Multipath Traffic Engineering
draft-kompella-teas-mpte-03
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Kireeti Kompella , Luay Jalil , Mazen Khaddam , Andy Smith | ||
| Last updated | 2026-07-06 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
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draft-kompella-teas-mpte-03
TEAS WG K. Kompella
Internet-Draft HPE
Intended status: Standards Track L. Jalil
Expires: 7 January 2027 Verizon
M. Khaddam
Cox Communications
A. J. Smith
Arrcus, Inc.
6 July 2026
Multipath Traffic Engineering
draft-kompella-teas-mpte-03
Abstract
Shortest path routing offers an easy-to-understand, easy-to-implement
method of establishing loop-free connectivity in a network, but
offers few other features. Equal-cost multipath (ECMP), a simple
extension, uses multiple equal-cost paths between any two points in a
network: at any node in a path (really, Directed Acyclic Graph),
traffic can be (typically equally) load-balanced among the next hops.
ECMP is easy to add on to shortest path routing, and offers a few
more features, such as resiliency and load distribution, but the
feature set is still quite limited.
Traffic Engineering (TE), on the other hand, offers a very rich
toolkit for managing traffic flows and the paths they take in a
network. A TE network can have link attributes such as bandwidth,
colors, risk groups and alternate metrics. A TE path can use these
attributes to include or avoid certain links, increase path
diversity, manage bandwidth reservations, improve service experience,
and offer protection paths. However, TE typically doesn't offer
multipathing as the tunnels used to implement TE usually take a
single path.
This memo proposes multipath traffic-engineering (MPTE), combining
the best of ECMP and TE. The multipathing proposed here need not be
strictly equal-cost, allowing for some "slack" to admit more paths.
The load balancing at each hop is optimally weighted to each next hop
rather than always being equally weighted. Moreover, traffic can
enter and leave an MPTE construct via multiple ingresses and
egresses. The proposal includes several choices of control and data
planes.
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Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.1. Definition of Commonly Used Terms . . . . . . . . . . 5
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Multipathing . . . . . . . . . . . . . . . . . . . . . . 7
2.1.1. ECMP (slack 0) from node 0 to node 5 . . . . . . . . 7
2.1.2. nECMP from node 0 to node 5 with slack 10 . . . . . . 8
2.1.3. Multipathing from node 0 to egresses {5, 8} . . . . . 8
2.1.4. MPTED from ingresses {0, 1} to egresses {5, 8} . . . 8
2.2. Load balancing . . . . . . . . . . . . . . . . . . . . . 8
2.2.1. Flow-aware load balancing . . . . . . . . . . . . . . 9
2.2.2. Per-packet load balancing . . . . . . . . . . . . . . 9
2.3. Constraints . . . . . . . . . . . . . . . . . . . . . . . 10
2.4. Protection . . . . . . . . . . . . . . . . . . . . . . . 10
2.5. Tunnels . . . . . . . . . . . . . . . . . . . . . . . . . 11
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2.6. Backward Compatibility . . . . . . . . . . . . . . . . . 12
3. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1. MPTED . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2. Tunnel Provisioning . . . . . . . . . . . . . . . . . . . 15
3.3. Signaling Overview . . . . . . . . . . . . . . . . . . . 15
4. Signaling . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1. Message Flow . . . . . . . . . . . . . . . . . . . . . . 16
4.2. Message Types . . . . . . . . . . . . . . . . . . . . . . 16
4.2.1. JUNCTION . . . . . . . . . . . . . . . . . . . . . . 16
4.2.2. LABEL . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2.3. NOTIFY . . . . . . . . . . . . . . . . . . . . . . . 17
4.3. Forwarding State . . . . . . . . . . . . . . . . . . . . 17
4.3.1. IP Tunnels . . . . . . . . . . . . . . . . . . . . . 17
4.3.2. Static Label . . . . . . . . . . . . . . . . . . . . 19
4.3.3. Signaled Label . . . . . . . . . . . . . . . . . . . 19
5. Graceful Restart . . . . . . . . . . . . . . . . . . . . . . 19
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 20
9.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
Operators managing traffic within their networks have several tools,
among them:
1. Equal-cost Multipath (ECMP): balance traffic along multiple
paths. This yields some resilience and some traffic management,
as traffic can be load-balanced across multiple paths. To use
ECMP effectively, one may have to adjust link metrics to allow
multiple paths to have the same overall distance.
2. Traffic Engineering (TE): state constraints for a path from an
ingress router to an egress router, and let a path computation
engine compute it. This gives much greater control over the
nodes and links traversed, but is usually limited to finding a
single path from ingress to egress [RFC2702].
