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Internet Engineering Task Force (IETF) K. Sriram
Request for Comments: 8704 D. Montgomery
BCP: 84 USA NIST
Updates: 3704 J. Haas
Category: Best Current Practice Juniper Networks, Inc.
ISSN: 2070-1721 February 2020
Enhanced Feasible-Path Unicast Reverse Path Forwarding
Abstract
This document identifies a need for and proposes improvement of the
unicast Reverse Path Forwarding (uRPF) techniques (see RFC 3704) for
detection and mitigation of source address spoofing (see BCP 38).
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 RFC 3704).
However, as shown in this document, the existing feasible-path uRPF
still has shortcomings. This document describes enhanced feasible-
path uRPF (EFP-uRPF) techniques that are more flexible (in a
meaningful way) about directionality than the feasible-path uRPF (RFC
3704). The proposed EFP-uRPF methods aim to significantly reduce
false positives regarding invalid detection in source address
validation (SAV). Hence, they can potentially alleviate ISPs'
concerns about the possibility of disrupting service for their
customers and encourage greater deployment of uRPF techniques. This
document updates RFC 3704.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8704.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Terminology
1.2. Requirements Language
2. Review of Existing Source Address Validation Techniques
2.1. SAV Using Access Control List
2.2. SAV Using Strict Unicast Reverse Path Forwarding
2.3. SAV Using Feasible-Path Unicast Reverse Path Forwarding
2.4. SAV Using Loose Unicast Reverse Path Forwarding
2.5. SAV Using VRF Table
3. SAV Using Enhanced Feasible-Path uRPF
3.1. Description of the Method
3.1.1. Algorithm A: Enhanced Feasible-Path uRPF
3.2. Operational Recommendations
3.3. A Challenging Scenario
3.4. Algorithm B: Enhanced Feasible-Path uRPF with Additional
Flexibility across Customer Cone
3.5. Augmenting RPF Lists with ROA and IRR Data
3.6. Implementation and Operations Considerations
3.6.1. Impact on FIB Memory Size Requirement
3.6.2. Coping with BGP's Transient Behavior
3.7. Summary of Recommendations
3.7.1. Applicability of the EFP-uRPF Method with Algorithm A
4. Security Considerations
5. IANA Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Source address validation (SAV) refers to the detection and
mitigation of source address (SA) spoofing [RFC2827]. This document
identifies a need for and proposes improvement of the unicast Reverse
Path Forwarding (uRPF) techniques [RFC3704] for SAV. 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 document, 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 enhanced feasible-path uRPF (EFP-uRPF)
techniques that aim 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.
Throughout this document, the routes under consideration are assumed
to have been vetted based on prefix filtering [RFC7454] and possibly
origin validation [RFC6811].
The EFP-uRPF methods aim to significantly reduce false positives
regarding invalid detection in SAV. They 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 Denial of Service (DoS) and Distributed DoS (DDoS)
prevention benefits network wide.
1.1. Terminology
The Reverse Path Forwarding (RPF) list is the list of permissible
source-address prefixes for incoming data packets on a given
interface.
Peering relationships considered in this document are provider-to-
customer (P2C), customer-to-provider (C2P), and peer-to-peer (P2P).
Here, "provider" refers to a transit provider. The first two are
transit relationships. A peer connected via a P2P link is known as a
lateral peer (non-transit).
AS A's customer cone is A plus all the ASes that can be reached from
A following only P2C links [Luckie].
A stub AS is an AS that does not have any customers or lateral peers.
In this document, a single-homed stub AS is one that has a single
transit provider and a multihomed stub AS is one that has multiple
(two or more) transit providers.
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Review of Existing Source Address Validation Techniques
There are various existing techniques for the mitigation of DoS/DDoS
attacks with spoofed addresses [RFC2827] [RFC3704]. SAV is performed
in network edge devices, such as border routers, Cable Modem
Termination Systems (CMTS) [RFC4036], and Packet Data Network
Gateways (PDN-GWs) in mobile networks [Firmin]. Ingress Access
Control List (ACL) and uRPF are techniques employed for implementing
SAV [RFC2827] [RFC3704] [ISOC].
