MPLS Segment Routing over IPAlibaba, Incxiaohu.xxh@alibaba-inc.comFuturewei Technologiesstewart.bryant@gmail.comOld Dog Consultingadrian@olddog.co.ukCiscoshassan@cisco.comNokiawim.henderickx@nokia.comHuaweilizhenbin@huawei.comMPLS-SR-over-IPSR-MPLS-over-IPMPLS-SR-over-UDPSR-MPLS-over-UDPMPLS Segment Routing (SR-MPLS) is a method of source routing a packet
through an MPLS data plane by imposing a stack of MPLS labels on the
packet to specify the path together with any packet-specific
instructions to be executed on it.
SR-MPLS can be leveraged to realize a source-routing mechanism across
MPLS, IPv4, and IPv6 data planes by using an MPLS label stack as a
source-routing instruction set while making no changes to SR-MPLS
specifications and interworking with SR-MPLS implementations.This document describes how SR-MPLS-capable routers and IP-only
routers can seamlessly coexist and interoperate through the use of
SR-MPLS label stacks and IP encapsulation/tunneling such as MPLS-over-UDP
as defined in RFC 7510.IntroductionMPLS Segment Routing (SR-MPLS) is a method of source routing a packet through an
MPLS data plane. This is achieved by the sender imposing a stack of MPLS
labels that partially or completely specify the path that the packet is
to take and any instructions to be executed on the packet as it passes
through the network.
SR-MPLS uses an MPLS label stack to encode a sequence of source-routing
instructions. This can be used to realize a source-routing mechanism
that can operate across MPLS, IPv4, and IPv6 data planes. This approach
makes no changes to SR-MPLS specifications and allows interworking with
SR-MPLS implementations. More specifically, the source-routing
instructions in a source-routed packet could be
uniformly encoded as an MPLS label stack regardless of whether the
underlay is IPv4, IPv6 (including Segment Routing for IPv6 (SRv6) ), or MPLS.This document describes how SR-MPLS-capable routers and IP-only
routers can seamlessly coexist and interoperate through the use of
SR-MPLS label stacks and IP encapsulation/tunneling such as MPLS-over-UDP
. describes various use
cases for tunneling SR-MPLS over IP. describes a typical application scenario and how the
packet forwarding happens.TerminologyThis memo makes use of the terms defined in and .
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 when, and only when, they appear in all capitals, as
shown here.
Use CasesTunneling SR-MPLS using IPv4 and/or IPv6 (including SRv6) tunnels is
useful at least in the use cases listed below. In all cases, this can be
enabled using an IP tunneling mechanism such as MPLS-over-UDP as described
in . The tunnel selected MUST have its remote
endpoint (destination) address equal to the address of the next
node capable of SR-MPLS identified as being on the SR path (i.e., the
egress of the active segment). The local endpoint (source) address is
set to an address of the encapsulating node.
gives further advice on how to set the source address if the UDP
zero-checksum mode is used with MPLS-over-UDP. Using UDP as the
encapsulation may be particularly beneficial because it is agnostic of
the underlying transport.
Incremental deployment of the SR-MPLS technology may be
facilitated by tunneling SR-MPLS packets across parts of a network
that are not SR-MPLS as shown in . This
demonstrates how islands of SR-MPLS may be connected across a legacy
network. It may be particularly useful for joining sites (such as
data centers).
If the encoding of entropy is desired, IP-tunneling mechanisms that allow the
encoding of entropy, such as MPLS-over-UDP encapsulation where the source port of the UDP
header is used as an entropy field, may be used to maximize the
utilization of Equal-Cost Multipath (ECMP) and/or Link Aggregation
Groups (LAGs), especially when it is difficult to make use of the
entropy-label mechanism. This is to be contrasted with where MPLS-over-IP does not provide
for an entropy mechanism. Refer to ) for more discussion about using entropy labels in
SR-MPLS.
Tunneling MPLS over IP provides a technology that enables Segment
Routing (SR) in an IPv4 and/or IPv6 network where the routers do not
support SRv6 capabilities and
where MPLS forwarding is not an option. This is shown in .
