Wed, 06/11/2008 - 15:19 by Damien Saucez • Categories:

The Farinacci et al. "LISP Alternative Topology (LISP-ALT)" draft proposes a mechanism to manage EID to RLOCs mapping in the LISP protocol.

The idea of LISP-ALT (ALT for Alternative Topology) propose to build an public Internet overlay to manage mapping in LISP.

The Locator/ID separation Protocol (LISP) proposes to separate Identifiers and locators. Endpoints are identified with EIDs (Endpoint IDentifiers) and routing between two ends is performed on RLOCs (Routing LOCators). LISP should limit the routing table size in the DFZ, permit renumbering, add TE capabilities and permit mobility without addresses changing.

Basically, an endpoint determines its destination EID and send packets as in today's Internet. When the packet arrives a source domain border router, it is encapsulated in a LISP packet. The source of this packet is a RLOC associated to the source domain (depends on the source EID) and the destination is a RLOC belonging to the destination domain (depends on the destination EID). Multiple RLOC can be assigned to a particular EID (EIDs can be prefixes while RLOCs are always addresses), it is up to the LISP engine to define which RLOC to use at time t for an EID.

The key (from my point of view) of LISP is the mapping system and how to manage mappings. LISP-ALT proposes to create an overlay above the Internet between LISP-ALT capable boxes. The topology is based on GRE tunnels between some of these boxes and eBGP sessions are used (on that logical topology) to build the mapping system hierarchy. When a packet has to be sent to an EID and the source domain does not have any mapping for that destination, the packet is sent to the destination EID through the overlay. The overlay is able to make communications between EIDs without passing through RLOCs with the help of BGP. The overlay is named LAT for LISP Alternate Topology.

When a LISP-ALT boxes receives a data message (the opposite of mapping messages) via the LAT, it sends a Map-Reply to the sender. The Map-Reply gives the mapping to RLOCs for the EID. As soon as the source domain has the mapping, it sends the packets through the Internet using the RLOCs.

In addition, it is possible for a LISP domain to send Map-Requests on the LAT, the destination replies with the Map-Reply corresponding to the request.

It is allow to send data packets on the LAT to avoid mapping resolution latency. Indeed, without that, if the mapping is unknown, packets cannot be sent to the destination (see comments later).

When thinking LISP-ALT, authors focused on the fact that the technology should limit as it maximum the modifications on the current infrastructure, this is why BGP and GRE are used.

This paper is related to our LISP researches (http://inl.info.ucl.ac.be/softwares/openlisp).

LISP-ALT draft at http://www.ietf.org/internet-drafts/draft-fuller-lisp-alt-02.txt
LISP draft at http://tools.ietf.org/html/draft-farinacci-lisp-07