| SPRING | C. Filsfils |
| Internet-Draft | P. Camarillo, Ed. |
| Intended status: Standards Track | Cisco Systems, Inc. |
| Expires: January 3, 2019 | J. Leddy |
| Comcast | |
| D. Voyer | |
| Bell Canada | |
| S. Matsushima | |
| SoftBank | |
| Z. Li | |
| Huawei Technologies | |
| July 2, 2018 |
SRv6 Network Programming
draft-filsfils-spring-srv6-network-programming-05
This document describes the SRv6 network programming concept and its most basic functions.
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.
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 January 3, 2019.
Copyright (c) 2018 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Segment Routing leverages the source routing paradigm. An ingress node steers a packet through a ordered list of instructions, called segments. Each one of these instructions represents a function to be called at a specific location in the network. A function is locally defined on the node where it is executed and may range from simply moving forward in the segment list to any complex user-defined behavior. The network programming consists in combining segment routing functions, both simple and complex, to achieve a networking objective that goes beyond mere packet routing.
This document illustrates the SRv6 Network Programming concept and aims at standardizing the main segment routing functions to enable the creation of interoperable overlays with underlay optimization and service programming.
Familiarity with the Segment Routing Header is assumed.
SRH is the abbreviation for the Segment Routing Header. We assume that the SRH may be present multiple times inside each packet.
NH is the abbreviation of the IPv6 next-header field.
NH=SRH means that the next-header field is 43 with routing type 4.
When there are multiple SRHs, they must follow each other: the next-header field of all SRH, except the last one, must be SRH.
The effective next-header (ENH) is the next-header field of the IP header when no SRH is present, or is the next-header field of the last SRH.
In this version of the document, we assume that there is no other extension header than the SRH. These will be lifted in future versions of the document.
SID: A Segment Identifier which represents a specific segment in segment routing domain. The SID type used in this document is IPv6 address (also referenced as SRv6 Segment or SRv6 SID).
A SID list is represented as <S1, S2, S3> where S1 is the first SID to visit, S2 is the second SID to visit and S3 is the last SID to visit along the SR path.
(SA,DA) (S3, S2, S1; SL) represents an IPv6 packet with:
SRH[SL] represents the SID pointed by the SL field in the first SRH. In our example, SRH[2] represents S1, SRH[1] represents S2 and SRH[0] represents S3.
FIB is the abbreviation for the forwarding table. A FIB lookup is a lookup in the forwarding table. When a packet is intercepted on a wire, it is possible that SRH[SL] is different from the DA.
An SRv6 Segment is a 128-bit value. "SID" (abbreviation for Segment Identifier) is often used as a shorter reference for "SRv6 Segment".
An SRv6-capable node N maintains a "My Local SID Table". This table contains all the SRv6 segments explicitly instantiated at node N. N is the parent node for these SIDs.
A local SID of N can be an IPv6 address associated to a local interface of N but it is not mandatory. Nor is the My Local SID table populated by default with all IPv6 addresses defined on node N.
In most use-cases, a local SID will NOT be an address associated to a local interface of N.
A local SID of N could be routed to N but it does not have to be. Most often, it is routed to N via a shorter-mask prefix.
Let's provide a classic illustration.
Node N is configured with a loopback0 interface address of C1::1/40 originated in its IGP. Node N is configured with two SIDs: C1::100 and C2::101.
The entry C1::1 is not defined explicitly as an SRv6 SID and hence does not appear in the "My Local SID Table". The entries C1::100 and C2::101 are defined explicitly as SRv6 SIDs and hence appear in the "My Local SID Table".
The network learns about a path to C1::/40 via the IGP and hence a packet destined to C1::100 would be routed up to N. The network does not learn about a path to C2::/40 via the IGP and hence a packet destined to C2::101 would not be routed up to N.
A packet could be steered to a non-routed SID C2::101 by using a SID list <...,C1::100,C2::101,...> where the non-routed SID is preceded by a routed SID to the same node. This is similar to the local vs global segments in SR-MPLS.
Every SRv6 SID instantiated has a specific instruction bound to it. This information is stored in the "My Local SID Table". The "My Local SID Table" has three main purposes:
We represent an SRv6 SID as LOC:FUNCT where LOC is the L most significant bits and FUNCT is the 128-L least significant bits. L is called the locator length and is flexible. Each operator is free to use the locator length it chooses. Most often the LOC part of the SID is routable and leads to the node which instantiates that SID.
Often, for simplicity of illustration, we will use a locator length of 64 bits. This is just an example. Implementations must not assume any a priori prefix length.
The FUNCT part of the SID is an opaque identification of a local function bound to the SID. Hence the name SRv6 Local SID.
A function may require additional arguments that would be placed in the rightmost-bits of the 128-bit space. In such case, the SRv6 SID will have the form LOC:FUNCT:ARGS.
These arguments may vary on a per-packet basis and may contain information related to the flow, service, or any other information required by the function associated to the SRv6 SID.
For this reason, the "My Local SID Table" matches on a per longest-prefix-match basis.
A node may receive a packet with an SRv6 SID in the DA without an SRH. In such case the packet should still be processed by the Segment Routing engine.
Each entry of the "My Local SID Table" indicates the function associated with the local SID and its parameters.
We define hereafter a set of well-known functions that can be associated with a SID.
End Endpoint function
The SRv6 instantiation of a prefix SID
End.X Endpoint function with Layer-3 cross-connect
The SRv6 instantiation of a Adj SID
End.T Endpoint function with specific IPv6 table lookup
End.DX2 Endpoint with decapsulation and Layer-2 cross-connect
L2VPN use-case
End.DX2V Endpoint with decapsulation and VLAN L2 table lookup
EVPN Flexible cross-connect use-cases
End.DT2U Endpoint with decaps and unicast MAC L2 table lookup
EVPN Bridging unicast use-cases
End.DT2M Endpoint with decapsulation and L2 table flooding
EVPN Bridging BUM use-cases with ESI filtering
End.DX6 Endpoint with decapsulation and IPv6 cross-connect
IPv6 L3VPN use (equivalent of a per-CE VPN label)
End.DX4 Endpoint with decapsulation and IPv4 cross-connect
IPv4 L3VPN use (equivalent of a per-CE VPN label)
End.DT6 Endpoint with decapsulation and IPv6 table lookup
IPv6 L3VPN use (equivalent of a per-VRF VPN label)
End.DT4 Endpoint with decapsulation and IPv4 table lookup
IPv4 L3VPN use (equivalent of a per-VRF VPN label)
End.DT46 Endpoint with decapsulation and IP table lookup
IP L3VPN use (equivalent of a per-VRF VPN label)
End.B6 Endpoint bound to an SRv6 policy
SRv6 instantiation of a Binding SID
End.B6.Encaps Endpoint bound to an SRv6 encapsulation Policy
SRv6 instantiation of a Binding SID
End.BM Endpoint bound to an SR-MPLS Policy
SRv6/SR-MPLS instantiation of a Binding SID
End.S Endpoint in search of a target in table T
The list is not exhaustive. In practice, any function can be attached to a local SID: e.g. a node N can bind a SID to a local VM or container which can apply any complex function on the packet.
We call N the node who has an explicitly instantiated SID S and we detail the function that N binds to S.
At the end of this section we also present some flavours of these well-known functions.
The Endpoint function ("End" for short) is the most basic function.
When N receives a packet whose IPv6 DA is S and S is a local End SID, N does:
1. IF NH=SRH and SL > 0
2. decrement SL
3. update the IPv6 DA with SRH[SL]
4. FIB lookup on the updated DA ;; Ref1
5. forward accordingly to the matched entry ;; Ref2
6. ELSE
7. drop the packet ;; Ref3
Ref1: The End function performs the FIB lookup in the forwarding table associated to the ingress interface
Ref2: If the FIB lookup matches a multicast state, then the related RPF check must be considered successful
Ref3: a local SID could be bound to a function which authorizes the decapsulation of an outer header (e.g. IPinIP) or the punting of the packet to TCP, UDP or any other protocol. This however needs to be explicitly defined in the function bound to the local SID. By default, a local SID bound to the well-known function "End" only allows the punting to OAM protocols and neither allows the decapsulation of an outer header nor the cleanup of an SRH. As a consequence, an End SID cannot be the last SID of an SRH and cannot be the DA of a packet without SRH.
