The MPLS WG Archive

[Eric W Gray <ewgray@mindspring.com>]

Hongwei,

    I've added some comments WRT David's responses below.  Most are
nits.

David Charlap wrote:

> Hongwei wrote:
> >
> > First thank David very much.  In RFC 3036, the basic discovery
> > mechanism is:
> > To engage in LDP Basic Discovery on an interface an LSR periodically
> > sends LDP Link Hellos out the interface.  LDP Link Hellos are sent
> > as UDP packets addressed to the well-known LDP discovery port for
> > the "all routers on this subnet" group multicast address.
> >
> > here I have something not understood:
> > 1. what does the term "interface" mean? is it a physical port to
> > which a link is connected to? if a router has four transimitters,
> > it has four interfaces. am I right?
>
> If they are four independantly addressible (not necessarily via IP
> address), then yes.
>
> I mention this qualifier to distinguish it from ports that are all
> electrically identical.  For instance, you might have a multi-port
> Ethernet card where all the ports are bridged together and therefore act
> as a single layer-3 interface as far as the routing infrastructure is
> concerned.

Interfaces can be anything that the local box is configured to treat as an
interface.  Specifically, an LSP or (possibly bidirectional) set of LSPs can

be treated as an interface.  As David points out multiple physical
interfaces
may be treated as a single logical 'interface'.  Also, one physical
interface
can be treated as multiple logical interfaces.

This is not always a good idea and should not be done without some good
reason since it adds complexity.  For example, two adjacent LSRs should
be in agreement as to what each thinks are the interfaces connecting them or

you can get interesting behavior.

This points to the 'independently addressable' aspect of an interface as
David mentions - as long as both ends are in agreement as to what an
interface consists of each can independently address the interface of the
other.

>
>
> > 2. what is the "all routers on this subnet" group multicast address?
>
> Multicast 101.  It is a well-known multicast IP address.  Its behavior
> is similar to broadcast addresses, except that only routers should
> process the packets - hosts should not.

It is 0xe0000002 (224.0.0.2).

>
>
> > 3. if a LSR's neighbour router is not LSR, it doesn't understand
> > the LDP Link Hellos, should it ignore the message or send a
> > notification message back? if it sends a notification message back
> > to the LSR,then how to make sure that the LSR understands the
> > notification message? it seems like a dead loop.
>
> The LDP Hello messages are sent to a specific wel-known UDP port.  If
> the receiving router isn't running LDP (the only legitimate reason to
> not understand an LDP Hello packet), then it won't be listening on that
> port, and the packet will get ignored.

LDP TCP/UDP well known ports are explicitly listed in RFC 3036 and in
assigned numbers. They are both 646.

>
>
> An LDP router sends its hello packets to the all-routers address on each
> interface.  If there are no other LDP routers attached, then it will get
> no answers.  If there are any LDP routers attached, then each one will
> get the Hello, and each will respond.

Each will be sending its own Hello messages (as opposed to responding to
those sent by the other).  The response to a hello message depends on the
determination of transport address as described in RFC 3036.

> Then the TCP sessions will be
> created according to the RFC.  Once the TCP sessions are established,
> the routers can be considered "LDP Peers", since they are now able to
> send state-changing messages to each other.

A really minor nit - they are peers from the moment that they discover
each other.  At least that is how the RFC is worded.  An LSR cannot
establish a session with a peer if the session must exist before the other
LSR is a peer.

>
>
> -- David