SRUTI '05 Paper
[SRUTI '05 Technical Program]
Using Routing and Tunneling to Combat DoS Attacks
Abstract:
Thorough defense against DoS attacks is extremely difficult without
incurring significant changes to the Internet architecture. We
present a series of changes aimed at establishing protection
boundaries to reduce the effectiveness of most flooding DoS attacks
against servers. Only minimal and local changes are required to
current network architectures. We show that our scheme is highly
beneficial even if deployed at a single ISP, with additional benefits
arising from multiple-ISP deployment. Finally, we show that the
changes can be implemented with off-the-shelf components.
Denial-of-Service (DoS) attacks have become a serious threat to the
future potential of the Internet. If the Internet is to become
critical infrastructure, as advocated by Internet telephony and
digital convergence proponents, then it must be robust to attack.
This is not to say that it must be impossible to disrupt Internet
systems, but that the probability of persistent disruption must be
low. In practice, this means deterring attack through legal means,
preventing attack by removing attack vectors, or enabling effective
rapid response in face of attack. In reality, all three of these are
likely to be needed.
In previous work[3], we examined possible changes
to the Internet architecture aimed at increasing robustness. The goal
was deliberately radical: to restrict the modes of operation of the
network to those that are actually desired, greatly reducing
the potential attack surface. We proposed separating the IP address
space into ``client'' and ``server'' spaces, allowing
communication between the two spaces, but not within each space, and we
advocated path-based addresses for clients, rather than globally
unique addresses.
The potential benefits would be significant: source address spoofing
and reflection attacks would be eliminated, and worms would find it
much more difficult to spread. Without spoofed addresses, reactive
response mechanisms could be automated simply and safely.
Unfortunately the proposal required
IPv6 to encode path-based addresses, HIP[7] to provide a stable
end-system binding, and new inter-domain path-encoders to build path
addresses. Peer-to-peer applications presented additional complexity. Even
if there were global agreement on such an architectural change,
implementation could not happen overnight.
Such practical constraints lead us to wonder how many of these
potential benefits could be obtained without change to the
end-systems, and with only minimal and local changes to the routing
infrastructure.
The issue of local change is important. Altruism is rarely an
important guiding principle for ISPs, so any changes must provide
short-term benefits. More customers must be attracted, more money
must be received from existing customers, or
running costs must be lower if a change is to be made.
Changes to the client-side of the Internet are likely to require
very widespread deployment before benefits are seen by servers, and so
they are unlikely to be deployed. The question then is
whether there is a server-side-first deployment strategy that leads in
the long run to a more robust Internet, has benefits for early
adopters, and incremental benefits as more adopters come on board.
In this paper we will explore just such a strategy. The plan we will
sketch out does not have all the benefits of our original
architecture; there are, nonetheless, benefits at each step along
the path for the ISPs doing the deployment. Most importantly, the
solutions are still very general, so they do not trade off short-term
expediency against the future ability to evolve the applications
running on the Internet.
DoS attacks can target scarce resources at any layer in the stack, but
in this paper we only address flooding attacks that target the IP and
transport layers (e.g. TCP, UDP, DNS). As such we only present an IP
layer solution rather than using an application layer overlay like the one
presented by [5]. Attacks targeting
higher-layer functionality are still important, but robust
applications are of little use if data cannot reach them. Before
examining solutions though, it is important to step back and evaluate
why this is a hard problem.
There are a number of factors that complicate the picture. The
simplest DoS attack consists of one host flooding traffic to another.
If the recipient cannot keep up due to CPU, memory, network bandwidth,
or other resource starvation, then the attack is likely to be
successful. The obvious question is: why can't the recipient simply
request that the traffic stop? The reasons are many and varied, and
shed light on the nature of the problem.
The natural expectation is that an end-system could say to the
network ``don't send me any more traffic from X''. There are
essentially two technical issues: where in the network this would be
enforced, and how to ensure that this mechanism can only be used
legitimately.
In the existing Internet, there are not many places where such packet
filters could be installed. To be effective, filtering must be
installed upstream of any bottleneck on the path to the recipient, and
ideally should be close to the source of the attack. Unfortunately,
tracing back the path taken from the attacker to the victim is not
trivial, given that wide-area Internet paths are almost
always asymmetric. As a result, to automatically find an appropriate
place to install such a filter requires mechanisms that do not
currently exist. Even if such mechanisms did exist, it is far from
obvious what incentive the ISP at the appropriate place for a filter
would have for installing it, since an ISP close to the attacker is
unlikely to have a direct business relationship with the victim.
