Network Working Group C. Filsfils, Ed.
Internet-Draft S. Previdi
Intended status: Informational Cisco Systems, Inc.
Expires: November 30, 2018 G. Dawra
LinkedIn
E. Aries
Juniper Networks
P. Lapukhov
Facebook
May 29, 2018
BGP-Prefix Segment in large-scale data centers
draft-ietf-spring-segment-routing-msdc-09
Abstract
This document describes the motivation and benefits for applying
segment routing in BGP-based large-scale data-centers. It describes
the design to deploy segment routing in those data-centers, for both
the MPLS and IPv6 dataplanes.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Large Scale Data Center Network Design Summary . . . . . . . 3
2.1. Reference design . . . . . . . . . . . . . . . . . . . . 4
3. Some open problems in large data-center networks . . . . . . 5
4. Applying Segment Routing in the DC with MPLS dataplane . . . 6
4.1. BGP Prefix Segment (BGP-Prefix-SID) . . . . . . . . . . . 6
4.2. eBGP Labeled Unicast (RFC8277) . . . . . . . . . . . . . 6
4.2.1. Control Plane . . . . . . . . . . . . . . . . . . . . 7
4.2.2. Data Plane . . . . . . . . . . . . . . . . . . . . . 8
4.2.3. Network Design Variation . . . . . . . . . . . . . . 9
4.2.4. Global BGP Prefix Segment through the fabric . . . . 10
4.2.5. Incremental Deployments . . . . . . . . . . . . . . . 10
4.3. iBGP Labeled Unicast (RFC8277) . . . . . . . . . . . . . 11
5. Applying Segment Routing in the DC with IPv6 dataplane . . . 13
6. Communicating path information to the host . . . . . . . . . 13
7. Addressing the open problems . . . . . . . . . . . . . . . . 14
7.1. Per-packet and flowlet switching . . . . . . . . . . . . 14
7.2. Performance-aware routing . . . . . . . . . . . . . . . . 15
7.3. Deterministic network probing . . . . . . . . . . . . . . 16
8. Additional Benefits . . . . . . . . . . . . . . . . . . . . . 17
8.1. MPLS Dataplane with operational simplicity . . . . . . . 17
8.2. Minimizing the FIB table . . . . . . . . . . . . . . . . 17
8.3. Egress Peer Engineering . . . . . . . . . . . . . . . . . 17
8.4. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . 18
9. Preferred SRGB Allocation . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
11. Manageability Considerations . . . . . . . . . . . . . . . . 19
12. Security Considerations . . . . . . . . . . . . . . . . . . . 20
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
15.1. Normative References . . . . . . . . . . . . . . . . . . 22
15.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
Segment Routing (SR), as described in
[I-D.ietf-spring-segment-routing] leverages the source routing
paradigm. A node steers a packet through an ordered list of
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instructions, called segments. A segment can represent any
instruction, topological or service-based. A segment can have a
local semantic to an SR node or global within an SR domain. SR
allows to enforce a flow through any topological path while
maintaining per-flow state only at the ingress node to the SR domain.
Segment Routing can be applied to the MPLS and IPv6 data-planes.
The use-cases described in this document should be considered in the
context of the BGP-based large-scale data-center (DC) design
described in [RFC7938]. This document extends it by applying SR both
with IPv6 and MPLS dataplane.
2. Large Scale Data Center Network Design Summary
This section provides a brief summary of the informational document
[RFC7938] that outlines a practical network design suitable for data-
centers of various scales:
o Data-center networks have highly symmetric topologies with
multiple parallel paths between two server attachment points. The
well-known Clos topology is most popular among the operators (as
described in [RFC7938]). In a Clos topology, the minimum number
of parallel paths between two elements is determined by the
"width" of the "Tier-1" stage. See Figure 1 below for an
illustration of the concept.
o Large-scale data-centers commonly use a routing protocol, such as
BGP-4 [RFC4271] in order to provide endpoint connectivity.
Recovery after a network failure is therefore driven either by
local knowledge of directly available backup paths or by
distributed signaling between the network devices.
o Within data-center networks, traffic is load-shared using the
Equal Cost Multipath (ECMP) mechanism. With ECMP, every network
device implements a pseudo-random decision, mapping packets to one
of the parallel paths by means of a hash function calculated over
certain parts of the packet, typically a combination of various
packet header fields.
The following is a schematic of a five-stage Clos topology, with four
devices in the "Tier-1" stage. Notice that number of paths between
Node1 and Node12 equals to four: the paths have to cross all of
Tier-1 devices. At the same time, the number of paths between Node1
and Node2 equals two, and the paths only cross Tier-2 devices. Other
topologies are possible, but for simplicity only the topologies that
have a single path from Tier-1 to Tier-3 are considered below. The
rest could be treated similarly, with a few modifications to the
logic.
