SAVNET                                                         W. Cheng
Internet-Draft                                             China Mobile
Intended status: Standards Track                                 C. Lin
Expires: December 30, 2025                         New H3C Technologies
                                                                 S. Yue
                                                           China Mobile
                                                          June 30, 2025



                     Intra-domain SAV Support via BGP
                draft-cheng-savnet-intra-domain-sav-bgp-03


Abstract

   This document describes a method for publishing source prefixes via
   the BGP protocol, iterating through the SAV table entries based on
   intra-domain next hop SAV rules. The generation of intra-domain next
   hop SAV rules is implemented by the intra-domain IGP protocol, and
   the BGP protocol inherits the source interface list from its next
   hop SAV rules to generate the SAV rule table for source prefixes.

Status of this Memo

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   This Internet-Draft will expire on December 30, 2025.

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   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document. Please review these documents



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   carefully, as they describe your rights and restrictions with
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Table of Contents


   1. Introduction...................................................2
   2. Terminology....................................................3
   3. Solution.......................................................3
      3.1. Overview..................................................3
      3.2. Intra-domain SAVNET.......................................6
      3.3. Procedure.................................................8
      3.4. Multi-Homed Scenarios.....................................8
      3.5. BGP Source Prefix Filtering..............................10
   4. Example.......................................................10
   5. Deployment Considerations.....................................11
      5.1. Intra-domain SAV Support via IGP and BGP.................11
   6. IANA Considerations...........................................11
   7. Security Considerations.......................................12
   8. References....................................................12
      8.1. Normative References.....................................12
      8.2. Informative References...................................12
   Acknowledgments..................................................12
   Authors' Addresses...............................................12

   1. Introduction

   As shown in Figure 1, the existing network has the following
   scenario: within the intra-domain network, topology information is
   disseminated via the IGP protocol, while prefix information is
   distributed via iBGP neighbors. All iBGP nodes establish iBGP
   neighbor relationships with Route Reflectors (RRs) and exchange
   source prefix information. The IGP protocol contains network
   topology information but lacks source prefix information, while the
   BGP protocol holds source prefix information but does not include
   network topology information. In this scenario, it is necessary to
   combine the network topology information from IGP with the source
   prefix information from the BGP protocol in order to compute the
   source prefix's associated source port information and generate SAV
   rules.

   +---------------------------------------------------------+
   |                                                   AS    |
   |                                                         |
   |      iBGP ===========  RR  =========== iBGP             |

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   |   +---------+    (           )      +---------+         |
   |   |iBGP Node|---( IGP Network )-----|iBGP Node|         |
   |   +---------+    (           )      +---------+         |
   +---------------------------------------------------------+

                Figure 1: The case of the SAVNET Procedure



   The scenario described in [draft-cheng-savnet-intra-domain-sav-IGP-
   00] and [I-D.lin-Intra-domain-savnet-method] involves the
   publication of SAV source prefix by the IGP protocol, and the
   generation of SAV rules based on the connectivity calculation using
   the IGP's topology information. However, for the scenario described
   in this document, where the source prefix information is published
   by the BGP protocol, it is unable to generate the required SAV
   rules.

   This document describes how to generate SAV rules using the topology
   information from the IGP protocol and the source prefix information
   from the BGP protocol in this network scenario.



   2. Terminology

   The following terminologies are used in this document.

   SAV Rule: The rule that indicates the source validity of a specific
   IP address or an IP prefix.

   SAV Table: The table or data structure that implements the SAV rules
   and is used for source address validation in the data plane.

   IGP: Interior Gateway Protocol.

   BGP: Border Gateway Protocol.

   Source prefix: The source prefixes are used to validate source
   addresses in the data plane.

   3. Solution

   3.1. Overview


   As shown in Figure  , [draft-ietf-savnet-intra-domain-architecture-
   00] describes SAV filtering on three types of interfaces: interfaces

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   on host-facing routers, such as interface 1 on Router C as shown in
   the figure; interfaces on customer-facing routers, such as interface
   1 on Router A and interface 1 on Router B as shown in the figure;
   and interfaces on AS border routers, such as interface 2 on Router D
   and interface 2 on Router E when facing another AS.
   For the first two types of interfaces, SAV filtering is primarily
   carried out using an allow-list. For the third type of interface,
   filtering is primarily implemented using a block-list.
   This document also describes SAV filtering on the interfaces of
   routers within the domain, such as Interface 1 of Router D and
   Interface 1 of Router E, as depicted in the diagram. Such interfaces
   employ allow-list based SAV filtering, with the computation process
   outlined in Section 3.3.
   The specific details about allow-list and block-list are detailed
   below.


