Segment Routing with the MPLS Data PlaneArrcusabashandy.ietf@gmail.comCisco Systems, Inc.BrusselsBelgiumcfilsfil@cisco.comCisco Systems, Inc.Italystefano@previdi.netOrangeFrancebruno.decraene@orange.comOrangeFranceslitkows.ietf@gmail.comGoogleUnited States of Americarobjs@google.com
Segment Routing (SR) leverages the source-routing paradigm. A node
steers a packet through a controlled set of instructions, called
segments, by prepending the packet with an SR header. In the MPLS
data plane, the SR header is instantiated through a label stack. This
document specifies the forwarding behavior to allow instantiating SR
over the MPLS data plane (SR-MPLS).Introduction
The Segment Routing architecture can be directly applied to
the MPLS architecture with no change in the MPLS forwarding plane.
This document specifies forwarding-plane behavior to allow
Segment Routing to operate on top of the MPLS data plane (SR-MPLS). This
document does not address control-plane behavior. Control-plane
behavior is specified in other documents such as , , and .
The Segment Routing problem statement is described in .
Coexistence of SR over the MPLS forwarding plane with LDP is
specified in .
Policy routing and traffic engineering using Segment Routing can be
found in .Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14
when, and only when, they appear in all capitals, as shown here.MPLS Instantiation of Segment Routing
MPLS instantiation of Segment Routing fits in the MPLS architecture
as defined in from both a control-plane and forwarding-plane
perspective:
From a control-plane perspective, does not mandate a
single signaling protocol. Segment Routing makes use of various
control-plane protocols such as link-state IGPs .
The flooding mechanisms of link-state IGPs fit very well with
label stacking on the ingress. A future control-layer protocol and/or
policy/configuration can be used to specify the label stack.
From a forwarding-plane perspective, Segment Routing does not
require any change to the forwarding plane because Segment IDs
(SIDs) are instantiated as MPLS labels, and the Segment Routing
header is instantiated as a stack of MPLS labels.
We call the "MPLS Control Plane Client (MCC)" any control-plane entity
installing forwarding entries in the MPLS data plane. Local
configuration and policies applied on a router are examples of MCCs.
In order to have a node segment reach the node, a network operator
SHOULD configure at least one node segment per routing instance,
topology, or algorithm. Otherwise, the node is not reachable within
the routing instance, within the topology,
or along the routing algorithm, which restricts
its ability to be used by an SR Policy and as a
Topology Independent Loop-Free Alternate (TI-LFA).Multiple Forwarding Behaviors for the Same Prefix
The SR architecture does not prohibit having more than one SID for
the same prefix. In fact, by allowing multiple SIDs for the same
prefix, it is possible to have different forwarding behaviors (such
as different paths, different ECMP and Unequal-Cost Multipath (UCMP) behaviors, etc.) for the
same destination.
Instantiating Segment Routing over the MPLS forwarding plane fits
seamlessly with this principle. An operator may assign multiple MPLS
labels or indices to the same prefix and assign different forwarding
behaviors to each label/SID. The MCC in the network downloads
different MPLS labels/SIDs to the FIB for different forwarding
behaviors. The MCC at the entry of an SR domain or at any point in
the domain can choose to apply a particular forwarding behavior to a
particular packet by applying the PUSH action to that packet using
the corresponding SID.SID Representation in the MPLS Forwarding Plane
When instantiating SR over the MPLS forwarding plane, a SID is
represented by an MPLS label or an index .
A global SID is a label, or an index that may be mapped to an
MPLS label within the Segment Routing Global Block (SRGB), of the node
that installs a global SID in its FIB and receives the labeled
packet. specifies the procedure to map a global segment
represented by an index to an MPLS label within the SRGB.
The MCC MUST ensure that any label value corresponding to any SID it
installs in the forwarding plane follows the rules below:
The label value MUST be unique within the router on which the MCC
is running, i.e., the label MUST only be used to represent the SID
and MUST NOT be used to represent more than one SID or for any
other forwarding purpose on the router.
The label value MUST NOT come from the range of special-purpose
labels .
Labels allocated in this document are considered per-platform downstream
allocated labels .Segment Routing Global Block and Local Block
The concepts of SRGB and global SID
are explained in . In general, the SRGB need not be a
contiguous range of labels.
For the rest of this document, the SRGB is specified by the list of
MPLS label ranges [Ll(1),Lh(1)], [Ll(2),Lh(2)],..., [Ll(k),Lh(k)]
where Ll(i) =< Lh(i).
The following rules apply to the list of MPLS ranges representing the
SRGB:
The list of ranges comprising the SRGB MUST NOT overlap.
Every range in the list of ranges specifying the SRGB MUST NOT
cover or overlap with a reserved label value or range ,
respectively.
If the SRGB of a node does not conform to the structure specified
in this section or to the previous two rules, the SRGB MUST
be completely ignored by all routers in the routing domain, and the
node MUST be treated as if it does not have an SRGB.
The list of label ranges MUST only be used to instantiate global
SIDs into the MPLS forwarding plane.
