OPSEC Working Group K. Sriram
Internet-Draft D. Montgomery
Updates: RFC3704 (if approved) USA NIST
Intended status: Best Current Practice J. Haas
Expires: January 9, 2020 Juniper Networks, Inc.
July 8, 2019
Enhanced Feasible-Path Unicast Reverse Path Filtering
draft-ietf-opsec-urpf-improvements-03
Abstract
This document identifies a need for improvement of the unicast
Reverse Path Filtering techniques (uRPF) (see BCP 84) for detection
and mitigation of source address spoofing (see BCP 38). The strict
uRPF is inflexible about directionality, the loose uRPF is oblivious
to directionality, and the current feasible-path uRPF attempts to
strike a balance between the two (see BCP 84). However, as shown in
this draft, the existing feasible-path uRPF still has shortcomings.
This document describes an enhanced feasible-path uRPF technique,
which aims to be more flexible (in a meaningful way) about
directionality than the feasible-path uRPF. It can potentially
alleviate ISPs' concerns about the possibility of disrupting service
for their customers, and encourage greater deployment of uRPF
techniques.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 9, 2020.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Review of Existing Source Address Validation Techniques . . . 4
2.1. SAV using Access Control List . . . . . . . . . . . . . . 4
2.2. SAV using Strict Unicast Reverse Path Filtering . . . . . 4
2.3. SAV using Feasible-Path Unicast Reverse Path Filtering . 5
2.4. SAV using Loose Unicast Reverse Path Filtering . . . . . 7
2.5. SAV using VRF Table . . . . . . . . . . . . . . . . . . . 7
3. SAV using Enhanced Feasible-Path uRPF . . . . . . . . . . . . 7
3.1. Description of the Method . . . . . . . . . . . . . . . . 7
3.1.1. Algorithm A: Enhanced Feasible-Path uRPF . . . . . . 9
3.2. Operational Recommendations . . . . . . . . . . . . . . . 10
3.3. A Challenging Scenario . . . . . . . . . . . . . . . . . 10
3.4. Algorithm B: Enhanced Feasible-Path uRPF with Additional
Flexibility Across Customer Cone . . . . . . . . . . . . 11
3.5. Augmenting RPF Lists with ROA and IRR Data . . . . . . . 12
3.6. Implementation and Operations Considerations . . . . . . 12
3.6.1. Impact on FIB Memory Size Requirement . . . . . . . . 12
3.6.2. Coping with BGP's Transient Behavior . . . . . . . . 14
3.7. Summary of Recommendations . . . . . . . . . . . . . . . 14
4. Security Considerations . . . . . . . . . . . . . . . . . . . 15
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Normative References . . . . . . . . . . . . . . . . . . 15
7.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
Source Address Validation (SAV) refers to the detection and
mitigation of source address spoofing [RFC2827]. This document
identifies a need for improvement of the unicast Reverse Path
Filtering (uRPF) techniques [RFC3704] for SAV. The strict uRPF is
inflexible about directionality (see [RFC3704] for definitions), the
loose uRPF is oblivious to directionality, and the current feasible-
path uRPF attempts to strike a balance between the two [RFC3704].
However, as shown in this draft, the existing feasible-path uRPF
still has shortcomings. Even with the feasible-path uRPF, ISPs are
often apprehensive that they may be dropping customers' data packets
with legitimate source addresses.
This document describes an enhanced feasible-path uRPF technique,
which aims to be more flexible (in a meaningful way) about
directionality than the feasible-path uRPF. It is based on the
principle that if BGP updates for multiple prefixes with the same
origin AS were received on different interfaces (at border routers),
then incoming data packets with source addresses in any of those
prefixes should be accepted on any of those interfaces (presented in
Section 3). For some challenging ISP-customer scenarios (see
Section 3.3), this document also describes a more relaxed version of
the enhanced feasible-path uRPF technique (presented in Section 3.4).
Implementation and operations considerations are discussed in
Section 3.6.
