Network C. Weiqiang
Internet-Draft China Mobile
Intended status: Standards Track G. Mirsky
Expires: January 3, 2021 ZTE Corp.
P. Shaofu
L. Aihua
ZTE Corporation
G. Mishra
Verizon Inc.
July 2, 2020
Unified Identifier in IPv6 Segment Routing Networks
draft-mirsky-6man-unified-id-sr-07
Abstract
Segment Routing architecture leverages the paradigm of source
routing. It can be realized in a network data plane by prepending
the packet with a list of instructions, a.k.a. segments. A segment
can be encoded as a Multi-Protocol Label Switching (MPLS) label, IPv4
address, or IPv6 address. Segment Routing can be applied in MPLS
data plane by encoding segments in the MPLS label stack. It also can
be applied to IPv6 data plane by encoding a list of segment
identifiers in IPv6 Segment Routing Extension Header (SRH). This
document extends the use of the SRH to unified segment identifiers
encoded, for example, as MPLS label or IPv4 address, to compress the
SRH, and support more detailed network programming and interworking
between SR-MPLS and SRv6 domains.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 3, 2021.
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Copyright Notice
Copyright (c) 2020 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . 3
1.1.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2. Requirements Language . . . . . . . . . . . . . . . . 4
2. Segment Routing Extension Header: Benefits and Challenges . . 4
3. Unified SIDs in IPv6 Segment Routing Extension Header . . . . 4
4. The Use Case of Unified Segment Identifier . . . . . . . . . 6
4.1. Interworking Between SR-MPLS and SRv6 Using U-SID . . . . 6
5. Operations with Unified Segment Identifier . . . . . . . . . 9
5.1. Procedures of SR-MPLS over IP . . . . . . . . . . . . . . 10
5.2. Packet Forwarding . . . . . . . . . . . . . . . . . . . . 10
6. Interworking Between Different UET domain Considerations . . 12
6.1. Interworking Using Binding SID . . . . . . . . . . . . . 12
6.2. Interworking Using Mixing U-SID . . . . . . . . . . . . . 12
6.2.1. UET capability Advertisement . . . . . . . . . . . . 13
6.2.2. SRv6 SID Allocated per UET . . . . . . . . . . . . . 13
6.2.3. Packets Forwarding Procedures . . . . . . . . . . . . 15
6.2.4. SRH with mixing elements Pseudo-code . . . . . . . . 18
7. Control Plane in Support of Unified SID . . . . . . . . . . . 19
8. U-SID supporting SRv6 programming . . . . . . . . . . . . . . 20
9. Implementation Considerations . . . . . . . . . . . . . . . . 20
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
11. Security Considerations . . . . . . . . . . . . . . . . . . . 21
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
14. Normative References . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
Segment Routing architecture [RFC8402] leverages the paradigm of
source routing. It can be realized in a network data plane by
prepending the packet with a list of instructions, a.k.a. segment
identifiers (SIDs). A segment can be encoded as a Multi-Protocol
Label Switching (MPLS) label, IPv4 address, or IPv6 address. Segment
Routing can be applied in MPLS data plane by encoding 20-bits SIDs in
MPLS label stack [RFC8660]. It also can be applied to IPv6 data
plane by encoding a list of 128-bits SIDs in IPv6 Segment Routing
Extension Header (SRH) [I-D.ietf-6man-segment-routing-header].
This document extends the use of the SRH
[I-D.ietf-6man-segment-routing-header] to unified identifiers encoded
as MPLS label or IPv4 address to support more detailed network
programming and interworking between SR-MPLS and SRv6 domains.
1.1. Conventions used in this document
1.1.1. Acronyms
SR: Segment Routing
SRH: Segment Routing Extension Header
MPLS: Multiprotocol Label Switching
SR-MPLS: Segment Routing using MPLS data plane
SID: Segment Identifier
IGP: Interior Gateway Protocol
DA: Destination Address
ILM: Incoming Label Map
FEC: Forwarding Equivalence Class
FTN: FEC-to-NHLFE map
OAM: Operation, Administration and Maintenance
TE: Traffic Engineering
SRv6: Segment Routing in IPv6
U-SID: Unified Segment Identifier
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PSP: Penultimate Segment Popping
FIB: Forwarding Information Base
UET: U-SID Encapsulation Type
1.1.2. Requirements Language
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Segment Routing Extension Header: Benefits and Challenges
Many functions related to Operation, Administration and Maintenance
(OAM) require identification of the SR tunnel ingress and the path,
constructed by segments, between the ingress and the egress SR nodes.
Combination of IPv6 encapsulation [RFC8200] and SRH
[I-D.ietf-6man-segment-routing-header], referred to as SRv6, comply
with these requirements while it is challenging when applying SR in
MPLS networks, also referred to as SR-MPLS.