A local segment MAY be allocated from the Segment Routing Local Block
(SRLB) or from any unused label as long as it does not use
a special-purpose label. The SRLB consists of the range of local
labels reserved by the node for certain local segments. In a
controller-driven network, some controllers or applications MAY use
the control plane to discover the available set of Local SIDs on a
particular router . The rules
applicable to the SRGB are also applicable to the SRLB, except the
SRGB MUST only be used to instantiate global
SIDs into the MPLS forwarding plane. The recommended, minimum, or
maximum size of the SRGB and/or SRLB is a matter of future study.Mapping a SID Index to an MPLS Label
This subsection specifies how the MPLS label value is calculated
given the index of a SID. The value of the index is determined by an
MCC such as IS-IS or OSPF
. This section only
specifies how to map the index to an MPLS label. The calculated MPLS
label is downloaded to the FIB, sent out with a forwarded packet, or
both.
Consider a SID represented by the index "I". Consider an SRGB as
specified in . The total size of the SRGB, represented by
the variable "Size", is calculated according to the formula: The following rules MUST be applied by the MCC when calculating the
MPLS label value corresponding to the SID index value "I".
0 =< I < size. If index "I" does not satisfy the previous inequality, then the label cannot be calculated.
The label value corresponding to the SID index "I" is calculated
as follows:
j = 1 , temp = 0
While temp + Lh(j)- Ll(j) < I
temp = temp + Lh(j)- Ll(j) + 1
j = j+1
label = I - temp + Ll(j)
An example for how a router calculates labels and forwards traffic
based on the procedure described in this section can be found in
.Incoming Label Collision
The MPLS Architecture defines the term Forwarding
Equivalence Class (FEC) as the set of packets with similar and/or
identical characteristics that are forwarded the same way and are
bound to the same MPLS incoming (local) label. In Segment Routing
MPLS, a local label serves as the SID for a given FEC.
We define SR FEC as one of the following:
(Prefix, Routing Instance, Topology, Algorithm) , where a
topology identifies a set of links with metrics. For the purpose
of incoming label collision resolution, the same Topology
numerical value SHOULD be used on all routers to identify the same
set of links with metrics. For MCCs where the "Topology" and/or
"Algorithm" fields are not defined, the numerical value of zero
MUST be used for these two fields. For the purpose of incoming
label collision resolution, a routing instance is identified by a
single incoming label downloader to the FIB. Two MCCs running on the
same router are considered different routing instances if the only
way the two instances know about each other's incoming labels
is through redistribution. The numerical value used to identify a
routing instance MAY be derived from other configuration or MAY be
explicitly configured. If it is derived from other configuration,
then the same numerical value SHOULD be derived from the same
configuration as long as the configuration survives router reload.
If the derived numerical value varies for the same configuration,
then an implementation SHOULD make the numerical value used to
identify a routing instance configurable.
(next hop, outgoing interface), where the outgoing interface is
physical or virtual.
(number of adjacencies, list of next hops, list of outgoing
interfaces IDs in ascending numerical order). This FEC represents
parallel adjacencies .
(Endpoint, Color). This FEC represents an SR Policy .
(Mirror SID). The Mirror SID (see ) is the IP
address advertised by the advertising node to identify the Mirror SID.
The IP address is encoded as specified in .
This section covers the RECOMMENDED procedure for handling the scenario
where, because of an error/misconfiguration, more than one SR FEC as
defined in this section maps to the same incoming MPLS label.
Examples illustrating the behavior specified in this section can be
found in .
An incoming label collision occurs if the SIDs of the set of FECs
{FEC1, FEC2, ..., FECk} map to the same incoming SR MPLS label "L1".
Suppose an anycast prefix is advertised with a Prefix-SID by some,
but not all, of the nodes that advertise that prefix. If the Prefix-SID
sub-TLVs result in mapping that anycast prefix to the same
incoming label, then the advertisement of the Prefix-SID by some, but
not all, of the advertising nodes MUST NOT be treated as a label
collision.
An implementation MUST NOT allow the MCCs belonging to the same
router to assign the same incoming label to more than one SR FEC.
The objective of the following steps is to deterministically install
in the MPLS Incoming Label Map, also known as label FIB, a single FEC
with the incoming label "L1". By "deterministically install", we mean
if the set of FECs {FEC1, FEC2,..., FECk} map to the same incoming SR
MPLS label "L1", then the steps below assign the same FEC to the
label "L1" irrespective of the order by which the mappings of this
set of FECs to the label "L1" are received. For example, first-
come, first-served tiebreaking is not allowed. The remaining FECs may
be installed in the IP FIB without an incoming label.
The procedure in this section relies completely on the local FEC and
label database within a given router.
The collision resolution procedure is as follows:
Given the SIDs of the set of FECs, {FEC1, FEC2,..., FECk} map to
the same MPLS label "L1".
Within an MCC, apply tiebreaking rules to select one FEC only, and
assign the label to it. The losing FECs are handled as if no
labels are attached to them. The losing FECs with algorithms other
than the shortest path first are not installed in the
FIB.
If the same set of FECs are attached to the same label "L1",
then the tiebreaking rules MUST always select the same FEC
irrespective of the order in which the FECs and the label "L1"
are received. In other words, the tiebreaking rule MUST be
deterministic.
If there is still collision between the FECs belonging to
different MCCs, then reapply the tiebreaking rules to the
remaining FECs to select one FEC only, and assign the label to that
FEC.
Install the selected FEC into the IP FIB and its incoming label into
the label FIB.
The remaining FECs with the default algorithm (see the
Prefix-SID algorithm specification ) may be installed
in the FIB natively, such as pure IP entries in case of Prefix
FEC, without any incoming labels corresponding to their SIDs. The
remaining FECs with algorithms other than the shortest path first
are not installed in the FIB.
