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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" submissionType="IETF" category="info" consensus="true" docName="draft-ietf-dmm-srv6-mobile-uplane-24" number="9433" ipr="trust200902" obsoletes="" updates="" submissionType="IETF" xml:lang="en" tocInclude="true" symRefs="true" sortRefs="true" version="3">
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  <front>

    <title abbrev="SRv6 Mobile User-Plane">
			Segment User Plane">Segment Routing over IPv6 for the Mobile
    User Plane</title>
    <seriesInfo name="Internet-Draft" value="draft-ietf-dmm-srv6-mobile-uplane-24"/> name="RFC" value="9433"/>
    <author fullname="Satoru Matsushima" initials="S." surname="Matsushima" role="editor">
      <organization abbrev="SoftBank">SoftBank</organization>
      <address>
        <postal>
          <street/>
          <city/>
          <region/>
          <code/>
          <country>Japan</country>
        </postal>
        <email>satoru.matsushima@g.softbank.co.jp</email>
      </address>
    </author>
    <author fullname="Clarence Filsfils" initials="C." surname="Filsfils">
      <organization abbrev="Cisco Systems, Inc.">
				Cisco Systems, Inc.</organization>
      <address>
        <postal>
          <street/>
          <city/>
          <region/>
          <code/>
          <country>Belgium</country>
        </postal>
        <email>cf@cisco.com</email>
      </address>
    </author>
    <author fullname="Miya Kohno" initials="M." surname="Kohno">
      <organization abbrev="Cisco Systems, Inc.">
		Cisco Systems, Inc.</organization>
      <address>
        <postal>
          <street/>
          <city/>
          <region/>
          <code/>
          <country>Japan</country>
        </postal>
        <email>mkohno@cisco.com</email>
      </address>
    </author>
    <author fullname="Pablo Camarillo Garvia" initials="P." surname="Camarillo" role="editor">
      <organization abbrev="Cisco Systems, Inc.">
			Cisco Systems, Inc.</organization>
      <address>
        <postal>
          <street/>
          <city/>
          <region/>
          <code/>
          <country>Spain</country>
        </postal>
        <email>pcamaril@cisco.com</email>
      </address>
    </author>
    <author fullname="Daniel Voyer" initials="D." surname="Voyer">
      <organization abbrev="Bell Canada">Bell Canada</organization>
      <address>
        <postal>
          <street/>
          <city/>
          <region/>
          <code/>
          <country>Canada</country>
        </postal>
        <email>daniel.voyer@bell.ca</email>
      </address>
    </author>
    <date year="2023"/>
    <workgroup>DMM Working Group</workgroup> year="2023" month="July" />
    <area>int</area>
    <workgroup>dmm</workgroup>

    <abstract>
      <t>
        This document discusses the applicability of SRv6 (Segment Segment Routing IPv6) over IPv6
        (SRv6) to the user-plane user plane of mobile networks. The network programming
        nature of SRv6 accomplishes mobile user-plane functions in a simple
        manner.  The statelessness of SRv6 and its ability to control both
        service layer path and underlying transport can be beneficial to the
        mobile
        user-plane, user plane, providing flexibility, end-to-end network slicing,
        and
        SLA Service Level Agreement (SLA) control for various
        applications. </t>
      <t>
        This document discusses how SRv6 (Segment Routing over IPv6) could be used as user-plane the user plane of
        mobile networks. This document also specifies the SRv6 Segment
        Endpoint behaviors Behaviors required for mobility use-cases. use cases.
      </t>
    </abstract>
  </front>
  <middle>

    <section numbered="true" toc="default">
      <name>Introduction</name>
      <t> In
      <t>In mobile networks, mobility systems provide connectivity over a
      wireless link to stationary and non-stationary nodes.  The user-plane user plane
      establishes a tunnel between the mobile node and its anchor node over
      IP-based backhaul and core networks. </t>
      <t> This
      <t>This document specifies the applicability of SRv6 (Segment
        Routing IPv6) <xref
      target="RFC8754" format="default"/><xref format="default"/> <xref target="RFC8986"
      format="default"/> to mobile networks. </t>

      <t>Segment Routing (SR) <xref target="RFC8402" format="default"/> is a source routing
      source-routing architecture: a node steers a packet through an ordered
      list of instructions called "segments". A segment can represent any
      instruction, topological or service based.</t>
      <t>SRv6 applied to mobile networks enables a source-routing based mobile architecture, architecture based
      on source routing, where operators can explicitly indicate a route for
      the packets to and from the mobile node.  The SRv6 Endpoint nodes serve
      as mobile user-plane anchors.</t>
    </section>
    <!-- End section "Introduction" -->

  <section numbered="true" toc="default">
      <name>Conventions and Terminology</name>
        <t>
    The key words "MUST", "MUST NOT",
        "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
        "RECOMMENDED", "NOT RECOMMENDED",
        "MAY", "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
    "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
    NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>",
    "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document are
    to be interpreted as described in BCP 14 BCP&nbsp;14 <xref target="RFC2119" format="default"/> target="RFC2119"/>
    <xref target="RFC8174" format="default"/> target="RFC8174"/> when, and only when, they appear in all capitals,
    as shown here.
        </t>
      <section anchor="terms" numbered="true" toc="default">
        <name>Terminology</name>
        <ul spacing="compact">
          <li>CNF: Cloud-native Network Function</li>
          <li>NFV:
        <dl spacing="normal" newline="false">
          <dt>CNF:</dt> <dd>Cloud-native Network Function</dd>
          <dt>NFV:</dt> <dd>Network Function Virtualization </li>
          <li>PDU: Packet </dd>
          <dt>PDU:</dt> <dd>Packet Data Unit</li>
          <li>PDU Session: Context Unit</dd>
          <dt>PDU Session:</dt> <dd>Context of a UE connected to a mobile network.</li>
          <li>UE: User Equipment</li>
          <li>gNB: gNodeB
          network</dd>
          <dt>UE:</dt> <dd>User Equipment</dd>
          <dt>gNB:</dt> <dd>gNodeB <xref target="TS.23501" format="default"/></li>
          <li>UPF: User
          format="default"/></dd>
          <dt>UPF:</dt> <dd>User Plane Function </li>
          <li>VNF: Virtual </dd>
          <dt>VNF:</dt> <dd>Virtual Network Function</li>
          <li>DN: Data Network</li>
          <li>Uplink: from Function</dd>
          <dt>DN:</dt> <dd>Data Network</dd>
          <dt>Uplink:</dt> <dd>from the UE towards the DN</li>
          <li>Downlink: from DN</dd>
          <dt>Downlink:</dt> <dd>from the DN towards the UE</li>
        </ul> UE</dd>
        </dl>
        <t>The following terms used within this document are defined in <xref
        target="RFC8402" format="default"/>: Segment Routing, SR Domain, domain,
        Segment ID (SID), SRv6, SRv6 SID, Active Segment, SR Policy, Prefix SID, Adjacency SID and Binding SID.</t>
        <t> The SID (BSID).</t>
        <t>The following terms used within this document are defined in <xref
        target="RFC8754" format="default"/>: SRH, SR Source Node, Transit Node, SR Segment Endpoint
            Node Routing Header (SRH) and Reduced
        SRH.</t>

        <t>The following terms used within this document are defined in <xref
        target="RFC8986" format="default"/>: NH, SL, FIB, SA, DA, SRv6 SID behavior, NH (next header), SL (the Segments
        Left field of the SRH), FIB (Forwarding Information Base), SA (Source
        Address), DA (Destination Address), and SRv6 Segment
        Endpoint Behavior.</t>
      </section>
      <!-- End subsection "Terminology" -->

        <section anchor="conventions" numbered="true" toc="default">
        <name>Conventions</name>
        <t>An SR Policy is resolved to a SID list. A SID list is represented as &lt;S1, S2, S3&gt; where S1 is the first SID to visit, S2 is the second SID to visit, and S3 is the last SID to visit along the SR path.</t>

        <t>(SA,DA) (S3, S2, S1; SL) represents an IPv6 packet with:</t> where:</t>
            <ul spacing="compact"> spacing="normal">
             <li>Source Address is SA, Destination Address is DA, and next-header
             next header is SRH</li>
          <li>SRH
	     <li><t>SRH with SID list &lt;S1, S2, S3&gt; with Segments Left = SL</li>
          <li>Note
	     SL</t>

	     <t>Note the difference between the &lt;&gt; and () symbols:
	     symbols. &lt;S1, S2, S3&gt; represents a SID list where S1 is the
	     first SID and S3 is the last SID to traverse.  (S3, S2, S1; SL)
	     represents the same SID list but encoded in the SRH format where
	     the rightmost SID in the SRH is the first SID and the leftmost
	     SID in the SRH is the last SID.  When referring to an SR policy Policy
	     in a high-level use-case, use case, it is simpler to use the &lt;S1, S2,
	     S3&gt; notation. When referring to an illustration of the
	     detailed packet behavior, the (S3, S2, S1; SL) notation is more convenient.</li>
	     convenient.</t>
             </li>
             <li>The payload of the packet is omitted.</li>
	    </ul>
        <t>(SA1,DA1) (SA2, DA2) represents an IPv6 packet with:</t> where:</t>
            <ul spacing="compact"> spacing="normal">
                <li>Source Address is SA1, Destination Address is DA1, and next-header
                next header is IP</li> IP.</li>
		<li>Source Address is SA2, and Destination Address is DA2.</li>
            </ul>
        <t>Throughout the document document, the representation SRH[n] is used as a
        shorter representation of Segment List[n], as defined in <xref
        target="RFC8754" format="default"/>.</t>

        <t>This document uses the following conventions throughout the
        different examples:</t>
        <ul spacing="compact">
          <li> gNB::1 spacing="normal">
          <li>gNB::1 is an IPv6 address (SID) assigned to the gNB.</li>
          <li> U1::1
          <li>U1::1 is an IPv6 address (SID) assigned to UPF1.</li>
          <li> U2::1
          <li>U2::1 is an IPv6 address (SID) assigned to UPF2.</li>
          <li> U2::
          <li>U2:: is the Locator of UPF2.</li>
        </ul>

      </section>
      <!-- End subsection  "Conventions" -->

        <section anchor="srv6-funcs" numbered="true" toc="default">
        <name>Predefined SRv6 Endpoint Behaviors</name>

<t>
	    The following SRv6 Endpoint Behaviors are used throughout this document. They are defined in
	    <xref target="RFC8986" format="default"/>.
        </t>
<ul spacing="compact">
          <li> End.DT4: spacing="normal">
          <li>End.DT4: Decapsulation and Specific IPv4 Table Lookup</li>
          <li> End.DT6:
          <li>End.DT6: Decapsulation and Specific IPv6 Table Lookup</li>
          <li> End.DT46:
          <li>End.DT46: Decapsulation and Specific IP Table Lookup</li>
          <li> End.DX4:
          <li>End.DX4: Decapsulation and IPv4 Cross-Connect</li>
          <li> End.DX6:
          <li>End.DX6: Decapsulation and IPv6 Cross-Connect</li>
          <li> End.DX2:
          <li>End.DX2: Decapsulation and L2 Cross-Connect</li>
          <li> End.T:
          <li>End.T: Endpoint with specific IPv6 Table Lookup</li>
        </ul>
        <t>
	    This document defines new SRv6 Segment Endpoint Behaviors in <xref target="srv6_functions" format="default"/>.</t>
      </section>
      <!-- End section "Predefined SRv6 Functions" -->
    </section>
    <!-- End section "Conventions and Terminology" -->

