SRv6 Path Egress Protection

3.1. Mechanism

Figure 1 is used to explain the mechanism of SR path egress node and link protection.¶

            *******  *******   SIDa
        [PE1]-----[P1]-----[PEA]---[CE2]    PEA Egress
        / |        |&        | \   /        PEB Backup Egress
       /  |        |&        |  \ /         CEx Customer Edge
  [CE1]   |        |&        |   X          Px  Non-Provider Edge
       \  |        |&        |  / \         *** SR Path
        \ |        |& &&&&&  | /   \        &&& Backup Path
        [PE2]-----[P2]-----[PEB]---[CE3]
                        Mirror SID

Where node PEA is the egress node (or egress for short) 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 node (or say protector or backup egress for short) to provide the fast protection for the egress node (or say primary egress node) PEA. Node P1 is the direct previous/upstream hop of egress node PEA and acts as PLR (refer to [I-D.ietf-rtgwg-segment-routing-ti-lfa]) to support the fast protection for PEA.¶

When PEB is selected as a backup egress node to protect the egress node PEA, a Mirror SID (refer to Section 5.1 of [RFC8402]) is configured on PEB to protect PEA. PEB MUST advertise 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. In some cases, PEB can figure out the behavior of the SIDa at PEA from other protocols. For example, when SIDa is a VPN SID of PEA, PEB can get the behavior of the SIDa at PEA based on the VPN SID distribution by [RFC9252] or [RFC4364]. If PEB cannot obtain the behavior of the SIDa at PEA from protocols, the behavior MUST be configured on PEB.¶

When PEB gets the forwarding behavior of the SIDa of PEA from PEA or other means, it MUST add 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:¶

o

RL = <Mirror SID> if the LFA is the next hop node to PEB along the shortest path to PEB; or¶

o

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 egress node PEA fails, P1 as PLR sends the packet with SIDa carried by the SR path to backup egress node PEB, but MUST encapsulate 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:¶

o

Pkt' = (T, Mirror SID) (S, SIDa)Pkt0 if RL = <Mirror SID>; or¶

o

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 [RFC8986]). 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:¶

    S01. If (Upper-Layer header type == 41(IPv6) ) {
    S02.    Remove the outer IPv6 header with all its extension headers
    S03.    Set the packet's associated FIB table to Tm
    S04.    Submit the packet to the egress IPv6 FIB lookup for
               transmission to the new destination
    S05. } Else {
    S06.    Process as per Section 4.1.1 of RFC8986
    S07. }

Egress link protection is similar to egress node protection [RFC8679]. When the egress link from egress node PEA to CE2 fails, PEA acting as a PLR reroutes the traffic to backup egress node PEB via a backup path. Specifically, PEA as a PLR pre-computes a Repair List RL (or say backup path) toward PEB after receiving <PEB, PEA, Mirror SID> and knowing that PEB wants to protect SIDa of PEA. When the link fails, PEA as PLR sends the packet with SIDa by executing H.Encaps with the Repair List RL.¶

3.2. Example

Figure 2 shows an example of fast protecting egress node PE3 of an SR path, which is from ingress node PE1 to egress node PE3.¶

                                Locator: A3:1::/64
              *******  *******  VPN SID: A3:1::B100
          [PE1]-----[P1]-----[PE3]---[CE2]      PE3 Egress
          / |        |&        | \   /          PE4 Backup Egress
         /  |        |&        |  \ /           CEx Customer Edge
    [CE1]   |        |&        |   X            Px  Non-Provider Edge
         \  |        |&        |  / \           *** SR Path
          \ |        |& &&&&&  | /   \          &&& Backup Path
          [PE2]-----[P2]-----[PE4]---[CE3]
                                Locator: A4:1::/64
                                VPN SID: A4:1::B100
                             Mirror SID: A4:1::3, protect A3:1::/64

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 Figure 2, 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:¶

o

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)¶

o

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.¶

4.1. Extensions to IS-IS

A new sub-TLV, called IS-IS SRv6 Mirror SID sub-TLV, is defined. It is used in the SRv6 Locator TLV defined in [RFC9352] 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.¶

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type (TBD1)   |    Length     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Reserved    |    SRv6 Endpoint Function     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SID (16 octets)                       |
 :                                                               :
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         sub-sub-TLVs                          |
 :                                                               :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type:

TBD1 (suggested value 8) is to be assigned by IANA.¶

Length:

variable.¶

Reserved:

1 octet. This octet MUST be set to zero on transmit, and ignored on receipt.¶

SRv6 Endpoint Function:

2 octets. It contains the endpoint function 74 for Mirror SID.¶

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.¶

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Type (TBD2)  |    Length     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Locator-Size  | Locator (variable)            ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                                                               :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Locator-Size  | Locator (variable)            ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type:

TBD2 (suggested value 1) is to be assigned by IANA.¶

Length:

variable.¶

Locator-Size:

1 octet. Number of bits (1 - 128) in the Locator field.¶

Locator:

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 MUST have an SRv6 Locator TLV containing 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.¶

4.2. Extensions to OSPF

Similarly, a new sub-TLV, called OSPF Mirror SID sub-TLV, is defined. It is used in the SRv6 Locator TLV defined in [I-D.ietf-lsr-ospfv3-srv6-extensions] to advertise SRv6 Mirror SID and the locators of the nodes to be protected. Its format is illustrated below.¶

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Type (TBD4)           |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Reserved          |    SRv6 Endpoint Function     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SID (16 octets)                       |
 :                                                               :
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                            sub-TLVs                           |
 :                                                               :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type:

TBD4 (suggested value 8) is to be assigned by IANA.¶

Length:

variable.¶

Reserved:

2 octets. It MUST be set to zero for transmission and ignored on reception.¶

SRv6 Endpoint Function:

2 octets. It contains the endpoint function 74 for End.M SID.¶

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.¶

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Type (TBD5)           |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Locator-Size  |  Locator (variable)           ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                                                               :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Locator-Size  |  Locator (variable)           ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type:

TBD5 (suggested value 1) is to be assigned by IANA.¶

Length:

variable.¶

Locator-Size:

1 octet. Number of bits (1 - 128) in the Locator field.¶

Locator:

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 LSA, the LSA MUST have an SRv6 Locator TLV containing an OSPF SRv6 Mirror SID sub-TLV, which includes the Mirror SID and node A's locator in an OSPF Protected Locators sub-TLV.¶

7.1. Normative References

[I-D.ietf-lsr-ospfv3-srv6-extensions]

Li, Z., Hu, Z., Talaulikar, K., and P. Psenak, "OSPFv3 Extensions for SRv6", Work in Progress, Internet-Draft, draft-ietf-lsr-ospfv3-srv6-extensions-15, 21 June 2023, <https://datatracker.ietf.org/doc/html/draft-ietf-lsr-ospfv3-srv6-extensions-15>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8400]

Chen, H., Liu, A., Saad, T., Xu, F., and L. Huang, "Extensions to RSVP-TE for Label Switched Path (LSP) Egress Protection", RFC 8400, DOI 10.17487/RFC8400, June 2018, <https://www.rfc-editor.org/info/rfc8400>.

[RFC8402]

Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, July 2018, <https://www.rfc-editor.org/info/rfc8402>.

[RFC8667]

Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C., Bashandy, A., Gredler, H., and B. Decraene, "IS-IS Extensions for Segment Routing", RFC 8667, DOI 10.17487/RFC8667, December 2019, <https://www.rfc-editor.org/info/rfc8667>.

[RFC8679]

Shen, Y., Jeganathan, M., Decraene, B., Gredler, H., Michel, C., and H. Chen, "MPLS Egress Protection Framework", RFC 8679, DOI 10.17487/RFC8679, December 2019, <https://www.rfc-editor.org/info/rfc8679>.

[RFC8986]

Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 (SRv6) Network Programming", RFC 8986, DOI 10.17487/RFC8986, February 2021, <https://www.rfc-editor.org/info/rfc8986>.

[RFC9256]

Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", RFC 9256, DOI 10.17487/RFC9256, July 2022, <https://www.rfc-editor.org/info/rfc9256>.

[RFC9352]

Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B., and Z. Hu, "IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352, February 2023, <https://www.rfc-editor.org/info/rfc9352>.

7.2. Informative References

[I-D.ietf-rtgwg-segment-routing-ti-lfa]

Litkowski, S., Bashandy, A., Filsfils, C., Francois, P., Decraene, B., and D. Voyer, "Topology Independent Fast Reroute using Segment Routing", Work in Progress, Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-11, 30 June 2023, <https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-segment-routing-ti-lfa-11>.

[RFC3107]

Rekhter, Y. and E. Rosen, "Carrying Label Information in BGP-4", RFC 3107, DOI 10.17487/RFC3107, May 2001, <https://www.rfc-editor.org/info/rfc3107>.

[RFC4364]

Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February 2006, <https://www.rfc-editor.org/info/rfc4364>.

[RFC5226]

Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 5226, DOI 10.17487/RFC5226, May 2008, <https://www.rfc-editor.org/info/rfc5226>.

[RFC9252]

Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene, B., Zhuang, S., and J. Rabadan, "BGP Overlay Services Based on Segment Routing over IPv6 (SRv6)", RFC 9252, DOI 10.17487/RFC9252, July 2022, <https://www.rfc-editor.org/info/rfc9252>.