Network Working Group Anil Kumar S N
INTERNET-DRAFT Gaurav Agrawal
Intended Status: Standard Track Vinod Kumar S
Expires: January 9, 2017 Huawei Technologies India
July 8, 2016
Maximally Redundant Trees in Segment Routing
draft-agv-rtgwg-spring-segment-routing-mrt-02
Abstract
This document presents a Fast Reroute (FRR) approach aimed at
providing link and node protection of node and adjacency segments
within the Segment Routing (SR) framework based on Maximally
Redundant Trees (MRT) FRR algorithm [RFC 7811].
Fast-Reroute with Maximally Redundant Trees (MRT-FRR) for Segment
routing network is to provide link-protection and node- protection
with 100% coverage in Segment routing network topology that is still
connected after the failure. MRT is computational efficient.
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 January 9, 2017.
Copyright Notice
Copyright (c) 2013 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
AGV Expires January 9, 2017 [Page 1]
INTERNET DRAFT MRT in Segment Routing July 8, 2016
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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Standard Terminology . . . . . . . . . . . . . . . . . . . . 3
2. Draft Specific Terminology . . . . . . . . . . . . . . . . . . 3
3. MRT segment routing requirements . . . . . . . . . . . . . . . 5
4. MRT segment routing overview . . . . . . . . . . . . . . . . . 5
5. Requirements for SR MRT implementation . . . . . . . . . . . . 7
3 Security Considerations . . . . . . . . . . . . . . . . . . . . 7
4 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
5 References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1 Normative References . . . . . . . . . . . . . . . . . . . 8
5.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
AGV Expires January 9, 2017 [Page 2]
INTERNET DRAFT MRT in Segment Routing July 8, 2016
1 Introduction
Segment routing MRT FRR is one among the local repair mechanisms
for Segment routing network. Another well known local repair
mechanism for SR is Topology Independent Fast Reroute which is also
capable of restoring end-to-end connectivity in case of a failure of
a link or a node, with guaranteed coverage properties. MRT guarantees
100% recovery for single failures when the network is 2-connected.
This guaranteed coverage does not depend on the link metrics, which
an operator may be using to traffic-engineer the IP network. The
link metrics and general network topology are largely decoupled from
the guaranteed coverage.
The advantage of MRT over TI-LFA would be the computation
complexities involved in MRT is much lesser then TI-LFA with
additional cost of memory usage. MRT is best suited for
access/aggregate ring network or low end devices which has low
computing capacity but could afford to have enough memory to hold
more FIB entries.
1.1 Standard Terminology
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. Draft Specific Terminology
For ease of reading, some of the terminology defined in [RFC 7811] is
repeated here.
Redundant Trees (RT): A pair of trees where the path from any node
X to the root R along the first tree is node-disjoint with the path
from the same node X to the root along the second tree. These can be
computed in 2-connected graphs.
Maximally Redundant Trees (MRT): A pair of trees where the path
from any node X to the root R along the first tree and the path from
the same node X to the root along the second tree share the minimum
number of nodes and the minimum number of links. Each such shared
node is a cut-vertex. Any shared links are cut-links. Any RT is an
MRT but many MRTs are not RTs. The two MRTs are referred to as MRT-
Blue and MRT-Red.
MRT-Red: MRT-Red is used to describe one of the two MRTs; it is
used to described the associated forwarding topology and MT-ID.
Specifically, MRT-Red is the decreasing MRT where links in the GADAG
AGV Expires January 9, 2017 [Page 3]
INTERNET DRAFT MRT in Segment Routing July 8, 2016
are taken in the direction from a higher topologically ordered node
to a lower one.
MRT-Blue: MRT-Blue is used to describe one of the two MRTs; it is
used to described the associated forwarding topology and MT-ID.
Specifically, MRT-Blue is the increasing MRT where links in the GADAG
are taken in the direction from a lower topologically ordered node to
a higher one.
Rainbow MRT MT-ID: It is useful to have an MT-ID that refers to the
multiple MRT topologies and to the default topology. This is
referred to as the Rainbow MRT MT-ID and is used by LDP to reduce
signaling and permit the same label to always be advertised to all
peers for the same (MT-ID, Prefix).
MRT Island: From the computing router, the set of routers that
support a particular MRT profile and are connected via MRT- eligible
links.
Island Border Router (IBR): A router in the MRT Island that is
connected to a router not in the MRT Island and both routers are in a
common area or level.