Procedures of SR-MPLS-over-IPThis section describes the construction of forwarding information
base (FIB) entries and the forwarding behavior that allow the deployment
of SR-MPLS when some routers in the network are IP only (i.e., do not
support SR-MPLS). Note that the examples in Sections and assume that
OSPF or IS-IS is enabled; in fact, other mechanisms of discovery and
advertisement could be used including other routing protocols (such as
BGP) or a central controller.Forwarding Entry ConstructionThis subsection describes how to construct the forwarding
information base (FIB) entry on an SR-MPLS-capable router when some or
all of the next hops along the shortest path towards a prefix Segment
Identifier (Prefix-SID) are IP-only routers.
provides a concrete example of how the process applies when using OSPF
or IS-IS.Consider router A that receives a labeled packet with top label
L(E) that corresponds to the Prefix-SID SID(E) of prefix P(E)
advertised by router E. Suppose the i-th next-hop router (termed NHi)
along the shortest path from router A toward SID(E) is not SR-MPLS
capable while both routers A and E are SR-MPLS capable. The following
processing steps apply:
Router E is SR-MPLS capable, so it advertises a Segment Routing
Global Block (SRGB). The SRGB is defined in .
There are a number of ways that the advertisement can be achieved
including IGPs, BGP, and configuration/management protocols. For
example, see .
When Router E advertises the Prefix-SID SID(E) of prefix P(E), it MUST
also advertise the egress endpoint address and the encapsulation type of any
tunnel used to reach E. This information is flooded domain wide.
If A and E are in different routing domains, then the information MUST
be flooded into both domains. How this is achieved depends on the
advertisement mechanism being used. The objective is that router A
knows the characteristics of router E that originated the
advertisement of SID(E).
Router A programs the FIB entry for prefix P(E) corresponding
to the SID(E) according to whether a pop or swap action is advertised
for the prefix. The resulting action may be:
pop the top label
swap the top label to a value equal to SID(E) plus the
lower bound of the SRGB of E
Once constructed, the FIB can be used by a router to tell it how to
process packets. It encapsulates the packets according to the
appropriate encapsulation advertised for the segment and then sends
the packets towards the next hop NHi.FIB Construction ExampleThis section is non-normative and provides a worked example of how
a FIB might be constructed using OSPF and IS-IS extensions. It is based
on the process described in .
Router E is SR-MPLS capable, so it advertises a Segment Routing
Global Block (SRGB) using
or
.
When Router E advertises the Prefix-SID SID(E) of prefix P(E),
it also advertises the encapsulation endpoint address and the tunnel
type of any tunnel used to reach E using
or
.
If A and E are in different domains, then the information is
flooded into both domains and any intervening domains.
The OSPF Tunnel Encapsulations TLV
or the IS-IS
Tunnel Encapsulation Type sub-TLV
is flooded
domain wide.
The OSPF SID/Label Range TLV
or
the IS-IS SR-Capabilities sub-TLV
is
advertised domain wide so that router A knows the
characteristics of router E.
When router E advertises the prefix P(E):
If router E is running IS-IS, it uses the extended
reachability TLV (TLVs 135, 235, 236, 237) and associates
the IPv4/IPv6 or IPv4/IPv6 Source Router ID sub-TLV(s)
.
If router E is running OSPF, it uses the OSPFv2 Extended
Prefix Opaque Link-State Advertisement (LSA) and sets the
flooding scope to Autonomous System (AS) wide.
If router E is running IS-IS and advertises the IS-IS
Router CAPABILITY TLV (TLV 242) , it sets the
"Router ID" field to a valid value or includes an IPv6
TE Router ID sub-TLV (TLV 12), or it does both. The "S" bit
(flooding scope) of the IS-IS Router CAPABILITY TLV (TLV 242) is set
to "1".
