Overview Of The SPP Architecture
This section gives an overview of the SPP architecture. It describes the key hardware and software features that make it possible to support the main abstractions provided to an SPP slice/user:
- Slice
- Fastpath
- Meta-Interface
- Packet queue and scheduling
- Filter
Coupled with these abstractions are the following system features:
- Resource virtualization
- Traffic isolation
- High performance
- Protocol extensibility
These features allow the SPP to support the concurrent operation of multiple high-speed, virtual routers and allows the user to add support for new protocols. For example, one PlanetLab user could be forwarding IPv4 traffic while a second one could be forwarding I3 traffic. Meanwhile a third user could be programming the SPP to support MPLS.
We begin with a very simple example of an IPv4 router to illustrate the SPP concepts. Then, we briefly review the SPP's main hardware components. Finally, we describe the architectural features in three parts. The first two parts emphasize the virtualization feature of the SPP while the third part emphasizes the extensibility of the SPP. Part I describes how packets travel through the SPP assuming that it has already been configured with a fastpath for an IPv4 router. Part II describes what happens when we create and configure the SPP abstractions (e.g., create a meta-interface and bind it to a queue) for the router in Part I. Part III sketches how the example would be different if the router handled a simple virtual circuit protocol instead of IPv4.
Contents
IPv4 Example
We begin with a simple example of two slices/users (A and B) concurrently using the same SPP as an IPv4 router (R). As shown in the figure (right), A's traffic is between the endhosts H1 and H3, and B's traffic is between H2 and H4. Traffic will travel over multiple Internet hops from an endhost to router R. Also, H1 and H2 are in the same stub network, and H3 and H4 are in the same stub network. (Note: The IP addresses in the figure were chosen for notational convenience and do not refer to existing networks.) Furthermore, both slices want 100 Mb/s of bandwidth in and out of R. We have purposely elected to make the logical views of the two slices as similar as possible to show how the SPP substrate can host this virtualization.
From a logical point of view, each user of router R needs a configuration (right) which includes, as a minimum, one fastpath consisting of three meta-interfaces (m0-m2), four queues (q0-q3), and six filters (f0-f5). Meta-interface m0 goes to R itself; m1 to H1 and H2; and m2 to H3 and H4.
|
|
The configuration for both slices will reflect these properties of the example:
- Both slices are concurrently using the same SPP;
- They have identical service demands;
- But they are communicating between different pairs of hosts (and socket addresses); and
- The SPP provides fair sharing of its bandwidth.
The differences will be reflected in each slice's configuration of R. The configuration of the two slices will be identical except for the following:
- The UDP port numbers of their meta-interfaces will be different so that the SPP can segregate traffic from the two slices coming in the same physical interface.
- Some fields in the filters will reflect the different socket addresses used at the endhosts.
Slices A and B will have the logical views shown in the tables (right). Note the following:
- The total bandwidth of the meta-interfaces (202 Mb/s) can not exceed the bandwidth of the fastpath (FP).
- There should be atleast one queue bound to each meta-interface (MI).
- The highest numbered queues are associated with meta-interface 0 which are for local delivery and exception traffic.
- The only difference between the two tables is that the UDP port number of the MI sockets are 22000 for slice A and 33000 for slice B.
| MIout | |||
|---|---|---|---|
| MIin | m0 | m1 | m2 |
| m0 | f0 | f1 | |
| m1 | f2 | f3 | |
| m2 | f4 | f5 | |
At the least, each slice will have six filters because there will be two filters for each meta-interface; i.e., one for each possible meta-interface destination. For example, traffic from m1 can go to m0 or m2. Although the general structure of the filters will be the same for both slices, the details will be different reflecting the difference between the endhosts used by each slice.
For example, the filter labeled f3 that forwards from m1 to m2 are forwarding traffic destined for host H3 for slice A and destined for host H4 for slice B. Since H3 and H4 have different IP addresses, the f3 filter for the two slices will be different.
So, how does the SPP make it appear that each slice has two dedicated 100 Mb/s paths through R even when traffic from both slices is coming in at the same time?
SPP Hardware Components
For a developer, the most important hardware components of an SPP are the Processing Engines (right). There are two types of Processing Engines (PEs): 1) a General-Purpose Processing Engine (GPE), and 2) a Network Processor Engine (NPE). A GPE is a conventional server blade running the standard PlanetLab operating system. A user can log into a GPE (using ssh) and can run processes that handle packets. An NPE includes two IXP 2850 network processors, each with 16 cores for processing packets and an xScale management processor. The NPE has a 10 GbE network connection and is capable of forwarding packets at 10 Gb/s.
All input and output passes through a Line Card (LC) which is an NPE that has been customized to route traffic between the external interfaces and the GPEs and NPEs. The LC has ten GbE interfaces, some of which will have public IP addresses while others will be used for direct connection to other SPP nodes.
The Control Processor (CP) configures application slices based on slice descriptions obtained from PlanetLab Central, a centralized database that is used to manage the global PlanetLab infrastructure. The CP also hosts a netFPGA, allowing application developers to implement processing in configurable hardware, as well as software.
[...[ FIGURE SHOWING 3 PKT PATHS ]...]
When we describe how the SPP processes packets, you will see that:
- For most packets, the main job of the LC is to forward packets to the NPE with the correct fastpath (FP).
- In most cases, packets going to the GPE are from the NPE although they can also come from the LC.
- Packets going to the GPE from an NPE are associated with a FP (e.g., IPv4 ICMP packet).
- Packets going to the GPE from the LC are going to an endpoint configured by a user that is NOT part of a FP.
Keeping these points in mind should make it easier to understand the partitioning of activities among SPP components.
Part I: IPv4 Packet Forwarding
Outer PlanetLab Headers And Inner Headers
[...[ FIGURE SHOWING ENCAPSULATION ]...]
Let's focus on slice A's traffic which is coming from socket (9.1.1.1, 11000) at H1 and going to socket (7.1.1.1, 5000) at H2. Conceptually, a transit packet coming to router R has the form (H, (H', D)) where H is the Planetlab (or outer) IP+UDP headers, H' is the inner IP+UDP headers, and D is the UDP data. The destination socket in H (outer headers) will reflect the UDP tunnel associated with m1 (MI 1); i.e., (9.3.1.1, 22000). While the source and destination sockets in H' (inner headers) will reflect the source and destination sockets of the application; i.e., (9.1.1.1, 11000) and (7.1.1.1, 5000).
Similarly, if we examine slice B's packets, the destination socket in H (outer headers) will be (9.3.1.1, 33000); i.e., the same IP address used by slice A but a different port number. The source and destination sockets in H' (inner headers) will be (9.2.1.1, 12000) and (7.1.1.2, 6000) which reflect the different application sockets used by slice B.
If we extend this example so that packets transit through multiple PlanetLab nodes, the outer headers H will change as packets pass through each node while the inner headers H' will remain fixed during transit. This situation is analogous to the case when an IP packet transits multiple ethernet networks where IP packets are encapsulated in an ethernet frame which changes as the packet moves through the networks but the IP packet header remains fixed.
In simple terms, the outer and inner headers reflect the two main concerns in an SPP node:
- The outer headers tell the SPP which virtual router should handle the packet.
- The inner and outer headers tell the virtual router which interface should be used for forwarding.
It is this second point that distinguishes an SPP node from other types of PlanetLab nodes since other PlanetLab nodes only have one interface.
Forwarding A Packet
The main job of a router is to forward packets out the appropriate interfaces. A simple IPv4 router has a routing table with each entry containing a network address (the lookup key) and an interface (the lookup result). The router examines the destination address from an incoming packet, searches the routing table for the best matching key, and returns the corresponding interface to be used for forwarding.
Because the SPP houses multiple, virtual routers, an incoming packet is first directed to the appropriate virtual router component which then uses a forwarding table built from the user's filters to determine the appropriate queue (and therefore, meta-interface) to use. This forwarding table has a richer semantics than the simple one described above. Furthermore, each virtual router has a fastpath and a slowpath which are handled by different hardware components (i.e., NPE and GPE respectively). For now, we focus on the fastpath.
In our example, the SPP processes an incoming transit packet in the following way:
- LC(in): Determine the NPE from the outer headers and forward the packet to the NPE.
- The destination socket in the outer header identifies the NPE.
- NPE: Lookup the MI, create new outer headers, enqueue the packet for an MI and forward the packet to the LC.
- Before the NPE can do a lookup, it has to extract from the incoming packet all of the fields that are used during lookup.
- The lookup key includes a slice (or virtual router) identifier.
- LC(out): Add the ethernet header based on the outer headers and transmit the frame.
Now, consider the NPE (right) as it processes an incoming packet from the LC. The Rx block stores the packet in a DRAM buffer, creates a buffer descriptor in SRAM, and sends a meta-packet containing a buffer handle to the next block. Various meta-packet formats are used for communicating between blocks. A buffer handle is a convenient representation for both the buffer address and the buffer descriptor address.