Island Neighbor (IN): A router that is not in the MRT Island but is
adjacent to an IBR and in the same area/level as the IBR..
AGV Expires January 9, 2017 [Page 4]
INTERNET DRAFT MRT in Segment Routing July 8, 2016
3. MRT segment routing requirements
To extend MRT support to Segment routing following requirement
need to be achieved :
1. SR MRT Capabilities must be advertised using IGP extension for SR
MRT. Also SR MRT capabilities must be in sync with IGP specific MRT
capabilities advertisement. If the peer has not advertised the SR MRT
capability, then it indicates that LSR does not support MRT
procedures.
2. As specified in MRT Architecture [RFC 7811], both Option 1A and
Option 1B can be used for the implementation of SR MRT.
For Option 1A, two additional Prefix SID's/Label for RED and BLUE MT
must be advertised in addition to default prefix SID/Label. The IGP
extension carrying prefix SID for RED and BLUE MT must have
corresponding MT-ID allocated by IANA for default MRT profile.
For Option 1B, Global Unique Context SID/Label for Red & Blue as
topology identifier must be used.
4. MRT segment routing overview
Segment routing devices has to undergo no changes with respect to
forwarding plane. Segment Routing (SR) allows 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). Prefix segments represent an ECMP-aware shortest-path to
a prefix (or a node), as per the state of the IGP topology.
Adjacency segments represent a hop over a specific adjacency between
two nodes in the IGP.
MRT FRR in segment routing network does not require any additional
signaling (other than IGP extensions).
Basically MRT Fast Reroute requires that packets to be forwarded not
only on the shortest-path tree, but also on two Maximally Redundant
Trees (MRTs), referred to as the MRT-Blue and the MRT-Red. A router
that experiences a local failure must also have predetermined which
alternate to use. The MRT algorithm is based on those presented in
[MRTLinear] and expanded in [EnyediThesis]. Default MRT Profile path
calculation uses Lowpoint algorithm to calculate Maximally Redundant
Trees.
Just as packets routed on a hop-by-hop basis require that each router
compute a shortest-path tree that is consistent, it is necessary for
AGV Expires January 9, 2017 [Page 5]
INTERNET DRAFT MRT in Segment Routing July 8, 2016
each router to compute the MRT-Blue next hops and MRT-Red next hops
in a consistent fashion.
A router's Labeled Forwarding Information Base (L-FIB) will continue
to contain primary next hops segment entries for the current
shortest-path tree for forwarding traffic. In addition, a router's
L-FIB will contain primary next hops segments for the MRT-Blue for
forwarding received traffic on the MRT-Blue and primary next hops
segments for the MRT-Red for forwarding received traffic on the MRT-
Red.
Within a link-state IGP domain, an SR-capable IGP node advertises
segments for its attached prefixes and adjacencies. These segments
are called IGP segments or IGP SIDs. They play a key role in Segment
Routing and use-cases as they enable the expression of any
topological path throughout the IGP domain. Such a topological path
is either expressed as a single IGP segment or a list of multiple IGP
segments. After running MRT lowpoint algorithm IGP will advertise two
more additional labels as MRT-BLUE and MRT-RED for each such IGP
segments.
Since segment Routing is directly applied to the MPLS architecture
with no change on the forwarding plane and The ingress node of an SR
domain encodes an ordered list of segments as a stack of labels. By
default The ingress node encode only default path labels. The
protecting router after detecting the node or link failure switches
the top label with MRT label[MRT-RED or MRT-BLUE is selected based on
algorithm] for the same destination. Till packet reaches the
destination MRT colored label path is followed by the packet.
AGV Expires January 9, 2017 [Page 6]
INTERNET DRAFT MRT in Segment Routing July 8, 2016
5. Requirements for SR MRT implementation
REQ1 : IGP Extension to carry the Segment Routing Node MRT Capability
in addition to exiting IGP extension carrying IGP MRT Capability
REQ2 : IGP Extension to carry Red & Blue MRT SR Segments in addition
to existing Default SR Segment
3 Security Considerations
None of the security consideration are identified
4 IANA Considerations
None of the IANA consideration are identified
AGV Expires January 9, 2017 [Page 7]
INTERNET DRAFT MRT in Segment Routing July 8, 2016
5 References
5.1 Normative References
[I-D.ietf-rtgwg-mrt-frr-algorithm]
Envedi, G., Csaszar, A., Atlas, A., Bowers, C., and A.