    <section anchor="motivations" numbered="true" toc="default">
      <name>Motivation</name>
      <t> Mobile networks are becoming more challenging to operate.  On one
      hand, traffic is constantly growing, and latency requirements are
      tighter; on the other-hand, other hand, there are new use-cases use cases like distributed NFV
      Infrastructure that are also challenging network operations. On top of
      this, the number of devices connected is steadily growing, causing
      scalability problems in mobile entities as the state to maintain keeps
      increasing.</t>

<t> The current architecture of mobile networks does not take into
      account the underlying transport. The user-plane user plane is rigidly fragmented
      into radio access, core core, and service networks, networks that connected by tunneling
      according to user-plane roles such as access and anchor nodes. These
      factors have made it difficult for the operator to optimize and operate
      the data-path. data path.
      </t>
      <t> In the meantime,
	applications have shifted to use IPv6, and network operators
	have started adopting IPv6 as their IP transport.
        SRv6, the IPv6 dataplane data plane instantiation of Segment Routing
	<xref target="RFC8402" format="default"/>, integrates both
	the application data-path data path and the underlying transport layer into
	a single protocol, allowing operators to optimize the network in a
	simplified manner and removing forwarding state from the network. It is also
  suitable for virtualized environments, like VNF/CNF to VNF/CNF VNF/CNF-to-VNF/CNF networking. SRv6 has been deployed in dozens of networks <xref target="I-D.matsushima-spring-srv6-deployment-status" format="default"/>.</t>
      <t> SRv6 defines the network-programming network programming concept <xref target="RFC8986" format="default"/>.
  Applied to mobility, SRv6 can provide the user-plane behaviors needed
  for mobility management.  SRv6 takes advantage of the underlying transport
  awareness and flexibility together with the ability to also include services to optimize the end-to-end mobile dataplane.</t> data plane.</t>
      <t>The use-cases use cases for SRv6 mobility are discussed in <xref target="I-D.camarilloelmalky-springdmm-srv6-mob-usecases" format="default"/>, and the architectural benefits are discussed in <xref target="I-D.kohno-dmm-srv6mob-arch" />. </t>
    </section>
    <!-- End section "Motivation" -->

    <section anchor="scenarios" numbered="true" toc="default">
      <name>3GPP Reference Architecture</name>
      <t> This section presents the 3GPP Reference Architecture reference architecture and possible deployment
        scenarios.</t>
      <t> <xref target="fig_5g-ref-arch" format="default"/> shows a reference diagram from
        the 5G packet core architecture <xref target="TS.23501" format="default"/>.</t>
      <t> The user plane described in this document does not depend on any
	specific architecture.  The 5G packet core architecture as shown is
	based on the 3GPP standards.</t>

<figure anchor="fig_5g-ref-arch">
<name>3GPP 5G Reference Architecture</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
                                +-----+
                                | AMF |
                               /+-----+
                              /    | [N11]
                       [N2]  /  +-----+
                     +------/   | SMF |
                    /           +-----+
                   /              / \
                  /              /   \  [N4]
                 /              /     \                    ________
                /              /       \                  /        \
+--+      +-----+ [N3] +------+  [N9]  +------+  [N6]    /          \
|UE|------| gNB |------| UPF1 |--------| UPF2 |--------- \    DN    /
+--+      +-----+      +------+        +------+           \________/
]]></artwork>
</figure>
      <ul spacing="compact">
        <li>UE: User Equipment</li>
        <li>gNB: gNodeB

      <dl spacing="normal" newline="false">
        <dt>UE:</dt>
	<dd>User Equipment</dd>
        <dt>gNB:</dt>
	<dd>gNodeB with N3 interface towards packet core (and N2 for control plane)</li>
        <li>UPF1: UPF
	plane)</dd>
        <dt>UPF1:</dt>
	<dd>UPF with Interfaces N3 and N9 (and N4 for control plane)</li>
        <li>UPF2: UPF plane)</dd>
        <dt>UPF2:</dt>
	<dd>UPF with Interfaces N9 and N6 (and N4 for control plane)</li>
        <li>SMF: Session plane)</dd>
        <dt>SMF:</dt>
	<dd>Session Management Function </li>
        <li>AMF: Access Function</dd>
        <dt>AMF:</dt>
	<dd>Access and Mobility Management Function </li>
        <li>DN: Data Network Function</dd>
        <dt>DN:</dt>
	<dd>Data Network, e.g., operator services, services and Internet access </li>
      </ul> </dd>
      </dl>
      <t> This reference diagram does not depict a UPF that is only connected
	to N9 interfaces, although the mechanisms defined in this document
	also work in such a case.</t>
      <t> Each session from a UE gets assigned to a UPF. Sometimes multiple
      UPFs may be used, providing richer service functions.  A UE gets its
      IPv4 address, or IPv6 prefix, from the DHCP block of its UPF. The UPF
      advertises that IP address block toward the Internet, ensuring that
      return traffic is routed to the right UPF. </t>
    </section>
    <!-- End section "A 3GPP Reference Architecture" -->

    <section anchor="uplane-functions" numbered="true" toc="default">
      <name>User-plane modes</name>
      <name>User-Plane Modes</name>
      <t>This section introduces an SRv6 based SRv6-based mobile user-plane.It user plane. It presents
      two different "modes" that vary with respect to the use of SRv6.
	The SRv6.</t>
      <t>The first one mode is the "Traditional mode", which inherits the current
      3GPP mobile architecture. In this mode mode, the <xref target="TS.29281"
      format="default">GTP-U protocol</xref> is replaced by SRv6, however SRv6. However, the
      N3, N9 N9, and N6 interfaces are still point-to-point interfaces with no
      intermediate waypoints as in the current mobile network
      architecture.</t>
      <t> The second mode is the "Enhanced mode".  This is an evolution from
      the "Traditional mode". In this mode mode, the N3, N9 N9, or N6 interfaces have
      intermediate waypoints -SIDs- (SIDs) that are used for Traffic Engineering traffic engineering or
      VNF purposes transparent to 3GPP functionalities. This results in
      optimal end-to-end policies across the mobile network with transport and
      services awareness.</t>
      <t>In both, both the Traditional and the Enhanced modes, this document assumes
      that the gNB as well as the UPFs are SR-aware (N3, N9 N9, and -potentially- potentially
      N6 interfaces are SRv6).</t>
      <t>In addition to those two modes, this document introduces three
      mechanisms for interworking with legacy access networks (those where the
      N3 interface is unmodified). In this document document, they are introduced as a
      variant to the Enhanced mode, however but they are equally applicable to the
      Traditional mode.</t>
      <t>One of these mechanisms is designed to interwork with legacy gNBs
      using GTP-U/IPv4. The second mechanism is designed to interwork with
      legacy gNBs using GTP-U/IPv6. The third of those mechanisms mechanism is another mode that
      allows deploying SRv6 when legacy gNBs and UPFs that still run GTP-U.</t>
      <t> This document uses the SRv6 Segment Endpoint Behaviors defined in
      <xref target="RFC8986" format="default"/> as well as the new SRv6 Segment
      Endpoint Behaviors designed for the mobile user plane that are
      defined in this document in <xref target="srv6_functions" format="default"/>. format="default"/> of this
      document.
      </t>
      <section anchor="traditional_mode" numbered="true" toc="default">
        <name>Traditional mode</name> Mode</name>
        <t> In the traditional Traditional mode, the existing mobile UPFs remain unchanged
        with the sole exception of the use of SRv6 as the data plane instead
        of GTP-U.  There is no impact to the rest of the mobile system.</t>
        <t> In existing 3GPP mobile networks, a PDU Session is mapped 1-for-1
        with a specific GTP-U tunnel (Tunnel Endpoint Identifier - TEID). (TEID)). This
        1-for-1 mapping is mirrored here to replace GTP-U encapsulation with
        the SRv6 encapsulation, while not changing anything else. There will
        be a unique SRv6 SID associated with each PDU Session, and the SID
        list only contains a single SID.</t>
        <t> The traditional Traditional mode minimizes the changes required changes to the mobile
        system; hence hence, it is a good starting point for forming a common
        ground.</t>
        <t> The gNB/UPF control-plane control plane (N2/N4 interface) is unchanged, specifically unchanged;
        specifically, a single IPv6 address is provided to the gNB. The same
        control plane signalling signaling is used, and the gNB/UPF decides to use SRv6
        based on signaled GTP-U parameters per local policy. The only
        information from the GTP-U parameters used for the SRv6 policy is the
        TEID, QFI -QoS (QoS Flow Identifier-, Identifier), and the IPv6 Destination Address.</t>
        <t> Our example topology is shown in <xref target="fig_traditional"
        format="default"/>.  The gNB and the UPFs are SR-aware.  In the
        descriptions of the uplink and downlink packet flow, A is an IPv6
        address of the UE, and Z is an IPv6 address reachable within the Data Network DN.  A
        End.MAP, a new SRv6 Endpoint Behavior, End.MAP, Behavior defined in <xref
        target="end-map-function" format="default"/>, is used.</t>

<figure anchor="fig_traditional">
<name>Traditional mode Mode - example topology</name> Example Topology</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
                                                           ________
                  SRv6           SRv6                     /        \
+--+      +-----+ [N3] +------+  [N9]  +------+  [N6]    /          \
|UE|------| gNB |------| UPF1 |--------| UPF2 |--------- \    DN    /
+--+      +-----+      +------+        +------+           \________/
         SRv6 node     SRv6 node       SRv6 node
]]></artwork>
</figure>

        <section anchor="traditional_up" numbered="true" toc="default">
          <name>Packet flow Flow - Uplink</name>
          <t> The uplink packet flow is as follows:</t>

<artwork align="center" name="" type="" alt=""><![CDATA[
UE_out  : (A,Z)
gNB_out : (gNB, U1::1) (A,Z)     -> H.Encaps.Red <U1::1>
UPF1_out: (gNB, U2::1) (A,Z)     -> End.MAP
UPF2_out: (A,Z)                  -> End.DT4 or End.DT6]]></artwork> End.DT6
]]></artwork>

          <t> When the UE packet arrives at the gNB, the gNB performs a an
          H.Encaps.Red operation. Since there is only one SID, there is no
          need to push an SRH (reduced SRH). gNB only adds an outer IPv6
          header with IPv6 DA U1::1. gNB obtains the SID U1::1 from the
          existing control plane (N2 interface). U1::1 represents an anchoring
          SID specific for that session at UPF1.</t>
          <t> When the packet arrives at UPF1, the SID U1::1 is associated
          with the End.MAP SRv6 Endpoint Behavior. End.MAP replaces U1::1 by with
          U2::1, that which belongs to the next UPF (U2).</t>
          <t> When the packet arrives at UPF2, the SID U2::1 corresponds to an
          End.DT4/End.DT6/End.DT46 SRv6 Endpoint Behavior. UPF2 decapsulates
          the packet, performs a lookup in a specific table associated with
          that mobile network network, and forwards the packet toward the data network (DN).</t> DN.</t>
        </section>
        <!-- End section "Packet flow - Uplink" -->

          <section anchor="traditional_dn" numbered="true" toc="default">
          <name>Packet flow Flow - Downlink</name>
          <t>The downlink packet flow is as follows:</t>

<artwork align="center" name="" type="" alt=""><![CDATA[
UPF2_in : (Z,A)
UPF2_out: (U2::, U1::2) (Z,A)    -> H.Encaps.Red <U1::2>
UPF1_out: (U2::, gNB::1) (Z,A)   -> End.MAP
gNB_out : (Z,A)                  -> End.DX4, End.DX6, End.DX2
]]></artwork>

