Network Working Group S. Kini, Ed.
Internet-Draft Ericsson
Intended status: Informational K. Kompella
Expires: April 2, 2015 Juniper
S. Sivabalan
Cisco
S. Litkowski
Orange
R. Shakir
B.T.
X. Xu
Huawei
W. Hendrickx
Alcatel-Lucent
J. Tantsura
Ericsson
September 29, 2014
Entropy labels for source routed stacked tunnels
draft-kini-mpls-spring-entropy-label-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 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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 2, 2015.
Kini, et al. Expires April 2, 2015 [Page 1]
Internet-Draft EL for source routed stacked tunnels September 2014
Copyright Notice
Copyright (c) 2014 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Recommended EL solution for SPRING . . . . . . . . . . . . . 4
5. Options considered . . . . . . . . . . . . . . . . . . . . . 5
5.1. Single EL at the bottom of the stack of tunnels . . . . . 5
5.2. An EL per tunnel in the stack . . . . . . . . . . . . . . 6
5.3. A re-usable EL for a stack of tunnels . . . . . . . . . . 7
5.3.1. EL at top of stack . . . . . . . . . . . . . . . . . 7
5.4. ELs at readable label stack depths . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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
Kini, et al. Expires April 2, 2015 [Page 2]
Internet-Draft EL for source routed stacked tunnels September 2014
[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. 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].
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
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
Kini, et al. Expires April 2, 2015 [Page 3]
Internet-Draft EL for source routed stacked tunnels September 2014
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>.
+-----+ +-----+
| 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
Kini, et al. Expires April 2, 2015 [Page 4]
Internet-Draft EL for source routed stacked tunnels September 2014
expressed in terms of the number of label stack entries that the LSR
can read and is henceforth referred to as the Readable Label Depth
(RLD) capability. 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. The recommendations for inserting <ELI,
EL> pairs are:
o <ELI, EL> pairs MUST be inserted below those labels that are
advertised with ELC.
o An LSR that is limited in the number of <ELI, EL> pairs that it
can insert SHOULD prefer to 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.
set current EL insertion point to the bottommost EL-capable location
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.
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
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
Kini, et al. Expires April 2, 2015 [Page 5]
Internet-Draft EL for source routed stacked tunnels September 2014
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, 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
restrict the number of tunnels that can form a LSP and constrain the
types of LSPs that can be created. This was considered unacceptable.
Kini, et al. Expires April 2, 2015 [Page 6]
Internet-Draft EL for source routed stacked tunnels September 2014
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, SD>. At P1 the outgoing label
stack would be <SS1, ELI, EL, S-SvcS1, 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 (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
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
Kini, et al. Expires April 2, 2015 [Page 7]
Internet-Draft EL for source routed stacked tunnels September 2014
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
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.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-00 (work in progress),
March 2014.
Kini, et al. Expires April 2, 2015 [Page 8]
Internet-Draft EL for source routed stacked tunnels September 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.
[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.
Authors' Addresses
Sriganesh Kini (editor)
Ericsson
Email: sriganesh.kini@ericsson.com
Kini, et al. Expires April 2, 2015 [Page 9]
Internet-Draft EL for source routed stacked tunnels September 2014
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
Kini, et al. Expires April 2, 2015 [Page 10]
