draft-ietf-opsec-urpf-improvements-04.txt   rfc8704.txt 
OPSEC Working Group K. Sriram Internet Engineering Task Force (IETF) K. Sriram
Internet-Draft D. Montgomery Request for Comments: 8704 D. Montgomery
BCP: 84 (if approved) USA NIST BCP: 84 USA NIST
Updates: 3704 (if approved) J. Haas Updates: 3704 J. Haas
Intended status: Best Current Practice Juniper Networks, Inc. Category: Best Current Practice Juniper Networks, Inc.
Expires: March 2, 2020 August 30, 2019 ISSN: 2070-1721 February 2020
Enhanced Feasible-Path Unicast Reverse Path Forwarding Enhanced Feasible-Path Unicast Reverse Path Forwarding
draft-ietf-opsec-urpf-improvements-04
Abstract Abstract
This document identifies a need for and proposes improvement of the This document identifies a need for and proposes improvement of the
unicast Reverse Path Forwarding (uRPF) techniques (see RFC 3704) for unicast Reverse Path Forwarding (uRPF) techniques (see RFC 3704) for
detection and mitigation of source address spoofing (see BCP 38). detection and mitigation of source address spoofing (see BCP 38).
The strict uRPF is inflexible about directionality, the loose uRPF is Strict uRPF is inflexible about directionality, the loose uRPF is
oblivious to directionality, and the current feasible-path uRPF oblivious to directionality, and the current feasible-path uRPF
attempts to strike a balance between the two (see RFC 3704). attempts to strike a balance between the two (see RFC 3704).
However, as shown in this document, the existing feasible-path uRPF However, as shown in this document, the existing feasible-path uRPF
still has shortcomings. This document describes enhanced feasible- still has shortcomings. This document describes enhanced feasible-
path uRPF (EFP-uRPF) techniques, which are more flexible (in a path uRPF (EFP-uRPF) techniques that are more flexible (in a
meaningful way) about directionality than the feasible-path uRPF (RFC meaningful way) about directionality than the feasible-path uRPF (RFC
3704). The proposed EFP-uRPF methods aim to significantly reduce 3704). The proposed EFP-uRPF methods aim to significantly reduce
false positives regarding invalid detection in source address false positives regarding invalid detection in source address
validation (SAV). Hence they can potentially alleviate ISPs' validation (SAV). Hence, they can potentially alleviate ISPs'
concerns about the possibility of disrupting service for their concerns about the possibility of disrupting service for their
customers, and encourage greater deployment of uRPF techniques. This customers and encourage greater deployment of uRPF techniques. This
document updates RFC 3704. document updates RFC 3704.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This memo documents an Internet Best Current Practice.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
This Internet-Draft will expire on March 2, 2020. 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 Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4 1.2. Requirements Language
2. Review of Existing Source Address Validation Techniques . . . 4 2. Review of Existing Source Address Validation Techniques
2.1. SAV using Access Control List . . . . . . . . . . . . . . 4 2.1. SAV Using Access Control List
2.2. SAV using Strict Unicast Reverse Path Forwarding . . . . 5 2.2. SAV Using Strict Unicast Reverse Path Forwarding
2.3. SAV using Feasible-Path Unicast Reverse Path Forwarding . 6 2.3. SAV Using Feasible-Path Unicast Reverse Path Forwarding
2.4. SAV using Loose Unicast Reverse Path Forwarding . . . . . 7 2.4. SAV Using Loose Unicast Reverse Path Forwarding
2.5. SAV using VRF Table . . . . . . . . . . . . . . . . . . . 8 2.5. SAV Using VRF Table
3. SAV using Enhanced Feasible-Path uRPF . . . . . . . . . . . . 8 3. SAV Using Enhanced Feasible-Path uRPF
3.1. Description of the Method . . . . . . . . . . . . . . . . 8 3.1. Description of the Method
3.1.1. Algorithm A: Enhanced Feasible-Path uRPF . . . . . . 10 3.1.1. Algorithm A: Enhanced Feasible-Path uRPF
3.2. Operational Recommendations . . . . . . . . . . . . . . . 10 3.2. Operational Recommendations
3.3. A Challenging Scenario . . . . . . . . . . . . . . . . . 11 3.3. 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 . . . . . . . . . . . . 12 Flexibility across Customer Cone
3.5. Augmenting RPF Lists with ROA and IRR Data . . . . . . . 12 3.5. Augmenting RPF Lists with ROA and IRR Data
3.6. Implementation and Operations Considerations . . . . . . 13 3.6. Implementation and Operations Considerations
3.6.1. Impact on FIB Memory Size Requirement . . . . . . . . 13 3.6.1. Impact on FIB Memory Size Requirement
3.6.2. Coping with BGP's Transient Behavior . . . . . . . . 14 3.6.2. Coping with BGP's Transient Behavior
3.7. Summary of Recommendations . . . . . . . . . . . . . . . 15 3.7. Summary of Recommendations
3.7.1. Applicability of the enhanced feasible-path uRPF 3.7.1. Applicability of the EFP-uRPF Method with Algorithm A
(EFP-uRPF) method with Algorithm A . . . . . . . . . 15 4. Security Considerations
4. Security Considerations . . . . . . . . . . . . . . . . . . . 16 5. IANA Considerations
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 6. References
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16 6.1. Normative References
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 6.2. Informative References
7.1. Normative References . . . . . . . . . . . . . . . . . . 17 Acknowledgements
7.2. Informative References . . . . . . . . . . . . . . . . . 17 Authors' Addresses
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 (SA) spoofing [RFC2827]. This document mitigation of source address (SA) spoofing [RFC2827]. This document
identifies a need for and proposes improvement of improvement of the identifies a need for and proposes improvement of the unicast Reverse
unicast Reverse Path Forwarding (uRPF) techniques [RFC3704] for SAV. Path Forwarding (uRPF) techniques [RFC3704] for SAV. Strict uRPF is
The strict uRPF is inflexible about directionality (see [RFC3704] for inflexible about directionality (see [RFC3704] for definitions), the
definitions), the loose uRPF is oblivious to directionality, and the loose uRPF is oblivious to directionality, and the current feasible-
current feasible-path uRPF attempts to strike a balance between the path uRPF attempts to strike a balance between the two [RFC3704].