This is the SRv6 instantiation of a Prefix SID.
The "Endpoint with cross-connect to an array of layer-3 adjacencies" function (End.X for short) is a variant of the End function.
When N receives a packet destined to S and S is a local End.X SID, N does:
1. IF NH=SRH and SL > 0
2. decrement SL
3. update the IPv6 DA with SRH[SL]
4. forward to layer-3 adjacency bound to the SID S ;; Ref1
5. ELSE
6. drop the packet ;; Ref2
Ref1: If an array of adjacencies is bound to the End.X SID, then one entry of the array is selected based on a hash of the packet's header.
Ref2: An End.X function only allows punting to OAM and does not allow decaps. An End.X SID cannot be the last SID of an SRH and cannot be the DA of a packet without SRH.
The End.X function is required to express any traffic-engineering policy.
This is the SRv6 instantiation of an Adjacency SID.
If a node N has 30 outgoing interfaces to 30 neighbors, usually the operator would explicitly instantiate 30 End.X SIDs at N: one per layer-3 adjacency to a neighbor. Potentially, more End.X could be explicitly defined (groups of layer-3 adjacencies to the same neighbor or to different neighbors).
Note that with SR-MPLS, an AdjSID is typically preceded by a PrefixSID. This is unlikely in SRv6 as most likely an End.X SID is globally routed to N.
Note that if N has an outgoing interface bundle I to a neighbor Q made of 10 member links, N may allocate up to 11 End.X local SIDs for that bundle: one for the bundle itself and then up to one for each member link. This is the equivalent of the L2-Link Adj SID in SR-MPLS [I-D.ietf-isis-l2bundles].
The "Endpoint with specific IPv6 table lookup" function (End.T for short) is a variant of the End function.
When N receives a packet destined to S and S is a local End.T SID, N does:
1. IF NH=SRH and SL > 0 ;; Ref1
2. decrement SL
3. update the IPv6 DA with SRH[SL]
4. lookup the next segment in IPv6 table T associated with the SID
5. forward via the matched table entry
6. ELSE
7. drop the packet
Ref1: The End.T SID must not be the last SID
The End.T is used for multi-table operation in the core.
The "Endpoint with decapsulation and Layer-2 cross-connect to OIF" function (End.DX2 for short) is a variant of the endpoint function.
When N receives a packet destined to S and S is a local End.DX2 SID, N does:
1. IF NH=SRH and SL > 0
2. drop the packet ;; Ref1
3. ELSE IF ENH = 59 ;; Ref2
4. pop the (outer) IPv6 header and its extension headers
5. forward the resulting frame via OIF associated to the SID
6. ELSE
7. drop the packet
Ref1: An End.DX2 SID must always be the last SID, or it can be the Destination Address of an IPv6 packet with no SRH header.
Ref2: We conveniently reuse the next-header value 59 allocated to IPv6 No Next Header [RFC8200]. When the SID corresponds to function End.DX2 and the Next-Header value is 59, we know that an Ethernet frame is in the payload without any further header.
An End.DX2 function could be customized to expect a specific VLAN format and rewrite the egress VLAN header before forwarding on the outgoing interface.
One of the applications of the End.DX2 function is the L2VPN use-case.
The "Endpoint with decapsulation and specific VLAN L2 table lookup" function (End.DX2V for short) is a variant of the endpoint function.
When N receives a packet destined to S and S is a local End.DX2V SID, N does:
1. IF NH=SRH and SL > 0
2. drop the packet ;; Ref1
3. ELSE IF ENH = 59 ;; Ref2
4. pop the (outer) IPv6 header and its extension headers
5. lookup the exposed inner VLANs in L2 table T
6. forward via the matched table entry
7. ELSE
8. drop the packet
Ref1: An End.DX2V SID must always be the last SID, or it can be the Destination Address of an IPv6 packet with no SRH header.
Ref2: We conveniently reuse the next-header value 59 allocated to IPv6 No Next Header [RFC8200]. When the SID corresponds to function End.DX2V and the Next-Header value is 59, we know that an Ethernet frame is in the payload without any further header.
An End.DX2V function could be customized to expect a specific VLAN format and rewrite the egress VLAN header before forwarding on the outgoing interface.
The End.DX2V is used for EVPN Flexible cross-connect use-cases.
The "Endpoint with decapsulation and specific unicast MAC L2 table lookup" function (End.DT2U for short) is a variant of the endpoint function.
When N receives a packet destined to S and S is a local End.DT2U SID, N does:
1. IF NH=SRH and SL > 0
2. drop the packet ;; Ref1
3. ELSE IF ENH = 59 ;; Ref2
4. pop the (outer) IPv6 header and its extension headers
5. learn the exposed inner MAC SA in L2 table T ;; Ref3
6. lookup the exposed inner MAC DA in L2 table T
7. forward via the matched T entry else to all L2OIF in T
8. ELSE
9. drop the packet
Ref1: An End.DT2U SID must always be the last SID, or it can be the Destination Address of an IPv6 packet with no SRH header.
Ref2: We conveniently reuse the next-header value 59 allocated to IPv6 No Next Header [RFC8200]. When the SID corresponds to function End.DT2U and the Next-Header value is 59, we know that an Ethernet frame is in the payload without any further header.
Ref3: In EVPN, the learning of the exposed inner MAC SA is done via control plane.
The End.DT2U is used for EVPN Bridging unicast use cases.
The "Endpoint with decapsulation and specific L2 table flooding" function (End.DT2M for short) is a variant of the endpoint function.
This function may take an argument: "Arg.FE2". It is an argument specific to EVPN ESI filtering. It is used to exclude a specific OIF from L2 table T flooding.
When N receives a packet destined to S and S is a local End.DT2M SID, N does:
1. IF NH=SRH and SL > 0
2. drop the packet ;; Ref1
3. ELSE IF ENH = 59 ;; Ref2
4. pop the (outer) IPv6 header and its extension headers
5. learn the exposed inner MAC SA in L2 table T ;; Ref3
6. forward on all L2OIF excluding the one specified in Arg.FE2
7. ELSE
8. drop the packet
Ref1: An End.DT2M SID must always be the last SID, or it can be the Destination Address of an IPv6 packet with no SRH header.
Ref2: We conveniently reuse the next-header value 59 allocated to IPv6 No Next Header [RFC8200]. When the SID corresponds to function End.DT2M and the Next-Header value is 59, we know that an Ethernet frame is in the payload without any further header.
Ref3: In EVPN, the learning of the exposed inner MAC SA is done via control plane
The End.DT2M is used for EVPN Bridging BUM use case with ESI filtering capability.
The "Endpoint with decapsulation and cross-connect to an array of IPv6 adjacencies" function (End.DX6 for short) is a variant of the End and End.X functions.
When N receives a packet destined to S and S is a local End.DX6 SID, N does:
1. IF NH=SRH and SL > 0
2. drop the packet ;; Ref1
3. ELSE IF ENH = 41 ;; Ref2
4. pop the (outer) IPv6 header and its extension headers
5. forward to layer-3 adjacency bound to the SID S ;; Ref3
6. ELSE
7. drop the packet
Ref1: The End.DX6 SID must always be the last SID, or it can be the Destination Address of an IPv6 packet with no SRH header.
Ref2: 41 refers to IPv6 encapsulation as defined by IANA allocation for Internet Protocol Numbers
Ref3: Selected based on a hash of the packet's header (at least SA, DA, Flow Label)
One of the applications of the End.DX6 function is the L3VPN use-case where a FIB lookup in a specific tenant table at the egress PE is not required. This would be equivalent to the per-CE VPN label in MPLS[RFC4364].
The "Endpoint with decapsulation and cross-connect to an array of IPv4 adjacencies" function (End.DX4 for short) is a variant of the End and End.X functions.