Definition of Reverse Path Filtering (RPF) list: The list of
permissible source address prefixes for incoming data packets on a
given interface.
Throughout this document, the routes under consideration are assumed
to have been vetted based on prefix filtering [RFC7454] and possibly
(in the future) origin validation [RFC6811].
The enhanced feasible-path uRPF methods described here are expected
to add greater operational robustness and efficacy to uRPF, while
minimizing ISPs' concerns about accidental service disruption for
their customers. It is expected that this will encourage more
deployment of uRPF to help realize its DDoS prevention benefits
network wide.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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2. Review of Existing Source Address Validation Techniques
There are various existing techniques for mitigation against DDoS
attacks with spoofed addresses [RFC2827] [RFC3704]. Source address
validation (SAV) is performed in network edge devices such as border
routers, Cable Modem Termination Systems (CMTS) [RFC4036], and Packet
Data Network (PDN) gateways in mobile networks [Firmin]. Ingress
Access Control List (ACL) and unicast Reverse Path Filtering (uRPF)
are techniques employed for implementing SAV [RFC2827] [RFC3704]
[ISOC].
2.1. SAV using Access Control List
Ingress/egress Access Control Lists (ACLs) are maintained which list
acceptable (or alternatively, unacceptable) prefixes for the source
addresses in the incoming/outgoing Internet Protocol (IP) packets.
Any packet with a source address that does not match the filter is
dropped. The ACLs for the ingress/egress filters need to be
maintained to keep them up to date. Updating the ACLs is an operator
driven manual process, and hence operationally difficult or
infeasible.
Typically, the egress ACLs in access aggregation devices (e.g. CMTS,
DSLAM) permit source addresses only from the address spaces
(prefixes) that are associated with the interface on which the
customer network is connected. Ingress ACLs are typically deployed
on border routers, and drop ingress packets when the source address
is spoofed (e.g., belongs to obviously disallowed prefix blocks, IANA
special-purpose prefixes [SPAR-v4][SPAR-v6], provider's own prefixes,
etc.).
2.2. SAV using Strict Unicast Reverse Path Filtering
Note: In the figures (scenarios) that follow, the following
terminology is used: "fails" means drops packets with legitimate
source addresses; "works (but not desirable)" means passes all
packets with legitimate source addresses but is oblivious to
directionality; "works best" means passes all packets with legitimate
source addresses with no (or minimal) compromise of directionality.
Further, the notation Pi[ASn ASm ...] denotes a BGP update with
prefix Pi and an AS_PATH as shown in the square brackets.
In the strict unicast Reverse Path Filtering (uRPF) method, an
ingress packet at border router is accepted only if the Forwarding
Information Base (FIB) contains a prefix that encompasses the source
address, and forwarding information for that prefix points back to
the interface over which the packet was received. In other words,
the reverse path for routing to the source address (if it were used
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as a destination address) should use the same interface over which
the packet was received. It is well known that this method has
limitations when networks are multi-homed, routes are not
symmetrically announced to all transit providers, and there is
asymmetric routing of data packets. Asymmetric routing occurs (see
Figure 1) when a customer AS announces one prefix (P1) to one transit
provider (ISP-a) and a different prefix (P2) to another transit
provider (ISP-b), but routes data packets with source addresses in
the second prefix (P2) to the first transit provider (ISP-a) or vice
versa.
+------------+ ---- P1[AS2 AS1] ---> +------------+
| AS2(ISP-a) | <----P2[AS3 AS1] ---- | AS3(ISP-b)|
+------------+ +------------+
/\ /\
\ /
\ /
\ /
P1[AS1]\ /P2[AS1]
\ /
+-----------------------+
| AS1(customer) |
+-----------------------+
P1, P2 (prefixes originated)
Consider data packets received at AS2
(1) from AS1 with source address in P2, or
(2) from AS3 that originated from AS1
with source address in P1:
* Strict uRPF fails
* Feasible-path uRPF fails
* Loose uRPF works (but not desirable)
* Enhanced Feasible-path uRPF works best
Figure 1: Scenario 1 for illustration of efficacy of uRPF schemes.
