< draft-ietf-mpls-spring-entropy-label-04.txt   draft-ietf-mpls-spring-entropy-label-05.txt >
Network Working Group S. Kini, Ed. Network Working Group S. Kini
Internet-Draft Internet-Draft
Intended status: Informational K. Kompella Intended status: Informational K. Kompella
Expires: January 9, 2017 Juniper Expires: October 26, 2017 Juniper
S. Sivabalan S. Sivabalan
Cisco Cisco
S. Litkowski S. Litkowski
Orange Orange
R. Shakir R. Shakir
Google
J. Tantsura J. Tantsura
July 8, 2016 April 24, 2017
Entropy labels for source routed tunnels with label stacks Entropy label for SPRING tunnels
draft-ietf-mpls-spring-entropy-label-04 draft-ietf-mpls-spring-entropy-label-05
Abstract Abstract
Source routed tunnels with label stacking is a technique that can be Source routed tunnels with label stacking is a technique that can be
leveraged to provide a method to steer a packet through a controlled leveraged to provide a method to steer a packet through a controlled
set of segments. This can be applied to the Multi Protocol Label set of segments. This can be applied to the Multi Protocol Label
Switching (MPLS) data plane. Entropy label (EL) is a technique used Switching (MPLS) data plane. Entropy label (EL) is a technique used
in MPLS to improve load balancing. This document examines and in MPLS to improve load balancing. This document examines and
describes how ELs are to be applied to source routed tunnels with describes how ELs are to be applied to source routed tunnels with
label stacks. label stacks.
skipping to change at page 1, line 44 skipping to change at page 1, line 44
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 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. Recommended EL solution for SPRING . . . . . . . . . . . . . 5 4. Entropy Readable Label Depth . . . . . . . . . . . . . . . . 5
5. Options considered . . . . . . . . . . . . . . . . . . . . . 6 5. Maximum SID Depth . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Single EL at the bottom of the stack of tunnels . . . . . 6 6. LSP stitching using the binding SID . . . . . . . . . . . . . 8
5.2. An EL per tunnel in the stack . . . . . . . . . . . . . . 7 7. Insertion of entropy labels for SPRING path . . . . . . . . . 10
5.3. A re-usable EL for a stack of tunnels . . . . . . . . . . 8 7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 10
5.3.1. EL at top of stack . . . . . . . . . . . . . . . . . 8 7.1.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . 11
5.4. ELs at readable label stack depths . . . . . . . . . . . 8 7.1.2. Example 2 . . . . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 7.2. Considerations for the placement of entropy labels . . . 12
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 9 7.2.1. ERLD value . . . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7.2.2. Segment type . . . . . . . . . . . . . . . . . . . . 14
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7.2.2.1. Node-SID . . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.2.2.2. Adjacency-SID representing an ECMP bundle . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . 10 7.2.2.3. Adjacency-SID representing a single IP link . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . 10 7.2.2.4. Adjacency-SID representing a single link within a
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 L2 bundle . . . . . . . . . . . . . . . . . . . . 15
7.2.2.5. Adjacency-SID representing a L2 bundle . . . . . 15
7.2.3. Maximizing number of LSRs that will loadbalance . . . 15
7.2.4. Preference for a part of the path . . . . . . . . . . 16
7.2.5. Combining criteria . . . . . . . . . . . . . . . . . 16
8. A simple algorithm example . . . . . . . . . . . . . . . . . 16
9. Deployment Considerations . . . . . . . . . . . . . . . . . . 17
10. Options considered . . . . . . . . . . . . . . . . . . . . . 18
10.1. Single EL at the bottom of the stack of tunnels . . . . 18
10.2. An EL per tunnel in the stack . . . . . . . . . . . . . 18
10.3. A re-usable EL for a stack of tunnels . . . . . . . . . 19
10.4. EL at top of stack . . . . . . . . . . . . . . . . . . . 20
10.5. ELs at readable label stack depths . . . . . . . . . . . 20
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
14. Security Considerations . . . . . . . . . . . . . . . . . . . 21
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
15.1. Normative References . . . . . . . . . . . . . . . . . . 21
15.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction 1. Introduction
The source routed tunnels with label stacking paradigm is leveraged The source routed tunnels with label stacking paradigm is leveraged
by techniques such as Segment Routing (SR) by techniques such as Segment Routing (SR)
[I-D.ietf-spring-segment-routing] to steer a packet through a set of [I-D.ietf-spring-segment-routing] to steer a packet through a set of
segments. This can be directly applied to the MPLS data plane, but segments. This can be directly applied to the MPLS data plane, but
it has implications on the label stack depth. it has implications on the label stack depth.
