Network Working Group S. Kini, Ed.
Internet-Draft Ericsson
Intended status: Informational K. Kompella
Expires: April 28, 2015 Juniper
S. Sivabalan
Cisco
S. Litkowski
Orange
R. Shakir
B.T.
X. Xu
Huawei
W. Hendrickx
Alcatel-Lucent
J. Tantsura
Ericsson
October 25, 2014
Entropy labels for source routed stacked tunnels
draft-kini-mpls-spring-entropy-label-02
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 and describes how ELs
are to be applied to source routed stacked tunnels.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 28, 2015.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Abbreviations and Terminology . . . . . . . . . . . . . . . . 3
3. Use-case for multipath load balancing in source stacked
tunnels . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Recommended EL solution for SPRING . . . . . . . . . . . . . 5
5. Options considered . . . . . . . . . . . . . . . . . . . . . 6
5.1. Single EL at the bottom of the stack of tunnels . . . . . 6
5.2. An EL per tunnel in the stack . . . . . . . . . . . . . . 7
5.3. A re-usable EL for a stack of tunnels . . . . . . . . . . 8
5.3.1. EL at top of stack . . . . . . . . . . . . . . . . . 8
5.4. ELs at readable label stack depths . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The source routed stacked tunnels paradigm is leveraged by techniques
such as Segment Routing (SR) [I-D.filsfils-spring-segment-routing] to
steer a packet through a set of segments. This can be directly
applied to the MPLS data plane, but it has implications on label
stack depth.
Clarifying statements on label stack depth have been provided in
[RFC7325] but they do not address the case of source routed stacked
MPLS tunnels as described in [I-D.gredler-spring-mpls] or
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[I-D.filsfils-spring-segment-routing] where deeper label stacks are
more prevalent.
Entropy label (EL) [RFC6790] is a technique used in the MPLS data
plane to provide entropy for load balancing. When using LSP
hierarchies there are implications on how [RFC6790] should be
applied. One such issue is addressed by
[I-D.ravisingh-mpls-el-for-seamless-mpls] but that is when different
levels of the hierarchy are created at different LSRs. The current
document addresses the case where the hierarchy is created at a
single LSR as required by source stacked tunnels.
A use-case requiring load balancing with source stacked tunnels is
given in Section 3. A recommended solution is described in Section 4
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 5.
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].
Although this document is not a protocol specification, the use of
this language clarifies the instructions to protocol designers
producing solutions that satisfy the requirements set out in this
document.
2. Abbreviations and Terminology
EL - Entropy Label
ELI - Entropy Label Identifier
ELC - Entropy Label Capability
SR - Segment Routing
ECMP - Equal Cost Multi Paths
MPLS - Multiprotocol Label Switching
SID - Segment Identifier
RLD - Readable Label Depth
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OAM - Operation, Administration and Maintenance
3. Use-case for multipath load balancing in source stacked tunnels
Source stacked tunnels have several use-cases, one of which is
service chaining [I-D.filsfils-spring-segment-routing-use-cases].
Consider the service-chaining network in Figure 1 that has MPLS as
the data plane. The requirement of the use-case is to create a LSP
from source LSR S, apply the services S1, S2 and finally terminate
the LSP at destination LSR D. Local load balancing is required
across the parallel links between P1 and S1. Local load balancing is
also required between the ECMP paths from S1 to S2 i.e., between the
paths S1-P1-P2-P3-S2 and S1-P1-P2-P4-S2. Segment routing can be used
to achieve this. A segment to S1 is stacked above the segment to S2
which in turn is stacked above the segment to D. Labels for service
instructions are also inserted in the stack at appropriate depths so
that services S1 and S2 are executed. To achieve local load
balancing the SIDs of specific interfaces is not specified. Since
entropy label is a standardized [RFC6790] mechanism defined for MPLS
it can be adapted to the case of source stacked tunnels. Multiple
ways to apply entropy labels exist and a recommended solution is
described in Section 4 and all the options considered are listed in
Section 5 along with their tradeoffs. We denote SN to be the node
SID of LSR N and SN{L1,L2,...} to denote the SID of the adjacency for
the set for links {L1,L2,...} of LSR N and S-SvcN to denote the SID
for a service at service LSR N. The label stack that the source LSR
S uses for the LSP can be <SS1, S-SvcS1, SS2, S-SvcS2, SD> or <SP1,
SP1{L1,L2}, S-SvcS1, SS2, S-SvcS2, SD>.