2.3. SAV using Feasible-Path Unicast Reverse Path Filtering
The feasible-path uRPF technique helps partially overcome the problem
identified with the strict uRPF in the multi-homing case. The
feasible-path uRPF is similar to the strict uRPF, but in addition to
inserting the best-path prefix, additional prefixes from alternative
announced routes are also included in the RPF list. This method
relies on either (a) announcements for the same prefixes (albeit some
may be prepended to effect lower preference) propagating to all
transit providers performing feasible-path uRPF checks, or (b)
announcement of an aggregate less specific prefix to all transit
providers while announcing more specific prefixes (covered by the
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less specific prefix) to different transit providers as needed for
traffic engineering. As an example, in the multi-homing scenario
(see Figure 2), if the customer AS announces routes for both prefixes
(P1, P2) to both transit providers (with suitable prepends if needed
for traffic engineering), then the feasible-path uRPF method works.
It should be mentioned that the feasible-path uRPF works in this
scenario only if customer routes are preferred at AS2 and AS3 over a
shorter non-customer route. However, the feasible-path uRPF method
has limitations as well. One form of limitation naturally occurs
when the recommendation (a) or (b) mentioned above regarding
propagation of prefixes is not followed. Another form of limitation
can be described as follows. In Scenario 2 (described above,
illustrated in Figure 2), it is possible that the second transit
provider (ISP-b or AS3) does not propagate the prepended route for
prefix P1 to the first transit provider (ISP-a or AS2). This is
because AS3's decision policy permits giving priority to a shorter
route to prefix P1 via a lateral peer (AS2) over a longer route
learned directly from the customer (AS1). In such a scenario, AS3
would not send any route announcement for prefix P1 to AS2 (over the
p2p link). Then a data packet with source address in prefix P1 that
originates from AS1 and traverses via AS3 to AS2 will get dropped at
AS2.
+------------+ routes for P1, P2 +-----------+
| AS2(ISP-a) |<-------------------->| AS3(ISP-b)|
+------------+ (p2p) +-----------+
/\ /\
\ /
P1[AS1]\ /P2[AS1]
\ /
P2[AS1 AS1 AS1]\ /P1[AS1 AS1 AS1]
\ /
+-----------------------+
| AS1(customer) |
+-----------------------+
P1, P2 (prefixes originated)
Consider data packets received at AS2 via AS3
that originated from AS1 and have source address in P1:
* Feasible-path uRPF works (if customer route to P1
is preferred at AS3 over shorter path)
* Feasible-path uRPF fails (if shorter path to P1
is preferred at AS3 over customer route)
* Loose uRPF works (but not desirable)
* Enhanced Feasible-path uRPF works best
Figure 2: Scenario 2 for illustration of efficacy of uRPF schemes.
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2.4. SAV using Loose Unicast Reverse Path Filtering
In the loose unicast Reverse Path Filtering (uRPF) method, an ingress
packet at the border router is accepted only if the FIB has one or
more prefixes that encompass the source address. That is, a packet
is dropped if no route exists in the FIB for the source address.
Loose uRPF sacrifices directionality. It only drops packets if the
spoofed address is unreachable in the current FIB (e.g., IANA
special-purpose prefixes [SPAR-v4][SPAR-v6], unallocated, allocated
but currently not routed).
2.5. SAV using VRF Table
The Virtual Routing and Forwarding (VRF) technology allows a router
to maintain multiple routing table instances, separate from the
global Routing Information Base (RIB) [Juniper][RFC4364]. External
BGP (eBGP) peering sessions send specific routes to be stored in a
dedicated VRF table. The uRPF process queries the VRF table (instead
of the FIB) for source address validation. A VRF table can be
dedicated per eBGP peer and used for uRPF for only that peer,
resulting in strict mode operation. For implementing loose uRPF on
an interface, the corresponding VRF table would be global, i.e.,
contains the same routes as in the FIB.
