RFC8662: Entropy Label for Source Packet Routing in Networking (SPRING) Tunnels
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Related keywords:
(ECMP (equal-cost multipath))
(Flow-aware load balancing)
Internet Engineering Task Force (IETF) S. Kini
Request for Comments: 8662
Category: Standards Track K. Kompella
ISSN: 2070-1721 Juniper
S. Sivabalan
Cisco
S. Litkowski
Orange
R. Shakir
Google
J. Tantsura
Apstra, Inc.
December 2019
Entropy Label for Source Packet Routing in Networking (SPRING) Tunnels
Abstract
Segment Routing (SR) leverages the source-routing paradigm. A node
steers a packet through an ordered list of instructions, called
segments. Segment Routing can be applied to the Multiprotocol Label
Switching (MPLS) data plane. Entropy labels (ELs) are used in MPLS
to improve load-balancing. This document examines and describes how
ELs are to be applied to Segment Routing MPLS.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8662.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Requirements Language
2. Abbreviations and Terminology
3. Use Case Requiring Multipath Load-Balancing
4. Entropy Readable Label Depth
5. Maximum SID Depth
6. LSP Stitching Using the Binding SID
7. Insertion of Entropy Labels for SPRING Path
7.1. Overview
7.1.1. Example 1: The Ingress Node Has a Sufficient MSD
7.1.2. Example 2: The Ingress Node Does Not Have a Sufficient
MSD
7.2. Considerations for the Placement of Entropy Labels
7.2.1. ERLD Value
7.2.2. Segment Type
7.2.3. Maximizing Number of LSRs That Will Load-Balance
7.2.4. Preference for a Part of the Path
7.2.5. Combining Criteria
8. A Simple Example Algorithm
9. Deployment Considerations
10. Options Considered
10.1. Single EL at the Bottom of the Stack
10.2. An EL per Segment in the Stack
10.3. A Reusable EL for a Stack of Tunnels
10.4. EL at Top of Stack
10.5. ELs at Readable Label Stack Depths
11. IANA Considerations
12. Security Considerations
13. References
13.1. Normative References
13.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
Segment Routing [RFC8402] is based on source-routed tunnels to steer
a packet along a particular path. This path is encoded as an ordered
list of segments. When applied to the MPLS data plane [RFC8660],
each segment is an LSP (Label Switched Path) with an associated MPLS
label value. Hence, label stacking is used to represent the ordered
list of segments, and the label stack associated with an SR tunnel
can be seen as nested LSPs (LSP hierarchy) in the MPLS architecture.
Using label stacking to encode the list of segments has implications
on the label stack depth.
Traffic load-balancing over ECMP (Equal-Cost Multipath) or LAGs (Link
Aggregation Groups) is usually based on a hashing function. The
local node that performs the load-balancing is required to read some
header fields in the incoming packets and then compute a hash based
on those fields. The result of the hash is finally mapped to a list
of outgoing next hops. The hashing technique is required to perform
a per-flow load-balancing and thus, prevents packet misordering. For
IP traffic, the usual fields that are hashed are the source address,
the destination address, the protocol type, and, if provided by the
upper layer, the source port and destination port.
The MPLS architecture brings some challenges when an LSR (Label
Switching Router) tries to look up at header fields. An LSR needs be
able to look up at header fields that are beyond the MPLS label stack
while the MPLS header does not provide any information about the
upper-layer protocol. An LSR must perform a deeper inspection
compared to an ingress router, which could be challenging for some
hardware. Entropy labels (ELs) [RFC6790] are used in the MPLS data
plane to provide entropy for load-balancing. The idea behind the
entropy label is that the ingress router computes a hash based on
several fields from a given packet and places the result in an
additional label named "entropy label". Then, this entropy label can
be used as part of the hash keys used by an LSR. Using the entropy
label as part of the hash keys reduces the need for deep packet
inspection in the LSR while keeping a good level of entropy in the
load-balancing. When the entropy label is used, the keys used in the
hashing functions are still a local configuration matter, and an LSR
may use solely the entropy label or a combination of multiple fields
from the incoming packet.
When using LSP hierarchies, there are implications on how [RFC6790]
should be applied. The current document addresses the case where a
hierarchy is created at a single LSR as required by Segment Routing.