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3. Multi-egress: allow traffic from an ingress router to a
destination dst to use several egress routers, all of which have
routes to that destination. dst may be an Internet prefix
[RFC4271], a VPN prefix [RFC4364], an EVPN address [RFC7432], a
VPLS site [RFC4761], [RFC4762] or some other service destination.
For BGP-signaled destinations, this requires that the BGP tie-
breaking algorithm yield multiple results (rather than a single
one), all of which become candidates for egress.
4. Multi-ingress: consider multiple ingress routers as "equivalent"
with respect to some of the traffic they sent to one or more
egress routers. For example, an eBGP peer router or a VPN site
may be multi-homed to several ingress routers, all of which would
send such traffic to the same set of egress routers.
[RFC2702] describes requirements for MPLS-based TE, and thus is
relevant to this memo. At the same time, the authors appear to
believe that one can either have TE or multipathing, but not both.
This is further emphasized by the notion of a Label Switched Path,
which is used to implement MPLS-based TE. RSVP-TE ([RFC3209]), the
protocol designed to meet the requirements of [RFC2702], builds a
single path from one ingress to one egress (for unicast traffic).
In order to satisfy the constraints, TE often uses non-shortest
paths. To do so without looplng packets, a tunnel is used. Such
tunnels have to be signaled. RSVP-TE is a signaling protocol for
MPLS-based tunnels.
In this memo, we introduce a new tool: multipath TE (MPTE). This
allows an operator to specify constraints for paths (as in TE),
specify multiple ingresses and egresses, and use multiple paths from
ingress to egress. Effectively, MPTE combines the advantages of the
four tools above. The resulting set of paths from ingresses to
egresses is a Directed Acyclic Graph (DAG), here called an MPTE DAG
or MPTED. Finally, this memo allows the use of multiple types of
tunnels. The main contribution of this memo is the notion of a
(multipath) unicast tunnel across an MPTED, called an MPTE tunnel or
MPTET, and an overview of how they are created. Protocols for
provisioning such tunnels will be specified in companion documents.
Another companion document defines how to distribute MPTE
capabilities in an IGP so that entities computing MPTEDs can know
which nodes to include in the DAG.
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1.1. Terminology
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.
1.1.1. Definition of Commonly Used Terms
This section provides definitions for terms and abbreviations that
have a specific meaning to the MPTE protocol and that are used
throughout this memo.
constraints: desired properties of paths between ingresses and
egresses.
constrained shortest path first (CSPF): A modification to SPF to
take into account TE constraints.
directed acyclic graph (DAG): a directed graph that has no cycles.
The result of a multipath SPF or CSPF computation is a DAG.
directed graph: a set of nodes and directed links. A network is
represented by a directed graph.
egress: an end node of an MPTE DAG.
equal-cost multipath (ECMP): a DAG consisting of shortest paths from
an ingress to an egress.
flow-aware load balancing (FALB): load balancing that maps packets
that belong to a given flow onto the same path. What consitutes a
flow depends on the type of traffic; for IP traffic, a flow is
typically defined by the 5-tuple <source IP, dest IP, protocol
type, source port, dest port>.
ingress: a starting node of an MPTE DAG.
label-switched path (LSP): an MPLS tunnel from an ingress to one or
more egresses.
link: A (directed) edge between two nodes. A pair of nodes may have
0 or more links between them. A link between nodes u and v will
be denoted by (u, v, i), where i is u's oif for the link. A link
may have associated attributes, in particular, a metric.
load balancing (LB): a method whereby traffic to an egress is
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distributed among multiple next hops at each node along the DAG.
metric: a positive number describing the contribution of a link to
the oveall path length.
MC: MPTED computer: the entity computing the MPTED, typically the
ingress (if there is a single ingress) or a Path Computation
Element
MPTE: multipath TE with path constraints (including a slack) using
nECMP paths from an ingress to one or more egresses.
MPTED: an MPTE DAG resulting from CSPF-type computation on MPTE
constraints.
MPTEP: MPTE protocol: the protocol used to signal MPTETs.
MPTET: MPTE tunnel: the signaled (and hence, forwarding) entity
associated with an MPTED. Note: this terminology dates back to
[RFC3209], where the signaled entity was a simple path with one
ingress and one egress. MPTE constructs a DAG, not a path. This
document will use the term DAG (MPTED) (rather than path) for the
result of a CSPF computation, and the term tunnel (MPTET) for the
signaled entity. If it helps visualization, imagine a multi-
branched tunnel in a cave with multiple entry and exit points.
non-equal-cost multipath (nECMP) (generally qualified by "with
slack s"): a DAG of paths from an ingress u to an egress v that are
within s of min(u, v).
node: a vertex of a graph. A node may have associated attributes.
outgoing interface (oif): a unique number (oif) assigned by a node
for each outgoing link it has.