                      +----------------------------------+
                      |            Other ASes            |
                      +----------------------------------+
                         |                            |
      +------------------|----------------------------|--------------+
      |  Intra-domain    |                            |              |
      |   iBGP           |                            |              |
      |                  |2                           |2             |
      |            +----+#+---+                 +----+#+---+         |
      |            | Router D |                 | Router E |         |
      |            +----+#+---+                 +----+#+---+         |
      |                  |1                           |1             |
      |                  |                            |              |
      |                  |                            |              |
      |            +----+#+--------------------------+#+----+        |
      |            |      Other intra-domain routers        |        |
      |            +-+#+---+#+-----------------------+#+----+        |
      |              /       \                        |              |
      |             /         \                       |              |
      |            /           \                      |              |
      |     +----+#+---+  +----+#+---+          +----+#+---+         |
      |     | Router A |  | Router B |          | Router C |         |
      |     +---+#+----+  +------+#+-+          +----+#+---+         |
      |           \1             /1                   |1             |
      +------------\------------/---------------------|--------------+
                    \          /                      |
                  +--------------+            +--------------+
                  |   Customer   |            |    Host      |
                  |   Network    |            |   Network    |
                  +--------------+            +--------------+
          Figure 2:  Overview of intra-domain SAVNET architecture

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   The term allow-list means that if a packet received from a specified
   interface has a source address found in the interface's allow-list,
   then the packet should be permitted to pass through. For example,
   for an allow-list entry (P1, A-1), if a packet received from
   interface A-1 has a source address within the range of P1, then the
   packet will be allowed to pass through. Otherwise, the packet will
   be discarded.

   The term block-list means that if a packet received from a specified
   interface has a source address found in the interface's block-list,
   then the packet must be discarded. Otherwise, the packet will be
   allowed to pass through.

   For the host-facing router interfaces, the allow-list can be
   generated for directly connected host segments, as shown in Figure
   2. If the interface 1 network segment address of Router C is P, the
   allow-list (P, C-1) can be generated for Router C's interface 1.



   For the ports of routers facing the customers, as shown in Figure 2,
   if the interface 1 network segment address of Router A is P1, and
   the interface 1 network segment address of Router B is P2, the
   allow-lists (P1, A-1) and (P2, A-1) can be generated for Router A's
   interface 1, and the allow-lists (P1, B-1) and (P2, B-1) can be
   generated for Router B's interface 1.

   For the router interfaces facing the customers, as shown in Figure
   2, where Router A's interface 1 belongs to network segment address
   P1 and Router B's interface 1 belongs to network segment address P2,
   the allow-lists (P1, A-1) and (P2, A-1) can be generated for Router
   A's interface 1, and the allow-lists (P1, B-1) and (P2, B-1) can be
   generated for Router B's interface 1.

   For the inter-domain router interfaces, as depicted in Figure 4,
   where Router D's interface 1 and Router E's interface 1 are
   involved, the calculation process described in section 4.3 can be
   used to generate the allow-lists for Router D's interface 1 and
   Router E's interface 1.

   For the interface facing another AS, as shown in Figure 2, such as
   Router D's interface 2 and Router E's interface 2, this scenario
   typically involves using block-lists for SAV filtering. The
   generation of block-lists on the edge interfaces connecting to
   external domains can be achieved by generating block-list entries on
   the upper interface when learning routes from the lower interface.

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   For example, if Router D learns the prefix P on interface 1, then a
   block-list entry (P, D-2) can be generated on interface 2.

   3.2. Intra-domain SAVNET

   This section introduces a new method for computing and generating
   SAV rules based on BGP source prefix and IGP topology information in
   an intra-domain scenario. This method relies on two fundamental
   pieces of information: the source prefix information and
   reachability information. The source prefix information can be
   transmitted through static configuration or the BGP protocol. This
   document addresses the scenario where source prefix information is
   transmitted via the BGP protocol.

   The source prefix information consists of the source prefix and the
   next-hop information for the prefix publication.

