Network Working Group S. Kini
Internet-Draft
Intended status: Informational K. Kompella
Expires: August 3, 2018 Juniper
S. Sivabalan
Cisco
S. Litkowski
Orange
R. Shakir
Google
J. Tantsura
January 30, 2018
Entropy label for SPRING tunnels
draft-ietf-mpls-spring-entropy-label-08
Abstract
Segment Routing (SR) leverages the source routing paradigm. A node
steers a packet through an ordered list of instructions, called
segments. Segment Routing can be applied to the Multi Protocol Label
Switching (MPLS) data plane. Entropy label (EL) is a technique used
in MPLS to improve load-balancing. This document examines and
describes how ELs are to be applied to Segment Routing when applied
to the MPLS dataplane.
Status of This Memo
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This Internet-Draft will expire on August 3, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Abbreviations and Terminology . . . . . . . . . . . . . . . . 4
3. Use-case requiring multipath load-balancing . . . . . . . . . 4
4. Entropy Readable Label Depth . . . . . . . . . . . . . . . . 5
5. Maximum SID Depth . . . . . . . . . . . . . . . . . . . . . . 7
6. LSP stitching using the binding SID . . . . . . . . . . . . . 8
7. Insertion of entropy labels for SPRING path . . . . . . . . . 10
7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1.1. Example 1 where the ingress node has a sufficient MSD 11
7.1.2. Example 2 where the ingress node has not a sufficient
MSD . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.2. Considerations for the placement of entropy labels . . . 12
7.2.1. ERLD value . . . . . . . . . . . . . . . . . . . . . 13
7.2.2. Segment type . . . . . . . . . . . . . . . . . . . . 14
7.2.2.1. Node-SID . . . . . . . . . . . . . . . . . . . . 14
7.2.2.2. Adjacency-set SID . . . . . . . . . . . . . . . . 15
7.2.2.3. Adjacency-SID representing a single IP link . . . 15
7.2.2.4. Adjacency-SID representing a single link within a
L2 bundle . . . . . . . . . . . . . . . . . . . . 15
7.2.2.5. Adjacency-SID representing a L2 bundle . . . . . 15
7.2.3. Maximizing number of LSRs that will load-balance . . 15
7.2.4. Preference for a part of the path . . . . . . . . . . 16
7.2.5. Combining criteria . . . . . . . . . . . . . . . . . 16
8. A simple example algorithm . . . . . . . . . . . . . . . . . 16
9. Deployment Considerations . . . . . . . . . . . . . . . . . . 17
10. Options considered . . . . . . . . . . . . . . . . . . . . . 18
10.1. Single EL at the bottom of the stack . . . . . . . . . . 18
10.2. An EL per segment in the stack . . . . . . . . . . . . . 18
10.3. A re-usable EL for a stack of tunnels . . . . . . . . . 19
10.4. EL at top of stack . . . . . . . . . . . . . . . . . . . 19
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10.5. ELs at readable label stack depths . . . . . . . . . . . 20
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
14. Security Considerations . . . . . . . . . . . . . . . . . . . 21
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
15.1. Normative References . . . . . . . . . . . . . . . . . . 21
15.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
Segment Routing [I-D.ietf-spring-segment-routing] is based on source
routed tunnels to steer a packet along a particular path. This path
is encoded as an ordered list of segments. When applied to the MPLS
dataplane [I-D.ietf-spring-segment-routing-mpls], each segment is an
LSP with an associated MPLS label value. Hence, label stacking is
used to represent the ordered list of segments and the label stack
associated with an SR tunnel can be seen as nested LSPs (LSP
hierarchy) in the MPLS architecture.
Using label stacking to encode the list of segment has implications
on the label stack depth.
Entropy label (EL) [RFC6790] is a technique used in the MPLS data
plane to provide entropy for load-balancing. When using LSP
hierarchies, there are implications on how [RFC6790] should be
applied. The current document addresses the case where a hierarchy
is created at a single LSR as required by Segment Routing.
A use-case requiring load-balancing with SR is given in Section 3. A
recommended solution is described in Section 7 keeping in
consideration the limitations of implementations when applying
[RFC6790] to deeper label stacks. Options that were considered to
arrive at the recommended solution are documented for historical
purposes in Section 10.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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2. Abbreviations and Terminology
EL - Entropy Label
ELI - Entropy Label Identifier
ELC - Entropy Label Capability
ERLD - Entropy Readable Label Depth
SR - Segment Routing
ECMP - Equal Cost Multi Path
LSR - Label Switch Router
MPLS - Multiprotocol Label Switching
MSD - Maximum SID Depth
SID - Segment Identifier
RLD - Readable Label Depth
OAM - Operation, Administration and Maintenance
3. Use-case requiring multipath load-balancing
+------+
| |
+-------| P3 |-----+
| +-----| |---+ |
L3| |L4 +------+ L1| |L2 +----+
| | | | +--| P4 |--+
+-----+ +-----+ +-----+ | +----+ | +-----+
| S |-----| P1 |------------| P2 |--+ +--| D |
| | | | | |--+ +--| |
+-----+ +-----+ +-----+ | +----+ | +-----+
+--| P5 |--+
+----+
S=Source LSR, D=Destination LSR, P1,P2,P3,P4,P5=Transit LSRs,
L1,L2,L3,L4=Links
Figure 1: Traffic engineering use-case
Traffic-engineering is one of the applications of MPLS and is also a
requirement for source routed tunnels with label stacks [RFC7855].
