SR-TE Path Midpoint Protection
draft-hu-spring-segment-routing-proxy-forwarding-13
Network Working Group Z. Hu
Internet-Draft Huawei Technologies
Intended status: Standards Track H. Chen
Expires: August 24, 2021 Futurewei
J. Yao
Huawei Technologies
C. Bowers
Juniper Networks
Y. Zhu
China Telecom
Y. Liu
China Mobile
February 20, 2021
SR-TE Path Midpoint Protection
draft-hu-spring-segment-routing-proxy-forwarding-13
Abstract
Segment Routing Traffic Engineering (SR-TE) supports explicit paths
using segment lists containing adjacency-SIDs, node-SIDs and binding-
SIDs. The current SR FRR such as TI-LFA provides fast re-route
protection for the failure of a node along a SR-TE path by the direct
neighbor or say point of local repair (PLR) to the failure. However,
once the IGP converges, the SR FRR is no longer sufficient to forward
traffic of the path around the failure, since the non-neighbors of
the failure will no longer have a route to the failed node. This
document describes a mechanism for fast re-route protection against
the failure of a SR-TE path after the IGP converges. It provides
fast re-route protection for an adjacency segment, a node segment and
a binding segment of the path.
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-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on August 24, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Proxy Forwarding . . . . . . . . . . . . . . . . . . . . . . 3
3. Extensions to IGP for Proxy Forwarding . . . . . . . . . . . 4
3.1. Extensions to OSPF . . . . . . . . . . . . . . . . . . . 4
3.1.1. Advertising Proxy Forwarding . . . . . . . . . . . . 4
3.1.2. Advertising Binding Segment . . . . . . . . . . . . . 7
3.2. Extensions to IS-IS . . . . . . . . . . . . . . . . . . . 10
3.2.1. Advertising Proxy Forwarding . . . . . . . . . . . . 10
3.2.2. Advertising Binding Segment . . . . . . . . . . . . . 12
4. Building Proxy Forwarding Table . . . . . . . . . . . . . . . 13
4.1. Advertising Proxy Forwarding . . . . . . . . . . . . . . 15
4.2. Building Proxy Forwarding Table . . . . . . . . . . . . . 15
5. Node Protection for Segment List . . . . . . . . . . . . . . 16
5.1. Next Segment is an Adjacency Segment . . . . . . . . . . 16
5.2. Next Segment is a Node Segment . . . . . . . . . . . . . 17
5.3. Next Segment is a Binding Segment . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7.1. OSPFv2 . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.2. OSPFv3 . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.3. IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Normative References . . . . . . . . . . . . . . . . . . 21
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9.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
Segment Routing Traffic Engineering (SR-TE) is a technology that
implements traffic engineering using a segment list. SR-TE supports
the creation of explicit paths using adjacency-SIDs, node-SIDs,
anycast-SIDs, and binding-SIDs. A node-SID in the segment list
defining an SR-TE path indicates a loose hop that the SR-TE path
should pass through. When the node fails, the network may no longer
be able to properly forward traffic on that SR-TE path.
[I-D.ietf-rtgwg-segment-routing-ti-lfa] describes an SR FRR mechanism
that provides fast re-route protection for the failure of a node on a
SR-TE path by the direct neighbor or say point of local repair (PLR)
to the failure. However, once the IGP converges, the SR FRR is no
longer sufficient to forward traffic of the path around the failure,
since the non-neighbors of the failure will no longer have a route to
the failed node and drop the traffic.
To solve this problem,
[I-D.ietf-spring-segment-protection-sr-te-paths] proposes that a hold
timer should be configured on every router in a network. After the
IGP converges on the event of a node failure, if the node-SID of the
failed node becomes unreachable, the forwarding changes should not be
communicated to the forwarding planes on all configured routers
(including PLRs for the failed node) until the hold timer expires.
This solution may not work for some cases such as some of nodes in
the network not supporting this solution.
This document describes a proxy protection/forwarding mechanism,
which provides more protection coverages. It considers the fast re-
route protection capability of every node in the network and supports
the fast re-route protection of the binding-SIDs on a failed node.
2. Proxy Forwarding
In the proxy forwarding mechanism, each neighbor of a possible failed
node advertises its SR proxy forwarding capability in its network
domain when it has the capability. This capability indicates that
the neighbor (the Proxy Forwarder) will forward traffic on behalf of
the failed node. A router receiving the SR Proxy Forwarding
capability from the neighbors of a failed node will send traffic
using the node-SID of the failed node to the nearest Proxy Forwarder
after the IGP converges on the event of the failure.
