| Internet-Draft | SAVI in an EVPN network | July 2023 |
| Levy-Abegnoli, et al. | Expires 11 January 2024 | [Page] |
Source Address Validation procedures have been specified in the SAVI Working Group and provide a set of mechanisms and state machines to verify Source Address ownership. The main mechanisms are described in [RFC6620] and [RFC7513].¶
[RFC7432] and furthermore [RFC9161] specify how an EVPN network could learn and distribute IP addresses. [RFC9161] describes a mechanism by which the PE can proxy some ND messages based on this information.¶
In this document, we review how these two sets of specifications and underlying mechanisms can interact to provide Source Address Validation in an EVPN network.¶
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[RFC6620] describes a mechanism that provides Source Address Validation Improvements (SAVI) for IPv6 networks based the First Come First Serve (FCFS) principle, applicable to any type of IPv6 addresses, including those assigned through IPv6 [RFC4291][RFC8200] Neighbor Discovery (ND) [RFC4861] Stateless Address Autoconfiguration (SLAAC) [RFC4862]. According to that specification, an IPv6 entry freshly snooped on a SAVI device needs to reach the “VALID“ state before traffic sourced with it can be forwarded.¶
Another SAVI specification, [RFC7513], describes a similar mechanism for addresses assigned by DHCPv6/DHCPv4 server. Again, traffic sourced which such addresses can only be forwarded when the address state is “BOUND”.¶
Section 10 of "BGP MPLS-Based Ethernet VPN" [RFC7432](EVPN) indicates that a Provider Edge (PE) router may learn IP addresses and advertise them to other PEs. EVPN allows PEs to execute a proxy ARP/ND function that is further detailed in "Operational Aspects of Proxy ARP/ND in Ethernet Virtual Private Network" [RFC9161]. According to section 3.2 of [RFC9161], IPv6 addresses should be learnt by snooping Neighbor Advertisements (NA), then advertised in an EVPN MAC/IP Advertisement route, and finally used on remote PEs to perform said proxy ARP/ND function.¶
Assuming one would want to perform Source Address Validation in an EVPN network, two models can be deployed:¶
These two models are reviewed in Section 3. Corresponding interactions between SAVI and EVPN are reviewed as well in this document.¶
This document uses the following abbreviations:¶
In model 1, a SAVI device is inserted between client (host or CE) and PE (or VTEP). This first model is straight forward and does not require any additional specification. It is presented in Figure 1.¶
The SAVI devices operate as a verification perimeter between untrusted clients (CEs or Hosts) and PEs (or VTEPs). As specified in [RFC6620] and [RFC7513], only validated addresses can inject traffic over the trusted perimeter. The mechanisms to validate addresses are specified in these two RFCs. Note that IPv6 DAD NS (IP source is set to the unspecified address, see section section-2.5.2 of [RFC4291] ), used by SAVI for validation, can still be forwarded by EVPN but are not used by EVPN proxy for learning. Therefore, as long as an address is not validated by SAVI, it remains unknown by EVPN.¶
protection perimeter
+- - - - - - - - - - - - - - - - - - - - - - -+
| |
| trusted perimeter |
| +- - - - - - - - - -+ |
| | | |
| | L3 NETWORK | |
| | VTEP VTEP | |
+-------+ | +------+ ++------+ +------++ +------+ | +-------+
| | | | SAVI | | | | | | SAVI | | | |
|HOST/CE| | | | |EVPN/PE| |EVPN/PE| | | | |HOST/CE|
| | | |device| | | | | |device| | | |
+-------+ | +------+ ++------+ +------++ +------+ | +-------+
| | | |
| | | |
| | | |
| | | |
| +- - - - - - - - - -+ |
| |
| |
+- - - - - - - - - - - - - - - - - - - - - - -+
In model 2, the SAVI device and the PE/VTEP are colapsed into the same box. SAVI device is referenced as a "SAVI instance". This model is presented in Figure 2.¶
Model 2 is fundamentally the same as the model 1. SAVI instance acts as a Security wall between untrusted clients and the VTEPs, and only allows validated sources to inject traffic beyond the VTEP. The injection happens over an API between SAVI instance and EVPN rather than over a physical wire. Furthermore, only validated sources are learnt per section 3.2 of [RFC9161].¶
protection perimeter
+- - - - - - - - - - - - - - - - - -- - -- - +
| |
| trusted perimeter |
| +- - - -- - - - - - -+ |
