draft-ietf-mpls-spring-entropy-label-06.txt   draft-ietf-mpls-spring-entropy-label-07.txt 
Network Working Group S. Kini Network Working Group S. Kini
Internet-Draft Internet-Draft
Intended status: Informational K. Kompella Intended status: Informational K. Kompella
Expires: November 4, 2017 Juniper Expires: April 20, 2018 Juniper
S. Sivabalan S. Sivabalan
Cisco Cisco
S. Litkowski S. Litkowski
Orange Orange
R. Shakir R. Shakir
Google Google
J. Tantsura J. Tantsura
May 3, 2017 October 17, 2017
Entropy label for SPRING tunnels Entropy label for SPRING tunnels
draft-ietf-mpls-spring-entropy-label-06 draft-ietf-mpls-spring-entropy-label-07
Abstract Abstract
Source routed tunnels with label stacking is a technique that can be Segment Routing (SR) leverages the source routing paradigm. A node
leveraged to steer a packet through a controlled set of segments. steers a packet through an ordered list of instructions, called
This can be applied to the Multi Protocol Label Switching (MPLS) data segments. Segment Routing can be applied to the Multi Protocol Label
plane. Entropy label (EL) is a technique used in MPLS to improve Switching (MPLS) data plane. Entropy label (EL) is a technique used
load-balancing. This document examines and describes how ELs are to in MPLS to improve load-balancing. This document examines and
be applied to source routed tunnels with label stacks. describes how ELs are to be applied to Segment Routing when applied
to the MPLS dataplane.
Status of This Memo Status of This Memo
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This Internet-Draft will expire on November 4, 2017. This Internet-Draft will expire on April 20, 2018.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Abbreviations and Terminology . . . . . . . . . . . . . . . . 3 2. Abbreviations and Terminology . . . . . . . . . . . . . . . . 4
3. Use-case requiring multipath load-balancing . . . . . . . . . 4 3. Use-case requiring multipath load-balancing . . . . . . . . . 4
4. Entropy Readable Label Depth . . . . . . . . . . . . . . . . 5 4. Entropy Readable Label Depth . . . . . . . . . . . . . . . . 5
5. Maximum SID Depth . . . . . . . . . . . . . . . . . . . . . . 7 5. Maximum SID Depth . . . . . . . . . . . . . . . . . . . . . . 7
6. LSP stitching using the binding SID . . . . . . . . . . . . . 8 6. LSP stitching using the binding SID . . . . . . . . . . . . . 8
7. Insertion of entropy labels for SPRING path . . . . . . . . . 10 7. Insertion of entropy labels for SPRING path . . . . . . . . . 10
7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . 11 7.1.1. Example 1 where the ingress node has a sufficient MSD 11
7.1.2. Example 2 . . . . . . . . . . . . . . . . . . . . . . 12 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. Considerations for the placement of entropy labels . . . 12
7.2.1. ERLD value . . . . . . . . . . . . . . . . . . . . . 13 7.2.1. ERLD value . . . . . . . . . . . . . . . . . . . . . 13
7.2.2. Segment type . . . . . . . . . . . . . . . . . . . . 14 7.2.2. Segment type . . . . . . . . . . . . . . . . . . . . 14
7.2.2.1. Node-SID . . . . . . . . . . . . . . . . . . . . 14 7.2.2.1. Node-SID . . . . . . . . . . . . . . . . . . . . 14
7.2.2.2. Adjacency-SID representing an ECMP bundle . . . . 14 7.2.2.2. Adjacency-set SID . . . . . . . . . . . . . . . . 15
7.2.2.3. Adjacency-SID representing a single IP link . . . 15 7.2.2.3. Adjacency-SID representing a single IP link . . . 15
7.2.2.4. Adjacency-SID representing a single link within 7.2.2.4. Adjacency-SID representing a single link within a
an L2 bundle . . . . . . . . . . . . . . . . . . 15 L2 bundle . . . . . . . . . . . . . . . . . . . . 15
7.2.2.5. Adjacency-SID representing an 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 7.2.3. Maximizing number of LSRs that will load-balance . . 15
7.2.4. Preference for a part of the path . . . . . . . . . . 16 7.2.4. Preference for a part of the path . . . . . . . . . . 16
7.2.5. Combining criteria . . . . . . . . . . . . . . . . . 16 7.2.5. Combining criteria . . . . . . . . . . . . . . . . . 16
8. A simple algorithm example . . . . . . . . . . . . . . . . . 16 8. A simple example algorithm . . . . . . . . . . . . . . . . . 16
9. Deployment Considerations . . . . . . . . . . . . . . . . . . 17 9. Deployment Considerations . . . . . . . . . . . . . . . . . . 17
10. Options considered . . . . . . . . . . . . . . . . . . . . . 18 10. Options considered . . . . . . . . . . . . . . . . . . . . . 18
10.1. Single EL at the bottom of the stack of tunnels . . . . 18 10.1. Single EL at the bottom of the stack . . . . . . . . . . 18
10.2. An EL per tunnel in 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 10.3. A re-usable EL for a stack of tunnels . . . . . . . . . 19
10.4. EL at top of stack . . . . . . . . . . . . . . . . . . . 20 10.4. EL at top of stack . . . . . . . . . . . . . . . . . . . 19
10.5. ELs at readable label stack depths . . . . . . . . . . . 20 10.5. ELs at readable label stack depths . . . . . . . . . . . 20
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21 12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
14. Security Considerations . . . . . . . . . . . . . . . . . . . 21 14. Security Considerations . . . . . . . . . . . . . . . . . . . 21
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
15.1. Normative References . . . . . . . . . . . . . . . . . . 21 15.1. Normative References . . . . . . . . . . . . . . . . . . 21
15.2. Informative References . . . . . . . . . . . . . . . . . 22 15.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction 1. Introduction
The source routed tunnels with label stacking paradigm is leveraged Segment Routing [I-D.ietf-spring-segment-routing] is based on source
by techniques such as Segment Routing (SR) routed tunnels to steer a packet along a particular path. This path
[I-D.ietf-spring-segment-routing] to steer a packet through a set of is encoded as an ordered list of segments. When applied to the MPLS
segments. This can be directly applied to the MPLS data plane, but dataplane [I-D.ietf-spring-segment-routing-mpls], each segment is an
it has implications on the label stack depth. 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.
Clarifying statements on label stack depth have been provided in Using label stacking to encode the list of segment has implications
[RFC7325] but the RFC does not address the case of source routed on the label stack depth.
stacked MPLS tunnels as described in
[I-D.ietf-spring-segment-routing] where deeper label stacks are more
prevalent.
