Echo-Request & Echo-Reply - PPP

This is a discussion on Echo-Request & Echo-Reply - PPP ; 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 ...

+ Reply to Thread
Results 1 to 5 of 5

Thread: Echo-Request & Echo-Reply

  1. Echo-Request & Echo-Reply

    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?...
    Thanks.

  2. Re: Echo-Request & Echo-Reply

    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/

  3. Re: Echo-Request & Echo-Reply

    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

  4. Re: Echo-Request & Echo-Reply

    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/

  5. Re: Echo-Request & Echo-Reply

    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

+ Reply to Thread