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Once the affected traffic reaches a Proxy Forwarder, it sends the
traffic on the post-failure shortest path to the node immediately
following the failed node in the segment list.
For a binding segment of a possible failed node, the node advertises
the information about the binding segment, including the binding SID
and the list of SIDs associated with the binding SID, to its direct
neighbors only. Note that the information is not advertised in the
network domain.
After the node fails and the IGP converges on the failure, the
traffic with the binding SID of the failed node will reach its
neighbor having SR Proxy Forwarding capability. Once receiving the
traffic, the neighbor swaps the binding SID with the list of SIDs
associated with the binding SID and sends the traffic along the post-
failure shortest path to the first node in the segment list.
3. Extensions to IGP for Proxy Forwarding
This section defines extensions to IGP for advertising the SR proxy
forwarding capability of a node in a network domain and the
information about each binding segment (including its binding SID and
the list of SIDs associated) of a node to its direct neighbors.
3.1. Extensions to OSPF
3.1.1. Advertising Proxy Forwarding
When a node P has the capability to do a SR proxy forwarding for all
its neighboring nodes for protecting the failures of these nodes,
node P advertises its SR proxy forwarding capability in its router
information opaque LSA, which contains a Router Functional
Capabilities TLV of the format as shown in Figure 1.
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Functional Capabilities |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Router Functional Capabilities TLV
One bit (called PF bit) in the Functional Capabilities field of the
TLV is used to indicate node P's SR proxy forwarding capability.
When this bit is set to one by node P, it indicates that node P is
capable of doing a SR proxy forwarding for its neighboring nodes.
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For a node X in the network, it learns the prefix/node SID of node N,
which is originated and advertised by node N. It creates a proxy
prefix/node SID of node N for node P if node P is capable of doing SR
proxy forwarding for node N. The proxy prefix/node SID of node N for
node P is a copy of the prefix/node SID of node N originated by node
N, but stored under (or say, associated with) node P.
In normal operations, node X prefers to use the prefix/node SID of
node N. When node N fails, node X prefers to use the proxy prefix/
node SID of node N. Thus node X will forward the traffic targeting
to the prefix/node SID of node N to node P when node N fails, and
node P will do a SR proxy forwarding for node N and forward the
traffic towards its final destination without going through node N.
After node N fails, node X will keep the FIB entry to the proxy
prefix/node SID of node N for a given period of time.
Note that the behaviors of normal IP forwarding and routing
convergences in a network are not changed at all by the SR proxy
forwarding. For example, the next hop used by BGP is an IP address
(or prefix). The IGP and BGP converge in normal ways for changes in
the network. The packet with its IP destination to this next hop is
forwarded according to the IP forwarding table (FIB) derived from IGP
and BGP routes.
If node P can not do a SR proxy forwarding for all its neighboring
nodes, but for some of them, then it advertises the node SID of each
of the nodes as a proxy node SID, indicating that it is able to do
proxy forwarding for the node SID.
A new TLV, called Proxy Node SIDs TLV, is defined for node P to
advertise the node SIDs of some of its neighboring nodes. It has the
format as shown in Figure 2.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Node SID Sub-TLVs |
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: OSPF Proxy Node SIDs TLV
The Type (TBD1) is to be assigned by IANA. The TLV contains a number
of Node SID Sub-TLVs. The Length is the total size of the Node SID
Sub-TLVs included in the TLV. A Node SID Sub-TLV is the Prefix SID
Sub-TLV defined in [RFC8665].
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A proxy forwarding node P originates an Extended Prefix Opaque LSA
containing this new TLV. The TLV includes the Node SID Sub-TLVs for
the node SIDs of some of P's neighboring nodes. For each of some of
P's neighboring nodes, the Node SID Sub-TLV for its prefix/node SID
is included the TLV. This prefix/node SID is called a proxy prefix/
node SID.
A proxy forwarding node will originate an Extended Prefix Opaque LSA,
which includes a Proxy Node SIDs TLV. The format of the LSA is shown
in Figure 3.