Entropy label (EL) [RFC6790] is a technique used in the MPLS data Entropy label (EL) [RFC6790] is a technique used in the MPLS data
plane to provide entropy for load-balancing. When using LSP plane to provide entropy for load-balancing. When using LSP
hierarchies, there are implications on how [RFC6790] should be hierarchies, there are implications on how [RFC6790] should be
applied. The current document addresses the case where the hierarchy applied. The current document addresses the case where a hierarchy
is created at a single LSR as required by source routed tunnels with is created at a single LSR as required by Segment Routing.
label stacks.
A use-case requiring load-balancing with source routed tunnels with A use-case requiring load-balancing with SR is given in Section 3. A
label stacks is given in Section 3. A recommended solution is recommended solution is described in Section 7 keeping in
described in Section 7 keeping in consideration the limitations of consideration the limitations of implementations when applying
implementations when applying [RFC6790] to deeper label stacks. [RFC6790] to deeper label stacks. Options that were considered to
Options that were considered to arrive at the recommended solution arrive at the recommended solution are documented for historical
are documented for historical purposes in Section 10. purposes in Section 10.
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Although this document is not a protocol specification, the use of Although this document is not a protocol specification, the use of
this language clarifies the instructions to protocol designers this language clarifies the instructions to protocol designers
producing solutions that satisfy the requirements set out in this producing solutions that satisfy the requirements set out in this
skipping to change at page 4, line 43 skipping to change at page 4, line 50
| S |-----| P1 |------------| P2 |--+ +--| D | | S |-----| P1 |------------| P2 |--+ +--| D |
| | | | | |--+ +--| | | | | | | |--+ +--| |
+-----+ +-----+ +-----+ | +----+ | +-----+ +-----+ +-----+ +-----+ | +----+ | +-----+
+--| P5 |--+ +--| P5 |--+
+----+ +----+
S=Source LSR, D=Destination LSR, P1,P2,P3,P4,P5=Transit LSRs, S=Source LSR, D=Destination LSR, P1,P2,P3,P4,P5=Transit LSRs,
L1,L2,L3,L4=Links L1,L2,L3,L4=Links
Figure 1: Traffic engineering use-case Figure 1: Traffic engineering use-case
Traffic-engineering (TE) is one of the applications of MPLS and is Traffic-engineering is one of the applications of MPLS and is also a
also a requirement for source routed tunnels with label stacks requirement for source routed tunnels with label stacks [RFC7855].
[RFC7855]. Consider the topology shown in Figure 1. The LSR S
requires data to be sent to LSR D along a traffic-engineered path Consider the topology shown in Figure 1. The LSR S requires data to
that goes over the link L1. Good load-balancing is also required be sent to LSR D along a traffic-engineered path that goes over the
across equal cost paths (including parallel links). To engineer link L1. Good load-balancing is also required across equal cost
traffic along a path that takes link L1, the label stack that LSR S paths (including parallel links). To engineer traffic along a path
creates consists of a label to the node SID of LSR P3, stacked over that takes link L1, the label stack that LSR S creates consists of a
the label for the adjacency SID of link L1 and that in turn is label to the node SID of LSR P3, stacked over the label for the
stacked over the label to the node SID of LSR D. For simplicity lets adjacency SID of link L1 and that in turn is stacked over the label
assume that all LSRs use the same label space (SRGB) for source to the node SID of LSR D. For simplicity lets assume that all LSRs
routed label stacks. Let L_N-Px denote the label to be used to reach use the same label space (SRGB) for source routed label stacks. Let
the node SID of LSR Px. Let L_A-Ln denote the label used for the L_N-Px denote the label to be used to reach the node SID of LSR Px.
adjacency SID for link Ln. The LSR S must use the label stack <L_N- Let L_A-Ln denote the label used for the adjacency SID for link Ln.
P3, L_A-L1, L_N-D> for traffic-engineering. However to achieve good The LSR S must use the label stack <L_N-P3, L_A-L1, L_N-D> for
load-balancing over the equal cost paths P2-P4-D, P2-P5-D and the traffic-engineering. However to achieve good load-balancing over the
parallel links L3, L4, a mechanism such as Entropy labels [RFC6790] equal cost paths P2-P4-D, P2-P5-D and the parallel links L3, L4, a
should be adapted for source routed label stacks. Indeed, the SPRING mechanism such as Entropy labels [RFC6790] should be adapted for
architecture with the MPLS dataplane uses nested MPLS LSPs composing source routed label stacks. Indeed, the SPRING architecture with the
the source routed label stacks. As each MPLS node may have MPLS dataplane ([I-D.ietf-spring-segment-routing-mpls]) uses nested
limitations in the number of labels it can push when it is ingress or MPLS LSPs composing the source routed label stacks. As each MPLS
inspect when doing load-balancing, an entropy label insertion node may have limitations in the number of labels it can push when it
strategy becomes important to keep the benefit of the load-balancing. is ingress or inspect when doing load-balancing, an entropy label
Multiple ways to apply entropy labels were considered and are insertion strategy becomes important to keep the benefit of the load-
documented in Section 10 along with their trade-offs. A recommended balancing. Multiple ways to apply entropy labels were considered and
solution is described in Section 7. are documented in Section 10 along with their trade-offs. A
recommended solution is described in Section 7.
4. Entropy Readable Label Depth 4. Entropy Readable Label Depth
The Entropy Readable Label Depth (ERLD) is defined as the number of The Entropy Readable Label Depth (ERLD) is defined as the number of
labels a router can both: labels a router can both:
a. Read in an MPLS packet received on its incoming interface(s) a. Read in an MPLS packet received on its incoming interface(s)
(starting from the top of the stack). (starting from the top of the stack).
b. Use in its load-balancing function. b. Use in its load-balancing function.
skipping to change at page 6, line 25 skipping to change at page 6, line 25
| EL | | ELI | | Label 30 | | Label 30 | | Label 30 | | EL | | ELI | | Label 30 | | Label 30 | | Label 30 |
+----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+
| ELI | | Label 20 | | Label 20 | | Label 20 | | Label 20 | | ELI | | Label 20 | | Label 20 | | Label 20 | | Label 20 |
+----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+
| Label 16 | | Label 16 | | Label 16 | | Label 16 | | Label 16 | P1 | Label 16 | | Label 16 | | Label 16 | | Label 16 | | Label 16 | P1
+----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+ +----------+
Packet 1 Packet 2 Packet 3 Packet 4 Packet 5 Packet 1 Packet 2 Packet 3 Packet 4 Packet 5
Figure 2: Label stacks with ELI/EL Figure 2: Label stacks with ELI/EL
In the figure below, we consider the displayed packets received on a In the figure 2, we consider the displayed packets received on a
router interface. We consider also a single ERLD value for the router interface. We consider also a single ERLD value for the
router. router.
o If the router has an ERLD of 3, it will be able to load-balance 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- 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 balancing keys. The ERLD value of 3 means that the router can
read and take into account the entropy label for load-balancing if read and take into account the entropy label for load-balancing if
it is placed between position 1 (top) and position 3. 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 o If the router has an ERLD of 5, it will be able to load-balance
skipping to change at page 7, line 6 skipping to change at page 7, line 6
load-balancing keys. load-balancing keys.
