One document matched: draft-ietf-rsvp-diagnostic-msgs-03.txt
Differences from draft-ietf-rsvp-diagnostic-msgs-02.txt
INTERNET-DRAFT Lixia Zhang
<draft-ietf-rsvp-diagnostic-msgs-03.txt> Andreas Terzis
Expiration: May 1998 UCLA
November 1997
RSVP Diagnostic Messages
<draft-ietf-rsvp-diagnostic-msgs-03.txt>
Status of this 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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Distribution of this memo is unlimited.
This Internet Draft expires in May, 1998.
Abstract
This document specifies the RSVP diagnosis facility. As the
deployment of RSVP is spreading out, it becomes clear that a method
for collecting information about the RSVP state along the path is
needed. This specification describes the functionality, diagnostic
message formats, and processing rules.
1. Introduction
In the original design of the RSVP protocol, error messages are the
only means for the end hosts to receive feedback information
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regarding a specific request that has failed, a failure in setting up
either a PATH state or reservation state. In the absence of
failures, 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 or after the failure. Such missing
information, however, can be highly desirable for debugging purpose,
or for network resource management in general.
This document specifies RSVP diagnostic messages that allows one 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 produce no side-effects. That is, they do
not change any RSVP state at either routers or hosts. Similarly,
they do not represent an error report but a collection of RSVP state
information as requested.
We have the following design goals in mind:
- To be able to collect RSVP state information at every hop along
the path where the PATH state has been set up, either for an
existing reservation or before a reservation request is made;
here the "hop" means RSVP-capable routers.
More specifically, we want to be able to collect information
about flowspec, refresh timer values, and reservation merging at
each hop along the path.
- To be able to collect the routing hop count for each non-RSVP
cloud.
- To avoid diagnostic packet implosion or explosion.
The following are specifically identified as non-goals:
- 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 the scope of RSVP.
2. Overview
We define two types of RSVP diagnostic packets, diagnostic request
(DREQ) and reply (DREP). This diagnostic tool can be invoked by a
client from any host that may or may not be a participant of the RSVP
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session to be diagnosed. Thus generally speaking three nodes are
involved in performing the diagnostic function: the requester, the
starting and the ending nodes of the diagnosis, as shown in Figure 1.
It is possible that the client invoking the diagnosis function may
reside directly on the LAST-HOP, in which case that the first two
nodes are the the same. The starting node of the diagnosis is named
"LAST-HOP", meaning the last-hop of the path segment to be diagnosed,
which can be either the receiving end or an intermediate router along
a reserved path. The ending node is the sender host in general,
although one can also limit the length of the path segment to be
diagnosed by specifying a hop-count limit for the diagnosis messages.
To avoid packet implosion or explosion, all diagnostic packets are
forwarded via unicast only.
A client invokes RSVP diagnostic functions by generating a DREQ
packet and sending to the LAST-HOP node which should be on the RSVP
path to be diagnosed. This DREQ packet specifies the RSVP session
and a sender host to that session. The DREQ packet starts collecting
information at the LAST-HOP node and proceeds toward the sender (see
Figure 1).
Receiver LAST-HOP Sender
__ __ __ __ __ __ __
| |---------| |------>| |-->| |-->| |-->| |---->| |
|__| |__| DREQ |__| |__| |__| |__| |__|
^
| RSVP routers
|
|request
_|_
| | Requester
|___|
Figure 1
Each RSVP-capable router receiving the DREQ packet adds to the packet
a response data object containing the router's RSVP state for the
specified RSVP session, and then forwards the request via unicast to
the router that it believes to be the previous hop for the given
sender. Each subsequent RSVP router attaches its own response data
object to the end of the DREQ packet, then forwards via unicast to
the previous hop. When the DREQ packet reaches the sender, the
sender changes the packet type to Diagnostic Reply (DREP) and sends
the completed response to the original requester. Partial response
may also be returned before the DREQ packet reaches the sender if any
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error condition along the path, such as "no path state", prevents
further forwarding of the DREQ packet, or if the specified hop-count
for the diagnosis has been reached.
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 the only choice if the packets have to cross firewalls,
due to the way that firewalls operate.
