SRv6 for Redundancy Protection

Abstract

Redundancy Protection is a generalized protection mechanism to achieve the high reliability of service transmission in Segment Routing network. The mechanism inherits the "Live-Live" methodology, targeting to enhance the functionalities of Segment Routing over IPv6. Inspired by DetNet Packet Replication and Packet Elimination functions, two new Segments are introduced to provide replication and elimination functions on specific network nodes by leveraging SRv6 Segment programming capabilities.¶

1. Introduction

Redundancy Protection is a generalized protection mechanism to achieve the high reliability of service transmission in Segment Routing network. Specifically, packets of flows are replicated at a network node into two or more copies, which are transported via different and disjoint paths in parallel. When copies of packets are received and merged at one network node, the redundant packets are determined and further eliminated to guarantee only one copy of packets is transmitted. The mechanism inherits the "Live-Live" methodology, targeting to enhance the functionalities of Segment Routing over IPv6 [RFC8986]. Inspired by DetNet [RFC8655] Packet Replication and Packet Elimination Functions, two new Segments are introduced to provide the replication and elimination functions on specific network nodes by leveraging SRv6 Segment programming capabilities. As it is unnecessary to perform switchover of recieving packets between different paths, redundancy protection can facilitate to achieve zero packet loss target when failure on either path happens.¶

Redundancy protection provides ultra reliable protection to many services, for example Cloud VR/Game, IPTV service and other type of video services, high value private line service etc. In this document, redundancy protection is applied to point-to-point service. The mechanism for point-to-multipoint service stays out of the scope of this document.¶

Segment Routing (SR) leverages the source routing paradigm. An ingress node steers a packet through an ordered list of instructions, called "segments". A segment can be associated to an arbitrary processing of the packet in the node identified by the segment.¶

This document extends the Segment Routing capabilities to support the redundancy protection in an SRv6 environment, including the definitions of two new Segments, meta data encapsulation, and a variation of Segment Routing Policy.¶

2. Terminology

3. Redundancy Protection in Segment Routing Scenario

              |                                              |
              |<--------------- SRv6 Domain ---------------->|
              |                                              |
              |                    +-----+                   |
              |              +-----+  R3 +-----+             |
              |              |     +-----+     |             |
           +-----+        +--+--+           +--+--+       +-----+
    -------+  R1 +--------+ Red |           | Mer +-------+  R2 +-------
           +-----+        +--+--+           +--+--+       +-----+
                            |      +-----+     |
                            +------+  R4 +-----+
                                   +-----+

Figure 1: Example Scenario of Redundancy Protection in SRv6 Domain¶

This figure shows an example of redundancy protection used in SRv6 domain. R1, R2, R3, R4, Red and Mer are SR-capable nodes. When a flow is sent into SRv6 domain, the process is:¶

1) R1 receives the traffic flow and encapsulates packets with a list of segments destined to R2, which is instantiated as an ordered list of SRv6 SIDs.¶

2) When the packet flow arrives at Red node, known as Redundancy node, each packet is replicated into two or more copies. Each copy of the packet is encapsulated with a new segment list, which represents different disjoint forwarding paths.¶

3) Meta data information such as flow identification (FID) and sequence number (SN) is used to facilitate the packet elimination on Merging node (Mer). Flow identification identifies the specific flow, and sequence number distinguishes the packet sequence of a flow. Meta data is either carried in the packet before it arrives at redundancy node, or added to each of the replicas at redundancy node.¶

4) The multiple replicas go through different paths until the Mer node, known as Merging node. The first received copy of each flow packet is transmitted from Merging node to R2, and the redundant packets are eliminated.¶

5) When there is any failures or packet loss in one path, the service transmission continues through the other path non-disruptively.¶

6) Sometimes, out-of-order packets may occur since service packets are recovered from different forwarding paths. In this case, the merging node or other network nodes behind merging node is desired to include a reordering function, which is implementation specific and out of the scope of this document.¶

In this example, service protection is supported by utilizing two packet flows transmitted over two forwarding paths. It is noted that there is no limitation of the number of replicas. For a unidirectional flow, Red node supports replication function, and Mer node supports elimination function. Reordering function MAY be required in combination of elimination function on merging node. To minimize the jitter caused by random packet loss, the disjoint paths are recommended to have similar path forwarding delay.¶

