Path Monitoring System/Head-end based MPLS Ping and Traceroute in Inter-domain Segment Routing Networks

Many network deployments have built their networks consisting of multiple ASes either for the ease of operations or as a result of network mergers and acquisitions. SR can be deployed in such scenarios to provide end-to-end paths, traversing multiple Autonomous systems(ASes). specifies SR with an MPLS data plane. describes BGP peering SIDs, and describes Egress Peer Engineering, which will help in steering packets from one AS to another. Using the above SR capabilities, paths that span across multiple ASes can be created.

+----------------+ | Controller/PMS | +----------------+ |---AS1-----| |------AS2------| |----AS3---| ASBR2----ASBR3 ASBR5------ASBR7 / \ / \ / \ / \ PE1----P1---P2 P3---P4---PE4 P5---P6--PE5 \ / \ / \ / \ / ASBR1----ASBR4 ASBR6------ASBR8 Autonomous System: AS1, AS2, AS3 Provider Edge: PE1, PE4, PE5 Provider: P1, P2, P3, P4, P5, P6 AS Boundary Router:ASBR1, ASBR2, ASBR3, ASBR4, ASBR5, ASBR6, ASBR7, ASBR8 For example, describes an inter-AS network scenario consisting of ASes AS1, AS2 and AS3. AS1, AS2 and AS3 are SR enabled and the egress links have PeerNode SID/PeerAdj SID/ PeerSet SID configured and advertised via . PeerNode SID/PeerAdj SID/PeerSet SID are referred to as Egress Peer Engineering SIDs (EPE-SIDs) in this document. The controller or the head-end can build an end-to-end Traffic-Engineered path consisting of Node-SIDs, Adjacency-SIDs and EPE-SIDs. It is useful for operators to be able to perform LSP ping and traceroute procedures on these inter-AS SR-MPLS paths, to detect and diagnose failed deliveries and to determine the actual path that traffic takes through the network. LSP ping/traceroute procedures use IP connectivity for echo reply to reach the head-end. In inter-AS networks, IP connectivity may not be there from each router in the path. For example, in , P3 and P4 may not have IP connectivity for PE1. It is not always possible to carry out LSP ping and traceroute functionality on these paths to verify basic connectivity and fault isolation using existing LSP ping and traceroute mechanisms( and ). That is because there might not always be IP connectivity from a responding node back to the source address of the ping packet when the responding node is in a different AS from the source of the ping. describes mechanisms to carry out MPLS ping/traceroute from a Path Monitoring System (PMS). It is possible to build GRE tunnels or static routes to each router in the network to get IP connectivity for the reverse path. This mechanism is operationally very heavy and requires the PMS to be capable of building a huge number of GRE tunnels or installing the necessary static routes, which may not be feasible. describes an Echo-relay based solution based on advertising a new Relay Node Address Stack TLV containing a stack of Echo-relay IP addresses. These mechanisms can be applied to SR networks as well. The mechanism requires the return ping packet to be processed on the slow path or as a bump-in-the-wire on every relay node. The motivation of the current document is to provide an alternate mechanism for ping/traceroute in inter-domain SR networks. The definition of the term "domain" as applicable to this document is defined in . This document describes a new mechanism that is efficient and simple and can be easily deployed in SR-MPLS networks. This mechanism uses MPLS paths and no changes are required in the forwarding path. Any MPLS-capable node will be able to forward the echo-reply packet in the fast path. The current document describes a mechanism that uses the Reply Path TLV to convey the reverse path. Three new sub-TLVs are defined for the Reply path TLV that facilitate encoding SR label stack. The return path can either be derived by a smart application or controller that has a full topology view. This document also proposes mechanisms to derive the return path dynamically during traceroute procedures. The current document is focused on the inter-domain use case. However, the protocol extensions described in this document may be applied to indicate the return path for other use cases as well which are out-of-scope for this document and therefore not further described in this document.