          <t> When the packet arrives at the UPF2, the UPF2 maps that flow
          into a PDU Session. This PDU Session is associated with the segment
          endpoint &lt;U1::2&gt;.  UPF2 performs
	      a an H.Encaps.Red operation,
          encapsulating the packet into a new IPv6 header with no SRH since
          there is only one SID.</t>
          <t> Upon packet arrival on UPF1, the SID U1::2 is a local SID
          associated with the End.MAP SRv6 Endpoint Behavior. It maps the SID
          to the next anchoring point and replaces U1::2 by with gNB::1, that which
          belongs to the next hop.</t>
          <t> Upon packet arrival on gNB, the SID gNB::1 corresponds to an
          End.DX4, End.DX6 End.DX6, or End.DX2 behavior (depending on the PDU Session
          Type). The gNB decapsulates the packet, removing the IPv6 header and
          all its extensions headers, and forwards the traffic toward the
          UE.</t>
        </section>
        <!-- End section "Packet flow - Downlink" -->

      </section>
      <!-- End section "Traditional mode" -->

      <section anchor="enhanced_mode" numbered="true" toc="default">
        <name>Enhanced mode</name> Mode</name>
        <t> Enhanced mode improves scalability, provides traffic engineering
        capabilities, and allows service programming <xref
        target="I-D.ietf-spring-sr-service-programming" format="default"/>,
        thanks to the use of multiple SIDs in the SID list (instead of a
        direct connectivity in between UPFs with no intermediate waypoints as
        in Traditional Mode).</t> mode).</t>
        <t>Thus, the main difference is that the SR policy MAY Policy <bcp14>MAY</bcp14>
        include SIDs for traffic engineering and service programming in
        addition to the anchoring SIDs at UPFs.</t>
        <t>Additionally
        <t>Additionally, in this mode mode, the operator may choose to aggregate
        several devices under the same SID list (e.g., stationary residential
        meters [water/energy] (water and energy) connected to the same cell) to improve
        scalability.</t>
        <t>The gNB/UPF control-plane control plane (N2/N4 interface) is unchanged, specifically unchanged;
        specifically, a single IPv6 address is provided to the gNB. A local
        policy instructs the gNB to use SRv6.</t>
        <t> The gNB resolves the IP address received via the control plane
        into a SID list. The resolution mechanism is out of the scope of this
        document.</t>
        <t> Note that the SIDs MAY <bcp14>MAY</bcp14> use the arguments argument <xref
        target="arguments-for-mobility" format="default">Args.Mob.Session
        </xref> if required by the UPFs.</t>
        <t> <xref target="fig_enhanced" format="default"/> shows an Enhanced
        mode topology.  The gNB and the UPF are SR-aware.  The Figure figure
	shows two service segments,
        S1 and C1.  S1 represents a VNF in the network, and C1
        represents an intermediate router used for Traffic Engineering traffic engineering
        purposes to enforce a low-latency path in the network.  Note that
        neither S1 nor C1 are required to have an N4 interface.</t>

<figure anchor="fig_enhanced">
<name>Enhanced mode Mode - Example topology</name> Topology</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
                                 +----+  SRv6               _______
                 SRv6          --| C1 |--[N3]              /       \
+--+    +-----+  [N3]         /  +----+  \  +------+ [N6] /         \
|UE|----| gNB |--       SRv6 /    SRv6    --| UPF1 |------\   DN    /
+--+    +-----+  \      [N3]/      TE       +------+       \_______/
       SRv6 node  \ +----+ /               SRv6 node
                   -| S1 |-
                    +----+
                   SRv6 node
                     VNF
]]></artwork>
</figure>

        <section anchor="enhanced_uplink" numbered="true" toc="default">
          <name>Packet flow Flow - Uplink</name>
          <t>The uplink packet flow is as follows:</t>

<artwork align="center" name="" type="" alt=""><![CDATA[
UE_out  : (A,Z)
gNB_out : (gNB, S1)(U1::1, C1; SL=2)(A,Z)->H.Encaps.Red<S1,C1,U1::1>
S1_out  : (gNB, C1)(U1::1, C1; SL=1)(A,Z)
C1_out  : (gNB, U1::1)(A,Z)              ->End with PSP
UPF1_out: (A,Z)                          ->End.DT4,End.DT6,End.DT2U
]]></artwork>

          <t> UE sends its packet (A,Z) on a specific bearer to its
          gNB.  gNB's control plane associates that session from the UE(A)
          with the IPv6 address B.  gNB resolves B into a SID list. list  &lt;S1,
          C1, U1::1&gt;. </t>
          <t> When gNB transmits the packet, it contains all the segments of
          the SR policy. Policy. The SR policy Policy includes segments for traffic
          engineering (C1) and for service programming (S1). </t>
          <t> Nodes S1 and C1 perform their related Endpoint functionality and
          forward the packet. The End "End with PSP PSP" functionality referes refers to the
          Endpoint behavior Behavior with Penultimate Segment Popping as defined in RFC8986.</t>
          <xref target="RFC8986" format="default"/>.</t>
          <t> When the packet arrives at UPF1, the active segment (U1::1) is an End.DT4/End.DT6/End.DT2U End.DT4/End.DT6/End.DT2U, which performs the
          decapsulation (removing the IPv6 header with all its extension
          headers) and forwards toward the data network.</t> DN.</t>
        </section>
        <!-- End section "Packet flow - Uplink" -->

        <section numbered="true" toc="default">
          <name>Packet flow Flow - Downlink</name>
          <t>The downlink packet flow is as follows:</t>

<artwork align="center" name="" type="" alt=""><![CDATA[
UPF1_in : (Z,A)                             ->UPF1 maps the flow w/
                                              SID list <C1,S1, gNB>
UPF1_out: (U1::1, C1)(gNB::1, S1; SL=2)(Z,A)->H.Encaps.Red
C1_out  : (U1::1, S1)(gNB::1, S1; SL=1)(Z,A)
S1_out  : (U1::1, gNB::1)(Z,A)              ->End with PSP
gNB_out : (Z,A)                             ->End.DX4/End.DX6/End.DX2
]]></artwork>

          <t>When the packet arrives at the UPF1, the UPF1 maps that
          particular flow into a UE PDU Session. This UE PDU Session is
          associated with the policy &lt;C1, S1, gNB&gt;. The UPF1 performs a
          H.Encaps.Red operation, encapsulating the packet into a new IPv6
          header with its corresponding SRH.</t>
          <t>The nodes C1 and S1 perform their related Endpoint processing.</t>
          <t>Once the packet arrives at the gNB, the IPv6 DA corresponds to an
          End.DX4, End.DX6 End.DX6, or End.DX2 behavior at the gNB (depending on the
          underlying traffic).  The gNB decapsulates the packet, removing the
          IPv6 header, and forwards the traffic towards the UE. The SID gNB::1
          is one example of a SID associated to this service.</t>
          <t>Note that there are several means to provide the UE session
          aggregation. The decision on about which one to use is a local decision
          made by the operator. One option is to use the <xref
          target="arguments-for-mobility" format="default">Args.Mob.Session
          </xref>. Another option comprises the gNB performing an IP lookup on
          the inner packet by using the End.DT4, End.DT6, and End.DT2U
          behaviors.</t>
        </section>
        <!-- End section "Packet flow - Downlink" -->

        <section numbered="true" toc="default">
          <name>Scalability</name>
          <t>The Enhanced Mode mode improves scalability since it allows the
          aggregation of several UEs under the same SID list. For example, in
          the case of stationary residential meters that are connected to the
          same cell, all such devices can share the same SID list.  This
          improves scalability compared to Traditional Mode mode (unique SID per
          UE) and compared to GTP-U (TEID per UE).</t>
        </section>
      </section>
      <!-- End section "Enhanced Mode" -->

      <section anchor="enhanced_gtp" numbered="true" toc="default">
        <name>Enhanced mode Mode with unchanged Unchanged gNB GTP-U behavior</name> Behavior</name>
        <t> This section describes two mechanisms for interworking with legacy
        gNBs that still use GTP-U: one for IPv4, IPv4 and another for IPv6.</t>
        <t> In the interworking scenarios as illustrated in <xref
        target="fig_interworking" format="default"/>, the gNB does not support
        SRv6.  The gNB supports GTP-U encapsulation over IPv4 or IPv6.  To
        achieve interworking, an SR Gateway (SRGW) entity is added. The SRGW
        is a new entity that maps the GTP-U traffic into SRv6. It is deployed
        at the boundary of the SR Domain domain and performs the mapping
        functionality for inbound/outbound inbound and outbound traffic.</t>
        <t> The SRGW is not an anchor point and maintains very little state.
	    For this reason,
	    both IPv4 and IPv6 methods scale to millions of UEs.</t>

<figure anchor="fig_interworking">
<name>Example topology Topology for interworking</name> Interworking</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
                                                          _______
                 IP GTP-U        SRv6                    /       \
+--+      +-----+ [N3] +------+  [N9]  +------+  [N6]   /         \
|UE|------| gNB |------| SRGW |--------| UPF  |---------\   DN    /
+--+      +-----+      +------+        +------+          \_______/
                      SR Gateway       SRv6 node
]]></artwork>
</figure>

        <t>Both of the mechanisms described in this section are applicable to either the Traditional Mode or mode and the Enhanced Mode.</t> mode.</t>
        <section numbered="true" toc="default">
          <name>Interworking with IPv6 GTP-U</name>
          <t>In this interworking mode mode, the gNB at the N3 interface uses GTP-U
	      over IPv6.</t>
          <t>Key points:
          </t>
          <ul spacing="compact"> spacing="normal">
            <li> The gNB is unchanged (control-plane (control plane or user-plane) user plane) and
            encapsulates into GTP-U (N3 interface is not modified).</li> <li>
            The 5G Control-Plane control plane towards the gNB (N2 interface) is unmodified,
            though multiple UPF addresses need to be used - one used. One IPv6 address (i.e.
            (i.e., a BSID at the SRGW) is needed per &lt;SLA, PDU session type&gt;. Session
            Type&gt;. The SRv6 SID is different depending on the required
            &lt;SLA, PDU session type&gt; Session Type&gt; combination.</li>
            <li> In the uplink, the SRGW removes the GTP-U header, finds the
            SID list related to the IPv6 DA, and adds SRH with the SID
            list.</li>
            <li> There is no state for the downlink at the SRGW.</li>
            <li> There is simple state in the uplink at the SRGW; using
            Enhanced mode results in fewer SR policies Policies on this node.  An SR policy
            Policy is shared across UEs as long as they belong to the same
            context (i.e., tenant). A set of many different policies (i.e.,
            different SLAs) increases the amount of state required.</li>
            <li> When a packet from the UE leaves the gNB, it is SR-routed.

