Fixed typos and missing steps in Behavior

Author: kellyyeh

doc/DHCPv6_relay/DHCPv6-relay-agent-High-Level-Design.md

@@ -41,7 +41,7 @@ DUID: DHCP Unique Identifier (Each DHCPv6 client and server has a DUID. DHCPv6 s
 
 SONiC currently supports DHCPv4 Relay via the use of open source ISC DHCP package. However, DHCPv6 specification does not define a way to communicate client link-layer address to the DHCP server where DHCP server is not connected to the same network link as DHCP client. DHCPv6 requires all clients prepare and send a DUID as the client identifier in all DHCPv6 message exchanges. However, these methods do not provide a simple way to extract a client's link-layer address. Providing option 79 in DHCPv6 Relay-Forward messages will help carry the client link-layer address explicitly. The server needs to know the client's MAC address to allow DHCP Reservation, which provides pre-set IP address to specific client based on its physical MAC address. The DHCPv6 relay agent is able to read the source MAC address of DHCPv6 messages that it received from client, and encapsulate these messages within a DHCPv6 Relay-Forward message, inserting the client MAC address as option 79 in the Relay-Forward header sent to the server.
 
-With heterogenous DHCP client implementation across the network, DUIDs could not resolve IP resource tracking issue. The two types of DUIDs, DUID-LL and DUID-LLT used to facilitate resource tracking both have link layer addresses embedded. The current client link-layer address option in DHCPv6 specification limits the DHCPv6 Relay to first hop to provide the client link layer address, which are relay agents that are connected to the same link as the client, and that limits SONiC DHCPv6 deployment to ToR/MoR switches for early stages. One olution would be to provide SONiC's own DHCPv6 relay agent feature.
+With heterogenous DHCP client implementation across the network, DUIDs could not resolve IP resource tracking issue. The two types of DUIDs, DUID-LL and DUID-LLT used to facilitate resource tracking both have link layer addresses embedded. The current client link-layer address option in DHCPv6 specification limits the DHCPv6 Relay to first hop to provide the client link layer address, which are relay agents that are connected to the same link as the client, and that limits SONiC DHCPv6 deployment to ToR/MoR switches for early stages. One solution would be to provide SONiC's own DHCPv6 relay agent feature.
 
 # DHCPv6
 
@@ -60,7 +60,7 @@ DHCP is a network protocol used to assign IP addresses and provide configuration
 
 # Why DHCPv6 relay agent
 
-Generally, the DHCPv6 clients get IP by multicasting the DHCP packets in the LAN, and the server will response clients' request. In this case, it would be necessary to keep the DHCPv6 server and clients in the same LAN. DHCPv6 relay agent is used to transmit different subnets' DHCPv6 packets, so that all subnets can share DHCPv6 server, and DHCPv6 server is not required on every LAN.
+Generally, the DHCPv6 clients get IP by multicasting the DHCP packets in the LAN, and the server will respond to clients' request. In this case, it would be necessary to keep the DHCPv6 server and clients in the same LAN. DHCPv6 relay agent is used to transmit different subnets' DHCPv6 packets, so that all subnets can share DHCPv6 server, and DHCPv6 server is not required on every LAN.
 
 A DHCPv6 client sends most messages using a reserved, link-scoped multicast destination address so that the client need not be configured with the address or addresses of DHCP servers.
 
@@ -100,19 +100,20 @@ The packets are forwarded to configurable IPv6 helpers addresses.
 
 # Relay Agent Behavior
 
-1. DHCPv6 client sends multicast SOLICITE message to ALL\_DHCP\_Relay\_Agents\_and\_Servers. Message received by relay agent.
+1. DHCPv6 client sends multicast SOLICIT message to ALL\_DHCP\_Relay\_Agents\_and\_Servers. Message received by relay agent.
   - Relay agent at default uses ALL\_DHCP\_Servers multicast address. It may be configured to use unicast addresses, or other addresses selected by the network administrator.
 2. DHCPv6 relay agent constructs a Relay-forward message copies the source address from header of the IP datagram to the peer-address field of the Relay-forward message and received DHCP message into Relay Message option, and relays this Relay-forward message to the DHCPv6 server in RELAY\_FORWARD message
   - DHCPv6 relay agent also places a global or site-scope address with a prefix assigned to the link on which the client should be assigned an address in the link-address field. (will be used by server to determine the link from which the client should be assigned an address)
   - Hop-count in Relay-forward message is set to 0.
   - If Relay Agent were to relay a message from a relay agent, it checks if the hop-count in the message is greater than or equal to HOP\_COUNT\_LIMIT, and discard if so. Else, hop\_count is incremented by 1.
 3. DHCPv6 server received the SOLICIT message, refers to the Relay Agent IP and select an IP address to allocate to the DHCPv6 client.
-4. The DHCPv6 server constructs a RELAY-REPLY message that embeds the advertise messages, and sends it to the DHCPv6 relay agent.
-5. DHCPv6 client receives ADVERTISE message and relays a REQUEST message to the DHCPv6 relay agent.
-6. DHCPv6 relay agent constructs REQUEST message into a RELAY-FORWARD message, and relays to DHCPv6 server.
-7. DHCPv6 server receives the REQUEST message, and sends a REPLY message to the relay agent. Server creates a Relay-reply message that includes a Relay Message option containing the the REPLY message and sends it to the relay agent.
-8. DHCPv6 relay agent extracts message and relays the message to the address contained in the peer-address field of the Relay-reply message.
-9. DHCPv6 client receives the REPLY message that contains the desired IP address.
+4. The DHCPv6 server constructs a RELAY-REPLY message that embeds the ADVERTISE messages, and sends it to the DHCPv6 relay agent.
+5. DHCPv6 relay agent extracts ADVERTISE message from RELAY-REPLY message and forwards it to the client.
+6. DHCPv6 client receives ADVERTISE message and relays a REQUEST message to the DHCPv6 relay agent.
+7. DHCPv6 relay agent constructs REQUEST message into a RELAY-FORWARD message, and relays to DHCPv6 server.
+8. DHCPv6 server receives the REQUEST message, and sends a REPLY message to the relay agent. Server creates a Relay-reply message that includes a Relay Message option containing the the REPLY message and sends it to the relay agent.
+9. DHCPv6 relay agent extracts message and relays the message to the address contained in the peer-address field of the Relay-reply message.
+10. DHCPv6 client receives the REPLY message that contains the desired IP address.
 
 ![image](https://user-images.githubusercontent.com/42761586/117859842-5fd13b00-b244-11eb-9297-c2674d128dd9.png)