One document matched: draft-ietf-ipngwg-pmtuv6-00.txt
INTERNET-DRAFT J. McCann, Digital Equipment Corporation
November 6, 1995 S. Deering, Xerox PARC
Path MTU Discovery for IP version 6
draft-ietf-ipngwg-pmtuv6-00.txt
Abstract
This document describes Path MTU Discovery for IP version 6. It is
largely derived from RFC-1191, which describes Path MTU Discovery for
IP version 4.
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
Distribution of this document is unlimited.
Expiration
May 6, 1996
draft-ietf-ipngwg-pmtuv6-00.txt [Page 1]
INTERNET-DRAFT draft-ietf-ipngwg-pmtuv6-00.txt November 6, 1995
Contents
Abstract........................................................1
Status of this Memo.............................................1
Contents........................................................2
1. Introduction.................................................3
2. Protocol overview............................................3
3. Protocol Requirements........................................4
4. Implementation suggestions...................................4
4.1. Layering...................................................5
4.2. Storing PMTU information...................................5
4.3. Purging stale PMTU information.............................7
4.4. TCP layer actions..........................................8
4.5. Issues for other transport protocols.......................9
4.6. Management interface......................................10
5. Security considerations.....................................10
Acknowledgements...............................................11
References.....................................................12
Authors' Addresses.............................................13
draft-ietf-ipngwg-pmtuv6-00.txt [Page 2]
INTERNET-DRAFT draft-ietf-ipngwg-pmtuv6-00.txt November 6, 1995
1. Introduction
When one IPv6 node has a large amount of data to send to another
node, the data is transmitted in a series of IPv6 packets. It is
usually preferable that these packets be of the largest size that can
successfully traverse the path from the source node to the
destination node. This packet size is referred to as the Path MTU
(PMTU), and it is equal to the minimum of the MTUs of the hops in a
path. IPv6 defines a standard mechanism for a node to discover the
PMTU of an arbitrary path.
A PMTU is associated with a path. In IPv6, a path is identified by a
particular combination of source and destination IPv6 addresses, flow
id, and perhaps IPv6 Routing header information.
Nodes not implementing Path MTU Discovery use the IPv6 minimum link
MTU as defined in [IPv6-SPEC] as the maximum packet size. In most
cases, this will result in the use of smaller packets than necessary,
because most paths have a PMTU greater than the IPv6 minimum link
MTU. A node sending packets much smaller than the Path MTU allows is
wasting network resources and probably getting suboptimal throughput.
2. Protocol overview
This memo describes a technique to dynamically discover the PMTU of a
path. The basic idea is that a source node initially assumes that
the PMTU of a path is the (known) MTU of the first hop in the path.
If any of the packets sent on that path are too large to be forwarded
by some router along the path, that router will discard them and
return ICMPv6 Packet Too Big messages [ICMPv6]. Upon receipt of such
a message, the source node reduces its assumed PMTU for the path to
be equal to the MTU of the constricting hop as reported in the Packet
Too Big message.
The PMTU discovery process ends when the node's estimate of the PMTU
is less than or equal to the actual PMTU. Note that several
iterations of the packet-sent/Packet-Too-Big-message-received cycle
may occur before the PMTU discovery process ends, as there may be
hops with smaller MTUs further along the path.
Alternatively, the node may elect to end the discovery process by
ceasing to send packets larger than the IPv6 minimum link MTU.
The PMTU of a path may change over time, due to changes in the
routing topology. Reductions of the PMTU are detected by Packet Too
Big messages. To detect increases in a path's PMTU, a node
periodically increases its assumed PMTU. This will almost always
result in packets being discarded and Packet Too Big messages being
generated, because in most cases the PMTU of the path will not have
changed. Therefore, attempts to detect increases in a path's PMTU
draft-ietf-ipngwg-pmtuv6-00.txt [Page 3]
INTERNET-DRAFT draft-ietf-ipngwg-pmtuv6-00.txt November 6, 1995
should be done infrequently.
3. Protocol Requirements
When a node receives a Packet Too Big message, it MUST reduce its
estimate of the PMTU for the relevant path, based on the value of the
MTU field in the message. The precise behavior of a node in this
circumstance is not specified, since different applications may have
different requirements, and since different implementation
architectures may favor different strategies.
