RFC 6830
The Locator/ID Separation Protocol (LISP)
6.2. Routing Locator Selection
Both the client-side and server-side may need control over the selection of RLOCs for conversations between them. This control is achieved by manipulating the 'Priority' and 'Weight' fields in EID-to-RLOC Map-Reply messages. Alternatively, RLOC information MAY be gleaned from received tunneled packets or EID-to-RLOC Map-Request messages. The following are different scenarios for choosing RLOCs and the controls that are available: o The server-side returns one RLOC. The client-side can only use one RLOC. The server-side has complete control of the selection. o The server-side returns a list of RLOCs where a subset of the list has the same best Priority. The client can only use the subset list according to the weighting assigned by the server-side. In this case, the server-side controls both the subset list and
load-splitting across its members. The client-side can use RLOCs
outside of the subset list if it determines that the subset list
is unreachable (unless RLOCs are set to a Priority of 255). Some
sharing of control exists: the server-side determines the
destination RLOC list and load distribution while the client-side
has the option of using alternatives to this list if RLOCs in the
list are unreachable.
o The server-side sets a Weight of 0 for the RLOC subset list. In
this case, the client-side can choose how the traffic load is
spread across the subset list. Control is shared by the server-
side determining the list and the client determining load
distribution. Again, the client can use alternative RLOCs if the
server-provided list of RLOCs is unreachable.
o Either side (more likely the server-side ETR) decides not to send
a Map-Request. For example, if the server-side ETR does not send
Map-Requests, it gleans RLOCs from the client-side ITR, giving the
client-side ITR responsibility for bidirectional RLOC reachability
and preferability. Server-side ETR gleaning of the client-side
ITR RLOC is done by caching the inner-header source EID and the
outer-header source RLOC of received packets. The client-side ITR
controls how traffic is returned and can alternate using an outer-
header source RLOC, which then can be added to the list the
server-side ETR uses to return traffic. Since no Priority or
Weights are provided using this method, the server-side ETR MUST
assume that each client-side ITR RLOC uses the same best Priority
with a Weight of zero. In addition, since EID-Prefix encoding
cannot be conveyed in data packets, the EID-to-RLOC Cache on
Tunnel Routers can grow to be very large.
o A "gleaned" Map-Cache entry, one learned from the source RLOC of a
received encapsulated packet, is only stored and used for a few
seconds, pending verification. Verification is performed by
sending a Map-Request to the source EID (the inner-header IP
source address) of the received encapsulated packet. A reply to
this "verifying Map-Request" is used to fully populate the
Map-Cache entry for the "gleaned" EID and is stored and used for
the time indicated from the 'TTL' field of a received Map-Reply.
When a verified Map-Cache entry is stored, data gleaning no longer
occurs for subsequent packets that have a source EID that matches
the EID-Prefix of the verified entry.
RLOCs that appear in EID-to-RLOC Map-Reply messages are assumed to be
reachable when the R-bit for the Locator record is set to 1. When
the R-bit is set to 0, an ITR or PITR MUST NOT encapsulate to the
RLOC. Neither the information contained in a Map-Reply nor that
stored in the mapping database system provides reachability
information for RLOCs. Note that reachability is not part of the mapping system and is determined using one or more of the Routing Locator reachability algorithms described in the next section.6.3. Routing Locator Reachability
Several mechanisms for determining RLOC reachability are currently defined: 1. 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. 2. An ITR may receive an ICMP Network Unreachable or Host Unreachable message for an RLOC it is using. This indicates that the RLOC is likely down. Note that trusting ICMP messages may not be desirable, but neither is ignoring them completely. Implementations are encouraged to follow current best practices in treating these conditions. 3. An ITR that participates in the global routing system can determine that an RLOC is down if no BGP Routing Information Base (RIB) route exists that matches the RLOC IP address. 4. An ITR may receive an ICMP Port Unreachable message from a destination host. This occurs if an ITR attempts to use interworking [RFC6832] and LISP-encapsulated data is sent to a non-LISP-capable site. 5. An ITR may receive a Map-Reply from an ETR in response to a previously sent Map-Request. The RLOC source of the Map-Reply is likely up, since the ETR was able to send the Map-Reply to the ITR. 6. When an ETR receives an encapsulated packet from an ITR, the source RLOC from the outer header of the packet is likely up. 7. An ITR/ETR pair can use the Locator reachability algorithms described in this section, namely Echo-Noncing or RLOC-Probing.
When determining Locator up/down reachability by examining the
Locator-Status-Bits from the LISP-encapsulated data packet, an ETR
will receive 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:
o Under normal circumstances, each ITR will advertise a default
route into the site IGP.
o If an ITR fails or if the upstream link to its PE 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.
