| SPRING Working Group | R. Bonica |
| Internet-Draft | S. Hegde |
| Intended status: Standards Track | Juniper Networks |
| Expires: January 9, 2020 | Y. Kamite |
| NTT Communications Corporation | |
| A. Alston | |
| D. Henriques | |
| Liquid Telecom | |
| J. Halpern | |
| Ericsson | |
| J. Linkova | |
| G. Chen | |
| Baidu | |
| July 8, 2019 |
IPv6 Support for Segment Routing: SRv6+
draft-bonica-spring-srv6-plus-04
This document describes SRv6+. SRv6+ is a Segment Routing (SR) solution that leverages IPv6. It supports a wide variety of use-cases while remaining in strict compliance with IPv6 specifications. SRv6+ is optimized for for ASIC-based forwarding devices that operate at high data rates.
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Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
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Network operators deploy Segment Routing (SR) so that they can forward packets through SR paths. An SR path provides unidirectional connectivity from its ingress node to its egress node. While an SR path can follow the least cost path from ingress to egress, it can also follow any other path.
An SR path contains one or more segments. A segment provides unidirectional connectivity from its ingress node to its egress node. It also includes a topological instruction that controls its behavior.
The topological instruction is executed on the segment ingress node. It determines the segment egress node and the method by which the segment ingress node forwards packets to the segment egress node.
Per-segment service instructions can augment a segment. Per-segment service instructions, if present, are executed on the segment egress node.
Likewise, a per-path service instruction can augment a path. The per-path service instruction, if present, is executed on the path egress node. Section 3 of this document illustrates the relationship between SR paths, segments and instructions.
A Segment Identifier (SID) identifies each segment. Because there is a one-to-one mapping between segments and the topological instructions that control them, the SID that identifies a segment also identifies the topological instruction that controls it.
A SID is different from the topological instruction that it identifies. While a SID identifies a topological instruction, it does not contain the topological instruction that it identifies. Therefore, a SID can be encoded in relatively few bits, while the topological instruction that it identifies may require many more bits for encoding.
An SR path can be represented by its ingress node as an ordered sequence of SIDs. In order to forward a packet through an SR path, the SR ingress node encodes the SR path into the packet as an ordered sequence of SIDs. It can also augment the packet with service instructions.
Because the SR ingress node is also the first segment ingress node, it executes the topological instruction associated with the first segment. This causes the packet to be forwarded to the first segment egress node. When the first segment egress node receives the packet, it executes any per-segment service instructions that augment the first segment.
If the SR path contains exactly one segment, the first segment egress node is also the path egress node. In this case, that node executes any per-path service instruction that augments the path, and SR forwarding is complete.
If the SR path contains multiple segments, the first segment egress node is also the second segment ingress node. In this case, that node executes the topological instruction associated with the second segment. The above-described procedure continues until the packet arrives at the SR egress node.
In the above-described procedure, only the SR ingress node maintains path information. Segment ingress and egress nodes maintain information regarding the segments in which they participate, but they do not maintain path information.
The SR architecture, described above, can leverage either an MPLS data plane or an IPv6 data plane. SR-MPLS leverages MPLS. SRv6 [I-D.ietf-6man-segment-routing-header] leverages IPv6.
This document describes SRv6+. SRv6+ is another SR variant that leverages IPv6. It supports a wide variety of use-cases while remaining in strict compliance with IPv6 specifications. SRv6+ is optimized for ASIC-based forwarding devices that operate at high data rates. Section 9 of this document highlights differences between SRv6 and SRv6+.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC8174] when, and only when, they appear in all capitals, as shown here.
An SRv6+ path is determined by the segments that it contains. It can be represented by its ingress node as an ordered sequence of SIDs.
A segment is determined by its ingress node and by the topological instruction that controls its behavior. The topological instruction determines the segment egress node and the method by which the segment ingress node forwards packets to the segment egress node.
Per-segment service instructions augment, but do not determine, segments. A segment ingress node can:
Likewise, per-path service instructions augment, but do not determine, paths.
---- ---- ---- ---- ---- ----
|Node|----|Node|----|Node|----|Node|----|Node|----|Node|
| A | | B | | C | | D | | E | | F |
---- ---- ---- ---- ---- ----
| | | |
-------------------| |-------------------|
Segment A-C |---------| Segment D-F
Segment C-D
| |
-------------------------------------------------
SRv6+ Path
Figure 1: Paths, Segments And Instructions
Figure 1 depicts an SRv6+ path. The path provides unidirectional connectivity from its ingress node (i.e., Node A) to its egress node (i.e., Node F). It contains Segment A-C, Segment C-D and Segment D-F.
