Segment Routing ArchitectureCisco Systems, Inc.BrusselsBEcfilsfil@cisco.comCisco Systems, Inc.Italystefano@previdi.netCisco Systems, Incginsberg@cisco.comOrangeFRbruno.decraene@orange.comOrangeFRstephane.litkowski@orange.comGoogle, Inc.1600 Amphitheatre ParkwayMountain View, CA94043USrobjs@google.comNetwork Working GroupSegment Routing (SR) leverages the source routing paradigm. A node
steers a packet through an ordered list of instructions, called
segments. A segment can represent any instruction, topological or
service-based. A segment can have a semantic local to an SR node or
global within an SR domain. SR allows to enforce a flow through any
topological path while maintaining per-flow state only at the ingress
nodes to the SR domain.Segment Routing can be directly applied to the MPLS architecture with
no change on the forwarding plane. A segment is encoded as an MPLS
label. An ordered list of segments is encoded as a stack of labels. The
segment to process is on the top of the stack. Upon completion of a
segment, the related label is popped from the stack.Segment Routing can be applied to the IPv6 architecture, with a new
type of routing header. A segment is encoded as an IPv6 address. An
ordered list of segments is encoded as an ordered list of IPv6 addresses
in the routing header. The active segment is indicated by the
Destination Address of the packet. The next active segment is indicated
by a pointer in the new routing header.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.Segment Routing (SR) leverages the source routing paradigm. A node
steers a packet through an SR Policy instantiated as an ordered list of
instructions called segments. A segment can represent any instruction,
topological or service-based. A segment can have a semantic local to an
SR node or global within an SR domain. SR supports per-flow explicit
routing while maintaining per-flow state only at the ingress nodes to
the SR domain.A segment is often referred by its Segment Identifier (SID).A segment may be associated with a topological instruction. A
topological local segment may instruct a node to forward the packet via
a specific outgoing interface. A topological global segment may instruct
an SR domain to forward the packet via a specific path to a destination.
Different segments may exist for the same destination, each with
different path objectives (e.g., which metric is minimized, what
constraints are specified).A segment may be associated with a service instruction (e.g. the
packet should be processed by a container or VM associated with the
segment). A segment may be associated with a QoS treatment (e.g., shape
the packets received with this segment at x Mbps).The SR architecture supports any type of instruction associated with
a segment.The SR architecture supports any type of control-plane: distributed,
centralized or hybrid.In a distributed scenario, the segments are allocated and signaled by
IS-IS or OSPF or BGP. A node individually decides to steer packets on a
source-routed policy (e.g., pre-computed local protection ) . A node individually
computes the source-routed policy.In a centralized scenario, the segments are allocated and
instantiated by an SR controller. The SR controller decides which nodes
need to steer which packets on which source-routed policies. The SR
controller computes the source-routed policies. The SR architecture does
not restrict how the controller programs the network. Likely options are
NETCONF, PCEP and BGP. The SR architecture does not restrict the number
of SR controllers. Specifically multiple SR controllers may program the
same SR domain. The SR architecture allows these SR controllers to
discover which SID’s are instantiated at which nodes and which
sets of local (SRLB) and global labels (SRGB) are available at which
node.A hybrid scenario complements a base distributed control-plane with a
centralized controller. For example, when the destination is outside the
IGP domain, the SR controller may compute a source-routed policy on
behalf of an IGP node. The SR architecture does not restrict how the
nodes which are part of the distributed control-plane interact with the
SR controller. Likely options are PCEP and BGP.Hosts MAY be part of an SR Domain. A centralized controller can
inform hosts about policies either by pushing these policies to hosts or
responding to requests from hosts.The SR architecture can be instantiated on various dataplanes. This
document introduces two dataplanes instantiations of SR: SR over MPLS
(SR-MPLS) and SR over IPv6 (SRv6).Segment Routing can be directly applied to the MPLS architecture with
no change on the forwarding plane A segment is
encoded as an MPLS label. An SR Policy is instantiated as a stack of
labels. The segment to process (the active segment) is on the top of the
