Subject: RE: How to know that egress router is reached?

From: "Hongwei" <[email-masked]>
Date: Tue, 21 Aug 2001 23:21:03 +0100

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?
2. what is the "all routers on this subnet" group multicast address?
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.

I appreciate help from every one.

--Hongwei

-----Original Message-----
From: David Charlap [mailto:[email-masked]]
Sent: 20 August 2001 17:31
To: Hongwei
Subject: Re: How to know that egress router is reached?

Hongwei wrote:
>
> Thank you David. I still feel some confused about a problem:
>
> how does a LSR know that the next hop router is an LDP peer or not?
> does the router administrator configure this information?

A box running LDP will have a TCP connection to each of its peers.  The
LDP discovery mechanism is used to establish these connections.

If the next hop is a router that you don't have such a connection to,
then it is not an LDP peer.

-- David

---

From: David Charlap <[email-masked]>
Date: Tue, 21 Aug 2001 18:24:28 -0400

Hongwei wrote:
> 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.
> 
> 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.

> 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.

> 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

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.

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.  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.

-- David

---

From: Eric W Gray <[email-masked]>
Date: Tue, 21 Aug 2001 20:48:35 -0400

Hongwei,

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

> as a single layer-3 interface as far as the routing infrastructure is

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.

> process the packets - hosts should not.

It is 0xe0000002 (224.0.0.2).

> 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.

> 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

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.

---

From: Eric W Gray <[email-masked]>
Date: Tue, 21 Aug 2001 20:48:35 -0400

Hongwei,