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 ETR decapsulates a packet, it will check for any change in
the 'Locator-Status-Bits' field. When a bit goes from 1 to 0, the
ETR, if acting also 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.
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.
When ITRs receive ICMP Network Unreachable or Host Unreachable
messages as a method to determine unreachability, they will refrain
from using Locators that are described in Locator lists of
Map-Replies. However, using this approach is unreliable because many
network operators turn off generation of ICMP Destination Unreachable
messages.
If an ITR does receive an ICMP Network Unreachable or Host Unreachable message, it MAY originate its own ICMP Destination Unreachable message destined for the host that originated the data packet the ITR encapsulated. Also, BGP-enabled ITRs can unilaterally examine the RIB to see if a locator address from a Locator-Set in a mapping entry matches a prefix. If it does not find one and BGP is running in the Default- Free Zone (DFZ), it can decide to not use the Locator even though the Locator-Status-Bits indicate that the Locator is up. In this case, the path from the ITR to the ETR that is assigned the Locator is not available. More details are in [LOC-ID-ARCH]. Optionally, an ITR can send a Map-Request to a Locator, and if a Map-Reply is returned, reachability of the Locator has been determined. Obviously, sending such probes increases the number of control messages originated by Tunnel Routers for active flows, so Locators are assumed to be reachable when they are advertised. This assumption does create a dependency: Locator unreachability is detected by the receipt of ICMP Host Unreachable messages. When a Locator has been determined to be unreachable, it is not used for active traffic; this is the same as if it were listed in a Map-Reply with Priority 255. The ITR can test the reachability of the unreachable Locator by sending periodic Requests. Both Requests and Replies MUST be rate- limited. Locator reachability testing is never done with data packets, since that increases the risk of packet loss for end-to-end sessions. When an ETR decapsulates a packet, it knows that it is reachable from the encapsulating ITR because that is how the packet arrived. 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.6.3.1. Echo Nonce Algorithm
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 [RFC4086] 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 next sends a data packet to the ITR, it 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 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 requests are sent is an implementation-specific setting. However, when an ITR is in the echo-nonce-request state, it can echo the ETR's nonce in the next set of packets that it encapsulates and subsequently continue sending echo-nonce-request packets. 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 only 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. Note that other Locator reachability mechanisms are being researched and can be used to compliment or even override the echo nonce algorithm. See the next section for an example of control-plane probing.
6.3.2. RLOC-Probing Algorithm
RLOC-Probing is a method that an ITR or PITR can use to determine the reachability status of one or more Locators that it has cached in a Map-Cache entry. The probe-bit of the Map-Request and Map-Reply messages is used for RLOC-Probing. RLOC-Probing is done in the control plane on a timer basis, where an ITR or PITR will originate a Map-Request destined to a locator address from one of its own locator addresses. A Map-Request used as an RLOC-probe is NOT encapsulated and NOT sent to a Map-Server or to the mapping database system as one would when soliciting mapping data. The EID record encoded in the Map-Request is the EID-Prefix of the Map-Cache entry cached by the ITR or PITR. The ITR may include a mapping data record for its own database mapping information that contains the local EID-Prefixes and RLOCs for its site. RLOC-probes are sent periodically using a jittered timer interval. When an ETR receives a Map-Request message with the probe-bit set, it returns a Map-Reply with the probe-bit set. The source address of the Map-Reply is set according to the procedure described in Section 6.1.5. The Map-Reply SHOULD contain mapping data for the EID-Prefix contained in the Map-Request. This provides the opportunity for the ITR or PITR that sent the RLOC-probe to get mapping updates if there were changes to the ETR's database mapping entries. There are advantages and disadvantages of RLOC-Probing. The greatest benefit of RLOC-Probing is that it can handle many failure scenarios allowing the ITR to determine when the path to a specific Locator is reachable or has become unreachable, thus providing a robust mechanism for switching to using another Locator from the cached Locator. RLOC-Probing can also provide rough Round-Trip Time (RTT) estimates between a pair of Locators, which can be useful for network management purposes as well as for selecting low delay paths. The major disadvantage of RLOC-Probing is in the number of control messages required and the amount of bandwidth used to obtain those benefits, especially if the requirement for failure detection times is very small. Continued research and testing will attempt to characterize the tradeoffs of failure detection times versus message overhead.
