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NOP
MSS
SACK
permitted
SACK
option
8 bits
00000000
Kind = 0
00000001
Kind = 1
00000010
Kind = 2
00000100
Kind = 4
00000100
Length = 4
00000010
Length = 2
Figure A6-5: TCP Options
MSS (16 bits)
Kind = 5
00000101
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Variable
Length
xxxxxxxx
- Padding (variable) which is comprised of 0s is used to ensure the TCP header is 32-bit aligned.
- The Type of Service (TOS) field in the IP header was intended to use for TOS routing, in which
a router would have separate routing tables for different TOS values. When forwarding a packet,
the router would first choose a routing table based on the packet’s TOS, followed by normal
routing table lookup. TOS routing has rarely been implemented in the Internet. Only 2 routing
protocols – OSPF and IS-IS have ever supported the calculation of separate paths based on TOS.
- The first 3 bits in the TOS field in the IP header were being used for IP Precedence, in which
values from 0 – 7 can be used to specify the transmission priority of packets at each router hop.
IP Precedence does not affect the path of a packet as with TOS. IP Precedence is being phased
out in favor of DSCP, but is supported by many applications and routers.
- Differentiated Services Code Point (DSCP) is a modification of the TOS field. 6 bits of the
field are being reallocated for use as the DSCP field. DSCP is not compatible with IP Precedence.
- An application can modify the handling of IP packets by extending the IP header with IP options.
IP options are rarely used for regular IP packets, as most routers are heavily optimized for
forwarding IP packets without IP options. The use of IP options introduces a potential DoS
vulnerability against routers due to the additional processing workload of packets with IP options.
- Most IP options (eg: the record-route and timestamp options) are used for statistics collection
and do not affect the forwarding path of packets. However, the strict-source route and
loose-source route options can be utilized by the originator of a packet to control the forwarding
path the packet.
- IP source routing is often considered as a security hole, as even with security is being provided
through address filtering, the final destination of a packet might buried in the IP options field.
As a result, most routers are configured to discard packets containing source routing options with
the no ip source-route global configuration command.
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Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com
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