To allow an efficient load-balancing based on entropy labels, a To allow an efficient load-balancing based on entropy labels, a
router running SPRING SHOULD advertise its ERLD (or ERLDs), so all router running SPRING SHOULD advertise its ERLD (or ERLDs), so all
the other SPRING routers in the network are aware of its capability. the other SPRING routers in the network are aware of its capability.
How this advertisement is done is outside the scope of this document. How this advertisement is done is outside the scope of this document.
To advertise an ERLD value, a SPRING router: To advertise an ERLD value, a SPRING router:
o MUST be entropy label capable and, as a consequence, MUST apply o MUST be entropy label capable and, as a consequence, MUST apply
all the procedures defined in [RFC6790]. the dataplane procedures defined in [RFC6790].
o MUST be able to read an ELI/EL which is located within its ERLD o MUST be able to read an ELI/EL which is located within its ERLD
value. value.
o MUST take into account this EL in its load-balancing function. o MUST take into account this EL in its load-balancing function.
5. Maximum SID Depth 5. Maximum SID Depth
The Maximum SID Depth defines the maximum number of labels that a The Maximum SID Depth defines the maximum number of labels that a
particular node can impose on a packet. This includes any kind of particular node can impose on a packet. This includes any kind of
skipping to change at page 7, line 50 skipping to change at page 7, line 50
/ \ / \
PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2 PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2
| \ | | \ |
----> P10 \ | ----> P10 \ |
IP Pkt | \ | IP Pkt | \ |
P11 --- P12 --- P13 P11 --- P12 --- P13
100 10000 100 10000
Figure 3 Figure 3
In the Figure 3, an IP packet comes in the MPLS network at PE1. All 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 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 P11-P12 which is 100. PE1 wants to steer the traffic using a SPRING
path to PE2 along path to PE2 along
PE1->P1->P7->P8->P9->P4->P5->P10->P11->P12->P13->PE2. By using PE1->P1->P7->P8->P9->P4->P5->P10->P11->P12->P13->PE2. By using
Adjacency SIDs only, PE1 will be required to push (as an I-LSR) 10 adjacency SIDs only, PE1 (acting as an I-LSR) will be required to
labels on the IP packet received and so requires an MSD of 10. If push 10 labels on the IP packet received and thus requires an MSD of
the IP packet should be carried over an MPLS service like a regular 10. If the IP packet should be carried over an MPLS service like a
layer 3 VPN, an additional service label will be imposed, requiring regular layer 3 VPN, an additional service label should be imposed,
an MSD of 11 for PE1. In addition, if PE1 wants to insert an ELI/EL requiring an MSD of 11 for PE1. In addition, if PE1 wants to insert
for load-balancing purpose, PE1 will need to push 13 labels on the IP an ELI/EL for load-balancing purpose, PE1 will need to push 13 labels
packet requiring an MSD of 13. on the IP packet requiring an MSD of 13.
In the SPRING architecture, Node SIDs or Binding SIDs can be used to 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 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 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 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 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. 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 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 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 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 label stack, P5's MSD needs also to be taken into account and may
limit the number of labels that could be imposed. limit the number of labels that could be imposed.
6. LSP stitching using the binding SID 6. LSP stitching using the binding SID
The binding SID allows binding a segment identifier to an existing The binding SID allows binding a segment identifier to an existing
LSP. As examples, the binding SID can represent an RSVP-TE tunnel, LSP. As examples, the binding SID can represent an RSVP-TE tunnel,
an LDP path (through the mapping server advertisement), a SPRING an LDP path (through the mapping server advertisement), or a SPRING
path... Each LSP associated with a binding SID has its own entropy path. Each LSP associated with a binding SID has its own entropy
label capability. label capability.
In the figure 3, if we consider that: In the figure 3, we consider that:
o P6, PE2, P10, P11, P12 are pure LDP routers. 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 PE1, P1, P2, P3, P4, P7, P8, P9 are pure SPRING routers.
o P5 is running SPRING and LDP. o P5 is running SPRING and LDP.
o P5 acts as a mapping server (MS) and advertises Prefix SIDs for o P5 acts as a mapping server and advertises Prefix SIDs for the LDP
the LDP FECs: an index value of 20 is used for PE2. FECs: an index value of 20 is used for PE2.
o All SPRING routers use an SRGB of [1000, 1999]. o All SPRING routers use an SRGB of [1000, 1999].
o P6 advertises label 20 for the PE2 FEC. o P6 advertises label 20 for the PE2 FEC.
o Traffic from PE1 to PE2 uses the shortest path. o Traffic from PE1 to PE2 uses the shortest path.
PE1 ----- P1 -- P2 -- P3 -- P4 ---- P5 --- P6 --- PE2 PE1 ----- P1 -- P2 -- P3 -- P4 ---- P5 --- P6 --- PE2
--> +----+ +----+ +----+ +----+ --> +----+ +----+ +----+ +----+
IP Pkt | IP | | IP | | IP | | IP | IP Pkt | IP | | IP | | IP | | IP |
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
|1020| |1020| | 20 | |1020| |1020| | 20 |
+----+ +----+ +----+ +----+ +----+ +----+
SPRING LDP SPRING LDP
In term of packet forwarding, by learning the MS advertisement from In term of packet forwarding, by learning the mapping-server
PE5, PE1 imposes a label 1020 to an IP packet destinated to PE2. advertisement from PE5, PE1 imposes a label 1020 to an IP packet
SPRING routers along the shortest path to PE2 will switch the traffic destinated to PE2. SPRING routers along the shortest path to PE2
until it reaches P5 which will perform the LSP stitching. P5 will will switch the traffic until it reaches P5 which will perform the
swap the SPRING label 1020 to the LDP label 20 advertised by the LSP stitching. P5 will swap the SPRING label 1020 to the LDP label
nexthop P6. P6 will then forward the packet using the LDP label 20 advertised by the nexthop P6. P6 will then forward the packet
towards PE2. using the LDP label towards PE2.
PE1 cannot push an ELI/EL for the binding SID without knowing that 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 the tail-end of the LSP associated with the binding (PE2) is entropy
label capable. label capable.
To accomodate the mix of signalling protocols involved during the To accomodate the mix of signalling protocols involved during the
stitching, the entropy label capability SHOULD be propagated between stitching, the entropy label capability SHOULD be propagated between
the signalling protocols. Each binding SID SHOULD have its own the signalling protocols. Each binding SID SHOULD have its own
entropy label capability that MUST be inherited from the entropy entropy label capability that MUST be inherited from the entropy
label capability of the associated LSP. If the router advertising label capability of the associated LSP. If the router advertising
skipping to change at page 10, line 9 skipping to change at page 10, line 9
The proposed solution only works if the SPRING router advertising the The proposed solution only works if the SPRING router advertising the
binding SID is also performing the dataplane LSP stitching. In our binding SID is also performing the dataplane LSP stitching. In our
example, if the mapping server function is hosted on P8 instead of 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 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. consequence, it does not set the ELC for the associated binding SID.
