Internet DRAFT - draft-arora-mpls-spring-ttl-procedures-srte-paths
draft-arora-mpls-spring-ttl-procedures-srte-paths
Routing area K. Arora
Internet-Draft S. Hegde
Intended status: Standards Track Juniper Networks Inc.
Expires: August 25, 2019 S. Aldrin
Google
S. Litkowski
Orange Business Service
M. Durrani
Equinix
February 21, 2019
TTL Procedures for SR-TE Paths in Label Switched Path Traceroute
Mechanisms
draft-arora-mpls-spring-ttl-procedures-srte-paths-01
Abstract
Segment routing supports the creation of explicit paths using
adjacency-sids, node-sids, and anycast-sids. The SR-TE paths are
built by stacking the labels that represent the nodes and links in
the explicit path. A very useful Operations And Maintenance
requirement is to be able to trace these paths as defined in
[RFC8029]. This document specifies a uniform mechanism to support
MPLS traceroute for the SR-TE paths when the nodes in the network are
following uniform mode or short-pipe mode [RFC3443].
Requirements Language
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 RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on August 25, 2019.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem with SR-TE Paths . . . . . . . . . . . . . . . . . . 3
2.1. Short Pipe model . . . . . . . . . . . . . . . . . . . . 4
2.2. Uniform Model . . . . . . . . . . . . . . . . . . . . . . 4
3. Detailed Solution For TTL procedures for SR-TE paths . . . . 5
3.1. P bit in DDMT TLV . . . . . . . . . . . . . . . . . . . . 5
3.1.1. Procedures for a PHP router of the tunnel being
traced . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.2. Procedures for a egress router of the tunnel being
traced . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.3. Procedures for a ingress router of the SR-TE path . . 5
3.1.4. Example describing the solution . . . . . . . . . . . 6
3.2. Procedures for handling binding-sids . . . . . . . . . . 7
3.2.1. Uniform Model . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Shortpipe Model . . . . . . . . . . . . . . . . . . . 8
4. Backward Compatibility . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The mechanisms to handle TTL procedures for SR-TE paths are described
in ([RFC8287]). Section 7.5 of ([RFC8287]) defines the TTL
manipulation procedures for short pipe model as below.The LSR
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initiating the traceroute SHOULD start by setting the TTL to 1 for
the tunnel in the LSP's label stack it wants to start the tracing
from, the TTL of all outer labels in the stack to the max value, and
the TTL of all the inner labels in the stack to zero. However this
mechanism has issues when the constituent tunnels are penultimate-
hop-popping(PHP). This document does not propose any change to
([RFC8287]) if the constituent tunnels are ultimate-hop-popping (UHP)
or Egress LSR advertizes explicit NULL.
Section 2 describes problems in tracing SR-TE paths and the need for
a specialized mechanism to trace SR-TE paths. Section 3 describes
the solution applied to mpls echo request/response to trace
adjacency-sids and node-sids trace SR-TE path in uniform model and
short pipe model.
2. Problem with SR-TE Paths
The topology shown in Figure 1. illustrates a example network
topology with SPRING enabled on each node.
Node Node Node Node
sid:1 sid:2 sid:3 sid:4
+----+ 10 +----+ 10 +----+ 10 +----+
| R1 |--------| R2 |--------| R3 |--------| R4 |
+----+ +----+ +----+ +----+
Label stack:
+------------+
| 1003 (top)|
+------------+
| 1004 |
+------------+
Figure 1: Example topology with SRGB 1000-2000
Consider an explicit path in the topology in Figure 1 from R1->R4 via
R1->R2->R3->R4. The label stack to instantiate this path contains
two node-sids 1003 and 1004. The 1003 label will take the packet
from R1 to R3. The next label in the stack 1004 will take the packet
from R3 to the destination R4. consider the mechanism below for the
TTL procedures specified in RFC 8287 for short pipe model and uniform
model for PHP LSPs.