Clarifying statements on label stack depth have been provided in Clarifying statements on label stack depth have been provided in
skipping to change at page 3, line 4 skipping to change at page 3, line 25
The source routed tunnels with label stacking paradigm is leveraged The source routed tunnels with label stacking paradigm is leveraged
by techniques such as Segment Routing (SR) by techniques such as Segment Routing (SR)
[I-D.ietf-spring-segment-routing] to steer a packet through a set of [I-D.ietf-spring-segment-routing] to steer a packet through a set of
segments. This can be directly applied to the MPLS data plane, but segments. This can be directly applied to the MPLS data plane, but
it has implications on the label stack depth. it has implications on the label stack depth.
Clarifying statements on label stack depth have been provided in Clarifying statements on label stack depth have been provided in
[RFC7325] but the RFC does not address the case of source routed [RFC7325] but the RFC does not address the case of source routed
stacked MPLS tunnels as described in stacked MPLS tunnels as described in
[I-D.ietf-spring-segment-routing] where deeper label stacks are more [I-D.ietf-spring-segment-routing] where deeper label stacks are more
prevalent. 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 the hierarchy
is created at a single LSR as required by source routed tunnels with is created at a single LSR as required by source routed tunnels with
label stacks. label stacks.
A use-case requiring load balancing with source routed tunnels with A use-case requiring load balancing with source routed tunnels with
label stacks is given in Section 3. A recommended solution is label stacks is given in Section 3. A recommended solution is
described in Section 4 keeping in consideration the limitations of described in Section 7 keeping in consideration the limitations of
implementations when applying [RFC6790] to deeper label stacks. implementations when applying [RFC6790] to deeper label stacks.
Options that were considered to arrive at the recommended solution Options that were considered to arrive at the recommended solution
are documented for historical purposes in Section 5. are documented for historical 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 RFC 2119 [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
document. document.
2. Abbreviations and Terminology 2. Abbreviations and Terminology
EL - Entropy Label EL - Entropy Label
ELI - Entropy Label Identifier ELI - Entropy Label Identifier
ELC - Entropy Label Capability ELC - Entropy Label Capability
ERLD - Entropy Readable Label Depth
SR - Segment Routing SR - Segment Routing
ECMP - Equal Cost Multi Paths ECMP - Equal Cost Multi Paths
LSR - Label Switch Router
MPLS - Multiprotocol Label Switching MPLS - Multiprotocol Label Switching
MSD - Maximum SID Depth
SID - Segment Identifier SID - Segment Identifier
RLD - Readable Label Depth RLD - Readable Label Depth
OAM - Operation, Administration and Maintenance OAM - Operation, Administration and Maintenance
3. Use-case requiring multipath load balancing 3. Use-case requiring multipath load balancing
+------+ +------+
| | | |
+-------| P3 |-----+ +-------| P3 |-----+
| +-----| |---+ | | +-----| |---+ |
L3| |L4 +------+ L1| |L2 +----+ L3| |L4 +------+ L1| |L2 +----+
| | | | +--| P4 |--+ | | | | +--| P4 |--+
+-----+ +-----+ +-----+ | +----+ | +-----+ +-----+ +-----+ +-----+ | +----+ | +-----+
| 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 (TE) is one of the applications of MPLS and is
also a requirement for source routed tunnels with label stacks. also a requirement for source routed tunnels with label stacks
Consider the topology shown in Figure 1. Lets say the LSR P1 has a
limitation that it can only look four labels deep in the stack to do
multipath decisions. All other transit LSRs in the figure can read
deep label stacks and the LSR S can insert as many <ELI, EL> pairs as
needed. 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 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.
Multiple ways to apply entropy labels were considered and are
documented in Section 5 along with their tradeoffs. A recommended
solution is described in Section 4.
