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TEST.MY.CORP. Instead, it must first query the name server to determine to which host (domain name) it needs to deliver the message. For message delivery, the name server stores resource records (RRs), known as Mail Exchange (MX) RRs. They map a domain name to two values: ► A preference value. Because multiple MX resource records can exist for the same domain name, a preference (priority) is assigned to them. The lowest preference value corresponds to the most preferred record. This is useful whenever the most preferred host is unreachable; the sending SMTP then tries to contact the next preferred host. ► A host name. It is also possible that the name server responds with an empty list of MX RRs. This means that the domain name is in the name server's authority, but has no MX assigned to it. In this case, the sending SMTP might try to establish the connection with the host name itself. An important recommendation is given in RFC 2821: After obtaining the MX records, the sending SMTP should query the name server for well-known services (WKS) records for this host and check that the referenced host has SMTP as a WKS entry. Note: Even though this is only a recommendation, it is widely used in most SMTP implementations. Here is an example containing MX resource records: mydiv.mycorp.com. IN MX 0 smtp1.mydiv.redbookscorp.com. IN MX 2 smtp2.mydiv.redbookscorp.com. IN MX 4 smtp3.mydiv.redbookscorp.com. IN WKS 10.12.34.56 TCP (SMTP) In this example, mail for mydiv.mycorp.com should be delivered to smtp1.mydiv.mycorp.com, because it has a preference of 0. If this host is unreachable, the mail should instead be delivered to the next preferred server, smtp2.mydiv.mycorp.com, which has a preference of 2. If this host is not available, the mail is delivered to the least preferred server, smtp3.mydiv.mycorp.com. A problem arises when using SMTP and MX records in mixed IPv4 and IPv6 environments. Specifically, a user sending an e-mail does not necessarily know if all of the servers and gateways the message will traverse are all IPv4 or IPv6 566 TCP/IP Tutorial and Technical Overview
capable. As IPv6 becomes more pervasive within the Internet community, the likelihood increases that: ► An IPv4 client will be directed to an IPv6-only server (or vice versa). ► An IPv4 client’s message will traverse an IPv6-only network segment (or vice versa). This impending problem is discussed in RFC 3974, which discusses best practices to circumvent the issue. The primary method for this is ensuring that all IPv6 capable entities also be IPv4 capable. Addressing mailboxes on server systems When a user employs a server system for all mail functions, the mailbox address seen by other SMTP users refers exclusively to the mail server system, not to the client system. For example, if two systems are named mydiv.redbookscorp.com and smtp1.mydiv.redbookscorp.com, and the first one is used as a client and the second as a server, the mailbox address might be: user@smtp1.mydiv.redbookscorp.com This mailbox address appears in the From: header field of all outgoing mail as well as in the SMTP commands to remote servers issued by the smtp1 server system. When the user uses a POP server (refer to 15.4, “Post Office Protocol (POP)” on page 589), however, the mailbox address on outbound mail items contains the workstation's host name (for example, user@mydiv.redbookscorp.com). In this case, the sender should include a Reply-To: field in the mail header to indicate that replies should not be sent to the originating mailbox. For example, the mail header might look like this: Date: Fri, 10 Feb 95 15:38:23 From: user@mydiv.redbookscorp.com To: "Your Email" Reply-To: user@smtp1.mydiv.redbookscorp.com Subject: Another sample email Note that the From: address reflects the address of the user’s client system, while the Reply-To: field reflects the address of the smtp1 server system. This is because messages sent to the client system will fail because no SMTP server is active there. Instead, messages sent back to the user will be sent to the Reply-To address, and the user’s client can then connect to the smtp1 server to retrieve its mail. For this reason, the receiving mail agent is expected to send replies to the Reply-To: address and not the From: address. Chapter 15. Mail applications 567
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TCP/IP Tutorial and Technical Overv
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Note: Before using this information
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2.7 Frame relay . . . . . . . . . .
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5.4.2 RIP-2 limitations . . . . . .
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8.4 RFCs relevant to this chapter .
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12.4.7 LDAP and DCE . . . . . . . .
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16.1 Web browsers . . . . . . . . .
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21.1 IPTV overview . . . . . . . .
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24.6.1 Introduction . . . . . . . .
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Trademarks The following terms are
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► TCP/IP and System p: http://www
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Thanks to the following people for
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xxiv TCP/IP Tutorial and Technical
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Finally, other standard protocols e
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1.1 TCP/IP architectural model 1.1.