A use case requiring load-balancing with SR is given in Section 3. A
recommended solution is described in Section 7 keeping in
consideration the limitations of implementations when applying
[RFC6790] to deeper label stacks. Options that were considered to
arrive at the recommended solution are documented for historical
purposes in Section 10.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Abbreviations and Terminology
Adj-SID Adjacency Segment Identifier
ECMP Equal-Cost Multipath
EL Entropy Label
ELI Entropy Label Indicator
ELC Entropy Label Capability
ERLD Entropy Readable Label Depth
FEC Forwarding Equivalence Class
LAG Link Aggregation Group
LSP Label Switched Path
LSR Label Switching Router
MPLS Multiprotocol Label Switching
MSD Maximum SID Depth
Node SID Node Segment Identifier
OAM Operations, Administration, and Maintenance
RLD Readable Label Depth
SID Segment Identifier
SPT Shortest Path Tree
SR Segment Routing
SRGB Segment Routing Global Block
VPN Virtual Private Network
3. Use Case Requiring Multipath Load-Balancing
Traffic engineering is one of the applications of MPLS and is also a
requirement for Segment Routing [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 steer traffic along a path that crosses 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 Adj-SID (Adjacency
Segment Identifier) 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 Segment Routing (as a reminder, it
is called the SRGB, Segment Routing Global Block). 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 Adj-SID for link Ln. In our example,
the LSR S must use the label stack <L_N-P3, L_A-L1, L_N-D>. However,
to achieve good load-balancing over the equal-cost paths P2-P4-D,
P2-P5-D, and the parallel links L3 and L4, a mechanism such as
entropy labels [RFC6790] should be adapted for Segment Routing.
Indeed, the Source Packet Routing in Networking (SPRING) architecture
with the MPLS data plane [RFC8660] uses nested MPLS LSPs composing
the source-routed label stack.
+------+
| |
+-------| P3 |-----+
| +-----| |---+ |
L3| |L4 +------+ L1| |L2 +----+
| | | | +--| P4 |--+
+-----+ +-----+ +-----+ | +----+ | +-----+
| S |-----| P1 |------------| P2 |--+ +--| D |
| | | | | |--+ +--| |
+-----+ +-----+ +-----+ | +----+ | +-----+
+--| P5 |--+
+----+
Key:
S = Source LSR
D = Destination LSR
P1, P2, P3, P4, P5 = Transit LSRs
L1, L2, L3, L4 = Links
Figure 1: Traffic-Engineering Use Case
An MPLS node may have limitations in the number of labels it can
push. It may also have a limitation in the number of labels it can
inspect when looking for hash keys during load-balancing. While the
entropy label is normally inserted at the bottom of the transport
tunnel, this may prevent an LSR from taking into account the EL in
its load-balancing function if the EL is too deep in the stack. In a
Segment Routing environment, it is important to define the
considerations that need to be taken into account when inserting an
EL. Multiple ways to apply entropy labels were considered and are
documented in Section 10 along with their trade-offs. A recommended
solution is described in Section 7.
4. Entropy Readable Label Depth
The Entropy Readable Label Depth (ERLD) is defined as the number of
labels a router can both:
a. Read in an MPLS packet received on its incoming interface(s)
(starting from the top of the stack).
b. Use in its load-balancing function.
The ERLD means that the router will perform load-balancing using the
EL if the EL is placed within the first ERLD labels.
A router capable of reading N labels but not using an EL located
within those N labels MUST consider its ERLD to be 0.
In a distributed switching architecture, each line card may have a
different capability in terms of ERLD. For simplicity, an
implementation MAY use the minimum ERLD of all line cards as the ERLD
value for the system.
There may also be a case where a router has a fast switching path
(handled by an Application-Specific Integrated Circuit, or ASIC, or
network processor) and a slow switching path (handled by a CPU) with
a different ERLD for each switching path. Again, for simplicity's
sake, an implementation MAY use the minimum ERLD as the ERLD value
for the system.
The drawback of using a single ERLD for a system lower than the
capability of one or more specific components is that it may increase
the number of ELI/ELs inserted. This leads to an increase of the
label stack size and may have an impact on the capability of the
ingress node to push this label stack.
Examples:
| 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 Figure 2, we consider the displayed packets received on a router
interface. We consider also a single ERLD value for the router.
* If the router has an ERLD of 3, it will be able to load-balance
Packet 1 displayed in Figure 2 using the EL as part of the load-
balancing keys. The ERLD value of 3 means that the router can
read and take into account the entropy label for load-balancing if
it is placed between position 1 (top of the MPLS label stack) and
position 3.
* If the router has an ERLD of 5, it will be able to load-balance
Packets 1 to 3 in Figure 2 using the EL as part of the load-
balancing keys. Packets 4 and 5 have the EL placed at a position
greater than 5, so the router is not able to read it and use it as
part of the load-balancing keys.
* If the router has an ERLD of 10, it will be able to load-balance
all the packets displayed in Figure 2 using the EL as part of the
load-balancing keys.
To allow an efficient load-balancing based on entropy labels, a
router running SPRING SHOULD advertise its ERLD (or ERLDs), so all
the other SPRING routers in the network are aware of its capability.
How this advertisement is done is outside the scope of this document
(see Section 7.2.1 for potential approaches).
To advertise an ERLD value, a SPRING router:
* MUST be entropy label capable and, as a consequence, MUST apply
the data-plane procedures defined in [RFC6790].
* MUST be able to read an ELI/EL, which is located within its ERLD
value.
* MUST take into account an EL within the first ERLD labels 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 can include any kind of
labels (service, entropy, transport, etc.). In an MPLS network, the
MSD is a limit of the head-end of an SR tunnel or a Binding SID
anchor node that performs imposition of additional labels on an
existing label stack.
Depending on the number of MPLS operations (POP, SWAP, etc.) to be
performed before the PUSH, the MSD can vary due to hardware or
software limitations. As for the ERLD, different MSD limits can
exist within a single node based on the line-card types used in a
distributed switching system. Thus, the MSD is a per link and/or
per-node property.
An external controller can be used to program a label stack on a
particular node. This node SHOULD advertise its MSD to the
controller in order to let the controller know the maximum label
stack depth of the path computed that is supported on the head-end.
How this advertisement is done is outside the scope of this document.
([RFC8476], [RFC8491], and [MSD-BGP] provide examples of
advertisement of the MSD.) As the controller does not have the
knowledge of the entire label stack to be pushed by the node, in
addition to the MSD value, the node SHOULD advertise the type of the
MSD. For instance, the MSD value can represent the limit for pushing
transport labels only while in reality the node can push an
additional service label. As another example, the MSD value can
represent the full limit of the node including all label types
(transport, service, entropy, etc.). This gives the ability for the
controller to program a label stack while leaving room for the local
node to add more labels (e.g., service, entropy, etc.) without
reaching the hardware/software limit. If the node does not provide
the meaning of the MSD value, the controller could program an LSP
using a number of labels equal to the full limit of the node. When
receiving this label stack from the controller, the ingress node may
not be able to add any service (L2VPN, L3VPN, EVPN, etc.) label on
top of this label stack. The consequence could be for the ingress
node to drop service packets that should have been forwarded over the
LSP.
P7 ---- P8 ---- P9
/ \
PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2
| \ |
----> P10 \ |
IP Pkt | \ |
P11 --- P12 --- P13
100 10000
Figure 3: Topology Illustrating Label Stack Reduction
In Figure 3, an IP packet comes into 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 Adj-SIDs only, PE1 (acting as an
ingress LSR, also known as an I-LSR) will be required to push 10
labels on the IP packet received and thus, requires an MSD of 10. If
the IP packet should be carried over an MPLS service like a regular
layer 3 VPN, an additional service label should be imposed requiring
an MSD of 11 for PE1. In addition, if PE1 wants to insert an ELI/EL
for load-balancing purposes, 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 could use the following label stack:
<Node_P9, Node_P5, Binding_P5, Node_PE2>. In this example, we
consider a combination of Node SIDs and a Binding SID advertised by
P5 that will stitch the traffic along the path P10 -> P11 -> P12 ->
P13. The instruction associated with the Binding SID at P5 is thus
to swap Binding_P5 to Adj_P12-P13 and then push <Adj_P11-P12,
Node_P11>. P5 acts as a stitching node that pushes additional labels
on an existing label stack; P5's MSD needs also to be taken into
account and may limit the number of labels that can be imposed.
6. LSP Stitching Using the Binding SID
The Binding SID allows binding a segment identifier to an existing
LSP. As examples, the Binding SID can represent an RSVP-TE tunnel,
an LDP path (through the Mapping Server Advertisement), or a SPRING
path. Each tail-end router of an MPLS LSP associated with a Binding
SID has its own entropy label capability. The entropy label
capability of the associated LSP is advertised in the control-plane
protocol used to signal the LSP.
In Figure 4, we consider that:
* P6, PE2, P10, P11, P12, and P13 are pure LDP routers.
* PE1, P1, P2, P3, P4, P7, P8, and P9 are pure SPRING routers.
* P5 is running SPRING and LDP.
* P5 acts as a Mapping Server and advertises Prefix-SIDs for the LDP
FECs: an index value of 20 is used for PE2.
* All SPRING routers use an SRGB of [1000, 1999].
* P6 advertises label 20 for the PE2 FEC.
* 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
Figure 4: Example Illustrating Need for ELC Propagation
In terms of packet forwarding, by learning the Mapping Server
Advertisement from P5, PE1 imposes a label 1020 to an IP packet
destined to PE2. SPRING routers along the shortest path to PE2 will
switch the traffic until it reaches P5. P5 will perform the LSP
stitching by swapping the SPRING label 1020 to the LDP label 20
advertised by the next hop P6. P6 will finally 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 accommodate the mix of signaling protocols involved during the
stitching, the entropy label capability SHOULD be propagated between
the signaling domains. Each Binding SID SHOULD have its own entropy
label capability that MUST be inherited from the entropy label
capability of the associated LSP. If the router advertising the
Binding SID does not know the ELC state of the target FEC, it MUST
NOT set the ELC for the Binding SID. An ingress node MUST NOT push
an ELI/EL associated with a Binding SID unless this Binding SID has
the entropy label capability. How the entropy label capability is
advertised for a Binding SID is outside the scope of this document
(see Section 7.2.1 for potential approaches).
In our example, if PE2 is LDP entropy label capable, it will add the
entropy label capability in its LDP advertisement. When P5 receives
the FEC/label binding for PE2, it learns about the ELC and can set
the ELC in the Mapping Server Advertisement. Thus, PE1 learns about
the ELC of PE2 and may push an ELI/EL associated with the Binding
SID.
The proposed solution only works if the SPRING router advertising the
Binding SID is also performing the data-plane LSP stitching. In our
example, if the Mapping Server function is hosted on P8 instead of
P5, P8 does not know about the ELC state of PE2's LDP FEC. As a
consequence, it does not set the ELC for the associated Binding SID.
7. Insertion of Entropy Labels for SPRING Path
7.1. Overview
The solution described in this section follows the data-plane
processing defined in [RFC6790]. Within a SPRING path, a node may be
ingress, egress, transit (regarding the entropy label processing
described in [RFC6790]), or it can be any combination of those. For
example:
* The ingress node of a SPRING domain can be an ingress node from an
entropy label perspective.
* Any LSR terminating a segment of the SPRING path is an egress node
(because it terminates the segment) but can also be a transit node
if the SPRING path is not terminated because there is a subsequent
SPRING MPLS label in the stack.
* 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 (the ERLD) on the
depth of the label stack that it can read and process in order to do
multipath load-balancing based on entropy labels.
If an EL does not occur within the ERLD of an LSR in the label stack
of an MPLS packet that it receives, then it would lead to poor load-
balancing at that LSR. Hence, an ELI/EL pair must be within the ERLD
of the LSR in order for the LSR to use the EL during load-balancing.
Adding a single ELI/EL pair for the entire SPRING path can also lead
to poor load-balancing as well because the ELI/EL may not occur
within the ERLD of some LSR on the path (if too deep) or may not be
present in the stack when it reaches some LSRs (if it is too
shallow).
In order for the EL to occur within the ERLD of LSRs along the path
corresponding to a SPRING label stack, multiple <ELI, EL> pairs MAY
be inserted in this label stack.
The insertion of an ELI/EL MUST occur only with a SPRING label
advertised by an LSR that advertised an ERLD (the LSR is entropy
label capable) or with a SPRING label associated with a Binding SID
that has the ELC set.
The ELs among multiple <ELI, EL> pairs inserted in the stack MAY be
the same or different. The LSR that inserts <ELI, EL> pairs can 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 limitations, the
inserted ELs are at positions such that an LSR along the path
receives an MPLS packet without an EL in the label stack within that
LSR's ERLD, then the load-balancing performed by that LSR would be
poor. An implementation MAY consider multiple criteria when
inserting <ELI, EL> pairs.
7.1.1. Example 1: The Ingress Node Has a Sufficient MSD
ECMP LAG LAG
PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2
Figure 5: Accommodating MSD Limitations
In Figure 5, PE1 wants to forward some MPLS VPN traffic over an
explicit path to PE2 resulting in the following label stack to be
pushed onto the received IP header: <Adj_P1P2, Adj_set_P2P3,
Adj_P3P4, Adj_P4P5, Adj_P5P6, Adj_P6PE2, VPN_label>. PE1 is limited
to push a maximum of 11 labels (MSD=11). P2, P3, and P6 have an ERLD
of 3 while others have an ERLD of 10.
PE1 can only add two ELI/EL pairs in the label stack due to its MSD
limitation. It should insert them strategically to benefit load-
balancing along the longest part of the path.
PE1 can take into account multiple parameters when inserting ELs; as
examples:
* The ERLD value advertised by transit nodes.
* The requirement of load-balancing for a particular label value.
* Any service provider preference: favor beginning of the path or
end of the path.
In Figure 5, a good strategy may be to use the following stack
<Adj_P1P2, Adj_set_P2P3, ELI1, EL1, Adj_P3P4, Adj_P4P5, Adj_P5P6,
Adj_P6PE2, ELI2, EL2, VPN_label>. The original stack requests P2 to
forward based on an L3 adjacency-set that will require load-
balancing. Therefore, it is important to ensure that P2 can load-
balance correctly. As P2 has a limited ERLD of 3, an ELI/EL must be
inserted just after the label that P2 will use to forward. On the
path to PE2, P3 has also a limited ERLD, but P3 will forward based on
a regular adjacency segment that may not require load-balancing.
Therefore, it does not seem important to ensure that P3 can do load-
balancing despite 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 so is expected
to load-balance. It becomes important to insert a new ELI/EL just
after the P6 forwarding label.
In the case above, the ingress node was able to support a sufficient
MSD to ensure end-to-end load-balancing while taking into account the
path attributes. However, there might be cases where the ingress
node may not have the necessary label imposition capacity.