		This simplifies network slicing
		<xref target="I-D.ietf-lsr-flex-algo" target="RFC9350" format="default"/>.</li>
            <li> In the uplink, the SRv6 BSID steers traffic
		into an SR policy Policy when it arrives at the SRGW.</li>
          </ul>
          <t> An example topology is shown in
	      <xref target="fig_interworking_ipv6" format="default"/>.</t>
          <t> S1 and C1 are two service segments.
	      S1 represents a VNF in the network, and C1 represents a router
	      configured for Traffic Engineering.</t> traffic engineering.</t>

<figure anchor="fig_interworking_ipv6">
<name>Enhanced mode Mode with unchanged Unchanged gNB IPv6/GTP-U behavior</name> Behavior</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
                               +----+
             IPv6/GTP-U       -| S1 |-                            ___
+--+  +-----+ [N3]           / +----+ \                          /
|UE|--| gNB |-         SRv6 /   SRv6   \ +----+   +------+ [N6] /
+--+  +-----+ \        [N9]/     VNF    -| C1 |---| UPF2 |------\  DN
        GTP-U  \ +------+ /              +----+   +------+       \___
                -| SRGW |-                SRv6      SRv6
                 +------+                  TE
                SR Gateway
]]></artwork>
</figure>

          <section numbered="true" toc="default">
            <name>Packet flow Flow - Uplink</name>
            <t>The uplink packet flow is as follows:</t>

<artwork align="center" name="" type="" alt=""><![CDATA[
UE_out  : (A,Z)
gNB_out : (gNB, B)(GTP: TEID T)(A,Z)       -> Interface N3 unmodified
                                              (IPv6/GTP)
SRGW_out: (SRGW, S1)(U2::T, C1; SL=2)(A,Z) -> B is an End.M.GTP6.D
                                              SID at the SRGW
S1_out  : (SRGW, C1)(U2::T, C1; SL=1)(A,Z)
C1_out  : (SRGW, U2::T)(A,Z)               -> End with PSP
UPF2_out: (A,Z)                            -> End.DT4 or End.DT6
]]></artwork>

            <t> The UE sends a packet destined to Z toward the gNB on a
            specific bearer for that session. The gNB, which is unmodified,
            encapsulates the packet into IPv6, UDP, and GTP-U headers.  The
            IPv6 DA B, B and the GTP-U TEID T are the ones received in the N2
            interface.</t>
            <t> The IPv6 address that was signaled over the N2 interface for
            that UE PDU Session, B, is now the IPv6 DA. B is an SRv6
            Binding SID at the SRGW.  Hence  Hence, the packet is routed to the
            SRGW.</t>
            <t> When the packet arrives at the SRGW, the SRGW identifies B as
            an End.M.GTP6.D Binding SID (see <xref target="End-M-GTP6-D"
            format="default"/>).  Hence, the SRGW removes the IPv6, UDP, and
            GTP-U headers, headers and pushes an IPv6 header with its own SRH
            containing the SIDs bound to the SR policy Policy associated with this BindingSID.
            Binding SID.  There is at least one instance of the End.M.GTP6.D SID
            per PDU type.</t>
            <t> S1 and C1 perform their related Endpoint functionality and
            forward the packet.</t>
            <t> When the packet arrives at UPF2, the active segment is (U2::T) (U2::T),
            which is bound to End.DT4/6.  UPF2 then decapsulates (removing the
            outer IPv6 header with all its extension headers) and forwards the
            packet toward the data network.</t> DN.</t>
          </section>
          <!-- End section "Packet flow - Uplink" -->

          <section numbered="true" toc="default">
            <name>Packet flow Flow - Downlink</name>
            <t>The downlink packet flow is as follows:</t>

<artwork align="center" name="" type="" alt=""><![CDATA[
UPF2_in : (Z,A)                           -> UPF2 maps the flow with
                                             <C1, S1, SRGW::TEID,gNB>
UPF2_out: (U2::1, C1)(gNB, SRGW::TEID, S1; SL=3)(Z,A) -> H.Encaps.Red
C1_out  : (U2::1, S1)(gNB, SRGW::TEID, S1; SL=2)(Z,A)
S1_out  : (U2::1, SRGW::TEID)(gNB, SRGW::TEID, S1, SL=1)(Z,A)
SRGW_out: (SRGW, gNB)(GTP: TEID=T)(Z,A)   -> SRGW/96 is End.M.GTP6.E
gNB_out : (Z,A)
]]></artwork>

            <t> When a packet destined to A arrives at the UPF2, the UPF2
		performs a lookup in the table associated to A and finds the
		SID list &lt;C1, S1, SRGW::TEID, gNB&gt;. The UPF2 performs
		an H.Encaps.Red operation, encapsulating the packet into
		a new IPv6 header with its corresponding SRH.</t>
            <t> C1 and S1 perform their related Endpoint processing.</t>
            <t> Once the packet arrives at the SRGW, the SRGW identifies the
		active SID as an End.M.GTP6.E function. The SRGW removes
		the IPv6 header and all its extensions headers. The SRGW
		generates new IPv6, UDP, and GTP-U headers. The new IPv6 DA
		is the gNB gNB, which is the last SID in the received SRH.
		The TEID in the generated GTP-U header is also an argument of the
		received End.M.GTP6.E SID. The SRGW pushes the headers to
		the packet and forwards the packet toward the gNB.  There
		is one instance of the End.M.GTP6.E SID per PDU type.</t>
            <t> Once the packet arrives at the gNB, the packet is a regular
		IPv6/GTP-U packet. The gNB looks for the specific radio bearer
		for that TEID and forwards it on the bearer. This gNB behavior
		is not modified from current and previous generations.</t>
          </section>
          <!-- End section "Packet flow - Downlink" -->

          <section numbered="true" toc="default">
            <name>Scalability</name>

<t> For the downlink traffic, the SRGW is stateless. All the state
            is in the SRH pushed by the UPF2.  The UPF2 must have the UE
		states
            state since it is the UE's session anchor point.</t>
            <t> For the uplink traffic, the state at the SRGW does not
            necessarily need to be unique per PDU Session; the SR policy Policy can
            be shared among UEs.  This enables more scalable SRGW deployments
            compared to a solution holding millions of states, one or more per
            UE.</t>
          </section>
          <!-- End section "Scalability" -->

        </section>
        <!-- End section "Interworking with IPv6 GTP" -->

        <section numbered="true" toc="default">
          <name>Interworking with IPv4 GTP-U</name>
          <t> In this interworking mode mode, the gNB uses GTP over IPv4 in the N3 interface</t>
          interface.</t>
          <t> Key points:
          </t>
          <ul spacing="compact"> spacing="normal">
            <li> The gNB is unchanged and encapsulates packets into GTP-U (the
            N3 interface is not modified).</li>
            <li>N2 signaling is not changed, though multiple UPF addresses
            need to be provided - -- one for each PDU Session Type.</li>
            <li> In the uplink, traffic is classified by SRGW's classification
            engine and steered into an SR policy. Policy.  The SRGW may be implemented
            in a UPF or as a separate entity. How the classification engine
            rules are set up is outside the scope of this document, though one
            example is using BGP signaling from a Mobile User Plane (MUP) Controller
            <xref target="I-D.mhkk-dmm-srv6mup-architecture"
            format="default"/>.</li>
            <li> SRGW removes the GTP-U header, finds the SID list related to DA,
            and adds an SRH with the SID list.</li>
          </ul>
          <t> An example topology is shown in <xref
          target="fig_interworking_ipv4" format="default"/>. In this mode mode, the
          gNB is an unmodified gNB using IPv4/GTP.  The UPFs are SR-aware.  As
          before, the SRGW maps the IPv4/GTP-U traffic to SRv6.</t>
          <t> S1 and C1 are two service segment endpoints.  S1 represents a
          VNF in the network, and C1 represents a router configured for Traffic Engineering.</t>
          traffic engineering.</t>

<figure anchor="fig_interworking_ipv4">
<name>Enhanced mode Mode with unchanged Unchanged gNB IPv4/GTP-U behavior</name> Behavior</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
                               +----+
             IPv4/GTP-U       -| S1 |-                            ___
+--+  +-----+ [N3]           / +----+ \                          /
|UE|--| gNB |-         SRv6 /   SRv6   \ +----+   +------+ [N6] /
+--+  +-----+ \        [N9]/     VNF    -| C1 |---| UPF2 |------\  DN
        GTP-U  \ +------+ /              +----+   +------+       \___
                -| UPF1 |-                SRv6      SRv6
                 +------+                  TE
                SR Gateway
]]></artwork>
</figure>

          <section numbered="true" toc="default">
            <name>Packet flow Flow - Uplink</name>
            <t>The uplink packet flow is as follows:</t>

<artwork align="center" name="" type="" alt=""><![CDATA[
gNB_out : (gNB, B)(GTP: TEID T)(A,Z)          -> Interface N3
                                                 unchanged IPv4/GTP
SRGW_out: (SRGW, S1)(U2::1, C1; SL=2)(A,Z)    -> H.M.GTP4.D function
S1_out  : (SRGW, C1)(U2::1, C1; SL=1)(A,Z)
C1_out  : (SRGW, U2::1) (A,Z)                 -> PSP
UPF2_out: (A,Z)                               -> End.DT4 or End.DT6
]]></artwork>

            <t> The UE sends a packet destined to Z toward the gNB on a
            specific bearer for that session. The gNB, which is unmodified,
            encapsulates the packet into a new IPv4, UDP, and GTP-Uheaders. GTP-U headers.
            The IPv4 DA, B, and the GTP-UTEID are the ones received at the N2
            interface.</t>
            <t> When the packet arrives at the SRGW for UPF1, the SRGW has an a
            classification engine rule for incoming traffic from the
		gNB, gNB that
            steers the traffic into an SR policy Policy by using the function
            H.M.GTP4.D. The SRGW removes the IPv4, UDP, and GTP headers and
            pushes an IPv6 header with its own SRH containing the SIDs related
            to the SR policy Policy associated with this traffic.  The SRGW forwards
            according to the new IPv6 DA.</t>
            <t> S1 and C1 perform their related Endpoint functionality and
            forward the packet.</t>
            <t> When the packet arrives at UPF2, the active segment is (U2::1) (U2::1),
            which is bound to End.DT4/6 End.DT4/6, which performs the decapsulation
            (removing the outer IPv6 header with all its extension headers)
            and forwards toward the data network.</t> DN.</t>
    <t>Note that the interworking mechanisms for IPv4/GTP-U and IPv6/GTP-U differs.
    differ. This is due to the fact that IPv6/GTP-U can leverage the remote
    steering capabilities provided by the Segment Routing BSID. In IPv4 IPv4, this
    construct is not available, and building a similar mechanism would require
    a significant address consumption.</t>
          </section>
          <!-- End section "Packet flow - Uplink" -->

          <section numbered="true" toc="default">
            <name>Packet flow Flow - Downlink</name>
            <t>The downlink packet flow is as follows:</t>

<artwork align="center" name="" type="" alt=""><![CDATA[
UPF2_in : (Z,A)                            -> UPF2 maps flow with SID
                                            <C1, S1,GW::SA:DA:TEID>
UPF2_out: (U2::1, C1)(GW::SA:DA:TEID, S1; SL=2)(Z,A) ->H.Encaps.Red
C1_out  : (U2::1, S1)(GW::SA:DA:TEID, S1; SL=1)(Z,A)
S1_out  : (U2::1, GW::SA:DA:TEID)(Z,A)
SRGW_out: (GW, gNB)(GTP: TEID=T)(Z,A)       -> End.M.GTP4.E
gNB_out : (Z,A)
]]></artwork>

            <t>When a packet destined to A arrives at the UPF2, the UPF2
            performs a lookup in the table associated to A and finds the SID
            list &lt;C1, S1, SRGW::SA:DA:TEID&gt;. The UPF2 performs
		a an
            H.Encaps.Red operation, encapsulating the packet into a new IPv6
            header with its corresponding SRH.</t>
            <t>The nodes C1 and S1 perform their related Endpoint
            processing.</t>

