📄 rfc2473.txt
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^ ^ ^ v
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--------------------#1-- -----Orig.Packet?--- - - - - - - -
#1 #3 Int.Error Code, #5 |
Int.Error Code,^ v Source Address, v v
ICMPv6 Payload | IPv6 | Orig. Packet | IPv4 |
+--------------+ +------------+ +------------+ + - - +
| | | | | |
| ICMP v6 | | ICMP v6 | | ICMP v4 | | |
| Input | | Err Report | | Err Report |
| - - - - +----+ - - - -| + - - - -+ + - - +
| | | |
| IPv6 Layer | | IPv4 Layer | | |
| | | |
+--------------------------------+ +------------+ + - - +
| | |
^ V V
#0 #4 #6
| | |
Tunnel ICMPv6 ICMPv6 ICMPv4
Message Message Message
| | |
Fig.7 Error Reporting Flow in a Node (IPv6 Tunneling Protocol Engine)
Fig.7 path #2 and Fig.8 (b) - The IPv6 tunnel error input
decapsulates the tunnel IPv6 packet, which is the ICMPv6 message
payload, obtaining the original packet, and thus the original headers
and dispatches the "internal error code", the source address from the
original packet header, and the original packet, down to the error
report block of the protocol identified by the Next Header field in
the tunnel header immediately preceding the original packet in the
ICMP message payload.
From here the processing depends on the protocol of the original
packet:
Conta & Deering Standards Track [Page 24]
RFC 2473 Generic Packet Tunneling in IPv6 December 1998
(a) - for an IPv6 original packet
Fig.7 path #3 and Fig.8 (c.1)- for an IPv6 original packet, the
ICMPv6 error report builds an ICMP message of a type and code
according to the "internal error code", containing the "original
packet" as ICMP payload.
Fig.7 path #4 and Fig.8 (d.1)- The ICMP message has the tunnel
entry-point node address as source address, and the original packet
source node address as destination address. The tunnel entry-point
node sends the ICMP message to the source node of the original
packet.
(b) - for an IPv4 original packet
Fig.7 path #5 and Fig.8 (c.2) - for an IPv4 original packet, the
ICMPv4 error report builds an ICMP message of a type and code
derived from the the "internal error code", containing the
"original packet" as ICMP payload.
Fig.7 path #6 and Fig.8 (d.2) - The ICMP message has the tunnel
entry-point node IPv4 address as source address, and the original
packet IPv4 source node address as destination address. The tunnel
entry-point node sends the ICMP message to the source node of the
original packet.
A graphical description of the header processing taking place is the
following:
Conta & Deering Standards Track [Page 25]
RFC 2473 Generic Packet Tunneling in IPv6 December 1998
< Tunnel Packet >
+--------+- - - - - -+--------+------------------------------//------+
| IPv6 | IPv6 | ICMP | Tunnel |
(a)| | Extension | | IPv6 |
| Header | Headers | Header | Packet in error |
+--------+- - - - - -+--------+------------------------------//------+
< Tunnel Headers > < Tunnel ICMP Message >
< ICMPv6 Message Payload >
|
v
< Tunnel ICMP Message >
< Tunnel IPv6 Packet in Error >
+--------+ +---------+ +----------+--------//------+
| ICMP | | Tunnel | | Original | Original |
(b) | | + | IPv6 | + | | Packet |
| Header | | Headers | | Headers | Payload |
+--------+ +---------+ +----------+--------//------+
| <Original Packet in Error >
----------------- |
| |
--------------|---------------
| |
V V
+---------+ +--------+ +-------------------//------+
| New | | ICMP | | |
(c.1) | IPv6 | + | | + | Orig. Packet in Error |
| Headers | | Header | | |
+---------+ +--------+ +-------------------//------+
|
v
+---------+--------+-------------------//------+
| New | ICMP | Original |
(d.1) | IPv6 | | |
| Headers | Header | Packet in Error |
+---------+--------+-------------------//------+
< New ICMP Message >
Conta & Deering Standards Track [Page 26]
RFC 2473 Generic Packet Tunneling in IPv6 December 1998
or for an IPv4 original packet
+---------+ +--------+ +-------------------//------+
| New | | ICMP | | |
(c.2) | IPv4 | + | | + | Orig. Packet in Error |
| Header | | Header | | |
+---------+ +--------+ +-------------------//------+
|
v
+---------+--------+-------------------//------+
| New | ICMP | Original |
(d.2) | IPv4 | | |
| Header | Header | Packet in Error |
+---------+--------+-------------------//------+
< New ICMP Message >
Fig.8 ICMP Error Reporting and Processing
8.1 Tunnel ICMP Messages
The tunnel ICMP messages that are reported to the source of the
original packet are:
hop limit exceeded
The tunnel has a misconfigured hop limit, or contains a
routing loop, and packets do not reach the tunnel exit-
point node. This problem is reported to the tunnel entry-
point node, where the tunnel hop limit can be reconfigured
to a higher value. The problem is further reported to the
source of the original packet as described in section 8.2,
or 8.3.