6.4. EID Reachability within a LISP Site
A site may be multihomed using two or more ETRs. The hosts and infrastructure within a site will be addressed using one or more EID-Prefixes that are mapped to the RLOCs of the relevant ETRs in the mapping system. One possible failure mode is for an ETR to lose reachability to one or more of the EID-Prefixes within its own site. When this occurs when the ETR sends Map-Replies, it can clear the R-bit associated with its own Locator. And when the ETR is also an ITR, it can clear its Locator-Status-Bit in the encapsulation data header. It is recognized that there are no simple solutions to the site partitioning problem because it is hard to know which part of the EID-Prefix range is partitioned and which Locators can reach any sub-ranges of the EID-Prefixes. This problem is under investigation with the expectation that experiments will tell us more. Note that this is not a new problem introduced by the LISP architecture. The problem exists today when a multihomed site uses BGP to advertise its reachability upstream.6.5. Routing Locator Hashing
When an ETR provides an EID-to-RLOC mapping in a Map-Reply message to a requesting ITR, the Locator-Set for the EID-Prefix may contain different Priority values for each locator address. When more than one best Priority Locator exists, the ITR can decide how to load- share traffic against the corresponding Locators. The following hash algorithm may be used by an ITR to select a Locator for a packet destined to an EID for the EID-to-RLOC mapping: 1. Either a source and destination address hash or the traditional 5-tuple hash can be used. The traditional 5-tuple hash includes the source and destination addresses; source and destination TCP, UDP, or Stream Control Transmission Protocol (SCTP) port numbers; and the IP protocol number field or IPv6 next-protocol fields of a packet that a host originates from within a LISP site. When a packet is not a TCP, UDP, or SCTP packet, the source and destination addresses only from the header are used to compute the hash. 2. Take the hash value and divide it by the number of Locators stored in the Locator-Set for the EID-to-RLOC mapping. 3. The remainder will yield a value of 0 to "number of Locators minus 1". Use the remainder to select the Locator in the Locator-Set.
Note that when a packet is LISP encapsulated, the source port number in the outer UDP header needs to be set. Selecting a hashed value allows core routers that are attached to Link Aggregation Groups (LAGs) to load-split the encapsulated packets across member links of such LAGs. Otherwise, core routers would see a single flow, since packets have a source address of the ITR, for packets that are originated by different EIDs at the source site. A suggested setting for the source port number computed by an ITR is a 5-tuple hash function on the inner header, as described above. Many core router implementations use a 5-tuple hash to decide how to balance packet load across members of a LAG. The 5-tuple hash includes the source and destination addresses of the packet and the source and destination ports when the protocol number in the packet is TCP or UDP. For this reason, UDP encoding is used for LISP encapsulation.6.6. Changing the Contents of EID-to-RLOC Mappings
Since the LISP architecture uses a caching scheme to retrieve and store EID-to-RLOC mappings, the only way an ITR can get a more up-to- date mapping is to re-request the mapping. However, the ITRs do not know when the mappings change, and the ETRs do not keep track of which ITRs requested its mappings. For scalability reasons, we want to maintain this approach but need to provide a way for ETRs to change their mappings and inform the sites that are currently communicating with the ETR site using such mappings. When adding a new Locator record in lexicographic order to the end of a Locator-Set, it is easy to update mappings. We assume that new mappings will maintain the same Locator ordering as the old mapping but will just have new Locators appended to the end of the list. So, some ITRs can have a new mapping while other ITRs have only an old mapping that is used until they time out. When an ITR has only an old mapping but detects bits set in the Locator-Status-Bits that correspond to Locators beyond the list it has cached, it simply ignores them. However, this can only happen for locator addresses that are lexicographically greater than the locator addresses in the existing Locator-Set. When a Locator record is inserted in the middle of a Locator-Set, to maintain lexicographic order, the SMR procedure in Section 6.6.2 is used to inform ITRs and PITRs of the new Locator-Status-Bit mappings. When a Locator record is removed from a Locator-Set, ITRs that have the mapping cached will not use the removed Locator because the xTRs will set the Locator-Status-Bit to 0. So, even if the Locator is in the list, it will not be used. For new mapping requests, the xTRs
can set the Locator AFI to 0 (indicating an unspecified address), as well as setting the corresponding Locator-Status-Bit to 0. This forces ITRs with old or new mappings to avoid using the removed Locator. If many changes occur to a mapping over a long period of time, one will find empty record slots in the middle of the Locator-Set and new records appended to the Locator-Set. At some point, it would be useful to compact the Locator-Set so the Locator-Status-Bit settings can be efficiently packed. We propose here three approaches for Locator-Set compaction: one operational mechanism and two protocol mechanisms. The operational approach uses a clock sweep method. The protocol approaches use the concept of Solicit-Map-Requests and Map-Versioning.6.6.1. Clock Sweep
The clock sweep approach uses planning in advance and the use of count-down TTLs to time out mappings that have already been cached. The default setting for an EID-to-RLOC mapping TTL is 24 hours. So, there is a 24-hour window to time out old mappings. The following clock sweep procedure is used: 1. 24 hours before a mapping change is to take effect, a network administrator configures the ETRs at a site to start the clock sweep window. 2. During the clock sweep window, ETRs continue to send Map-Reply messages with the current (unchanged) mapping records. The TTL for these mappings is set to 1 hour. 3. 24 hours later, all previous cache entries will have timed out, and any active cache entries will time out within 1 hour. During this 1-hour window, the ETRs continue to send Map-Reply messages with the current (unchanged) mapping records with the TTL set to 1 minute. 4. At the end of the 1-hour window, the ETRs will send Map-Reply messages with the new (changed) mapping records. So, any active caches can get the new mapping contents right away if not cached, or in 1 minute if they had the mapping cached. The new mappings are cached with a TTL equal to the TTL in the Map-Reply.
