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Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
Understanding DHCP Relay Agents
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Understanding DHCP Relay Agents

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  • 1. Netmanias Technical Document: Understanding DHCP Relay Agents 가 www.nmcgroups.com Understanding DHCP Relay Agents Table of Contents I. Overview II. Why are DHCP Relay Agents Needed in DHCP Operations? III. Basic Operations of DHCP Relay Agents Appendix − Format of DHCP Messages in a Network with DHCP Relay Agents This document is the third in our DHCP technical documents series and explains the basic operations of a DHCP relay agent. It covers the procedures of IP address allocation/lease, IP address renewal and IP address release using a DHCP relay agent when a device and a DHCP server are located in different subnets. It also describes how the parameters of DHCP messages are replaced by a DHCP relay agent during such procedures. November 5, 2013 www.netmanias.com NMC Consulting Group (tech@netmanias.com) About NMC Consulting Group NMC Consulting Group is an advanced and professional network consulting company, specializing in IP network areas (e.g., FTTH, Metro Ethernet and IP/MPLS), service areas (e.g., IPTV, IMS and CDN), and wireless network areas (e.g., Mobile WiMAX, LTE and Wi-Fi) since 2002. Copyright © 2002-2013 NMC Consulting Group. All rights reserved.
  • 2. Understanding DHCP Relay Agents Abbreviations ACK DA DHCP IHL IP MAC SA TOS TTL UDP Acknowledgement Destination Address Dynamic Host Configuration Protocol Internet Header Length Internet Protocol Medium Access Control Source Address Type of Service Time To Live User Datagram Protocol 1
  • 3. Understanding DHCP Relay Agents I. Overview This document provides a technical understanding of a DHCP relay agent that is required in a multisubnet environment where a DHCP client and a DHCP server reside in different subnets. Chapter II explains why these DHCP relay agents are needed in DHCP operations. Chapter III describes the basic principles of DHCP operations using a DHCP relay agent. Finally, in Appendix, specific message parameters used by DHCP relay agents in each DHCP procedure will be presented. Before you read this document it is recommended that you refer to the companion documents, “Understanding the Basic Operations of DHCP” [2] and “Understanding the Detailed Operations of DHCP” [3]. II. Why are DHCP Relay Agents Needed in DHCP Operations? Generally, DHCP messages are broadcasted. So, in order for the messages to be exchanged between a DHCP client (PC) and DHCP server, both the client and server have to reside on the same subnet. That is because routers do not forward any broadcast IP packet (i.e. one with a destination MAC address of FF:FF:FF:FF:FF:FF and a destination IP address of 255.255.255.255) to other interfaces. Thus a broadcast DHCP packet sent by a DHCP client cannot be delivered to DHCP server(s) on different subnet(s) through a router (shown in Figure 1 (a)). This restriction requires all individual subnets have its own DHCP server for DHCP operation, which is practically not feasible in network operators' networks or corporate computer networks (too many DHCP servers are required in the network!). To address this problem, the concept of a DHCP relay agent has long been adopted [1]. As shown in Figure 1 (b), enabling the DHCP relay agent function in the router allows DHCP messages to be exchanged between a DHCP client and DHCP server residing on different subnets.1 The core function of this DHCP relay agent is to convert a broadcast DHCP packet into a unicast one, and forward it to a DHCP server. 1 Generally, routers and L3 switches support all of DHCP relay agent functions. 2