When N receives a packet destined to S and S is a local End.DX4 SID, N does:
1. IF NH=SRH and SL > 0
2. drop the packet ;; Ref1
3. ELSE IF ENH = 4 ;; Ref2
4. pop the (outer) IPv6 header and its extension headers
5. forward to layer-3 adjacency bound to the SID S ;; Ref3
6. ELSE
7. drop the packet
Ref1: The End.DX4 SID must always be the last SID, or it can be the Destination Address of an IPv6 packet with no SRH header.
Ref2: 4 refers to IPv4 encapsulation as defined by IANA allocation for Internet Protocol Numbers
Ref3: Selected based on a hash of the packet's header (at least SA, DA, Flow Label)
One of the applications of the End.DX4 function is the L3VPN use-case where a FIB lookup in a specific tenant table at the egress PE is not required. This would be equivalent to the per-CE VPN label in MPLS[RFC4364].
The "Endpoint with decapsulation and specific IPv6 table lookup" function (End.DT6 for short) is a variant of the End function.
When N receives a packet destined to S and S is a local End.DT6 SID, N does:
1. IF NH=SRH and SL > 0
2. drop the packet ;; Ref1
3. ELSE IF ENH = 41 ;; Ref2
4. pop the (outer) IPv6 header and its extension headers
5. lookup the exposed inner IPv6 DA in IPv6 table T
6. forward via the matched table entry
7. ELSE
8. drop the packet
Ref1: the End.DT6 SID must always be the last SID, or it can be the Destination Address of an IPv6 packet with no SRH header.
Ref2: 41 refers to IPv6 encapsulation as defined by IANA allocation for Internet Protocol Numbers
One of the applications of the End.DT6 function is the L3VPN use-case where a FIB lookup in a specific tenant table at the egress PE is required. This would be equivalent to the per-VRF VPN label in MPLS[RFC4364].
Note that an End.DT6 may be defined for the main IPv6 table in which case and End.DT6 supports the equivalent of an IPv6inIPv6 decaps (without VPN/tenant implication).
The "Endpoint with decapsulation and specific IPv4 table lookup" function (End.DT4 for short) is a variant of the End function.
When N receives a packet destined to S and S is a local End.DT4 SID, N does:
1. IF NH=SRH and SL > 0
2. drop the packet ;; Ref1
3. ELSE IF ENH = 4 ;; Ref2
4. pop the (outer) IPv6 header and its extension headers
5. lookup the exposed inner IPv4 DA in IPv4 table T
6. forward via the matched table entry
7. ELSE
8. drop the packet
Ref1: the End.DT4 SID must always be the last SID, or it can be the Destination Address of an IPv6 packet with no SRH header.
Ref2: 4 refers to IPv4 encapsulation as defined by IANA allocation for Internet Protocol Numbers
One of the applications of the End.DT4 is the L3VPN use-case where a FIB lookup in a specific tenant table at the egress PE is required. This would be equivalent to the per-VRF VPN label in MPLS[RFC4364].
Note that an End.DT4 may be defined for the main IPv4 table in which case and End.DT4 supports the equivalent of an IPv4inIPv6 decaps (without VPN/tenant implication).
The "Endpoint with decapsulation and specific IP table lookup" function (End.DT46 for short) is a variant of the End function.
When N receives a packet destined to S and S is a local End.DT46 SID, N does:
1. IF NH=SRH and SL > 0
2. drop the packet ;; Ref1
3. ELSE IF ENH = 4 ;; Ref2
4. pop the (outer) IPv6 header and its extension headers
5. lookup the exposed inner IPv4 DA in IPv4 table T
6. forward via the matched table entry
7. ELSE IF ENH = 41 ;; Ref2
8. pop the (outer) IPv6 header and its extension headers
9. lookup the exposed inner IPv6 DA in IPv6 table T
10. forward via the matched table entry
11. ELSE
12. drop the packet
Ref1: the End.DT46 SID must always be the last SID, or it can be the Destination Address of an IPv6 packet with no SRH header.
Ref2: 4 and 41 refer to IPv4 and IPv6 encapsulation respectively as defined by IANA allocation for Internet Protocol Numbers
One of the applications of the End.DT46 is the L3VPN use-case where a FIB lookup in a specific tenant table at the egress PE is required. This would be equivalent to the per-VRF VPN label in MPLS[RFC4364].
Note that an End.DT46 may be defined for the main IP table in which case and End.DT46 supports the equivalent of an IPinIPv6 decaps (without VPN/tenant implication).
The "Endpoint bound to an SRv6 Policy" is a variant of the End function.
When N receives a packet destined to S and S is a local End.B6 SID, N does:
1. IF NH=SRH and SL > 0 ;; Ref1
2. do not decrement SL nor update the IPv6 DA with SRH[SL]
3. insert a new SRH ;; Ref2
4. set the IPv6 DA to the first segment of the SRv6 Policy
5. forward according to the first segment of the SRv6 Policy
6. ELSE
7. drop the packet
Ref1: An End.B6 SID, by definition, is never the last SID.
Ref2: In case that an SRH already exists, the new SRH is inserted in between the IPv6 header and the received SRH
Note: Instead of the term "insert", "push" may also be used.
The End.B6 function is required to express scalable traffic-engineering policies across multiple domains. This is the SRv6 instantiation of a Binding SID.
This is an optimization of the End.B6 function.
End.B6.Red will reduce the size of the SRH by one segment by avoiding the insertion of the first SID in the pushed SRH. In this way, the first segment is only introduced in the DA and the packet is forwarded according to it.
Note that SL value is initially pointing to a non-existing segment in the SRH.
This is a variation of the End.B6 behavior where the SRv6 Policy also includes an IPv6 Source Address A.
When N receives a packet destined to S and S is a local End.B6.Encaps SID, N does:
1. IF NH=SRH and SL > 0
2. decrement SL and update the IPv6 DA with SRH[SL]
3. push an outer IPv6 header with its own SRH
4. set the outer IPv6 SA to A
5. set the outer IPv6 DA to the first segment of the SRv6 Policy
6. forward according to the first segment of the SRv6 Policy
7. ELSE
8. drop the packet
Instead of simply inserting an SRH with the policy (End.B6), this behavior also adds an outer IPv6 header. The source address defined for the outer header does not have to be a local SID of the node.
This is an optimization of the End.B6.Encaps function.
End.B6.Encaps.Red will reduce the size of the SRH by one segment by avoiding the insertion of the first SID in the pushed SRH. In this way, the first segment is only introduced in the DA and the packet is forwarded according to it.
Note that SL value is initially pointing to a non-existing segment in the SRH.
The "Endpoint bound to an SR-MPLS Policy" is a variant of the End.B6 function.
When N receives a packet destined to S and S is a local End.BM SID, N does:
1. IF NH=SRH and SL > 0 ;; Ref1
2. decrement SL and update the IPv6 DA with SRH[SL]
3. push an MPLS label stack <L1, L2, L3> on the received packet
4. forward according to L1
5. ELSE
6. drop the packet
Ref1: an End.BM SID, by definition, is never the last SID.
The End.BM function is required to express scalable traffic-engineering policies across multiple domains where some domains support the MPLS instantiation of Segment Routing.
This is an SRv6 instantiation of a SR-MPLS Binding SID.
The "Endpoint in search of a target in Table T" function (End.S for short) is a variant of the End function.
When N receives a packet destined to S and S is a local End.S SID, N does:
1. IF NH=SRH and SL = 0 ;; Ref1
2. drop the packet
3. ELSE IF match(last SID) in specified table T
4. forward accordingly
5. ELSE
6. apply the End behavior
Ref1: By definition, an End.S SID cannot be the last SID, as the last SID is the targeted object.
The End.S function is required in information-centric networking (ICN) use-cases where the last SID in the SRv6 SID list represents a targeted object. If the identification of the object would require more than 128 bits, then obvious customization of the End.S function may either use multiple SIDs or a TLV of the SR header to encode the searched object ID.
Generally, any SR-aware application can be bound to an SRv6 SID. This application could represent anything from a small piece of code focused on topological/tenant function to a much larger process focusing on higher-level applications (e.g. video compression, transcoding etc.).
