Network Working Group S. Kini, Ed.
Internet-Draft Ericsson
Intended status: Informational K. Kompella
Expires: March 02, 2014 Juniper
S. Sivabalan
Cisco
August 29, 2013
Entropy labels for source routed stacked tunnels
draft-kini-mpls-entropy-label-src-stacked-tunnels-01
Abstract
Source routed tunnel stacking is a technique that can be leveraged to
provide a method to steer a packet through a controlled set of
segments. This can be applied to the Multi Protocol Label Switching
(MPLS) data plane. Entropy label (EL) is a technique used in MPLS to
improve load balancing. This document examines how ELs are to be
applied to source routed stacked tunnels.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Abbreviations and Terminology . . . . . . . . . . . . . . . . 2
3. Entropy Labels for source routed stacked tunnels . . . . . . 3
3.1. Single EL at the bottom of the stack of tunnels . . . . . 4
3.2. An EL per tunnel in the stack . . . . . . . . . . . . . . 4
3.3. A re-usable EL for a stack of tunnels . . . . . . . . . . 4
3.4. ELs at readable label stack depths . . . . . . . . . . . 5
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
The source routed stacked tunnels paradigm is leveraged by techniques
such as Segment Routing (SR) [I-D.filsfils-rtgwg-segment-routing] to
steer a packet through a set of segments. This can be directly
applied to the MPLS data plane. Entropy labels (EL) [RFC6790] is a
technique used by the MPLS data plane to do load balancing. Applying
ELs to stacked tunnels brings up some issues and these are documented
in Section 3.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Abbreviations and Terminology
EL - Entropy Label
ELI - Entropy Label Identifier
SR - Segment Routing
ECMP - Equal Cost Multi Paths
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MPLS - Multi Protocol Label Switching
SID - Segment Identifier
3. Entropy Labels for source routed stacked tunnels
Stacked tunnels have several use-cases, one of which is service
chaining [I-D.filsfils-rtgwg-segment-routing-use-cases]. Consider a
service-chaining network in Figure 1. The source LSR S wants to send
traffic to destination LSR D. This traffic is required to go through
service nodes S1 and S2 to produce the service chain S-S1-S2-D.
Segment Routing can be used to achieve this. Load balancing is
required across the parallel links between P1 and S1. Load balancing
is also required between the ECMP paths from S1 to S2, S1-P1-P2-P3-S2
and S1-P1-P2-P4-S2. The source LSR wants the intermediate LSRs P1
and P2 to take local load balancing decisions and does not specify
the Segment Identifiers (SIDs) of specific interfaces. Entropy
labels should be used to achieve the desired load balancing. Two
possible ways to use the entropy labels and their associated
tradeoffs are discussed below. We denote SN to be the node segment
identifier (SID) of LSR N and SN{L1,L2,...} to denote the SID of the
adjacency set for links {L1,L2,...} of LSR N and S-N to denote the
SID for a service at service node N. The label stack that the source
LSR S uses for the service chain can be <SS1, S-S1, SS2, S-S2, SD> or
<SP1, SP1{L1,L2}, S-S1, SS2, S-S2, SD>. The issues discussed in this
document are equally applicable to both of these options.
+-----+ +-----+
| S1 | +------| P3 |------+
+-----+ | +-----+ |
L1| |L2 | |
+-----+ +-----+ +-----+ +-----+
| S |-----| P1 |-----| P2 | | S2 |
+-----+ +-----+ +-----+ +-----+
| |
| +-----+ |
+------| P4 |------+
+-----+
|
+-----+
| D |
+-----+
S=Source LSR, D=Destination LSR, S1,S2=service-nodes, L1,L2=links,
P1,P2,P3,P4=Transit LSRs
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Figure 1: Service chaining use-case
3.1. Single EL at the bottom of the stack of tunnels
In this option a single EL is used for the entire label stack. The
source LSR S encodes the entropy label (EL) below the labels of all
the stacked tunnels. In Figure 1 label stack at LSR S would look
like <SP1, SP1{L1,L2}, SS1, S-S1, SS2, S-S2, SD, 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
correspondingly goes deeper in the label stack. As a result,
intermediate LSRs (such as P1) that have to walk the label stack at
least until the EL to perform load balancing decisions have to access
a larger number of bytes in the packet header when making forwarding
decisions. A network design using this approach, should ensure that
all intermediate LSRs have the capability to traverse the maximum
label stack depth in order to do effective load balancing. The use-
case for which the tunnel stacking is applied would determine the
maximum label stack depth.