  • 4. Understanding DHCP Relay Agents (a) Network w/o DHCP relay agent PC Broadcast Domain (1 Subnet) DHCP Server DHCP server unable to receive any DHCP messages L2 SW Internet Broadcast Domain (1 Subnet) L2 SW Router Broadcast packets discarded by router DHCP Discover/Request (Broadcast) (b) Network w/ DHCP relay agent PC1 Broadcast Domain (1.1.1.0/24) L2 SW 1.1.1.254/24 Broadcast Domain (2.1.1.0/254) DHCP messages delivered to DHCP server through DHCP relay agent 100.1.1.254/24 Internet Router DHCP relay agent, converting DHCP Relay Agent broadcast DHCP messages to unicast ones and forwarding to DHCP server L2 SW DHCP Discover/Request (Broadcast) 100.1.1.1 DHCP Server DHCP Discover/Request (Unicast) Figure 1. Comparison of DHCP operations between networks with and without a DHCP relay agent III. Basic Operations of DHCP Relay Agents This chapter describes how a PC (e.g. PC1) on “1.1.1.0/24” subnet, as shown in Figure 1 - (b), can communicate with the DHCP server using a DHCP relay agent for all DHCP operations, such as IP address allocation/lease, IP address renewal and IP address release. 3.1 IP Address Allocation/Lease Procedure The DHCP relay agent is located between a PC and DHCP server as shown in Figure 2. The DHCP relay agent receives DHCP Discover and Request messages broadcasted by the PC, and unicasts them directly to the DHCP server. At this point, the DHCP relay agent stores its IP address (the interface address at which it received the DHCP Discover/Request messages) in the “Relay Agent IP (=Gateway IP=giaddr)” field of the DHCP message to be relayed. The DHCP server unicasts a DHCP Offer/Ack message, with the destination IP address set as the relay agent IP address, to the DHCP relay agent. The DHCP relay agent, after checking the “Broadcast Flag” field of the received message, replaces the destination IP address with the IP address of the PC (Broadcast Flag=0) or with the broadcast IP address (Broadcast Flag=1) depending on the “Broadcast Flag” value. It also replaces the source IP address with the IP address of the DHCP relay agent, and forwards the modified message to the PC. 3
  • 5. Understanding DHCP Relay Agents Configuration: DHCP Server IP=100.1.1.1 PC MAC=m1 L2 Network PC DHCP Server IP=100.1.1.1 DHCP Server MAC=m5 Downlink MAC=m2 Uplink MAC=m3 Downlink IP=1.1.1.254 Relay Uplink IP=100.1.1.254 (Relay Agent IP) 1a DHCP Discover (Broadcast) 1b DHCP Discover (Unicast) Ethernet Header {DA=FF:FF:FF:FF:FF:FF, SA=m1}, IP Header {SIP=0.0.0.0, DIP=255.255.255.255}, DHCP Payload {Broadcast Flag=0 or 1, Relay Agent IP=0.0.0.0, Client MAC=m1} Ethernet Header {DA=m5, SA=m3}, IP Header {SIP=100.1.1.254, DIP=100.1.1.1}, DHCP Payload {Broadcast Flag=0 or 1, Relay Agent IP=1.1.1.254, Client MAC=m1} 2b DHCP Offer (Unicast or Broadcast) 2a DHCP Offer (Unicast) Ethernet Header {DA=m1 or FF:FF:FF:FF:FF:FF, SA=m2}, IP Header {SIP=1.1.1.254, DIP=1.1.1.10 or 255.255.255.255}, DHCP Payload {Broadcast Flag=0 or 1, Your IP=1.1.1.10, Relay Agent IP=1.1.1.254, Client MAC=m1, DHCP Server Identifier(54)=100.1.1.1, Options} Ethernet Header {DA=m3, SA=m5}, IP Header {SIP=100.1.1.1, DIP=1.1.1.254}, DHCP Payload {Broadcast Flag=0 or 1, Your IP=1.1.1.10, Relay Agent IP=1.1.1.254, Client MAC=m1, DHCP Server Identifier(54)=100.1.1.1, Options} 3a DHCP Request (Broadcast) 3b DHCP Request (Unicast) Ethernet Header {DA=FF:FF:FF:FF:FF:FF, SA=m1}, IP Header {SIP=0.0.0.0, DIP=255.255.255.255}, DHCP Payload {Broadcast Flag=0 or 1, Relay Agent IP=0.0.0.0, Client MAC=m1, DHCP Server Identifier(54)=100.1.1.1} Ethernet Header {DA=m5, SA=m3}, IP Header {SIP=100.1.1.254, DIP=100.1.1.1}, DHCP Payload {Broadcast Flag=0 or 1, Relay Agent IP=1.1.1.254, Client MAC=m1, DHCP Server Identifier(54)=100.1.1.1} 4b DHCP Ack (Unicast or Broadcast) 4a DHCP Ack (Unicast) Ethernet Header {DA=m1 or FF:FF:FF:FF:FF:FF, SA=m2}, IP Header {SIP=1.1.1.254, DIP=1.1.1.10 or 