Path Computation Element (PCE): an entity capable of performing CSPF
on behalf of another node, the path computation client.
path length: the sum of the metrics of the links that constitute
path p, denoted by len(p)
shared risk group (SRG): nodes and/or links that share "risk" (e.g.,
have a common power feed, or use a common fiber conduit)
shortest path: a path between a pair of nodes u, v with minimum
length. The set of shortest paths between u and v is a DAG,
denoted by sp(u, v). The length of a shortest path from u to v is
denoted by min(u, v)
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shortest path first (SPF): an algorithm for computing the shortest
path DAG from an ingress to an egress; typically refers to Dijkstra's
algorithm for computing shortest paths between a given pair of nodes,
or pairwise between all nodes.
signaling source (SS): an entity responsible for signaling an MPTET
slack: a path p from u to v has slack s if len(p) = min(u, v) + s.
traffic engineering (TE): a methodology for mapping traffic trunks to
given paths or DAGs across a network.
traffic trunk: a unidirectional aggregate of traffic flows from an
ingress to a set of egresses that is treated identically in the
forwarding plane.
tunnel originator (TO): entity having the specifications of the
MPTET
2. Overview
Consider Figure 1:
r
2 == 3 Link Metrics (symm): 0-2: 100; 0-4: 200; 0-6: 110
r/ r\ r\\ 1-2 (not shown): 110; 1-4 (not sh): 100; 1-6: 100
0 -- 4 -- 5 2-3: (100, 100); 2-4: 100; 3-5: (100, 110)
\ / \ / \ 4-5: 100; 4-6: 110; 4-7: 50
1 - 6 = 7 -- 8 5-7: 100; 5-8: 10; 6-7: (100, 110); 7-8: 50
r Node pairs 2-3, 3-5 and 6-7 each have two links.
Links marked with 'r' have color red.
Figure 1: Network 1
2.1. Multipathing
2.1.1. ECMP (slack 0) from node 0 to node 5
There are 4 ECMP paths from node 0 to node 5:
1. 0-2=3-5 (2 paths)
2. 0-2-4-5
3. 0-4-5
These 4 distinct paths all have length 300.
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2.1.2. nECMP from node 0 to node 5 with slack 10
There are 7 nECMP paths with slack 10 to node 5:
1. 0-2=3=5 (4 paths)
2. 0-2-4-5
3. 0-4-5
4. 0-6-7-5
These 7 paths have lengths 300 or 310. Thus, allowing nECMP paths a
slack of 10 has yielded 3 additional paths, which provide increased
diversity and load balancing, and possibly decreased congestion.
2.1.3. Multipathing from node 0 to egresses {5, 8}
If, for some traffic trunk that starts at node 0, nodes 5 and 8 are
equally good as egresses, then one can compute an ECMP DAG from 0 to
{5, 8}; this yields 4 paths to 5 and 6 paths to 8, for a total of 10
paths this traffic trunk can take. Similarly, a nECMP DAG to {5, 8}
with slack 10 has 15 paths, whereas one with slack 5 has the same 10
paths as with slack 0.
2.1.4. MPTED from ingresses {0, 1} to egresses {5, 8}
If traffic from node 0 to nodes {5, 8} and from node 1 to nodes {5,
8} have common characteristics, it may make sense to compute a single
DAG from {0, 1} to {5, 8}. Doing so allows the operator to view this
entire DAG as one logical entity; a nice side benefit is reduced
control and data plane state due to state sharing.
2.2. Load balancing
Nodes in a netword have a Forwarding Information Base (FIB). A FIB
maps a packet's destination address da to one or more "next hops".
When a packet with address da arrives at n, n sends the packet to one
of the next hops. n typically will distribute packets in a given
ratio among the next hops. This is load balancing.
The main goal of ECMP/nECMP is to supply as many nodes as possible in
the MPTED with multiple next hops on which to forward the traffic
trunk. At such nodes, traffic belonging to the trunk can be
distributed among the next hops instead of going to a single next
hop. This has the potential to reduce congestion and provide better
utilization of available links.
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2.2.1. Flow-aware load balancing
When load balancing packets from a traffic trunk, it is often
required that packets from a given flow be sent to the same next hop.
This improves the probability of in-order delivery of packets in that
flow, which is important for certain types of traffic. This is
called flow-aware load balancing (FALB). The most common flow in IP
traffic is defined by a 5-tuple (typically from the innermost IP
header) consisting of the source IP address, the destination IP
address, the protocol, the source port and the destination port. A
hash (usually 16- or 20-bit) of this 5-tuple is called the packet's
entropy.