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Consider the topology shown in Figure 1. The LSR S requires data to
be sent to LSR D along a traffic-engineered path that goes over the
link L1. Good load-balancing is also required across equal cost
paths (including parallel links). To engineer traffic along a path
that takes link L1, the label stack that LSR S creates consists of a
label to the node SID of LSR P3, stacked over the label for the
adjacency SID of link L1 and that in turn is stacked over the label
to the node SID of LSR D. For simplicity lets assume that all LSRs
use the same label space (SRGB) for source routed label stacks. Let
L_N-Px denote the label to be used to reach the node SID of LSR Px.
Let L_A-Ln denote the label used for the adjacency SID for link Ln.
The LSR S must use the label stack <L_N-P3, L_A-L1, L_N-D> for
traffic-engineering. However to achieve good load-balancing over the
equal cost paths P2-P4-D, P2-P5-D and the parallel links L3, L4, a
mechanism such as Entropy labels [RFC6790] should be adapted for
source routed label stacks. Indeed, the SPRING architecture with the
MPLS dataplane ([I-D.ietf-spring-segment-routing-mpls]) uses nested
MPLS LSPs composing the source routed label stacks. As each MPLS
node may have limitations in the number of labels it can push when it
is ingress or inspect when doing load-balancing, an entropy label
insertion strategy becomes important to keep the benefit of the load-
balancing. Multiple ways to apply entropy labels were considered and
are documented in Section 10 along with their trade-offs. A
recommended solution is described in Section 7.
4. Entropy Readable Label Depth
The Entropy Readable Label Depth (ERLD) is defined as the number of
labels a router can both:
a. Read in an MPLS packet received on its incoming interface(s)
(starting from the top of the stack).
b. Use in its load-balancing function.
The ERLD means that the router will perform load-balancing using the
EL label if the EL is placed within the ERLD first labels.
A router capable of reading N labels but not using an EL located
within those N labels MUST consider its ERLD to be 0. In a
distributed switching architecture, each linecard may have a
different capability in terms of ERLD. For simplicity, an
implementation MAY use the minimum ERLD between each linecard as the
ERLD value for the system.
Examples:
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| Payload |
+----------+
| Payload | | EL | P7
+----------+ +----------+
| Payload | | EL | | ELI |
+----------+ +----------+ +----------+
| Payload | | EL | | ELI | | Label 50 |
+----------+ +----------+ +----------+ +----------+
| Payload | | EL | | ELI | | Label 40 | | Label 40 |
+----------+ +----------+ +----------+ +----------+ +----------+
| EL | | ELI | | Label 30 | | Label 30 | | Label 30 |
+----------+ +----------+ +----------+ +----------+ +----------+
| ELI | | Label 20 | | Label 20 | | Label 20 | | Label 20 |
+----------+ +----------+ +----------+ +----------+ +----------+
| Label 16 | | Label 16 | | Label 16 | | Label 16 | | Label 16 | P1
+----------+ +----------+ +----------+ +----------+ +----------+
Packet 1 Packet 2 Packet 3 Packet 4 Packet 5
Figure 2: Label stacks with ELI/EL
In the figure 2, we consider the displayed packets received on a
router interface. We consider also a single ERLD value for the
router.
o If the router has an ERLD of 3, it will be able to load-balance
Packet 1 displayed in Figure 2 using the EL as part of the load-
balancing keys. The ERLD value of 3 means that the router can
read and take into account the entropy label for load-balancing if
it is placed between position 1 (top) and position 3.
o If the router has an ERLD of 5, it will be able to load-balance
Packets 1 to 3 in Figure 2 using the EL as part of the load-
balancing keys. Packets 4 and 5 have the EL placed at a position
greater than 5, so the router is not able to read it and use as
part of the load-balancing keys.
o If the router has an ERLD of 10, it will be able to load-balance
all the packets displayed in Figure 2 using the EL as part of the
load-balancing keys.
To allow an efficient load-balancing based on entropy labels, a
router running SPRING SHOULD advertise its ERLD (or ERLDs), so all
the other SPRING routers in the network are aware of its capability.
How this advertisement is done is outside the scope of this document.
