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INTERNET-DRAFT                                             Michael Welzl
draft-welzl-pmtud-options-00.txt                 University of Innsbruck
                                           Institute of Computer Science
Experimental                                           24. November 2003


             PMTU-Options: Path MTU Discovery Using Options


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

Abstract

   This document describes an experimental enhancement of Path MTU
   Discovery that has the potential of reducing loss, speeding up
   convergence, reducing load in routers which would otherwise need to
   generate a large amount of ICMP messages, and alleviating certain
   additional problems (interactions with tunnels, Black Hole
   Detection). The idea is to use an IP Option which queries routers for
   their MTU before starting a Path MTU Discovery process. The result
   retrieved in this is used as an upper limit for Path MTU Discovery.
   To this end, it is fed back to the source either at the packetization
   layer (recommended) or at the IP layer.


Table of Contents

   Status of this Memo .......................................   1
   1.      Definitions .......................................   2



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   2.      Introduction ......................................   3
   3.      Specification .....................................   3
   3.1.      Probe MTU Option Format for IPv4 ................   3
   3.2.      Feedback Format .................................   4
   3.2.1       IP ............................................   5
   3.2.2       TCP ...........................................   5
   3.2.3       SCTP ..........................................   6
   3.2.4       DCCP ..........................................   6
   3.3.      Host Operation ..................................   7
   3.4.      IPv6 Usage ......................................   8
   3.4.1       Probe MTU Option Format for IPv6 ..............   9
   3.4.2       Feedback Format: IP ...........................  10
   3.4.3       Feedback Format: TCP ..........................  10
   3.4.4       Feedback Format: SCTP .........................  11
   3.4.5       Feedback Format: DCCP .........................  12
   3.5.      IP Tunnels ......................................  12
   4.      Advantages ........................................  13
   4.1.      Reducing Packet Loss ............................  13
   4.2.      Circumventing Black Hole Detection ..............  13
   4.3.      Other Problems with ICMP Fragmentation Needed ...  14
   4.4.      Circumventing Problems with IP Tunnels ..........  14
   5.      Discussion of Issues ..............................  15
   5.1.      Slow Path Processing ............................  15
   5.2.      Placing Additional Burden on Routers ............  16
   5.3.      Motivating Deployment ...........................  16
   6.      Discussion of Usage Scenarios .....................  16
   6.1.      Detecting PMTU Increases ........................  16
   6.2.      RTT-Robust Transport Protocols ..................  17
   6.3.      Tunnels .........................................  17
   7.      Related Work ......................................  17
   8.      Security Considerations ...........................  18
   9.      IANA Considerations ...............................  18
   10.     Normative References ..............................  19
   11.     Informative References ............................  19
   12.     Acknowledgements ..................................  20
   13.     Author's Address ..................................  21
   14.     Full Copyright Statement ..........................  22

1. Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.


   Throughout this document, "Path MTU Discovery" (PMTUD) refers to the
   mechanism described in RFC 1191 [RFC1191] and "Packetization Layer
   Path MTU Discovery" (PLPMTUD) refers to the mechanism described in



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   [draft-PLPMTUD]. The mechanism described in this document is called
   "Path MTU Discovery using Options" (PMTU-Options).


   In the IP architecture, the choice of what size datagram to send is
   made by a protocol at a layer above IP. We refer 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).


2. Introduction


   This memo specifies how options can be used as an enhancement for
   PMTUD and PLPMTUD. The method resembles the mechanism described in
   [RFC1063]: a sender includes an IP Option containing the MTU of its
   outgoing link.  Upon forwarding, each router compares the value with
   the MTUs of the links which are traversed by the datagram and updates
   the field if one of the MTUs of its links is smaller. If all routers
   support this scheme, the receiver has the correct MTU in the option
   and can communicate it back to the sender (preferably at the
   packetization layer instead of the IP layer as specified in
   [RFC1063]).

   The main difference is that the mechanism described in this document
   does not rely on all routers along a path to support the IP option.
   Instead, when this scheme is carried out just before doing PMTUD or
   PLPMTUD, the result is used as an upper limit for the MTU of the path
   (i.e. the Path MTU will definitely not exceed the value obtained with
   this mechanism), no matter how many routers support it. This method
   has several potential advantages over standard PMTUD or PLPMTUD
   (listed in section 4) but also some issues (listed in section 5).
   Being an experimental specification, it is mainly intended for
   special usage scenarios, some of which are described in section 6.