For a proxy forwarding node P, having a number of neighboring nodes,
P originates and maintains an Extended Prefix Opaque LSA, which
includes a Proxy Node SIDs TLV. The TLV contains the Prefix/Node SID
Sub-TLV for each of some of the neighboring nodes after node P
creates the corresponding proxy forwarding entries for protecting the
failure of some of the neighboring nodes.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | LS Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Type(7)| Opaque ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: TLVs :
: (including Proxy Node SIDs TLV) :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: OSPFv2 Extended Prefix Opaque LSA
When an neighboring node fails, P maintains the LSA with the TLV
containing the Prefix/Node SID Sub-TLV for the neighboring node for a
given period of time. After the given period of time, the Prefix/
Node SID Sub-TLV for the neighboring node is removed from the TLV in
the LSA and then after a given time the corresponding proxy
forwarding entries for protecting the failure of the neighboring node
is removed.
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For a node X in the network, it learns the prefix/node SID of node N
and the proxy prefix/node SID of node N. The former is originated
and advertised by node N, and the latter is originated and advertised
by the proxy forwarding node P of node N. Note that the proxy
Prefix/Node SID Sub-TLV for node N does not contain a prefix of node
N, and the prefix is the prefix associated with the prefix/node SID
of node N originated by node N.
In normal operations, node X prefers to use the prefix/node SID of
node N. When node N fails, node X prefers to use the proxy prefix/
node SID of node N. Thus node X will forward the traffic targeting
to node N to node P when node N fails, and node P will do a proxy
forwarding for node N and forward the traffic towards its destination
without going through node N.
3.1.2. Advertising Binding Segment
For a binding segment (or binding for short) on a node A, which
consists of a binding SID and a list of segments, node A advertises
an LSA containing the binding (i.e., the binding SID and the list of
the segments). The LSA is advertised only to each of the node A's
neighboring nodes. For OSPFv2, the LSA is a opaque LSA of LS type 9
(i.e., a link local scope LSA).
A binding segment is represented by binding segment TLV of the format
as shown in Figure 4.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |BindingSID Type| SIDs Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Binding SID Sub-TLV/value ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SID Sub-TLVs/values ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: OSPF Binding Segment TLV
It comprises a binding SID and a list of segments (SIDs). The fields
of this TLV are defined as follows:
Type: 2 octets, its value (TBD2) is to be assigned by IANA.
Length: 2 octets, its value is (4 + length of Sub-TLVs/values).
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Binding SID Type (BT): 1 octet indicates whether the binding SID is
represented by a Sub-TLV or a value included in the TLV. For the
binding SID represented by a value, it indicates the type of binding
SID. The following BT values are defined:
o BT = 0: The binding SID is represented by a Sub-TLV (i.e., Binding
SID Sub-TLV) in the TLV. A binding SID Sub-TLV is a SID/Label Sub-
TLV defined in [RFC8665]. BT != 0 indicates that the binding SID is
represented by a value.
o BT = 1: The binding SID value is a label, which is represented by
the 20 rightmost bits. The length of the value is 3 octets.
o BT = 2: The binding SID value is a 32-bit SID. The length of the
value is 4 octets.
SIDs Type (ST): 1 octet indicates whether the list of segments (SIDs)
are represented by Sub-TLVs or values included in the TLV. For the
SIDs represented by values, it indicates the type of SIDs. The
following ST values are defined:
o ST = 0: The SIDs are represented by Sub-TLVs (i.e., SID Sub-TLVs)
in the TLV. A SID Sub-TLV is an Adj-SID Sub-TLV, a Prefix-SID Sub-
TLV or a SID/Label Sub-TLV defined in [RFC8665]. ST != 0 indicates
that the SIDs are represented by values.
o ST = 1: Each of the SID values is a label, which is represented by
the 20 rightmost bits. The length of the value is 3 octets.
o ST = 2: Each of the SID values is a 32-bit SID. The length of the
value is 4 octets.
The opaque LSA of LS Type 9 containing the binding segment (i.e., the
binding SID and the list of the segments) has the format as shown in
Figure 5. It may have Opaque Type of x (the exact type is to be
assigned by IANA) for Binding Segment Opaque LSA.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | LS Type (9) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Type(x)| Opaque ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Binding Segment TLVs :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: OSPFv2 Binding Segment Opaque LSA
For every binding on a node A, the LSA originated by A contains a
binding segment TLV for it.