Tiebreaking Rules
The default tiebreaking rules are specified as follows:
Determine the lowest administrative distance among the competing FECs as defined in the section below. Then filter away all the competing FECs with a higher administrative distance.
If more than one competing FEC remains after step 1, select the
smallest numerical FEC value. The numerical value of the FEC is
determined according to the FEC encoding described later in this
section.
These rules deterministically select which FEC to install in the MPLS
forwarding plane for the given incoming label.
This document defines the default tiebreaking rules that SHOULD be
implemented. An implementation MAY choose to support different tiebreaking
rules and MAY use one of these instead of the default
tiebreaking rules. To maximize MPLS forwarding consistency in case
of a SID configuration error, the network operator MUST deploy, within
an IGP flooding area, routers implementing the same tiebreaking
rules.
Each FEC is assigned an administrative distance. The FEC
administrative distance is encoded as an 8-bit value. The lower the
value, the better the administrative distance.
The default FEC administrative distance order starting from the
lowest value SHOULD be:
Explicit SID assignment to a FEC that maps to a label outside the
SRGB irrespective of the owner MCC. An explicit SID assignment is
a static assignment of a label to a FEC such that the assignment
survives a router reboot.
An example of explicit SID allocation is static assignment of
a specific label to an Adj-SID.
An implementation of explicit SID assignment MUST guarantee
collision freeness on the same router.
Dynamic SID assignment:
All FEC types, except for the SR Policy, are
ordered using the default administrative distance
defined by the implementation.
The Binding SID assigned to the SR Policy always has a
higher default administrative distance than the default
administrative distance of any other FEC type.
To maximize MPLS forwarding consistency, if the same FEC is advertised
in more than one protocol, a user MUST ensure that the administrative
distance preference between protocols is the same on all routers of
the IGP flooding domain. Note that this is not really new as this
already applies to IP forwarding.
The numerical sort across FECs SHOULD be performed as follows:
Each FEC is assigned a FEC type encoded in 8 bits. The type codepoints
for each SR FEC defined at the beginning
of this section are as follows:
120:
(Prefix, Routing Instance, Topology, Algorithm)
130:
(next hop, outgoing interface)
140:
Parallel Adjacency
150:
SR Policy
160:
Mirror SID
The numerical values above are mentioned to guide
implementation. If other numerical values are used, then the
numerical values must maintain the same greater-than ordering
of the numbers mentioned here.
The fields of each FEC are encoded as follows:
All fields in all FECs are encoded in big endian order.
The Routing Instance ID is represented by 16 bits. For routing
instances that are identified by less than 16 bits, encode the
Instance ID in the least significant bits while the most
significant bits are set to zero.
The address family is represented by 8 bits, where IPv4 is encoded as
100, and IPv6 is encoded as 110. These numerical values are
mentioned to guide implementations. If other numerical values
are used, then the numerical value of IPv4 MUST be less than
the numerical value for IPv6.
All addresses are represented in 128 bits as follows:
The IPv6 address is encoded natively.
The IPv4 address is encoded in the most significant bits, and
the remaining bits are set to zero.
All prefixes are represented by (8 + 128) bits.
A prefix is encoded in the most significant bits, and the
remaining bits are set to zero.
The prefix length is encoded before the prefix in an 8-bit field.
The Topology ID is represented by 16 bits. For routing instances
that identify topologies using less than 16 bits, encode the
topology ID in the least significant bits while the most
significant bits are set to zero.
The Algorithm is encoded in a 16-bit field.
The Color ID is encoded using 32 bits.
Choose the set of FECs of the smallest FEC type codepoint.
Out of these FECs, choose the FECs with the smallest address
family codepoint.
Encode the remaining set of FECs as follows:
(Prefix, Routing Instance, Topology, Algorithm) is encoded as
(Prefix Length, Prefix, routing_instance_id, Topology, SR
Algorithm).
(next hop, outgoing interface) is encoded as (next hop,
outgoing_interface_id).
(number of adjacencies, list of next hops in ascending
numerical order, list of outgoing interface IDs in ascending
numerical order) is used to encode a parallel
adjacency .
(Endpoint, Color) is encoded as (Endpoint_address, Color_id).
(IP address) is the encoding for a Mirror SID FEC. The IP
address is encoded as described above in this section.
Select the FEC with the smallest numerical value.
The numerical values mentioned in this section are for guidance only.
If other numerical values are used, then the other numerical values
MUST maintain the same numerical ordering among different SR FECs.Redistribution between Routing Protocol Instances
The following rule SHOULD be applied when redistributing SIDs with
prefixes between routing protocol instances:
If the SRGB of the receiving instance is the same as the SRGB of the origin
instance, then:
the index is redistributed with the route.
Else,
the index is not redistributed and if the receiving instance
decides to advertise an index with the redistributed route, it
is the duty of the receiving instance to allocate a fresh
index relative to its own SRGB. Note that in this case, the
receiving instance MUST compute the local label it assigns to
the route according to and install it in FIB.
It is outside the scope of this document to define local node
behaviors that would allow the mapping of the original index into a new index
in the receiving instance via the addition of an offset or other
policy means.IllustrationConsider the simple topology above, where:
A and B are in the IS-IS domain with SRGB = [16000-17000]
B and C are in the OSPF domain with SRGB = [20000-21000]
B redistributes 192.0.2.1/32 into the IS-IS domain
In this case, A learns 192.0.2.1/32 as an IP leaf connected to B, which is
usual for IP prefix redistributionHowever, according to the redistribution rule above, B
decides not to advertise any index with 192.0.2.1/32 into IS-IS
because the SRGB is not the same.Illustration 2
Consider the example in the illustration described in .