            <t>Once the packet arrives at the SRGW, the SRGW identifies the
            active SID as an End.M.GTP4.E function. The SRGW removes the IPv6
            header and all its extensions headers. The SRGW generates an IPv4,
            UDP, and GTP-U headers. The IPv4 SA and DA are received as SID
            arguments.  The TEID in the generated GTP-U header is also the arguments
            argument of the received End.M.GTP4.E SID.  The SRGW pushes the
            headers to the packet and forwards the packet toward the gNB.</t>
            <t> When the packet arrives at the gNB, the packet is a regular
            IPv4/GTP-U packet. The gNB looks for the specific radio bearer for
            that TEID and forwards it on the bearer. This gNB behavior is not
            modified from current and previous generations.</t>
          </section>
          <!-- End section "Packet flow - Downlink" -->

          <section numbered="true" toc="default">
            <name>Scalability</name>
            <t>For the downlink traffic, the SRGW is stateless. All the state
            is in the SRH pushed by the UPF2. The UPF must have this UE-base
            state anyway (since it is its anchor point).</t>
            <t>For the uplink traffic, the state at the SRGW is dedicated on a
		per UE/session
            per-UE/session basis according to a classification engine.  There
            is state for steering the different sessions in the form of an SR
            Policy. However, SR policies Policies are shared among several
            UE/sessions.</t>
          </section>
          <!-- End section "Scalability" -->
        </section>
        <!-- End section "Interworking with IPv4 GTP" -->

        <section numbered="true" toc="default">
          <name>Extensions to the interworking mechanisms</name> Interworking Mechanisms</name>
          <t>This section presents two mechanisms for interworking with gNBs
          and UPFs that do not support SRv6. These mechanisms are used to
          support GTP-U over IPv4 and IPv6.</t>
          <t>Even

          <t> Even though these methods are presented as an extension to the "Enhanced mode", it is straightforward in its
	      applicability
          Enhanced mode, they are also applicable to the "Traditional mode".</t> Traditional mode.
	  </t>
        </section>
        <!-- End section "Extensions .. interworking mechanisms" -->
      </section>
      <!-- End "Enhanced mode with unchanged gNB GTP-U ..." -->

      <section anchor="drop_in" numbered="true" toc="default">
        <name>SRv6 Drop-in Drop-In Interworking</name>
        <t>This section introduces another mode useful for legacy gNB and UPFs
        that still operate with GTP-U.  This mode provides an SRv6-enabled
        user plane in between two GTP-U tunnel endpoints.</t>
        <t>This mode employs two SRGWs that map GTP-U traffic to SRv6 and vice-versa.</t>
        vice versa.</t>
        <t>Unlike other interworking modes, in this mode mode, both of the mobility
        overlay endpoints use GTP-U.  Two SRGWs are deployed in either an N3 or
        N9 interface to realize an intermediate SR policy.</t> Policy.</t>

<figure anchor="fig_drop_in">
<name>Example topology Topology for SRv6 Drop-in mode</name> Drop-In Mode</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
                            +----+
                           -| S1 |-
+-----+                   / +----+ \
| gNB |-            SRv6 /   SRv6   \ +----+   +--------+    +-----+
+-----+  \              /     VNF    -| C1 |---| SRGW-B |----| UPF |
   GTP[N3]\ +--------+ /              +----+   +--------+    +-----+
           -| SRGW-A |-                SRv6   SR Gateway-B     GTP
            +--------+                  TE
           SR Gateway-A
]]></artwork>
</figure>

        <t>The packet flow of <xref target="fig_drop_in" format="default"/> is
        as follows:</t>

<artwork align="center" name="" type="" alt=""><![CDATA[
gNB_out : (gNB, U::1)(GTP: TEID T)(A,Z)
GW-A_out: (GW-A, S1)(U::1, SGB::TEID, C1; SL=3)(A,Z)->U::1 is an
                                                      End.M.GTP6.D.Di
                                                      SID at SRGW-A
S1_out  : (GW-A, C1)(U::1, SGB::TEID, C1; SL=2)(A,Z)
C1_out  : (GW-A, SGB::TEID)(U::1, SGB::TEID, C1; SL=1)(A,Z)
GW-B_out: (GW-B, U::1)(GTP: TEID T)(A,Z)            ->SGB::TEID is an
                                                      End.M.GTP6.E
                                                      SID at SRGW-B
UPF_out : (A,Z)
]]></artwork>

        <t>When a packet destined to Z is sent to the gNB, which is unmodified (control-plane
        (control plane and user-plane user plane remain GTP-U), gNB performs
        encapsulation into a new IP, UDP, and GTP-U headers.  The IPv6 DA, U::1,
        and the GTP-U TEID are the ones received at the N2 interface.</t>
        <t>The IPv6 address that was signaled over the N2 interface for that
        PDU Session, U::1, is now the IPv6 DA.  U::1 is an SRv6 Binding
        SID at SRGW-A.  Hence  Hence, the packet is routed to the SRGW.</t>
        <t>When the packet arrives at SRGW-A, the SRGW identifies U::1 as an
        End.M.GTP6.D.Di Binding SID (see <xref target="End-M-GTP6-D-Di"
        format="default"/>).  Hence, the SRGW removes the IPv6, UDP, and GTP-U headers,
        headers and pushes an IPv6 header with its own SRH containing the
        SIDs bound to the SR
          policy Policy associated with this Binding SID. There is
        one instance of the End.M.GTP6.D.Di SID per PDU type.</t>
        <t>S1 and C1 perform their related Endpoint functionality and forward
          the packet.</t>
        <t>Once the packet arrives at SRGW-B, the SRGW identifies the active
        SID as an End.M.GTP6.E function. The SRGW removes the IPv6 header and
        all its extensions headers. The SRGW generates new IPv6, UDP, and GTP
        headers. The new IPv6 DA is U::1 U::1, which is the last SID in the
        received SRH. The TEID in the generated GTP-U header is an argument of
        the received End.M.GTP6.E SID. The SRGW pushes the headers to the
        packet and forwards the packet toward UPF.  There is one instance of
        the End.M.GTP6.E SID per PDU type.</t>
        <t>Once the packet arrives at UPF, the packet is a regular IPv6/GTP
        packet. The UPF looks for the specific rule for that TEID to forward
        the packet. This UPF behavior is not modified from current and
        previous generations.</t>
      </section>
      <!-- End section "SRv6 Drop-in Interworking" -->
    </section>
    <!-- End section "User-plane behaviors" -->

    <section anchor="srv6_functions" numbered="true" toc="default">
      <name>SRv6 Segment Endpoint Mobility Behaviors</name>
      <!-- Add text on functions used on UPF1, UPF2,... -->

      <t>This section introduces new SRv6 Segment Endpoint Behaviors for the
      mobile user-plane. user plane. The behaviors described in this document are
      compatible with the NEXT and REPLACE flavors defined in <xref
      target="I-D.ietf-spring-srv6-srh-compression" format="default" />.</t>
      <section anchor="arguments-for-mobility" numbered="true" toc="default">
        <name>Args.Mob.Session</name>
        <t>Args.Mob.Session provide provides per-session information for charging, buffering
        buffering, or other purposes required by some mobile nodes.  The
        Args.Mob.Session argument format is used in combination with the End.Map,
          End.DT4/End.DT6/End.DT46
        End.DT4/End.DT6/End.DT46, and End.DX4/End.DX6/End.DX2 behaviors. Note
        that proposed format is applicable for 5G networks, while similar
        formats could be used for legacy networks.
        </t>

<figure>
<name>Args.Mob.Session format</name> Format</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   QFI     |R|U|                PDU Session ID                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PDU Sess(cont')|
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
        <ul spacing="compact">
          <li> QFI: QoS

        <dl spacing="normal" newline="false">
          <dt>QFI:</dt>
	  <dd>QoS Flow Identifier <xref target="TS.38415" format="default"/></li>
          <li> R: Reflective
	  format="default"/>.</dd>
          <dt>R:</dt>

	  <dd>Reflective QoS Indication <xref target="TS.23501"
	  format="default"/>. This parameter indicates the activation of
	  reflective QoS towards the UE for the transferred packet. Reflective
	  QoS enables the UE to map UL User Plane uplink user-plane traffic to QoS Flows flows without SMF provided
	  SMF-provided QoS rules.</li>
          <li>U: Unused rules.</dd>
          <dt>U:</dt>
	  <dd>Unused and for future use. MUST <bcp14>MUST</bcp14> be 0 on
	  transmission and ignored on receipt.</li>
          <li>PDU receipt.</dd>
          <dt>PDU Session ID: Identifier ID:</dt>
	  <dd>Identifier of PDU Session. The GTP-U equivalent is TEID.</li>
        </ul> TEID.</dd>
        </dl>
        <t>Args.Mob.Session is required in case that one SID aggregates
        multiple PDU Sessions. Since the SRv6 SID is likely NOT to be
        instantiated per PDU session, Session, Args.Mob.Session helps the UPF to
        perform the behaviors which that require granularity per QFI and/or per PDU Session
      granularity.</t> Session.</t>
        <t>Note that the encoding of user-plane messages (e.g., Echo Request,
        Echo Reply, Error Indication Indication, and End Marker) is out of the scope of
        this draft. document. <xref
        target="I-D.murakami-dmm-user-plane-message-encoding" /> defines one
        possible encoding.</t> encoding method.</t>
      </section>
      <section anchor="end-map-function" numbered="true" toc="default">
        <name>End.MAP</name>
        <t>The "Endpoint behavior
        <t>End.MAP (Endpoint Behavior with SID mapping" behavior (End.MAP for
	    short) mapping)
	   is used in several scenarios. Particularly in mobility,
      End.MAP is used by the intermediate UPFs.</t>
        <t>When node N receives a packet whose IPv6 DA is D and D is a local End.MAP SID, N does:</t>
        <artwork name="" type="" align="left" alt=""><![CDATA[ does the following:</t>

<sourcecode type="pseudocode"><![CDATA[
S01. If (IPv6 Hop Limit <= 1) {
S02.    Send an ICMP Time Exceeded message to the Source Address, Address with
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing, and discard the packet.
S03. }
S04. Decrement IPv6 Hop Limit by 1
S05. Update the IPv6 DA with the new mapped SID
S06. Submit the packet to the egress IPv6 FIB lookup for
        transmission to the new destination
                    ]]></artwork>
        <t>Notes:
]]></sourcecode>

        <t>Note: The SRH is not modified (neither the SID, SID nor the SL
        value).</t>
      </section>
      <section anchor="End-M-GTP6-D" numbered="true" toc="default">
        <name>End.M.GTP6.D</name>
        <t>The "Endpoint behavior
        <t>End.M.GTP6.D (Endpoint Behavior with IPv6/GTP-U decapsulation into SR policy"
	    behavior (End.M.GTP6.D for short)
        Policy) is used in the interworking
        scenario for the uplink towards SRGW from the legacy gNB using
        IPv6/GTP.  Any SID instance of this behavior is associated with an SR
        Policy B and an IPv6 Source Address S.
        </t>
        <t>When the SR Gateway node N receives a packet destined to D D, and D
        is a local End.M.GTP6.D SID, N does:</t>
        <artwork name="" type="" align="left" alt=""><![CDATA[ does the following:</t>

<sourcecode type="pseudocode"><![CDATA[
S01. When an SRH is processed {
S02.   If (Segments Left != 0) {
S03.      Send an ICMP Parameter Problem to the Source Address, Address with
             Code 0 (Erroneous header field encountered), encountered) and
             Pointer set to the Segments Left field,
             interrupt packet processing, and discard the packet.
S04.   }
S05.   Proceed to process the next header in the packet
S06. }
                ]]></artwork>
]]></sourcecode>

