draft-ietf-opsec-urpf-improvements-03.txt   draft-ietf-opsec-urpf-improvements-04.txt 
OPSEC Working Group K. Sriram OPSEC Working Group K. Sriram
Internet-Draft D. Montgomery Internet-Draft D. Montgomery
Updates: RFC3704 (if approved) USA NIST BCP: 84 (if approved) USA NIST
Intended status: Best Current Practice J. Haas Updates: 3704 (if approved) J. Haas
Expires: January 9, 2020 Juniper Networks, Inc. Intended status: Best Current Practice Juniper Networks, Inc.
July 8, 2019 Expires: March 2, 2020 August 30, 2019
Enhanced Feasible-Path Unicast Reverse Path Filtering Enhanced Feasible-Path Unicast Reverse Path Forwarding
draft-ietf-opsec-urpf-improvements-03 draft-ietf-opsec-urpf-improvements-04
Abstract Abstract
This document identifies a need for improvement of the unicast This document identifies a need for and proposes improvement of the
Reverse Path Filtering techniques (uRPF) (see BCP 84) for detection unicast Reverse Path Forwarding (uRPF) techniques (see RFC 3704) for
and mitigation of source address spoofing (see BCP 38). The strict detection and mitigation of source address spoofing (see BCP 38).
uRPF is inflexible about directionality, the loose uRPF is oblivious The strict uRPF is inflexible about directionality, the loose uRPF is
to directionality, and the current feasible-path uRPF attempts to oblivious to directionality, and the current feasible-path uRPF
strike a balance between the two (see BCP 84). However, as shown in attempts to strike a balance between the two (see RFC 3704).
this draft, the existing feasible-path uRPF still has shortcomings. However, as shown in this document, the existing feasible-path uRPF
This document describes an enhanced feasible-path uRPF technique, still has shortcomings. This document describes enhanced feasible-
which aims to be more flexible (in a meaningful way) about path uRPF (EFP-uRPF) techniques, which are more flexible (in a
directionality than the feasible-path uRPF. It can potentially meaningful way) about directionality than the feasible-path uRPF (RFC
alleviate ISPs' concerns about the possibility of disrupting service 3704). The proposed EFP-uRPF methods aim to significantly reduce
for their customers, and encourage greater deployment of uRPF false positives regarding invalid detection in source address
techniques. 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 Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 9, 2020. This Internet-Draft will expire on March 2, 2020.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Review of Existing Source Address Validation Techniques . . . 4 2. Review of Existing Source Address Validation Techniques . . . 4
2.1. SAV using Access Control List . . . . . . . . . . . . . . 4 2.1. SAV using Access Control List . . . . . . . . . . . . . . 4
2.2. SAV using Strict Unicast Reverse Path Filtering . . . . . 4 2.2. SAV using Strict Unicast Reverse Path Forwarding . . . . 5
2.3. SAV using Feasible-Path Unicast Reverse Path Filtering . 5 2.3. SAV using Feasible-Path Unicast Reverse Path Forwarding . 6
2.4. SAV using Loose Unicast Reverse Path Filtering . . . . . 7 2.4. SAV using Loose Unicast Reverse Path Forwarding . . . . . 7
2.5. SAV using VRF Table . . . . . . . . . . . . . . . . . . . 7 2.5. SAV using VRF Table . . . . . . . . . . . . . . . . . . . 8
3. SAV using Enhanced Feasible-Path uRPF . . . . . . . . . . . . 7 3. SAV using Enhanced Feasible-Path uRPF . . . . . . . . . . . . 8
3.1. Description of the Method . . . . . . . . . . . . . . . . 7 3.1. Description of the Method . . . . . . . . . . . . . . . . 8
3.1.1. Algorithm A: Enhanced Feasible-Path uRPF . . . . . . 9 3.1.1. Algorithm A: Enhanced Feasible-Path uRPF . . . . . . 10
3.2. Operational Recommendations . . . . . . . . . . . . . . . 10 3.2. Operational Recommendations . . . . . . . . . . . . . . . 10
3.3. A Challenging Scenario . . . . . . . . . . . . . . . . . 10 3.3. A Challenging Scenario . . . . . . . . . . . . . . . . . 11
3.4. Algorithm B: Enhanced Feasible-Path uRPF with Additional 3.4. Algorithm B: Enhanced Feasible-Path uRPF with Additional
Flexibility Across Customer Cone . . . . . . . . . . . . 11 Flexibility Across Customer Cone . . . . . . . . . . . . 12
3.5. Augmenting RPF Lists with ROA and IRR Data . . . . . . . 12 3.5. Augmenting RPF Lists with ROA and IRR Data . . . . . . . 12
3.6. Implementation and Operations Considerations . . . . . . 12 3.6. Implementation and Operations Considerations . . . . . . 13
3.6.1. Impact on FIB Memory Size Requirement . . . . . . . . 12 3.6.1. Impact on FIB Memory Size Requirement . . . . . . . . 13
3.6.2. Coping with BGP's Transient Behavior . . . . . . . . 14 3.6.2. Coping with BGP's Transient Behavior . . . . . . . . 14
3.7. Summary of Recommendations . . . . . . . . . . . . . . . 14 3.7. Summary of Recommendations . . . . . . . . . . . . . . . 15
4. Security Considerations . . . . . . . . . . . . . . . . . . . 15 3.7.1. Applicability of the enhanced feasible-path uRPF
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 (EFP-uRPF) method with Algorithm A . . . . . . . . . 15
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15 4. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7.1. Normative References . . . . . . . . . . . . . . . . . . 15 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
7.2. Informative References . . . . . . . . . . . . . . . . . 16 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 7.1. Normative References . . . . . . . . . . . . . . . . . . 17
7.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction 1. Introduction
Source Address Validation (SAV) refers to the detection and Source Address Validation (SAV) refers to the detection and
mitigation of source address spoofing [RFC2827]. This document mitigation of source address (SA) spoofing [RFC2827]. This document
identifies a need for improvement of the unicast Reverse Path identifies a need for and proposes improvement of improvement of the
Filtering (uRPF) techniques [RFC3704] for SAV. The strict uRPF is unicast Reverse Path Forwarding (uRPF) techniques [RFC3704] for SAV.
inflexible about directionality (see [RFC3704] for definitions), the The strict uRPF is inflexible about directionality (see [RFC3704] for
loose uRPF is oblivious to directionality, and the current feasible- definitions), the loose uRPF is oblivious to directionality, and the
path uRPF attempts to strike a balance between the two [RFC3704]. current feasible-path uRPF attempts to strike a balance between the
However, as shown in this draft, the existing feasible-path uRPF two [RFC3704]. However, as shown in this document, the existing
still has shortcomings. Even with the feasible-path uRPF, ISPs are feasible-path uRPF still has shortcomings. Even with the feasible-
often apprehensive that they may be dropping customers' data packets path uRPF, ISPs are often apprehensive that they may be dropping
with legitimate source addresses. customers' data packets with legitimate source addresses.
