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[RRG] Mapping model discussion - push, pull, hybrids and notify
- To: Routing Research Group <email@example.com>
- Subject: [RRG] Mapping model discussion - push, pull, hybrids and notify
- From: Robin Whittle <firstname.lastname@example.org>
- Date: Tue, 11 Mar 2008 00:42:15 +1100
- Organization: First Principles
- User-agent: Thunderbird 22.214.171.124 (Windows/20080213)
As noted in a recent message ("Agenda") there is an item "Mapping
Model Discussion" for discussion on Tuesday:
I am not sure what this discussion will entail. Perhaps it will be
theoretical discussion of different ways of getting the mapping data
where it is needed, without pushing it everywhere - whilst avoiding
discussion of current proposals.
In case it is a discussion about push, pull, hybrids etc. here is
some stuff which might be of interest - my analysis of the mapping
distribution aspects of the major proposals.
LISP-NERD Pure push.
The entire mapping database is pushed, relatively slowly, to
every ITR. ITRs do this by initiating downloads of files from
some servers, such as by using HTTP.
(NERD also contemplates a hybrid of push and pull, where some
likely to be frequently used parts of the mapping database
are downloaded - pushed - and others are not. This means that
for some EIDs the ITR is a full database ITR and for others
it is a caching ITR, relying on some query server system, such
as ALT or CONS. But if at least some ITRs can get the full
database, why not make them, or servers near them into query
servers, so nearby partly caching ITRs can query them? See:
Major strengths: No delay of packets.
Major critique: Lots of mapping data to push and store at
every ITR. Also NERD's "push" system
is not very fast or efficient. For instance
unless there is some other elaboration,
a hundred ITRs in one are of the Net will each
be doing independent downloads, rather than
using some nearby staging point where the data
is fanned out, or getting the data one from
LISP-ALT Pure pull.
All ITRs are caching ITRs and use a global network to send
queries (typically in the form of traffic packets which will
be delivered to the correct ETR) to the authoritative query
servers, which are typically ETRs.
Major strengths: Can scale to any number of EIDs without any
extra burden on ITRs.
Major critiques: Initial packets will frequently be delayed due
to the time it takes to look up the mapping or
to deliver the packet (much the same thing)
over the global ALT network. Furthermore,
unless there is some elaboration, the path
through the ALT network's routers will not
be optimised for geography, and could involve
many long trips back and forth around the
TRRP Pure pull - but with two incomplete "notify" techniques
All ITRs are caching ITRs and use DNS to look up mapping. So
this is another global query server system.
The two approaches to pushing an alert about updated mapping
are incomplete. One involves sending messages to the querier,
but the querier may not be reachable, especially if the query
went via a caching DNS server. The other involves the ITR
signalling unreachability of the destination, which may prompt
the ITR to do a fresh mapping request. Also, there is an initial
packet delivery system (waypoint routers) which may not scale
well. See: messages 450, 475 etc.
Major strengths: Like ALT, can cope with arbitrary numbers of
EIDs/micronets without any extra burden on
Major critiques: Slowness of query server network due to global
nature and need for two lookups, at least.
Resolvable by converting the whole DNS server
system into locally replicated server farms
which answer the query directly. This somewhat
resembles the hybrid push-pull nature of APT
or Ivip. See message 497.
APT Hybrid push-pull (slow push) with local notify
Full mapping database (for the APT island) pushed to all Default
Mappers (DMs) which are full database ITRs and full database
query servers. Push is relatively slow, via BGP flooding.
The rest of the ITRs are caching ITRs and get their mapping
quickly from a DM, by sending a traffic packet to the DM,
which also tunnels it to the correct ETR. DM can notify
ITRs of changed mapping.
Major strengths: Most ITRs can be cheaper caching ITRs, but
the get mapping quickly and reliably, without
delaying packet delivery, thanks to the local
nature of the query servers they rely on.
Greatly reduces the number of places the full
mapping database needs to be pushed to, compared
to NERD's full push.
Major critiques: Since there is no unified global mapping system
the APT system starts off as individual islands,
which can't support end-user networks properly
if those networks use longer prefixes than the
/24 which is the practical limit in today's
IPv4 network. See messages 472, 485-87 etc.
(To be continued.)
Slow rate of push compared to NERD or Ivip.
Ivip Hybrid push-pull (fast push) with local notify.
Fast push to full database ITRs and query servers, located
wherever operators want them. Unified but decentralised
global push system enables charging per mapping change, which
is difficult or impossible to achieve with BGP, and could be
tricky to implement with APT.
Beyond the full database ITRs and query servers, ITRs are caching
- including caching ITRs in sending hosts. ITRs query the local
full database query servers directly, or via one or more levels
of caching query server. Query servers notify recent queriers of
mapping changes, using UDP packets, and expect acknowledgement.
Notify message carries the new mapping information - it is not
just cache invalidation. Notify is secured by using the nonce
from the initial query.
Major benefits: Like APT, but with greater flexibility,
enables operators to choose how far to push
the full database into their networks. Good
support for micronets down to /32.
The fast (~5 second) nature of the push system
means the mapping can be simple, only a single
ETR address. End-users do their own multihoming
monitoring and make their own decisions about
service restoration, so the system is modular
rather than monolithic. Also, as a result of
the single ETR mapping data, less mapping to
store and simpler functionality for ITRs and
ETRs, which don't need to do reachability stuff.
Fast push also enables the system to be used
for IPv4 and IPv6 mobility with generally
optimal path lengths, no upgrades for
correspondent hosts and few for the mobile
hosts. See Ivip-summary.pdf for full list
of benefits of fast push.
Major critiques: No explicit TE function in the ITR, so user
has to do it by splitting traffic over
multiple micronets, and mapping each micronet
to different ETRs.
Most folks seem to think fast push is difficult
or impossible, and are wary of any system which
pushes the full database anywhere. But it
looks feasible to me - and there have been
no detailed critiques of the db-fast-push ID at:
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