2.1. SAV Using Access Control List
Ingress/egress ACLs are maintained to 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 fails the filtering criteria 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; hence, it is operationally difficult or infeasible.
Typically, the egress ACLs in access aggregation devices (e.g., CMTS,
PDN-GW) 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 Forwarding
Note: In the figures (scenarios) in this section and the subsequent
sections, 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.
* 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 uRPF method, an ingress packet at a 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 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 multihomed,
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. Then, data packets with a source address in
prefix P2 that are received at AS2 directly from AS1 will get
dropped. Further, data packets with a source address in prefix P1
that originate from AS1 and traverse via AS3 to AS2 will also get
dropped at AS2.
+------------+ ---- 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 a source address (SA) in P2, or
(2) from AS3 that originated from AS1 with a SA 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 Forwarding
The feasible-path uRPF technique helps partially overcome the problem
identified with the strict uRPF in the multihoming 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
less-specific prefix) to different transit providers as needed for
traffic engineering.
As an example, in the multihoming scenario (see Scenario 2 in
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 here, 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 a 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 a source address in P1:
* Feasible-path uRPF works (if the customer route to P1
is preferred at AS3 over the shorter path)
* Feasible-path uRPF fails (if the shorter path to P1
is preferred at AS3 over the 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
2.4. SAV Using Loose Unicast Reverse Path Forwarding
In the loose 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 source 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 [RFC4364]
[Juniper] allows a router to maintain multiple routing table
instances separate from the global Routing Information Base (RIB).
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
The 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 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, in its Adj-RIBs-In [RFC4271], a border router of ISP-A
has 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.
Thus, the EFP-uRPF method gathers feasible paths for customer
interfaces in a more precise way (as compared to the 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 also be extended to create the RPF lists
for lateral peer interfaces. That is, the EFP-uRPF method can be
applied (and loose uRPF avoided) on lateral peer interfaces. That
will help to avoid compromising directionality for lateral peer
interfaces (which is inevitable with loose uRPF; see Section 2.4).
Looking back at Scenarios 1 and 2 (Figures 1 and 2), the 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 EFP-uRPF at ISP4 (AS4). ISP4 learns a route for
prefix P1 via a 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 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 and given the commonality of the
origin AS across the routes for P1, P2, and P3, AS4 includes all of
these prefixes in the RPF list for the customer interfaces (from AS2
and AS3).
+----------+ 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 a 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
(Section 3.1) can be specified as follows (to be implemented in a
transit AS):
1. Create the set of unique origin ASes considering only the routes
in the Adj-RIBs-In of customer interfaces. Call it Set A = {AS1,
AS2, ..., ASn}.
2. Considering all routes in Adj-RIBs-In 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 the RPF list on all customer interfaces on
which one or more of the prefixes in Set X1 were received.
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 also be extended to apply the EFP-uRPF method
to lateral peer interfaces. However, it is left up to the operator
to decide whether they should apply the 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 multihomed stub AS:
* A multihomed 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:
* A non-stub AS should also announce at least one of the prefixes it
originates to each of its transit provider ASes.
* 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, which poses a
challenge for the enhanced feasible-path uRPF (as well as for
traditional feasible-path uRPF), may be constructed. 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].)
+----------+
| 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 a 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-RIBs-In. Including the
additional degree of flexibility, the modified algorithm called
Algorithm B (implemented in a transit AS) can be described as
follows:
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-RIBs-In for the interfaces in Set I. Call it Set P = {P1,
P2, ..., Pm}.
3. Create the set of all unique origin ASes seen in the routes that
exist in Adj-RIBs-In 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-RIBs-In 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 the proposed
EFP-uRPF method with Algorithm B is still a much better choice (for
the scenario under consideration) than applying the loose uRPF
method, which is oblivious to directionality.
So, applying the EFP-uRPF method with Algorithm B is recommended on
customer interfaces for the challenging scenarios, such as those
described in Section 3.3.
3.5. Augmenting RPF Lists with ROA and IRR Data
It is worth emphasizing that an indirect part of the proposal in this
document 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. Special care should be exercised when using IRR
data because it is not always accurate or trusted. In the EFP-uRPF
method with Algorithm A (see Section 3.1.1), if a ROA includes prefix
Pi and ASj, then augment the RPF list of each customer interface on
which at least one route with origin ASj was received with prefix Pi.
In the EFP-uRPF method with Algorithm B, if ASj belongs in Set A (see
Step #3 Section 3.4) and if a ROA includes prefix Pi and ASj, then
augment the RPF list Z in Step 5 of Algorithm B with prefix Pi.
Similar procedures can be followed with reliable IRR data as well.
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
RPF lists that are not necessarily the same as the existing FIB
contents. In the algorithms (Sections 3.1.1 and 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 the 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., ternary content-addressable memory
(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 equal the total
number of originated prefixes from ASes in its customer cone
(assuming Algorithm B in Section 3.4 is used). (Note: EFP-uRPF with
Algorithm A (see Section 3.1.1) requires much less memory than EFP-
uRPF with Algorithm B.)
The following table shows the measured customer cone sizes in number
of prefixes originated (from all ASes in the customer cone) for
various types of ISPs [Sriram-RIPE63]:
+------------+---------------------------------------+
| Type of | Measured Customer Cone Size in # |
| ISP | Prefixes (in turn this is an estimate |
| | for RPF list size on the line card) |
+============+=======================================+
| Very Large | 32393 |
| Global ISP | |
| #1 | |
+------------+---------------------------------------+
| Very Large | 29528 |
| Global ISP | |
| #2 | |
+------------+---------------------------------------+
| Large | 20038 |
| Global ISP | |
+------------+---------------------------------------+
| Mid-size | 8661 |
| Global ISP | |
+------------+---------------------------------------+
| Regional | 1101 |
| ISP (in | |
| Asia) | |
+------------+---------------------------------------+
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.
A very large global ISP's router line card is likely to have a FIB
size large enough to accommodate 2 million routes [Cisco1].
Similarly, the line cards in routers corresponding to a large global
ISP, a midsize 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 midsize 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 reannounced 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 predetermined 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, the AS under consideration SHOULD perform ACL-based
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:
* The EFP-uRPF method with Algorithm B (see Section 3.4) SHOULD
be applied on customer interfaces.
* The loose uRPF method SHOULD be applied on lateral peer and
transit provider interfaces.
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 pertaining RPF lists (see Section 3.5 for
details).
3.7.1. Applicability of the EFP-uRPF Method with Algorithm A
The EFP-uRPF method with Algorithm A is not mentioned in the above
set of recommendations. It is an alternative to EFP-uRPF with
Algorithm B and can be used in limited circumstances. The EFP-uRPF
method with Algorithm A is expected to work fine if an ISP deploying
it has only multihomed stub customers. It is trivially equivalent to
strict uRPF if an ISP deploys it for a single-homed stub customer.
More generally, it is also expected to work fine when there is
absence of limitations, such as those described in Section 3.3.
However, caution is required for use of EFP-uRPF with Algorithm A
because even if the limitations are not expected at the time of
deployment, the vulnerability to change in conditions exists. It may
be difficult for an ISP to know or track the extent of use of
NO_EXPORT (see Section 3.3) on routes within its customer cone. If
an ISP decides to use EFP-uRPF with Algorithm A, it should make its
direct customers aware of the operational recommendations in
Section 3.2. This means that the ISP notifies direct customers that
at least one prefix originated by each AS in the direct customer's
customer cone must propagate to the ISP.
On a lateral peer interface, an ISP may choose to apply the EFP-uRPF
method with Algorithm A (with appropriate modification of the
algorithm). This is because stricter forms of uRPF (than the loose
uRPF) may be considered applicable by some ISPs on interfaces with
lateral peers.
4. Security Considerations
The security considerations in BCP 38 [RFC2827] and RFC 3704
[RFC3704] apply for this document as well. In addition, if
considering using the EFP-uRPF method with Algorithm A, an ISP or AS
operator should be aware of the applicability considerations and
potential vulnerabilities discussed in Section 3.7.1.
In augmenting RPF lists with ROA (and possibly reliable IRR)
information (see Section 3.5), a trade-off is made in favor of
reducing false positives (regarding invalid detection in SAV) at the
expense of another slight risk. The other risk being that a
malicious actor at another AS in the neighborhood within the customer
cone might take advantage (of the augmented prefix) to some extent.
This risk also exists even with normal announced prefixes (i.e.,
without ROA augmentation) for any uRPF method other than the strict
uRPF. However, the risk is mitigated if the transit provider of the
other AS in question is performing SAV.
Though not within the scope of this document, security hardening of
routers and other supporting systems (e.g., Resource PKI (RPKI) and
ROA management systems) against compromise is extremely important.
The compromise of those systems can affect the operation and
performance of the SAV methods described in this document.
5. IANA Considerations
This document has no IANA actions.
6. References
6.1. 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>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
6.2. Informative References
[CAIDA] CAIDA, "Information for AS 174 (COGENT-174)", October
2019, <https://spoofer.caida.org/as.php?asn=174>.
[Cisco1] Cisco, "Internet Routing Table Growth Causes %ROUTING-FIB-
4-RSRC_LOW Message on Trident-Based Line Cards", 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, "Cisco Nexus 7000 Series NX-OS Unicast Routing
Configuration Guide, Release 5.x (Chapter 15: 'Managing
the Unicast RIB and FIB')", March 2018,
<https://www.cisco.com/c/en/us/td/docs/switches/
datacenter/sw/5_x/nx-
os/unicast/configuration/guide/l3_cli_nxos/
l3_NewChange.html>.
[Firmin] Firmin, F., "The Evolved Packet Core",
<https://www.3gpp.org/technologies/keywords-acronyms/100-
the-evolved-packet-core>.
[ISOC] Internet Society, "Addressing the challenge of IP
spoofing", September 2015,
<https://www.internetsociety.org/resources/doc/2015/
addressing-the-challenge-of-ip-spoofing/>.
[Juniper] Juniper Networks, "Creating Unique VPN Routes Using VRF
Tables", May 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>.
[Luckie] Luckie, M., Huffaker, B., Dhamdhere, A., Giotsas, V., and
kc. claffy, "AS Relationships, customer cones, and
validation", In Proceedings of the 2013 Internet
Measurement Conference, DOI 10.1145/2504730.2504735,
October 2013,
<https://dl.acm.org/doi/10.1145/2504730.2504735>.
[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, "IANA IPv4 Special-Purpose Address Registry",
<https://www.iana.org/assignments/iana-ipv4-special-
registry/>.
[SPAR-v6] IANA, "IANA IPv6 Special-Purpose Address Registry",
<https://www.iana.org/assignments/iana-ipv6-special-
registry/>.
[Sriram-RIPE63]
Sriram, K. and R. Bush, "Estimating CPU Cost of BGPSEC on
a Router", Presented at RIPE 63 and at the SIDR WG meeting
at IETF 83, March 2012,
<http://www.ietf.org/proceedings/83/slides/slides-83-sidr-
7.pdf>.
[Sriram-URPF]
Sriram, K., Montgomery, D., and J. Haas, "Enhanced
Feasible-Path Unicast Reverse Path Filtering", Presented
at the OPSEC WG meeting at IETF 101, March 2018,
<https://datatracker.ietf.org/meeting/101/materials/
slides-101-opsec-draft-sriram-opsec-urpf-improvements-00>.
Acknowledgements
The authors would like to thank Sandy Murphy, Alvaro Retana, 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. The
comments and suggestions received from the IESG reviewers are also
much appreciated.
Authors' Addresses
Kotikalapudi Sriram
USA National Institute of Standards and Technology
100 Bureau Drive
Gaithersburg, MD 20899
United States of America
Email: ksriram@nist.gov
Doug Montgomery
USA National Institute of Standards and Technology
100 Bureau Drive
Gaithersburg, MD 20899
United States of America
Email: dougm@nist.gov
Jeffrey Haas
Juniper Networks, Inc.
1133 Innovation Way
Sunnyvale, CA 94089
United States of America
Email: jhaas@juniper.net