To facilitate the latter case, a DREQ packet may optionally carry a
ROUTE object, which is a list of router addresses that the DREQ
packet has passed through on the way to the sender; this ROUTE object
is built incrementally as the DREQ packet passes through the
intermediate routers. The DREP packet can then be returned to the
requester by reversing the path.
When the path consists of many hops, it is possible that the total
length of a DREP packet will exceed the path MTU size before reaching
the sender, thus the packet has to be fragmented. Relying on IP
fragmentation and reassembly, however, can be problematic, especially
when DREP packets are returned to the requester hop-by-hop, in which
case fragmentation/reassembly would have to be performed at every
hop. To avoid such excessive overhead, we let the requester define a
default path MTU size which is carried in every DREQ packet. If an
intermediate router finds that the default MTU size is bigger than
that of the outgoing link, it returns the DREQ packet with the
corresponding error bit set. If an intermediate router detects that
a DREQ packet size reaches the MTU size, it sends a partial DREP,
consisting of the collected responses back, to the requester and then
continues to forward the trimmed DREQ packet to the next hop towards
the sender.
Through out this document we use the word "DREQ packet", rather the
word "message" to call a diagnostic request since it always consists
of a single packet. On the other hand, one DREQ packet can generate
multiple DREP packets, each containing a fragment of the total reply.
Furthermore, when discussing diagnostic packet handling, the
terminology used refers to the direction of data packet flow, thus
"outgoing interface" of a router is the interface a DREQ packet comes
from. THE DREQ then gets forwarded to an "incoming interface",
because DREQ packets travel in the reverse direction of the data
flow.
Notice that one can forward DREQ packets only after the RSVP PATH
state has been set up. If no PATH state exists, one may resort to
the traceroute or mtrace facility to examine whether the
unicast/multicast routing is working correctly.
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3. Diagnostic Packet Format
A diagnostic packet consists of the following parts:
+-----------------------------------+
| RSVP common header |
+-----------------------------------+
| Diagnostic packet header object |
+-----------------------------------+
| session object |
+-----------------------------------+
| (optional) SELECT object |
+-----------------------------------+
| (optional) ROUTE object |
+-----------------------------------+
| zero or more Response Object |
+-----------------------------------+
3.1. RSVP Message Common Header
In the RSVP message common header,
0 1 2 3
+-------------+-------------+-------------+-------------+
| Vers | Flags| Type | RSVP Checksum |
+-------------+-------------+-------------+-------------+
| Send_TTL | reserved | RSVP Length |
+-------------+-------------+-------------+-------------+
The Flags field is unused for now and must be set to zero.
Type = 8: DREQ
Type = 9: DREP
The RSVP Checksum is the 16-bit one's complement of the one's
complement sum of the whole diagnosis message (including this
header). For computing the checksum, the checksum field is set to
zero. When receiving packets, the checksum MUST be verified before
processing a packet.
Send_TTL: the TTL value that a router puts in the IP packet header
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when forwarding the DREQ packet to the previous hop.
RSVP length: the total length of this diagnostic packet in bytes,
including the common header. If this is a DREP packet and the MF
flag in the diagnostic packet header (see below) is set, this length
field indicate the length of this single DREP fragment, rather than
the total length of the the complete DREP reply (which may not be
known in advance).
3.2. RSVP Diagnostic Packet Header Object
Both DREQ and DREP headers are 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 | class | c-type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max-RSVP-hops | RSVP-hop-count| Reserved |H|MF|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+---------------+---------------+---------------+---------------+
| path MTU | Fragment offset |
+---------------+---------------+---------------+---------------+
| |
| Sender Filter-Spec |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LAST-HOP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Response Address Filter-Spec |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Next-Hop RSVP_HOP Object |
| |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
+ Followed by RSVP Session Object |
| |
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Length is the length of this diagnostic header object.
Class = 30.
C-type field is used to distinguish between IPv4 (C-type = 1) and
IPv6 (Ctype = 2). In the IPv6 case addresses will be 16 bytes each.
