SRv6 Path Egress Protection

The fast protection of a transit node of a Segment Routing (SR) path or tunnel is described in . specifies the fast protection of egress node(s) of an MPLS TE LSP tunnel including P2P TE LSP tunnel and P2MP TE LSP tunnel in details. However, these documents do not discuss the fast protection of the egress node of a Segment Routing for IPv6 (SRv6) path or tunnel. This document fills that void and presents protocol extensions for the fast protection of the egress node of an SRv6 path or tunnel. Egress node and egress, fast protection and protection as well as SRv6 path and SRv6 tunnel will be used exchangeably below. There are a number of topics related to the egress protection, which include the detection of egress node failure, the relation between egress protection and global repair, and so on. These are discussed in details in .

The following terminologies are used in this document. Segment Routing SR for IPv6 Segment Routing Header Segment Identifier Link State Advertisement in OSPF Label Switched Path in MPLS or Link State Protocol PDU in IS-IS Protocol Data Unit Link Sate, which is LSA in OSPF or LSP in IS-IS Traffic Engineering Source Address Destination Address Point-to-MultiPoint Point-to-Point Customer Edge Provider Edge Loop-Free Alternate Topology Independent LFA Bidirectional Forwarding Detection Virtual Private Network Layer 3 VPN Virtual Routing and Forwarding Forwarding Information Base Point of Local Repair Border Gateway Protocol Interior Gateway Protocol Open Shortest Path First Intermediate System to Intermediate System

This section describes the mechanism of SR path egress protection and illustrates it through an example.

is used to explain the mechanism of SR path egress node protection.

Where node PEA is the egress of the SR path from PE1 to PEA, and has SIDa which is the active segment in the packet from the SR path at PEA. Node PEB is the backup egress (or say protector) to provide the protection for egress (or say primary egress) PEA. Node P1 is the direct previous/upstream hop of egress PEA and acts as PLR (refer to ) to support the protection for PEA. When PEB is selected as a backup egress to protect the egress PEA, a Mirror SID (refer to Section 5.1 of ) is configured on PEB to protect PEA. PEB advertises this information through IGP, which includes the Mirror SID and the egress PEA. The information is represented by <PEB, PEA, Mirror SID>, which indicates that PEB protects PEA with Mirror SID. After PEA receives the information <PEB, PEA, Mirror SID>, it may send the forwarding behavior of the SIDa at PEA to PEB with the Mirror SID using some protocols such as BGP if PEB can not obtain this behavior from other approaches and PEB wants to protect SIDa of PEA. How to send the forwarding behavior of the SIDa to PEB is out scope of this document. When PEB gets the forwarding behavior of the SIDa of PEA from PEA or other means, it adds a forwarding entry for the SIDa according to the behavior into the forwarding table for node PEA. This table is identified by the Mirror SID, which indicates node PEA's context. Using the forwarding entry for SIDa in this table, a packet with SIDa will be transmitted by PEB to the same destination as it is transmitted by PEA. For example, assume that the packet with SIDa is transmitted by PEA to CE2 through the forwarding behavior of the SIDa in PEA. The packet will be transmitted by PEB to the same CE2 through looking up the table identified by the Mirror SID. After P1 as PLR receives the information <PEB, PEA, Mirror SID> and knows that PEB wants to protect SIDa of PEA, it computes an LFA for PEA assuming that PEA and PEB have a same anycast address. A Repair List RL (or say backup path) is obtained based on the LFA. It is one of the following: RL = <Mirror SID> if the LFA is the next hop node to PEB along the shortest path to PEB; or RL = <S1, ..., Sn, Mirror SID> if the LFA is a TI-LFA, where <S1, ..., Sn> is the TI-LFA Repair List to PEB computed by P1. When PEA fails, P1 as PLR sends the packet with SIDa carried by the SR path to PEB, but encapsulates the packet before sending it by executing H.Encaps with the Repair List RL and a Source Address T. P1 as PLR needs to retain the route to PEA for a period of time after its IGP converges on the failure of PEA. Thus the backup path for PEA will be used when the other nodes (such as PE1) still send the packet to PEA via P1 since their IGPs do not converge on the failure. Suppose that the packet received by P1 is represented by Pkt = (S, SIDa)Pkt0, where SA = S and DA = SIDa, and Pkt0 is the rest of the packet. The execution of H.Encaps pushes an IPv6 header to Pkt and sets some fields in the outer and inner IPv6 header to produce an encapsulated packet Pkt'. Pkt' will be one of the following: Pkt' = (T, Mirror SID) (S, SIDa)Pkt0 if RL = <Mirror SID>; or Pkt' = (T, S1)(Mirror SID, Sn, ..., S1; SL=n) (S, SIDa)Pkt0 if RL = <S1, ..., Sn, Mirror SID>. When PEB receives the re-routed packet, which is (T, Mirror SID) (S, SIDa)Pkt0, it decapsulates the packet and forwards the decapsulated packet using the FIB table Tm identified by the Mirror SID as a variant of End.DT6 SID. The Mirror SID is called End.M. It obtains the Mirror SID in the outer IPv6 header of the packet, removes this outer IPv6 header with all its extension headers, and then processes the inner IPv6 packet (i.e., (S, SIDa)Pkt0, the packet without the outer IPv6 header). PEB finds the FIB table Tm for node PEA using the Mirror SID as the context ID, and submits the packet to this FIB table lookup and transmission to the same destination as PEA does. The behavior of Mirror SID (End.M for short) is a variant of the End.DT6 behavior (refer to Section 4.6 of ). The End.M SID MUST be the last segment in an SR path, and a SID instance is associated with an IPv6 FIB table Tm. When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.M SID, N does the following:

shows an example of protecting egress PE3 of a SR path, which is from ingress PE1 to egress PE3.

Where node P1's pre-computed backup path for PE3 is from P1 to PE4 via P2. In normal operations, after receiving a packet with destination PE3, P1 forwards the packet to PE3 according to its FIB. When PE3 receives the packet, it sends the packet to CE2. When PE3 fails, P1 as PLR detects the failure through using a failure detection mechanism such as BFD and forwards the packet to PE4 via the backup path. When PE4 receives the packet, it sends the packet to the same CE2. When P1's IGP converges on the failure of PE3, P1 as PLR needs to retain the route to PE3 for a period of time. Thus the backup path for PE3 will be used when the other nodes (such as PE1) still send the packet to PE3 via P1 since their IGPs do not converge on the failure. In , Both CE2 and CE3 are dual home to PE3 and PE4. PE3 has a locator A3:1::/64 and a VPN SID A3:1::B100. PE4 has a locator A4:1::/64 and VPN SID A4:1::B100. A Mirror SID A4:1::3 is configured on PE4 for protecting PE3 with locator A3:1::/64. After the configuration, PE4 advertises this information through an IGP LS (i.e., LSA in OSPF or LSP in IS-IS), which includes PE3's locator and Mirror SID A4:1::3. Every node in the SR domain will receive this IGP LS, which indicates that PE4 wants to protect PE3 (indicated by PE3's locator) with Mirror SID A4:1::3. When PE4 (e.g., BGP on PE4) receives a prefix whose VPN SID belongs to PE3 that is protected by PE4 through Mirror SID A4:1::3, it finds PE4's VPN SID corresponding to PE3's VPN SID. For example, local PE4 has Prefix 1.1.1.1 with VPN SID A4:1::B100, when PE4 receives prefix 1.1.1.1 with remote PE3's VPN SID A3:1::B100, it knows that they are for the same VPN. The forwarding behaviors for these two VPN SIDs are the same from function's point of view. If the behavior for PE3's VPN SID in PE3 forwards the packet with it to CE2, then the behavior for PE4's VPN SID in PE4 forwards the packet to the same CE2; and vice versa. PE4 creates a forwarding entry for PE3's VPN SID A3:1::B100 in the FIB table identified by Mirror SID A4:1::3 according to the forwarding behavior for PE4's VPN SID A4:1::B100. Node P1's pre-computed backup path for destination PE3 is from P1 to PE4 having mirror SID A4:1::3. When P1 receives a packet destined to PE3's VPN SID A3:1::B100, in normal operations, it forwards the packet with source A1:1:: and destination PE3's VPN SID A3:1::B100 according to the FIB using the destination PE3's VPN SID A3:1::B100. When PE3 fails, P1 as PLR sends the packet to PE4 via the backup path pre-computed. P1 encapsulates the packet using H.Encaps before sending it to PE4. Suppose that the packet received by P1 is represented by Pkt = (SA = A1:1::, DA = A3:1::B100)Pkt0, where DA = A3:1::B100 is PE3's VPN SID, and Pkt0 is the rest of the packet. The encapsulated packet Pkt' will be one of the following: Pkt' = (T, Mirror SID A4:1::3) (A1:1::, A3:1::B100)Pkt0 if the LFA is the next hop node to PE4 along the shortest path to PE4; or (otherwise) Pkt' = (T, S1)(Mirror SID A4:1::3, Sn, ..., S1; SL=n) (A1:1::, A3:1::B100)Pkt0. where T is a Source Address, <S1, ..., Sn> is the TI-LFA Repair List to PE4 computed by P1. When PE4 receives the re-routed packet, it decapsulates the packet and forwards the decapsulated packet by executing the behavior of End.M for the Mirror SID that is associated with the IPv6 FIB table for PE3. The packet received by PE4 is (T, Mirror SID A4:1::3) (A1:1::, PE3's VPN SID A3:1::B100)Pkt0. PE4 obtains Mirror SID A4:1::3 in the outer IPv6 header of the packet, removes this outer IPv6 header, and then processes the inner IPv6 packet (A1:1::, A3:1::B100)Pkt0. It finds the FIB table for PE3 using Mirror SID A4:1::3 as the context ID, gets the forwarding entry for PE3's VPN SID A3:1::B100 from the table, and forwards the packet to CE2 using the entry.