The term domain used in this document implies an IGP domain where every node is visible to every other node for shortest path computation. The domain implies an IGP area or level. An AS consists of one or more IGP domains. The procedures described in this document apply to paths built across multiple domains which include inter-area as well as inter-AS paths. The procedures and deployment scenarios described in this document apply to inter-AS paths where the participating ASes belong to closely coordinating administrations or to single ownership. This document applies to SR-MPLS networks where all nodes in each of the domains are SR capable. It also applies to SR-MPLS networks where SR acts as an overlay having SR-incapable underlay nodes. In such networks, the traceroute procedure is executed only on the overlay SR nodes.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

When the network consists of a large number of nodes, the nodes are segregated into multiple IGP domains as shown in . The connectivity to the remote PEs can be achieved using BGP-Labeled Unicast (BGP-LU) or by stacking the labels for each domain as described in .

|-Domain 1|-------Domain 2-----|--Domain 3-| PE1------ABR1--------P--------ABR2------PE4 \ / \ /\ / -------- ----------------- ------- BGP-LU BGP-LU BGP-LU It is useful to support MPLS ping and traceroute mechanisms for these networks. The procedures described in this document for constructing Reply Path TLV and its use in echo reply are equally applicable to networks consisting of multiple IGP domains that use BGP-LU or label stacking.

Reply Path (RP) TLv is defined in . SR networks statically assign the labels to nodes and a PMS/Head-end may know the entire database. The reverse path can be built from the PMS/Head-end by stacking segments for the reverse path. Reply Path TLV as defined in is used to carry the return path. Reply mode 5, Reply via Specified Path is defined in section 4.1 of . While using the procedures described in this document, the reply mode is set to 5 (Reply via Specified Path), and Reply Path TLV is included in the echo request message as described in . The Reply Path TLV is constructed as per Section 4.2 of . This document defines three new sub-TLVs to encode the SR path. The type of segment that the head-end chooses to send in the Reply Path TLV is governed by local policy. Implementations MAY provide Command Line Interface (CLI) input parameters in the form of Labels/IPv4 addresses/IPv6 addresses or a combination of these which get encoded in the Reply Path TLV. Implementations MAY also provide mechanisms to acquire the database of remote domains and compute the return path based on the acquired database. For traceroute purposes, the return path will have to consider the reply being sent from every node along the path. The return path changes when the traceroute progresses and crosses each domain. One of the ways this can be implemented on the headend is to acquire the entire database (of all domains) and build a return path for every node along the SR-MPLS path based on the knowledge of the database. Another mechanism is to use a dynamically computed return path as described in Some networks may consist of pure IPv4 domains and pure IPv6 domains. Handling end-to-end MPLS OAM for such networks is out of the scope of this document. It is recommended to use dual-stack in such cases and use end-to-end IPv6 addresses for MPLS ping and traceroute procedures.

Section 4 of defines various segment types. The types of segments applicable to this document have been defined in this section for the use of MPLS OAM. The intention was to keep the definitions as close to those in as possible with modifications only when needed. One or more segment sub-TLVs can be included in the Reply Path TLV. The Segment sub-TLVs included in a Reply Path TLV MAY be of different types. Below types of segment sub-TLVs are apply to the Reply Path TLV. The code points for the sub-TLVs are taken from the IANA registry common to TLVs 1, 16 and 21. This document defines the Segment sub-TLVs usage and processing when it appears in TLV 21. If these sub-TLVs appear in TLVs 1 or 16, appropriate error codes MUST be returned as defined in . Type-A: SID only, in the form of MPLS Label Type-C: IPv4 Node Address with optional SID Type-D: IPv6 Node Address with optional SID for SR MPLS

The Type A Segment Sub-TLV encodes a single SID in the form of an MPLS label. The format is as follows:

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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | RESERVED | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Label | TC |S| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where: Type: TBD1(to be assigned by IANA from the registry "Sub-TLVs for TLV Types 1, 16, and 21"). Length: 2 octets. Carries value 8. The length excludes the length of the Type and Length Fields. Flags: 1 octet of flags as defined in . RESERVED: 3 octets of reserved bits. MUST be set to zero when sending; MUST be ignored on receipt.. Label: 20 bits of label value. TC: 3 bits of traffic class S: 1 bit Reserved TTL: 1 octet of TTL. The following applies to the Type-A Segment sub-TLV: The S bit MUST be zero upon transmission, and MUST be ignored upon reception. If the originator wants the receiver to choose the TC value, it MUST set the Traffic Class(TC) field to zero. If the originator wants the receiver to choose the TTL value, it MUST set the TTL field to 255. If the originator wants to recommend a value for these fields, it MUST put those values in the TC and/or TTL fields. The receiver MAY override the originator's values for these fields. This would be determined by local policy at the receiver. One possible policy would be to override the fields only if the fields have the default values specified above.