   The IGP's topology information includes the connectivity details
   between nodes and the IGP prefix information published by each node.

   As depicted in Figure 3, source prefix information is disseminated
   via the BGP protocol, where Router C advertises the source prefix
   Prefix1 with a next hop of Router 2, Router D advertises the source
   prefix Prefix2 with a next hop of Router 3, and Router E advertises
   the source prefix Prefix3 with a next hop of Router 4. Router B,
   serving as the BGP Route Reflector, is responsible for collecting
   and reflecting all BGP source prefix information.

   Based on the IGP's topology information, the interface list
   corresponding to the IGP prefix can be calculated. The specific
   calculation process can be found in [draft-cheng-savnet-intra-
   domain-sav-BGP-00]. The first-level NextHop SAV rule table is
   generated based on this information in the form of (IGP-Prefix, if).

   The calculation of the next-hop SAV rule is not limited to IGP and
   can involve other new extended protocols not described in this
   document.

   Subsequently, using the source prefix information distributed via
   the iBGP protocol, a match is made against the first-level SAV rule
   table based on the source prefix information. Once a match is found,
   the interface list "if" is inherited to produce the second-level SAV
   rule table (BGP-Prefix, if). This document relies on the generation
   of SAV rules based on the next hop derived from the IGP protocol.
   The relationships of the generated SAV rule table are illustrated in
   Figure 4.

   +---------------------------------------------------------+

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   |                                                   AS    |
   |                    iBGP Router1                         |
   | SAV Rule:          +--------3-+                         |
   | (Prefix1, A-1)     | Router A |                         |
   | (Prefix2, A-1)     +1--------2+                         |
   | (Prefix3, A-2)      /\       /\                         |
   |                     /         \                         |
   |                    /           \                        |
   |                   /            \
   |                  /              \
   |   RR       +----------+      +----------+               |
   |iBGP Router5| Router B |      | Router E | iBGP Router6  |
   |            +1--------2+      +---------1+               |
   |             /\       /\                /\               |
   |             /         \                 \               |
   |            /           \                 \              |
   |      +----------+   +----------+      +----------+      |
   |      | Router C |   | Router D |      | Router F |      |
   |      +----------+   +----------+      +----------+      |
   |          |               |                  |           |
   |   iBGP Router2      iBGP Router3      iBGP Router4      |
   |    Prefix1           Prefix2           Prefix3          |
   +---------------------------------------------------------+
               Figure 3:  Example 1 of Topology Calculation


   This approach enables automatic adjustment of SAV table entries
   based on topological changes, thereby achieving secure protection
   for source addresses within the domain.





        +----------+                 +----------+
        | BGP Route|                 | IGP LSDB |
        +----------+                 +----------+
         |      |                         |
         |      |                         V
         |      V                    +------------+
         |  NextHop  <- - - - ->     |NextHop SAV |
         |      |                    +------------+
         |      |
         V      V
       Source  NextHop
       Prefix  If
             |
             V

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       +------------+
       | BGP SAV    |
       +------------+
            Figure 4:  The calculation process of BGP SAV table


   3.3. Procedure

   The calculation process for intra-domain SAV rules based on BGP is
   as follows:

   Step 1: Perform calculation based on the LSDB of the IGP protocol
   and generate first-level SAV rules using the prefix information
   published by the IGP nodes. The generated SAV rule takes the form:
   (IGP-Prefix, if). This also forms the next-hop SAV table required
   for BGP.

   Step 2: Iterate through all source prefix information distributed by
   the BGP protocol. For each source prefix, match it with the
   corresponding next-hop information of the publisher. Then, search
   and match this next-hop address in the SAV rules generated in Step
   1. Obtain and utilize the inherited interface list from the first-
   level SAV rules to generate second-level SAV rules. The generated
   rules take the form: (BGP-Prefix, if).

   Step 3: If there are changes in the topological information of the
   IGP protocol, repeat the calculation in Step 1. If there are changes
   in the SAV rules generated in Step 1, the BGP protocol refreshes the
   (BGP-Prefix, if) rule table based on the next-hop associated SAV
   table, thus skipping Step 2.

   Step 4: If there are changes in the source prefix information
   distributed by the BGP protocol, skip Step 1 and proceed with the
   calculation according to Step 2, refreshing the rule list generated
   by BGP.