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+-----+ +-----+
| S1 | +------| P3 |------+
+-----+ | +-----+ |
L1| |L2 | |
+-----+ +-----+ +-----+ +-----+
| S |-----| P1 |-----| P2 | | S2 |
+-----+ +-----+ +-----+ +-----+
| |
| +-----+ |
+------| P4 |------+
+-----+
|
+-----+
| D |
+-----+
S=Source LSR, D=Destination LSR, S1,S2=service-LSRs, L1,L2=links,
P1,P2,P3,P4=Transit LSRs
Figure 1: Service chaining use-case
4. Recommended EL solution for SPRING
The solution described in this section follows [RFC6790].
An LSR may have a limitation in its ability to read and process the
label stack in order to do multipath load balancing. This limitation
expressed in terms of the number of label stack entries that the LSR
can read is henceforth referred to 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. 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
corresponding to a label stack, multiple <ELI, EL> pairs MAY be
inserted in the label stack as long as the labels below which they
are inserted are entropy label capable. 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 RLD, then the load balancing performed by
that LSR would be poor. Special attention should be paid when a
forwarding adjacency LSP (FA-LSP) [RFC4206] is used as a link along
the path of a source stacked LSP, since the labels of the FA-LSP
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would additionally count towards the depth of the label stack when
calculating the appropriate positions to insert the ELs. The
recommendations for inserting <ELI, EL> pairs are:
o An LSR that is limited in the number of <ELI, EL> pairs that it
can insert SHOULD insert such pairs deeper in the stack.
o An LSR SHOULD try to insert an <ELI, EL> pair within the RLD of
the maximum number of LSRs along the path as it can.
o An LSR SHOULD try to insert the minimum number of such pairs while
trying to satisfy the above criteria.
A sample algorithm to insert ELs is shown below. Implementations can
choose any algorithm as long as it follows the above recommendations.
Initialize the current EL insertion point to the
bottommost label in the stack that is EL-capable
while local-node can push more labels OR
top of stack has been reached {
insert an ELI+EL at current insertion point
move insertion point up until current EL is out of RLD
AND
insertion point is EL-capable
set current insertion point to new insertion point
}
Figure 2: Algorithm to insert <ELI, EL> pairs in a label stack
The RLD can be advertised via protocols and those extensions would be
described in a separate document.
The recommendations above are not expected to bring any additional
OAM considerations beyond those described in section 6 of [RFC6790].
However, the OAM requirements and solutions for source stacked
tunnels are still under discussion and future revisions of this
document will address those if needed.
5. Options considered
5.1. 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, SS1, S-SvcS1, SS2, S-SvcS2, SD, ELI, EL> <remaining packet
header>. Note that the notation in [RFC6790] is used to describe the
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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 (if
found) to perform load balancing decisions have to access a larger
number of bytes in the packet header when making forwarding
decisions. A load balanced network design using this approach must
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.
In the case where the hardware is capable of pushing a single <ELI,
EL> pair at any depth, this option is the same as the recommended
solution in Section 4.
This option was discounted since there exist a number of hardware
implementations which have a low maximum readable label depth.
Choosing this option can lead to a loss of load-balancing using EL in
a significant part of the network but that is a critical requirement
in a service provider network.
5.2. 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
<SS1, ELI, EL1, S-SvcS1, SS2, ELI, EL2, S-SvcS2, SD> where all the
ELs can 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.
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
no limit on how many it can insert then this option is the same as
the recommended solution in Section 4.
This option was discounted 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
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restrict the number of tunnels that can form a LSP and constrain the
types of LSPs that can be created. This was considered unacceptable.