The Type-C Segment sub-TLV encodes an IPv4 node address, SR Algorithm and an optional SID in the form of an MPLS label. The format is as follows:

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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | RESERVED (MBZ) | SR Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 Node Address (4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID (optional, 4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where: Type: TBD2 (to be assigned by IANA from the registry "Sub-TLVs for TLV Types 1, 16, and 21"). Length: 2 octets. Carries value 8 or 12. Flags: 1 octet of flags as defined in . SR Algorithm: 1 octet specifying SR Algorithm as described in section 3.1.1 in , when A-Flag as defined in is present. SR Algorithm is used by the receiver to derive the Label. When A-flag is unset, this field has no meaning and thus MUST be set to zero on transmission and ignored on receipt. RESERVED: 2 octets of reserved bits. MUST be set to zero when sending; MUST be ignored on receipt. IPv4 Node Address: 4-octet IPv4 address representing a node. SID: optional: 4-octet field containing label, TC, S and TTL as defined in . When the SID field is present, it MUST be used for constructing the Reply Path. The following applies to the Type-C Segment sub-TLV: The IPv4 Node Address MUST be present. The SID is optional and specifies a 4-octet MPLS SID containing label, TC, S and TTL as defined in . If the length is 8, then only the IPv4 Node Address is present. If the length is 12, then the IPv4 Node Address and the MPLS SID are present.

The Type-D segment sub-TLV encodes an IPv6 node address, SR Algorithm and an optional SID in the form of an MPLS label. The format is as follows:

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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | RESERVED(MBZ) | SR Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // IPv6 Node Address (16 octets) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID (optional, 4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where: Type: TBD3 (to be assigned by IANA from the registry "Sub-TLVs for TLV Types 1, 16, and 21"). Length: 2 Octets. carries value 20 or 24. Flags: 1 octet of flags as defined in . SR Algorithm: 1 octet specifying SR-Algorithm as described in section 3.1.1 in , when A-Flag as defined in is present. SR-Algorithm is used by the receiver to derive the label.When A-flag is unset, this field has no meaning and thus MUST be set to zero (MBZ) on transmission and ignored on receipt. RESERVED: 2-octet of reserved bits. MUST be set to zero when sending; MUST be ignored on receipt. IPv6 Node Address: 16-octet IPv6 address representing a node. SID: optional: 4-octet field containing label, TC, S and TTL as defined in The following applies to the Type-D segment sub-TLV: The IPv6 Node Address MUST be present. The SID is optional and specifies a 4-octet MPLS SID containing label, TC, S and TTL as defined in .When the SID field is present, it MUST be used for constructing the Reply Path. If the length is 20, then only the IPv6 Node Address is present. If the length is 24, then the IPv6 Node Address and the MPLS SID are present.

The Segment Types described above contain the following flags in the "Flags" field (codes to be assigned by IANA from the new registry "Segment Sub-TLV Flags" )

0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | |A| | | | | | | +-+-+-+-+-+-+-+-+ where: A-Flag: This flag indicates the presence of SR Algorithm ID in the "SR-Algorithm" field applicable to various Segment Types. Unused bits in the Flag octet MUST be set to zero upon transmission and MUST be ignored upon receipt. The following applies to the Segment Flags: A-Flag applies to Segment Type-C and Type-D. If A-Flag appears with Type-A Segment Type, it MUST be ignored.

SRv6 data plane is not in the scope of this document.