Gopalan, "Algorithms for computing Maximally Redundant
Trees for IP/LDP Fast- Reroute", draft-ietf-rtgwg-mrt-frr-
algorithm-05 (work in progress), July 2015.
[I-D.ietf-rtgwg-mrt-frr-architecture]
Atlas, A., Kebler, R., Bowers, C., Envedi, G., Csaszar,
A., Tantsura, J., and R. White, "An Architecture for IP/
LDP Fast-Reroute Using Maximally Redundant Trees", draft-
ietf-rtgwg-mrt-frr-architecture-05 (work in progress),
January 2015.
[RFC5036]
Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <http://www.rfc-editor.org/info/rfc5036>.
[RFC5561]
Thomas, B., Raza, K., Aggarwal, S., Aggarwal, R., and JL.
Le Roux, "LDP Capabilities", RFC 5561, DOI
10.17487/RFC5561, July 2009, <http://www.rfc-
editor.org/info/rfc5561>.
[RFC6420]
Cai, Y. and H. Ou, "PIM Multi-Topology ID (MT-ID) Join
Attribute", RFC 6420, DOI 10.17487/RFC6420, November 2011,
<http://www.rfc-editor.org/info/rfc6420>.
[RFC7307]
Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
King, "LDP Extensions for Multi-Topology", RFC 7307, DOI
10.17487/RFC7307, July 2014, <http://www.rfc-
editor.org/info/rfc7307>.
AGV Expires January 9, 2017 [Page 8]
INTERNET DRAFT MRT in Segment Routing July 8, 2016
[EnyediThesis]
Enyedi, G., "Novel Algorithms for IP Fast Reroute",
Department of Telecommunications and Media Informatics,
Budapest University of Technology and Economics Ph.D.
Thesis, February 2011,
<https://repozitorium.omikk.bme.hu/bitstream/
handle/10890/1040/ertekezes.pdf>.
[MRTLinear]
Enyedi, G., Retvari, G., and A. Csaszar, "On Finding
Maximally Redundant Trees in Strictly Linear Time", IEEE
Symposium on Computers and Communications (ISCC), 2009,
<http://opti.tmit.bme.hu/~enyedi/ipfrr/
distMaxRedTree.pdf>.
AGV Expires January 9, 2017 [Page 9]
INTERNET DRAFT MRT in Segment Routing July 8, 2016
5.2. Informative References
[I-D.atlas-rtgwg-mrt-mc-arch]
Atlas, A., Kebler, R., Wijnands, I., Csaszar, A., and G.
Envedi, "An Architecture for Multicast Protection Using
Maximally Redundant Trees", draft-atlas-rtgwg-mrt-mc-
arch-02 (work in progress), July 2013.
[I-D.ietf-isis-mrt]
Li, Z., Wu, N., Zhao, Q., Atlas, A., Bowers, C., and J.
Tantsura, "Intermediate System to Intermediate System (IS-
IS) Extensions for Maximally Redundant Trees (MRT)",
draft-ietf-isis-mrt-00 (work in progress), February 2015.
[I-D.ietf-ospf-mrt]
Atlas, A., Hegde, S., Bowers, C., Tantsura, J., and Z. Li,
"OSPF Extensions to Support Maximally Redundant Trees",
draft-ietf-ospf-mrt-00 (work in progress), January 2015.
[I-D.wijnands-mpls-mldp-node-protection]
Wijnands, I., Rosen, E., Raza, K., Tantsura, J., Atlas,
A., and Q. Zhao, "mLDP Node Protection", draft-wijnands-
mpls-mldp-node-protection-04 (work in progress), June
2013.
[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>.
AGV Expires January 9, 2017 [Page 10]
INTERNET DRAFT MRT in Segment Routing July 8, 2016
Authors' Addresses
Anil Kumar S N
Huawei Technologies India Pvt. Ltd,
Near EPIP Industrial Area,
Kundalahalli Village,
Whitefield,
Bangalore - 560066
EMail: anil.ietf@gmail.com
Gaurav Agrawal
Huawei Technologies India Pvt. Ltd,
Near EPIP Industrial Area,
Kundalahalli Village,
Whitefield,
Bangalore - 560066
EMail: gaurav.agrawal@huawei.com
Vinod Kumar S
Huawei Technologies India Pvt. Ltd,
Near EPIP Industrial Area,
Kundalahalli Village,
Whitefield,
Bangalore - 560066
EMail: vinods.kumar@huawei.com
AGV Expires January 9, 2017 [Page 11]