The ways in which an SR-aware application can binds itself on a local SID depends on the operating system. Let us consider an SR-aware application running on a Linux operating system. A possible approach is to associate an SRv6 SID to a target (virtual) interface, so that packets with IP DA corresponding to the SID will be sent to the target interface. In this approach, the SR-aware application can simply listen to all packets received on the interface.
A different approach for the SR-aware app is to listen to packets received with a specific SRv6 SID as IPv6 DA on a given transport port (i.e. corresponding to a TCP or UDP socket). In this case, the app can read the SRH information with a getsockopt Linux system call and can set the SRH information to be added to the outgoing packets with a setsocksopt system call.
[I-D.xuclad-spring-sr-service-programming] defines a set of additional functions in order to enable non SR-aware applications to be associated with a SRv6 Local SID.
We present the PSP and USP variants of the functions End, End.X and End.T. For each of these functions these variants can be enabled or disabled either individually or together.
After the instruction 'update the IPv6 DA with SRH[SL]' is executed, the following instructions must be added:
1. IF updated SL = 0 & PSP is TRUE & O-bit = 0 & A-bit = 0 ;; Ref1
2. pop the top SRH ;; Ref2
Ref1: If the SRH.Flags.O-bit or SRH.Flags.A-bit is set, PSP of the SRH is disabled. Section 6.1 specifies the pseudocode needed to process the SRH.Flags.O-bit.
Ref2: The received SRH had SL=1. When the last SID is written in the DA, the End, End.X and End.T functions with the PSP flavour pop the first (top-most) SRH. Subsequent stacked SRH's may be present but are not processed as part of the function.
We insert at the beginning of the pseudo-code the following instructions:
1. IF NH=SRH & SL = 0 & USP=TRUE ;; Ref1
2. pop the top SRH
3. restart the function processing on the modified packet ;; Ref2
Ref1: The next header is an SRH header
Ref2: Typically SL of the exposed SRH is > 0 and hence the restarting of the complete function would lead to decrement SL, update the IPv6 DA with SRH[SL], FIB lookup on updated DA and forward accordingly to the matched entry.
We define hereafter the set of basic transit behaviors.
T Transit behavior
T.Insert Transit behavior with insertion of an SRv6 policy
T.Insert.Red Transit behavior with reduced insert of an SRv6 policy
T.Encaps Transit behavior with encapsulation in an SRv6 policy
T.Encaps.Red Transit behavior with reduced encaps in an SRv6 policy
T.Encaps.L2 T.Encaps behavior of the received L2 frame
T.Encaps.L2.Red Transit with reduce encaps of received L2 frame
This list can be expanded in case any new functionality requires it.
As per [RFC8200], if a node N receives a packet (A, S2)(S3, S2, S1; SL=2) and S2 is neither a local address nor a local SID of N then N forwards the packet without inspecting the SRH.
This means that N treats the following two packets with the same performance:
A transit node does not need to count by default the amount of transit traffic with an SRH extension header. This accounting might be enabled as an optional behavior leveraging SEC4 behavior described later in this document.Section 7.4
A transit node MUST include the outer flow label in its ECMP hash[RFC6437].
Node N receives two packets P1=(A, B2) and P2=(A,B2)(B3, B2, B1; SL=1). B2 is neither a local address nor SID of N.
N steers the transit packets P1 and P2 into an SRv6 Policy with one SID list <S1, S2, S3>.
The "T.Insert" transit insertion behavior is defined as follows:
1. insert the SRH (B2, S3, S2, S1; SL=3) ;; Ref1, Ref1bis
2. set the IPv6 DA = S1
3. forward along the shortest path to S1
Ref1: The received IPv6 DA is placed as last SID of the inserted SRH.
Ref1bis: The SRH is inserted before any other IPv6 Routing Extension Header.
After the T.Insert behavior, P1 and P2 respectively look like:
The T.Insert.Red behavior is an optimization of the T.Insert behavior. It is defined as follows:
1. insert the SRH (B2, S3, S2; SL=3)
2. set the IPv6 DA = S1
3. forward along the shortest path to S1
T.Insert.Red will reduce the size of the SRH by one segment by avoiding the insertion of the first SID in the pushed SRH. In this way, the first segment is only introduced in the DA and the packet is forwarded according to it.
Note that SL value is initially pointing to a non-existing segment in the SRH.
After the T.Insert.Red behavior, P1 and P2 respectively look like:
Node N receives two packets P1=(A, B2) and P2=(A,B2)(B3, B2, B1; SL=1). B2 is neither a local address nor SID of N.
N steers the transit packets P1 and P2 into an SR Encapsulation Policy with a Source Address T and a Segment list <S1, S2, S3>.
The T.Encaps transit encapsulation behavior is defined as follows:
1. push an IPv6 header with its own SRH (S3, S2, S1; SL=2)
2. set outer IPv6 SA = T and outer IPv6 DA = S1
3. set outer payload length, traffic class and flow label ;; Ref1
4. update the Next-Header value ;; Ref1
5. decrement inner Hop Limit or TTL ;; Ref1
6. forward along the shortest path to S1
After the T.Encaps behavior, P1 and P2 respectively look like:
The T.Encaps behavior is valid for any kind of Layer-3 traffic. This behavior is commonly used for L3VPN with IPv4 and IPv6 deployements.
The SRH MAY be omitted when the SRv6 Policy only contains one segment and there is no need to use any flag, tag or TLV.
Ref 1: As described in [RFC2473] (Generic Packet Tunneling in IPv6 Specification)
The T.Encaps.Red behavior is an optimization of the T.Encaps behavior. It is defined as follows:
1. push an IPv6 header with its own SRH (S3, S2; SL=2)
2. set outer IPv6 SA = T and outer IPv6 DA = S1
3. set outer payload length, traffic class and flow label ;; Ref1
4. update the Next-Header value ;; Ref1
5. decrement inner Hop Limit or TTL ;; Ref1
6. forward along the shortest path to S1
Ref 1: As described in [RFC2473] (Generic Packet Tunneling in IPv6 Specification)
T.Encaps.Red will reduce the size of the SRH by one segment by avoiding the insertion of the first SID in the SRH of the pushed IPv6 packet. In this way, the first segment is only introduced in the DA and the packet is forwarded according to it.
Note that SL value is initially pointing to a non-existing segment in the SRH.
After the T.Encaps.Red behavior, P1 and P2 respectively look like:
The SRH MAY be omitted when the SRv6 Policy only contains one segment and there is no need to use any flag, tag or TLV.
While T.Encaps encapsulates the received IP packet, T.Encaps.L2 encapsulates the received L2 frame (i.e. the received ethernet header and its optional VLAN header is in the payload of the outer packet).
If the outer header is pushed without SRH then the DA must be a SID of type End.DX2, End.DX2V, End.DT2U or End.DT2M and the next-header must be 59 (IPv6 NoNextHeader). The received Ethernet frame follows the IPv6 header and its extension headers.
Else, if the outer header is pushed with an SRH, then the last SID of the SRH must be of type End.DX2, End.DX2V, End.DT2U or End.DT2M and the next-header of the SRH must be 59 (IPv6 NoNextHeader). The received Ethernet frame follows the IPv6 header and its extension headers.
The T.Encaps.L2.Red behavior is an optimization of the T.Encaps.L2 behavior.
T.Encaps.L2.Red will reduce the size of the SRH by one segment by avoiding the insertion of the first SID in the SRH of the pushed IPv6 packet. In this way, the first segment is only introduced in the DA and the packet is forwarded according to it.
Note that SL value is initially pointing to a non-existing segment in the SRH.
The SRH MAY be omitted when the SRv6 Policy only contains one segment and there is no need to use any flag, tag or TLV.
Any SRv6 capable node SHOULD implement the following set of combined counters (packets and bytes):
Furthermore, an SRv6 capable node maintains an aggregate counter CNT3 tracking the IPv6 traffic that was received with a destination address matching a local interface address that is not a local SID and the next-header is SRH with SL>0. We remind that this traffic is dropped as an interface address is not a local SID by default. A SID must be explicitly instantiated.
