The MPLS WG Archive

["Sharma, Prem" <p_sharma@trillium.com>]

In my opinion, the followings endorse the statement that MPLS provides the good separation between control and forwarding plane:

• the forwarding tables contain the information that is oblivious to the control plane that has provided that information.
• the forwarding information present in the forwarding tables is/may be sufficient enough to take the data through the network backbone.
• the semantics of the forwarding information may not be known to the forwarding plane. They complexities related to the semantics could be just confined to control plane.
• the entries in the forwarding tables need not necessarily be populated through control protocols.

Any comments/corrections are welcome.

 

Thanks.

Prem

 

 

-----Original Message-----
From: Sukanta Ganguly [mailto:sganguly@opulentsystems.com]
Sent: Tuesday, September 04, 2001 8:41 AM
To: David Escobar; Hongwei
Cc: mpls@UU.NET
Subject: Re: Control and Forwarding functions

To some extent the function is the same. In case of a standard IP router the routing protocol does the job of route updates, which is the control paths responsibility. The control paths learn about the routes via the OSPF, BGP etc... and then keep an updated lookup table so that the data path can perform the lookup and figure out the outgoing port.

With MPLS, there are the control portion which is handled via the LDP, CR-LDP etc... After learning the labels the data path uses them for its lookup process and does the swapping.

 Only thing that is worth noting is that this done over the IP network (atleast in the Internet), so we can say MPLS overlays on top of an IP network and hence creates a tunnel over IP routing.

 

SG

*********** REPLY SEPARATOR ***********

On 9/4/2001 at 10:20 AM David Escobar wrote:

Hi Hongwei:

I though that a router's Control function was the distribution of information among routers to build the routing tables and that the Forwarding function was the routing table look up to find the output port, output label, etc. I think that the control function in your definition is the signaling proccess to establish an SVC, so I think we are talking about different things

Thanks for answering.

----- Original Message -----

From: Hongwei

To: David Escobar

Cc: mpls@UU.NET

Sent: Sunday, September 02, 2001 2:59 PM

Subject: RE: Control and Forwarding functions

Hi David,

 

As much as I understand, MPLS technology is connection-oriented, and is below or at the same level of IP. Like all connection-oriented protocols, MPLS needs a set-up procedure before traffic begins. PVC or SVC is established then. So called Control is the setup procedure(label requesting and label mapping in LDP and CR-LDP). It establishes the forwarding table,and it is before the traffic begins. So called Forwarding is in the traffic stage. The outgoing port is easily got from the forwarding table just because the setup procedure has established an explicit VC. So the traffic becomes easy. Label swapping is needed because the labels in the MPLS shim are locally significant, not globally(IP address is globally important). The locally important label can be dynamically allocated and released by the router. This adds flexibility. ATM also needs VCI/VPI swapping. If globally important identifiers such as (source IP, destination IP) pair are used as label of VC, multiple traffic between different processes in the source machine and different processes in the destination machine can't be distinguished.

 

While IP has no setup procedure because it is a connectionless technology. So I just see its Forwarding function. At every hop, every IP packet has to find next hop's IP address and its corresponding MAC address by ARP.

 

It is true that both IP and MPLS need consult the table, so MPLS forwarding is not necessarily faster than IP if packet's destination IP address can always match an entry in IP routing table. And now it is more apparent that MPLS is intended to combine the advantages of ATM and IP. MPLS can help to realize traffic engineering over IP networks, such as explicit routing can help allocate traffic evenly, and turn to alternative route quickly in case of network failure.

 

I am just brave(rash?) enough to make above comments. I will appreciate any correction of my mistakes. THX in advance:)

 

--Hongwei

 

-----Original Message-----
From: owner-mpls@UU.NET [mailto:owner-mpls@UU.NET]On Behalf Of David Escobar
Sent: 02 September 2001 18:32
To: mpls@UU.NET
Subject: Control and Forwarding functions

Hi:

It is said that MPLS makes a good separation between the Control and the Forwarding functions. It is also said that MPLS may use extensions of existing IP protocols to piggyback label distribution (MPLS-BGP, MPLS-RSVP-TUNNELS).

What is the meaning of good separation between the Control and the Forwarding functions? MPLS still uses the same Control protocols, just a little altered to provide label distribution by piggybacking and the Forwarding function still needs to make table look up to find the next hop. Even worse, it needs to make label swapping. Conventional IP also makes table look up but with the advantage of not requiring label swapping. Why can be inferred that conventional IP does not make a good separation of the Control and Forwarding function while MPLS does?