LISP Generic Protocol Extension

Introduction The LISP Data-Plane is defined in . It specifies an encapsulation format that carries IPv4 or IPv6 packets (henceforth jointly referred to as IP) in a LISP header and outer UDP/IP transport. The LISP Data-Plane header does not specify the protocol being encapsulated and therefore is currently limited to encapsulating only IP packet payloads. Other protocols, most notably Virtual eXtensible Local Area Network (VXLAN) (which defines a similar header format to LISP), are used to encapsulate Layer-2 (L2) protocols such as Ethernet. This document defines an extension for the LISP header, as defined in , to indicate the inner protocol, enabling the encapsulation of Ethernet, IP or any other desired protocol all the while ensuring compatibility with existing LISP deployments. A flag in the LISP header, called the P-bit, is used to signal the presence of the 8-bit Next Protocol field. The Next Protocol field, when present, uses 8 bits of the field that was allocated to the echo-noncing and map-versioning features in . Those two features are no longer available when the P-bit is used. However, appropriate LISP-GPE (LISP Generic Protocol Extension) shim headers can be defined to specify capabilities that are equivalent to echo-noncing and/or map-versioning. Since all of the reserved bits of the LISP Data-Plane header have been allocated, LISP-GPE can also be used to extend the LISP Data-Plane header by defining Next Protocol "shim" headers that implements new data plane functions not supported in the LISP header. For example, the use of the Group-Based Policy (GBP) header or of the In-situ Operations, Administration, and Maintenance (IOAM) header with LISP-GPE, can be considered an extension to add support in the Data-Plane for Group-Based Policy functionalities or IOAM metadata.

Conventions 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.

Definition of Terms This document uses terms already defined in .

LISP Header Without Protocol Extensions As described in , the LISP header has no protocol identifier that indicates the type of payload being carried. Because of this, LISP is limited to carrying IP payloads. The LISP header contains a series of flags (some defined, some reserved), a Nonce/Map-version field and an instance ID/Locator-status-bit field. The flags provide flexibility to define how the various fields are encoded. Notably, Flag bit 5 is the last reserved bit in the LISP header.

LISP Header

Generic Protocol Extension for LISP (LISP-GPE) This document defines two changes to the LISP header in order to support multi-protocol encapsulation: the introduction of the P-bit and the definition of a Next Protocol field. This document specifies the protocol behavior when the P-bit is set to 1, no changes are introduced when the P-bit is set to 0. The LISP-GPE header is shown in and described below.

LISP-GPE Header

P-Bit:

Flag bit 5 is defined as the Next Protocol bit. The P-bit is set to 1 to indicate the presence of the 8 bit Next Protocol field. If the P-bit is clear (0) the LISP header is bit-by-bit equivalent to the definition in . When the P-bit is set to 1, bits N, E, V, and bits 8-23 of the 'Nonce/Map-Version/Next Protocol' field MUST be set to zero on transmission and MUST be ignored on receipt. Features equivalent to those that were implemented with bits N,E and V in , such as echo-noncing and map-versioning, can be implemented by defining appropriate LISP-GPE shim headers. When the P-bit is set to 1, the LISP-GPE header is encoded as:

LISP-GPE with P-bit set to 1

Next Protocol:

When the P-bit is set to 1, the lower 8 bits of the first 32-bit word are used to carry a Next Protocol. This Next Protocol field contains the protocol of the encapsulated payload packet. This document defines the following Next Protocol values:

0x00 :: Reserved
0x01 :: IPv4
0x02 :: IPv6
0x03 :: Ethernet
0x04 :: Network Service Header (NSH)
0x05 to 0x7D:: Unassigned
0x7E, 0x7F:: Experimentation and testing
0x80 to 0xFD:: Unassigned (shim headers)
0xFE, 0xFF:: Experimentation and testing (shim headers)

The values are tracked in the IANA LISP-GPE Next Protocol Registry as described in .

Next protocol values 0x7E, 0x7F and 0xFE, 0xFF are assigned for experimentation and testing as per . Next protocol values from Ox80 to 0xFD are assigned to protocols encoded as generic "shim" headers. All shim protocols MUST use the header structure in , which includes a Next Protocol field. When shim headers are used with other protocols identified by next protocol values from 0x00 to 0x7F, all the shim headers MUST come first. Shim headers can be used to incrementally deploy new GPE features, keeping the processing of shim headers known to a given xTR implementation in the 'fast' path (typically an ASIC), while punting the processing of the remaining new GPE features to the 'slow' path. Shim protocols MUST have the first 32 bits defined as:

Shim Header Where:

Type:: This field identifies the different messages of this protocol.
Length:: The length, in 4-octet units, of this protocol message not including the first 4 octets.
Reserved:: The use of this field is reserved to the protocol defined in this message.
Next Protocol Field:: The next protocol field contains the protocol of the encapsulated payload. The values are tracked in the IANA LISP-GPE Next Protocol Registry as described in .