255.255.255.255}, DHCP Payload {Broadcast Flag=0 or 1, Your IP=1.1.1.10, Relay Agent IP=1.1.1.254, Client MAC=m1, DHCP Server Identifier(54)=100.1.1.1, Options} DHCP Ethernet Header {DA=m3, SA=m5}, IP Header {SIP=100.1.1.1, DIP=1.1.1.254}, DHCP Payload {Broadcast Flag=0 or 1, Your IP=1.1.1.10, Relay Agent IP=1.1.1.254, Client MAC=m1, DHCP Server Identifier(54)=100.1.1.1, Options} IP address=1.1.1.10 allocation completed Internet Access SIP=1.1.1.10 Figure 2. IP address allocation/lease procedure in the network with a DHCP relay agent 1. DHCP Discover As described in the references [2], “Understanding the Basic Operations of DHCP”, and [3], “Understanding the Detailed Operations of DHCP”, a DHCP client broadcasts a DHCP Discover message on the physical Ethernet subnet to discover all DHCP servers available on the subnet. Upon receiving packets that have a UDP destination port set to 67 (DHCP Discover/Request), a DHCP relay agent replaces the values in the fields of the packets as follows, and then unicasts the modified message to a DHCP server:  Destination MAC address: The broadcast MAC address sent by the PC is replaced with the DHCP server MAC address (m5).  Source MAC address: The PC MAC address (m1) is replaced with the uplink MAC address of the DHCP relay agent (m3).  Destination IP address: The broadcast IP address (255.255.255.255) sent by the PC is replaced with the DHCP server IP address (100.1.1.1).  Source IP address: The IP address sent by the PC (0.0.0.0) is replaced with the uplink IP address of the DHCP relay agent (100.1.1.254).  Relay Agent IP address: The IP address sent by the PC (0.0.0.0) is replaced with the downlink IP address of the DHCP relay agent (1.1.1.254), the IP address of the relay agent’s interface at which the DHCP Discover message was received. 4
  • 6. Understanding DHCP Relay Agents 2. DHCP Offer The DHCP server, by referring to the relay agent IP address (giaddr) in a DHCP Discover message, selects an IP address to allocate to the DHCP client from an IP pool, and sends a DHCP Offer message with the destination IP address set as the relay agent IP address2. The DHCP relay agent, when receiving the message, replaces the values in the fields of the packets as follows, and then sends the modified message on to the DHCP client (PC):  Destination MAC address: If the “Broadcast Flag” field of the DHCP Offer message sent by a DHCP server is 0 (zero), the DHCP relay agent replaces the value in this field with the PC MAC address (Client MAC field: m1), and unicasts the message. However, if the “Broadcast Flag” value is 1 (one), the relay agent replaces it with the broadcast MAC address (FF:FF:FF:FF:FF:FF), and broadcasts the message.  Source MAC address: The DHCP server MAC address (m5) is replaced with the downlink MAC address of the DHCP relay agent (m2).  Destination IP address: The DHCP server sends the DHCP Offer message, with the destination address set as the relay agent IP address in the DHCP Discover message (1.1.1.254), to a DHCP relay agent. If the “Broadcast Flag” value in the message is 0, the relay agent replaces the value with the IP address allocated to the PC (Your IP field: 1.1.1.10) for unicasting. However, if the “Broadcast Flag” value is 1, the relay agent replaces it with the broadcast IP address (255.255.255.255) for broadcasting.  Source IP address: The DHCP server IP address (100.1.1.1) is replaced with the downlink IP address of the DHCP relay agent (1.1.1.254). 3. DHCP Request The DHCP client (PC) which received the DHCP Offer message broadcasts a DHCP Request message on the physical Ethernet subnet to request network information data such as IP addresses. The DHCP relay agent, upon receiving this message, replaces the values in the fields (same as in the DHCP Discover message) of the packets as follows, and then unicasts the message to the DHCP server:  Destination MAC address: The broadcast MAC address sent by the PC is replaced with the DHCP server MAC address (m5).  