SRIFT: Segment Routing in Fat Trees
draft-zzhang-rift-sr-04
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Zhaohui (Jeffrey) Zhang , Jeff Tantsura , Jordan Head , Don Fedyk | ||
| Last updated | 2022-05-26 | ||
| Stream | (None) | ||
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draft-zzhang-rift-sr-04
RIFT Z. Zhang
Internet-Draft J. Tantsura
Intended status: Standards Track J. Head
Expires: 27 November 2022 Juniper Networks
D. Fedyk
Individual
26 May 2022
SRIFT: Segment Routing in Fat Trees
draft-zzhang-rift-sr-04
Abstract
This document specifies signaling procedures for Segment Routing in
RIFT. Each node's loopback address, Segment Routing Global Block
(SRGB) and Node Segment Identifier (Node-SID), which are typically
assigned by a configuration management system and distibuted by
routing protocols, are distributed southbound from the Top Of Fabric
(TOF) nodes via RIFT's Key-Value distribution mechanism, so that each
node can compute how to reach a segment represented by the active SID
in a packet. An SR controller signals SR policies to ingress nodes
so that they can send packets with a desired segment list to steer
traffic.
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 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-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on 27 November 2022.
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Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. SR in RIFT (SRIFT) . . . . . . . . . . . . . . . . . . . . . 4
3. Well-Known KV Registry Values . . . . . . . . . . . . . . . . 6
3.1. SRIFT Node Key-Type . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Before we discuss the SR procedures for RIFT, let us first review how
SR works with OSPF [RFC8665] and IS-IS [RFC8667].
Each node is provisioned with a loopback address as well as SRGB and
Node-SID values. The loopback address and Node-SID are centrally
coordinated and are unique per-node within the SR network. These
values are then communicated to each node out-of-band and stored as
configuration information. Communication could be done via primitive
pen and paper or via modern signaling (Netconf/YANG) from a
configuration management system.
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SRGB information represents the label range of the "global" labels
that can be allocated on a particular node for SR. SRGB could have
more than one contiguous range of labeks allocated to it. It is
comprised of the first available label value and the total number of
available labels per range. While in modern networks it is common
for each node to have identical SRGB values so that a Node-SID will
correspond to the same label on each node, this is not required as to
allow for flexible label allocation. In either scenario, SRGB is
part of each node's configuration. In today's networks, it is likely
pushed to nodes by a configuration management system.
Each node then signals its SRGB and Node-SID to the other nodes. A
Node-SID is an index value assigned to a node (say node X), and
another node (say node Y) uses the Node-SID to derive (from Y's SRGB)
the label to use when sending traffic to node X.
Consider the following example illustrating Node A's computed IP
route and label values.
B
* *
* *
* *
A D
* *
* *
* *
C
Node Name Loopback Node SID SRGB Label Base SRGB Label Range
--------- -------- -------- --------------- ----------------
A 10.1.1.1 1 100 50
B 10.1.1.2 2 100 50
C 10.1.1.3 3 200 50
D 10.1.1.4 4 100 50
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Destination Next Hop
----------- --------
10.1.1.1 local
10.1.1.2 if_ab
10.1.1.3 if_ac
10.1.1.4 if_ab, if_ac
Label Next Hop
----- --------
100 (La_a) pop and look up next header
101 (Lb_a) swap to 101 (Lb_b), via if_ab
102 (Lc_a) swap to 202 (Lc_c), via if_ac
103 (Ld_a) swap to 103 (Ld_b), via if_ab
swap to 203 (Ld_c), via if_ac
The specific notation Lb_a refers to the label derived for node B,
using B's Node-SID as index into A's SRGB. Similarly, Ld_c refers to
the label derived for Node D, using D's Node-SID as index into C's
SRGB.
Node A computes the route to Node D's loopback address. The next
hops are Node B (via if_ab) and Node C (via if_ac). Node A uses Node
D's Node-SID (which was advertised along with the loopback address)
to index into its local SRGB to obtain a label value of 103 (Ld_a).
Furthermore, Node A also uses Node D's Node-SID to derive label
values for Node B and Node C, 103 (Ld_b) and 203 (Ld_c) respectively,
using D's Node-SIDs as index into B and C' SRGBs respectively.
Notice that Node C's SRGB is different from the other nodes. Node A
can now program its label forwarding state with (Ld_a --> (via if_ab
swap to Ld_b, via if_ac swap to Ld_c)).
Similarly, Node B computes the route to Node D's loopback address,
but this time finds that the next hop is Node D itself (via if_bd).
Node B will also use Node D's Node-SID (again, advertised with the
loopback address) to index into its local SRGB and obtain a label
value of 103 (Ld_b) and index into Node D's SRGB and obtain a label
value of 103 (Ld_d). The label forwarding state can be programmed
with (Ld_b --> via if_bd swap to Ld_d). Finally, Node D programs its
label forwarding state with (Ld_d -> pop and lookup next header).
