| BESS Workgroup | T. Yu |
| Internet-Draft | |
| Intended status: Standards Track | H. Wang |
| Expires: June 30, 2019 | Huawei Technologies |
| December 27, 2018 |
EVPN Layer 2 Attributes Extended Community Usage in EVPN
draft-yu-bess-evpn-l2-attributes-02
This document adopts Layer 2 Attributes Extended Community defined in [RFC8214] to EVPN ELAN. Interoperability mechanism of control word is described for EVPN ELAN and VPWS. Also flow label mechanism is described for EVPN ELAN and VPWS.
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EVPN [RFC7432] lacks of a negotiation mechanism on L2 capabilities. If the L2 capabilities between PE devices are different, they are not able to communicate properly. [RFC8214] defines Layer 2 Attributes Extended Community but there is no description on how to be use in EVPN ELAN. This documents describes how Layer 2 Attributes Extended Community is used in EVPN ELAN.
Considering some devices have no capability to support control word and there are legacy devices without control word function supported, an interoperability mechanism on control word is described.
To achieve better load balancing on EVPN ELAN and VPWS services, flow label function in EVPN is described in this document.
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 specifies EVPN defined in [RFC7432].
EVI: An EVPN Instance spanning the Provider Edge (PE) devices participating in that EVPN ELAN.
EVPN VPWS: EVPN Virtual Private Wire Service, refers to [RFC8214].
EVPN E-Tree: Ethernet VPN Ethernet-Tree defined in [RFC8317].
CW: Control Word defined in [RFC4448].
CI: Control Word Indicator, defined in Section 4 of this document.
PE: Provider Edge device.
FL: Flow-Aware Transport Label, defined in [RFC6391].
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 when used in EVPN ELAN. BUM traffic SHOULD NOT include control word when forwarded no matter C bit is set to 1 or 0 when working in EVPN ELAN.
CI bit is defined to achieve interoperability between devices with and without control word capability. Control Word Indicator is a MPLS label. CI bit MUST NOT set 1 if C bit is set 0.
CI label MUST NOT be an MPLS reserved label [RFC3032] and with TTL=1. CI label MUST NOT be used for directing forwarding behavior in any cases.
When a PE sends packet with CI label, it MUST keep CI label value consistentfor traffic towards one ES. EVI service label value, including type1 and type2, MAY be used as CI value directly.
The usage of CI bit is described in Section 5.
F bit is defined to achieve flow label 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 SHOULD be same across all Ethernet Segments within one EVI on a local PE when used in EVPN ELAN. BUM traffic SHOULD NOT include Flow label when forwarded no matter F bit is set to 1 or 0 when working in ELAN mode.
C bit and F bit MUST NOT set 1 together.
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 bit enabled, the behavior SHOULD be spread to all MAC tables towards the corresponding ES.
The description on EVPN ELAN CW mechanism is based on the network topology showed in Figure 3:
+--------+ +--------+
| PE1 | | PE2 |
| (CW) |-------| (CW) |
+--------+ +--------+
\ /
\ /
+--------+
| PE3 |
| (NCW) |
+--------+
Figure 3: Network Topology for EVPN ELAN Control Word
PE1 and PE2 are routers with CW enabled and PE3 is router CW disabled or without CW capability. It is assumed that PE1, PE2 and PE3 are working in the same EVI.
When an EVPN ELAN working in deterministic mode, each PE advertises CW capability based on local status in auto-discovery route via l2 attribute. CI bit is not used in this mode. In such case, C bit of devices in Figure 3 is: C bit of PE1 and PE2 is 1, C bit of PE3 is 0. After a PE receives EVPN auto-discovery route, it compares 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 treats PE2 and PE3 as valid destination.
PE2 treats PE1 and PE3 as valid destination.
PE3 treats PE1 and PE2 as 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 being different.
Considering some devices has no capability of adding and parsing CW, there are two ways to interoperate such devices:
1. Disable CW function on all devices and keep CW status consistent within EVI, then the EVI can work in the deterministic mode.
2. An interoperable mode based on CI MAY be implemented to achieve interoperability with such devices.
Considering EVPN label is "upstream-assigned", a PE can recieve packet with same label but with different CW status from different remote PEs. This brings challenge on CW interoperate mechanism. To overcome this challenge, CI is introduced to allow a PE identifying CW status on forwarding plane.
When an EVPN ELAN working in interoperable mode, if CW on a PE is enabled, the PE MUST set both C and CI bit = 1. If CW on a PE is disabled, the PE MUST set both C and CI bit = 0.
After a PE receives EVPN auto-discovery route, it compares the value of C bit and CI:
After such process, PE1, PE2 and PE3 treat each other as valid EVPN destination.
When local PE forwards a packet to remote PE, if remote PE is control word enabled (C=1), 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 (C=0), 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 at EVPN label, if it is bottom of stack, it indicates CW is not included behind. If it is not bottom of stack, it indicates CW is included behind CI.
When working in interoperable mode, impact of lacking complete CW between PEs SHOULD be evaluated based on section 8 of this document.
The description on EVPN VPWS CW mechanism is based on the network topology showed in Figure 4:
+--------+ +--------+
| PE1 | | PE2 |
| (CW) |-------| (CW) |
+--------+ +--------+
\
\
+--------+
| PE3 |
| (NCW) |
+--------+
Figure 4: Network Topology for EVPN ELAN Control Word
PE1 and PE2 are routers with CW enabled and PE3 is router CW disabled or without CW capability. It is assumed that there is one EVPN VPWS between PE1 and PE2 and another VPWS between PE1 and PE3.
