SPRING F. Clad, Ed.
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track X. Xu, Ed.
Expires: March 12, 2021 Alibaba
C. Filsfils
Cisco Systems, Inc.
D. Bernier
Bell Canada
C. Li
Huawei
B. Decraene
Orange
S. Ma
Mellanox
C. Yadlapalli
AT&T
W. Henderickx
Nokia
S. Salsano
Universita di Roma "Tor Vergata"
September 08, 2020
Service Programming with Segment Routing
draft-ietf-spring-sr-service-programming-03
Abstract
This document defines data plane functionality required to implement
service segments and achieve service programming in SR-enabled MPLS
and IPv6 networks, as described in the Segment Routing architecture.
Status of This Memo
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This Internet-Draft will expire on March 12, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Classification and Steering . . . . . . . . . . . . . . . . . 4
4. Service Segments . . . . . . . . . . . . . . . . . . . . . . 5
4.1. SR-Aware Services . . . . . . . . . . . . . . . . . . . . 5
4.2. SR-Unaware Services . . . . . . . . . . . . . . . . . . . 6
5. SR Service Policies . . . . . . . . . . . . . . . . . . . . . 7
5.1. SR-MPLS Data Plane . . . . . . . . . . . . . . . . . . . 8
5.2. SRv6 Data Plane . . . . . . . . . . . . . . . . . . . . . 10
6. SR Proxy Behaviors . . . . . . . . . . . . . . . . . . . . . 11
6.1. Static SR Proxy . . . . . . . . . . . . . . . . . . . . . 14
6.1.1. SR-MPLS Pseudocode . . . . . . . . . . . . . . . . . 16
6.1.2. SRv6 Pseudocode . . . . . . . . . . . . . . . . . . . 17
6.2. Dynamic SR Proxy . . . . . . . . . . . . . . . . . . . . 23
6.2.1. SR-MPLS Pseudocode . . . . . . . . . . . . . . . . . 24
6.2.2. SRv6 Pseudocode . . . . . . . . . . . . . . . . . . . 24
6.3. Shared Memory SR Proxy . . . . . . . . . . . . . . . . . 25
6.4. Masquerading SR Proxy . . . . . . . . . . . . . . . . . . 25
6.4.1. SRv6 Masquerading Proxy Pseudocode . . . . . . . . . 27
6.4.2. Destination NAT Flavor . . . . . . . . . . . . . . . 28
6.4.3. Caching Flavor . . . . . . . . . . . . . . . . . . . 28
7. Metadata . . . . . . . . . . . . . . . . . . . . . . . . . . 29
7.1. MPLS Data Plane . . . . . . . . . . . . . . . . . . . . . 29
7.2. IPv6 Data Plane . . . . . . . . . . . . . . . . . . . . . 30
7.2.1. SRH TLV Objects . . . . . . . . . . . . . . . . . . . 30
7.2.2. SRH Tag . . . . . . . . . . . . . . . . . . . . . . . 31
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 31
8.1. SR-Aware Services . . . . . . . . . . . . . . . . . . . . 32
8.2. Proxy Behaviors . . . . . . . . . . . . . . . . . . . . . 32
9. Related Works . . . . . . . . . . . . . . . . . . . . . . . . 32
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
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10.1. SRv6 Endpoint Behaviors . . . . . . . . . . . . . . . . 33
10.2. Segment Routing Header TLVs . . . . . . . . . . . . . . 33
11. Security Considerations . . . . . . . . . . . . . . . . . . . 33
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 34
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
14.1. Normative References . . . . . . . . . . . . . . . . . . 35
14.2. Informative References . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
1. Introduction
Segment Routing (SR) [RFC8402] is an architecture based on the source
routing paradigm that seeks the right balance between distributed
intelligence and centralized programmability. SR can be used with an
MPLS or an IPv6 data plane to steer packets through an ordered list
of instructions, called segments. These segments may encode simple
routing instructions for forwarding packets along a specific network
path, but also steer them through Virtual Network Functions (VNFs) or
physical service appliances available in the network.
In an SR network, each of these services, running either on a
physical appliance or in a virtual environment, are associated with a
segment identifier (SID). These service SIDs are then leveraged as
part of a SID-list to steer packets through the corresponding
services. Service SIDs may be combined together in a SID-list to
achieve service programming, but also with other types of segments as
defined in [RFC8402]. SR thus provides a fully integrated solution
for overlay, underlay and service programming. Furthermore, the IPv6
instantiation of SR (SRv6) [I-D.ietf-spring-srv6-network-programming]
supports metadata transportation in the Segment Routing Header
[I-D.ietf-6man-segment-routing-header], either natively in the tag
field or with extensions such as TLVs.
This document describes how a service can be associated with a SID,
including legacy services with no SR capabilities, and how these
service SIDs are integrated within an SR policy. The definition of
an SR Policy and the traffic steering mechanisms are covered in
[I-D.ietf-spring-segment-routing-policy] and hence outside the scope
of this document.
The definition of control plane components, such as service segment
discovery, is outside the scope of this data plane document. For
reference, the option of using BGP extensions to support SR service
programming is proposed in [I-D.dawra-idr-bgp-sr-service-chaining].
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2. Terminology
This document leverages the terminology proposed in [RFC8402],
[RFC8660], [I-D.ietf-6man-segment-routing-header],
[I-D.ietf-spring-srv6-network-programming] and
[I-D.ietf-spring-segment-routing-policy]. It also introduces the
following new terms.
Service segment: A segment associated with a service. The service
may either run on a physical appliance or in a virtual environment
such as a virtual machine or container.
SR-aware service: A service that is fully capable of processing SR
traffic. An SR-aware service can be directly associated with a
service segment.
SR-unaware service: A service that is unable to process SR traffic or
may behave incorrectly due to presence of SR information in the
packet headers. An SR-unaware service can be associated with a
service segment through an SR proxy function.
3. Classification and Steering
Classification and steering mechanisms are defined in section 8 of
[I-D.ietf-spring-segment-routing-policy] and are independent from the
purpose of the SR policy. From the perspective of a headend node
classifying and steering traffic into an SR policy, there is no
difference whether this policy contains IGP, BGP, peering, VPN or
service segments, or any combination of these.
As documented in the above reference, traffic is classified when
entering an SR domain. The SR policy headend may, depending on its
capabilities, classify the packets on a per-destination basis, via
simple FIB entries, or apply more complex policy routing rules
requiring to look deeper into the packet. These rules are expected
to support basic policy routing such as 5-tuple matching. In
addition, the IPv6 SRH tag field defined in
[I-D.ietf-6man-segment-routing-header] can be used to identify and
classify packets sharing the same set of properties. Classified
traffic is then steered into the appropriate SR policy and forwarded
as per the SID-list(s) of the active candidate path.
SR traffic can be re-classified by an SR endpoint along the original
SR policy (e.g., DPI service) or a transit node intercepting the
traffic. This node is the head-end of a new SR policy that is
imposed onto the packet, either as a stack of MPLS labels or as an
IPv6 SRH.
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4. Service Segments
In the context of this document, the term service refers to a
physical appliance running on dedicated hardware, a virtualized
service inside an isolated environment such as a Virtual Machine
(VM), container or namespace, or any process running on a compute
element. A service may also comprise multiple sub-components running
in different processes or containers. Unless otherwise stated, this
document does not make any assumption on the type or execution
environment of a service.
