| BESS Workgroup | T. Yu |
| Internet-Draft | Huawei Technologies |
| Intended status: Standards Track | December 5, 2018 |
| Expires: June 8, 2019 |
EVPN Layer 2 Attributes Extended Community Usage in EVPN
draft-yu-bess-evpn-l2-attributes-01
This document aims to define a negotiation mechanism for L2 capabilities in an EVPN scenario.
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EVPN [RFC7432] is lacking a negotiation mechanism on L2 capabilities. If the L2 capabilities between PE devices are different, they are not able to communicate properly.
This document aims to define a negotiation mechanism for L2 capabilities in an EVPN scenario.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT","SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
EVPN: BGP MPLS-Based Ethernet VPN defined in [RFC7432]
EVPN ELAN: In order to distinguish EVPN VPWS, EVPN ELAN speficies EVPN defined in [RFC7432]
CW: Control Word defined in [RFC4448]
CI: Control word indicator, defined in Section 4 of this document
PE: Provider Edge
FL: Flow label, defined in [RFC6391]
EVPN Layer 2 Attributes Extended Community is defined in EVPN VPWS [RFC8214]. This document describes the behaviors how it adapts to EVPN ELAN. A mechanism to achieve interoperability between devices with and without CW capabilities is defined. Flow Label mechanism is defined. EVPN Layer 2 Attributes Extended Community is advertised along with Ethernet Auto-discovery. The definition of EVPN Layer 2 Attributes Extended Community is same with [RFC8214]. it is listed as below for convenience.
+-------------------------------------------+ | Type (0x06) / Sub-type (0x04) (2 octets) | +-------------------------------------------+ | Control Flags (2 octets) | +-------------------------------------------+ | L2 MTU (2 octets) | +-------------------------------------------+ | Reserved (2 octets) | +-------------------------------------------+
Figure 1: EVPN Layer 2 Attributes Extended Community
The definition of Control Flags is as below:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+ | MBZ |CI|F|C|P|B| (MBZ = MUST Be Zero) +-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+
Figure 2: Control Flags
The P bit and B bit defined in [RFC8214] must be zero when used in EVPN ELAN mode. This is because [RFC7432] has defined ESI Label Extended Community to achieve single-active redundancy mode.
C bit indicates the control word enable status of known unicast traffic. C bit MUST set 0 when advertising A-D route if PE has no capability of processing control word. C bit SHOULD be same across all Ethernet Segments within one EVI on a local PE. BUM traffic SHOULD NOT include control word when forwarded no matter C bit is set to 1 or 0 when working in ELAN mode.
CI bit is defined to achieve interoperability between devices with and without control word capability. Control Word Indicator is one octet with value 0xFF. CI bit MUST NOT set 1 if C bit is set 0. The usage of CI bit is described in Section 4.
F bit is defined to achieve Flow-Aware Transport Labels [RFC6391] in EVPN. It can be used in both EVPN ELAN and VPWS. When F bit is set to 1, the PE announces it has the capability of both sending and receiving flow label. F bit MUST be same across all Ethernet Segments within one EVI on a local PE. BUM traffic SHOULD NOT include Flow label when forwarded no matter F bit is set to 1 or 0 when working in ELAN mode.
Other bits in Control Flags are reserved for future investigation and MUST be zero.
L2 MTU is a 2-octet value indicating the CE-PE link MTU in bytes. It MUST be same across all ES within one EVI on a local PE.
If local PE does not support EVPN Layer 2 Attributes Extended Community, this community MUST be ignored.
When a PE receives A-D routes with C, CI or F bits enabled, the behavior SHOULD be spreaded to all MAC tables towards the corresponding ES when applicable.
The description below is based on the network topology showed in Figure 3:
+--------+ +--------+
| PE1 | | PE2 |
| (CW) |-------| (CW) |
+--------+ +--------+
\ /
\ /
+--------+
| PE3 |
| (NCW) |
+--------+
Figure 3: Network Topology for Control Word Interoperability
PE1 and PE2 are routers with control word capability and PE3 is router control word disabled or without control word capability. It is assumed that PE1, PE2 and PE3 are working in same EVI.
When local PE receives auto-discovery routes from remote PE, if CI=0, then goes process A, if CI=1, then goes to process B.
Process A:
Compare the value of C bit with local control word enable status. If same, local PE treats remote PE as a valid EVPN destination. If not same, local PE treats remote PE as an invalid EVPN destination. After such process, PE1 treat PE2 as a valid destination and PE3 as an invalid destination. PE2 treat PE1 as a valid destination and PE3 as an invalid destination. PE3 treat both PE1 and PE2 as an invalid destination. PE1 and PE2 will forward traffic with control word encapsulated. PE1 and PE2 will not be able to interoperate with PE3, due to control word status difference.