This section describes extensions to IS-IS and OSPF for advertising the information about SRv6 path egress protection.

A new sub-TLV, called IS-IS SRv6 Mirror SID sub-TLV, is defined. It is used in the SRv6 Locator TLV defined in to advertise SRv6 Mirror SID and the locators of the nodes to be protected. The SRv6 Mirror SID inherit the topology/algorithm from the parent locator. The format of the sub-TLV is illustrated below.

TBD1 (suggested value 8) is to be assigned by IANA. variable. 1 octet. No flags are currently defined. 2 octets. It contains the endpoint function 74 for Mirror SID. 16 octets. This field contains the SRv6 Mirror SID to be advertised. A protected locators sub-sub-TLV is defined and used to carry the Locators of the egress nodes to be protected by the SRv6 mirror SID. It has the following format.

TBD2 (suggested value 1) is to be assigned by IANA. variable. 1 octet. Number of bits (1 - 128) in the Locator field. 1-16 octets. This field encodes an SRv6 Locator of an egress node to be protected by the SRv6 mirror SID. The Locator is encoded in the minimal number of octets for the given number of bits. When node B advertises that B wants to protect node A with a Mirror SID through an LSP, the LSP contains an IS-IS SRv6 Mirror SID sub-TLV, which includes the Mirror SID and node A's locator in an IS-IS Protected locators sub-sub-TLV.

Similarly, a new sub-TLV, called OSPF Mirror SID sub-TLV, is defined. It is used to advertise SRv6 Mirror SID and the locators of the nodes to be protected. Its format is illustrated below.

TBD4 (suggested value 8) is to be assigned by IANA. variable. 1 octet. No flags are currently defined. 1 octet. It MUST be set to zero for transmission and ignored on reception. 2 octets. It contains the endpoint function 74 for End.M SID. 16 octets. This field contains the SRv6 Mirror SID to be advertised. A protected locators sub-TLV is defined and used to carry the locators of the nodes to be protected by the SRv6 Mirror SID. It has the following format.

TBD5 (suggested value 1) is to be assigned by IANA. variable. 1 octet. Number of bits (1 - 128) in the Locator field. 1-16 octets. This field encodes an SRv6 Locator of an egress node to be protected by the SRv6 mirror SID. The Locator is encoded in the minimal number of octets for the given number of bits.

The security about the egress protection is described in in details in . The extensions to OSPF and IS-IS described in this document for SRv6 path egress protection should not cause extra security issues.

Under sub-registry "SRv6 Endpoint Behaviors" , IANA has assigned the following for End.M Endpoint Behavior:

Under "IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability registry", IANA is requested to add the following new Sub-TLV:

IANA is requested to create and maintain a new registry for sub-sub-TLVs of the SRv6 Mirror SID Sub-TLV. The suggested registry name is Sub-Sub-TLVs for SRv6 Mirror SID Sub-TLV Initial values for the registry are given below. The future assignments are to be made through IETF Review .

Under registry "OSPFv3 Locator LSA Sub-TLVs" , IANA is requested to assign the following new Sub-TLVs:

Acknowledgments The authors would like to thank Acee Lindem, Peter Psenak, Yimin Shen, Jie Dong, Zhenqiang Li, Alexander Vainshtein, Greg Mirsky, Bruno Decraene, Jeff Tantsura, Chris Bowers, Ketan Talaulikar and Bob Halley for their comments to this work.

Contributors' Addresses

Huanan Chen China Telecom 109, West Zhongshan Road, Tianhe District Guangzhou 510000 China Email: chenhn8.gd@chinatelecom.cn Peng Wu Huawei Huawei Bld., No.156 Beiqing Rd. Beijing 100095 China Email: baggio.wupeng@huawei.com Lei Liu Fujitsu United States of America Email: liulei.kddi@gmail.com Xufeng Liu Alef Edge United States of America Email: xufeng.liu.ietf@gmail.com