two [RFC3704]. However, as shown in this document, the existing However, as shown in this document, the existing feasible-path uRPF
feasible-path uRPF still has shortcomings. Even with the feasible- still has shortcomings. Even with the feasible-path uRPF, ISPs are
path uRPF, ISPs are often apprehensive that they may be dropping often apprehensive that they may be dropping customers' data packets
customers' data packets with legitimate source addresses. with legitimate source addresses.
This document describes an enhanced feasible-path uRPF (EFP-uRPF) This document describes enhanced feasible-path uRPF (EFP-uRPF)
technique, which aims to be more flexible (in a meaningful way) about techniques that aim 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.
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
origin validation [RFC6811]. origin validation [RFC6811].
The EFP-uRPF methods aim to significantly reduce false positives The EFP-uRPF methods aim to significantly reduce false positives
regarding invalid detection in SAV. They are expected to add greater regarding invalid detection in SAV. They are expected to add greater
operational robustness and efficacy to uRPF, while minimizing ISPs' operational robustness and efficacy to uRPF while minimizing ISPs'
concerns about accidental service disruption for their customers. It concerns about accidental service disruption for their customers. It
is expected that this will encourage more deployment of uRPF to help is expected that this will encourage more deployment of uRPF to help
realize its DDoS prevention benefits network wide. realize its Denial of Service (DoS) and Distributed DoS (DDoS)
prevention benefits network wide.
1.1. Terminology 1.1. Terminology
Reverse Path Forwarding (RPF) list: The list of permissible source- The Reverse Path Forwarding (RPF) list is the list of permissible
address prefixes for incoming data packets on a given interface. source-address prefixes for incoming data packets on a given
interface.
Peering relationships considered in this document are provider-to- Peering relationships considered in this document are provider-to-
customer (P2C), customer-to-provider (C2P), and peer-to-peer (p2p). customer (P2C), customer-to-provider (C2P), and peer-to-peer (P2P).
Provider here refers to transit provider. The first two are transit Here, "provider" refers to a transit provider. The first two are
relationships. A peer connected via a p2p link is known as a lateral transit relationships. A peer connected via a P2P link is known as a
peer (non-transit). lateral peer (non-transit).
Customer Cone: AS A's customer cone is A plus all the ASes that can AS A's customer cone is A plus all the ASes that can be reached from
be reached from A following only P2C links [Luckie]. A following only P2C links [Luckie].
A stub AS is an AS that does not have any customers or lateral peers. 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 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 transit provider and a multihomed stub AS is one that has multiple
(two or more) transit providers. (two or more) transit providers.
1.2. Requirements Language 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", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. 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 the mitigation of DoS/DDoS
attacks with spoofed addresses [RFC2827] [RFC3704]. Source address attacks with spoofed addresses [RFC2827] [RFC3704]. SAV is performed
validation (SAV) is performed in network edge devices such as border in network edge devices, such as border routers, Cable Modem
routers, Cable Modem Termination Systems (CMTS) [RFC4036], and Packet Termination Systems (CMTS) [RFC4036], and Packet Data Network
Data Network gateways (PDN-GW) in mobile networks [Firmin]. Ingress Gateways (PDN-GWs) in mobile networks [Firmin]. Ingress Access
Access Control List (ACL) and unicast Reverse Path Forwarding (uRPF) Control List (ACL) and uRPF are techniques employed for implementing
are techniques employed for implementing SAV [RFC2827] [RFC3704] 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 to list Ingress/egress ACLs are maintained to list acceptable (or
acceptable (or alternatively, unacceptable) prefixes for the source alternatively, unacceptable) prefixes for the source addresses in the
addresses in the incoming/outgoing Internet Protocol (IP) packets. incoming/outgoing Internet Protocol (IP) packets. Any packet with a
Any packet with a source address that fails the filtering criteria is source address that fails the filtering criteria is dropped. The
dropped. The ACLs for the ingress/egress filters need to be ACLs for the ingress/egress filters need to be maintained to keep
maintained to keep them up to date. Updating the ACLs is an them up to date. Updating the ACLs is an operator-driven manual
operator-driven manual process, and hence operationally difficult or process; hence, it is 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,
PDN-GW) 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
is spoofed (e.g., belongs to obviously disallowed prefix blocks, IANA 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 Forwarding 2.2. SAV Using Strict Unicast Reverse Path Forwarding
Note: In the figures (scenarios) in this section and the subsequent Note: In the figures (scenarios) in this section and the subsequent
sections, the following terminology is used: "fails" means drops sections, the following terminology is used:
packets with legitimate source addresses; "works (but not desirable)"
means passes all packets with legitimate source addresses but is
oblivious to directionality; "works best" means passes all packets
with legitimate source addresses with no (or minimal) compromise of
directionality. Further, the notation Pi[ASn ASm ...] denotes a BGP
update with prefix Pi and an AS_PATH as shown in the square brackets.
