After a number of exchanges with Robin and Noel, here is a revision of
LISP critique (sorry Noel I'm one day late -- it took much longer than
I thought to put things together and I may have missed some worthwhile
point due to word limit)
This is largely a revision from Noel's critiques, combined with
comments from Robin and myself. The focus is on LISP as a routing
scalability solution; I did not include any comments on the mobility
support part
Comments welcome!
Lixia
-----------
LISP Critique
LISP continues to go through changes based on lessons learned in
actual deployment. This critique is based on the (understanding of)
description at the time of this writing. This critique includes issues
from fundamental architectural limitations, potential problems that
require co-ordinated change, and new issues as the results of the
design; in addition it also includes a clarification of some basic
definitions.
First, two basic terms in LISP needs clarification: as state in LISP
drafts:
"LISP specifies an architecture and mechanism for replacing the
addresses currently used by IP with two separate name spaces:
Endpoint IDS (EIDs), used within sites, and Routing Locators
(RLOCs),
used on the transit networks that make up the Internet
infrastructure."
Thus "EID" in LISP is not a host identifier, but IP addresses used
within a site for packet delivery. Furthermore, an RLOC is not simply
any IP address reachable in the global default free zone; it is an
special address that binds to an attachment point of a site.
Regarding the architecture, LISP's most serious challenges are due to
the fact that it effectively divides today's routed IP address space
into two, edges and the core, which comes with all the challenges that
such a grand division brings; the list below attempts to capture the
major ones that have been identified (not in priority order).
The first question is whether, or how, one can draw a clear boundary
to sort existing networks into the core and edges. For example where
should one put those transit networks that do not provide global
connectivity (e.g. Internet2)? Do such networks belong to the core or
edges? Does the core represent a connected cloud (bar transient
failures)?
The second class of challenges arises from the fact that the
reachability to each edge destination is now a combined result of 3
major components: the mapping database that captures connectivity
between an edge site and its TRs, realtime status between an edge site
and its TRs, and the connectivity between ITR and ETR to encapsulate
packets over. In designing these components, three goals are often in
conflict: minimizing overhead, minimizing complexity, and maximizing
performance.
Because the mapping database will be very large in size, LISP lets
ITRs query mapping on demand, which brings up the question of how to
handle packets while ITR waiting for the mapping information. The
current decision (dropping packets) favors simplicity at the cost of
data performance. Would be feasible to buffer the packets? How deep
such a buffer could be? Such questions need future research.
Another issue arises from caching the mapping information: caching
improves the performance, but introduces the problem of detecting, and
replacing, outdated mappings. This is a very lengthy topic with many
subtly different failure modes, which cannot be covered here in any
detail.
Because of this caching effect, and the fact that the ETR to a
multihomed destination site is chosen at ITR, LISP design also faces
challenges of response to component failures. LISP cannot easily test
reachability of ultimate destinations (e.g. behind an ETR).
Regarding the mapping system, the ALT design has potential performance
and scaling issues (e.g. concentration of request load at the top-
level nodes); an interface has been built to allow replacement of the
mapping system. Another issue is potential identification (EIDs)
namespace provider lock-in, unless some mechanism can be worked out to
allow multiple competing providers to provide resolutions from EIDs to
ETRs (perhaps as part of a new mapping system). This last point
touches on an even more important issue: an ISP's performance
critically depends on the performance of the mapping system, a single
mapping system to serve all seems problematic.
Another class of issues relates to network management and operations.
- Although LISP does provide significant tools for multi-homing,
load-sharing, optimal-entry-selection, etc, these currently depend
on correct
configuration; response to component failures is also limited.
- LISP is currently working through NAT boxes, but only in limited
configurations. In particular, due to the use of fixed UDP ports,
it is not
currently possible to support more than one ETR behind a NAT box.
- Encapsulations of data packets increase the packet size and may lead
to PMTU problems.
Last but not least: One would not be able to see global routing table
size reduction unless/until LISP has been adopted by significant
number of networks. On the other hand, LISP is potentially a useful
tool in data centers where its one-level of indirection may help
significantly simplify the support for virtual servers.
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