Network Working Group G. Chen
Internet-Draft H. Deng
Intended status: Informational B. Zhou
Expires: January 12, 2010 CMCC, Inc.
M. Xu
D. Huo
Y. Cao
Tsinghua University
July 11, 2009
An Incremental Deployable Mapping Service for Scalable Routing
Architecture
draft-chen-lisp-er-mo-01
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Abstract
This document describes a mechanism of providing mapping service for
LISP-like architecture. The mapping service comprises of EID Router
(ER) mechanism and supplementary DHT Mapping Overlay (MO), in which
ER mechanism is for reducing forwarding entries in routers while
driving the packets to the destination through tunnels, and the DHT
MO serves as a supplement that provides specific mappings to reduce
the number of tunnels. The mechanism is flexibly deployable for ISPs
since it costs little and is easy to progress.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definition of Terms . . . . . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. When an ITR meets packets . . . . . . . . . . . . . . . . . . 6
5. Utilization of current BGP system . . . . . . . . . . . . . . 7
5.1. Automatic Mapping obtainment and storage . . . . . . . . . 7
5.2. Mapping propagation by BGP . . . . . . . . . . . . . . . . 7
6. EID Router mechanism . . . . . . . . . . . . . . . . . . . . . 9
6.1. Address aggregation policy . . . . . . . . . . . . . . . . 9
6.2. EID Router . . . . . . . . . . . . . . . . . . . . . . . . 9
6.3. When an ER meets packets . . . . . . . . . . . . . . . . . 9
7. Supplementary DHT Mapping Overlay (MO) . . . . . . . . . . . . 11
7.1. Mapping Node (MN) and Mapping Server (MS) . . . . . . . . 11
7.2. MNID Assignment and K-bucket Table . . . . . . . . . . . . 11
7.3. LOOKUP Process . . . . . . . . . . . . . . . . . . . . . . 12
7.4. Security Consideration of Mapping Storage . . . . . . . . 12
7.5. Self-adaptive Capability . . . . . . . . . . . . . . . . . 13
7.6. Dynamic Adjustment of K value and m value . . . . . . . . 13
7.7. Mapping Storing and Exchanging in Multi-homing Scenario . 13
8. Incremental Deployment . . . . . . . . . . . . . . . . . . . . 14
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
12.1. Normative References . . . . . . . . . . . . . . . . . . . 18
12.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
LISP [I-D.farinacci-lisp] is an architecture for scalable routing.
It defines two address spaces: Routing Locators (RLOC) and Endpoint
Identifiers (EID). LISP uses EIDs as lookup keys for a new EID-to-
RLOC mapping database, in which way several mapping services are
built such as [I-D.fuller-lisp-alt] and [I-D.meyer-lisp-cons]. In
these mapping service solutions, different kinds of overlays are
designed and built as database for storing mapping information, as
well as providing mapping lookup results for mapping queries.
The problem they commonly share is that packets without any caches on
current ITR have to be waiting for the reply of mapping lookup query,
or simply be dropped by this ITR as long as no relevant cache exists
on this ITR.
One solution to this problem could be that, instead of sending lookup
queries to the Mapping Overlay (MO), data packet itself is sent to
the MO as a query (e.g., "Data Probe" in [I-D.fuller-lisp-alt]>) and
get forwarded in the MO to the final ETR linked to the site in which
the destination EID resides. But usually when a packet is going
through the MO, long latency becomes a remarkable problem then.
In this draft we describe an incremental deployable mapping service
for LISP. This mapping service comprises of EID Router (ER)
mechanism and supplementary DHT Mapping Overlay (MO). The ER
mechanism is designed for reducing forwarding entries in routers,
while driving the packets to the destination through tunnels. The
DHT MO serves as a supplement that provides specific mappings to
reduce the number of tunnels along the path to the destination. Note
that an ER can be deployed unilaterally in an AS for it's own
benefits and the DHT MO is unitedly built among ASes however whether
to join the MO is not compulsory to an AS (it can still benefit from
deploying the ER).
The remainder of this document is organized as follows: Section 2
provides the definitions of terms in this document. Section 3
sketches an overview of the mapping service. Section 4 describes how
an ITR handles the packets. Section 5 describes how to utilize
current BGP system in the mapping service. Section 6 describes how
the EID Router mechanism works, and Section 7 describes how to build
the DHT Mapping Overlay and how to retrieve mappings in it. And
Section 8 shows the steps for deploying the mapping service
incrementally.
