Term
| IP is a best-effort, connectionless datagram delivery service. Explain what best-effort and connectionless mean. |
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Definition
a. Best-effort means there are no guarantees that a datagram will successfully reach its destination.
b. Connectionless means that IP does not hold any connection state in any of the network devices (i.e. routers); each datagram is handled independently. |
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Term
| List the fields and their sizes within an IPv4 header. |
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Definition
a. Version - 4 bits
b. IHL - 4 bits
c. DSField - 6 bits
d. ECN - 2 bits
e. Total Length - 16 bits
f. Identification - 16 bits
g. Flags - 3 bits
h. Fragment Offset - 13 bits
i. TTL - 8 bits
j. Protocol - 8 bits
k. Header Checksum - 16 bits
l. Source IP - 32 bits
m. Destination IP - 32 bits
n. Options (if any) - variable up to 320 bits/40 bytes
o. IP Data (if any) - variable up to 65515 bytes |
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Term
| List the fields and their sizes within an IPv6 header. |
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Definition
a. Version - 4 bits
b. DSField - 6 bits
c. ECN - 2 bits
d. Flow Label - 20 bits
e. Payload Length - 16 bits
f. Next Header - 8 bits
g. Hop Limit - 8 bits
h. Source IP - 128 bits
i. Dest IP - 128 bits |
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Term
| What is the size of a typical IPv4 (no options) and IPv6 header (no extension headers)? |
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Definition
a. IPv4 - 20 bytes
b. IPv6 - 40 bytes |
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Term
| What is the 4 bit version field in an IPv4/IPv6 header? |
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Definition
| It contains the version number of the IP datagram i.e. IPv4 = 4, IPv6 = 6. |
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Term
| Describe the 4 bit IPv4 IHL field. |
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Definition
| This specifies the number of 32 bit words in the IPv4 header, incuding options. As this is a 4 bit field, the IPv4 header is limited to a max of 15 32-bit words (60 bytes). An IPv4 header with no options contains 5 32-bit words (20 bytes). |
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Term
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Definition
| The 8-bit Type of Service byte was originally used to signify the traffic class. Use of this feature never became widespread so it was split into the 6-bit DS Field and 2-bit ECN field. |
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Term
| What is the 16-bit IPv4 Total Length field and why is it important? |
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Definition
a. This defines the total length of the IPv4 datagram.
b. When used with the IHL, the start of the data field can be calculated. This is important because some L2 protocols may not accurately convey the the size of the encapsulated IP packet e.g. an IP datagram may be smaller than the 46-byte minimum Ethernet payload size, so padding will be required. The IP layer on the receiver needs to be able to detect this padding. |
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Term
| If the IPv4 Total Length field is 16 bits, what is the maximum size of an IP datagram? |
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Definition
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Term
| What is the maximum size IPv4 datagram that a host is obliged to receive? |
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Definition
| 576 bytes. Many apps (DNS, DHCP) use a datagram size of 512 bytes to avoid this limit. |
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Term
| What is the maximum size IPv6 datagram that a host is obliged to receive? |
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Definition
| An IPv6 host must be able to proceess a datagram at least as large as the MTU of the link to which it is attached, and the minimum link MTU is 1280 bytes. |
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Term
| Describe the IPv6 Payload Length field. |
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Definition
a. Defines the length of the payload and Extension Headers, but not the 40-byte IPv6 header.
b. 16-bit field, therfore the payload and Extension Headers are limited to 65535 bytes. |
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Term
| What is the theoretical max size of an IPv6 jumbogram? |
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Definition
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Term
| Describe the IPv4 Identification field, and name its IPv6 counterpart. |
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Definition
a. 16 bit field used to identify each IP packet, very important for fragmentation.
b. IPv6 uses the Fragmentation Extension Header. |
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Term
| What happens when the IPv4 TTL or IPv6 Hop Limit field reaches 0. |
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Definition
| The packet is discarded and the sender is notified with an ICMP message. |
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Term
| Describe the IPv4 Protocol field and list some common values. |
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Definition
a. 8-bit field used to describe the encapsulated protocol.
b. TCP-6, UDP-17, IPv4-4, IPv6-6, EIGRP-88, OSPF-89, ICMP-1. |
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Term
| What is the IPv6 equivalent of the IPv4 Protocol field? |
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Definition
| The 8-bit Next Header field. This will define the type of header that follows the IPv6 header. This could be the encapsulated protocol or an IPv6 Extension Field. |
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Term
| Describe the IPv4 Checksum field. |
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Definition
a. 16 bit field that calculates the checksum of the IPv4 header only.
b. A received checksum with no errors should have a value of 0.
c. No error messages are sent when a bad packet is discarded. |
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Term
| What is the IPv6 equivalent of the IPv4 Checksum field? |
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Definition
| There isn't one, data integrity is left to the upper layers. |
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Term
| Describe the IPv4 options field. |
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Definition
a. Variable length up to 40 bytes.
b. Must always end on a 32-bit boundary, padding is used if necessary.
c. Options are identified by an 8-bit type field.
d. Some options only consist of the 8-bit type field, others are longer. |
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Term
| Describe the IPv4 option 8-bit Type field. |
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Definition
a. 3 subfields
i. Copy (1 bit) - defines if the option should be copied to fragments.
ii. Class (2 bits) - 0=control, 2=debugging and measurement, 1&3=reserved.
iii. Number (5 bits) - identifies the option. |
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Term
| Why are the Source and Record Route options not practical for Internet traffic? |
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Definition
| These options require IPv4 addresses to be placed in the IPv4 header. As there is limited space in the header (60 bytes, minus the 20-byte regular header), it is not feasible to record IPv4 addresses across an Internet with an average of 15 hops. |
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Term
| Describe the makeup of a variable length IPv4 Option field. |
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Definition
a. 8-bit Type field.