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2.1. Reference design
Tier-1
+-----+
|NODE |
+->| 5 |--+
| +-----+ |
Tier-2 | | Tier-2
+-----+ | +-----+ | +-----+
+------------>|NODE |--+->|NODE |--+--|NODE |-------------+
| +-----| 3 |--+ | 6 | +--| 9 |-----+ |
| | +-----+ +-----+ +-----+ | |
| | | |
| | +-----+ +-----+ +-----+ | |
| +-----+---->|NODE |--+ |NODE | +--|NODE |-----+-----+ |
| | | +---| 4 |--+->| 7 |--+--| 10 |---+ | | |
| | | | +-----+ | +-----+ | +-----+ | | | |
| | | | | | | | | |
+-----+ +-----+ | +-----+ | +-----+ +-----+
|NODE | |NODE | Tier-3 +->|NODE |--+ Tier-3 |NODE | |NODE |
| 1 | | 2 | | 8 | | 11 | | 12 |
+-----+ +-----+ +-----+ +-----+ +-----+
| | | | | | | |
A O B O <- Servers -> Z O O O
Figure 1: 5-stage Clos topology
In the reference topology illustrated in Figure 1, It is assumed:
o Each node is its own AS (Node X has AS X). 4-byte AS numbers are
recommended ([RFC6793]).
* For simple and efficient route propagation filtering, Node5,
Node6, Node7 and Node8 use the same AS, Node3 and Node4 use the
same AS, Node9 and Node10 use the same AS.
* In case of 2-byte autonomous system numbers are used and for
efficient usage of the scarce 2-byte Private Use AS pool,
different Tier-3 nodes might use the same AS.
* Without loss of generality, these details will be simplified in
this document and assume that each node has its own AS.
o Each node peers with its neighbors with a BGP session. If not
specified, eBGP is assumed. In a specific use-case, iBGP will be
used but this will be called out explicitly in that case.
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o Each node originates the IPv4 address of its loopback interface
into BGP and announces it to its neighbors.
* The loopback of Node X is 192.0.2.x/32.
In this document, the Tier-1, Tier-2 and Tier-3 nodes are referred to
respectively as Spine, Leaf and ToR (top of rack) nodes. When a ToR
node acts as a gateway to the "outside world", it is referred to as a
border node.
3. Some open problems in large data-center networks
The data-center network design summarized above provides means for
moving traffic between hosts with reasonable efficiency. There are
few open performance and reliability problems that arise in such
design:
o ECMP routing is most commonly realized per-flow. This means that
large, long-lived "elephant" flows may affect performance of
smaller, short-lived "mouse" flows and reduce efficiency of per-
flow load-sharing. In other words, per-flow ECMP does not perform
efficiently when flow lifetime distribution is heavy-tailed.
Furthermore, due to hash-function inefficiencies it is possible to
have frequent flow collisions, where more flows get placed on one
path over the others.
o Shortest-path routing with ECMP implements an oblivious routing
model, which is not aware of the network imbalances. If the
network symmetry is broken, for example due to link failures,
utilization hotspots may appear. For example, if a link fails
between Tier-1 and Tier-2 devices (e.g. Node5 and Node9), Tier-3
devices Node1 and Node2 will not be aware of that, since there are
other paths available from perspective of Node3. They will
continue sending roughly equal traffic to Node3 and Node4 as if
the failure didn't exist which may cause a traffic hotspot.
o Isolating faults in the network with multiple parallel paths and
ECMP-based routing is non-trivial due to lack of determinism.
Specifically, the connections from HostA to HostB may take a
different path every time a new connection is formed, thus making
consistent reproduction of a failure much more difficult. This
complexity scales linearly with the number of parallel paths in
the network, and stems from the random nature of path selection by
the network devices.
Further in this document (Section 7), it is demonstrated how these
problems could be addressed within the framework of Segment Routing.
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First, it will be explained how to apply SR in the DC, for MPLS and
IPv6 data-planes.
4. Applying Segment Routing in the DC with MPLS dataplane
4.1. BGP Prefix Segment (BGP-Prefix-SID)
A BGP Prefix Segment is a segment associated with a BGP prefix. A
BGP Prefix Segment is a network-wide instruction to forward the
packet along the ECMP-aware best path to the related prefix.
The BGP Prefix Segment is defined as the BGP-Prefix-SID Attribute in
[I-D.ietf-idr-bgp-prefix-sid] which contains an index. Throughout
this document the BGP Prefix Segment Attribute is referred as the
BGP-Prefix-SID and the encoded index as the label-index.
In this document, the network design decision has been made to assume
that all the nodes are allocated the same SRGB (Segment Routing
Global Block), e.g. [16000, 23999]. This provides operational
simplification as explained in Section 9, but this is not a
requirement.
For illustration purpose, when considering an MPLS data-plane, it is
assumed that the label-index allocated to prefix 192.0.2.x/32 is X.
As a result, a local label (16000+x) is allocated for prefix
192.0.2.x/32 by each node throughout the DC fabric.
When IPv6 data-plane is considered, it is assumed that Node X is
allocated IPv6 address (segment) 2001:DB8::X.