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2. Definition of Terms
Mapping: an EID-to-ELOC mapping.
EID aggregated prefix: an aggregated prefix which covers some EID
blocks.
EID+RLOC aggregated prefix: an aggregated prefix which covers some
EID block(s) and RLOC(s).
EID Router (ER): a new introduced router which keeps entries to all
EID aggregated prefixes.
Mapping Node (MN): an entity used for storing a mapping. Each MN
holds and can only hold one mapping, and each mapping is related
to only one MN. It can be implemented as a process in a MS, which
has a data structure to store the mapping as well as the ability
to manage and retrieve the mappings.
Master Mapping Node (MMN): a chosen Mapping Node used to be the
representative among redundant MNs. It is in charge of initiating
mapping query and exchanging mappings.
Mapping Server (MS): a server specified to physically store
mappings. Each MS can hold more than one Mapping Nodes.
Mapping Overlay (MO): a DHT overlay, which is designed for storing
the distributed mapping information. Only one MO exists among
ISPs in the Internet.
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3. Overview
The mechanism described in this draft aims:
o to eliminate all forwarding entries to distant customer ASes in P
routers;
o to eliminate the forwarding entries, targeted to distant customer
ASes not behind the border routers, in the border routers;
o to be deployed incrementally;
o to help reduce the number of tunnels.
To achieve the four aims above, the mechanism described in this draft
mainly comprises of the following two parts:
o EID Router (ER) mechanism for non-cached packets tunneling, and
o DHT Mapping Overlay (MO) as a supplement, which provides specific
mappings to reduce tunneling cost.
The EID Router mechanism is designed for the first three aims, and
the DHT MO is designed for the last aim.
In EID Router mechanism, by manually or automatically setting the
default route to an ER (each AS at least has one ER), all forwarding
entries to distant customer ASes in P routers, and a part of
forwarding entries (targeted to distant customer ASes not behind the
border routers) in the border routers can be eliminated.
The current running Border Gateway Protocol [RFC4271] is mainly
utilized to propagate mappings through the current running BGP
speaking system. The most important reason to use the current
running BGP speaking system is to make the deployment backward
compatible, so that incremental deployment can be achieved.
The DHT Mapping Overlay can help reduce the number of tunnels which
result from deploying the ER mechanism. It is optional for ISPs and
only needs a little investment on it.
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4. When an ITR meets packets
When an ITR receives a packet originated from a customer site, it
checks whether a copy of mapping exists in its cache first.
If the mapping exists, the ITR encapsulates the packet in a LISP
header, putting the RLOC extracted from the mapping onto the outer
destination address, meanwhile selecting one of the ITR's RLOC as the
outer source address.
Else if cache misses (i.e., no relevant copy of mapping exists in the
ITR), two concurrent events occur:
o Data Plane Traffic: the packet simply follows a default route
preset manually or automatically to an ER in current AS. Since ER
knows whole global mapping information, it can forward every
packet to the right ETR by encapsulating the packet in LISP header
with the ITR's RLOC in the outer source address and the ETR's RLOC
in the outer destination address.
o Control Plane Traffic: the ITR sends a Mapping Query to its
default Mapping Server (MS) in the AS. And then a mapping LOOKUP
process (details of mapping lookup process are shown in Section 7)
is launched in the Mapping Overlay (MO) by the Master Mapping Node
(MMN) of the ITR. After the MMN receives a copy of queried
mapping from the MO, it returns the copy to the ITR which
initiated the Mapping Query, and is cached for a period of time.
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5. Utilization of current BGP system
The BGP is an inter-Autonomous System routing protocol. The primary
function of a BGP speaking system is to exchange network reachability
information with other BGP systems. This network reachability
information includes information on the list of ASes that
reachability information traverses. This information is sufficient
for constructing a graph of AS connectivity for this reachability, as
well as inevitable for constructing the mappings from EIDs onto RLOCs
automatically. Moreover, especially for incremental deployment
requirement, which means ASes deployed new mechanism must work along
with those not deployed ones, it is necessary to design mapping
service inherently adaptable for the current running BGP system
(i.e., the BGP system we use for basic routing and forwarding today).
The BGP in the mapping service has two functions: to obtain the
mappings automatically, and to propagate mappings to ERs in other
ASes. They're both based on current running BGP system.