For node A running OSPFv3, it originates a link-local scoping LSA of
a new LSA function code (TBD3) containing binding segment TLVs for
the bindings on it. The format of the LSA is illustrated in
Figure 6.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age |0|0|0| BS-LSA (TBD3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Binding Segment TLVs :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: OSPFv3 Binding Segment Opaque LSA
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The U-bit is set to 0, and the scope is set to 00 for link-local
scoping.
3.2. Extensions to IS-IS
3.2.1. Advertising Proxy Forwarding
When a node P has the capability to do a SR proxy forwarding for its
neighboring nodes for protecting the failures of them, node P
advertises its SR proxy forwarding capability in its LSP, which
contains a Router Capability TLV of Type 242 including a SR
capabilities sub-TLV of sub-Type 2.
One bit (called PF bit as shown in Figure 7) in the Flags field of
the SR capabilities sub-TLV is defined to indicate node P's SR proxy
forwarding capability. When this bit is set to one by node P, it
indicates that node P is capable of doing a SR proxy forwarding for
its neighboring nodes.
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 (2) | Length | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Range |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// SID/Label Sub-TLV (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3 4 5 6 7
+--+--+--+--+--+--+--+--+
| I| V|PF| |
+--+--+--+--+--+--+--+--+
Flags
Figure 7: SR Capabilities sub-TLV
If node P can not do a SR proxy forwarding for all its neighboring
nodes, but for some of them, then it advertises the node SID of each
of the nodes as a proxy node SID, indicating that it is able to do
proxy forwarding for the node SID.
The IS-IS SID/Label Binding TLV (suggested value 149) is defined in
[RFC8667]. A Proxy Forwarder uses the SID/Label Binding TLV to
advertise the node SID of its neighboring node. The Flags field of
the SID/Label Binding TLV is extended to include a P flag as shown in
Figure 8. The prefix/node SID in prefix/node SID Sub-TLV included in
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SID/Label Binding TLV is identified as a proxy forwarding prefix/node
SID.
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 | Length | Flags | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Range | Prefix Length | Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Prefix (continued, variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SubTLVs (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|F|M|S|D|A|P| |
+-+-+-+-+-+-+-+-+
Flags
Figure 8: SID/Label Binding TLV
Where:
P-Flag: Proxy forwarding flag. If set, this prefix/node SID is
advertised by the proxy node. This TLV is used to announce that the
node has the ability to proxy forward the prefix/node SID.
When the P-flag is set in the SID/Label Binding TLV, the following
usage rules apply.
The Range, Prefix Length and Prefix field are not used. They should
be set to zero on transmission and ignored on receipt.
SID/Label Binding TLV contains a number of prefix/node SID Sub-TLVs.
The TLV advertised by a proxy forwarding node P contains prefix/node
SID Sub-TLVs for the node SIDs of P's neighbor nodes. Each of the
Sub-TLVs is a prefix/node SID Sub-TLV defined in [RFC8667]. From the
SID in a prefix/node SID Sub-TLV advertised by the Proxy Forwarding
node, its prefix can be obtained through matching corresponding
prefix/node SID advertised by the neighbor/protected node using
TLV-135 (or 235, 236, or 237) together with the prefix/node SID Sub-
TLV.
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3.2.2. Advertising Binding Segment
[I-D.ietf-spring-segment-routing-policy] has defined the usage of
binding-SID. For supporting binding SID proxy forwarding, a new IS-
IS TLV, called Binding Segment TLV, is defined. It contains a
binding SID and a list of segments (SIDs). This TLV may be
advertised in IS-IS Hello (IIH) PDUs, LSPs, or in Circuit Scoped Link
State PDUs (CS-LSP) [RFC7356]. Its format is shown in Figure 9.
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 | Length |BindingSID Type| SIDs Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Binding SID value/Sub-TLV ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SID values/Sub-TLVs ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: IS-IS Binding Segment TLV
The fields of this TLV are defined as follows:
Type: 1 octet Suggested value 152 (to be assigned by IANA)
Length: 1 octet (2 + length of Sub-TLVs/values).
Binding SID Type (BT): 1 octet indicates whether the binding SID is
represented by a Sub-TLV or a value included in the TLV. For the
binding SID represented by a value, it indicates the type of binding
SID. The following BT values are defined:
o BT = 0: The binding SID is represented by a Sub-TLV (i.e., binding
SID Sub-TLV) in the TLV. A binding SID Sub-TLV is a SID/Label Sub-
TLV defined in [RFC8667]. BT != 0 indicates that the binding SID is
represented by a value.
o BT = 1: The binding SID value is a label, which is represented by
the 20 rightmost bits. The length of the value is 3 octets.
o BT = 2: The binding SID value is a 32-bit SID. The length of the
value is 4 octets.