When an EVPN VPWS working in deterministic mode, each PE advertises CW capability based on local status in auto-discovery route via l2 attribute. In such case, C bit of devices in Figure 4 is: C bit of PE1 is 1, C bit of PE2 is 0. After a PE receives EVPN auto-discovery route, it compares the value of C bit with local control word enable status. If not same, local PE MUST NOT bring up the EVPN VPWS.
After such process:
EVPN VPWS between PE1 and PE2 is up.
EVPN VPWS between PE1 and PE3 is down.
PE1 and PE2 will forward traffic with control word encapsulated.
Considering some devices has no capability of adding and parsing CW, there are two ways to interoperate such devices:
1. Disable CW function on PE1 and keep CW status consistent within EVPN VPWS towards PE3, then the EVPN VPWS can work in the deterministic mode.
2. An interoperable mode MAY be implemented to achieve interoperability with such devices.
When an EVPN VPWS working in interoperable mode, in case of C bit status is not consistent on both ends, VPWS MUST tear up.
In such case:
EVPN VPWS between PE1 and PE2 is up.
EVPN VPWS between PE1 and PE3 is up.
PE1 and PE2 will forward traffic with CW encapsulated. PE1 and PE3 forward traffic without CW encapsulated.
When working in interoperable mode, impact of lacking complete CW between PEs SHOULD be evaluated based on section 8 of this document.
CI MUST NOT set 1 when working in EVPN VPWS.
The procedure described in section 5.1 applies to EVPN E-Tree, both "two RTs" and "single RT" via "leaf" flag solution.
When using "single RT" E-Tree, there is a paticular case that devices with and without CW capability can interoperate without CI involved. This case is: root device is CW disabled but (part of) the leaf devices CW enabled. In such case, leaf PE SHOULD treat root and other leafs as valid EVPN destination, even C bit status is not consistent. Leaf PEs with CW enabled forward known unicast traffic without CW encapsulated towards root. Similarly when an E-tree is working in interoperable mode instead of deterministic mode, impact of lacking complete CW between PEs SHOULD be evaluated based on section 8 of this document.
+--------+ +--------+
| PE1 | | PE2 |
| (FL) |-------| (FL) |
+--------+ +--------+
\ /
\ /
+--------+
| PE3 |
| (NFL) |
+--------+
Figure 5: Network Topology for Flow Label Usage
Flow label is introduced allowing to achieve 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 encapsulation 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 towardsPE1, packet encapsulation is as below:
PE2->PE1: Transport Label + EVPN Label to PE1 + FL + Payload
PE3->PE1: Transport Label + EVPN Label to PE1 + Payload
When PE1 receives a packet, after looking at EVPN label, if it is bottom of stack, it indicates FL is not included behind. If it is not bottom of stack, it indicates FL is included behind.
The flow label mechanism described above is applicable to both EVPN ELAN, E-Tree and EVPN VPWS.
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 ELAN or VPWS destination.
In EVPN, CW is used avoid misordering caused by transit node using MPLS payload as hash factor. The detailed reason for 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 SHOULD 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 SHOULD be disabled, otherwise control word will be treated as part of MAC address mistakenly.
Similarly, entropy label [RFC6790] SHOULD NOT be used in EVPN carrying services sensitive to misordering.
When data plane is not using MPLS, C, F and CI bits in control flags MUST be 0. Control word and Flow Label are only applicable to MPLS data plane.
When working with legacy devices without l2 attribute in EVPN ELAN, local PE MAY assume the behavior of all remote PE is same with local. Also a route policy MAY be implemented to specify L2 behavior manually in case l2 attribute is absent. This function SHOULD be used only for interoperability. A PE SHOULD NOT overwrite existing l2 attribute in a received route.
This document updates the behavior specified in [RFC7432] and [RFC8214]. The security considerations listed in these two documents apply. However, there are no new security considerations due to the text of this document.
This document does not make any requests from IANA.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC7432] | Sajassi, A., Aggarwal, R., Bitar, N., Isaac, A., Uttaro, J., Drake, J. and W. Henderickx, "BGP MPLS-Based Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February 2015. |
| [RFC8214] | Boutros, S., Sajassi, A., Salam, S., Drake, J. and J. Rabadan, "Virtual Private Wire Service Support in Ethernet VPN", RFC 8214, DOI 10.17487/RFC8214, August 2017. |
| [RFC8317] | Sajassi, A., Salam, S., Drake, J., Uttaro, J., Boutros, S. and J. Rabadan, "Ethernet-Tree (E-Tree) Support in Ethernet VPN (EVPN) and Provider Backbone Bridging EVPN (PBB-EVPN)", RFC 8317, DOI 10.17487/RFC8317, January 2018. |
| [RFC8469] | Bryant, S., Malis, A. and I. Bagdonas, "Recommendation to Use the Ethernet Control Word", RFC 8469, DOI 10.17487/RFC8469, November 2018. |
| [RFC3032] | Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001. |
| [RFC4448] | Martini, L., Rosen, E., El-Aawar, N. and G. Heron, "Encapsulation Methods for Transport of Ethernet over MPLS Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006. |
| [RFC6391] | Bryant, S., Filsfils, C., Drafz, U., Kompella, V., Regan, J. and S. Amante, "Flow-Aware Transport of Pseudowires over an MPLS Packet Switched Network", RFC 6391, DOI 10.17487/RFC6391, November 2011. |
| [RFC6790] | Kompella, K., Drake, J., Amante, S., Henderickx, W. and L. Yong, "The Use of Entropy Labels in MPLS Forwarding", RFC 6790, DOI 10.17487/RFC6790, November 2012. |