McKay wrote:
> Hi,
> Can anyone tell me what is the difference between the ICMP
> Echo-Request & Echo-Reply and LCP Echo-Request & Echo-Reply... If they
> serve for the same aim why does a PPP implementation need this echo
> options although ICMP implemented above it also contains that?. If
> they are different what is the difference although the names makes us
> to think that they are same?...

ICMP Echo-Request & Echo-Reply are done at the network layer while
PPP Echo-Request & Echo-Reply are done at the link layer.

The ISP might block ICMP ping Echo-Requests and/or Echo-Replies.
In that case you need the PPP version link layer version.

--
Clifford Kite Email: "echo xvgr_yvahk-ccc@ri1.arg|rot13"
PPP-Q&A links, downloads: http://ckite.no-ip.net/

Clifford Kite writes:
> McKay wrote:
> > Hi,
> > Can anyone tell me what is the difference between the ICMP
> > Echo-Request & Echo-Reply and LCP Echo-Request & Echo-Reply... If they

Nit picking: the messages from RFC 792 are ICMP Echo and ICMP Echo
Reply (not Echo-Request).

> > serve for the same aim why does a PPP implementation need this echo
> > options although ICMP implemented above it also contains that?. If
> > they are different what is the difference although the names makes us
> > to think that they are same?...
>
> ICMP Echo-Request & Echo-Reply are done at the network layer while
> PPP Echo-Request & Echo-Reply are done at the link layer.

There are many important implications from this:

- ICMP Echo and Echo Reply are (naturally) available only when
you implement IPv4 or IPv6. If you don't have IP (and PPP
is most certainly not limited to just IP), then you can't
use ICMP.

- Since ICMP is implemented at or above[1] the network layer,
it can be used end-to-end over an internet (a collection of
networks), but PPP's Echo-Request and Echo-Reply messages
cannot be used end-to-end between hosts. [Tunneling
notwithstanding.]

- ICMP/IP messages may travel over load-balanced links using
multiple paths. LCP Echo-Request and Echo-Reply are
intended to be used on a single link.

- There are usually user-based tools to generate ICMP Echo
messages ("ping"), but typically no diagnostic tools are
provided for LCP Echo-Request. (An implementation *could*
do so if it wanted, but none that I know of do this.)

> The ISP might block ICMP ping Echo-Requests and/or Echo-Replies.
> In that case you need the PPP version link layer version.

It depends on what you're doing with it. ICMP Echo is sometimes
worthwhile for manually testing a path between two IP nodes. It's
seldom right for testing a specific interface (that is to say, if it
fails, you don't necessarily know anything about the interface being
"tested") or for automated testing. The PPP echo mechanism operates
on the link itself, so it can be useful for testing the actual link.

In practical terms, it's common (and not wrong) to use LCP
Echo-Request and Echo-Reply at frequent intervals to test the
integrity of the underlying physical layer (assuming that layer
doesn't provide positive notification of failure) and of the peer's
status. It's good to do this, because otherwise a failed (but
undetected) link becomes a black hole in the network. On the other
hand, using frequent pings as a means to detect liveness is usually
somewhere between "wrong" and "very wrong," as it tells you nothing
about transport or application liveness, and it imposes extra load on
the network at large for no reason.

They're different in usage and other considerations, and most
implementations need both.


[1] There are many utterly worthless arguments based on the OSI
layering model available here. Pick the one you like. It
won't make any difference in the code.

--
James Carlson, IP Systems Group
Sun Microsystems / 1 Network Drive 71.234W Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757 42.497N Fax +1 781 442 1677

James Carlson wrote:
> Clifford Kite writes:
>> McKay wrote:
>> > Hi,
>> > Can anyone tell me what is the difference between the ICMP
>> > Echo-Request & Echo-Reply and LCP Echo-Request & Echo-Reply... If they

> Nit picking: the messages from RFC 792 are ICMP Echo and ICMP Echo
> Reply (not Echo-Request).

Hmm.. Then I guess Stevens has it wrong; he said the ping program
sends an ICMP echo request message (type 8).

Or maybe you object to the separation with "-", but it was easier to
just cut-and-paste his references and not bother with replacing the "-"
with a space.

--
Clifford Kite Email: "echo xvgr_yvahk-ccc@ri1.arg|rot13"
PPP-Q&A links, downloads: http://ckite.no-ip.net/

Clifford Kite writes:
> James Carlson wrote:
> > Clifford Kite writes:
> >> McKay wrote:
> >> > Hi,
> >> > Can anyone tell me what is the difference between the ICMP
> >> > Echo-Request & Echo-Reply and LCP Echo-Request & Echo-Reply... If they
>
> > Nit picking: the messages from RFC 792 are ICMP Echo and ICMP Echo
> > Reply (not Echo-Request).
>
> Hmm.. Then I guess Stevens has it wrong; he said the ping program
> sends an ICMP echo request message (type 8).
>
> Or maybe you object to the separation with "-", but it was easier to
> just cut-and-paste his references and not bother with replacing the "-"
> with a space.

I did say I was picking nits, right?

Here's the text from RFC 792:

Echo or Echo Reply Message
[...]
Type

8 for echo message;

0 for echo reply message.
[...]
Description

The data received in the echo message must be returned in the echo
reply message.

And in the IANA's icmp-parameters registry:

0 Echo Reply [RFC792]
[...]
8 Echo [RFC792]

Yes, it really is defined as "Echo" and "Echo Reply" for ICMP.

The text does go on to refer to "echo requests," but the form of the
text doesn't appear (at least to me) to make that a term d'art:

The identifier and sequence number may be used by the echo sender
to aid in matching the replies with the echo requests. For
example, the identifier might be used like a port in TCP or UDP to
identify a session, and the sequence number might be incremented
on each echo request sent. The echoer returns these same values
in the echo reply.

--
James Carlson, IP Systems Group
Sun Microsystems / 1 Network Drive 71.234W Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757 42.497N Fax +1 781 442 1677