In the inter-AS scenario, the procedures described in this document are used to specify the return path, if IP connectivity to the initiator is not available, and may be used in any case. LSP ping initiator MUST set the Reply Mode of the echo request to 5 "Reply via Specified Path", and a Reply Path TLV MUST be carried in the echo request message correspondingly. The Reply Path TLV MUST contain the Segment Routing Path in the reverse direction encoded as an ordered list of segments. The first Segment MUST correspond to the top Segment in the MPLS header that the responder MUST use while sending the echo reply.

As described in , when Reply mode is set to 5 (Reply via Specified Path), the echo request must contain the Reply path TLV. Absence of Reply Path TLV is treated as a malformed echo request. When an echo request is received, if the egress LSR does not know the Reply Mode 5 defined in , an echo reply with the return code set to "Malformed echo request received" and the Subcode set to zero must be sent back to the ingress LSR according to the rules of . If the echo request message contains a malformed segment sub-TLV, an echo reply with return code set to "Malformed echo request received" and the Subcode set to zero must be sent back to the ingress LSR. When a Reply Path TLV is received, the responder that supports processing it, MUST use the segments in Reply Path TLV to build the echo reply. The responder MUST follow the normal FEC validation procedures as described in and and this document does not suggest any change to those procedures. When the echo reply has to be sent out the Reply Path TLV MUST be used to construct the MPLS packet to send out.

The echo reply message is sent as an MPLS packet with an MPLS label stack. The echo reply message MUST be constructed as described in the . An MPLS packet is constructed with an echo reply in the payload. The top label MUST be constructed from the first Segment from the Reply Path TLV. The remaining labels MUST follow the order from the Reply Path TLV. The responder MAY check the reachability of the top label in its own Label Forwarding Information Base (LFIB) before sending the echo reply and provide necessary log information in case of unreachability. In certain scenarios, the head-end MAY choose to send Type-C/Type-D segments consisting of IPV4 addresses or IPv6 addresses, when it is unable to derive the SID from available topology information. Optionally SID may also be associated with the Type-C/Type-D segment, if such information is available from the controller or via operator input. In such cases, the node sending the echo reply MUST derive the MPLS labels based on Node-SIDs associated with the IPv4 /IPv6 addresses or from the optional MPLS SIDs in the Type-C/Type-D segments and encode the echo reply with MPLS labels. The reply path return code is set as described in section 7.4 of . According to section 5.3 of , the Reply Path TLV is included in an echo reply indicating the specified return path that the echo reply message is required to follow. When the node is configured to dynamically create a return path for the next echo request, the procedures described in MUST be used. The reply path return code MUST be set to TBA1 and the same Reply Path TLV or a new Reply Path TLV MUST be included in the echo reply.

The rules and processes defined in Section 4.6 of and Section 5.4 of apply here. In addition, if the Reply Path return code is "Use Reply Path TLV in the echo reply for building the next echo request" (defined in this document), the Reply Path TLV from the echo Reply MUST be sent in the next echo request with TTL incremented by 1. If the TTL is already 255, the traceroute procedure MUST be ended with an appropriate log message.

The example topology given in will be used in the below sections to explain LSP ping and traceroute procedures. The PMS/Head-end has a complete view of topology. PE1, P1, P2, ASBR1 and ASBR2 are in AS1. Similarly ASBR3, ASBR4, P3, P4 and PE4 are in AS2. AS1 and AS2 have SR enabled. IGPs like OSPF/ISIS are used to flood SIDs in each AS. The ASBR1, ASBR2, ASBR3 and ASBR4 advertise BGP EPE-SIDs for the inter-AS links. Topology of AS1 and AS2 are advertised via BGP-Link State (BGP-LS) to the controller/PMS or Head-end node. The EPE-SIDs are also advertised via BGP-LS as described in . The example uses EPE-SIDs for the inter-AS links but the same could be achieved using adjacency-SIDs advertised for a passive IGP link. The description in the document uses below notations for Segment Identifiers(SIDs). Node SIDs: N-PE1, N-P1, N-ASBR1 etc. Adjacency SIDs: Adj-PE1-P1, Adj-P1-P2 etc. EPE-SIDs: EPE-ASBR2-ASBR3, EPE-ASBR1-ASBR4, EPE-ASBR3-ASBR2 etc.