When a flow-based selection within a set needs to be performed, the source address, the destination address and the flow-label MUST be included in the flow-based hash.
This occurs when the destination address is updated, a FIB lookup is performed and multiple ECMP paths exist to the updated destination address.
This occurs when End.X is bound to an array of adjacencies.
This occurs when the packet is steered in an SR policy whose selected path has multiple SID lists [I-D.filsfils-spring-segment-routing-policy].
[I-D.ali-spring-srv6-oam] defines the OAM behavior for SRv6. This includes the definition of the SRH Flag 'O-bit', as well as additional OAM Endpoint functions.
We use the following terminology:
The purpose of this section is to document how a domain of trust can operate SRv6-based services for internal traffic while preventing any external traffic from accessing the internal SRv6-based services.
It is expected that future documents will detail enhanced security mechanisms for SRv6 (e.g. how to allow external traffic to leverage internal SRv6 services).
An SRv6 router MUST support an ACL on the external interface that drops any traffic with SA or DA in the internal SID space.
A provider would generally do this for its internal address space to prevent access to internal addresses and in order to prevent spoofing. The technique is extended to the local SID space.
The typical counters of an ACL are expected.
An SRv6 router MUST support an ACL with the following behavior:
1. IF (DA == LocalSID) && (SA != internal address or SID space)
2. drop
This prevents access to local SIDs from outside the operator's infrastructure. Note that this ACL may not be enabled in all cases. For example, specific SIDs can be used to provide resources to devices that are outside of the operator's infrastructure.
When an SID is in the form of LOC:FUNCT:ARGS the DA match should be implemented as a prefix match covering the argument space of the specific SID i.s.o. a host route.
The typical counters of an ACL are expected.
As per the End definition, an SRv6 router MUST only implement the End behavior on a local IPv6 address if that address has been explicitly enabled as a segment.
This address may or may not be associated with an interface. This address may or may not be routed. The only thing that matters is that the local SID must be explicitly instantiated and explicitly bound to a function (the default function is the End function).
An SRv6 router should support Unicast-RPF on source address on external interface.
This is a generic provider technique applied to the internal address space. It is extended to the internal SID space.
The typical counters to validate such filtering are expected.
In an SDN environment, one expects the controller to explicitly provision the SIDs and/or discover them as part of a service discovery function. Applications residing on top of the controller could then discover the required SIDs and combine them to form a distributed network program.
The concept of "SRv6 network programming" refers to the capability for an application to encode any complex program as a set of individual functions distributed through the network. Some functions relate to underlay SLA others to overlay/tenant, others to complex applications residing in VM and containers.
The specification of the SRv6 control-plane is outside the scope of this document.
We limit ourselves to a few important observations.
The End and End.X SIDs express topological functions and hence are expected to be signaled in the IGP together with the flavours PSP and USP [I-D.bashandy-isis-srv6-extensions].
The presence of SIDs in the IGP do not imply any routing semantics to the addresses represented by these SIDs. The routing reachability to an IPv6 address is solely governed by the classic, non-SID-related, IGP information. Routing is not governed neither influenced in any way by a SID advertisement in the IGP.
These two SIDs provide important topological functions for the IGP to build FRR/TI-LFA solution and for TE processes relying on IGP LSDB to build SR policies.
BGP-LS is expected to be the key service discovery protocol. Every node is expected to advertise via BGP-LS its SRv6 capabilities (e.g. how many SIDs in can insert as part of an T.Insert behavior) and any locally instantiated SID[I-D.ietf-idr-bgp-ls-segment-routing-ext][I-D.ietf-idr-te-lsp-distribution].
The End.DX4, End.DX6, End.DT4, End.DT6, End.DT46 and End.DX2 SIDs are expected to be signaled in BGP[I-D.dawra-idr-srv6-vpn].
The following table summarizes which SID would be signaled in which signaling protocol.
| IGP | BGP-LS | BGP IP/VPN | |
|---|---|---|---|
| End (PSP, USP) | X | X | |
| End.X (PSP, USP) | X | X | |
| End.T (PSP, USP) | X | X | |
| End.DX2 | X | X | |
| End.DX2V | X | X | |
| End.DT2U | X | X | |
| End.DT2M | X | X | |
| End.DX6 | X | X | X |
| End.DX4 | X | X | |
| End.DT6 | X | X | X |
| End.DT4 | X | X | |
| End.DT46 | X | X | |
| End.B6 | X | ||
| End.B6.Encaps | X | ||
| End.B6.BM | X | ||
| End.S | X |
The following table summarizes which transit capability would be signaled in which signaling protocol.
| IGP | BGP-LS | BGP IP/VPN | |
|---|---|---|---|
| T | X | ||
| T.Insert | X | ||
| T.Encaps | X | ||
| T.Encaps.L2 | X |
The previous table describes generic capabilities. It does not describe specific instantiated SID.
For example, a BGP-LS advertisement of the T capability of node N would indicate that node N supports the basic transit behavior. The T.Insert behavior would describe the capability of node N to instantiation a T.Insert behavior, specifically it would describe how many SIDs could be inserted by N without significant performance degradation. Same for T.Encaps (the number potentially lower as the overhead of the additional outer IP header is accounted).
The reader should also remember that any specific instantiated SR policy (via T.Insert or T.Encaps) is always assigned a Binding SID. They should remember that BSIDs are advertised in BGP-LS as shown in Table 1. Hence, it is normal that Table 2 only focuses on the generic capabilities related to T.Insert and T.Encaps as Table 1 advertises the specific instantiated BSID properties.
We introduce a simplified SID allocation technique to ease the reading of the text. We document the reference diagram. We then illustrate the network programming concept through different use-cases. These use-cases have been thought to allow straightforward combination between each other.
To simplify the illustration, we assume:
Each node K has:
Let us assume the following topology where all the links have IGP metric 10 except the link 23 which is 100.
Nodes A, 1 to 8 and B are considered within the network domain while nodes CE-A and CE-B are outside the domain.
4------5---9
/ | \ /
3 | 6
\ | /
A--1--- 2------7---8--B
/ \
CE-A CE-B
Tenant100 Tenant100 with
IPv4 20/8
Figure 1: Reference topology
Any edge node such as 1 would be configured with an ACL on any of its external interface (e.g. from CE-A) which drops any traffic with SA or DA in A::/4. See SEC 1.
Any core node such as 6 could be configured with an ACL with the SEC2 behavior "IF (DA == LocalSID) && (SA is not in A::/4 or B::/4) THEN drop".
SEC 3 protection is a default property of SRv6. A SID must be explicitly instantiated. In our illustration, the only available SIDs are those explicitly instantiated.
Any edge node such as 1 would be configured with Unicast-RPF on source address on external interface (e.g. from CE-A). See SEC 4.
Let us illustrate the SR-IPVPN use-case applied to IPv4.
Nodes 1 and 8 are configured with a tenant 100, each respectively connected to CE-A and CE-B.
Node 8 is configured with a local SID A8::D100 of function End.DT4 bound to tenant IPv4 table 100.
Via BGP signaling or an SDN-based controller, Node 1's tenant-100 IPv4 table is programmed with an IPv4 SR-VPN route 20/8 via SRv6 policy <A8::D100>.
When 1 receives a packet P from CE-A destined to 20.20.20.20, 1 looks up P in its tenant-100 IPv4 table and finds an SR-VPN entry 20/8 via SRv6 policy <A8::D100>. As a consequence, 1 pushes an outer IPv6 header with SA=A1::0, DA=A8::D100 and NH=4. 1 then forwards the resulting packet on the shortest path to A8::/40.
When 8 receives the packet, 8 matches the DA in its My LocalSID table, finds the bound function End.DT4(100) and confirms NH=4. As a result, 8 decaps the outer header, looks up the inner IPv4 DA in tenant-100 IPv4 table, and forward the (inner) IPv4 packet towards CE-B.