In the strict unicast Reverse Path Forwarding (uRPF) method, an * "fails" means drops packets with legitimate source addresses.
ingress packet at a border router is accepted only if the Forwarding
Information Base (FIB) contains a prefix that encompasses the source * "works (but not desirable)" means passes all packets with
address, and forwarding information for that prefix points back to legitimate source addresses but is oblivious to directionality.
the interface over which the packet was received. In other words,
the reverse path for routing to the source address (if it were used * "works best" means passes all packets with legitimate source
as a destination address) should use the same interface over which addresses with no (or minimal) compromise of directionality.
the packet was received. It is well known that this method has
limitations when networks are multi-homed, routes are not * The notation Pi[ASn ASm ...] denotes a BGP update with prefix Pi
symmetrically announced to all transit providers, and there is and an AS_PATH as shown in the square brackets.
asymmetric routing of data packets. Asymmetric routing occurs (see
Figure 1) when a customer AS announces one prefix (P1) to one transit In the strict uRPF method, an ingress packet at a border router is
provider (ISP-a) and a different prefix (P2) to another transit accepted only if the Forwarding Information Base (FIB) contains a
provider (ISP-b), but routes data packets with source addresses in prefix that encompasses the source address and forwarding information
the second prefix (P2) to the first transit provider (ISP-a) or vice for that prefix points back to the interface over which the packet
versa. Then data packets with source address in prefix P2 that are was received. In other words, the reverse path for routing to the
received directly from AS1 will get dropped. Further, data packets source address (if it were used as a destination address) should use
with source address in prefix P1 that originate from AS1 and traverse the same interface over which the packet was received. It is well
via AS3 to AS2 will also get dropped at AS2. 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] ---> +------------+ +------------+ ---- 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 (SA) in P2, or (1) from AS1 with a source address (SA) in P2, or
(2) from AS3 that originated from AS1 with SA in P1: (2) from AS3 that originated from AS1 with a SA 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 Forwarding 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 multihoming 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.
(see Scenario 2 in Figure 2), if the customer AS announces routes for
both prefixes (P1, P2) to both transit providers (with suitable As an example, in the multihoming scenario (see Scenario 2 in
prepends if needed for traffic engineering), then the feasible-path Figure 2), if the customer AS announces routes for both prefixes (P1,
uRPF method works. It should be mentioned that the feasible-path P2) to both transit providers (with suitable prepends if needed for
uRPF works in this scenario only if customer routes are preferred at traffic engineering), then the feasible-path uRPF method works. It
AS2 and AS3 over a shorter non-customer route. However, the should be mentioned that the feasible-path uRPF works in this
feasible-path uRPF method has limitations as well. One form of scenario only if customer routes are preferred at AS2 and AS3 over a
limitation naturally occurs when the recommendation (a) or (b) shorter non-customer route. However, the feasible-path uRPF method
mentioned above regarding propagation of prefixes is not followed. 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 Another form of limitation can be described as follows. In Scenario
2 (described here, illustrated in Figure 2), it is possible that the 2 (described here, illustrated in Figure 2), it is possible that the
second transit provider (ISP-b or AS3) does not propagate 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 prepended route for prefix P1 to the first transit provider (ISP-a or
AS2). This is because AS3's decision policy permits giving priority 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 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 longer route learned directly from the customer (AS1). In such a
scenario, AS3 would not send any route announcement for prefix P1 to scenario, AS3 would not send any route announcement for prefix P1 to
AS2 (over the p2p link). Then a data packet with source address in AS2 (over the P2P link). Then, a data packet with a source address
prefix P1 that originates from AS1 and traverses via AS3 to AS2 will in prefix P1 that originates from AS1 and traverses via AS3 to AS2
get dropped at AS2. will 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]
\ / \ /
+-----------------------+ +-----------------------+
| AS1(customer) | | AS1(customer) |
+-----------------------+ +-----------------------+
P1, P2 (prefixes originated) P1, P2 (prefixes originated)
Consider data packets received at AS2 via AS3 Consider data packets received at AS2 via AS3
that originated from AS1 and have source address in P1: that originated from AS1 and have a source address in P1:
* Feasible-path uRPF works (if customer route to P1 * Feasible-path uRPF works (if the customer route to P1
is preferred at AS3 over shorter path) is preferred at AS3 over the shorter path)
* Feasible-path uRPF fails (if shorter path to P1 * Feasible-path uRPF fails (if the shorter path to P1
is preferred at AS3 over customer route) is preferred at AS3 over the 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 Forwarding 2.4. SAV Using Loose Unicast Reverse Path Forwarding
In the loose unicast Reverse Path Forwarding (uRPF) method, an In the loose uRPF method, an ingress packet at the border router is
ingress packet at the border router is accepted only if the FIB has accepted only if the FIB has one or more prefixes that encompass the
one or more prefixes that encompass the source address. That is, a source address. That is, a packet is dropped if no route exists in
packet is dropped if no route exists in the FIB for the source the FIB for the source address. Loose uRPF sacrifices
address. Loose uRPF sacrifices directionality. It only drops directionality. It only drops packets if the source address is
packets if the source address is unreachable in the current FIB unreachable in the current FIB (e.g., IANA special-purpose prefixes
(e.g., IANA special-purpose prefixes [SPAR-v4][SPAR-v6], unallocated, [SPAR-v4][SPAR-v6], unallocated, allocated 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 [RFC4364] The Virtual Routing and Forwarding (VRF) technology [RFC4364]
[Juniper] allows a router to maintain multiple routing table [Juniper] allows a router to maintain multiple routing table
instances separate from the global Routing Information Base (RIB). instances separate from the global Routing Information Base (RIB).