7.1.2. Example 2: The Ingress Node Does Not Have a Sufficient MSD
ECMP LAG ECMP ECMP
PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- P7 --- P8 --- PE2
Figure 6: MSD Considerations
In Figure 6, PE1 wants to forward MPLS VPN traffic over an explicit
path to PE2 resulting in the following label stack to be pushed onto
the IP header: <Adj_P1P2, Adj_set_P2P3, Adj_P3P4, Adj_P4P5, Adj_P5P6,
Adj_set_P6P7, Adj_P7P8; Adj_set_P8PE2, VPN_label>. PE1 is limited to
push a maximum of 11 labels. P2, P3, and P6 have an ERLD of 3 while
others have an ERLD of 15.
Using a similar strategy as the previous case may lead to a dilemma,
as PE1 can only push a single ELI/EL while we may need a minimum of
three to load-balance the end-to-end path. An optimized stack that
would enable end-to-end load-balancing may be: <Adj_P1P2,
Adj_set_P2P3, ELI1, EL1, Adj_P3P4, Adj_P4P5, Adj_P5P6, Adj_set_P6P7,
ELI2, EL2, Adj_P7P8, Adj_set_P8PE2, ELI3, EL3, VPN_label>.
A decision needs to be taken to favor some part of the path for load-
balancing considering that load-balancing may not work on the other
parts. A service provider may decide to place the ELI/EL after the
P6 forwarding label as it will allow P4 and P6 to load-balance.
Placing the ELI/EL at the bottom of the stack is also a possibility
enabling load-balancing for P4 and P8.
7.2. Considerations for the Placement of Entropy Labels
The sample cases described in the previous section showed that ELI/EL
placement 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 an implementation MAY
take into account when placing ELI/ELs. This list of criteria is not
considered exhaustive and an implementation MAY take into account
additional criteria or tiebreakers that are not documented here. As
the insertion of ELI/ELs is performed by the ingress node, having
ingress nodes that do not use the same criteria does not cause an
interoperability issue. However, from a network design and operation
perspective, it is better to have all ingress routers using the same
criteria.
An implementation SHOULD try to maximize the possibility of load-
balancing along the path by inserting an ELI/EL where multiple equal-
cost paths are available and minimize the number of ELI/ELs that need
to be inserted. In case of a trade-off, an implementation SHOULD
provide flexibility to the operator to select the criteria to be
considered when placing ELI/ELs or specify a subobjective for
optimization.
2 2
PE1 -- P1 -- P2 --P3 --- P4 --- P5 -- ... -- P8 -- P9 -- PE2
| |
P3'--- P4'--- P5'
Figure 7: MSD Trade-Offs
Figure 7 will be used as reference in the following subsections. All
metrics are equal to 1 except P3-P4 and P4-P5, which have a metric 2.
We consider the MSD of nodes to be the full limit of label imposition
(including service labels, entropy labels, and transport labels).
7.2.1. ERLD Value
As mentioned in Section 7.1, the ERLD value is an important parameter
to consider when inserting an ELI/EL. If an ELI/EL does not fall
within the ERLD of a node on the path, the node will not be able to
load-balance the traffic efficiently.
The ERLD value can be advertised via protocols, and those extensions
are described in separate documents (for instance, [ISIS-ELC] and
[OSPF-ELC]).
Let's consider a path from PE1 to PE2 using the following stack
pushed by PE1: <Adj_P1P2, Node_P9, Adj_P9PE2, Service_label>.
Using the ERLD as an input parameter can 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 has advertised the adjacency segment. In the example
above, the ERLD associated with Adj_P1P2 would be the ERLD of router
P1, as P1 will perform the forwarding based on the Adj_P1P2 label.
For a label bound to a node segment, multiple strategies MAY be
implemented. An implementation MAY try to evaluate the minimum ERLD
value along the node segment path. If an implementation cannot find
the minimum ERLD along the path of the segment or does not support
the computation of the minimum ERLD, it SHOULD instead use the ERLD
of the tail-end node. Using the ERLD of the tail end of the node
segment mimics the behavior of [RFC6790] where the ingress takes only
care of the egress of the LSP. In the example above, if the
implementation supports computation of minimum ERLD along the path,
the ERLD associated with label Node_P9 would be the minimum ERLD
between nodes {P2,P3,P4 ..., P8}. If the implementation does not
support the computation of minimum ERLD, it will consider the ERLD of
P9 (tail-end node of Node_P9 SID). While providing the more optimal
ELI/EL placement, evaluating the minimum ERLD increases the
complexity of ELI/EL insertion. As the path to the Node SID may
change over time, a recomputation of the minimum ERLD is required for
each topology change. This recomputation may require the positions
of the ELI/ELs to change.
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 or does not support this
evaluation, it SHOULD instead use the ERLD of the node advertising
the Binding SID. As for the node segment, evaluating the minimum
ERLD adds complexity in the ELI/EL insertion process.
7.2.2. Segment Type
Depending on the type of segment a particular label is bound to, an
implementation can deduce that this particular label will be subject
to load-balancing on the path.
7.2.2.1. Node SID
An MPLS label bound to a Node SID represents a path that may cross
multiple hops. Load-balancing may be needed on the node starting
this path but also on any node along the path.
In Figure 7, let's consider a path from PE1 to PE2 using the
following stack pushed by PE1: <Adj_P1P2, Node_P9, Adj_P9PE2,
Service_label>.
If, for example, PE1 is limited to push 6 labels, it can add a single
ELI/EL within the label stack. An operator may want to favor a
placement that would allow load-balancing along the Node SID path.
In Figure 7, P3, which is along the Node SID path, requires load-
balancing between two equal-cost paths.
An implementation MAY try to evaluate if load-balancing is really
required within a node segment path. This could be done by running
an additional SPT (Shortest Path Tree) computation and analyzing of
the node segment path to prevent a node segment that does not really
require load-balancing from being preferred when placing ELI/ELs.
Such inspection may be time consuming for implementations and without
a 100% guarantee, as a node segment path may use LAGs that are
invisible to the IP topology. As a simpler approach, an
implementation MAY consider that a label bound to a Node SID will be
subject to load-balancing and require an ELI/EL.
7.2.2.2. Adjacency-Set SID
An adjacency-set is an Adj-SID that refers to a set of adjacencies.
When an adjacency-set segment is used within a label stack, an
implementation can deduce that load-balancing is expected at the node
that advertised this adjacency segment. An implementation MAY favor
the insertion of an ELI/EL after the Adj-SID representing an
adjacency-set.
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.
An implementation MAY NOT place an ELI/EL after a regular Adj-SID in
order to favor the insertion of ELI/ELs following other segments.
Readers should note that an adjacency segment representing a single
IP link may require load-balancing. This is the case when a LAG (L2
bundle) is implemented between two IP nodes and the L2 bundle SR
extensions [RFC8668] are not implemented. In such a case, it could
be useful to insert an ELI/EL in a readable position for the LSR
advertising the label associated with the adjacency segment. To
communicate the requirement for load-balancing for a particular
Adjacency SID to ingress nodes, a user can enforce the use of the L2
bundle SR extensions defined in [RFC8668] or can declare the single
adjacency as an adjacency-set.
7.2.2.4. Adjacency SID Representing a Single Link within an L2 Bundle
When the L2 bundle SR extensions [RFC8668] 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 MAY NOT insert an
ELI/EL after the corresponding label.
7.2.2.5. Adjacency SID Representing an L2 Bundle
When the L2 bundle SR extensions [RFC8668] 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 MAY insert an ELI/EL after the
corresponding label.
7.2.3. Maximizing Number of LSRs That Will Load-Balance
When placing ELI/ELs, an implementation MAY optimize the number of
LSRs that both need to load-balance (i.e., have ECMPs) and that will
be able to perform load-balancing (i.e., the EL is within their
ERLD).
Let's consider a path from PE1 to PE2 using the following stack
pushed by PE1: <Adj_P1P2, Node_P9, Adj_P9PE2, Service_label>. All
routers have an ERLD of 10 except P1 and P2, which have an ERLD of 4.
PE1 is able to push 6 labels, so only a single ELI/EL can be added.
In the example above, adding an ELI/EL after Adj_P1P2 will only allow
load-balancing at P1, while inserting it after Adj_PE2P9 will allow
load-balancing at P2, P3 ... P9 and maximize the number of LSRs that
can perform load-balancing.
7.2.4. Preference for a Part of the Path
An implementation MAY allow the user to favor a part of the end-to-
end path when the number of ELI/ELs that can be pushed is not enough
to cover the entire path. As an example, a service provider may want
to favor load-balancing at the beginning of the path or at the end of
the path, so the implementation favors putting the ELI/ELs near the
top or the bottom of the stack.
7.2.5. Combining Criteria
An implementation MAY combine multiple criteria to determine the best
ELI/ELs placement. However, combining too many criteria could lead
to implementation complexity and high resource consumption. Each
time the network topology changes, a new evaluation of the ELI/EL
placement will be necessary for each impacted LSP.
8. A Simple Example Algorithm
A simple implementation might take into account the ERLD when placing
ELI/EL while trying to minimize the number of ELI/ELs inserted and
trying to maximize the number of LSRs that can load-balance.
The example algorithm is based on the following considerations:
* An LSR that can insert a limited number of <ELI, EL> pairs should
insert such pairs deeper in the stack.
* An LSR should try to insert <ELI, EL> pairs at positions to
maximize the number of transit LSRs for which the EL occurs within
the ERLD of those LSRs.
* An LSR should try to insert the minimum number of such pairs while
trying to satisfy the above criteria.
The pseudocode of the example algorithm is shown below.
Initialize the current EL insertion point to the
bottom-most label in the stack that is EL-capable
while (local-node can push more <ELI,EL> pairs OR
insertion point is not above label stack) {
insert an <ELI,EL> pair below current insertion point
move new insertion point up from current insertion point until
((last inserted EL is below the ERLD) AND (ERLD > 2)
AND
(new insertion point is EL-capable))
set current insertion point to new insertion point
}
Figure 8: Example Algorithm to Insert <ELI, EL> Pairs in a Label
Stack
When this algorithm is applied to the example described in Section 3,
it will result in ELs being inserted in two positions; one after the
label L_N-D and another after L_N-P3. Thus, the resulting label
stack would be <L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI, EL>.
9. Deployment Considerations
As long as LSR node data-plane capabilities are limited (number of
labels that can be pushed or number of labels that can be inspected),
hop-by-hop load-balancing of SPRING-encapsulated flows will require
trade-offs.
The entropy label is still a good and usable solution as it allows
load-balancing without having to perform deep packet inspection on
each LSR: It does not seem reasonable to have an LSR inspecting UDP
ports within a GRE tunnel carried over a 15-label SPRING tunnel.
Due to the limited capacity of reading a deep stack of MPLS labels,
multiple ELI/ELs may be required within the stack, which directly
impacts the capacity of the head-end to push a deep stack: each ELI/
EL inserted requires two additional labels to be pushed.
Placement strategies of ELI/ELs are required to find the best trade-
off. Multiple criteria could be taken into account, and some level
of customization (by the user) is required to accommodate different
deployments. Since analyzing the path of each destination to
determine the best ELI/EL 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 the future, hardware and software capacity may increase data-plane
capabilities and may remove some of these limitations, increasing
load-balancing capability using entropy labels.
10. Options Considered
Different options that were considered to arrive at the recommended
solution are documented in this section.
These options are detailed here only for historical purposes.
10.1. Single EL at the Bottom of the Stack
In this option, a single EL is used for the entire label stack. The
source LSR S encodes the entropy label at the bottom of the label
stack. In the example described in Section 3, it will result in the
label stack at LSR S to look like <L_N-P3, L_A-L1, L_N-D, ELI, EL>
<remaining packet header>. Note that the notation in [RFC6790] is
used to describe the label stack. An issue with this approach is
that as the label stack grows due an increase in the number of SIDs,
the EL goes correspondingly deeper in the label stack. Hence,
transit LSRs have to access a larger number of bytes in the packet
header when making forwarding decisions. In the example described in
Section 3, if we consider that the LSR P1 has an ERLD of 3, P1 would
load-balance traffic poorly on the parallel links L3 and L4 since the
EL is below the ERLD of P1. A load-balanced network design using
this approach must ensure that all intermediate LSRs have the
capability to read the maximum label stack depth as required for the
application that uses source-routed stacking.
This option was rejected since there exist a number of hardware
implementations that have a low maximum readable label depth.
Choosing this option can lead to a loss of load-balancing using EL in
a significant part of the network when that is a critical requirement
in a service-provider network.
10.2. An EL per Segment in the Stack
In this option, each segment/label in the stack can be given its own
EL. When load-balancing is required to direct traffic on a segment,
the source LSR pushes an <ELI, EL> before pushing the label
associated to this segment. In the example described in Section 3,
the source label stack that is LSR S encoded 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
implementations that can push a limited number of labels on the label
stack. Choosing this option would result in a hardware requirement
to push two additional labels per tunnel label. Hence, it would
restrict the number of tunnels that can be stacked in an LSP and
hence, constrain the types of LSPs that can be created. This was
considered unacceptable.
10.3. A Reusable EL for a Stack of Tunnels
In this option, an LSR that terminates a tunnel reuses the EL of the
terminated tunnel for the next inner tunnel. It does this by storing
the EL from the outer tunnel when that tunnel is terminated and
reinserting it below the next inner tunnel label during the label-
swap operation. The LSR that stacks tunnels should insert an EL
below the outermost tunnel. It should not insert ELs for any inner
tunnels. Also, the penultimate hop LSR of a segment must not pop the
ELI and EL even though they are exposed as the top labels since the
terminating LSR of that segment would reuse the EL for the next
segment.
In Section 3, the source label stack that is LSR S encoded would be
<L_N-P3, ELI, EL, L_A-L1, L_N-D>. At P1, the outgoing label stack
would be <L_N-P3, ELI, EL, L_A-L1, L_N-D> after it has load-balanced
to one of the links L3 or L4. At P3, the outgoing label stack would
be <L_N-D, ELI, EL>. At P2, the outgoing label stack would be <L_N-
D, ELI, EL> and it would load-balance to one of the next-hop LSRs P4
or P5. Accessing the EL at an intermediate LSR (e.g., P1) is
independent of the depth of the label stack and hence, independent of
the specific use case to which the label stack is applied.
This option was rejected due to the significant change in label-swap
operations that would be required for existing hardware.
10.4. EL at Top of Stack
A slight variant of the reusable EL option is to keep the EL at the
top of the stack rather than below the tunnel label. In this case,
each LSR that is not terminating a segment should continue to keep
the received EL at the top of the stack when forwarding the packet
along the segment. An LSR that terminates a segment should use the
EL from the terminated segment at the top of the stack when
forwarding onto the next segment.