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To be able to identify a host withi
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est-effort service. As a result, ap
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The client/server model TCP is a pe
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Gateway Interconnects networks at h
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1.2.1 ARPANET the last release of t
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Integrated Digital Network Exchange
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1.2.4 Internet2 coordination. RIPE
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1.2.5 The Open Systems Interconnect
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The process of standardization is s
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Protocol status can be any of the f
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1.4 Future of the Internet Trying t
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28 TCP/IP Tutorial and Technical Ov
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2.1 Ethernet and IEEE 802 local are
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► Introduced in 1985, RFC 948 - T
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The mapping of 32-bit Internet addr
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There are a large number of propose
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2.5 Integrated Services Digital Net
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► The value hex 81 (binary 100000
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process the incoming packet (refer
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frame size, retransmission timer, a
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2.8.1 Physical layer PPP presents a
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Basic InATMARP operates essentially
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The client handles the table update
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To be exact, the TCP/IP PDU is enca
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The address resolution in an ATM ne
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LANs. Each separate LE layer needs
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services of a classic LAN; thus, th
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2.11.3 MPOA functional components T
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Destination resolution The action o
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3.1 Internet Protocol (IP) 3.1.1 IP
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There are five classes of IP addres
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3.1.2 IP subnets Special use IP add
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Therefore, it is normal to use a co
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Because of the all bits 0 and all b
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► Hosts or networks for which the
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► The default route that contains
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To differentiate between subnets, t
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3.1.4 Methods of delivery: Unicast,
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Anycasting Sometimes, the same IP s
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212.0.0 - 213.255.255 RIPE NCC 214.
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Originally, NAT was suggested as a
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packet, the destination address is
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As shown in Figure 3-14, Network Ad
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class of the network number (thus t
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3.1.9 IP datagram Therefore, a larg
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► Service Type: The service type
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► Destination IP Address: The 32-
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Fragmentation When an IP datagram t
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Pointer This field points to the op
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► Because IP routers are not requ
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3.2.1 ICMP messages ICMP messages a
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5 Source route failed 6 Destination
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Figure 3-34 ICMP: Router Solicitati
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Timestamp Request (13) and Timestam
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Record routes Record the route per
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An exception to the rule is the asy
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ARP packet reception When a host re
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different physical device, it will
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BOOTP is a draft standard protocol.
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Length Hardware address length in b
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3. The value of the hops field is i
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3.7.1 The DHCP message format The f
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Options The first four bytes of the
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3. The client receives one or more
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Figure 3-46 shows the DHCP process
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administration workload and removes
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► RFC 2474 - Definition of the Di
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4.1 Ports and sockets 4.1.1 Ports T
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► An association is the 5-tuple t
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The UDP datagram has an 8-byte head
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4.3.1 TCP concept Communication, or
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The window principle A simple trans
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Imagine some special cases: ► Pac
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TCP segment format Figure 4-10 show
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158 TCP/IP Tutorial and Technical O
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SACK option Selective Acknowledgmen
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A problem now arises, because the s
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► TIME-WAIT: FINs have been recei
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sender, while the advertised window
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most one segment each round-trip ti
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2. Each time another duplicate ACK
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Figure 5-1 shows an environment whe
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Figure 5-2 on page 173 depicts the
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► To provide more efficient resou
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Figure 5-3 Shortest-Path First (SPF
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5.2.5 Hybrid routing ► Unless the
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The RIP packet format is shown in F
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Figure 5-5 illustrates the distance
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can be considerable. Figure 5-7 ill
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The limitation to this rule is that
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► Support for multicasting: RIP-2
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5.4.2 RIP-2 limitations Authenticat
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The use of the command field and th
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5.6 Open Shortest Path First (OSPF)
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Intra-area, area border, and AS bou
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► Hello and dead intervals: The r