3. Specification

3.1. Probe MTU Option Format for IPv4

   The "Probe MTU Option" for IPv4 that has routers update the MTU value
   (IP option number 11) is specified as in [RFC1063], with the
   exception of an additional "MTU Nonce" field:







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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 11   |   Size = 8    |              MTU              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           MTU Nonce                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This option always contains the lowest MTU of all the links that have
   been traversed so far by the datagram.

   A host that sends this option must initialize the MTU field to be the
   MTU of the directly-connected network. If the host is multi-homed,
   this should be for the first-hop network.

   Each router that receives a datagram containing this option must
   compare the MTU field with the MTUs of the inbound and outbound links
   for the datagram. If either MTU is lower than the value in the MTU
   field of the option, the MTU field should be set to the lower MTU.
   (Note that routers conforming to RFC-1812 may not know either the
   inbound interface or the outbound interface at the time that IP
   options are processed. Accordingly, support for this option may
   require major router software changes).

   Any host receiving a datagram containing this option should confirm
   that value of the MTU field of the option is less than or equal to
   that of the inbound link, and if necessary, reduce the MTU field
   value, before processing the option.

   If the receiving host is not able to accept datagrams as large as
   specified by the value of the MTU field of the option, then it should
   reduce the MTU field to the size of the largest datagram it can
   accept.

   The MTU Nonce field is a means to prevent attackers from hiding the
   fact that a router has updated the MTU field and provide some
   protection against broken downstream routers.  It MUST be initialized
   with a random non-zero 32-bit number by the sender. The number MUST
   be kept for later comparison with the Nonce value which is fed back
   to the sender. A router which updates the MTU field in the option
   MUST set the MTU Nonce field to 0.


3.2. Feedback Format

   IP Option processing is known to be a costly operation.  To avoid
   placing this burden on routers along the backward path, feedback
   SHOULD be stored at the packetization layer; it MAY be stored at the



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   IP layer in special cases (e.g.  if PMTU-Options is used by a UDP
   based application).  Header extensions are specified for IP, TCP,
   SCTP and DCCP. A host implementing PMTU-Options SHOULD react to this
   feedback in all of the supported protocols and provide the MTU value
   to the local PMTUD or PLPMTUD instance. The PMTUD or PLPMTUD instance
   MUST NOT increase a cached PMTU value in response to PMTU-Options
   feedback. If the MTU value from this feedback is greater than or
   equal to the cached MTU value and the MTU Nonce is set to 0, there is
   a chance that an intermediate node or the receiver misbehaved (due to
   broken software or because of an attack).


3.2.1. IP


   The Reply MTU Option for IPv4 (IP option number 12) is specified as
   in [RFC1063], with the exception of an additional "MTU Nonce" field:


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 12   |    Size = 8   |              MTU              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           MTU Nonce                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This option is used to return the value learned from a Probe MTU
   Option to the sender of the Probe MTU Option at the IP level.  Since
   it causes unnecessary processing overhead in routers along the
   backward path, its usage is only recommended for rare special cases.
   In particular, it may be helpful for UDP-based applications which
   utilize PMTU-Options.


3.2.2. TCP


   The Reply MTU Option format for TCP over IPv4 is:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Kind      |   Length = 8  |              MTU              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           MTU Nonce                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   The first octet of this option contains the option kind, identifying
   the TCP option (to be specified by IANA).  The second octet of this
   option contains the length field, specifying the option length in
   octets. The length field is set to 8. The next two fields (two and
   four octets, respectively) contain the result of the MTU-Options
   process and the MTU Nonce; both of these fields must be copied from
   the IPv4 Probe MTU Option by the receiver.


3.2.3. SCTP


   In SCTP [RFC2960], the sender is informed by the receiver about PMTU-
   Options feedback by including the Reply MTU chunk. This chunk looks
   as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Chunk Type  | Flags=00000000|      Chunk Length = 12        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              MTU              |          MTU Nonce            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     MTU Nonce (continued)     |         Padding (0)           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Note: The Reply MTU chunk is considered a Control chunk.

   The Chunk Type field identifies the chunk; it consists of two high-
   order bits "11", indicating that a processing SCTP node which does
   not recognize this chunk type must skip this chunk and continue
   processing, but report in an ERROR Chunk using the "Unrecognized
   Chunk Type" cause of error.  The sender of the Reply MTU chunk SHOULD
   send the MTU feedback via some other means (via a different active
   protocol or an IP option) in response to this ERROR Chunk.  The
   trailing six bits are currently undefined (to be specified by IANA).
   Since this chunk has no specific flags, the Flags field is set to 0.
   The Chunk Length field is set to 12 (the length of the chunk
   including the Chunk Type, Flags and Length fields).

   The MTU and MTU Nonce fields are similar to the specification in
   sections 3.2.1 and 3.2.2; since the length of a SCTP chunk must be a
   multiple of 4 octets, the last octet is filled with 0 (padding).


3.2.4. DCCP





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   The Feedback MTU Option format for DCCP over IPv4 is:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length = 8  |              MTU              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           MTU Nonce                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Type field indentifies the option (number to be specified by
   IANA). The rest of the option is similar to the TCP option defined in
   section 3.2.2.



3.3 Host Operation


   Generally, the PMTU-Options process is carried out before initiating
   a regular PMTUD or PLPMTUD process.

   Since IP options require processing at every router along a path, it
   is important to ensure that no packets unnecessarily include IP
   options. Thus, a host implementing PMTU-Options must keep state in
   routing table entries and only initiate a PMTUD or PLPMTUD (and
   accompanying PMTU-Options) process when this information becomes
   stale. The process of purging stale PMTU information is specified in
   [RFC1191], sections 6.2 and 6.3. Additionally, the number of
   datagrams carrying IP options may be reduced by using PMTU-Options in
   special situations only; a discussion of such potential usage
   scenarios is provided in section 6.

   It is assumed that the datagram size used when doing PMTU-Options is
   already some sensible value (e.g., the result of a recent PMTU
   process or the first-hop data-link MTU). When a Probe MTU Option is
   added to a datagram, it is prolonged by 8 octets (16 octets with
   IPv6).  This number must therefore be taken into account when
   generating a datagram -- when doing PMTU-Options, the size of the
   datagram including the Probe MTU Option must not exceed the recent
   PMTU value. The correct size must be communicated to the
   packetization layer protocol (see [RFC1191], section 6.4, for a
   suggestion of how such communication could be implemented).

   A host receiving a packet that carries the Probe MTU Option MUST feed
   back this information to the sender by copying the MTU and MTU Nonce
   fields as specified in section 3.2 to the next return datagram. To
   this end, it SHOULD inform a packetization layer protocol which is



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   communicating with the originator of the Probe MTU Option.
   Alternatively, a host receiving a packet that carries the Probe MTU
   Option MAY use the MTU Reply IP Option; this method is not
   recommended because it involves unnecessary processing overhead in
   routers on the return path.

   A host MUST be able to accept feedback from IP and SHOULD be able to
   accept feedback from all packetization layer protocols. This feedback
   contains a MTU Nonce value which either has the same value as the MTU
   Nonce field in the original Probe MTU Option (a random number
   initialized by the sender), meaning that the initial MTU value in the
   Probe MTU Option was not changed by routers, or 0, meaning that the
   initial MTU value in the Probe MTU Option was changed by routers. If
   the PMTU upper limit from PMTU-Options feedback is greater than the
   initial PMTU value stored at the sender, the latter value MUST be
   used. If, in such a case, the MTU Nonce was changed, there is a
   chance that an intermediate node or the receiver misbehaved (due to
   broken software or because of an attack).

   The resulting PMTU upper limit at the PMTU-Options sender MUST be
   communicated to the local PMTUD or PLPMTUD instance. Note that
   because the options are placed on unreliable datagrams, the original
   sender will have to resend probes (possibly once per window of data)
   until it receives feedback.

   A host that implements PMTU-Options SHOULD wait for feedback to
   arrive before initiating the subsequent PMTUD or PLPMTUD process,
   which remains unchanged except for one difference: the MTU during the
   process SHOULD not exceed the value received from PMTU-Options.
   Feedback from PMTU-Options SHOULD not be kept any longer -- it is
   only intended as an aid for the subsequent PMTUD or PLPMTUD process.
   Since packet drops can substantially delay the reception of feedback,
   a host MAY use a timer to initiate PTMUD or PLPMTUD even when no
   feedback has arrived; if such a timer is implemented, a means to
   configure this timer MUST be provided.


3.4. IPv6 Usage

   Path MTU Discovery for IPv6 is specified in [RFC1981]. PMTU-Options
   can be combined with this PMTUD variant just like regular PMTUD or
   PLPMTUD; the mechanism should work with IPv6 without requiring any
   substantial changes. The following sections describe the IPv6 formats
   for the Probe MTU Option and feedback -- these formats differ from
   the IPv4 variants in that the MTU field is larger (4 instead of 2
   octets) to support IPv6 jumbograms [RFC2675] and the MTU Nonce field
   is larger (8 instead of 4 octets) to ensure 8-octet alignment without
   wasting space for padding octets.



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   3.4.1. Probe MTU Option Format for IPv6

   The option format specified in section 3.1 is a Hop-by-Hop Options
   header in the IPv6 case.  The PMTU-Options Hop-by-Hop Options header
   is identified by a Next Header value of 0 in the IPv6 header, and has
   the following format:

    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
                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |  Option Type  |Opt Data Len=12|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              MTU                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    8-octet MTU Nonce field                    +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Option Type field identifies the option; it consists of two high-
   order bits "00", indicating that a processing IPv6 node which does
   not recognize this option type must skip over this option and
   continue processing the header. The following bit "1" indicates that
   the Option Data field (MTU and MTU Nonce) may change en-route.  The
   trailing five bits are currently undefined (to be specified by IANA).
   The MTU field was stretched to 4 octets to support IPv6 jumbograms
   [RFC2675].

   Since it may be assumed that, when either of the option-bearing
   headers are present, they carry a very small number of options --
   usually only one [RFC2460] -- the MTU Nonce field was stretched to
   fit the 8-octet alignment (otherwise, a PadN Option of 6 octets would
   have to be used in most cases). This way, the security of PMTU-
   Options is enhanced.

   A complete Hop-by-Hop Options header containing this one option would
   look as follows:
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  | Hdr Ext Len=1 |  Option Type  |Opt Data Len=12|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              MTU                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    8-octet MTU Nonce field                    +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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3.4.2. Feedback Format: IP


   The option format specified in section 3.2.1 is a Destination Options
   header in the IPv6 case.  The PMTU-Options Destination Options header
   is identified by a Next Header value of 60 in the immediately
   preceding header, and has the following format:

    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
                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |  Option Type  |Opt Data Len=12|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              MTU                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    8-octet MTU Nonce field                    +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Option Type field identifies the option; it consists of two high-
   order bits "00", indicating that a processing IPv6 node which does
   not recognize this option type must skip over this option and
   continue processing the header. The following bit "0" indicates that
   the Option Data field (MTU and MTU Nonce) does not change en-route.
   The trailing five bits are currently undefined (to be specified by
   IANA).

   A complete Destination Options header containing this one option
   would look as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  | Hdr Ext Len=1 |  Option Type  |Opt Data Len=12|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              MTU                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    8-octet MTU Nonce field                    +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


3.4.3. Feedback Format: TCP


   The Reply MTU Option format for TCP over IPv6 is:



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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Kind      |  Length = 14  |              MTU              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         MTU (continued)       |           MTU Nonce           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      MTU Nonce (continued)    |     MTU Nonce (continued)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      MTU Nonce (continued)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The first octet of this option contains the option kind, identifying
   the TCP option. The second octet of this option contains the length
   field, specifying the total option length in octets. The length field
   is set to 14. The next two fields (four and eight octets,
   respectively) contain the result of the MTU-Options process and the
   MTU Nonce; both of these fields must be copied from the IPv6 Probe
   MTU Option by the receiver.



3.4.4. Feedback Format: SCTP


   In SCTP [RFC2960], the sender is informed by the receiver about PMTU-
   Options feedback by including the Reply MTU chunk. For IPv6, this
   chunk looks as follows:


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Chunk Type  | Flags=00000000|      Chunk Length = 16        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              MTU                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    8-octet MTU Nonce field                    +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Note: The Reply MTU chunk is considered a Control chunk.

   The Chunk Type field identifies the chunk; it consists of two high-
   order bits "11", indicating that a processing SCTP node which does
   not recognize this chunk type must skip this chunk and continue



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   processing, but report in an ERROR Chunk using the "Unrecognized
   Chunk Type" cause of error.  The sender of the Reply MTU chunk SHOULD
   send the MTU feedback via some other means (via a different active
   protocol or an IP option) in response to this ERROR Chunk.  The
   trailing six bits are currently undefined (to be specified by IANA).
   Since this chunk has no specific flags, the Flags field is set to 0.
   The Chunk Length field is set to 16 (the length of the chunk
   including the Chunk Type, Flags and Length fields).

   The MTU and MTU Nonce fields are similar to the specification in
   section 3.4.2.


3.4.5. Feedback Format: DCCP


   The Reply MTU Option format for DCCP over IPv6 is:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Length = 14  |              MTU              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         MTU (continued)       |           MTU Nonce           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      MTU Nonce (continued)    |     MTU Nonce (continued)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      MTU Nonce (continued)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Type field indentifies the option (number to be
   specified by IANA). The rest of the option is similar
   to the TCP option defined in section 3.4.3.



3.5. IP Tunnels

   If a network node that performs IP-in-IP tunneling (be it because
   of IPsec, IPv6 or for any other purpose) encapsulates a datagram
   carrying the Probe MTU Option, it should copy this option to
   the outer IP header, no matter how many headers there are
   in between. If the network node at the "other side of the tunnel"
   (the node that unpackages an IP datagram by removing the outer
   header) receives a datagram carrying the Probe MTU Option in the
   (outer) IP header, it should subtract the length of the outer and
   all intermediate headers from the value in the (outer header) option
   and then copy it into the inner header option on unpackaging.



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   Such nodes SHOULD NOT change the MTU Nonce field.


4. Advantages

   The PMTU-Options mechanism has a number of potential advantages:


4.1. Reducing Packet Loss

   Probing for the Path MTU means that at some point, a datagram with
   a size exceeding the Path MTU must be sent with the DF bit set to 1.
   Such a datagram will be dropped, which normally requires the
   data it carried to be retransmitted. This retransmission is
   additional traffic which is caused by PMTUD or PLPMTUD.

   The result of PMTU-Options is to be interpreted as an upper limit
   for the Path MTU. If it turns out to be the Path MTU during the
   subsequent PMTUD or PLPMTUD process, the Path MTU is detected
   without causing packet loss.


4.2. Circumventing Black Hole Detection

   PMTUD is known to have a problem called "Black Hole Detection"
   [RFC2923], which happens when a PMTU sender does not receive an
   ICMP Fragmentation Needed message even though the size of a
   datagram with DF set to 1 has exceeded the MTU of a link. This
   may happen if, for instance, routers or firewalls are misconfigured.
   PLPMTUD is robust against this failure because it does not rely
   on ICMP.

   Since the information from PMTU-Options is explicit even when
   the size of a datagram has not exceeded the MTU of a link, the
   "Black Hole Detection" problem of PMTUD could be circumvented
   in a special case that is best explained by means of a simple
   example:


           MTU A  ------  MTU B  ------  MTU C  ------  MTU A
   Sender --------| R1 |---------| R2 |---------| R3 |--------- Receiver
                  ------         ------         ------

   In this example, a datagram traverses routers R1, R2 and R3, and MTU
   A > MTU B > MTU C. PMTUD has converged a long time ago, and a path
   change has occured. Not having noticed any problems, the host now
   probes for a larger MTU. Routers R1 and R2 are misconfigured not to
   send ICMP "Fragmentation Needed" messages -- the size is increased



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   and exceeds MTU B, but the sender never notices this because R1
   simply drops the datagram.


   Now let us assume that PTMU-Options is used. The sender first submits
   a datagram carrying a Probe MTU Option; R3, which is properly
   configured, updates the value in the option (originally A) with C.
   Since PMTUD uses this value as an upper limit, it never exceeds the
   MTU of the link between routers R1 and R2, and Black Hole Detection
   does not occur.



4.3. Other Problems with ICMP Fragmentation Needed

   In addition to the danger of leading to the Black Hole Detection
   problem, utilizing ICMP Fragmentation Needed messages has the
   following additional problems:

   o  Generating an ICMP packet requires memory to be allocated, header
      fields to be initialized etc., which is a costly operation for a
      router.
   o  An ICMP message is additional signaling traffic that consumes
      network capacity.
   o  An ICMP message can be dropped, e.g. as the result of congestion
      on the return path.

   By replacing the generation of an ICMP Fragmentation Needed message
   with a simple option field update, PMTU-Options circumvents these
   problems.


4.4. Circumventing Problems with IP Tunnels

   Sometimes, the DF bit is ignored by network nodes that encapsulate IP
   packets: the total length of a packet with additional headers may
   exceed the Path MTU of the tunnel even if this is not the case
   without the extra headers.  Also, the inner nodes of a tunnel are
   often invisible to the data flow that is carried through the tunnel.
   It would therefore be up to the network nodes at the edge of the
   tunnel to perform PMTUD or PLPMTUD and fragment a packet
   appropriately, if necessary. Simply ignoring the DF bit is an
   attractive alternative.

   If the PMTU-Options mechanism is supported by routers along a tunnel
   path and IP Options are copied to the outer IP header, it is possible
   to detect a potentially smaller MTU in the tunnel, thereby decreasing
   the chance of fragmentation in the tunnel.



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5. Discussion of Issues

   In what follows, we discuss some obvious problems with PMTU-Options:


5.1. Slow Path Processing

   IP packets carrying options are known to be processed in the "Slow
   Path" (software, as opposed to the hardware-only "Fast Path") in most
   normal IP router.  This means that packets carrying the Probe MTU
   Option will experience a notable delay along the forward path.  It is
   therefore not recommended to use datagrams that belong to a PMTU-
   Options process for round-trip time estimation; this means that it is
   questionable whether PMTU-Options should be used at the beginning of
   a TCP connection.

   In IETF and IRTF mailing lists, the impact of option processing has
   been claimed to be immense (e.g., slowing down packets by factor 100)
   on several occasions. While this may be true for packet processing in
   a single router, a recent measurement study has shown that the impact
   on end-to-end delay can be much less severe [WeRo].  The study
   encompassed two separate tests -- one in September/October 2002 and
   one in August 2003. In each of these tests, a ping carrying a NOP IP
   Option was sent to a host from the list at [TBIT], followed by a ping
   without the option; this process was repeated 100 times per host, and
   there was a pause of 1 second in between all pings -- thus, these
   measurements do not say anything about the (possibly different)
   behavior of routers when they are flooded with a large number of
   packets that contain IP options. Additionally, a traceroute was
   carried out for each host.

   4427 and 4401 hosts reacted, with a number of hops ranging from 6 to
   34 (most hosts were in the range of 14 - 25 hops). The pings
   traversed 72143 unique router addresses (not necessarily as many
   different machines because different interfaces on a single router
   may show up as different addresses in traceroute) in 2002 and 80293
   in 2003. The measurements from hosts that did not answer with options
   (approximately 3/4) were weighted based on the frequency of occurence
   (a router which shows up 100 times in the measurements is 100 times
   less important than a router which shows up only once), path length
   and variance (to diminish the effect of queuing delay).

   The final result was that each router caused an additional delay of
   1% (2002) and 0.8 (2003) if a packet contains a NOP option.



5.2. Placing Additional Burden on Routers



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   PMTUD and PLPMTUD only require the "problematic" router (router
   attached to a link with an MTU that was just exceeded) to do
   substantial extra work (notably, all the routers on the return path
   from the "problematic" router to the sender are involved in
   forwarding an ICMP Fragmentation Needed message in the case of
   standard PMTUD [RFC1191]). PMTU-Options involves all routers along
   the path by having them process IP options. In other words, while the
   burden for an individual router is smaller (processing of the Probe
   MTU Option is probably a less costly operation than generating an
   ICMP Fragmentation Needed message), the burden for the whole network
   is perhaps greater.


5.3. Motivating Deployment

   From the perspective of a single node, there is no immediate gain
   when deploying PMTU-Options; at least the sender, receiver and a
   router (ideally attached to the link with the Path MTU -- otherwise
   the only benefit of PMTU-Options could be faster PMTUD convergence
   because it starts with a smaller value) must participate for the
   mechanism to be beneficial.  However, the same is true for Explicit
   Congestion Notification (ECN) [RFC3168], a deployment overview of
   which is given at [ECN].

   PMTU-Options has the following motivating deployment factor: a router
   with a particularly small MTU will typically need to send a large
   number of ICMP packets. This is where PMTU-Options deployment would
   be most beneficial because it might lead to reduced CPU load. From
   the perspective of an end host where PMTU-Options is used, such
   routers are exactly the ones that should be updated because they have
   small MTUs.


6. Discussion of Usage Scenarios

   Since the Probe MTU Option places an additional burden on routers via
   IP option processing and the additional delay from Slow Path
   processing can falsify a round-trip time estimation, it is
   questionable whether the mechanism should be used at the beginning of
   a standard TCP connection. Being an experimental PMTUD enhancement,
   PMTU-Options is rather intended to be used under special
   circumstances -- depending on the importance of the aforementioned
   advantages as opposed to the gravity of the aforementioned issues
   with the mechanism. In what follows, some examples of potential usage
   scenarios are given:


6.1. Detecting PMTU Increases



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   Since the PMTU may change (e.g., when a routing change occurs) and
   even become larger while a long-lasting connection is active,
   [RFC1191] describes a method to probe for increased MTUs (which
   should be done rarely).  The recommended method increases the packet
   size according to a table specified in [RFC1191]; alternatively, the
   "aged" cached PMTU values may be reset to the first-hop data-link
   MTU. Since chances are high that the PMTU did not change and either
   of these process will therefore immediately exceed the current PMTU,
   it may be recommendable to use PMTU-Options before increasing a
   cached PMTU value.



6.2. RTT-robust Transport Protocols

   Transport protocols that do not rely on round-trip time estimates as
   heavily as TCP or SCTP may be a good fit for PMTU-Options. In
   particular, this includes DCCP [draft-DCCP], where the importance of
   round-trip time estimates depends on the Congestion Control ID
   (CCID), and UDP, where no round-trip time estimation is specified.
   Currently, PMTUD is unavailable for applications that utilize UDP --
   it is up to such applications to find out about the ideal size of a
   datagram. PMTUD or PLPMTUD may however be provided to such
   applications in the future (the issue has been discussed in the PMTUD
   Working Group). Assuming that it is available to applications running
   on top of UDP, it may be recommendable for such an application to use
   PMTU-Options depending on its requirements.


6.3. Tunnels

   The operation of PMTU-Options across tunnels is specified in section
   3.5, the potential advantage of this kind of operation is described
   in section 4.4.  In addition to the viewpoint of an application
   traversing a tunnel without wanting PMTUD to fail, the endpoints of a
   tunnel may sometimes also need to determine the MTU in between them
   (the viewpoint being a tunnel with endpoints which do not care about
   actual application endpoints) [draft-TunnelMTU]. In such a case,
   using PMTU-Options may be recommendable due to the potentially
   controlled (or known) tunnel environment and sporadic MTU
   determination at tunnel endpoints.


7. Related Work

   This work can be seen as an extension of the basic idea in [RFC1063].
   A related document is [draft-PMTUDv6], where the idea of utilizing
   options for PMTUD is described for IPv6. This is the only other text



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   that the author is aware of where a Probe MTU option is combined with
   regular PMTUD. Some of the design choices in this document (e.g., the
   MTU Nonce) were based on [RFC3168] and [draft-QS].


8. Security Considerations

   Since MTU-Options requires routers to change a value in packets and
   must therefore always be placed in the outermost IP header to remain
   functioning, it cannot be fully protected with IPsec; however, as the
   explicit MTU update to the sender originates from the receiver of an
   end-to-end connection and not an intermediate router as with ICMP
   Fragmentation Needed messages, the receiver can be authenticated
   using the Encapsulation Security Payload (ESP) header [RFC2406] or
   the Authentication Header (AH) [RFC2402].  For this purpose, the
   Probe MTU Option is classified as mutable and the Reply MTU Option is
   classified as immutable.  The mutable header field (MTU field in the
   Probe MTU Option) is where IPsec cannot help -- it cannot prevent
   intermediate attackers from sending false data. The following attacks
   from such a malicious node must be considered:

   o  Sending a MTU value that is too large:

      A host MUST ignore feedback containing an MTU that is larger than
      the MTU it initially wrote into the option. If a router reduces
      the MTU value, it MUST set the MTU Nonce to 0 -- to hide this
      fact, a malicious node would have to guess the original MTU Nonce,
      which has a 1/(2^32) chance of success (1/2^128 with IPv6).  If a
      malicious node ignores the MTU Nonce and still increases the MTU
      value, the attacker can only succeed if the new value is smaller
      than the (unknown) original value from the sender. Even then, the
      value received from this option is only used as an upper limit for
      PMTUD and PLPMTUD, which will still work in case of such an
      attack. Only the benefit of PMTU-Options can be lost.

   o  Sending a MTU value that is too small:

      This attack, which is similar to an attack with ICMP
      "Fragmentation needed" messages carrying a value that is too
      small, can degrade the performance of a sender. PMTU-Options does
      not provide a mechanism to prevent this attack. This is one of the
      most important reasons to consider it experimental: the result of
      PMTU-Options should merely be used as a hint.


9. IANA Considerations

   This specification reuses the obsolete IPv4 option numbers 11 and 12.



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   It requires two IPv6 Option Type numbers (with leading bits "001" and
   "000"), a TCP option number, a SCTP Chunk Type number (with leading
   bits "00") and a DCCP option number.



10. Normative References

   [RFC1191] Mogul, J.C., and Deering, S.E., "Path MTU discovery", RFC
   1191, November 1990.

   [RFC1981] McCann, J., Deering, S. and Mogul, J., "Path MTU Discovery
   for IP version 6", RFC 1981, August 1996.

   [draft-PLPMTUD] Mathis, M., Heffner, J. and Lahey, K., "Path MTU
   Discovery", Internet-draft draft-ietf-pmtud-method-00.txt, October
   19, 2003.

   [RFC1435] Knowles, S., "IESG Advice from Experience with Path MTU
   Discovery.", RFC 1435, March 1993.

   [RFC2460] Deering, S., and Hinden, R., "Internet Protocol, Version 6
   (IPv6)", RFC 2460, December 1998.

   [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC
   2923, September 2000.

   [RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
   Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and
   Paxson, V., "Stream Control Transmission Protocol", RFC 2960, October
   2000.

   [draft-DCCP] Kohler, E., Handley, M., Floyd, S., and Padhye, J.,
   "Datagram Congestion Control Protocol (DCCP)", Internet-draft draft-
   ietf-dccp-spec-05.txt, October 2003.

   [RFC2402] Kent, S., and Atkinson, R., "IP Authentication Header", RFC
   2402, November 1998


11. Informative References

   [RFC1063] Mogul, J.C., Kent, C.A., Partridge, C., and McCloghrie, K.,
   "IP MTU discovery options.", RFC 1063, July, 1988.

   [WeRo] Welzl, M., and Rossi, M., "On The Impact of IP Option
   Processing", Preprint-Reihe des Fachbereichs Mathematik-Informatik
   (technical report), No. 15, 2003.  work in progress -- will soon be



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   available from http://www.welzl.at/ip-options

   [RFC2675] Borman, D., Deering, S., and Hinden, R., "IPv6 Jumbograms",
   RFC 2675, August 1999.

   [draft-TunnelMTU] Templin, F., "Dynamic MTU Determination for
   IPv6-in-IPv4 Tunnels", Internet-draft draft-ietf-templin-
   tunnelmtu-06.txt, November 2003. Currently available from:
   http://www.geocities.com/osprey67/tunnelmtu-06.txt

   [RFC3168] Ramakrishnan, K., Floyd, S., and Black, D., "The Addition
   of Explicit Congestion Notification (ECN) to IP", RFC 3168, September
   2001.

   [ECN] The ECN website, http://www.icir.org/floyd/ecn.html

   [draft-PMTUDv6] Park, S. D., and Lee, H., "The PMTU Discovery for
   IPv6 Using Hop-by-Hop Option Header", Internet-draft draft-park-pmtu-
   ipv6-option-header-00.txt, March 2003 (expired).

   [draft-QS] Jain, A., and Floyd, S., "Quick-Start for TCP and IP",
   Internet-draft draft-amit-quick-start-02.txt, October 2002 (expired).

   [TBIT] The TBIT website, http://www.icir.org/tbit/

   [RFC2406] Kent, S., and Atkinson, R., "IP Encapsulating Security
   Payload (ESP)", RFC 2406, November 1998.



12. Acknowledgements

   The author would like to thank everybody who contributed to this
   document via helpful discussions. In particular, this includes: Eddie
   Kohler, Matt Mathis, Michael Richardson and Fred Templin.
















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13. Author's  Address

   Michael Welzl
   University of Innsbruck
   Institute fuer Informatik
   Technikerstr. 25/7
   A-6020 Innsbruck, Austria

   Phone: +43 (512) 507-6110
   Fax: +43 (5122) 507-2977
   Email: michael.welzl@uibk.ac.at
   Web: http://www.welzl.at







































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14. Full Copyright Statement

Copyright (C) The Internet Society (1997).  All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implmentation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works.  However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
























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