stack. Upon completion of a segment, the related label is popped from
the stack.Segment Routing can be applied to the IPv6 architecture with a new
type of routing header called the SR header (SRH) . An instruction is
associated with a segment and encoded as an IPv6 address. An SRv6
segment is also called an SRv6 SID. An SR Policy is instantiated as an
ordered list of SRv6 SID’s in the routing header. The active
segment is indicated by the Destination Address(DA) of the packet. The
next active segment is indicated by the SegmentsLeft (SL) pointer in the
SRH. When an SRv6 SID is completed, the SL is decremented and the next
segment is copied to the DA. When a packet is steered on an SR policy,
the related SRH is added to the packet.In the context of an IGP-based distributed control-plane, two
topological segments are defined: the IGP adjacency segment and the IGP
prefix segment.In the context of a BGP-based distributed control-plane, two
topological segments are defined: the BGP peering segment and the BGP
prefix segment.The headend of an SR Policy binds a SID (called Binding segment or
BSID) to its policy. When the headend receives a packet with active
segment matching the BSID of a local SR Policy, the headend steers the
packet into the associated SR Policy.This document defines the IGP, BGP and Binding segments for the
SR-MPLS and SRv6 dataplanes.SR-MPLS: the instantiation of SR on the MPLS dataplaneSRv6: the instantiation of SR on the IPv6 dataplane.Segment: an instruction a node executes on the incoming packet (e.g.:
forward packet according to shortest path to destination, or, forward
packet through a specific interface, or, deliver the packet to a given
application/service instance).SID: a segment identifier. Note that the term SID is commonly used in
place of the term Segment, though this is technically imprecise as it
overlooks any necessary translation.SR-MPLS SID: an MPLS label or an index value into an MPLS label space
explicitly associated with the segment.SRv6 SID: an IPv6 address explicitly associated with the segment.Segment Routing Domain (SR Domain): the set of nodes participating in
the source based routing model. These nodes may be connected to the same
physical infrastructure (e.g.: a Service Provider's network). They may
as well be remotely connected to each other (e.g.: an enterprise VPN or
an overlay). If multiple protocol instances are deployed, the SR domain
most commonly includes all of the protocol instances in a single SR
domain. However, some deployments may wish to sub-divide the network
into multiple SR domains, each of which includes one or more protocol
instances. It is expected that all nodes in an SR Domain are managed by
the same administrative entity.Active Segment: the segment that MUST be used by the receiving router
to process the packet. In the MPLS dataplane it is the top label. In the
IPv6 dataplane it is the destination address. .PUSH: the instruction consisting of the insertion of a segment at the
top of the segment list. In SR-MPLS the top of the segment list is the
topmost (outer) label of the label stack. In SRv6, the top of the
segment list is represented by the first segment in the Segment Routing
Header as defined in .NEXT: when the active segment is completed, NEXT is the instruction
consisting of the inspection of the next segment. The next segment
becomes active. In SR-MPLS, NEXT is implemented as a POP of the top
label. In SRv6, NEXT is implemented as the copy of the next segment from
the SRH to the Destination Address of the IPv6 header.CONTINUE: the active segment is not completed and hence remains
active. In SR-MPLS, CONTINUE instruction is implemented as a SWAP of the
top label. In SRv6, this is the plain IPv6
forwarding action of a regular IPv6 packet according to its Destination
Address.SR Global Block (SRGB): the set of global segments in the SR Domain.
If a node participates in multiple SR domains, there is one SRGB for
each SR domain. In SR-MPLS, SRGB is a local property of a node and
identifies the set of local labels reserved for global segments. In
SR-MPLS, using the same SRGB on all nodes within the SR Domain is
strongly recommended. Doing so eases operations and troubleshooting as
the same label represents the same global segment at each node. In SRv6,
the SRGB is the set of global SRv6 SIDs in the SR Domain.SR Local Block (SRLB): local property of an SR node. If a node
participates in multiple SR domains, there is one SRLB for each SR
domain. In SR-MPLS, SRLB is a set of local labels reserved for local
segments. In SRv6, SRLB is a set of local IPv6 addresses reserved for
local SRv6 SID’s. In a controller-driven network, some controllers
or applications MAY use the control plane to discover the available set
of local segments.Global Segment: a segment which is part of the SRGB of the domain.