Implementation and Deployment Considerations

Applicability Statement LISP-GPE conforms, as an UDP-based encapsulation protocol, to the UDP usage guidelines as specified in . The applicability of these guidelines are dependent on the underlay IP network and the nature of the encapsulated payload. outlines two applicability scenarios for UDP applications, 1) general Internet and 2) controlled environment. The controlled environment means a single administrative domain or adjacent set of cooperating domains. A network in a controlled environment can be managed to operate under certain conditions whereas in general Internet this cannot be done. Hence requirements for a tunnel protocol operating under a controlled environment can be less restrictive than the requirements of general internet. LISP-GPE scope of applicability is the same set of use cases covered by for the LISP dataplane protocol. The common property of these use cases is a large set of cooperating entities seeking to communicate over the public Internet or other large underlay IP infrastructures, while keeping the addressing and topology of the cooperating entities separate from the underlay and Internet topology, routing, and addressing. LISP-GPE is meant to be deployed in network environments operated by a single operator or adjacent set of cooperating network operators that fits with the definition of controlled environments in . For the purpose of this document, a traffic-managed controlled environment (TMCE), outlined in , is defined as an IP network that is traffic-engineered and/or otherwise managed (e.g., via use of traffic rate limiters) to avoid congestion. Significant portions of text in this Section are based on . It is the responsibility of the network operators to ensure that the guidelines/requirements in this section are followed as applicable to their LISP-GPE deployments

Congestion Control Functionality LISP-GPE does not natively provide congestion control functionality and relies on the payload protocol traffic for congestion control. As such LISP-GPE MUST be used with congestion controlled traffic or within a network that is traffic managed to avoid congestion (TMCE). An operator of a traffic managed network (TMCE) may avoid congestion by careful provisioning of their networks, rate-limiting of user data traffic and traffic engineering according to path capacity. Keeping in mind the reccomendation above, new encapsulated payloads, when registered with LISP-GPE, MUST be accompained by a set of guidelines derived from . Such new protocols should be designed for explicit congestion signals to propagate consistently from lower layer protocols into IP. Then the IP internetwork layer can act as a portability layer to carry congestion notification from non-IP-aware congested nodes up to the transport layer (L4). By following the guidelines in , subnetwork designers can enable a layer-2 protocol to participate in congestion control without dropping packets via propagation of explicit congestion notification (ECN ) to receivers.

UDP Checksum For IP payloads, specifies how to handle UDP Checksums encouraging implementors to consider UDP checksum usage guidelines in when it is desirable to protect UDP and LISP headers against corruption. In order to provide integrity of LISP-GPE headers, options and payload, for example to avoid mis-delivery of payload to different tenant systems in case of data corruption, outer UDP checksum SHOULD be used with LISP-GPE when transported over IPv4. The UDP checksum provides a statistical guarantee that a payload was not corrupted in transit. These integrity checks are not strong from a coding or cryptographic perspective and are not designed to detect physical-layer errors or malicious modification of the datagram (see ). In deployments where such a risk exists, an operator SHOULD use additional data integrity mechanisms such as offered by IPSec. An operator MAY choose to disable UDP checksum and use zero checksum if LISP-GPE packet integrity is provided by other data integrity mechanisms such as IPsec or additional checksums or if one of the conditions in a, b, c are met.

UDP Zero Checksum Handling with IPv6 By default, UDP checksum MUST be used when LISP-GPE is transported over IPv6. A tunnel endpoint MAY be configured for use with zero UDP checksum if additional requirements described in this section are met. When LISP-GPE is used over IPv6, UDP checksum is used to protect IPv6 headers, UDP headers and LISP-GPE headers and payload from potential data corruption. As such by default LISP-GPE MUST use UDP checksum when transported over IPv6. An operator MAY choose to configure to operate with zero UDP checksum if operating in a traffic managed controlled environment as stated in if one of the following conditions are met:

It is known that the packet corruption is exceptionally unlikely (perhaps based on knowledge of equipment types in their underlay network) and the operator is willing to take a risk of undetected packet corruption
It is judged through observational measurements (perhaps through historic or current traffic flows that use non zero checksum) that the level of packet corruption is tolerably low and where the operator is willing to take the risk of undetected corruption
LISP-GPE payload is carrying applications that are tolerant of misdelivered or corrupted packets (perhaps through higher layer checksum validation and/or reliability through retransmission)

In addition LISP-GPE tunnel implementations using Zero UDP checksum MUST meet the following requirements:

Use of UDP checksum over IPv6 MUST be the default configuration for all LISP-GPE tunnels
If LISP-GPE is used with zero UDP checksum over IPv6 then such xTR implementation MUST meet all the requirements specified in and requirements 1 as specified in
The ETR that decapsulates the packet SHOULD check the source and destination IPv6 addresses are valid for the LISP-GPE tunnel that is configured to receive Zero UDP checksum and discard other packets for which such check fails
The ITR that encapsulates the packet MAY use different IPv6 source addresses for each LISP-GPE tunnel that uses Zero UDP checksum mode in order to strengthen the decapsulator's check of the IPv6 source address (i.e the same IPv6 source address is not to be used with more than one IPv6 destination address, irrespective of whether that destination address is a unicast or multicast address). When this is not possible, it is RECOMMENDED to use each source address for as few LISP-GPE tunnels that use zero UDP checksum as is feasible
Measures SHOULD be taken to prevent LISP-GPE traffic over IPv6 with zero UDP checksum from escaping into the general Internet. Examples of such measures include employing packet filters at the PETR and/or keeping logical or physical separation of LISP network from networks carrying General Internet