To advertise an ERLD value, a SPRING router:
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o MUST be entropy label capable and, as a consequence, MUST apply
the dataplane procedures defined in [RFC6790].
o MUST be able to read an ELI/EL which is located within its ERLD
value.
o MUST take into account this EL in its load-balancing function.
5. Maximum SID Depth
The Maximum SID Depth defines the maximum number of labels that a
particular node can impose on a packet. This includes any kind of
labels (service, entropy, transport...). In an MPLS network, the MSD
is a limit of the Ingress LSR (I-LSR) or any stitching node that
would perform an imposition of additional labels on an existing label
stack.
Depending of the number of MPLS operations (POP, SWAP...) to be
performed before the PUSH, the MSD may vary due to the hardware or
software limitations. As for the ERLD, there may also be different
MSD limits based on the linecard type used in a distributed switching
system.
When an external controller is used to program a label stack on a
particular node, this node MAY advertise its MSD value or a subset of
its MSD value to the controller. How this advertisement is done is
outside the scope of this document. As the controller does not have
the knowledge of the entire label stack to be pushed by the node, the
node may advertise an MSD value which is lower than its actual limit.
This gives the ability for the controller to program a label stack up
to the advertised MSD value while leaving room for the local node to
add more labels (e.g., service, entropy, transport...) without
reaching the hardware/software limit.
P7 ---- P8 ---- P9
/ \
PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2
| \ |
----> P10 \ |
IP Pkt | \ |
P11 --- P12 --- P13
100 10000
Figure 3
In the figure 3, an IP packet comes in the MPLS network at PE1. All
metrics are considered equal to 1 except P12-P13 which is 10000 and
P11-P12 which is 100. PE1 wants to steer the traffic using a SPRING
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path to PE2 along
PE1->P1->P7->P8->P9->P4->P5->P10->P11->P12->P13->PE2. By using
adjacency SIDs only, PE1 (acting as an I-LSR) will be required to
push 10 labels on the IP packet received and thus requires an MSD of
10. If the IP packet should be carried over an MPLS service like a
regular layer 3 VPN, an additional service label should be imposed,
requiring an MSD of 11 for PE1. In addition, if PE1 wants to insert
an ELI/EL for load-balancing purpose, PE1 will need to push 13 labels
on the IP packet requiring an MSD of 13.
In the SPRING architecture, Node SIDs or Binding SIDs can be used to
reduce the label stack size. As an example, to steer the traffic on
the same path as before, PE1 may be able to use the following label
stack: <Node_P9, Node_P5, Binding_P5, Node_PE2>. In this example we
consider a combination of Node SIDs and a Binding SID advertised by
P5 that will stitch the traffic along the path P10->P11->P12->P13.
The instruction associated with the binding SID at P5 is thus to swap
Binding_P5 to Adj_P12-P13 and then push <Adj_P11-P12, Node_P11>. P5
acts as a stitching node that pushes additional labels on an existing
label stack, P5's MSD needs also to be taken into account and may
limit the number of labels that could be imposed.
6. LSP stitching using the binding SID
The binding SID allows binding a segment identifier to an existing
LSP. As examples, the binding SID can represent an RSVP-TE tunnel,
an LDP path (through the mapping server advertisement), or a SPRING
path. Each LSP associated with a binding SID has its own entropy
label capability.
In the figure 3, we consider that:
o P6, PE2, P10, P11, P12, P13 are pure LDP routers.
o PE1, P1, P2, P3, P4, P7, P8, P9 are pure SPRING routers.
o P5 is running SPRING and LDP.
o P5 acts as a mapping server and advertises Prefix SIDs for the LDP
FECs: an index value of 20 is used for PE2.
o All SPRING routers use an SRGB of [1000, 1999].
o P6 advertises label 20 for the PE2 FEC.
o Traffic from PE1 to PE2 uses the shortest path.
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PE1 ----- P1 -- P2 -- P3 -- P4 ---- P5 --- P6 --- PE2
--> +----+ +----+ +----+ +----+
IP Pkt | IP | | IP | | IP | | IP |
+----+ +----+ +----+ +----+
|1020| |1020| | 20 |
+----+ +----+ +----+
SPRING LDP
In term of packet forwarding, by learning the mapping-server
advertisement from PE5, PE1 imposes a label 1020 to an IP packet
destinated to PE2. SPRING routers along the shortest path to PE2
will switch the traffic until it reaches P5 which will perform the
LSP stitching. P5 will swap the SPRING label 1020 to the LDP label
20 advertised by the nexthop P6. P6 will then forward the packet
using the LDP label towards PE2.
PE1 cannot push an ELI/EL for the binding SID without knowing that
the tail-end of the LSP associated with the binding (PE2) is entropy
label capable.
To accomodate the mix of signalling protocols involved during the
stitching, the entropy label capability SHOULD be propagated between
the signalling protocols. Each binding SID SHOULD have its own
entropy label capability that MUST be inherited from the entropy
label capability of the associated LSP. If the router advertising
the binding SID does not know the ELC state of the target FEC, it
MUST NOT set the ELC for the binding SID. An ingress node MUST NOT
push an ELI/EL associated with a binding SID unless this binding SID
has the entropy label capability. How the entropy label capability
is advertised for a binding SID is outside the scope of this
document.
