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+
+SR-MPLS: Segment Routing for MPLS
+=================================
+
+This is a memo intended to contain documentation of the VPP SR-MPLS
+implementation. Everything that is not directly obvious should come
+here. For any feedback on content that should be explained please
+mailto:pcamaril@cisco.com
+
+Segment Routing
+---------------
+
+Segment routing is a network technology focused on addressing the
+limitations of existing IP and Multiprotocol Label Switching (MPLS)
+networks in terms of simplicity, scale, and ease of operation. It is a
+foundation for application engineered routing as it prepares the
+networks for new business models where applications can control the
+network behavior.
+
+Segment routing seeks the right balance between distributed intelligence
+and centralized optimization and programming. It was built for the
+software-defined networking (SDN) era.
+
+Segment routing enhances packet forwarding behavior by enabling a
+network to transport unicast packets through a specific forwarding path,
+different from the normal path that a packet usually takes (IGP shortest
+path or BGP best path). This capability benefits many use cases, and one
+can build those specific paths based on application requirements.
+
+Segment routing uses the source routing paradigm. A node, usually a
+router but also a switch, a trusted server, or a virtual forwarder
+running on a hypervisor, 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 local semantic to a
+segment-routing node or global within a segment-routing network. Segment
+routing allows an operator to enforce a flow through any topological
+path and service chain while maintaining per-flow state only at the
+ingress node to the segment-routing network. Segment routing also
+supports equal-cost multipath (ECMP) by design.
+
+Segment routing can operate with either an MPLS or an IPv6 data plane.
+All the currently available MPLS services, such as Layer 3 VPN (L3VPN),
+L2VPN (Virtual Private Wire Service [VPWS], Virtual Private LAN Services
+[VPLS], Ethernet VPN [E-VPN], and Provider Backbone Bridging Ethernet
+VPN [PBB-EVPN]), can run on top of a segment-routing transport network.
+
+**The implementation of Segment Routing in VPP covers both the IPv6 data
+plane (SRv6) as well as the MPLS data plane (SR-MPLS). This page
+contains the SR-MPLS documentation.**
+
+Segment Routing terminology
+---------------------------
+
+-  SegmentID (SID): is an MPLS label.
+-  Segment List (SL) (SID List): is the sequence of SIDs that the packet
+   will traverse.
+-  SR Policy: is a set of candidate paths (SID list+weight). An SR
+   policy is uniquely identified by its Binding SID and associated with
+   a weighted set of Segment Lists. In case several SID lists are
+   defined, traffic steered into the policy is unevenly load-balanced
+   among them according to their respective weights.
+-  BindingSID: a BindingSID is a SID (only one) associated one-one with
+   an SR Policy. If a packet arrives with MPLS label corresponding to a
+   BindingSID, then the SR policy will be applied to such packet.
+   (BindingSID is popped first.)
+
+SR-MPLS features in VPP
+-----------------------
+
+The SR-MPLS implementation is focused on the SR policies, as well on its
+steering. Others SR-MPLS features, such as for example AdjSIDs, can be
+achieved using the regular VPP MPLS implementation.
+
+The Segment Routing Policy
+(*draft-filsfils-spring-segment-routing-policy*) defines SR Policies.
+
+Creating a SR Policy
+--------------------
+
+An SR Policy is defined by a Binding SID and a weighted set of Segment
+Lists.
+
+A new SR policy is created with a first SID list using:
+
+::
+
+   sr mpls policy add bsid 40001 next 16001 next 16002 next 16003 (weight 5)
+
+-  The weight parameter is only used if more than one SID list is
+   associated with the policy.
+
+An SR policy is deleted with:
+
+::
+
+   sr mpls policy del bsid 40001
+
+The existing SR policies are listed with:
+
+::
+
+   show sr mpls policies
+
+Adding/Removing SID Lists from an SR policy
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+An additional SID list is associated with an existing SR policy with:
+
+::
+
+   sr mpls policy mod bsid 40001 add sl next 16001 next 16002 next 16003 (weight 3)
+
+Conversely, a SID list can be removed from an SR policy with:
+
+::
+
+   sr mpls policy mod bsid 4001 del sl index 1
+
+Note that this CLI cannot be used to remove the last SID list of a
+policy. Instead the SR policy delete CLI must be used.
+
+The weight of a SID list can also be modified with:
+
+::
+
+   sr mpls policy mod bsid 40001 mod sl index 1 weight 4
+
+SR Policies: Spray policies
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Spray policies are a specific type of SR policies where the packet is
+replicated on all the SID lists, rather than load-balanced among them.
+
+SID list weights are ignored with this type of policies.
+
+A Spray policy is instantiated by appending the keyword **spray** to a
+regular SR-MPLS policy command, as in:
+
+::
+
+   sr mpls policy add bsid 40002 next 16001 next 16002 next 16003 spray
+
+Spray policies are used for removing multicast state from a network core
+domain, and instead send a linear unicast copy to every access node. The
+last SID in each list accesses the multicast tree within the access
+node.
+
+Steering packets into a SR Policy
+---------------------------------
+
+Segment Routing supports three methods of steering traffic into an SR
+policy.
+
+Local steering
+~~~~~~~~~~~~~~
+
+In this variant incoming packets match a routing policy which directs
+them on a local SR policy.
+
+In order to achieve this behavior the user needs to create an ‘sr
+steering policy via sr policy bsid’.
+
+::
+
+   sr mpls steer l3 2001::/64 via sr policy bsid 40001
+   sr mpls steer l3 2001::/64 via sr policy bsid 40001 fib-table 3
+   sr mpls steer l3 10.0.0.0/16 via sr policy bsid 40001
+   sr mpls steer l3 10.0.0.0/16 via sr policy bsid 40001 vpn-label 500
+
+Remote steering
+~~~~~~~~~~~~~~~
+
+In this variant incoming packets have an active SID matching a local
+BSID at the head-end.
+
+In order to achieve this behavior the packets should simply arrive with
+an active SID equal to the Binding SID of a locally instantiated SR
+policy.
+
+Automated steering
+~~~~~~~~~~~~~~~~~~
+
+In this variant incoming packets match a BGP/Service route which
+recurses on the BSID of a local policy.
+
+In order to achieve this behavior the user first needs to color the SR
+policies. He can do so by using the CLI:
+
+::
+
+   sr mpls policy te bsid xxxxx endpoint x.x.x.x color 12341234
+
+Notice that an SR policy can have a single endpoint and a single color.
+Notice that the *endpoint* value is an IP46 address and the color a u32.
+
+Then, for any BGP/Service route the user has to use the API to steer
+prefixes:
+
+::
+
+   sr steer l3 2001::/64 via next-hop 2001::1 color 1234 co 2
+   sr steer l3 2001::/64 via next-hop 2001::1 color 1234 co 2 vpn-label 500
+
+Notice that *co* refers to the CO-bits (values [0|1|2|3]).
+
+Notice also that a given prefix might be steered over several colors
+(same next-hop and same co-bit value). In order to add new colors just
+execute the API several times (or with the del parameter to delete the
+color).
+
+This variant is meant to be used in conjunction with a control plane
+agent that uses the underlying binary API bindings of
+*sr_mpls_steering_policy_add*/*sr_mpls_steering_policy_del* for any BGP
+service route received.