In Segment A-C, Node A is the ingress node, Node B is a transit node, and Node C is the egress node. Therefore, the topological instruction that controls the segment is executed on Node A, while per-segment service instructions that augment the segment (if any exist) are executed on Node C.
In Segment C-D, Node C is the ingress node and Node D is the egress node. Therefore, the topological instruction that controls the segment is executed on Node C, while per-segment service instructions that augment the segment (if any exist) are executed on Node D.
In Segment D-F, Node D is the ingress node, Node E is a transit node, and Node F is the egress node. Therefore, the topological instruction that controls the segment is executed on Node D, while per-segment service instructions that augment the segment (if any exist) are executed on Node F.
Node F is also the path egress node. Therefore, if a per-path service instruction augments the path, it is executed on Node F.
Segments A-C, C-D and D-F are also contained by other paths that are not included in the figure.
SRv6+ supports the following segment types:
Strictly routed segments forward packets through a specified link that connects the segment ingress node to the segment egress node. Loosely routed segments forward packets through the least cost path from the segment ingress node to the segment egress node.
Each segment type is described below.
When a packet is submitted to a strictly routed segment, the topological instruction associated with that segment operates upon the packet. The topological instruction executes on the segment ingress node and accepts the following parameters:
The topological instruction behaves as follows:
When a packet is submitted to a loosely routed segment, the topological instruction associated with that segment operates upon the packet. The topological instruction executes on the segment ingress node and accepts an IPv6 address as a parameter. The IPv6 address identifies an interface on the segment egress node.
The topological instruction behaves as follows:
A Segment Identifier (SID) is an unsigned integer that identifies a segment. Because there is a one-to-one mapping between segments and the topological instructions that control them, the SID that identifies a segment also identifies the topological instruction that controls it.
A SID is different from the topological instruction that it identifies. While a SID identifies a topological instruction, it does not contain the topological instruction that it identifies. Therefore, a SID can be encoded in relatively few bits, while the topological instruction that it identifies may require many more bits for encoding.
SIDs have node-local significance. This means that a segment ingress node MUST identify each segment that it originates with a unique SID. However, a SID that is used by one segment ingress node to identify a segment that it originates can be used by another segment ingress node to identify another segment.
Although SIDs have node-local significance, an SRv6+ path can be uniquely identified by its ingress node and an ordered sequence of SIDs. This is because the topological instruction associated with each segment determines the ingress node of the next segment (i.e., the node upon which the next SID has significance.)
Although SIDs have node-local significance, they can be assigned in a manner that facilitates debugging. See Section 5.2 and Section 5.3 for details.
SID values range from 0 to a configurable Maximum SID Value (MSV). The values 0 through 15 are reserved for future use. The following are valid MSVs:
packet encoding, network operators can configure all nodes within an SRv6+ domain to have the smallest feasible MSV. The following paragraphs explain how an operator determines the smallest feasible MSV.
In order to optimize
Consider an SRv6+ domain that contains 5,000 nodes connected to one another by point-to-point infrastructure links. The network topology is not a full-mesh. In fact, each node supports 200 point-to-point infrastructure links or fewer. Given this SRv6+ domain, we will determine the smallest feasible MSV under the following conditions:
If an SRv6+ domain contains strictly routed segments only, and each node creates a strictly routed segment to each of its neighbors, each node will create 200 segments or fewer and consume 200 SIDs or fewer. This is because each node has 200 neighbors or fewer. Because SIDs have node-local significance (i.e., they can be reused across nodes), the smallest feasible MSV is 65,535.
Adding nodes to this SRv6+ domain will not increase the smallest feasible MSV, so long as each node continues to support 65,519 point-to-point infrastructure links or fewer. If a single node is added to the domain and that node supports 240 infrastructure links, the smallest feasible MSV will increase to 65,535.
If an SRv6+ domain contains loosely routed segments only, and every node creates a loosely routed segment to every other node, every node will create 4,999 segments and consume 4,999 SIDs. This is because the domain contains 5,000 nodes. Because SIDs have node-local significance (i.e., they can be reused across nodes), the smallest feasible MSV is 65,535.
Adding nodes to this SRv6+ domain will not increase the smallest feasible MSV until the number of nodes exceeds 65,519. When the smallest feasible MSV increases, it becomes 4,294,967,295.