Process B:
In this process, if C is set 0, then remote PE is treated as valid EVPN destination. If C is set 1, then only when CI and C bit status is same with local, remote PE is treated as a valid EVPN destination. When local PE forwards a packet to remote PE, if remote PE is control word enabled, then the unicast packet MUST be encapsulated with a control word indicator before control word. If remote PE is control word disabled or no control word capability, then unicast packet is directly encapsulated after MPLS label as payload.
For example:
PE1 advertises A-D route with CI and C bit set 1, packet forwarding behavior is as below:
PE2->PE1: Transport Label + EVPN Label to PE1 + CI + CW + Payload
PE3->PE1: Transport Label + EVPN Label to PE1 + Payload
PE2 advertises A-D route with CI and C bit set 1, PE3 advertise A-D route with CI and C bit set 0, packet forwarding from PE1 to PE2 and PE3 is as below::
PE1->PE2: Transport Label + EVPN Label to PE2 + CI + CW + Payload
PE1->PE3: Transport Label + EVPN Label to PE3 + Payload
After a PE receives a packet, after looking into EVPN label, if the first byte is "0xFF", it indicates control word is included behind. If the first byte is not "0xFF", it indicates control word is not included behind.
The control word interoperability mechanism described above is not applicable to EVPN VPWS, it is applicable to EVPN ELAN.
When working in EVPN VPWS mode, in case of C bit status is not consistent on both ends, VPWS SHOULD be teared up and behave like control word not enabled.
+--------+ +--------+
| PE1 | | PE2 |
| (FL) |-------| (FL) |
+--------+ +--------+
\ /
\ /
+--------+
| PE3 |
| (NFL) |
+--------+
Figure 4: Network Topology for Flow Label Interoperability
Flow label is introduced allowing to archieve better load-balancing in the network in conditions mentioned below:
PE1 and PE2 are routers with flow label capability and flow label enabled and PE3 is router flow label disabled or without flow label capability. It is assumed that PE1, PE2 and PE3 are working in same EVI.
When local PE receives auto-discovery routes from remote PE, F bit does not impact validation process of EVPN auto-discovery routes. Even F bit of remote PE does not match with local status, local PE SHOULD still add remote PE as a valid EVPN destination.
When sending traffic from PE1 towards PE2 and PE3, as PE1 has already learnt the FL status of PE2 and PE3, packet encapulation is as below:
PE1->PE2: Transport Label + EVPN Label to PE2 + FL + Payload
PE1->PE3: Transport Label + EVPN Label to PE3 + Payload
When sending traffic from PE2 and PE3 towarding PE1, packet encapulation is as below:
PE2->PE1: Transport Label + EVPN Label to PE1 + FL + Payload
PE3->PE1: Transport Label + EVPN Label to PE1 + Payload
When PE1 recieves packets, after looking at EVPN label, it can be both bottom of stack or not. When EVPN label is bottom of stack, and flow label enabled locally, packet MUST be treated as valid and parse following bytes as payload.
The flow label mechanism described above is applicable to both EVPN ELAN and EVPN VPWS.
When interoperating with devices not supporting EVPN Layer 2 Attributes Extended Community, A-D routes received will not contain such community. Local PE MAY assume the behavior of all remote PE is same with local.
When data plane is not using MPLS, all bits in control flags MUST be 0. Control word and Flow Label are only applicable to MPLS data plane.
If a non-zero MTU attribute is received, it MUST be checked against local MTU value if the local value is not zero. If there is a mismatch, the local PE MUST NOT add the remote PE as the EVPN destination.
In EVPN, CW is used avoid misordering caused by transit node using MPLS payload as hash factor. The detailed reason of this refers to [RFC8469]. CW is mandatory for an EVPN carrying misordering sensitive service, when CW is not available, mitigations described in section 6 of [RFC8469] also apply to EVPN.
Flow Label MUST NOT be used in EVPN carrying services sensitive to misordering. Flow Label MAY be used to achieve better load-balancing in network, when transit node has no capability to look inside MPLS payload as hash factor.
It has been recognized that some transit nodes treat payload after MPLS label as MAC address info and use as hash factor directly. When it is bearing services sensitive to misordering, such load balancing function MUST be disabled, otherwise control word will be treated as part of MAC address mistakenly.
Similarly, entropy label [RFC6790] MUST NOT be used in EVPN carrying services sensitive to misordering.
There are no new security considerations due to the text of this document.
This document does not make any requests from IANA.