CS475 – Networks
Lecture 19
Chapter 5: End-to-End Protocols
Assignments
• Homework 6 posted, due
5.3 Remote Procedure Call (RPC)
TCP and UDP provide byte stream channels. Another comm.
pattern is the request/reply or _______________ transaction
paradigm.
The Remote Procedure Call (RPC) transport protocol more
closely matches the needs of an application wanting to use
request/reply than either UDP or TCP.
5.3.1 RPC Fundamentals
RPC is commonly used in building distributed systems. RPC
RPC therefore involves two components:
allows a procedure to be either local or remote. An application
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A protocol to manage message passing between the
client (caller) and server.
calls the procedure as if it were local. The calling application
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___________________ until the procedure returns.
Programming language and compiler (a ____________
compiler) support to translate a procedure call on the
Remote procedure calls are more complex than local procedure
client into a request message and translate the message
calls:
into arguments on the server. The return value is
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handled similarly.
Messages may be limited in size. They may also be lost
or reordered.
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Different computers may have different OSes,
architectures, and data representations.
The term RPC refers to a ______________ of protocol rather
than a specific standard like TCP. There is no one dominant
RPC protocol. We will look at several protocol design choices.
We will look at only the protocol related aspects of RPC.
Translation of arguments into messages and vice versa is
covered in Chapter 7.
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5.3.1 RPC Fundamentals - Identifiers in RPC
RPC must provide a name space for identifying the procedure to An RPC protocol must also match a reply message to the
be called.
corresponding request. This is done by including a message ID
The name space can be either flat or ______________________. in both the request and reply.
A flat name space requires central coordination to prevent
A client ____________________ may be used as part of the
assigning the same ID to different procedures. The ID can be
message ID in order to ensure that the correct match in the event
carried in a single field in an RPC request.
that the client reboots with an outstanding request.
5.3.1 RPC Fundamentals - Overcoming Network Limitations
RPC can implement reliability using
RPC reliability may implement at-most-once semantics in which
ACKs. Each side has a retransmit
there is a guarantee that no more than one request is delivered to
timer that causes the message to be
the server.
resent in the event of a time out. A
Implementation of zero-or-more or __________________
reply can be used as an implicit ACK.
semantics is simpler and sufficient for applications in which
_________________ requests can be
multiple requests have the same effect as one request.
implemented using logical channels.
5.3.2 RPC Implementations - SunRPC
SunRPC (aka ONC RPC) was developed by Sun as part of their 8).
Network File System (____________). SunRPC can be
Different RPC servers are dynamically assigned TCP/UDP port
implemented over several different transport protocols (SunRPC numbers. The port mapper RPC server listens on well-known
is also considered a transport protocol).
port 111. RPC clients can query the _____________________
SunRPC uses a 32-bit program number and a 32-bit procedure
to determine the port number assigned to a program.
number to identify a procedure (the NFS server has program ID SunRPC does not implement its own reliability or fragmentation
0x100003, the NFS read procedure has ID 6 while write has ID
methods. It relies on the underlying protocol.
The XID field is a transaction ID that is unique to a request/reply
pair. The Program and Procedure contain the corresponding
32-bit IDs. The Version field specifies a version of a program.
Multiple versions of a program may be running on the server.
The variable length Credentials and Verifier fields are used by
the client to _____________________ itself to the server.
SunRPC request and reply headers
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5.3.2 RPC Implementations - DCE-RPC
DCE-RPC is used in Microsoft's
DCE-RPC supports very large messages and implements its own
DCOM and ActiveX technologies. It
fragmentation scheme. Selective acknowledgment is used
is also used in ________________, a
allowing only missing fragments to be retransmitted instead of
standard for distributed object-
the entire message.
oriented systems.
A typical DCE-RPC exchange is
shown at right. If the server responds
quickly enough, no Pings are sent.
DCE-RPC supports multiple logical channels known as
_______________________ and there is an ActivityID field in
the header. A SequenceNum field distinguishes between calls in
an activity. The sequence number is remembered at the server to
Selective acknowledgment is shown above.
ensure at-most-once semantics.
A WindowSize is used for flow-control.
5.4 Transport for Real Time Apps (RTP)
Multimedia applications can be categorized as either streaming
RTP can run over many lower level protocols,
(audio, video streams) or interactive (VoIP, teleconferencing).
but is typically run on top of UDP. RTP is still
Interactive applications have the strictest real-time requirements. considered a _______________ protocol.
5.4.1 Requirements
A multimedia protocol must allow applications to interoperate.
UDP does not provide congestion control and this is desirable in
One approach is to specify a particular audio and video coding
many real-time apps. The receiver must notify the sender that
scheme. RTP allows the sender to indicate which coding method losses are occurring.
it wants to use.
A real-time protocol should provide some indication of
Our protocol must support playback synchronization to prevent
_______________ boundaries.
______________ and provide means for audio and video
We should be able to associate a particular user (rather than just
synchronization.
a host) with a stream.
The protocol must provide some means to indicate that a packet Finally our protocol should use BW efficiently.
is lost, so that the receiver can take appropriate action.
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5.4.2 RTP Design
RTP supports a variety of applications. For each class (audio) it The extension (X) bit indicates the presence of an extended
defines a ________________ and one or more formats.
header following the main header (rarely used).
A profile defines the fields in the RTP header. A format defines
The 4-bit CC field indicates the number of contributing sources.
how the data after the header is to be interpreted (simple audio
The mark (M) bit denotes the start of a ____________. The 7-bit
samples or an MPEG video stream).
payload type (PT) field indicates the type of payload data. The
M and PT fields are precisely defined by the application profile.
The Sequence number is used at the receiver to detect
______________ or out-of-order packets. The application
decides what to do in the case of a missing packet, not RTP.
The Timestamp allows samples to be played back at the
appropriate interval and allows for synchronization between
Header Format
different streams. It is the number of ticks from the first sample.
The first two bits specify the RTP ________________. The P
The synch. source (SSRC) identifies a stream ______________.
bit indicates if padding is used. The last byte of padding
A node with multiple cameras would have a different ID for each
contains the pad count.
camera.
The contributing source (CSRC) is used when several streams
pass through a mixer (combining multiple audio streams). The
SSRC is then the ID of the mixer.
5.4.3 Control Protocol
A control stream (RTCP) is associated with a data stream (RTP). RTCP defines several different packet types:
RTCP (1) provides feedback on performance, (2) correlates and
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synchronizes different streams from a sender, (3) conveys info
on the identity of the sender.
sender reports contain transmission and reception
statistics
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Multiple streams from a sender are associated with a
receiver reports (from non-senders) contain reception
statistics.
_____________________ name (CNAME) that is assigned to
the sender. Association with a CNAME allows different sources
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source descriptions carry __________________.
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application specific packet
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Refer to the rpc and xdr man pages for more
information.
Start homework.
to be synchronized.
In Class Exercises
• Run the command “rpcinfo -p” on Linux.
• Check the portmap and rpcinfo man pages.
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