SIDs Type (ST): 1 octet indicates whether the SIDs are represented by
Sub-TLVs or values included in the TLV. For the SIDs represented by
values, it indicates the type of SIDs. The following ST values are
defined:
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o ST = 0: The SIDs are represented by Sub-TLVs (i.e., SID Sub-TLVs)
in the TLV. A SID Sub-TLV is an Adj-SID Sub-TLV, a Prefix-SID Sub-
TLV or a SID/Label Sub-TLV defined in [RFC8667]. ST != 0 indicates
that the SIDs are represented by values.
o ST = 1: Each of the SID values is a label, which is represented by
the 20 rightmost bits. The length of the value is 3 octets.
o ST = 2: Each of the SID values is a 32-bit SID. The length of the
value is 4 octets.
4. Building Proxy Forwarding Table
Figure 10 is used to illustrate the SR proxy forwarding approach.
Each node N has SRGB = [N000-N999]. RT1 is an ingress node of SR
domain. RT3 is a failure node. RT2 is a Point of Local Repair (PLR)
node, i.e., a proxy forwarding node. Three label stacks are shown in
the figure. Label Stack 1 uses only adjacency-SIDs and represents
the path RT1->RT2->RT3->RT4->RT5. Label Stack 2 uses only node-SIDs
and represents the ECMP-aware path RT1->RT3->RT4->RT5. Label Stack 3
uses a node-SID and a binding SID. The Binding-SID with label=100 at
RT3 represents the ECMP-aware path RT3->RT4->RT5. So Label Stack 3,
which consists of the node-SID for RT3 following by Binding-SID=100,
represents the ECMP-aware path RT1->RT3->RT4->RT5.
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Node SID:2 Node SID:3
+-----+ +-----+
| |----------+ |
/ |RT2 | | RT3 |\
/ +-----+ +-----+ \
/ | \ /| \
/ | \ / | \
/ | \ / | \
/ | \ / | \
/ | \ / | \
Node SID:1 | \ / | \Node SID:4 Node SID:5
+-----+ | \ / | +-----+ +-----+
| | | X | | |-------| |
| RT1 | | / \ | | RT4 | | RT5 |
+-----+ | / \ | +-----+ +-----+
\ | / \ | /
\ | / \ | /
\ | / \ | /
\ | / \ | /
\ | / \| /
\ |/ | /
\ +-----+ +-----+ /
\ | | | |/
\ | RT6 |-----------| RT7 |
+-----+ +-----+
Node SID:6 Node SID:7
+-----------------+ +--------------+
| Node SRGB | | Adj-SID | +-------+ +-------+ +-------+
+-----------------+ +--------------+ |Label | |Label | |Label |
| RT1:[1000-1999] | |RT1->RT2:10012| |Stack 1| |Stack 2| |Stack 3|
+-----------------+ +--------------+ +-------+ +-------+ +-------+
| RT2:[2000-2999] | |RT2->RT3:20023| | 10012 | | 1003 | | 1003 |
+-----------------+ +--------------+ +-------+ +-------+ +-------+
| RT3:[3000-3999] | |RT3->RT6:30036| | 20023 | | 3004 | | 100 |
+-----------------+ +--------------+ +-------+ +-------+ +-------+
| RT4:[4000=4999] | |RT3->RT7:30037| | 30034 | | 4005 | 100 is
+-----------------+ +--------------+ +-------+ +-------+ binding SID
| RT5:[5000-5999] | |RT3->RT4:30034| | 40045 | to
+-----------------+ +--------------+ +-------+ {30034,40045}
| RT6:[6000-6999] | |RT7->RT4:70074|
+-----------------+ +--------------+
| RT7:[7000-7999] | |RT4->RT5:40045|
+-----------------+ +--------------+
Figure 10: Topology of SR-TE Path
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4.1. Advertising Proxy Forwarding
If the Point of Local Repair (PLR), for example, RT2, has the
capability to do a SR proxy forwarding for all its neighboring nodes,
it must advertise this capability. If the PLR can not do a SR proxy
forwarding for all its neighboring nodes, but for some of them, for
example, RT3, then it uses proxy Node SIDs TLV to advertise the
prefix-SID learned from RT3. The TLV contains the Sub-TLV/value for
the prefix/node SID of RT3 as a proxy SID. When RT3 fails, RT2 needs
to maintain the Sub-TLV/value for a period of time. When the proxy
forwarding table corresponding to the fault node is deleted (see
section 3.2), the Sub-TLV/value is withdrawn. The nodes in the
network (for example, RT1) learn the prefix/node SID TLV advertised
by RT3 and the proxy Node SIDs TLV advertised by RT2. When RT3 is
normal, the nodes prefer prefix/node SID TLV. When the RT3 fails,
the proxy prefix/node SIDs TLV advertised by RT2 is preferred.