The instruction associated to the segment is defined at the SR Domain
level. A topological shortest-path segment to a given destination within
an SR domain is a typical example of a global segment.Local Segment: In SR-MPLS, this is a local label outside the SRGB. It
MAY be part of the explicitly advertised SRLB. In SRv6, this can be any
IPv6 address i.e., the address MAY be part of the SRGB but used such
that it has local significance. The instruction associated to the
segment is defined at the node level.IGP Segment: the generic name for a segment attached to a piece of
information advertised by a link-state IGP, e.g. an IGP prefix or an IGP
adjacency.IGP-Prefix Segment: an IGP-Prefix Segment is an IGP Segment
representing an IGP prefix. When an IGP-Prefix Segment is global within
the SR IGP instance/topology it identifies an instruction to forward the
packet along the path computed using the routing algorithm specified in
the algorithm field, in the topology and the IGP instance where it is
advertised. Also referred to as Prefix Segment.Prefix SID: the SID of the IGP-Prefix Segment.IGP-Anycast Segment: an IGP-Anycast Segment is an IGP-Prefix Segment
which identify an anycast prefix advertised by a set of routers.Anycast-SID: the SID of the IGP-Anycast Segment.IGP-Adjacency Segment: an IGP-Adjacency Segment is an IGP Segment
attached to a unidirectional adjacency or a set of unidirectional
adjacencies. By default, an IGP-Adjacency Segment is local (unless
explicitly advertised otherwise) to the node that advertises it. Also
referred to as Adjacency Segment.Adj-SID: the SID of the IGP-Adjacency Segment.IGP-Node Segment: an IGP-Node Segment is an IGP-Prefix Segment which
identifies a specific router (e.g., a loopback). Also referred to as
Node Segment.Node-SID: the SID of the IGP-Node Segment.SR Policy: an ordered list of segments. The headend of an SR Policy
steers packets onto the SR policy. The list of segments can be specified
explicitly in SR-MPLS as a stack of labels and in SRv6 as an ordered
list of SRv6 SID’s. Alternatively, the list of segments is
computed based on a destination and a set of optimization objective and
constraints (e.g., latency, affinity, SRLG, ...). The computation can be
local or delegated to a PCE server. An SR policy can be configured by
the operator, provisioned via NETCONF or provisioned via PCEP . An SR policy can be used for traffic-engineering,
OAM or FRR reasons.Segment List Depth: the number of segments of an SR policy. The
entity instantiating an SR Policy at a node N should be able to discover
the depth insertion capability of the node N. For example, the PCEP SR
capability advertisement described in is one means of discovering this
capability.Forwarding Information Base (FIB): the forwarding table of a nodeWithin an SR 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 path throughout the SR
domain. Such a path is either expressed as a single IGP segment or a
list of multiple IGP segments.Advertisement of IGP segments requires extensions in link-state IGP
protocols. These extensions are defined in An IGP-Prefix segment is an IGP segment attached to an IGP prefix.
An IGP-Prefix segment is global (unless explicitly advertised
otherwise) within the SR domain. The context for an IGP-Prefix segment
includes the prefix, topology, and algorithm. Multiple SIDs MAY be
allocated to the same prefix so long as the tuple <prefix,
topology, algorithm> is unique.Multiple instances and topologies are defined in IS-IS and OSPF in:
, , and .Segment Routing supports the use of multiple routing algorithms
i.e, different constraint based shortest path calculations can be
supported. An algorithm identifier is included as part of a
Prefix-SID advertisement.This document defines two algorithms:"Shortest Path": this algorithm is the default behavior. The
packet is forwarded along the well known ECMP-aware SPF
algorithm employed by the IGPs. However it is explicitly allowed
for a midpoint to implement another forwarding based on local
policy. The "Shortest Path" algorithm is in fact the default and
current behavior of most of the networks where local policies
may override the SPF decision."Strict Shortest Path": This algorithm mandates that the
packet is forwarded according to ECMP-aware SPF algorithm and
instructs any router in the path to ignore any possible local
policy overriding the SPF decision. The SID advertised with
"Strict Shortest Path" algorithm ensures that the path the
packet is going to take is the expected, and not altered, SPF
path. Note that Fast Reroute (FRR)
mechanisms are still compliant with the Strict Shortest Path. In
other words, a packet received with a Strict-SPF SID may be
rerouted through a FRR mechanism.An IGP-Prefix Segment identifies the path, to the related prefix,
computed as per the associated algorithm. A packet injected anywhere
within the SR domain with an active Prefix-SID is expected to be
forwarded along a path computed using the specified algorithm.