The reader can easily infer all the other SR-IPVPN IP instantiations:
+---------------------------------+----------------------------------+
| Route at ingress PE(1) | SR-VPN Egress SID of egress PE(8)|
+---------------------------------+----------------------------------+
| IPv4 tenant route with egress | End.DT4 function bound to |
| tenant table lookup | IPv4-tenant-100 table |
+---------------------------------+----------------------------------+
| IPv4 tenant route without egress| End.DX4 function bound to |
| tenant table lookup | CE-B (IPv4) |
+---------------------------------+----------------------------------+
| IPv6 tenant route with egress | End.DT6 function bound to |
| tenant table lookup | IPv6-tenant-100 table |
+---------------------------------+----------------------------------+
| IPv6 tenant route without egress| End.DX6 function bound to |
| tenant table lookup | CE-B (IPv6) |
+---------------------------------+----------------------------------+
Let us illustrate the SR-Ethernet-VPWS use-case.
Node 1 is configured with an ethernet VPWS service:
Node 8 is configured with an ethernet VPWS service:
These configurations can either be programmed by an SDN controller or partially derived from a BGP-based signaling and discovery service.
When 1 receives a frame F from CE-A, 1 pushes an outer IPv6 header with SA=A1::0, DA=A8::DC2B and NH=59. Note that no additional header is pushed. 1 then forwards the resulting packet on the shortest path to A8::/40.
When 8 receives the packet, 8 matches the DA in its My LocalSID table and finds the bound function End.DX2. After confirming that the next-header=59, 8 decaps the outer IPv6 header and forwards the inner Ethernet frame towards CE-B.
The reader can easily infer the Ethernet VPWS use-case:
+------------------------+-----------------------------------+
| Route at ingress PE(1) | SR-VPN Egress SID of egress PE(8) |
+------------------------+-----------------------------------+
| Ethernet VPWS | End.DX2 function bound to |
| | CE-B (Ethernet) |
+------------------------+-----------------------------------+
Let us illustrate the SR-EVPN-FXC use-case (Flexible cross-connect service).
Node 1 is configured with an EVPN-FXC service:
Node 8 is configured with an EVPN-FXC service:
These configurations can either be programmed by an SDN controller or derived from a BGP-based EVPN-FXC service. EVPN route Type-1 is used for that purpose.
When node 1 receives a frame F from CE-A, it pushes an outer IPv6 header with SA=A1::0, DA=A8::DC2B and NH=59. Note that no additional header is pushed. Node 1 then forwards the resulting packet on the shortest path to A8::/40.
When node 8 receives the packet, it matches the IP DA in its My LocalSID table and finds the bound function End.DX2V. After confirming that the next-header=59, node 8 decaps the outer IPv6 header, performs a VLAN loopkup in table T1 and forwards the inner Ethernet frame to matching interface e.g. for VLAN 100, packet is forwarded to CE1-B and for VLAN 200, frame is forwarded to CE2-B.
The reader can easily infer the Ethernet FXC use-case:
+---------------------------------+------------------------------------+
| Route at ingress PE (1) | SR-VPN Egress SID of egress PE (8) |
+---------------------------------+------------------------------------+
| EVPN-FXC | End.DX2V function bound to |
| | CE1-B / CE2-B (Ethernet) |
+---------------------------------+------------------------------------+
There are few use cases to illustrate under SR-EVPN: bridging (unicast and multicast), multi-homing ESI filtering, EVPN L3, EVPN-IRB.
Node 1 is configured with an EVPN bridging service (E-LAN service):
Node 4 is configured with an EVPN bridging service:
Node 8 is configured with an EVPN bridging service:
The End.DT2M SID are exchanged between nodes via BGP-based EVPN Type-3 route.
Upon reception of EVPN Type-3 routes, each node build its own replication list per layer2 table T1.
On node 1, the replication list looks like: A4::D2BF:0, A8::D2CF:0. On node 4, the replication list looks like: A1::D2AF:0, A8::D2CF:0. On node 8, the replication list looks like: A1:D2AF:0, A4:D2BF:0. In the case of ingress replication, Ingress PE transmitting the BUM traffic stream replicates the traffic using that list.
When node 1 receives a BUM frame F from CE-A, it replicates that frame to remote nodes. For node 4, it pushes an outer IPv6 header with SA=A1::0, DA=A4::D2BF:0 and NH=59. For node 8, it performs the same operation but DA=A8::D2CF:0. Note that no additional header is pushed. Node 1 then forwards the resulting packet on the shortest path for each replication e.g. A4::D2BF:0/112 and A8::D2CF:0/112.
When node 4 receives the packet, it matches the DA in its My LocalSID table and finds the bound function End.DT2M and its related layer2 table T1. After confirming that the next-header=59, node 4 decaps the outer IPv6 header and forwards the inner Ethernet frame to all layer-2 output interface found to table T1. Similar processing is also performed by node 8 upon packet reception. This example is the same for any BUM stream coming from CE-B and CE-C.
Node 1,4 and 8 are also performing software MAC learning to exchange MAC reachability information (unicast traffic) via BGP among themselves.
Each MAC being learned in software are exchanged using BGP-based EVPN route type-2.
When node 1 receives an unicast frame F from CE-A, it learns its MAC-SA=CEA in software. Node 1 transmits that MAC and its associated SID A1::D2AA using BGP-based EVPN route-type 2 to all remote nodes.
When node 4 receives an unicast frame F from CE-B destinated to MAC-DA=CEA, it performs a L2 table T1 MAC-DA lookup to find the associated SID. It pushes an outer IPv6 header with SA=A4::0, DA=A1::D2AA and NH=59. Note that no additional header is pushed. Node 4 then forwards the resulting packet on the shortest path to A1::/40. Similar processing is also performed by node 8.
In L2 network, traffic loop avoidance is a MUST. In EVPN all-active multi-homing scenario, ESI filtering feature enforce that requirement.
Node 1 and node 2 are peering partners of a redundancy group where the access CE-A is connected in an all-active multi-homing way with these two nodes.
Node 1 is configured with an EVPN bridging service (E-LAN service):
Node 2 is configured with an EVPN bridging service:
The End.DT2M SID are exchanged between nodes via BGP-based EVPN route type-3.
Upon reception of EVPN Type-3 routes, each node build its own replication list per layer2 table T1.
The End.DT2M SID arguments Arg.FE2 are exchange between nodes via BGP ESI-filtering extended community attached to BGP-based EVPN route type-1.
Upon reception of EVPN route type-1 and type-3, node 1 merges the End.DT2M SID and the Arg.FE2 argument from node 2; its peering partner. Its replication list looks like A2::D2BF:C1. Similar procedure is performed by node 2.
When node 1 receives a BUM frame F from CE-A, it replicates that frame to remote nodes. For node 2, it pushes an outer IPv6 header with SA=A1::0, DA=A2::D2BF:C1 and NH=59. Note that no additional header is pushed. Node 1 then forwards the resulting packet on the shortest path for each replication e.g. A2::D2BF:00/112. Again, similar processing is also performed by node 8 upon packet reception
EVPN layer-3 works exactly in the same way of IPVPN. Please refer to SR-IPVPN section
EVPN IRB brings Layer-2 and Layer-3 together. It uses BGP-based EVPN route type-2 to achieve Layer-2 intra-subnet and Layer-3 inter-subnet forwarding. The EVPN route type-2 maintain the associated of a MAC/IP association.
Node 1 is configured with an EVPN IRB service:
Node 8 is configured with an EVPN IRB service:
Each ARP/ND request learned by each node are exchanged using BGP-based EVPN route type-2.
When node 1 receives an ARP/ND packet P from a host (10.10.10.10) on CE-A destined to 20.20.20.20, it learns its MAC-SA=CEA in software. It also learns the ARP/ND entry (IP SA=10.10.10.10) in its cache. Node 1 transmits that MAC/IP and its associated L2 SID A1::D2AA and L3 SID A1::D3AA using BGP-based EVPN route-type 2 to all remote nodes.
When node 8 receives a packet P from CE-C destined to 10.10.10.10 from a host (20.20.20.20), P looks up its tenant-100 IPv4 table and finds an SR-VPN entry for that prefix. As a consequence, node 8 pushes an outer IPv6 header with SA=A8::0, DA=A1::D3AA and NH=4. Node 8 then forwards the resulting packet on the shortest path to A1::/40. EVPN inter-subnet forwarding is then achieved.
