Several data plane mechanisms for determining RLOC reachability are currently defined. Please note that additional reachability mechanisms based on the control plane are defined in [RFC 9301
An ETR MAY examine the Locator-Status-Bits in the LISP header of an encapsulated data packet received from an ITR. If the ETR is also acting as an ITR and has traffic to return to the original ITR site, it can use this status information to help select an RLOC.
When an ETR receives an encapsulated packet from an ITR, the source RLOC from the outer header of the packet is likely to be reachable. Please note that in some scenarios the RLOC from the outer header can be a spoofable field.
An ITR/ETR pair can use the Echo-Noncing Locator reachability algorithms described in this section.
When determining Locator up/down reachability by examining the Locator-Status-Bits from the LISP-encapsulated data packet, an ETR will receive an up-to-date status from an encapsulating ITR about reachability for all ETRs at the site. CE-based ITRs at the source site can determine reachability relative to each other using the site IGP as follows:
Under normal circumstances, each ITR will advertise a default route into the site IGP.
If an ITR fails or if the upstream link to its Provider Edge fails, its default route will either time out or be withdrawn.
Each ITR can thus observe the presence or lack of a default route originated by the others to determine the Locator-Status-Bits it sets for them.
When ITRs at the site are not deployed in CE routers, the IGP can still be used to determine the reachability of Locators, provided they are injected into the IGP. This is typically done when a /32 address is configured on a loopback interface.
RLOCs listed in a Map-Reply are numbered with ordinals 0 to n-1. The Locator-Status-Bits in a LISP-encapsulated packet are numbered from 0 to n-1 starting with the least significant bit. For example, if an RLOC listed in the 3rd position of the Map-Reply goes down (ordinal value 2), then all ITRs at the site will clear the 3rd least significant bit (xxxx x0xx) of the 'Locator-Status-Bits' field for the packets they encapsulate.
When an xTR decides to use Locator-Status-Bits to affect reachability information, it acts as follows: ETRs decapsulating a packet will check for any change in the 'Locator-Status-Bits' field. When a bit goes from 1 to 0, the ETR, if also acting as an ITR, will refrain from encapsulating packets to an RLOC that is indicated as down. It will only resume using that RLOC if the corresponding Locator-Status-Bit returns to a value of 1. Locator-Status-Bits are associated with a Locator-Set per EID-Prefix. Therefore, when a Locator becomes unreachable, the Locator-Status-Bit that corresponds to that Locator's position in the list returned by the last Map-Reply will be set to zero for that particular EID-Prefix.
Locator-Status-Bits MUST NOT
be used over the public Internet and SHOULD
only be used in trusted and closed deployments. In addition, Locator-Status-Bits SHOULD
be coupled with Map-Versioning [RFC 9302
] to prevent race conditions where Locator-Status-Bits are interpreted as referring to different RLOCs than intended. Refer to Section 16
for security issues regarding this mechanism.
If an ITR encapsulates a packet to an ETR and the packet is received and decapsulated by the ETR, it is implied, but not confirmed by the ITR, that the ETR's RLOC is reachable. In most cases, the ETR can also reach the ITR but cannot assume this to be true, due to the possibility of path asymmetry. In the presence of unidirectional traffic flow from an ITR to an ETR, the ITR SHOULD NOT
use the lack of return traffic as an indication that the ETR is unreachable. Instead, it MUST
use an alternate mechanism to determine reachability.
The security considerations of Section 16
related to data plane reachability apply to the data plane RLOC reachability mechanisms described in this section.
When data flows bidirectionally between Locators from different sites, a data plane mechanism called "nonce echoing" can be used to determine reachability between an ITR and ETR. When an ITR wants to solicit a nonce echo, it sets the N- and E-bits and places a 24-bit nonce [RFC 4086
] in the LISP header of the next encapsulated data packet.
When this packet is received by the ETR, the encapsulated packet is forwarded as normal. When the ETR is an xTR (co-located as an ITR), it then sends a data packet to the ITR (when it is an xTR co-located as an ETR) and includes the nonce received earlier with the N-bit set and E-bit cleared. The ITR sees this "echoed nonce" and knows that the path to and from the ETR is up.
The ITR will set the E-bit and N-bit for every packet it sends while in the Echo-Nonce-request state. The time the ITR waits to process the echoed nonce before it determines that the path is unreachable is variable and is a choice left for the implementation.
If the ITR is receiving packets from the ETR but does not see the nonce echoed while being in the Echo-Nonce-request state, then the path to the ETR is unreachable. This decision MAY
be overridden by other Locator reachability algorithms. Once the ITR determines that the path to the ETR is down, it can switch to another Locator for that EID-Prefix.
Note that "ITR" and "ETR" are relative terms here. Both devices MUST
be implementing both ITR and ETR functionality for the Echo-Nonce mechanism to operate.
The ITR and ETR MAY
both go into the Echo-Nonce-request state at the same time. The number of packets sent or the time during which Echo-Nonce request packets are sent is an implementation-specific setting. In this case, an xTR receiving the Echo-Nonce request packets will suspend the Echo-Nonce state and set up an 'Echo-Nonce-request-state' timer. After the 'Echo-Nonce-request-state' timer expires, it will resume the Echo-Nonce state.
This mechanism does not completely solve the forward path reachability problem, as traffic may be unidirectional. That is, the ETR receiving traffic at a site MAY
not be the same device as an ITR that transmits traffic from that site, or the site-to-site traffic is unidirectional so there is no ITR returning traffic.
The Echo-Nonce algorithm is bilateral. That is, if one side sets the E-bit and the other side is not enabled for Echo-Noncing, then the echoing of the nonce does not occur and the requesting side may erroneously consider the Locator unreachable. An ITR SHOULD
set the E-bit in an encapsulated data packet when it knows the ETR is enabled for Echo-Noncing. This is conveyed by the E-bit in the Map-Reply message.
Many implementations default to not advertising that they are Echo-Nonce capable in Map-Reply messages, and so RLOC-Probing tends to be used for RLOC reachability.
The Echo-Nonce mechanism MUST NOT
be used over the public Internet and MUST
only be used in trusted and closed deployments. Refer to Section 16
for security issues regarding this mechanism.