| Internet-Draft | Enhanced Performance Measurement in SR | August 2021 |
| Gandhi, et al. | Expires 25 February 2022 | [Page] |
Segment Routing (SR) leverages the source routing paradigm. SR is applicable to both Multiprotocol Label Switching (SR-MPLS) and IPv6 (SRv6) data planes. This document defines procedure for Enhanced Performance Measurement of end-to-end SR paths including SR Policies for both SR-MPLS and SRv6 data planes using Simple Two-Way Active Measurement Protocol (STAMP) defined in RFC 8762. The procedure reduces the deployment and operational complexities in a network.¶
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Segment Routing (SR) leverages the source routing paradigm and greatly simplifies network operations for Software Defined Networks (SDNs). SR is applicable to both Multiprotocol Label Switching (SR-MPLS) and IPv6 (SRv6) data planes [RFC8402]. SR Policies as defined in [I-D.ietf-spring-segment-routing-policy] are used to steer traffic through a specific, user-defined paths using a stack of Segments. Built-in Performance Measurement (PM) for delay and packet loss as well as Connectivity Verification (CV) are essential requirements to provide Service Level Agreements (SLAs) in SR networks.¶
The Simple Two-Way Active Measurement Protocol (STAMP) provides capabilities for the measurement of various performance metrics in IP networks [RFC8762] without the use of a control channel to pre-signal session parameters. As described in [I-D.ietf-spring-stamp-srpm], the STAMP can be used for performance measurement for delay and packet loss of end-to-end SR paths.¶
Seamless Bidirectional Forwarding Detection (S-BFD) [RFC7880] provides a simplified mechanism for using BFD for path monitoring with a large proportion of negotiation aspects eliminated. The S-BFD can be used for connectivity verification of end-to-end SR paths.¶
Both STAMP and S-BFD require protocol support on the far-end Reflector node to process the received packets, and hence the received packets need to be punted from the forwarding fast path and return packets need to be generated. This limits the scale for number sessions and the ability to provide faster detection interval.¶
Enabling multiple protocols, S-BFD for connectivity verification and STAMP for performance measurement increases the deployment and operational complexities in a network. Also, implementing multiple protocols in a hardware significantly increases the development cost.¶
This document defines procedure for Enhanced Performance Measurement of end-to-end SR paths including SR Policies for both SR-MPLS and SRv6 data planes, using Simple Two-Way Active Measurement Protocol (STAMP) defined in [RFC8762]. The procedure reduces the deployment and operational complexities in a network.¶
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] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
S-BFD: Seamless Bidirectional Forwarding Detection.¶
BSID: Binding Segment ID.¶
ECMP: Equal Cost Multi-Path.¶
EB: Endpoint Behaviour.¶
HMAC: Hashed Message Authentication Code.¶
MBZ: Must be Zero.¶
MPLS: Multiprotocol Label Switching.¶
PM: Performance Measurement.¶
PTP: Precision Time Protocol.¶
SID: Segment ID.¶
SL: Segment List.¶
SR: Segment Routing.¶
SRH: Segment Routing Header.¶
SR-MPLS: Segment Routing with MPLS data plane.¶
SRv6: Segment Routing with IPv6 data plane.¶
STAMP: Simple Two-way Active Measurement Protocol.¶
TC: Traffic Class.¶
TTL: Time To Live.¶
In the reference topology shown in Figure 1, the STAMP Session-Sender [RFC8762] S1 initiates a Session-Sender test packet and the Session-Reflector R1 returns the test packet. The return test packet is transmitted back to the Session-Sender S1 on the same path (same set of links and nodes) or a different path in the reverse direction from the path taken towards the Session-Reflector R1.¶
The Session-Sender S1 and Session-Reflector R1 are connected via an SR path [RFC8402]. The SR path can be an SR Policy [I-D.ietf-spring-segment-routing-policy] on node S1 (called head-end) with destination to node R1 (called tail-end).¶
T1 T2
/ \
+-------+ STAMP Test Packet +-------+
| | - - - - - - - - - - - | |
| S1 |======================|| R1 |
| |<- - - - - - - - - - - | |
+-------+ Return Test Packet +-------+
\
T4
Session-Sender Session-Reflector
(Timestamp,
Pop and Forward)
As described in [I-D.ietf-spring-stamp-srpm], in loopback mode, the STAMP Session-Sender S1 initiates Session-Sender test packets and the Session-Reflector R1 forwards them back to the Session-Sender S1. The received STAMP test packets are not punted out of the fast path in forwarding at the Session-Reflector. At the Session-Reflector, the loopback function simply makes the necessary changes to the encapsulation including IP and UDP headers to return the STAMP test packet to the Session-Sender S1. No STAMP test session is created on the Session-Reflector R1.¶