The execution of a service can be integrated as part of an SR policy
by assigning a segment identifier, or SID, to the service and
including this service SID in the SR policy SID-list. Such a service
SID may be of local or global significance. In the former case,
other segments, such as prefix or adjacency segments, can be used to
steer the traffic up to the node where the service segment is
instantiated. In the latter case, the service is directly reachable
from anywhere in the routing domain. This is realized with SR-MPLS
by assigning a SID from the global label block
([I-D.ietf-spring-segment-routing-mpls]), or with SRv6 by advertising
the SID locator in the routing protocol
([I-D.ietf-spring-srv6-network-programming]). It is up to the
network operator to define the scope and reachability of each service
SID. This decision can be based on various considerations such as
infrastructure dynamicity, available control plane or orchestration
system capabilities.
This document categorizes services in two types, depending on whether
they are able to behave properly in the presence of SR information or
not. These are respectively named SR-aware and SR-unaware services.
4.1. SR-Aware Services
An SR-aware service can process the SR information in the packets it
receives. This means being able to identify the active segment as a
local instruction and move forward in the segment list, but also that
the service's own behavior is not hindered due to the presence of SR
information. For example, an SR-aware firewall filtering SRv6
traffic based on its final destination must retrieve that information
from the last entry in the SRH rather than the Destination Address
field of the IPv6 header.
An SR-aware service is associated with a locally instantiated service
segment, which is used to steer traffic through it.
If the service is configured to intercept all the packets passing
through the appliance, the underlying routing system only has to
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implement a default SR endpoint behavior (e.g., SR-MPLS node segment
or SRv6 End behavior), and the corresponding SID will be used to
steer traffic through the service.
If the service requires the packets to be directed to a specific
virtual interface, networking queue or process, a dedicated SR
behavior may be required to steer the packets to the appropriate
location. The definition of such service-specific functions is out
of the scope of this document.
SR-aware services also enable advanced network programming
functionalities such as conditional branching and jumping to
arbitrary SIDs in the segment list. In addition, SRv6 provides
several ways of passing and exchanging information between services
(e.g., SID arguments, tag field and TLVs). An example scenario
involving these features is described in [IFIP18], which discusses
the implementation of an SR-aware Intrusion Detection System.
Examples of SR-aware services are provided in section Section 8.1.
4.2. SR-Unaware Services
Any service that does not meet the above criteria for SR-awareness is
considered as SR-unaware.
An SR-unaware service is not able to process the SR information in
the traffic that it receives. It may either drop the traffic or take
erroneous decisions due to the unrecognized routing information. In
order to include such services in an SR policy, it is thus required
to remove the SR information as well as any other encapsulation
header before the service receives the packet, or to alter it in such
a way that the service can correctly process the packet.
In this document, we define the concept of an SR proxy as an entity,
separate from the service, that performs these modifications and
handle the SR processing on behalf of a service. The SR proxy can
run as a separate process on the service appliance, on a virtual
switch or router on the compute node or on a different host.
An SR-unaware service is associated with a service segment
instantiated on the SR proxy, which is used to steer traffic through
the service. Section 6 describes several SR proxy behaviors to
handle the encapsulation headers and SR information under various
circumstances.
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5. SR Service Policies
An SR service policy is an SR policy, as defined in
[I-D.ietf-spring-segment-routing-policy], that includes at least one
service. This service is represented in the SID-list by its
associated service SID. In case the policy should include several
services, the service traversal order is indicated by the relative
position of each service SID in the SID-list. Using the mechanisms
described in [I-D.ietf-spring-segment-routing-policy], it is possible
to load balance the traffic over several services, or instances of
the same service, by associating with the SR service policy a
weighted set of SID-lists, each containing a possible sequence of
service SIDs to be traversed. Similarly, several candidate paths can
be specified for the SR service policy, each with its own set of SID-
lists, for resiliency purposes.
Furthermore, binding SIDs (BSIDs) can be leveraged in the context of
service policies to reduce the number of SIDs imposed by the headend,
provide opacity between domains and improve scalability, as described
in [I-D.filsfils-spring-sr-policy-considerations]. For example, a
network operator may want a policy in its core domain to include
services that are running in one of its datacenters. One option is
to define an SR policy at ingress edge of the core domain that
explicitly includes all the SIDs needed to steer the traffic through
the core and in the DC, but that may result in a long SID-list and
requires to update the ingress edge configuration every time the DC
part of the policy is modified. Alternatively, a separate policy can
be defined at the ingress edge of the datacenter with only the SIDs
that needs to be executed there and its BSID included in the core
domain policy. That BSID remains stable when the DC policy is
modified and can even be shared among several core domain policies
that would require the same type of processing in the DC.
This section describes how services can be integrated within an SR-
MPLS or SRv6 service policy.
+------------------------------------------+
| SR network |
| |
+----+----+ +---------+ +----+-----+
| H +----------+ S +-----------+ E |
|(headend)| |(service)| |(endpoint)|
+----+----+ +---------+ +----+-----+
| =====================================> |
| P1(H,E,C) |
+------------------------------------------+
Figure 1: SR service policy
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Figure 1 illustrates a basic SR service policy instantiated on a
headend node H towards an endpoint E and traversing a service S. The
SR policy may also include additional requirements, such as traffic
engineering or VPN. On the head-end H, the SR policy P1 is created
with a color C and endpoint E and associated with an SR path that can
either be explicitly configured, dynamically computed on H or
provisioned by a network controller.
In its most basic form, the SR policy P1 would be resolved into the
SID-list < SID(S), SID(E) >. This is assuming that SID(S) and SID(E)
are directly reachable from H and S, respectively, and that the
forwarding path meets the policy requirement. However, depending on
the dataplane and the segments available in the network, additional
SIDs may be required to enforce the SR policy.
This model applies regardless of the SR-awareness of the service. If
it is SR-unaware, then S simply represents the proxy that takes care
of transmitting the packet to the actual service.
Traffic can then be steered into this policy using any of the
mechanisms described in [I-D.ietf-spring-segment-routing-policy].
The following subsections describe the specificities of each SR
dataplane.
5.1. SR-MPLS Data Plane
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+-----------------------------------------------+
| SR-MPLS network |
| |
+----+----+ ------> +---------+ ------> +----+-----+
| H +-------------+ S +-------------+ E |
|(headend)| |(service)| |(endpoint)|
+----+----+ +---------+ +----+-----+
| (1) (2) (3) (4) |
|+---------+ +---------+ +---------+ +---------+|
|| ... | | L(S) | | ... | | L(E) ||
|+---------+ +---------+ +---------+ +---------+|
|| L(S) | | ... | | L(E) | |Inner pkt||
|+---------+ +---------+ +---------+ +---------+|
|| ... | | L(E) | |Inner pkt| |
|+---------+ +---------+ +---------+ |
|| L(E) | |Inner pkt| |
|+---------+ +---------+ |
||Inner pkt| |
|+---------+ |
+-----------------------------------------------+
Figure 2: Packet walk in an SR-MPLS network
In an SR-MPLS network, the SR policy SID-list is encoded as a stack
of MPLS labels[I-D.ietf-spring-segment-routing-mpls] and pushed on
top of the packet.
In the example shown on Figure 2, the SR policy should steer the
traffic from the head-end H to the endpoint E via a service S. This
translates into an MPLS label stack that includes at least a label
L(S) associated to service S and a label L(E) associated to the
endpoint E. The label stack may also include additional intermediate
SIDs if these are required for traffic engineering (e.g., to encode a
low latency path between H and S and / or between S and E) or simply
for reachability purposes. Indeed, the service SID L(S) may be taken
from the global or local SID block of node S and, in the latter case,
one or more SIDs might be needed before L(S) in order for the packet
to reach node S (e.g., a prefix-SID of S), where L(S) can be
interpreted. The same applies for the SID L(E) at the SR policy
endpoint.
Special consideration must be taken into account when using Local
SIDs for service identification due to increased label stack depth
and the associated impacts.