Clearly, if not all SR capable nodes in an SR Domain support a given
algorithm it is not possible to guarantee that the packet will
follow a path consistent with the associated algorithm.A router MUST drop any SR traffic associated with an SR
algorithm, if the nexthop router has not advertised support for the
SR algorithm.The ingress node of an SR domain SHOULD validate that the path to
a prefix, advertised with a given algorithm, includes nodes all
supporting the advertised algorithm. If this constraint cannot be
met the packet SHOULD be dropped by the ingress node. Note that in
the special case of "Shortest Path", all nodes (SR Capable or not)
are assumed to support this algorithm.When SR is used over the MPLS dataplane SIDs are an MPLS label or
an index into an MPLS label space (either SRGB or SRLB). An
SRGB/SRLB is advertised as an ordered set of ranges which has the
following properties:Each range specifies a starting label and the number of
labels in that rangeThe set of ranges advertised by a given node MUST be
disjointWhen the SID is an index, the mapping of the index to a label is
computed using the following algorithm:Where possible, it is recommended that a consistent SRGB be
configured on all nodes in an SR Domain. This simplifies
troubleshooting as the same label will be associated with the same
prefix on all nodes. In addition, it simplifies support for anycast
as detailed in Section 3.3.The following behaviors are associated with SR operating over the
MPLS dataplane:the IGP signaling extension for IGP-Prefix segment includes a
flag to indicate whether directly connected neighbors of the
node on which the prefix is attached should perform the NEXT
operation or the CONTINUE operation when processing the SID.
This behavior is equivalent to Penultimate Hop Popping (NEXT) or
Ultimate Hop Popping (CONTINUE) in MPLS.A Prefix-SID is allocated in the form of an MPLS label (or an
index in the SRGB) according to a process similar to IP address
allocation. Typically, the Prefix-SID is allocated by policy by
the operator (or NMS) and the SID very rarely changes.While SR allows to attach a local segment to an IGP prefix,
it is specifically assumed that when the terms "IGP-Prefix
Segment" and "Prefix-SID" are used, the segment is global (the
SID is allocated from the SRGB or as an index into the
advertised SRGB). This is consistent with all the described
use-cases that require global segments attached to IGP
prefixes.The allocation process MUST NOT allocate the same Prefix-SID
to different IP prefixes.If a node learns a Prefix-SID having a value that falls
outside the locally configured SRGB range, then the node MUST
NOT use the Prefix-SID and SHOULD issue an error log reporting a
misconfiguration.If a node N advertises Prefix-SID SID-R for a prefix R that
is attached to N, if N specifies CONTINUE as the operation to be
performed by directly connected neighbors, N MUST maintain the
following FIB entry:A remote node M MUST maintain the following FIB entry for any
learned Prefix-SID SID-R attached to IP prefix R: When SR is used over the IPv6 dataplane: A Prefix-SID is an IPv6 address..An operator MUST explicitly instantiate an SRv6 SID. IPv6
node addresses are not SRv6 SIDs by default.A node N advertising an IPv6 address R usable as a segment
identifier MUST maintain the following FIB entry:Independent of Segment Routing support, any remote IPv6 node will
maintain a plain IPv6 FIB entry for any prefix, no matter if the
represent a segment or not. This allows forwarding of packets to the
node which owns the SID even by nodes which do not support Segment
Routing.An IGP Node-SID MUST NOT be associated with a prefix that is owned
by more than one router within the same routing domain.An "Anycast Segment" or "Anycast SID" enforces the ECMP-aware
shortest-path forwarding towards the closest node of the anycast set.
This is useful to express macro-engineering policies or protection
mechanisms.An IGP-Anycast segment MUST NOT reference a particular node.Within an anycast group, all routers in an SR domain MUST advertise
the same prefix with the same SID value.The figure above describes a network example with two groups of
transit devices. Group A consists of devices {A1, A2, A3 and A4}.