When router B redistributes the prefix 192.0.2.1/32, router B decides
to allocate and advertise the same index 1 with the prefix
192.0.2.1/32.
Within the SRGB of the IS-IS domain, index 1 corresponds to the local
label 16001. Hence, according to the redistribution rule above, router B
programs the incoming label 16001 in its FIB to match traffic
arriving from the IS-IS domain destined to the prefix
192.0.2.1/32.Effect of Incoming Label Collision on Outgoing Label Programming
When determining what outgoing label to use, the ingress node
that pushes new segments, and hence a stack of MPLS labels, MUST use, for
a given FEC, the label that has been selected by the node
receiving the packet with that label exposed as the top label. So in case
of incoming label collision on this receiving node, the ingress node
MUST resolve this collision by using this same "Incoming Label Collision resolution procedure" and by using the data of the receiving node.
In the general case, the ingress node may not have the exact same
data as the receiving node, so the result may be different. This is
under the responsibility of the network operator. But in a typical
case, e.g., where a centralized node or a distributed link-state IGP
is used, all nodes would have the same database. However, to minimize
the chance of misforwarding, a FEC that loses its incoming label to
the tiebreaking rules specified in MUST NOT be
installed in FIB with an outgoing Segment Routing label based on the
SID corresponding to the lost incoming label.
Examples for the behavior specified in this section can be found in
.PUSH, CONTINUE, and NEXT
PUSH, NEXT, and CONTINUE are operations applied by the forwarding
plane. The specifications of these operations can be found in
. This subsection specifies how to implement each of these
operations in the MPLS forwarding plane.PUSH
As described in , PUSH corresponds to pushing one or more
labels on top of an incoming packet then sending it out of a
particular physical interface or virtual interface, such as a UDP
tunnel or the Layer 2 Tunneling Protocol version 3 (L2TPv3) , towards a particular
next hop.
When pushing labels onto a packet's label stack, the Time-to-Live
(TTL) field and the Traffic Class (TC)
field of each label stack entry must, of
course, be set. This document does not specify any set of rules for
setting these fields; that is a matter of local policy. Sections and specify additional details about forwarding
behavior.CONTINUE
As described in , the CONTINUE operation corresponds to
swapping the incoming label with an outgoing label. The value of the
outgoing label is calculated as specified in Sections and .NEXT
As described in , NEXT corresponds to popping the topmost
label. The action before and/or after the popping depends on the
instruction associated with the active SID on the received packet
prior to the popping. For example, suppose the active SID in the
received packet was an Adj-SID ; on receiving the
packet, the node applies the NEXT operation, which corresponds to popping
the topmost label, and then sends the packet out of the physical or
virtual interface (e.g., the UDP tunnel or L2TPv3 tunnel
) towards the next hop corresponding to the Adj-SID.Mirror SID
If the active SID in the received packet was a Mirror SID (see ) allocated by the receiving router, the receiving
router applies the NEXT operation, which corresponds to popping the topmost
label, and then performs a lookup using the contents of the packet
after popping the outermost label in the mirrored forwarding table.
The method by which the lookup is made, and/or the actions applied to
the packet after the lookup in the mirror table, depends on the
contents of the packet and the mirror table. Note that the packet
exposed after popping the topmost label may or may not be an MPLS
packet. A Mirror SID can be viewed as a generalization of the context
label in because a Mirror SID does not make any
assumptions about the packet underneath the top label.MPLS Label Downloaded to the FIB for Global and Local SIDs
The label corresponding to the global SID "Si", which is represented by the
global index "I" and downloaded to the FIB, is used to match packets whose
active segment (and hence topmost label) is "Si". The value of this
label is calculated as specified in .
For Local SIDs, the MCC is responsible for downloading the correct
label value to the FIB. For example, an IGP with SR extensions downloads the MPLS label corresponding to an Adj-SID .Active Segment
When instantiated in the MPLS domain, the active segment on a packet
corresponds to the topmost label and is calculated
according to the procedure specified in Sections and . When
arriving at a node, the topmost label corresponding to the active SID
matches the MPLS label downloaded to the FIB as specified in .Forwarding Behavior for Global SIDs
This section specifies the forwarding behavior, including the calculation
of outgoing labels, that corresponds to a global SID when applying
the PUSH, CONTINUE, and NEXT operations in the MPLS forwarding plane.
This document covers the calculation of the outgoing label for the
top label only. The case where the outgoing label is not the top
label and is part of a stack of labels that instantiates a routing
policy or a traffic-engineering tunnel is outside the scope of this
document and may be covered in other documents such as .Forwarding for PUSH and CONTINUE of Global SIDs
Suppose an MCC on router "R0" determines that, before sending the packet towards a neighbor "N", the PUSH or CONTINUE
operation is to be applied to an incoming packet related to the global SID "Si".
SID "Si" is represented by the global index "I" and owned by the router Ri. Neighbor "N" may be directly
connected to "R0" through either a physical or a virtual interface (e.g.,
UDP tunnel or L2TPv3 tunnel ).