6.6.2. Solicit-Map-Request (SMR)
Soliciting a Map-Request is a selective way for ETRs, at the site where mappings change, to control the rate they receive requests for Map-Reply messages. SMRs are also used to tell remote ITRs to update the mappings they have cached. Since the ETRs don't keep track of remote ITRs that have cached their mappings, they do not know which ITRs need to have their mappings updated. As a result, an ETR will solicit Map-Requests (called an SMR message) from those sites to which it has been sending encapsulated data for the last minute. In particular, an ETR will send an SMR to an ITR to which it has recently sent encapsulated data. An SMR message is simply a bit set in a Map-Request message. An ITR or PITR will send a Map-Request when they receive an SMR message. Both the SMR sender and the Map-Request responder MUST rate-limit these messages. Rate-limiting can be implemented as a global rate- limiter or one rate-limiter per SMR destination. The following procedure shows how an SMR exchange occurs when a site is doing Locator-Set compaction for an EID-to-RLOC mapping: 1. When the database mappings in an ETR change, the ETRs at the site begin to send Map-Requests with the SMR bit set for each Locator in each Map-Cache entry the ETR caches. 2. A remote ITR that receives the SMR message will schedule sending a Map-Request message to the source locator address of the SMR message or to the mapping database system. A newly allocated random nonce is selected, and the EID-Prefix used is the one copied from the SMR message. If the source Locator is the only Locator in the cached Locator-Set, the remote ITR SHOULD send a Map-Request to the database mapping system just in case the single Locator has changed and may no longer be reachable to accept the Map-Request. 3. The remote ITR MUST rate-limit the Map-Request until it gets a Map-Reply while continuing to use the cached mapping. When Map-Versioning as described in Section 6.6.3 is used, an SMR sender can detect if an ITR is using the most up-to-date database mapping. 4. The ETRs at the site with the changed mapping will reply to the Map-Request with a Map-Reply message that has a nonce from the SMR-invoked Map-Request. The Map-Reply messages SHOULD be rate- limited. This is important to avoid Map-Reply implosion.
5. The ETRs at the site with the changed mapping record the fact
that the site that sent the Map-Request has received the new
mapping data in the Map-Cache entry for the remote site so the
Locator-Status-Bits are reflective of the new mapping for packets
going to the remote site. The ETR then stops sending SMR
messages.
Experimentation is in progress to determine the appropriate rate-
limit parameters.
For security reasons, an ITR MUST NOT process unsolicited
Map-Replies. To avoid Map-Cache entry corruption by a third party, a
sender of an SMR-based Map-Request MUST be verified. If an ITR
receives an SMR-based Map-Request and the source is not in the
Locator-Set for the stored Map-Cache entry, then the responding
Map-Request MUST be sent with an EID destination to the mapping
database system. Since the mapping database system is a more secure
way to reach an authoritative ETR, it will deliver the Map-Request to
the authoritative source of the mapping data.
When an ITR receives an SMR-based Map-Request for which it does not
have a cached mapping for the EID in the SMR message, it MAY not send
an SMR-invoked Map-Request. This scenario can occur when an ETR
sends SMR messages to all Locators in the Locator-Set it has stored
in its map-cache but the remote ITRs that receive the SMR may not be
sending packets to the site. There is no point in updating the ITRs
until they need to send, in which case they will send Map-Requests to
obtain a Map-Cache entry.
6.6.3. Database Map-Versioning
When there is unidirectional packet flow between an ITR and ETR, and
the EID-to-RLOC mappings change on the ETR, it needs to inform the
ITR so encapsulation to a removed Locator can stop and can instead be
started to a new Locator in the Locator-Set.
An ETR, when it sends Map-Reply messages, conveys its own Map-Version
Number. This is known as the Destination Map-Version Number. ITRs
include the Destination Map-Version Number in packets they
encapsulate to the site. When an ETR decapsulates a packet and
detects that the Destination Map-Version Number is less than the
current version for its mapping, the SMR procedure described in
Section 6.6.2 occurs.