When the packet arrives at S, this node determines the MPLS payload
type and the appropriate behavior for processing the packet based on
the semantic locally associated to the top label L(S). If S is an
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SR-aware service, the SID L(S) may provide additional context or
indication on how to process the packet (e.g., a firewall SID may
indicate which rule set should be applied onto the packet). If S is
a proxy in front of an SR-unaware service, L(S) indicates how and to
which service attached to this proxy the packet should be
transmitted. At some point in the process, L(S) is also popped from
the label stack in order to expose the next SID, which may be L(E) or
another intermediate SID.
5.2. SRv6 Data Plane
+-----------------------------------------------+
| SRv6 network |
| |
+----+----+ ------> +---------+ ------> +----+-----+
| H +-------------+ S +-------------+ E |
|(headend)| |(service)| |(endpoint)|
+----+----+ +---------+ +----+-----+
| (1) (2) (3) (4) |
|+---------+ +---------+ +---------+ +---------+|
||IP6(H,..)| |IP6(H, S)| |IP6(H,..)| |IP6(H, E)||
|+---------+ +---------+ +---------+ +---------+|
||SRH(E,..,| |SRH(E,..,| |SRH(E,..,| |SRH(E,..,||
|| S,..;| | S,..;| | S,..;| | S,..;||
|| SL=i)| | SL=j)| | SL=k)| | SL=0)||
|+---------+ +---------+ +---------+ +---------+|
||Inner pkt| |Inner pkt| |Inner pkt| |Inner pkt||
|+---------+ +---------+ +---------+ +---------+|
+-----------------------------------------------+
Figure 3: Packet walk in an SRv6 network
In an SRv6 network, the SR Policy is encoded into the packet as an
IPv6 header possibly followed by a Segment Routing Header (SRH)
[I-D.ietf-6man-segment-routing-header].
In the example shown on Figure 3, the SR policy should steer the
traffic from the head-end H to the endpoint E via a service S. This
translates into Segment-List that includes at least a segment SID(S)
to the service, or service proxy, S and a segment SID(E) to the
endpoint E. The Segment-List may also include additional
intermediate SIDs if these are required for traffic engineering
(e.g., the encode a low latency path between H and S and / or between
S and E) or simply for reachability purposes. Indeed, the service
SID locator may or may not be advertised in the routing protocol and,
in the latter case, one or more SIDs might be needed before SID(S) in
order to bring the packet up to node S, where SID(S) can be
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interpreted. The same applies for the segment SID(E) at the SR
policy endpoint.
When the packet arrives at S, this node determines how to process the
packet based on the semantic locally associated to the active segment
SID(S). If S is an SR-aware service, then SID(S) may provide
additional context or indication on how to process the packet (e.g.,
a firewall SID may indicate which rule set should be applied onto the
packet). If S is a proxy in front of an SR-unaware service, SID(S)
indicates how and to which service attached to this proxy the packet
should be transmitted. At some point in the process, the SRv6 End
function is also applied in order to make the next SID, which may be
SID(E) or another intermediate SID, active.
The "Inner pkt" on Figure 3 represents the SRv6 payload, which may be
an encapsulated IP packet, an Ethernet frame or a transport-layer
payload, for example.
6. SR Proxy Behaviors
This section describes several SR proxy behaviors designed to enable
SR service programming through SR-unaware services. A system
implementing one of these behaviors may handle the SR processing on
behalf of an SR-unaware service and allows the service to properly
process the traffic that is steered through it.
A service may be located at any hop in an SR policy, including the
last segment. However, the SR proxy behaviors defined in this
section are dedicated to supporting SR-unaware services at
intermediate hops in the segment list. In case an SR-unaware service
is at the last segment, it is sufficient to ensure that the SR
information is ignored (IPv6 routing extension header with Segments
Left equal to 0) or removed before the packet reaches the service
(MPLS PHP, SRv6 decapsulation behavior or PSP flavor).
As illustrated on Figure 4, the generic behavior of an SR proxy has
two parts. The first part is in charge of passing traffic from the
network to the service. It intercepts the SR traffic destined for
the service via a locally instantiated service segment, modifies it
in such a way that it appears as non-SR traffic to the service, then
sends it out on a given interface, IFACE-OUT, connected to the
service. The second part receives the traffic coming back from the
service on IFACE-IN, restores the SR information and forwards it
according to the next segment in the list. IFACE-OUT and IFACE-IN
are respectively the proxy interface used for sending traffic to the
service and the proxy interface that receives the traffic coming back
from the service. These can be physical interfaces or sub-interfaces
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(VLANs) and, unless otherwise stated, IFACE-OUT and IFACE-IN can
represent the same interface.
+----------------------------+
| |
| Service |
| |
+----------------------------+
^ Non SR |
| traffic |
| v
+-----------+----------+
+--| IFACE OUT | IFACE IN |--+
SR traffic | +-----------+----------+ | SR traffic
---------->| SR proxy |---------->
| |
+----------------------------+
Figure 4: Generic SR proxy
In the next subsections, the following SR proxy mechanisms are
defined:
o Static proxy
o Dynamic proxy
o Shared-memory proxy
o Masquerading proxy
Each mechanism has its own characteristics and constraints, which are
summarized in the below table. It is up to the operator to select
the best one based on the proxy node capabilities, the service
behavior and the traffic type. It is also possible to use different
proxy mechanisms within the same service policy.
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+-----+-----+-----+-----+
| | | | M |
| | | S | a |
| | | h | s |
| | | a | q |
| | | r | u |
| | D | e | e |
| S | y | d | r |
| t | n | | a |
| a | a | m | d |
| t | m | e | i |
| i | i | m | n |
| c | c | . | g |
+---------------------------------------+-----+-----+-----+-----+
| | SR-MPLS | Y | Y | Y | - |
| | | | | | |
| SR flavors | Inline SRv6 | P | P | P | Y |
| | | | | | |
| | SRv6 encapsulation | Y | Y | Y | - |
+----------------+----------------------+-----+-----+-----+-----+
| Chain agnostic configuration | N | N | Y | Y |
+---------------------------------------+-----+-----+-----+-----+
| Transparent to chain changes | N | Y | Y | Y |
+----------------+----------------------+-----+-----+-----+-----+
| | DA modification | Y | Y | Y | NAT |
| | | | | | |
| | Payload modification | Y | Y | Y | Y |
| | | | | | |
|Service support | Packet generation | Y | Y |cache|cache|
| | | | | | |
| | Packet deletion | Y | Y | Y | Y |
| | | | | | |
| | Packet re-ordering | Y | Y | Y | Y |
| | | | | | |
| | Transport endpoint | Y | Y |cache|cache|
+----------------+----------------------+-----+-----+-----+-----+
| | Ethernet | Y | Y | Y | - |
| Supported | | | | | |
| traffic | IPv4 | Y | Y | Y | - |
| | | | | | |
| | IPv6 | Y | Y | Y | Y |
+----------------+----------------------+-----+-----+-----+-----+
Figure 5: SR proxy summary
Note: The use of a shared memory proxy requires both the service
(VNF) and the proxy to be running on the same node.
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6.1. Static SR Proxy
The static proxy is an SR endpoint behavior for processing SR-MPLS or
SRv6 encapsulated traffic on behalf of an SR-unaware service. This
proxy thus receives SR traffic that is formed of an MPLS label stack
or an IPv6 header on top of an inner packet, which can be Ethernet,
IPv4 or IPv6.
A static SR proxy segment is associated with the following mandatory
parameters
o INNER-TYPE: Inner packet type
o NH-ADDR: Next hop Ethernet address (only for inner type IPv4 and
IPv6)
o IFACE-OUT: Local interface for sending traffic towards the service
o IFACE-IN: Local interface receiving the traffic coming back from
the service
o CACHE: SR information to be attached on the traffic coming back
from the service, including at least
* CACHE.SA: IPv6 source address (SRv6 only)
* CACHE.LIST: Segment list expressed as MPLS labels or IPv6
address
A static SR proxy segment is thus defined for a specific service,
inner packet type and cached SR information. It is also bound to a
pair of directed interfaces on the proxy. These may be both
directions of a single interface, or opposite directions of two
different interfaces. The latter is recommended in case the service
is to be used as part of a bi-directional SR service policy. If the
proxy and the service both support 802.1Q, IFACE-OUT and IFACE-IN can
also represent sub-interfaces.