There are two common ways to achieve FALB of IP traffic. One is to
do a "deepish" packet inspection (dPI), find the relevant 5-tuple,
and use that to compute the packet's entropy. The entropy is then
used to ensure that packets in the flow are sent to the same next
hop. This memo suggests sending TE traffic over a tunnel (see
{tunnels}); this can make the identification of IP flows expensive
and error-prone.
Another way of accomplishing this is to insert the entropy in the
tunnel header. Many of the tunnels suggested in this memo have such
a field. The ingress is in a good position to identify flows, and,
when encapsulating the packet into the tunnel, can insert the entropy
in the header. The heavy lifting of identifying flows is thus placed
on the ingress. Transit nodes can simply use the entropy field to
correctly map packets in a flow to the same next hop, thus ensuring
FALB.
2.2.2. Per-packet load balancing
FALB is often required and is a good default behavior, especially as
end applications may be expecting packets in a flow to be delivered
in order. However, FALB has the issue that it attempts
(statistically) to place roughly the number of flows in the given
ratio on the outgoing links; that may not place traffic in the same
ratio, as flows need not carry the same amount of traffic. In some
cases (typically when configured to), one can do per-packet load
balancing (PPLB), meaning that load balancing is no longer flow
aware. This can be done when the end applications do not require
packets in a flow to be in order, or if some (bookended) devices
outside the network put the packets back in order before delivering
them to the applications (typically by addind a sequence number).
When feasible, PPLB gives much better load distribution, and is
currently the subject of investigation, implementation and
standardization.
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One can achieve this by configuring each router in the DAG to do PPLB
for the traffic trunks in the MPTET, or more simply by the ingress
router assigning entropy at random to each packet it places in the
MPTET. The latter approach keeps the decision of which DAGs (and
corresponding traffic trunks) should be flow-aware and which not at
the ingress; all other nodes simply do what the entropy fields tells
them to do.
2.3. Constraints
Constraints are an intent-based specification of acceptable paths
that a traffic trunk may take from ingress(es) to egress(es).
Constraints are thus an abstract way to control the resources that a
particular traffic trunk uses.
One way to do this is to add "resource class attributes" or "colors"
[RFC2702] to links, and then specify "include" and "exclude" sets.
An include set means that all links that a path traverses must
contain at least one element of the include set. An exclude set
means that no link in the path can contain any color from the exclude
set.
Another way is to specify a (maximum) bandwidth that a traffic trunk
can carry. This means that all links in the path must have that much
available capacity. Packets exceeding the bandwidth can be forwarded
normally, marked as droppable, or dropped.
Let's add some simple constraints to our DAG. We associate the color
red to one of the links from 2 to 3, and to the shorter of the links
from 6 to 7. Then, we constrain the paths to "include red or blue",
meaning only use links with either color red or blue. This yields
the following:
* ECMP from node 0 to node 5 with constraints "include red or blue"
yields a single path.
2.4. Protection
One very useful aspect of TE is the ability to specify that a path
must be link- or node- or shared-risk-disjoint from another path.
That means that the two paths do not have links or nodes or "shared
risk groups". Additionally, one can build protection paths for an
existing path to protect against link or node failures [RFC4090].
This is especially important as traditional TE currently takes a
single path through the network, meaning that a link or node failure
will result in dropped traffic until the TE path is restored.
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While not quite as crucial in the case of an MPTET, since ideally,
there will be multiple nexthops at each node, there will be cases
where a node has a single next hop, or all next hops share a common
failure mode. Identifying these cases and building protection paths
for such nodes will be described in a future version of this memo.
2.5. Tunnels
The shortest path first algorithm [SPF] is an easy-to-implement and
very efficient algorithm whereby all routers in a network can agree
on the path that a packet to a particular destination should take.
That means if all routers are agreed (roughly) on the topology and
metrics of the network, they will forward packets in a loop-free
manner to all destinations -- without the need for signaling or
tunnels. However, an MPTED will not contain the same paths -- some
paths may be rejected as they don't satisfy the constraints; other
paths may be used even though they are not shortest paths. Thus, to
route packets in a traffic trunk over a computed MPTED, a tunnel is
typically used. This tunnel will have to be signaled to the MPTED
nodes. The tunnel may be MPLS- or IP-based.
A few things are important about tunnel headers: whether they carry
an entropy field (EF), whether they have a "discriminator" (D) that
allows multiple tunnels between an ingress-egress pair, whether they
allow multiple egresses (ME), and whether they allow multiple
ingresses (MI). These will be discussed in the description of the
tunnels below.