External BGP (eBGP) peering sessions send specific routes to be External BGP (eBGP) peering sessions send specific routes to be
stored in a dedicated VRF table. The uRPF process queries the VRF stored in a dedicated VRF table. The uRPF process queries the VRF
table (instead of the FIB) for source address validation. A 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 table can be dedicated per eBGP peer and used for uRPF for only that
peer, resulting in strict mode operation. For implementing loose peer, resulting in strict mode operation. For implementing loose
uRPF on an interface, the corresponding VRF table would be global, uRPF on an interface, the corresponding VRF table would be global,
i.e., 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 The 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 compromising directionality. The method
method is based on the principle that if BGP updates for multiple is based on the principle that if BGP updates for multiple prefixes
prefixes with the same origin AS were received on different with the same origin AS were received on different interfaces (at
interfaces (at border routers), then incoming data packets with border routers), then incoming data packets with source addresses in
source addresses in any of those prefixes should be accepted on any any of those prefixes should be accepted on any of those interfaces.
of those interfaces. The EFP-uRPF method can be best explained with The EFP-uRPF method can be best explained with an example, as
an example as follows: follows:
Let us say, a border router of ISP-A has in its Adj-RIBs-In [RFC4271] Let us say, in its Adj-RIBs-In [RFC4271], a border router of ISP-A
the set of prefixes {Q1, Q2, Q3} each of which has AS-x as its origin has the set of prefixes {Q1, Q2, Q3}, each of which has AS-x as its
and AS-x is in ISP-A's customer cone. In this set, the border router origin and AS-x is in ISP-A's customer cone. In this set, the border
received the route for prefix Q1 over a customer facing interface, router received the route for prefix Q1 over a customer-facing
while it learned the routes for prefixes Q2 and Q3 from a lateral interface while it learned the routes for prefixes Q2 and Q3 from a
peer and an upstream transit provider, respectively. In this example lateral peer and an upstream transit provider, respectively. In this
scenario, the enhanced feasible-path uRPF method requires Q1, Q2, and example scenario, the enhanced feasible-path uRPF method requires Q1,
Q3 be included in the RPF list for the customer interface under Q2, and Q3 be included in the RPF list for the customer interface
consideration. under consideration.
Thus, the enhanced feasible-path uRPF (EFP-uRPF) method gathers Thus, the EFP-uRPF method gathers feasible paths for customer
feasible paths for customer interfaces in a more precise way (as interfaces in a more precise way (as compared to the feasible-path
compared to feasible-path uRPF) so that all legitimate packets are uRPF) so that all legitimate packets are accepted while the
accepted while the directionality property is not compromised. 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 create the RPF lists for customer interfaces. It can also be extended to create the RPF lists
lateral peer interfaces also. That is, the EFP-uRPF method can be for lateral peer interfaces. 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 to avoid compromising 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 (Figures 1 and 2), the EFP-uRPF
enhanced feasible-path uRPF (EFP-uRPF) method works better than the method works better than the other uRPF methods. Scenario 3
other uRPF methods. Scenario 3 (Figure 3) further illustrates the (Figure 3) further illustrates the enhanced feasible-path uRPF method
enhanced feasible-path uRPF method with a more concrete example. In with a more concrete example. In this scenario, the focus is on
this scenario, the focus is on operation of the feasible-path uRPF at operation of the EFP-uRPF at ISP4 (AS4). ISP4 learns a route for
ISP4 (AS4). ISP4 learns a route for prefix P1 via a customer-to- prefix P1 via a C2P interface from customer ISP2 (AS2). This route
provider (C2P) interface from customer ISP2 (AS2). This route for P1 for P1 has origin AS1. ISP4 also learns a route for P2 via another
has origin AS1. ISP4 also learns a route for P2 via another C2P C2P interface from customer ISP3 (AS3). Additionally, AS4 learns a
interface from customer ISP3 (AS3). Additionally, AS4 learns a route route for P3 via a lateral P2P interface from ISP5 (AS5). Routes for
for P3 via a lateral peer-to-peer (p2p) interface from ISP5 (AS5). all three prefixes have the same origin AS (i.e., AS1). Using the
Routes for all three prefixes have the same origin AS (i.e., AS1). enhanced feasible-path uRPF scheme and given the commonality of the
Using the enhanced feasible-path uRPF scheme, given the commonality origin AS across the routes for P1, P2, and P3, AS4 includes all of
of the origin AS across the routes for P1, P2 and P3, AS4 includes these prefixes in the RPF list for the customer interfaces (from AS2
all of these prefixes in the RPF list for the customer interfaces 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) /
/ \ / / \ /
+----------+ +----------+ / +----------+ +----------+ /
| AS2(ISP2)| | AS3(ISP3)| / | AS2(ISP2)| | AS3(ISP3)| /
+----------+ +----------+ / +----------+ +----------+ /
/\ /\ / /\ /\ /
\ / / \ / /
P1[AS1]\ /P2[AS1] /P3[AS1] P1[AS1]\ /P2[AS1] /P3[AS1]
(C2P)\ /(C2P) /(C2P) (C2P)\ /(C2P) /(C2P)
\ / / \ / /
+----------------+ / +----------------+ /
| AS1(customer) |/ | AS1(customer) |/
+----------------+ +----------------+
P1, P2, P3 (prefixes originated) P1, P2, P3 (prefixes originated)
Consider that data packets (sourced from AS1) Consider that data packets (sourced from AS1)
may be received at AS4 with source address may be received at AS4 with a 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 The underlying algorithm in the solution method described above
(Section 3.1) can be specified as follows (to be implemented in a (Section 3.1) can be specified as follows (to be implemented in a
transit AS): transit AS):
1. Create the set of unique origin ASes considering only the routes 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, in the Adj-RIBs-In of customer interfaces. Call it Set A = {AS1,
AS2, ..., ASn}. AS2, ..., ASn}.