4.2. eBGP Labeled Unicast (RFC8277)
Referring to Figure 1 and [RFC7938], the following design
modifications are introduced:
o Each node peers with its neighbors via a eBGP session with
extensions defined in [RFC8277] (named "eBGP8277" throughout this
document) and with the BGP-Prefix-SID attribute extension as
defined in [I-D.ietf-idr-bgp-prefix-sid].
o The forwarding plane at Tier-2 and Tier-1 is MPLS.
o The forwarding plane at Tier-3 is either IP2MPLS (if the host
sends IP traffic) or MPLS2MPLS (if the host sends MPLS-
encapsulated traffic).
Figure 2 zooms into a path from server A to server Z within the
topology of Figure 1.
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+-----+ +-----+ +-----+
+---------->|NODE | |NODE | |NODE |
| | 4 |--+->| 7 |--+--| 10 |---+
| +-----+ +-----+ +-----+ |
| |
+-----+ +-----+
|NODE | |NODE |
| 1 | | 11 |
+-----+ +-----+
| |
A <- Servers -> Z
Figure 2: Path from A to Z via nodes 1, 4, 7, 10 and 11
Referring to Figure 1 and Figure 2 and assuming the IP address with
the AS and label-index allocation previously described, the following
sections detail the control plane operation and the data plane states
for the prefix 192.0.2.11/32 (loopback of Node11)
4.2.1. Control Plane
Node11 originates 192.0.2.11/32 in BGP and allocates to it a BGP-
Prefix-SID with label-index: index11 [I-D.ietf-idr-bgp-prefix-sid].
Node11 sends the following eBGP8277 update to Node10:
. IP Prefix: 192.0.2.11/32
. Label: Implicit-Null
. Next-hop: Node11's interface address on the link to Node10
. AS Path: {11}
. BGP-Prefix-SID: Label-Index 11
Node10 receives the above update. As it is SR capable, Node10 is
able to interpret the BGP-Prefix-SID and hence understands that it
should allocate the label from its own SRGB block, offset by the
Label-Index received in the BGP-Prefix-SID (16000+11 hence 16011) to
the NLRI instead of allocating a non-deterministic label out of a
dynamically allocated portion of the local label space. The
implicit-null label in the NLRI tells Node10 that it is the
penultimate hop and must pop the top label on the stack before
forwarding traffic for this prefix to Node11.
Then, Node10 sends the following eBGP8277 update to Node7:
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. IP Prefix: 192.0.2.11/32
. Label: 16011
. Next-hop: Node10's interface address on the link to Node7
. AS Path: {10, 11}
. BGP-Prefix-SID: Label-Index 11
Node7 receives the above update. As it is SR capable, Node7 is able
to interpret the BGP-Prefix-SID and hence allocates the local
(incoming) label 16011 (16000 + 11) to the NLRI (instead of
allocating a "dynamic" local label from its label manager). Node7
uses the label in the received eBGP8277 NLRI as the outgoing label
(the index is only used to derive the local/incoming label).
Node7 sends the following eBGP8277 update to Node4:
. IP Prefix: 192.0.2.11/32
. Label: 16011
. Next-hop: Node7's interface address on the link to Node4
. AS Path: {7, 10, 11}
. BGP-Prefix-SID: Label-Index 11
Node4 receives the above update. As it is SR capable, Node4 is able
to interpret the BGP-Prefix-SID and hence allocates the local
(incoming) label 16011 to the NLRI (instead of allocating a "dynamic"
local label from its label manager). Node4 uses the label in the
received eBGP8277 NLRI as outgoing label (the index is only used to
derive the local/incoming label).
Node4 sends the following eBGP8277 update to Node1:
. IP Prefix: 192.0.2.11/32
. Label: 16011
. Next-hop: Node4's interface address on the link to Node1
. AS Path: {4, 7, 10, 11}
. BGP-Prefix-SID: Label-Index 11
Node1 receives the above update. As it is SR capable, Node1 is able
to interpret the BGP-Prefix-SID and hence allocates the local
(incoming) label 16011 to the NLRI (instead of allocating a "dynamic"
local label from its label manager). Node1 uses the label in the
received eBGP8277 NLRI as outgoing label (the index is only used to
derive the local/incoming label).
4.2.2. Data Plane
Referring to Figure 1, and assuming all nodes apply the same
advertisement rules described above and all nodes have the same SRGB
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(16000-23999), here are the IP/MPLS forwarding tables for prefix
192.0.2.11/32 at Node1, Node4, Node7 and Node10.
-----------------------------------------------
Incoming label | outgoing label | Outgoing
or IP destination | | Interface
------------------+----------------+-----------
16011 | 16011 | ECMP{3, 4}
192.0.2.11/32 | 16011 | ECMP{3, 4}
------------------+----------------+-----------
Figure 3: Node1 Forwarding Table
-----------------------------------------------
Incoming label | outgoing label | Outgoing
or IP destination | | Interface
------------------+----------------+-----------
16011 | 16011 | ECMP{7, 8}
192.0.2.11/32 | 16011 | ECMP{7, 8}
------------------+----------------+-----------
Figure 4: Node4 Forwarding Table
-----------------------------------------------
Incoming label | outgoing label | Outgoing
or IP destination | | Interface
------------------+----------------+-----------
16011 | 16011 | 10
192.0.2.11/32 | 16011 | 10
------------------+----------------+-----------
Figure 5: Node7 Forwarding Table
-----------------------------------------------
Incoming label | outgoing label | Outgoing
or IP destination | | Interface
------------------+----------------+-----------
16011 | POP | 11
192.0.2.11/32 | N/A | 11
------------------+----------------+-----------
Node10 Forwarding Table
4.2.3. Network Design Variation
A network design choice could consist of switching all the traffic
through Tier-1 and Tier-2 as MPLS traffic. In this case, one could
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filter away the IP entries at Node4, Node7 and Node10. This might be
beneficial in order to optimize the forwarding table size.
A network design choice could consist in allowing the hosts to send
MPLS-encapsulated traffic based on the Egress Peer Engineering (EPE)
use-case as defined in [I-D.ietf-spring-segment-routing-central-epe].
For example, applications at HostA would send their Z-destined
traffic to Node1 with an MPLS label stack where the top label is
16011 and the next label is an EPE peer segment
([I-D.ietf-spring-segment-routing-central-epe]) at Node11 directing
the traffic to Z.
4.2.4. Global BGP Prefix Segment through the fabric
When the previous design is deployed, the operator enjoys global BGP-
Prefix-SID and label allocation throughout the DC fabric.
A few examples follow:
o Normal forwarding to Node11: a packet with top label 16011
received by any node in the fabric will be forwarded along the
ECMP-aware BGP best-path towards Node11 and the label 16011 is
penultimate-popped at Node10 (or at Node 9).
o Traffic-engineered path to Node11: an application on a host behind
Node1 might want to restrict its traffic to paths via the Spine
node Node5. The application achieves this by sending its packets
with a label stack of {16005, 16011}. BGP Prefix SID 16005 directs
the packet up to Node5 along the path (Node1, Node3, Node5). BGP-
Prefix-SID 16011 then directs the packet down to Node11 along the
path (Node5, Node9, Node11).
4.2.5. Incremental Deployments
The design previously described can be deployed incrementally. Let
us assume that Node7 does not support the BGP-Prefix-SID and let us
show how the fabric connectivity is preserved.
From a signaling viewpoint, nothing would change: even though Node7
does not support the BGP-Prefix-SID, it does propagate the attribute
unmodified to its neighbors.
From a label allocation viewpoint, the only difference is that Node7
would allocate a dynamic (random) label to the prefix 192.0.2.11/32
(e.g. 123456) instead of the "hinted" label as instructed by the BGP-
Prefix-SID. The neighbors of Node7 adapt automatically as they
always use the label in the BGP8277 NLRI as outgoing label.
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Node4 does understand the BGP-Prefix-SID and hence allocates the
indexed label in the SRGB (16011) for 192.0.2.11/32.
As a result, all the data-plane entries across the network would be
unchanged except the entries at Node7 and its neighbor Node4 as shown
in the figures below.
The key point is that the end-to-end Label Switched Path (LSP) is
preserved because the outgoing label is always derived from the
received label within the BGP8277 NLRI. The index in the BGP-Prefix-
SID is only used as a hint on how to allocate the local label (the
incoming label) but never for the outgoing label.
------------------------------------------
Incoming label | outgoing | Outgoing
or IP destination | label | Interface
-------------------+----------------------
12345 | 16011 | 10
Figure 7: Node7 Forwarding Table
------------------------------------------
Incoming label | outgoing | Outgoing
or IP destination | label | Interface
-------------------+----------------------
16011 | 12345 | 7
Figure 8: Node4 Forwarding Table
The BGP-Prefix-SID can thus be deployed incrementally one node at a
time.
When deployed together with a homogeneous SRGB (same SRGB across the
fabric), the operator incrementally enjoys the global prefix segment
benefits as the deployment progresses through the fabric.
4.3. iBGP Labeled Unicast (RFC8277)
The same exact design as eBGP8277 is used with the following
modifications:
All nodes use the same AS number.
Each node peers with its neighbors via an internal BGP session
(iBGP) with extensions defined in [RFC8277] (named "iBGP8277"
throughout this document).
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Each node acts as a route-reflector for each of its neighbors and
with the next-hop-self option. Next-hop-self is a well known
operational feature which consists of rewriting the next-hop of a
BGP update prior to send it to the neighbor. Usually, it's a
common practice to apply next-hop-self behavior towards iBGP peers
for eBGP learned routes. In the case outlined in this section it
is proposed to use the next-hop-self mechanism also to iBGP
learned routes.
Cluster-1
+-----------+
| Tier-1 |
| +-----+ |
| |NODE | |
| | 5 | |
Cluster-2 | +-----+ | Cluster-3
+---------+ | | +---------+
| Tier-2 | | | | Tier-2 |
| +-----+ | | +-----+ | | +-----+ |
| |NODE | | | |NODE | | | |NODE | |
| | 3 | | | | 6 | | | | 9 | |
| +-----+ | | +-----+ | | +-----+ |
| | | | | |
| | | | | |
| +-----+ | | +-----+ | | +-----+ |
| |NODE | | | |NODE | | | |NODE | |
| | 4 | | | | 7 | | | | 10 | |
| +-----+ | | +-----+ | | +-----+ |
+---------+ | | +---------+
| |
| +-----+ |
| |NODE | |
Tier-3 | | 8 | | Tier-3
+-----+ +-----+ | +-----+ | +-----+ +-----+
|NODE | |NODE | +-----------+ |NODE | |NODE |
| 1 | | 2 | | 11 | | 12 |
+-----+ +-----+ +-----+ +-----+
Figure 9: iBGP Sessions with Reflection and Next-Hop-Self
For simple and efficient route propagation filtering and as
illustrated in Figure 9:
Node5, Node6, Node7 and Node8 use the same Cluster ID (Cluster-
1)
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Node3 and Node4 use the same Cluster ID (Cluster-2)
Node9 and Node10 use the same Cluster ID (Cluster-3)
The control-plane behavior is mostly the same as described in the
previous section: the only difference is that the eBGP8277 path
propagation is simply replaced by an iBGP8277 path reflection with
next-hop changed to self.
The data-plane tables are exactly the same.
5. Applying Segment Routing in the DC with IPv6 dataplane
The design described in [RFC7938] is reused with one single
modification. It is highlighted using the example of the
reachability to Node11 via spine node Node5.
Node5 originates 2001:DB8::5/128 with the attached BGP-Prefix-SID for
IPv6 packets destined to segment 2001:DB8::5
([I-D.ietf-idr-bgp-prefix-sid]).
Node11 originates 2001:DB8::11/128 with the attached BGP-Prefix-SID
advertising the support of the SRH for IPv6 packets destined to
segment 2001:DB8::11.
The control-plane and data-plane processing of all the other nodes in
the fabric is unchanged. Specifically, the routes to 2001:DB8::5 and
2001:DB8::11 are installed in the FIB along the eBGP best-path to
Node5 (spine node) and Node11 (ToR node) respectively.
An application on HostA which needs to send traffic to HostZ via only
Node5 (spine node) can do so by sending IPv6 packets with a Segment
Routing header (SRH, [I-D.ietf-6man-segment-routing-header]). The
destination address and active segment is set to 2001:DB8::5. The
next and last segment is set to 2001:DB8::11.
The application must only use IPv6 addresses that have been
advertised as capable for SRv6 segment processing (e.g. for which the
BGP prefix segment capability has been advertised). How applications
learn this (e.g.: centralized controller and orchestration) is
outside the scope of this document.
6. Communicating path information to the host
There are two general methods for communicating path information to
the end-hosts: "proactive" and "reactive", aka "push" and "pull"
models. There are multiple ways to implement either of these
methods. Here, it is noted that one way could be using a centralized
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controller: the controller either tells the hosts of the prefix-to-
path mappings beforehand and updates them as needed (network event
driven push), or responds to the hosts making request for a path to
specific destination (host event driven pull). It is also possible
to use a hybrid model, i.e., pushing some state from the controller
in response to particular network events, while the host pulls other
state on demand.
It is also noted, that when disseminating network-related data to the
end-hosts a trade-off is made to balance the amount of information
Vs. the level of visibility in the network state. This applies both
to push and pull models. In the extreme case, the host would request
path information on every flow, and keep no local state at all. On
the other end of the spectrum, information for every prefix in the
network along with available paths could be pushed and continuously
updated on all hosts.
7. Addressing the open problems
This section demonstrates how the problems described above (in
section 3) could be solved using the segment routing concept. It is
worth noting that segment routing signaling and data-plane are only
parts of the solution. Additional enhancements, e.g., such as the
centralized controller mentioned previously, and host networking
stack support are required to implement the proposed solutions. Also
the applicability of the solutions described below are not restricted
to the data-center alone, the same could be re-used in context of
other domains as well
7.1. Per-packet and flowlet switching
A flowlet is defined as a burst of packets from the same flow
followed by an idle interval.
With some ability to choose paths on the host, one may go from per-
flow load-sharing in the network to per-packet or per-flowlet. The
host may select different segment routing instructions either per
packet, or per flowlet, and route them over different paths. This
allows for solving the "elephant flow" problem in the data-center and
avoiding link imbalances.
Note that traditional ECMP routing could be easily simulated with on-
host path selection, using method proposed in [GREENBERG09]. The
hosts would randomly pick a Tier-2 or Tier-1 device to "bounce" the
packet off of, depending on whether the destination is under the same
Tier-2 nodes, or has to be reached across Tier-1. The host would use
a hash function that operates on per-flow invariants, to simulate
per-flow load-sharing in the network.
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Using Figure 1 as reference, let us illustrate this concept assuming
that HostA has an elephant flow to HostZ called Flow-f.
Normally, a flow is hashed on to a single path. Let's assume HostA
sends its packets associated with Flow-f with top label 16011 (the
label for the remote ToR, Node11, where HostZ is connected) and Node1
would hash all the packets of Flow-F via the same next-hop (e.g.
Node3). Similarly, let's assume that leaf Node3 would hash all the
packets of Flow-F via the same next-hop (e.g.: spine node Node5).
This normal operation would restrict the elephant flow on a small
subset of the ECMP paths to HostZ and potentially create imbalance
and congestion in the fabric.
Leveraging the flowlet proposal, assuming HostA is made aware of 4
disjoint paths via intermediate segment 16005, 16006, 16007 and 16008
(the BGP prefix SID's of the 4 spine nodes) and also made aware of
the prefix segment of the remote ToR connected to the destination
(16011), then the application can break the elephant flow F into
flowlets F1, F2, F3, F4 and associate each flowlet with one of the
following 4 label stacks: {16005, 16011}, {16006, 16011}, {16007,
16011} and {16008, 16011}. This would spread the load of the elephant
flow through all the ECMP paths available in the fabric and re-
balance the load.
7.2. Performance-aware routing
Knowing the path associated with flows/packets, the end host may
deduce certain characteristics of the path on its own, and
additionally use the information supplied with path information
pushed from the controller or received via pull request. The host
may further share its path observations with the centralized agent,
so that the latter may keep up-to-date network health map to assist
other hosts with this information.
For example, an application A.1 at HostA may pin a flow destined to
HostZ via Spine node Node5 using label stack {16005, 16011}. The
application A.1 may collect information on packet loss or other
metrics. A.1 may additionally publish this information to a
centralized agent, e.g. after a flow completes, or periodically for
longer lived flows. Next, using both local and/or global performance
data, application A.1 as well as other applications sharing the same
resources in the DC fabric may pick up the best path for the new
flow, or update an existing path (e.g.: when informed of congestion
on an existing path). The mechanisms for collecting the flow
metrics, their publishing to a centralized agent and the decision
process at the centralized agent and the application/host to pick a
path through the network based on this collected information is
outside the scope of this document.
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One particularly interesting instance of performance-aware routing is
dynamic fault-avoidance. If some links or devices in the network
start discarding packets due to a fault, the end-hosts could probe
and detect the path(s) that are affected and hence steer the affected
flows away from the problem spot. Similar logic applies to failure
cases where packets get completely black-holed, e.g., when a link
goes down and the failure is detected by the host while probing the
path.
For example, an application A.1 informed about 5 paths to Z {16005,
16011}, {16006, 16011}, {16007, 16011}, {16008, 16011} and {16011}
might use the last one by default (for simplicity). When performance
is degrading, A.1 might then start to pin flows to each of the 4
other paths (each via a distinct spine) and monitor the performance.
It would then detect the faulty path and assign a negative preference
to the faulty path to avoid further flows using it. Gradually, over
time, it may re-assign flows on the faulty path to eventually detect
the resolution of the trouble and start reusing the path. The
mechanisms for monitoring performance for a specific flow and for the
various paths and the deduction of optimal paths to improve the same
for the flow are outside the scope of this document.
By leveraging Segment Routing, one avoids issues associated with
oblivious ECMP hashing. For example, if in the topology depicted on
Figure 1 a link between spine node Node5 and leaf node Node9 fails,
HostA may exclude the segment corresponding to Node5 from the prefix
matching the servers under Tier-2 devices Node9. In the push path
discovery model, the affected path mappings may be explicitly pushed
to all the servers for the duration of the failure. The new mapping
would instruct them to avoid the particular Tier-1 node until the
link has recovered. Alternatively, in pull path, the centralized
controller may start steering new flows immediately after it
discovers the issue. Until then, the existing flows may recover
using local detection of the path issues.
7.3. Deterministic network probing
Active probing is a well-known technique for monitoring network
elements' health, constituting of sending continuous packet streams
simulating network traffic to the hosts in the data-center. Segment
routing makes possible to prescribe the exact paths that each probe
or series of probes would be taking toward their destination. This
allows for fast correlation and detection of failed paths, by
processing information from multiple actively probing agents. This
complements the data collected from the hosts routing stacks as
described in Section 7.2.
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For example, imagine a probe agent sending packets to all machines in
the data-center. For every host, it may send packets over each of
the possible paths, knowing exactly which links and devices these
packets will be crossing. Correlating results for multiple
destinations with the topological data, it may automatically isolate
possible problem to a link or device in the network.
8. Additional Benefits
8.1. MPLS Dataplane with operational simplicity
As required by [RFC7938], no new signaling protocol is introduced.
The BGP-Prefix-SID is a lightweight extension to BGP Labeled Unicast
[RFC8277]. It applies either to eBGP or iBGP based designs.
Specifically, LDP and RSVP-TE are not used. These protocols would
drastically impact the operational complexity of the Data Center and
would not scale. This is in line with the requirements expressed in
[RFC7938].
Provided the same SRGB is configured on all nodes, all nodes use the
same MPLS label for a given IP prefix. This is simpler from an
operation standpoint, as discussed in Section 9
8.2. Minimizing the FIB table
The designer may decide to switch all the traffic at Tier-1 and Tier-
2's based on MPLS, hence drastically decreasing the IP table size at
these nodes.
This is easily accomplished by encapsulating the traffic either
directly at the host or the source ToR node by pushing the BGP-
Prefix-SID of the destination ToR for intra-DC traffic, or the BGP-
Prefix-SID for the the border node for inter-DC or DC-to-outside-
world traffic.
8.3. Egress Peer Engineering
It is straightforward to combine the design illustrated in this
document with the Egress Peer Engineering (EPE) use-case described in
[I-D.ietf-spring-segment-routing-central-epe].
In such case, the operator is able to engineer its outbound traffic
on a per host-flow basis, without incurring any additional state at
intermediate points in the DC fabric.
For example, the controller only needs to inject a per-flow state on
the HostA to force it to send its traffic destined to a specific
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Internet destination D via a selected border node (say Node12 in
Figure 1 instead of another border node, Node11) and a specific
egress peer of Node12 (say peer AS 9999 of local PeerNode segment
9999 at Node12 instead of any other peer which provides a path to the
destination D). Any packet matching this state at host A would be
encapsulated with SR segment list (label stack) {16012, 9999}. 16012
would steer the flow through the DC fabric, leveraging any ECMP,
along the best path to border node Node12. Once the flow gets to
border node Node12, the active segment is 9999 (because of PHP on the
upstream neighbor of Node12). This EPE PeerNode segment forces
border node Node12 to forward the packet to peer AS 9999, without any
IP lookup at the border node. There is no per-flow state for this
engineered flow in the DC fabric. A benefit of segment routing is
the per-flow state is only required at the source.
As well as allowing full traffic engineering control such a design
also offers FIB table minimization benefits as the Internet-scale FIB
at border node Node12 is not required if all FIB lookups are avoided
there by using EPE.
8.4. Anycast
The design presented in this document preserves the availability and
load-balancing properties of the base design presented in
[I-D.ietf-spring-segment-routing].
For example, one could assign an anycast loopback 192.0.2.20/32 and
associate segment index 20 to it on the border Node11 and Node12 (in
addition to their node-specific loopbacks). Doing so, the EPE
controller could express a default "go-to-the-Internet via any border
node" policy as segment list {16020}. Indeed, from any host in the DC
fabric or from any ToR node, 16020 steers the packet towards the
border Node11 or Node12 leveraging ECMP where available along the
best paths to these nodes.
9. Preferred SRGB Allocation
In the MPLS case, it is recommend to use same SRGBs at each node.
Different SRGBs in each node likely increase the complexity of the
solution both from an operational viewpoint and from a controller
viewpoint.
From an operation viewpoint, it is much simpler to have the same
global label at every node for the same destination (the MPLS
troubleshooting is then similar to the IPv6 troubleshooting where
this global property is a given).
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From a controller viewpoint, this allows us to construct simple
policies applicable across the fabric.
Let us consider two applications A and B respectively connected to
Node1 and Node2 (ToR nodes). A has two flows FA1 and FA2 destined to
Z. B has two flows FB1 and FB2 destined to Z. The controller wants
FA1 and FB1 to be load-shared across the fabric while FA2 and FB2
must be respectively steered via Node5 and Node8.
Assuming a consistent unique SRGB across the fabric as described in
the document, the controller can simply do it by instructing A and B
to use {16011} respectively for FA1 and FB1 and by instructing A and
B to use {16005 16011} and {16008 16011} respectively for FA2 and
FB2.
Let us assume a design where the SRGB is different at every node and
where the SRGB of each node is advertised using the Originator SRGB
TLV of the BGP-Prefix-SID as defined in
[I-D.ietf-idr-bgp-prefix-sid]: SRGB of Node K starts at value K*1000
and the SRGB length is 1000 (e.g. Node1's SRGB is [1000, 1999],
Node2's SRGB is [2000, 2999], ...).
In this case, not only the controller would need to collect and store
all of these different SRGB's (e.g., through the Originator SRGB TLV
of the BGP-Prefix-SID), furthermore it would need to adapt the policy
for each host. Indeed, the controller would instruct A to use {1011}
for FA1 while it would have to instruct B to use {2011} for FB1
(while with the same SRGB, both policies are the same {16011}).
Even worse, the controller would instruct A to use {1005, 5011} for
FA1 while it would instruct B to use {2011, 8011} for FB1 (while with
the same SRGB, the second segment is the same across both policies:
16011). When combining segments to create a policy, one need to
carefully update the label of each segment. This is obviously more
error-prone, more complex and more difficult to troubleshoot.
10. IANA Considerations
This document does not make any IANA request.
11. Manageability Considerations
The design and deployment guidelines described in this document are
based on the network design described in [RFC7938].
The deployment model assumed in this document is based on a single
domain where the interconnected DCs are part of the same
administrative domain (which, of course, is split into different
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autonomous systems). The operator has full control of the whole
domain and the usual operational and management mechanisms and
procedures are used in order to prevent any information related to
internal prefixes and topology to be leaked outside the domain.
As recommended in [I-D.ietf-spring-segment-routing], the same SRGB
should be allocated in all nodes in order to facilitate the design,
deployment and operations of the domain.
When EPE ([I-D.ietf-spring-segment-routing-central-epe]) is used (as
explained in Section 8.3, the same operational model is assumed. EPE
information is originated and propagated throughout the domain
towards an internal server and unless explicitly configured by the
operator, no EPE information is leaked outside the domain boundaries.
12. Security Considerations
This document proposes to apply Segment Routing to a well known
scalability requirement expressed in [RFC7938] using the BGP-Prefix-
SID as defined in [I-D.ietf-idr-bgp-prefix-sid].
It has to be noted, as described in Section 11 that the design
illustrated in [RFC7938] and in this document, refer to a deployment
model where all nodes are under the same administration. In this
context, it is assumed that the operator doesn't want to leak outside
of the domain any information related to internal prefixes and
topology. The internal information includes prefix-sid and EPE
information. In order to prevent such leaking, the standard BGP
mechanisms (filters) are applied on the boundary of the domain.
Therefore, the solution proposed in this document does not introduce
any additional security concerns from what expressed in [RFC7938] and
[I-D.ietf-idr-bgp-prefix-sid]. It is assumed that the security and
confidentiality of the prefix and topology information is preserved
by outbound filters at each peering point of the domain as described
in Section 11.
13. Acknowledgements
The authors would like to thank Benjamin Black, Arjun Sreekantiah,
Keyur Patel, Acee Lindem and Anoop Ghanwani for their comments and
review of this document.
14. Contributors
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Gaya Nagarajan
Facebook
US
Email: gaya@fb.com
Gaurav Dawra
Cisco Systems
US
Email: gdawra.ietf@gmail.com
Dmitry Afanasiev
Yandex
RU
Email: fl0w@yandex-team.ru
Tim Laberge
Cisco
US
Email: tlaberge@cisco.com
Edet Nkposong
Salesforce.com Inc.
US
Email: enkposong@salesforce.com
Mohan Nanduri
Microsoft
US
Email: mnanduri@microsoft.com
James Uttaro
ATT
US
Email: ju1738@att.com
Saikat Ray
Unaffiliated
US
Email: raysaikat@gmail.com
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Jon Mitchell
Unaffiliated
US
Email: jrmitche@puck.nether.net
15. References
15.1. Normative References
[I-D.ietf-idr-bgp-prefix-sid]
Previdi, S., Filsfils, C., Lindem, A., Sreekantiah, A.,
and H. Gredler, "Segment Routing Prefix SID extensions for
BGP", draft-ietf-idr-bgp-prefix-sid-21 (work in progress),
May 2018.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-15 (work
in progress), January 2018.
[I-D.ietf-spring-segment-routing-central-epe]
Filsfils, C., Previdi, S., Dawra, G., Aries, E., and D.
Afanasiev, "Segment Routing Centralized BGP Egress Peer
Engineering", draft-ietf-spring-segment-routing-central-
epe-10 (work in progress), December 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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/info/rfc4271>.
[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.
[RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address
Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
<https://www.rfc-editor.org/info/rfc8277>.
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15.2. Informative References
[GREENBERG09]
Greenberg, A., Hamilton, J., Jain, N., Kadula, S., Kim,
C., Lahiri, P., Maltz, D., Patel, P., and S. Sengupta,
"VL2: A Scalable and Flexible Data Center Network", 2009.
[]
Previdi, S., Filsfils, C., Leddy, J., Matsushima, S., and
d. daniel.voyer@bell.ca, "IPv6 Segment Routing Header
(SRH)", draft-ietf-6man-segment-routing-header-13 (work in
progress), May 2018.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012,
<https://www.rfc-editor.org/info/rfc6793>.
Authors' Addresses
Clarence Filsfils (editor)
Cisco Systems, Inc.
Brussels
BE
Email: cfilsfil@cisco.com
Stefano Previdi
Cisco Systems, Inc.
Italy
Email: stefano@previdi.net
Gaurav Dawra
LinkedIn
USA
Email: gdawra.ietf@gmail.com
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Ebben Aries
Juniper Networks
1133 Innovation Way
Sunnyvale CA 94089
US
Email: exa@juniper.net
Petr Lapukhov
Facebook
US
Email: petr@fb.com
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