Notation: ((X,Y),(Z,W)) refers to a label stack whose top label stack
entry has the label corresponding to the node-SID of X, with TTL Y,
and whose second label stack entry has the label corresponding to the
node-SID of Z, with TTL W.
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According to the procedure in Section 7.5 of [RFC8287], the LSP
traceroute is done as follows in short pipe model and uniform model:
2.1. Short Pipe model
Refer the diagram in Figure 1.
1. Ingress R1 sends mpls LSP Echo Request with label stack of
((1003,1),(1004,0)) to R2.
2. Since R2 receives mpls LSP Echo Request with TTL as 1 for outer
most label, R2's local software processes the Lsp traceroute packet
and R2 sends an echo reply to R1 with return code as 'transit'.
3. R1 receives the LSP Echo Reply from R2, and then sends next LSP
Echo Request with label stack ((1003,2),(1004,0)).
4. R2 forwards packet to R3 as ((1004,0)) (i.e. R2 being PHP, pops
the label 1003 and does not propagate TTL)
5. R3 receives a packet with TTL=0 at the top of the stack. Receipt
of a packet with TTL=0 may cause R3 to drop the packet or rate limit
it.
6. Even if R3's local software processes the packet and validates
the FEC for 1003 and sends egress code in echo-reply, the next packet
will have ((1003,255), (1004, 1)) which causes TTL to expire again on
R3 as the 1003 label is popped at the penultimate.
RFC 8287 suggests that when R1's LSP Echo Request has reached the
egress of the outer tunnel, R1 should begin to trace the inner tunnel
by sending a LSP Echo Request with label stack ((1003,255),(1004,1)).
However, as explained in step 6, the traceroute procedure does not
work correctly.
2.2. Uniform Model
1. Ingress R1 sends mpls LSP Echo Request with label stack of
((1003,1),(1004,0)) to R2.
2. Since R2 receives mpls LSP Echo Request with TTL as 1 for outer
most label, R2's local software processes the Lsp ping packet and R2
sends an echo reply to R1 with return code as 'transit'.
3. R1 receives the LSP Echo Reply from R2, and then sends next LSP
Echo Request with label stack ((1003,2),(1004,0)).
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4. It is expected that R2 should propogate the TTL of outer label to
inner label before forwarding the packet to R3. However most of the
PFEs implementations generally do not increase a label stack entry's
TTL when they do TTL propagation. So when (1003,2) is popped, we
might still end up with (1004,0) at R3, even if we have TTL
propagation configured. Increasing the TTL of a packet is not a good
practice as it can result in forwarding loops.
5. R3 receives a packet with TTL=0 at the top of the stack. Receipt
of a packet with TTL=0 will cause R3 to drop the packet or rate limit
it.
6. Even if R3's local software processes the packet and validates
the FEC for 1003 and sends egress code in echo-reply, the next packet
will have ((1003,255), (1004, 1)) which causes TTL to expire again on
R3 as the 1003 label is popped at the penultimate.
So in either case (uniform model or short pipe model) traceroute may
not work for SR-TE paths with PHP Lsps.
3. Detailed Solution For TTL procedures for SR-TE paths
3.1. P bit in DDMT TLV
DS flags has 4 unused bits from position '0' to '3'. This document
uses bit '3' in DS flags of downstream mapping TLV.
3.1.1. Procedures for a PHP router of the tunnel being traced
When a LSR receives an echo request it MUST validate the outermost
FEC in the echo request. LSR SHOULD set the 'P' bit in the DS flags
of downstream mapping TLV if its a PHP router for the outermost FEC.
Other cases it should work as explained in [RFC8029] and [RFC8287].
3.1.2. Procedures for a egress router of the tunnel being traced
When a LSR receives an echo request it MUST validate the outermost
FEC in the echo request. Egress cases should work as explained in
[RFC8029] and [RFC8287].