7. Insertion of entropy labels for SPRING path 7. Insertion of entropy labels for SPRING path
7.1. Overview 7.1. Overview
The solution described in this section follows [RFC6790]. Within a The solution described in this section follows the dataplane
SPRING path, a node may be ingress, egress, transit (regarding the processing defined in [RFC6790]. Within a SPRING path, a node may be
entropy label processing described in [RFC6790]), or it can be any ingress, egress, transit (regarding the entropy label processing
combination of those. For example: 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 o The ingress node of a SPRING domain may be an ingress node from an
entropy label perspective. entropy label perspective.
o Any LSR terminating a segment of the SPRING path is an egress node 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 (because it terminates the segment) but may also be a transit node
if the SPRING path is not terminated because there is a subsequent if the SPRING path is not terminated because there is a subsequent
SPRING MPLS label in the stack. SPRING MPLS label in the stack.
o Any LSR processing a binding SID may be a transit node and an o Any LSR processing a binding SID may be a transit node and an
ingress node (because it may push additional labels when ingress node (because it may push additional labels when
processing the binding SID). processing the binding SID).
As described earlier, an LSR may have a limitation, ERLD, on the 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 depth of the label stack that it can read and process in order to do
multipath load-balancing based on entropy labels. multipath load-balancing based on entropy labels.
If an EL does not occur within the ERLD of an LSR in the label stack 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- 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 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. 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 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 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 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. 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 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 corresponding to a SPRING label stack, multiple <ELI, EL> pairs MAY
be inserted in this label stack. be inserted in this label stack.
The insertion of the ELI/EL SHOULD occur only with a SPRING label 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 advertised by an LSR that advertised an ERLD (the LSR is entropy
label capable) or with a SPRING label associated with a binding SID label capable) or with a SPRING label associated with a binding SID
that has the ELC set. that has the ELC set.
The ELs among multiple <ELI, EL> pairs inserted in the stack MAY be 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 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 limitations on the number of such pairs that it can insert and also
the depth at which it can insert them. If due to limitations, the 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 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 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 LSR's ERLD, then the load-balancing performed by that LSR would be
poor. An implementation MAY consider multiple criteria when poor. An implementation MAY consider multiple criteria when
inserting <ELI, EL> pairs. inserting <ELI, EL> pairs.
7.1.1. Example 1 7.1.1. Example 1 where the ingress node has a sufficient MSD
ECMP LAG LAG ECMP LAG LAG
PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2 PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2
Figure 4 Figure 4
In the Figure 4, PE1 wants to forward some MPLS VPN traffic over an 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 explicit path to PE2 resulting in the following label stack to be
pushed onto the received IP header: {VPN_label, Adj_P6PE2, Adj_P5P6, pushed onto the received IP header: <Adj_P1P2, Adj_set_P2P3,
Adj_P4P5, Adj_P3P4, Adj_Bundle_P2P3, Adj_P1P2}. PE1 is limited to Adj_P3P4, Adj_P4P5, Adj_P5P6, Adj_P6PE2, VPN_label>. PE1 is limited
push a maximum of 11 labels (MSD=11). P2, P3 and P6 have an ERLD of to push a maximum of 11 labels (MSD=11). P2, P3 and P6 have an ERLD
3 while others have an ERLD of 10. 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 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- limitation. It should insert them strategically to benefit load-
balancing along the longest part of the path. balancing along the longest part of the path.
PE1 may take into account multiple parameters when inserting ELs, as PE1 may take into account multiple parameters when inserting ELs, as
examples: examples:
o The ERLD value advertised by transit nodes. o The ERLD value advertised by transit nodes.
o The requirement of load-balancing for a particular label value. o The requirement of load-balancing for a particular label value.
o Any service provider preference: favor beginning of the path or o Any service provider preference: favor beginning of the path or
end of the path. end of the path.
In the Figure 4, a good strategy may be to use the following stack In the figure 4, a good strategy may be to use the following stack
{VPN_label, ELI2,EL2, Adj_P6PE2, Adj_P5P6, Adj_P4P5, Adj_P3P4, ELI1, <Adj_P1P2, Adj_set_P2P3, ELI1, EL1, Adj_P3P4, Adj_P4P5, Adj_P5P6,
EL1, Adj_Bundle_P2P3, Adj_P1P2}. The original stack requests P2 to Adj_P6PE2, VPN_label>. The original stack requests P2 to forward
forward based on a bundle Adjacency segment that will require load- based on a L3 adjacency set that will require load-balancing.
balancing. Therefore it is important to ensure that P2 can load- Therefore it is important to ensure that P2 can load-balance
balance correctly. As P2 has a limited ERLD of 3, ELI/EL must be correctly. As P2 has a limited ERLD of 3, ELI/EL must be inserted
inserted just next to the label that P2 will use to forward. On the just next to the label that P2 will use to forward. On the path to
path to PE2, P3 has also a limited ERLD, but P3 will forward based on PE2, P3 has also a limited ERLD, but P3 will forward based on a basic
a basic adjacency segment that may require no load-balancing. adjacency segment that may require no load-balancing. Therefore it
Therefore it does not seem important to ensure that P3 can do load- does not seem important to ensure that P3 can do load-balancing
balancing despite of its limited ERLD. The next nodes along the despite of its limited ERLD. The next nodes along the forwarding
forwarding path have a high ERLD that does not cause any issue, path have a high ERLD that does not cause any issue, except P6,
except P6, moreover P6 is using some LAGs to PE2 and so is expected moreover P6 is using some LAGs to PE2 and so is expected to load-
to load-balance. It becomes important to insert a new ELI/EL just balance. It becomes important to insert a new ELI/EL just next to P6
next to P6 forwarding label. forwarding label.
In the case above, the ingress node had enough label push capacity to In the case above, the ingress node had enough label push capacity to
ensure end-to-end load-balancing taking into the path attributes. ensure end-to-end load-balancing taking into the path attributes.
There might be some cases, where the ingress node may not have the There might be some cases, where the ingress node may not have the
necessary label imposition capacity. necessary label imposition capacity.
7.1.2. Example 2 7.1.2. Example 2 where the ingress node has not a sufficient MSD
ECMP LAG ECMP ECMP ECMP LAG ECMP ECMP
PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- P7 --- P8 --- PE2 PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- P7 --- P8 --- PE2
Figure 5 Figure 5
In the Figure 5, PE1 wants to forward MPLS VPN traffic over an 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 explicit path to PE2 resulting in the following label stack to be
pushed onto the IP header: {VPN_label, Adj_Bundle_P8PE2, Adj_P7P8, pushed onto the IP header: <Adj_P1P2, Adj_set_P2P3, Adj_P3P4,
Adj_Bundle_P6P7, Adj_P5P6, Adj_P4P5, Adj_P3P4, Adj_Bundle_P2P3, Adj_P4P5, Adj_P5P6, Adj_set_P6P7, Adj_P7P8; Adj_set_P8PE2,
Adj_P1P2}. PE1 is limited to push a maximum of 11 labels, P2, P3 and VPN_label>. PE1 is limited to push a maximum of 11 labels, P2, P3
P6 have an ERLD of 3 while others have an ERLD of 15. 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, 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 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 three to load-balance the end-to-end path. An optimized stack that
would enable end-to-end load-balancing may be: {VPN_label, ELI3, EL3, would enable end-to-end load-balancing may be: <Adj_P1P2,
Adj_Bundle_P8PE2, Adj_P7P8, ELI2, EL2, Adj_Bundle_P6P7, Adj_P5P6, Adj_set_P2P3, ELI1, EL1, Adj_P3P4, Adj_P4P5, Adj_P5P6, Adj_set_P6P7,
Adj_P4P5, Adj_P3P4, ELI1, EL1, Adj_Bundle_P2P3, Adj_P1P2}. 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- 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 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 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 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 the ELI/EL at bottom of the stack is also a possibility enabling
load-balancing for P4 and P8. load-balancing for P4 and P8.
7.2. Considerations for the placement of entropy labels 7.2. Considerations for the placement of entropy labels
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limited is not an easy decision and multiple criteria may be taken limited is not an easy decision and multiple criteria may be taken
into account. into account.
This section describes some considerations that could be taken into This section describes some considerations that could be taken into
account when placing ELI/ELs. This list of criteria is not account when placing ELI/ELs. This list of criteria is not
considered as exhaustive and an implementation MAY take into account considered as exhaustive and an implementation MAY take into account
additional criteria or tie-breakers that are not documented here. additional criteria or tie-breakers that are not documented here.
An implementation SHOULD try to maximize the load-balancing where An implementation SHOULD try to maximize the load-balancing where
multiple ECMP paths are available and minimize the number of EL/ELIs multiple ECMP paths are available and minimize the number of EL/ELIs
that need to be inserted. In case of trade-off, an implementation 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 MAY provide flexibility to the operator to select the criteria to be
considered when placing EL/ELIs or the sub-objective for which to considered when placing EL/ELIs or the sub-objective for which to
optimize. optimize.
PE1 -- P1 -- P2 -- P3 -- P4 -- P5 -- ... -- P8 -- P9 -- PE2 2 2
PE1 -- P1 -- P2 --P3 --- P4 --- P5 -- ... -- P8 -- P9 -- PE2
| | | |
P3'--- P4'--- P5' P3'--- P4'--- P5'
Figure 6 Figure 6
The figure above will be used as reference in the following The figure above will be used as reference in the following
subsections. subsections. All metrics are equal to 1, except P3-P4 and P4-P5
which have a metric 2.
7.2.1. ERLD value 7.2.1. ERLD value
As mentioned in Section 7.1, the ERLD value is an important parameter As mentioned in Section 7.1, the ERLD value is an important parameter
to consider when inserting ELI/EL as if an ELI/EL does not fall to consider when inserting ELI/EL. If an ELI/EL does not fall within
within the ERLD of a node on the path, the node will not be able to the ERLD of a node on the path, the node will not be able to load-
load-balance the traffic efficiently. balance the traffic efficiently.
The ERLD value can be advertised via protocols and those extensions The ERLD value can be advertised via protocols and those extensions
are described in separate documents [I-D.ietf-isis-mpls-elc] and are described in separate documents [I-D.ietf-isis-mpls-elc] and
[I-D.ietf-ospf-mpls-elc]. [I-D.ietf-ospf-mpls-elc].
Let's consider a path from PE1 to PE2 using the following stack Let's consider a path from PE1 to PE2 using the following stack
pushed by PE1: {Service_label, Adj_PE2P9, Node_P9, Adj_P1P2}. pushed by PE1: <Adj_P1P2, Node_P9, Adj_P9PE2, Service_label>.
Using the ERLD as an input parameter may help to minimize the number 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 of required ELI/EL pairs to be inserted. An ERLD value must be
retrieved for each SPRING label in the label stack. retrieved for each SPRING label in the label stack.
For a label bound to an adjacency segment, the ERLD is the ERLD of For a label bound to an adjacency segment, the ERLD is the ERLD of
the node that advertised the adjacency segment. In the example the node that advertised the adjacency segment. In the example
above, the ERLD associated with Adj_P1P2 would be the ERLD of router 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. 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 For a label bound to a node segment, multiple strategies MAY be
implemented. An implementation may try to evaluate the minimum ERLD implemented. An implementation may try to evaluate the minimum ERLD
value along the node segment path. If an implementation cannot find 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 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 of the starting node instead. In the example above, if the
implementation supports computation of minimum ERLD along the path, implementation supports computation of minimum ERLD along the path,
the ERLD associated to label Node_P9 would be the minimum ERLD the ERLD associated with label Node_P9 would be the minimum ERLD
between nodes {P2,P3,P4 ..., P8}. If an implementation does not between nodes {P2,P3,P4 ..., P8}. If an implementation does not
support the computation of minimum ERLD, it should consider the ERLD support the computation of minimum ERLD, it should consider the ERLD
of P2 (starting node that will forward based on the Node_P9 label). 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 For a label bound to a binding segment, if the binding segment
describes a path, an implementation may also try to evaluate the 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 this path. If the implementation cannot find the
minimum ERLD along the path of the segment, it can use the ERLD of minimum ERLD along the path of the segment, it can use the ERLD of
the starting node instead. the starting node instead.
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Depending of the type of segment a particular label is bound to, an Depending of the type of segment a particular label is bound to, an
implementation may deduce that this particular label will be subject implementation may deduce that this particular label will be subject
to load-balancing on the path. to load-balancing on the path.
7.2.2.1. Node-SID 7.2.2.1. Node-SID
An MPLS label bound to a Node-SID represents a path that may cross 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 multiple hops. Load-balancing may be needed on the node starting
this path but also on any node along the path. this path but also on any node along the path.
Let's consider a path from PE1 to PE2 using the following stack In the figure 6, let's consider a path from PE1 to PE2 using the
pushed by PE1: {Service_label, Adj_PE2P9, Node_P9, Adj_P1P2}. following stack pushed by PE1: <Adj_P1P2, Node_P9, Adj_P9PE2,
Service_label>.
If, for example, PE1 is limited to pushing 6 labels, it can add a If, for example, PE1 is limited to push 6 labels, it can add a single
single ELI/EL within the label stack. An operator may want to favor ELI/EL within the label stack. An operator may want to favor a
a placement that would allow load-balancing along the Node-SID path. placement that would allow load-balancing along the Node-SID path.