b. 8 Bit Length.
c. Option data with padding if necessary. |
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Term
| Who can process IPv6 Extension Headers? |
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Definition
| Only the end hosts, with the exception of the Routing Header, which needs to be processed by certain routers in the path. |
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Term
| In what order must the IPv6 Extension Headers be processed? |
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Definition
| The order of IPv6 Extension Headers is only a recommendation, except in the case of the Hop-by-Hop Options (HOPOPT). These must immediately follow the IPv6 header. |
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Term
| Describe the 2 types of IPv6 options. |
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Definition
a. Hop-by-Hop options must be processed by every router a packet encounters.
b. Destination options are only processed by the destination. |
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Term
| Describe how IPv6 options are encoded. |
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Definition
a. 8-bit option Type, which has 3 subfields:
i. 2-bit Action field - defines what an IPv6 node should do if it doesn't recognise the option.
ii. 1-bit Change field - set to 1 when the option data can be modified as the datagram is forwarded.
iii. 5-bit Type field - defines the option.
b. 8-bit Option Data Length - defines the size of the option data in bytes.
c. Variable Option Data - the option data. |
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Term
| Describe the possible values for the IPv6 2-bit Option Type, Action subfield. |
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Definition
00 - Skip option, continue processing.
01 - Discard datagram silently.
10 - Discard datagram, send ICMPv6 Parameter Problem to source.
11 - Same as 10, but only send ICMPv6 message if offending packet was not multicast. |
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Term
| Briefly describe the IPv6 Pad1 Option. |
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Definition
a. Type 0
b. 1-byte of padding to ensure the IPv6 option is aligned to an 8-byte offset. |
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Term
| Briefly describe the IPv6 PadN Option. |
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Definition
a. Type 1
b. 2 or more bytes of padding to ensure the IPv6 option is aligned to an 8-byte offset. |
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Term
| Briefly describe the IPv6 Jumbo Payload Option. |
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Definition
a. Type 194
b. 32-bit field to specify the payload length of an IPv6 datagram which has a payload greater than 65535 bytes.
c. When this option is used, TCP must use this value and not the Length field in the base header. |
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Term
| Briefly describe the IPv6 Tunnel Encapsulation Limit Option. |
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Definition
a. Type 4
b. Allows a sender to define how many times the packet can be tunnelled.
c. Limit is decremented by 1 every time the packet is tunnelled. |
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Term
| Briefly describe the IPv6 Router Alert Option. |
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Definition
a. Type 5
b. Indicates a datagram that needs to be processed by a router. |
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Term
| Briefly describe the IPv6 Quick Start Option. |
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Definition
a. Type 6.
b. Allows all nodes in a path to agree on a predetermined transmission rate.
c. QS will only be enabled if all nodes are able to participate. |
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Term
| Briefly describe the IPv6 CALIPSO Option. |
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Definition
a. Type 7.
b. Allows a datagram to be marked with a security level label.
c. May be useful for government, military or financial organisations. |
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Term
| Briefly describe the IPv6 Home Address Option. |
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Definition
a. Type 201.
b. Allows a sender to specify its home address when using IP Mobility. |
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Term
| Describe the purpose of the IPv6 Routing Header. |
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Definition
a. Permits the sender to control, in part, the path a datagram takes through the network by specifying waypoints.
b. 2 types:
i. Type 0 - RH0 - deprecated as it can be used in a DoS attack.
ii. Type 2 - RH2 - only has room for a single address. |
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Term
| Describe the makeup of an IPv6 Routing Header Type 0 Extension. |
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Definition
a. 8-bit Next Header field.
b. 8-bit Header Extension Length - defines length of extension.
c. 8-bit routing type - 0 for RH0.
d. 8-bit Segments Left field - defines how many waypoints remain.
e. 32-bit reserved - set to 0.
f. N x 128-bit IPv6 addresses - the defined waypoints. |
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Term
| How does an IPv6 RH2 differ from a RH0. |
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Definition
a. The RH2 only has room for a single address to be defined.
b. The Routing Type field will have a value of 2. |
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Term
| Who will process the Routing Header |
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Definition
| Only the nodes that are defined in the Destination IP address field. Each of these nodes will replace the destination IP with the next address and decrement the Segments Left field. |
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Term
| Who is permitted to fragment with IPv4 and IPv6? |
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Definition
a. IPv4 - any host or router.
b. IPv6 - only the sender. |
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Term
| Describe the IPv6 Fragmentation Header format. |
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Definition
8-byte header, formatted as follows:
a. 8-bit Next Header field.
b. 8-bit reserved field.
c. 13-bit Fragment Offset field - defines where the fragment payload sits within the original packet. Defined in 8-byte units.
d. 2-bit Res field.
e. 1-bit M field - indicates if more fragments follow this datagram.
f. 32-bit Identification field - permits double the amount of fragments as IPv4. |
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Term
| Define the "fragmentable" and "unfragmentable" parts of an IPv6 datagram. |
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Definition
a. unfragmentable - IPv6 header, Extension headers that need to be processed by intermediate nodes (up to and including the Routing Header if present). Each fragment will contain the unfragmentable info in its header.
b. fragmentable part - Destination Options Header, upper layer headers, payload data. |
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Term
Define the following IPv6 Next Header values:
IPv6 Header
Hop-by-Hop Options (HOPOPT)
Destination Options
Routing
Fragment
No Next Header
ICMPv6 |
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Definition
IPv6 Header - 41
Hop-by-Hop Options (HOPOPT) - 0
Destination Options - 60
Routing - 43
Fragment - 44
No Next Header - 59
ICMPv6 - 58 |
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