4.2. Building Proxy Forwarding Table
A SR proxy node P needs to build an independent proxy forwarding
table for each neighbor N. The proxy forwarding table for node N
contains the following information:
1: Node N's SRGB range and the difference between the SRGB start
value of node P and that of node N;
2: All adjacency-SID of N and Node-SID of the node pointed to by node
N's adjacency-SID.
3: The binding-SID of N and the label stack associated with the
binding-SID.
Node P (PLR) uses a proxy forwarding table based on the next segment
to find a node N as a backup forwarding entry to the adj-SID and
Node-SID of node N. When node N fails, the proxy forwarding table
needs to be maintained for a period of time, which is recommended for
30 minutes.
Node RT3 in the topology of Figure 1 is node N, and node RT2 is node
P (PLR). RT2 builds the proxy forwarding table for RT3. RT2
calculates the proxy forwarding table for RT3, as shown in Figure 11.
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+==========+===============+============+=============+==============+
| In-label | SRGBDiffValue | Next Label | Action | Map Label |
+==========+===============+============+=============+==============+
| 2003 | -1000 | 30034 | Fwd to RT4 | 2004 |
+----------+---------------+------------+-------------+--------------+
| 30036 | Fwd to RT6 | 2006 |
+------------+-------------+--------------+
| 30037 | Fwd to RT7 | 2007 |
+------------+-------------+--------------+
| 100 | Swap to { 30034, 40045 } |
+------------+-------------+--------------+
Figure 11: RT2's Proxy Forwarding Table for RT3
5. Node Protection for Segment List
Segment Routing Traffic Engineering supports the creation of explicit
paths using adjacency-SIDs, node-SIDs, and binding-SIDs. The label
stack is a combination of one or more of adjacency-SIDs, node-SIDs,
and binding-SIDs. This Section shows how a proxy node uses the SR
proxy forwarding mechanism to protect traffic to the destination node
when the next segment of label stack is adjacency-SIDs, node-SIDs, or
binding-SIDs, respectively.
5.1. Next Segment is an Adjacency Segment
As shown in Figure 1, Label Stack 1 {10012, 20023, 30034, 40045}
represents SR-TE strict explicit path RT1->RT2->RT3->RT4->RT5. When
RT3 fails, node RT2 acts as a PLR, and uses next adj-SID (30034) of
the label stack to lookup the proxy forwarding table built by RT2
locally for RT3. The path returned is the label forwarding path to
RT3's next hop node RT4, which bypasses RT3. The specific steps are
as follows:
a. RT1 pops top adj-SID 10012, and forwards the packet to RT2;
b. RT2 uses the label 20023 to identify the next hop node RT3, which
has failed. RT2 pops label 20023 and queries the Proxy Forwarding
Table corresponding to RT3 with label 30034. The query result is
2004. RT2 uses 2004 as the incoming label to query the label
forwarding table. The next hop is RT7, and the incoming label is
changed to 7004.
c. So the packet leaves RT2 out the interface to RT7 with label
stack {7004, 40045}. RT4 forwards it to RT4, where the original path
is rejoined.
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d. RT2 forwards packets to RT7. RT7 queries the local routing table
to forward the packet to RT4.
5.2. Next Segment is a Node Segment
As shown in Figure 1, Label Stack 2 {1003, 3004, 4005} represents SR-
TE loose path RT1->RT3->RT4->RT5, where 1003 is the node SID of RT3.