In the memo, we consider the following tunnel types:
1. IP-in-IP: [RFC2003] encapsulation allows the creation of an
"outer" IP header (IPv4 or IPv6) to carry a payload packet (which
is typically an IP payload). The outer IP header's protocol
field indicates the "protocol" of the inner payload packet. The
outer header of IP-in-IPv4 tunnel doesn't contain an EF; transit
nodes can either spray packets across outgoing next hops, attempt
to do dPI, or use the same next hop for all packets. For IP-in-
IPv6, the outer header's flow label can be used for the EF. To
accommodate ME, the egresses have to have the same (anycast) IP
address which would be used as the destination IP of the tunnel.
MI is not possible.
2. GRE: Generic Routing Encapsulation. We include in this
definition [RFC2784] and [RFC2890] with the Key Present (bit 2)
set to 0. This is similar to IP-in-IP; however, the payload is
not required to be IP. There is no EF in the header. D, ME and
MI same as for IP-in-IP.
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3. GRE-E: GRE with Key Present; the Key value is the EF. D, ME and
MI same as for IP-in-IP.
4. GRE6: GRE with IPv6 addresses. The EF is the Flow Label field of
the IPv6 header. D, ME and MI same as for IP-in-IP.
5. G-in-U: GRE-in-UDP [RFC8086]. The UDP source port is the EF; the
GRE Key, if present, can be ignored from a load balancing point
of view. D, ME and MI as in IP-in-IP.
6. MPLS-in-UDP [RFC7510]. The UDP source port is the EF; D, ME and
MI as in IP-in-IP.
7. SRv6 [RFC8754]. The EF is the Flow Label in the IPv6 header.
8. SigLab (signaled label switching). The labels to be used are
signaled. Signaling proceeds from egress(es) to ingress(es). An
entropy label can be used as the EF. At each node, a different
label is used for each MPTED; this is the discriminator. ME and
MI are both allowed.
9. StatLab (static label). A single statically-assigned label
defines the tunnel throughout the MPTED. Here, a block of MPLS
labels is given to a label allocator; these labels MUST NOT be
allocated by any node in the network. EF, D, ME and MI are as
for SigLab. The MPTED computer (MC) must interact with the
allocator when creating or deleting an MPTED.
2.6. Backward Compatibility
Introducing a new idea to the network (and thus new protocols, new
extenstion and new software) is typically done incrementally. Thus,
in a network transitioning to MPTE, there will be some nodes that are
MPTE-capable, and others that are not.
In Figure 1 above, if node 4 is not MPTE-capable, it can either be
left out of the MPTED, or a "classical" tunnel can be constructed
from (say) node 2 to node 5, allowing hybrid paths 0-2-(4)-5 and 0-2-
(4)-5-8 for a DAG from {0} to {5, 8}. The signaling specs will say
whether this is possible, and if so, how it can be achieved.
3. Operation
Here are the steps to create an MPTE tunnel:
1. Define the traffic trunk for the MPTET. Examples include "BGP
destinations with community xyz" or "gold class traffic belonging
to VPN foo" or "AI workload bar from DC A to DC B".
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2. Define the constraints of the traffic trunk, including:
1. ingresses, and the bandwidth entering the DAG at each
ingress;
2. egresses;
3. metric to minimize (perhaps with slack) -- this could capture
delay or fiber length;
4. criteria of acceptable nodes and links for the DAG, including
link colors and shared risk groups (SRGs).
This information is given to the Tunnel Originator (TO).
3. The TO sends this information to the MPTE Computer (MC).
4. The MC computes a DAG that satisfies the constraints. The DAG
consists of a set of junctions; these are sent to the Signaling
Source (SS).
5. The SS instantiates the MPTET by sending signaling messages to
all the junctions.
6. When ready, the SS tells each ingress that the MPTET meeting the
DAG constraints is ready for traffic.
7. The ingresses map traffic matching the traffic trunk to the
MPTET.
Computation (possibly using a variant of CSPF) of an MPTED is done by
the MC, which may be an ingress or a PCE [RFC4655]. (This memo does
not specify such an algorithm.) Signaling primarily occurs between
the SS and each junction node. Auxiliary signaling may occur among
junction nodes.
3.1. MPTED
In this memo, a node is identified by its IP loopback address. A
link from node u to node v is identified by u's loopback address and
its (4-octet) outgoing interface index (oif), a unique identifier for
the link allocated by u. oifs are usually exchanged in the TE
extensions of an IGP. (A link also has a (4-octet) incoming
interface index, the iif. For neighbors u and v, the correlation
between u's oif and v's iif is typically done by the IGP. iifs are
not used in this memo.) For now, this memo only deals with point-to-
point links; a future revision will describe the use of multi-access
links.