4. Recommended EL solution for SPRING [RFC7855]. 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 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 loadbalancing, entropy labels insertion strategy
becomes important to keep benefit of the loadbalancing. Multiple
ways to apply entropy labels were considered and are documented in
Section 10 along with their tradeoffs. A recommended solution is
described in Section 7.
The solution described in this section follows [RFC6790]. 4. Entropy Readable Label Depth
An LSR may have a limitation on the depth of the label stack that it The Entropy Readable Label Depth (ERLD) is defined as the number of
can read and process in order to do multipath load balancing. This labels a router can:
limitation expressed in terms of the number of label stack entries
that the LSR can read is defined as the Readable Label Depth (RLD)
capability of that LSR. If an EL does not occur within the RLD of an
LSR in the label stack of the 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 RLD of the LSR in order for the LSR to use the EL
during load balancing. The RLD of an LSR is a characteristic of the
forwarding plane of that LSR's implementation and determining it is
outside the scope of this document.
In order for the EL to occur within the RLD of LSRs along the path a. read in an MPLS packet received on its incoming interface
corresponding to a label stack, multiple <ELI, EL> pairs MAY be (starting from the top of the stack) and
inserted in the label stack as long as the tunnel's label below which
they are inserted are advertised with entropy label capability b. use in its loadbalancing function.
enabled. The ELs among multiple <ELI, EL> pairs inserted in the
stack may be same or different. The LSR that inserts <ELI, EL> pairs The ERLD means that the router will perform load-balancing using the
MAY have limitations on the number of such pairs that it can insert EL label if the EL is placed within the ERLD first labels.
and also the depth at which it can insert them. If due to any
limitation, the inserted ELs are at positions such that an LSR along A router capable of reading N labels but not using an EL located
the path receives an MPLS packet without an EL in the label stack within those N labels MUST consider its ERLD to be 0. In a
within that LSR's RLD, then the load balancing performed by that LSR distributed switching architecture, each linecard may have a
would be poor. Special attention should be paid when a forwarding different capability in term of ERLD. For simplicity reason, an
adjacency LSP (FA-LSP) [RFC4206] is used as a link along the path of implementation MAY use the minimum ERLD between each linecard as the
a source routed LSP's label stack, since the labels of the FA-LSP ERLD value for the system.
would additionally count towards the depth of the label stack when
calculating the appropriate positions to insert the ELs. The Examples:
recommendations for inserting <ELI, EL> pairs are:
| 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 below, 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 loadbalance the
Packet 1 displayed in the Figure 2 using the EL as part of the
loadbalancing keys. The ERLD value of 3 means that the router can
read and take into account the entropy label for loadbalancing 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 loadbalance the
Packet 1 to 3 in the Figure 2 using the EL as part of the
loadbalancing keys. The Packet 4 and 5 have the EL placed at a
position greater than 5, so the router is not able to read it and
take it into account during the hashing.
o If the router has an ERLD of 10, it will be able to loadbalance
all the packets displayed in the Figure 2 using the EL as part of
the loadbalancing keys.
To allow an efficient loadbalancing 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 out of scope of this document.
To advertise an ERLD value, a SPRING router:
o MUST be entropy label capable and as a consequence MUST apply all
the 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
out of 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 real 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
path to PE2 along
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
labels on the IP packet received, it so 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 will be imposed, requiring
an MSD of 11 for PE1. In addition, if PE1 wants to insert an ELI/EL
for loadbalancing 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 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 to bind 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), a SPRING
path... Each LSP associated with a binding SID has its own entropy
label capability.
In the figure 3, if we consider that:
o P6, PE2, P10, P11, P12 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 (MS) 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.
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 MS 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 to a binding SID unless this binding SID
has the entropy label capability. How the entropy label capability
is advertised for a binding SID is out of 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 works only 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 LDP FEC. As a
consequence, it does not set the ELC on the associated binding SID.
7. Insertion of entropy labels for SPRING path
7.1. Overview
The solution described in this section follows [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 ended here 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 on the depth of
the label stack that it can read and process in order to do multipath
load balancing based on entropy labels: we called it the ERLD.