Max-RSVP-hops specifies the maximum number of RSVP hops that the
requester wants to collect information from. In case an error
condition in the middle of the path prevents the DREQ packet from
reaching the specified sender, one may use this field to perform an
expanding-length search to reach the point just before the problem.
The fragment offset field indicates where in the total reply this
fragment belongs. The fragment offset is measured in octets. The
first fragment has offset zero.
RSVP-hop-count field records the number of RSVP hops that have been
traversed so far.
The H flag indicates how the reply should be returned. When H = 0,
DREP packets should be sent to the response address directly. If H =
1, DREP packets must be returned to the LAST-HOP address in a hop-
by-hop way. The node specified by the LAST-HOP address then forwards
DREP packets to the response address.
The MF flag means "more fragments". It must be set to zero (0) on
all DREQ packets, and set to one (1) on all DREP packets that carry
partial results and are returned by intermediate routers due to the
MTU limit. When the sender converts a DREQ packet to DREP, the MF
flag remains zero. An intermediate router may also converts a DREQ
packet to DREP when the DREQ packet has traversed the specified
number of Max-RSVP-hops, in which case the MF flag remains zero.
Message ID identifies an individual DREQ packet and the corresponding
reply (or all the fragments of the reply). A possible way of using
the message ID is the 16 bits to specify the ID of the process doing
the query and the low 16 bits to be the sequence number of the query.
This way processes on the same machine can distinguish between each
other's replies and between different copies of the same query.
The path MTU is a 16-bit field that specifies a default MTU size, in
number of bytes, that all diagnostic packets must fit within.
Sender Filter-Spec is the IP address plus the port of the sender
being traced. The DREQ packet proceeds hop-by-hop towards this
address.
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LAST-HOP address is the IP address of the last hop at the receiving
end for the path being traced. The DREQ packet starts collecting
information at this node and proceeds toward the sender.
Response Address Filter-Spec contains the IP address and the port to
which the DREP packet(s) should be sent. This Response Address
Filter-Spec specifies the process originating the request.
The Next-Hop RSVP_HOP object carries the IP address of the interface
to which the DREQ must be forwarded to. This object is updated on a
hop by hop basis, and is used for the same reasons that a RESV
message contains an RSVP_HOP object. That is, to distinguish logical
interfaces and avoid problems caused by routing asymmetries.
The session object identifies the RSVP session for which the state
information is being collected.
Optionally, the diagnostic packet may contain a SELECT object which
carries a list of [Class, C-type] pairs, each pair specifies one type
of RSVP object the diagnosis invoking client wants to examine. When
a SELECT object is included in the DREQ packet, each RSVP router
along the way should attach to the response object each type of the
objects specified in the SELECT list. In the absence of a SELECT
object, the router will attach a set of default objects.
The SELECT object has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | class | c-type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| class | c-type | class | c-type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ................... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Length field represents the total length of the object in number
of bytes.
Class = 33
C-type field is not used at the moment and must be set to zero.
The object payload part carries a list of [Class, C-type] pairs. In
case where the requested number of objects is an odd number, the last
two bytes must be set to zero.
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Optionally, the diagnostic packet may also contain a ROUTE object, as
defined below. The ROUTE object is to be used to return DREP packets
hop-by-hop.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | class | c-type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reserved | R-pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ List of RSVP routers |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Length field represents the total length of the object in number
of bytes, from which the number of addresses in the RSVP router list
can be easily computed.
Class = 31.
C-type field is used to distinguish between IPv4 (C-type = 1) and
IPv6 (Ctype = 2) ROUTE object.
R-pointer is used in DREP packets only (see Section 4.2 for details),
but is incremented as each hop adds its incoming interface address in
the ROUTE object.
In a DREQ packet, the List of RSVP routers lists all the RSVP hops
between the LAST-HOP address, as specified in the Diagnostic packet
header object, and the last RSVP router the DREQ packet has visited.
In a DREP packet, List of RSVP routers lists all the RSVP hops
between the LAST-HOP and the router that returns this DREP packet.
3.3. Response Data Object
When receiving a DREQ packet, each RSVP router attaches a "response
data" object to it before forwarding on. The response data object is
defined as follows:
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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-err| K | timer value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| (TUNNEL object) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Tspec object |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| filter spec object |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| flowspec object |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Class = 32.