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An MPTED is identified by a unique (4-octet) ID (the MID) assigned to
the MPTED by the MC. As an MPTED can change over its lifetime, it is
assigned a version number starting at 0 and incremented every time
the MPTED is recomputed. Thus, a full MPTED ID (the FID) consists of
<MC, MID, version>.
An MPTED consists of two or more "junction nodes". A junction node
can have one of five types:
1. a pure ingress node has zero incoming links and one or more
outgoing links in the MPTED. Traffic routed on a MPTED enters at
the ingress.
2. a pure egress node has one or more incoming links and zero
outgoing links in the MPTED. Traffic routed on a MPTED leaves at
an egress.
3. a transit ingress node where traffic can either enter the MPTED
or arrive from another ingress node to continue on in the MPTED.
4. a transit egress node where traffic can either exit the MPTED or
go on to another egress node.
5. a "regular" junction node has one or more incoming links and one
or more outgoing links. Traffic does not enter or leave at such
a node: it comes from a phop and goes to an nhop.
A junction node v consists of v, its previous hops (phops) and its
next hops (nhops). A phop is specified by an incoming link of v: (u,
v, oif1); an nhop by an outgoing link of v: (v, w, oif2). Note that,
since links are point-to-point, it is sufficient to specify (u, oif1)
((v, oif2)) for a phop (nhop, respectively). The nodes u (and w) are
loosely referred to as a phop (and nhop) of v, although strictly
speaking the link should be included. A pure ingress has no phops
and a pure egress has no nhops.
The MPTED is broken down into a set of junction nodes. A junction
node v is specified by:
1. bandwidth (coming in to and going out of v)
2. a list of phops of v
3. a list of nhops of v, with corresponding load balancing shares
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3.2. Tunnel Provisioning
A designated entity, the Tunnel Originator (TO), is given the
specifications of the MPTET: the ingresses, the egresses and the
constraints. The TO is typically one of the tunnel ingresses or a
PCE. The TO sends the tunnel specification to the MC. The MC
computes the MPTED (as a list of junctions) and returns this to the
TO. The TO then sends the list of junctions to the Signaling Source
(SS) which provisions the tunnel.
Note that TO, MC and SS are functional blocks; they may reside on
separate nodes or co-reside on the same node. For example, a single
node X may be the TO and SS but decide to delegate computation to a
(remote) PCE. X then gets the results via PCEP and signals the
tunnel. Other permutations are possible.
3.3. Signaling Overview
The SS signals the creation or update of an MPTE tunnnel by sending
to each junction node v a JUNCTION message consisting of:
1. the MPTET ID
2. the junction node specification
3. the tunnel type
4. some flags
After v parses this specification, for all tunnel types other than
SigLab, it installs FIB state for the junction.
For tunnel type SigLab, v allocates an incoming MPLS label L_u for
each phop u (u, oif), and sends a LABEL message to u containing:
1. the MPTET ID
2. the phop (u's loopback + u's oif for the link)
3. the allocated label L_u
u records label L_u as part of its own junction state.
When v receives a LABEL message from all its nhops, it installs swap
state in its LFIB.
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4. Signaling
Several signaling protocols are being extended to provision MPTETs:
RSVP-TE [I-D.kbr-teas-mptersvp], PCEP [I-D.beeram-pce-pcep-mpted] and
BGP [I-D.zzhang-idr-mpte-signaling], among others. The details of
each will be specified in companion documents; this memo restricts
itself to an overview of the common elements.
4.1. Message Flow
Provisioning messages (to create, update and delete a tunnel) are
sent from the Signaling Source (SS) to each junction node (including
possibly other ingresses). Notifications are sent from each junction
node to the SS to send updates on the state of that node with respect
to the MPTET. Label messages (when needed) are sent hop-by-hop from
egresses to their phops and further upstream in an ordered fashion.
In special scenarios, a node may send a message to one or more of its
nhops.
4.2. Message Types
4.2.1. JUNCTION
A JUNCTION message contains the following information elements:
MPTET ID: a unique identifier for an MPTE tunnel. This usually
consists of the TO ID and a unique ID in the namespace of the TO.
It also includes a version number to distinguish among instances
of a tunnel as it is undergoes updates. The companion signaling
documents will describe the MPTET ID in more detail.
Tunnel Type: various types of tunnels are used, so each node must be
told which type of tunnel this MPTET consists of.