The Decap block extracts some fields from the outer header and outputs some internal identifiers in preparation for the Parse block. For example, it determines the slice ID (sID), the incoming MI (rxMI), and the code option (copt). In our example, both slices will be using the IPv4 code option. But in general, the SPP supports multiple code options to handle different header formats.
The main part of forwarding is done by the Parse, Lookup and Header Format blocks. The Parse block forms the lookup key to be used in the Lookup block. The Lookup block uses the TCAM to determine the disposition of the packet; i.e., the scheduling queue and the destination socket of the outgoing tunnel. And the Header Format block creates the new outer header for the outgoing packet and sends a meta-packet to the Queue Manager.
Meanwhile, the Queue Manager is scheduling packets according to a Deficit Round Robin (DRR) algorithm to provide bandwidth sharing. When a packet should be transmitted, the Queue Manager sends a meta-packet to the Tx block. Finally, the Tx block sends the packet with its new outer header to the LC.
An examination of the Lookup block and the TCAM will explain how multiple virtual routers and multiple code options can be concurrently supported.
XXXXX
The Lookup block treats this lookup key as a generic bit string which it uses to query the TCAM.
The result returned by the TCAM is passed onto the
Header format block which does whatever post-lookup processing is required, such as modifying fields in the outgoing packet headers. Each slice has its own set of filters in the TCAM (each filter includes a slice id as a hidden part of its lookup key, allowing the Lookup block to restrict lookups to the keys that are relevant for a particular packet), and has the freedom to define the semantics of the Lookup key and result in whatever way is appropriate to it. Software running in a GPE on behalf of a slice can insert filters into an NPE using a generic interface that treats the filter as an unstructured bit string. We generally expect slice developers to provide more structured interfaces that are semantically meaningful to their higher level software, and implement those interfaces using the lower level generic interface provided by the SPP.
Code options are implemented within the Parse and Header Format blocks, which precede
and follow the Lookup block.
The Parse block extracts the appropriate header fields and forms a 112 bit Lookup key which
it passes on to the Lookup block. The Lookup block treats this as a generic bit string,
which it uses to query the TCAM. The result returned by the TCAM is passed onto the
Header format block which does whatever post-lookup processing is required, such as
modifying fields in the outgoing packet headers.
Each slice has its own set of filters in the TCAM (each filter includes a slice id as a hidden
part of its lookup key, allowing the Lookup block to restrict lookups to the keys that are
relevant for a particular packet), and has the freedom to define the semantics of the Lookup key
and result in whatever way is appropriate to it. Software running in a GPE on behalf of a slice
can insert filters into an NPE using a generic interface that treats the filter as an
unstructured bit string. We generally expect slice developers to provide more structured interfaces
that are semantically meaningful to their higher level software, and implement those interfaces using
the lower level generic interface provided by the SPP.
XXXXX
Internal Identifiers
Before describing how these steps are carried out in the SPP, it is worth noting that most of the abstractions presented to a user (e.g., queues, filters) are implemented with corresponding actual SPP components. A user's logical view (e.g., MI 3) is supported by mapping the user's logical resources onto the SPPs actual resources. A user's queues, filters and meta-interfaces are mapped to the SPP's actual queues, filters and meta-interfaces that have been assigned to the user as part of its context. The user's abstract components and actual components have non-negative identifiers; e.g., a queue with QID 3. So, in our example, even though both users have a queue with queue ID 3, they are mapped to different actual queues with their own unique queue IDs. During packet processing, the user's logical identifiers are mapped to the SPP's actual (or internal) identifiers.
These mappings are carried out by the TCAM (Ternary Content-Addressable Memory) in each NPE. Basically, the TCAM is a database of key-result pairs. It selects an entry whose key matches selected packet information and outputs the corresponding result.
The TCAM in the LC determines the slice context and related internal and external identifiers based on the PlanetLab headers of an incoming packet. These identifiers include:
- FPid: Fastpath ID
- Copt: Code option
- rxMIid: Incoming Meta-Interface ID
- VLANtag: VLAN tag to be used to identify fastpath
[...[ IPv4 TCAM FILTER FIGURE ]...]
The figure (right) shows the format of the NPE's TCAM key and result fields. The key includes the key fields from the user's filters (e.g., destination IP address) and outputs a result that includes primarily forwarding information from the user's filter (e.g., --txdaddr).
XXX
Part II: Configuring the SPP
xxx
Part III: A Virtual Circuit Router
xxx