Source MAC address: The PC MAC address (m1) is replaced with the uplink MAC address of the DHCP relay agent (m3).  Destination IP address: The broadcast IP address (255.255.255.255) sent by the PC is replaced with the DHCP server IP address (100.1.1.1).  Source IP address: The IP address sent by the PC (0.0.0.0) is replaced with the uplink IP address of the DHCP relay agent (100.1.1.254)  Relay Agent IP address: The IP address sent by the PC (0.0.0.0) is replaced with the downlink IP address of the DHCP relay agent (1.1.1.254), the IP address of the relay agent’s interface at which the DHCP Discover message was received. 2 Unless the IP address of a DHCP relay agent is set as “0.0.0.0”, a DHCP server always unicasts a DHCP Offer message to the DHCP relay agent regardless of the Broadcast Flag value. 5
  • 7. Understanding DHCP Relay Agents 4. DHCP Ack The DHCP server sends a DHCP Ack message with the destination IP address set as the relay agent IP address (giaddr)3. The DHCP relay agent, upon receiving this message, replaces the values in the fields of the packets as follows, and then unicasts the message to the DHCP client (PC):  Destination MAC address: If the “Broadcast Flag” field of the DHCP Ack message sent by the DHCP server is 0 (zero), the DHCP relay agent replaces the value in this field with the PC MAC address (Client MAC field: m1), and unicasts the message. However, if the “Broadcast Flag” value is 1 (one), the relay agent replaces it with the broadcast MAC address (FF:FF:FF:FF:FF:FF), and broadcasts the message.  Source MAC address: The DHCP server MAC address (m5) is replaced with the downlink MAC address of the DHCP relay agent (m2).  Destination IP address: The DHCP server sends the DHCP Ack message, with the destination address set as the relay agent IP address in the DHCP Request message (1.1.1.254), to a DHCP relay agent. If the “Broadcast Flag” value in the message is 0, the relay agent replaces the value with the IP address allocated to the PC (Your IP field: 1.1.1.10) for unicasting. However, if the “Broadcast Flag” value is 1, the relay agent replaces it with the broadcast IP address (255.255.255.255) for broadcasting.  Source IP address: The DHCP server IP address (100.1.1.1) is replaced with the downlink IP address of the DHCP relay agent (1.1.1.254). 3.2 IP Address Renewal Procedure According to the reference [1], a DHCP client (PC) keeps/stores the DHCP server IP address acquired through a DHCP Ack message (in the DHCP Server Identifier field) during the IP address allocation procedure. Then, if it needs to use the IP address beyond the lease duration, it sends a DHCP Request message to the DHCP server, through unicasting, not broadcasting. And the DHCP server, in respond to the message, unicasts a DHCP Ack message to the DHCP client. As such, in case DHCP messages are to be unicasted, the DHCP relay agent does not need to play its role (of converting a broadcast message to a unicast one) for DHCP operations. So, as can be seen in Figure 3, the DHCP relay agent is not involved in any DHCP operations during the IP address renewal procedure. 3 Unless the IP address of a DHCP relay agent is set as “0.0.0.0”, a DHCP server always unicasts a DHCP Ack message to the DHCP relay agent regardless of the Broadcast Flag value. 6