Finally there is the issue of authenticating that the request to
install such a filter really did come from the victim. No appropriate
authentication infrastructure currently exists, and without good
authentication, any process that automates the installation of filters
may be exploited as a DoS mechanism in its own right.
All these problems are made worse by Distributed
Denial-of-Service attacks (DDoS), where many hosts are
compromised and flood a victim in unison. With DDoS it
becomes very important that any filters are as far upstream as
possible. The sheer number of filters that the victim
might need to be installed could also be a problem with existing
hardware.
Any automated filter mechanism that installs filters for a specific
{source, destination} pair of IP addresses will be useless against
an attacker that spoofs the source address of packets. Not only can
the attacker simply change address to avoid existing filters, but he
might be able to spoof a legitimate machine's source address in
``attack'' packets, with the goal of causing a filter to be installed
that prevents legitimate communication.
Recent data[2] from security vendors indicates that source
address spoofing is rarely used today in DDoS attacks since it is not
needed for them to be effective. Botnets are relatively easy to create, and since there is no
automatic way to silence compromised hosts, there is little incentive
to spoof. In fact, just enough edge-sites employ ingress filtering on
outgoing traffic that a botnet actually has slightly greater firepower
if address spoofing is not used. However if an automatic
filtering mechanism were deployed, the prevalence of address spoofing
would no doubt increase significantly. At present, an interesting
vicious cycle exists:
- Attacks do not source
address spoof because there is no automated filtering mechanism.
- There is relatively little deployment of ingress filtering,
because attacks do not source address spoof.
- There is no automated filtering mechanism because attacks could
source-address spoof to bypass it.
One way to break such a cycle would be to deploy a mechanism to
effectively defend against non-spoofed DoS attacks, while not being
vulnerable to abuse by spoofed traffic. Such a mechanism would
not by itself defend against spoofed traffic, but it would shift
the balance. There would be more incentive for ingress filtering,
although this seems unlikely to ever be ubiquitous. Most importantly,
it opens the door for additional mechanisms to be designed that can
detect the difference between spoofed and non-spoofed traffic.
In this paper we sketch out one such solution. It is not intended to
be a panacea, but rather to raise the bar somewhat, and to do so with
off-the-shelf technologies.
There are two important points in defending against DoS: the point
of detection of an attack, and the point of control where
the effects of an attack can be mitigated.
The process of attack detection is not simple. Some attacks such
as SYN-flooding are easy to detect, at least at the recipient. Other
attacks such as making a large number of HTTP requests to a web server
are harder, since they require knowledge of what normal traffic looks
like, and the purpose normally served by that traffic. Detection,
however, is not the focus of this paper. Products from vendors such as Arbor
Networks are available that can perform this sort of analysis, so we
will assume that a detection infrastructure exists at
important servers.
Our focus is on establishing points of control, once an attack
has been detected. The current Internet has no real concept of a
point of control, beyond what can be done using traditional firewalls
and routing protocols. Generally firewalls are too close to the
victim to be useful against DoS, and destination-based
routing does not provide sufficient discrimination of traffic flows.
The requirement is for a victim to be able to
communicate with a point of control upstream of local bottleneck
links, and to be able to request instantiation of filters
that prevent hostile traffic reaching its subnet. Such filters must
be installed quickly, without human
intervention. In addition, the point of control must be able to
validate the filter request, so filters cannot be
installed maliciously by third parties.
None of this is terribly radical, and what remains is engineering
rather than science. How can the victim rapidly and reliably discover
the existence of a point of control on the path from the
attacker? How can it do this when there are many
thousands of attackers? How can the point of control validate the
filter request?
Our solution answers these questions through segmentation of the address
space, careful control of routing information, and
encapsulation. These are the three key IP-level building
blocks we have to work with in the Internet today.