Ctype 1 and 2 specify whether this is an IPv4 or IPv6 response data,
respectively.
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 IP address of the interface
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on which packets from the sender, as defined in the Diagnostic Packet
Header, are expected to arrive, or 0 if unknown.
Outgoing Interface Address specifies the IP address of the interface
from which the DREQ packet comes, and to which packets from the given
sender and for the specified session address flow, or 0 if unknown.
Previous-RSVP-Hop Router Address specifies the router from which this
router receives RSVP PATH messages for this source, or 0 if unknown.
Notice that the response object format as shown above assumes IPv4
addresses of 4-byte each; in case of IPv6 (indicated by C-type = 2),
these three addresses will be 16 bytes each.
Reservation style is the 4-byte value of RSVP Style Object as defined
in the RSVP specification.
D-TTL contains the routing hop count this DREQ packet traveled from
the down-stream RSVP router to the current router.
M is a single-bit flag which indicates whether the reservation, as
described by the objects below, is merged with reservations from
other downstream interfaces when being forwarded upstream.
R-error is a 3-bit field that indicates error conditions at a router.
Currently defined values are
0x00: no error
0x01: no PATH state
0x02: MTU too big
0x04: ROUTE object too big
K is the refresh timer parameter defined in RSVP, and timer value is
the local refresh timer value in seconds.
The next part, TUNNEL object, is an optional one which should be
inserted when a DREQ packet arrives at an RSVP router that acts as a
tunnel exit point. The TUNNEL object provides mapping between the
end-to-end RSVP session that is being diagnosed and the RSVP session
over the tunnel. This mapping information allows the diagnosis client
to conduct diagnosis over the involved tunnel session when so
desired, by invoking a separate Diagnostic query for the
corresponding Tunnel Session and Tunnel Sender. Keep in mind,
however, that multiple end-to-end sessions may all map to one pre-
configured tunnel session which may have totally different parameter
settings.
The tunnel object is defined in the RSVP Tunnel Specification
[RSVPTUN], with the following format:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | class | c-type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Session object (for the end-to-end session) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Sender Filter-Spec (for the tunnel sender) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SESSION_ASSOC Object
Class=192. Ctype 1 specifies IPv4 sessions, Ctype 2 specifies IPv6
sessions, and Ctypes 3 and 4 specify sessions with IPSEC Generalized
Port Id for IPv4 and IPv6 respectively.
The remaining 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 DREQ forwarding). In the case of a SE
reservation the filter spec is actually the set of all filter specs
that share the reservation. The flowspec describes the actual
reservation in place.
Also note that the length of these object is varying so the lengths
used on the diagram above are not representative.
4. Diagnostic Packet Forwarding Rules
4.1. DREQ Packet Forwarding
DREQ packets are forwarded via hop-by-hop unicast from the LAST-HOP
address to the Sender address as specified in the diagnostic packet
header. Each hop performs the following processing before forwarding
the packet to the next hop towards the sender:
1. Compute the routing hop count from the previous RSVP hop. This
is done by subtracting the value of the TTL value in the IP
header from Send_TTL in RSVP common header. The result is then
saved in the D-TTL field of the response data object.
2. If no PATH state exists for the specified session, set R-error =
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0x01 in the Response Data object.
3. If the path MTU value is too large, set "MTU too large" error
bit, and change the MTU value to the MTU value of the incoming
interface for PATH messages for the current router.
4. Attach the response data object to the end of the DREQ packet.
If the DREQ packet contains a SELECT object, attach one copy of
each of the objects specified in the SELECT. Otherwise attach
Tspec, filter spec, and flowspec objects to the response object.
Tspec, filter spec, and flowspec objects describe the
reservation in place at the Outgoing Interface for the specified
session.
If no reservation state exists for the specified RSVP session,
the response object will contain no filter-spec or flowspec
object.
If neither PATH nor reservation state exists for the specified
RSVP session, then the response object contains none of the
Tspec, filter or flow spec object.