2. SR in RIFT (SRIFT)
In referring to the previous section, it is clear that each RIFT node
participating in a SR domain requires the following information:
* SRGB values of all adjacent nodes
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* Node-SID values of all nodes participating in the routing domain
* Loopback addresses or System IDs of all other nodes
In OSPF and IS-IS, each node's SR information is simply flooded.
With RIFT, Node TIEs could be used to flood SR information, but each
node would have to learn its own SR information first. With RIFT's
Key-Value mechanism, KV-TIEs can be used for TOF nodes to flood all
nodes' SR information that it learns from an SR controller, therefore
accommodating both provisioning and signalling of SR. The non-TOF
nodes do not need any SR related provisioning, which goes very well
with RIFT's ZTP concept.
ToF nodes in an SR domain MUST populate KV South TIEs with the
minimum required SR information for each node. Specifically SRGB
Label Base, SRGB Label Range, Node-SID, RIFT System ID, and Loopback
Address. While the Loopback Address must be included, it MAY be set
to an empty value in cases if loopbacks are not configured for nodes.
Traffic forwarding in an SR network is typically done in two ways.
The first option is to use Prefix-SIDs and allow traffic to follow
the shortest paths for the prefixes. Prefix-SIDs for node prefixes,
i.e. Node-SIDs (for loopback addresses), can be used both for
encapsulating service traffic to service nodes (e.g. VPN PEs) and
for SR-TE traffic steering purposes (see below), but the benefits of
other Prefix-SIDs are not clear, so currently only Node-SIDs are
supported with RIFT.
The second option is to use SR-TE and follow a specific segment list
in the packet header. Each node in the path steers the packet to the
currently active segment in the list, following the natural path for
that segment (see above). Since a node only has the full topology
south of it, and a leaf node does not have any south topology, the
traffic steering information (i.e. the segment list) must be
programmed by controllers into ingress nodes via SR policies.
Support for Adjacency SIDs will be considered in future revisions.
Consider the following 4-level topology:
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ToF1 ToF2
Spine1_11 Spine1_21 | Spine1_21 Spine1_22
|
Spine2_11 Spine2_21 | Spine2_21 Spine2_22
|
Leaf11 Leaf12 | Leaf21 Leaf22
Suppose the TE controller instructs Leaf11 to send a packet to
Spine2_11 with label stack (Label_TOF2, Label_Spine2_21,
Label_Leaf21). Spine2_11 recognizes that Label_TOF2 maps to node
TOF2 and it should not simply follow the default route (because the
default route could lead to an unintended path via TOF1). In other
words, each node needs to have a specific route to every node (that
may appear in the segment list). That means for RIFT the southbound
distance vector routing needs to additionally advertise routes for
the nodes in the north, and they must be propagated all the way down.
Each node originates a route for its own loopback address and
advertises it southbound, with a special marking that allows a south
node to re-advertise it further south.
If loopback addresses are not used, similar "routes" for System IDs
must be used. It is RECOMMENDED to use loopback addresses to reuse
existing mechanisms.
3. Well-Known KV Registry Values
This section requests an entry from the RIFT Well-Known Key-Type
Registry for networks that use SR along with suggested values in
accordance with RIFT-KV-REGISTRY [RIFT-KV-REGISTRY].
+============+=======+==================================+
| Name | Value | Description |
+============+=======+==================================+
| SRIFT Node | TBD | Key-Type describing a SRIFT node |
+------------+-------+----------------------------------+
Table 1: Requested Entries
3.1. SRIFT Node Key-Type
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD2 | SRIFT Node |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (System ID, |
| Loopback Address, |
| SRGB Label Base, |
| SRGB Label Range, |
| Node-SID,) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
System ID:
A node's 64-bit RIFT System ID.
Loopback Address:
A node's loopback address. This MAY be set to 0 if loopback
addresses are not used.
SRGB Label Base:
The first valid label within the corresponding node's SRGB.
SRGB Label Range:
The total number of valid labels in the corresponding node's
SRGB.
Node-SID:
The corresponding node's Node-SID value.
4. Security Considerations
This document does not introduce any new security concerns with RIFT
or any other referenced protocols. RIFT KV TIEs are already
extensively secured via RIFT's specification.
5. Acknowledgements
The authors thank Bruno Rijsman and Antoni Przygenda for their review
and suggestions.
6. References
6.1. Normative References
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[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>.
[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>.
[RIFT] Przygienda, T., Sharma, A., Thubert, P., Rijsman, B., and
D. Afanasiev, "RIFT: Routing in Fat Trees", Work in
Progress, draft-ietf-rift-rift-12, May 2020.
[RIFT-KV-REGISTRY]
Przygienda, T., "RIFT Keys Structure and Well-Known
Registry in Key Value TIE", Work in Progress, draft-
przygienda-rift-kv-registry-00, December 2020.
6.2. Informative References
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
[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>.
Authors' Addresses
Zhaohui Zhang
Juniper Networks
Email: zzhang@juniper.net
Jeff Tantsura
Juniper Networks
Email: jefftant.ietf@gmail.com
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Jordan Head
Juniper Networks
Email: jhead@juniper.net
Don Fedyk
Individual
Email: don.fedyk@gmail.com
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