Ideally we would like to protect all Internet hosts, but realistically
it is usually servers that present the biggest target. We propose,
consequently, to provide a protected virtual net for those servers
that wish to be better defended. An ISP providing such protection
could charge a premium fee for the service, giving a clear incentive
for deployment. In [10] the author presents a
similar approach that for small DDoS attacks in the context
of a single ISP. Our solution is intended to extend to a network of
many collaborating ISPs, but in this section we first examine how our
solution might initially be deployed at a single ISP, before examining
options how multiple ISPs might cooperate.
The first step is to designate certain subnets of the IP address
space as server subnets: these will receive
additional protection from attack. We refer to these subnets
collectively as the server-net; conceptually they are
within a protection boundary ringed by control points.
Traffic from the public Internet must traverse one of these
control points on its way into the server-net.
A condition of being a server-net host is not being permitted to
send directly to other server-net hosts. This constraint
prevents hosts inside the server-net from attacking other
hosts inside the server-net, and prevents server-net hosts being
exploited as relays in reflection attacks on other server-net hosts.
It also helps slow the spread of worms within the server-net boundary.
The basic functions of a server-net boundary control point are encapsulation and filtering. At an encapsulator, packets
destined for a server are encapsulated IP-in-IP, and sent to a
decapsulator located in the server's co-location facility. An ISP
must have at least one encapsulator, but maximum benefit will be
gained with one encapsulator associated with each PoP or peering link.
The principal advantage of this architecture is that when a server is
attacked, the decapsulator knows precisely which encapsulators the
malicious traffic traversed. As a result, it can ask them to filter
traffic, stopping the attack some distance upstream of the victim. We
note that encapsulation isn't the only technique by which this could
be achieved. In particular, MPLS tunneling might also be used for
this purpose. However IP-in-IP encapsulation has advantages over
MPLS. First, the address of the encapsulator can be directly obtained
by the decapsulator, rather than needing additional mechanisms to
reverse map the MPLS labels, but perhaps more importantly it is much
harder to extend an MPLS solution inter-domain, which is our eventual
goal.
Causing incoming traffic, even traffic originating from within the local ISP,
to traverse an encapsulator requires careful control of routing.
Routes to the server-net subnets should only be advertised from the
encapsulators themselves, to ensure that there is no way to bypass
them and send directly to the servers. In addition, the decapsulator
addresses should be taken from the ISP's infrastructure address space,
which (according to best current practice) should never be advertised
outside of the ISP's own network. This prevents an attacker directly
flooding a decapsulator associated with a server.
Figure 1:
Scenarios for single ISP architecture.
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Possible communication paths within this architecture are illustrated
in Figure 1. Flows 1 and 2 show the typical
scenario with
packets from a client passing through an encapsulator, being tunneled
to a decapsulator, and finally arriving at the servers. Flow 3 is
client to client and is unaffected. Flow 4, from one protected server
to another, is disallowed to prevent reflection attacks and the spread
of worms. Finally, flow 5 shows a protected server choosing to perform
decapsulation itself.
Serverclient traffic could be sent via the reverse of the
incoming tunnel, or it could be forwarded natively. Either reverse
path for the traffic is feasible with the proposed architecture, it
should be an operation decision as to which is used. Tunneling has the
benefit that a smart encapsulator can view both directions of a flow,
allowing it to monitor and filter traffic
more intelligently. However, this requires forwarding state at the
decapsulator that is set up based on observed incoming traffic, and this
state might be vulnerable to DoS if source address spoofing is used.
One solution is for the server to switch dynamically from a native to
a tunneled reverse path once a connection is fully established and is
therefore known not to be spoofed. Traffic with a tunneled reverse path can
then be forwarded from encapsulator to decapsulator with higher
diffserv priority, which might lessen the effect of flooding attacks
that spoof source addresses.
The goal of a server-net when deployed at a single ISP is to allow
automated filtering of unwanted traffic at the ingress point of that
ISP. To perform such automated filtering requires a detection
infrastructure in place, located so that it can monitor traffic to the
server. Possible locations for this would be in the decapsulator, in
the server, or on the path between the two.
Once a flow has been identified as hostile, the decapsulator
needs to be informed, and it in turn informs the encapsulator, which
installs the appropriate filter. The use of infrastructure addresses
between decapsulator and encapsulator is the first line of defense
against subversion of the filtering capabilities, as it should simply
not be possible for normal Internet hosts to send filtering requests directly to an
encapsulator. Behind this first line of defense, the signaling
channel between the decapsulator and the encapsulator should
be secured. Simple nonce exchange may be sufficient to
protect against off-path attacks, given that a compromised router on
the path can already cause DoS. Public-key-based
solutions are, of course, also possible.