This option was rejected due to the significant change in label swap
operations that would be required for existing hardware.
10.5. ELs at Readable Label Stack Depths
In this option, the source LSR inserts ELs for tunnels in the label
stack at depths such that each LSR along the path that must load-
balance is able to access at least one EL. Note that the source LSR
may have to insert multiple ELs in the label stack at different
depths for this to work since intermediate LSRs may have differing
capabilities in accessing the depth of a label stack. The label
stack depth access value of intermediate LSRs must be known to create
such a label stack. How this value is determined is outside the
scope of this document. This value can be advertised using a
protocol such as an IGP.
Applying this method to the example in Section 3, if LSR P1 needs to
have the EL within a depth of 4, then the source label stack that is
LSR S encoded would be <L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI, EL>,
where all the ELs would typically have the same value.
In the case where the ERLD has different values along the path and
the LSR that is inserting <ELI, EL> pairs has no limit on how many
pairs it can insert, and it knows the appropriate positions in the
stack where they should be inserted, this option is the same as the
recommended solution in Section 7.
Note that a refinement of this solution, which balances the number of
pushed labels against the desired entropy, is the solution described
in Section 7.
11. IANA Considerations
This document has no IANA actions.
12. Security Considerations
Compared to [RFC6790], this document introduces the notion of ERLD
and MSD, and may require an ingress node to push multiple ELIs/ELs.
These changes do not introduce any new security considerations beyond
those already listed in [RFC6790].
13. References
13.1. Normative References
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
<https://www.rfc-editor.org/info/rfc6790>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Litkowski, S., and R. Shakir, "Segment Routing with the
MPLS Data Plane", RFC 8660, DOI 10.17487/RFC8660, December
2019, <https://www.rfc-editor.org/info/rfc8660>.
13.2. Informative References
[ISIS-ELC] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
and M. Bocci, "Signaling Entropy Label Capability and
Entropy Readable Label Depth Using IS-IS", Work in
Progress, Internet-Draft, draft-ietf-isis-mpls-elc-10, 21
October 2019,
<https://tools.ietf.org/html/draft-ietf-isis-mpls-elc-10>.
[OSPF-ELC] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
and M. Bocci, "Signaling Entropy Label Capability and
Entropy Readable Label-stack Depth Using OSPF", Work in
Progress, Internet-Draft, draft-ietf-ospf-mpls-elc-12, 25
October 2019,
<https://tools.ietf.org/html/draft-ietf-ospf-mpls-elc-12>.
[RFC8668] Ginsberg, L., Bashandy, A., Filsfils, C., Nanduri, M., and
E. Aries, "Advertising Layer 2 Bundle Member Link
Attributes in IS-IS", RFC 8668, DOI 10.17487/RFC8668,
December 2019, <https://www.rfc-editor.org/info/rfc8668>.
[RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
Litkowski, S., Horneffer, M., and R. Shakir, "Source
Packet Routing in Networking (SPRING) Problem Statement
and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
2016, <https://www.rfc-editor.org/info/rfc7855>.
[RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
"Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
DOI 10.17487/RFC8476, December 2018,
<https://www.rfc-editor.org/info/rfc8476>.
[RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
DOI 10.17487/RFC8491, November 2018,
<https://www.rfc-editor.org/info/rfc8491>.
[MSD-BGP] Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G.,
and N. Triantafillis, "Signaling MSD (Maximum SID Depth)
using Border Gateway Protocol Link-State", Work in
Progress, Internet-Draft, draft-ietf-idr-bgp-ls-segment-
routing-msd-09, 15 October 2019,
<https://tools.ietf.org/html/draft-ietf-idr-bgp-ls-
segment-routing-msd-09>.
Acknowledgements
The authors would like to thank John Drake, Loa Andersson, Curtis
Villamizar, Greg Mirsky, Markus Jork, Kamran Raza, Carlos Pignataro,
Bruno Decraene, Chris Bowers, Nobo Akiya, Daniele Ceccarelli, and Joe
Clarke for their review, comments, and suggestions.
Contributors
Xiaohu Xu
Huawei
Email: xuxiaohu@huawei.com
Wim Hendrickx
Nokia
Email: wim.henderickx@nokia.com
Gunter Van de Velde
Nokia
Email: gunter.van_de_velde@nokia.com
Acee Lindem
Cisco
Email: acee@cisco.com
Authors' Addresses
Sriganesh Kini
Email: sriganeshkini@gmail.com
Kireeti Kompella
Juniper
Email: kireeti@juniper.net
Siva Sivabalan
Cisco
Email: msiva@cisco.com
Stephane Litkowski
Orange
Email: slitkows.ietf@gmail.com
Rob Shakir
Google
Email: robjs@google.com
Jeff Tantsura
Apstra, Inc.
Email: jefftant.ietf@gmail.com