In the Service Location Protocol (SLPv2 [RFC2608]), Directory Agents
(DAs) accept service registrations from Service Agents (SAs) and
answer queries from User Agents (UAs); they enhance the performance
and scalability of SLPv2. The use of scopes in SLPv2 further
improves its scalability. In general, a DA can serve multiple
scopes, and a scope can be served by multiple DAs. When multiple DAs
are present for a scope, how should they interact with each other?
This document describes the Mesh-enhanced Service Location Protocol
(mSLP), addressing this open issue in SLPv2.
mSLP defines a scope-based fully-meshed peering DA architecture: for
each scope, all DAs serving the scope form a fully-meshed peer
relationship (similar to IBGP [RFC1771]). Peer DAs exchange new
service registrations in shared scopes via anti-entropy [EPID-
ALGO,UPDA-PROP] and direct forwarding. mSLP improves the reliability
and consistency of SLP DA services, and simplifies SA registrations
in systems with multiple DAs.
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 BCP 14, RFC 2119
[RFC2119].
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RFC 3528 Mesh-enhanced Service Location Protocol (mSLP) April 2003
Peer DAs (or Peers)
DAs that share one or multiple scopes are peers.
Peering Connection
A persistent connection (e.g., TCP) that provides reliable and
ordered transfers between two peers. The closing of a peering
connection terminates the peer relationship.
Mesh-enhanced DA (MDA)
An MDA carries the "mesh-enhanced" attribute keyword in its DA
Advertisement (DAAdvert) message, maintains peering connections to
all peers, and properly interacts with peers.
Mesh-enhanced SA (MSA)
An MSA uses the Mesh Forwarding extension (Section 4.3) when it
registers with MDAs.
Registration Update
A registration update refers to a Service Registration (SrvReg) or
Service Deregistration (SrvDeReg) message.
Registration State
A registration state refers to an entry in the registration
database.
Accept DA
When a DA accepts a registration update from an SA, the DA is the
accept DA for the update.
Accept Timestamp
The arrival timestamp of a registration update at its accept DA is
the accept timestamp of the update. All accept timestamps
assigned by the same DA MUST be monotonically increasing.
Version Timestamp
When an MSA sends a registration update to an MDA, the MSA assigns
a version timestamp to the update. All version timestamps
assigned by the same MSA MUST be monotonically increasing.
mSLP is designed as a lightweight enhancement to SLPv2. It is
backward compatible with SLPv2. mSLP defines two enhanced entities:
MDAs and MSAs. They can be deployed incrementally. An enhanced
entity supports extended operations without affecting its original
functionality as defined in RFC 2608 [RFC2608]. For simplicity and
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compatibility, an enhanced entity works as a non-enhanced entity to
interact with non-enhanced entities. Table 1 summarizes all
interactions involving an MDA or MSA.
Interaction Equivalent To Defined In
----------------------------------------------
MDA <--> MDA mSLP
MDA <--> MSA mSLP
MDA <--> DA DA <--> DA RFC 2608
MDA <--> SA DA <--> SA RFC 2608
MDA <--> UA DA <--> UA RFC 2608
MSA <--> DA SA <--> DA RFC 2608
MSA <--> UA SA <--> UA RFC 2608
Table 1. Interactions involving an MDA or MSA
(x,y)
+--------------------------------------------------+
| +------------+ |
| | MDA4 (z) | |
| +------------+ |
| | (z) |
+------------+ (y) +------------+ (y) +------------+
| MDA1 (x,y) | ---------- | MDA3 (y,z) | ---------- | MDA2 (x,y) |
+------------+ +------------+ +------------+
Figure 1. A scope-based fully-meshed peering DA architecture
mSLP employs a scope-based fully-meshed peering DA architecture. For
each scope, all MDAs that serve the scope form a fully-meshed peer
relationship. Figure 1 shows an example for four MDAs and three
scopes (x, y, and z). Note that a single peering connection is
needed between two peers for exchanging all service registrations in
their shared scopes.
This architecture enhances SLP DA services. First, it improves the
consistency among peer DAs by automatically reconciling inconsistent
states among them. Second, it enables newly booted and rebooted MDAs
to catch up on all new registrations at once from their peers, purely
through DA interaction, without involving SAs.