They are all provisioned with the anycast address 192.0.2.10/32 and
the anycast SID 100.Similarly, group B consists of devices {B1, B2, B3 and B4} and
are all provisioned with the anycast address 192.0.2.1/32, anycast
SID 200. In the above network topology, each PE device has a path to
each of the groups A and B.PE1 can choose a particular transit device group when sending
traffic to PE3 or PE4. This will be done by pushing the anycast SID
of the group in the stack.Processing the anycast, and subsequent segments, requires special
care.Considering an MPLS deployment, in the above topology, if device
PE1 (or PE2) requires to send a packet to the device PE3 (or PE4) it
needs to encapsulate the packet in an MPLS payload with the
following stack of labels. Label allocated by R1 for anycast SID 100 (outer label).Label allocated by the nearest router in group A for SID 30
(for destination PE3).While the first label is easy to compute, in this case since
there are more than one topologically nearest devices (A1 and A2),
unless A1 and A2 allocated the same label value to the same prefix,
determining the second label is impossible. Devices A1 and A2 may be
devices from different hardware vendors. If both don't allocate the
same label value for SID 30, it is impossible to use the anycast
group "A" as a transit anycast group towards PE3. Hence, PE1 (or
PE2) cannot compute an appropriate label stack to steer the packet
exclusively through the group A devices. Same holds true for devices
PE3 and PE4 when trying to send a packet to PE1 or PE2.To ease the use of anycast segment in a short term, it is
recommended to configure the same SRGB on all nodes of a particular
anycast group. Using this method, as mentioned above, computation of
the label following the anycast segment is straightforward.Using anycast segment without configuring the same SRGB on nodes
belonging to the same device group may lead to misrouting (in an
MPLS VPN deployment, some traffic may leak between VPNs).The adjacency is formed by the local node (i.e., the node
advertising the adjacency in the IGP) and the remote node (i.e., the
other end of the adjacency). The local node MUST be an IGP node. The
remote node may be an adjacent IGP neighbor or a non-adjacent neighbor
(e.g.: a Forwarding Adjacency, ).A packet injected anywhere within the SR domain with a segment list
{SN, SNL}, where SN is the Node-SID of node N and SNL is an Adj-SID
attached by node N to its adjacency over link L, will be forwarded
along the shortest-path to N and then be switched by N, without any IP
shortest-path consideration, towards link L. If the Adj-SID identifies
a set of adjacencies, then the node N load-balances the traffic among
the various members of the set.Similarly, when using a global Adj-SID, a packet injected anywhere
within the SR domain with a segment list {SNL}, where SNL is a global
Adj-SID attached by node N to its adjacency over link L, will be
forwarded along the shortest-path to N and then be switched by N,
without any IP shortest-path consideration, towards link L. If the
Adj-SID identifies a set of adjacencies, then the node N does
load-balance the traffic among the various members of the set. The use
of global Adj-SID allows to reduce the size of the segment list when
expressing a path at the cost of additional state (i.e.: the global
Adj-SID will be inserted by all routers within the area in their
forwarding table).An "IGP Adjacency Segment" or "Adj-SID" enforces the switching of
the packet from a node towards a defined interface or set of
interfaces. This is key to theoretically prove that any path can be
expressed as a list of segments.The encodings of the Adj-SID include a set of flags supporting the
following functionalities:Eligible for Protection (e.g.: using IPFRR or MPLS-FRR)Indication whether the Adj-SID has local or global scope.