In our example, if PE2 is LDP entropy label capable, it will add the
entropy label capability in its LDP advertisement. When P5 receives
the FEC/label binding for PE2, it learns about the ELC and can set
the ELC in the mapping server advertisement. Thus PE1 learns about
the ELC of PE2 and may push an ELI/EL associated with the binding
SID.
The proposed solution only works if the SPRING router advertising the
binding SID is also performing the dataplane LSP stitching. In our
example, if the mapping server function is hosted on P8 instead of
P5, P8 does not know about the ELC state of PE2's LDP FEC. As a
consequence, it does not set the ELC for the associated binding SID.
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7. Insertion of entropy labels for SPRING path
7.1. Overview
The solution described in this section follows the dataplane
processing defined in [RFC6790]. Within a SPRING path, a node may be
ingress, egress, transit (regarding the entropy label processing
described in [RFC6790]), or it can be any combination of those. For
example:
o The ingress node of a SPRING domain may be an ingress node from an
entropy label perspective.
o Any LSR terminating a segment of the SPRING path is an egress node
(because it terminates the segment) but may also be a transit node
if the SPRING path is not terminated because there is a subsequent
SPRING MPLS label in the stack.
o Any LSR processing a binding SID may be a transit node and an
ingress node (because it may push additional labels when
processing the binding SID).
As described earlier, an LSR may have a limitation, ERLD, on the
depth of the label stack that it can read and process in order to do
multipath load-balancing based on entropy labels.
If an EL does not occur within the ERLD of an LSR in the label stack
of an MPLS packet that it receives, then it would lead to poor load-
balancing at that LSR. Hence an ELI/EL pair must be within the ERLD
of the LSR in order for the LSR to use the EL during load-balancing.
Adding a single ELI/EL pair for the entire SPRING path may lead also
to poor load-balancing as well because the EL/ELI may not occur
within the ERLD of some LSR on the path (if too deep) or may not be
present in the stack when it reaches some LSRs if it is too shallow.
In order for the EL to occur within the ERLD of LSRs along the path
corresponding to a SPRING label stack, multiple <ELI, EL> pairs MAY
be inserted in this label stack.
The insertion of the ELI/EL SHOULD occur only with a SPRING label
advertised by an LSR that advertised an ERLD (the LSR is entropy
label capable) or with a SPRING label associated with a binding SID
that has the ELC set.
The ELs among multiple <ELI, EL> pairs inserted in the stack MAY be
the same or different. The LSR that inserts <ELI, EL> pairs MAY have
limitations on the number of such pairs that it can insert and also
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the depth at which it can insert them. If, due to limitations, the
inserted ELs are at positions such that an LSR along the path
receives an MPLS packet without an EL in the label stack within that
LSR's ERLD, then the load-balancing performed by that LSR would be
poor. An implementation MAY consider multiple criteria when
inserting <ELI, EL> pairs.
7.1.1. Example 1 where the ingress node has a sufficient MSD
ECMP LAG LAG
PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2
Figure 4
In the figure 4, PE1 wants to forward some MPLS VPN traffic over an
explicit path to PE2 resulting in the following label stack to be
pushed onto the received IP header: <Adj_P1P2, Adj_set_P2P3,
Adj_P3P4, Adj_P4P5, Adj_P5P6, Adj_P6PE2, VPN_label>. PE1 is limited
to push a maximum of 11 labels (MSD=11). P2, P3 and P6 have an ERLD
of 3 while others have an ERLD of 10.
PE1 can only add two ELI/EL pairs in the label stack due to its MSD
limitation. It should insert them strategically to benefit load-
balancing along the longest part of the path.
PE1 may take into account multiple parameters when inserting ELs, as
examples:
o The ERLD value advertised by transit nodes.
o The requirement of load-balancing for a particular label value.
o Any service provider preference: favor beginning of the path or
end of the path.
In the figure 4, a good strategy may be to use the following stack
<Adj_P1P2, Adj_set_P2P3, ELI1, EL1, Adj_P3P4, Adj_P4P5, Adj_P5P6,
Adj_P6PE2, VPN_label>. The original stack requests P2 to forward
based on a L3 adjacency set that will require load-balancing.
Therefore it is important to ensure that P2 can load-balance
correctly. As P2 has a limited ERLD of 3, ELI/EL must be inserted
just next to the label that P2 will use to forward. On the path to
PE2, P3 has also a limited ERLD, but P3 will forward based on a basic
adjacency segment that may require no load-balancing. Therefore it
does not seem important to ensure that P3 can do load-balancing
despite of its limited ERLD. The next nodes along the forwarding
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path have a high ERLD that does not cause any issue, except P6,
moreover P6 is using some LAGs to PE2 and so is expected to load-
balance. It becomes important to insert a new ELI/EL just next to P6
forwarding label.