An ITR, when it encapsulates packets to ETRs, can convey its own Map-Version Number. This is known as the Source Map-Version Number. When an ETR decapsulates a packet and detects that the Source Map-Version Number is greater than the last Map-Version Number sent in a Map-Reply from the ITR's site, the ETR will send a Map-Request to one of the ETRs for the source site. A Map-Version Number is used as a sequence number per EID-Prefix, so values that are greater are considered to be more recent. A value of 0 for the Source Map-Version Number or the Destination Map-Version Number conveys no versioning information, and an ITR does no comparison with previously received Map-Version Numbers. A Map-Version Number can be included in Map-Register messages as well. This is a good way for the Map-Server to assure that all ETRs for a site registering to it will be synchronized according to Map-Version Number. See [RFC6834] for a more detailed analysis and description of Database Map-Versioning.7. Router Performance Considerations
LISP is designed to be very "hardware-based forwarding friendly". A few implementation techniques can be used to incrementally implement LISP: o When a tunnel-encapsulated packet is received by an ETR, the outer destination address may not be the address of the router. This makes it challenging for the control plane to get packets from the hardware. This may be mitigated by creating special Forwarding Information Base (FIB) entries for the EID-Prefixes of EIDs served by the ETR (those for which the router provides an RLOC translation). These FIB entries are marked with a flag indicating that control-plane processing should be performed. The forwarding logic of testing for particular IP protocol number values is not necessary. There are a few proven cases where no changes to existing deployed hardware were needed to support the LISP data- plane. o On an ITR, prepending a new IP header consists of adding more octets to a MAC rewrite string and prepending the string as part of the outgoing encapsulation procedure. Routers that support Generic Routing Encapsulation (GRE) tunneling [RFC2784] or 6to4 tunneling [RFC3056] may already support this action.
o A packet's source address or interface the packet was received on
can be used to select VRF (Virtual Routing/Forwarding). The VRF's
routing table can be used to find EID-to-RLOC mappings.
For performance issues related to map-cache management, see
Section 12.
8. Deployment Scenarios
This section will explore how and where ITRs and ETRs can be deployed
and will discuss the pros and cons of each deployment scenario. For
a more detailed deployment recommendation, refer to [LISP-DEPLOY].
There are two basic deployment tradeoffs to consider: centralized
versus distributed caches; and flat, Recursive, or Re-encapsulating
Tunneling. When deciding on centralized versus distributed caching,
the following issues should be considered:
o Are the Tunnel Routers spread out so that the caches are spread
across all the memories of each router? A centralized cache is
when an ITR keeps a cache for all the EIDs it is encapsulating to.
The packet takes a direct path to the destination Locator. A
distributed cache is when an ITR needs help from other
re-encapsulating routers because it does not store all the cache
entries for the EIDs it is encapsulating to. So, the packet takes
a path through re-encapsulating routers that have a different set
of cache entries.
o Should management "touch points" be minimized by only choosing a
few Tunnel Routers, just enough for redundancy?
o In general, using more ITRs doesn't increase management load,
since caches are built and stored dynamically. On the other hand,
using more ETRs does require more management, since EID-Prefix-to-
RLOC mappings need to be explicitly configured.
When deciding on flat, Recursive, or Re-encapsulating Tunneling, the
following issues should be considered:
o Flat tunneling implements a single tunnel between the source site
and destination site. This generally offers better paths between
sources and destinations with a single tunnel path.
o Recursive Tunneling is when tunneled traffic is again further
encapsulated in another tunnel, either to implement VPNs or to
perform Traffic Engineering. When doing VPN-based tunneling, the
site has some control, since the site is prepending a new tunnel
header. In the case of TE-based tunneling, the site may have
control if it is prepending a new tunnel header, but if the site's
ISP is doing the TE, then the site has no control. Recursive
Tunneling generally will result in suboptimal paths but with the
benefit of steering traffic to parts of the network that have more
resources available.
o The technique of re-encapsulation ensures that packets only
require one tunnel header. So, if a packet needs to be re-routed,
it is first decapsulated by the ETR and then re-encapsulated with
a new tunnel header using a new RLOC.
The next sub-sections will examine where Tunnel Routers can reside in
the network.
8.1. First-Hop/Last-Hop Tunnel Routers
By locating Tunnel Routers close to hosts, the EID-Prefix set is at
the granularity of an IP subnet. So, at the expense of more
EID-Prefix-to-RLOC sets for the site, the caches in each Tunnel
Router can remain relatively small. But caches always depend on the
number of non-aggregated EID destination flows active through these
Tunnel Routers.