On the other hand, the size of IPv6 SID presents a scaling challenge
to use topological instructions that define strict explicit traffic-
engineered (TE) path or support network programming in combination
with service-based instructions. At the same time, that is where SR-
MPLS approach provides better results due to smaller SID length. It
can be used to compress the SRv6 header size when a smaller namespace
of available SIDs is sufficient for addressing the particular
network.
SR-MPLS is broadly used in metro networks. With the gradual
deployment of SRv6 in the core networks, supporting interworking
between SR-MPLS and SRv6 becomes the necessity for operators. It is
operationally more efficient and straightforward if SRv6 can use the
same size SIDs as in SR-MPLS. The SRH can be extended to define the
same as in SR-MPLS SID length to support the unified segment
identifier (U-SID). As a result, end-to-end SR tunnel may use U-SIDs
across SR-MPLS and SRv6 domains.
3. Unified SIDs in IPv6 Segment Routing Extension Header
SRH format has been defined in Section 3 of
[I-D.ietf-6man-segment-routing-header] as presented in Figure 1
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last Entry | Flags | Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[0] (128 bits IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
...
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[n] (128 bits IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// //
// Optional Type Length Value objects (variable) //
// //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: SRH format
This document defines a new field Size in the SRH Flags field as a
two-bits field, termed as UET (U-SID Encapsulation Type), with the
following values:
0b00 - 128-bits SID, an IPv6 address.
0b01 - 32-bits SID. In some environments, the context could be of
IPv4 address, while in some other cases, it could represent an
index of list or range of IPv4/IPv6 addresses. Another
interpretation of 32-bits SID could be as a complementary element
of an IPv4/IPv6 prefix. The setting of the interpretation might
be done through the control plane based signaling and is outside
the scope of this document. If this SID represents a
complementary part of an IPv4/IPv6 prefix, the original IP address
can be re-constructed by using, for example, mapping, stitching,
shifting or translating operation. Specification of such a
mechanism is outside the scope of this document.
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0b10 - 32-bits SID, which includes an MPLS label in the leftmost
20-bits as displayed in Figure 2. Information in the Context
field could be interpreted as a flavor of a particular network
programming behavior. Specification of the network programming
using this type of U-SID is outside the scope of this document.
[Ed.note. Replace with a reference to the U-SID network
programming document.]
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPLS Label | Context |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format of Unified SID with MPLS Label
0b11 - Reserved for future use.
This document also introduce a compatible operation on Segment Left
field, also termed as SRH.SL. That is, if SRH.UET Flag is 0b00,
SRH.SL represents the count of 128bits-SID items in SRH, if SRH.UET
Flag is 0b01 or 0b10, SRH.SL represents the count of 32bits-SID items
in SRH.
4. The Use Case of Unified Segment Identifier
U-SID can be used for interworking between SR-MPLS and SRv6 domains.
SR-MPLS is often used in a metro network, for example, in the
backhaul metro network of CMCC. If the core network uses SRv6, for
example, the core network of the same operator, U-SID can be used in
the SRv6 domain to interwork with SR-MPLS in the metro network to
form an end-to-end tunnel.
4.1. Interworking Between SR-MPLS and SRv6 Using U-SID
SR-MPLS uses SR SIDs as MPLS label in MPLS stack, and the SIDs are
32-bits long. SRv6 uses SR SIDs as IPv6 extension header in SRH, and
the SIDs are 128-bits long.
The U-SID uses the same 32-bits long SIDs in MPLS stack and SRH.
Thus, four 32-bits long U-SIDs can be placed in the space of a single
128-bits long header. The encapsulation is illustrated in Figure 3.
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+---------+ +----------------------------------+
| | | IPv6 header |
| Ethernet| +----------------------------------+
| | | SRH |
+---------+ +----------------------------------+
| USID1 | | USID1 | USID2 | ... | USID4 |
+---------+ +----------------------------------+
| USID2 | | USID5 |... | USIDn | Null |
+---------+ +----------------------------------+
| ... | + Payload |
+---------+ +----------------------------------+
| USIDn |
+---------+
| Payload |
+---------+
Figure 3: 32-bits long U-SIDs Encapsulation
The SR-MPLS and SRv6 interworking is illustrated in Figure 4. An
end-to-end SR tunnel from A to F crosses the SR-MPLS and SRv6
domains. The SR-MPLS domain could be using IPv4 or IPv6 address
family. The SRv6 border nodes (E/G) receive SR-MPLS packets and
forward them into the SRv6 domain using an SR-MPLS Binding SID
[RFC8660].