In the case above, the ingress node had enough label push capacity to
ensure end-to-end load-balancing taking into the path attributes.
There might be some cases, where the ingress node may not have the
necessary label imposition capacity.
7.1.2. Example 2 where the ingress node has not a sufficient MSD
ECMP LAG ECMP ECMP
PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- P7 --- P8 --- PE2
Figure 5
In the figure 5, PE1 wants to forward MPLS VPN traffic over an
explicit path to PE2 resulting in the following label stack to be
pushed onto the IP header: <Adj_P1P2, Adj_set_P2P3, Adj_P3P4,
Adj_P4P5, Adj_P5P6, Adj_set_P6P7, Adj_P7P8; Adj_set_P8PE2,
VPN_label>. PE1 is limited to push a maximum of 11 labels, P2, P3
and P6 have an ERLD of 3 while others have an ERLD of 15.
Using a similar strategy as the previous case may lead to a dilemma,
as PE1 can only push a single ELI/EL while we may need a minimum of
three to load-balance the end-to-end path. An optimized stack that
would enable end-to-end load-balancing may be: <Adj_P1P2,
Adj_set_P2P3, ELI1, EL1, Adj_P3P4, Adj_P4P5, Adj_P5P6, Adj_set_P6P7,
ELI2, EL2, Adj_P7P8; Adj_set_P8PE2, ELI3, EL3, VPN_label>.
A decision needs to be taken to favor some part of the path for load-
balancing considering that load-balancing may not work on the other
part. A service provider may decide to place the ELI/EL after the P6
forwarding label as it will allow P4 and P6 to load-balance. Placing
the ELI/EL at bottom of the stack is also a possibility enabling
load-balancing for P4 and P8.
7.2. Considerations for the placement of entropy labels
The sample cases described in the previous section showed that
placing the ELI/EL when the maximum number of labels to be pushed is
limited is not an easy decision and multiple criteria may be taken
into account.
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This section describes some considerations that could be taken into
account when placing ELI/ELs. This list of criteria is not
considered as exhaustive and an implementation MAY take into account
additional criteria or tie-breakers that are not documented here.
An implementation SHOULD try to maximize the load-balancing where
multiple ECMP paths are available and minimize the number of EL/ELIs
that need to be inserted. In case of a trade-off, an implementation
MAY provide flexibility to the operator to select the criteria to be
considered when placing EL/ELIs or the sub-objective for which to
optimize.
2 2
PE1 -- P1 -- P2 --P3 --- P4 --- P5 -- ... -- P8 -- P9 -- PE2
| |
P3'--- P4'--- P5'
Figure 6
The figure above will be used as reference in the following
subsections. All metrics are equal to 1, except P3-P4 and P4-P5
which have a metric 2.
7.2.1. ERLD value
As mentioned in Section 7.1, the ERLD value is an important parameter
to consider when inserting ELI/EL. If an ELI/EL does not fall within
the ERLD of a node on the path, the node will not be able to load-
balance the traffic efficiently.
The ERLD value can be advertised via protocols and those extensions
are described in separate documents [I-D.ietf-isis-mpls-elc] and
[I-D.ietf-ospf-mpls-elc].
Let's consider a path from PE1 to PE2 using the following stack
pushed by PE1: <Adj_P1P2, Node_P9, Adj_P9PE2, Service_label>.
Using the ERLD as an input parameter may help to minimize the number
of required ELI/EL pairs to be inserted. An ERLD value must be
retrieved for each SPRING label in the label stack.
For a label bound to an adjacency segment, the ERLD is the ERLD of
the node that advertised the adjacency segment. In the example
above, the ERLD associated with Adj_P1P2 would be the ERLD of router
P1 as P1 will perform the forwarding based on the Adj_P1P2 label.
For a label bound to a node segment, multiple strategies MAY be
implemented. An implementation may try to evaluate the minimum ERLD
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value along the node segment path. If an implementation cannot find
the minimum ERLD along the path of the segment, it can use the ERLD
of the starting node instead. In the example above, if the
implementation supports computation of minimum ERLD along the path,
the ERLD associated with label Node_P9 would be the minimum ERLD
between nodes {P2,P3,P4 ..., P8}. If an implementation does not
support the computation of minimum ERLD, it should consider the ERLD
of P2 (starting node that will forward based on the Node_P9 label).
For a label bound to a binding segment, if the binding segment
describes a path, an implementation may also try to evaluate the
minimum ERLD along this path. If the implementation cannot find the
minimum ERLD along the path of the segment, it can use the ERLD of
the starting node instead.
7.2.2. Segment type
Depending of the type of segment a particular label is bound to, an
implementation may deduce that this particular label will be subject
to load-balancing on the path.
