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Versions: 00 01 02 03 04 05 06 07 08 rfc2745                            
Internet Draft                                               Lixia Zhang, UCLA
Expiration: August 1996
File: draft-ietf-rsvp-diagnostic-msgs-00.txt

                        RSVP Diagnostic Messages

Status of Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
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     This draft describes the RSVP diagnosis facility.  As
     the deployment of RSVP has spread, it has become clear that
     a method for collecting information about the RSVP state along
     the way from a receiver to a particular source is needed.
     This specification describes the required functionality, packet
     format, and processing rules.

1.  Introduction

In the original design of RSVP, error messages are the only means for
the end hosts to receive feedback information regarding a specific
request that has failed, a failure in setting up either a PATH state
or reservation state.  In absence of a failure, one receives no
feedback regarding the details of a reservation that has been put in
place, such as whether, or where, or how, one's own reservation
request is merged with that of others.  In case of a failure, the
error message carries back only the information from the failed point,
without any information about the state at other hops before the
failure.  Such missing information, however, can be highly desirable
for debugging purpose, or even be interesting to end applications.

In this memo we specify an RSVP diagnostic facility to collect
information of RSVP state along the path from a receiver to a specific
sender.  Diagnostic messages are independent from any other RSVP
control messages and are side-effect free.  That is, they do not change
any RSVP state at either routers or hosts.  A diagnostic reply is not
an error report, but a collection of RSVP state information as

We have the following design goals in mind:
- To be able to collect RSVP state information at every hop along the
  path once the PATH state has been set up, either for an existing
  reservation or before one has made any reservation request.
- More specifically, to be able to collect information about flowspec,
  timer values, and reservation merging at each hop along the path.
- To be able to collect hop count for each non-RSVP cloud the path
- To avoid packet implosion or explosion.
Here the "hop" is defined as RSVP-capable routers.

The following are specifically identified as non-goals for the time
- Checking the resource availability along a path.  Such functionality
  may be useful for future reservation requests, but would require
  modifications to existing admission control module which is beyond
  our control.

2. Overview

We define two types of diagnostic packets, diagnostic request (DREQ)
and reply (DREP).  To avoid packet implosion or explosion, we restrict
diagnostic packets to unicast only (but see Section 5.2 on firewall
issues).  The requesting host is not necessarily have the
receiving end of the delivery path that is to be queried.  The
requester simply sends an RSVP Diagnostic request packet (DREQ) to
the last-hop router of the path.  The DREQ packet specifies the RSVP
session and a sender host to that session. The last-hop router adds
to the DREQ packet a response data block containing its RSVP state
for the specified RSVP session, and then forwards the request via
unicast to the router that it believes is the proper previous hop for
the given source.  Each subsequent hop adds its own response data to
the end of the request packet, then unicast forwards to the previous
hop.  When the DREQ packet reaches the sender, the sender then changes
the packet type to Diagnostic Reply (DREP) and sends the completed
response to the original requester.  The response may also be returned
before reaching the sender if any error condition along the path, such
as "no path state", prevents further forwarding of the request packet.

DREP packets can be unicast back to the requester either directly, or
in a hop-by-hop manner by reversing the exact path that the DREQ
packet has taken.  The former is faster and more efficient, but the
latter may be needed when the packets have to go across firewalls.  To
facilitate the latter case, DREQ packet may carry an optional ROUTING
object, which is a list of router addresses that the packet has passed
through on the way to the sender.  The DREP packet can then be
returned to the requester by revering the path.

When the path consists many hops, it is possible that the total length
of a DREP packet will exceed the path MTU size and the packet has to
be fragmented.  Relying on IP fragmentation and reassembly, however,
is troublesome, especially when DREP packets are returned to the
requester hop-by-hop, in which case fragmentation/reassembly would
have to be performed at each hop.  To avoid such excessive overhead,
We propose to define a default MTU value, and once an intermediate
router detects that a DREQ packet size reaches the pre-defined MTU
size, it returns the partial result to the requester, and then
forwards the trimmed DREQ packet to the next hop towards the sender.
Therefore through out this memo we use the word "DREQ packet", rather
the word "message" to call a diagnostic request which always consists
of a single packet.  On the other hand, one diagnostic request can
generate multiple DREP packets, each containing a fragment of the
total reply.