2. Considering all routes in Adj-RIBs-In 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 the RPF list on all customer interfaces on
interfaces on which one or more of the prefixes in set X1 were 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).
The above algorithm can be extended to apply EFP-uRPF method to The above algorithm can also be extended to apply the EFP-uRPF method
lateral peer interfaces also. However, it is left up to the operator to lateral peer interfaces. However, it is left up to the operator
to decide whether they should apply EFP-uRPF or loose uRPF method on to decide whether they should apply the EFP-uRPF or loose uRPF method
lateral peer interfaces. The loose uRPF method is recommended to be on lateral peer interfaces. The loose uRPF method is recommended to
applied on transit provider interfaces. be 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 multihomed stub AS:
o A multi-homed stub AS should announce at least one of the prefixes * A multihomed 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 * 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 * 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, which poses a
which poses a challenge for the enhanced feasible-path uRPF (as well challenge for the enhanced feasible-path uRPF (as well as for
as for traditional feasible-path uRPF). In the scenario illustrated traditional feasible-path uRPF), may be constructed. In the scenario
in Figure 4, since routes for neither P1 nor P2 are propagated on the illustrated in Figure 4, since routes for neither P1 nor P2 are
AS2-AS4 interface (due to the presence of NO_EXPORT Community), the propagated on the AS2-AS4 interface (due to the presence of NO_EXPORT
enhanced feasible-path uRPF at AS4 will reject data packets received Community), the enhanced feasible-path uRPF at AS4 will reject data
on that interface with source addresses in P1 or P2. (For a little packets received on that interface with source addresses in P1 or P2.
more complex example scenario, see slide #10 in [sriram-urpf].) (For a little 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) / \
+----------+ +----------+ +----------+ +----------+
skipping to change at page 11, line 39 skipping to change at line 497
\ / P1[AS1] \ / P1[AS1]
P1[AS1] NO_EXPORT \ / P2[AS1] P1[AS1] NO_EXPORT \ / P2[AS1]
P2[AS1] NO_EXPORT \ / (C2P) P2[AS1] NO_EXPORT \ / (C2P)
(C2P) \ / (C2P) \ /
+----------------+ +----------------+
| AS1(customer) | | AS1(customer) |
+----------------+ +----------------+
P1, P2 (prefixes originated) P1, P2 (prefixes originated)
Consider that data packets (sourced from AS1) Consider that data packets (sourced from AS1)
may be received at AS4 with source address may be received at AS4 with a source address
in P1 or P2 via AS2: in P1 or P2 via AS2:
* Feasible-path uRPF fails * Feasible-path uRPF fails
* Loose uRPF works (but not desirable) * Loose uRPF works (but not desirable)
* Enhanced Feasible-path uRPF with Algorithm A fails * Enhanced feasible-path uRPF with Algorithm A fails
* Enhanced Feasible-path uRPF with Algorithm B works best * Enhanced feasible-path uRPF with Algorithm B works best
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-RIBs-In. Including the
additional degree of flexibility, the modified algorithm called additional degree of flexibility, the modified algorithm called
Algorithm B (implemented in a transit AS) can be described as Algorithm B (implemented in a transit AS) can be described as
follows: 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-RIBs-In 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-RIBs-In 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}.
skipping to change at page 12, line 38 skipping to change at line 539
Adj-RIBs-In 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 the proposed
proposed EFP-uRPF method with Algorithm B is a much better choice EFP-uRPF method with Algorithm B is still a much better choice (for
(for the scenario under consideration) than applying the loose uRPF 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 the 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. described in Section 3.3.