The above requirements do not change either the requirements specified in as modified by or the requirements specified in . The requirement to check the source IPv6 address in addition to the destination IPv6 address, plus the recommendation against reuse of source IPv6 addresses among LISP-GPE tunnels collectively provide some mitigation for the absence of UDP checksum coverage of the IPv6 header. A traffic-managed controlled environment that satisfies at least one of three conditions listed at the beginning of this section provides additional assurance.

DSCP, ECN, TTL, and 802.1Q When encapsulating IP (including over Ethernet) packets provides guidance for mapping DSCP between inner and outer IP headers. The Pipe model typically fits better Network virtualization. The DSCP value on the tunnel header is set based on a policy (which may be a fixed value, one based on the inner traffic class, or some other mechanism for grouping traffic). Some aspects of the Uniform model (which treats the inner and outer DSCP value as a single field by copying on ingress and egress) may also apply, such as the ability to remark the inner header on tunnel egress based on transit marking. However, the Uniform model is not conceptually consistent with network virtualization, which seeks to provide strong isolation between encapsulated traffic and the physical network. describes the mechanism for exposing ECN capabilities on IP tunnels and propagating congestion markers to the inner packets. This behavior MUST be followed for IP packets encapsulated in LISP-GPE. Though Uniform or Pipe models could be used for TTL (or Hop Limit in case of IPv6) handling when tunneling IP packets, Pipe model is more aligned with network virtualization. provides guidance on handling TTL between inner IP header and outer IP tunnels; this model is more aligned with the Pipe model and is recommended for use with LISP-GPE for network virtualization applications. When a LISP-GPE router performs Ethernet encapsulation, the inner 802.1Q 3-bit priority code point (PCP) field MAY be mapped from the encapsulated frame to the DSCP codepoint of the DS field defined in . When a LISP-GPE router performs Ethernet encapsulation, the inner header 802.1Q VLAN Identifier (VID) MAY be mapped to, or used to determine the LISP Instance IDentifier (IID) field. Refer to for consideration about the use of integrity protection for deployments, such as the public Internet, concerned with on-path attackers.

Backward Compatibility LISP-GPE uses the same UDP destination port (4341) allocated to LISP. When encapsulating IP packets to a non LISP-GPE capable router the P-bit MUST be set to 0. That is, the encapsulation format defined in this document MUST NOT be sent to a router that has not indicated that it supports this specification because such a router would ignore the P-bit (as described in ) and so would misinterpret the other LISP header fields possibly causing significant errors.

Detection of ETR Capabilities The discovery of xTR capabilities to support LISP-GPE is out of the scope of this document. Given that the applicability domain of LISP-GPE is a traffic-managed controlled environment, ITR/ETR (xTR) configuration mechanisms may be used for this purpose.

IANA Considerations

LISP-GPE Next Protocol Registry IANA is requested to set up a registry of LISP-GPE "Next Protocol". These are 8-bit values. Next Protocol values in the table below are defined in this document. New values are assigned under the Specification Required policy . The protocols that are being assigned values do not themselves need to be IETF standards track protocols.

Next Protocol	Description	Reference
0x0	Reserved	This Document
0x1	IPv4	This Document
0x2	IPv6	This Document
0x3	Ethernet	This Document
0x4	NSH	This Document
0x05..0x7D	Unassigned
0x7E..0x7F	Experimentation and testing	This Document
0x80..0xFD	Unassigned (shim headers)
0x8E..0x8F	Experimentation and testing (shim headers)	This Document

Security Considerations LISP-GPE security considerations are similar to the LISP security considerations and mitigation techniques documented in . LISP-GPE, as many encapsulations that use optional extensions, is subject to on-path adversaries that can make arbitrary modifications to the packet (including the P-Bit) to change or remove any part of the payload, or claim to encapsulate any protocol payload type. Typical integrity protection mechanisms (such as IPsec) SHOULD be used in combination with LISP-GPE by those protocol extensions that want to protect from on-path attackers. With LISP-GPE, issues such as data-plane spoofing, flooding, and traffic redirection may depend on the particular protocol payload encapsulated.

Acknowledgements and Contributors A special thank you goes to for his guidance and detailed review. Thanks to for the suggestion to assign codepoints for experimentations and testing. This Working Group (WG) document originated as draft-lewis-lisp-gpe; the following are its coauthors and contributors along with their respective affiliations at the time of WG adoption. The editor of this document would like to thank and recognize them and their contributions. These coauthors and contributors provided invaluable concepts and content for this document's creation. , Cisco Systems, Inc. Cisco Systems, Inc. , Cisco Systems, Inc. , Cisco Systems, Inc. , Cisco Systems, Inc. , Broadcom , Innovium