This document defines a new STAMP measurement mode, enhanced loopback mode, that is loopback mode enabled with network programming function. In this mode, both transmit (T1) and receive (T2) timestamps in data plane are collected by the Session-Sender test packets as shown in Figure 1. The network programming function optimizes the "operations of punt test packet and generate return test packet" on the Session-Reflector as timestamping is implemented in forwarding fast path in hardware. This helps to achieve higher STAMP test session scale and faster detection interval.¶
The Session-Sender adds transmit timestamp (T1) in the payload of the Session-Sender test packet. The Session-Reflector adds the receive timestamp (T2) in the payload of the received test packet in forwarding fast path in hardware without punting the test packet (e.g. to slow path or control-plane). The network programming function enables Session-Reflector to add the receive timestamp (T2) at a specific offset in the payload which is locally provisioned, consistently in the network.¶
The Session-Reflector only adds the receive timestamp if the source IP address (in case of SR-MPLS) or outer IPv6 header destination IP address (in case of SRv6) in the test packet matches the local node address to ensure that the test packet reaches the intended Session-Reflector and the receive timestamp is returned by the intended Session-Reflector.¶
An example provisioning model and typical measurement parameters are shown in Figure 2:¶
+------------+
| Controller |
+------------+
STAMP Mode / \ Timestamp Label/SRv6 EB
Enhanced Loopback Mode / \ Timestamp Offset
Timestamp Label/SRv6 EB / \ Timestamp Format
Timestamp Format / \
Missed Packet Count (N) / \
Delay Threshold/Count(TH/M) / \
Packet Loss Threshold(XofY) / \
v v
+-------+ +-------+
| | | |
| S1 |==========| R1 |
| | | |
+-------+ +-------+
Session-Sender Session-Reflector
Example of a STAMP mode is enhanced loopback mode defined in this document. The values for Timestamp Label and SRv6 Endpoint Behaviour may be provisioned as described in this document. Example of Timestamp Format is 64-bit PTPv2 [IEEE1588]. Example of Timestamp Offset is 16 and 32 bytes for the unauthenticated and authenticated STAMP Session-Sender test packets, respectively. Example of threshold values configured for generating notifications are: Missed Packet Count (N), Delay Exceeded Threshold and Packet Count (TH/M) and Packet Loss Threshold (XofY), as described in this document.¶
The mechanisms to provision the Session-Sender and Session-Reflector are outside the scope of this document.¶
For enhanced performance monitoring of an end-to-end SR path including SR Policy, STAMP Session-Sender test packets are transmitted in loopback mode enabled with network programming function to timestamp and forward the packet.¶
For SR Policy, the Session-Sender test packets are transmitted using the Segment List (SL) of the Candidate-Path [I-D.ietf-spring-segment-routing-policy]. When a Candidate-Path has more than one Segment Lists, multiple Session-Sender test packets MUST be transmitted, one using each Segment List.¶
The STAMP Session-Sender test packets is transmitted using an MPLS header for each Label Stack of the SR-MPLS Policy Candidate-Path(s) as shown in Figure 3.¶
The SR-MPLS header can contain the MPLS label stack of the forward path or both forward and the reverse direction paths. In the former case, the return test packets are received by the Session-Sender via IP/UDP [RFC0768] return path and the MPLS header is removed by the Session-Reflector.¶
In the latter case, the Segment List of the reverse direction SR path is added in the Session-Sender test packet header to receive the return test packet on a specific path, either using the Binding SID [I-D.ietf-pce-binding-label-sid] or Segment List of the Reverse SR Policy [I-D.ietf-pce-sr-bidir-path]. In this case, the MPLS header is not removed by the Session-Reflector.¶
In both cases, the Session-Sender MUST set the Destination Address equal to the Session-Sender address and the Source Address equal to the Session-Reflector address in the IP header of the test packets.¶
In this document, two new Timestamp Labels are defined for SR-MPLS data plane to enable network programming function for "timestamp, pop and forward" the received test packet, one for unauthenticated mode and one for authenticated mode.¶