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neighbors. This store and forward a
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All OSPF packets share the common h
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Step 1: Database exchange process T
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Area 1 Area 0 Figure 5-18 OSPF virt
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In this example, the ASBR is redist
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In this figure, the ASBR is adverti
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EIGRP processes the information in
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5.9.1 BGP concepts and terminology
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► Routes and paths: A route assoc
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► Full mesh of BGP sessions withi
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RFC 4271 recommends a 90 second hol
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Each path attribute is placed into
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► LOCAL_PREF (local preference):
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5.9.6 BGP aggregation ► Redistrib
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ecause AS 2 does not know this aggr
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Figure 5-29 also illustrates the in
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► Ease of implementation: Distanc
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6.1 Multicast addressing Multicast
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address. In this situation, multica
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The fields in the IGMP message cont
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In Figure 6-4 on page 245, the Type
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To join a group, the host sends an
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amount of processing involved in re
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6.4 Multicast forwarding algorithms
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The disadvantage to the center-base
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When the routers exchange their rou
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6.5.3 DVMRP tunnels Some IP routers
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6.6.2 MOSPF and multiple OSPF areas
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6.7.1 PIM dense mode The PIM-DM pro
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Building the PIM-SM multicast deliv
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RP selection An RP is selected as p
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See Figure 6-14 for an overview of
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6.9.1 MBONE routing MBONE will most
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of multicast systems has accelerate
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7.1 Mobile IP overview Mobile IP en
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Figure 7-1 shows a mobile IP operat
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The prefix lengths extension can fo
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D Decapsulation by mobile node. The
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7.2.1 Tunneling The home agent exam
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When a mobile node moves from its h
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8.1 Why QoS? In the Internet and in
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8.2.1 Service classes The Integrate
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a specific bandwidth, a maximum pac
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obeying a token bucket (r,b) and be
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If the path message reaches the fir
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Figure 8-6 shows the reservation pr
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Path and reservation states can als
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RSVP message format An RSVP message
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Style The Style object defines the
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The RSVP Resv messages looks simila
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Integrated Services, described in 8
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different priorities, the DS field
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Figure 8-15 shows the use of bounda
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The traffic conditioner is mainly u
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Figure 8-18 shows the traffic condi
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Intserv model end-to-end across a n
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2. HostA generates a RSVP PATH mess
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Figure 8-21 shows the cooperation o
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► RFC 2212 - Specification of Gua
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9.1 IPv6 introduction 9.1.1 IP grow
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Eventually, the specification for I
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vers Figure 9-3 IPv6 packet contain
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Where: Type The type of the option.
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source routing, which operates in a
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► It improves multicast scalabili
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Global unicast address format IPv6
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Scope 4-bit value indicating the sc
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9.2.4 Flow labels Figure 9-10 Traff
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9.2.6 Packet sizes If the algorithm
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9.3 Internet Control Message Protoc
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Address resolution Figure 9-15 show
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The response to the neighbor solici
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IP Header Option 2 Figure 9-18 Rout
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(for example, a new workstation tha
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outer A has forwarded the packet to
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The stateless autoconfiguration pro
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Note the following fields in the IP
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The following extensions are specif
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Site X is multihomed to two provide
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DHCP Advertise This is a unicast me
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On top of the native IPv6 support t
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For further information about mobil
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9.7.3 New research and development
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► IPv4/IPv6 header translation.Th
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► If the destination is not on th
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IPv6/IPv4 Host Ethernet IPv6/IPv4 R
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It is, of course, possible that aft
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direction, the router adds the ::FF
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► RFC 2675 - IPv6 Jumbograms, Aug
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10.1 Wireless concepts Given the di
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Isotropic radiation Actual radiatio
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10.2.2 Reachability 10.2.3 Scalabil
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802.11a An extension to 802.11 that
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10.4 WiMax confidentiality function
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10.5 Applications of wireless netwo
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Protocol, which is defined by the O