When the node RT3 fails, the proxy forwarding TLV advertised by the
RT2 is preferred to direct the traffic of the RT1 to the PLR node
RT2. Node RT2 acts as a PLR node and queries the proxy forwarding
table locally built for RT3. The path returned is the label
forwarding path to RT3's next hop node RT4, which bypasses RT3. The
specific steps are as follows:
a. RT1 swaps label 1003 to out-label 2003 to RT3.
b. RT2 receives the label forwarding packet whose top label of label
stack is 2003, and searches for the local Routing Table, the behavior
found is to lookup Proxy Forwarding table due to RT3 failure, RT2
pops label 2003.
c. RT2 uses 3004 as the in-label to lookup Proxy Forwarding table,
The value of Map Label calculated based on SRGBDiffValue is 2004.
and the query result is forwarding the packet to RT4.
d. Then RT2 queries the Routing Table to RT4, using the primary or
backup path to RT4. The next hop is RT7.
e. RT2 forwards the packet to RT7. RT7 queries the local routing
table to forward the packet to RT4.
f. After RT1 convergences, node SID 1003 is preferred to the proxy
SID implied/advertised by RT2.
5.3. Next Segment is a Binding Segment
As shown in Figure 1, Label Stack 3 {1003, 100} represents SR-TE
loose path RT1->RT3->RT4->RT5, where 100 is a Binding-SID, which
represents segment list {30034, 40045}.
When the node RT3 fails, the proxy forwarding SID implied or
advertised by the RT2 is preferred to forward the traffic of the RT1
to the PLR node RT2. Node RT2 acts as a PLR node and uses Binding-
SID to query the proxy forwarding table locally built for RT3. The
path returned is the label forwarding path to RT3's next hop node
(RT4), which bypasses RT3. The specific steps are as follows:
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a. RT1 swaps label 1003 to out-label 2003 to RT3.
b. RT2 receives the label forwarding packet whose top label of label
stack is 2003, and searches for the local Routing Table, the behavior
found is to lookup Proxy Forwarding table due to RT3 failure.
c. RT2 uses Binding-SID:100 (label 2003 has pop) as the in-label to
lookup the Next Label record of the Proxy Forwarding Table, the
behavior found is to swap to Segment list {30034, 40045}.
d. RT2 swaps Binding-SID:100 to Segment list {30034, 40045}, and
uses the 3034 to lookup the Next Label record of the Proxy Forwarding
table again. The behavior found is to forward the packet to RT4.
e. RT2 queries the Routing Table to RT4, using primary or backup
path to RT4. The next hop is RT7.
f. RT2 forwards packets to RT7. RT7 queries the local routing table
to forward the packet to RT4.
6. Security Considerations
The extensions to OSPF and IS-IS described in this document result in
two types of behaviors in data plane when a node in a network fails.
One is that for a node, which is a upstream (except for the direct
upstream) node of the failed node along a SR-TE path, it continues to
send the traffic to the failed node along the SR-TE path for an
extended period of time. The other is that for a node, which is the
direct upstream node of the failed node, it fast re-routes the
traffic around the failed node to the direct downstream node of the
failed node along the SR-TE path. These behaviors are internal to a
network and should not cause extra security issues.
7. IANA Considerations
7.1. OSPFv2
Under Subregistry Name "OSPF Router Functional Capability Bits"
within the "Open Shortest Path First v2 (OSPFv2) Parameters"
[RFC7770], IANA is requested to assign one bit for Proxy Forwarding
Capability as follows:
+============+==================+===================+
| Bit number | Capability Name | Reference |
+============+==================+===================+
| 31 | Proxy Forwarding | This document |
+------------+------------------+-------------------+
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Under Registry Name "OSPFv2 Extended Prefix Opaque LSA TLVs"
[RFC7684], IANA is requested to assign one new TLV value for OSPF
Proxy Node SIDs as follows:
+============+=====================+================+
| TLV Value | TLV Name | Reference |
+============+=====================+================+
| 2 | Proxy Node SIDs TLV | This document |
+------------+---------------------+----------------+
Under Registry Name "Opaque Link-State Advertisements (LSA) Option
Types" [RFC5250], IANA is requested to assign new Opaque Type
registry values for Binding Segment LSA as follows:
+================+==================+================+
| Registry Value | Opaque Type | Reference |
+================+==================+================+
| 10 | Binding Segment | This document |
+----------------+------------------+----------------+
IANA is requested to create and maintain new registries:
o OSPFv2 Binding Segment Opaque LSA TLVs
Initial values for the registry are given below. The future
assignments are to be made through IETF Review [RFC5226].