This document describes an enhanced feasible-path uRPF technique, This document describes an enhanced feasible-path uRPF (EFP-uRPF)
which aims to be more flexible (in a meaningful way) about technique, which aims to be more flexible (in a meaningful way) about
directionality than the feasible-path uRPF. It is based on the directionality than the feasible-path uRPF. It is based on the
principle that if BGP updates for multiple prefixes with the same principle that if BGP updates for multiple prefixes with the same
origin AS were received on different interfaces (at border routers), origin AS were received on different interfaces (at border routers),
then incoming data packets with source addresses in any of those then incoming data packets with source addresses in any of those
prefixes should be accepted on any of those interfaces (presented in prefixes should be accepted on any of those interfaces (presented in
Section 3). For some challenging ISP-customer scenarios (see Section 3). For some challenging ISP-customer scenarios (see
Section 3.3), this document also describes a more relaxed version of Section 3.3), this document also describes a more relaxed version of
the enhanced feasible-path uRPF technique (presented in Section 3.4). the enhanced feasible-path uRPF technique (presented in Section 3.4).
Implementation and operations considerations are discussed in Implementation and operations considerations are discussed in
Section 3.6. 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 Throughout this document, the routes under consideration are assumed
to have been vetted based on prefix filtering [RFC7454] and possibly to have been vetted based on prefix filtering [RFC7454] and possibly
(in the future) origin validation [RFC6811]. origin validation [RFC6811].
The enhanced feasible-path uRPF methods described here are expected The EFP-uRPF methods aim to significantly reduce false positives
to add greater operational robustness and efficacy to uRPF, while regarding invalid detection in SAV. They are expected to add greater
minimizing ISPs' concerns about accidental service disruption for operational robustness and efficacy to uRPF, while minimizing ISPs'
their customers. It is expected that this will encourage more concerns about accidental service disruption for their customers. It
deployment of uRPF to help realize its DDoS prevention benefits is expected that this will encourage more deployment of uRPF to help
network wide. realize its DDoS prevention benefits network wide.
1.1. Requirements Language 1.1. Terminology
Reverse Path Forwarding (RPF) list: 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).
Provider here refers to transit provider. The first two are transit
relationships. A peer connected via a p2p link is known as a lateral
peer (non-transit).
Customer Cone: 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 multi-homed 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", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in RFC 2119 [RFC2119]. "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 2. Review of Existing Source Address Validation Techniques
There are various existing techniques for mitigation against DDoS There are various existing techniques for mitigation against DDoS
attacks with spoofed addresses [RFC2827] [RFC3704]. Source address attacks with spoofed addresses [RFC2827] [RFC3704]. Source address
validation (SAV) is performed in network edge devices such as border validation (SAV) is performed in network edge devices such as border
routers, Cable Modem Termination Systems (CMTS) [RFC4036], and Packet routers, Cable Modem Termination Systems (CMTS) [RFC4036], and Packet
Data Network (PDN) gateways in mobile networks [Firmin]. Ingress Data Network gateways (PDN-GW) in mobile networks [Firmin]. Ingress
Access Control List (ACL) and unicast Reverse Path Filtering (uRPF) Access Control List (ACL) and unicast Reverse Path Forwarding (uRPF)
are techniques employed for implementing SAV [RFC2827] [RFC3704] are techniques employed for implementing SAV [RFC2827] [RFC3704]
[ISOC]. [ISOC].
2.1. SAV using Access Control List 2.1. SAV using Access Control List
Ingress/egress Access Control Lists (ACLs) are maintained which list Ingress/egress Access Control Lists (ACLs) are maintained to list
acceptable (or alternatively, unacceptable) prefixes for the source acceptable (or alternatively, unacceptable) prefixes for the source
addresses in the incoming/outgoing Internet Protocol (IP) packets. addresses in the incoming/outgoing Internet Protocol (IP) packets.
Any packet with a source address that does not match the filter is Any packet with a source address that fails the filtering criteria is
dropped. The ACLs for the ingress/egress filters need to be dropped. The ACLs for the ingress/egress filters need to be
maintained to keep them up to date. Updating the ACLs is an operator maintained to keep them up to date. Updating the ACLs is an
driven manual process, and hence operationally difficult or operator-driven manual process, and hence operationally difficult or
infeasible. infeasible.
Typically, the egress ACLs in access aggregation devices (e.g. CMTS, Typically, the egress ACLs in access aggregation devices (e.g., CMTS,
DSLAM) permit source addresses only from the address spaces PDN-GW) permit source addresses only from the address spaces
(prefixes) that are associated with the interface on which the (prefixes) that are associated with the interface on which the
customer network is connected. Ingress ACLs are typically deployed customer network is connected. Ingress ACLs are typically deployed
on border routers, and drop ingress packets when the source address on border routers, and drop ingress packets when the source address
is spoofed (e.g., belongs to obviously disallowed prefix blocks, IANA is spoofed (e.g., belongs to obviously disallowed prefix blocks, IANA
special-purpose prefixes [SPAR-v4][SPAR-v6], provider's own prefixes, special-purpose prefixes [SPAR-v4][SPAR-v6], provider's own prefixes,
etc.). etc.).
2.2. SAV using Strict Unicast Reverse Path Filtering 2.2. SAV using Strict Unicast Reverse Path Forwarding
Note: In the figures (scenarios) that follow, the following Note: In the figures (scenarios) in this section and the subsequent
terminology is used: "fails" means drops packets with legitimate sections, the following terminology is used: "fails" means drops
source addresses; "works (but not desirable)" means passes all packets with legitimate source addresses; "works (but not desirable)"
packets with legitimate source addresses but is oblivious to means passes all packets with legitimate source addresses but is
directionality; "works best" means passes all packets with legitimate oblivious to directionality; "works best" means passes all packets
source addresses with no (or minimal) compromise of directionality. with legitimate source addresses with no (or minimal) compromise of
Further, the notation Pi[ASn ASm ...] denotes a BGP update with directionality. Further, the notation Pi[ASn ASm ...] denotes a BGP
prefix Pi and an AS_PATH as shown in the square brackets. update with prefix Pi and an AS_PATH as shown in the square brackets.