Default scope SHOULD be Local.A weight (as described below) is also associated with the Adj-SID
advertisement.A node SHOULD allocate one Adj-SID for each of its adjacencies.A node MAY allocate multiple Adj-SIDs for the same adjacency. An
example is to support an Adj-SID which is eligible for protection and
an Adj-SID which is NOT eligible for protection.A node MAY associate the same Adj-SID to multiple adjacencies.In order to be able to advertise in the IGP all the Adj-SIDs
representing the IGP adjacencies between two nodes, parallel adjacency
suppression MUST NOT be performed by the IGP.When a node binds an Adj-SID to a local data-link L, the node MUST
install the following FIB entry:The Adj-SID implies, from the router advertising it, the forwarding
of the packet through the adjacency(ies) identified by the Adj-SID,
regardless of its IGP/SPF cost. In other words, the use of adjacency
segments overrides the routing decision made by the SPF algorithm.Adj-SIDs can be used in order to represent a set of parallel
interfaces between two adjacent routers.A node MUST install a FIB entry for any locally originated
adjacency segment (Adj-SID) of value W attached to a set of links B
with:When parallel adjacencies are used and associated to the same
Adj-SID, and in order to optimize the load balancing function, a
"weight" factor can be associated to the Adj-SID advertised with
each adjacency. The weight tells the ingress (or an
SDN/orchestration system) about the load-balancing factor over the
parallel adjacencies. As shown in , A
and B are connected through two parallel adjacenciesNode A advertises following Adj-SIDs and weights: Link-1: Adj-SID 1000, weight: 1Link-2: Adj-SID 1000, weight: 2Node S receives the advertisements of the parallel
adjacencies and understands that by using Adj-SID 1000 node A will
load-balance the traffic across the parallel links (link-1 and
link-2) according to a 1:2 ratio i.e., twice as many packets will
flow over Link-2 as compared to Link-1.In LAN subnetworks, link-state protocols define the concept of
Designated Router (DR, in OSPF) or Designated Intermediate System
(DIS, in IS-IS) that conduct flooding in broadcast subnetworks and
that describe the LAN topology in a special routing update (OSPF
Type2 LSA or IS-IS Pseudonode LSP).The difficulty with LANs is that each router only advertises its
connectivity to the DR/DIS and not to each of the individual nodes
in the LAN. Therefore, additional protocol mechanisms (IS-IS and
OSPF) are necessary in order for each router in the LAN to advertise
an Adj-SID associated to each neighbor in the LAN. These extensions
are defined in IGP SR extensions documents.In the following example diagram it is assumed that the all areas
are part of a single SR Domain.The example here below assumes the IPv6 control plane with the MPLS
dataplane.In area 2, node Z allocates Node-SID 150 to his local IPv6 prefix
2001:DB8::2:1/128.ABRs G and J will propagate the prefix and its SIDs into the
backbone area by creating a new instance of the prefix according to
normal inter-area/level IGP propagation rules.Nodes C and I will apply the same behavior when leaking prefixes
from the backbone area down to area 1. Therefore, node S will see
prefix 2001:DB8::2:1/128 with Prefix-SID 150 and advertised by nodes C
and I.It therefore results that a Prefix-SID remains attached to its
related IGP Prefix through the inter-area process.When node S sends traffic to 2001:DB8::2:1/128, it pushes
Node-SID(150) as active segment and forward it to A.When packet arrives at ABR I (or C), the ABR forwards the packet
according to the active segment (Node-SID(150)). Forwarding continues
across area borders, using the same Node-SID(150), until the packet
reaches its destination.BGP segments may be allocated and distributed by BGP.A BGP-Prefix segment is a BGP segment attached to a BGP prefix.A BGP-Prefix segment is global (unless explicitly advertised
otherwise) within the SR domain.The BGP Prefix SID is the BGP equivalent to the IGP Prefix
Segment.A likely use-case for the BGP Prefix Segment is an IGP-free
hyper-scale spine-leaf topology where connectivity is learned solely
via BGP In the context of BGP Egress Peer Engineering (EPE), as described
in , an
EPE enabled Egress PE node MAY advertise segments corresponding to its
attached peers. These segments are called BGP peering segments or BGP
peering SIDs. They enable the expression of source-routed inter-domain
paths.An ingress border router of an AS may compose a list of segments to
steer a flow along a selected path within the AS, towards a selected
egress border router C of the AS and through a specific peer. At
minimum, a BGP peering Engineering policy applied at an ingress PE
involves two segments: the Node SID of the chosen egress PE and then
the BGP peering segment for the chosen egress PE peer or peering
interface.Three types of BGP peering segments/SIDs are defined: PeerNode SID,
PeerAdj SID and PeerSet SID. PeerNode SID: a BGP PeerNode segment/SID is a local segment. At
the BGP node advertising it, its semantics is: SR header operation: NEXT.Next-Hop: the connected peering node to which the segment
is related.PeerAdj SID: a BGP PeerAdj segment/SID is a local segment. At
the BGP node advertising it, the semantic is: SR header operation: NEXT.Next-Hop: the peer connected through the interface to which