   3.4. Multi-Homed Scenarios

   In the multi-homed scenario depicted in Figure 5, the prefix
   10.0.0.0/16 is accessed via nodes D and E, with the subnet
   10.0.1.0/24 being accessed via node D, and 10.0.0.0/24 being
   accessed via node E. Through IGP connectivity calculation, the SAV
   table entries derived on node A are: (10.0.1.0/24, A-1),
   (10.0.0.0/24, A-2).

   +---------------------------------------------------+
   |                   iBGP 11.11.11.11           AS   |
   | Node A SAV Rule:   +--------+                     |

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   | (10.0.1.0/24, A-1) | Node A |                     |
   | (10.0.0.0/24, A-2) +1------2+                     |
   |                     /      \                      |
   |                    /        \                     |
   |                   /          \                    |
   |            +--------+      +--------+             |
   |            | Node B |      | Node D |             |
   |            +-+------+      +---+----+             |
   |              |                 |                  |
   |              |                 |                  |
   |              |                 |                  |
   |         +----+2--+         +---+2---+             |
   |         | Node C |         | Node E |             |
   |         +----+1--+         +---+1---+             |
   |              |                 |                  |
   |        iBGP 22.22.22.22   iBGP 33.33.33.33        |
   |         (10.0.1.0/24     10.0.0.0/24)             |
   |           (      10.0.0.0/16      )               |
   +---------------------------------------------------+

                 Figure 5: The case of Multi-homing Access

   In the scenario of single-homed user network access to the local
   network, ACL or strict uRPF can generally provide effective
   protection. However, in the case of multi-homed user network access,
   there may be asymmetric routing and the prefixes of asymmetric
   routes on the router may dynamically change to achieve dynamic load
   balancing. In this case, ACL or strict uRPF cannot be directly used
   to protect the user access interface as a allow-list, as this may
   lead to issues of misfiltering.

   In the mentioned network, Node C could receive packets with a source
   address of 10.0.0.0/24 via interface C-1, and Node A could receive
   similar packets via interface A-1, potentially causing misfiltering.
   Similarly, Node E could receive packets with a source address of
   10.0.1.0/24 via interface E-1, and Node A could receive similar
   packets via interface A-2, leading to potential misfiltering of
   packets.

   To address this issue , one approach is to configure a static route
   (10.0.0.0/24, C-1) on the interface C-1 of Node C, and when
   advertising this route through BGP, specify a higher Prefer value
   than the Prefer value of this route on Node E. Similarly, on Node E,
   configure a static route (10.0.1.0/24, E-1) and when advertising it
   through BGP, specify a higher Prefer value than the Prefer value of
   this route on Node C.

   The resulting SAV entries are as follows:

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   Entries on Node C: (10.0.1.0/24, C-1), (10.0.0.0/24, C-1)
   Entries on Node E: (10.0.0.0/24, E-1), (10.1.0.0/24, E-1)
   Entries on Node A: (10.0.1.0/24, A-1), (10.0.0.0/24, A-1)
                      (10.0.1.0/24, A-2), (10.0.0.0/24, A-2)


   3.5. BGP Source Prefix Filtering

   In actual network deployment, some source prefixes do not need to be
   filtered. To distinguish which source prefixes require filtering,
   BGP community attributes can be used to identify the source prefixes
   that require filtration.

   When the BGP protocol advertises a source prefix, it includes the
   specified community attribute to indicate that this source prefix
   requires SAV calculation. On the devices performing SAV calculation,
   SAV rules are generated only for routes with the specified community
   attribute.

   4. Example

   +---------------------------------------------------------+
   |                                                   AS    |
   |                   iBGP 11.11.11.11                      |
   | SAV Rule:          +--------3-+                         |
   | (10.0.0.0/24, A-1) | Router A |                         |
   | (20.0.0.0/24, A-1) +1--------2+                         |
   | (30.0.0.0/24, A-2)  /\       /\                         |
   |                     /         \                         |
   |                    /           \                        |
   |                   /            \
   |                  /              \
   |   RR       +----------+      +----------+               |
   |iBGP        | Router B |      | Router E | iBGP          |
   |55.55.55.55 +1--------2+      +---------1+ 66.66.66.66   |
   |             /\       /\                /\               |
   |             /         \                 \               |
   |            /           \                 \              |
   |      +----------+   +----------+      +----------+      |
   |      | Router C |   | Router D |      | Router F |      |
   |      +----------+   +----------+      +----------+      |
   |          |               |                  |           |
   |   iBGP 22.22.22.22  iBGP 33.33.33.33  iBGP 44.44.44.44  |
   |    P: 10.0.0.0/24   P: 20.0.0.0/24    P:30.0.0.0/24     |
   +---------------------------------------------------------+
               Figure 7:  Example 3 of Topology Calculation