The first part of this behavior is triggered when the proxy node
receives a packet whose active segment matches a segment associated
with the static proxy behavior. It removes the SR information from
the packet then sends it on a specific interface towards the
associated service. This SR information corresponds to the full
label stack for SR-MPLS or to the encapsulation IPv6 header with any
attached extension header in the case of SRv6.
The second part is an inbound policy attached to the proxy interface
receiving the traffic returning from the service, IFACE-IN. This
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policy attaches to the incoming traffic the cached SR information
associated with the SR proxy segment. If the proxy segment uses the
SR-MPLS data plane, CACHE contains a stack of labels to be pushed on
top of the packets. With the SRv6 data plane, CACHE is defined as a
source address, an active segment and an optional SRH (tag, segments
left, segment list and metadata). The proxy encapsulates the packets
with an IPv6 header that has the source address, the active segment
as destination address and the SRH as a routing extension header.
After the SR information has been attached, the packets are forwarded
according to the active segment, which is represented by the top MPLS
label or the IPv6 Destination Address. An MPLS TTL or IPv6 Hop Limit
value may also be configured in CACHE. If it is not, the proxy
should set these values according to the node's default setting for
MPLS or IPv6 encapsulation.
In this scenario, there are no restrictions on the operations that
can be performed by the service on the stream of packets. It may
operate at all protocol layers, terminate transport layer
connections, generate new packets and initiate transport layer
connections. This behavior may also be used to integrate an
IPv4-only service into an SRv6 policy. However, a static SR proxy
segment can be used in only one service policy at a time. As opposed
to most other segment types, a static SR proxy segment is bound to a
unique list of segments, which represents a directed SR service
policy. This is due to the cached SR information being defined in
the segment configuration. This limitation only prevents multiple
segment lists from using the same static SR proxy segment at the same
time, but a single segment list can be shared by any number of
traffic flows. Besides, since the returning traffic from the service
is re-classified based on the incoming interface, an interface can be
used as receiving interface (IFACE-IN) only for a single SR proxy
segment at a time. In the case of a bi-directional SR service
policy, a different SR proxy segment and receiving interface are
required for the return direction.
The static proxy behavior may also be used for sending traffic
through "bump in the wire" services that are transparent to the IP
and Ethernet layers. This type of processing is assumed when the
inner traffic type is Ethernet, since the original destination
address of the Ethernet frame is preserved when the packet is steered
into the SR Policy and likely associated with a node downstream of
the policy tail-end. In case the inner type is IP (IPv4 or IPv6),
the NH-ADDR parameter may be set to a dummy or broadcast Ethernet
address, or simply to the address of the proxy receiving interface
(IFACE-IN).
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6.1.1. SR-MPLS Pseudocode
6.1.1.1. Static Proxy for Inner Type Ethernet
When processing an MPLS packet whose top label matches a locally
instantiated MPLS static proxy SID for Ethernet traffic, the
following pseudocode is executed.
S01. POP all labels in the MPLS label stack.
S02. Submit the frame to the Ethernet module for transmission via
interface IFACE-OUT.
Figure 6: SID processing for MPLS static proxy (Ethernet)
When processing an Ethernet frame received on the interface IFACE-IN
and with a destination MAC address that is neither a broadcast
address nor matches the address of IFACE-IN, the following pseudocode
is executed.
S01. Retrieve the CACHE entry associated with IFACE-IN.
S02. If the CACHE entry is not empty {
S03. Remove the preamble or Frame Check Sequence (FCS).
S04. PUSH all labels from the retrieved CACHE entry.
S05. Submit the packet to the MPLS module for transmission as per
the top label in the MPLS label stack.
S06. }
Figure 7: Inbound policy for MPLS static proxy (Ethernet)
6.1.1.2. Static Proxy for Inner Type IPv4
When processing an MPLS packet whose top label matches a locally
instantiated MPLS static proxy SID for IPv4 traffic, the following
pseudocode is executed.
S01. POP all labels in the MPLS label stack.
S02. Submit the packet to the IPv4 module for transmission on
interface IFACE-OUT via NH-ADDR.
Figure 8: SID processing for MPLS static proxy (IPv4)
When processing an IPv4 packet received on the interface IFACE-IN and
with a destination address that does not match any address of IFACE-
IN, the following pseudocode is executed.
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S01. Retrieve the CACHE entry associated with IFACE-IN.
S02. If the CACHE entry is not empty {
S03. Decrement the TTL and adjust the checksum accordingly.
S04. PUSH all labels from the retrieved CACHE entry.
S05. Submit the packet to the MPLS module for transmission as per
the top label in the MPLS label stack.
S06. }
Figure 9: Inbound policy for MPLS static proxy (IPv4)
6.1.1.3. Static Proxy for Inner Type IPv6
When processing an MPLS packet whose top label matches a locally
instantiated MPLS static proxy SID for IPv6 traffic, the following
pseudocode is executed.
S01. POP all labels in the MPLS label stack.
S02. Submit the packet to the IPv6 module for transmission on
interface IFACE-OUT via NH-ADDR.
Figure 10: SID processing for MPLS static proxy (IPv6)
When processing an IPv6 packet received on the interface IFACE-IN and
with a destination address that does not match any address of IFACE-
IN, the following pseudocode is executed.
S01. Retrieve the CACHE entry associated with IFACE-IN.
S02. If the CACHE entry is not empty {
S03. Decrement the Hop Limit.
S04. PUSH all labels from the retrieved CACHE entry.
S05. Submit the packet to the MPLS module for transmission as per
the top label in the MPLS label stack.
S06. }
Figure 11: Inbound policy for MPLS static proxy (IPv6)
6.1.2. SRv6 Pseudocode
6.1.2.1. Static Proxy for Inner Type Ethernet
When processing an IPv6 packet matching a FIB entry locally
instantiated as an SRv6 static proxy SID for Ethernet traffic, the
following pseudocode is executed.
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S01. When an SRH is processed {
S02. If (Segments Left == 0) {
S03. Proceed to process the next header in the packet.
S04. }
S05. If (IPv6 Hop Limit <= 1) {
S06. Send an ICMP Time Exceeded message to the Source Address,
Code 0 (hop limit exceeded in transit),
Interrupt packet processing and discard the packet.
S07. }
S08. max_last_entry = (Hdr Ext Len / 2) - 1
S09. If ((Last Entry > max_last_entry) or
(Segments Left > (Last Entry + 1))) {
S10. Send an ICMP Parameter Problem message to the Source Address,
Code 0 (Erroneous header field encountered),
Pointer set to the Segments Left field,
Interrupt packet processing and discard the packet.
S11. }
S12. Decrement Hop Limit by 1.
S13. Decrement Segments Left by 1.
S14. Copy Segment List[Segments Left] from the SRH to the
Destination Address of the IPv6 header.
S15. If (Upper-layer header type != 143 (Ethernet)) {
S16. Resubmit the packet to the IPv6 module for transmission to
the new destination.
S17. }
S18. Perform IPv6 decapsulation.
S19. Submit the frame to the Ethernet module for transmission via
interface IFACE-OUT.
S20. }
Figure 12: SID processing for SRv6 static proxy (Ethernet)
S15: 143 (Ethernet) refers to the value assigned by IANA for
"Ethernet" in the "Internet Protocol Numbers" registry.