Router A programs the FIB entry for prefix P(E) corresponding
to the SID(E) according to whether a pop or swap action is advertised
for the prefix as follows:
If the No-PHP (NP) Flag in OSPF or the Persistent (P) Flag in IS-IS is clear:
pop the top label
If the No-PHP (NP) Flag in OSPF or the Persistent (P) Flag in IS-IS is set:
swap the top label to a value equal to SID(E) plus the
lower bound of the SRGB of E
When forwarding the packet according to the constructed FIB entry, the
router encapsulates the packet according to the encapsulation as advertised
using the mechanisms described in
or . It then sends the
packets towards the next hop NHi.Note that specifies the use of port number 6635
to indicate that the payload of a UDP packet is MPLS, and port number 6636 for
MPLS-over-UDP utilizing DTLS. However,
and provide dynamic protocol
mechanisms to configure the use of any Dynamic Port for a tunnel that uses UDP
encapsulation. Nothing in this document prevents the use of an IGP or any other
mechanism to negotiate the use of a Dynamic Port when UDP encapsulation is used
for SR-MPLS, but if no such mechanism is used, then the port numbers specified in
are used.Packet-Forwarding Procedures specifies an IP-based encapsulation for
MPLS, i.e., MPLS-over-UDP. This approach is applicable where IP-based
encapsulation for MPLS is required and further fine-grained load
balancing of MPLS packets over IP networks over
ECMP and/or LAGs is also required. This
section provides details about the forwarding procedure when
UDP encapsulation is adopted for SR-MPLS-over-IP. Other encapsulation
and tunneling mechanisms can be applied using similar techniques,
but for clarity, this section uses UDP encapsulation as the exemplar.Nodes that are SR-MPLS capable can process SR-MPLS packets. Not all
of the nodes in an SR-MPLS domain are SR-MPLS capable. Some nodes may
be "legacy routers" that cannot handle SR-MPLS packets but can forward
IP packets. A node capable of SR-MPLS MAY advertise its capabilities
using the IGP as described in . There are six
types of nodes in an SR-MPLS domain:
Domain ingress nodes that receive packets and encapsulate them
for transmission across the domain. Those packets may be any
payload protocol including native IP packets or packets that are
already MPLS encapsulated.
Legacy transit nodes that are IP routers but that are not
SR-MPLS capable (i.e., are not able to perform Segment
Routing).
Transit nodes that are SR-MPLS capable but that are not
identified by a SID in the SID stack.
Transit nodes that are SR-MPLS capable and need to perform
SR-MPLS routing because they are identified by a SID in the SID
stack.
The penultimate node capable of SR-MPLS on the path that processes
the last SID on the stack on behalf of the domain egress node.
The domain egress node that forwards the payload packet for
ultimate delivery.
Packet Forwarding with Penultimate Hop PoppingThe description in this section assumes that the label associated
with each Prefix-SID is advertised by the owner of the Prefix-SID as
a Penultimate Hop-Popping (PHP) label. That is, if one of the IGP
flooding mechanisms is used, the NP-Flag in OSPF or the P-Flag in
IS-IS associated with the Prefix-SID is not set.In the example shown in , assume that
routers A, E, G, and H are capable of SR-MPLS 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, such as 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 the explicit path
{E->G->H}. Router A will impose an MPLS label stack on the
packet that corresponds to that explicit path. Since the next hop
toward router E is only IP capable (B is a legacy transit node),
router A replaces the top label (that indicated router E) with a
UDP-based tunnel for MPLS (i.e., MPLS-over-UDP ) to router E and then sends the packet. In other
words, router A pops the top label and then encapsulates the MPLS
packet in a UDP tunnel to router E.When the IP-encapsulated MPLS packet arrives at router E (which
is a transit node capable of SR-MPLS), router E strips the IP-based
tunnel header and then processes the decapsulated MPLS packet. The top
label indicates that the packet must be forwarded toward router G.