  • 8. Understanding DHCP Relay Agents PC MAC=m1 DHCP Server IP=100.1.1.1 PC Relay DHCP T1 Timer Expired (IP Address Renewal Time) 1 DHCP Request (Unicast) IP Header {SIP=1.1.1.10, DIP=100.1.1.1}, DHCP Payload {Client IP=1.1.1.10, Relay Agent IP=0.0.0.0, Client MAC=m1} 2 DHCP Ack (Unicast) IP Header {SIP=100.1.1.1, DIP=1.1.1.10}, DHCP Payload {Client IP=1.1.1.10, Your IP=1.1.1.10, Relay Agent IP=0.0.0.0, Client MAC=m1, DHCP Server Identifier(54)=100.1.1.1, Options} IP address=1.1.1.10 renewal completed Internet Access SIP=1.1.1.10 Figure 3. IP address renewal procedure in the network with a DHCP relay agent 1. DHCP Request The DHCP client (PC) unicasts a DHCP Request message with the destination IP address set as the IP address of a DHCP server. Thus, a DHCP relay agent does not receive this message. In other words, no field of the DHCP Request message is replaced by the DHCP relay agent during the IP address renewal procedure. 2. DHCP Ack The DHCP server unicasts a DHCP Ack message with the destination IP address set as the IP address of the DHCP client (PC). Again, a DHCP relay agent does not receive this message. In other words, no field of the DHCP Ack message is replaced by the DHCP relay agent during the IP address renewal procedure. 3.3 IP Address Release Procedure According to the reference [1], RFC 1542, when an IP address is released, a DHCP client (PC) unicasts a DHCP Release message to a DHCP server directly. Thus, a DHCP relay agent is not involved in the IP address release procedure as shown in Figure 4. PC MAC=m1 DHCP Server IP=100.1.1.1 PC Relay DHCP Internet Access SIP=1.1.1.10 DHCP client shut down (or “ipconfig /release”) 1 DHCP Release (Unicast) IP Header {SIP=1.1.1.10, DIP=100.1.1.1}, DHCP Payload {Client IP=1.1.1.10, Relay Agent IP=0.0.0.0, Client MAC=m1, DHCP Server Identifier(54)=100.1.1.1} IP address=1.1.1.10 release completed Figure 4. IP address release procedure in the network with a DHCP relay agent 7
  • 9. Understanding DHCP Relay Agents 1. DHCP Release A DHCP client unicasts a DHCP Release message with the destination IP address set as the IP address of a DHCP server. Thus a DHCP relay agent does not receive this message. In other words, no fields of the DHCP Ack message is replaced by the DHCP relay agent during the IP address release procedure. References [1] W. Wimer, “Clarifications and Extensions for the Bootstrap Protocol”, RFC 1542, Standard, October 1993. [2] Netmanias Technical Document, “Understanding the Basic Operations of DHCP”, October 2013, [3] Netmanias Technical Document, “Understanding the Detailed Operations of DHCP”, October 2013, 8
  • 10. Understanding DHCP Relay Agents Appendix − Format of DHCP Messages in a Network with DHCP Relay Agents This appendix provides specific examples of DHCP message parameters that are replaced by a DHCP relay agent during DHCP procedures. However, in case of IP address renewal and release procedures, a DHCP relay agent does NOT replace any part of DHCP messages. Thus, all messages related to those procedures are excluded in this appendix. DHCP Discover Message Downlink IP=1.1.1.254 Uplink IP=100.1.1.254 Downlink MAC=m2 Relay Downlink MAC=m3 L2 Network PC MAC=m1 PC DHCP Discover Message from PC to DHCP Relay Agent 0B 2B DHCP Server IP=100.1.1.1 DHCP Server MAC=m5 DHCP DHCP Discover Message from DHCP Relay Agent to DHCP Server 4B 0B 2B 4B Source MAC Address = PC MAC Address (m1) Source MAC Address = DHCP Relay Agent Uplink MAC (m3) Ethernet Destination MAC Address = DHCP Server MAC (m5) EtherType = 0x0800 (IP) Ver = 4 IHL = 5 TOS Identification IP EtherType = 0x0800 (IP) Total Length Flags Ver = 4 IHL = 5 Fragment Offset Protocol=17(UDP) Header Checksum Source IP Address = 0.0.0.0 TOS Identification TTL Total Length Flags Fragment Offset Protocol=17(UDP) Header Checksum Source IP Address = DHCP Relay Agent Uplink IP (100.1.1.254) Destination IP Address = 255.255.255.255 (Broadcast) Destination IP Address = DHCP Server IP (100.1.1.1) Source Port = 68 (bootpc) Destination Port = 67 (bootps) UDP Length UDP Destination Port = 67 (bootps) UDP Checksum UDP Length UDP Checksum HOPS OP code=1(Req.) HW Type=Ethernet HW Length = 6 Transaction ID (xid) Seconds Broadcast Flag (1b) = 1 Client IP address (ciaddr) = 0.0.0.0 HOPS