In the strict unicast Reverse Path Filtering (uRPF) method, an In the strict unicast Reverse Path Forwarding (uRPF) method, an
ingress packet at border router is accepted only if the Forwarding ingress packet at a border router is accepted only if the Forwarding
Information Base (FIB) contains a prefix that encompasses the source Information Base (FIB) contains a prefix that encompasses the source
address, and forwarding information for that prefix points back to address, and forwarding information for that prefix points back to
the interface over which the packet was received. In other words, the interface over which the packet was received. In other words,
the reverse path for routing to the source address (if it were used the reverse path for routing to the source address (if it were used
as a destination address) should use the same interface over which as a destination address) should use the same interface over which
the packet was received. It is well known that this method has the packet was received. It is well known that this method has
limitations when networks are multi-homed, routes are not limitations when networks are multi-homed, routes are not
symmetrically announced to all transit providers, and there is symmetrically announced to all transit providers, and there is
asymmetric routing of data packets. Asymmetric routing occurs (see asymmetric routing of data packets. Asymmetric routing occurs (see
Figure 1) when a customer AS announces one prefix (P1) to one transit 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-a) and a different prefix (P2) to another transit
provider (ISP-b), but routes data packets with source addresses in provider (ISP-b), but routes data packets with source addresses in
the second prefix (P2) to the first transit provider (ISP-a) or vice the second prefix (P2) to the first transit provider (ISP-a) or vice
versa. versa. Then data packets with source address in prefix P2 that are
received directly from AS1 will get dropped. Further, data packets
with source address in prefix P1 that originate from AS1 and traverse
via AS3 to AS2 will also get dropped at AS2.
+------------+ ---- P1[AS2 AS1] ---> +------------+ +------------+ ---- P1[AS2 AS1] ---> +------------+
| AS2(ISP-a) | <----P2[AS3 AS1] ---- | AS3(ISP-b)| | AS2(ISP-a) | <----P2[AS3 AS1] ---- | AS3(ISP-b)|
+------------+ +------------+ +------------+ +------------+
/\ /\ /\ /\
\ / \ /
\ / \ /
\ / \ /
P1[AS1]\ /P2[AS1] P1[AS1]\ /P2[AS1]
\ / \ /
+-----------------------+ +-----------------------+
| AS1(customer) | | AS1(customer) |
+-----------------------+ +-----------------------+
P1, P2 (prefixes originated) P1, P2 (prefixes originated)
Consider data packets received at AS2 Consider data packets received at AS2
(1) from AS1 with source address in P2, or (1) from AS1 with source address (SA) in P2, or
(2) from AS3 that originated from AS1 (2) from AS3 that originated from AS1 with SA in P1:
with source address in P1:
* Strict uRPF fails * Strict uRPF fails
* Feasible-path uRPF fails * Feasible-path uRPF fails
* Loose uRPF works (but not desirable) * Loose uRPF works (but not desirable)
* Enhanced Feasible-path uRPF works best * Enhanced Feasible-path uRPF works best
Figure 1: Scenario 1 for illustration of efficacy of uRPF schemes. Figure 1: Scenario 1 for illustration of efficacy of uRPF schemes.
2.3. SAV using Feasible-Path Unicast Reverse Path Filtering 2.3. SAV using Feasible-Path Unicast Reverse Path Forwarding
The feasible-path uRPF technique helps partially overcome the problem The feasible-path uRPF technique helps partially overcome the problem
identified with the strict uRPF in the multi-homing case. The identified with the strict uRPF in the multi-homing case. The
feasible-path uRPF is similar to the strict uRPF, but in addition to feasible-path uRPF is similar to the strict uRPF, but in addition to
inserting the best-path prefix, additional prefixes from alternative inserting the best-path prefix, additional prefixes from alternative
announced routes are also included in the RPF list. This method announced routes are also included in the RPF list. This method
relies on either (a) announcements for the same prefixes (albeit some relies on either (a) announcements for the same prefixes (albeit some
may be prepended to effect lower preference) propagating to all may be prepended to effect lower preference) propagating to all
transit providers performing feasible-path uRPF checks, or (b) transit providers performing feasible-path uRPF checks, or (b)
announcement of an aggregate less specific prefix to all transit announcement of an aggregate less specific prefix to all transit
providers while announcing more specific prefixes (covered by the providers while announcing more specific prefixes (covered by the
less specific prefix) to different transit providers as needed for less specific prefix) to different transit providers as needed for
traffic engineering. As an example, in the multi-homing scenario traffic engineering. As an example, in the multi-homing scenario
(see Figure 2), if the customer AS announces routes for both prefixes (see Scenario 2 in Figure 2), if the customer AS announces routes for
(P1, P2) to both transit providers (with suitable prepends if needed both prefixes (P1, P2) to both transit providers (with suitable
for traffic engineering), then the feasible-path uRPF method works. prepends if needed for traffic engineering), then the feasible-path
It should be mentioned that the feasible-path uRPF works in this uRPF method works. It should be mentioned that the feasible-path
scenario only if customer routes are preferred at AS2 and AS3 over a uRPF works in this scenario only if customer routes are preferred at
shorter non-customer route. However, the feasible-path uRPF method AS2 and AS3 over a shorter non-customer route. However, the
has limitations as well. One form of limitation naturally occurs feasible-path uRPF method has limitations as well. One form of
when the recommendation (a) or (b) mentioned above regarding limitation naturally occurs when the recommendation (a) or (b)
propagation of prefixes is not followed. Another form of limitation mentioned above regarding propagation of prefixes is not followed.
can be described as follows. In Scenario 2 (described above, Another form of limitation can be described as follows. In Scenario
illustrated in Figure 2), it is possible that the second transit 2 (described here, illustrated in Figure 2), it is possible that the
provider (ISP-b or AS3) does not propagate the prepended route for second transit provider (ISP-b or AS3) does not propagate the
prefix P1 to the first transit provider (ISP-a or AS2). This is prepended route for prefix P1 to the first transit provider (ISP-a or
because AS3's decision policy permits giving priority to a shorter AS2). This is because AS3's decision policy permits giving priority
route to prefix P1 via a lateral peer (AS2) over a longer route to a shorter route to prefix P1 via a lateral peer (AS2) over a
learned directly from the customer (AS1). In such a scenario, AS3 longer route learned directly from the customer (AS1). In such a
would not send any route announcement for prefix P1 to AS2 (over the scenario, AS3 would not send any route announcement for prefix P1 to
p2p link). Then a data packet with source address in prefix P1 that AS2 (over the p2p link). Then a data packet with source address in
originates from AS1 and traverses via AS3 to AS2 will get dropped at prefix P1 that originates from AS1 and traverses via AS3 to AS2 will
AS2. get dropped at AS2.