The benefits are clearly greatest for large ISPs hosting server farms
and running a large number of encapsulators. Such ISPs will typically
have many peering points with other large ISPs, and these peering
points will be both geographically and topologically distributed.
This traffic from a large distributed attack will be spread across
many encapsulators because it will be entering the network from many
neighboring ISPs, and so it will be stopped closer to its origin,
before it has aggregated to the point where it can cause serious
damage.
Smaller ISPs still benefit because their customers can control their
degree of exposure, but a large enough attack is likely to overwhelm
all incoming links. Nothing an ISP can do by itself will help
in such circumstances.
Figure 2:
Route propagation for single ISP server-net
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The benefits of a server-net increase as ISPs
co-operate. Extending the protection boundary of the
server-net to include the server-nets of co-operating ISPs moves the
control points nearer to the sources of the DoS traffic. As we extend
the protection boundary, distributed attacks become less concentrated
at any particular control point, since traffic from each attacking host
enters the server-net through its local encapsulator. Traffic that
would previously have traversed the peering link uncontrolled now
traverses the peering link encapsulated, within the server-net
control boundary.
The general idea is that traffic enters the server-net at the
server-net ISP closest to the traffic source, and is then tunneled
from that ISP's encapsulator directly to a decapsulator at the
destination ISP border. At the destination ISP, the traffic is
decapsulated and re-encapsulated to get it to the final decapsulator
near to the server. In principle it would be possible to tunnel
direct from the remote ISP to the server subnet, but this assumes a
degree of trust between ISPs that seems unlikely and unnecessary.
Traffic originating within an immediate neighbor ISP is forwarded
natively to the border of the destination ISP
where it is encapsulated as in the single ISP case.
To achieve DoS resistance for the server-net, we need careful control
over the propagation of routing information. We also need appropriate
filtering capability, a filter request channel, and sufficient
encapsulation capacity. We will discuss first the single
ISP scenario, then explore the options for extending this to multiple
ISPs, and finish by discussing feedback and encapsulation.
The basic requirements of routing to implement a server-net within a
single ISP are:
- Server-net addresses must only be advertised to the rest of the
Internet from the encapsulators.
- The encapsulators need to learn which subnets are in the
server-net space, and which decapsulator is associated with each.
- The addresses used by the encapsulator and
decapsulator must not be advertised to the outside world.
- A server-net subnet must not be advertised to server-net hosts
on other server-net subnets.
We believe these requirements can be satisfied by current
BGP[9] implementations on current router hardware.
The server-net could be manually configured, but
in a large ISP this will probably be unfeasible. There are many
ways to do this dynamically, but one possible solution is
illustrated in Figure 2 and elaborated below.
- The decapsulator associated with a server-net
subnet advertises that subnet into I-BGP. It
advertises its decapsulation address as the BGP nexthop router
address. The route is tagged with a special BGP server-net community. The route is also tagged with
the ``do not export'' community that the ISP normally uses to
indicate internal infrastructure routes that should not be exported to
peers.
- All border routers in the domain are already configured to not
propagate routes to their external peers that have been tagged with
the do not export community, so server-net routes will
not leak to the outside world.
- All encapsulators in the domain receive the server-net routes, and
match on the server-net community. They then remove the do not export
community and the server-net community from matching routes, and
re-advertise them to external peers with the nexthop rewritten to be
their own address. This will draw external traffic to the
encapsulators.
- The encapsulators also re-advertise any routes tagged with the
server-net community into their IGP routing. IGP routes are generally preferred over
I-BGP routes on the basis of administrative distance[1],
so this will cause traffic from clients within the ISP's domain to be
drawn to the encapsulators rather than directly to the decapsulators.
- The decapsulator routers also receive routes containing the server-net community that have been advertised by other
decapsulators. The decapsulator installs a black-hole route for these
subnets to prevent server-net to server-net communication.
Using routing in this way should satisfy our requirements.
It is likely that slightly simpler solutions are possible if we
can add BGP Path Attributes, but this is probably best
done in the light of deployment experience.