The method by which the MCC on router "R0" determines that the PUSH or
CONTINUE operation must be applied using the SID "Si" is beyond the
scope of this document.
An example of a method to determine the SID
"Si" for the PUSH operation is the case where IS-IS
receives the Prefix-SID "Si" sub-TLV
advertised with the prefix "P/m" in TLV 135, and the prefix "P/m" is the longest matching
network prefix for the incoming IPv4 packet.
For the CONTINUE operation, an example of a method used to determine the SID
"Si" is the case where IS-IS receives the Prefix-SID "Si" sub-TLV advertised with
prefix "P" in TLV 135, and the top label of the incoming packet
matches the MPLS label in the FIB corresponding to the SID "Si" on
router "R0".
The forwarding behavior for PUSH and CONTINUE corresponding to the
SID "Si" is as follows:
If neighbor "N" does not support SR or advertises an invalid
SRGB or a SRGB that is too small for the SID "Si", then:
If it is possible to send the packet towards neighbor "N"
using standard MPLS forwarding behavior as specified in
and , forward the packet. The method
by which a router decides whether it is possible to send the
packet to "N" or not is beyond the scope of this document. For
example, the router "R0" can use the downstream label
determined by another MCC, such as LDP , to send the
packet.
Else, if there are other usable next hops, use them to forward the incoming packet.
The method by which the
router "R0" decides on the possibility of using other next hops
is beyond the scope of this document. For example, the
MCC on "R0" may chose the send an IPv4 packet without pushing
any label to another next hop.
Otherwise, drop the packet.
Else,
Calculate the outgoing label as specified in using
the SRGB of neighbor "N".
Determine the outgoing label stack
If the operation is PUSH:
Push the calculated label according to the MPLS label
pushing rules specified in .
Else,
swap the incoming label with the calculated label
according to the label-swapping rules in .
Send the packet towards neighbor "N".
Forwarding for the NEXT Operation for Global SIDs
As specified in , the NEXT operation corresponds to popping
the topmost label. The forwarding behavior is as follows:
Pop the topmost label
Apply the instruction associated with the incoming label that has
been popped
The action on the packet after popping the topmost label depends on
the instruction associated with the incoming label as well as the
contents of the packet right underneath the top label that was
popped. Examples of the NEXT operation are described in Forwarding Behavior for Local SIDs
This section specifies the forwarding behavior for Local SIDs when SR
is instantiated over the MPLS forwarding plane.Forwarding for the PUSH Operation on Local SIDs
Suppose an MCC on router "R0" determines that the PUSH operation is to
be applied to an incoming packet using the Local SID "Si" before
sending the packet towards neighbor "N", which is directly connected to R0
through a physical or virtual interface such as a UDP tunnel
or L2TPv3 tunnel .
An example of such a Local SID is an Adj-SID allocated and advertised
by IS-IS . The method by
which the MCC on "R0" determines that the PUSH operation is to be applied
to the incoming packet is beyond the scope of this document. An
example of such a method is the backup path used to protect against a
failure using TI-LFA .
As mentioned in , a Local SID is specified by an MPLS label.
Hence, the PUSH operation for a Local SID is identical to the label push
operation using any MPLS label . The forwarding action after
pushing the MPLS label corresponding to the Local SID is also
determined by the MCC. For example, if the PUSH operation was done to
forward a packet over a backup path calculated using TI-LFA, then the
forwarding action may be sending the packet to a certain neighbor
that will in turn continue to forward the packet along the backup
path.Forwarding for the CONTINUE Operation for Local SIDs
A Local SID on router "R0" corresponds to a local label.
In such a
scenario, the outgoing label towards next hop "N" is determined by
the MCC running on the router "R0", and the forwarding behavior for the
CONTINUE operation is identical to the swap operation on an
MPLS label .Outgoing Label for the NEXT Operation for Local SIDs
The NEXT operation for Local SIDs is identical to the NEXT operation for
global SIDs as specified in .IANA Considerations
This document has no IANA actions.Manageability Considerations
This document describes the applicability of Segment Routing over the
MPLS data plane. Segment Routing does not introduce any change in
the MPLS data plane. Manageability considerations described in
apply to the MPLS data plane when used with Segment
Routing. SR Operations, Administration, and Maintenance (OAM) use cases for the MPLS data plane are defined in
. SR OAM procedures for the MPLS data plane are defined in
.Security Considerations
This document does not introduce additional security requirements and
mechanisms other than the ones described in .ReferencesNormative ReferencesInformative ReferencesIS-IS Extensions for Segment RoutingOSPFv3 Extensions for Segment RoutingOSPF Extensions for Segment RoutingSegment Routing MPLS Interworking with LDPTopology Independent Fast Reroute using Segment RoutingSegment Routing Policy ArchitectureExamplesIGP Segment Examples
Consider the network diagram of and the IP addresses and IGP
segment allocations of . Assume that the network is running
IS-IS with SR extensions ,
and all links have the same metric. The following examples can be
constructed.IGP Segments -- IllustrationIGP Address and Segment Allocation -- Illustration
Suppose R1 wants to send IPv4 packet P1 to R8. In this case, R1
needs to apply the PUSH operation to the IPv4 packet.
Remember that the SID index "8" is a global IGP segment attached to
the IP prefix 192.0.2.8/32. Its semantic is global within the IGP
domain: any router forwards a packet received with active segment 8
to the next hop along the ECMP-aware shortest path to the related
prefix.