5. If any error bit is set, change the type field in RSVP common
header from DREQ to DREP, recompute the checksum and send the
packet back to either the LAST-HOP address (if H = 1), or to the
response address directly via unicast (if H = 0).
6. Increment the RSVP-hop-count field in the diagnostic packet
header by one.
If the resulting value is equal to that of Max-RSVP-hops, 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.
7. If the resulting DREQ packet size exceeds the MTU limit, minus
some margin to hold the address list object as described below,
go to Send_DREP().
8. If no error bit set ,then if the H-bit is set, append the
"Incoming Interface Address" to the end of the ROUTE object,
increment R-Pointer by one, update the Next-Hop RSVP_HOP object
to be the Previous Hop from the Path State and update the packet
length field in the RSVP common header accordingly. Finally
forward the DREQ packet to the next hop towards the source,
after recomputing the checksum.
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Send_DREP():
1. If the H flag in the Diagnostic Header header is off, set
target=response address given in the DREQ header, else set
target = the last address in ROUTE.
2. Make a copy of the DREQ packet and change the type field in RSVP
common header from DREQ to DREP. If this host is not the source
set the MF flag on.
If the ROUTE object is so large such that (size of ROUTE + size
of response data object) > path MTU, then set the "route too
big" error bit, recompute the checksum, send the response packet
and go to 4, else recompute the checksum and send the response
packet.
3. If this host is not the source, then trim off all the response
data objects from the original DREQ packet, adjust the "Fragment
offset" value in the RSVP common header accordingly and forward
the modified DREQ packet towards the source, after recomputing
the checksum.
4. 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 towards 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 LAST-HOP router receives a DREP packet, it sends the packet
to the Response address.
4.3. MTU Selection and Adjustment
Because the DREQ packet carries the allowed MTU size of previous hops
that the DREP packets will later traverse, this unique feature allows
the easy semantic fragmentation as described above. Whenever the
DREQ packet grows to approach the size of MTU, it can be trimmed
before being forwarded again.
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When a requester sends a DREQ packet, the path MTU field in the RSVP
Diagnostic Packet header can be set to a configured default value.
Whenever a DREQ packet size approaches the specified MTU value, an
intermediate RSVP router makes a copy of the packet, converts it to a
DREP packet to send back, and then trims off the partial results from
DREQ packet and forwards it.
It is possible that the original MTU value is chosen larger than the
actual MTU value along some portion of the path being traced.
Therefore each intermediate RSVP router must check the MTU value when
processing a DREQ packet. If the specified MTU value is larger than
the MTU of the incoming interface (that the DREQ packet will be
forwarded to), the router
(1) sets the R-error value,
(2) changes the MTU value in the header to the smaller value, and
(3) converts the DREQ packet to a DREP and sends it back to the
requester.
In the rare case where some intermediate routers do not check, or
enforce upon, the MTU value carried in the diagnostic packets, it is
possible that on the way back to the requester, a DREP packet may
encounter a link of smaller MTU.
When this happens, the router follows steps (1) and (2) as outlined
above, and trims off the extra part of the DREP packet to fit in the
smaller MTU of the link. The trimming must be done at response
object boundaries. Such trimming of packets results in information
loss. However because the requester learns what is the available MTU
size, it can either ignore the loss, or otherwise try again with the
smaller MTU value.
4.4. 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
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5.1. Across Firewalls
Firewalls may cause problems in diagnostic packet forwarding. Let us
look at two different cases.
First, let us assume that the querier resides on 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 on.
If the requester is a third party host and is separated from the
LAST-HOP address by a firewall (either the requester is behind a
firewall, or the LAST-HOP is a router behind a firewall, or both), at
this time we do not know any other solution but to change the LAST-
HOP to a node that is on the same side of the firewall as the
requester.
5.2. Examination of RSVP Timers
One can easily collect 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.
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5.4. Discovering Reservation Merges
The flowspec value in a response data block specifies the amount of
resources 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 sender and itself, (2) how many
of the hops already have some reservation by other receivers, and (3)
possibly a rough prediction of 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 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 that
caused RSVP message losses, or whether there are non-RSVP clouds, and
where they are, that may have caused the performance problem.