If an EL does not occur within the ERLD of an LSR in the label stack
of the 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 loadbalancing 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
anymore in the stack for some LSRs if 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
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
the depth at which it can insert them. If due to any limitation, 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 criterias when
inserting <ELI, EL> pairs.
7.1.1. Example 1
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: {VPN_label, Adj_P6PE2, Adj_P5P6,
Adj_P4P5, Adj_P3P4, Adj_Bundle_P2P3, Adj_P1P2}. 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 place them in a smart way to benefit of load
balancing along the longest part of the path.
PE1 may take into account multiple parameters when placing the 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
{VPN_label, ELI2,EL2, Adj_P6PE2, Adj_P5P6, Adj_P4P5, Adj_P3P4, ELI1,
EL1, Adj_Bundle_P2P3, Adj_P1P2}. The original stack requests P2 to
forward based on a bundle Adjacency segment that will require load
balancing. Therefore it is important to ensure that P2 can
loadbalance correctly. As P2 has a limited ERLD of 3, ELI/EL must be
inserted just next to the label 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 path have a high ERLD that does not cause any issue,
except P6, moreover P6 is using some LAGs to PE2 and is so expected
to loadbalance. 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 push capacity.
7.1.2. Example 2
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 : {VPN_label, Adj_Bundle_P8PE2, Adj_P7P8,
Adj_Bundle_P6P7, Adj_P5P6, Adj_P4P5, Adj_P3P4, Adj_Bundle_P2P3,
Adj_P1P2}. PE1 is limited to push a maximum of 11 labels, P2, P3 and
P6 have a ERLD of 3 while others have a 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 loadbalance the end to end path. An optimized stack that
would enable end-to-end load balancing may be: {VPN_label, ELI3, EL3,
Adj_Bundle_P8PE2, Adj_P7P8, ELI2, EL2, Adj_Bundle_P6P7, Adj_P5P6,
Adj_P4P5, Adj_P3P4, ELI1, EL1, Adj_Bundle_P2P3, Adj_P1P2}.
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 P6
forwarding label as it will allow P4 and P6 to loadbalance. 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 shown 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.
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 trade-off, an implementation
MAY provide flexibility to operator to select the criteria to be
taken into account when placing EL/ELIs or the sub-objective to be
optimized for.
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 sub
sections.
7.2.1. ERLD value
As mentioned in Section 7.1, the ERLD value is an important parameter
to take into account when inserting ELI/EL as if an ELI/EL does not
fall within the ERLD of a node on the path, the node will not be able
to loadbalance the traffic in an efficient way.
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: {Service_label, Adj_PE2P9, Node_P9, Adj_P1P2}.
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 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
value along the node segment 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. In the example above, if the
implementation supports computation of minimum ERLD along the path,
the ERLD associated to label Node_P9 would be the minimum ERLD
between nodes {P2,P3,P4 ..., P8}. If the implementation does not
support the computation of minimum ERLD, it should consider the ERLD
of P2 (starting node that will forward based on 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.
Let's consider a path from PE1 to PE2 using the following stack
pushed by PE1: {Service_label, Adj_PE2P9, Node_P9, Adj_P1P2}.
If, for example, PE1 is limited to 6 labels to be pushed, 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
additional SPT computation and by analysis the node segment path. So
a node segment that does not really require load balancing may not be
preferred when placing EL/ELIs. Such inspection may be time
consuming for implementations without 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 so requires an
EL/ELI.
7.2.2.2. Adjacency-SID representing an ECMP bundle
When an adjacency segment representing an ECMP bundle 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 L2 bundle SR
extensions [I-D.ietf-isis-l2bundles] are not implemented. In such
case, it may be interesting to keep the possibility to insert an EL/
ELI in a readable position for the LSR advertising the label
associated to 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 loadbalance
When placing ELI/ELs, an implementation may try to maximize the
number of LSRs that LSRs that both needs to load balance (i.e. have
ECMP paths) and that will be able to perform load balancing (i.e. EL
label is within their ERLD).
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
routers have ERLD of 10, expect P1 and P2 which have a 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 so 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. But combining too much criteria may lead to
implementation complexity and high control plane resource
consumption. Each time, the network topology will change, a new
evaluation of the EL/ELI placement will be necessary.