Notice that one can forward DREQ packets only after the path state has
been set up.  Otherwise one may resort back to the traceroute
facility to examine whether the unicast/multicast routing is working

3. DREQ / DREP Packet Format

A diagnostic packet consists of three parts: the RSVP common header,
diagnostic packet header, and response data object.

3.1 RSVP Message Common Header

In the RSVP message common header,

                0             1              2             3
         | Vers | Flags|    Type     |       RSVP Checksum       |
         |         RSVP Length       |  reserved   |   Send_TTL  |
         |                     Message ID                        |
         |Resved    |MF|             Fragment offset             |

Flags field is unused for now.

Type = 8: DREQ
Type = 9: DREP

RSVP Checksum covers the entire packet body including this header.

Send_TTL holds IP TTL value that a router puts in the IP header when a
DREQ packet is forwarded to the previous hop.

Message ID identifies an individual DREQ packet and corresponding
reply (or all the fragments of the reply).

MF flag is on for all but the last packet (fragment) in a Diagnostic Reply.

Fragment Offset field gives the byte offset of the current fragment
in the complete Diagnostic Reply.

3.2 RSVP Diagnostic Packet Header Object

Both the DREQ/DREP header is a concatenation of Diagnostic Packet
Header Object and an RSVP Session object, as defined below:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|        length = 20            |  class        |     c-type    |
|   M-TTL       |  hop-count    |  Error code   |  XXXX |     |H|
|                     Source Address                            |
|                   Destination Address                         |
|                     Response Address                          |
|                                                               |
+                     RSVP Session Object                       |
|                                                               |

Class field is unused for the moment and must be set to zero.

C-type is used to distinguish IPv4 and IPv6.

M-TTL specifies the maximum number of hops that the requester
wants to collect information for.  In case that some error condition in
the middle of the path keeps the DREQ packet from reaching the
specified sender, this field can be used to perform an
expanding-length search to reach the point just before the problem.

Hop-count field records the number of RSVP hops that have been
traversed so far.

Error code expresses the errors encountered that prevented DREQ from
reaching the source.  Currently defined values are
        0x00: no error
        0x01: lack of PATH state

A possible use of the 4 bit value XXXX is discussed in Section 5.2
when the "Response Address" is a multicast address.  Otherwise the
value is 0.

H flag indicates how the reply should be returned to the requester.
When H = 0, DREP packets should be sent to the requester directly via
unicast; when H = 1, DREP packets should be returned to the request
in a hop-by-hop way.

Source address specifies the IP address of the source for the
path being traced.  The DREQ packet proceeds hop-by-hop towards this

Destination address field specifies the IP address of the receiving
end for the path being traced.  The DREQ packet starts at this node
and proceeds toward the source.

Response Address field specifies the address to which the DREP
packet(s) gets sent.

The session object identifies the session for which the RSVP state
information is being collected.

Optionally, the packet header may also contain a ROUTE Object, as
defined below, right after the Session object (to be used to return
DREP packets hop-by-hop)

|          length               |  class        | R-pointer     |
|                                                               |
+                     List of RSVP routers                      |
|                                                               |

Length field represents the total length of the object in number of
bytes, from which the number of addresses in the router list can be
easily computed.

R-pointer is used in DREP packets only (see Section 4.2), and must be
zero in all DREQ packets.  (notice that this is a violation of the
common object format defined in RSVP spec)

List of RSVP routers lists all the RSVP hops between the Destination
and the router that returns this DREP packet in a DREP packet, or the
last router that has updated this list in a DREQ packet.

3.3 Response Data

Each RSVP router adds a "response data" segment to the DREQ packet
before it forwards it on.  The response data looks like this:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|          length               |  class        |   C-type      |
|                       DREQ Arrival Time                       |
|                  Incoming Interface Address                   |
|                  Outgoing Interface Address                   |
|                 Previous-RSVP-Hop Router Address              |
|                      reservation style                        |
|   D-TTL     |M|R-error|   K   |     timer value               |
|                                                               |
|                       Tspec object                            |
|                                                               |
|                                                               |
|                     filter spec object                        |
|                                                               |
|                                                               |
|                   flowspec object                             |
|                                                               |

DREQ Arrival Time is a 32-bit NTP timestamp specifying the arrival
time of the DREQ packet at this router.  The 32-bit form of an NTP
timestamp consists of the middle 32 bits of the full 64-bit form; that
is, the low 16 bits of the integer part and the high 16 bits of the
fractional part.