5.1. Automatic Mapping obtainment and storage
When an customer AS advertise an BGP UPDATE message to homed (no
matter single-homed or multi-homed) provider AS which is deployed the
DHT mapping server described in Section 7, the provider AS would set
or update the relevant mapping information according to the
advertised route to the customer AS. The announced prefix is treat
as the EID in the mapping <EID, RLOC> and the address of the ETR
which directly receives BGP announcement from the customer AS is
chosen as the RLOC.
This mapping could be stored both in MN (Mapping Node) and ER (EID
Router) concurrently. In the former case, one mapping refers to one
MN and vice versa as described in Section 7. However in the latter
case, the mapping is not only stored in the ER in current provider
AS, but also propagated to distant provider ASes by BGP
advertisements and stored in ERs at those ASes.
Note that the mappings obtained so far are original specific
mappings. In DHT MO, these original specific mappings are stored on
MNs and no changes on mapping granularity. However in ER mechanism,
during the mapping propagation by BGP, mapping granularity is changed
once a prefix aggregation occurs in an AS (details are shown in
Section 5.2).
5.2. Mapping propagation by BGP
BGP speakers work as what they act today, in addition that mapping
information is affiliated in BGP UPDATE message. Each BGP speaker on
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the route SHALL keep the originality of the mappings (i.e., the
mappings stay untouched during propagation), except that it
aggregates some prefixes into one. New mapping SHOULD be formed when
such aggregation occurs, in which case both EID and RLOC in mapping
<EID, RLOC> are updated, that EID is set to the new aggregated EID
block which covers more prefixes while RLOC is set to the address of
either ER (if ER is deployed) or border router (if no ER is deployed)
in current AS.
Note that since aggregation is permitted during the mapping
propagation, the number of mappings stored on the ERs would be far
more less than the number of mappings stored in the MO.
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6. EID Router mechanism
6.1. Address aggregation policy
All addresses from edge customer ASes can be seen as the EIDs. EID
prefixes can be aggregated to EID aggregated prefix. Moreover we
allow EIDs to be aggregated with RLOCs to EID+RLOC aggregated prefix.
For example, suppose two EID blocks 166.111.8/24 and 166.111.9/24
belong to two customer ASes homed to a provider AS which has some
RLOCs range from 166.111.10/24 to 166.111.11/24, the provider AS can
aggregate either to an EID aggregated prefix 166.111.8/23 or to an
EID+RLOC aggregated prefix 166.111.8/22.
6.2. EID Router
An EID Router is no particular than a legacy router, except that
special configuration is applied. It is configured to act as an eBGP
speaker, and only loads the forwarding entries to all EID aggregated
prefixes. Note that the EID+RLOC aggregated prefixes don't have to
be loaded in EID Routers, since the RLOCs in the EID+RLOC aggregated
prefixes are supposed be reachable (i.e., forwarding entries to these
prefixes should be preserved in the P routers).
So the ideal situation becomes:
o the EID Routers load the forwarding entries to all EID aggregated
prefixes,
o the P routers load the forwarding entries to all RLOCs and all
EID+RLOC aggregated prefixes, and
o the border routers load the forwarding entries to all RLOCs and
the prefixes (i.e., EID aggregated prefixes and EID+RLOC
aggregated prefixes) of the distant ASes behind the border
routers.
So due to deploying the EID Router mechanism, P routers and border
routers can get their FIB (Forwarding Information Base) size reduced.
6.3. When an ER meets packets
When an ER receives a packet, it matches the destination address with
entries in its forwarding table (that can be seen as the mapping
table). Since the ER holds whole mapping table (from its angle of
view), this packet can be encapsulated in a LISP header and sent out.
The tunnel end point may be one of the following four kinds of
routers:
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o the border router of the peering AS on the path to the
destination, in which case aggregation occurs in this peering AS
or this peering AS didn't pass the mapping information to the
current AS.
o the border router of the non-peering AS on the path to the
destination, in which case aggregation occurs in this non-peering
AS.
o the EID Router of a distant AS (either peering or non-peering) on
the path to the destination, in which case the downstream AS
didn't pass the mapping information to this distant AS so that the
ER in this distant AS created a new mapping (the ER's RLOC is set
in the mapping).
o the destination ETR, in which case the originality of the mapping
is maintained.
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7. Supplementary DHT Mapping Overlay (MO)
The DHT Mapping Overlay (MO) is based on [Kademlia], a highly
efficient protocol of Distributed Hash Table (DHT) overlay for Peer-
to-Peer network, which applies XOR as metric to measure distance.
