ISP
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... traffic leaving an edge network and entering an ISP can be
limited to traffic it is legitimately sending, attacks ...
...
However, Ingress Access Lists are typically maintained manually; for
example, forgetting to have the list updated at the ISPs if the set
of prefixes changes (e.g., as a result of multihoming ...
... common at edge network interfaces to their ISP, it is in no sense
common between ISPs, which normally use asymmetrical "hot ...
... network interfaces to their ISP, it is in no sense
common between ISPs, which normally use asymmetrical "hot potato"
routing ...
... prefixes and some legitimate
prefixes are not being advertised or not being accepted by the ISP
under its policy, the effect is the same as ingress filtering using
...
... RPF
work better in the case of asymmetric or multihomed traffic. The ISP
assigns a better metric which is not propagated outside of the
router ...
... edge routers; in
particular, when applied to multihoming to different ISPs, this
assumption may fail.
...
... routers performing Feasible RPF checking. For example, this may
not hold e.g., in the case where a secondary ISP does not propagate
the BGP advertisement to the primary ISP ...
... ISP does not propagate
the BGP advertisement to the primary ISP e.g., due to route-maps or
other routing policies ...
... ingress
filtering mechanism.
Also, many ISPs use default routes for various purposes such as
collecting illegitimate traffic ...
... traffic they do not have a "real" route to, and
smaller ISPs may well purchase transit capabilities and use a default
route from a larger provider. At least some implementations of Loose
...
... RPF, this is useful in places where asymmetric routing is
found, such as on inter-ISP links. However, like Loose RPF, since it
...
... ingress filtering is not
applied only at the "last-mile" interface between the ISP and the end
user. It's perfectly fine, and recommended, to also perform ingress
filtering at the edges ...
... user. It's perfectly fine, and recommended, to also perform ingress
filtering at the edges of ISPs where appropriate, at the routers
connecting LANs ...
... not just between the first two parties. That is, if a user
negotiates a special ingress filtering arrangement with his ISP, he
should also ensure (or make sure the ISP ensures) that the same
...
... ingress filtering arrangement with his ISP, he
should also ensure (or make sure the ISP ensures) that the same
arrangements also apply to the ISP's upstream ...
... should also ensure (or make sure the ISP ensures) that the same
arrangements also apply to the ISP's upstream and peering links, if
...
... ingress filtering is being used there -- or will get used, at some
point in the future; similarly with the upstream ISPs and peers.
In consequence, manual models which do not automatically propagate
...
... ingress filtering
may not be readily apparent. The routers and other ISP
infrastructure are vulnerable to several kinds of attacks. The
...
... Ingress filtering on peering links, whether by ISPs or by end-sites,
is not really that much different from the more typical "downstream"
...
... network might:
o use two ISPs for backing up the Internet connectivity to ensure
robustness,
...
... o need several points of access to the Internet in places where no
one ISP offers service, or
...
...
5. Ensure, by BGP or by contract, that each ISP's ingress filter is
complete, as described in Section 4.2.
...
... edge networks only deliver traffic to their ISPs that
will in fact pass the ingress filter, as described in Section
...
...
The fourth and the fifth must be ensured in the upstream ISPs as
well, as described in Section 3.1.
...
... edge network and the ISP, since it loses the directionality of the
test. This argues in favor of either using a complete filter in the
...
... multihoming is being used for robustness or
to change routing from time to time depending on measured ISP
behavior, the simplest approach will be to ensure that its ISPs in
...
... change routing from time to time depending on measured ISP
behavior, the simplest approach will be to ensure that its ISPs in
fact carry its addresses in routing ...
... provider-independent prefixes and exchange routes
with its ISPs with BGP, to ensure that its prefix is carried upstream ...
... prefix is carried upstream
to the major transit ISPs. Of necessity, this implies that the edge
network ...
...
There are a number of techniques which make it easier to ensure the
ISP's ingress filter is complete. Feasible RPF and Strict RPF ...
... RPF with
operational techniques both work quite well for multihomed or
asymmetric scenarios between the ISP and an edge network.
...
... provider-based addressing and
whose ISPs implement ingress filters (which they should do), the
third option is to route ...
... edge network may choose to
connect to each of its ISPs through two or more different Points of
Presence (POPs), so that if one POP or line experiences an outage,
...
... POP or line experiences an outage,
another link to the same ISP can be used. Alternatively, a set of
tunnels could be configured instead of multiple connections ...
... tunnels could be configured instead of multiple connections to the
same ISP [4][5]. This way the edge routers ...
... edge routers are configured to first
inspect the source address of a packet destined to an ISP and shunt
it into the appropriate tunnel or interface ...
... it into the appropriate tunnel or interface toward the ISP.
If such a scenario is applied exhaustively, so that an exit router ...
... originating from any other prefix can be summarily discarded instead
of sending it to an ISP.
...
... the most bulletproof when done properly; typically, ingress access
lists are best fit between the edge and the ISP when the
configuration is not too dynamic if strict RPF is not an option,
...
... configuration is not too dynamic if strict RPF is not an option,
between ISPs if the number of used prefixes is low, or as an
additional layer ...
... edge network and the
ISP. In many cases, a simple strict RPF can be augmented by
operational procedures in the case of asymmetric traffic ...
... security properties of a more relaxed approach should
be carefully considered before applying it. Especially when applied
by an ISP towards an edge network, there don't seem to be many
...
... networks in particular.
It is important for ISPs to implement ingress filtering to prevent
spoofed addresses ...
... RPF or similar Strict RPF
techniques could almost always be applied between the ISP and
multi-homed edge networks ...
...
o Some form of ingress filtering is also reasonable between ISPs,
especially if the number of prefixes is low.
...