R2 is the next hop along the shortest path towards R8. By applying
the steps in , the outgoing label downloaded to R1's FIB
corresponding to the global SID index "8" is 1008 because the SRGB of
R2 = [1000,5000] as shown in .
Because the packet is IPv4, R1 applies the PUSH operation using the
label value 1008 as specified in . The resulting MPLS
header will have the "S" bit set because it is followed
directly by an IPv4 packet.
The packet arrives at router R2.
Because top label 1008
corresponds to the IGP SID index "8", which is the Prefix-SID attached to
the prefix 192.0.2.8/32 owned by Node R8, the instruction
associated with the SID is "forward the packet using one of the ECMP interfaces or next hops along the shortest path(s) towards R8". Because R2 is not the penultimate hop, R2
applies the CONTINUE operation to the packet and sends it to R3 using
one of the two links connected to R3 with top label 1008 as specified
in .
R3 receives the packet with top label 1008. Because top label
1008 corresponds to the IGP SID index "8", which is the Prefix-SID attached
to the prefix 192.0.2.8/32 owned by Node R8, the instruction
associated with the SID is "send the packet using one of the ECMP interfaces and next hops along the shortest path towards R8". Because R3
is the penultimate hop, we assume that R3 performs penultimate hop
popping, which corresponds to the NEXT operation; the packet is then sent to
R8. The NEXT operation results in popping the outer label
and sending the packet as a pure IPv4 packet to R8.
In conclusion, the path followed by P1 is R1-R2--R3-R8. The ECMP
awareness ensures that the traffic is load-shared between any ECMP
path; in this case, it's the two links between R2 and R3.Incoming Label Collision Examples
This section outlines several examples to illustrate the handling of
label collision described in .
For the examples in this section, we assume that Node A has the
following:
OSPF default admin distance for implementation=50
IS-IS default admin distance for implementation=60
Example 1
The following example illustrates incoming label collision resolution for the same FEC
type using MCC administrative distance.
FEC1:
Node A receives an OSPF Prefix-SID Advertisement from Node B for 198.51.100.5/32 with index=5.
Assuming that OSPF SRGB on Node A = [1000,1999], the incoming label is 1005.
FEC2:
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for 203.0.113.105/32
with index=5. Assuming that IS-IS SRGB on Node A = [1000,1999], the incoming label is 1005.
FEC1 and FEC2 both use dynamic SID assignment.
Since neither of the
FECs are of type 'SR Policy', we use the default admin distances of 50 and
60 to break the tie. So FEC1 wins.Example 2
The following example Illustrates incoming label collision resolution for different FEC
types using the MCC administrative distance.
FEC1:
Node A receives an OSPF Prefix-SID Advertisement from Node B for
198.51.100.6/32 with index=6.
Assuming that OSPF SRGB on Node A = [1000,1999],
the incoming label on Node A corresponding to
198.51.100.6/32 is 1006.
FEC2:
IS-IS on Node A assigns label 1006 to the globally significant
Adj-SID (i.e., when advertised, the L-Flag is clear in the Adj-SID
sub-TLV as described in ). Hence, the incoming label corresponding
to this Adj-SID is 1006. Assume Node A allocates this Adj-SID
dynamically, and it may differ across router reboots.
FEC1 and FEC2 both use dynamic SID assignment. Since neither of the
FECs are of type 'SR Policy', we use the default admin distances of 50 and
60 to break the tie. So FEC1 wins.Example 3
The following example illustrates incoming label collision resolution based on
preferring static over dynamic SID assignment.
FEC1:
OSPF on Node A receives a Prefix-SID Advertisement from Node B for
198.51.100.7/32 with index=7. Assuming that the OSPF SRGB on Node A
= [1000,1999], the incoming label corresponding to 198.51.100.7/32
is 1007.
FEC2:
The operator on Node A configures IS-IS on Node A to assign label
1007 to the globally significant Adj-SID (i.e., when advertised, the
L-Flag is clear in the Adj-SID sub-TLV as described in ).
Node A assigns this Adj-SID explicitly via configuration, so the Adj-SID
survives router reboots.
FEC1 uses dynamic SID assignment, while FEC2 uses explicit SID
assignment. So FEC2 wins.Example 4
The following example illustrates incoming label collision resolution using FEC type
default administrative distance.
FEC1:
OSPF on Node A receives a Prefix-SID Advertisement from Node B for
198.51.100.8/32 with index=8. Assuming that OSPF SRGB on Node A =
[1000,1999], the incoming label corresponding to 198.51.100.8/32 is
1008.
FEC2:
Suppose the SR Policy Advertisement from the controller to Node A for the
policy identified by (Endpoint = 192.0.2.208, color = 100) that
consists of SID-List=<S1, S2> assigns the globally significant
Binding-SID label 1008.
From the point of view of Node A, FEC1 and FEC2 both use dynamic SID
assignment. Based on the default administrative distance outlined in
, the Binding SID has a higher administrative distance
than the Prefix-SID; hence, FEC1 wins.Example 5
The following example illustrates incoming label collision resolution based on FEC type
preference.
FEC1:
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.110/32 with index=10. Assuming that the IS-IS SRGB on Node A
= [1000,1999], the incoming label corresponding to 203.0.113.110/32
is 1010.
FEC2:
IS-IS on Node A assigns label 1010 to the globally significant
Adj-SID (i.e., when advertised, the L-Flag is clear in the Adj-SID
sub-TLV as described in ).