3.1.3. Procedures for a ingress router of the SR-TE path
When an ingress LSR receives an echo response it MUST behave as
defined below depending on the return code in the echo response.
1. When an ingress LSR receives an echo response with return code as
8 (Label switched at stack-depth), Ingress LSR MUST check if the LSR
that sent the echo response is PHP for the outermost FEC in the FEC
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stack. If the LSR that sent the echo response is PHP for the
outermost FEC then while sending next echo request Ingress LSR MUST
increase the TTL value of inner label also (if exists) in addition to
increasing the TTL value of the tunnel it is tracing. Ingress LSR
can detect that LSR that sent the echo response is a PHP router for
the outermost FEC, either by looking at 'P' bit set in the DS flags
of downstream mapping TLV or if Ingress LSR has received LABEL '3' in
the label stack TLV of downstream detailed mapping TLV. For all
other cases ingress should work as explained in [RFC8029] and
[RFC8287].
2. When an Ingress LSR receives an echo response with return code as
3 (Replying router is an egress for the FEC at stack-depth) for the
outermost FEC and this is not the only FEC in the FEC stack, then
ingress LSR SHOULD remove the outermost FEC from the FEC stack and
send the next traceroute request with the same TTL value for all the
labels in the label stack as the previous echo request. This will
ensure the egress of the tunnel is visited twice, once as egress for
top label and again as a transit for next tunnel.
3.1.4. Example describing the solution
This section provides a detailed description of how PHP router helps
ingress in handling TTL procedures for SR-TE paths. Below are the
procedures performed by PHP router and ingress router to perform TTL
procedure for mpls traceroute for SR-TE paths. Below solution works
for both uniform model and short pipe model.
1. Ingress R1 sends mpls LSP Echo Request with label stack of
((1003,1),(1004,0)) to R2.
2. Since R2 receives mpls LSP Echo Request with TTL as 1 for outer
most label, R2's local software processes the Lsp ping packet. R2's
local software validates the outermost FEC and looking at the FEC R2
knows that its the PHP router for outermost FEC (Node-Sid R3).
3. R2 sets a bit in the DS flags in the DDMT TLV in echo response (P
bit, One of the reserved bits).
4. When R1 looks at the echo response from R2 it sees P bit in DDMT
TLV.
5. So R1 increments the TTL value of Node-R3 by 1 (make it 2) and
TTL value of next element in the label stack also
6. R1 should send the next mpls LSP Echo Request with label stack
((1003,2),(1004,1)).
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7. R2 being PHP pops the outermost label from the label stack and
forwards the packet to R3 with with label (1004, 1)
8. R3 receives mpls LSP Echo Request with TTL as 1 for outer most
label, R3's local software processes the echo request.
9. R3 validates the outermost FEC and sends echo response to R1 with
return code as the egress for outermost FEC (Node-Sid R3).
10. When R1 receives echo response with return code as egress, R1
should remove outermost FEC (Node-Sid R3) from the FEC stack and send
the next echo request with the same TTL value as the previous one i.e
((1003,2),(1004,1)).
11. Since R3 is the PHP router for FEC (Node-Sid R4) in the label
stack. R3 should set 'P' bit in the in the DS flags in the DDMT TLV
in echo response with return code as Transit.
12. R1 should send the next mpls LSP Echo Request with label stack
((1003,2),(1004,2)) with FEC Node-Sid-R4 .
13. R2 pops the first label from the label stack and R3 pops the
second label from the label stack.
14. R4 receives an unlabelled packet with RA bit set in ip options.
R4 delivers the packet to local software for processing.
15. R4's local software validates the ouetmost FEC as 'egress' and
sends an echo reply with return code as egress.
17. R1 receives an echo reply with return code as egress for the
last FEC in the FEC stack TLV and completes the traceroute.
3.2. Procedures for handling binding-sids
Inorder to provide greater scalability, network opacity, and service
independence, SR architecture [RFC8402] defines a Binding SID (BSID).