| | | |
| | L3 NETWORK | |
| VTEP/PE | | VTEP/PE |
+-------+ | +--------+-+------+ +------+-+--------+ | +-------+
| | | |SAVI | | | | SAVI | | | |
|HOST/CE| | | API EVPN | | EVPN API | | |HOST/CE|
| | | |instance| | | |instance| | | |
+-------+ | +--------+-+------+ +------+-+--------+ | +-------+
| | | |
| | | |
| | | |
| | | |
| +- - - - ---- - - - -+ |
| |
| |
+- - - - - - - - - - - - - - - - - - - - - - +
protection perimeter
In this model however, it is possible and even desirable to leverage the knowledge of remote IP entries stored in the VTEP BGP tables to make SAVI validation process more efficient. This is described in section Section 6¶
The next section review interactions between SAVI and EVPN for different type of addresses¶
The "SAVI Solution for DHCP" [RFC7513] specification describes a mechanism that provides source address validation for IPv6 networks where addresses are assigned by DHCPv6 server. The address validation is achieved by snooping DHCPv6 address assignments, which is known as DHCP Snooping, or validating discovered address with the DHCP server which is described as Data Snooping process. Both processes are described in detail in [RFC7513].¶
During DHCP Snooping process and according to associated state machine, an IP entry freshly snooped on a SAVI device progresses from NO_BIND to BOUND.¶
The whole DHCP address assignment procedure is performed using IPv6 Link-Local addresses, which are expected to be VALID prior to the beginning of this process. The DHCP server or relay can then be located anywhere in the EVPN network without the DHCP messages being blocked by SAVI. EVPN itself does not learn from DHCP, so until the address is assigned, DAD completed and the host have sent an NA (response to a lookup or unsolicited NA), EVPN is not aware of the assigned address. After the NA is sent, the address can be learnt by EVPN, stored locally by the EVPN proxy-ND, and distributed by BGP per [RFC9161].¶
According to [RFC6620], any address, used as the source of a packet or showing up as a target of a Neighbor Advertisement (NA) or a target of a DAD Neighbor Solicitation (NS), which is not locally known and validated, is treated as a TENTATIVE address and a DAD NS (Duplicate Address Detection Neighbor Solicitation) message with this address in the target field is originated by the SAVI device and broadcasted to any validated or trusted port. A response received implies a duplication and/or IP theft and on the contrary, no response allow to progress the state of the address from TENTATIVE to VALID.¶
Main interactions are listed below:¶
The flow below shows two sequences, A and B, with A where Host does not respond to DAD sent by SAVI and B where host does respond. Sequence goes like this for A:¶
In the absence of a response from HOST, the state of the "S" entry at the ingress SAVI progresses to VALID.¶
For B, after step 3:¶
Upon receiving the NA, the ingress SAVI instance move the entry to NO_BIND and does not allow traffic from this source to be forwarded.¶
layer3 network
+-----------+
VTEP | | VTEP
+------+ +--------+-----+--+ +-+------+--------+ +-------+
| | | | | | | | | |
| HOST | | SAVI | EVPN | | EVPN | SAVI | |HOST |
| | | | | | | | | |
+----+-+ +----+---+---+-+--+ +-+--+---+---+----+ +---+---+
| | | | | | | |
| | | +-----------+ | | |
(1)data, src=S | | | S in VALID |
+------------>| | | state |
| | | | | |
| S: TENTATIVE | | | |
| | |(2)DAD NS, target=S | |
| +- +-------+---------------+------->|(3)DAD NS |
| | | | | +---------->|
| | timeout | | | |
| | | | | | |
| A | | | | | |
| | S: VALID | | | |
| | | | | | |
| | | | | | |
| +- | | | | |
| | |(2)DAD NS, target=S | |
| +- +-------+---------------+------->|(3)DAD NS |
| | | | | +---------->|
| | | |(4)NA to all_nodes(4) | |
| B | |<------+---------------+--------+-----------+
| | | | | | |
| | S: NO_BIND | | | |
| +- | | | | |
| | | | | |
| | | | | |
In model 2 where SAVI is co-located with BGP and EVPN proxy table, upon starting the validation process for an address, SAVI MAY perform a lookup into EVPN/BGP proxy table, where entry can be 1) not found, 2) local or 3) remote.¶
If the entry is local, this is an error as it is expected only VALID entries should be learnt by EVPN ND proxy. The entry in the proxy table MUST be deleted.¶
Other cases are described in the next sections¶