If an SRv6+ domain contains both strictly and loosely routed segments, each node will create 5,199 segments or fewer and consume 5,199 SIDs or fewer. This value is the sum of the following:
Because SIDs have node-local significance (i.e., they can be reused across nodes), the smallest feasible MSV is 65,535.
Adding nodes to this SRv6+ domain will not increase the smallest feasible MSV until the number of nodes plus the maximum number of infrastructure links per node exceeds 65,519. When the smallest feasible MSV increases, it becomes 4,294,967,295.
Network operators can establish conventions by which they assign SIDs to strictly routed segments. These conventions can facilitate debugging.
For example, a network operator can reserved a range of SIDs for strictly routed segments. It can further divide that range into subranges, so that all segments sharing a common egress node are identified by SIDs from the same subrange.
In order to facilitate debugging, all loosely routed segments that share a common egress node are identified by the same SID. In order to maintain this discipline, network wide co-ordination is required.
For example, assume that an SRv6+ domain contains N nodes. Network administrators reserve a block of N SIDs and configure one of those SIDs on each node. Each node advertises its SID into the control plane. When another node receives that advertisement, it creates a loosely routed segment between itself and the advertising node. It also associates the SID that it received in the advertisement with the newly created segment. See [I-D.bonica-lsr-crh-isis-extensions] for details.
SRv6+ supports the following service instruction types:
Each is described below.
Per-segment service instructions can augment a segment. Per-segment service instructions, if present, are executed on the segment egress node. Because the path egress node is also a segment egress node, it can execute per-segment service instructions.
The following are examples of per-segment service instructions:
Per-segment Service Instruction Identifiers identify a set of service instructions. Per-segment Service Instruction Identifiers are allocated and distributed by a controller. They have domain-wide significance.
A per-path service instruction can augment a path. The per-path service instruction, if present, is executed on the path egress node.
The following are examples of per-path service instructions:
Per-path Service Instruction Identifiers identify per-path service instructions. Per-path Service Instruction Identifiers are allocated and distributed by the processing node (i.e., the path egress node). They have node-local significance. This means that the path egress node MUST allocate a unique Per-path Service Instruction Identifier for each per-path service instruction that it instantiates.
SRv6+ ingress nodes generate IPv6 header chains that represent SRv6+ paths. An IPv6 header chain contains an IPv6 header. It can also contain one or more extension headers.
An extension header chain that represents an SRv6+ path can contain any valid combination of IPv6 extension headers. The following bullet points describe how SRv6+ leverages IPv6 extension headers:
The following subsections describe how SRv6+ uses the Routing header and the Destination Options header.
SRv6+ defines a new Routing header type, called the Compressed Routing Header (CRH). The CRH contains the following fields:
As per [RFC8200], when an IPv6 node receives a packet, it examines the packet's destination address. If the destination address represents an interface belonging to the node, the node processes the next header. If the node encounters and recognizes the CRH, it processes the CRH as follows:
When the packet arrives at the segment egress node, the above-described procedure is repeated.
According to [RFC8200], the Destination Options header contains one or more IPv6 options. It can occur twice within a packet, once before a Routing header and once before an upper-layer header. The Destination Options header that occurs before a Routing header is processed by the first destination that appears in the IPv6 Destination Address field plus subsequent destinations that are listed in the Routing header. The Destination Options header that occurs before an upper-layer header is processed by the packet's final destination only.
Therefore, SRv6+ defines the following new IPv6 options:
The SRv6+ Per-Segment Service Instruction Option is encoded in a Destination Options header that precedes the CRH. Therefore, it is processed by every segment egress node. It includes a Per-Segment Service Instruction Identifier and causes segment egress nodes to execute per-segment service instructions.
The SRv6+ Per-Path Service Instruction Option is encoded in a Destination Options header that precedes the upper-layer header. Therefore, it is processed by the path egress node only. It includes a Per-Path Service Instruction Identifier and causes the path egress node to execute a per-path service instruction.
IS-IS extensions have been defined for the following purposes:BGP extensions are being defined so that SRv6+ path egress nodes can associated path-terminating service instructions with Network Layer Reachability Information (NLRI).
SRv6 defines a Routing header type, called the Segment Routing Header (SRH). The SRH contains a field that represents the SRv6 path as an ordered sequence of SIDs. Each SID contained by that field is 128 bits long.