7.2.2.1. Node-SID
An MPLS label bound to a Node-SID represents a path that may cross
multiple hops. Load-balancing may be needed on the node starting
this path but also on any node along the path.
In the figure 6, let's consider a path from PE1 to PE2 using the
following stack pushed by PE1: <Adj_P1P2, Node_P9, Adj_P9PE2,
Service_label>.
If, for example, PE1 is limited to push 6 labels, it can add a single
ELI/EL within the label stack. An operator may want to favor a
placement that would allow load-balancing along the Node-SID path.
In the figure above, P3 which is along the Node-SID path requires
load-balancing on two equal-cost paths.
An implementation may try to evaluate if load-balancing is really
required within a node segment path. This could be done by running
an additional SPT computation and analysis of the node segment path
to prevent a node segment that does not really require load-balancing
from being preferred when placing EL/ELIs. Such inspection may be
time consuming for implementations and without a 100% guarantee, as a
node segment path may use LAG that could be invisible from the IP
topology. A simpler approach would be to consider that a label bound
to a Node-SID will be subject to load-balancing and requires an EL/
ELI.
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7.2.2.2. Adjacency-set SID
An adjacency-set is an adjacency SID that refers to a set of
adjacencies. When an adjacency-set segment is used within a label
stack, an implementation can deduce that load-balancing is expected
at the node that advertised this adjacency segment. An
implementation could then favor this particular label value when
placing ELI/ELs.
7.2.2.3. Adjacency-SID representing a single IP link
When an adjacency segment representing a single IP link is used
within a label stack, an implementation can deduce that load-
balancing may not be expected at the node that advertised this
adjacency segment.
The implementation could then decide to place ELI/ELs to favor other
LSRs than the one advertising this adjacency segment.
Readers should note that an adjacency segment representing a single
IP link may require load-balancing. This is the case when a LAG (L2
bundle) is implemented between two IP nodes and the L2 bundle SR
extensions [I-D.ietf-isis-l2bundles] are not implemented. In such a
case, it may be useful to insert an EL/ELI in a readable position for
the LSR advertising the label associated with the adjacency segment.
7.2.2.4. Adjacency-SID representing a single link within a L2 bundle
When L2 bundle SR extensions [I-D.ietf-isis-l2bundles] are used,
adjacency segments may be advertised for each member of the bundle.
In this case, an implementation can deduce that load-balancing is not
expected on the LSR advertising this segment and could then decide to
place ELI/ELs to favor other LSRs than the one advertising this
adjacency segment.
7.2.2.5. Adjacency-SID representing a L2 bundle
When L2 bundle SR extensions [I-D.ietf-isis-l2bundles] are used, an
adjacency segment may be advertised to represent the bundle. In this
case, an implementation can deduce that load-balancing is expected on
the LSR advertising this segment and could then decide to place ELI/
ELs to favor this LSR.
7.2.3. Maximizing number of LSRs that will load-balance
When placing ELI/ELs, an implementation may try to maximize the
number of LSRs that both need to load-balance (i.e., have ECMP paths)
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and that will be able to perform load-balancing (i.e., the EL label
is within their ERLD).
Let's consider a path from PE1 to PE2 using the following stack
pushed by PE1: <Adj_P1P2, Node_P9, Adj_P9PE2, Service_label>. All
routers have an ERLD of 10, expect P1 and P2 which have an ERLD of 4.
PE1 is able to push 6 labels, so only a single ELI/EL can be added.
In the example above, adding ELI/EL next to Adj_P1P2 will only allow
load-balancing at P1 while inserting it next to Adj_PE2P9, will allow
load-balancing at P2,P3 ... P9 and maximizing the number of LSRs that
could perform load-balancing.
7.2.4. Preference for a part of the path
An implementation may propose to favor a part of the end-to-end path
when the number of EL/ELI that can be pushed is not enough to cover
the entire path. As example, a service provider may want to favor
load-balancing at the beginning of the path or at the end of path, so
the implementation should prefer putting the ELI/ELs near the top or
near of the bottom of the stack.
7.2.5. Combining criteria
An implementation can combine multiple criteria to determine the best
EL/ELIs placement. However, combining too many criteria may lead to
implementation complexity and high resource consumption. Each time
the network topology changes, a new evaluation of the EL/ELI
placement will be necessary for each impacted LSPs.
8. A simple example algorithm
A simple implementation might take into account ERLD when placing
ELI/EL while trying to minimize the number of EL/ELIs inserted and
trying to maximize the number of LSRs that can load-balance.
The example algorithm is based on the following considerations:
o An LSR that is limited in the number of <ELI, EL> pairs that it
can insert SHOULD insert such pairs deeper in the stack.
o An LSR should try to insert <ELI, EL> pairs at positions so that
for the maximum number of transit LSRs, the EL occurs within the
ERLD of those LSRs.
o An LSR should try to insert the minimum number of such pairs while
trying to satisfy the above criteria.