+------------+ routes for P1, P2 +-----------+ +------------+ routes for P1, P2 +-----------+
| AS2(ISP-a) |<-------------------->| AS3(ISP-b)| | AS2(ISP-a) |<-------------------->| AS3(ISP-b)|
+------------+ (p2p) +-----------+ +------------+ (p2p) +-----------+
/\ /\ /\ /\
\ / \ /
P1[AS1]\ /P2[AS1] P1[AS1]\ /P2[AS1]
\ / \ /
P2[AS1 AS1 AS1]\ /P1[AS1 AS1 AS1] P2[AS1 AS1 AS1]\ /P1[AS1 AS1 AS1]
\ / \ /
skipping to change at page 7, line 5 skipping to change at page 7, line 40
that originated from AS1 and have source address in P1: that originated from AS1 and have source address in P1:
* Feasible-path uRPF works (if customer route to P1 * Feasible-path uRPF works (if customer route to P1
is preferred at AS3 over shorter path) is preferred at AS3 over shorter path)
* Feasible-path uRPF fails (if shorter path to P1 * Feasible-path uRPF fails (if shorter path to P1
is preferred at AS3 over customer route) is preferred at AS3 over customer route)
* Loose uRPF works (but not desirable) * Loose uRPF works (but not desirable)
* Enhanced Feasible-path uRPF works best * Enhanced Feasible-path uRPF works best
Figure 2: Scenario 2 for illustration of efficacy of uRPF schemes. Figure 2: Scenario 2 for illustration of efficacy of uRPF schemes.
2.4. SAV using Loose Unicast Reverse Path Filtering 2.4. SAV using Loose Unicast Reverse Path Forwarding
In the loose unicast Reverse Path Filtering (uRPF) method, an ingress In the loose unicast Reverse Path Forwarding (uRPF) method, an
packet at the border router is accepted only if the FIB has one or ingress packet at the border router is accepted only if the FIB has
more prefixes that encompass the source address. That is, a packet one or more prefixes that encompass the source address. That is, a
is dropped if no route exists in the FIB for the source address. packet is dropped if no route exists in the FIB for the source
Loose uRPF sacrifices directionality. It only drops packets if the address. Loose uRPF sacrifices directionality. It only drops
spoofed address is unreachable in the current FIB (e.g., IANA packets if the source address is unreachable in the current FIB
special-purpose prefixes [SPAR-v4][SPAR-v6], unallocated, allocated (e.g., IANA special-purpose prefixes [SPAR-v4][SPAR-v6], unallocated,
but currently not routed). allocated but currently not routed).
2.5. SAV using VRF Table 2.5. SAV using VRF Table
The Virtual Routing and Forwarding (VRF) technology allows a router The Virtual Routing and Forwarding (VRF) technology [RFC4364]
to maintain multiple routing table instances, separate from the [Juniper] allows a router to maintain multiple routing table
global Routing Information Base (RIB) [Juniper][RFC4364]. External instances separate from the global Routing Information Base (RIB).
BGP (eBGP) peering sessions send specific routes to be stored in a External BGP (eBGP) peering sessions send specific routes to be
dedicated VRF table. The uRPF process queries the VRF table (instead stored in a dedicated VRF table. The uRPF process queries the VRF
of the FIB) for source address validation. A VRF table can be table (instead of the FIB) for source address validation. A VRF
dedicated per eBGP peer and used for uRPF for only that peer, table can be dedicated per eBGP peer and used for uRPF for only that
resulting in strict mode operation. For implementing loose uRPF on peer, resulting in strict mode operation. For implementing loose
an interface, the corresponding VRF table would be global, i.e., uRPF on an interface, the corresponding VRF table would be global,
contains the same routes as in the FIB. i.e., contains the same routes as in the FIB.
3. SAV using Enhanced Feasible-Path uRPF 3. SAV using Enhanced Feasible-Path uRPF
3.1. Description of the Method 3.1. Description of the Method
Enhanced feasible-path uRPF (EFP-uRPF) method adds greater Enhanced feasible-path uRPF (EFP-uRPF) method adds greater
operational robustness and efficacy to existing uRPF methods operational robustness and efficacy to existing uRPF methods
discussed in Section 2. That is because it avoids dropping discussed in Section 2. That is because it avoids dropping
legitimate data packets and avoids compromising directionality. The legitimate data packets and avoids compromising directionality. The
method is based on the principle that if BGP updates for multiple method is based on the principle that if BGP updates for multiple
prefixes with the same origin AS were received on different prefixes with the same origin AS were received on different
interfaces (at border routers), then incoming data packets with interfaces (at border routers), then incoming data packets with
source addresses in any of those prefixes should be accepted on any source addresses in any of those prefixes should be accepted on any
of those interfaces. The EFP-uRPF method can be best explained with of those interfaces. The EFP-uRPF method can be best explained with
an example as follows: an example as follows:
Let us say, a border router of ISP-A has in its Adj-RIB-Ins [RFC4271] Let us say, a border router of ISP-A has in its Adj-RIBs-In [RFC4271]
the set of prefixes {Q1, Q2, Q3} each of which has AS-x as its origin 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 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, received the route for prefix Q1 over a customer facing interface,
while it learned the routes for prefixes Q2 and Q3 from a lateral while it learned the routes for prefixes Q2 and Q3 from a lateral
peer and an upstream transit provider, respectively. In this example peer and an upstream transit provider, respectively. In this example
scenario, the enhanced feasible-path uRPF method requires Q1, Q2, and scenario, the enhanced feasible-path uRPF method requires Q1, Q2, and
Q3 be included in the RPF list for the customer interface under Q3 be included in the RPF list for the customer interface under
consideration. consideration.