When processing the Upper-layer header of a packet matching a FIB
entry locally instantiated as an SRv6 static proxy SID for Ethernet
traffic, the following pseudocode is executed.
S01. If (Upper-layer header type != 143 (Ethernet)) {
S02. Process as per [I-D.ietf-spring-srv6-network-programming] Section 4.1.1
S03. }
S04. Perform IPv6 decapsulation.
S05. Submit the frame to the Ethernet module for transmission via
interface IFACE-OUT.
Figure 13: Upper-layer header processing for SRv6 static proxy
(Ethernet)
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When processing an Ethernet frame received on the interface IFACE-IN
and with a destination MAC address that is neither a broadcast
address nor matches the address of IFACE-IN, the following pseudocode
is executed.
S01. Retrieve the CACHE entry associated with IFACE-IN.
S02. If the CACHE entry is not empty {
S03. Remove the preamble or Frame Check Sequence (FCS).
S04. Perform IPv6 encapsulation with an SRH
Source Address of the IPv6 header is set to CACHE.SA,
Destination Address of the IPv6 header is set to
CACHE.LIST[0],
Next Header of the SRH is set to 143 (Ethernet),
Segment List of the SRH is set to CACHE.LIST.
S05. Submit the packet to the IPv6 module for transmission to the
next destination.
S06. }
Figure 14: Inbound policy for SRv6 static proxy (Ethernet)
S04: CACHE.LIST[0] represents the first entry in CACHE.LIST. Unless
a local configuration indicates otherwise, the SIDs in CACHE.LIST
should be encoded in the Segment List field in reversed order, the
Segment Left and Last Entry values should be set of the length of
CACHE.LIST minus 1. If CACHE.LIST contains a single entry, the SRH
can be omitted and the Next Header field of the IPv6 header set to
143 (Ethernet).
6.1.2.2. Static Proxy for Inner Type IPv4
When processing an IPv6 packet matching a FIB entry locally
instantiated as an SRv6 static proxy SID for IPv4 traffic, the
following pseudocode is executed.
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S01. When an SRH is processed {
S02. If (Segments Left == 0) {
S03. Proceed to process the next header in the packet.
S04. }
S05. If (IPv6 Hop Limit <= 1) {
S06. Send an ICMP Time Exceeded message to the Source Address,
Code 0 (hop limit exceeded in transit),
Interrupt packet processing and discard the packet.
S07. }
S08. max_last_entry = (Hdr Ext Len / 2) - 1
S09. If ((Last Entry > max_last_entry) or
(Segments Left > (Last Entry + 1))) {
S10. Send an ICMP Parameter Problem message to the Source Address,
Code 0 (Erroneous header field encountered),
Pointer set to the Segments Left field,
Interrupt packet processing and discard the packet.
S11. }
S12. Decrement Hop Limit by 1.
S13. Decrement Segments Left by 1.
S14. Copy Segment List[Segments Left] from the SRH to the
Destination Address of the IPv6 header.
S15. If (Upper-layer header type != 4 (IPv4)) {
S16. Resubmit the packet to the IPv6 module for transmission to
the new destination.
S17. }
S18. Perform IPv6 decapsulation.
S19. Submit the packet to the IPv4 module for transmission on
interface IFACE-OUT via NH-ADDR.
S20. }
Figure 15: SID processing for SRv6 static proxy (IPv4)
When processing the Upper-layer header of a packet matching a FIB
entry locally instantiated as an SRv6 static proxy SID for IPv4
traffic, the following pseudocode is executed.
S01. If (Upper-layer header type != 4 (IPv4)) {
S02. Process as per [I-D.ietf-spring-srv6-network-programming] Section 4.1.1
S03. }
S04. Perform IPv6 decapsulation.
S05. Submit the packet to the IPv4 module for transmission on
interface IFACE-OUT via NH-ADDR.
Figure 16: Upper-layer header processing for SRv6 static proxy (IPv4)
When processing an IPv4 packet received on the interface IFACE-IN and
with a destination address that does not match any address of IFACE-
IN, the following pseudocode is executed.
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S01. Retrieve the CACHE entry associated with IFACE-IN.
S02. If the CACHE entry is not empty {
S03. Decrement the TTL and adjust the checksum accordingly.
S04. Perform IPv6 encapsulation with an SRH
Source Address of the IPv6 header is set to CACHE.SA,
Destination Address of the IPv6 header is set to
CACHE.LIST[0],
Next Header of the SRH is set to 4 (IPv4),
Segment List of the SRH is set to CACHE.LIST.
S05. Submit the packet to the IPv6 module for transmission to the
next destination.
S06. }
Figure 17: Inbound policy for SRv6 static proxy (IPv4)
S04: CACHE.LIST[0] represents the first entry in CACHE.LIST. Unless
a local configuration indicates otherwise, the SIDs in CACHE.LIST
should be encoded in the Segment List field in reversed order, the
Segment Left and Last Entry values should be set of the length of
CACHE.LIST minus 1. If CACHE.LIST contains a single entry, the SRH
can be omitted and the Next Header field of the IPv6 header set to 4
(IPv4).
6.1.2.3. Static Proxy for Inner Type IPv6
When processing an IPv6 packet matching a FIB entry locally
instantiated as an SRv6 static proxy SID for IPv6 traffic, the
following pseudocode is executed.
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S01. When an SRH is processed {
S02. If (Segments Left == 0) {
S03. Proceed to process the next header in the packet.
S04. }
S05. If (IPv6 Hop Limit <= 1) {
S06. Send an ICMP Time Exceeded message to the Source Address,
Code 0 (hop limit exceeded in transit),
Interrupt packet processing and discard the packet.
S07. }
S08. max_last_entry = (Hdr Ext Len / 2) - 1
S09. If ((Last Entry > max_last_entry) or
(Segments Left > (Last Entry + 1))) {
S10. Send an ICMP Parameter Problem message to the Source Address,
Code 0 (Erroneous header field encountered),
Pointer set to the Segments Left field,
Interrupt packet processing and discard the packet.
S11. }
S12. Decrement Hop Limit by 1.
S13. Decrement Segments Left by 1.
S14. Copy Segment List[Segments Left] from the SRH to the
Destination Address of the IPv6 header.
S15. If (Upper-layer header type != 41 (IPv6)) {
S16. Resubmit the packet to the IPv6 module for transmission to
the new destination.
S17. }
S18. Perform IPv6 decapsulation.
S19. Submit the packet to the IPv6 module for transmission on
interface IFACE-OUT via NH-ADDR.
S20. }
Figure 18: SID processing for SRv6 static proxy (IPv6)
When processing the Upper-layer header of a packet matching a FIB
entry locally instantiated as an SRv6 static proxy SID for IPv6
traffic, the following pseudocode is executed.
S01. If (Upper-layer header type != 41 (IPv6)) {
S02. Process as per [I-D.ietf-spring-srv6-network-programming] Section 4.1.1
S03. }
S04. Perform IPv6 decapsulation.
S05. Submit the packet to the IPv6 module for transmission on
interface IFACE-OUT via NH-ADDR.
Figure 19: Upper-layer header processing for SRv6 static proxy (IPv6)
When processing an IPv6 packet received on the interface IFACE-IN and
with a destination address that does not match any address of IFACE-
IN, the following pseudocode is executed.
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S01. Retrieve the CACHE entry associated with IFACE-IN.
S02. If the CACHE entry is not empty {
S03. Decrement the Hop Limit.
S04. Perform IPv6 encapsulation with an SRH
Source Address of the IPv6 header is set to CACHE.SA,
Destination Address of the IPv6 header is set to
CACHE.LIST[0],
Next Header of the SRH is set to 41 (IPv6),
Segment List of the SRH is set to CACHE.LIST.
S05. Submit the packet to the IPv6 module for transmission to the
next destination.
S06. }
Figure 20: Inbound policy for SRv6 static proxy (IPv6)
S04: CACHE.LIST[0] represents the first entry in CACHE.LIST. Unless
a local configuration indicates otherwise, the SIDs in CACHE.LIST
should be encoded in the Segment List field in reversed order, the
Segment Left and Last Entry values should be set of the length of
CACHE.LIST minus 1. If CACHE.LIST contains a single entry, the SRH
can be omitted and the Next Header field of the (outer) IPv6 header
set to 41 (IPv6).
6.2. Dynamic SR Proxy
The dynamic proxy is an improvement over the static proxy that
dynamically learns the SR information before removing it from the
incoming traffic. The same information can then be re-attached to
the traffic returning from the service. As opposed to the static SR
proxy, no CACHE information needs to be configured. Instead, the
dynamic SR proxy relies on a local caching mechanism on the node
instantiating this segment.
Upon receiving a packet whose active segment matches a dynamic SR
proxy function, the proxy node pops the top MPLS label or applies the
SRv6 End behavior, then compares the updated SR information with the
cache entry for the current segment. If the cache is empty or
different, it is updated with the new SR information. The SR
information is then removed and the inner packet is sent towards the
service.
The cache entry is not mapped to any particular packet, but instead
to an SR service policy identified by the receiving interface (IFACE-
IN). Any non-link-local IP packet or non-local Ethernet frame
received on that interface will be re-encapsulated with the cached
headers as described in Section 6.1. The service may thus drop,
modify or generate new packets without affecting the proxy.
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6.2.1. SR-MPLS Pseudocode
The dynamic proxy SR-MPLS pseudocode is obtained by inserting the
following instructions at the beginning of the static SR-MPLS
pseudocode (Section 6.1.1).
S01. If the top label S bit is different from 0 {
S02. Discard the packet.
S03. }
S04. POP the top label.
S05. Copy the IPv6 encapsulation in a CACHE entry associated with the
interface IFACE-IN.
Figure 21: SID processing for MPLS dynamic proxy
S01: As mentioned at the beginning of Section 6, an SR proxy is not
needed to include an SR-unaware service at the end of an SR policy.
S05: An implementation may optimize the caching procedure by copying
information into the cache only if it is different from the current
content of the cache entry. Furthermore, a TTL margin can be
configured for the top label stack entry to prevent constant cache
updates when multiple equal-cost paths with different hop counts are
used towards the SR proxy node. In that case, a TTL difference
smaller than the configured margin should not trigger a cache update
(provided that the labels are the same).
When processing an Ethernet frame, an IPv4 packet or an IPv6 packet
received on the interface IFACE-IN and with a destination address
that does not match any address of IFACE-IN, the pseudocode reported
in Figure 7, Figure 9 or Figure 11, respectively, is executed.
6.2.2. SRv6 Pseudocode
When processing an IPv6 packet matching a FIB entry locally
instantiated as an SRv6 dynamic proxy SID, the same pseudocode as
described in Figure 12, Figure 15 and Figure 18, respectively for
Ethernet, IPv4 and IPv6 traffic, is executed with the following
addition between lines S17 and S18.
(... S17. })
S17.1. Copy the IPv6 encapsulation in a CACHE entry associated with
the interface IFACE-IN.
(S18. Perform IPv6 decapsulation...)
Figure 22: SID processing for SRv6 dynamic proxy
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An implementation may optimize the caching procedure by copying
information into the cache only if it is different from the current
content of the cache entry. A Hop Limit margin can be configured to
prevent constant cache updates when multiple equal-cost paths with
different hop counts are used towards the SR proxy node. In that
case, a Hop Limit difference smaller than the configured margin
should not trigger a cache update. Similarly, the Flow Label value
can be ignored when comparing the current packet IPv6 header with the
cache entry. In this case, the Flow Label should be re-computed by
the proxy node when it restores the IPv6 encapsulation from the cache
entry.
When processing the Upper-layer header of a packet matching a FIB
entry locally instantiated as an SRv6 dynamic proxy SID, process the
packet as per [I-D.ietf-spring-srv6-network-programming]
Section 4.1.1.
When processing an Ethernet frame, an IPv4 packet or an IPv6 packet
received on the interface IFACE-IN and with a destination address
that does not match any address of IFACE-IN, the same pseudocode as
in Figure 14, Figure 17 or Figure 20, respectively, is executed.
6.3. Shared Memory SR Proxy
The shared memory proxy is an SR endpoint behavior for processing SR-
MPLS or SRv6 encapsulated traffic on behalf of an SR-unaware service.
This proxy behavior leverages a shared-memory interface with a
virtualized service (VNF) in order to hide the SR information from an
SR-unaware service while keeping it attached to the packet. We
assume in this case that the proxy and the VNF are running on the
same compute node. A typical scenario is an SR-capable vrouter
running on a container host and forwarding traffic to VNFs isolated
within their respective container.
6.4. Masquerading SR Proxy
The masquerading proxy is an SR endpoint behavior for processing SRv6
traffic on behalf of an SR-unaware service. This proxy thus receives
SR traffic that is formed of an IPv6 header and an SRH on top of an
inner payload. The masquerading behavior is independent from the
inner payload type. Hence, the inner payload can be of any type but
it is usually expected to be a transport layer packet, such as TCP or
UDP.
A masquerading SR proxy segment is associated with the following
mandatory parameters:
o NH-ADDR: Next hop Ethernet address
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o IFACE-OUT: Local interface for sending traffic towards the service
o IFACE-IN: Local interface receiving the traffic coming back from
the service
A masquerading SR proxy segment is thus defined for a specific
service and bound to a pair of directed interfaces or sub-interfaces
on the proxy. As opposed to the static and dynamic SR proxies, a
masquerading segment can be present at the same time in any number of
SR service policies and the same interfaces can be bound to multiple
masquerading proxy segments. The only restriction is that a
masquerading proxy segment cannot be the last segment in an SR
service policy.
The first part of the masquerading behavior is triggered when the
proxy node receives an IPv6 packet whose Destination Address matches
a masquerading proxy SID. The proxy inspects the IPv6 extension
headers and substitutes the Destination Address with the last SID in
the SRH attached to the IPv6 header, which represents the final
destination of the IPv6 packet. The packet is then sent out towards
the service.
The service receives an IPv6 packet whose source and destination
addresses are respectively the original source and final destination.
It does not attempt to inspect the SRH, as RFC8200 specifies that
routing extension headers are not examined or processed by transit
nodes. Instead, the service simply forwards the packet based on its
current Destination Address. In this scenario, we assume that the
service can only inspect, drop or perform limited changes to the
packets. For example, Intrusion Detection Systems, Deep Packet
Inspectors and non-NAT Firewalls are among the services that can be
supported by a masquerading SR proxy. Flavors of the masquerading
behavior are defined in Section 6.4.2 and Section 6.4.3 to support a
wider range of services.
The second part of the masquerading behavior, also called de-
masquerading, is an inbound policy attached to the proxy interface
receiving the traffic returning from the service, IFACE-IN. This
policy inspects the incoming traffic and triggers a regular SRv6
endpoint processing (End) on any IPv6 packet that contains an SRH.
This processing occurs before any lookup on the packet Destination
Address is performed and it is sufficient to restore the right active
SID as the Destination Address of the IPv6 packet.
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6.4.1. SRv6 Masquerading Proxy Pseudocode
Masquerading: When processing an IPv6 packet matching a FIB entry
locally instantiated as an SRv6 masquerading proxy SID, the following
pseudocode is executed.
S01. When an SRH is processed {
S02. If (Segments Left == 0) {
S03. Proceed to process the next header in the packet.
