Internet DRAFT - draft-kini-mpls-spring-entropy-label
draft-kini-mpls-spring-entropy-label
Network Working Group S. Kini, Ed.
Internet-Draft Ericsson
Intended status: Informational K. Kompella
Expires: September 4, 2015 Juniper
S. Sivabalan
Cisco
S. Litkowski
Orange
R. Shakir
B.T.
X. Xu
Huawei
W. Hendrickx
Alcatel-Lucent
J. Tantsura
Ericsson
March 3, 2015
Entropy labels for source routed stacked tunnels
draft-kini-mpls-spring-entropy-label-03
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 4, 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 requiring 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 . . . . . . . . . . 7
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 requiring multipath load balancing in source stacked
tunnels
+------+
| |
+-------| P3 |-----+
| +-----| |---+ |
L3| |L4 +------+ L1| |L2 +----+
| | | | +--| P4 |--+
+-----+ +-----+ +-----+ | +----+ | +-----+
| S |-----| P1 |------------| P2 |--+ +--| D |
| | | | | |--+ +--| |
+-----+ +-----+ +-----+ | +----+ | +-----+
+--| P5 |--+
+----+
S=Source LSR, D=Destination LSR, P1,P2,P3,P4,P5=Transit LSRs,
L1,L2,L3,L4=Links
Figure 1: Traffic engineering use-case
Traffic-engineering (TE) is one of the applications of MPLS and is
also a requirement for source stacked tunnels. 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 stacked
tunnels. Lets L_N-P denote the label to be used to reach the node
SID of LSR P. 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 stacked tunnels. 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.
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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 tunnel's label below which
they are inserted are advertised with entropy label capability
enabled. 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 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 <ELI, EL> pairs at positions so that
for the maximum number of transit LSRs, the EL occurs within the
RLD of the incoming packet to that LSR.
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.
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Initialize the current EL insertion point to the
bottommost 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 RLD) AND (RLD > 2)
AND
(new 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
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
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>
The RLD can be advertised via protocols and those extensions would be
described in separate documents [I-D.xu-isis-mpls-elc] and
[I-D.xu-ospf-mpls-elc].
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 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 the LSR P1 would poorly load-balance traffic
on the 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 must ensure that all intermediate LSRs have the capability
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to traverse the maximum label stack depth as required for that
application that uses source routed stacking.
In the case where the hardware is capable of pushing a single <ELI,
EL> pair at any depth, this option is the same as the recommended
solution in Section 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. In the
example described in Section 3, the source LSR S encoded label stack
would be <L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI, EL> where all the ELs
can be the same. Accessing the EL at an intermediate LSR is
independent of the depth of the label stack and hence independent of
the specific application that uses source stacking on that network.
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
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
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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.
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
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 nexthop 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 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
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 Section 3 above, if LSR P1
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,
EL> where all the ELs would typically have the same value.
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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, Loa Andersson, Curtis
Villamizar, Greg Mirsky, Markus Jork, Kamran Raza and Nobo Akiya for
their review comments and suggestions.
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.
[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-04 (work in progress), October 2014.
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[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-01 (work in progress),
September 2014.
[I-D.xu-ospf-mpls-elc]
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.
[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.
9.2. Informative References
[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.ietf-isis-segment-routing-extensions]
Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS
Extensions for Segment Routing", draft-ietf-isis-segment-
routing-extensions-03 (work in progress), October 2014.
[I-D.ietf-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-ietf-ospf-segment-
routing-extensions-04 (work in progress), February 2015.
[RFC7325] Villamizar, C., Kompella, K., Amante, S., Malis, A., and
C. Pignataro, "MPLS Forwarding Compliance and Performance
Requirements", RFC 7325, August 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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