Consider an SR-MPLS path from PE1 to PE4 consisting of a label stack [N-P1, N-ASBR1, EPE-ASBR1-ASBR4, N-PE4] from . In order to perform MPLS ping procedures on this path, the remote end (PE4) needs IP connectivity to head end PE1, for the echo reply to travel back to PE1. In a deployment that uses a controller-computed inter-domain path, there may be no IP connectivity from PE4 to PE1 as they lie in different ASes. PE1 sends an echo request message to the end-point PE4 along the path that consists of label stacks [N-P1, N-ASBR1, EPE-ASBR1-ASBR4, N-PE4]. PE1 adds the return path from PE4 to PE1 in the echo request message in the Reply Path TLV. As an example, Reply Path TLV for PE1 to PE4 for LSP ping is [N-ASBR4, EPE-ASBR4-ASBR1, N-PE1]. This example path provides the entire return path up to the head-end node PE1. The mechanism used to construct the return path is implementation-dependent. An implementation may also build a return Path consisting of labels to reach its own AS. Once the label stack is popped off the echo reply message will be exposed. The further packet forwarding will be based on IP lookup. An example return Path for this case could be [N-ASBR4, EPE-ASBR4-ASBR1]. On receiving MPLS echo request PE4 first validates FEC in the echo request. PE4 then builds a label stack to send the response from PE4 to PE1 by copying the labels from Reply Path TLV. PE4 builds the echo reply packet with the MPLS label stack constructed and imposes MPLS headers on top of the echo reply packet and sends out the packet to PE1. This Segment List stack can successfully steer reply back to the Head-end node (PE1).

The traceroute procedure involves visiting every node on the path and echo replies sent from every node. In this section, we describe the traceroute mechanisms when the headend/PMS has complete visibility of the database. Headend/PMS computes the return path from each node in the entire SR-MPLS path that is being tracerouted. The return path computation is implementation-dependent. As the headend/PMS completely controls the return path, it can use proprietary computations to build the return path. One of the ways the return path can be built is to use the principle of building label stacks by adding each domain border node's Node SID on the return path label stack as the traceroute progresses. For inter-AS networks, in addition to the border node's Node-SID, EPE-SID in the reverse direction also needs to be added to the label stack. The Inter-domain/inter-as traceroute procedure uses the TTL expiry mechanism as specified in and . Every echo request packet Headend/PMS will include the appropriate return path in the Reply Path TLV. The node that receives the echo request will follow procedures described in section and section to send out an echo reply. For Example: Let us consider a topology from . Let us consider a SR-MPLS path [N-P1, N-ASBR1, EPE-ASBR1-ASBR4, N-PE4]. The traceroute is being executed for this inter-AS path for destination PE4. PE1 sends the first echo request with TTL set to 1 and includes Reply Path TLV consisting of Type-A Segment containing label derived from its own SR Global Block (SRGB). Note that the type of segment used in constructing the return Path is local policy. If the entire network has the same SRGB configured, Type-A segments can be used. The TTL expires on P1 and P1 sends an echo reply using the return path. Note that implementations may choose to exclude the Reply Path TLV until traceroute reaches the first domain border as the return IP path to PE1 is expected to be available inside the first domain. TTL is set to 2 and the next the echo request is sent out. Until the traceroute procedure reaches the domain border node ASBR1, the same return path TLV consisting of single Label (PE1's node Label) is used. When an echo request reaches the border node ASBR1, and an echo reply is received from ASBR1, the next echo request needs to include an additional label as ASBR1 is a border node. The head-end node has complete visibility of the network database learned via BGP-LS and and can derive the details of Autonomous System Boundary Router (ASBR) nodes. The Reply Path TLV is built based on the forward path. As the forward path consists of EPE-ASBR1-ASBR4, an EPE-SID in the reverse direction is included in the Reply Path TLV. The return path now consists of two labels [EPE-ASBR4-ASBR1, N-PE1]. The echo reply from ASBR4 will use this return path to send the reply. The next echo request after visiting the border node ASBR4 will update the return path with the Node-SID label of ASBR4. The return path beyond ASBR4 will be [N-ASBR4, EPE-ASBR4-ASBR1, N-PE1]. This same return path is used until the traceroute procedure reaches the next set of border nodes. When there are multiple ASes the traceroute procedure will continue by adding a set of Node-SIDs and EPE-SIDs as the border nodes are visited. Note that the above return path-building procedure requires the database of all the domains to be available at the headend/PMS.

assumes the same SRGB is configured on all nodes along the path. The SRGB may differ from one node to another node and the SR architecture allows the nodes to use different SRGB. In such scenarios, PE1 sends Type-C (or Type-D in case of IPv6 networks) segment with the Node address of PE1 and with optional MPLS SID associated with the Node address. The receiving node derives the label for the return path based on its own SRGB. When the traceroute procedure crosses the border ASBR1, headend PE1 should send a Type-A segment for N-PE1 based on the label derived from ASBR1's SRGB. This is required because, ASBR4, P3, P4 etc may not have the topology information to derive SRGB for PE1. After the traceroute procedure reaches ASBR4 the return path will be [N-PE1 (Type-A with label based on ASBR1's SRGB), EPE-ASBR4-ASBR1, N-ASBR4 (Type-C)]. To extend the example to multiple ASes consisting of 3 or more ASes, let us consider a traceroute from PE1 to PE5 in . In this example, the PE1 to PE5 path has to cross 3 domains AS1, AS2 and AS3. Let us consider a path from PE1 to PE5 that goes through [PE1, ASBR1, ASBR4, ASBR6, ASBR8,PE5]. When the traceroute procedure is visiting the nodes in AS1, the Reply Path TLV sent from the headend consists of [N-PE1]. When the traceroute procedure reaches the ASBR4, the return Path consists of [N-PE1, EPE-ASBR4-ASBR1]. While visiting nodes in AS2, the traceroute procedure consists of Reply Path TLV [N-PE1, EPE-ASBR4-ASBR1, N-ASBR4]. similarly, while visiting the ASBR8 Reply Path TLV adds the EPE-SID from ASBR8 to ASBR6. While visiting nodes in AS3 Node-SId of ASBR8 would also be added which makes the return Path [N-PE1, EPE-ASBR4-ASBR1, N-ASBR4, EPE-ASBR8-ASBR6, N-ASBR8] Let us consider another example from topology . This topology consists of multi-domain IGP with a common border node between the domains. This could be achieved with multi-area or multi-level IGP or multiple instances of IGP deployed on the same node. The return path computation for this topology is similar to the multi-AS computation except that the return path consists of a single border node label. When the traceroute procedure visits node P, the return path consists of [N-PE1, N-ABR1].

In some cases, the head-end may not have complete visibility of Inter-AS/Inter-domain topology. In such cases, it can rely on downstream routers to build the reverse path for MPLS traceroute procedures. For this purpose, Reply Path TLV in the echo reply corresponds to the return path to be used in building the next echo request. Value Meaning ------ ---------------------- TBA1 Use Reply Path TLV in the echo reply for building the next echo request.

To dynamically build the return Path for the traceroute procedures, the domain border nodes along the path being traced should support the procedures described in this section. Local policy on the domain border nodes should determine whether the domain border node participates in building the return path dynamically during traceroute. The headend/PMS node may include its node label while initiating the traceroute procedure. When an Area Border Router (ABR) receives the echo request, if the local policy implies building a dynamic return path, ABR should include its Node label in the reply path TLV and send it in the echo reply. If there is a Reply Path TLV included in the received echo request message, the ABR's node label is added before the existing segments. The type of segment added is based on local policy. In cases when SRGB is not uniform across the network, it is RECOMMENDED to add a Type-C or a Type-D segment, but implementations MAY safely use other approaches if they see benefits in doing so. If the existing segment in the Reply Path TLV is a Type-C/Type-D segment, that segment should be converted to a Type-A segment based on ABR's own SRGB. This is because downstream nodes will not know what SRGB to use to translate the IP address to a label. As the ABR added its own Node label, it is guaranteed that this ABR will be in the return path and will be forwarding the traffic based on the next label after its label. When an ASBR receives an echo request from another AS, and ASBR is configured to build the return path dynamically, ASBR should build a Reply Path TLV and include it in the echo reply. The Reply Path TLV should consist of its node label and an EPE-SID to the AS from where the traceroute message was received. A Reply path return code of TBA1 should be set in the echo reply to indicate that the next echo request should use the return Path from the Reply Path TLV in the echo reply. ASBR should locally decide the outgoing interface for the echo reply packet. Generally, remote ASBR will choose the interface on which the incoming OAM packet was received to send the echo reply out. Reply Path TLV is built by adding two segment sub TLVs. The top segment sub TLV consists of the ASBR's Node SID and the second segment consists of the EPE-SID in the reverse direction to reach the AS from which the OAM packet was received. The type of segment chosen to build Reply Path TLV is a local policy. It is recommended to use the Type-C/Type-D segment for the top segment when the SRGB is not guaranteed to be uniform in the domain. Irrespective of which type of segment is included in the Reply Path TLV, the responder of echo requests should always translate the Reply Path TLV to a label stack and build an MPLS header for the echo reply packet. This procedure can be applied to an end-to-end path consisting of multiple ASes. Each ASBR that receives an echo request from another AS adds its Node-SID and EPE-SID on top of existing segments in the Reply Path TLV. An ASBR that receives the echo request from a neighbor belonging to the same AS, MUST look at the Reply Path TLV received in the echo request. If the Reply Path TLV consists of a Type-C/Type-D segment, it MUST convert the Type-C/Type-D segment to a Type-A segment by deriving a label from its own SRGB. The ASBR MUST set the reply path return code to TBA1 and send the newly constructed Reply Path TLV in the echo reply. Internal nodes or nondomain border nodes MAY not set the Reply Path TLV return code to TBA1 in the echo reply message as there is no change in the return Path. In these cases, the headend node/PMS that initiates the traceroute procedure MUST continue to send previously sent Reply Path TLV in the echo request message in every next echo request. Note that an ASBR's local policy may prohibit it from participating in the dynamic traceroute procedures. If such an ASBR is encountered in the forward path, dynamic return path-building procedures will fail. In such cases, ASBR that supports this document MUST set the return code TBA2 to indicate local policies do not allow the dynamic return path building. Value Meaning ------ --------------------------------------------------- TBA2 Local policy does not allow dynamic Return Path building.

Let us consider a topology from . Let us consider an SR policy path built from PE1 to PE4 with a label stack as below. N-P1, N-ASBR1, EPE-ASBR1-ASBR4, N-PE4. PE1 begins traceroute with TTL set to 1 and includes [N-PE1] in the Reply Path TLV. The traceroute packet TTL expires on P1 and P1 processes the traceroute as per the procedures described in and . P1 sends an echo reply with the same Reply Path TLV with the reply path return code set to 6. The return code of the echo reply itself is set to the return code as per and . This traceroute doesn't need any changes to the Reply Path TLV till it leaves AS1. The same Reply Path TLV that is received may be included in the echo reply by P1 and P2 or no Reply Path TLV included so that headend continues to use the same return path in the echo request that it used to send the previous echo request. When ASBR1 receives the echo request, in case it receives the Type-C/Type-D segment in the Reply Path TLV in the echo request, converts that Type-C/Type-D segment to Type-A based on its own SRGB. When ASBR4 receives the echo request, it should form this Reply Path TLV using its Node SID (N-ASBR4) and EPE-SID (EPE-ASRB4-ASBR1) labels and set the reply path return code to TBA1. Then PE1 should use this Reply Path TLV in subsequent echo requests. In this example, when the subsequent echo request reaches P3, it should use this Reply Path TLV for sending the echo reply. The same Reply Path TLV is sufficient for any router in AS2 to send the reply. Because the first label(N-ASBR4) can direct echo reply to ASBR4 and the second one (EPE-ASBR4-ASBR1) to direct echo reply to AS1. Once the echo reply reaches AS1, normal IP forwarding or the N-PE1 helps it to reach PE1. The example described in the above paragraphs can be extended to multiple ASes by following the same procedure of each ASBR adding Node-SID and EPE-SID on receiving echo requests from neighboring AS. Let us consider a topology from . It consists of multiple IGP domains with multiple areas/levels or separate IGP instances. There is a single border node that separates the two domains. In this case, PE1 sends a traceroute packet with TTL set to 1 and includes N-PE1 in the Reply Path TLV. ABR1 receives the echo request and while sending the echo reply adds its node Label to the Reply Path TLV and sets the Reply path return code to TBA1. The Reply Path TLV in the echo reply from ABR1 consists of [N-ABR1, N-PE1]. The next echo request with TTL 2 reaches the P node. It is an internal node so it does not change the return Path. Echo request with TTL 3 reaches ABR2 and it adds its Node label so the Reply Path TLV sent in echo reply will be [N-ABR2, N-ABR1, N-PE1]. echo request with TTL 4 reaches PE4 and it sends an echo reply return code as Egress. PE4 does not include any Reply Path TLV in the echo reply. The above example assumes uniform SRGB throughout the domain. In the case of different SRGBs, the top segment will be a Type-C/Type-D segment and all other segments will be Type-A. Each border node converts the Type-C/Type-D segment to Type-A before adding its segment to the Reply Path TLV.

The procedures described in this document enable LSP ping and traceroute to be executed across multiple IGP domains or multiple ASes that belong to the same administration or closely cooperating administration. It is assumed that sharing domain internal information across such domains does not pose a security risk. However, procedures described in this document may be used by an attacker to extract the domain's internal information. An operator MUST deploy appropriate filter policies as described in to restrict the LSP ping/traceroute packets based on origin. It is also RECOMMENDED that an operator deploy security mechanisms such as MACsec on inter-domain links or security-vulnerable links to prevent spoofing attacks. All the security considerations defined in will be applicable for this document. Appropriate filter policies SHOULD be applied at the edges to prevent attackers from getting into the network. In the event of such a security breach, the network devices MUST have mechanisms to prevent Denial-of-service attacks as described in .

IANA should assign three new sub-TLVs from the "sub-TLVs for TLV Types 1, 16, and 21" sub-registry of the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry. Sub-Type Sub-TLV Name Reference -------- ----------------- ------------ TBD1 SID only in the form of MPLS Section 4.1 label of this document TBD2 IPv4 Node Address with Section 4.2 optional SID for SR-MPLS of this document TBD3 IPv6 Node Address with Section 4.3 optional SID for SR-MPLS of this document The allocation of code points for the segment sub-TLVs should be done from the Standards Action range (0-16383)

IANA should create a new "Segment ID Sub-TLV flags" (see Section ) registry under the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry. This registry tracks the assignment of 8 flags in the Segment ID sub-TLV flags field. The flags are numbered from 0 (most significant bit, transmitted first) to 8. New entries are assigned by Standards Action. Initial entries in the registry are as follows: Bit number | Name | Reference ------------+----------------------------+-------------- 1 | A Flag | Section 4.4 | | of this document

IANA should assign new return codes in the "Reply path return code" registry under the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry. Value Meaning Reference -------- ----------------- ------------ TBA1 Use Reply Path TLV This document from this echo reply for building next echo request. TBA2 Local policy does This document not allow dynamic return Path building. The return codes should be assigned from the Standards Action range (0x0000-0xFFFB).

1.Carlos Pignataro NC State University cpignata@gmail.com 2. Zafar Ali Cisco Systems, Inc. zali@cisco.com

This section is to be removed before publishing as an RFC. RFC-Editor: Please clean up the references cited by this section before publication. This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in . The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.

Organization: Juniper Networks Implementation: JUNOS. Description: Implementation for sending Return path TLV with Type-A segment subTLV Maturity Level: Prototype Coverage: Partial. Type-A SIDs in Return Path TLV Contact: shraddha@juniper.net

Thanks to Bruno Decraene for suggesting the use of generic segment sub-TLV. Thanks to Adrian Farrel, Huub van Helvoort, Dhruv Dhody, Dongjie, for careful review and comments. Thanks to Mach Chen for suggesting to use Reply Path TLV. Thanks to Gregory Mirsky for the detailed review which helped improve the readability of the document to a great extent.