S04. }
S05. If (IPv6 Hop Limit <= 1) {
S06. Send an ICMP Time Exceeded message to the Source Address,
Code 0 (hop limit exceeded in transit),
Interrupt packet processing and discard the packet.
S07. }
S08. max_last_entry = (Hdr Ext Len / 2) - 1
S09. If ((Last Entry > max_last_entry) or
(Segments Left > (Last Entry + 1))) {
S10. Send an ICMP Parameter Problem message to the Source Address,
Code 0 (Erroneous header field encountered),
Pointer set to the Segments Left field,
Interrupt packet processing and discard the packet.
S11. }
S12. Decrement Hop Limit by 1.
S13. Decrement Segments Left by 1.
S14. Copy Segment List[0] from the SRH to the Destination Address
of the IPv6 header.
S15. Submit the packet to the IPv6 module for transmission on
interface IFACE-OUT via NH-ADDR.
S16. }
Figure 23: SID processing for SRv6 masquerading proxy
When processing the Upper-layer header of a packet matching a FIB
entry locally instantiated as an SRv6 masquerading proxy SID, process
the packet as per [I-D.ietf-spring-srv6-network-programming]
Section 4.1.1.
De-masquerading: When processing an IPv6 packet received on the
interface IFACE-IN and with a destination address that does not match
any address of IFACE-IN, the following pseudocode is executed.
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S01. When an SRH is processed {
S02. If (IPv6 Hop Limit <= 1) {
S03. Send an ICMP Time Exceeded message to the Source Address,
Code 0 (hop limit exceeded in transit),
Interrupt packet processing and discard the packet.
S04. }
S05. If (Segments Left != 0) {
S06. max_last_entry = (Hdr Ext Len / 2) - 1
S07. If ((Last Entry > max_last_entry) or
(Segments Left > Last Entry)) {
S08. Send an ICMP Parameter Problem message to the Source Address,
Code 0 (Erroneous header field encountered),
Pointer set to the Segments Left field,
Interrupt packet processing and discard the packet.
S09. }
S10. Copy Segment List[Segments Left] from the SRH to the
Destination Address of the IPv6 header.
S11. }
S12. Decrement Hop Limit by 1.
S13. Submit the packet to the IPv6 module for transmission to the
next destination.
S14. }
Figure 24: Inbound policy for SRv6 masquerading proxy
6.4.2. Destination NAT Flavor
Services modifying the destination address in the packets they
process, such as NATs, can be supported by reporting the updated
Destination Address back into the Segment List field of the SRH.
The Destination NAT flavor of the SRv6 masquerading proxy is enabled
by adding the following instruction between lines S09 and S10 of the
de-masquerading pseudocode in Figure 24.
(... S09. })
S09.1. Copy the Destination Address of the IPv6 header to the
Segment List[0] entry of the SRH.
(S10. Copy Segment List[Segments Left] from the SRH to the
Destination Address of the IPv6 header...)
6.4.3. Caching Flavor
Services generating packets or acting as endpoints for transport
connections can be supported by adding a dynamic caching mechanism
similar to the one described in Section 6.2.
The caching flavor of the SRv6 masquerading proxy is enabled by:
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o Adding the following instruction between lines S14 and S15 of the
masquerading pseudocode in Figure 23.
(... S14. Copy Segment List[0] from the SRH to the Destination
Address of the IPv6 header.
S14.1. Copy the IPv6 encapsulation in a CACHE entry associated with
the interface IFACE-IN.
(S15. Submit the packet to the IPv6 module for transmission on
interface IFACE-OUT via NH-ADDR.)
o Updating the de-masquerading pseudocode such that, in addition to
the SRH processing in Figure 24, the following pseudocode is
executed when processing an IPv6 packet (received on the interface
IFACE-IN and with a destination address that does not match any
address of IFACE-IN) that does not contain an SRH.
S01. Retrieve the CACHE entry associated with IFACE-IN.
S02. If the CACHE entry is not empty {
S03. If (IPv6 Hop Limit <= 1) {
S04. Send an ICMP Time Exceeded message to the Source Address,
Code 0 (hop limit exceeded in transit),
Interrupt packet processing and discard the packet.
S05. }
S06. Decrement Hop Limit by 1.
S07. Update the IPv6 encapsulation according to the retrieved CACHE
entry.
S08. Submit the packet to the IPv6 module for transmission to the
next destination.
S09. }
7. Metadata
7.1. MPLS Data Plane
Metadata can be carried for SR-MPLS traffic in a Segment Routing
Header inserted between the last MPLS label and the MPLS payload.
When used solely as a metadata container, the SRH does not carry any
segment but only the mandatory header fields, including the tag and
flags, and any TLVs that is required for transporting the metadata.
Since the MPLS encapsulation has no explicit protocol identifier
field to indicate the protocol type of the MPLS payload, how to
indicate the presence of metadata in an MPLS packet is a potential
issue to be addressed. One possible solution is to add the
indication about the presence of metadata in the semantic of the
SIDs. Note that only the SIDs whose behavior involves looking at the
metadata or the MPLS payload would need to include such semantic
(e.g., service segments). Other segments, such as topological
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segments, are not affected by the presence of metadata. Another,
more generic, solution is to introduce a protocol identifier field
within the MPLS packet as described in
[I-D.xu-mpls-payload-protocol-identifier].
7.2. IPv6 Data Plane
7.2.1. SRH TLV Objects
The IPv6 SRH TLV objects are designed to carry all sorts of metadata.
TLV objects can be imposed by the ingress edge router that steers the
traffic into the SR service policy.
An SR-aware service may impose, modify or remove any TLV object
attached to the first SRH, either by directly modifying the packet
headers or via a control channel between the service and its
forwarding plane.
An SR-aware service that re-classifies the traffic and steers it into
a new SR service policy (e.g. DPI) may attach any TLV object to the
new SRH.
Metadata imposition and handling will be further discussed in a
future version of this document.
7.2.1.1. Opaque Metadata TLV
This document defines an SRv6 TLV called Opaque Metadata TLV. This
is a fixed-length container to carry any type of Service Metadata.
No assumption is made by this document on the structure or the
content of the carried metadata. The Opaque Metadata TLV has the
following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| Service Metadata |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
o Type: to be assigned by IANA.
o Length: 14.
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o Service Metadata: 14 octets of opaque data.
7.2.1.2. NSH Carrier TLV
This document defines an SRv6 TLV called NSH Carrier TLV. It is a
container to carry Service Metadata in the form of Variable-Length
Metadata as defined in [RFC8300] for NSH MD Type 2. The NSH Carrier
TLV has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Service Metadata //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
o Type: to be assigned by IANA.
o Length: the total length of the TLV.
o Flags: 8 bits. No flags are defined in this document. SHOULD be
set to 0 on transmission and MUST be ignored on receipt.
o Service Metadata: a list of Service Metadata TLV as defined in
[RFC8300] for NSH MD Type 2.
7.2.2. SRH Tag
The SRH tag identifies a packet as part of a group or class of
packets [I-D.ietf-6man-segment-routing-header].
In the context of service programming, this field can be used to
encode basic metadata in the SRH. An example use-case is to leverage
the SRH tag to encode a policy ID. This policy ID can then be used
by an SR-aware function to identify a particular processing policy to
be applied on that packet.
8. Implementation Status
This section is to be removed prior to publishing as an RFC.
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8.1. SR-Aware Services
Specific SRv6 support has been implemented for the below open-source
services:
o Iptables (1.6.2 and later) [IPTABLES]
o Nftables (0.8.4 and later) [NFTABLES]
o Snort [SNORT]
In addition, any service relying on the Linux kernel, version 4.10
and later, or FD.io VPP for packet forwarding can be considered as
SR-aware.
