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2.2.1 Routing and Traffic Engineering
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Из курса от партнера University of Illinois at Urbana-Champaign
Cloud Networking
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Week 2
This week, we will dive further into the data center network stack, looking at routing and switching for physical and virtual machines and congestion control. We’ll examine what concerns routing needs to address in these environments and how it’s done in practice. We’ll also see how the network is moving deeper into the physical hosts in order to address the networking needs of virtual machines. With regards to congestion control, we’ll learn what problems TCP’s congestion control faces in data centers and how these are being addressed.
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P. Brighten GodfreyAssociate Professor
Department of Computer Science
Ankit SinglaAssistant Professor
Department of Computer Science, ETH Zürich
[MUSIC]
Today, we will discuss routing.
Routing solves the problem of how to find paths between sources and
destinations in the network.
For example, here you have a source and a destination,
and the source wonders what path it can use to reach the destination.
So you want to install, for example,
some routing information in switches to be able to find those paths.
>> Now, one of the things that makes this a kind of tricky problem is that we want
to do this in a distributed way.
The switches and routers in the network will be making independent decisions
about where to send traffic.
And when we first start up the network, or if there's some change, we don't
necessarily at any local point, we don't have a view of the entire network.
Okay, so if there are these entities that are making individual decisions,
we might make bad decisions if we don't design a good protocol.
We might make decisions where we don't get a shortest path, don't get an efficient
path, or the worst case would be we get a path that actually results in a loop.
Where trapped packets will follow a infinite loop or
at least a loop until the TTL drops down to zero.
So, that is one of the key insights in a very early and
important layer two routing protocol, which is the spanning tree protocol.
>> The spanning tree protocol was designed by Radia Perlman in the 1980's
to address some of these issues.
So it achieves loop free routing in simple networks.
The core idea of the spanning tree protocol,
is to use a subset off the network links to route on.
The subset is a tree, so there are no loops.
So you pick a tree, and then you use that tree to route back its own.
What this means is you are giving up some of the network's capacity.
You're not using the other links.
You can bring those links up in case of failure.
>> Now as we'll see this, for a modern network, is a simplistic protocol.
It's not good enough.
But the advantage and
the real impact of this protocol is that it is a simple protocol.
In the 1980's, it had a huge effect on accelerating the deployment of networks,
because you can just plug in devices without any configuration, and
the network works.
>> Precisely, also it's still being used in enterprise networks quite heavily.
>> So, the simplification that the expanding tree protocol makes is it takes
a large network and it, kind of, down samples it to a tree.
Okay. So
this works well if your network looks like a tree.
As in perhaps a traditional data center of ten, 15 years ago,
for an enterprise where if you squint a little bit this looks like a tree.
Okay, it's not a tree, there's some redundancy for
failover or links that you might bundle together and treat as one.
But you can see the tree like structure,
and spanning tree can be made to work well here.
But now if we're moving to a data center that needs to carry much more traffic.
East-West traffic between the hosts at the leaves.
Then we're putting a lot of load on the top of the tree, the root of the tree.
And this means we need very large, physically large and expensive,
switches to support that large volume of traffic,
increasingly large volume in a very small number of switches.
>> As a result, what people are doing in design now is building topologies that
scale out horizontally, as opposed to vertically.
So, you're not building the tree such that the top tier is very big,
in terms of using large and expensive switches.
What you're doing is using numerous Smaller, cheaper switches to build, for
example, close like networks, as in this example here.
Spanning tree protocol is a pretty bad fit in these cases.
Because if you think about it, if you take just a tree subset of this network,
you'll be throwing away most of the capacity.
>> So there will be many links left unused, and we can't do multi-pathing.
Which we want for both load balance, and capacity,
as well as fast fail over, as we'll see.
>> Right.
Also, the spanning tree is quite a fragile protocol,
so if one device fails, your network is essentially partitioned.
You have the tree, vine device fails, the tree is partitioned.
Further, spanning tree depends on broadcasting information throughout
the network.
What this means is there can be scenarios where due to misconfiguration you can have
a broadcast storm.
So the packet simply keep looping throughout the network.
And you have no capacity left for actual traffic.
>> So this is a increasing problem as we build higher performance networks.
And there's a alphabet soup of proposes fixes.
It's hard to tell which of these is winning, exactly.
>> If any, really.
It's hard to gain an idea of what is getting traction.
Network operators are very conservative in their choice of moving to a new protocol.
Because critical operations really depend on these networks.
Further, there's lots of confusion.
Propriety flavors of these standards, for example Brocade's VCS,
Cisco's FabricPath, these are flavors of the TRILL standard.
These just add to the mix and create confusion.
There is in fact significant debate among operators as to which of these
protocols might eventually replace spanning tree in deployments.
Let's look briefly at Trill's design.
This is just one of the proposed replacements, but
it illustrates how one might get around these problems.
What TRILL does is run a link state protocol between switches.
So every switch learns the topology of the network.
Now every switch knows how to reach every other switch.
And that's how you do routing.
How you learn where the end hosts are connected
has still to be managed through a separate protocol.
For example, you might maintain a directory of which hosts are connected to
which switch, or perhaps you flood that information as well.
Given this setup, let's look at what Trill achieves.
All links become usable, because you have the entire topology.
And multi-path routing also becomes possible.
Let's look at an example of Trill working.
So in this case, the host sends its packet to the switch,
which is in Trill's case called an R bridge.
And this switch encapsulates this packet
adding information about which switch it needs to go to.
So this is the destination switch.
The host itself need not be aware of this topology information.
The r wedge on the drill switch has that information.
Then it forwards the switch to the next hub
that'll help reach that destination switch along the shortest path.
Every successive switch does the same.
And finally, this reaches the destination switch.
There the packet is decapsulated and then delivered to the end host.
>> So Trill fixes an important problem with the spanning tree protocol,
which is that it now allows us to use any link in the network.
However, there's still a single failure domain.
And it requires new hardware to support this new protocol.
And if you're running a large Enterprise network, you're
making some decisions that you probably want to have take some conservative
choices about what's going to be supported and interoperable years into the future.
>> For these reasons, one of the options under consideration is OSPF.
OSPF has been used for awhile in ISP networks, for example.
OSPF is Open Shortest Path First.
It's also a link state protocol, but it works at layer three.
So what that means is, it routes on IP addresses instead of MAC identifiers.
The protocol itself is quite similar.
It's just a link state protocol flooding off the topology information, and
then use of that topology information independently at nodes.
OSPF conveniently sidesteps many of the difficulties that spanning tree protocol
faces, as we just discussed.
Being a link state protocol, it has all of the advantages of Trill, but
none of the newness.
So it uses traditional routing.
This is well established, it's been used for a while,
operators are comfortable with it.
The disadvantage, however, is that it routes on layer there,
which means that you need a lot more configuration.
You have to assign for example IP addresses, and sub-nets, and
manage all of this instead of it being a plug and play protocol.
Like with the spanning tree.
>> And we do, however,
get better scalability, because you can get that aggregation at layer three.
Still however, with some scalability problems as you grow larger and larger.
You're still flooding information about the whole topology everywhere.
So that brings us to a more hierarchical version of OSPF.
>> That's right.
Another disadvantage of OSPF
is the lack of freedom in terms of traffic engineering.
Even though you have these multiple paths and
you can use simple protocols like ECMP, to use those paths.
There's very little you can do to manipulate which routes are being used
between which sources and destinations.
So you can only, for example, meddle with link costs.
So if you're using the link-state protocol the link-state protocol essentially
calculates the least cost path and costs are based on edge costs, right?
For example, this might be a notion of distance in the network.
If far everyone has a higher cost range.
So, you can meddle with the link cost in manipulating OSPF.
But that's all you have freedom to do, more or less.
>> So, having mentioned OSPF.
There's another natural question, which is.
Well, what about all of the other routing protocols we use in Enterprises and
on the Internet.
Namely BGP.
So let's review what BGP is.
BGP is the border gateway protocol.
So this is the standard global scale Internet routing protocol.
And what BGP does is it allows us to essentially, do something very simple.
If I'm a router in part of 1AS, and you're a router in another AS.
I can offer you routes to certain destinations,
along with some metadata like the sequence of AS's along the path and
maybe some tags and community information.
And then, you will collect a series of routes.
A bunch of options at the various destinations and you got to pick one of
them, which of your neighbors you'd like to forward the information to.
So this is called a path vector protocol because you get
a vector which is the encoding of the path for each destination.
You have these options.
And the path being recorded there in the announcement
allows us to make sure that there aren't any loops in it.
>> Precisely.
Let's walk through the simple example to see how BGP works in practice.
This is a very high level discussion, we'll be omitting factors that are not
going to be relevant to the data center for example.
On the internet, if I must say one of the most important
features BGP is being able to use policy that's economic policy.
We have relationships with other ISPs that you're connected to, use that
policy to distinguish who you are going to forward or expose routing information too.
So that's something that we'll ignore here.
We'll just focus on BGP's path vector protocol measure,
te very basics of how it propagates routes.
So in this example we have a few different autonomous systems,
AS's, which are connected to each other in this network.
And this AS's that we are going to look at AS1 is going to propagate some
announcements for this IP prefix to other nodes, to other neighboring AS's.
So it sends out these announcements to its neighbors.
The neighbors then further might choose to propagate those announcements
as shown here.
Now AS2 has already seen a shorter path
to this IP prefix because its connected directly to S1.
So it'll ignore the announcement from AS5.
Further, AS2 can forward that announcement to the other two AS's, AS3 and AS4.
At this point, everyone has reachability to that prefix.
So this is a summary of how BGP works.
Notice that the parts are stored as a vector as Brighton mentioned,
in the routing table itself.
>> BGP, in fact, EBGP, which is the inter domain part of the protocol.
Is now being used in a number of major data centers
as the routing protocol between switches within the data center.
That's, at some level, it's kind of surprising that we take this Internet
scale protocol and Use it there, but what does that get us?
Well, first of all it's IP level, so
we do get that aggregation across groups of IP addresses.
So, it avoids the layer
two broadcast scalability issues, again since this is a layer three protocol.
BGP is a very well tested protocol that is in many vendor implementations.
And, allows us to use ECMP like several of the other options that we just
talked about.
Just to build off what Brighton said, unlike OSPF,
BGP was designed to be used in such a way, as to be interoperable.
So I could be an ISP in Switzerland.
Brighton could run an ISP in the US.
And BGP should work fine.
Regardless of how our implementations of BGP ended up being built.
So they could be built completely independently of each other and
they have to work together.
So this is something that vendors ensure.
So BGP can provide much greater interoperability than something
like OSPF which might be designed for one domain in many cases.
>> We do also have to acknowledge some disadvantages of
using a protocol like BGP here.
Because it's layer three and
not layer two, virtual machine migration is going to be more complex.
And we discussed that in another module.
There will be slower convergence,
particularly in BGP, not a protocol that converges at microsecond scales.
You can tune it.
But there's still going to be delays as this
distributed system finds new paths in the face of a change.
And finally, when you're deploying BGP,
there's along with the flexibility and control over routing,
comes with a large set of possible knobs and
dials to tune the protocol that makes configuration potentially more complex.
So, let's take a step back and talk about what we've seen.
So, we saw a handful of different protocols that looked very different,
Trill at layer two and OSPF, link state and BGP.
A protocol traditionally designed for
the Internet now being used even in a network within one building.
So what's going on there?
Well, there's actually some commonality between all of these designs.
If you think about what was insufficient with the spanning tree protocol.
That was a reasonable protocol if we have a tree-like network,
where the problem of finding paths is a lot simpler.
But now as we get larger scale data centers to support lost of
East-West traffic.
We've basically got a pretty complex network, especially if you think about
conditions like failure where the topology becomes irregular.
Got a pretty complex network now, even within one building.
So that's why it makes sense to take a general purpose
Internet scale routing protocol and bring it into the data center and use it there.
And that's kind of a feature of all the designs we talked about.
That whether it's at layer two or layer three,
there's basically now a pretty good general purpose routing protocol
that we're using between switches in the data center.
>> What Brighton was saying also leads us into a separate point
about exploiting the richness of these complex systems.
So you have these topologies that no longer look like trees.
They have multiple paths between various sources and destinations.
And we've installed them perhaps using these general purpose
routing protocols into the routing tables.
Now, how do you use those multiple paths?
How do you decide what traffic goes on this path versus the other path?
You need some idea of what the network's conditions look like
to be able to make these decisions.
If some paths have failed, perhaps you can fall back on other paths without waiting
for the routing protocol to converge.
As we noted earlier, routing protocols can take on the order of a second in
the case of BGP particularly to converge.
So using this backup connectivity is really important.
So, this is what we look at in the next lesson.
How to use multi-path routing in realtime to work on these problems.
[MUSIC]
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