With more Tunnel Routers doing encapsulation, the increase in control
traffic grows as well: since the EID granularity is greater, more
Map-Requests and Map-Replies are traveling between more routers.
The advantage of placing the caches and databases at these stub
routers is that the products deployed in this part of the network
have better price-memory ratios than their core router counterparts.
Memory is typically less expensive in these devices, and fewer routes
are stored (only IGP routes). These devices tend to have excess
capacity, both for forwarding and routing states.
LISP functionality can also be deployed in edge switches. These
devices generally have layer-2 ports facing hosts and layer-3 ports
facing the Internet. Spare capacity is also often available in these
devices.
8.2. Border/Edge Tunnel Routers
Using Customer Edge (CE) routers for tunnel endpoints allows the EID
space associated with a site to be reachable via a small set of RLOCs
assigned to the CE routers for that site. This is the default
behavior envisioned in the rest of this specification.
This offers the opposite benefit of the first-hop/last-hop Tunnel Router scenario: the number of mapping entries and network management touch points is reduced, allowing better scaling. One disadvantage is that fewer network resources are used to reach host endpoints, thereby centralizing the point-of-failure domain and creating network choke points at the CE router. Note that more than one CE router at a site can be configured with the same IP address. In this case, an RLOC is an anycast address. This allows resilience between the CE routers. That is, if a CE router fails, traffic is automatically routed to the other routers using the same anycast address. However, this comes with the disadvantage where the site cannot control the entrance point when the anycast route is advertised out from all border routers. Another disadvantage of using anycast Locators is the limited advertisement scope of /32 (or /128 for IPv6) routes.8.3. ISP Provider Edge (PE) Tunnel Routers
The use of ISP PE routers as tunnel endpoint routers is not the typical deployment scenario envisioned in this specification. This section attempts to capture some of the reasoning behind this preference for implementing LISP on CE routers. The use of ISP PE routers as tunnel endpoint routers gives an ISP, rather than a site, control over the location of the egress tunnel endpoints. That is, the ISP can decide whether the tunnel endpoints are in the destination site (in either CE routers or last-hop routers within a site) or at other PE edges. The advantage of this case is that two tunnel headers can be avoided. By having the PE be the first router on the path to encapsulate, it can choose a TE path first, and the ETR can decapsulate and re-encapsulate for a tunnel to the destination end site. An obvious disadvantage is that the end site has no control over where its packets flow or over the RLOCs used. Other disadvantages include difficulty in synchronizing path liveness updates between CE and PE routers. As mentioned in earlier sections, a combination of these scenarios is possible at the expense of extra packet header overhead; if both site and provider want control, then Recursive or Re-encapsulating Tunnels are used.
8.4. LISP Functionality with Conventional NATs
LISP routers can be deployed behind Network Address Translator (NAT) devices to provide the same set of packet services hosts have today when they are addressed out of private address space. It is important to note that a locator address in any LISP control message MUST be a globally routable address and therefore SHOULD NOT contain [RFC1918] addresses. If a LISP router is configured with private addresses, they MUST be used only in the outer IP header so the NAT device can translate properly. Otherwise, EID addresses MUST be translated before encapsulation is performed. Both NAT translation and LISP encapsulation functions could be co-located in the same device. More details on LISP address translation can be found in [RFC6832].8.5. Packets Egressing a LISP Site
When a LISP site is using two ITRs for redundancy, the failure of one ITR will likely shift outbound traffic to the second. This second ITR's cache may not be populated with the same EID-to-RLOC mapping entries as the first. If this second ITR does not have these mappings, traffic will be dropped while the mappings are retrieved from the mapping system. The retrieval of these messages may increase the load of requests being sent into the mapping system. Deployment and experimentation will determine whether this issue requires more attention.9. Traceroute Considerations
When a source host in a LISP site initiates a traceroute to a destination host in another LISP site, it is highly desirable for it to see the entire path. Since packets are encapsulated from the ITR to the ETR, the hop across the tunnel could be viewed as a single hop. However, LISP traceroute will provide the entire path so the user can see 3 distinct segments of the path from a source LISP host to a destination LISP host:
Segment 1 (in source LISP site based on EIDs):
source host ---> first hop ... next hop ---> ITR
Segment 2 (in the core network based on RLOCs):
ITR ---> next hop ... next hop ---> ETR
Segment 3 (in the destination LISP site based on EIDs):
ETR ---> next hop ... last hop ---> destination host
For segment 1 of the path, ICMP Time Exceeded messages are returned
in the normal manner as they are today. The ITR performs a TTL
decrement and tests for 0 before encapsulating. Therefore, the ITR's
hop is seen by the traceroute source as having an EID address (the
address of the site-facing interface).