3. SAV using Enhanced Feasible-Path uRPF
3.1. Description of the Method
Enhanced feasible-path uRPF (EFP-uRPF) method adds greater
operational robustness and efficacy to existing uRPF methods
discussed in Section 2. That is because it avoids dropping
legitimate data packets and avoids compromising directionality. The
method is based on the principle that if BGP updates for multiple
prefixes with the same origin AS were received on different
interfaces (at border routers), then incoming data packets with
source addresses in any of those prefixes should be accepted on any
of those interfaces. The EFP-uRPF method can be best explained with
an example as follows:
Let us say, a border router of ISP-A has in its Adj-RIB-Ins [RFC4271]
the set of prefixes {Q1, Q2, Q3} each of which has AS-x as its origin
and AS-x is in ISP-A's customer cone. In this set, the border router
received the route for prefix Q1 over a customer facing interface,
while it learned the routes for prefixes Q2 and Q3 from a lateral
peer and an upstream transit provider, respectively. In this example
scenario, the enhanced feasible-path uRPF method requires Q1, Q2, and
Q3 be included in the RPF list for the customer interface under
consideration.
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Thus, the enhanced feasible-path uRPF (EFP-uRPF) method gathers
feasible paths for customer interfaces in a more precise way (as
compared to feasible-path uRPF) so that all legitimate packets are
accepted while the directionality property is not compromised.
The above described EFP-uRPF method is recommended to be applied on
customer interfaces. It can be extended to design the RPF lists for
lateral peer interfaces also. That is, the EFP-uRPF method can be
applied (and loose uRPF avoided) on lateral peer interfaces. That
will help avoid compromise of directionality for lateral peer
interfaces (which is inevitable with loose uRPF; see Section 2.4).
Looking back at Scenarios 1 and 2 (Figure 1 and Figure 2), the
enhanced feasible-path uRPF (EFP-uRPF) method works better than the
other uRPF methods. Scenario 3 (Figure 3) further illustrates the
enhanced feasible-path uRPF method with a more concrete example. In
this scenario, the focus is on operation of the feasible-path uRPF at
ISP4 (AS4). ISP4 learns a route for prefix P1 via a customer-to-
provider (C2P) interface from customer ISP2 (AS2). This route for P1
has origin AS1. ISP4 also learns a route for P2 via another C2P
interface from customer ISP3 (AS3). Additionally, AS4 learns a route
for P3 via a lateral peer-to-peer (p2p) interface from ISP5 (AS5).
Routes for all three prefixes have the same origin AS (i.e., AS1).
Using the enhanced feasible-path uRPF scheme, given the commonality
of the origin AS across the routes for P1, P2 and P3, AS4 includes
all of these prefixes to the RPF list for the customer interfaces
(from AS2 and AS3).
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+----------+ P3[AS5 AS1] +------------+
| AS4(ISP4)|<---------------| AS5(ISP5) |
+----------+ (p2p) +------------+
/\ /\ /\
/ \ /
P1[AS2 AS1]/ \P2[AS3 AS1] /
(C2P)/ \(C2P) /
/ \ /
+----------+ +----------+ /
| AS2(ISP2)| | AS3(ISP3)| /
+----------+ +----------+ /
/\ /\ /
\ / /
P1[AS1]\ /P2[AS1] /P3[AS1]
(C2P)\ /(C2P) /(C2P)
\ / /
+----------------+ /
| AS1(customer) |/
+----------------+
P1, P2, P3 (prefixes originated)
Consider that data packets (sourced from AS1)
may be received at AS4 with source address
in P1, P2 or P3 via any of the neighbors (AS2, AS3, AS5):
* Feasible-path uRPF fails
* Loose uRPF works (but not desirable)
* Enhanced Feasible-path uRPF works best
Figure 3: Scenario 3 for illustration of efficacy of uRPF schemes.
