SPRING WG Ting. Liao
Internet-Draft Ting. Ao
Intended status: Standards Track Fangwei. Hu
Expires: April 21, 2016 ZTE Corporation
October 19, 2015
SPRING SID Calculation
draft-lt-spring-sid-calculation-00.txt
Abstract
Segment Routing (SR) allows for a flexible definition of end-to-end
paths within IGP topologies by encoding paths as sequences of
topological sub-paths, called "segments". These segments are
advertised by the link-state routing protocols (IS-IS and OSPF). And
a segment is identified by a Segment Routing ID (SID). This document
proposes a method to calculate SID forwarding entry within SR
topology in a SR domain, and to guarantee the consistency of SR
encapsulation forwarding.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Abbreviations . . . . . . . . . . . . . . . . 3
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. SID calculation . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. SR independent topology . . . . . . . . . . . . . . . . . 4
4.2. SID calculation . . . . . . . . . . . . . . . . . . . . . 5
4.3. With SRMN or SRMS . . . . . . . . . . . . . . . . . . . . 5
4.3.1. Default label . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Normative References . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Segment Routing (SR) allows for a flexible definition of end-to-end
paths within IGP topologies by encoding paths as sequences of
topological sub-paths, called "segments". The segments for its
attached prefixes and adjacencies are advertised by the link-state
routing protocols (IS-IS and OSPF). These segments are called IGP
segments or IGP SIDs. And an IGP-Prefix Segment is an IGP segment
attached to an IGP prefix. The segments forwarding entries are
learnt from its attached prefixes' egress information as described in
[I-D.ietf-spring-segment-routing], where wrote: A node N attaching a
Prefix-SID SID-R to its attached prefix R MUST maintain the following
FIB entry: Incoming Active Segment: SID-R Ingress Operation: NEXT
Egress interface: NULL A remote node M MUST maintain the following
FIB entry for any learned Prefix-SID SID-R attached to IP prefix R:
Incoming Active Segment: SID-R Ingress Operation: If the next-hop of
R is the originator of R and instructed to remove the active segment:
NEXT Else: CONTINUE Egress interface: the interface towards the next-
hop along the shortest-path to prefix R.
In some situations such as the SR need to be calculated as CSPF
calculation, the path calculated is not the shortest path, so the SR
needs to have the dependent topology. with the knowledge learnt from
[I-D.ietf-spring-segment-routing], the SR node does not have the
state about its neighbours. With the ldp interoperability as
described in [I-D.filsfils-spring-segment-routing-ldp-interop]
section 4.2, P6's next-hop for the IGP route "PE3" is not SR capable
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(P7 does not advertise the SR capability). However, P6 has an LDP
label binding from that next-hop for the same FEC (e.g. LDP label
1037). Hence, P6 swaps 103 for 1037 and forwards to P7. The SR node
needs to judge its neighbour is SR or LDP. So it is still necessary
to maintain a SR topology to do the sid calculation.
This draft describes a mechanism to do the sid calculation.
2. Conventions and Abbreviations
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 [RFC2119] .
The following notations and abbreviations are used throughout this
draft.
SID: Segment Identifiers.
3. Motivation
As shown in the figure 1.
+--+ +--+ +--+
----|P1|---|P2|---|P3|----
/ +--+ +--+ +--+ \
/ \
+---+ +---+ +---+ +---+
|CE1|----|PE1| |PE2|----|CE2|
+---+ +---+ +---+ +---+
\ /
\ +--+ +--+ /
----|R5|----------|R6|----
+--+ +--+
Figure 1 Scenario 1
PE1\P1\P2\P3\PE2 are routers supporting mpls forwarding, while R5\R6
are routers do not enable mpls or occurred some error the mpls
neighbour between R5\R6 is broken. Such as the SID assigned to PE1
10001, P1 10002, P2 10003, P3 10004 and PE2 10005. From PE1 to PE2,
each link cost between the nodes is 1. So the shortest path is
PE1-R5-R6-PE2, with SR forwarding, the next hop with the forwarding
entry on PE1 to PE2 10005 is to R5 currently. It will causing the
mpls encapsulation packets forwarding to R5 who does not support mpls
forwarding, the packets will be dropped.