Node A allocates this Adj-SID dynamically, and it may differ across
router reboots. Hence, both FEC1 and FEC2 both use dynamic SID
assignment.
Since both FECs are from the same MCC, they have the same default
admin distance. So we compare the FEC type codepoints. FEC1 has FEC type
codepoint=120, while FEC2 has FEC type codepoint=130. Therefore,
FEC1 wins.Example 6
The following example illustrates incoming label collision resolution based on address
family preference.
FEC1:
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.111/32 with index=11. Assuming that the IS-IS SRGB on Node A
= [1000,1999], the incoming label on Node A for 203.0.113.111/32 is
1011.
FEC2:
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
2001:DB8:1000::11/128 with index=11. Assuming that the IS-IS SRGB on
Node A = [1000,1999], the incoming label on Node A for
2001:DB8:1000::11/128 is 1011.
FEC1 and FEC2 both use dynamic SID assignment. Since both FECs are
from the same MCC, they have the same default admin distance. So we
compare the FEC type codepoints. Both FECs have FEC type codepoint=120.
So we compare the address family. Since IPv4 is preferred over IPv6, FEC1
wins.Example 7
The following example illustrates incoming label collision resolution based on prefix
length.
FEC1:
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.112/32 with index=12. Assuming that IS-IS SRGB on Node A =
[1000,1999], the incoming label for 203.0.113.112/32 on Node A is
1012.
FEC2:
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.128/30 with index=12. Assuming that the IS-IS SRGB on Node A
= [1000,1999], the incoming label for 203.0.113.128/30 on Node A is
1012.
FEC1 and FEC2 both use dynamic SID assignment. Since both FECs are
from the same MCC, they have the same default admin distance. So we
compare the FEC type codepoints. Both FECs have FEC type codepoint=120.
So we compare the address family. Both are a part of the IPv4 address family, so we
compare the prefix length. FEC1 has prefix length=32, and FEC2 has
prefix length=30, so FEC2 wins.Example 8
The following example illustrates incoming label collision resolution based on the
numerical value of the FECs.
FEC1:
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.113/32 with index=13. Assuming that IS-IS SRGB on Node A =
[1000,1999], the incoming label for 203.0.113.113/32 on Node A
is 1013.
FEC2:
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.213/32 with index=13. Assuming that IS-IS SRGB on Node A =
[1000,1999], the incoming label for 203.0.113.213/32 on Node A
is 1013.
FEC1 and FEC2 both use dynamic SID assignment. Since both FECs are
from the same MCC, they have the same default admin distance. So we
compare the FEC type codepoints. Both FECs have FEC type codepoint=120.
So we compare the address family. Both are a part of the IPv4 address family, so we
compare the prefix length. Prefix lengths are the same, so we compare
the prefix. FEC1 has the lower prefix, so FEC1 wins.Example 9
The following example illustrates incoming label collision resolution based on the Routing
Instance ID.
FEC1:
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.114/32 with index=14. Assume that this IS-IS instance on
Node A has Routing Instance ID = 1000 and SRGB = [1000,1999]. Hence,
the incoming label for 203.0.113.114/32 on Node A is 1014.
FEC2:
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.114/32 with index=14. Assume that this is another instance
of IS-IS on Node A but Routing Instance ID = 2000 is different and
SRGB = [1000,1999] is the same. Hence, the incoming label for 203.0.113.114/32 on
Node A is 1014.
These two FECs match all the way through the prefix length and
prefix. So the Routing Instance ID breaks the tie, and FEC1 wins.Example 10
The following example illustrates incoming label collision resolution based on the topology
ID.
FEC1:
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.115/32 with index=15. Assume that this IS-IS instance on
Node A has Routing Instance ID = 1000. Assume that the prefix
advertisement of 203.0.113.115/32 was received in the IS-IS Multi-topology
advertisement with ID = 50. If the IS-IS SRGB for this routing
instance on Node A = [1000,1999], then the incoming label of
203.0.113.115/32 for topology 50 on Node A is 1015.
FEC2:
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.115/32 with index=15. Assume that it has the same Routing
Instance ID = 1000, but 203.0.113.115/32 was advertised with
IS-IS Multi-topology ID = 40, which is different. If the IS-IS SRGB on Node A =
[1000,1999], then the incoming label of 203.0.113.115/32 for topology 40
on Node A is also 1015.
Since these two FECs match all the way through the prefix length, prefix,
and Routing Instance ID, we compare the IS-IS Multi-topology ID, so FEC2
wins.Example 11
The following example illustrates incoming label collision for resolution based on
the algorithm ID.
FEC1:
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.116/32 with index=16. Assume that IS-IS on Node A has Routing
Instance ID = 1000. Assume that Node B advertised 203.0.113.116/32
with IS-IS Multi-topology ID = 50 and SR algorithm = 0. Assume that
the IS-IS SRGB on Node A = [1000,1999]. Hence, the incoming label
corresponding to this advertisement of 203.0.113.116/32 is 1016.
FEC2:
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.116/32 with index=16. Assume that it is the same IS-IS
instance on Node A with Routing Instance ID = 1000. Also assume that
Node C advertised 203.0.113.116/32 with IS-IS Multi-topology ID = 50
but with SR algorithm = 22. Since it is the same routing instance,
the SRGB on Node A = [1000,1999]. Hence, the incoming label
corresponding to this advertisement of 203.0.113.116/32 by Node C is
also 1016.