In the figure above, P3 which is along the Node-SID path requires In the figure above, P3 which is along the Node-SID path requires
load-balancing on two equal-cost paths. load-balancing on two equal-cost paths.
An implementation may try to evaluate if load-balancing is really An implementation may try to evaluate if load-balancing is really
required within a node segment path. This could be done by running required within a node segment path. This could be done by running
an additional SPT computation and analysis of the node segment path an additional SPT computation and analysis of the node segment path
to prevent a node segment that does not really require load-balancing to prevent a node segment that does not really require load-balancing
from being preferred when placing EL/ELIs. Such inspection may be from being preferred when placing EL/ELIs. Such inspection may be
time consuming for implementations and without a 100% guarantee, as a time consuming for implementations and without a 100% guarantee, as a
node segment path may use LAG that could be invisible from the IP 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 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/ to a Node-SID will be subject to load-balancing and requires an EL/
ELI. ELI.
7.2.2.2. Adjacency-SID representing an ECMP bundle 7.2.2.2. Adjacency-set SID
When an adjacency segment representing an ECMP bundle is used within An adjacency-set is an adjacency SID that refers to a set of
a label stack, an implementation can deduce that load-balancing is adjacencies. When an adjacency-set segment is used within a label
expected at the node that advertised this adjacency segment. An 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 implementation could then favor this particular label value when
placing ELI/ELs. placing ELI/ELs.
7.2.2.3. Adjacency-SID representing a single IP link 7.2.2.3. Adjacency-SID representing a single IP link
When an adjacency segment representing a single IP link is used When an adjacency segment representing a single IP link is used
within a label stack, an implementation can deduce that load- within a label stack, an implementation can deduce that load-
balancing may not be expected at the node that advertised this balancing may not be expected at the node that advertised this
adjacency segment. adjacency segment.
The implementation could then decide to place ELI/ELs to favor other The implementation could then decide to place ELI/ELs to favor other
LSRs than the one advertising this adjacency segment. LSRs than the one advertising this adjacency segment.
Readers should note that an adjacency segment representing a single Readers should note that an adjacency segment representing a single
IP link may require load-balancing. This is the case when a LAG (L2 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 bundle) is implemented between two IP nodes and the L2 bundle SR
extensions [I-D.ietf-isis-l2bundles] are not implemented. In such extensions [I-D.ietf-isis-l2bundles] are not implemented. In such a
case, it may be interesting to keep the possibility to insert an EL/ case, it may be useful to insert an EL/ELI in a readable position for
ELI in a readable position for the LSR advertising the label the LSR advertising the label associated with the adjacency segment.
associated with the adjacency segment.
7.2.2.4. Adjacency-SID representing a single link within an L2 bundle 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, When L2 bundle SR extensions [I-D.ietf-isis-l2bundles] are used,
adjacency segments may be advertised for each member of the bundle. adjacency segments may be advertised for each member of the bundle.
In this case, an implementation can deduce that load-balancing is not In this case, an implementation can deduce that load-balancing is not
expected on the LSR advertising this segment and could then decide to expected on the LSR advertising this segment and could then decide to
place ELI/ELs to favor other LSRs than the one advertising this place ELI/ELs to favor other LSRs than the one advertising this
adjacency segment. adjacency segment.
7.2.2.5. Adjacency-SID representing an L2 bundle 7.2.2.5. Adjacency-SID representing a L2 bundle
When L2 bundle SR extensions [I-D.ietf-isis-l2bundles] are used, an When L2 bundle SR extensions [I-D.ietf-isis-l2bundles] are used, an
adjacency segment may be advertised to represent the bundle. In this adjacency segment may be advertised to represent the bundle. In this
case, an implementation can deduce that load-balancing is expected on case, an implementation can deduce that load-balancing is expected on
the LSR advertising this segment and could then decide to place ELI/ the LSR advertising this segment and could then decide to place ELI/
ELs to favor this LSR. ELs to favor this LSR.
7.2.3. Maximizing number of LSRs that will load-balance 7.2.3. Maximizing number of LSRs that will load-balance
When placing ELI/ELs, an implementation may try to maximize the 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) number of LSRs that both need to load-balance (i.e., have ECMP paths)
and that will be able to perform load-balancing (i.e., the EL label and that will be able to perform load-balancing (i.e., the EL label
is within their ERLD). is within their ERLD).
Let's consider a path from PE1 to PE2 using the following stack Let's consider a path from PE1 to PE2 using the following stack
pushed by PE1: {Service_label, Adj_PE2P9, Node_P9, Adj_P1P2}. All 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. 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. 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 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 P1 while inserting it next to Adj_PE2P9, will allow
load-balancing at P2,P3 ... P9 and maximizing the number of LSRs that load-balancing at P2,P3 ... P9 and maximizing the number of LSRs that
could perform load-balancing. could perform load-balancing.
7.2.4. Preference for a part of the path 7.2.4. Preference for a part of the path
An implementation may propose to favor a part of the end-to-end 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 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 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 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 the implementation should prefer putting the ELI/ELs near the top or
near of the bottom of the stack. near of the bottom of the stack.
7.2.5. Combining criteria 7.2.5. Combining criteria
An implementation can combine multiple criteria to determine the best An implementation can combine multiple criteria to determine the best
EL/ELIs placement. But combining too much criteria may lead to EL/ELIs placement. However, combining too many criteria may lead to
implementation complexity and high control plane resource implementation complexity and high resource consumption. Each time
consumption. Each time the network topology changes, a new the network topology changes, a new evaluation of the EL/ELI
evaluation of the EL/ELI placement will be necessary for each placement will be necessary for each impacted LSPs.
impacted LSPs.
8. A simple algorithm example 8. A simple example algorithm
A simple implementation can only take into account ERLD when placing A simple implementation might take into account ERLD when placing
ELI/EL while keep minimizing the number of EL/ELIs inserted and ELI/EL while trying to minimize the number of EL/ELIs inserted and
maximizing the number of LSRs that can load-balance. trying to maximize the number of LSRs that can load-balance.
The algorithm example is based on the following considerations: The example algorithm is based on the following considerations:
o An LSR that is limited in the number of <ELI, EL> pairs that it 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. can insert SHOULD insert such pairs deeper in the stack.
o An LSR should try to insert <ELI, EL> pairs at positions so that 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 for the maximum number of transit LSRs, the EL occurs within the
ERLD of those LSRs. ERLD of those LSRs.
o An LSR should try to insert the minimum number of such pairs while o An LSR should try to insert the minimum number of such pairs while
trying to satisfy the above criteria. trying to satisfy the above criteria.
The pseudocode of the example is shown below. The pseudocode of the example algorithm is shown below.