        <t>When processing the Upper-layer Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.M.GTP6.D SID, N does:</t>
        <artwork name="" type="" align="left" alt=""><![CDATA[ does the following:</t>

<sourcecode type="pseudocode"><![CDATA[
S01. If (Next Header (NH) == UDP & UDP_Dest_port == GTP) {
S02.    Copy the GTP-U TEID and QFI to buffer memory
S03.    Pop the IPv6, UDP, and GTP-U Headers headers
S04.    Push a new IPv6 header with its own SRH containing B
S05.    Set the outer IPv6 SA to S
S06.    Set the outer IPv6 DA to the first SID of B
S07.    Set the outer Payload Length, Traffic Class, Flow Label,
           Hop Limit, and Next-Header Next Header (NH) fields
S08.    Write in the SRH[0] the Args.Mob.Session based on
           the information of in buffer memory
S09.    Submit the packet to the egress IPv6 FIB lookup and for
           transmission to the new destination
S10. } Else {
S11.    Process as per [RFC8986] [RFC8986], Section 4.1.1
S12. }
                ]]></artwork>
        <t>Notes:
        S07. The
]]></sourcecode>

       <t>Notes:</t>
       <ul spacing="normal">
        <li>In line S07, the NH is set based on the SID parameter. There is one
          instantiation of the End.M.GTP6.D SID per PDU Session Type,
          hence Type;
          hence, the NH is already known in advance. For In addition, for the IPv4v6 PDU
          Session Type, in addition the router inspects the first nibble of the
          PDU to know the NH value.</t>
        <t>The value.</li>
        <li>The last segment SHOULD <bcp14>SHOULD</bcp14> be
        followed by an Args.Mob.Session argument space space, which is used to
        provide the session identifiers, as shown in line S08.</t> S08.</li>
      </ul>
      </section>
      <!-- End section "End.M.GTP6.D" -->

     <section anchor="End-M-GTP6-D-Di" numbered="true" toc="default">
        <name>End.M.GTP6.D.Di</name>
        <t>The "Endpoint behavior
        <t>End.M.GTP6.D.Di (Endpoint Behavior with IPv6/GTP-U decapsulation into SR policy Policy
        for Drop-in Mode" behavior (End.M.GTP6.D.Di for short) Mode) is used in the SRv6
        drop-in interworking scenario described in <xref target="drop_in"
        format="default"/>. The difference between End.M.GTP6.D as another
        variant of the IPv6/GTP decapsulation function is that the original IPv6
        DA of the GTP-U packet is preserved as the last SID in SRH.</t>
        <t>Any SID instance of this behavior is associated with an SR Policy B
        and an IPv6 Source Address S.</t>
        <t>When the SR Gateway node N receives a packet destined to D D, and
        D is a local End.M.GTP6.D.Di SID, N does:</t>
        <artwork name="" type="" align="left" alt=""><![CDATA[ does the following:</t>

<sourcecode type="pseudocode"><![CDATA[
S01. When an SRH is processed {
S02.   If (Segments Left != 0) {
S03.      Send an ICMP Parameter Problem to the Source Address, Address with
             Code 0 (Erroneous header field encountered), encountered) and
             Pointer set to the Segments Left field,
             interrupt packet processing, and discard the packet.
S04.   }
S05.   Proceed to process the next header in the packet
S06. }
                ]]></artwork>
]]></sourcecode>

        <t>When processing the Upper-layer Upper-Layer header of a packet matching a FIB
        entry locally instantiated as an End.M.GTP6.Di SID, N does:</t>
        <artwork name="" type="" align="left" alt=""><![CDATA[ does the
        following:</t>

<sourcecode type="pseudocode"><![CDATA[
S01. If (Next Header = UDP & UDP_Dest_port = GTP) {
S02.    Copy D to buffer memory
S03.    Pop the IPv6, UDP, and GTP-U Headers headers
S04.    Push a new IPv6 header with its own SRH containing B
S05.    Set the outer IPv6 SA to S
S06.    Set the outer IPv6 DA to the first SID of B
S07.    Set the outer Payload Length, Traffic Class, Flow Label,
           Hop Limit, and Next-Header Next Header fields
S08.    Prepend D to the SRH (as SRH[0]) and set SL accordingly
S09.    Submit the packet to the egress IPv6 FIB lookup and for
           transmission to the new destination
S10. } Else {
S11.    Process as per [RFC8986] [RFC8986], Section 4.1.1
S12. }
                ]]></artwork>
        <t>Notes:
        S07. The
]]></sourcecode>

<t>Notes:</t>
<ul spacing="normal">
<li>In line S07, the NH is set based on the SID parameter. There is one
        instantiation of the End.M.GTP6.Di SID per PDU Session Type,
          hence Type; hence,
        the NH is already known in advance. For In addition, for the IPv4v6 PDU Session Type, in addition
        the router inspects the first nibble of the PDU to know
        the NH value.</t>
        <t>S SHOULD value.</li>
        <li>S <bcp14>SHOULD</bcp14> be an End.M.GTP6.E SID instantiated
	at the SR gateway.</t> Gateway.</li>
</ul>
      </section>
      <!-- End section "End.M.GTP6.D.Di" -->

      <section numbered="true" toc="default">
        <name>End.M.GTP6.E</name>
        <t>The "Endpoint behavior
        <t>End.M.GTP6.E (Endpoint Behavior with encapsulation for IPv6/GTP-U tunnel"
        behavior (End.M.GTP6.E for short)
        behavior) is used among others in the
        interworking scenario for the downlink toward the legacy gNB using
        IPv6/GTP.</t>
        <t>The prefix of End.M.GTP6.E SID MUST <bcp14>MUST</bcp14> be followed by
        the Args.Mob.Session argument space space, which is used to provide the
        session identifiers.</t>
        <t>When the SR Gateway node N receives a packet destined to D, and
        D is a local End.M.GTP6.E SID, N does the following:</t>
        <artwork name="" type="" align="left" alt=""><![CDATA[

<sourcecode type="pseudocode"><![CDATA[
S01. When an SRH is processed {
S02.   If (Segments Left != 1) {
S03.      Send an ICMP Parameter Problem to the Source Address, Address with
             Code 0 (Erroneous header field encountered), encountered) and
             Pointer set to the Segments Left field,
             interrupt packet processing, and discard the packet.
S04.   }
S05.   Proceed to process the next header in the packet
S06. }
                ]]></artwork>
]]></sourcecode>

        <t>When processing the Upper-layer Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.M.GTP6.E SID, N does:</t>
        <artwork name="" type="" align="left" alt=""><![CDATA[ does the following:</t>

<sourcecode type="pseudocode"><![CDATA[
S01.    Copy SRH[0] and D to buffer memory
S02.    Pop the IPv6 header and all its extension headers
S03.    Push a new IPv6 header with a UDP/GTP-U Header header
S04.    Set the outer IPv6 SA to S
S05.    Set the outer IPv6 DA from buffer memory
S06.    Set the outer Payload Length, Traffic Class, Flow Label,
           Hop Limit, and Next-Header Next Header fields
S07.    Set the GTP-U TEID (from buffer memory)
S08.    Submit the packet to the egress IPv6 FIB lookup and for
           transmission to the new destination
                ]]></artwork>
        <t>Notes:
        An
]]></sourcecode>

<t>Notes:</t>
<ul spacing="normal">
        <li>An End.M.GTP6.E SID MUST <bcp14>MUST</bcp14> always be the penultimate SID.
        The TEID is extracted from the argument space of the current
        SID.</t>
        <t>
        SID.</li>
        <li> The source address S SHOULD <bcp14>SHOULD</bcp14> be an End.M.GTP6.D SID instantiated at the egress SR gateway.</t> Gateway.</li>
</ul>
      </section>
      <!-- End section "End.M.GTP6.E" -->

      <section numbered="true" toc="default">
        <name>End.M.GTP4.E</name>
        <t>The "Endpoint behavior
        <t>End.M.GTP4.E (Endpoint Behavior with encapsulation for IPv4/GTP-U tunnel"
	    behavior (End.M.GTP4.E for short)
        tunnel) is used in the downlink when doing interworking with legacy
        gNB using IPv4/GTP.</t>
        <t>When the SR Gateway node N receives a packet destined to S S, and S
	    is a local End.M.GTP4.E SID, N does:</t>
        <artwork name="" type="" align="left" alt=""><![CDATA[ does the following:</t>

<sourcecode type="pseudocode"><![CDATA[
S01. When an SRH is processed {
S02.   If (Segments Left != 0) {
S03.      Send an ICMP Parameter Problem to the Source Address, Address with
             Code 0 (Erroneous header field encountered), encountered) and
             Pointer set to the Segments Left field,
             interrupt packet processing, and discard the packet.
S04.   }
S05.   Proceed to process the next header in the packet
S06. }
                ]]></artwork>
]]></sourcecode>

        <t>When processing the Upper-layer Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.M.GTP4.E SID, N does:</t>
        <artwork name="" type="" align="left" alt=""><![CDATA[ does the following:</t>

<sourcecode type="pseudocode"><![CDATA[
S01.    Store the IPv6 DA and SA in buffer memory
S02.    Pop the IPv6 header and all its extension headers
S03.    Push a new IPv4 header with a UDP/GTP-U Header header
S04.    Set the outer IPv4 SA and DA (from buffer memory)
S05.    Set the outer Total Length, DSCP, Time To Live, and
           Next-Header
           Next Header fields
S06.    Set the GTP-U TEID (from buffer memory)
S07.    Submit the packet to the egress IPv4 FIB lookup and for
           transmission to the new destination
                ]]></artwork>
        <t>Notes:

        The
]]></sourcecode>

<t>Notes:</t>
<ul spacing="normal">
  <li><t>The End.M.GTP4.E SID in S has the following format:</t>

<figure>
<name>End.M.GTP4.E SID Encoding</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
0                                                         127
+-----------------------+-------+----------------+---------+
|  SRGW-IPv6-LOC-FUNC   |IPv4DA |Args.Mob.Session|0 Padded |
+-----------------------+-------+----------------+---------+
       128-a-b-c            a            b           c
]]></artwork>
</figure>
        <t>The
  </li>

       <li><t>The IPv6 Source Address has the following format:</t>

<figure>
<name>IPv6 SA Encoding for End.M.GTP4.E</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
0                                                         127
+----------------------+--------+--------------------------+
|  Source UPF Prefix   |IPv4 SA | any bit pattern(ignored) |
+----------------------+--------+--------------------------+
         128-a-b            a                  b
]]></artwork>
</figure>
       </li>
</ul>
      </section>
      <!-- End section "End.M.GTP4.E" -->

      <section numbered="true" toc="default">
        <name>H.M.GTP4.D</name>
        <t>The "SR
        <t>H.M.GTP4.D (SR Policy Headend with tunnel decapsulation and map to an SRv6
        policy" behavior (H.M.GTP4.D for short)
        policy) is used in the direction
        from the legacy IPv4 user-plane user plane to the SRv6 user-plane network.</t>
        <t>When the SR Gateway node N receives a packet destined to a
        SRGW-IPv4-Prefix, N does:</t> does the following:</t>

<artwork align="left" name="" type="" alt=""><![CDATA[
S01. IF Payload == UDP/GTP-U THEN
S02.    Pop the outer IPv4 header and UDP/GTP-U headers
S03.    Copy IPv4 DA, DA and TEID to form SID B
S04.    Copy IPv4 SA to form IPv6 SA B'
S05.    Encapsulate the packet into a new IPv6 header
S06.    Set the IPv6 DA = B
S07.    Forward along the shortest path to B
S08. ELSE
S09.    Drop the packet
]]></artwork>

        <t>The SID B has the following format:</t>

<figure>
<name>H.M.GTP4.D SID Encoding</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
0                                                         127
+-----------------------+-------+----------------+---------+
|Destination UPF Prefix |IPv4DA |Args.Mob.Session|0 Padded |
+-----------------------+-------+----------------+---------+
       128-a-b-c            a            b           c
]]></artwork>
</figure>

        <t> The SID B MAY <bcp14>MAY</bcp14> be an SRv6 Binding SID instantiated at the first
        UPF (U1) to bind an SR policy Policy <xref target="RFC9256" format="default"/>.</t>
      </section>
      <!-- End section "T.M.Tmap" -->

      <section numbered="true" toc="default">
        <name>End.Limit: Rate Limiting behavior</name>
        <name>End.Limit</name>
        <t> The mobile user-plane user plane requires a rate-limit feature. For this
        purpose, this document defines a new behavior behavior, called "End.Limit".
        The "End.Limit" behavior encodes in its arguments the
          rate limiting rate-limiting
        parameter that should be applied to this packet.  Multiple flows of
        packets should have the same group identifier in the SID when those
        flows are in the same AMBR (Aggregate Maximum Bit Rate) group.  The
        encoding format of the rate limit rate-limit segment SID is as follows:</t>

<figure>
<name>End.Limit: Rate limiting behavior argument format</name> Rate-Limiting Behavior Argument Format</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
+----------------------+----------+-----------+
| LOC+FUNC rate-limit  | group-id | limit-rate|
+----------------------+----------+-----------+
      128-i-j                i          j
]]></artwork>
</figure>

        <t> If the limit-rate bits are set to zero, the node should
            not do rate limiting unless static configuration or
            control-plane
            control plane sets the limit rate associated to the SID.</t>
      </section>
      <!-- End section "End.Limit: Rate Limiting function" -->
    </section>
    <!-- End section "" -->

    <section anchor="pdu_sessions" numbered="true" toc="default">
      <name>SRv6 supported
      <name>SRv6-Supported 3GPP PDU session types</name> Session Types</name>
      <t>The 3GPP <xref target="TS.23501" format="default"/> defines the
      following PDU session
		types: Session Types:
      </t>
      <ul spacing="compact"> spacing="normal">
        <li>IPv4</li>
        <li>IPv6</li>
        <li>IPv4v6</li>
        <li>Ethernet</li>
        <li>Unstructured</li>
      </ul>

      <t> SRv6 supports the 3GPP PDU session types Session Types without any protocol
	    overhead by using the corresponding SRv6 behaviors (End.DX4, behaviors:</t>
	    <ul spacing="normal">
	      <li>End.DX4 and End.DT4 for IPv4 PDU sessions; End.DX6, Sessions</li>
	      <li>End.DX6, End.DT6, and End.T for IPv6 PDU sessions; End.DT46 Sessions</li>
	      <li>End.DT46 for IPv4v6 PDU sessions; End.DX2 Sessions</li>
	      <li>End.DX2 for L2 and Unstructured PDU sessions).</t> Sessions</li>
	    </ul>
    </section>
    <!-- End section "SRv6 supported PDU session types" -->

    <section anchor="netslice" numbered="true" toc="default">
      <name>Network Slicing Considerations</name>
      <t>A mobile network may be required to implement "network slices",
	    which logically separate network resources within the same SR Domain.</t> domain.</t>

	    <t><xref target="RFC9256" format="default"/>
	    describes a solution to build basic network slices with SR.
	    Depending on the requirements, these slices can be further
	    refined by adopting the mechanisms from:
      </t>
      <ul spacing="compact"> spacing="normal">
        <li>IGP Flex-Algo
		<xref target="I-D.ietf-lsr-flex-algo" target="RFC9350" format="default"/></li>
        <li>Inter-Domain policies
	       <xref target="RFC9087" format="default"/></li>
      </ul>

      <t>Furthermore, these can be combined with ODN/AS (On Demand Nexthop/Automated (On-Demand Next Hop / Automated Steering)
	    <xref target="RFC9256" format="default"/> for
	    automated slice provisioning and traffic steering.</t>
      <t>Further details on how these tools can be used to create
          end to end
          end-to-end network slices are documented in
          <xref target="I-D.ali-spring-network-slicing-building-blocks" target="I-D.ali-teas-spring-ns-building-blocks" format="default"/>.</t>
    </section>
    <!-- End section "Network Slicing Considerations" -->

    <section anchor="c-plane" numbered="true" toc="default">
      <name>Control Plane Considerations</name>
      <t>This document focuses on user-plane behavior and its independence
      from the control plane.  While the SRv6 mobile user-plane behaviors may
      be utilized in emerging architectures, such as architectures (for example, those described in
      <xref target="I-D.gundavelli-dmm-mfa" format="default"/>, format="default"/> and <xref
      target="I-D.mhkk-dmm-srv6mup-architecture" format="default"/> for example, require control plane support for the user-plane, format="default"/>), this
      document does not impose any change to the existent mobility control plane.</t>
      plane.
      </t>
      <t> <xref target="IANA" format="default"/> allocates SRv6
	    Segment
      Endpoint Behavior codepoints for the new behaviors defined in this
      document.</t>
    </section>
    <!-- End section "Control Plane Considerations" -->

    <section numbered="true" toc="default">
      <name>Security Considerations</name>
      <t> The security considerations for Segment Routing are discussed in
      <xref target="RFC8402" format="default"/>.  More specifically specifically, for SRv6 SRv6,
      the security considerations and the mechanisms for securing an SR domain
      are discussed in <xref target="RFC8754" format="default"/>.  Together,
      they describe the required security mechanisms that allow establishment
      of an SR domain of trust to operate SRv6-based services for internal
      traffic while preventing any external traffic from accessing or
      exploiting the SRv6-based services.</t>
      <t>The technology described in this document is applied to a mobile
      network that is within the SR Domain. domain. It's important to note the ressemblance
      resemblance between the SR Domain domain and the 3GPP Packet Core Domain.</t>
      <t>This document introduces new SRv6 Endpoint Behaviors. Those behaviors
      operate on control plane information, including information within the
      received SRH payload on which the behaviors operate. Altering the
      behaviors requires that an attacker alter the SR Domain domain as defined in
      <xref target="RFC8754" format="default"/>.  Those behaviors do not need
      any special security consideration given that it is they are deployed within that
      SR Domain.</t> domain.</t>
    </section>
    <!-- End section "Security Considerations" -->

    <section anchor="IANA" numbered="true" toc="default">
      <name>IANA Considerations</name>
      <t> The

      <t>The following values have been allocated within in the "SRv6 Endpoint
      Behaviors" <xref target="RFC8986" format="default"/>
	  sub-registry belonging to subregistry
      within the top-level "Segment Routing Parameters" registry:</t>
      <table anchor="endpoint_opcodes" align="center">
        <name>SRv6 Mobile User-plane User-Plane Endpoint Behavior Types</name>
        <thead>
          <tr>
            <th align="left">Value</th>
            <th align="center">Hex</th>
            <th align="center">Endpoint behavior</th> Behavior</th>
            <th align="center">Reference</th>
            <th align="center">Change Controller</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td align="left">40</td>
            <td align="center">0x0028</td>
            <td align="center">End.MAP</td>
            <td align="center">[This.ID]</td> align="center">RFC 9433</td>
            <td align="center">IETF</td>
          </tr>
          <tr>
            <td align="left">41</td>
            <td align="center">0x0029</td>
            <td align="center">End.Limit</td>
            <td align="center">[This.ID]</td> align="center">RFC 9433</td>
            <td align="center">IETF</td>
          </tr>
          <tr>
            <td align="left">69</td>
            <td align="center">0x0045</td>
            <td align="center">End.M.GTP6.D</td>
            <td align="center">[This.ID]</td> align="center">RFC 9433</td>
            <td align="center">IETF</td>
          </tr>
          <tr>
            <td align="left">70</td>
            <td align="center">0x0046</td>
            <td align="center">End.M.GTP6.Di</td>
            <td align="center">[This.ID]</td> align="center">RFC 9433</td>
            <td align="center">IETF</td>
          </tr>
          <tr>
            <td align="left">71</td>
            <td align="center">0x0047</td>
            <td align="center">End.M.GTP6.E</td>
            <td align="center">[This.ID]</td> align="center">RFC 9433</td>
            <td align="center">IETF</td>
          </tr>
          <tr>
            <td align="left">72</td>
            <td align="center">0x0048</td>
            <td align="center">End.M.GTP4.E</td>
            <td align="center">[This.ID]</td> align="center">RFC 9433</td>
            <td align="center">IETF</td>
          </tr>
        </tbody>
      </table>
    </section>
    <!-- End section "IANA Considerations" -->

    <section numbered="true" toc="default">
      <name>Contributors</name>
      <t>Kentaro Ebisawa
      Toyota Motor Corporation
      Japan</t>
      <t>Email: ebisawa@toyota-tokyo.tech</t>
      <t>Tetsuya Murakami
      Arrcus, Inc.
      United States of America</t>
      <t>Email: tetsuya.ietf@gmail.com</t>
      <t>Charles E. Perkins
      Lupin Lodge
      United States of America</t>
      <t>Email: charliep@computer.org</t>
      <t>Jakub Horn
      Cisco Systems, Inc.
      United States of America</t>
      <t>Email: jakuhorn@cisco.com</t>
    </section>
    <!-- End section "Contributors" -->

    <section anchor="acknowledge" numbered="true" toc="default">
      <name>Acknowledgements</name>
      <t>The authors would like to thank Daisuke Yokota, Bart Peirens,
	  Ryokichi Onishi, Kentaro Ebisawa, Peter Bosch, Darren Dukes,
	  Francois Clad, Sri Gundavelli, Sridhar Bhaskaran, Arashmid Akhavain,
    Ravi Shekhar, Aeneas Dodd-Noble, Carlos Jesus Bernardos, Dirk v. Hugo
     and Jeffrey Zhang for their useful comments of this work.</t>
    </section>
    <!-- End section "Acknowledgements" -->

  </middle>
  <back>

<displayreference target="I-D.ietf-spring-sr-service-programming" to="SR-SERV-PROG"/>
<displayreference target="I-D.camarilloelmalky-springdmm-srv6-mob-usecases" to="SRV6-MOB-USECASES"/>
<displayreference target="I-D.ali-teas-spring-ns-building-blocks" to="NETWORK-SLICE"/>
<displayreference target="I-D.mhkk-dmm-srv6mup-architecture" to="MUP-SR-ARCH"/>
<displayreference target="I-D.matsushima-spring-srv6-deployment-status" to="SRV6-DEPLOY-STAT"/>
<displayreference target="I-D.kohno-dmm-srv6mob-arch" to="SRV6-MOB-ARCH-DISCUSS"/>
<displayreference target="I-D.gundavelli-dmm-mfa" to="MFA"/>
<displayreference target="I-D.murakami-dmm-user-plane-message-encoding" to="SRV6-UP-MSG-ENCODING"/>
<displayreference target="I-D.ietf-spring-srv6-srh-compression" to="SRV6-SRH-COMPRESSION"/>

    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8402.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8986.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8754.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.9256.xml"/>

        <reference anchor="TS.23501">
          <front>
            <title>System Architecture architecture for the 5G System</title>
            <author surname="3GPP" fullname="3GPP"> System (5GS)</title>
            <author>
	      <organization>3GPP</organization>
                </author>
            <date month="November" year="2017"/> month="June" year="2023"/>
          </front>
          <seriesInfo name="3GPP TS 23.501" value="15.0.0"/> TS" value="23.501"/>
	  <refcontent>Version 17.9.0</refcontent>
        </reference>

      </references>
      <references>

        <name>Informative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.9087.xml"/>
        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-ietf-lsr-flex-algo.xml"/>
        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-ietf-spring-sr-service-programming.xml"/>
        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-camarilloelmalky-springdmm-srv6-mob-usecases.xml"/>
        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-ali-spring-network-slicing-building-blocks.xml"/>
        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-mhkk-dmm-srv6mup-architecture.xml"/> href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.9350.xml"/>

<!-- [I-D.ietf-spring-sr-service-programming] IESG state I-D Exists. Updated to long version because missing editor role for Clad and Xu.  -->

<reference anchor="I-D.ietf-spring-sr-service-programming" target="https://datatracker.ietf.org/doc/html/draft-ietf-spring-sr-service-programming-07">
<front>
<title>Service Programming with Segment Routing</title>
<author initials="F." surname="Clad" fullname="Francois Clad" role="editor">
<organization>Cisco Systems, Inc.</organization>
</author>
<author initials="X." surname="Xu" fullname="Xiaohu Xu" role="editor">
<organization>China Mobile</organization>
</author>
<author initials="C." surname="Filsfils" fullname="Clarence Filsfils">
<organization>Cisco Systems, Inc.</organization>
</author>
<author initials="D." surname="Bernier" fullname="Daniel Bernier">
<organization>Bell Canada</organization>
</author>
<author initials="C." surname="Li" fullname="Cheng Li">
<organization>Huawei</organization>
</author>
<author initials="B." surname="Decraene" fullname="Bruno Decraene">
<organization>Orange</organization>
</author>
<author initials="S." surname="Ma" fullname="Shaowen Ma">
<organization>Mellanox</organization>
</author>
<author initials="C." surname="Yadlapalli" fullname="Chaitanya Yadlapalli">
<organization>AT&amp;T</organization>
</author>
<author initials="W." surname="Henderickx" fullname="Wim Henderickx">
<organization>Nokia</organization>
</author>
<author initials="S." surname="Salsano" fullname="Stefano Salsano">
<organization>Universita di Roma "Tor Vergata"</organization>
</author>
<date month="February" day="15" year="2023"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-spring-sr-service-programming-07"/>
</reference>

<!-- [I-D.camarilloelmalky-springdmm-srv6-mob-usecases] IESG state Expired. Updated to long version because missing editor role for Camarillo and Elmalky. -->

<reference anchor="I-D.camarilloelmalky-springdmm-srv6-mob-usecases" target="https://datatracker.ietf.org/doc/html/draft-camarilloelmalky-springdmm-srv6-mob-usecases-02">
<front>
<title>SRv6 Mobility Use-Cases</title>
<author initials="P." surname="Camarillo" fullname="Pablo Camarillo" role="editor">
<organization>Cisco Systems, Inc.</organization>
</author>
<author initials="C." surname="Filsfils" fullname="Clarence Filsfils">
<organization>Cisco Systems, Inc.</organization>
</author>
<author initials="H." surname="Elmalky" fullname="Hani Elmalky" role="editor">
<organization>Individual</organization>
</author>
<author initials="S." surname="Matsushima" fullname="Satoru Matsushima">
<organization>SoftBank</organization>
</author>
<author initials="D." surname="Voyer" fullname="Daniel Voyer">
<organization>Bell Canada</organization>
</author>
<author initials="A." surname="Cui" fullname="Anna Cui">
<organization>AT&amp;T</organization>
</author>
<author initials="B." surname="Peirens" fullname="Bart Peirens">
<organization>Proximus</organization>
</author>
<date month="August" day="15" year="2019"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-camarilloelmalky-springdmm-srv6-mob-usecases-02"/>
</reference>

<!-- [I-D.ali-teas-spring-ns-building-blocks] IESG state Expired -->

        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-matsushima-spring-srv6-deployment-status.xml"/> href="https://bib.ietf.org/public/rfc/bibxml3/reference.I-D.ali-teas-spring-ns-building-blocks.xml"/>

<!-- [I-D.mhkk-dmm-srv6mup-architecture] IESG state I-D Exists -->

        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-kohno-dmm-srv6mob-arch.xml"/> href="https://bib.ietf.org/public/rfc/bibxml3/reference.I-D.mhkk-dmm-srv6mup-architecture.xml"/>

<!-- [I-D.matsushima-spring-srv6-deployment-status] IESG state Expired -->

        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-gundavelli-dmm-mfa.xml"/> href="https://bib.ietf.org/public/rfc/bibxml3/reference.I-D.matsushima-spring-srv6-deployment-status.xml"/>

<!-- [I-D.kohno-dmm-srv6mob-arch] IESG state Exists. Updated to long version because xi:include shows March 12, 2023, as the date when it's axtually march 9, 2023 -->

<reference anchor="I-D.kohno-dmm-srv6mob-arch" target="https://datatracker.ietf.org/doc/html/draft-kohno-dmm-srv6mob-arch-06">
<front>
<title>Architecture Discussion on SRv6 Mobile User plane</title>
<author initials="M." surname="Kohno" fullname="Miya Kohno">
<organization>Cisco Systems, Inc.</organization>
</author>
<author initials="F." surname="Clad" fullname="Francois Clad">
<organization>Cisco Systems, Inc.</organization>
</author>
<author initials="P." surname="Camarillo" fullname="Pablo Camarillo">
<organization>Cisco Systems, Inc.</organization>
</author>
<author initials="Z." surname="Ali" fullname="Zafar Ali">
<organization>Cisco Systems, Inc.</organization>
</author>
<date month="March" day="9" year="2023"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-kohno-dmm-srv6mob-arch-06"/>
</reference>

<!-- [I-D.gundavelli-dmm-mfa] IESG state Expired -->

        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-murakami-dmm-user-plane-message-encoding.xml"/> href="https://bib.ietf.org/public/rfc/bibxml3/reference.I-D.gundavelli-dmm-mfa.xml"/>

<!-- [I-D.murakami-dmm-user-plane-message-encoding] IESG state Expired -->

        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-ietf-spring-srv6-srh-compression.xml"/> href="https://bib.ietf.org/public/rfc/bibxml3/reference.I-D.murakami-dmm-user-plane-message-encoding.xml"/>

<!-- [I-D.ietf-spring-srv6-srh-compression] IESG state I-D Exists. Updated to long version because missing editor role for Cheng and Clad-->

<reference anchor="I-D.ietf-spring-srv6-srh-compression" target="https://datatracker.ietf.org/doc/html/draft-ietf-spring-srv6-srh-compression-05">
<front>
<title>Compressed SRv6 Segment List Encoding in SRH</title>
<author initials="W." surname="Cheng" fullname="Weiqiang Cheng" role="editor">
<organization>China Mobile</organization>
</author>
<author initials="C." surname="Filsfils" fullname="Clarence Filsfils">
<organization>Cisco Systems</organization>
</author>
<author initials="Z." surname="Li" fullname="Zhenbin Li">
<organization>Huawei Technologies</organization>
</author>
<author initials="B." surname="Decraene" fullname="Bruno Decraene">
<organization>Orange</organization>
</author>
<author initials="F." surname="Clad" fullname="Francois Clad" role="editor">
<organization>Cisco Systems</organization>
</author>
<date month="June" day="20" year="2023"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-spring-srv6-srh-compression-05"/>
</reference>

        <reference anchor="TS.29281">
          <front>
            <title>General Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U)</title>
            <author surname="3GPP" fullname="3GPP">
            <author>
	      <organization>3GPP</organization>
                </author>
            <date month="December" year="2017"/> month="September" year="2022"/>
          </front>
          <seriesInfo name="3GPP TS 29.281" value="15.1.0"/> TS" value="29.281"/>
          <refcontent>Version 17.4.0</refcontent>
        </reference>

        <reference anchor="TS.38415">
          <front>
            <title>Draft Specification for 5GS container (TS 38.415)</title>
            <author surname="3GPP" fullname="3GPP">
            <title>PDU session user plane protocol</title>
            <author>
	      <organization>3GPP</organization>
                </author>
            <date month="August" year="2017"/> month="April" year="2022"/>
          </front>
          <seriesInfo name="3GPP R3-174510" value="0.0.0"/> TS" value="38.415"/>
	  <refcontent>Version 17.0.0</refcontent>
        </reference>

      </references>
    </references>

    <section anchor="Implementations" numbered="true" anchor="acknowledge" numbered="false" toc="default">
      <name>Implementations</name>
      <t>RFC Editor: Please remove this section prior
      <name>Acknowledgements</name>
      <t>The authors would like to publication.</t>
      <t>This document introduces new SRv6 Endpoint Behaviors. These behaviors have an
	  open-source P4 implementation available in
	  <eref target="https://github.com/ebiken/p4srv6"/>.</t>
      <t>Additionally, a full open-source implementation thank <contact fullname="Daisuke Yokota"/>,
      <contact fullname="Bart Peirens"/>, <contact fullname="Ryokichi
      Onishi"/>, <contact fullname="Kentaro Ebisawa"/>, <contact
      fullname="Peter Bosch"/>, <contact fullname="Darren Dukes"/>, <contact
      fullname="Francois Clad"/>, <contact fullname="Sri Gundavelli"/>,
      <contact fullname="Sridhar Bhaskaran"/>, <contact fullname="Arashmid
      Akhavain"/>, <contact fullname="Ravi Shekhar"/>, <contact
      fullname="Aeneas Dodd-Noble"/>, <contact fullname="Carlos Jesus
      Bernardos"/>, <contact fullname="Dirk von Hugo"/>, and <contact
      fullname="Jeffrey Zhang"/> for their useful comments of this document is available in Linux Foundation FD.io VPP project since release 20.05. More information available here:
    <eref target="https://docs.fd.io/vpp/20.05/d7/d3c/srv6_mobile_plugin_doc.html"/>.</t>
      <t>There are also experimental implementations in M-CORD NGIC and Open Air Interface (OAI).</t> work.</t>
    </section>

    <section numbered="false" toc="default">
      <name>Contributors</name>

      <contact fullname="Kentaro Ebisawa">
      <organization>Toyota Motor Corporation</organization>
      <address>
	<postal>
	  <country>Japan</country>
	</postal>
	<email>ebisawa@toyota-tokyo.tech</email>
      </address>
    </contact>

      <contact fullname="Tetsuya Murakami" >
        <organization>Arrcus, Inc.</organization>
        <address>
          <postal>
            <street></street>
            <city></city>
            <country>United States of America</country>
          </postal>
          <email>tetsuya.ietf@gmail.com</email>
        </address>
      </contact>

      <contact fullname="Charles E. Perkins" >
        <organization>Lupin Lodge</organization>
        <address>
          <postal>
            <street></street>
            <city></city>
            <country>United States of America</country>
          </postal>
          <email>charliep@computer.org</email>
        </address>
      </contact>

      <contact fullname="Jakub Horn" >
        <organization>Cisco Systems, Inc.</organization>
        <address>
          <postal>
            <street></street>
            <city></city>
            <country>United States of America</country>
          </postal>
          <email>jakuhorn@cisco.com</email>
        </address>
      </contact>

    </section>
    <!-- End section "Implementations" -->

  </back> </rfc>