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authorNathan Skrzypczak <nathan.skrzypczak@gmail.com>2021-10-08 14:01:27 +0200
committerDave Wallace <dwallacelf@gmail.com>2021-10-13 15:32:33 +0000
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Type: improvement Change-Id: If453321785b04f9c16e8cea36fb1910efaeb2c59 Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>
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-# SR-MPLS: Segment Routing for MPLS {#srmpls_doc}
-
-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 <a href="https://datatracker.ietf.org/doc/draft-filsfils-spring-segment-routing-policy/">Segment Routing Policy (*draft-filsfils-spring-segment-routing-policy*)</a> 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 methos 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. \ No newline at end of file