A Binding SID is bound to an SR policy which typically involves a
list of SIDs. These Binding SIDs may appear in another SR Policy or
may be used to steer service traffic from the service origin. The
TTL handling mechanisms for MPLS traceroute procedures involving
Binding SIDs is described below.
3.2.1. Uniform Model
When the node advertising the Binding SID is operating in uniform
mode [RFC3443], it SHOULD send FEC stack change sub-TLV as in sec
4.5.1 of [RFC8029]. The ingress node SHOULD increment the TTL of
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Binding SID label at every step until "egress" return code is sent
for all the new FECs included due to FEC stack change and all the
Tunnels replaced by the Binding SID are completely traced. It is
required that all the label popping nodes involved in these tunnels
MUST support uniform model and copy the TTL to bottom label when the
label is popped.
3.2.2. Shortpipe Model
When the node advertising the Binding SID is operating in short pipe
model [RFC3443], it SHOULD not send FEC stack change sub-TLV. The
Binding SID is treated as single hop and the nodes internal to the
Tunnel represented by Binding SID SHOULD NOT be traced.
4. Backward Compatibility
The extension proposed in this document is backward compatible with
procedures described in [RFC8029] and [RFC8287]. If the LSR with the
proposed solution is the Ingress and all other LSR in the SR tunnel
are not with the extension, Then no LSR is going to set 'P' bit so
ingress LSR with new extension will work as per [RFC8029] and
[RFC8287].If the LSR with the proposed extension is the one of the
transit router and if its the PHP then it may set 'P' bit based on
the section 3. Ingress may not react to the 'P' bit and traceroute
will continue to work as per [RFC8029] and [RFC8287].
5. Security Considerations
TBD
6. IANA Considerations
IANA has created and now maintains a registry entitled "DS Flags".
The registration policy for this registry is Standards Action
[RFC5226]. IANA has made the following assignments:
Bit Number Name Reference
---------- ------------------------------------------- ---------
7 N: Treat as a Non-IP Packet [RFC8029]
6 I: Interface and Label Stack Object Request [RFC8029]
5 E: ELI/EL push indicator [RFC8012]
4 L: Label-based load balance indicator [RFC8012]
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3 P: Penulimate Hop router
2-0 Unassigned
7. Acknowledgements
Thanks to Przemyslaw Krol for careful review and comments.
8. References
8.1. Normative References
[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>.
[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>.
[RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
Switched (MPLS) Data-Plane Failures", RFC 8029,
DOI 10.17487/RFC8029, March 2017,
<https://www.rfc-editor.org/info/rfc8029>.
[RFC8287] Kumar, N., Ed., Pignataro, C., Ed., Swallow, G., Akiya,
N., Kini, S., and M. Chen, "Label Switched Path (LSP)
Ping/Traceroute for Segment Routing (SR) IGP-Prefix and
IGP-Adjacency Segment Identifiers (SIDs) with MPLS Data
Planes", RFC 8287, DOI 10.17487/RFC8287, December 2017,
<https://www.rfc-editor.org/info/rfc8287>.
[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>.
8.2. Informative References
[RFC3443] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
in Multi-Protocol Label Switching (MPLS) Networks",
RFC 3443, DOI 10.17487/RFC3443, January 2003,
<https://www.rfc-editor.org/info/rfc3443>.
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Authors' Addresses
Kapil Arora
Juniper Networks Inc.
Exora Business Park
Bangalore, KA 560103
India
Email: kapilaro@juniper.net
Shraddha Hegde
Juniper Networks Inc.
Exora Business Park
Bangalore, KA 560103
India
Email: shraddha@juniper.net
Sam Aldrin
Google
Email: aldrin.ietf@gmail.com
Stephane Litkowski
Orange Business Service
Email: stephane.litkowski@orange.com
Muhammad Durrani
Equinix
Email: mdurrani@equinix.com
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