Here in the MO, it is adapted to meet several requirements below:
o MO should be scalable;
o MO should have a good ability of redundancy;
o MO should be self-adaptive for mapping adding or failure;
o MO should be flexible for balancing performance and overhead;
o MO should support multi-homing scenario.
The benefit of deploying the MO is that, it provides specific
mappings since it doesn't aggregate prefixes (i.e., mappings stored
in MO are finest-granulated that each mapping refers to one relation
between a customer AS and one of its provider site). Due to the
large number of such fines-granulated mappings, the MO should be
scalable and capable for redundancy. So DHT is chosen as the means
of distributing the mappings.
7.1. Mapping Node (MN) and Mapping Server (MS)
As described in Section 5.1, a mapping is automatically obtained from
the BGP advertisement through the ETR. Afer that it is sent to a MS
in current provider AS and then stored in a new created MN (or
manually set on the MN). Note that each mapping can only be
initially stored on one MN in the MO, and each MS can accommodate
more than one MNs. For example, an ISP is accessed by 5 customer
ASes labeled as a, b, c, d, e, whose corresponding EIDs are v, w, x,
y, z respectively. These five EID prefixes of customer ASes are one-
to-one mapped, forming five MNs physically existed on one or multiple
MSes administrated by the ISP.
7.2. MNID Assignment and K-bucket Table
In the MO, each MN is assigned a 160 bit ID. The DHT MO utilizes the
highest numerical IP address reserved in customer ASes as a MNID.
For example, assume a customer AS with a prefix 162.137.2/24 is
mapped to the RLOC 134.121.3.56. The lower 32 bits of the MNID of
the corresponding Master Mapping Node (MMN) is 0xA28902FE (i.e.,
162.137.2.254), and the rest 128 bits are all 0. The mapping will be
stored on this MMN and several (at least one) other MNs whose MNIDs
are closest to the MNID 0xA28902FE.
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Each MN manages a K-bucket table of its own that keeps the
information how it can reach other MNs (i.e., the RLOCs of the
resident MSes of these MNs). Each MN's reachability imformation is
stored on a node in K-bucket. The table of a MN N consists of 160
rows in which the i-th row (0 <= i < 160) preserves the reachability
information of some MNs (i.e., the RLOCs of the resident MSes of
these MNs) which are at a distance range 2^I ~ 2^(i+1) from N. If i
becomes quite large, the number of nodes that the i-th row preserves
is limited to K at most.
7.3. LOOKUP Process
LOOKUP process needs to call FIND_MAP with MNID of destination MN as
parameter. Here describes the FIND_MAP procedure (MN B is the
destination MN):
1. MN A calculate the distance D from A to B (D = A XOR B);
2. Fetch m MNs from the right row of K-bucket table of MN A and then
query them (call FIND_MAP for every one of these m MNs);
3. MN A set a timer waiting reply for each MN that a called
FIND_MAP. If it expires, then delete information of
corresponding MN in K-bucket table.
4. Each MN who received FIND_MAP call will check if it is one of the
closest MNs destined to B. If so then return mapping to MN A;
else like in step 1 and 2, calculates distance D and fetches m
closer MNs, then return them to MN A.
5. MN A continues to send FIND_MAP calls to those returned MNs until
mapping returned or find K closest MNs (which means no such
mapping existed).
7.4. Security Consideration of Mapping Storage
In native Kademlia, any MN can initiate a STORE call to put the <key,
value> pair on other K closest nodes. But for the reason that it
could probably cause security problem, for instance a malicious MN
store a wrong mapping in other MNs, a mapping can only initially
stored on one or more MNs (a MMN is chosen) which are under
supervision of the ISP who in fact controls this mapping. And only
the MMN is authorized to call STORE. After running for hours, MNs in
some other autonomous systems could keep cache of the mapping.
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7.5. Self-adaptive Capability
Comparing to other non-DHT mapping system, the DHT MO is more
adaptive for MN failure and dynamic MN joining.
Assume an ISP deploys multiple MSes for the address block of a
customer AS in one or multiple provider ASes it administrates. When
some of MNs go down, as long as at least one MN is healthy, mappings
service can be normally provided without manually configuration.
Even if they're all out of health temporarily, mapping information
cached on other MNs could also be available in a period of time
(cache updating period).
When a new customer site accesses to some ISP, a new mapping is
required to be added in the MO. It needs to add a new MN u into the
MO and put this mapping in MN u. At first, an existing MN w in MO
should be known and w is put into u's K-bucket table. Then do a
LOOKUP process with u's MNID as parameter. Finally information in
K-bucket table of MN u can be built up and meanwhile other MNs update
their K-bucket table as well during the LOOUP process.