8.2. Proxy Behaviors
The static SR proxy is available for SR-MPLS and SRv6 on various
Cisco hardware and software platforms. Furthermore, the following
proxies are available on open-source software.
+-------------+-------------+
| VPP | Linux |
+---+-----------------------------------+-------------+-------------+
| M | Static proxy | Available | In progress |
| P | | | |
| L | Dynamic proxy | In progress | In progress |
| S | | | |
| | Shared memory proxy | In progress | In progress |
+---+-----------------------------------+-------------+-------------+
| | Static proxy | Available | In progress |
| S | | | |
| R | Dynamic proxy | Available | Available |
| v | | | |
| 6 | Shared memory proxy | In progress | In progress |
| | | | |
| | Masquerading proxy | Available | Available |
+---+-----------------------------------+-------------+-------------+
Figure 25: Open-source implementation status table
9. Related Works
The Segment Routing solution addresses a wide problem that covers
both topological and service policies. The topological and service
instructions can be either deployed in isolation or in combination.
SR has thus a wider applicability than the architecture defined in
[RFC7665]. Furthermore, the inherent property of SR is a stateless
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network fabric. In SR, there is no state within the fabric to
recognize a flow and associate it with a policy. State is only
present at the ingress edge of the SR domain, where the policy is
encoded into the packets. This is completely different from other
proposals such as [RFC8300] and the MPLS label swapping mechanism
described in [I-D.ietf-mpls-sfc], which rely on state configured at
every hop of the service chain.
10. IANA Considerations
10.1. SRv6 Endpoint Behaviors
This I-D requests the IANA to allocate, within the "SRv6 Endpoint
Behaviors" sub-registry belonging to the top-level "Segment-routing
with IPv6 dataplane (SRv6) Parameters" registry, the following
allocations:
Value Description Reference
--------------------------------------------------------------
TBA1-1 End.AN - SR-aware function (native) [This.ID]
TBA1-2 End.AS - Static proxy [This.ID]
TBA1-3 End.AD - Dynamic proxy [This.ID]
TBA1-4 End.AM - Masquerading proxy [This.ID]
TBA1-5 End.AM - Masquerading proxy with NAT [This.ID]
TBA1-6 End.AM - Masquerading proxy with Caching [This.ID]
TBA1-7 End.AM - Masquerading proxy with NAT & [This.ID]
Caching
10.2. Segment Routing Header TLVs
This I-D requests the IANA to allocate, within the "Segment Routing
Header TLVs" registry, the following allocations:
Value Description Reference
----------------------------------------------
TBA2-1 Opaque Metadata TLV [This.ID]
TBA2-2 NSH Carrier TLV [This.ID]
11. Security Considerations
The security requirements and mechanisms described in [RFC8402],
[I-D.ietf-6man-segment-routing-header] and
[I-D.ietf-spring-srv6-network-programming] also apply to this
document.
This document does not introduce any new security vulnerabilities.
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12. Acknowledgements
The authors would like to thank Thierry Couture, Ketan Talaulikar,
Loa Andersson, Andrew G. Malis, Adrian Farrel, Alexander Vainshtein
and Joel M. Halpern for their valuable comments and suggestions on
the document.
13. Contributors
The following people have contributed to this document:
Pablo Camarillo
Cisco Systems, Inc.
Spain
Email: pcamaril@cisco.com
Bart Peirens
Proximus
Belgium
Email: bart.peirens@proximus.com
Dirk Steinberg
Lapishills Consulting Limited
Cyprus
Email: dirk@lapishills.com
Ahmed AbdelSalam
Cisco Systems, Inc.
Italy
Email: ahabdels@cisco.com
Gaurav Dawra
LinkedIn
United States of America
Email: gdawra@linkedin.com
Stewart Bryant
Futurewei Technologies Inc
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Email: stewart.bryant@gmail.com
Hamid Assarpour
Broadcom
Email: hamid.assarpour@broadcom.com
Himanshu Shah
Ciena
Email: hshah@ciena.com
Luis M. Contreras
Telefonica I+D
Spain
Email: luismiguel.contrerasmurillo@telefonica.com
Jeff Tantsura
Individual
Email: jefftant@gmail.com
Martin Vigoureux
Nokia
Email: martin.vigoureux@nokia.com
Jisu Bhattacharya
Cisco Systems, Inc.
United States of America
Email: jisu@cisco.com
14. References
14.1. Normative References
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[]
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-spring-segment-routing-mpls]
Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing with MPLS
data plane", draft-ietf-spring-segment-routing-mpls-22
(work in progress), May 2019.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", draft-
ietf-spring-segment-routing-policy-08 (work in progress),
July 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-18 (work in
progress), August 2020.
[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>.
14.2. Informative References
[I-D.dawra-idr-bgp-sr-service-chaining]
Dawra, G., Filsfils, C., daniel.bernier@bell.ca, d.,
Uttaro, J., Decraene, B., Elmalky, H., Xu, X., Clad, F.,
and K. Talaulikar, "BGP Control Plane Extensions for
Segment Routing based Service Chaining", draft-dawra-idr-
bgp-sr-service-chaining-02 (work in progress), January
2018.
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[I-D.filsfils-spring-sr-policy-considerations]
Filsfils, C., Talaulikar, K., Krol, P., Horneffer, M., and
P. Mattes, "SR Policy Implementation and Deployment
Considerations", draft-filsfils-spring-sr-policy-
considerations-05 (work in progress), April 2020.
[I-D.ietf-mpls-sfc]
Farrel, A., Bryant, S., and J. Drake, "An MPLS-Based
Forwarding Plane for Service Function Chaining", draft-
ietf-mpls-sfc-07 (work in progress), March 2019.
[I-D.xu-mpls-payload-protocol-identifier]
Xu, X., Assarpour, H., Ma, S., and F. Clad, "MPLS Payload
Protocol Identifier", draft-xu-mpls-payload-protocol-
identifier-06 (work in progress), March 2019.
[IFIP18] Abdelsalam, A., Salsano, S., Clad, F., Camarillo, P., and
C. Filsfils, "SEgment Routing Aware Firewall For Service
Function Chaining scenarios", IFIP Networking conference ,
May 2018.
[IPTABLES]
"iptables-1.6.2 changes", February 2018,
<https://netfilter.org/projects/iptables/files/changes-
iptables-1.6.2.txt>.
[NFTABLES]
"nftables-0.8.4 changes", May 2018,
<https://netfilter.org/projects/nftables/files/changes-
nftables-0.8.4.txt>.
[RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC7665, October 2015,
<https://www.rfc-editor.org/info/rfc7665>.
[RFC8300] Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
"Network Service Header (NSH)", RFC 8300,
DOI 10.17487/RFC8300, January 2018,
<https://www.rfc-editor.org/info/rfc8300>.
[SNORT] "SR-Snort", March 2018,
<https://github.com/SRouting/SR-Snort>.
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Authors' Addresses
Francois Clad (editor)
Cisco Systems, Inc.
France
Email: fclad@cisco.com
Xiaohu Xu (editor)
Alibaba
Email: xiaohu.xxh@alibaba-inc.com
Clarence Filsfils
Cisco Systems, Inc.
Belgium
Email: cf@cisco.com
Daniel Bernier
Bell Canada
Canada
Email: daniel.bernier@bell.ca
Cheng Li
Huawei
Email: chengli13@huawei.com
Bruno Decraene
Orange
France
Email: bruno.decraene@orange.com
Shaowen Ma
Mellanox
Email: mashaowen@gmail.com
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Chaitanya Yadlapalli
AT&T
USA
Email: cy098d@att.com
Wim Henderickx
Nokia
Belgium
Email: wim.henderickx@nokia.com
Stefano Salsano
Universita di Roma "Tor Vergata"
Italy
Email: stefano.salsano@uniroma2.it
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