3.2. An EL per tunnel in the stack
In this option each tunnel in the stack can be given its own EL. The
source LSR pushes an <ELI, EL> before pushing a tunnel label when
load balancing is required to direct traffic on that tunnel. For the
same Figure 1 above, the source LSR S encoded label stack would be
<SP1, SP1{L1,L2}, ELI, EL1, SS1, S-S1, SS2, ELI, EL2, SD> where all
the ELs would typically have the same value. Accessing the EL at an
intermediate LSR is independent of the depth of the label stack and
hence independent of the specific use-case to which the stacked
tunnels are applied. 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
should have the capability to push such a deep label stack. Also,
the bandwidth overhead and potential MTU issues of deep label stacks
should be accounted for in the network design.
3.3. A re-usable EL for a stack of tunnels
In this option an LSR that terminates a tunnel re-uses the EL of the
terminated tunnel for the next inner tunnel. It does this by storing
the EL from the outer tunnel when that tunnel is terminated and re-
inserting it below the next inner tunnel label during the label swap
operation. The LSR that stacks tunnels SHOULD insert an EL below the
outermost tunnel. It SHOULD NOT insert ELs for any inner tunnels.
For the same Figure 1 above, the source LSR S encoded label stack
would be <SP1, ELI, EL, SP1{L1,L2}, SS1, S-S1, SS2, SD>. At P1 the
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outgoing label stack would be <SS1, ELI, EL, S-S1, SS2, SD> after it
has load balanced to one of the links L1 or L2. At S1 the outgoing
label stack would be <SS2, ELI, EL, SD>. At P2 the outgoing label
stack would be <SS2, ELI, EL, SD> and it would load balance to one of
the nexthop LSRs P3 or P4. Accessing the EL at an intermediate LSR
is independent of the depth of the label stack and hence independent
of the specific use-case to which the stacked tunnels are applied.
3.4. 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. Details of this will follow in subsequent
versions if this option is found to be worth pursuing. For the same
Figure 1 above, if LSR P1 needs to have the EL within a depth of 4,
then the source LSR S encoded label stack would be <SP1, SP1{L1,L2},
ELI, EL1, SS1, S-S1, SS2, ELI, EL2, SD> where all the ELs would
typically have the same value.
4. Acknowledgements
The authors would like to thank Rob Shakir and TBD for their
comments.
5. IANA Considerations
This memo includes no request to IANA.
6. Security Considerations
7. References
7.1. Normative References
[I-D.filsfils-rtgwg-segment-routing-use-cases]
Filsfils, C., Francois, P., Previdi, S., Decraene, B.,
Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
"Segment Routing Use Cases", draft-filsfils-rtgwg-segment-
routing-use-cases-01 (work in progress), July 2013.
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[I-D.filsfils-rtgwg-segment-routing]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
"Segment Routing Architecture", draft-filsfils-rtgwg-
segment-routing-00 (work in progress), June 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, November 2012.
7.2. Informative References
[I-D.previdi-isis-segment-routing-extensions]
Previdi, S., Filsfils, C., Bashandy, A., Gredler, H., and
S. Litkowski, "IS-IS Extensions for Segment Routing",
draft-previdi-isis-segment-routing-extensions-02 (work in
progress), July 2013.
[I-D.psenak-ospf-segment-routing-extensions]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H., and R.
Shakir, "OSPF Extensions for Segment Routing", draft-
psenak-ospf-segment-routing-extensions-02 (work in
progress), July 2013.
Authors' Addresses
Sriganesh Kini (editor)
Ericsson
Email: sriganesh.kini@ericsson.com
Kireeti Kompella
Juniper
Email: kireeti@juniper.net
Siva Sivabalan
Cisco
Email: msiva@cisco.com
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