Transaction ID (xid) Seconds Broadcast Flag (1b) = 1 Client IP address (ciaddr) = 0.0.0.0 Your IP Address (yiaddr) = 0.0.0.0 Server IP address (siaddr) = 0.0.0.0 Server IP address (siaddr) = 0.0.0.0 Gateway IP Address (giaddr) = 0.0.0.0 Gateway IP Address (giaddr) = DHCP Relay Agent Downlink IP (1.1.1.254) Client Hardware Address (16B) (chaddr) = PC MAC Address (m1) Client Hardware Address (16B) (chaddr) = PC MAC Address (m1) Server Name (64B) (sname) Server Name (64B) (sname) Filename (128B) Filename (128B) DHCP Message Type (Option 53) = 1 (DHCP Discover) DHCP Message Type (Option 53) = 1 (DHCP Discover) Client Identifier (Option 51) = PC MAC Address (m1) Client Identifier (Option 51) = PC MAC Address (m1) Parameter Request List (Option 55) = Option 1, 3, 6, ... Parameter Request List (Option 55) = Option 1, 3, 6, ... DHCP Message Payload Your IP Address (yiaddr) = 0.0.0.0 DHCP Message Payload UDP Source Port = 68 (bootpc) OP code=1(Req.) HW Type=Ethernet HW Length = 6 IP TTL Ethernet Destination MAC Address = FF:FF:FF:FF:FF:FF (Broadcast) Figure 5. DHCP Discover message in IP address allocation/lease procedure Ethernet Header  Destination MAC Address: The Broadcast MAC address (0xFFFFFFFFFFFF) is replaced with the DHCP server MAC address (m5).  Source MAC Address: The PC MAC address (m1) is replaced with the uplink MAC address of the DHCP relay agent (m3). IP Header  Source IP Address: The IP address (0.0.0.0) is replaced with the uplink IP address of the DHCP relay agent (100.1.1.254).  Destination IP Address: The broadcast IP address (255.255.255.255) is replaced with the DHCP server IP address (100.1.1.1). DHCP Message Payload  Gateway IP Address (giaddr): The IP address (0.0.0.0) is replaced with the downlink IP address of the DHCP relay agent (1.1.1.254) that receives a DHCP Discover message from PC. 9
  • 11. Understanding DHCP Relay Agents DHCP Offer Message Uplink IP=100.1.1.254 Downlink IP=1.1.1.254 Downlink MAC=m2 Relay Downlink MAC=m3 L2 Network PC MAC=m1 PC DHCP Offer Message from DHCP Relay Agent to PC 0B 2B DHCP Server IP=100.1.1.1 DHCP Server MAC=m5 DHCP DHCP Offer Message from DHCP Server to DHCP Relay Agent 4B 0B 2B 4B Source MAC Address = DHCP Relay Agent MAC Address (m2) Ethernet Destination MAC Address = DHCP Relay Agent Uplink MAC (m3) Source MAC Address = DHCP Server MAC (m5) EtherType = 0x0800 (IP) Ver = 4 IHL = 5 TOS Identification IP EtherType = 0x0800 (IP) Total Length Flags Ver = 4 IHL = 5 Fragment Offset Protocol=17(UDP) Header Checksum TOS Identification TTL Total Length Flags Fragment Offset Protocol=17(UDP) Header Checksum Source IP Address = DHCP Relay Agent Downlink IP (1.1.1.254) Source IP Address = DHCP Server IP Address (100.1.1.1) Destination IP Address = 255.255.255.255 (Broadcast) Destination IP Address = DHCP Relay Agent Downlink IP (1.1.1.254) Destination Port = 68 (bootpc) Source Port = 67 (bootps) Destination Port = 68 (bootpc) UDP Length UDP Checksum UDP Length UDP Checksum OP code=2(Reply) HW Type=Ethernet HW Length = 6 HOPS OP code=2(Reply) HW Type=Ethernet HW Length = 6 Transaction ID (xid) Seconds Broadcast Flag (1b) = 1 Client IP address (ciaddr) = 0.0.0.0 UDP Source Port = 67 (bootps) UDP IP TTL Ethernet Destination MAC Address = FF:FF:FF:FF:FF:FF (Broadcast) HOPS Transaction ID (xid) Seconds Broadcast Flag (1b) = 1 Client IP address (ciaddr) = 0.0.0.0 Gateway IP Address (giaddr) = DHCP Relay Agent Downlink IP (1.1.1.254) Gateway IP Address (giaddr) = DHCP Relay Agent Downlink IP (1.1.1.254) Client Hardware Address (16B) (chaddr) = PC MAC Address (m1) Client Hardware Address (16B) (chaddr) = PC MAC Address (m1) Server Name (64B) (sname) Server Name (64B) (sname) Filename (128B) Filename (128B) DHCP Message Type (Option 53) = 2 (DHCP Offer) DHCP Message Type (Option 53) = 2 (DHCP Offer) Subnet Mask (Option 1) = 255.255.255.0 (/24) Subnet