7.6. Dynamic Adjustment of K value and m value
After one LOOKUP, if the time of this LOOKUP is greater than
threshold t (manually configured by ISP), which implies that this
LOOKUP spent too long time, then increase K by 1. At the same time,
if 2m < K then m = 2m, otherwise increase m by 1. Consequently, more
queries will be sent to MNs during this LOOKUP process. However if
the time of this LOOKUP is no greater than t, K value and m value
stay not changed.
When congestion occurs in some AS, K value and m value both decrease
by 1 to suppress number of updates that used to keep in touch with
other MNs.
7.7. Mapping Storing and Exchanging in Multi-homing Scenario
Suppose a scenario that a customer site accesses to more than one
ISP, which is called multi-homing. When a new MMN x puts the new
mapping in the mapping system, another MMN y with the same MNID will
be probed in the MO. Different to native Kademlia protocol, no "ID
Collision Error" occurs. Instead x tells y this new mapping and
meanwhile obtains mapping information existed already. Finally x and
y both know all mapping information about how to destine for the
customer AS. Of course x and y will probe each other to ensure
availability every period of time.
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8. Incremental Deployment
This mechanism is practical for incremental deployment, since no big
changes introduced on existing routers. Instead of deploying an
imperative third-party infrastructure over current Internet, an ISP
only puts one or more MSes in its domain and configures it to join
the MO if it wants to benefit from deploying the DHT MO.
An ISP could start from deploying an ER in its domain, through which
way the number of entries in other routers in this domain could be
reduced however the length of the intra-domain route grows. It's up
to ISPs to decide whether to tolerate such length-stretch to obtain
decrease of FIB (Forwarding Information Base) size.
As time goes by, suppose more and more ISPs have deployed ERs. Some
of them may then deploy the DHT MO to benefit from specific mappings
(that can decrease number of tunnels needed in each data
transmission) by simply putting MSes in their ASes and let them join
the MO automatically as described in Section 7.
There're no new particular devices or functions required to support
backward-compatibility.
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9. Acknowledgements
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10. Security Considerations
The ERs can apply any existing security mechanisms for BGP to enhance
the security. And for DHT MO, existing authentication methods for
DHT (especially for Kademlia) can be adapted to enhance its security.
Other new security enhancements are expected to design to support the
mechanism in this draft in future.
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11. IANA Considerations
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12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
12.2. Informative References
[I-D.farinacci-lisp]
Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol (LISP)",
draft-farinacci-lisp-12 (work in progress), March 2009.
[I-D.fuller-lisp-alt]
Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "LISP
Alternative Topology (LISP+ALT)", draft-fuller-lisp-alt-05
(work in progress), February 2009.
[I-D.meyer-lisp-cons]
Brim, S., "LISP-CONS: A Content distribution Overlay
Network Service for LISP", draft-meyer-lisp-cons-04 (work
in progress), April 2008.
[Kademlia]
Maymounkov, P. and D. Mazieres, "Kademlia: A Peer-to-peer
Information System Based on the XOR Metric", IPTPS'02,
Boston, 2002.
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Authors' Addresses
Gang Chen
CMCC, Inc.
53A, Xibianmennei Ave.,
Xuanwu District
Beijing 100053
P.R.China
Phone: +86-10-1391-071-0674
Email: phdgang@gmail.com
Hui Deng
CMCC, Inc.
53A, Xibianmennei Ave.,
Xuanwu District
Beijing 100053
P.R.China
Phone: +86-10-1391-075-0201
Email: denghui02@gmail.com
Bo Zhou
CMCC, Inc.
53A, Xibianmennei Ave.,
Xuanwu District
Beijing 100053
P.R.China
Phone: +86-10-1381-194-8723
Email: zhouboyj@chinamobile.com
Mingwei Xu
Tsinghua University
Department of Computer Science, Tsinghua University
Beijing 100084
P.R.China
Phone: +86-10-6278-5822
Email: xmw@csnet1.cs.tsinghua.edu.cn
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Dong Huo
Tsinghua University
Department of Computer Science, Tsinghua University
Beijing 100084
P.R.China
Phone: +86-10-6278-5822
Email: dhuo.thu@gmail.com
Yu Cao
Tsinghua University
Department of Computer Science, Tsinghua University
Beijing 100084
P.R.China
Phone: +86-10-6278-5822
Email: cyanalyst@126.com
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