8. A simple algorithm example
A simple implementation can only take into account ERLD when placing
ELI/EL while keep minimizing the number of EL/ELIs inserted and
maximizing the number of LSRs that can loadbalance.
The algorithm example 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
RLD of the incoming packet to that LSR. ERLD of the incoming packet to that LSR.
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.
A sample algorithm that follows the above recommendations to insert The pseudocode of the example is shown below.
ELs is shown below. For node SIDs, the minimum value of RLDs of LSRs
on that node segment should be used.
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 RLD) AND (RLD > 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 2: Algorithm to insert <ELI, EL> pairs in a label stack 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 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}
The RLD can be advertised via protocols and those extensions are 9. Deployment Considerations
described in separate documents [I-D.xu-isis-mpls-elc] and
[I-D.xu-ospf-mpls-elc].
It should be noted that the above algorithm is a sample and other As long as LSR node dataplane capabilities with be limited (number of
algorithms that optimize for other criteria and provide additional labels that can be pushed, or number of labels that can be
tuning parameters to give operators the control of the optimization inspected), hop-by-hop load balancing of SPRING encapsulated flows
criteria may be developed. will require trade-offs.
The recommendations above are not expected to bring any additional Entropy label is still a good and usable solution as it allows load
OAM considerations beyond those described in section 6 of [RFC6790]. balancing without having to perform a deep packet inspection on each
However, the OAM requirements and solutions for source routed tunnels LSR: it does not sound reasonable to have a LSR inspecting UDP port
formed by label stacking are still under discussion and future within a GRE tunnel carried over a 15 labels SPRING tunnel.
revisions of this document will address those if needed.
5. Options considered 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 requiring two additional labels to be pushed.
Placement strategies of EL/ELIs are so required to find the best
trade-off. Multiple criteria may be taken into account and some
level of customization (by 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.
In future, hardware and software capacity may increase dataplane
capabilities and may be remove some of those limits: this may
increase the capacity of load balancing using entropy labels.
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.
5.1. Single EL at the bottom of the stack of tunnels 10.1. Single EL at the bottom of the stack of tunnels
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 (EL) at the bottom of the
label stack. In the example described in Section 3, it will result 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, 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] EL} {remaining packet header}. Note that the notation in [RFC6790]
is used to describe the label stack. An issue with this approach is 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 transit the EL goes correspondingly deeper in the label stack. Hence transit
LSRs have to access a larger number of bytes in the packet header LSRs have to access a larger number of bytes in the packet header
when making forwarding decisions. In the example described in when making forwarding decisions. In the example described in
Section 3, the LSR P1 would poorly load-balance traffic on the Section 3, the LSR P1 would poorly load-balance traffic on the
parallel links L3, L4 since the EL is below the RLD of the packet parallel links L3, L4 since the EL is below the RLD of the packet
received by P1. A load balanced network design using this approach received by P1. A load balanced network design using this approach
must ensure that all intermediate LSRs have the capability to must ensure that all intermediate LSRs have the capability to
traverse the maximum label stack depth as required for that traverse the maximum label stack depth as required for that
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, 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 EL> pair at any depth, this option is the same as the recommended
solution in Section 4. 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 but that is a critical requirement a significant part of the network but that is a critical requirement
in a service provider network. in a service provider network.
5.2. An EL per tunnel in the stack 10.2. An EL per tunnel in the stack
In this option each tunnel in the stack can be given its own EL. The 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 source LSR pushes an <ELI, EL> before pushing a tunnel label when
load balancing is required to direct traffic on that tunnel. In the load balancing is required to direct traffic on that tunnel. In the
example described in Section 3, the source LSR S encoded label stack 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 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 can be the same. Accessing the EL at an intermediate LSR 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 application that uses source routed tunnels with label the specific application that uses source routed tunnels with label
stacking in that network. A drawback is that the depth of the label stacking in that network. A drawback is that the depth of the label
stack grows significantly, almost 3 times as the number of labels in stack grows significantly, almost 3 times as the number of labels in
the label stack. The network design should ensure that source LSRs the label stack. The network design should ensure that source LSRs
should have the capability to push such a deep label stack. Also, should have the capability to push such a deep label stack. Also,
the bandwidth overhead and potential MTU issues of deep label stacks the bandwidth overhead and potential MTU issues of deep label stacks
should be accounted for in the network design. should be accounted for in the network design.