After receiving a Packet Too Big message, a node MUST attempt to
avoid eliciting more such messages in the near future. The node MUST
reduce the size of the packets it is sending along the path. Using a
PMTU estimate larger than the IPv6 minimum link MTU may continue to
elicit Packet Too Big messages. Since each of these messages (and
the dropped packets they respond to) consume network resources, the
node MUST force the PMTU Discovery process to end.
Nodes using PMTU Discovery MUST detect decreases in Path MTU as fast
as possible. Nodes MAY detect increases in Path MTU, but because
doing so requires sending packets larger than the current estimated
PMTU, and because the likelihood is that the PMTU will not have
increased, this MUST be done at infrequent intervals. An attempt to
detect an increase (by sending a packet larger than the current
estimate) MUST NOT be done less than 5 minutes after a Packet Too Big
message has been received for the given path. The recommended
setting for this timer is twice its minimum value (10 minutes).
A node MUST NOT reduce its estimate of the Path MTU below the IPv6
minimum link MTU [IPv6].
A node MUST NOT increase its estimate of the Path MTU in response to
the contents of a Packet Too Big message. A message purporting to
announce an increase in the Path MTU might be a stale packet that has
been floating around in the network, a false packet injected as part
of a denial-of-service attack, or the result of having multiple paths
to the destination.
4. Implementation suggestions
This section discusses how PMTU Discovery may be implemented. This
is not a specification, but rather a set of suggestions.
The issues include:
- What layer or layers implement PMTU Discovery?
- Where is the PMTU information cached?
draft-ietf-ipngwg-pmtuv6-00.txt [Page 4]
INTERNET-DRAFT draft-ietf-ipngwg-pmtuv6-00.txt November 6, 1995
- How is stale PMTU information removed?
- What must transport and higher layers do?
4.1. Layering
In the IP architecture, the choice of what size packet to send is
made by a protocol at a layer above IP. This memo refers to such a
protocol as a "packetization protocol". Packetization protocols are
usually transport protocols (for example, TCP) but can also be
higher-layer protocols (for example, protocols built on top of UDP).
Implementing PMTU Discovery in the packetization layers simplifies
some of the inter-layer issues, but has several drawbacks: the
implementation may have to be redone for each packetization protocol,
it becomes hard to share PMTU information between different
packetization layers, and the connection-oriented state maintained by
some packetization layers may not easily extend to save PMTU
information for long periods.
It is therefore suggested that the IP layer store PMTU information
and that the ICMP layer process received Packet Too Big messages.
The packetization layers may respond to changes in the PMTU, by
changing the size of the messages they send. To support this
layering, packetization layers require a way to learn of changes in
the value of MMS_S, the "maximum send transport-message size". The
MMS_S is derived from the Path MTU by subtracting the size of the
IPv6 header plus space reserved by the IP layer for additional
headers (if any).
It is possible that a packetization layer, perhaps a UDP application
outside the kernel, is unable to change the size of messages it
sends. This may result in a packet size that exceeds the Path MTU.
To accommodate such situations, IPv6 defines a mechanism that allows
large payloads to be divided into fragments, with each fragment sent
in a separate packet (see [IPv6-SPEC] section "Fragment Header").
However, packetization layers are encouraged to avoid sending
messages that will require fragmentation (for the case against
fragmentation, see [FRAG]).
4.2. Storing PMTU information
In general, each PMTU value learned should be associated with a
specific path. A path is identified by a source IPv6 address, a
destination IPv6 address, a flow id, and possibly IPv6 Routing header
information.
Note: Some paths may be further distinguished by different
security classifications. The details of such classifications are
draft-ietf-ipngwg-pmtuv6-00.txt [Page 5]
INTERNET-DRAFT draft-ietf-ipngwg-pmtuv6-00.txt November 6, 1995
beyond the scope of this memo.
The obvious place to store this association is as a field in the
routing table entries. A node will not have a route for every
possible destination, but it should be able to cache a per-
destination route for every active destination. (This requirement is
already imposed by the need to process ICMP Redirect messages.)
When the first packet is sent to a destination for which no per-
destination route exists, a route is chosen from the set of more
aggregated routes, for example a subnet route or a default route.
The PMTU fields in these route entries should be initialized to be
the MTU of the associated first-hop link, and must never be changed
by the PMTU Discovery process. (PMTU Discovery only creates or
changes entries for per-destination routes). Until a Packet Too Big
message is received, the PMTU associated with the initially chosen
route is presumed to be accurate.
When a Packet Too Big message is received, the ICMP layer determines
a new estimate for the Path MTU (from the value in the MTU field in
the Packet Too Big message). If a per-destination route for this
path does not exist, then one is created (the new route uses the same
first-hop router as the current route). If the PMTU estimate
associated with the per-destination route is higher than the new
estimate, then the value in the routing entry is changed.
The packetization layers must be notified about decreases in the
PMTU. Any packetization layer instance (for example, a TCP
connection) that is actively using the path must be notified if the
PMTU estimate is decreased.
Note: even if the Packet Too Big message contains an Original
Packet Header that refers to a UDP packet, the TCP layer must be
notified if any of its connections use the given path.
Also, the instance that sent the packet that elicited the Packet Too
Big message should be notified that its packet has been dropped, even
if the PMTU estimate has not changed, so that it may retransmit the
dropped data.
Note: An implementation can avoid the use of an asynchronous
notification mechanism for PMTU decreases by postponing
notification until the next attempt to send a packet larger than
the PMTU estimate. In this approach, when an attempt is made to
SEND a packet that is larger than the PMTU estimate, the SEND
function should fail and return a suitable error indication. This
approach may be more suitable to a connectionless packetization
layer (such as one using UDP), which (in some implementations) may
be hard to "notify" from the ICMP layer. In this case, the normal
timeout-based retransmission mechanisms would be used to recover
from the dropped packets.
draft-ietf-ipngwg-pmtuv6-00.txt [Page 6]
INTERNET-DRAFT draft-ietf-ipngwg-pmtuv6-00.txt November 6, 1995
It is important to understand that the notification of the
packetization layer instances using the path about the change in the
PMTU is distinct from the notification of a specific instance that a
packet has been dropped. The latter should be done as soon as
practical (i.e., asynchronously from the point of view of the
packetization layer instance), while the former may be delayed until
a packetization layer instance wants to create a packet.
Retransmission should be done for only for those packets that are
known to be dropped, as indicated by a Packet Too Big message.
4.3. Purging stale PMTU information
Internetwork topology is dynamic; routes change over time. The PMTU
discovered for a given destination may be wrong if a new route comes
into use. Thus, PMTU information cached by a node can become stale.
If the stale PMTU value is too large, this will be discovered almost
immediately once a large enough packet is sent to the given
destination. No such mechanism exists for realizing that a stale
PMTU value is too small, so an implementation should "age" cached
values. When a PMTU value has not been decreased for a while (on the
order of 10 minutes), the PMTU estimate should be set to the MTU of
the first-hop link, and the packetization layers should be notified
of the change. This will cause the complete PMTU Discovery process
to take place again.
Note: an implementation should provide a means for changing the
timeout duration, including setting it to "infinity". For
example, nodes attached to an FDDI link which is then attached to
the rest of the Internet via a small MTU serial line are never
going to discover a new non-local PMTU, so they should not have to
put up with dropped packets every 10 minutes.
An upper layer must not retransmit data in response to an increase in
the PMTU estimate, since this increase never comes in response to an
indication of a dropped packet.
One approach to implementing PMTU aging is to add a timestamp field
to the routing table entry. This field is initialized to a
"reserved" value, indicating that the PMTU has never been changed.
Whenever the PMTU is decreased in response to a Packet Too Big
message, the timestamp is set to the current time.
Once a minute, a timer-driven procedure runs through the routing
table, and for each entry whose timestamp is not "reserved" and is
older than the timeout interval:
- The PMTU estimate is set to the MTU of the first hop link.
- Packetization layers using this route are notified of the increase.
draft-ietf-ipngwg-pmtuv6-00.txt [Page 7]
INTERNET-DRAFT draft-ietf-ipngwg-pmtuv6-00.txt November 6, 1995
PMTU estimates may disappear from the routing table if the per-
destination routes are removed; this can happen in response to an
ICMPv6 Redirect message, or because certain routing-table daemons
delete old routes after several minutes. Also, on a multi-homed node
a topology change may result in the use of a different source
interface. When this happens, if the packetization layer is not
notified then it may continue to use a cached PMTU value that is now
too small. One solution is to notify the packetization layer of a
possible PMTU change whenever a Redirect message causes a route
change, and whenever a route is simply deleted from the routing
table.
4.4. TCP layer actions
The TCP layer must track the PMTU for the destination of a
connection; it should not send segments that would result in packets
larger than the PMTU. A simple implementation could ask the IP layer
for this value each time it created a new segment, but this could be
inefficient. Moreover, TCP implementations that follow the "slow-
start" congestion-avoidance algorithm [CONG] typically calculate and
cache several other values derived from the PMTU. It may be simpler
to receive asynchronous notification when the PMTU changes, so that
these variables may be updated.
A TCP implementation must also store the MSS value received from its
peer, and must not send any segment larger than this MSS, regardless
of the PMTU. In 4.xBSD-derived implementations, this may require
adding an additional field to the TCP state record.
The value sent in the TCP MSS option is independent of the PMTU.
This MSS option value is used by the other end of the connection,
which may be using an unrelated PMTU value. See [IPv6-SPEC] sections
"Packet Size Issues" and "Maximum Upper-Layer Payload Size" for
information on selecting a value for the TCP MSS option.
When a Packet Too Big message is received, it implies that a packet
was dropped by the router that sent the ICMP message. It is
sufficient to treat this as any other dropped segment, and wait until
the retransmission timer expires to cause retransmission of the
segment. If the PMTU Discovery process requires several steps to
find the PMTU of the full path, this could delay the connection by
many round-trip times.
Alternatively, the retransmission could be done in immediate response
to a notification that the Path MTU has changed, but only for the
specific connection specified by the Packet Too Big message. The
packet size used in the retransmission should, of course, be no
larger than the new PMTU.
Note: A packetization layer must not retransmit in response to
draft-ietf-ipngwg-pmtuv6-00.txt [Page 8]
INTERNET-DRAFT draft-ietf-ipngwg-pmtuv6-00.txt November 6, 1995
every Packet Too Big message, since a burst of several oversized
segments will give rise to several such messages and hence several
retransmissions of the same data. If the new estimated PMTU is
still wrong, the process repeats, and there is an exponential
growth in the number of superfluous segments sent!
This means that the TCP layer must be able to recognize when a
Packet Too Big notification actually decreases the PMTU that it
has already used to send a packet on the given connection, and
should ignore any other notifications.
Modern TCP implementations incorporate "congestion avoidance" and
"slow-start" algorithms to improve performance [CONG]. Unlike a
retransmission caused by a TCP retransmission timeout, a
retransmission caused by a Packet Too Big message should not change
the congestion window. It should, however, trigger the slow-start
mechanism (i.e., only one segment should be retransmitted until
acknowledgements begin to arrive again).
TCP performance can be reduced if the sender's maximum window size is
not an exact multiple of the segment size in use (this is not the
congestion window size, which is always a multiple of the segment
size). In many systems (such as those derived from 4.2BSD), the
segment size is often set to 1024 octets, and the maximum window size
(the "send space") is usually a multiple of 1024 octets, so the
proper relationship holds by default. If PMTU Discovery is used,
however, the segment size may not be a submultiple of the send space,
and it may change during a connection; this means that the TCP layer
may need to change the transmission window size when PMTU Discovery
changes the PMTU value. The maximum window size should be set to the
greatest multiple of the segment size that is less than or equal to
the sender's buffer space size.
4.5. Issues for other transport protocols
Some transport protocols (such as ISO TP4 [ISOTP]) are not allowed to
repacketize when doing a retransmission. That is, once an attempt is
made to transmit a segment of a certain size, the transport cannot
split the contents of the segment into smaller segments for
retransmission. In such a case, the original segment can be
fragmented by the IP layer during retransmission. Subsequent
segments, when transmitted for the first time, should be no larger
than allowed by the Path MTU.
The Sun Network File System (NFS) uses a Remote Procedure Call (RPC)
protocol [RPC] that, in many cases, sends payloads that must be
fragmented even for the first-hop link. This might improve
performance in certain cases, but it is known to cause reliability
and performance problems, especially when the client and server are
separated by routers.
draft-ietf-ipngwg-pmtuv6-00.txt [Page 9]
INTERNET-DRAFT draft-ietf-ipngwg-pmtuv6-00.txt November 6, 1995
It is recommended that NFS implementations use PMTU Discovery
whenever routers are involved. Most NFS implementations allow the
RPC datagram size to be changed at mount-time (indirectly, by
changing the effective file system block size), but might require
some modification to support changes later on.
Also, since a single NFS operation cannot be split across several UDP
datagrams, certain operations (primarily, those operating on file
names and directories) require a minimum payload size that if sent in
a single packet would exceed the PMTU. NFS implementations should
not reduce the payload size below this threshold, even if PMTU
Discovery suggests a lower value. (Of course, in this case the
payload will be fragmented by the IP layer.)
4.6. Management interface
It is suggested that an implementation provide a way for a system
utility program to:
- Specify that PMTU Discovery not be done on a given route.
- Change the PMTU value associated with a given route.
The former can be accomplished by associating a flag with the routing
entry; when a packet is sent via a route with this flag set, the IP
layer does not send packets larger than the IPv6 minimum link MTU.
These features might be used to work around an anomalous situation,
or by a routing protocol implementation that is able to obtain Path
MTU values.
The implementation should also provide a way to change the timeout
period for aging stale PMTU information.
5. Security considerations
This Path MTU Discovery mechanism makes possible two denial-of-
service attacks, both based on a malicious party sending false Packet
Too Big messages to a node.
In the first attack, the false message indicates a PMTU much smaller
than reality. This should not entirely stop data flow, since the
victim node should never set its PMTU estimate below the IPv6 minimum
link MTU. It will, however, result in suboptimal performance.
In the second attack, the false message indicates a PMTU larger than
reality. If believed, this could cause temporary blockage as the
victim sends packets that will be dropped by some router. Within one
round-trip time, the node would discover its mistake (receiving
draft-ietf-ipngwg-pmtuv6-00.txt [Page 10]
INTERNET-DRAFT draft-ietf-ipngwg-pmtuv6-00.txt November 6, 1995
Packet Too Big messages from that router), but frequent repetition of
this attack could cause lots of packets to be dropped. A node,
however, should never raise its estimate of the PMTU based on a
Packet Too Big message, so should not be vulnerable to this attack.
A malicious party could also cause problems if it could stop a victim
from receiving legitimate Packet Too Big messages, but in this case
there are simpler denial-of-service attacks available.
Acknowledgements
We would like to acknowledge the authors of and contributors to
[RFC-1191], from which the majority of this document was derived.
draft-ietf-ipngwg-pmtuv6-00.txt [Page 11]
INTERNET-DRAFT draft-ietf-ipngwg-pmtuv6-00.txt November 6, 1995
References
[CONG] Van Jacobson. Congestion Avoidance and Control. Proc.
SIGCOMM '88 Symposium on Communications Architectures and
Protocols, pages 314-329. Stanford, CA, August, 1988.
[FRAG] C. Kent and J. Mogul. Fragmentation Considered Harmful.
In Proc. SIGCOMM '87 Workshop on Frontiers in Computer
Communications Technology. August, 1987.
[ICMPv6] A. Conta and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", June 1995
<draft-ietf-ipngwg-icmp-02.txt>
[IPv6-SPEC] S. Deering and R. Hinden, "Internet Protocol Version 6
[IPv6] Specification" Internet Draft, June 1995
<draft-ietf-ipngwg-ipv6-spec-02.txt>
[ISOTP] ISO. ISO Transport Protocol Specification: ISO DP 8073.
RFC 905, SRI Network Information Center, April, 1984.
[RFC-1191] J. Mogul and S. Deering, "Path MTU Discovery",
November 1990
[RPC] Sun Microsystems, Inc. RPC: Remote Procedure Call
Protocol. RFC 1057, SRI Network Information Center,
June, 1988.
draft-ietf-ipngwg-pmtuv6-00.txt [Page 12]
INTERNET-DRAFT draft-ietf-ipngwg-pmtuv6-00.txt November 6, 1995
Authors' Addresses
Jack McCann
Digital Equipment Corporation
110 Spitbrook Road, ZKO3-3/U14
Nashua, NH 03062
Phone: +1 603 881 2608
Fax: +1 603 881 0120
Email: mccann@zk3.dec.com
Stephen E. Deering
Xerox Palo Alto Research Center
3333 Coyote Hill Road
Palo Alto, CA 94304
Phone: +1 415 812 4839
Fax: +1 415 812 4471
Email: deering@parc.xerox.com
Expiration
May 6, 1996
draft-ietf-ipngwg-pmtuv6-00.txt [Page 13]
| PAFTECH AB 2003-2026 | 2026-04-23 06:23:02 |