3.1.1. Algorithm A: Enhanced Feasible-Path uRPF
The underlying algorithm in the solution method described above can
be specified as follows (to be implemented in a transit AS):
1. Create the list of unique origin ASes considering only the routes
in the Adj-RIB-Ins of customer interfaces. Call it Set A = {AS1,
AS2, ..., ASn}.
2. Considering all routes in Adj-RIB-Ins for all interfaces
(customer, lateral peer, and transit provider), form the set of
unique prefixes that have a common origin AS1. Call it Set X1.
3. Include set X1 in Reverse Path Filter (RPF) list on all customer
interfaces on which one or more of the prefixes in set X1 were
received.
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4. Repeat Steps 2 and 3 for each of the remaining ASes in Set A
(i.e., for ASi, where i = 2, ..., n).
The above algorithm can be extended to apply EFP-uRPF method to
lateral peer interfaces also. However, it is left up to the operator
to decide whether they should apply EFP-uRPF or loose uRPF method on
lateral peer interfaces. The loose uRPF method is recommended to be
applied on transit provider interfaces.
3.2. Operational Recommendations
The following operational recommendations will make the operation of
the enhanced feasible-path uRPF robust:
For multi-homed stub AS:
o A multi-homed stub AS SHOULD announce at least one of the prefixes
it originates to each of its transit provider ASes. (It is
understood that a single-homed stub AS would announce all prefixes
it originates to its sole transit provider AS.)
For non-stub AS:
o A non-stub AS SHOULD also announce at least one of the prefixes it
originates to each of its transit provider ASes.
o Additionally, from the routes it has learned from customers, a
non-stub AS SHOULD announce at least one route per origin AS to
each of its transit provider ASes.
3.3. A Challenging Scenario
It should be observed that in the absence of ASes adhering to above
recommendations, the following example scenario may be constructed
which poses a challenge for the enhanced feasible-path uRPF (as well
as for traditional feasible-path uRPF). In the scenario illustrated
in Figure 4, since routes for neither P1 nor P2 are propagated on the
AS2-AS4 interface (due to the presence of NO_EXPORT Community), the
enhanced feasible-path uRPF at AS4 will reject data packets received
on that interface with source addresses in P1 or P2. (For a little
more complex example scenario see slide #10 in [sriram-urpf].)
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+----------+
| AS4(ISP4)|
+----------+
/\ /\
/ \ P1[AS3 AS1]
P1 and P2 not / \ P2[AS3 AS1]
propagated / \ (C2P)
(C2P) / \
+----------+ +----------+
| AS2(ISP2)| | AS3(ISP3)|
+----------+ +----------+
/\ /\
\ / P1[AS1]
P1[AS1] NO_EXPORT \ / P2[AS1]
P2[AS1] NO_EXPORT \ / (C2P)
(C2P) \ /
+----------------+
| AS1(customer) |
+----------------+
P1, P2 (prefixes originated)
Consider that data packets (sourced from AS1)
may be received at AS4 with source address
in P1 or P2 via AS2:
* Feasible-path uRPF fails
* Loose uRPF works (but not desirable)
* Enhanced Feasible-path uRPF with Algorithm A fails
* Enhanced Feasible-path uRPF with Algorithm B works best
Figure 4: Illustration of a challenging scenario.
3.4. Algorithm B: Enhanced Feasible-Path uRPF with Additional
Flexibility Across Customer Cone
Adding further flexibility to the enhanced feasible-path uRPF method
can help address the potential limitation identified above using the
scenario in Figure 4 (Section 3.3). In the following, "route" refers
to a route currently existing in the Adj-RIB-in. Including the
additional degree of flexibility, the modified algorithm (implemented
in a transit AS) can be described as follows (we call this Algorithm
B):
1. Create the set of all directly-connected customer interfaces.
Call it Set I = {I1, I2, ..., Ik}.
2. Create the set of all unique prefixes for which routes exist in
Adj-RIB-Ins for the interfaces in Set I. Call it Set P = {P1,
P2, ..., Pm}.