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The pseudocode of the example algorithm is shown below.
Initialize the current EL insertion point to the
bottommost label in the stack that is EL-capable
while (local-node can push more <ELI,EL> pairs OR
insertion point is not above label stack) {
insert an <ELI,EL> pair below current insertion point
move new insertion point up from current insertion point until
((last inserted EL is below the ERLD) AND (ERLD > 2)
AND
(new insertion point is EL-capable))
set current insertion point to new insertion point
}
Figure 7: Example algorithm to insert <ELI, EL> pairs in a label
stack
When this algorithm is applied to the example described in Section 3,
it will result in ELs being inserted in two positions, one below the
label L_N-D and another below L_N-P3. Thus the resulting label stack
would be <L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI, EL>
9. Deployment Considerations
As long as LSR node dataplane capabilities are limited (number of
labels that can be pushed, or number of labels that can be
inspected), hop-by-hop load-balancing of SPRING encapsulated flows
will require trade-offs.
Entropy label is still a good and usable solution as it allows load-
balancing without having to perform a deep packet inspection on each
LSR: it does not seem reasonable to have an LSR inspecting UDP ports
within a GRE tunnel carried over a 15 label SPRING tunnel.
Due to the limited capacity of reading a deep stack of MPLS labels,
multiple EL/ELIs may be required within the stack which directly
impacts the capacity of the head-end to push a deep stack: each EL/
ELI inserted requires two additional labels to be pushed.
Placement strategies of EL/ELIs are required to find the best trade-
off. Multiple criteria may be taken into account and some level of
customization (by the user) may be required to accommodate the
different deployments. Analyzing the path of each destination to
determine the best EL/ELI placement may be time consuming for the
control plane, we encourage implementations to find the best trade-
off between simplicity, resource consumption, and load-balancing
efficiency.
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In future, hardware and software capacity may increase dataplane
capabilities and may be remove some of these limitations, increasing
load-balancing capability using entropy labels.
10. Options considered
Different options that were considered to arrive at the recommended
solution are documented in this section.
These options are detailed here only for historical purposes.
10.1. Single EL at the bottom of the stack
In this option, a single EL is used for the entire label stack. The
source LSR S encodes the entropy label at the bottom of the label
stack. In the example described in Section 3, it will result in the
label stack at LSR S to look like <L_N-P3, L_A-L1, L_N-D, ELI, EL>
<remaining packet header>. Note that the notation in [RFC6790] is
used to describe the label stack. An issue with this approach is
that as the label stack grows due an increase in the number of SIDs,
the EL goes correspondingly deeper in the label stack. Hence,
transit LSRs have to access a larger number of bytes in the packet
header when making forwarding decisions. In the example described in
Section 3, if we consider that the LSR P1 has an ERLD of 3, P1 would
load-balance traffic poorly on the parallel links L3 and L4 since the
EL is below the ERLD of P1. A load-balanced network design using
this approach must ensure that all intermediate LSRs have the
capability to read the maximum label stack depth as required for the
application that uses source routed stacking.
This option was rejected since there exist a number of hardware
implementations which have a low maximum readable label depth.
Choosing this option can lead to a loss of load-balancing using EL in
a significant part of the network when that is a critical requirement
in a service-provider network.
10.2. An EL per segment in the stack
In this option, each segment/label in the stack can be given its own
EL. When load-balancing is required to direct traffic on a segment,
the source LSR pushes an <ELI, EL> before pushing the label
associated to this segment . In the example described in Section 3,
the source LSR S encoded label stack would be <L_N-P3, ELI, EL, L_A-
L1, L_N-D, ELI, EL> where all the ELs can be the same. Accessing the
EL at an intermediate LSR is independent of the depth of the label
stack and hence independent of the specific application that uses
source routed tunnels with label stacking. A drawback is that the
depth of the label stack grows significantly, almost 3 times as the
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number of labels in the label stack. The network design should
ensure that source LSRs have the capability to push such a deep label
stack. Also, the bandwidth overhead and potential MTU issues of deep
label stacks should be considered in the network design.
This option was rejected due to the existence of hardware
implementations that can push a limited number of labels on the label
stack. Choosing this option would result in a hardware requirement
to push two additional labels per tunnel label. Hence it would
restrict the number of tunnels that can be stacked in a LSP and hence
constrain the types of LSPs that can be created. This was considered
unacceptable.
10.3. A re-usable EL for a stack of tunnels
In this option an LSR that terminates a tunnel re-uses the EL of the
terminated tunnel for the next inner tunnel. It does this by storing
the EL from the outer tunnel when that tunnel is terminated and re-
inserting it below the next inner tunnel label during the label swap
operation. The LSR that stacks tunnels should insert an EL below the
outermost tunnel. It should not insert ELs for any inner tunnels.