Figure 3:
Route propagation for multi-ISP server-net
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To protect its own server subnets, each ISP joining a
multi-ISP server-net first runs the mechanisms described for a single-ISP
server-net. To peer within a multi-ISP server-net, the following changes are
needed:
- Instead of an encapsulator
at the border of the ISP, as shown in figure 2, a
router with both encapsulation and decapsulation capability is used.
- The encap/decap router does not remove the
server-net and do not export communities when
transmitting the route to a cooperating neighboring
ISP.
If the number of server subnets in the multi-ISP server-net were small,
then the neighboring ISP can do exactly as described in the single ISP
solution. However, in a large server-net, the number of
server subnets is likely to exceed the number of routes that can be
safely redistributed into the IGP. In addition, even cooperating ISPs
are likely to be wary of providing another ISP with a way to inject routes
into their IGP. Thus we need to modify the mechanism a little, as
shown in figure 3.
On receipt at the neighboring ISP, the border router will match on the
server-net community, and add an additional server-net-transit community to these routes, distinguishing them from
locally originated server-net routes. Routes with this additional
community will not be redistributed into the IGP routing by
encapsulators.
The result is that traffic from clients within the neighboring
ISP's network (C2 in Figure 3) will reach the first
encapsulator in the destination ISP using native forwarding. On the
other hand, traffic from clients that would transit the neighboring ISP (C4
in Figure 3) will be encapsulated by the neighboring
ISP's encapsulator and tunneled to the decapsulator at the destination
ISP, before being immediately re-encapsulated and sent on to the
destination server subnet's decapsulator.
Where more than two ISPs peer within a server-net, routes
distributed onward to other server-net ISPs are sent with the
BGP nexthop unchanged. Thus traffic will only ever be
encapsulated and decapsulated twice:
- Encapsulated at the first encapsulator in the nearest server-net
domain to the client.
- Decapsulated and immediately re-encapsulated at the first
encapsulator in the destination ISP.
- Decapsulated at the destination subnet.
There is one issue remaining with regards to figure
3. There is no guarantee that traffic from clients
C3 will traverse encapsulator E2 on its way to R1, and hence be encapsulated.
To ensure this does happen requires controlling the inter-domain
distribution of routes for R1. In fact the requirement is that routes
for such decapsulators only transit between ISPs at the same peerings
that server-net routes transit. A very similar use of an
additional community tag can be made to preserve this congruence. The
principal difference is that such routes are never propagated outside
the server-net boundary.
IP-in-IP encapsulation is a standard feature on most routers. Juniper
Networks ship a hardware tunnel interface module[4]
capable of encapsulation at 10Gb/s that supports 8,000 tunnel virtual
interfaces; other vendors no doubt have similar products. Thus
current hardware is capable of performing fast tunneling to enough
destinations to satisfy even large server-nets.
Most backbone routers also support packet filtering capability. It
seems likely that they support sufficient filter rules to cope with
attacks on the scales currently seen. In a multi-ISP server-net, any
one attack is spread across many encapsulators, making it even harder
for an attacker to saturate the filtering capability.
A cheaper solution is to use PC hardware. 400 Mb/s
forwarding was possible in 2000[6] with minimum sized
packets, limited largely by the PCI bus. A modern PC with PCI-Express is
likely to be capable of in excess of 1 Gb/s with a
very large number of hashed filter rules.
No standard exists for automated pushback of filters, but one would
likely emerge if server-nets were widely deployed. In the meantime, a
signaling channel would have to work around what is currently
available.
Bro[8] can enable filter rules via the command line interface
of Cisco routers. This is clunky, but works. In a
similar manner, a server-net could use buddy-hosts co-located with
each encapsulator. A buddy host would validate that a filter request
came from the correct server-net decapsulator address by checking the
BGP routing table, and then performing a handshake to prevent
spoofing. It would install the filter rule in its local encapsulator
using the command line interface. In the long run, we expect routers would directly
support such a signaling channel.
The minimum filter granularity would likely be {source address,
destination prefix} to prevent the targeting of other hosts on a
victim's subnet. Also possible is {source prefix, destination
prefix} to avoid an attacker spoofing multiple hosts on the same
source subnet.
