obonaventure · obonaventure · Jan 2, 2019 · Nov 28, 2018
diff --git a/book-2nd/protocols/ipv6b.rst b/book-2nd/protocols/ipv6b.rst
@@ -47,7 +47,7 @@ IPv6 hosts and routers frequently interact with the datalink layer service. To u
 
 .. index:: link-local IPv6 address
 
-Hosts ``A`` and ``B`` are attached to the same datalink layer network. They can thus exchange frames by using the MAC addresses shown in the figure above. To be able to use IPv6 to exchange packets, they need to have an IPv6 address. One possibility would be to manually configure an IPv6 address on each host. However, IPv6 provides a better solution thanks to the `link-local` IPv6 addresses. A `link-local` IPv6 address is an address that is composed by concatenating the ``fe80:://64`` prefix with the MAC address of the device. In the example above, host A would use IPv6 `link-local` address ``fe80::0223:45FF:FE67:89ab`` and host B ``fe80::0234:5678:9aFF:FEbc:dede``. With these two IPv6 addresses, the hosts can exchange IPv6 packets.
+Hosts ``A`` and ``B`` are attached to the same datalink layer network. They can thus exchange frames by using the MAC addresses shown in the figure above. To be able to use IPv6 to exchange packets, they need to have an IPv6 address. One possibility would be to manually configure an IPv6 address on each host. However, IPv6 provides a better solution thanks to the `link-local` IPv6 addresses. A `link-local` IPv6 address is an address that is composed by concatenating the ``fe80:://64`` prefix with the MAC address of the device. In the example above, host A would use IPv6 `link-local` address ``fe80::0223:45FF:FE67:89ab`` and host B ``fe80::0234:56FF:FE78:9abc``. With these two IPv6 addresses, the hosts can exchange IPv6 packets.
 
 .. note:: Converting MAC addresses in host identifiers
 
@@ -58,7 +58,7 @@ Hosts ``A`` and ``B`` are attached to the same datalink layer network. They can
 
      A MAC address
 
- MAC addresses are allocated in blocks of :math:`2^{20}`. When a company registers for a block of MAC addresses, it receives an identifier. company identifier is then used to populated the `c` bits of the MAC addresses. The company can allocate all addresses in starting with this prefix and mangages the `m` bits as it wishes. 
+ MAC addresses are allocated in blocks of :math:`2^{20}`. When a company registers for a block of MAC addresses, it receives an identifier. company identifier is then used to populated the `c` bits of the MAC addresses. The company can allocate all addresses in starting with this prefix and manages the `m` bits as it wishes. 
 
   .. figure:: pkt/macaddr-eui64.png
      :align: center
@@ -156,11 +156,11 @@ Several options can be included in the Router Advertisement message. The simples
 
 The key information placed in this option are the prefix and its length. This allows the hosts attached to the subnet to automatically configure their own IPv6 address. The `Valid` and `Preferred` `Lifetimes` provide information about the expected lifetime of the prefixes. Associating some time validity to prefixes is a good practice from an operational viewpoint. There are some situations where the prefix assigned to a subnet needs to change without impacting the hosts attached to the subnet. This is often called the IPv6 renumbering problem in the literature :rfc:`7010`. A very simple scenario is the following. An SME subscribes to one ISP. Its router is attached to another router of this ISP and advertises a prefix assigned by the ISP. The SME is composed of a single subnet and all its hosts rely on stateless address configuration. After a few years, the SME decides to change of network provider. It connects its router to the second ISP and receives a different prefix from this ISP. At this point, two prefixes are advertised on the SME's subnet. The old prefix can be advertised with a short lifetime to ensure that hosts will stop using it while the new one is advertised with a longer lifetime. After sometime, the router stops advertising the old prefix and the hosts stop using it. The old prefix can now be returned back to the first ISP. In larger networks, renumbering an IPv6 remains a difficult operational problem [LeB2009]_.
 
-Upon reception of this message, the host can derive its global IPv6 address by concatenating its 64 bits identifier with the received prefix. It concludes the SLAAC by sending a Neighbour Solicitation message targeted at its global IPv6 address to ensure that no other host is not using the same IPv6 address.
+Upon reception of this message, the host can derive its global IPv6 address by concatenating its 64 bits identifier with the received prefix. It concludes the SLAAC by sending a Neighbour Solicitation message targeted at its global IPv6 address to ensure that no other host is using the same IPv6 address.
 
 .. note:: Router Advertisements and Hop Limits
 
-  ICMPv6 Router Advertisements messages are regularly sent by routers. They are destined to all devices attached to the local subnet and no router should ever forward them to another subnet. Still, these messages are sent inside IPv6 packets whose `Hop Limit` is always set to ``255``. Given that the packet should not the forwarded outside of the local subnet, the reader could expect instead a `Hop Limit` set to ``1``. Using a `Hop Limit` set to ``255`` provides one important benefit from a security viewpoint and this hack has been adapted in several Internet protocols. When a host receives a `Router Advertisement` message, it expects that this message has been generated by a router attached to the same subnet. Using a `Hop Limit` of ``255`` provides a simple check for this. If the message was generated by an attacker outside the subnet, it would reach the subnet with a decremented `Hop Limit`. Checking that the `Hop Limit` is set to ``255`` is a simple [#fsend]_ verification that the packet was generated on this particular subnet. :rfc:`5082` provides other examples of protocols that use this hack and discuss its limitations.
+  ICMPv6 Router Advertisements messages are regularly sent by routers. They are destined to all devices attached to the local subnet and no router should ever forward them to another subnet. Still, these messages are sent inside IPv6 packets whose `Hop Limit` is always set to ``255``. Given that the packet should not be forwarded outside of the local subnet, the reader could expect instead a `Hop Limit` set to ``1``. Using a `Hop Limit` set to ``255`` provides one important benefit from a security viewpoint and this hack has been adapted in several Internet protocols. When a host receives a `Router Advertisement` message, it expects that this message has been generated by a router attached to the same subnet. Using a `Hop Limit` of ``255`` provides a simple check for this. If the message was generated by an attacker outside the subnet, it would reach the subnet with a decremented `Hop Limit`. Checking that the `Hop Limit` is set to ``255`` is a simple [#fsend]_ verification that the packet was generated on this particular subnet. :rfc:`5082` provides other examples of protocols that use this hack and discuss its limitations.
 
 
 Routers regularly send Router Advertisement messages. These messages are triggered by a timer that is often set at approximately 30 seconds. Usually, hosts wait for the arrival of a Router Advertisement message to configure their address. This implies that hosts could sometimes need to wait 30 seconds before being able to configure their address. If this delay is too long, a host can also send a `Router Solicitation` message. This message is sent towards the multicast address that corresponds to all IPv6 routers (i.e. ``FF01::2``) and the default router will reply.