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 this It is worth emphasizing that an indirect part of the proposal in this
document 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. Special care should be exercised when using IRR Registry (IRR) data. Special care should be exercised when using IRR
data because it not always accurate or trusted. In the EFP-uRPF 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 method with Algorithm A (see Section 3.1.1), if a ROA includes prefix
Pi and ASj, then augment with Pi the RPF list of each customer Pi and ASj, then augment the RPF list of each customer interface on
interface on which at least one route with origin ASj was received. 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 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 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 augment the RPF list Z in Step 5 of Algorithm B with prefix Pi.
procedures can be followed with reliable IRR data as well. This will Similar procedures can be followed with reliable IRR data as well.
help make the RPF lists more robust about source addresses that may This will help make the RPF lists more robust about source addresses
be legitimately used by customers of the ISP. 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 (Sections 3.1.1 and 3.4) described here,
described here, the RPF lists are constructed by applying a set of the RPF lists are constructed by applying a set of rules to all
rules to all received BGP routes (not just those selected as best received BGP routes (not just those selected as best path and
path and installed in the FIB). The concept of uRPF querying an RPF installed in the FIB). The concept of uRPF querying an RPF list
list (instead of the FIB) is similar to uRPF querying a VRF table (instead of the FIB) is similar to uRPF querying a VRF table (see
(see (Section 2.5). 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., ternary content addressable memory be additional memory (i.e., ternary content-addressable memory
(TCAM)) available to store the RPF lists in line cards. For an ISP's (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 AS, the RPF list size for each line card will roughly equal the total
number of originated prefixes from ASes in its customer cone number of originated prefixes from ASes in its customer cone
(assuming Algorithm B in Section 3.4 is used). (Note: EFP-uRPF with (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- Algorithm A (see Section 3.1.1) requires much less memory than EFP-
uRPF with Algorithm B.) uRPF with Algorithm B.)
The following table shows the measured customer cone sizes in number The following table shows the measured customer cone sizes in number
of prefixes originated (from all ASes in the customer cone) for 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 | Measured Customer Cone Size in # |
| | # Prefixes (in turn this is an | | ISP | Prefixes (in turn this is an estimate |
| | estimate for RPF list size on | | | for RPF list size on the line card) |
| | the line card) | +============+=======================================+
+---------------------------------+---------------------------------+ | Very Large | 32393 |
| Very Large Global ISP #1 | 32393 | | Global ISP | |
| ------------------------------- | ------------------------------- | | #1 | |
| Very Large Global ISP #2 | 29528 | +------------+---------------------------------------+
| ------------------------------- | ------------------------------- | | Very Large | 29528 |
| Large Global ISP | 20038 | | Global ISP | |
| ------------------------------- | ------------------------------- | | #2 | |
| Mid-size Global ISP | 8661 | +------------+---------------------------------------+
| ------------------------------- | ------------------------------- | | Large | 20038 |
| Regional ISP (in Asia) | 1101 | | Global ISP | |
+---------------------------------+---------------------------------+ +------------+---------------------------------------+
| Mid-size | 8661 |
| Global ISP | |
+------------+---------------------------------------+
| Regional | 1101 |
| ISP (in | |
| Asia) | |
+------------+---------------------------------------+
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
at ASes at the edges of the Internet where the customer cone sizes ASes at the edges of the Internet where the customer cone sizes are
are smaller as shown in Table 1. 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 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 midsize 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 midsize 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
effective closer to the edges of the Internet. effective closer to the edges of the Internet.
3.6.2. Coping with BGP's Transient Behavior 3.6.2. Coping with BGP's Transient Behavior
BGP routing announcements can exhibit transient behavior. Routes may BGP routing announcements can exhibit transient behavior. Routes may
be withdrawn temporarily and then re-announced due to transient be withdrawn temporarily and then reannounced due to transient
conditions such as BGP session reset or link failure-recovery. To conditions, such as BGP session reset or link failure recovery. To
cope with this, hysteresis should be introduced in the maintenance of cope with this, hysteresis should be introduced in the maintenance of
the RPF lists. Deleting entries from the RPF lists SHOULD be delayed the RPF lists. Deleting entries from the RPF lists SHOULD be delayed
by a pre-determined amount (the value based on operational by a predetermined amount (the value based on operational experience)
experience) when responding to route withdrawals. This should help when responding to route withdrawals. This should help suppress the
suppress the effects due to the transients in BGP. 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, the AS under consideration SHOULD perform ACL-based to the AS, the AS under consideration SHOULD perform ACL-based
source address validation (SAV). 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:
* The enhanced feasible-path uRPF (EFP-uRPF) method with * The EFP-uRPF method with Algorithm B (see Section 3.4) SHOULD
Algorithm B (see Section 3.4) SHOULD be applied on customer be applied on customer interfaces.
interfaces.
* Loose uRPF method SHOULD be applied on lateral peer and * The loose uRPF method SHOULD be applied on lateral peer and
transit provider interfaces. transit provider interfaces.
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 pertaining RPF lists (see Section 3.5 for used to augment the pertaining RPF lists (see Section 3.5 for
details). details).