Tunnel Information: provides details for the MPTET. For example,
for an MPLS tunnel with a statically assigned label, the Tunnel
Information is the label. For IP-based tunnels, the Tunnel
Information is the source and destination IP addresses (plus
optional other information).
Junction Bandwidth: specifies the bandwidth incoming to the junction
in Megabits per second (Mbps).
nhop share: a 2-octet share of the outgoing bandwidth per nhop. A
Junction should attempt to send a ratio of (share n)/(sum (share
i)) of the incoming bandwidth to nhop #n.
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4.2.2. LABEL
A LABEL message is used to let each junction know what to use to
forward packets in the MPTET. A LABEL message is sent from an egress
junction node to each of its phops. A pure ingress node never sends
a LABEL message as it has no phops. The LABEL message carries the
MPTET ID and a label, which can be an MPLS label or an IP destination
address.
4.2.3. NOTIFY
A NOTIFY is sent from a junction node to the SS to let the SS know
the state of the MPTET at that node. This could be the labels it
assigned to its phops, or error conditions.
4.3. Forwarding State
From a forwarding point of view, an ingress's job is to:
1. identify the traffic trunk, i.e., the set of packets that are to
be sent via the MPTET;
2. encapsulate the packets in the signaled tunnel type;
3. forward the packet to the ingress's next hops, in accordance with
the computed weights.
FIB entries have a lookup portion (the "routes") and a next hop
portion. In all cases, the next hop at junction J is a weighted list
of J's nhops as specified by the SS in the JUNCTION message.
For an ingress node, the routes define the traffic trunk meant to be
carried by the MPTET.
For a non-ingress node v, the routes identify the MPTET from its
phops.
4.3.1. IP Tunnels
If the MPTET is an IP tunnel, all junctions will need to know the
MPTET type, the MPTET source IP src and destination IP dst.
4.3.1.1. MPTET Type
MPTET signaling determines the tunnel type. This is typically
configured at the TO.
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4.3.1.2. MPTET Destination IP Address dst
Consider first the case that the MPTET has a single egress E. Here
are two ways to support multiple MPTETs to the egress:
1. E has an Egress Address Block (EAB) consisting of a set of IPv4
and/or IPv6 prefixes and/or SRv6 locators
[I-D.saad-teas-rsvpte-ip-tunnels]. E chooses an element of the
appropriate EAB, dst, and signals dst in its LABEL message to its
phops.
2. E uses an IP tunnel that has a discriminator field D, and uses
that in conjunction with its loopback dst. The LABEL message
consists of <dst, D>.
If there are multiple egresses, dst MUST be a common (anycast)
address amomg all the egresses. One of the egresses, E, is chosen to
pick dst (using either of the methods above), and signals this to its
phops, as well as all other egresses.
4.3.1.3. MPTET Source IP Address src
If there is a single ingress I, src is set to I's loopback.
If there are multiple ingresses, and dst was chosen from an EAB, each
ingress sets src to its own loopback.
If there are multiple ingresses, and a discriminator D is being used,
there MUST be a designated ingress I who picks src, and sends it to
all other ingresses.
4.3.1.4. IP MPTET FIB Entries
In the case where an EAB was used to pick dst, the route consists
simply of dst.
In the case where a discriminator D is being used for the MPTET, the
route consists of <dst, src, D> at each junction. Otherwise, the
route is <dst, src>. (Note: this last case only allows one MPTET
between a src-dst pair, for each tunnel type.)
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4.3.2. Static Label
For a MPTET with a "statically assigned" label (typically by a PCE),
the route consists of the assigned label, and the nexthops are a set
of interfaces. In the simplest case, the entire DAG has a single
label; if so, the label operation is null. A variant allows for
different controller-assigned labels for each junction node; in this
case, the forwarding state is as for "signaled" labels, where the
incoming label is swapped to the correct outgoing label.
4.3.3. Signaled Label
For signaled labels, the routes for node v are the labels v sent to
its phops. Each nexthop is a swap of the incoming label to the label
sent by v's nhops.
5. Graceful Restart
A node N is capable of Graceful Restart if a) it can maintain control
plane state across restarts; and b) it can maintain forwarding state
across restarts. If N is capable of Graceful Restart, an MPTE DAG
going through N can continue functioning while N restarts. While N
is restarting, new JUNCTION/LABEL messages will be dropped or
ignored; new MPTE DAGs passing through N will not be established.
Once restart is complete, N will send an OPEN message and re-
establish connections will all its peers (or all the MPTEP
Reflectors). Thereafter, N can participate in new DAGs passing
through it by processing received JUNCTION messages.
More details will be described in a future version.
6. IANA Considerations
None. The related protocol documents will have IANA requirements.