Initialize the current EL insertion point to the Initialize the current EL insertion point to the
bottommost label in the stack that is EL-capable bottommost label in the stack that is EL-capable
while (local-node can push more <ELI,EL> pairs OR while (local-node can push more <ELI,EL> pairs OR
insertion point is not above label stack) { insertion point is not above label stack) {
insert an <ELI,EL> pair below current insertion point insert an <ELI,EL> pair below current insertion point
move new insertion point up from current insertion point until move new insertion point up from current insertion point until
((last inserted EL is below the ERLD) AND (ERLD > 2) ((last inserted EL is below the ERLD) AND (ERLD > 2)
AND AND
(new insertion point is EL-capable)) (new insertion point is EL-capable))
set current insertion point to new insertion point set current insertion point to new insertion point
} }
Figure 7: Example algorithm to insert <ELI, EL> pairs in a label Figure 7: Example algorithm to insert <ELI, EL> pairs in a label
stack stack
When this algorithm is applied to the example described in Section 3, 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 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 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} would be <L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI, EL>
9. Deployment Considerations 9. Deployment Considerations
As long as LSR node dataplane capabilities with be limited (number of As long as LSR node dataplane capabilities are limited (number of
labels that can be pushed, or number of labels that can be labels that can be pushed, or number of labels that can be
inspected), hop-by-hop load-balancing of SPRING encapsulated flows inspected), hop-by-hop load-balancing of SPRING encapsulated flows
will require trade-offs. will require trade-offs.
Entropy label is still a good and usable solution as it allows load- Entropy label is still a good and usable solution as it allows load-
balancing without having to perform a deep packet inspection on each balancing without having to perform a deep packet inspection on each
LSR: it does not seem reasonable to have an LSR inspecting UDP ports LSR: it does not seem reasonable to have an LSR inspecting UDP ports
within a GRE tunnel carried over a 15 label SPRING tunnel. within a GRE tunnel carried over a 15 label SPRING tunnel.
Due to the limited capacity of reading a deep stack of MPLS labels, Due to the limited capacity of reading a deep stack of MPLS labels,
skipping to change at page 18, line 6 skipping to change at page 18, line 6
Placement strategies of EL/ELIs are required to find the best trade- Placement strategies of EL/ELIs are required to find the best trade-
off. Multiple criteria may be taken into account and some level of off. Multiple criteria may be taken into account and some level of
customization (by the user) may be required to accommodate the customization (by the user) may be required to accommodate the
different deployments. Analyzing the path of each destination to different deployments. Analyzing the path of each destination to
determine the best EL/ELI placement may be time consuming for the determine the best EL/ELI placement may be time consuming for the
control plane, we encourage implementations to find the best trade- control plane, we encourage implementations to find the best trade-
off between simplicity, resource consumption, and load-balancing off between simplicity, resource consumption, and load-balancing
efficiency. efficiency.
In future, hardware and software capacity may increase dataplane In future, hardware and software capacity may increase dataplane
capabilities and may be remove some of these limits, increasing load- capabilities and may be remove some of these limitations, increasing
balancing capability using entropy labels. load-balancing capability using entropy labels.
10. Options considered 10. Options considered
Different options that were considered to arrive at the recommended Different options that were considered to arrive at the recommended
solution are documented in this section. solution are documented in this section.
10.1. Single EL at the bottom of the stack of tunnels 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 In this option, a single EL is used for the entire label stack. The
source LSR S encodes the entropy label (EL) at the bottom of the source LSR S encodes the entropy label at the bottom of the label
label stack. In the example described in Section 3, it will result stack. In the example described in Section 3, it will result in the
in the label stack at LSR S to look like {L_N-P3, L_A-L1, L_N-D, ELI, label stack at LSR S to look like <L_N-P3, L_A-L1, L_N-D, ELI, EL>
EL} {remaining packet header}. Note that the notation in [RFC6790] <remaining packet header>. Note that the notation in [RFC6790] is
is used to describe the label stack. An issue with this approach 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, that as the label stack grows due an increase in the number of SIDs,
the EL goes correspondingly deeper in the label stack. Hence, the EL goes correspondingly deeper in the label stack. Hence,
transit LSRs have to access a larger number of bytes in the packet transit LSRs have to access a larger number of bytes in the packet
header when making forwarding decisions. In the example described in header when making forwarding decisions. In the example described in
Section 3, the LSR P1 would load-balance traffic poorly on the Section 3, if we consider that the LSR P1 has an ERLD of 3, P1 would
parallel links L3 and L4 since the EL is below the ERLD of the packet load-balance traffic poorly on the parallel links L3 and L4 since the
received by P1. A load-balanced network design using this approach EL is below the ERLD of P1. A load-balanced network design using
must ensure that all intermediate LSRs have the capability to this approach must ensure that all intermediate LSRs have the
traverse the maximum label stack depth as required for the capability to read the maximum label stack depth as required for the
application that uses source routed stacking. application that uses source routed stacking.
In the case where the hardware is capable of pushing a single <ELI,
EL> pair at any depth, this option is the same as the recommended
solution in Section 7.
This option was rejected since there exist a number of hardware This option was rejected since there exist a number of hardware
implementations which have a low maximum readable label depth. implementations which have a low maximum readable label depth.
Choosing this option can lead to a loss of load-balancing using EL in 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 a significant part of the network when that is a critical requirement
in a service-provider network. in a service-provider network.
10.2. An EL per tunnel in the stack 10.2. An EL per segment in the stack
In this option, each tunnel in the stack can be given its own EL.
The source LSR pushes an <ELI, EL> before pushing a tunnel label when
load-balancing is required to direct traffic on that tunnel. 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 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.
In the case where the RLD is the minimum value (3) for all LSRs, all In this option, each segment/label in the stack can be given its own
LSRs are EL capable and the LSR that is inserting <ELI, EL> pairs has EL. When load-balancing is required to direct traffic on a segment,
no limit on how many it can insert then this option is the same as the source LSR pushes an <ELI, EL> before pushing the label
the recommended solution in Section 7. 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
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 This option was rejected due to the existence of hardware
implementations that can push a limited number of labels on the label implementations that can push a limited number of labels on the label
stack. Choosing this option would result in a hardware requirement stack. Choosing this option would result in a hardware requirement
to push two additional labels per tunnel label. Hence it would 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 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 constrain the types of LSPs that can be created. This was considered
unacceptable. unacceptable.
10.3. A re-usable EL for a stack of tunnels 10.3. A re-usable EL for a stack of tunnels
skipping to change at page 19, line 38 skipping to change at page 19, line 31
the EL from the outer tunnel when that tunnel is terminated and re- 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 inserting it below the next inner tunnel label during the label swap
operation. The LSR that stacks tunnels should insert an EL below the operation. The LSR that stacks tunnels should insert an EL below the
outermost tunnel. It should not insert ELs for any inner tunnels. 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 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 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 terminating LSR of that segment would re-use the EL for the next
segment. segment.