Value TLV Name Definition
----- ----------------------- ----------
0 Reserved
1 Binding Segment TLV This Document
2-32767 Unassigned
32768-65535 Reserved
7.2. OSPFv3
Under Registry Name "OSPFv3 LSA Function Codes", IANA is requested to
assign new registry values for Binding Segment LSA as follows:
+========+========================+================+
| Value | LSA Function Code Name | Reference |
+========+========================+================+
| 16 | Binding Segment LSA | This document |
+--------+------------------------+----------------+
IANA is requested to create and maintain new registries:
o OSPFv3 Binding Segment LSA TLVs
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Initial values for the registry are given below. The future
assignments are to be made through IETF Review [RFC5226].
Value TLV Name Definition
----- ----------------------- ----------
0 Reserved
1 Binding Segment TLV This Document
2-32767 Unassigned
32768-65535 Reserved
7.3. IS-IS
Under Registration "Segment Routing Capability" in the "sub-TLVs for
TLV 242" registry [RFC8667], IANA is requested to assign one bit flag
for Proxy Forwarding Capability as follows:
+============+=======================+===============+
| Bit number | Capability Name | Reference |
+============+=======================+===============+
| 2 | Proxy Forwarding (PF) | This document |
+------------+-----------------------+---------------+
Under Registration "Segment Identifier/Label Binding TLV 149"
[RFC8667], IANA is requested to assign one bit P-Flag as follows:
+============+=================+===============+
| Bit number | Flag Name | Reference |
+============+=================+===============+
| 5 | P-Flag | This document |
+------------+-----------------+---------------+
Under Registry Name: IS-IS TLV Codepoints, IANA is requested to
assign one new TLV value for IS-IS Binding Segment as follows:
+========+======================+===============+
| Value | TLV Name | Reference |
+========+======================+===============+
| 152 | Binding Segment TLV | This Document |
+--------+----------------------+---------------+
8. Acknowledgements
The authors would like to thank Peter Psenak, Acee Lindem, Les
Ginsberg, Bruno Decraene and Jeff Tantsura for their comments to this
work.
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9. References
9.1. 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>.
[RFC5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
July 2008, <https://www.rfc-editor.org/info/rfc5250>.
[RFC7356] Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding
Scope Link State PDUs (LSPs)", RFC 7356,
DOI 10.17487/RFC7356, September 2014,
<https://www.rfc-editor.org/info/rfc7356>.
[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.
[RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
February 2016, <https://www.rfc-editor.org/info/rfc7770>.
[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>.
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9.2. Informative References
[I-D.ietf-rtgwg-segment-routing-ti-lfa]
Litkowski, S., Bashandy, A., Filsfils, C., Decraene, B.,
and D. Voyer, "Topology Independent Fast Reroute using
Segment Routing", draft-ietf-rtgwg-segment-routing-ti-
lfa-05 (work in progress), November 2020.
[I-D.ietf-spring-segment-protection-sr-te-paths]
Hegde, S., Bowers, C., Litkowski, S., Xu, X., and F. Xu,
"Segment Protection for SR-TE Paths", draft-ietf-spring-
segment-protection-sr-te-paths-00 (work in progress),
September 2020.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", draft-
ietf-spring-segment-routing-policy-09 (work in progress),
November 2020.
[I-D.sivabalan-pce-binding-label-sid]
Sivabalan, S., Filsfils, C., Tantsura, J., Hardwick, J.,
Previdi, S., and C. Li, "Carrying Binding Label/Segment-ID
in PCE-based Networks.", draft-sivabalan-pce-binding-
label-sid-07 (work in progress), July 2019.
[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
2009, <https://www.rfc-editor.org/info/rfc5462>.
Authors' Addresses
Zhibo Hu
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Beijing 100095
China
Email: huzhibo@huawei.com
Huaimo Chen
Futurewei
Boston, MA
USA
Email: Huaimo.chen@futurewei.com
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Junda Yao
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Beijing 100095
China
Email: yaojunda@huawei.com
Chris Bowers
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
USA
Email: cbowers@juniper.net
Yongqing
China Telecom
109, West Zhongshan Road, Tianhe District
Guangzhou 510000
China
Email: zhuyq8@chinatelecom.cn
Yisong
China Mobile
510000
China
Email: liuyisong@chinamobile.com
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