In the Session-Sender test packets for SR-MPLS Policies, a Timestamp Label is added in the MPLS header as shown in Figure 3, to collect "Receive Timestamp" field in the payload of the test packet. The Label Stack for the reverse direction SR-MPLS path can be added after the Timestamp Label (not shown in the Figure) to receive the return test packet on a specific path. When a Session-Reflector receives a packet with Timestamp Label, after timestamping the packet at a specific offset, the Session-Reflector pops the Timestamp Label and forwards the packet using the next label or IP header in the packet (just like the data packets for the normal traffic).¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Label(1) | TC |S| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Label(n) | TC |S| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extension Label (15) | TC |S| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Timestamp Label (TBA1 or TBA2) | TC |S| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IP Header | . Source IP Address = Session-Reflector IPv4 or IPv6 Address . . Destination IP Address = Session-Sender IPv4 or IPv6 Address . . Protocol = UDP . . . +---------------------------------------------------------------+ | UDP Header | . Source Port = As chosen by Session-Sender . . Destination Port = As chosen by Session-Sender . . . +---------------------------------------------------------------+ | Payload = Test Packet as specified in Section 3 of RFC 8972 | . in Figure 1 and Figure 3 . . . +---------------------------------------------------------------+
The timestamp Labels for STAMP test packets in unauthenticated and authenticated modes can be allocated using one of the following methods:¶
The STAMP Session-Sender needs to know if the Session-Reflector can process the Timestamp Label to avoid dropping test packets. The signaling extension for this capability exchange is outside the scope of this document.¶
The STAMP Session-Sender test packets for SRv6 data plane is transmitted using a Segment Routing Header (SRH) [RFC8754] for each Segment List of the SRv6 Policy Candidate-Path(s) as shown in Figure 4.¶
The SRH can contain the Segment List of the forward path or both forward and the reverse direction paths. In the former case, an inner IPv6 header (after SRH and before UDP header) MUST be added that contains the Destination Address equal to the Session-Sender address and the Source Address equal to the Session-Reflector address as shown in Figure 4. In this case, the SRH is removed by the Session-Reflector and IP/UDP return path is used.¶
In the latter case, the Segment List of the reverse direction SR path is added in the SRH to receive the return test packet on a specific path, either using the Binding SID [I-D.ietf-pce-binding-label-sid] or Segment List of the Reverse SR Policy [I-D.ietf-pce-sr-bidir-path]. In this case, the SRH is not removed by the Session-Reflector and an inner IPv6 header is not required. When the return test packet contains an SRH at the Session-Sender, the procedure defined for upper-layer header processing for SRv6 SIDs in [RFC8986] MUST be used to process the UDP header in the received test packets.¶
The [RFC8986] defines SRv6 Endpoint Behaviours (EB) for SRv6 nodes. In this document, two new Timestamp Endpoint Behaviours are defined for Segment Routing Header (SRH) [RFC8754] to enable "Timestamp and Forward (TSF)" function for the received test packets, one for unauthenticated mode and one for authenticated mode.¶
In the Session-Sender test packets for SRv6 Policies, Timestamp Endpoint Function (End.TSF) is carried with the target Segment Identifier (SID) in SRH [RFC8754] as shown in Figure 4, to collect "Receive Timestamp" field in the payload of the test packet. The Segment List for the reverse direction path can be added after the target SID to receive the return test packet on a specific path. When a Session-Reflector receives a packet with Timestamp Endpoint (End.TSF) for the target SID which is local, after timestamping the packet at a specific offset, the Session-Reflector forwards the packet using the next SID in the SRH or inner IPv6 header in the packet (just like the data packets for the normal traffic).¶