Likewise, SRv6+ defines a Routing Header Type, called the Compressed Routing Header (CRH). The CRH contains a field that represents the SRv6+ path as an ordered sequence of SIDs. Within that field, SIDs can be 16 or 32 bits long.
| SIDs | SRv6 SRH (128-bit SID) | SRv6+ CRH (16-bit SID) | SRv6+ CRH (32-bit SID) |
|---|---|---|---|
| 1 | 24 | 16 | 16 |
| 2 | 40 | 16 | 16 |
| 3 | 56 | 16 | 24 |
| 4 | 72 | 16 | 24 |
| 5 | 88 | 24 | 32 |
| 6 | 104 | 24 | 32 |
| 7 | 120 | 24 | 40 |
| 8 | 136 | 24 | 40 |
| 9 | 152 | 32 | 48 |
| 10 | 168 | 32 | 48 |
| 11 | 184 | 32 | 56 |
| 12 | 200 | 32 | 56 |
| 13 | 216 | 40 | 64 |
| 14 | 232 | 40 | 64 |
| 15 | 248 | 40 | 72 |
| 16 | 264 | 40 | 72 |
Table 1 reflects Routing header size as a function of Routing header type and Number of SIDs contained by the Routing header.
Large Routing headers are undesirable for the following reasons:
SRv6+ decouples topological instructions from service instructions. Topological instructions are invoked at the segment ingress node, as a result of CRH processing, while service instructions are invoked at the segment egress node, as a result of Destination Option processing. Therefore, network operators can use SRv6+ mechanisms to support topological instructions, service instructions, or both.
---------- ---------- ----------
| Ethernet | | Ethernet | | Ethernet |
---------- ---------- ----------
Service | VXLAN | | Dest | | Dest |
Instruction ---------- | | | |
| UDP | | Option | | Option |
---------- ---------- ----------
Topological | | | | |
Instructions | CRH | | | | CRH |
---------- | | ----------
| IPv6 | | IPv6 | | IPv6 |
---------- ---------- ----------
Option 1 Option 2 Option 3
Figure 2: EVPN Design Alternatives
Figure 2 illustrates this point by depicting design options available to network operators offering Ethernet Virtual Private Network services over Virtual eXtensible Local Area Network (VXLAN). In Option 1, the network operator encodes topological instructions in the CRH, while encoding service instructions in a VXLAN header. In Option 2, the network operator encodes service instructions in a Destination Options header, while allowing traffic to traverse the least cost path between the ingress and egress Provider Edge (PE) routers. In Option 3, the network operator encodes topological instructions in the CRH, and encodes service instructions in a Destination Options header.
The IPv6 Authentication Header (AH) can be used to authenticate SRv6+ packets. However, AH processing is not defined in SRv6.
SRv6+ supports traffic engineering solutions that rely exclusively upon strictly routed segments. For example, consider an SRv6+ network whose diameter is 12 hops and whose minimum feasible MSV is 65,525. In that network, in the worst case, SRv6+ overhead is 72 bytes (i.e., a 40-byte IPv6 header and a 32-byte CRH).
SRv6 also supports traffic engineering solutions that rely exclusively upon strictly routed segments (i.e., END.X SIDs). However, SRv6 overhead may be prohibitive. For example, consider an SRv6 network whose diameter is 12 hops. In the worst case, SRv6 overhead is 240 bytes (i.e., a 40 byte IPv6 header and a 200-byte SRH).
In SRv6, an IPv6 address can represent either of the following:
In SRv6+ an IPv6 address always represents a network interface, as per [RFC4291].
In order to be compliant with this specification, an SRv6+ implementation MUST:
Additionally, an SRv6+ implementation MAY:
Ping and Traceroute both operate correctly in SRv6+ (i.e., in the presence of the CRH).
As per [RFC4443], SRv6+ nodes rate limit the ICMPv6 messages that they emit.
An SRv6+ implementation MAY include a configuration option that determines how it encodes SIDs (i.e., in 16 or 32 bits). In order to reduce operational complexity, network operators typically configure their networks so that every node encodes SIDs identically.
As a network grows, its minimum feasible MSV may increase. In this case, the network may need to migrate from one SID encoding to another. The following bullet points describe a migration strategy for an SRv6+ network that is migrating from 16-bit SIDs to 32-bit SIDs:.
SID values 0-15 are reserved for future use. They may be assigned by IANA, based on IETF Consensus.
IANA is requested to establish a "Registry of SRv6+ Reserved SIDs". Values 0-15 are reserved for future use.
SRv6+ domains MUST NOT span security domains. In order to enforce this requirement, security domain edge routers MUST do one of the following:
The authors wish to acknowledge Dr. Vanessa Ameen and John Scudder.