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   After conducting internal IGP calculations, on router A, it is
   determined that BGP neighbors reachable via A-1 are 22.22.22.22 and
   33.33.33.33. BGP neighbor 44.44.44.44 is reachable via A-2.

   Following the source prefix calculation in BGP, inheriting the
   outgoing interface information from the connectivity calculation,
   router A can compute the following sav table entries: (10.0.0.0/24,
   A-1) obtained from BGP neighbor 22.22.22.22, (20.0.0.0/24, A-1)
   obtained from BGP neighbor 33.33.33.33, and (30.0.0.0/24, A-2)
   obtained from BGP neighbor 44.44.44.44.

   5. Deployment Considerations

   5.1. Intra-domain SAV Support via IGP and BGP

   If the network topology information and source prefix information
   within the domain are both conveyed by the IGP protocol, SAV rules
   can be automatically generated following the calculation method
   described in [draft-cheng-savnet-intra-domain-sav-IGP-00] or [I-
   D.lin-Intra-domain-savnet-method].

   If in the network, the intra-domain network topology information is
   conveyed by the IGP protocol, while the intra-domain source prefix
   information is transmitted via the BGP protocol, this SAV
   calculation method can be deployed to generate SAV rules for
   preventing source address attacks in outbound and inbound traffic.

   If the intra-domain source prefixes are transmitted via BGP, while
   network connectivity information is conveyed by protocols other than
   IGP, this deployment can still be used to calculate SAV rules. The
   BGP protocol simply inherits the interfaces from the topological
   calculation into the final generated SAV rules, based on the next-
   hop information in the source prefixes.

   Furthermore, it is also possible to plan a separate BGP domain
   within the intra-domain, using BGP RR to reflect and propagate all
   intra-domain source prefixes. First, through IGP or other extended
   technologies, the SAV table entries corresponding to the next hops
   of BGP source prefixes are calculated. Finally, through the next hop
   of BGP, the SAV table entries of the next hops are obtained to
   generate the BGP-published source prefix SAV table entries,
   ultimately achieving BGP calculation SAVNET functionality within the
   intra-domain.

   6. IANA Considerations

   This document does not involve IANA.


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   7. Security Considerations

   TBD

   8. References

   8.1. Normative References

   [I-D.ietf-savnet-intra-domain-architecture]
             Li, D., Wu, J., Huang, M., Chen, L., Geng, N., Qin, L.,
             and F. Gao, "Intra-domain Source Address Validation
             (SAVNET) Architecture", Work in Progress, Internet-Draft,
             draft-li-savnet-intra-domain-architecture-03, 25 July
             2023, <https://datatracker.ietf.org/doc/html/draft-li-
             savnet-intra-domain-architecture-06>.

   [I-D.lin-Intra-domain-savnet-method] D. Li,"Intra-domain SAVNET
             method", Work in Progress,
             <https://www.ietf.org/archive/id/draft-lin-savnet-lsr-
             intra-domain-method-03.txt>



   8.2. Informative References

    [I-D.ietf-savnet-intra-domain-problem-statement]

             Li, D., Wu, J., Qin, L., Huang, M., and N. Geng, "Source
             Address Validation in Intra-domain Networks Gap Analysis,
             Problem Statement, and Requirements", Work in Progress,
             Internet-Draft, draft-ietf-savnet-intra-domain-problem-
             statement-02, 17 August 2023,
             <https://datatracker.ietf.org/doc/html/draft-ietf-savnet-
             intra-domain-problem-statement-02>.

   Acknowledgments

   TBD

   Authors' Addresses

   Weiqiang Cheng
   China Mobile
   China

   Email: chengweiqiang@chinamobile.com



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   Changwang Lin
   New H3C Technologies
   China

   Email: linchangwang.04414@h3c.com



   Shengnan Yue
   China Mobile
   China
   yueshengnan@chinamobile.com




































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