Since these two FECs match all the way through in terms of the prefix length, prefix,
Routing Instance ID, and Multi-topology ID, we compare the SR
algorithm IDs, so FEC1 wins.Example 12
The following example illustrates incoming label collision resolution based on the FEC
numerical value, independent of how the SID is assigned to the
colliding FECs.
FEC1:
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.117/32 with index=17. Assume that the IS-IS SRGB on Node A
= [1000,1999]; thus, the incoming label is 1017.
FEC2:
Suppose there is an IS-IS Mapping Server Advertisement (SID / Label
Binding TLV) from Node D that has range = 100 and prefix = 203.0.113.1/32.
Suppose this Mapping Server Advertisement generates 100 mappings, one
of which maps 203.0.113.17/32 to index=17.
Assuming that it is the
same IS-IS instance, the SRGB = [1000,1999] and hence the
incoming label for 1017.
Even though FEC1 comes from a normal Prefix-SID Advertisement and
FEC2 is generated from a Mapping Server Advertisement, it is not used as
a tiebreaking parameter. Both FECs use dynamic SID assignment, are
from the same MCC, and have the same FEC type codepoint=120. Their
prefix lengths are the same as well. FEC2 wins based on its lower
numerical prefix value, since 203.0.113.17 is less than
203.0.113.117.Example 13
The following example illustrates incoming label collision resolution based on address
family preference.
FEC1:
SR Policy Advertisement from the controller to Node A. Endpoint
address=2001:DB8:3000::100, color=100, SID-List=<S1, S2>, and the
Binding-SID label=1020.
FEC2:
SR Policy Advertisement from controller to Node A. Endpoint
address=192.0.2.60, color=100, SID-List=<S3, S4>, and the Binding-SID
label=1020.The FEC tiebreakers match, and they have the
same FEC type codepoint=140. Thus, FEC2 wins based on the IPv4 address family
being preferred over IPv6.Example 14
The following example illustrates incoming label resolution based on the numerical value of
the policy endpoint.
FEC1:
SR Policy Advertisement from the controller to Node A. Endpoint
address=192.0.2.70, color=100, SID-List=<S1, S2>, and Binding-SID
label=1021.
FEC2:
SR Policy Advertisement from the controller to Node A. Endpoint
address=192.0.2.71, color=100, SID-List=<S3, S4>, and Binding-SID
label=1021.
The FEC tiebreakers match, and they have the
same address family. Thus, FEC1 wins by having the lower numerical endpoint
address value.Examples for the Effect of Incoming Label Collision on an Outgoing Label
This section presents examples to illustrate the effect of incoming
label collision on the selection of the outgoing label as described in
.Example 1
The following example illustrates the effect of incoming label resolution on the
outgoing label.
FEC1:
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.122/32 with index=22. Assuming that the IS-IS SRGB on Node A
= [1000,1999], the corresponding incoming label is 1022.
FEC2:
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.222/32 with index=22. Assuming that the IS-IS SRGB on Node A
= [1000,1999], the corresponding incoming label is 1022.
FEC1 wins based on the lowest numerical prefix value. This means that
Node A installs a transit MPLS forwarding entry to swap incoming
label 1022 with outgoing label N and to use outgoing interface I. N is
determined by the index associated with FEC1 (index=22) and the SRGB
advertised by the next-hop node on the shortest path to reach
203.0.113.122/32.
Node A will generally also install an imposition MPLS forwarding
entry corresponding to FEC1 for incoming prefix=203.0.113.122/32
pushing outgoing label N, and using outgoing interface I.
The rule in means Node A MUST NOT install an ingress
MPLS forwarding entry corresponding to FEC2 (the losing FEC, which
would be for prefix 203.0.113.222/32).Example 2
The following example illustrates the effect of incoming label collision resolution on
outgoing label programming on Node A.
FEC1:SR Policy Advertisement from the controller to Node A.
Endpoint address=192.0.2.80, color=100, SID-List=<S1, S2>, and
Binding-SID label=1023.
FEC2:
SR Policy Advertisement from controller to Node A.
Endpoint address=192.0.2.81, color=100, SID-List=<S3, S4>, and
Binding-SID label=1023.
FEC1 wins by having the lower numerical endpoint address value. This
means that Node A installs a transit MPLS forwarding entry to swap
incoming label=1023 with outgoing labels, and the outgoing interface
is determined by the SID-List for FEC1.
In this example, we assume that Node A receives two BGP/VPN routes:
R1 with VPN label=V1, BGP next hop = 192.0.2.80, and color=100
R2 with VPN label=V2, BGP next hop = 192.0.2.81, and color=100
We also assume that Node A has a BGP policy that matches color=100
and allows its usage as Service Level Agreement (SLA) steering information. In this case,
Node A will install a VPN route with label stack = <S1,S2,V1>
(corresponding to FEC1).
The rule described in means that Node A MUST NOT install
a VPN route with label stack = <S3,S4,V1> (corresponding to FEC2.)Acknowledgements
The authors would like to thank Les Ginsberg, Chris Bowers, Himanshu
Shah, Adrian Farrel, Alexander Vainshtein, Przemyslaw Krol, Darren
Dukes, Zafar Ali, and Martin Vigoureux for their valuable comments on
this document.Contributors
The following contributors have substantially helped the definition
and editing of the content of this document: