Alibaba, Inc

xiaohu.xxh@alibaba-inc.com

Futurewei Technologies

stewart.bryant@gmail.com

Old Dog Consulting

adrian@olddog.co.uk

Cisco

shassan@cisco.com

Nokia

wim.henderickx@nokia.com

Huawei

lizhenbin@huawei.com

MPLS-SR-over-IP, SR-MPLS-over-IP, MPLS-SR-over-UDP, SR-MPLS-over-UDP MPLS Segment Routing (SR-MPLS) is a method of source routing a packet through an MPLS data plane by imposing a stack of MPLS labels on the packet to specify the path together with any packet-specific instructions to be executed on it. SR-MPLS can be leveraged to realize a source-routing mechanism across MPLS, IPv4, and IPv6 data planes by using an MPLS label stack as a source-routing instruction set while making no changes to SR-MPLS specifications and interworking with SR-MPLS implementations. This document describes how SR-MPLS-capable routers and IP-only routers can seamlessly coexist and interoperate through the use of SR-MPLS label stacks and IP encapsulation/tunneling such as MPLS-over-UDP as defined in RFC 7510.

Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

Copyright Notice Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents () in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• .  Introduction
  • .  Terminology
• .  Use Cases
• .  Procedures of SR-MPLS-over-IP
  • .  Forwarding Entry Construction
    • .  FIB Construction Example
  • .  Packet-Forwarding Procedures
    • .  Packet Forwarding with Penultimate Hop Popping
    • .  Packet Forwarding without Penultimate Hop Popping
    • .  Additional Forwarding Procedures
• .  IANA Considerations
• .  Security Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Contributors
• Authors' Addresses

Introduction MPLS Segment Routing (SR-MPLS) is a method of source routing a packet through an MPLS data plane. This is achieved by the sender imposing a stack of MPLS labels that partially or completely specify the path that the packet is to take and any instructions to be executed on the packet as it passes through the network. SR-MPLS uses an MPLS label stack to encode a sequence of source-routing instructions. This can be used to realize a source-routing mechanism that can operate across MPLS, IPv4, and IPv6 data planes. This approach makes no changes to SR-MPLS specifications and allows interworking with SR-MPLS implementations. More specifically, the source-routing instructions in a source-routed packet could be uniformly encoded as an MPLS label stack regardless of whether the underlay is IPv4, IPv6 (including Segment Routing for IPv6 (SRv6) ), or MPLS. This document describes how SR-MPLS-capable routers and IP-only routers can seamlessly coexist and interoperate through the use of SR-MPLS label stacks and IP encapsulation/tunneling such as MPLS-over-UDP . describes various use cases for tunneling SR-MPLS over IP. describes a typical application scenario and how the packet forwarding happens.

Terminology This memo makes use of the terms defined in and . 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.

Use Cases Tunneling SR-MPLS using IPv4 and/or IPv6 (including SRv6) tunnels is useful at least in the use cases listed below. In all cases, this can be enabled using an IP tunneling mechanism such as MPLS-over-UDP as described in . The tunnel selected MUST have its remote endpoint (destination) address equal to the address of the next node capable of SR-MPLS identified as being on the SR path (i.e., the egress of the active segment). The local endpoint (source) address is set to an address of the encapsulating node. gives further advice on how to set the source address if the UDP zero-checksum mode is used with MPLS-over-UDP. Using UDP as the encapsulation may be particularly beneficial because it is agnostic of the underlying transport.

• Incremental deployment of the SR-MPLS technology may be facilitated by tunneling SR-MPLS packets across parts of a network that are not SR-MPLS as shown in . This demonstrates how islands of SR-MPLS may be connected across a legacy network. It may be particularly useful for joining sites (such as data centers).

  SR-MPLS-over-UDP to Tunnel between SR-MPLS Sites ________________________ _______ ( ) _______ ( ) ( IP Network ) ( ) ( SR-MPLS ) ( ) ( SR-MPLS ) ( Network ) ( ) ( Network ) ( -------- -------- ) ( | Border | SR-in-UDP Tunnel | Border | ) ( | Router |========================| Router | ) ( | R1 | | R2 | ) ( -------- -------- ) ( ) ( ) ( ) ( ) ( ) ( ) (_______) ( ) (_______) (________________________)

• If the encoding of entropy is desired, IP-tunneling mechanisms that allow the encoding of entropy, such as MPLS-over-UDP encapsulation where the source port of the UDP header is used as an entropy field, may be used to maximize the utilization of Equal-Cost Multipath (ECMP) and/or Link Aggregation Groups (LAGs), especially when it is difficult to make use of the entropy-label mechanism. This is to be contrasted with where MPLS-over-IP does not provide for an entropy mechanism. Refer to ) for more discussion about using entropy labels in SR-MPLS.
• Tunneling MPLS over IP provides a technology that enables Segment Routing (SR) in an IPv4 and/or IPv6 network where the routers do not support SRv6 capabilities and where MPLS forwarding is not an option. This is shown in .

  SR-MPLS Enabled within an IP Network __________________________________ __( IP Network )__ __( )__ ( -- -- -- ) -------- -- -- |SR| -- |SR| -- |SR| -- -------- | Ingress| |IR| |IR| | | |IR| | | |IR| | | |IR| | Egress| -->| Router |===========| |======| |======| |======| Router|--> | SR | | | | | | | | | | | | | | | | | | SR | -------- -- -- | | -- | | -- | | -- -------- (__ -- -- -- __) (__ __) (__________________________________) Key: IR : IP-only Router SR : SR-MPLS-capable Router == : SR-MPLS-over-UDP Tunnel

Procedures of SR-MPLS-over-IP This section describes the construction of forwarding information base (FIB) entries and the forwarding behavior that allow the deployment of SR-MPLS when some routers in the network are IP only (i.e., do not support SR-MPLS). Note that the examples in Sections and assume that OSPF or IS-IS is enabled; in fact, other mechanisms of discovery and advertisement could be used including other routing protocols (such as BGP) or a central controller.

Forwarding Entry Construction This subsection describes how to construct the forwarding information base (FIB) entry on an SR-MPLS-capable router when some or all of the next hops along the shortest path towards a prefix Segment Identifier (Prefix-SID) are IP-only routers. provides a concrete example of how the process applies when using OSPF or IS-IS. Consider router A that receives a labeled packet with top label L(E) that corresponds to the Prefix-SID SID(E) of prefix P(E) advertised by router E. Suppose the i-th next-hop router (termed NHi) along the shortest path from router A toward SID(E) is not SR-MPLS capable while both routers A and E are SR-MPLS capable. The following processing steps apply:

• Router E is SR-MPLS capable, so it advertises a Segment Routing Global Block (SRGB). The SRGB is defined in . There are a number of ways that the advertisement can be achieved including IGPs, BGP, and configuration/management protocols. For example, see .
• When Router E advertises the Prefix-SID SID(E) of prefix P(E), it MUST also advertise the egress endpoint address and the encapsulation type of any tunnel used to reach E. This information is flooded domain wide.
• If A and E are in different routing domains, then the information MUST be flooded into both domains. How this is achieved depends on the advertisement mechanism being used. The objective is that router A knows the characteristics of router E that originated the advertisement of SID(E).
• Router A programs the FIB entry for prefix P(E) corresponding to the SID(E) according to whether a pop or swap action is advertised for the prefix. The resulting action may be:
  • pop the top label
  • swap the top label to a value equal to SID(E) plus the lower bound of the SRGB of E

Once constructed, the FIB can be used by a router to tell it how to process packets. It encapsulates the packets according to the appropriate encapsulation advertised for the segment and then sends the packets towards the next hop NHi.

FIB Construction Example This section is non-normative and provides a worked example of how a FIB might be constructed using OSPF and IS-IS extensions. It is based on the process described in .

• Router E is SR-MPLS capable, so it advertises a Segment Routing Global Block (SRGB) using or .
• When Router E advertises the Prefix-SID SID(E) of prefix P(E), it also advertises the encapsulation endpoint address and the tunnel type of any tunnel used to reach E using or .
• If A and E are in different domains, then the information is flooded into both domains and any intervening domains.
  • The OSPF Tunnel Encapsulations TLV or the IS-IS Tunnel Encapsulation Type sub-TLV is flooded domain wide.
  • The OSPF SID/Label Range TLV or the IS-IS SR-Capabilities sub-TLV is advertised domain wide so that router A knows the characteristics of router E.
  • When router E advertises the prefix P(E):
    • If router E is running IS-IS, it uses the extended reachability TLV (TLVs 135, 235, 236, 237) and associates the IPv4/IPv6 or IPv4/IPv6 Source Router ID sub-TLV(s) .
    • If router E is running OSPF, it uses the OSPFv2 Extended Prefix Opaque Link-State Advertisement (LSA) and sets the flooding scope to Autonomous System (AS) wide.
  • If router E is running IS-IS and advertises the IS-IS Router CAPABILITY TLV (TLV 242) , it sets the "Router ID" field to a valid value or includes an IPv6 TE Router ID sub-TLV (TLV 12), or it does both. The "S" bit (flooding scope) of the IS-IS Router CAPABILITY TLV (TLV 242) is set to "1".
• Router A programs the FIB entry for prefix P(E) corresponding to the SID(E) according to whether a pop or swap action is advertised for the prefix as follows:
  • If the No-PHP (NP) Flag in OSPF or the Persistent (P) Flag in IS-IS is clear:
    • pop the top label
  • If the No-PHP (NP) Flag in OSPF or the Persistent (P) Flag in IS-IS is set:
    • swap the top label to a value equal to SID(E) plus the lower bound of the SRGB of E

When forwarding the packet according to the constructed FIB entry, the router encapsulates the packet according to the encapsulation as advertised using the mechanisms described in or . It then sends the packets towards the next hop NHi. Note that specifies the use of port number 6635 to indicate that the payload of a UDP packet is MPLS, and port number 6636 for MPLS-over-UDP utilizing DTLS. However, and provide dynamic protocol mechanisms to configure the use of any Dynamic Port for a tunnel that uses UDP encapsulation. Nothing in this document prevents the use of an IGP or any other mechanism to negotiate the use of a Dynamic Port when UDP encapsulation is used for SR-MPLS, but if no such mechanism is used, then the port numbers specified in are used.

Packet-Forwarding Procedures specifies an IP-based encapsulation for MPLS, i.e., MPLS-over-UDP. This approach is applicable where IP-based encapsulation for MPLS is required and further fine-grained load balancing of MPLS packets over IP networks over ECMP and/or LAGs is also required. This section provides details about the forwarding procedure when UDP encapsulation is adopted for SR-MPLS-over-IP. Other encapsulation and tunneling mechanisms can be applied using similar techniques, but for clarity, this section uses UDP encapsulation as the exemplar. Nodes that are SR-MPLS capable can process SR-MPLS packets. Not all of the nodes in an SR-MPLS domain are SR-MPLS capable. Some nodes may be "legacy routers" that cannot handle SR-MPLS packets but can forward IP packets. A node capable of SR-MPLS MAY advertise its capabilities using the IGP as described in . There are six types of nodes in an SR-MPLS domain:

• Domain ingress nodes that receive packets and encapsulate them for transmission across the domain. Those packets may be any payload protocol including native IP packets or packets that are already MPLS encapsulated.
• Legacy transit nodes that are IP routers but that are not SR-MPLS capable (i.e., are not able to perform Segment Routing).
• Transit nodes that are SR-MPLS capable but that are not identified by a SID in the SID stack.
• Transit nodes that are SR-MPLS capable and need to perform SR-MPLS routing because they are identified by a SID in the SID stack.
• The penultimate node capable of SR-MPLS on the path that processes the last SID on the stack on behalf of the domain egress node.
• The domain egress node that forwards the payload packet for ultimate delivery.

Packet Forwarding with Penultimate Hop Popping The description in this section assumes that the label associated with each Prefix-SID is advertised by the owner of the Prefix-SID as a Penultimate Hop-Popping (PHP) label. That is, if one of the IGP flooding mechanisms is used, the NP-Flag in OSPF or the P-Flag in IS-IS associated with the Prefix-SID is not set.

Packet-Forwarding Example with PHP +-----+ +-----+ +-----+ +-----+ +-----+ | A +-------+ B +-------+ C +-------+ D +-------+ H | +-----+ +--+--+ +--+--+ +--+--+ +-----+ | | | | | | +--+--+ +--+--+ +--+--+ | E +-------+ F +-------+ G | +-----+ +-----+ +-----+ +--------+ |IP(A->E)| +--------+ +--------+ +--------+ | UDP | |IP(E->G)| |IP(G->H)| +--------+ +--------+ +--------+ | L(G) | | UDP | | UDP | +--------+ +--------+ +--------+ | L(H) | | L(H) | |Exp Null| +--------+ +--------+ +--------+ | Packet | ---> | Packet | ---> | Packet | +--------+ +--------+ +--------+

In the example shown in , assume that routers A, E, G, and H are capable of SR-MPLS while the remaining routers (B, C, D, and F) are only capable of forwarding IP packets. Routers A, E, G, and H advertise their Segment Routing related information, such as via IS-IS or OSPF. Now assume that router A (the Domain ingress) wants to send a packet to router H (the Domain egress) via the explicit path {E->G->H}. Router A will impose an MPLS label stack on the packet that corresponds to that explicit path. Since the next hop toward router E is only IP capable (B is a legacy transit node), router A replaces the top label (that indicated router E) with a UDP-based tunnel for MPLS (i.e., MPLS-over-UDP ) to router E and then sends the packet. In other words, router A pops the top label and then encapsulates the MPLS packet in a UDP tunnel to router E. When the IP-encapsulated MPLS packet arrives at router E (which is a transit node capable of SR-MPLS), router E strips the IP-based tunnel header and then processes the decapsulated MPLS packet. The top label indicates that the packet must be forwarded toward router G. Since the next hop toward router G is only IP capable, router E replaces the current top label with an MPLS-over-UDP tunnel toward router G and sends it out. That is, router E pops the top label and then encapsulates the MPLS packet in a UDP tunnel to router G. When the packet arrives at router G, router G will strip the IP-based tunnel header and then process the decapsulated MPLS packet. The top label indicates that the packet must be forwarded toward router H. Since the next hop toward router H is only IP capable (D is a legacy transit router), router G would replace the current top label with an MPLS-over-UDP tunnel toward router H and send it out. However, since router G reaches the bottom of the label stack (G is the penultimate node capable of SR-MPLS on the path), this would leave the original packet that router A wanted to send to router H encapsulated in UDP as if it was MPLS (i.e., with a UDP header and destination port indicating MPLS) even though the original packet could have been any protocol. That is, the final SR-MPLS has been popped exposing the payload packet. To handle this, when a router (here it is router G) pops the final SR-MPLS label, it inserts an explicit NULL label before encapsulating the packet in an MPLS-over-UDP tunnel toward router H and sending it out. That is, router G pops the top label, discovers it has reached the bottom of stack, pushes an explicit NULL label, and then encapsulates the MPLS packet in a UDP tunnel to router H.

Packet Forwarding without Penultimate Hop Popping demonstrates the packet walk in the case where the label associated with each Prefix-SID advertised by the owner of the Prefix-SID is not a Penultimate Hop-Popping (PHP) label (e.g., the NP-Flag in OSPF or the P-Flag in IS-IS associated with the Prefix-SID is set). Apart from the PHP function, the roles of the routers are unchanged from .

Packet-Forwarding Example without PHP +-----+ +-----+ +-----+ +-----+ +-----+ | A +-------+ B +-------+ C +--------+ D +--------+ H | +-----+ +--+--+ +--+--+ +--+--+ +-----+ | | | | | | +--+--+ +--+--+ +--+--+ | E +-------+ F +--------+ G | +-----+ +-----+ +-----+ +--------+ |IP(A->E)| +--------+ +--------+ | UDP | |IP(E->G)| +--------+ +--------+ +--------+ | L(E) | | UDP | |IP(G->H)| +--------+ +--------+ +--------+ | L(G) | | L(G) | | UDP | +--------+ +--------+ +--------+ | L(H) | | L(H) | | L(H) | +--------+ +--------+ +--------+ | Packet | ---> | Packet | ---> | Packet | +--------+ +--------+ +--------+

As can be seen from the figure, the SR-MPLS label for each segment is left in place until the end of the segment where it is popped and the next instruction is processed.

Additional Forwarding Procedures

Non-MPLS Interfaces:

Although the description in the previous two sections is based on the use of Prefix-SIDs, tunneling SR-MPLS packets is useful when the top label of a received SR-MPLS packet indicates an Adjacency SID and the corresponding adjacent node to that Adjacency SID is not capable of MPLS forwarding but can still process SR-MPLS packets. In this scenario, the top label would be replaced by an IP tunnel toward that adjacent node and then forwarded over the corresponding link indicated by the Adjacency SID.

When to Use IP-Based Tunnels:

The description in the previous two sections is based on the assumption that an MPLS-over-UDP tunnel is used when the next hop towards the next segment is not MPLS enabled. However, even in the case where the next hop towards the next segment is MPLS capable, an MPLS-over-UDP tunnel towards the next segment could still be used instead due to local policies. For instance, in the example as described in , assume F is now a transit node capable of SR-MPLS while all the other assumptions remain unchanged; since F is not identified by a SID in the stack and an MPLS-over-UDP tunnel is preferred to an MPLS LSP according to local policies, router E replaces the current top label with an MPLS-over-UDP tunnel toward router G and sends it out. (Note that if an MPLS LSP was preferred, the packet would be forwarded as native SR-MPLS.)

IP Header Fields:

When encapsulating an MPLS packet in UDP, the resulting packet is further encapsulated in IP for transmission. IPv4 or IPv6 may be used according to the capabilities of the network. The address fields are set as described in . The other IP header fields (such as the ECN field , the Differentiated Services Code Point (DSCP) , or IPv6 Flow Label) on each UDP-encapsulated segment SHOULD be configurable according to the operator's policy; they may be copied from the header of the incoming packet; they may be promoted from the header of the payload packet; they may be set according to instructions programmed to be associated with the SID; or they may be configured dependent on the outgoing interface and payload. The TTL field setting in the encapsulating packet header is handled as described in , which refers to .

Entropy and ECMP:

When encapsulating an MPLS packet with an IP tunnel header that is capable of encoding entropy (such as ), the corresponding entropy field (the source port in the case of a UDP tunnel) MAY be filled with an entropy value that is generated by the encapsulator to uniquely identify a flow. However, what constitutes a flow is locally determined by the encapsulator. For instance, if the MPLS label stack contains at least one entropy label and the encapsulator is capable of reading that entropy label, the entropy label value could be directly copied to the source port of the UDP header. Otherwise, the encapsulator may have to perform a hash on the whole label stack or the five-tuple of the SR-MPLS payload if the payload is determined as an IP packet. To avoid recalculating the hash or hunting for the entropy label each time the packet is encapsulated in a UDP tunnel, it MAY be desirable that the entropy value contained in the incoming packet (i.e., the UDP source port value) is retained when stripping the UDP header and is reused as the entropy value of the outgoing packet.

Congestion Considerations:

provides a detailed analysis of the implications of congestion in MPLS-over-UDP systems and builds on , which describes the congestion implications of UDP tunnels. All of those considerations apply to SR-MPLS-over-UDP tunnels as described in this document. In particular, it should be noted that the traffic carried in SR-MPLS flows is likely to be IP traffic.

IANA Considerations This document has no IANA actions.

Security Considerations The security consideration of (which redirects the reader to ) and apply. DTLS SHOULD be used where security is needed on an SR-MPLS-over-UDP segment including when the IP segment crosses the public Internet or some other untrusted environment. provides security considerations for Segment Routing, and is particularly applicable to SR-MPLS. It is difficult for an attacker to pass a raw MPLS-encoded packet into a network, and operators have considerable experience in excluding such packets at the network boundaries, for example, by excluding all packets that are revealed to be carrying an MPLS packet as the payload of IP tunnels. Further discussion of MPLS security is found in . It is easy for a network ingress node to detect any attempt to smuggle an IP packet into the network since it would see that the UDP destination port was set to MPLS, and such filtering SHOULD be applied. If, however, the mechanisms described in or are applied, a wider variety of UDP port numbers might be in use making port filtering harder. SR packets not having a destination address terminating in the network would be transparently carried and would pose no different security risk to the network under consideration than any other traffic. Where control-plane techniques are used (as described in ), it is important that these protocols are adequately secured for the environment in which they are run as discussed in and .

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies the architecture for Multiprotocol Label Switching (MPLS). [STANDARDS-TRACK] This document specifies the encoding to be used by an LSR in order to transmit labeled packets on Point-to-Point Protocol (PPP) data links, on LAN data links, and possibly on other data links as well. This document also specifies rules and procedures for processing the various fields of the label stack encoding. [STANDARDS-TRACK] Various applications of MPLS make use of label stacks with multiple entries. In some cases, it is possible to replace the top label of the stack with an IP-based encapsulation, thereby enabling the application to run over networks that do not have MPLS enabled in their core routers. This document specifies two IP-based encapsulations: MPLS-in-IP and MPLS-in-GRE (Generic Routing Encapsulation). Each of these is applicable in some circumstances. [STANDARDS-TRACK] The functionality provided by IPv6's Type 0 Routing Header can be exploited in order to achieve traffic amplification over a remote path for the purposes of generating denial-of-service traffic. This document updates the IPv6 specification to deprecate the use of IPv6 Type 0 Routing Headers, in light of this security concern. [STANDARDS-TRACK] This document redefines how the explicit congestion notification (ECN) field of the IP header should be constructed on entry to and exit from any IP-in-IP tunnel. On encapsulation, it updates RFC 3168 to bring all IP-in-IP tunnels (v4 or v6) into line with RFC 4301 IPsec ECN processing. On decapsulation, it updates both RFC 3168 and RFC 4301 to add new behaviours for previously unused combinations of inner and outer headers. The new rules ensure the ECN field is correctly propagated across a tunnel whether it is used to signal one or two severity levels of congestion; whereas before, only one severity level was supported. Tunnel endpoints can be updated in any order without affecting pre-existing uses of the ECN field, thus ensuring backward compatibility. Nonetheless, operators wanting to support two severity levels (e.g., for pre-congestion notification -- PCN) can require compliance with this new specification. A thorough analysis of the reasoning for these changes and the implications is included. In the unlikely event that the new rules do not meet a specific need, RFC 4774 gives guidance on designing alternate ECN semantics, and this document extends that to include tunnelling issues. [STANDARDS-TRACK] This document specifies version 1.2 of the Datagram Transport Layer Security (DTLS) protocol. The DTLS protocol provides communications privacy for datagram protocols. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. The DTLS protocol is based on the Transport Layer Security (TLS) protocol and provides equivalent security guarantees. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document updates DTLS 1.0 to work with TLS version 1.2. [STANDARDS-TRACK] This document specifies an IP-based encapsulation for MPLS, called MPLS-in-UDP for situations where UDP (User Datagram Protocol) encapsulation is preferred to direct use of MPLS, e.g., to enable UDP-based ECMP (Equal-Cost Multipath) or link aggregation. The MPLS- in-UDP encapsulation technology must only be deployed within a single network (with a single network operator) or networks of an adjacent set of cooperating network operators where traffic is managed to avoid congestion, rather than over the Internet where congestion control is required. Usage restrictions apply to MPLS-in-UDP usage for traffic that is not congestion controlled and to UDP zero checksum usage with IPv6. OSPFv2 requires functional extension beyond what can readily be done with the fixed-format Link State Advertisements (LSAs) as described in RFC 2328. This document defines OSPFv2 Opaque LSAs based on Type-Length-Value (TLV) tuples that can be used to associate additional attributes with prefixes or links. Depending on the application, these prefixes and links may or may not be advertised in the fixed-format LSAs. The OSPFv2 Opaque LSAs are optional and fully backward compatible. This document introduces new sub-TLVs to support advertisement of IPv4 and IPv6 prefix attribute flags and the source router ID of the router that originated a prefix advertisement. This document defines a new optional Intermediate System to Intermediate System (IS-IS) TLV named CAPABILITY, formed of multiple sub-TLVs, which allows a router to announce its capabilities within an IS-IS level or the entire routing domain. This document obsoletes RFC 4971. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Segment Routing (SR) leverages the source routing paradigm. A node steers a packet through an ordered list of instructions, called "segments". A segment can represent any instruction, topological or service based. A segment can have a semantic local to an SR node or global within an SR domain. SR provides a mechanism that allows a flow to be restricted to a specific topological path, while maintaining per-flow state only at the ingress node(s) to the SR domain. SR can be directly applied to the MPLS architecture with no change to the forwarding plane. A segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels. The segment to process is on the top of the stack. Upon completion of a segment, the related label is popped from the stack. SR can be applied to the IPv6 architecture, with a new type of routing header. A segment is encoded as an IPv6 address. An ordered list of segments is encoded as an ordered list of IPv6 addresses in the routing header. The active segment is indicated by the Destination Address (DA) of the packet. The next active segment is indicated by a pointer in the new routing header. Informative References Data centers are critical components of the infrastructure used by network operators to provide services to their customers. Data centers are attached to the Internet or a backbone network by gateway routers. One data center typically has more than one gateway for commercial, load balancing, and resiliency reasons. Segment Routing is a popular protocol mechanism for use within a data center, but also for steering traffic that flows between two data center sites. In order that one data center site may load balance the traffic it sends to another data center site, it needs to know the complete set of gateway routers at the remote data center, the points of connection from those gateways to the backbone network, and the connectivity across the backbone network. Segment Routing may also be operated in other domains, such as access networks. Those domains also need to be connected across backbone networks through gateways. This document defines a mechanism using the BGP Tunnel Encapsulation attribute to allow each gateway router to advertise the routes to the prefixes in the Segment Routing domains to which it provides access, and also to advertise on behalf of each other gateway to the same Segment Routing domain. Work in Progress Segment Routing can be applied to the IPv6 data plane using a new type of Routing Extension Header called the Segment Routing Header. This document describes the Segment Routing Header and how it is used by Segment Routing capable nodes. Work in Progress Some networks use tunnels for a variety of reasons. A large variety of tunnel types are defined and the ingress needs to select a type of tunnel which is supported by the egress. This document defines how to advertise egress tunnel capabilities in IS-IS Router Capability TLV. Work in Progress Networks use tunnels for a variety of reasons. A large variety of tunnel types are defined and the tunnel encapsulator router needs to select a type of tunnel which is supported by the tunnel decapsulator router. This document defines how to advertise, in OSPF Router Information Link State Advertisement (LSAs), the list of tunnel encapsulations supported by the tunnel decapsulator. Work in Progress This document considers the interaction of Differentiated Services (diffserv) with IP tunnels of various forms. This memo provides information for the Internet community. This document provides a security framework for Multiprotocol Label Switching (MPLS) and Generalized Multiprotocol Label Switching (GMPLS) Networks. This document addresses the security aspects that are relevant in the context of MPLS and GMPLS. It describes the security threats, the related defensive techniques, and the mechanisms for detection and reporting. This document emphasizes RSVP-TE and LDP security considerations, as well as inter-AS and inter-provider security considerations for building and maintaining MPLS and GMPLS networks across different domains or different Service Providers. This document is not an Internet Standards Track specification; it is published for informational purposes. Load balancing is a powerful tool for engineering traffic across a network. This memo suggests ways of improving load balancing across MPLS networks using the concept of "entropy labels". It defines the concept, describes why entropy labels are useful, enumerates properties of entropy labels that allow maximal benefit, and shows how they can be signaled and used for various applications. This document updates RFCs 3031, 3107, 3209, and 5036. [STANDARDS-TRACK] Different routing protocols employ different mechanisms for securing protocol packets on the wire. While most already have some method for accomplishing cryptographic message authentication, in many cases the existing methods are dated, vulnerable to attack, and employ cryptographic algorithms that have been deprecated. The "Keying and Authentication for Routing Protocols" (KARP) effort aims to overhaul and improve these mechanisms. This document does not contain protocol specifications. Instead, it defines the areas where protocol specification work is needed. This document is a companion document to RFC 6518, "Keying and Authentication for Routing Protocols (KARP) Design Guidelines"; together they form the guidance and instruction KARP design teams will use to review and overhaul routing protocol transport security. The User Datagram Protocol (UDP) provides a minimal message-passing transport that has no inherent congestion control mechanisms. This document provides guidelines on the use of UDP for the designers of applications, tunnels, and other protocols that use UDP. Congestion control guidelines are a primary focus, but the document also provides guidance on other topics, including message sizes, reliability, checksums, middlebox traversal, the use of Explicit Congestion Notification (ECN), Differentiated Services Code Points (DSCPs), and ports. Because congestion control is critical to the stable operation of the Internet, applications and other protocols that choose to use UDP as an Internet transport must employ mechanisms to prevent congestion collapse and to establish some degree of fairness with concurrent traffic. They may also need to implement additional mechanisms, depending on how they use UDP. Some guidance is also applicable to the design of other protocols (e.g., protocols layered directly on IP or via IP-based tunnels), especially when these protocols do not themselves provide congestion control. This document obsoletes RFC 5405 and adds guidelines for multicast UDP usage. The Source Packet Routing in Networking (SPRING) architecture describes how Segment Routing can be used to steer packets through an IPv6 or MPLS network using the source routing paradigm. This document illustrates some use cases for Segment Routing in an IPv6-only environment.