4. Segment to Support Redundancy Protection

To achieve the packet replication and elimination functions, Redundancy Segment and Merging Segment, as well as the related SRv6 Endpoint Behavior are introduced.¶

S01. When an SRH is processed {
S02.   If (Segments Left>0)   {
S03.     Decrement IPv6 Hop Limit by 1
S04.     Decrement Segments Left by 1
S05.     Update IPv6 DA with Segment List[Segments Left]
S06.     Add flow identification and sequence number if indicated*
S07.     Duplicate the packets (as number of active SID lists in B)
S08.     Push the new IPv6 headers to each replica. The IPv6 header
         contains an SRH with the SID list in B
S09.     Set the outer IPv6 SA to A
S10.     Set the outer IPv6 DA to the first SID of new SRH SL
S11.     Set the outer Payload Length, Traffic Class, Flow Label,
         Hop Limit and Next-Header fields
S12.     Submit the packet to the egress IPv6 FIB lookup
         for transmission to the new destination
S13.   }
S14. }
* Adding flow identification and sequence number is an optional behavior
for Redundancy Segment. The instruction execution is determined and
explicitly indicated by SR policy or Segment itself.

S01. When an SRH is processed {
S02.  If (Segments Left> or ==0)   {
S03.    Acquire the sequence number of received packet and
        look it up in table
S04.      If (this sequence number does not exist in the table) {
S05.       Store this sequence number in table
S06.       Remove the outer IPv6+SRH header
S07.       Decrement IPv6 Hop Limit by 1 in inner SRH
S08.       Decrement Segments Left by 1 in inner SRH
S09.       Update IPv6 DA with Segment List[Segments Left] in inner SRH
S10.       Submit the packet to the egress IPv6 FIB lookup and transmit
S11.      }
S12.      ELSE {
S13.           Drop the packet
S14.      }
S15.    }
S16. }

5. Meta Data to Support Redundancy Protection

To support the redundancy protection function, flow identification and sequence number are added in the packet and further used at merging node when the merging function is executed. Flow identification identifies one specific flow of redundancy protection, and is usually allocated from centralized controller to SR ingress node or redundancy node in SR network. Note that flow identification can also be allocated and advertised by merging node. BGP, PCEP or Netconf protocols can facilitate the advertisement and distribution of flow identification among controller, redundancy node and merging node. Sequence number distinguishes the packets within a flow by specifying the order of packets. Not like the uniqueness of flow identification to one specific flow, sequence number keeps changing to each packet within a flow. It is RECOMMENDED to add the sequence number in forwarding plane as performance and scalability is required.¶

Figure 4 suggests an encapsulation of flow identification and sequence number in Segment Routing Header (SRH)[RFC8754] when redundancy protection is used in SRv6 network.¶

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Header   |  Hdr Ext Len  |  Routing Type | Segments Left |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Last Entry |    Flags      |     Tag (Sequence Number)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~          Merging Segment (Locator+Function+Arg:Flow ID)       ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                              ...                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                  Segment List[n] (128 bits)                   ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4 Encapsulation of Flow Identification and Sequence Number¶

Since the flow identification is only used at merging node to identify the specific flow of redundancy protection, it is RECOMMENDED to be encapsulated in the Arguments of Merging Segment in SRH. The length of flow identification is not limited, however in practice it is suggested to be 16 bits.¶

All the duplicates of the same packet need to be tagged for deduplication at the merging node. For this purpose, we will use a sequence number. It is RECOMMENDED to encode the seq number in the Tag field of the SRH, with a length of 16bits.¶

6. Segment Routing Policy to Support Redundancy Protection

Redundancy Policy is a variation of SR Policy to conduct the replicas to multiple disjoint paths for redundancy protection. It extends SR policy [I-D.ietf-spring-segment-routing-policy] to include more than one active and parallel ordered lists of segments between redundancy node and merging node, and all the ordered lists of segments are used at the same time to steer each copy of flow into different disjoint paths.¶

7. IANA Considerations

This document requires registration of End.R behavior and End.M behavior in "SRv6 Endpoint Behaviors" sub-registry of "Segment Routing Parameters" registry.¶

8. Security Considerations

TBD¶

9. Acknowledgements

The authors would like to thank Bruno Decraene, Ron Bonica, James Guichard, Jeffrey Zhang, Balazs Varga for their valuable comments and discussions.¶

10. References