7. Security Considerations
TBD
8. Acknowledgements
Many thanks to Kostas Zorbadelos for his careful reading and useful
suggestions, and his continued interest in this technology. Many
thanks as well to the MPTE group at HPE Networking for the active
discussions that advance the technology and make it implementable.
9. References
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9.1. Normative References
[I-D.beeram-pce-pcep-mpted]
Beeram, V. P., Kompella, K., and A. Stone, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Multipath Traffic Engineered Directed
Acyclic Graph (MPTED) Tunnels", Work in Progress,
Internet-Draft, draft-beeram-pce-pcep-mpted-01, 1 March
2026, <https://datatracker.ietf.org/doc/html/draft-beeram-
pce-pcep-mpted-01>.
[I-D.kbr-teas-mptersvp]
Kompella, K., Beeram, V. P., and C. Ramachandran, "RSVP-TE
Extensions for Multipath Traffic Engineered Directed
Acyclic Graph Tunnels", Work in Progress, Internet-Draft,
draft-kbr-teas-mptersvp-03, 2 March 2026,
<https://datatracker.ietf.org/doc/html/draft-kbr-teas-
mptersvp-03>.
[I-D.saad-teas-rsvpte-ip-tunnels]
Saad, T., Beeram, V. P., and A. Smith, "IP RSVP-TE:
Extensions to RSVP for P2P IP-TE LSP Tunnels", Work in
Progress, Internet-Draft, draft-saad-teas-rsvpte-ip-
tunnels-03, 5 July 2026,
<https://datatracker.ietf.org/doc/html/draft-saad-teas-
rsvpte-ip-tunnels-03>.
[I-D.zzhang-idr-mpte-signaling]
Zhang, Z. J., Kompella, K., Mahale, A., Bhargava, R., and
A. Zhang, "BGP Signaling for Multipath Traffic Engineering
Junction States", Work in Progress, Internet-Draft, draft-
zzhang-idr-mpte-signaling-00, 2 March 2026,
<https://datatracker.ietf.org/doc/html/draft-zzhang-idr-
mpte-signaling-00>.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
DOI 10.17487/RFC2003, October 1996,
<https://www.rfc-editor.org/rfc/rfc2003>.
[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/rfc/rfc2119>.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
DOI 10.17487/RFC2784, March 2000,
<https://www.rfc-editor.org/rfc/rfc2784>.
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[RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
RFC 2890, DOI 10.17487/RFC2890, September 2000,
<https://www.rfc-editor.org/rfc/rfc2890>.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/rfc/rfc3209>.
[RFC7510] Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
"Encapsulating MPLS in UDP", RFC 7510,
DOI 10.17487/RFC7510, April 2015,
<https://www.rfc-editor.org/rfc/rfc7510>.
[RFC8086] Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
March 2017, <https://www.rfc-editor.org/rfc/rfc8086>.
[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/rfc/rfc8174>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/rfc/rfc8754>.
9.2. Informative References
[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
McManus, "Requirements for Traffic Engineering Over MPLS",
RFC 2702, DOI 10.17487/RFC2702, September 1999,
<https://www.rfc-editor.org/rfc/rfc2702>.
[RFC4090] Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
DOI 10.17487/RFC4090, May 2005,
<https://www.rfc-editor.org/rfc/rfc4090>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/rfc/rfc4271>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/rfc/rfc4364>.
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[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/rfc/rfc4655>.
[RFC4761] Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
LAN Service (VPLS) Using BGP for Auto-Discovery and
Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
<https://www.rfc-editor.org/rfc/rfc4761>.
[RFC4762] Lasserre, M., Ed. and V. Kompella, Ed., "Virtual Private
LAN Service (VPLS) Using Label Distribution Protocol (LDP)
Signaling", RFC 4762, DOI 10.17487/RFC4762, January 2007,
<https://www.rfc-editor.org/rfc/rfc4762>.
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/rfc/rfc7432>.
[SPF] Dijkstra, E. W., "A note on two problems in connexion with
graphs", 1 December 1959,
<https://doi.org/10.1007/BF01386390>.
Authors' Addresses
Kireeti Kompella
HPE
Sunnyvale, California 94089
United States of America
Email: kireeti.ietf@gmail.com
Luay Jalil
Verizon
Richardson, Texas 75081
United States of America
Email: luay.jalil@verizon.com
Mazen Khaddam
Cox Communications
Atlanta, Georgia 30328
United States of America
Email: mazen.khaddam@cox.com
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Andy J. Smith
Arrcus, Inc.
Philadelphia, Pennsylvania 19104
United States of America
Email: andy@arrcus.com
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