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+-----+ +-----+ +-----+ +-----+
| A +-----------+ B +-----------+ E +-----------+ F |
+-----+ +--+--+ +--+--+ +--+--+
| SR-MPLS | | SRv6 |
| | | |
+-----+ +--+--+ +--+--+ +--+--+
| C |-----------| D +-----------+ G +-----------+ H |
+-----+ +-----+ +-----+ +-----+
+--------------+
| Eth(E->G) |
+--------------+ +--------------+
| Eth(A->B) | |IPv6 DA:G.intf|
+--------------+ +--------------+ +--------------+
| USID(B) | | Eth(B->E) | |SRH |
+--------------+ +--------------+ |NH:MPLS SL:2|
| USID(E1) | | USID(E1) | |USID(ADJ E->G)|
+--------------+ +--------------+ |USID(ADJ G->H)|
| USID(E2) | | USID(E2) | |USID(ADJ H->F)|
+--------------+ +--------------+ +--------------+
|Label(service)| |Label(service)| |Label(service)|
+--------------+ +--------------+ +--------------+
| Payload | -> | Payload | -> | Payload |
+--------------+ +--------------+ +--------------+
Figure 4: SR-MPLS and SRv6 interworking
The SRv6 edge node E assigns two SIDs, e.g., E1 and E2, E1 is an SR-
MPLS Node-SID, E2 is an SR-MPLS Binding-SID, which represents an SRv6
policy (from E to F, via segment list E-G-H-F) with U-SID
encapsulation. At the headend A, the end-to-end segment list could
be B-E1-E2. Figure 6 demonstrates an example of the packet
forwarding, where U-SID is an MPLS label.
The reverse interworking is illustrated in Figure 5. An end-to-end
SR tunnel from F to A crosses the SRv6 and SR-MPLS domains. The SRv6
border nodes (E/G) receive SRv6 packets and forward them into the SR-
MPLS domain using an SR-MPLS Binding SID or normal Prefix/Adjacency
SID.
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+-----+ +-----+ +-----+ +-----+
| A +-----------+ B +-----------+ E +-----------+ F |
+-----+ +--+--+ +--+--+ +--+--+
| SR-MPLS | | SRv6 |
| | | |
+-----+ +--+--+ +--+--+ +--+--+
| C |-----------| D +-----------+ G +-----------+ H |
+-----+ +-----+ +-----+ +-----+
+--------------+
| Eth(F->H) |
+--------------+
|IPv6 DA:H.intf|
+--------------+
|SRH |
|NH:MPLS SL:2|
|USID(ADJ F->H)|
+--------------+ |USID(ADJ H->G)|
| Eth(E->B) | |USID(ADJ G->E)|
+--------------+ +--------------+ +--------------+
| Eth(B->A) | | USID(B) | | USID(B) |
+--------------+ +--------------+ +--------------+
| USID(A) | | USID(A) | | USID(A) |
+--------------+ +--------------+ +--------------+
|Label(service)| |Label(service)| |Label(service)|
+--------------+ +--------------+ +--------------+
| Payload | <- | Payload | <- | Payload |
+--------------+ +--------------+ +--------------+
Figure 5: SR-MPLS and SRv6 reverse interworking
The SRv6 edge node F assigns an SR-MPLS Binding-SID F2, which
represents an SRv6 policy (from F to E, via segment list F-H-G-E)
with U-SID encapsulation. At the headend F, the end-to-end segment
list could be F2-B-A.
5. Operations with Unified Segment Identifier
When SRH is used to include 32-bits long U-SIDs, the ingress and
transit nodes of an SR tunnel act as described in Section 5.1 and
Section 5.2 of [I-D.ietf-6man-segment-routing-header] respectively.
If U-SID is used to support interworking between SR-MPLS and SRv6
domains, it is beneficial that U-SID type matches to an MPLS label.
In that case, an ILM (Incoming Label Map) entry can be used to map a
U-SID to an IPv6 address. As a result, it is not necessary to
introduce a new type of index-based mapping table. For ILM entry of
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Adjacency-SID, the mapping result copied to DA (Destination Address)
is the remote interface IPv6 address, for ILM entry of Node-SID, the
mapping result that is copied into DA is a remote node loopback IPv6
address.
Operations on an MPLS label of U-SID type are the same as those
defined in [RFC8663]. However, SR-MPLS over SRH has the following
advantages compared with SR-MPLS over UDP:
o SRH is flexible to extend flags or sub-TLVs for service
requirements, but UDP not.
o Labels in SRH can meet 8 bytes alignment requirements as per
[RFC8200], but UDP not.
o The source address of the SR policy is not discarded, but UDP not.