For segment 2 of the path, ICMP Time Exceeded messages are returned
to the ITR because the TTL decrement to 0 is done on the outer
header, so the destinations of the ICMP messages are the ITR RLOC
address and the source RLOC address of the encapsulated traceroute
packet. The ITR looks inside of the ICMP payload to inspect the
traceroute source so it can return the ICMP message to the address of
the traceroute client and also retain the core router IP address in
the ICMP message. This is so the traceroute client can display the
core router address (the RLOC address) in the traceroute output. The
ETR returns its RLOC address and responds to the TTL decrement to 0,
as the previous core routers did.
For segment 3, the next-hop router downstream from the ETR will be
decrementing the TTL for the packet that was encapsulated, sent into
the core, decapsulated by the ETR, and forwarded because it isn't the
final destination. If the TTL is decremented to 0, any router on the
path to the destination of the traceroute, including the next-hop
router or destination, will send an ICMP Time Exceeded message to the
source EID of the traceroute client. The ICMP message will be
encapsulated by the local ITR and sent back to the ETR in the
originated traceroute source site, where the packet will be delivered
to the host.
9.1. IPv6 Traceroute
IPv6 traceroute follows the procedure described above, since the
entire traceroute data packet is included in the ICMP Time Exceeded
message payload. Therefore, only the ITR needs to pay special
attention to forwarding ICMP messages back to the traceroute source.
9.2. IPv4 Traceroute
For IPv4 traceroute, we cannot follow the above procedure, since IPv4 ICMP Time Exceeded messages only include the invoking IP header and 8 octets that follow the IP header. Therefore, when a core router sends an IPv4 Time Exceeded message to an ITR, all the ITR has in the ICMP payload is the encapsulated header it prepended, followed by a UDP header. The original invoking IP header, and therefore the identity of the traceroute source, is lost. The solution we propose to solve this problem is to cache traceroute IPv4 headers in the ITR and to match them up with corresponding IPv4 Time Exceeded messages received from core routers and the ETR. The ITR will use a circular buffer for caching the IPv4 and UDP headers of traceroute packets. It will select a 16-bit number as a key to find them later when the IPv4 Time Exceeded messages are received. When an ITR encapsulates an IPv4 traceroute packet, it will use the 16-bit number as the UDP source port in the encapsulating header. When the ICMP Time Exceeded message is returned to the ITR, the UDP header of the encapsulating header is present in the ICMP payload, thereby allowing the ITR to find the cached headers for the traceroute source. The ITR puts the cached headers in the payload and sends the ICMP Time Exceeded message to the traceroute source retaining the source address of the original ICMP Time Exceeded message (a core router or the ETR of the site of the traceroute destination). The signature of a traceroute packet comes in two forms. The first form is encoded as a UDP message where the destination port is inspected for a range of values. The second form is encoded as an ICMP message where the IP identification field is inspected for a well-known value.9.3. Traceroute Using Mixed Locators
When either an IPv4 traceroute or IPv6 traceroute is originated and the ITR encapsulates it in the other address family header, one cannot get all 3 segments of the traceroute. Segment 2 of the traceroute cannot be conveyed to the traceroute source, since it is expecting addresses from intermediate hops in the same address format for the type of traceroute it originated. Therefore, in this case, segment 2 will make the tunnel look like one hop. All the ITR has to do to make this work is to not copy the inner TTL to the outer, encapsulating header's TTL when a traceroute packet is encapsulated using an RLOC from a different address family. This will cause no TTL decrement to 0 to occur in core routers between the ITR and ETR.
10. Mobility Considerations
There are several kinds of mobility, of which only some might be of concern to LISP. Essentially, they are as follows.10.1. Site Mobility
A site wishes to change its attachment points to the Internet, and its LISP Tunnel Routers will have new RLOCs when it changes upstream providers. Changes in EID-to-RLOC mappings for sites are expected to be handled by configuration, outside of LISP.10.2. Slow Endpoint Mobility
An individual endpoint wishes to move but is not concerned about maintaining session continuity. Renumbering is involved. LISP can help with the issues surrounding renumbering [RFC4192] [LISA96] by decoupling the address space used by a site from the address spaces used by its ISPs [RFC4984].10.3. Fast Endpoint Mobility
Fast endpoint mobility occurs when an endpoint moves relatively rapidly, changing its IP-layer network attachment point. Maintenance of session continuity is a goal. This is where the Mobile IPv4 [RFC5944] and Mobile IPv6 [RFC6275] [RFC4866] mechanisms are used and primarily where interactions with LISP need to be explored. The problem is that as an endpoint moves, it may require changes to the mapping between its EID and a set of RLOCs for its new network location. When this is added to the overhead of Mobile IP binding updates, some packets might be delayed or dropped. In IPv4 mobility, when an endpoint is away from home, packets to it are encapsulated and forwarded via a home agent that resides in the home area the endpoint's address belongs to. The home agent will encapsulate and forward packets either directly to the endpoint or to a foreign agent that resides where the endpoint has moved to. Packets from the endpoint may be sent directly to the correspondent node, may be sent via the foreign agent, or may be reverse-tunneled back to the home agent for delivery to the mobile node. As the mobile node's EID or available RLOC changes, LISP EID-to-RLOC
mappings are required for communication between the mobile node and
the home agent, whether via the foreign agent or not. As a mobile
endpoint changes networks, up to three LISP mapping changes may be
required:
o The mobile node moves from an old location to a new visited
network location and notifies its home agent that it has done so.