Since the next hop toward router G is only IP capable, router E
replaces the current top label with an MPLS-over-UDP tunnel toward
router G and sends it out. That is, router E pops the top label and
then encapsulates the MPLS packet in a UDP tunnel to router G.When the packet arrives at router G, router G will strip the
IP-based tunnel header and then process the decapsulated MPLS
packet. The top label indicates that the packet must be forwarded
toward router H. Since the next hop toward router H is only
IP capable (D is a legacy transit router), router G would replace
the current top label with an MPLS-over-UDP tunnel toward router H
and send it out. However, since router G reaches the bottom of the
label stack (G is the penultimate node capable of SR-MPLS on the path),
this would leave the original packet that router A wanted to send to
router H encapsulated in UDP as if it was MPLS (i.e., with a UDP
header and destination port indicating MPLS) even though the
original packet could have been any protocol. That is, the final
SR-MPLS has been popped exposing the payload packet.To handle this, when a router (here it is router G) pops the
final SR-MPLS label, it inserts an explicit NULL label before encapsulating the packet in an
MPLS-over-UDP tunnel toward router H and sending it out. That is,
router G pops the top label, discovers it has reached the bottom of
stack, pushes an explicit NULL label, and then encapsulates the MPLS
packet in a UDP tunnel to router H.Packet Forwarding without Penultimate Hop Popping demonstrates the packet walk in the
case where the label associated with each Prefix-SID advertised by
the owner of the Prefix-SID is not a Penultimate Hop-Popping (PHP)
label (e.g., the NP-Flag in OSPF or the P-Flag in IS-IS
associated with the Prefix-SID is set). Apart from the PHP function,
the roles of the routers are unchanged from .As can be seen from the figure, the SR-MPLS label for each
segment is left in place until the end of the segment where it is
popped and the next instruction is processed.Additional Forwarding Procedures
Non-MPLS Interfaces:
Although the description in
the previous two sections is based on the use of Prefix-SIDs,
tunneling SR-MPLS packets is useful when the top label of a
received SR-MPLS packet indicates an Adjacency SID and the
corresponding adjacent node to that Adjacency SID is not capable
of MPLS forwarding but can still process SR-MPLS packets. In
this scenario, the top label would be replaced by an IP tunnel
toward that adjacent node and then forwarded over the
corresponding link indicated by the Adjacency SID.
When to Use IP-Based Tunnels:
The description in
the previous two sections is based on the assumption that
an MPLS-over-UDP tunnel is used when the next hop towards the next
segment is not MPLS enabled. However, even in the case where the
next hop towards the next segment is MPLS capable, an
MPLS-over-UDP tunnel towards the next segment could still be
used instead due to local policies. For instance, in the example
as described in , assume F is now a
transit node capable of SR-MPLS while all the other assumptions
remain unchanged; since F is not identified by a SID in the stack
and an MPLS-over-UDP tunnel is preferred to an MPLS LSP
according to local policies, router E replaces the current
top label with an MPLS-over-UDP tunnel toward router G and sends
it out. (Note that if an MPLS LSP was preferred, the packet
would be forwarded as native SR-MPLS.)
IP Header Fields:
When encapsulating an MPLS
packet in UDP, the resulting packet is further encapsulated in
IP for transmission. IPv4 or IPv6 may be used according to the
capabilities of the network. The address fields are set as
described in . The other IP header
fields (such as the ECN field , the
Differentiated Services Code Point (DSCP) , or IPv6 Flow Label) on each UDP-encapsulated
segment SHOULD be configurable according to the operator's
policy; they may be copied from the header of the incoming
packet; they may be promoted from the header of the payload
packet; they may be set according to instructions programmed to
be associated with the SID; or they may be configured dependent
on the outgoing interface and payload. The TTL field setting in
the encapsulating packet header is handled as described in
, which refers to .
Entropy and ECMP:
When encapsulating an MPLS
packet with an IP tunnel header that is capable of encoding
entropy (such as ), the corresponding
entropy field (the source port in the case of a UDP tunnel) MAY
be filled with an entropy value that is generated by the
encapsulator to uniquely identify a flow. However, what
constitutes a flow is locally determined by the encapsulator. For
instance, if the MPLS label stack contains at least one entropy
label and the encapsulator is capable of reading that entropy
label, the entropy label value could be directly copied to the
source port of the UDP header. Otherwise, the encapsulator may
have to perform a hash on the whole label stack or the five-tuple
of the SR-MPLS payload if the payload is determined as an IP packet.
To avoid recalculating the hash or hunting for the entropy label
each time the packet is encapsulated in a UDP tunnel, it MAY be
desirable that the entropy value contained in the incoming
packet (i.e., the UDP source port value) is retained when
stripping the UDP header and is reused as the entropy value of
the outgoing packet.
Congestion Considerations:
provides a detailed analysis of the
implications of congestion in MPLS-over-UDP systems and builds
on , which describes
the congestion implications of UDP tunnels. All of those
considerations apply to SR-MPLS-over-UDP tunnels as described
in this document. In particular, it should be noted that the
traffic carried in SR-MPLS flows is likely to be IP traffic.
IANA ConsiderationsThis document has no IANA actions.Security ConsiderationsThe security consideration of (which redirects
the reader to ) and
apply. DTLS SHOULD be used where security is
needed on an SR-MPLS-over-UDP segment including when the IP segment crosses
the public Internet or some other untrusted environment.
provides security considerations for Segment Routing, and is particularly applicable to SR-MPLS.It is difficult for an attacker to pass a raw MPLS-encoded packet
into a network, and operators have considerable experience in excluding
such packets at the network boundaries, for example, by excluding all
packets that are revealed to be carrying an MPLS packet as the payload
of IP tunnels. Further discussion of MPLS security is found in
.It is easy for a network ingress node to detect any attempt to smuggle an IP
packet into the network since it would see that the UDP destination port
was set to MPLS, and such filtering SHOULD be applied. If, however, the
mechanisms described in
or are applied,
a wider variety of UDP port numbers might be in use making port filtering
harder.SR packets not having a destination address terminating in the network
would be transparently carried and would pose no different security risk to
the network under consideration than any other traffic.Where control-plane techniques are used (as described in ), it is important that these protocols are adequately
secured for the environment in which they are run as discussed in
and .ReferencesNormative ReferencesSegment Routing with the MPLS Data PlaneInformative ReferencesOSPF Extensions for Segment RoutingIS-IS Extensions for Segment RoutingEntropy Label for Source Packet Routing in Networking (SPRING) TunnelsAcknowledgementsThanks to Joel Halpern, Bruno Decraene, Loa Andersson,
Ron Bonica, Eric Rosen, Jim Guichard, Gunter Van De Velde,
Andy Malis, Robert Sparks, and Al Morton for their insightful
comments on this document.Additional thanks to Mirja Kuehlewind, Alvaro Retana, Spencer Dawkins,
Benjamin Kaduk, Martin Vigoureux, Suresh Krishnan, and Eric Vyncke
for careful reviews and resulting comments.Contributors
Ahmed Bashandy
Individual
Email: abashandy.ietf@gmail.com
Clarence Filsfils
Cisco
Email: cfilsfil@cisco.com
John Drake
Juniper
Email: jdrake@juniper.net
Shaowen Ma
Mellanox Technologies
Email: mashaowen@gmail.com
Mach Chen
Huawei
Email: mach.chen@huawei.com
Hamid Assarpour
Broadcom
Email:hamid.assarpour@broadcom.com
Robert Raszuk
Bloomberg LP
Email: robert@raszuk.net
Uma Chunduri
Huawei
Email: uma.chunduri@gmail.com
Luis M. Contreras
Telefonica I+D
Email: luismiguel.contrerasmurillo@telefonica.com
Luay Jalil
Verizon
Email: luay.jalil@verizon.com
Gunter Van De Velde
Nokia
Email: gunter.van_de_velde@nokia.com
Tal Mizrahi
Marvell
Email: talmi@marvell.com
Jeff Tantsura
Apstra, Inc.
Email: jefftant.ietf@gmail.com