As described in [RFC6620], when validating a source address that is not known in the local SAVI table, a multicast DAD NS message is sent to all remote SAVI instances to check for the presence of this address on any of these instances. This process can be optimized by leveraging the content of EVPN / BGP tables on VTEPs and "unicast-forward" the DAD to the known address owner, if any, more in [RFC6085].¶
Depending on the presence of the address in the BGP table and if present, whether the address is active or not, there are three possible scenarios as described below.¶
When the source address that is being validated by SAVI process is not present in the BGP / EVPN table, it means that is was not learned on any of the remote VTEPs which indicates that none of the remote SAVI devices have the entry for that source address in VALID state. Based on the validation process described in [RFC6620], in such scenario the multicast NS DAD MAY NOT be forwarded over the EVPN network and the SAVI entry MAY move directly to VALID state.¶
Note that to prevent race condition where a host is VALID in the local SAVI instance but not yet present in the EVPN proxy table, SAVI MAY signal the entry to EVPN as soon as it becomes VALID.¶
When the source address that is being validated by SAVI process is present in the BGP / EVPN table, then the multicast NS DAD sent by SAVI can be "unicast-forwarded" per section 3.4. "Unicast-Forward Sub-function" of [RFC9161].¶
If the address is no longer active on the remote location, there will be no response and the validation process will follow the steps as specified in [RFC6620] to move the entry to VALID state. Once the entry is in VALID state, the traffic received from that address will be forwarded into EVPN network and the IP address will be learned by the EVPN. This is shown in Figure 4¶
SAVI instance MAY signal the entry to EVPN as soon as it becomes VALID.¶
This would be an example of host mobility use case.¶
local VTEP remote VTEP
+ - - - - - - - - - - - - - + +--- - - - - - - +
+------+ | +---------+ +---------+ | | +-----+ +----+ | +------+
| | | | local | | | | | | | | | | | |
|Client| | | SAVI |<API>| EVPN | | | |EVPN | |SAVI| | |Client|
+------+ | +---------+ +---------+ | | +-----+ +----+ | +------+
| + - - | - - - - - - - | - - + +--- - --- - | --+ |
| src=IP | | | IP in VALID |
+---------->+ | | state |
| | | | | |
| | Lookup EVPN/BGP| | | |
| | for IP | | | |
| +--------------->+ | | |
| | IP FOUND, MAC<>IP | | |
| <----------------+ | | |
| | | | | |
| | unicast NS DAD to MAC | | |
| +--------------------------------------> ------->|
| | | | | |
| |No response | | No response
| |entry moved to VALID | |
| + | | |
| NA is allowed to reach EVPN| | |
+--------------------------->+--------------------->
+ | IP learned | IP in NO_BIND
| | in BGP | state
+------------->
As in previous section, local SAVI sends the multicast NS DAD to the known (by EVPN, as a remote entry) target but this time, the target responds with a multicast NA. The validation process will follow the steps as specified in [RFC6620] and move the entry to NO_BIND state. Any NA received on this local SAVI instance from the validating port will not be signalled to EVPN and will not be learnt by EVPN ND proxy. This is a IP duplication / theft use case and should be signalled through logging or by triggering an alarm.¶
local VTEP remote VTEP
+ - - - - - - - - - - - - - + +--- - - - - - - +
+------+ | +---------+ +---------+ | | +-----+ +----+ | +------+
| | | | local | | | | | | | | | | | |
|Client| | | SAVI |<API>| EVPN | | | |EVPN | |SAVI| | |Client|
+------+ | +---------+ +---------+ | | +-----+ +----+ | +------+
| + - - | - - - - - - - | - - + +--- - --- - | -+ |
| src=IP | | | IP in VALID |
+---------->+ | | state |
| | | | | |
| | Lookup EVPN/BGP| | | |
| | for IP | | | |
| +--------------->+ | | |
| | IP FOUND, MAC<>IP | | |
| <----------------+ | | |
| | | | | |
| | unicast NS DAD to MAC | | |
| +--------------------------------------> ------> |
| | | | | NA |
| | | | allowed|<-------|
| |<--------------------------------------| |
| Entry in NO_BIND state | | | |
| | | | | |
| NA, target=IP | | | |
|---------->| | | | |
| Traffic from src/target IP| | | |
| is DROPPED and not visible| | | |
| to EVPN/BGP | | | |
This specification does not require IANA action.¶