The MPLS WG Archive

[David Escobar <c-david.escobar@wcom.com>]

Hi Aijun:

Again, I think we are talking about different things. Some books mention that routers have two functions: The Control and the Forwarding. I understand that the Control function is the distribution of information among routers to build the routing tables while the forwarding function is using those tables to find the output port, etc. Books also say that MPLS makes a good separation between those two functions. What I would like to understand is how is that different to IP. How this separation is not well done in IP while it is very well done in MPLS.

Thanks for answering.

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

From: Ding Aijun

To: Hongwei ; David Escobar

Cc: mpls@UU.NET

Sent: Monday, September 03, 2001 12:19 AM

Subject: Re: Control and Forwarding functions

Hi,

 

MPLS is a connection-oriented. So, it needs a data plane, a control plane. ATM network architecture is a good reference.

 

Forwarding lies in the data plane, while signaling in the control plane. Signaling, as well as call admission control, helps to set up the virtual link. And then, data forwarding happens on the pre-established link. This is a rough procedure. I hope it helps to explain the seperation between the control and forwarding function.

 

regards,

aijun

 

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

From: Hongwei

To: David Escobar

Cc: mpls@UU.NET

Sent: Monday, September 03, 2001 3:59 AM

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?