5.1. Procedures of SR-MPLS over IP
Procedures of SR-MPLS over IP of [RFC8663] described how to construct
an adjusted SR-MPLS FTN (FEC-to-NHLFE map) and ILM entry towards a
prefix-SID when next-hops are IP-only routers. The action of FTN and
ILM entry will steer the packet along an outer tunnel to the
destination node that has originated the FEC (Forwarding Equivalence
Class). UDP header is removed and put again at the each segment
endpoint. However, for SR-MPLS over SRH in this document we don't
try to depend on that adjusted FIB (Forwarding Information Base)
entry, because there are not any actions needed to get from the FIB
entry, a traditional ILM entry (maybe without out-label because of
IP-only next-hop) is enough to get the FEC information, i.e., to map
a U-SID to an IPv6 address and copy to DA. Note that an
implementation can get both FEC and next-hop/interface forwarding
information from the ILM entry, to avoid extra FIB lookup. An SRv6
policy chosen to encapsulate U-SID list within SRH is determined at
the ingress node of this SRv6 policy, SRH is preserved along the SR
to egress, though PSP (Penultimate Segment Popping) may be used, that
is different from SR-MPLS over IP/UDP method [RFC8663], so the source
address (i.e., the ingress of the SRv6 policy) is not discarded.
5.2. Packet Forwarding
U-SID based packet forwarding is similar to the processing described
in [RFC8663]. But it differs from that in FIB action and segment
list processing. For completeness, we repeat the description of
[RFC8663] with modification as follows.
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+-----+ +-----+ +-----+ +-----+ +-----+
| A +-------+ B +-------+ C +--------+ D +--------+ H |
+-----+ +--+--+ +--+--+ +--+--+ +-----+
| | |
| | |
+--+--+ +--+--+ +--+--+
| E +-------+ F +--------+ G |
+-----+ +-----+ +-----+
+--------+ +--------+ +--------+
|IP(A->E)| |IP(A->G)| |IP(A->G)|
+--------+ +--------+ +--------+
|SRH | |SRH | |SRH |(or PSP)
| SL:2 | | SL:1 | | SL:0 |
| L(E) | | L(E) | | L(E) |
| L(G) | | L(G) | | L(G) |
| L(H) | | L(H) | | L(H) |
+--------+ +--------+ +--------+
| Packet | ---> | Packet | ---> | Packet |
+--------+ +--------+ +--------+
Figure 6: Packet Forwarding Example
In the example shown in Figure 6, assume that routers A, E, G, and H
are U-SID capable (i.e., both SR-MPLS and SRv6 capable ) while the
remaining routers (B, C, D, and F) are only capable of forwarding IP
packets. Routers A, E, G, and H advertise their Segment Routing
related information via IS-IS or OSPF.
Now assume that router A (the Domain ingress) wants to send a packet
to router H (the Domain egress) via an SRv6 policy with the explicit
path {E->G->H}. Router A will impose an MPLS label stack within SRH
on the packet that corresponds to that explicit path. Router A
searches ILM entry by the top label (that indicated router E), get
the FEC information and next-hop/interface forwarding information, a
loopback IPv6 address of E, and then copy to DA and sends the packet.
The value of SRH.SL is 2.
When the IPv6 packet arrives at router E, router E picks the next
segment (label) within SRH based on the SRH.SL value of 2, searches
ILM entry by the next label, get the FEC information and next-hop/
interface forwarding information, a loopback IPv6 address of G, and
then copy to DA and sends the packet. The value of SRH.SL is 1.
When the IPv6 packet arrives at router G, router G gets the next
segment (label) within SRH based on the SRH.SL value of 1, looks up
ILM entry by the next label, gets the FEC information and next-hop/
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interface forwarding information, a loopback IPv6 address of H, and
then copies it to IP DA and transmits the packet. Because the value
of SRH.SL is 0, the SRH can be removed if the behavior flavor
codepoint of next segment (label) is set to PSP.
6. Interworking Between Different UET domain Considerations
6.1. Interworking Using Binding SID
In Figure 4 and Figure 5, We have seen a simple interworking solution
based on Binding SID.
This document RECOMMOND this method for interworking, because Binding
SID is very simple to hide the difference U-SID type of different
domain. Especially, a headend only with classical SRv6 SRH
encapsulation capability, i.e., no capability to put multiple short
U-SIDs to a single 128bits item, will not need to upgrade.
Altough Binding SID that is allocated for the specific SR policy
instance will bring more states on some domain boder nodes, the SR
policy instance itself maybe pre-exist due to other requirements.
The SR policy is created within each UET domain which is upgraded
separately.
In order to do interwork, as mentioned before, an MPLS Binding SID
could be allocated for an SRv6 policy, used to hide the details of
UET-0b00 domain (classical SRv6) for a traditional MPLS Label stack.
Similarly, an SRv6 Binding SID could be allocated for an SR-MPLS
policy, used to hide the details of UET-0b10 domain for a traditional
SRv6 SRH. And, an SRv6 Binding SID allocated for an SRv6 policy
which enable UET-0b01 compression style, will hide the details of
UET-0b01 domain for a traditional SRv6 SRH. There maybe other
combinations, and not list one by one.
Note that in some cases, Binding SID will cause multiple SRH to be
inserted in IPv6 header.