5.6. Crossing "Legacy" RSVP Routers
Given that this diagnosis function is developed and added to RSVP
after a number of RSVP implementations have been in place, it is
possible, or even likely, that when performing RSVP diagnosis, one
may encounter one or more RSVP-capable routers that do not understand
diagnostic packets, thus drop them. When this happens, the invoking
client will get no response from its requests.
One way to by-pass such "legacy" RSVP routers is running an iteration
of RSVP diagnosis by using information from traceroute, or mtrace in
case of multicast. When an RSVP diagnostic query times out (see next
section), one may first use traceroute to get the list of routers
along the path, and then gradually increases the value of Max-RSVP-
hops field in the DREQ packet, starting from a low value until one no
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longer receives a response. One can then try RSVP diagnosis again by
starting with the first router (which is further upstream towards the
sender) after the unresponding one.
6. Comments on Diagnostic Client Implementation.
Following the design principle that routers in the network should not
hold more than necessary state, RSVP nodes are responsible only for
forwarding Diagnostic packets and filling Response Data Objects.
Additional diagnostic functionalities should be carried out by the
Diagnostic Clients. Furthermore, if the diagnostic function is
invoked from a third-party host, we should not not require that host
be running RSVP daemon to perform the function. Below we sketch out
the basic functions that a diagnostic client daemon should carry out.
1. Take input from the user about the session to be diagnosed, the
last-hop and the sender address, the Max-RSVP-hops, and possibly
the SELECT list, create a DREQ packet and send to the LAST-HOP
RSVP node using raw IP packet with protocol number 46 (RSVP).
The port of the UDP socket that the Diagnostic Client is
listening to for replies, should be included in the Response
Address Filter-Spec.
2. Set a retransmission timer, waiting for the reply (one or more
DREP packets). Listen to the UDP port specified in the Response
Address Filter-Spec for responses from the LAST-HOP RSVP node.
The LAST-HOP RSVP node upon receiving DREP packets sends them to
the the Diagnostic Client as UDP packets, using the port
supplied to in the Response Address Filter-Spec.
3. Upon receiving a DREP packet to an outstanding diagnostic
request, the client should clear the retransmission timer, check
to see if the reply contains the complete result of the
requested diagnosis. If so, it should pass the result up to the
invoking entity immediately.
4. Reassemble DREP fragments. If the first reply to an outstanding
diagnostic request contains only a fragment of the expected
result, the client should set up a reassembly timer in a way
similar to IP packet reassembly timer. If the timer goes off
before all fragments arrive, the client should pass the partial
result to the invoking entity.
5. Use retransmission and reassembly timers to gracefully handle
packet losses and reply fragment scenarios. In the absence of
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INTERNET-DRAFT November 1997
response to the first diagnostic request, a client should
retransmit the request a few times. If all the retransmissions
also fail, the client should invoke traceroute or mtrace to
obtain the list of hops along the path segment to be diagnosed,
and then perform an iteration of diagnosis with increasing hop
count as suggested in Section 5.6 in order to cross RSVP-capable
but diagnosis-incapable routers.
6. If all the above efforts fail, the client must notify the
invoking entity.
7. Acknowledgments
The idea of developing a diagnostic facility for RSVP was first
suggested 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. Lee Breslau, Bob Braden,
and John Krawczyk contributed further comments after March 1996 IETF.
Vincent Subramaniam and Steven Berson provided valuable comments on
variable drafts of the memo. We would also like to acknowledge Intel
for providing a research grant as a partial support for this work.
8. References
[RSVPTUN] L. Zhang, A. Terzis, "RSVP Operation Over IP Tunnels ",
Internet Draft draft-ietf-rsvp-tunnel-02.txt, November, 1997.
9. Authors' Addresses
Lixia Zhang
UCLA
4531G Boelter Hall
Los Angeles, CA 90095
Phone: 310-825-2695
EMail: lixia@cs.ucla.edu
Andreas Terzis
UCLA
4677 Boelter Hall
Los Angeles, CA 90095
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Phone: 310-267-2190
Email: terzis@cs.ucla.edu
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