Thus, the enhanced feasible-path uRPF (EFP-uRPF) method gathers Thus, the enhanced feasible-path uRPF (EFP-uRPF) method gathers
feasible paths for customer interfaces in a more precise way (as feasible paths for customer interfaces in a more precise way (as
compared to feasible-path uRPF) so that all legitimate packets are compared to feasible-path uRPF) so that all legitimate packets are
accepted while the directionality property is not compromised. accepted while the directionality property is not compromised.
The above described EFP-uRPF method is recommended to be applied on The above described EFP-uRPF method is recommended to be applied on
customer interfaces. It can be extended to design the RPF lists for customer interfaces. It can be extended to create the RPF lists for
lateral peer interfaces also. That is, the EFP-uRPF method can be lateral peer interfaces also. That is, the EFP-uRPF method can be
applied (and loose uRPF avoided) on lateral peer interfaces. That applied (and loose uRPF avoided) on lateral peer interfaces. That
will help avoid compromise of directionality for lateral peer will help avoid compromise of directionality for lateral peer
interfaces (which is inevitable with loose uRPF; see Section 2.4). interfaces (which is inevitable with loose uRPF; see Section 2.4).
Looking back at Scenarios 1 and 2 (Figure 1 and Figure 2), the Looking back at Scenarios 1 and 2 (Figure 1 and Figure 2), the
enhanced feasible-path uRPF (EFP-uRPF) method works better than the enhanced feasible-path uRPF (EFP-uRPF) method works better than the
other uRPF methods. Scenario 3 (Figure 3) further illustrates the other uRPF methods. Scenario 3 (Figure 3) further illustrates the
enhanced feasible-path uRPF method with a more concrete example. In enhanced feasible-path uRPF method with a more concrete example. In
this scenario, the focus is on operation of the feasible-path uRPF at 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- ISP4 (AS4). ISP4 learns a route for prefix P1 via a customer-to-
provider (C2P) interface from customer ISP2 (AS2). This route for P1 provider (C2P) interface from customer ISP2 (AS2). This route for P1
has origin AS1. ISP4 also learns a route for P2 via another C2P has origin AS1. ISP4 also learns a route for P2 via another C2P
interface from customer ISP3 (AS3). Additionally, AS4 learns a route interface from customer ISP3 (AS3). Additionally, AS4 learns a route
for P3 via a lateral peer-to-peer (p2p) interface from ISP5 (AS5). 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). Routes for all three prefixes have the same origin AS (i.e., AS1).
Using the enhanced feasible-path uRPF scheme, given the commonality Using the enhanced feasible-path uRPF scheme, given the commonality
of the origin AS across the routes for P1, P2 and P3, AS4 includes 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 all of these prefixes in the RPF list for the customer interfaces
(from AS2 and AS3). (from AS2 and AS3).
+----------+ P3[AS5 AS1] +------------+ +----------+ P3[AS5 AS1] +------------+
| AS4(ISP4)|<---------------| AS5(ISP5) | | AS4(ISP4)|<---------------| AS5(ISP5) |
+----------+ (p2p) +------------+ +----------+ (p2p) +------------+
/\ /\ /\ /\ /\ /\
/ \ / / \ /
P1[AS2 AS1]/ \P2[AS3 AS1] / P1[AS2 AS1]/ \P2[AS3 AS1] /
(C2P)/ \(C2P) / (C2P)/ \(C2P) /
/ \ / / \ /
skipping to change at page 9, line 37 skipping to change at page 10, line 7
may be received at AS4 with source address may be received at AS4 with source address
in P1, P2 or P3 via any of the neighbors (AS2, AS3, AS5): in P1, P2 or P3 via any of the neighbors (AS2, AS3, AS5):
* Feasible-path uRPF fails * Feasible-path uRPF fails
* Loose uRPF works (but not desirable) * Loose uRPF works (but not desirable)
* Enhanced Feasible-path uRPF works best * Enhanced Feasible-path uRPF works best
Figure 3: Scenario 3 for illustration of efficacy of uRPF schemes. Figure 3: Scenario 3 for illustration of efficacy of uRPF schemes.
3.1.1. Algorithm A: Enhanced Feasible-Path uRPF 3.1.1. Algorithm A: Enhanced Feasible-Path uRPF
The underlying algorithm in the solution method described above can The underlying algorithm in the solution method described above
be specified as follows (to be implemented in a transit AS): (Section 3.1) can be specified as follows (to be implemented in a
transit AS):
1. Create the list of unique origin ASes considering only the routes 1. Create the set of unique origin ASes considering only the routes
in the Adj-RIB-Ins of customer interfaces. Call it Set A = {AS1, in the Adj-RIBs-In of customer interfaces. Call it Set A = {AS1,
AS2, ..., ASn}. AS2, ..., ASn}.
2. Considering all routes in Adj-RIB-Ins for all interfaces 2. Considering all routes in Adj-RIBs-In for all interfaces
(customer, lateral peer, and transit provider), form the set of (customer, lateral peer, and transit provider), form the set of
unique prefixes that have a common origin AS1. Call it Set X1. 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 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 interfaces on which one or more of the prefixes in set X1 were
received. received.
4. Repeat Steps 2 and 3 for each of the remaining ASes in Set A 4. Repeat Steps 2 and 3 for each of the remaining ASes in Set A
(i.e., for ASi, where i = 2, ..., n). (i.e., for ASi, where i = 2, ..., n).
skipping to change at page 10, line 21 skipping to change at page 10, line 39
lateral peer interfaces. The loose uRPF method is recommended to be lateral peer interfaces. The loose uRPF method is recommended to be
applied on transit provider interfaces. applied on transit provider interfaces.
3.2. Operational Recommendations 3.2. Operational Recommendations
The following operational recommendations will make the operation of The following operational recommendations will make the operation of
the enhanced feasible-path uRPF robust: the enhanced feasible-path uRPF robust:
For multi-homed stub AS: For multi-homed stub AS:
o A multi-homed stub AS SHOULD announce at least one of the prefixes 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 it originates to each of its transit provider ASes. (It is
understood that a single-homed stub AS would announce all prefixes understood that a single-homed stub AS would announce all prefixes
it originates to its sole transit provider AS.) it originates to its sole transit provider AS.)