6.2. Interworking Using Mixing U-SID
U-SRH can also provide an alternate interworking scheme to support an
end-to-end SR tunnel or policy using mixing type of U-SIDs, if more
headend nodes have be upgraded to support encapsulating mixing U-SID
in SRH. For example, an SID list could contain some 128bits-SIDs,
some 32bits-SIDs, some 32bits-Labels. For this purpose, we need
explicitly define U-SID types, in other words, the type is just UET.
The interworking of different UET domain is illustrated in Figure 7.
An end-to-end SR tunnel or policy from S to D with segment list <X,
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ABR1, Y, ABR2, Z, D>, crosses the UET-0b00 domain, UET-0b01 domain
and UET-0b10 domain. Note that any order of UET domains are also
possible and similar with the following illustration.
..................... .................. ...................
: : : : : :
+----+ +----+ +----+ +----+ +----+ +----+ +----+
| S +-----+ X +-----+ABR1+-----+ Y +-----+ABR2+-----+ Y +-----+ D |
+----+ +----+ +----+ +----+ +----+ +----+ +----+
: : : : : :
......UET(0b00)...... .....UET(0b01).... .....UET(0b10).....
Figure 7: Interworking between different UET SID
6.2.1. UET capability Advertisement
In SRv6 network, each node can configure its UET capability, and
advertise it to other nodes. Controller can also collect UET
capability information of all nodes. Typical UET capability is:
UET-0: Support classical 128bits SRv6 SID.
UET-1: Support shorter 32bits IPv4 address or number.
UET-2: Support shorter 32bits MPLS label.
UET-3: Support shorter 16bits number.
Each node can support one or more than one UET capabilitys. Refer to
Figure 7, node S/X/ABR1 can configure to support UET-0 capability,
node ABR1/Y/ABR2 can configure to support UET-1 capability, and node
ABR2/Y/D can configure to support UET-2 capability.
6.2.2. SRv6 SID Allocated per UET
An SRv6 SID is allocated per UET capability. In control plane, an
SRv6 SID is always 128bits, however:
An SRv6 SID that has UET-0 attribute means it is allocated for
traditional 128bits encapsulation purpose, that means in SRH the
next SID is also a 128bits SID.
An SRv6 SID that has UET-1 attribute means it is allocated for
32bits IPv4 encapsulation purpose, that means in SRH the next SID
is a 32bits IPv4 address.
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An SRv6 SID that has UET-2 attribute means it is allocated for
32bits MPLS encapsulation purpose, that means in SRH the next SID
is a 32bits MPLS Label.
An SRv6 SID that has UET-3 attribute means it is allocated for
16bits number encapsulation purpose, that means in SRH the next
SID is a 16bits number.
For the local SID entry of each SRv6 SID allocated per UET
capability, it will explicitly give the UET attribute information.
Each node allocate its SRv6 SID per UET capability, and advertise it
to other nodes with additional UET-Flavor. Controller can also
collect these SIDs used for E2E SID List programming.
In order to save label resources, MPLS label is not allocated per
UET. The UET information can be directly inserted in the context
field of the label item in SRH.
For example, refer to Figure 7, each node allocate the following SRv6
SID per UET.
Node S: 128bits-END-SID-S for UET-0.
NOde X: 128bits-END-SID-X for UET-0.
NOde ABR1: 128bits-END-SID-ABR1 for UET-0, and 128bits-END-SID-
ABR1' for UET-1.
NOde Y: 128bits-END-SID-Y for UET-1.
NOde ABR2: 128bits-END-SID-ABR2 for UET-1, and 128bits-END-SID-
ABR2' for UET-2.
NOde Z: 32bits-PREFIX-SID-Z. Note that MPLS Label allocation is
independent with UET.
NOde D: 32bits-PREFIX-SID-D. Note that MPLS Label allocation is
independent with UET.
Note that the above SRv6 SID itself is always a 128bits IPv6 address,
no relationship with its UET attribute. The UET attribute indicate
the next SID type, i.e., 128bits classical SID, 32bits IPv4 address,
or 32bits MPLS Label, etc.
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6.2.3. Packets Forwarding Procedures
Controller compute an E2E segment list <X, ABR1, Y, ABR2, Z, D>.
Controller knows that ABR1 and ABR2 are the border nodes between
different UET domain for the above segment list.
So, the SID list informed to headend is:
No.1 SID: 128bits-END-SID-X (with UET-0 Flag, and BL|NBL info)
No.2 SID: 128bits-END-SID-ABR1' (with UET-1 Flag, and BL|NBL info)
No.3 SID: 128bits-END-SID-Y (with UET-1 Flag, and BL|NBL info)
No.4 SID: 128bits-END-SID-ABR2' (with UET-2 Flag, and BL|NBL info)
No.5 SID: 32bits-PREFIX-SID-Z, (with UET-2 Flag)
No.6 SID: 32bits-PREFIX-SID-D, (with UET-0 Flag)
Note: BL is Block Length, NBL is non-Block Length.
The headend analysis how to get the compressed SID List.
The No.1 SID, 128bits-END-SID-X, has UET-0 Flag, so keep 128bits.
The No.1 SID, 128bits-END-SID-X, has UET-0 Flag, that means the next
SID, 128bits-END-SID-ABR1', need also keep 128bits.
The No.2 SID, 128bits-END-SID-ABR1' has UET-1 Flag, that means the
next SID, 128bits-END-SID-Y, need to be compressed as 32bits IPv4
address.
The No.3 SID, 128bits-END-SID-Y, has UET-1 Flag, that means the next
SID, 128bits-END-SID-ABR2', need to be compressed as 32bits IPv4
address.
The No.4 SID, 128bits-END-SID-ABR2', has UET-2 Flag, that means the
next SID, 32bits-PREFIX-SID-Z, need keep 32bits.
The No.5 SID, 32bits-PREFIX-SID-Z, has UET-2 Flag, that means the
next SID, 32bits-PREFIX-SID-D, need keep 32bits.
The No.6 SID, 32bits-PREFIX-SID-D, has UET-0 Flag, that means the
next SID, maybe a VPN service SRv6 SID, need keep 128bits.
So, the headend can get the following compressed SID List:
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128bits-END-SID-X for UET-0
128bits-END-SID-ABR1' for UET-1
32bits of 128bits-END-SID-Y for UET-1
32bits of 128bits-END-SID-ABR2' for UET-2
32bits-PREFIX-SID-Z (with UET-2 in context.field)
32bits-PREFIX-SID-D (with UET-0 in context.field)
At headend, the encapsulated SRH could be:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last Entry | Flags |UET| | Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ 128bits VPN-SID ~ [0]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
| 32bits-PREFIX-SID-D (with UET-0 in context.field) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 32bits-PREFIX-SID-Z (with UET-2 in context.field) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ [1]
| 32bits of 128bits-END-SID-ABR2' for UET-2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 32bits of 128bits-END-SID-Y for UET-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
~ 128bits-END-SID-ABR1' for UET-1 ~ [2]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
~ 128bits-END-SID-X for UET-0 ~ [3]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// //
// Optional Type Length Value objects (variable) //
// //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: SRH including different UET SID
The initial SRH.SL is set to 4, that is the count of 128bits based
SIDs in SRH, and the initial SRH.UET is set to 0b00, that represent
the first UET domain.
During the process of packets passing through multiple UET domains,
if SRH.UET change from 0b00 to 0b01 or 0b10, SRH.SL will quadruple,
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i.e., SRH.SL = SRH.SL * 4, that is the count of 32bits based SIDs in
SRH. When SRH.UET changed from 0b01 or 0b10 to 0b00, SRH.SL will
revert to its original size, i.e., SRH.SL = SRH.SL / 4, that is the
count of 128bits based SIDs in SRH.
Refer to Figure 7, next we will describe the process of packets
passing through each UET domain.
At the headend S, when packets sent to the No.1 segment node X, it
will decrement SRH.SL by 1, get the first 128bits SID from
SRH.List[], 128bits-END-SID-X for UET-0, copy to DA, and lookup FIB
to send packets. At this time, SRH.SL is 3, SRH.UET is 0b00.
At the No.1 segment node X, the local SID matched by DA has UET-0
attribute, that is, SRH.UET has no change. It will continue to
decrement SRH.SL by 1, get the next 128bits SID from SRH.List[],
128bits-END-SID-ABR1' for UET-1, copy to DA, and lookup FIB to send
packets. At this time, SRH.SL is 2, SRH.UET is 0b00.
At the No.2 segment node ABR1, the local SID matched by DA has UET-1
attribute, that is, SRH.UET has changed from UET-0 to UET-1. It will
firstly let SRH.SL * 4, then decrement SRH.SL by 1, get the next
32bits SID from SRH.List[], 32bits of 128bits-END-SID-Y for UET-1,
convert it to a complete IPv6 SID, copy to DA, and lookup FIB to send
packets. At this time, SRH.SL is 7, SRH.UET is 0b01.
At the No.3 segment node Y, the local SID matched by DA has UET-1
attribute, that is, SRH.UET has no change. It will continue to
decrement SRH.SL by 1, get the next 32bits SID from SRH.List[],
32bits of 128bits-END-SID-ABR2' for UET-2, convert it to a complete
IPv6 SID, copy to DA, and lookup FIB to send packets. At this time,
SRH.SL is 6, SRH.UET is 0b01.
At the No.4 segment node ABR2, the local SID matched by DA has UET-2
attribute, that is, SRH.UET has changed from UET-1 to UET-2. Because
the size of SID has no change, it will continue to decrement SRH.SL
by 1, get the next 32bits SID from SRH.List[], 32bits-PREFIX-SID-Z
(with UET-2 in context.field), map it to a complete IPv6 prefix FEC
by ILM entry, copy to DA, and lookup FIB (or directly get forwarding
information from ILM entry) to send packets. Note that the UET
information in context.field need update to SRH.UET again and see if
it changes, no change at this time, so there is no additional
processing. At this time, SRH.SL is 5, SRH.UET is 0b02.