Mask (Option 1) = 255.255.255.0 (/24) Router IP (Option 3) = 1.1.1.254 Router IP (Option 3) = 1.1.1.254 Domain Name Server IP (Option 6) = 10.1.1.1, 10.1.1.2 Domain Name Server IP (Option 6) = 10.1.1.1, 10.1.1.2 IP Address Lease Time (Option 51) = 3,600 seconds (1 hour) IP Address Lease Time (Option 51) = 3,600 seconds (1 hour) DHCP Server Identifier (Option 54) = 100.1.1.1 DHCP Server Identifier (Option 54) = 100.1.1.1 DHCP Message Payload Your IP Address (yiaddr) = 1.1.1.10 Server IP address (siaddr) = 0.0.0.0 DHCP Message Payload Your IP Address (yiaddr) = 1.1.1.10 Server IP address (siaddr) = 0.0.0.0 Figure 6. DHCP Offer message in IP address allocation/lease procedure Ethernet Header  Destination MAC Address: The uplink MAC address of the DHCP relay agent (m3) is replaced with the broadcast MAC address (0xFFFFFFFFFFFF). Note: In this example, as we assumed that the “Broadcast Flag” value is set to 1, the relay agent broadcasts the message.  Source MAC Address: The DHCP server MAC address (m5) is replaced with the downlink MAC address of the DHCP relay agent (m2). IP Header  Source IP Address: The DHCP server IP address (100.1.1.1) is replaced with the downlink IP address of the DHCP relay agent (1.1.1.254).  Destination IP Address: The downlink IP address of the DHCP relay agent (giaddr=1.1.1.254) is replaced with the broadcast IP address (255.255.255.255). Note: In this example, as we assumed that the “Broadcast Flag” value is set to 1, the relay agent broadcasts the message. 10
  • 12. Understanding DHCP Relay Agents DHCP Request Message Uplink IP=100.1.1.254 Downlink IP=1.1.1.254 Downlink MAC=m2 Relay Downlink MAC=m3 L2 Network PC MAC=m1 PC DHCP Request Message from PC to DHCP Relay Agent 0B 2B DHCP Server IP=100.1.1.1 DHCP Server MAC=m5 DHCP DHCP Request Message from DHCP Relay Agent to DHCP Server 4B 0B 2B 4B Source MAC Address = PC MAC Address (m1) Source MAC Address = DHCP Relay Agent Uplink MAC (m3) Ethernet Destination MAC Address = DHCP Server MAC (m5) EtherType = 0x0800 (IP) Ver = 4 IHL = 5 TOS Identification IP EtherType = 0x0800 (IP) Total Length Flags Ver = 4 IHL = 5 Fragment Offset Protocol=17(UDP) Header Checksum Source IP Address = 0.0.0.0 TOS Identification TTL Total Length Flags Fragment Offset Protocol=17(UDP) Header Checksum Source IP Address = DHCP Relay Agent Uplink IP (100.1.1.254) Destination IP Address = 255.255.255.255 (Broadcast) Destination IP Address = DHCP Server IP (100.1.1.1) Source Port = 68 (bootpc) Destination Port = 67 (bootps) UDP Length UDP Destination Port = 67 (bootps) UDP Checksum UDP Length UDP Checksum HOPS OP code=1(Req.) HW Type=Ethernet HW Length = 6 Transaction ID (xid) Seconds Broadcast Flag (1b) = 1 Client IP address (ciaddr) = 0.0.0.0 HOPS Transaction ID (xid) Seconds Broadcast Flag (1b) = 1 Client IP address (ciaddr) = 0.0.0.0 Your IP Address (yiaddr) = 0.0.0.0 Your IP Address (yiaddr) = 0.0.0.0 Server IP address (siaddr) = 0.0.0.0 Server IP address (siaddr) = 0.0.0.0 Gateway IP Address (giaddr) = DHCP Relay Agent Downlink IP (1.1.1.254) Client Hardware Address (16B) (chaddr) = PC MAC Address (m1) Server Name (64B) (sname) Server Name (64B) (sname) Filename (128B) Filename (128B) DHCP Message Type (Option 53) = 3 (DHCP Request) DHCP Message Type (Option 53) = 3 (DHCP Request) Client Identifier (Option 51) = PC MAC Address (m1) Requested IP Address (Option 50) = 1.1.1.10 Requested IP Address (Option 50) = 1.1.1.10 DHCP Server Identifier (Option 54) = 100.1.1.1 DHCP Server Identifier (Option 54) = 100.1.1.1 Parameter Request List (Option 55) = Option 1, 3, 6, ... Parameter Request List (Option 55) = Option 1, 3, 6, ... DHCP Message Payload Gateway IP Address (giaddr) = 0.0.0.0 Client Hardware Address (16B) (chaddr) = PC MAC Address (m1) Client Identifier (Option 51) = PC MAC Address (m1) DHCP Message Payload UDP Source Port = 68 (bootpc) OP code=1(Req.) HW Type=Ethernet HW Length = 6 IP TTL Ethernet Destination MAC Address = FF:FF:FF:FF:FF:FF (Broadcast) Figure 7. DHCP Request message in IP address allocation/lease procedure Ethernet Header  Destination MAC Address: The broadcast MAC address (0xFFFFFFFFFFFF) is replaced with the DHCP server MAC address (m5).  