Incoming Interface Address specifies the address of the interface on
which packets from this source are expected to arrive, or 0 if

Outgoing Interface Address specifies the address of the interface on
which packets from this source flow to the specified destination, or 0
if unknown.

Previous-RSVP-Hop Router Address specifies the router from which this
router receives PATH packets from this source, or 0 if unknown.

Reservation style is the 4 byte value of RSVP Style Object as defined in
RSVP specification.

D-TTL specifies the hop count this DREQ packet traveled from the
down-stream RSVP router to the current router.

M-flag indicates whether the reservation, as described by the objects
below, is merged with reservations from other downstream interfaces
when being forwarded upstream.

R-error indicates error conditions at a router.  Currently defined
values are
        0x00: no error
        0x01: no reservation

K is the parameter defined in RSVP, and timer value is the local
refresh timer value in second.

The rest parts, Tspec, filter spec, and flowspec objects follow the
definitions given in RSVP specification.  The latter two may be absent
(see Section 4.1 on DERQ forwarding).

4. Diagnostic Packet Forwarding Rules

4.1 DREQ Forwarding

DREQ packets are forwarded hop-by-hop via unicast from the destination
address to the source address as specified in the diagnostic packet

Each hop carries out the following processing before forwarding the
packet to the next hop towards the source:
- Compute the routing hop count from the previous RSVP hop.
  This is done by comparing the value of Send_TTL with the TTL value
  in the IP header.  The result is then put into the D-TTL field of
  the response data.
- Attach its response data block to the end of the DREQ packet.
  Tspec, filter spec, and flowspec objects in the data block describe
  the reservation in place at the Outgoing Interface for the specified
  If there is no reservation state, then the response data block will
  contain no filter-spec or flowspec; it should still have the Tspec
  value for the specified sender that has been carried to the router
  by PATH messages.
- increment hop-count by one.
- If the hop-count value is equal to that of M-TTL, or if the current
  hop is the sender as identified by the "Source Address" in the RSVP
  diagnostic header, go to Send_DREP(), and then return.
- If the resulting DREQ packet size exceeds a pre-defined MTU limit
  (minus some margin to hold the address list, see blow), go to
- Count the number or response data blocks, N, increase the "Fragment
  offset" value in RSVP common header by (24N + N X M)).
  (Section 3.3 defines the first 24 bytes in the response block, M is
   the length of Tspec + Filterspec + Flowspec)
- Trim off all the response data blocks from the original DREQ
  packet, then forward to the next hop towards the source.

1. If the H flag in the Diagnostic Header header is off,
  o Make a copy of the packet, change the packet type from DREQ to
    DREP, and send it to the response address given in the DREQ
  o Return.

2. Create a ROUTE Object (as defined in Section 3.2.2), Rnew.
   R-pointer value is filled by counting the number of response
   data blocks in the DREQ packet.  The list of RSVP routers is
   filled by taking the "Incoming Interface Address" from each of the
   response data block.

3. If the DREQ packet already contains a Router Object, Rold, merge
   Rnew with Rold by adding the R-point in Rnew to that in Rold, and
   attach the Router list of Rnew to the end of that in Rold.

4. Insert the resulting Route object from Steps 2 & 3 between
   Session object and the first response data block.

5. Make a copy of the resulting DREQ packet, change the packet type
   from DREQ to DREP, and send the packet to the last address in ROUTE

6. Return.

4.2 DREP Forwarding

When the H flag is off, DREP packets are sent directly to the original
requester.  When H flag is on, however, they are forwarded hop-by-hop
to the requester, by reversing the route as listed in the Route object.

When a router receives a DREP packet, it simply decreases R-pointer by
one (address length), and forward the packet to the address pointed by
R-pointer in the route list.

When the destination router receives a DREP packet, it sends the
packet to the Response address.

4.3 Errors

If an error condition prevents a DREP packet from being forwarded
further, the packet is simply dropped.

If an error condition, such as lack of PATH state, prevents a DREQ
packet from being forwarded further, the router must change the
current packet to DREP type and return it to the response address.

5. Problem Diagnosis by Using RSVP Diagnostic Facility

5.1 Broken Intermediate Router

A broken (or legacy) intermediate RSVP router may simply not
understand diagnostic packets, and drop them.  The querier would then
get no response at all from its requests.  It may then choose to first
do a multicast traceroute (in case of multicast) to get information
about the route length, and then perform an RSVP diagnosis search
by gradually increasing the value of M_TTL field until it no longer
receives a response.