Our initial plans are to work with ISPs to evaluate the feasibility of
deploying a trial server-net in the wild. Based on this as well as any
problems experienced, we will investigate whether it is worth
standardizing additional BGP Path Attributes to simplify deployment.
A server-net neutralizes whole classes of attack, and attackers will
adapt. Attacks requiring a full connection will have to become more
subtle to avoid detection, and brute-force attacks will need to employ
spoofing. To provide full protection, a server-net clearly needs to
tackle spoofing; in the short term, servers might signal encapsulators
to prioritize bidirectional flows.
In the longer term, server-net control points might be enhanced to
validate source addresses. A stateful control point could perform a
nonce-exchange with a source before allowing packets downstream. More
specifically, in response to a data packet, a control point might send an ICMP nonce
packet back to the source. The source would then need to echo the
nonce in the data packet to allow the packet to pass and to
instantiate state. For IPv6, the nonce could be carried in a data
packet using a destination option. For IPv4, this is harder, as IP
options are virtually useless. However, the IPSEC authentication
header might be generalized into a form similar to the IPv6
destination option. Although such extensions are a long-term
proposal, there might be incentive for deployment: a server-net under
attack would give much higher priority to connection setup packets that were
re-sent with the correct nonce echo.
A server-net is a protected region of the Internet that can provide
upstream filtering capability for servers under
DoS attack. We have sketched out how to
build such a capability using current off-the-shelf router hardware
and software components. The scheme we presented provides a workable
solution using tunneling and careful control of routing to provide points of
control in the network; filters may then be installed at these points to
drop DoS traffic close to its source. Further work is needed to test
real deployment of the scheme at ISPs, to understand the economics of
deployment, and to limit source address spoofing.
This work is not intended to be a complete solution to DoS attacks in
the Internet. It only protects servers, and only those that do not
need to communicate frequently with other similarly-protected servers.
In its current form, it only protects against non-spoofed attacks.
However, this is still a significant improvement over unprotected
directly-connected servers, especially when defending against attacks
on higher-level functionality which cannot be spoofed because they
require a full TCP connection to be setup. More complete
solutions are likely to require significant architectural change to
the Internet, and so will take very much longer to come to consensus
and deploy.
- 1
-
Cisco Systems.
What is administrative distance?
https://www.cisco.com/warp/public/105/admin_distance.html.
- 2
-
Communications Innovation Institute.
Summary of the initial meeting of the DoS-resistant Internet
working group.
https://www.thecii.org/dos-resistant/meeting-1/summary.html, January
2005.
- 3
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Mark Handley and Adam Greenhalgh.
Steps towards a DoS-resistant Internet architecture.
In Workshop on Future Directions in Network Architecture (FDNA
2004). ACM SIGCOMM, September 2004.
- 4
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Juniper Networks.
Tunnel services PIC datasheet.
https://www.juniper.net/products/modules/tunnel_pic.html.
- 5
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A. Keromytis, V. Misra, and D. Rubenstein.
SOS: Secure Overlay Services.
In Proceedings of ACM SIGCOMM, August 2002.
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E. Kohler, Robert Morris, Benjie Chen, John Jannotti, and M. Frans Kaashoek.
The Click modular router.
ACM Trans. on Computer Systems, 18(3):263-297, August 2000.
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R. Moskowitz, P. Nikander, and P. Jokela.
Host Identity Protocol.
draft-moskowitz-hip-09.txt, work-in-progress, IETF, February 2004.
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Vern Paxson.
Bro: A system for detecting network intruders in real-time.
In Computer Networks, 31(23-24), pages 2435-2463, December
1999.
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Y. Rekther and T. Li.
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https://www.ietf.org/rfc/rfc1771.txt, March 1995.
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In Proceedings of the 9th USENIX Security Symposium, August
2000.
Footnotes
- ... I-BGP
- I-BGP: Internal BGP
- using BGP to carry routing information between routers within a domain.
- ... community
- A
community is a locally defined BGP routing tag. The
actual value of community to use can be locally decided by
the ISP.
- ... routing
- IGP:
Interior Gateway Protocol - the intra-domain routing within an ISP,
typically OSPF or IS-IS.
- ...
ISP
- This assumes that both ISPs use the same community values
to indicate the server-net and do-not-export. If not, it will have
to translate to the neighboring ISPs values.
Adam Greenhalgh
2005-05-24
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