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11.1 Characteristics of application
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11.2 Application programming interf
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Definition of fields: sockfd This i
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An example of a client/server scena
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Figure 11-3 shows the conceptual mo
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- DES authentication: In addition t
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SNMP agent implementations provide
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11.2.4 REXX sockets Note: SET reque
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12.1 Domain Name System (DNS) The D
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We discuss this hierarchical struct
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12.1.6 Mapping IP addresses to doma
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Note: Although domains within the n
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Domain name stub resolver Figure 12
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When a domain is registered with th
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TTL The time-to-live (TTL) time in
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Figure 12-5 DNS message format Head
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Question section The next section c
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Where the fields before the TTL fie
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They are interconnected by an IP ga
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; 4.1.112.129.in-addr.arpa. IN PTR
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12.1.13 Transport Domain Name Syste
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In addition to TSIG, and GSS-TSIG,
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NOTIFY. Based on the RRs contained
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12.4.1 LDAP: Lightweight access to
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LDAP server must communicate using
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12.4.4 LDAP models distinguished na
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classes that a directory server can
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It is usual to follow either a geog
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Some example searches expressed inf
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12.4.6 LDAP URLs agree on a common
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LDAP interface for the GDA One way
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key piece to building intelligent n
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► RFC 4505 - Anonymous Simple Aut
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13.1 Telnet Telnet is a standard pr
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► The NVT provides a local echo f
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Number Name State RFC STD 12 Output
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The Interpret As Command (IAC) char
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Send Reply Meaning The terminal typ
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locks following the command. Howeve
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Principle of operation REXECD is a
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13.3.1 DCE directory service When w
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In order to find a resource in anot
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Login facility Provides the environ
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4. Now the user needs the authoriza
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e defined as single threading. A th
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environment. It also provides a way
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13.4.1 File naming DFS follows the
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typically provided by an operating
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14.1 File Transfer Protocol (FTP) F
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Page 622 and 623: - EXPUNGE: Permanently removes all
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computer, an AIX 5L or UNIX machine
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alternative dynamic portion each ti
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HTTP-Based Client Java Application
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Given these circumstances, this cha
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Group name Description of objects w
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As an example, consider the object
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This if further illustrated in Figu
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getBulkRequest Performs the same fu
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Messages sent between SNMP agents a
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To illustrate the process of queryi
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17.1.6 SNMP traps In this illustrat
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► User-based Security Model (USM)
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► A single authentication protoco
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InformRequest An InformRequest is g
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SnmpAuthMsg This field is used as a
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Data origin authentication Provided
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Local Socket The local IP address a
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► RFC 1213 - Management Informati
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advance, we now have fourth generat
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► The Extended Hypertext Markup L
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User Agent Profile Manager Used mai
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Here, we give a brief description o
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18.6 WAP push architecture The desi
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► Client control As owner of the
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18.6.4 Service indication The servi
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18.7 The Wireless Application Envir
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► WAP-specific data, such as the
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The primitive types and their abbre
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implementing an end-to-end connecti
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and allows WPT-TCP to send acknowle
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datagrams. For that functionality,
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Example of a WSP-WTP sequence flow
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exchange of information and control
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peer layer). The dashed line repres
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The peer who starts the negotiation
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2. The server uses the S-Connect.re
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Normal session establishment Figure
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Normal method invocation Figure 18-
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Wireless client HTTP WTLS WP-TCP Fi
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WTLS layer goals The primary goal i
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CipherSpec Specifies the bulk data
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WIM is used to store permanent priv
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► WAP-229_001-HTTP-20011031-a - W
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The following terminology is used w
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Changes to presence information are
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19.2 Presence Information Data Form