At the No.5 segment node Z, the normal address route entry matched by
DA has not any further UET attribute, that is, SRH.UET has no change.
It will continue to decrement SRH.SL by 1, get the next 32bits SID
from SRH.List[], 32bits-PREFIX-SID-D (with UET-0 in context.field),
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map it to a complete IPv6 prefix FEC by ILM entry, copy to DA, and
lookup FIB (or directly get forwarding information from ILM entry) to
send packets. Note that the UET information in context.field need
update to SRH.UET again and see if it changes, it is changed from
UET-2 to UET-0, so SRH.SL will be revert to its original size, i.e.,
let SRH.SL / 4. At this time, SRH.SL is 1, SRH.UET is 0b00.
At the No.6 segment node D, the normal address route entry matched by
DA has not any further UET attribute, that is, SRH.UET has no change.
It will continue to decrement SRH.SL by 1, get the next 128bits SID
from SRH.List[], 128bits VPN-SID, and follow the rest process
described in [I-D.ietf-spring-srv6-network-programming].
6.2.4. SRH with mixing elements Pseudo-code
Processing of SRH with mixing elements is demonstrated in the
following pseudo-code:
Headend sending packet:
S01. set initial SRH.UET, respond to the UET type of the first SID;
S02. set initial SRH.SL, it is the count of 128bits-based SID;
S03. if (SRH.UET == 0b00) {
S04. SRH.SL --;
S05. Get SRH.List[SRH.SL], 128bits, copy to IPv6 Header DA; Or,
headend know the first SID before SRH encapsulation,
just copy it to DA.
S06. FIB lookup according to DA, and forward packet;
S07. }
S08. else if (SRH.UET == 0b01) {
S09. SRH.SL = SRH.SL * 4;
S10. SRH.SL --;
S11. Get SRH.List[SRH.SL], 32bits, convert to 128bits SRv6 SID, copy
to IPv6 Header DA; Or, headend know the first SID before SRH
encapsulation, just copy it to DA;
S12. FIB lookup according to DA, and forward packet;
S13. }
S14. else if (SRH.UET == 0b10) {
S15. SRH.SL = SRH.SL * 4;
S16. SRH.SL --;
S17. Get SRH.List[SRH.SL], 32bits, lookup ILM entry and map it to 128
IPv6 address, copy it to IPv6 Header DA; Or, headend know
the first SID before SRH encapsulation, just copy it to DA;
S18. FIB lookup according to DA, or, directly get forwarding information
from ILM entry, and forward packet;
S19. }
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Transit/Egress receive packets:
S01. If DA matched local SID entry, copy the UET attr of local SID entry
to SRH.UET, check when SRH.UET changed from 0b00 to 0b01 or 0b10,
SRH.SL*4, when from 0b01 or 0b10 to 0b00, SRH.SL / 4;
Else If DA matched normal address route entry, SRH.UET no update;
S02. if (SRH.SL == 0) {
S03. process the inner payload;
S04. }
S05. else {
S06. if (SRH.UET == 0b00) {
S07. SRH.SL -- ;
S08. Get SRH.List[SRH.SL], 128bits, copy it to IPv6 Header DA;
S09. FIB lookup according to DA, and forward packet;
S10. }
S11. else if (SRH.UET == 0b01) {
S12. SRH.SL -- ;
S13. Get SRH.List[SRH.SL], 32bits, convert to 128bits SRv6 SID, copy
to IPv6 Header DA;
S14. FIB lookup according to DA, and forward packet;
S15. }
S16. else if (SRH.UET == 0b10) {
S17. SRH.SL --
S18. Get SRH.List[SRH.SL], 32bits, lookup ILM entry, map it to
128bits IPv6 address, copy it to IPv6 Header DA;
S19. Get UET info from SRH.List[SRH.SL] Context Field, copy it to
SRH.UET. Check if SRH.UET changed from 0b10 to 0b00, SRH.SL/4;
S20. FIB lookup according to DA, or, directly get forwarding
information from ILM entry, and forward packet;
S21. }
S22. }
7. Control Plane in Support of Unified SID
The introduction of the Unified Identifier may rely on the existing
SR extensions to the routing protocols. But some enhancements in the
control plane are still required. This section references to the
existing protocols and identifies necessary extensions.
Each node in the SRv6 domain need advertise its U-SID Encapsulation
capability, this information can be carried within SRv6-Capabilities
sub-TLV defined in [I-D.ietf-lsr-isis-srv6-extensions] and SRv6
Capabilities TLV defined in [I-D.ietf-lsr-ospfv3-srv6-extensions].
