SAVI in an EVPN network

Introduction 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 Neighbor Discovery (ND) Stateless Address Autoconfiguration (SLAAC) . 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, , 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"(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". According to section 3.2 of , 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:

the SAVI function runs on SAVI switches external to PEs
the SAVI function is co-located with the PEs

These two models are reviewed in . Corresponding interactions between SAVI and EVPN are reviewed as well in this document.

Acronyms This document uses the following abbreviations:

CE:: Customer Edge (router)
DAD:: Duplicate Address Detection
NA:: Neighbor Advertisement (message)
ND:: Neighbor Discovery (protocol)
NDP:: Neighbor Discovery Protocol
NS:: Neighbor Solicitation (message)
PE:: Provider Edge (router)
VLAN:: Virtual Local Area Network
VxLAN:: Virtual Extensible LAN
EVPN:: Ethernet Virtual Private Network
VTEP:: VXLAN Tunnel EndPoint (node)
BGP:: Border Gateway Protocol

Deployment models 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 . The SAVI devices operate as a verification perimeter between untrusted clients (CEs or Hosts) and PEs (or VTEPs). As specified in and , 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 ), 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.

Deployment model 1 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 . 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 .

Deployment model 2 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 The next section review interactions between SAVI and EVPN for different type of addresses

DHCP assigned addresses

SAVI-DHCP background The "SAVI Solution for DHCP" 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 . 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.

Interactions between RFC7513 and EVPN 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 .

SLAAC and other IPv6 addresses

SAVI-FCFS background According to , 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.

Interactions between SAVI-FCFS and EVPN Main interactions are listed below:

Sources not validated by SAVI are not learnt nor distributed by EVPN: SAVI does not allow NA to be forwarded (model 1) or signaled (model 2) to EVPN.
During the validation process for a Source, SAVI (ingress) originates a broadcast DAD, with target set to “Source” and forwards (model 1) or signals (model 2) it to EVPN, which delivers it to remote SAVI devices (over the wire or an API). Only SAVI devices that have a local validated entry or a trusted interface forward the DAD to enable the target to defend itself.
In case a response is sent by one of the DAD recipients, it is a broadcast NA (to “all-nodes”), processed by SAVI (egress). The source is expected to be a SAVI validated source, and as such, delivered from (egress) SAVI to EVPN for delivery to all VTEPs and finally to (ingress) SAVI to complete the validation process of the target “Source”.
The first NA sent by this Source triggers EVPN proxy learning and EVPN distribution to all VTEPs per .

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:

Data sourced with S is received on the ingress SAVI instance. An entry for S is created in TENTATIVE state.
A DAD is built with target=S and broadcasted to all SAVI instances over the EVPN network.
Only instances that have the entry "S" in VALID state or trusted ports forward the DAD.

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:

HOST responds to DAD with an NA.

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.

FCFS-SAVI | | | 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 | | | | | +- | | | | | | | | | | | | | | | | | ]]>

Enhanced SAVI/EVPN interactions 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 , 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 . 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.

Address not found in BGP/EVPN table.
Address found in BGP/EVPN table, remote not active.
Address found in BGP/EVPN table, remote active.

Address not found 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 , 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.

Address found in BGP/EVPN table, remote not active 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 . 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 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 SAVI instance MAY signal the entry to EVPN as soon as it becomes VALID. This would be an example of host mobility use case.

Remote not responding - Mobility | 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 +-------------> ]]>

Address found in BGP/EVPN table, remote active 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 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. This is shown in

Remote active - IP theft | 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 | | | | ]]>

Security Considerations

IANA Considerations This specification does not require IANA action.

Contributors

Acknowledgments