The Mobile IPv4 control packets the mobile node sends pass through
one of the new visited network's ITRs, which needs an EID-to-RLOC
mapping for the home agent.
o The home agent might not have the EID-to-RLOC mappings for the
mobile node's "care-of" address or its foreign agent in the new
visited network, in which case it will need to acquire them.
o When packets are sent directly to the correspondent node, it may
be that no traffic has been sent from the new visited network to
the correspondent node's network, and the new visited network's
ITR will need to obtain an EID-to-RLOC mapping for the
correspondent node's site.
In addition, if the IPv4 endpoint is sending packets from the new
visited network using its original EID, then LISP will need to
perform a route-returnability check on the new EID-to-RLOC mapping
for that EID.
In IPv6 mobility, packets can flow directly between the mobile node
and the correspondent node in either direction. The mobile node uses
its "care-of" address (EID). In this case, the route-returnability
check would not be needed but one more LISP mapping lookup may be
required instead:
o As above, three mapping changes may be needed for the mobile node
to communicate with its home agent and to send packets to the
correspondent node.
o In addition, another mapping will be needed in the correspondent
node's ITR, in order for the correspondent node to send packets to
the mobile node's "care-of" address (EID) at the new network
location.
When both endpoints are mobile, the number of potential mapping
lookups increases accordingly.
As a mobile node moves, there are not only mobility state changes in
the mobile node, correspondent node, and home agent, but also state
changes in the ITRs and ETRs for at least some EID-Prefixes.
The goal is to support rapid adaptation, with little delay or packet loss for the entire system. Also, IP mobility can be modified to require fewer mapping changes. In order to increase overall system performance, there may be a need to reduce the optimization of one area in order to place fewer demands on another. In LISP, one possibility is to "glean" information. When a packet arrives, the ETR could examine the EID-to-RLOC mapping and use that mapping for all outgoing traffic to that EID. It can do this after performing a route-returnability check, to ensure that the new network location does have an internal route to that endpoint. However, this does not cover the case where an ITR (the node assigned the RLOC) at the mobile-node location has been compromised. Mobile IP packet exchange is designed for an environment in which all routing information is disseminated before packets can be forwarded. In order to allow the Internet to grow to support expected future use, we are moving to an environment where some information may have to be obtained after packets are in flight. Modifications to IP mobility should be considered in order to optimize the behavior of the overall system. Anything that decreases the number of new EID-to-RLOC mappings needed when a node moves, or maintains the validity of an EID-to-RLOC mapping for a longer time, is useful.10.4. Fast Network Mobility
In addition to endpoints, a network can be mobile, possibly changing xTRs. A "network" can be as small as a single router and as large as a whole site. This is different from site mobility in that it is fast and possibly short-lived, but different from endpoint mobility in that a whole prefix is changing RLOCs. However, the mechanisms are the same, and there is no new overhead in LISP. A map request for any endpoint will return a binding for the entire mobile prefix. If mobile networks become a more common occurrence, it may be useful to revisit the design of the mapping service and allow for dynamic updates of the database. The issue of interactions between mobility and LISP needs to be explored further. Specific improvements to the entire system will depend on the details of mapping mechanisms. Mapping mechanisms should be evaluated on how well they support session continuity for mobile nodes.
10.5. LISP Mobile Node Mobility
A mobile device can use the LISP infrastructure to achieve mobility by implementing the LISP encapsulation and decapsulation functions and acting as a simple ITR/ETR. By doing this, such a "LISP mobile node" can use topologically independent EID IP addresses that are not advertised into and do not impose a cost on the global routing system. These EIDs are maintained at the edges of the mapping system (in LISP Map-Servers and Map-Resolvers) and are provided on demand to only the correspondents of the LISP mobile node. Refer to [LISP-MN] for more details.
(page 64 continued on part 4)