Also, the penultimate hop LSR of a segment must not pop the ELI and
EL even though they are exposed as the top labels since the
terminating LSR of that segment would re-use the EL for the next
segment.
In Section 3 above, the source LSR S encoded label stack would be
<L_N-P3, ELI, EL, L_A-L1, L_N-D>. At P1, the outgoing label stack
would be <L_N-P3, ELI, EL, L_A-L1, L_N-D> after it has load-balanced
to one of the links L3 or L4. At P3 the outgoing label stack would
be <L_N-D, ELI, EL>. At P2, the outgoing label stack would be <L_N-
D, ELI, EL> and it would load-balance to one of the nexthop LSRs P4
or P5. Accessing the EL at an intermediate LSR (e.g., P1) is
independent of the depth of the label stack and hence independent of
the specific use-case to which the label stack is applied.
This option was rejected due to the significant change in label swap
operations that would be required for existing hardware.
10.4. EL at top of stack
A slight variant of the re-usable EL option is to keep the EL at the
top of the stack rather than below the tunnel label. In this case,
each LSR that is not terminating a segment should continue to keep
the received EL at the top of the stack when forwarding the packet
along the segment. An LSR that terminates a segment should use the
EL from the terminated segment at the top of the stack when
forwarding onto the next segment.
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This option was rejected due to the significant change in label swap
operations that would be required for existing hardware.
10.5. ELs at readable label stack depths
In this option the source LSR inserts ELs for tunnels in the label
stack at depths such that each LSR along the path that must load
balance is able to access at least one EL. Note that the source LSR
may have to insert multiple ELs in the label stack at different
depths for this to work since intermediate LSRs may have differing
capabilities in accessing the depth of a label stack. The label
stack depth access value of intermediate LSRs must be known to create
such a label stack. How this value is determined is outside the
scope of this document. This value can be advertised using a
protocol such as an IGP.
Applying this method to the example in Section 3 above, if LSR P1
needs to have the EL within a depth of 4, then the source LSR S
encoded label stack would be <L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI,
EL> where all the ELs would typically have the same value.
In the case where the ERLD has different values along the path and
the LSR that is inserting <ELI, EL> pairs has no limit on how many
pairs it can insert, and it knows the appropriate positions in the
stack where they should be inserted, this option is the same as the
recommended solution in Section 7.
Note that a refinement of this solution which balances the number of
pushed labels against the desired entropy is the solution described
in Section 7.
11. Acknowledgements
The authors would like to thank John Drake, Loa Andersson, Curtis
Villamizar, Greg Mirsky, Markus Jork, Kamran Raza, Carlos Pignataro,
Bruno Decraene, Chris Bowers and Nobo Akiya for their review comments
and suggestions.
12. Contributors
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Xiaohu Xu
Huawei
Email: xuxiaohu@huawei.com
Wim Hendrickx
Nokia
Email: wim.henderickx@nokia.com
Gunter Van De Velde
Nokia
Email: gunter.van_de_velde@nokia.com
Acee Lindem
Cisco
Email: acee@cisco.com
13. IANA Considerations
This memo includes no request to IANA. Note to RFC Editor: Remove
this section before publication.
14. Security Considerations
This document does not introduce any new security considerations
beyond those already listed in [RFC6790].
15. References
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
<https://www.rfc-editor.org/info/rfc6790>.
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[RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
Litkowski, S., Horneffer, M., and R. Shakir, "Source
Packet Routing in Networking (SPRING) Problem Statement
and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
2016, <https://www.rfc-editor.org/info/rfc7855>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-15 (work
in progress), January 2018.
[I-D.ietf-spring-segment-routing-mpls]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing with MPLS
data plane", draft-ietf-spring-segment-routing-mpls-11
(work in progress), October 2017.
15.2. Informative References
[I-D.ietf-isis-mpls-elc]
Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability and
Readable Label-stack Depth Using IS-IS", draft-ietf-isis-
mpls-elc-03 (work in progress), January 2018.
[I-D.ietf-ospf-mpls-elc]
Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability and
Readable Label-stack Depth Using OSPF", draft-ietf-ospf-
mpls-elc-05 (work in progress), January 2018.
[I-D.ietf-isis-l2bundles]
Ginsberg, L., Bashandy, A., Filsfils, C., Nanduri, M., and
E. Aries, "Advertising L2 Bundle Member Link Attributes in
IS-IS", draft-ietf-isis-l2bundles-07 (work in progress),
May 2017.
Authors' Addresses
Sriganesh Kini
EMail: sriganeshkini@gmail.com
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Kireeti Kompella
Juniper
EMail: kireeti@juniper.net
Siva Sivabalan
Cisco
EMail: msiva@cisco.com
Stephane Litkowski
Orange
EMail: stephane.litkowski@orange.com
Rob Shakir
Google
EMail: rjs@rob.sh
Jeff Tantsura
EMail: jefftant@gmail.com
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