the segment is related.PeerSet SID. a BGP PeerSet segment/SID is a local segment. At
the BGP node advertising it, the semantic is: SR header operation: NEXT.Next-Hop: load-balance across any connected interface to
any peer in the related group.A peer set could be all the connected peers from the same
AS or a subset of these. A group could also span across AS. The
group definition is a policy set by the operator.The BGP extensions necessary in order to signal these BGP peering
segments are defined in An SR Policy is bound to a so-called Binding SID (BSID). Any packets
received with active segment = BSID are steered onto the bound SR
Policy.A BSID may either be a local or a global SID. If local, a BSID SHOULD
be allocated from the SRLB. If global, a BSID MUST be allocated from the
SRGB.One of the possible use cases for a BSID is to overcome a Segment
List Depth limitation on a given node by requiring that node only to
impose a BSID which could be SWAPPED on downstream nodes with a set of
SIDs associated with an SR policy.Another use case for a Binding Segment is to provide support for an
IGP node to advertise its ability to process traffic originally
destined to another IGP node, called the Mirrored node and identified
by an IP address or a Node-SID, provided that a "Mirroring Context"
segment be inserted in the segment list prior to any service segment
local to the mirrored node.When a given node B wants to provide egress node A protection, it
advertises a segment identifying node's A context. Such segment is
called "Mirror Context Segment" and identified by the Mirror SID.The Mirror SID is advertised using the binding segment defined in
SR IGP protocol extensions .In the event of a failure, a point of local repair (PLR) diverting
traffic from A to B does a PUSH of the Mirror SID on the protected
traffic. B, when receiving the traffic with the Mirror SID as the
active segment, uses that segment and processes underlying segments in
the context of A.Segment Routing is defined for unicast. The application of the
source-route concept to Multicast is not in the scope of this
document.This document does not require any action from IANA.Segment Routing is applicable to both MPLS and IPv6 data planes.Segment Routing adds some meta-data (instructions) on the packet,
with the list of forwarding path elements (e.g.: nodes, links, services,
etc.) that the packet must traverse. It has to be noted that the
complete source routed path may be represented by a single segment. This
is the case of the Binding SID.When applied to the MPLS data plane, Segment Routing does not
introduce any new behavior or any change in the way MPLS data plane
works. Therefore, from a security standpoint, this document does not
define any additional mechanism in the MPLS data plane.SR allows the expression of a source routed path using a single
segment (the Binding SID). Compared to RSVP-TE which also provides
explicit routing capability, there are no fundamental differences in
term of information provided. Both RSVP-TE and Segment Routing may
express a source routed path using a single segment.When a path is expressed using a single label, the syntax of the
meta-data is equivalent between RSVP-TE and
SR.When a source routed path is expressed with a list of segments
additional meta-data is added to the packet consisting of the source
routed path the packet must follow expressed as a segment list.When a path is expressed using a label stack, if one has access to
the meaning (i.e.: the Forwarding Equivalence Class) of the labels,
one has the knowledge of the explicit path. For the MPLS data plane,
as no data plane modification is required, there is no fundamental
change of capability. Yet, the occurrence of label stacking will
increase.From a network protection standpoint, there is an assumed trust
model such that any node imposing a label stack on a packet is assumed
to be allowed to do so. This is a significant change compared to plain
IP offering shortest path routing but not fundamentally different
compared to existing techniques providing explicit routing capability
such as RSVP-TE. By default, the explicit routing information MUST NOT
be leaked through the boundaries of the administered domain. Segment
Routing extensions that have been defined in various protocols,
leverage the security mechanisms of these protocols such as
encryption, authentication, filtering, etc.In the general case, a segment routing capable router accepts and
install labels, only if these labels have been previously advertised
by a trusted source. The received information is validated using
existing control plane protocols providing authentication and security
mechanisms. Segment Routing does not define any additional security
mechanism in existing control plane protocols.Segment Routing does not introduce signaling between the source and
the mid points of a source routed path. With SR, the source routed
path is computed using SIDs previously advertised in the IP control
plane. Therefore, in addition to filtering and controlled
advertisement of SIDs at the boundaries of the SR domain, filtering in
the data plane is also required. Filtering MUST be performed on the
forwarding plane at the boundaries of the SR domain and may require
looking at multiple labels/instruction.For the MPLS data plane, there are no new requirement as the
existing MPLS architecture already allows such source routing by
stacking multiple labels. And for security protection, and already call for the
filtering of MPLS packets on trust boundaries.When applied to the IPv6 data plane, Segment Routing does introduce
the Segment Routing Header (SRH, ) which is a type of
Routing Extension header as defined in .The SRH adds some meta-data on the IPv6 packet, with the list of
forwarding path elements (e.g.: nodes, links, services, etc.) that the
packet must traverse and that are represented by IPv6 addresses. A
complete source routed path may be encoded in the packet using a
single segment (single IPv6 address).From a network protection standpoint, there is an assumed trust
model such that any node adding an SRH to the packet is assumed to be
allowed to do so. Therefore, by default, the explicit routing
information MUST NOT be leaked through the boundaries of the
administered domain. Segment Routing extensions that have been defined
in various protocols, leverage the security mechanisms of these
protocols such as encryption, authentication, filtering, etc.In the general case, an SR IPv6 router accepts and install segments
identifiers (in the form of IPv6 addresses), only if these SIDs are
advertised by a trusted source. The received information is validated
using existing control plane protocols providing authentication and
security mechanisms. Segment Routing does not define any additional
security mechanism in existing control plane protocols.In addition, SR domain boundary routers, by default, MUST apply
data plane filters so to only accept packets whose DA and SRH (if any)
contain addresses previously advertised as SIDs.There are a number of security concerns with source routing at the
IPv6 data plane . The new IPv6-based segment
routing header is defined in . This document also
discusses the above security concerns.In SR enabled networks, the path the packet takes is encoded in the
header. As the path is not signaled through a protocol, OAM mechanisms
are necessary in order for the network operator to validate the
effectiveness of a path as well as to check and monitor its liveness and
performance. However, it has to be noted that SR allows to reduce
substantially the number of states in transit nodes and hence the number
of elements that a transit node has to manage is smaller.SR OAM use cases and requirements for the MPLS data plane are defined
in and . SR OAM procedures for the
MPLS data plane are defined in .SR routers receive advertisements of SIDs (index, label or IPv6
address) from the different routing protocols being extended for SR.
Each of these protocols have monitoring and troubleshooting mechanisms
to provide operation and management functions for IP addresses that MUST
be extended in order to include troubleshooting and monitoring functions
of the SID.SR architecture introduces the usage of global segments. Each global
segment must be bound to a unique index or address within an SR domain.
The management of the allocation of such index or address by the
operator is critical for the network behavior to avoid situations like
mis-routing. In addition to the allocation policy/tooling that the
operator will have in place, an implementation SHOULD protect the
network in case of conflict detection by providing a deterministic
resolution approach.When a path is expressed using a label stack, the occurrence of label
stacking will increase. A node may want to signal in the control plane
its ability in terms of size of the label stack it can support.A YANG data model for segment routing
configuration and operations has been defined in .When Segment Routing is applied to the IPv6 data plane, segments are
identified through IPv6 addresses. The allocation, management and
troubleshooting of segment identifiers is no different than the existing
mechanisms applied to the allocation and management of IPv6
addresses.The DA of the packet gives the active segment address. The segment
list in the SRH gives the entire path of the packet. The validation of
the source routed path is done through inspection of DA and SRH present
in the packet header matched to the equivalent routing table
entries.In the context of SR over the IPv6 data plane, the source routed path
is encoded in the SRH as described in . The SR IPv6 source
routed path is instantiated into the SRH as a list of IPv6 address where
the active segment is in the Destination Address (DA) field of the IPv6
packet header. Typically, by inspecting in any node the packet header,
it is possible to derive the source routed path it belongs to. Similar
to the context of SR over MPLS data plane, an implementation may
originate path control and monitoring packets where the source routed
path is inserted in the SRH and where each segment of the path inserts
in the packet the relevant data in order to measure the end to end path
and performance.The following people have substantially contributed to the definition
of the Segment Routing architecture and to the editing of this
document:We would like to thank Dave Ward, Peter Psenak, Dan Frost, Stewart
Bryant, Pierre Francois, Thomas Telkamp, Ruediger Geib, Hannes Gredler,
Pushpasis Sarkar, Eric Rosen, Chris Bowers and Alvaro Retana for their
comments and review of this document.