5.3. A re-usable EL for a stack of tunnels
In this option an LSR that terminates a tunnel re-uses the EL of the
terminated tunnel for the next inner tunnel. It does this by storing
the EL from the outer tunnel when that tunnel is terminated and re-
inserting it below the next inner tunnel label during the label swap
operation. The LSR that stacks tunnels SHOULD insert an EL below the
outermost tunnel. It SHOULD NOT insert ELs for any inner tunnels.
Also, the penultimate hop LSR of a segment MUST NOT pop the ELI and
EL even though they are exposed as the top labels since the
terminating LSR of that segment would re-use the EL for the next
segment.
For the same Figure 1 above, the source LSR S encoded label stack
would be <SS11, ELI, EL, S-SvcS1, SS2, S-SvcS2, SD>. At P1 the
outgoing label stack would be <SS1, ELI, EL, S-SvcS1, SS2, S-SvcS2,
SD> after it has load balanced to one of the links L1 or L2. At S1
the outgoing label stack would be <SS2, S-SvS2, ELI, EL, SD>. At P2
the outgoing label stack would be <SS2, ELI, EL, S-SvcS2, SD> and it
would load balance to one of the nexthop LSRs P3 or P4. Accessing
the EL at an intermediate LSR (e.g. P3) is independent of the depth
of the label stack and hence independent of the specific use-case to
which the stacked tunnels are applied.
This option was discounted due to the significant change in label
swap operations that would be required for existing hardware.
5.3.1. EL at top of stack
A slight variant of the re-usable EL option is to keep the EL at the
top of the stack rather than below the tunnel label. In this case
each LSR that is not terminating a segment should continue to keep
the received EL at the top of the stack when forwarding the packet
along the segment. An LSR that terminates a segment should use the
EL from the terminated segment at the top of the stack when
forwarding onto the next segment.
This option was discounted due to the significant change in label
swap operations that would be required for existing hardware.
5.4. 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
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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. 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 <SS1, S-SvcS1, ELI, EL2, SS2, SD> where
all the ELs would typically have the same value.
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
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
recommended solution in Section 4.
A variant of this solution was selected which balances the number of
labels that need to be pushed against the requirement for entropy.
6. Acknowledgements
The authors would like to thank John Drake and Loa Andersson for
their comments.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
This document does not introduce any new security considerations
beyond those already listed in [RFC6790].
9. References
9.1. Normative References
[I-D.filsfils-spring-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-spring-
segment-routing-04 (work in progress), July 2014.
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[I-D.filsfils-spring-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., Kini, S., and E.
Crabbe, "Segment Routing Use Cases", draft-filsfils-
spring-segment-routing-use-cases-01 (work in progress),
October 2014.
[I-D.gredler-spring-mpls]
Gredler, H., Rekhter, Y., Jalil, L., Kini, S., and X. Xu,
"Supporting Source/Explicitly Routed Tunnels via Stacked
LSPs", draft-gredler-spring-mpls-06 (work in progress),
May 2014.
[I-D.ravisingh-mpls-el-for-seamless-mpls]
Singh, R., Shen, Y., and J. Drake, "Entropy label for
seamless MPLS", draft-ravisingh-mpls-el-for-seamless-
mpls-02 (work in progress), July 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching
(GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, November 2012.
[RFC7325] Villamizar, C., Kompella, K., Amante, S., Malis, A., and
C. Pignataro, "MPLS Forwarding Compliance and Performance
Requirements", RFC 7325, August 2014.
9.2. Informative References
[I-D.previdi-isis-segment-routing-extensions]
Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
Litkowski, S., and J. Tantsura, "IS-IS Extensions for
Segment Routing", draft-previdi-isis-segment-routing-
extensions-05 (work in progress), February 2014.
[I-D.psenak-ospf-segment-routing-extensions]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", draft-psenak-ospf-
segment-routing-extensions-05 (work in progress), June
2014.
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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
Stephane Litkowski
Orange
Email: stephane.litkowski@orange.com
Rob Shakir
B.T.
Email: rob.shakir@bt.com
Xiaohu Xu
Huawei
Email: xuxiaohu@huawei.com
Wim Hendrickx
Alcatel-Lucent
Email: wim.henderickx@alcatel-lucent.com
Jeff Tantsura
Ericsson
Email: jeff.tantsura@ericsson.com
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