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+--+ +--+ +--+ +--+
----|P1|---|P2|---|P3|---|P4|----
/ sr +--+ sr+--+ sr+--+ sr+--+ sr \
/ \
+---+ +---+ +---+ +---+
|CE1|----|PE1| |PE2|----|CE2|
+---+ +---+ +---+ +---+
\ /
\sr +--+ ldp +--+ igp +--+ igp/
----|P5|------|P6|------|R7|----
+--+ +--+ +--+
Figure 2 Scenario 2
PE1\P1\P2\P3\P4\P5\P6\PE2 are routers supporting MPLS forwarding,
Such as the SID assigned to PE1 10001, P1 10002, P2 10003, P3 10004,
PE2 10005, P4 10006, P5 10007. While P5 and P6 is LDP neighbour, and
P6 and R7 is LDP neighbour , R7 and PE2 is LDP neighbour, with some
mistake, the LDP neighbour between P6-R7, R7-PE2 are off, and the IGP
neighbour between P6-R7, R7-PE2 are still on. From PE1 to PE2, each
link cost between the nodes is 1. So the shortest path is
PE1-P5-P6-R7-PE2, with SR forwarding, the next hop with the
forwarding entry on PE1 to PE2 10005 is to P5 currently. It will
causing the MPLS encapsulation packets forwarding to P5 with the
10005 in the outer label, and switches to the LDP label which the P6
assigned to PE2, and the MPLS packet forwarding to P6, P6 accept the
mpls packets, and find that the neighbour of the next hop is not ldp
neighbour, so the packets will be dropped either.
The problem in the SR dependent IGP topology is that the ingress
nodes has not have the full information with all the nodes on the
path. While the list includes nodes' information, all this nodes in
the list is SR enabled nodes, but the nodes' information is not all
the nodes on the path. So it is necessary to have the independent SR
topology to support the mixed network. After all, the network where
some nodes or links are not enabled mpls capabilities is commonly.
4. SID calculation
In the proposed mechanism, the SR will be an independent topology,
and the SID calculation will be calculated based in the topology.
4.1. SR independent topology
The SR topology will be an independent topology based on the SR-
Capabilities Sub-TLV (as described in
[I-D.ietf-isis-segment-routing-extensions] and
[I-D.ietf-ospf-segment-routing-extensions]. ) carried in each
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router's LSP. If SR node A accepted an lsp sending from node B, with
the SR-Capabilities Sub-TLV carried in node B's LSP, the node A will
know that the node B is a SR capable node, and in node A's LSDB, it
will has an independent SR topology.
4.2. SID calculation
With the independent SR topology, SID calculation will be calculated
in the topology, and the SID forwarding entry will be maintained with
SID calculation.
As figure 1 described, PE1 will have an independent topology which
includes the SID information of PE1\P1\P2\P3\PE2, and will not
include the information of R5\R6, so when PE1 calculate the SID to
10005, even though the IGP shortest-path is through R5\R6, PE1 will
calculate the path to 10005 only in the SR topology.
4.3. With SRMN or SRMS
Optionally, If there are some SRMNs or SRMSes assigned by the NMS in
the network, they assign SID to other SR or non-SR nodes, with the SR
nodes, the SR nodes will sending out an TLV with it's SID and the SR-
Capabilities Sub-TLV, then other nodes will learn which nodes are the
SR nodes. With the non-SR nodes, this nodes can not sending out the
SR-Capabilities Sub-TLV, but other SR nodes need to learn this SIDs'
information, in this case, the SRMNs or SRMSes will sending an
default label TLV extension out, using the default label to instruct
the packet with the SID assigned to non-SR node forwarding to itself,
and then it is responsible for sending the packet to the non-SR node.
4.3.1. Default label
Optionally, the default label could either be an reserved and unused
label, or be configured by the NMS and will be known by each SR node.
5. Security Considerations
TBD.
6. Acknowledgements
In progress.
7. IANA Considerations
TBD.
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8. Normative References
[I-D.filsfils-spring-segment-routing-ldp-interop]
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 interoperability with LDP", draft-
filsfils-spring-segment-routing-ldp-interop-03 (work in
progress), March 2015.
[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-05 (work in progress), June 2015.
[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-05 (work in progress), June 2015.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
and r. rjs@rob.sh, "Segment Routing Architecture", draft-
ietf-spring-segment-routing-06 (work in progress), October
2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, DOI 10.17487/RFC4915, June 2007,
<http://www.rfc-editor.org/info/rfc4915>.
[RFC4970] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 4970, DOI 10.17487/RFC4970, July
2007, <http://www.rfc-editor.org/info/rfc4970>.
[RFC5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
July 2008, <http://www.rfc-editor.org/info/rfc5250>.
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Authors' Addresses
Ting Liao
ZTE Corporation
No.50 Software Avenue
Nanjing, Jiangsu 210012
China
Phone: +86 25 88016576
Email: liao.ting@zte.com.cn
Ting Ao
ZTE Corporation
No.889 Bibo Rd
Shanghai 201203
China
Phone: +86 21 68896273
Email: ao.ting@zte.com.cn
Fangwei Hu
ZTE Corporation
No.889 Bibo Rd
Shanghai 201203
China
Phone: +86 21 68896273
Email: hu.fangwei@zte.com.cn
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