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3. Create the set of all unique origin ASes seen in the routes that
exist in Adj-RIB-Ins for the interfaces in Set I. Call it Set A
= {AS1, AS2, ..., ASn}.
4. Create the set of all unique prefixes for which routes exist in
Adj-RIB-Ins of all lateral peer and transit provider interfaces
such that each of the routes has its origin AS belonging in Set
A. Call it Set Q = {Q1, Q2, ..., Qj}.
5. Then, Set Z = Union(P,Q) is the RPF list that is applied for
every customer interface in Set I.
When Algorithm B (which is more flexible than Algorithm A) is
employed on customer interfaces, the type of limitation identified in
Figure 4 (Section 3.3) is overcome and the method works. The
directionality property is minimally compromised, but still the
proposed EFP-uRPF method with Algorithm B is a much better choice
(for the scenario under consideration) than applying the loose uRPF
method which is oblivious to directionality.
So, applying EFP-uRPF method with Algorithm B is recommended on
customer interfaces for the challenging scenarios such as those
described in Section 3.3. Further, it is recommended that loose uRPF
method for SAV should be applied on lateral peer and transit provider
interfaces.
3.5. Augmenting RPF Lists with ROA and IRR Data
It is worth emphasizing that an indirect part of the proposal in the
draft is that RPF filters may be augmented from secondary sources.
Hence, the construction of RPF lists using a method proposed in this
document (Algorithm A or B) can be augmented with data from Route
Origin Authorization (ROA) [RFC6482] as well as Internet Routing
Registry (IRR) data. Prefixes from registered ROAs and IRR route
objects that include ASes in an ISP's customer cone SHOULD be used to
augment the appropriate RPF lists. (Note: The ASes in a customer
cone are obtained in Step 3 of Algorithm B in Section 3.4.) This
will help make the RPF lists more robust about source addresses that
may be legitimately used by customers of the ISP.
3.6. Implementation and Operations Considerations
3.6.1. Impact on FIB Memory Size Requirement
The existing RPF checks in edge routers take advantage of existing
line card implementations to perform the RPF functions. For
implementation of the enhanced feasible-path uRPF, the general
necessary feature would be to extend the line cards to take arbitrary
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RPF lists that are not necessarily the same as the existing FIB
contents. In the algorithms (Section 3.1.1 and Section 3.4)
described here, the RPF lists are constructed by applying a set of
rules to all received BGP routes (not just those selected as best
path and installed in FIB). The concept of uRPF querying an RPF list
(instead of the FIB) is similar to uRPF querying a VRF table (see
(Section 2.5).
The techniques described in this document require that there should
be additional memory (i.e., TCAM) available to store the RPF lists in
line cards. For an ISP's AS, the RPF list size for each line card
will roughly and conservatively equal the total number of prefixes in
its customer cone (assuming Algorithm B in Section 3.4 is used).
(Note: Most ISP customer cone scenarios would not require Algorithm
B, but instead be served best by Algorithm A (see Section 3.1.1)
which requires much less FIB memory. This is because Algorithm B is
applicable for the less common scenarios such as Scenario 4 in
Figure 4 while Algorithm A is applicable for the more common
scenarios such as Scenario 3 in Figure 3.)
The following table shows the measured customer cone sizes for
various types of ISPs [sriram-ripe63]:
+---------------------------------+---------------------------------+
| Type of ISP | Measured Customer Cone Size in |
| | # Prefixes (in turn this is an |
| | estimate for RPF list size on |
| | line card) |
+---------------------------------+---------------------------------+
| Very Large Global ISP | 32392 |
| ------------------------------- | ------------------------------- |
| Very Large Global ISP | 29528 |
| ------------------------------- | ------------------------------- |
| Large Global ISP | 20038 |
| ------------------------------- | ------------------------------- |
| Mid-size Global ISP | 8661 |
| ------------------------------- | ------------------------------- |
| Regional ISP (in Asia) | 1101 |
+---------------------------------+---------------------------------+
Table 1: Customer cone sizes (# prefixes) for various types of ISPs.