It need also allocate SRv6 SID (Topology type and Service Function
type) per UET and advertise to other nodes, the advertisement of SRv6
END SID, END.X SID, LAN END.X SID defined in
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[I-D.ietf-lsr-isis-srv6-extensions] and
[I-D.ietf-lsr-ospfv3-srv6-extensions] need to be extended to carry
UET-Flavor information. These information can be collected and sent
to central controller through BGP-LS. Then, controller can send the
computed segment list to the headend through BGP or PCEP, and each
segment will include an explicit UET information. All these will be
defined in other documents.
The SR-MPLS extensions to Interior Gateway Protocols (IGP), IS-IS
[RFC8667], OSPF [RFC8665], and OSPFv3 [RFC8666], defined how 20-bits
and 32-bits SIDs advertised and bound to SR objects and/or
instructions. Extensions to BGP Link-state address family
[I-D.ietf-idr-bgp-ls-segment-routing-ext] enabled propagation of
segment information of variable length via BGP. The existed SR-MPLS
extensions can be used to get MPLS U-SID mapping FIB entry, and it
can coexist with SRv6 extensions to the same IGP/BGP-LS instance.
For simplicity purpose, tihs document suggest to use the existed
mature SR-MPLS control plane and FIB entry to server the MPLS U-SID
advertisement and mapping entry. However, it is possible to based on
SRv6 related TLVs/sub-TLVs to advertise the MPLS U-SID, and that will
be discussed in another document.
8. U-SID supporting SRv6 programming
U-SID can support SRv6 programming defined by
[I-D.ietf-spring-srv6-network-programming]. The details will be
described in another document.
9. Implementation Considerations
The Unified SID solution has been already implemented and tested by
two companies:
o Centec has conducted its PoC, and the report is available at
https://cloud.tencent.com/developer/article/1540023.
o Broadcom, in its lab, also conducted PoC testing of the U-SID
solution.
10. IANA Considerations
IANA is requested to allocate from the Segment Routing Header Flags
registry the two-bits long field referred to as Size.
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11. Security Considerations
This specification inherits all security considerations of [RFC8402]
and [I-D.ietf-6man-segment-routing-header].
12. Contributors
Wan Xiaolan
New H3C Technologies Co. Ltd
No.8, Yongjia Road, Haidian District
Beijing
China
Email: wxlan@h3c.com
Cheng Wei
Centec
Building B, No.5 Xing Han Street, Suzhou Industrial Park
Suzhou
China
Email: Chengw@centecnetworks.com
S.Zadok
Broadcom
Israel
Email: shay.zadok@broadcom.com
13. Acknowledgements
TBD
14. Normative References
[]
Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", draft-ietf-6man-segment-routing-header-26 (work in
progress), October 2019.
[I-D.ietf-idr-bgp-ls-segment-routing-ext]
Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
and M. Chen, "BGP Link-State extensions for Segment
Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16
(work in progress), June 2019.
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[I-D.ietf-lsr-isis-srv6-extensions]
Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
Z. Hu, "IS-IS Extension to Support Segment Routing over
IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-08
(work in progress), April 2020.
[I-D.ietf-lsr-ospfv3-srv6-extensions]
Li, Z., Hu, Z., Cheng, D., Talaulikar, K., and P. Psenak,
"OSPFv3 Extensions for SRv6", draft-ietf-lsr-
ospfv3-srv6-extensions-00 (work in progress), February
2020.
[I-D.ietf-spring-srv6-network-programming]
Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
Matsushima, S., and Z. Li, "SRv6 Network Programming",
draft-ietf-spring-srv6-network-programming-16 (work in
progress), June 2020.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
[RFC8663] Xu, X., Bryant, S., Farrel, A., Hassan, S., Henderickx,
W., and Z. Li, "MPLS Segment Routing over IP", RFC 8663,
DOI 10.17487/RFC8663, December 2019,
<https://www.rfc-editor.org/info/rfc8663>.
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[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>.
[RFC8666] Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions
for Segment Routing", RFC 8666, DOI 10.17487/RFC8666,
December 2019, <https://www.rfc-editor.org/info/rfc8666>.
[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, December 2019,
<https://www.rfc-editor.org/info/rfc8667>.
Authors' Addresses
Cheng Weiqiang
China Mobile
Beijing
China
Email: chengweiqiang@chinamobile.com
Greg Mirsky
ZTE Corp.
Email: gregimirsky@gmail.com
Peng Shaofu
ZTE Corporation
No.50 Software Avenue, Yuhuatai District
Nanjing
China
Email: peng.shaofu@zte.com.cn
Liu Aihua
ZTE Corporation
Zhongxing Industrial Park, Nanshan District
Shenzhen
China
Email: liu.aihua@zte.com.cn
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Gyan S. Mishra
Verizon Inc.
13101 Columbia Pike
Silver Spring MD 20904
United States of America
Phone: 301 502-1347
Email: gyan.s.mishra@verizon.com
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