For non-stub AS: For non-stub AS:
o A non-stub AS SHOULD also announce at least one of the prefixes it o A non-stub AS should also announce at least one of the prefixes it
originates to each of its transit provider ASes. originates to each of its transit provider ASes.
o Additionally, from the routes it has learned from customers, a o Additionally, from the routes it has learned from customers, a
non-stub AS SHOULD announce at least one route per origin AS to non-stub AS SHOULD announce at least one route per origin AS to
each of its transit provider ASes. each of its transit provider ASes.
3.3. A Challenging Scenario 3.3. A Challenging Scenario
It should be observed that in the absence of ASes adhering to above It should be observed that in the absence of ASes adhering to above
recommendations, the following example scenario may be constructed recommendations, the following example scenario may be constructed
which poses a challenge for the enhanced feasible-path uRPF (as well which poses a challenge for the enhanced feasible-path uRPF (as well
as for traditional feasible-path uRPF). In the scenario illustrated as for traditional feasible-path uRPF). In the scenario illustrated
in Figure 4, since routes for neither P1 nor P2 are propagated on the 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 AS2-AS4 interface (due to the presence of NO_EXPORT Community), the
enhanced feasible-path uRPF at AS4 will reject data packets received enhanced feasible-path uRPF at AS4 will reject data packets received
on that interface with source addresses in P1 or P2. (For a little on that interface with source addresses in P1 or P2. (For a little
more complex example scenario see slide #10 in [sriram-urpf].) more complex example scenario, see slide #10 in [sriram-urpf].)
+----------+ +----------+
| AS4(ISP4)| | AS4(ISP4)|
+----------+ +----------+
/\ /\ /\ /\
/ \ P1[AS3 AS1] / \ P1[AS3 AS1]
P1 and P2 not / \ P2[AS3 AS1] P1 and P2 not / \ P2[AS3 AS1]
propagated / \ (C2P) propagated / \ (C2P)
(C2P) / \ (C2P) / \
+----------+ +----------+ +----------+ +----------+
| AS2(ISP2)| | AS3(ISP3)| | AS2(ISP2)| | AS3(ISP3)|
skipping to change at page 11, line 42 skipping to change at page 12, line 12
Figure 4: Illustration of a challenging scenario. Figure 4: Illustration of a challenging scenario.
3.4. Algorithm B: Enhanced Feasible-Path uRPF with Additional 3.4. Algorithm B: Enhanced Feasible-Path uRPF with Additional
Flexibility Across Customer Cone Flexibility Across Customer Cone
Adding further flexibility to the enhanced feasible-path uRPF method Adding further flexibility to the enhanced feasible-path uRPF method
can help address the potential limitation identified above using the can help address the potential limitation identified above using the
scenario in Figure 4 (Section 3.3). In the following, "route" refers scenario in Figure 4 (Section 3.3). In the following, "route" refers
to a route currently existing in the Adj-RIB-in. Including the to a route currently existing in the Adj-RIB-in. Including the
additional degree of flexibility, the modified algorithm (implemented additional degree of flexibility, the modified algorithm called
in a transit AS) can be described as follows (we call this Algorithm Algorithm B (implemented in a transit AS) can be described as
B): follows:
1. Create the set of all directly-connected customer interfaces. 1. Create the set of all directly-connected customer interfaces.
Call it Set I = {I1, I2, ..., Ik}. Call it Set I = {I1, I2, ..., Ik}.
2. Create the set of all unique prefixes for which routes exist in 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, Adj-RIBs-In for the interfaces in Set I. Call it Set P = {P1,
P2, ..., Pm}. P2, ..., Pm}.
3. Create the set of all unique origin ASes seen in the routes that 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 exist in Adj-RIBs-In for the interfaces in Set I. Call it Set A
= {AS1, AS2, ..., ASn}. = {AS1, AS2, ..., ASn}.
4. Create the set of all unique prefixes for which routes exist in 4. Create the set of all unique prefixes for which routes exist in
Adj-RIB-Ins of all lateral peer and transit provider interfaces Adj-RIBs-In of all lateral peer and transit provider interfaces
such that each of the routes has its origin AS belonging in Set such that each of the routes has its origin AS belonging in Set
A. Call it Set Q = {Q1, Q2, ..., Qj}. A. Call it Set Q = {Q1, Q2, ..., Qj}.
5. Then, Set Z = Union(P,Q) is the RPF list that is applied for 5. Then, Set Z = Union(P,Q) is the RPF list that is applied for
every customer interface in Set I. every customer interface in Set I.
When Algorithm B (which is more flexible than Algorithm A) is When Algorithm B (which is more flexible than Algorithm A) is
employed on customer interfaces, the type of limitation identified in employed on customer interfaces, the type of limitation identified in
Figure 4 (Section 3.3) is overcome and the method works. The Figure 4 (Section 3.3) is overcome and the method works. The
directionality property is minimally compromised, but still the directionality property is minimally compromised, but still the
proposed EFP-uRPF method with Algorithm B is a much better choice proposed EFP-uRPF method with Algorithm B is a much better choice
(for the scenario under consideration) than applying the loose uRPF (for the scenario under consideration) than applying the loose uRPF
method which is oblivious to directionality. method which is oblivious to directionality.
So, applying EFP-uRPF method with Algorithm B is recommended on So, applying EFP-uRPF method with Algorithm B is recommended on
customer interfaces for the challenging scenarios such as those customer interfaces for the challenging scenarios such as those
described in Section 3.3. Further, it is recommended that loose uRPF described in Section 3.3.
method for SAV should be applied on lateral peer and transit provider
interfaces.
3.5. Augmenting RPF Lists with ROA and IRR Data 3.5. Augmenting RPF Lists with ROA and IRR Data
It is worth emphasizing that an indirect part of the proposal in the It is worth emphasizing that an indirect part of the proposal in this
draft is that RPF filters may be augmented from secondary sources. document is that RPF filters may be augmented from secondary sources.
Hence, the construction of RPF lists using a method proposed in this Hence, the construction of RPF lists using a method proposed in this
document (Algorithm A or B) can be augmented with data from Route document (Algorithm A or B) can be augmented with data from Route
Origin Authorization (ROA) [RFC6482] as well as Internet Routing Origin Authorization (ROA) [RFC6482] as well as Internet Routing
Registry (IRR) data. Prefixes from registered ROAs and IRR route Registry (IRR) data. Special care should be exercised when using IRR
objects that include ASes in an ISP's customer cone SHOULD be used to data because it not always accurate or trusted. In the EFP-uRPF
augment the appropriate RPF lists. (Note: The ASes in a customer method with Algorithm A (see Section 3.1.1), if a ROA includes prefix
cone are obtained in Step 3 of Algorithm B in Section 3.4.) This Pi and ASj, then augment with Pi the RPF list of each customer
will help make the RPF lists more robust about source addresses that interface on which at least one route with origin ASj was received.
may be legitimately used by customers of the ISP. 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 with Pi the RPF list Z in Step 5 of Algorithm B. 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. Implementation and Operations Considerations
3.6.1. Impact on FIB Memory Size Requirement 3.6.1. Impact on FIB Memory Size Requirement
The existing RPF checks in edge routers take advantage of existing The existing RPF checks in edge routers take advantage of existing
line card implementations to perform the RPF functions. For line card implementations to perform the RPF functions. For
implementation of the enhanced feasible-path uRPF, the general implementation of the enhanced feasible-path uRPF, the general
necessary feature would be to extend the line cards to take arbitrary necessary feature would be to extend the line cards to take arbitrary
RPF lists that are not necessarily the same as the existing FIB RPF lists that are not necessarily the same as the existing FIB
contents. In the algorithms (Section 3.1.1 and Section 3.4) contents. In the algorithms (Section 3.1.1 and Section 3.4)
described here, the RPF lists are constructed by applying a set of described here, the RPF lists are constructed by applying a set of
rules to all received BGP routes (not just those selected as best 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 path and installed in the FIB). The concept of uRPF querying an RPF
(instead of the FIB) is similar to uRPF querying a VRF table (see list (instead of the FIB) is similar to uRPF querying a VRF table
(Section 2.5). (see (Section 2.5).
The techniques described in this document require that there should The techniques described in this document require that there should
be additional memory (i.e., TCAM) available to store the RPF lists in be additional memory (i.e., ternary content addressable memory
line cards. For an ISP's AS, the RPF list size for each line card (TCAM)) available to store the RPF lists in line cards. For an ISP's
will roughly and conservatively equal the total number of prefixes in AS, the RPF list size for each line card will roughly equal the total
its customer cone (assuming Algorithm B in Section 3.4 is used). number of originated prefixes from ASes in its customer cone
(Note: Most ISP customer cone scenarios would not require Algorithm (assuming Algorithm B in Section 3.4 is used). (Note: EFP-uRPF with
B, but instead be served best by Algorithm A (see Section 3.1.1) Algorithm A (see Section 3.1.1) requires much less memory than EFP-
which requires much less FIB memory. This is because Algorithm B is uRPF with Algorithm B.)
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 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]: various types of ISPs [sriram-ripe63]:
+---------------------------------+---------------------------------+ +---------------------------------+---------------------------------+
| Type of ISP | Measured Customer Cone Size in | | Type of ISP | Measured Customer Cone Size in |
| | # Prefixes (in turn this is an | | | # Prefixes (in turn this is an |
| | estimate for RPF list size on | | | estimate for RPF list size on |
| | line card) | | | the line card) |
+---------------------------------+---------------------------------+ +---------------------------------+---------------------------------+
| Very Large Global ISP | 32392 | | Very Large Global ISP #1 | 32393 |
| ------------------------------- | ------------------------------- | | ------------------------------- | ------------------------------- |
| Very Large Global ISP | 29528 | | Very Large Global ISP #2 | 29528 |
| ------------------------------- | ------------------------------- | | ------------------------------- | ------------------------------- |
| Large Global ISP | 20038 | | Large Global ISP | 20038 |
| ------------------------------- | ------------------------------- | | ------------------------------- | ------------------------------- |
| Mid-size Global ISP | 8661 | | Mid-size Global ISP | 8661 |
| ------------------------------- | ------------------------------- | | ------------------------------- | ------------------------------- |
| Regional ISP (in Asia) | 1101 | | Regional ISP (in Asia) | 1101 |
+---------------------------------+---------------------------------+ +---------------------------------+---------------------------------+
Table 1: Customer cone sizes (# prefixes) for various types of ISPs. Table 1: Customer cone sizes (# prefixes) for various types of ISPs.
For some super large global ISPs that are at the core of the 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 Internet, the customer cone size (# prefixes) can be as high as a few
hundred thousand [CAIDA]. But uRPF is most effective when deployed hundred thousand [CAIDA]. But uRPF is most effective when deployed
at ASes at the edges of the Internet where the customer cone sizes at ASes at the edges of the Internet where the customer cone sizes
are smaller as shown in Table 1. are smaller as shown in Table 1.
A very large global ISP's router line card is likely to have a FIB 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]. size large enough to accommodate 2 million routes [Cisco1].
Similarly, the line cards in routers corresponding to a large global 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 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 sizes large enough to accommodate about 1 million, 0.5 million, and
100K routes, respectively [Cisco2]. Comparing these FIB size numbers 100K routes, respectively [Cisco2]. Comparing these FIB size numbers
with the corresponding RPF list size numbers in Table 1, it can be with the corresponding RPF list size numbers in Table 1, it can be
surmised that the conservatively estimated RPF list size is only a surmised that the conservatively estimated RPF list size is only a
small fraction of the anticipated FIB memory size under relevant ISP small fraction of the anticipated FIB memory size under relevant ISP
scenarios. What is meant here by relevant ISP scenarios is that only scenarios. What is meant here by relevant ISP scenarios is that only
smaller ISPs (and possibly some mid-size and regional ISPs) are smaller ISPs (and possibly some mid-size and regional ISPs) are
expected to implement the proposed EFP-uRPF method since it is most expected to implement the proposed EFP-uRPF method since it is most
skipping to change at page 14, line 36 skipping to change at page 15, line 13
by a pre-determined amount (the value based on operational by a pre-determined amount (the value based on operational
experience) when responding to route withdrawals. This should help experience) when responding to route withdrawals. This should help
suppress the effects due to the transients in BGP. suppress the effects due to the transients in BGP.
3.7. Summary of Recommendations 3.7. Summary of Recommendations
Depending on the scenario, an ISP or enterprise AS operator should Depending on the scenario, an ISP or enterprise AS operator should
follow one of the following recommendations concerning uRPF/SAV: follow one of the following recommendations concerning uRPF/SAV:
1. For directly connected networks, i.e., subnets directly connected 1. For directly connected networks, i.e., subnets directly connected
to the AS and not multi-homed, the AS under consideration SHOULD to the AS, the AS under consideration SHOULD perform ACL-based
perform ACL-based source address validation (SAV). source address validation (SAV).
2. For a directly connected single-homed stub AS (customer), the AS 2. For a directly connected single-homed stub AS (customer), the AS
under consideration SHOULD perform SAV based on the strict uRPF under consideration SHOULD perform SAV based on the strict uRPF
method. method.
3. For all other scenarios: 3. For all other scenarios:
* If the scenario does not involve complexity such as NO_EXPORT * The enhanced feasible-path uRPF (EFP-uRPF) method with
of routes (see Section 3.3, Figure 4), then the enhanced Algorithm B (see Section 3.4) SHOULD be applied on customer
feasible-path uRPF method in Algorithm A (see Section 3.1.1) interfaces.
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.
* In general, loose uRPF method for SAV SHOULD be applied on * Loose uRPF method SHOULD be applied on lateral peer and
lateral peer and transit provider interfaces. However, for transit provider interfaces.
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 It is also recommended that prefixes from registered ROAs and IRR
route objects that include ASes in an ISP's customer cone SHOULD be route objects that include ASes in an ISP's customer cone SHOULD be
used to augment the appropriate RPF lists. used to augment the pertaining RPF lists (see Section 3.5 for
details).
3.7.1. Applicability of the enhanced feasible-path uRPF (EFP-uRPF)
method with Algorithm A
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
multi-homed 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 4. Security Considerations
The security considerations in BCP 38 [RFC2827] and BCP 84 [RFC3704] The security considerations in BCP 38 [RFC2827] and BCP 84 [RFC3704]
apply for this document as well. In addition, AS operator should apply for this document as well. In addition, if considering using
apply the uRPF method that performs best (i.e., with zero or EFP-uRPF method with Algorithm A, an ISP or AS operator should be
insignificant possibility of dropping legitimate data packets) for aware of the applicability considerations and potential
the type of peer (customer, transit provider, etc.) and the nature of vulnerabilities discussed in Section 3.7.1.
customer cone scenario that apply (see Section 3.1.1 and
Section 3.4). 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 a slight other risk. The other risk being 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.
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 5. IANA Considerations
This document does not request new capabilities or attributes. It This document does not request new capabilities or attributes. It
does not create any new IANA registries. does not create any new IANA registries.
6. Acknowledgements 6. Acknowledgements
The authors would like to thank Sandy Murphy, Job Snijders, Marco The authors would like to thank Sandy Murphy, Alvaro Retana, Job
Marzetti, Marco d'Itri, Nick Hilliard, Gert Doering, Fred Baker, Igor Snijders, Marco Marzetti, Marco d'Itri, Nick Hilliard, Gert Doering,
Gashinsky, Igor Lubashev, Andrei Robachevsky, Barry Greene, Amir Fred Baker, Igor Gashinsky, Igor Lubashev, Andrei Robachevsky, Barry
Herzberg, Ruediger Volk, Jared Mauch, Oliver Borchert, Mehmet Greene, Amir Herzberg, Ruediger Volk, Jared Mauch, Oliver Borchert,
Adalier, and Joel Jaeggli for comments and suggestions. Mehmet Adalier, and Joel Jaeggli for comments and suggestions. The
comments and suggestions received from the IESG reviewers are also
much appreciated.
7. References 7. References
7.1. Normative References 7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
skipping to change at page 16, line 14 skipping to change at page 17, line 30
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
2004, <https://www.rfc-editor.org/info/rfc3704>. 2004, <https://www.rfc-editor.org/info/rfc3704>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271, Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006, DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>. <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>.
7.2. Informative References 7.2. Informative References
[CAIDA] "Information for AS 174 (COGENT-174)", CAIDA Spoofer [CAIDA] "Information for AS 174 (COGENT-174)", CAIDA Spoofer
Project , <https://spoofer.caida.org/as.php?asn=174>. Project , <https://spoofer.caida.org/as.php?asn=174>.
[Cisco1] "Internet Routing Table Growth Causes ROUTING-FIB- [Cisco1] "Internet Routing Table Growth Causes ROUTING-FIB-
4-RSRC_LOW Message on Trident-Based Line Cards", Cisco 4-RSRC_LOW Message on Trident-Based Line Cards", Cisco
Trouble-shooting Tech-notes , January 2014, Trouble-shooting Tech-notes , January 2014,
<https://www.cisco.com/c/en/us/support/docs/routers/asr- <https://www.cisco.com/c/en/us/support/docs/routers/asr-
9000-series-aggregation-services-routers/116999-problem- 9000-series-aggregation-services-routers/116999-problem-
skipping to change at page 17, line 5 skipping to change at page 18, line 28
spoofing", ISOC report , September 2015, spoofing", ISOC report , September 2015,
<https://www.internetsociety.org/resources/doc/2015/ <https://www.internetsociety.org/resources/doc/2015/
addressing-the-challenge-of-ip-spoofing/>. addressing-the-challenge-of-ip-spoofing/>.
[Juniper] "Creating Unique VPN Routes Using VRF Tables", Juniper [Juniper] "Creating Unique VPN Routes Using VRF Tables", Juniper
Networks TechLibrary , March 2019, Networks TechLibrary , March 2019,
<https://www.juniper.net/documentation/en_US/junos/topics/ <https://www.juniper.net/documentation/en_US/junos/topics/
topic-map/l3-vpns-routes-vrf-tables.html#id-understanding- topic-map/l3-vpns-routes-vrf-tables.html#id-understanding-
virtual-routing-and-forwarding-tables>. 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 ACM Internet
Measurement Conference (IMC), DOI 10.1145/2504730.2504735,
October 2013,
<http://www.caida.org/~amogh/papers/asrank-IMC13.pdf>.
[RFC4036] Sawyer, W., "Management Information Base for Data Over [RFC4036] Sawyer, W., "Management Information Base for Data Over
Cable Service Interface Specification (DOCSIS) Cable Modem Cable Service Interface Specification (DOCSIS) Cable Modem
Termination Systems for Subscriber Management", RFC 4036, Termination Systems for Subscriber Management", RFC 4036,
DOI 10.17487/RFC4036, April 2005, DOI 10.17487/RFC4036, April 2005,
<https://www.rfc-editor.org/info/rfc4036>. <https://www.rfc-editor.org/info/rfc4036>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>. 2006, <https://www.rfc-editor.org/info/rfc4364>.
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