In Section 3 above, the source LSR S encoded label stack would be 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 <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 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 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- 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 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 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 independent of the depth of the label stack and hence independent of
the specific use-case to which the label stack is applied. the specific use-case to which the label stack is applied.
This option was rejected due to the significant change in label swap This option was rejected due to the significant change in label swap
operations that would be required for existing hardware. operations that would be required for existing hardware.
10.4. EL at top of stack 10.4. EL at top of stack
A slight variant of the re-usable EL option is to keep the EL at the A slight variant of the re-usable EL option is to keep the EL at the
skipping to change at page 20, line 33 skipping to change at page 20, line 23
may have to insert multiple ELs in the label stack at different may have to insert multiple ELs in the label stack at different
depths for this to work since intermediate LSRs may have differing depths for this to work since intermediate LSRs may have differing
capabilities in accessing the depth of a label stack. The label capabilities in accessing the depth of a label stack. The label
stack depth access value of intermediate LSRs must be known to create stack depth access value of intermediate LSRs must be known to create
such a label stack. How this value is determined is outside the such a label stack. How this value is determined is outside the
scope of this document. This value can be advertised using a scope of this document. This value can be advertised using a
protocol such as an IGP. protocol such as an IGP.
Applying this method to the example in Section 3 above, if LSR P1 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 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, 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. EL> where all the ELs would typically have the same value.
In the case where the RLD has different values along the path and the In the case where the ERLD has different values along the path and
LSR that is inserting <ELI, EL> pairs has no limit on how many pairs the LSR that is inserting <ELI, EL> pairs has no limit on how many
it can insert, and it knows the appropriate positions in the stack pairs it can insert, and it knows the appropriate positions in the
where they should be inserted, this option is the same as the stack where they should be inserted, this option is the same as the
recommended solution in Section 7. recommended solution in Section 7.
Note that a refinement of this solution which balances the number of Note that a refinement of this solution which balances the number of
pushed labels against the desired entropy is the solution described pushed labels against the desired entropy is the solution described
in Section 7. in Section 7.
11. Acknowledgements 11. Acknowledgements
The authors would like to thank John Drake, Loa Andersson, Curtis The authors would like to thank John Drake, Loa Andersson, Curtis
Villamizar, Greg Mirsky, Markus Jork, Kamran Raza, Carlos Pignataro, Villamizar, Greg Mirsky, Markus Jork, Kamran Raza, Carlos Pignataro,
Bruno Decraene and Nobo Akiya for their review comments and Bruno Decraene, Chris Bowers and Nobo Akiya for their review comments
suggestions. and suggestions.
12. Contributors 12. Contributors
Xiaohu Xu Xiaohu Xu
Huawei Huawei
Email: xuxiaohu@huawei.com Email: xuxiaohu@huawei.com
Wim Hendrickx Wim Hendrickx
Nokia Nokia
Email: wim.henderickx@nokia.com Email: wim.henderickx@nokia.com
skipping to change at page 21, line 44 skipping to change at page 21, line 41
This document does not introduce any new security considerations This document does not introduce any new security considerations
beyond those already listed in [RFC6790]. beyond those already listed in [RFC6790].
15. References 15. References
15.1. Normative References 15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and [RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding", L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012, RFC 6790, DOI 10.17487/RFC6790, November 2012,
<http://www.rfc-editor.org/info/rfc6790>. <https://www.rfc-editor.org/info/rfc6790>.
[RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B., [RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
Litkowski, S., Horneffer, M., and R. Shakir, "Source Litkowski, S., Horneffer, M., and R. Shakir, "Source
Packet Routing in Networking (SPRING) Problem Statement Packet Routing in Networking (SPRING) Problem Statement
and Requirements", RFC 7855, DOI 10.17487/RFC7855, May and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
2016, <http://www.rfc-editor.org/info/rfc7855>. 2016, <https://www.rfc-editor.org/info/rfc7855>.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
and R. Shakir, "Segment Routing Architecture", draft-ietf-
spring-segment-routing-12 (work in progress), June 2017.
[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-10
(work in progress), June 2017.
15.2. Informative References 15.2. Informative References
[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP) [RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching Hierarchy with Generalized Multi-Protocol Label Switching
(GMPLS) Traffic Engineering (TE)", RFC 4206, (GMPLS) Traffic Engineering (TE)", RFC 4206,
DOI 10.17487/RFC4206, October 2005, DOI 10.17487/RFC4206, October 2005,
<http://www.rfc-editor.org/info/rfc4206>. <https://www.rfc-editor.org/info/rfc4206>.
[RFC7325] Villamizar, C., Ed., Kompella, K., Amante, S., Malis, A., [RFC7325] Villamizar, C., Ed., Kompella, K., Amante, S., Malis, A.,
and C. Pignataro, "MPLS Forwarding Compliance and and C. Pignataro, "MPLS Forwarding Compliance and
Performance Requirements", RFC 7325, DOI 10.17487/RFC7325, Performance Requirements", RFC 7325, DOI 10.17487/RFC7325,
August 2014, <http://www.rfc-editor.org/info/rfc7325>. August 2014, <https://www.rfc-editor.org/info/rfc7325>.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
and R. Shakir, "Segment Routing Architecture", draft-ietf-
spring-segment-routing-11 (work in progress), February
2017.
[I-D.ietf-isis-mpls-elc] [I-D.ietf-isis-mpls-elc]
Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S. Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability Using IS- Litkowski, "Signaling Entropy Label Capability Using IS-
IS", draft-ietf-isis-mpls-elc-02 (work in progress), IS", draft-ietf-isis-mpls-elc-02 (work in progress),
October 2016. October 2016.
[I-D.ietf-ospf-mpls-elc] [I-D.ietf-ospf-mpls-elc]
Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S. Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability Using Litkowski, "Signaling Entropy Label Capability Using
OSPF", draft-ietf-ospf-mpls-elc-04 (work in progress), OSPF", draft-ietf-ospf-mpls-elc-04 (work in progress),
November 2016. November 2016.
[I-D.ietf-isis-l2bundles] [I-D.ietf-isis-l2bundles]
Ginsberg, L., Bashandy, A., Filsfils, C., Nanduri, M., and Ginsberg, L., Bashandy, A., Filsfils, C., Nanduri, M., and
E. Aries, "Advertising L2 Bundle Member Link Attributes in E. Aries, "Advertising L2 Bundle Member Link Attributes in
IS-IS", draft-ietf-isis-l2bundles-04 (work in progress), IS-IS", draft-ietf-isis-l2bundles-07 (work in progress),
April 2017. May 2017.
Authors' Addresses Authors' Addresses
Sriganesh Kini Sriganesh Kini
EMail: sriganeshkini@gmail.com EMail: sriganeshkini@gmail.com
Kireeti Kompella Kireeti Kompella
Juniper Juniper
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