+---------------------------------------------------------------+ | IP Header | . Source IP Address = Session-Sender IPv6 Address . . Destination IP Address = Destination IPv6 Address . . . +---------------------------------------------------------------+ | SRH as specified in RFC 8754 | . <Segment List> . . SRv6 Endpoint End.TSF (value TBA3 or TBA4) . . . +---------------------------------------------------------------+ | IP Header | . Source IP Address = Session-Reflector IPv6 Address . . Destination IP Address = Session-Sender IPv6 Address . . . +---------------------------------------------------------------+ | UDP Header | . Source Port = As chosen by Session-Sender . . Destination Port = As chosen by Session-Sender . . . +---------------------------------------------------------------+ | Payload = Test Packet as specified in Section 3 of RFC 8972 | . in Figure 1 and Figure 3 . . . +---------------------------------------------------------------+
The Timestamp Endpoint Functions for "Timestamp and Forward" can be signaled using one of the following methods:¶
The STAMP Session-Sender needs to know if the Session-Reflector can process the Timestamp Endpoint Function to avoid dropping test packets. The signaling extension for this capability exchange is outside the scope of this document.¶
The timestamps T1 and T2 are used to measure the one-way delay. The delay metrics for an end-to-end SR path are notified, for example, when consecutive M number of test packets have measured delay values exceed the user-configured threshold TH, where M (Delay Exceeded Packet Count) and TH (Absolute and Percentage Delay Exceeded Threshold) are also locally provisioned values.¶
The round-trip packet loss for an end-to-end SR path is calculated using the Sequence Number in the Session-Sender test packets. The packet loss metric is notified when X number of Session-Sender test packets were lost out of last Y number of test packets transmitted by the Session-Sender, where Threshold XofY is locally provisioned value.¶
STAMP session state as UP (i.e. Connectivity verification success) for an end-to-end SR path is initially notified as soon as one or more return test packets are received at the Session-Sender.¶
STAMP session state as DOWN (i.e. Connectivity verification failure) for an end-to-end SR path is notified when consecutive N number of return test packets are not received at the Session-Sender, where N (Missed Packet Count) is a locally provisioned value.¶
In the loopback mode, a connectivity verification failure on the reverse direction path can cause the return test packets to not reach the Session-Sender. This is also true in the case where the return test packets are generated by the stateless Session-Reflector in two-way measurement. The stateful Session-Reflector can solve this issue by maintaining the forwarding direction state and notifying a connectivity verification success and failure to the Session-Sender.¶
The STAMP protocol is intended for deployment in limited domains [RFC8799]. As such, it assumes that a node involved in the STAMP protocol operation has previously verified the integrity of the path and the identity of the far-end Session-Reflector.¶
The security considerations specified in [RFC8762] and [RFC8972] also apply to the procedures defined in this document. Specifically, the message integrity protection using HMAC, as defined in Section 4.4 of [RFC8762] also apply to the procedure described in this document.¶
IANA maintains the "Special-Purpose Multiprotocol Label Switching (MPLS) Label Values" registry (see <https://www.iana.org/assignments/mpls-label-values/mpls-label-values.xml>). IANA is requested to allocate Timestamp Label value from the "Extended Special-Purpose MPLS Label Values" registry:¶
+-------------+---------------------------------+---------------+ | Value | Description | Reference | +-------------+---------------------------------+---------------+ | TBA1 | Timestamp Label | This document | | | for offset 16 for STAMP | | | | in Unauthenticated Mode | | +-------------+---------------------------------+---------------+ | TBA2 | Timestamp Label | This document | | | for offset 32 for STAMP | | | | in Authenticated Mode | | +-------------+---------------------------------+---------------+¶
IANA is requested to allocate, within the "SRv6 Endpoint Behaviors Registry" sub-registry belonging to the top-level "Segment Routing Parameters" registry [RFC8986], the following allocation:¶
+-------------+---------------------------------+---------------+ | Value | Endpoint Behavior | Reference | +-------------+---------------------------------+---------------+ | TBA3 | End.TSF (Timestamp and Forward) | This document | | | for offset 16 for STAMP | | | | in Unauthenticated Mode | | +-------------+---------------------------------+---------------+ | TBA4 | End.TSF (Timestamp and Forward) | This document | | | for offset 32 for STAMP | | | | in Authenticated Mode | | +-------------+---------------------------------+---------------+¶
The authors would like to thank Greg Mirsky, Kireeti Kompella, and Adrian Farrel for providing useful comments.¶