In the case where the RLD is the minimum value (3) for all LSRs, all In the case where the RLD is the minimum value (3) for all LSRs, all
LSRs are EL capable and the LSR that is inserting <ELI, EL> pairs has LSRs are EL capable and the LSR that is inserting <ELI, EL> pairs has
no limit on how many it can insert then this option is the same as no limit on how many it can insert then this option is the same as
the recommended solution in Section 4. the recommended solution in Section 7.
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.
5.3. A re-usable EL for a stack of tunnels 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 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 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- 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-D, 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 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 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.
5.3.1. 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
top of the stack rather than below the tunnel label. In this case 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 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 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 along the segment. An LSR that terminates a segment should use the
EL from the terminated segment at the top of the stack when EL from the terminated segment at the top of the stack when
forwarding onto the next segment. forwarding onto the next segment.
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.
5.4. ELs at readable label stack depths 10.5. ELs at readable label stack depths
In this option the source LSR inserts ELs for tunnels in the label 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 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 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 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 RLD has different values along the path and the
LSR that is inserting <ELI, EL> pairs has no limit on how many pairs 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 it can insert, and it knows the appropriate positions in the stack
where they should be inserted, then this option is the same as the where they should be inserted, then this option is the same as the
recommended solution in Section 4. 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 4. in Section 7.
6. 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 and Nobo Akiya for Villamizar, Greg Mirsky, Markus Jork, Kamran Raza, Carlos Pignataro,
their review comments and suggestions. Bruno Decraene and Nobo Akiya for their review comments and
suggestions.
7. Contributors 12. Contributors
Xiaohu Xu Xiaohu Xu
Huawei Huawei
Email: xuxiaohu@huawei.com Email: xuxiaohu@huawei.com
Wim Hendrickx Wim Hendrickx
Alcatel-Lucent Nokia
Email: wim.henderickx@alcatel-lucent.com Email: wim.henderickx@nokia.com
8. IANA Considerations 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 memo includes no request to IANA. Note to RFC Editor: Remove
this section before publication. this section before publication.
9. Security Considerations 14. Security Considerations
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].
10. References 15. References
10.1. 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>. <http://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>. <http://www.rfc-editor.org/info/rfc6790>.
10.2. Informative References [RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
Litkowski, S., Horneffer, M., and R. Shakir, "Source
[I-D.ietf-spring-segment-routing] Packet Routing in Networking (SPRING) Problem Statement
Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
and R. Shakir, "Segment Routing Architecture", draft-ietf- 2016, <http://www.rfc-editor.org/info/rfc7855>.
spring-segment-routing-09 (work in progress), July 2016.
[I-D.xu-isis-mpls-elc]
Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability Using IS-
IS", draft-xu-isis-mpls-elc-02 (work in progress), April
2015.
[I-D.xu-ospf-mpls-elc] 15.2. Informative References
Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability Using
OSPF", draft-xu-ospf-mpls-elc-01 (work in progress),
October 2014.
[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>. <http://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, <http://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]
Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability Using IS-
IS", draft-ietf-isis-mpls-elc-02 (work in progress),
October 2016.
[I-D.ietf-ospf-mpls-elc]
Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability Using
OSPF", draft-ietf-ospf-mpls-elc-04 (work in progress),
November 2016.
[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-04 (work in progress),
April 2017.
Authors' Addresses Authors' Addresses
Sriganesh Kini (editor) Sriganesh Kini
Email: sriganeshkini@gmail.com EMail: sriganeshkini@gmail.com
Kireeti Kompella Kireeti Kompella
Juniper Juniper
Email: kireeti@juniper.net EMail: kireeti@juniper.net
Siva Sivabalan Siva Sivabalan
Cisco Cisco
Email: msiva@cisco.com EMail: msiva@cisco.com
Stephane Litkowski Stephane Litkowski
Orange Orange
Email: stephane.litkowski@orange.com EMail: stephane.litkowski@orange.com
Rob Shakir Rob Shakir
Google
Email: rjs@rob.sh EMail: rjs@rob.sh
Jeff Tantsura Jeff Tantsura
Email: jefftant@gmail.com EMail: jefftant@gmail.com
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