unreachable node
One of the nodes in the tunnel is not or is no longer
reachable. This problem is reported to the tunnel entry-
point node, which should be reconfigured with a valid and
active path between the entry and exit-point of the tunnel.
The problem is further reported to the source of the
original packet as described in section 8.2, or 8.3.
parameter problem
A Parameter Problem ICMP message pointing to a valid Tunnel
Encapsulation Limit Destination header with a Tun Encap Lim
field value set to one is an indication that the tunnel
Conta & Deering Standards Track [Page 27]
RFC 2473 Generic Packet Tunneling in IPv6 December 1998
packet exceeded the maximum number of encapsulations
allowed. The problem is further reported to the source of
the original packet as described in section 8.2, or 8.3.
The above three problems detected inside the tunnel, which are a
tunnel configuration and a tunnel topology problem, are reported to
the source of the original IPv6 packet, as a tunnel generic
"unreachable" problem caused by a "link problem" - see section 8.2
and 8.3.
packet too big
The tunnel packet exceeds the tunnel Path MTU.
The information carried by this type of ICMP message is
used as follows:
- by a receiving tunnel entry-point node to set or adjust
the tunnel MTU
- by a sending tunnel entry-point node to indicate to the
source of an original packet the MTU size that should be
used in sending IPv6 packets towards the tunnel entry-point
node.
8.2 ICMP Messages for IPv6 Original Packets
The tunnel entry-point node builds the ICMP and IPv6 headers of the
ICMP message that is sent to the source of the original packet as
follows:
IPv6 Fields:
Source Address
A valid unicast IPv6 address of the outgoing
interface.
Destination Address
Copied from the Source Address field of the Original
IPv6 header.
ICMP Fields:
For any of the following tunnel ICMP error messages:
"hop limit exceeded"
Conta & Deering Standards Track [Page 28]
RFC 2473 Generic Packet Tunneling in IPv6 December 1998
"unreachable node"
"parameter problem" - pointing to a valid Tunnel Encapsulation
Limit destination header with the Tun Encap Lim field set to a
value zero:
Type 1 - unreachable node
Code 3 - address unreachable
For tunnel ICMP error message "packet too big":
Type 2 - packet too big
Code 0
MTU The MTU field from the tunnel ICMP message minus
the length of the tunnel headers.
According to the general rules described in 7.1, an ICMP "packet too
big" message is sent to the source of the original packet only if the
original packet size is larger than the minimum link MTU size
required for IPv6 [IPv6-Spec].
8.3 ICMP Messages for IPv4 Original Packets
The tunnel entry-point node builds the ICMP and IPv4 header of the
ICMP message that is sent to the source of the original packet as
follows:
IPv4 Fields:
Source Address
A valid unicast IPv4 address of the outgoing
interface.
Destination Address
Copied from the Source Address field of the Original
IPv4 header.
ICMP Fields:
For any of the following tunnel ICMP error messages:
"hop limit exceeded"
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