake
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: jdrake@juniper.net
Shane Amante
Level 3 Communications, LLC
1025 Eldorado Blvd
Broomfield, CO 80021
US
Email: shane@level3.net
Wim Henderickx
Alcatel-Lucent
Copernicuslaan 50
2018 Antwerp
Belgium
Email: wim.henderickx@alcatel-lucent.com
Lucy Yong
Huawei USA
1700 Alma Dr. Suite 500
Plano, TX 75075
US
Email: lucyyong@huawei.com
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