This architecture also simplifies SA registrations. In SLPv2, an SA
needs to discover and register with all existing DAs in its scopes,
and re-register when new DAs are discovered or old DAs are found to
have rebooted. In mSLP, for all MDAs, an MSA only needs to discover
and register with a sufficient number of them, such that the union of
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their scopes covers its scopes; the registrations will then be
propagated automatically to other MDAs in the registration scopes.
For example, in Figure 2, MSA1 only needs to discover and register
with MDA2, or with both MDA1 and MDA3.
(option2) +------------+ (option2)
+----------------- | MSA1 (x,y) | -----------------+
| +------------+ |
| | (option1) |
V V V
+----------+ +------------+ +----------+
| MDA1 (x) | ----------- | MDA2 (x,y) | ----------- | MDA3 (y) |
+----------+ +------------+ +----------+
Figure 2. Options for registering with MDAs
Furthermore, this architecture provides scaling advantages. Consider
a scope that has N SAs and M DAs, and assume N > M >= 2. Although
mSLP and SLPv2 need the same number of SLP messages to distribute
registrations from N SAs to M DAs, mSLP can reduce the number of
agents needed for taking care of registration distribution, and
reduce the number of TCP connections needed if each SA uses TCP for
its registrations. More specifically, the agents that need to take
care of registration distribution are all N SAs in SLPv2, but only M
DAs in mSLP. Also, the number of needed TCP connections is N*M in
SLPv2 as each SA has to connect with each DA and register, but only
N+M*(M-1)/2 in mSLP as each SA only needs to connect to one
contacting DA of a full mesh of M node and register, then
registrations are propagated through the DA mesh. For N=100 and
M=10, SLPv2 needs 1000 TCP connections, but mSLP only needs 145 such
connections.
Note that as mSLP employs full-mesh topology, which is mainly for
simplicity and reliability, it cannot scale to a large number of MDAs
in a single mesh. In general, mSLP can be applied if the number of
MDAs in a mesh is on the order of tens or below. One way to avoid
having a large number of MDAs in a mesh is to split the scope into
several finer scopes. For example, if we have N MDAs for scope "x",
and N is too large, then we can split "x" into two finer scopes:
"x-1" and "x-2", with N1 MDAs for "x-1" only, N2 MDAs for "x-2" only,
N3 MDAs for both "x-1" and "x-2", and N1+N2+N3=N. Thus, instead of
having a large full mesh of size N, we now have two smaller full
meshes of size N1+N3 and N2+N3, respectively. Accordingly, a service
registration that previously targets for scope "x", now needs to be
registered under both "x-1" and "x-2".
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An MDA can learn about new peers via static configuration, DHCP
[RFC2610], and DAAdvert multicast and unicast. In any case, an MDA
MUST get a peer's DAAdvert before establishing a peer relationship to
the peer.
After getting a new peer's DAAdvert, an MDA establishes a peering
connection (if such a connection does not exist yet) to the peer, and
sends its DAAdvert via the connection (Figure 3). An MDA can
identify a peering connection initiated by a peer by receiving the
peer's DAAdvert from the connection. Normally, a single peering
connection is set up between two peers, but there is a small
possibility that a pair of peering connections might be created
between two peers if they try to initiate a connection to each other
at almost the same time. Thus, when an MDA identifies a new peering
connection initiated by a peer, it SHOULD check whether it has
initiated another peering connection to the peer. If this is the
case, and it has a lower-numbered IP address than the peer, then the
MDA SHOULD terminate the connection it has initiated.
+------+ (1) MDA2's DAAdvert | +------+
| | <----------------------+ | |
| MDA1 | (2) Create a Peering Connection | MDA2 |
| | ---------------------------------------> | |
+------+ (3) MDA1's DAAdvert +------+
Figure 3. Establishing a peering connection
After establishing a peering connection, two peers (say, MDA1 and
MDA2) exchange information about their existing peers by forwarding
peers' DAAdverts via the peering connection (Figure 4). MDA1 will
forward the DAAdvert of a peer (say, MDA3) to MDA2 if:
(1) MDA3 shares scopes with MDA2, and
(2) MDA3 is an active peer of MDA1 (i.e., there is a peering
connection between MDA3 and MDA1) or an accept DA for
registrations currently maintained by MDA1 (i.e., MDA1
has registrations originally accepted by MDA3).
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MDA2 operates similarly. Note that all DAAdverts can be sent as one
TCP stream for efficiency. Exchanging information about existing
peers enables an MDA to learn about new peers incrementally.