Acknowledgements Thanks to Joel Halpern, Bruno Decraene, Loa Andersson, Ron Bonica, Eric Rosen, Jim Guichard, Gunter Van De Velde, Andy Malis, Robert Sparks, and Al Morton for their insightful comments on this document. Additional thanks to Mirja Kuehlewind, Alvaro Retana, Spencer Dawkins, Benjamin Kaduk, Martin Vigoureux, Suresh Krishnan, and Eric Vyncke for careful reviews and resulting comments.

Contributors Ahmed Bashandy Individual Email: abashandy.ietf@gmail.com Clarence Filsfils Cisco Email: cfilsfil@cisco.com John Drake Juniper Email: jdrake@juniper.net Shaowen Ma Mellanox Technologies Email: mashaowen@gmail.com Mach Chen Huawei Email: mach.chen@huawei.com Hamid Assarpour Broadcom Email:hamid.assarpour@broadcom.com Robert Raszuk Bloomberg LP Email: robert@raszuk.net Uma Chunduri Huawei Email: uma.chunduri@gmail.com Luis M. Contreras Telefonica I+D Email: luismiguel.contrerasmurillo@telefonica.com Luay Jalil Verizon Email: luay.jalil@verizon.com Gunter Van De Velde Nokia Email: gunter.van_de_velde@nokia.com Tal Mizrahi Marvell Email: talmi@marvell.com Jeff Tantsura Apstra, Inc. Email: jefftant.ietf@gmail.com

Authors' Addresses Alibaba, Inc

xiaohu.xxh@alibaba-inc.com

Futurewei Technologies

stewart.bryant@gmail.com

Old Dog Consulting

adrian@olddog.co.uk

Cisco

shassan@cisco.com

Nokia

wim.henderickx@nokia.com

Huawei

lizhenbin@huawei.com