When node 8 receives a packet P from CE-C destined to 10.10.10.10 from a host (10.10.10.11), P looks up its L2 table T1 MAC-DA lookup to find the associated SID. It pushes an outer IPv6 header with SA=A8::0, DA=A1::D2AA and NH=59. Note that no additional header is pushed. Node 8 then forwards the resulting packet on the shortest path to A1::/40. EVPN intra-subnet forwarding is then achieved.
Let's assume that node 1's tenant-100 IPv4 route "20/8 via A8::D100" is programmed with a color/community that requires low-latency underlay optimization [I-D.filsfils-spring-segment-routing-policy].
In such case, node 1 either computes the low-latency path to the egress node itself or delegates the computation to a PCE.
In either case, the location of the egress PE can easily be found by looking for who originates the SID block comprising the SID A8::D100. This can be found in the IGP's LSDB for a single domain case, and in the BGP-LS LSDB for a multi-domain case.
Let us assume that the TE metric encodes the per-link propagation latency. Let us assume that all the links have a TE metric of 10, except link 27 which has TE metric 100.
The low-latency path from 1 to 8 is thus 1245678.
This path is encoded in a SID list as: first a hop through A4::C5 and then a hop to 8.
As a consequence the SR-VPN entry 20/8 installed in the Node1's Tenant-100 IPv4 table is: T.Encaps with SRv6 Policy <A4::C5, A8::D100>.
When 1 receives a packet P from CE-A destined to 20.20.20.20, P looks up its tenant-100 IPv4 table and finds an SR-VPN entry 20/8. As a consequence, 1 pushes an outer header with SA=A1::0, DA=A4::C5, NH=SRH followed by SRH (A8::D100, A4::C5; SL=1; NH=4). 1 then forwards the resulting packet on the interface to 2.
2 forwards to 4 along the path to A4::/40.
When 4 receives the packet, 4 matches the DA in its My LocalSID table and finds the bound function End.X to neighbor 5. 4 notes the PSP capability of the SID A4::C5. 4 sets the DA to the next SID A8::D100. As 4 is the penultimate segment hop, it performs PSP and pops the SRH. 4 forwards the resulting packet to 5.
5, 6 and 7 forwards along the path to A8::/40.
When 8 receives the packet, 8 matches the DA in its My LocalSID Table and finds the bound function End.DT(100). As a result, 8 decaps the outer header, looks up the inner IPv4 DA in tenant-100 IPv4 table, and forward the (inner) IPv4 packet towards CE-B.
Let us analyze a policy applied at a midpoint on a packet without SRH.
Packet P1 is (A1::, A8::D100).
Let us consider P1 when it is received by node 2 and let us assume that that node 2 is configured to steer A8::/40 in a transit behavior T.Insert associated with SR policy <A4::C5>.
In such a case, node 2 would send the following modified packet P1 on the link to 4:
(A1::, A4::C5)(A8::D100, A4::C5; SL=1).
The rest of the processing is similar to the previous section.
Let us analyze a policy applied at a midpoint on a packet with an SRH.
Packet P2 is (A1::, A7::1)(A8::D100, A7::1; SL=1).
Let us consider P2 when it is received by node 2 and let us assume that node 2 is configured to steer A7::/40 in a transit behavior T.Insert associated with SR policy <A4::C5, A9::1>.
In such a case, node 2 would send the following modified packet P2 on the link to 4:
(A1::, A4::C5)(A7::1, A9::1, A4::C5; SL=2)(A8::D100, A7::1; SL=1)
Node 4 would send the following packet to 5: (A1::, A9::1)(A7::1, A9::1, A4::C5; SL=1)(A8::D100, A7::; SL=1)
Node 5 would send the following packet to 9: (A1::, A9::1)(A7::1, A9::1, A4::C5; SL=1)(A8::D100, A7::1; SL=1)
Node 9 would send the following packet to 6: (A1::, A7::1)(A8::D100, A7::1; SL=1)
Node 6 would send the following packet to 7: (A1::, A7::1)(A8::D100, A7::1; SL=1)
Node 7 would send the following packet to 8: (A1::, A8::D100)
Let us now describe a case where the ingress VPN edge node steers the packet destined to 20.20.20.20 towards the egress edge node connected to the tenant100 site with 20/8, but via an intermediate SR Policy represented by a single routable Binding SID. Let us illustrate this case with an intermediate policy which both encodes underlay optimization for low-latency and the service programming via two SR-aware container-based apps.
Let us assume that the End.B6 SID A2::B1 is configured at node 2 and is associated with midpoint T.Insert policy <A4::C5, A9::A1, A6::A2>.
A4::C5 realizes the low-latency path from the ingress PE to the egress PE. This is the underlay optimization part of the intermediate policy.
A9::A1 and A6::A2 represent two SR-aware NFV applications residing in containers respectively connected to node 9 and 6.
Let us assume the following ingress VPN policy for 20/8 in tenant 100 IPv4 table of node 1: T.Encaps with SRv6 Policy <A2::B1, A8::D100>.
This ingress policy will steer the 20/8 tenant-100 traffic towards the correct egress PE and via the required intermediate policy that realizes the SLA and NFV requirements of this tenant customer.
Node 1 sends the following packet to 2: (A1::, A2::B1) (A8::D100, A2::B1; SL=1)
Node 2 sends the following packet to 4: (A1::, A4::C5) (A6::A2, A9::A1, A4::C5; SL=2)(A8::D100, A2::B1; SL=1)
Node 4 sends the following packet to 5: (A1::, A9::A1) (A6::A2, A9::A1, A4::C5; SL=1)(A8::D100, A2::B1; SL=1)
Node 5 sends the following packet to 9: (A1::, A9::A1) (A6::A2, A9::A1, A4::C5; SL=1)(A8::D100, A2::B1; SL=1)
Node 9 sends the following packet to 6: (A1::, A6::A2) (A8::D100, A2::B1; SL=1)
Node 6 sends the following packet to 7: (A1::, A8::D100)
Node 7 sends the following packet to 8: (A1::, A8::D100) which decaps and forwards to CE-B.
The benefits of using an intermediate Binding SID are well-known and key to the Segment Routing architecture: the ingress edge node needs to push fewer SIDs, the ingress edge node does not need to change its SR policy upon change of the core topology or re-homing of the container-based apps on different servers. Conversely, the core and service organizations do not need to share details on how they realize underlay SLA's or where they home their NFV apps.
Let us assume two packets P1 and P2 received by node 2 exactly when the failure of link 27 is detected.
Node 2's pre-computed TI-LFA backup path for the destination A7:: is <A4::C5>. It is installed as a T.Insert transit behavior.
Node 2 protects the two packets P1 and P2 according to the pre-computed TI-LFA backup path and send the following modified packets on the link to 4:
Node 4 then sends the following modified packets to 5:
Then these packets follow the rest of their post-convergence path towards node 7 and then go to node 8 for the VPN decaps.
We have illustrated the service programming through SR-aware apps in a previous section.
We illustrate the use of End.AS function [I-D.xuclad-spring-sr-service-programming] to service chain an IP flow bound to the internet through two SR-unaware applications hosted in containers.
Let us assume that servers 20 and 70 are respectively connected to nodes 2 and 7. They are respectively configured with SID spaces A020::/40 and A070::/40. Their connected routers advertise the related prefixes in the IGP. Two SR-unaware container-based applications App2 and App7 are respectively hosted on server 20 and 70. Server 20 (70) is configured explicitly with an End.AS SID A020::2 for App2 (A070::7 for App7).
Let us assume a broadband customer with a home gateway CE-A connected to edge router 1. Router 1 is configured with an SR policy which encapsulates all the traffic received from CE-A into a T.Encaps policy <A020::2, A070::7, A8::D0> where A8::D0 is an End.DT4 SID instantiated at node 8.
P1 is a packet sent by the broadband customer to 1: (X, Y) where X and Y are two IPv4 addresses.
1 sends the following packet to 2: (A1::0, A020::2)(A8::D0, A070::7, A020::2; SL=2; NH=4)(X, Y).