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19.3 Presence protocols The Extensi
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19.3.1 Binding to TCP client-to-ser
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networks in such a manner, organiza
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20.1 Voice over IP (VoIP) introduct
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20.1.2 VoIP functional components F
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is a set of international CODEC sta
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Operational support system OSS prov
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► The proxy server provides appli
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Figure 20-2 An example of SIP reque
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► Stream Control Transmission Pro
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MDCX “ModifyConnection” changes
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Terminals Terminals are the LAN cli
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RTP RTCP audio G.711, G.723.1, etc.
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► RFC 3266 - Support for IPv6 in
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21.1 IPTV overview IPTV is an IP ne
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► Communication context This is s
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21.2 Functional components Figure 2
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21.2.4 IP (TV) transport IPTV requi
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► Congestion control SCTP maintai
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Type Description s Session name i*
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RTP header format The header of an
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1 1016 25 CelB 2 G726-32 26 JPEG 3
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Application RTP 12 1 10 2 9 3 8 4 7
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Receiver report An RTCP receiver re
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Length Contains the packet length.
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It allows weighted prediction, enab
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21.4 RFCs relevant to this chapter
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22.1 Security exposures and solutio
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Problem/exposure Remedy How to ensu
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Application proxy Access control En
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Encryption and decryption: Cryptogr
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eighth bit serving as parity bit. F
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cleartext clea cle Cleartext Alice'
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e and d are called the public and p
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A hash function that takes a key as
Page 814 and 815:
Examples of hash functions The most
Page 816 and 817:
Digital Signature Standard (DSS) As
Page 818 and 819:
public key and identification, a di
Page 820 and 821:
22.3 Firewalls In September 1998, t
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time. The network administrator mus
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Service level filtering Because mos
Page 826 and 827:
etween the secure and the non-secur
Page 828 and 829:
In normal mode, the FTP client firs
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circuit-level gateway (see Figure 2
Page 832 and 833:
Generally, a packet-filtering firew
Page 834 and 835:
Internal DNS and Mail server Secure
Page 836 and 837:
22.4.1 Concepts IPSec adds integrit
Page 838 and 839:
Tunneling Tunneling or encapsulatio
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AH format The AH format is describe
Page 842 and 843:
AH in transport mode In this mode,
Page 844 and 845:
connectionless, in that they operat
Page 846 and 847:
Padding Most encryption algorithms
Page 848 and 849:
The tunnel mode is used whenever ei
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When designing a VPN, limit the num
Page 852 and 853:
Figure 22-33 illustrates the simple
Page 854 and 855:
Figure 22-36 shows in detail how th
Page 856 and 857:
Before describing the details of th
Page 858 and 859:
Permanent identifiers The IKE proto
Page 860 and 861:
Note: For ISAKMP phase 1 messages,
Page 862 and 863:
Page 864 and 865:
authority using a protocol such as
Page 866 and 867:
IKE phase 2: Setting up protocol Se
Page 868 and 869:
Hash A Hash payload must immediatel
Page 870 and 871:
Generating the keys (phase 2) Using
Page 872 and 873:
22.5 SOCKS intranet is considered t
Page 874 and 875:
22.5.1 SOCKS Version 5 (SOCKSv5) SO
Page 876 and 877:
Where: VER Indicates the version of
Page 878 and 879:
X'03' Network unreachable X'04' Hos
Page 880 and 881:
X Window System’s clients. SSH fo
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22.7.2 SSL protocol Authentication
Page 884 and 885:
CipherSpec message after processing
Page 886 and 887:
The client then sends a finished me
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In this chapter, we begin by defini
Page 890 and 891:
usiness partner's and supplier's VP
Page 892 and 893:
name the principal. Both the realm
Page 894 and 895:
The authentication process consists
Page 896 and 897:
After the server has validated a cl
Page 898 and 899:
► Authorization and accounting in
Page 900 and 901:
modem connection is made, the NAS p
Page 902 and 903:
22.14.1 Terminology Before describi
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PPP Data Figure 22-54 L2TP packet c
Page 906 and 907:
complementary use of IPSec, an L2TP
Page 908 and 909:
The diagram shows the following tra
Page 910 and 911:
Key management on such a large scal
Page 912 and 913:
► RFC 2405 - The ESP DES-CBC Ciph
Page 914 and 915:
Page 916 and 917:
23.1 Port based network access cont
Page 918 and 919:
23.3 Port based network access cont
Page 920 and 921:
Figure 23-3 illustrates the control
Page 922 and 923:
802.1x authentication process The a
Page 924 and 925:
Figure 23-6 and Figure 23-7 show sn
Page 926 and 927:
Request and response type Value TLS
Page 928 and 929:
Figure 23-15 EAP-Success packet EAP
Page 930 and 931:
Figure 23-21 shows the sniffer trac
Page 932 and 933:
► Multicast propagation If authen
Page 934 and 935:
The Internet business has grown so
Page 936 and 937:
Generic techniques to enhance scala
Page 938 and 939:
24.5 Virtualization Virtualization
Page 940 and 941:
Network virtualization Network virt
Page 942 and 943:
24.6.2 VRRP definitions 24.6.3 VRRP
Page 944 and 945:
24.6.4 Sample configuration Figure
Page 946 and 947:
The fields of the VRRP header are d
Page 948 and 949:
24.8.1 Network Address Translation
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24.9 RFCs relevant to this chapter
Page 952 and 953:
A.1 MPLS: An introduction The idea
Page 954 and 955:
MPLS also provides the ability to a
Page 956 and 957:
Layer 2 Header MPLS Header Figure A
Page 958 and 959:
Label switched path (LSP) An LSP re
Page 960 and 961:
► The packet does not contain a l
Page 962 and 963:
7. R22 reviews the level-2 label ap
Page 964 and 965:
A.2.5 Label distribution protocols
Page 966 and 967:
Emulated Ethernet MUX MPLS Figure A
Page 968 and 969:
► The effort can provide a single
Page 970 and 971:
A generalized label only carries a
Page 972 and 973:
Fault management The LMP fault mana
Page 974 and 975:
Note: GMPLS allows SP to dynamicall
Page 976 and 977:
Furthermore, because the labels can
Page 978 and 979:
A.5 RFCs relevant to this chapter T
Page 980 and 981:
DLCI Data Link Connection Identifie
Page 982 and 983:
MTU Maximum Transmission Unit MVS M
Page 984 and 985:
VRML Virtual Reality Modeling Langu
Page 986 and 987:
► TCP/IP and System i http://www.
Page 988 and 989:
Page 990 and 991:
DHCP interoperability 140 forwardin
Page 992 and 993:
fingerprint 785 firewall 12, 776, 8
Page 994 and 995:
Security Parameter Index (SPI) 810
Page 996 and 997:
latency 61 Layer 2 Forwarding (L2F)
Page 998 and 999:
RFC 1483 53 RFC 1492 873 RFC 1510 8
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subnet number 73 subnets 72 subnett
Page 1004:
TCP/IP Tutorial and Technical Overv
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