Network Working Group Vijay Kumar Vasantha(Huawei)
Internet Draft Aug 4, 2008
<draft-kumar-ipv6-pmtu-using-routing-proto-00.txt>
Intended status: Proposed Standard
Expires: Feb 5, 2009
IPv6 Path MTU computation using routing protocol.
Status of This Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
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.
This Internet-Draft will expire on February 5, 2009.
Abstract
This document describes a mechanism for dynamically computing IPv6
PMTU and the modifications needed in IPv6 to support the solution.
Specification of Requirements
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 [RFC2119].
Table of Contents
Status of This Memo...................................................1
Abstract .............................................................1
Table of Contents.....................................................1
1. Introduction .....................................................2
2. PMTU computation in routing domain ...............................2
2.1 PMTU calculation within a routing protocol area.................3
2.2 PMTU calculation across a routing protocol area.................3
2.3 PMTU calculation across an autonomous system ...................4
2.4 Interoperability with PMTU non-computational routers ...........4
3. Modification in ICMPv6 error message .............................5
3.1 Packet Too Big Message .........................................5
4. Acknowledgments ..................................................5
5. Normative References .............................................5
6. Informative References ...........................................5
7. Authors' Addresses ...............................................5
Full Copyright Statement .............................................6
1. Introduction
The current IPv6 PMTU discovery has the following drawbacks,
1. The IPv6 PMTU discovery is done by trial and error method, which
can result in inefficient forwarding such as described below and
this in turn can result in delay in packet transmission.
. Packets may be dropped because of packet too big reason by any
intermediate router.
. Packets that are very small in size may be forwarded for
considerable amount of time resulting in inefficient usage of
available bandwidth.
2. The source comes to know about the packet drop only by ICMPv6
packet too big error. But this error packet will have to travel
from the problem occurred router to the source of the packet,
which consumes considerable amount of bandwidth on all the
intermediate links between the originator and the problem
occurred node.
This document defines a method in which an optimal PTMU is
dynamically computed using linked state routing protocols on routers
and modification needed in IPv6 to convey PMTU information to the
hosts.
The method by which PMTU can be calculated for each IPv6 route in an
IS-IS routing domain can be seen in [ISISPMTU]. The same computation
can be supported by other linked state routing protocols as well and
the overhead incurred in doing so is minimal as PMTU computation can
be closely associated with route computation.
In the remainder of this document, the key words "MUST", "MUST NOT"
"REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULDNOT",
"RECOMMENDED","MAY", and "OPTIONAL" are to be interpreted as
described in [RFC2119].
2. PMTU computation in routing domain
Internet is composed of many Autonomous systems and each autonomous
system is in turn composed of many areas. The below section
describes the overview of PMTU computation at each of these
hierarchy in the routing domain.
2.1 PMTU calculation within a routing protocol area
--------------
//// \\\\
// \\
| |
| AREA K |
| |
\\ //
\\\\ ////
--------------
In any routing area all the routers will have complete path to reach
all intra-area routes. The MTU to a particular destination along the
shortest path tree is considered as the PMTU to that particular
destination. Thus all the intra-area routes can be associated with
an optimal PMTU value. [ISISPMTU] describes an algorithm to achieve
the same and the same algorithm can be used in any other link state
IGP.
2.2 PMTU calculation across a routing protocol area
----------- -----------
//// \\\\ //// \\\\
// \\ // \\
| || |
| RTA |RTB| RTC |
| || |
\\ // \\ //
\\\\ //// \\\\ ////
----------- -----------
AREA K AREA L
As areas are created for hierarchical routing, the inter-area
route's path won't be completely visible to intra-area routers.
Consider a destination X originated by the router RTC, the
intra-area router RTA will know about the destination X only through
the area border router RTB and the path information of the
destination X on RTA will be confined only till the area border
router RTB.
As described in sec 2.1 the intra-area router like RTA can compute
PMTU for the inter-area destination like X considering only the
intra-area path information. But PMTU so computed won't reflect the
correct value till destination. Thus for the intra-area router like
RTA additional information is needed to compute the correct value of
PMTU for inter-area routes.
The ABR RTB provides the additional PMTU information from itself to
the destination when transferring route between areas and the
intra-area router can use the additional information and calculate
optimal PMTU.
[ISISPMTU] describes an algorithm to achieve the same and the same
algorithm can be used in any other link state IGP.
2.3 PMTU calculation across an autonomous system
----------- -----------
//// \\\\ //// \\\\
// \\ // \\
| || |
| AS1 |ASBR| AS2 |
| || |
\\ // \\ //
\\\\ //// \\\\ ////
----------- -----------
The problem described in section 2.2 is equally applicable when
dealing with inter AS routes. If each AS is operated under a
different protocol then intra-AS router cannot know the full path to
inter-AS routes, hence the ASBR should support the transformation of
PMTU across AS.
To support the PMTU computation across the AS all related protocols
should support the PMTU computation and PMTU information transfer
across ASBR. The method by which this is done in all the protocols
is out of scope of this document and the method by which IS-IS
supports the calculation and transfers the information across
protocol can be seen in [ISISPMTU].
Any change in PMTU of a route should be dynamically detected in
routing domain and PMTU should be recomputed. Thus all the routes in
a routing domain should be associated with a PMTU value and this
PMTU information should be downloaded to forwarding information
base.
2.4 Interoperability with PMTU non-computational routers
While interacting with PMTU non-computational routers the complete
information to compute the PMTU may not be available and the
interaction with PMTU non-computational router can occur within an
area, across an area and/or across an autonomous system.
[ISISPMTU] addresses this problem by carrying out best effort PMTU
computation at each hierarchy and the same algorithm can be applied
to other link state protocols as well. If the so computed PMTU is
not optimal then the originator of the IPv6 packet can know the
actual PMTU through the existing ICMPv6 packet too big messages.
3. Modification in ICMPv6 error message
3.1 Packet Too Big Message
The router should generate ICMPv6 packet too big error whenever it
detects that the size of the packet is greater than the computed
PMTU to the destination. The ICMPv6 packet too big error message
should convey the computed PMTU to the destination rather than the
outgoing interface's MTU value.
If a router does not have a computed PMTU value for a destination
then the forwarding behavior is unchanged.
4. Acknowledgements
The author would like to thank Saravana Kumar and K.L.Srini
5. Normative References
[ISISPMTU] Vijay Kumar Vasantha
"draft-kumar-isis-path-mtu-00.txt".
6. Informative References
[ISO10589] ISO. Intermediate System to Intermediate System
Routing Exchange Protocol for Use in Conjunction with
the Protocol for Providing the Connectionless-Mode
Network Service. ISO 10589, 1992.
[RFC1981] McCann, Deering & Mogul RFC 1981 Path MTU Discovery for
IP version 6, August 1996
7. Authors' Addresses
Vijay Kumar Vasantha
Huawei Technologies India Private Limited
Bangalore, India - 560008
vijaykumar@huawei.com
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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.
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology
described in this document or the extent to which any license
under such rights might or might not be available; nor does it
represent that it has made any independent effort to identify any
such rights. Information on the procedures with respect to rights
in RFC documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention
any copyrights, patents or patent applications, or other
proprietary rights that may cover technology that may be required
to implement this standard. Please address the information to the
IETF at ietf-ipr@ietf.org.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.