5.2 Across Firewalls

Firewalls can cause problems in diagnostic packet forwarding.  Let us
look at two different cases.

First, let us assume that the querier is a receiving host of the
session to be examined.  In this case, firewalls should not prevent
the forwarding of the diagnostic packets in a hop-by-hop manner,
assuming that proper holes have been punched on the firewall to allow
hop-by-hop forwarding of other RSVP packets.  The querier may start by
setting the H flag off, which can give a faster response delivery and
reduced overhead at intermediate routers.  However if no response is
received, the querier may resend the DREQ packet with H flag turned

If the requester is a third party host and is separated from the
destination address by a firewall (either the requester is behind a
firewall, or the destination is a router behind a firewall, or even
both), at this time I do not know any other solution but attempting to
use multicast.

To send a DREQ packet across a firewall (or firewalls), the request
should be multicast to the group being examined (since the last hop
router listens to that group).  All routers except the correct last
hop router, as identified by the destination address in the DREQ
header, should ignore any DREQ request received via multicast.

To receive a DREP packet across a firewall (or firewalls), the querier
should set the response address to a wellknown multicast address
allocated specifically for DREP packets.  In this case, all the reply
packets will be first unicast to the destination address, which in
turn multicasts them out, with the TTL value specified in the XXXX
field in the diagnostic header.  This response TTL should be set to a
value sufficient for the response from the destination router to reach
the querier.  However we choose to physically limit this value to be
no more than 15, because there is only one wellknown multicast
address for this purpose, therefore all the queriers from all other
sessions will receive the multicast DREP packets as well.  If the
querier still cannot receive the DREP packet when the TTL reaches the
limit, then one must consider using a node closer to the destination

5.3 Examination of RSVP Timers

One easily collects information about the current timer value at each
RSVP hop along the way.  This will be very helpful in situations when
the reservation state goes up and down frequently, to find out whether
the state changes are due to improper setting of timer values, or K
values (when across lossy links), or frequent routing changes.

5.3 Discovering Non-RSVP Clouds

The D-TTL field in each response data block shows the number of
routing hops between adjacent RSVP routers.  Therefore any value
greater than one indicates a non-RSVP clouds in between.  Together
with the arrival timestamps (assuming NTP works), this value can also
give some vague, though not necessarily accurate, indication of how
big that cloud might be.  One might also find out all the intermediate
non-RSVP routers by running either unicast or multicast trace route.

5.4 Discovering Reservation Merges

The flowspec value in a response data block specifies the amount of
resources (whatever that means by the yet to be defined flowspec)
being reserved for the data stream defined by the filter spec in the
same data block.  When this value of adjacent response data blocks
differs, that is, a downstream router Rd has a smaller value than its
immediate upstream router Ru, it indicates a merge of reservation with
RSVP request(s) from other down stream interface(s) at Rd.
Further, in case of SE style reservation, one can examine how the
different SE scopes get merged at each hop.

In particular, if a receiver sends a DREQ packet before sending its
own reservation, it can discover (1)how many RSVP hops there are along
the path between the specified source and itself, (2)how many of
the hops already have some reservation by other receivers, and
(3)possibly foresee how its reservation request might get merged with
other existing ones.

5.5 Error Diagnosis

In addition to examining the state of a working reservation, RSVP
diagnostic packets are more likely to be invoked when things are not
working or working correctly.  For example, a receiver has reserved an
adequate pipe for a specified incoming data stream, yet the observed
delay or loss ratio is much higher than expected.  In this case the
receiver can use the diagnostic facility to examine the reservation
state at each RSVP hop along the way to find out whether the RSVP state is
set up correctly, whether there is any blackhole along the way, or
whether there are non-RSVP clouds, and where, that may have caused the
performance problem.

6. Acknowledgment

The idea of developing a diagnostic facility for RSVP was first
proposed by Mark Handley of UCL.  Many thanks to Lee Breslau of Xerox
PARC and John Krawczyk of Baynetworks for their valuable comments on
the first draft of this memo.

Authors' Addresses

   Lixia Zhang
   4531G Boelter Hall
   Los Angeles, CA  90024

   Phone:    310-825-2695
   EMail:    lixia@cs.ucla.edu