3.7.1. Applicability of the enhanced feasible-path uRPF (EFP-uRPF) 3.7.1. Applicability of the EFP-uRPF Method with Algorithm A
method with Algorithm A
EFP-uRPF method with Algorithm A is not mentioned in the above set of The EFP-uRPF method with Algorithm A is not mentioned in the above
recommendations. It is an alternative to EFP-uRPF with Algorithm B set of recommendations. It is an alternative to EFP-uRPF with
and can be used in limited circumstances. The EFP-uRPF method with Algorithm B and can be used in limited circumstances. The EFP-uRPF
Algorithm A is expected to work fine if an ISP deploying it has only method with Algorithm A is expected to work fine if an ISP deploying
multi-homed stub customers. It is trivially equivalent to strict it has only multihomed stub customers. It is trivially equivalent to
uRPF if an ISP deploys it for a single-homed stub customer. More strict uRPF if an ISP deploys it for a single-homed stub customer.
generally, it is also expected to work fine when there is absence of More generally, it is also expected to work fine when there is
limitations such as those described in Section 3.3. However, caution absence of limitations, such as those described in Section 3.3.
is required for use of EFP-uRPF with Algorithm A because even if the However, caution is required for use of EFP-uRPF with Algorithm A
limitations are not expected at the time of deployment, the because even if the limitations are not expected at the time of
vulnerability to change in conditions exists. It may be difficult deployment, the vulnerability to change in conditions exists. It may
for an ISP to know or track the extent of use of NO_EXPORT (see be difficult for an ISP to know or track the extent of use of
Section 3.3) on routes within its customer cone. If an ISP decides NO_EXPORT (see Section 3.3) on routes within its customer cone. If
to use EFP-uRPF with Algorithm A, it should make its direct customers an ISP decides to use EFP-uRPF with Algorithm A, it should make its
aware of the operational recommendations in Section 3.2. This means direct customers aware of the operational recommendations in
that the ISP notifies direct customers that at least one prefix Section 3.2. This means that the ISP notifies direct customers that
originated by each AS in the direct customer's customer cone must at least one prefix originated by each AS in the direct customer's
propagate to the ISP. customer cone must propagate to the ISP.
On a lateral peer interface, an ISP may choose to apply the EFP-uRPF On a lateral peer interface, an ISP may choose to apply the EFP-uRPF
method with Algorithm A (with appropriate modification of the method with Algorithm A (with appropriate modification of the
algorithm). This is because stricter forms of uRPF (than the loose algorithm). This is because stricter forms of uRPF (than the loose
uRPF) may be considered applicable by some ISPs on interfaces with uRPF) may be considered applicable by some ISPs on interfaces with
lateral peers. 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 RFC 3704
apply for this document as well. In addition, if considering using [RFC3704] apply for this document as well. In addition, if
EFP-uRPF method with Algorithm A, an ISP or AS operator should be considering using the EFP-uRPF method with Algorithm A, an ISP or AS
aware of the applicability considerations and potential operator should be aware of the applicability considerations and
vulnerabilities discussed in Section 3.7.1. potential vulnerabilities discussed in Section 3.7.1.
In augmenting RPF lists with ROA (and possibly reliable IRR) In augmenting RPF lists with ROA (and possibly reliable IRR)
information (see Section 3.5), a trade-off is made in favor of information (see Section 3.5), a trade-off is made in favor of
reducing false positives (regarding invalid detection in SAV) at the reducing false positives (regarding invalid detection in SAV) at the
expense of a slight other risk. The other risk being a malicious expense of another slight risk. The other risk being that a
actor at another AS in the neighborhood within the customer cone malicious actor at another AS in the neighborhood within the customer
might take advantage (of the augmented prefix) to some extent. This cone might take advantage (of the augmented prefix) to some extent.
risk also exists even with normal announced prefixes (i.e., without This risk also exists even with normal announced prefixes (i.e.,
ROA augmentation) for any uRPF method other than the strict. without ROA augmentation) for any uRPF method other than the strict
However, the risk is mitigated if the transit provider of the other uRPF. However, the risk is mitigated if the transit provider of the
AS in question is performing SAV. other AS in question is performing SAV.
Though not within the scope of this document, security hardening of Though not within the scope of this document, security hardening of
routers and other supporting systems (e.g., Resource PKI (RPKI) and routers and other supporting systems (e.g., Resource PKI (RPKI) and
ROA management systems) against compromise is extremely important. ROA management systems) against compromise is extremely important.
The compromise of those systems can affect the operation and The compromise of those systems can affect the operation and
performance of the SAV methods described in this document. 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 has no IANA actions.
does not create any new IANA registries.
6. 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.
7. References 6. References
7.1. Normative References 6.1. 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>.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <https://www.rfc-editor.org/info/rfc2827>. May 2000, <https://www.rfc-editor.org/info/rfc2827>.
skipping to change at page 17, line 34 skipping to change at line 763
[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 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References 6.2. Informative References
[CAIDA] "Information for AS 174 (COGENT-174)", CAIDA Spoofer [CAIDA] CAIDA, "Information for AS 174 (COGENT-174)", October
Project , <https://spoofer.caida.org/as.php?asn=174>. 2019, <https://spoofer.caida.org/as.php?asn=174>.
[Cisco1] "Internet Routing Table Growth Causes ROUTING-FIB- [Cisco1] Cisco, "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", January
Trouble-shooting Tech-notes , January 2014, 2014, <https://www.cisco.com/c/en/us/support/docs/routers/
<https://www.cisco.com/c/en/us/support/docs/routers/asr- asr-9000-series-aggregation-services-routers/116999-
9000-series-aggregation-services-routers/116999-problem- problem-line-card-00.html>.
line-card-00.html>.
[Cisco2] "Cisco Nexus 7000 Series NX-OS Unicast Routing [Cisco2] Cisco, "Cisco Nexus 7000 Series NX-OS Unicast Routing
Configuration Guide, Release 5.x (Chapter 15: Managing the Configuration Guide, Release 5.x (Chapter 15: 'Managing
Unicast RIB and FIB)", Cisco Configuration Guides , March the Unicast RIB and FIB')", March 2018,
2018, <https://www.cisco.com/c/en/us/td/docs/switches/data <https://www.cisco.com/c/en/us/td/docs/switches/
center/sw/5_x/nx- datacenter/sw/5_x/nx-
os/unicast/configuration/guide/l3_cli_nxos/ os/unicast/configuration/guide/l3_cli_nxos/
l3_NewChange.html>. l3_NewChange.html>.
[Firmin] Firmin, F., "The Evolved Packet Core", 3GPP The Mobile [Firmin] Firmin, F., "The Evolved Packet Core",
Broadband Standard , <https://www.3gpp.org/technologies/ <https://www.3gpp.org/technologies/keywords-acronyms/100-
keywords-acronyms/100-the-evolved-packet-core>. the-evolved-packet-core>.
[ISOC] Vixie (Ed.), P., "Addressing the challenge of IP [ISOC] Internet Society, "Addressing the challenge of IP
spoofing", ISOC report , September 2015, spoofing", 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] Juniper Networks, "Creating Unique VPN Routes Using VRF
Networks TechLibrary , March 2019, Tables", May 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 [Luckie] Luckie, M., Huffaker, B., Dhamdhere, A., Giotsas, V., and
kc. claffy, "AS Relationships, Customer Cones, and kc. claffy, "AS Relationships, customer cones, and
Validation", In Proceedings of the 2013 ACM Internet validation", In Proceedings of the 2013 Internet
Measurement Conference (IMC), DOI 10.1145/2504730.2504735, Measurement Conference, DOI 10.1145/2504730.2504735,
October 2013, October 2013,
<http://www.caida.org/~amogh/papers/asrank-IMC13.pdf>. <https://dl.acm.org/doi/10.1145/2504730.2504735>.
[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>.
skipping to change at page 19, line 14 skipping to change at line 828
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R. [RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811, Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013, DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>. <https://www.rfc-editor.org/info/rfc6811>.
[RFC7454] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations [RFC7454] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454, and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
February 2015, <https://www.rfc-editor.org/info/rfc7454>. February 2015, <https://www.rfc-editor.org/info/rfc7454>.
[SPAR-v4] "IANA IPv4 Special-Purpose Address Registry", IANA , [SPAR-v4] IANA, "IANA IPv4 Special-Purpose Address Registry",
<https://www.iana.org/assignments/iana-ipv4-special- <https://www.iana.org/assignments/iana-ipv4-special-
registry/iana-ipv4-special-registry.xhtml>. registry/>.
[SPAR-v6] "IANA IPv6 Special-Purpose Address Registry", IANA , [SPAR-v6] IANA, "IANA IPv6 Special-Purpose Address Registry",
<https://www.iana.org/assignments/iana-ipv6-special- <https://www.iana.org/assignments/iana-ipv6-special-
registry/iana-ipv6-special-registry.xhtml>. registry/>.
[sriram-ripe63] [Sriram-RIPE63]
Sriram, K. and R. Bush, "Estimating CPU Cost of BGPSEC on Sriram, K. and R. Bush, "Estimating CPU Cost of BGPSEC on
a Router", Presented at RIPE-63; also, at IETF-83 SIDR WG a Router", Presented at RIPE 63 and at the SIDR WG meeting
Meeting, March 2012, at IETF 83, March 2012,
<http://www.ietf.org/proceedings/83/slides/ <http://www.ietf.org/proceedings/83/slides/slides-83-sidr-
slides-83-sidr-7.pdf>. 7.pdf>.
[sriram-urpf] [Sriram-URPF]
Sriram et al., K., "Enhanced Feasible-Path Unicast Reverse Sriram, K., Montgomery, D., and J. Haas, "Enhanced
Path Filtering", Presented at the OPSEC WG Meeting, Feasible-Path Unicast Reverse Path Filtering", Presented
IETF-101 London , March 2018, at the OPSEC WG meeting at IETF 101, March 2018,
<https://datatracker.ietf.org/meeting/101/materials/ <https://datatracker.ietf.org/meeting/101/materials/
slides-101-opsec-draft-sriram-opsec-urpf-improvements-00>. 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 Authors' Addresses
Kotikalapudi Sriram Kotikalapudi Sriram
USA National Institute of Standards and Technology USA National Institute of Standards and Technology
100 Bureau Drive 100 Bureau Drive
Gaithersburg MD 20899 Gaithersburg, MD 20899
USA United States of America
Email: ksriram@nist.gov Email: ksriram@nist.gov
Doug Montgomery Doug Montgomery
USA National Institute of Standards and Technology USA National Institute of Standards and Technology
100 Bureau Drive 100 Bureau Drive
Gaithersburg MD 20899 Gaithersburg, MD 20899
USA United States of America
Email: dougm@nist.gov Email: dougm@nist.gov
Jeffrey Haas Jeffrey Haas
Juniper Networks, Inc. Juniper Networks, Inc.
1133 Innovation Way 1133 Innovation Way
Sunnyvale CA 94089 Sunnyvale, CA 94089
USA United States of America
Email: jhaas@juniper.net Email: jhaas@juniper.net
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