PIM Working Group Ying-Dar. Lin
Internet-Draft Nai-Bin. Hsu
Expires: April 22, 2002 Dept of Comp. and Info. Sci.,
National Chiao Tung Uni.
Ren-Hung. Hwang
Dept of Comp Sci and Info Eng,
National Chung Cheng Uni.
October 22, 2001
RP Relocation Extension to PIM-SM Multicast Routing
draft-ydlin-pim-sm-rp-01
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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 April 22, 2002.
Abstract
The Protocol Independent Multicast-Sparse Mode (PIM-SM) protocol
establishes core-base tree to forward multicast datagrams in a
network. In PIM-SM, the core or Rendezvous Point (RP) of a group is
determined at each multicast router by hashing a group address, i.e.,
a class-D IP address, to one of the candidate RPs. The hash function
is characterized by its ability to evenly and uniquely choose the
core for a group and remains insensitive to the geographic
distribution of the group members and the sources. However, it may
result in a multicast tree with high cost.
This draft presents a relocation mechanism which is extension to PIM-
Lin, et. al. Expires April 22, 2002 [Page 1]
Internet-Draft RP Relocation Extension October 2001
SM, in which RP could be relocated periodically. When a new RP is
found, the original RP informs all members to re-join to the new RP.
1. Introduction
Multicast routing protocols can be categorized as source-based tree
and shared-tree protocols. A source-based tree protocol builds
separate trees for each (source, group) pair, that is, each source
has its own tree that reaches the active group members, such as DVMRP
[1], PIM-DM [2], and MOSPF [3]. On the other hand, shared-based
protocols such as PIM-SM [4] and CBT [5] build distributed trees
having a central point (or core) to whom all receivers attached.
Typically, a source sends datagrams to the RP and RP forwards them to
all members through the RP-based multicast tree. Therefore, a
shared-tree router only needs to maintain state information for each
group instead of for each (source, group) pair.
In PIM-SM, new members wanting to join a group send Join messages to
a core, called Rendezvous Point (RP), of the distribution tree. The
RP administers the specific multicast group(s), and facilitates the
joining/leaving of the group members. The address of the RP is
determined at each multicast router by mapping a group to one of the
candidate RPs with a hash function. However, the chosen RP may be
inappropriate for its group, for example, causes more tree cost. The
hash function is characterized by its ability to evenly and uniquely
choose a candidate RP to be a core in order to balance the service
load of each candidate RP in the network.
This draft proposes an RP relocation mechanism which is extension to
the PIM-SM multicast routing protocol. The hash function of PIM-SM
is initially used to obtain an RP for the multicast group. As the
sources come and go, RP relocates its location periodically
afterward. An attempt is made to obtain an appropriate core location
by using an estimated tree cost function to evaluate the
appropriateness of the candidates. When RP relocation is determined,
the original RP multicasts NEW_RP message to all members to inform
them to re-join to the new RP. Consequently, a new distribution tree
is built with the least cost.
2. Issues and Motivations
PIM-SM is a commonly used multicast routing protocol that provides
efficient communication for multicast groups with sparsely
distributed members. The designers observed that several hosts
wanting to participate in a multicast conference do not justify
having their group's multicast traffic periodically broadcast across
the entire network. To eliminate the scaling problem, PIM-SM is
designed to limit multicast traffic so that only routers interested
Lin, et. al. Expires April 22, 2002 [Page 2]
Internet-Draft RP Relocation Extension October 2001
in receiving traffic for a particular group will receive it. By
unicast routing, the source router knows how to reach and forward the
traffic to the RP. Then, RP distributes the traffic to all the
members through the RP-based multicast tree.
In order to broadcast the set of candidate RPs to the network nodes,
the bootstrap router (BSR) is elected for the domain. BSR originates
Bootstrap messages to distribute the set of RPs information, which
are distributed hop-by-hop throughout the domain. There is only one
RP-set per PIM-SM domain. By using a hash function, say Hash(), each
router can uniquely map a group address G to one of the routers in
the RP-set. That is, candidate C_k is chosen as RP if C_k yields
highest hash value Hash(G, M, C_i), for all C_i belong to the RPset.
The hash mark, M, allows a number of consecutive groups to resolve to
the same RP.
The shared tree, although provides better scalability, does not
optimize the delivery path through the network. RP for the group is
typically designed without respect to its location. Thus, an RP
could be located for away from all group members, resulting in
inefficient transmission.
Steiner tree [6] is a well-known example of finding a minimum
multicast tree. However, the multicast tree T(S, R) in PIM-SM, the
set of sources, S, and the set of members, R, are dynamic changed.
That is, after sources and/or receivers come and go, the least cost
multicast tree is different. Therefore, considering a single group
in which no member switches over to the shortest path tree (SPT),
there is a sequence of least cost multicast trees, {T0, T1, T2, ...
}, where Ti=Tree(S ->RPi) U Tree(RPi -> R). Since the tree Ti is
rooted at the RPi the problem of finding the sequence {Ti} is reduced
to the problem of finding the sequence {RPi}, where the RP0 is the
initial hash RP.
Therefore, in this study, assume RP_i is the current RP of G, we
propose an RP relocation mechanism which relocates RP when the tree
cost reduction, TC(RP_i) - TC(RP_{i+1}), if it is larger than a pre-
defined threshold, q. In addition, three control messages are needed
to migrate to the new RP and reliably maintain the membership of the
group. For convenience of our description, Table 1 summarizes the
symbols used in this draft.
Lin, et. al. Expires April 22, 2002 [Page 3]
Internet-Draft RP Relocation Extension October 2001
TABLE 1. Symbol Definitions
---------------------------------------------------------------------
R: set of receiving routers currentRP: current RP of G
S: set of source routers newRP: RP be migrated
M: hash mask hashRP: initial hashed RP of G
m: new member oldRP: previous RP of G
tr: RP relocation interval candRP: candidate RP
G: multicast group q: cost reduction threshold
G.RP: RP addr. of group G TC: cost of multicast tree
G.rFlag: relocation flag of G C_i: ith candidate RP, in RPset
RPset: set of candidate RPs C_k: min cost candidate RP, in RPset
---------------------------------------------------------------------
3. RP Relocation Algorithm
Of priority concern in a distribution tree, the lower the tree costs
of hop count and delay implies better routing paths. Tree cost is
defined as the sum of the costs of all links of the multicast tree.
Our mechanism attempts to reduce the tree cost by relocating the RP
for a multicast group. To map an appropriate RP in a group, %this
study uses an estimated function can be used. For example, computing
by equations in [7] we can obtain a RP with the nearly minimum tree
cost from the RPset of a group. Equation (3) calculates the
distribution tree cost if the candidate C_i is taken as the RP,
TC(C_i), by taking the average of Eq. (1) and (2), i.e., the maximum
and minimum bounds on tree cost. These equations use the distance
(i.e. cost:E -> R+) for each possible destination as the metric.
Notably, the distance information is already available to routers.
TCmin(C_i)= min. tree cost (1)
TCmax(C_i)= max. tree cost (2)
TC(C_i) =[ TCmin(C_i)+ TCmax(C_i) ]/2 (3)
The best-case tree is linear if a lower bound on the cost, TCmin, of
a tree rooted at some node is obtained. The cost of the tree is
simply the maximum distance from RP to any sender. When using hop
count (i.e. cost:E -> 1) as the metric, the function can obtain a
tighter bound by adding the number of senders that are at an equal
distance. This bound is owing to that the distribution tree cannot
be completely linear, but must have at least an additional link.
Lin, et. al. Expires April 22, 2002 [Page 4]
Internet-Draft RP Relocation Extension October 2001
--------------------------------------------
Algorithm RPIM-SM ( G )
set_of_sources S;
set_of_members R;
member_of_G m;
relocation_flag rFlag;
relocation_timer tr;
tree_cost_function TC;
Begin
while ( 1 ) do
case hashRP:
if receive 'Join' from a new member m
// hashRP found by Hash function
if (G.rFlag==true) then
unicasts 'NEW_RP(currentRP)' to m
// m send 'Join' to currentRP
// m send 'Prune' to hashRP
endif
endif
case currentRP:
if ( tr expired and S changed ) then
newRP = Select C_k from RPset,
where TC(C_k) is minimum
if reduction of TC(C_k) > q then
multicasts 'NEW_RP(currentRP, newRP)'
to {hashRP, sources, members} of G
endif
endif
endwhile
End
--------------------------------------------
Figure 1: The RP Relocation for the Group G.
Opposite to the lower bound of the cost, the upper bound on the cost,
TCmax, of a tree rooted at some node is that no links are shared
among the paths to each sender. Therefore, the maximum tree cost is
the sum of the sender distances. Also, if the number of group
senders is greater than the root degree, the bound is tightened by
subtracting the difference to calculate the cost for the sharing
links. The estimation function is thus defined as the average of the
sum of the minimum cost and maximum cost.
As shown in the algorithm of Fig.1, a new member m that joins a group
sends a Join message to the hashed RP. The hashRP verifies whether
the RP of this group has been migrated (i.e. the rFlag is true). If
so, hashRP redirects m to the new RP by sending the NEW_RP message.
Thus, m sends a Join message and established (*, G) state along the
path towards the new RP. In addition, m sends a Prune message to the
Lin, et. al. Expires April 22, 2002 [Page 5]
Internet-Draft RP Relocation Extension October 2001
hashRP to discard the previous path. The currentRP also confirms
whether if the relocation timer tr has expired and the set of sources
of the group has been changed. If it has, a new RP with significant
cost reduction is computed by the above cost functions TC for each
candidate RP.
--------------------------------------------
Algorithm ChangeRP(currentRP, newRP, G)
set_of_members R;
member_of_G m;
relocation_flag rFlag;
Begin
if (newRP == hashRP) then
G.rFlag = false /* return back to initial RP */
else
G.rFlag = true
endif
G.oldRP=G.RP
G.RP=newRP
case newRP:
send I_AM_RP message to hashRP
case source:
re-register to the newRP
case member:
send 'Join' to newRP
send 'Prune' to currentRP
end
-----------------------------------------------------------
Figure 2: The RP Migration of Group G.
TABLE 2. Relocation State Table
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| G | rFlag | G.RP |iif| nbrRPF | G.oldRP |iif | nbrRPF |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| G1| true | RP1 |i_1| v_1 | RP1' |i_1'| v_1' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| G2| true | RP2 |i_2| v_2 | RP2' |i_2'| v_2' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | ..... | | | | --- | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gn| false | RPn |i_n| v_n | --- | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Once the RP with least cost for a group is found, the function
ChangeRP is invoked, as shown in Fig.2. The currentRP first checks
whether the new RP is the hashRP. If it is, the rFlag is reset to
Lin, et. al. Expires April 22, 2002 [Page 6]
Internet-Draft RP Relocation Extension October 2001
false since the currentRP returns back to the original hashed RP. To
migrate the distribution tree, message NEW_RP is advertised by the
currentRP to inform the newRP, hashRP, sources, and all members of
the group.
For keeping this relocation state, each node with (*, G) state
maintains a relocation table to record the state of RP relocation, as
well as the addresses of the RP and old RP, as shown in Table 2, G.RP
and G.oldRP, respectively. The rFlag indicates whether if RP of this
group has been relocated, i.e., not the original hashed RP. When a
new RP is obtained, the current RP becomes the oldRP and its address
is moved to the G.oldRP; the new RP address is then saved at the
G.RP. Notably, the corresponding incoming interface (iif) and the
Reverse Path Forwarding (RPF) neighbor (nbrRPF) of the
G.newRP/G.oldRP are also saved. State information of the relocating
process prevents the chaos caused by RPF checking during the
transition between the two consecutive multicast trees.
For example, a node with (*, G1) state continues to receive and
distribute packets from the old RP (RP1') at the interface i_1',
until this node receives packets from the new RP (RP1) at the
interface i_1. Furthermore, in order to migrate the distribution
tree of the group G1, the current RP (RP1') of G1 multicasts the
encoded NEW_RP message towards the newRP, all sources (First-hop
routers) and members (Last-hop routers) of group G1. This message
redirects all sources to re-register, all members to re-join to the
new RP. Note that in case of Gn, since the rFlag is false, this
entry records hashRP of Gn at the G.RP field.
Figure 3 shows the RP state transition of a specific group G. At the
system start up, the RP is in the $Hashing$ state and enters the
$Selection$ state as timer tr expires. If the selected candidate
(C_k) can significantly reduce the current tree cost, migration of RP
tree is invoked and the $Transition$ state is entered, and thereafter
transits further to the $Relocation$ state or $Hashing$ state,
depending on whether the newly selected RP is the original hashed RP.
Returning back to the $Hashing$ state means that the original hashed
RP turns out to be the best RP for G again. On the other hand, if
the reduction in tree cost does not exceed the threshold, q, the RP
enters the $Relocation$ state or $Hashing$ state, depending on the
value of rFlag where true means this RP is a relocated, instead of
hashed, one. Note that the RPIM-SM verifies the source list, S, and
the tree cost of G every tr time unless the system re-startup.
Lin, et. al. Expires April 22, 2002 [Page 7]
Internet-Draft RP Relocation Extension October 2001
start up +-------+ (tr expired) & (S changed)
+-----------> +Hashing+------------------+
| [rFlag=0] +---+---+ (reduction<q) |
| ^ ^----------------+ |
| | &(rFlag=false)| |
| (tr expired)| | V
+----+-----+ -------------------------> +---+-+-----+
|Relocation|& (S changed) | Selection |
+----+-----+ | (reduction<q) +-----+-----+
^ ^----------------------------+ |
| | &(rFlag=true) |
| | |
| |Ck==hashRP |
| |[rFlag=false, |
| |currentRP=Ck] |
| +-----+----+ |
+---------- +Transition+ <---------------+
Ck!=hashRP +----------+ reduction>q
[rFlag=true, [changeRP]
currentRP=Ck]
Figure 3. State transition diagram of the RP Relocation.
4. Control Messages and Timers
4.1 Receiving Join Messages
This message is sent toward the RP or a source periodically, to
create or refresh a branch of a multicast distribution tree.
4.1.1 At the hashRP
if (G.rFlag == true) then
send NEW_RP(G.RP) to receiving router
else
handles it as in PIM-SM.
endif
4.1.2 At the currentRP
handles this message as in PIM-SM.
Lin, et. al. Expires April 22, 2002 [Page 8]
Internet-Draft RP Relocation Extension October 2001
4.2 Receiving Prune Messages
This message is sent toward a source by a router when it wish to
leave the multicast group. This message is also sent toward the RP
when a router switches from the RP tree to the source-rooted shortest
path tree. RPIM-SM handles this message as in PIM-SM.
4.3 Receiving NEW_RP Messages
This message is used to advertise to the group that who is the new RP
from now on. When the computation of tree cost in Fig. 1 is done
and the decision of the RP relocation is positive, the current RP
sends the message NEW_RP (G, newRP) to the newRP, hashRP, sources,
and multicasts it to all members of the group. Then, the migration
process is triggered, i.e., the sources re-register to the newRP and
members re-join to the newRP.
4.3.1 At the candRP
send I_AM_RP message to hashRP
G.RP=newRP
4.3.2 At the source router
send register message to newRP
G.oldRP=G.RP
G.RP=newRP
continue to send packet to G.oldRP for Td time period
4.3.3 At the receiving router
send Join message to newRP
G.oldRP=G.RP
G.RP=newRP
upon receiving packet from G.RP
send Prune message to G.oldRP
Lin, et. al. Expires April 22, 2002 [Page 9]
Internet-Draft RP Relocation Extension October 2001
4.3.4 At the hashRP
if (newRP==hashRP) then
G.rFlag=false;
else
G.RP=hashRP;
endif
4.3.5 Message Format
The format of this message is shown as following:
* PIM Ver: 2
* Type: 9
* Reserved=0, ignored on receipt.
* Checksum: standard IP checksum.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Unicast-Upstream Neighbor Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Multicast Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-newRP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.4 Receiving I_AM_RP Messages
Using this message, newRP informs hashRP that íşI am the RP of the
group at this time.íż Without this information at the hashRP, the
incoming receivers could not find the current RP to whom the Join
message send.
4.4.1 At the hashRP
G.RP=newRP
send RP_CONFIRM message to G.RP
restart Reset Timer Th
4.5 Receiving RP_CONFIRM Messages
Upon receiving I_AM_RP message, hashRP updates the corresponding
relocation state of G, and acknowledge the newRP with the RP_CONFIRM
Lin, et. al. Expires April 22, 2002 [Page 10]
Internet-Draft RP Relocation Extension October 2001
message.
4.5.1 At the currentRP
restart Expiry Timer Te
4.5.2 Message Format
The format of I_AM_RP and RP_CONFIRM messages is shown as following:
* PIM Ver: 2
* Type: 10 (I_AM_RP)/11 (RP_CONFIRM)
* Reserved=0, ignored on receipt.
* Checksum: standard IP checksum.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Unicast-Upstream Neighbor Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Multicast Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-RP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.6 Relocation Timer Tr Expires at the currentRP
if (S changed) then
newRP = Select C_k from RPset,
where TreeCost(C_k) is minimum
if reduction of TreeCost(C_k) > q then
send NEW_RP to { newRP, hashRP, S, R }
restart Relocation Timer Tr
if(currentRP is hashRP)
G.rFlag=true;
endif
endif
4.7 Expiry Timer Te Expires at the currentRP
send I_AM_RP message to hashRP
Lin, et. al. Expires April 22, 2002 [Page 11]
Internet-Draft RP Relocation Extension October 2001
4.8 Reset Timer Th Expires at the hashRP
send NEW_RP message to { S, R }
restart Relocation Timer Tr
G.rFlag=false;
4.9 Duplicate Timer Td Expires at the source router
stop sending packet to G.oldRP
5. Summary
This draft proposes a dynamic mechanism, RPIM-SM, which is an
extension to the PIM-SM multicast routing protocol. The original
hashed Rendezvous Point (RP) location may be inappropriate for the
group. Therefore, according to an estimated tree cost, the PIM-SM is
extended by relocating the RP periodically. In addition, the
candidate RP with minimum tree cost is selected to be the new RP of
the group. To migrate to the new RP, a additional message, NEW_RP,
is needed to notify sources to re-register, and members to re-join
the new RP.
6. Security Considerations
In addition to PIM-SM, the control messages, NEW_RP, I_AM_RP and
RP_CONFIRM may use IPsec to address security concerns.
Lin, et. al. Expires April 22, 2002 [Page 12]
Internet-Draft RP Relocation Extension October 2001
7. References
[1] D. Waitzman, C. Partridge, S.E. Deering, "Distance Vector Multicast
Routing Protocol", RFC1075, November 1988.
[2] S. Deering, D. Estrin, D. Farinacci, V. Jacobson, A. Helmy, L. Wei,
"Protocol Indepenent Multicast Version 2, Dense Mode Specification",
Internet Draft, Work in progress, draft-ietf-idmr-pim-dm-05.txt,
May 21, 1995.
[3] J. Moy, "Multicast Extensions to OSPF", RFC1584, March 1994.
[4] D. Estrin, D. Farinacci, A. Helmy, D. Thaler, S. Deering, M.
Handley, V. Jacobson, C. Liu, P. Sharma, L. Wei, "Protocol Independent
Multicast-Sparse Mode (PIM-SM): Protocol Specification", RFC2362, June
1998.
[5] A. Ballardie, "Core Based Trees (CBT) Multicast Routing
Architecture", RFC2201, September. 1997.
[6] S.L.Hakimi, "Steiner Problem in Graphs and Its Implications," Networks
VOL. 1 (1971) pp. 113-133.
[7] David G. Thaler and Chinya V. Ravishankar, "Distributed Center-
Location Algorithms", IEEE Journal on Selected Areas in Communications,
VOL. 15, No. 3, April 1997.
Authors' Addresses
Ying-Dar Lin
Dept of Comp. and Info. Sci., National Chiao Tung Uni.
No. 1001, Ta-Hsueh Road
Hsinchu, Taiwan 30050
ROC
Phone: +886 3 5731899
EMail: ydlin@cis.nctu.edu.tw
URI: http://speed.cis.nctu.edu.tw/~ydlin/
Lin, et. al. Expires April 22, 2002 [Page 13]
Internet-Draft RP Relocation Extension October 2001
Nai-Bin Hsu
Dept of Comp. and Info. Sci., National Chiao Tung Uni.
No. 43, Lane 486, Ming-Hu Road
Hsinchu, Taiwan 30050
ROC
Phone: +886 3 4245612
EMail: naibin@ms7.hinet.net
URI: http://speed.cis.nctu.edu.tw/
Ren-Hung Hwang
Dept of Comp Sci and Info Eng, National Chung Cheng Uni.
No. 160, SAN-HSING, MING-HSIUNG
Chiayi county, Taiwan 621
ROC
Phone: +886 5 2720411
EMail: rhhwang@cs.ccu.edu.tw
URI: http://exodus.cs.ccu.edu.tw/~rhhwang/
Lin, et. al. Expires April 22, 2002 [Page 14]