For some super large global ISPs that are at the core of the
Internet, the customer cone size (# prefixes) can be as high as a few
hundred thousand [CAIDA]. But uRPF is most effective when deployed
at ASes at the edges of the Internet where the customer cone sizes
are smaller as shown in Table 1.
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A very large global ISP's router line card is likely to have a FIB
size large enough to accommodate 2 to 6 million routes [Cisco1].
Similarly, the line cards in routers corresponding to a large global
ISP, a mid-size global ISP, and a regional ISP are likely to have FIB
sizes large enough to accommodate about 1 million, 0.5 million, and
100K routes, respectively [Cisco2]. Comparing these FIB size numbers
with the corresponding RPF list size numbers in Table 1, it can be
surmised that the conservatively estimated RPF list size is only a
small fraction of the anticipated FIB memory size under relevant ISP
scenarios. What is meant here by relevant ISP scenarios is that only
smaller ISPs (and possibly some mid-size and regional ISPs) are
expected to implement the proposed EFP-uRPF method since it is most
effective closer to the edges of the Internet.
3.6.2. Coping with BGP's Transient Behavior
BGP routing announcements can exhibit transient behavior. Routes may
be withdrawn temporarily and then re-announced due to transient
conditions such as BGP session reset or link failure-recovery. To
cope with this, hysteresis should be introduced in the maintenance of
the RPF lists. Deleting entries from the RPF lists SHOULD be delayed
by a pre-determined amount (the value based on operational
experience) when responding to route withdrawals. This should help
suppress the effects due to the transients in BGP.
3.7. Summary of Recommendations
Depending on the scenario, an ISP or enterprise AS operator should
follow one of the following recommendations concerning uRPF/SAV:
1. For directly connected networks, i.e., subnets directly connected
to the AS and not multi-homed, the AS under consideration SHOULD
perform ACL-based source address validation (SAV).
2. For a directly connected single-homed stub AS (customer), the AS
under consideration SHOULD perform SAV based on the strict uRPF
method.
3. For all other scenarios:
* If the scenario does not involve complexity such as NO_EXPORT
of routes (see Section 3.3, Figure 4), then the enhanced
feasible-path uRPF method in Algorithm A (see Section 3.1.1)
SHOULD be applied on customer interfaces.
* Else, if the scenario involves the complexity then the
enhanced feasible-path uRPF method in Algorithm B (see
Section 3.4) SHOULD be applied on customer interfaces.
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* In general, loose uRPF method for SAV SHOULD be applied on
lateral peer and transit provider interfaces. However, for
lateral peer interfaces, in some cases an operator MAY apply
EFP-uRPF method with Algorithm A if they deem it suitable.
It is also recommended that prefixes from registered ROAs and IRR
route objects that include ASes in an ISP's customer cone SHOULD be
used to augment the appropriate RPF lists.
4. Security Considerations
The security considerations in BCP 38 [RFC2827] and BCP 84 [RFC3704]
apply for this document as well. In addition, AS operator should
apply the uRPF method that performs best (i.e., with zero or
insignificant possibility of dropping legitimate data packets) for
the type of peer (customer, transit provider, etc.) and the nature of
customer cone scenario that apply (see Section 3.1.1 and
Section 3.4).
5. IANA Considerations
This document does not request new capabilities or attributes. It
does not create any new IANA registries.
6. Acknowledgements
The authors would like to thank Sandy Murphy, Job Snijders, Marco
Marzetti, Marco d'Itri, Nick Hilliard, Gert Doering, Fred Baker, Igor
Gashinsky, Igor Lubashev, Andrei Robachevsky, Barry Greene, Amir
Herzberg, Ruediger Volk, Jared Mauch, Oliver Borchert, Mehmet
Adalier, and Joel Jaeggli for comments and suggestions.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <https://www.rfc-editor.org/info/rfc2827>.
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[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
2004, <https://www.rfc-editor.org/info/rfc3704>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
7.2. Informative References
[CAIDA] "Information for AS 174 (COGENT-174)", CAIDA Spoofer
Project , <https://spoofer.caida.org/as.php?asn=174>.
[Cisco1] "Internet Routing Table Growth Causes ROUTING-FIB-
4-RSRC_LOW Message on Trident-Based Line Cards", Cisco
Trouble-shooting Tech-notes , January 2014,
<https://www.cisco.com/c/en/us/support/docs/routers/asr-
9000-series-aggregation-services-routers/116999-problem-
line-card-00.html>.
[Cisco2] "Cisco Nexus 7000 Series NX-OS Unicast Routing
Configuration Guide, Release 5.x (Chapter 15: Managing the
Unicast RIB and FIB)", Cisco Configuration Guides , March
2018, <https://www.cisco.com/c/en/us/td/docs/switches/data
center/sw/5_x/nx-
os/unicast/configuration/guide/l3_cli_nxos/
l3_NewChange.html>.
[Firmin] Firmin, F., "The Evolved Packet Core", 3GPP The Mobile
Broadband Standard , <https://www.3gpp.org/technologies/
keywords-acronyms/100-the-evolved-packet-core>.
[ISOC] Vixie (Ed.), P., "Addressing the challenge of IP
spoofing", ISOC report , September 2015,
<https://www.internetsociety.org/resources/doc/2015/
addressing-the-challenge-of-ip-spoofing/>.
[Juniper] "Creating Unique VPN Routes Using VRF Tables", Juniper
Networks TechLibrary , March 2019,
<https://www.juniper.net/documentation/en_US/junos/topics/
topic-map/l3-vpns-routes-vrf-tables.html#id-understanding-
virtual-routing-and-forwarding-tables>.
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[RFC4036] Sawyer, W., "Management Information Base for Data Over
Cable Service Interface Specification (DOCSIS) Cable Modem
Termination Systems for Subscriber Management", RFC 4036,
DOI 10.17487/RFC4036, April 2005,
<https://www.rfc-editor.org/info/rfc4036>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.
[RFC6482] Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
Origin Authorizations (ROAs)", RFC 6482,
DOI 10.17487/RFC6482, February 2012,
<https://www.rfc-editor.org/info/rfc6482>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.
[RFC7454] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
February 2015, <https://www.rfc-editor.org/info/rfc7454>.
[SPAR-v4] "IANA IPv4 Special-Purpose Address Registry", IANA ,
<https://www.iana.org/assignments/iana-ipv4-special-
registry/iana-ipv4-special-registry.xhtml>.
[SPAR-v6] "IANA IPv6 Special-Purpose Address Registry", IANA ,
<https://www.iana.org/assignments/iana-ipv6-special-
registry/iana-ipv6-special-registry.xhtml>.
[sriram-ripe63]
Sriram, K. and R. Bush, "Estimating CPU Cost of BGPSEC on
a Router", Presented at RIPE-63; also, at IETF-83 SIDR WG
Meeting, March 2012,
<http://www.ietf.org/proceedings/83/slides/
slides-83-sidr-7.pdf>.
[sriram-urpf]
Sriram et al., K., "Enhanced Feasible-Path Unicast Reverse
Path Filtering", Presented at the OPSEC WG Meeting,
IETF-101 London , March 2018,
<https://datatracker.ietf.org/meeting/101/materials/
slides-101-opsec-draft-sriram-opsec-urpf-improvements-00>.
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Authors' Addresses
Kotikalapudi Sriram
USA National Institute of Standards and Technology
100 Bureau Drive
Gaithersburg MD 20899
USA
Email: ksriram@nist.gov
Doug Montgomery
USA National Institute of Standards and Technology
100 Bureau Drive
Gaithersburg MD 20899
USA
Email: dougm@nist.gov
Jeffrey Haas
Juniper Networks, Inc.
1133 Innovation Way
Sunnyvale CA 94089
USA
Email: jhaas@juniper.net
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