+------+ DAAdverts of MDA1's existing peers +------+
| | ------------------------------------------> | |
| MDA1 | (Peering Connection) | MDA2 |
| | <------------------------------------------ | |
+------+ DAAdverts of MDA2's existing peers +------+
Figure 4. Exchanging information about existing peers
An MDA SHOULD tear down a peer relationship when it finds that the
peer has closed the peering connection, when it receives a DAAdvert
multicast from the peer with a DA stateless boot timestamp set to 0
(meaning that the peer is going to shutdown), or when it has not
received the peer's DAAdvert for more than CONFIG_DA_TIMEOUT seconds.
When an MDA accepts a registration update from an MSA, the MDA
assigns a unique accept ID to the update. An accept ID has two
components: an accept DA URL and an accept timestamp. The accept
timestamp is a 64-bit integer representing elapsed microseconds since
00:00 Coordinated Universal Time (UTC), January 1, 1900. Figure 6
shows the format for an accept ID entry. A registration state has
the same accept ID as that of the latest update applied to it.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Accept Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Accept Timestamp, cont'd. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Accept DA URL | Accept DA URL \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6. Accept ID entry
An MDA MUST propagate registrations in the increasing order of their
accept IDs, i.e., registrations having the same accept DA MUST be
propagated in the increasing order of their accept timestamps. Note
that registrations having different accept DAs MAY be propagated in
any order.
When registrations are propagated among MDAs, their arrival
timestamps at MDAs cannot be used for version resolution. For
example, assume that MSA1 sends a registration (R1) to MDA1 first,
and a new version of the same registration (R2) to MDA2 later. When
R1 and R2 are propagated, the arrival timestamp of R1 at MDA2 is
later than that of R2, but R1 SHOULD NOT overwrite R2 at MDA2 as R2
is a newer version.
mSLP resolves registration versions using version timestamps. When
an MSA sends a registration update to an MDA, the MSA assigns a
version timestamp to the update. The version timestamp is a 64-bit
integer representing elapsed microseconds since 00:00 UTC, January 1,
1900. mSLP assumes that each registration is updated only by one SA,
thus an MDA does not need to compare version timestamps from
different MSAs. An MDA installs a registration update if the update
has a newer version timestamp (from an MSA), or the update does not
have the Mesh Forwarding extension (from a non-MSA).
The Mesh Forwarding (MeshFwd) extension carries a version timestamp
and an accept ID entry. Figure 7 shows its format and two defined
Forwarding IDs (Fwd-IDs).
The MeshFwd extension is used with a Srv(De)Reg message, but it can
only be used with a fresh SrvReg, or a complete SrvDeReg.
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RFC 3528 Mesh-enhanced Service Location Protocol (mSLP) April 2003
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MeshFwd Extension ID = 0x0006 | Next Extension Offset (NEO) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NEO, cont'd. | Fwd-ID | Version Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version Timestamp, cont'd. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version Timestamp, cont'd. | Accept ID Entry \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fwd-ID Abbreviation
1 RqstFwd
2 Fwded
Figure 7. MeshFwd extension and its Fwd-IDs
An MSA uses the RqstFwd MeshFwd extension (Fwd-ID = RqstFwd, accept
timestamp = 0) in a Srv(De)Reg to explicitly request an MDA (the
accept DA) to forward the message.
An MDA uses the Fwded MeshFwd extension (Fwd-ID = Fwded, accept
timestamp != 0) in each Srv(De)Reg sent from it to another MDA,
either forwarding a Srv(De)Reg received from an MSA (if the message
has the RqstFwd MeshFwd extension), or propagating a registration
state in its database.
An MDA uses a summary vector to represent its received Srv(De)Reg(s)
that have a MeshFwd extension. This summary vector records the
latest accept timestamp for each accept DA that appears in the
MeshFwd extension. For example, consider n MDAs for a scope, if MDAi
has a summary vector as ((MDA1, T1), (MDA2, T2), ..., (MDAn, Tn)),
then MDAi has received all registrations originally accepted by MDAj
up to timestamp Tj, where 1<=i,j<=n.
An MDA updates its summary vector when it receives a Srv(De)Reg that
has a MeshFwd extension. The MDA adds a new accept ID to its summary
vector if the Srv(De)Reg has a new accept DA; the MDA updates the
accept timestamp of an existing accept ID in its summary vector if
the Srv(De)Reg has an existing accept DA.
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RFC 3528 Mesh-enhanced Service Location Protocol (mSLP) April 2003
When an MDA receives a SrvDeReg that has a MeshFwd extension, it
SHOULD retain the corresponding registration in the database, and
mark it as deleted. This way, the registration will not appear in
any query reply, and an earlier SrvReg will not mistakenly cause the
registration to reappear in the database. A registration state will
be purged from the database when it expires.
The Anti-entropy Request (AntiEtrpRqst) message carries an anti-
entropy type ID and a list of accept ID entries. Figure 8 shows its
format and two defined anti-entropy type IDs.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location Header (AntiEtrpRqst Function-ID = 12) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Anti-Entropy Type ID | Number of Accept ID Entries |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Accept ID Entry 1 . . . Accept ID Entry k \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Anti-Entropy Type Type ID
selective 1
complete 2
Figure 8. AntiEtrpRqst message and anti-entropy types
The AntiEtrpRqst message is used by an MDA to request new
registration states from a peer. The anti-entropy type is either
selective or complete. If the anti-entropy type is selective, only
registration states that have an accept ID greater than any specified
accept ID in the message are requested. If the anti-entropy type is
complete, all registration states that have an accept ID greater than
any specified accept ID in the message or have an accept DA not
specified in the message are requested.
For example, consider three MDAs (MDA1, MDA2, and MDA3) for a scope.
MDA2 has registration states originally accepted by MDA1, MDA2, and
MDA3. If MDA1 sends a selective AntiEtrpRqst to MDA2 using an accept
ID list as ((MDA2, T2)), then MDA1 only requests registration states
that are originally accepted by MDA2, and have an accept timestamp
greater than T2. If MDA1 sends a complete AntiEtrpRqst to MDA2 using
an accept ID list as ((MDA2, T2)), then MDA1 requests all
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RFC 3528 Mesh-enhanced Service Location Protocol (mSLP) April 2003
registration states originally accepted by MDA1 and MDA3, plus those
originally accepted by MDA2 and having an accept timestamp greater
than T2.
Anti-entropy is used for exchanging initial registration states when
two peers recognize each other for the first time, and for updating
new registration states after failures.
When an MDA receives an AntiEtrpRqst from a peer, it sends the
requested new registration states in the increasing order of their
accept IDs. At last a Service Acknowledgment (SrvAck) message is
sent to indicate that the processing of a corresponding AntiEtrpRqst
has been completed (Figure 9). A new registration state is sent as a
fresh SrvReg with its remaining lifetime. A newly deregistered state
is propagated as a SrvDeReg. Note that multiple Srv(De)Reg(s) can be
sent as one TCP stream for efficiency.
+------+ AntiEtrpRqst +------+
| | -------------------------------------------> | |
| MDA1 | (Peering Connection) | MDA2 |
| | <------------------------------------------- | |
+------+ New States via Srv(De)Reg(s) + SrvAck +------+
Figure 9. Anti-entropy via AntiEtrpRqst, Srv(De)Reg(s) and SrvAck
+------+ RqstFwd Srv(De)Reg +------+ Fwded Srv(De)Reg +------+
| | ---------------------> | | --------------------> | |
| MSA1 | | MDA1 | | MDA2 |
| | <--------------------- | | | |
+------+ SrvAck +------+ +------+
Figure 10. Direct forwarding of a Srv(De)Reg
After sending all new registration states accepted by itself to a
peer (via anti-entropy), an MDA directly forwards newly received
registration updates from MSAs to the peer until a failure occurs.
In Figure 10, when a Srv(De)Reg is directly forwarded from MDA1 to
MDA2, its Fwd-ID is set to Fwded, and its accept timestamp is set to
its arrival timestamp at MDA1. Note that a direct forwarding is
performed asynchronously: MDA1 can send a SrvAck to MSA1 before it
forwards the Srv(De)Reg to MDA2. Also note that the direct
forwarding of a Srv(De)Reg goes only one-hop from its accept DA (say,
MDA1) to all MDA1's peers that are in the registration scopes.
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RFC 3528 Mesh-enhanced Service Location Protocol (mSLP) April 2003
According to [RFC2608], a DA MUST reply with a SrvAck to a Srv(De)Reg
when the message is received from an SA. However, an MDA SHOULD NOT
reply with a SrvAck to a Srv(De)Reg if the message is received from a
peer. This is for efficiency because peers exchange Srv(De)Reg
messages via reliable peering connections. Note that an MDA MUST
reply with a SrvAck to an AntiEtrpRqst.
For each registration entry, an MDA maintains the following control
information: an accept ID (for registration propagation), a version
timestamp (for registration version resolution - rejecting previous
updates), and a deletion flag (deregistered or not).
For all registration entries, an MDA maintains a summary vector to
reflect its received registrations so far.
mSLP extends SLPv2 with three new definitions: a new attribute -
"mesh-enhanced" for DAAdvert, a new message extension - MeshFwd, and
a new message type - AntiEtrpRqst.
A UA MAY prefer an MDA to a non-MDA since an MDA is more likely to
reliably contain the complete set of current service registrations
for the UA's scopes.
A non-MSA needs to discover and register with all DAs in its scopes.
It does not use the MeshFwd extension.
A non-MDA accepts Srv(De)Reg(s) from SAs. It does not forward them.
For all MDAs, an MSA only needs to discover and register with
sufficient number of them, such that they cover its scopes. It uses
the MeshFwd extension when it registers with MDAs.
An MDA carries the "mesh-enhanced" attribute keyword in its DAAdvert.
It maintains a peer relationship to each peer. It accepts
registrations from SAs and peers, propagates registrations via anti-
entropy and direct forwarding to peers.
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RFC 3528 Mesh-enhanced Service Location Protocol (mSLP) April 2003
Interval Name Default Value Defined in
------------------- ------------- -----------
CONFIG_DA_KEEPALIVE 200 seconds Section 3.4
CONFIG_DA_TIMEOUT 300 seconds Section 3.4
The Mesh Forwarding (MeshFwd) extension ID, 0x0006, described in
Section 4.3, has been assigned by IANA out of the SLP extension space
(RFC 2608, Section 9.1) reserved for "optional to implement"
extensions (i.e., the 0x0000-0x3FFF range).
The function-ID of the Anti-entropy Request (AntiEtrpRqst) message
type, 12, described in Section 4.6, has been assigned by IANA (RFC
2608, Section 15).
mSLP uses standard SLPv2 authentication. First, an MDA SHOULD
authenticate other MDAs before setting up a peer relationship with
them so as to prevent any malicious MDA from joining the DA mesh.
Second, as a successful attack at an MDA may affect all MDAs in the
DA mesh, an MDA SHOULD authenticate MSAs before accepting and
forwarding their Srv(De)Reg messages to prevent illegitimate
modification or elimination of service registrations. Third, as an
MSA depends on the MDA with which it registers to forward its
Srv(De)Reg messages, it SHOULD authenticate the MDA to avoid using a
malicious MDA.
[RFC2608] Guttman, E., Perkins, C., Veizades, J. and M. Day,
"Service Location Protocol, Version 2", RFC 2608, June
1999.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Zhao, et al. Experimental [Page 13]
RFC 3528 Mesh-enhanced Service Location Protocol (mSLP) April 2003
[RFC1771] Rekhter, R. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[RFC2610] Perkins, C. and E. Guttman, "DHCP Options for Service
Location Protocol", RFC 2610, June, 1999.
[EPID-ALGO] A. Demers, D. Greene, C. Hauser, W. Irish, J. Larson, S.
Shenker, H. Sturgis, D. Swinehart and D. Terry, "Epidemic
algorithms for replicated database maintenance", the
sixth ACM symposium on principles of distributed
computing, Vancouver, Canada, 1987.
[UPDA-PROP] K. Petersen, M. Spreizer, D. Terry, M. Theimer and A.
Demers, "Flexible update propagation for weakly
consistent replication", the sixteenth ACM symposium on
operating systems principles, Saint Malo, France, 1997.
Weibin Zhao
Department of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027-7003
EMail: zwb@cs.columbia.edu
Henning Schulzrinne
Department of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027-7003
EMail: hgs@cs.columbia.edu
Erik Guttman
Sun Microsystems
Eichhoelzelstr. 7
74915 Waibstadt
Germany
EMail: Erik.Guttman@sun.com
Zhao, et al. Experimental [Page 14]
RFC 3528 Mesh-enhanced Service Location Protocol (mSLP) April 2003
Copyright (C) The Internet Society (2003). All Rights Reserved.
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Acknowledgement
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Zhao, et al. Experimental [Page 15]