2 forwards the packet to server 20.
20 receives the packet (A1::0, A020::2)(A8::D0, A070::7, A020::2; SL=2; NH=4)(X, Y) and forwards the inner IPv4 packet (X,Y) to App2. App2 works on the packet and forwards it back to 20. 20 pushes the outer IPv6 header with SRH (A1::0, A070::7)(A8::D0, A070::7, A020::2; SL=1; NH=4) and sends the (whole) IPv6 packet with the encapsulated IPv4 packet back to 2.
2 and 7 forward to server 70.
70 receives the packet (A1::0, A070::7)(A8::D0, A070::7, A020::2; SL=1; NH=4)(X, Y) and forwards the inner IPv4 packet (X,Y) to App7. App7 works on the packet and forwards it back to 70. 70 pushes the outer IPv6 header with SRH (A1::0, A8::D0)(A8::D0, A070::7, A020::2; SL=0; NH=4) and sends the (whole) IPv6 packet with the encapsulated IPv4 packet back to 7.
7 forwards to 8.
8 receives (A1::0, A8::D0)(A8::D0, A070::7, A020::2; SL=0; NH=4)(X, Y) and performs the End.DT4 function and sends the IP packet (X, Y) towards its internet destination.
The VPN use-case can be realized with SRv6 capability deployed solely at the ingress and egress PE's.
The SRTE/underlay-SLA use-case can be realized with SRv6 capability deployed at few strategic nodes.
The TI-LFA benefits are collected incrementally as SRv6 capabilities are deployed.
The service programming use-case can be realized with SRv6 capability deployed at few strategic nodes.
The SRv6 network programming concept allows integrating all the application and service requirements: multi-domain underlay SLA optimization with scale, overlay VPN/Tenant, sub-50msec automated FRR, security and service programming.
The combination of well-known techniques (SEC1, SEC2, SEC4) and carefully chosen architectural rules (SEC3) ensure a secure deployment of SRv6 inside a multi-domain network managed by a single organization.
Inter-domain security will be described in a companion document.
This document requests the following new IANA registries:
| Range | Hex | Registration proceadure | Notes |
|---|---|---|---|
| 0 | 0x0000 | Reserved | Invalid |
| 1-32767 | 0x0001-0x7FFF | IETF review | Draft Specifications |
| 32768-49151 | 0x8000-0xBFFF | Reserved for experimental use | |
| 49152-65534 | 0xC000-0xFFFE | Reserved for private use | |
| 65535 | 0xFFFF | Reserved | Wildcard |
The initial registrations for the "Draft Specifications" portion of the sub-registry are as follows:
| Value | Hex | Endpoint function | Reference |
|---|---|---|---|
| 1 | 0x0001 | End (no PSP, no USP) | [This.ID] |
| 2 | 0x0002 | End with PSP | [This.ID] |
| 3 | 0x0003 | End with USP | [This.ID] |
| 4 | 0x0004 | End with PSP&USP | [This.ID] |
| 5 | 0x0005 | End.X (no PSP, no USP) | [This.ID] |
| 6 | 0x0006 | End.X with PSP | [This.ID] |
| 7 | 0x0007 | End.X with USP | [This.ID] |
| 8 | 0x0008 | End.X with PSP&USP | [This.ID] |
| 9 | 0x0009 | End.T (no PSP, no USP) | [This.ID] |
| 10 | 0x000A | End.T with PSP | [This.ID] |
| 11 | 0x000B | End.T with USP | [This.ID] |
| 12 | 0x000C | End.T with PSP&USP | [This.ID] |
| 13 | 0x000D | End.B6 | [This.ID] |
| 14 | 0x000E | End.B6.Encaps | [This.ID] |
| 15 | 0x000F | End.BM | [This.ID] |
| 16 | 0x0010 | End.DX6 | [This.ID] |
| 17 | 0x0011 | End.DX4 | [This.ID] |
| 18 | 0x0012 | End.DT6 | [This.ID] |
| 19 | 0x0013 | End.DT4 | [This.ID] |
| 20 | 0x0014 | End.DT46 | [This.ID] |
| 21 | 0x0015 | End.DX2 | [This.ID] |
| 22 | 0x0016 | End.DX2V | [This.ID] |
| 23 | 0x0017 | End.DT2U | [This.ID] |
| 24 | 0x0018 | End.DT2M | [This.ID] |
| 25 | 0x0019 | End.S | [This.ID] |
| 26 | 0x001A | End.B6.Red | [This.ID] |
| 27 | 0x001B | End.B6.Encaps.Red | [This.ID] |
We are working on a extension of this document to provide Yang modelling for all the functionality described in this document. This work is ongoing in [I-D.raza-spring-srv6-yang].
The authors would like to acknowledge Stefano Previdi, Dave Barach, Mark Townsley, Peter Psenak, Paul Wells, Robert Hanzl, Dan Ye, Gaurav Dawra, Faisal Iqbal, Jaganbabu Rajamanickam, David Toscano, Asif Islam, Jianda Liu, Yunpeng Zhang, Jiaoming Li, Narendra A.K, Mike Mc Gourty, Bhupendra Yadav, Sherif Toulan, Satish Damodaran, John Bettink, Kishore Nandyala Veera Venk, Jisu Bhattacharya and Saleem Hafeez.
Daniel Bernier
Bell Canada
Canada
Email: daniel.bernier@bell.ca
Dirk Steinberg
Steinberg Consulting
Germany
Email: dws@dirksteinberg.de
Robert Raszuk
Bloomberg LP
United States of America
Email: robert@raszuk.net
Bruno Decraene
Orange
Frence
Email: bruno.decraene@orange.com
Bart Peirens
Proximus
Belgium
Email: bart.peirens@proximus.com
Hani Elmalky
Ericsson
United States of America
Email: hani.elmalky@gmail.com
Prem Jonnalagadda
Barefoot Networks
United States of America
Email: prem@barefootnetworks.com
Milad Sharif
Barefoot Networks
United States of America
Email: msharif@barefootnetworks.com
David Lebrun
Universite catholique de Louvain
Belgium
Email: david.lebrun@uclouvain.be
Stefano Salsano
Universita di Roma "Tor Vergata"
Italy
Email: stefano.salsano@uniroma2.it
Ahmed AbdelSalam
Gran Sasso Science Institute
Italy
Email: ahmed.abdelsalam@gssi.it
Gaurav Naik
Drexel University
United States of America
Email: gn@drexel.edu
Arthi Ayyangar
Arista
United States of America
Email: arthi@arista.com
Satish Mynam
Innovium Inc.
United States of America
Email: smynam@innovium.com
Wim Henderickx
Nokia
Belgium
Email: wim.henderickx@nokia.com
Shaowen Ma
Juniper
Singapore
Email: mashao@juniper.net
Ahmed Bashandy
Individual
United States of America
Email: abashandy.ietf@gmail.com
Francois Clad
Cisco Systems, Inc.
France
Email: fclad@cisco.com
Kamran Raza
Cisco Systems, Inc.
Canada
Email: skraza@cisco.com
Darren Dukes
Cisco Systems, Inc.
Canada
Email: ddukes@cisco.com
Patrice Brissete
Cisco Systems, Inc.
Canada
Email: pbrisset@cisco.com
Zafar Ali
Cisco Systems, Inc.
United States of America
Email: zali@cisco.com
| [I-D.ali-spring-srv6-oam] | Ali, Z., Filsfils, C., Kumar, N., Pignataro, C., faiqbal@cisco.com, f., Gandhi, R., Leddy, J., Matsushima, S., Raszuk, R., Peirens, B. and G. Naik, "Operations, Administration, and Maintenance (OAM) in Segment Routing Networks with IPv6 Data plane (SRv6)", Internet-Draft draft-ali-spring-srv6-oam-00, February 2018. |
| [I-D.ietf-6man-segment-routing-header] | Filsfils, C., Previdi, S., Leddy, J., Matsushima, S. and d. daniel.voyer@bell.ca, "IPv6 Segment Routing Header (SRH)", Internet-Draft draft-ietf-6man-segment-routing-header-14, June 2018. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |