Lecture 19
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Homework 6 posted, due next Tuesday.
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Questions?
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In-class Exercises
Log on locally under Linux or log on remotely to
csserver to answer the following questions:
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How do we send out-of-band data via TCP? (man send)
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How do we receive out-of-band data? (man recv)
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Which of the four TCP extensions described in class are
supported under Linux? (man tcp)
What acronym is used for the TCP extension that helps to
determine if the sequence number has wrapped around? What
does this acronym stand for?
Is there a way to disable Nagle's algorithm so that segments
are sent immediately? If so, how?
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In-class Exercises
The Linux /proc pseudo-filesystem interface can
be used to tune many of the TCP algorithms.
Changing the parameters requires system
administration privileges. Use cat to examine
appropriate /proc file contents (man tcp) and
determine the answers to the following:
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Is the optional SACK extension enabled?
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What is the default receive buffer size?
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Is the optional window scaling extension enabled?
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What is the default congestion control algorithm? Which
algorithms are available for use?
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Outline
Chapter 5 - End-to-End Protocols
5.1 Simple Demultiplexer (UDP)
5.2 Reliable Byte Stream (TCP)
5.3 Remote Procedure Call (RPC)
5.4 Transport for Real-Time Applications (RTP)
5.5 Summary
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Remote Procedure Call (RPC)
TCP and UDP provide
byte stream channels.
Another comm. pattern
is the request/reply or
message 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.
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RPC Fundamentals
RPC is commonly used in building distributed
systems. RPC allows a procedure to be either
local or remote. An application calls the
procedure as if it were local. The calling
application blocks until the procedure returns.
Remote procedure calls are more complex than
local procedure calls:
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Messages may be limited in size. They may also be
lost or reordered.
Different computers may have different OSes,
architectures, and data representations.
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RPC Fundamentals
RPC therefore involves two components:
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A protocol to manage message passing
between the client (caller) and server.
Programming language and compiler (a stub
compiler) support to translate a procedure call
on the client into a request message and
translate the message into arguments on the
server. The return value is handled similarly.
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RPC Fundamentals
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RPC Fundamentals
The term RPC refers to a type 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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RPC Fundamentals - Identifiers
RPC must provide a name space for identifying
the procedure to be called.
The name space can be either flat or hierarchical.
A flat name space requires central coordination to
prevent assigning the same ID to different
procedures. The ID can be carried in a single
field in an RPC request.
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RPC Fundamentals - Identifiers
An RPC protocol must also match a reply
message to the corresponding request. This is
done by including a message ID in both the
request and reply.
A client boot ID may be used as part of the
message ID in order to ensure that the correct
match in the event that the client reboots with an
outstanding request.
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RPC Fundamentals - Overcoming
Network Limitations
RPC can implement
reliability using ACKs. Each
side has a retransmit timer
that causes the message to
be resent in the event of a
time out. A reply can be
used as an implicit ACK.
Concurrent requests can be
implemented using logical
channels.
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RPC Fundamentals - Overcoming
Network Limitations
RPC reliability may implement at-most-once
semantics in which there is a guarantee that no
more than one request is delivered to the server.
Implementation of zero-or-more or idempotent
semantics is simpler and sufficient for applications
in which multiple requests have the same effect
as one request.
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RPC Implementations - SunRPC
SunRPC (aka ONC RPC) was developed by Sun
as part of their Network File System (NFS).
SunRPC can be implemented over several
different transport protocols (SunRPC is also
considered a transport protocol).
SunRPC uses a 32-bit program number and a 32bit procedure number to identify a procedure (the
NFS server has program ID 0x100003, the NFS
read procedure has ID 6 while write has ID 8).
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RPC Implementations - SunRPC
Different RPC servers are dynamically assigned
TCP/UDP port numbers. The port mapper RPC
server listens on well-known port 111. RPC
clients can query the port mapper to determine
the port number assigned to a program.
SunRPC does not implement its own reliability or
fragmentation methods. It relies on the underlying
protocol.
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RPC Implementations - SunRPC
SunRPC request and reply headers
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RPC Implementations - SunRPC
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 authenticate itself
to the server.
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RPC Implementations - DCE-RPC
DCE-RPC is used in
Microsoft's DCOM and
ActiveX technologies. It is
also used in CORBA, a
standard for distributed
object-oriented systems.
A typical DCE-RPC exchange
is shown at right. If the server
responds quickly enough, no
Pings are sent.
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RPC Implementations - DCE-RPC
DCE-RPC supports multiple logical channels
known as activities 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
ensure at-most-once semantics.
DCE-RPC supports very large messages and
implements its own fragmentation scheme.
Selective acknowledgment is used allowing only
missing fragments to be retransmitted instead of
the entire message.
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RPC Implementations - DCE-RPC
Selective acknowledgment is
shown at left. A WindowSize
is used for flow-control.
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Transport for Real-Time Apps (RTP)
Multimedia applications can be categorized as
either streaming (audio, video streams) or
interactive (VoIP, teleconferencing). Interactive
applications have the strictest real-time
requirements.
RTP can run over many
lower level protocols, but is
typically run on top of UDP.
RTP is still considered a
transport protocol.
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Requirements
A multimedia protocol must allow applications to
interoperate. One approach is to specify a
particular audio and video coding scheme. RTP
allows the sender to indicate which coding
method it wants to use.
Our protocol must support playback
synchronization to prevent jitter and provide
means for audio and video synchronization.
The protocol must provide some means to
indicate that a packet is lost, so that the receiver
can take appropriate action.
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Requirements
UDP does not provide congestion control and this
is desirable in many real-time apps. The receiver
must notify the sender that losses are occurring.
A real-time protocol should provide some
indication of frame boundaries.
We should be able to associate a particular user
(rather than just a host) with a stream.
Finally our protocol should use BW efficiently.
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RTP Design
RTP supports a variety of applications. For each
class (audio) it defines a profile and one or more
formats.
A profile defines the fields in the RTP header. A
format defines how the data after the header is to
be interpreted (simple audio samples or an MPEG
video stream).
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RTP Design - Header Format
The first two bits specify the RTP version. The P
bit indicates if padding is used. The last byte of
padding contains the pad count.
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RTP Design - Header Format
The extension (X) bit indicates the presence of an
extended header following the main header
(rarely used).
The 4-bit CC field indicates the number of
contributing sources.
The mark (M) bit denotes the start of a frame. The
7-bit payload type (PT) field indicates the type of
payload data. The M and PT fields are precisely
defined by the application profile.
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RTP Design - Header Format
The Sequence number is used at the receiver to
detect missing 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 different streams. It is
the number of ticks from the first sample.
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RTP Design - Header Format
The synch. source (SSRC) identifies a stream
source. A node with multiple cameras would have
a different ID for each 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.
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Control Protocol
A control stream (RTCP) is associated with a data
stream (RTP). RTCP (1) provides feedback on
performance, (2) correlates and synchronizes
different streams from a sender, (3) conveys info
on the identity of the sender.
Multiple streams from a sender are associated
with a canonical name (CNAME) that is assigned
to the sender. Association with a CNAME allows
different sources to be synchronized.
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Control Protocol
RTCP defines several different packet types:
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sender reports contain transmission and
reception statistics
receiver reports (from non-senders) contain
reception statistics.
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source descriptions carry CNAMEs.
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application specific packets
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In-class Exercises
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Run the command “rpcinfo -p” on Linux.
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Check the portmap and rpcinfo man pages.
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Refer to the rpc and xdr man pages for more
information.
Start homework.
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