Source MAC Address: The PC MAC address (m1) is replaced with the uplink MAC address of the DHCP relay agent (m3). IP Header  Source IP Address: The IP address (0.0.0.0) is replaced with the uplink IP address of the DHCP relay agent (100.1.1.254).  Destination IP Address: The broadcast IP address (255.255.255.255) is replaced with the DHCP server IP address (100.1.1.1). DHCP Message Payload  Gateway IP Address (giaddr): The IP address (0.0.0.0) is replaced with the downlink IP address of the DHCP relay agent (1.1.1.254) that receives a DHCP Request message from PC. 11
  • 13. Understanding DHCP Relay Agents DHCP Ack Message Uplink IP=100.1.1.254 Downlink IP=1.1.1.254 Downlink MAC=m2 Relay Downlink MAC=m3 L2 Network PC MAC=m1 PC DHCP Ack Message from DHCP Relay Agent to PC 0B 2B DHCP Server IP=100.1.1.1 DHCP Server MAC=m5 DHCP DHCP Ack Message from DHCP Server to DHCP Relay Agent 4B 0B 2B 4B Source MAC Address = DHCP Relay Agent MAC Address (m2) Ethernet Destination MAC Address = DHCP Relay Agent Uplink MAC (m3) Source MAC Address = DHCP Server MAC (m5) EtherType = 0x0800 (IP) Ver = 4 IHL = 5 TOS IP Flags Ver = 4 IHL = 5 Fragment Offset Protocol=17(UDP) Header Checksum TOS Identification TTL Total Length Flags Fragment Offset Protocol=17(UDP) Header Checksum Source IP Address = DHCP Relay Agent Downlink IP (1.1.1.254) Source IP Address = DHCP Server IP Address (100.1.1.1) Destination IP Address = 255.255.255.255 (Broadcast) Destination IP Address = DHCP Relay Agent Downlink IP (1.1.1.254) Destination Port = 68 (bootpc) Source Port = 67 (bootps) Destination Port = 68 (bootpc) UDP Length UDP Checksum UDP Length UDP Checksum OP code=2(Reply) HW Type=Ethernet HW Length = 6 HOPS OP code=2(Reply) HW Type=Ethernet HW Length = 6 Transaction ID (xid) Seconds Broadcast Flag (1b) = 1 Client IP address (ciaddr) = 0.0.0.0 UDP Source Port = 67 (bootps) UDP IP TTL EtherType = 0x0800 (IP) Total Length Identification Ethernet Destination MAC Address = FF:FF:FF:FF:FF:FF (Broadcast) HOPS Transaction ID (xid) Seconds Broadcast Flag (1b) = 1 Client IP address (ciaddr) = 0.0.0.0 Gateway IP Address (giaddr) = DHCP Relay Agent Downlink IP (1.1.1.254) Gateway IP Address (giaddr) = DHCP Relay Agent Downlink IP (1.1.1.254) Client Hardware Address (16B) (chaddr) = PC MAC Address (m1) Client Hardware Address (16B) (chaddr) = PC MAC Address (m1) Server Name (64B) (sname) Server Name (64B) (sname) Filename (128B) Filename (128B) DHCP Message Type (Option 53) = 5 (DHCP Ack) DHCP Message Type (Option 53) = 5 (DHCP Ack) Subnet Mask (Option 1) = 255.255.255.0 (/24) Subnet Mask (Option 1) = 255.255.255.0 (/24) Router IP (Option 3) = 1.1.1.254 Router IP (Option 3) = 1.1.1.254 Domain Name Server IP (Option 6) = 10.1.1.1, 10.1.1.2 Domain Name Server IP (Option 6) = 10.1.1.1, 10.1.1.2 IP Address Lease Time (Option 51) = 3,600 seconds (1 hour) IP Address Lease Time (Option 51) = 3,600 seconds (1 hour) DHCP Server Identifier (Option 54) = 100.1.1.1 DHCP Server Identifier (Option 54) = 100.1.1.1 DHCP Message Payload Your IP Address (yiaddr) = 1.1.1.10 Server IP address (siaddr) = 0.0.0.0 DHCP Message Payload Your IP Address (yiaddr) = 1.1.1.10 Server IP address (siaddr) = 0.0.0.0 Figure 8. DHCP Ack message in IP address allocation/lease procedure Ethernet Header  Destination MAC Address: The uplink MAC address of the DHCP relay agent is replaced with the broadcast MAC address (0xFFFFFFFFFFFF). Note: In this example, as we assumed that the “Broadcast Flag” is set to 1, the relay agent broadcasts the message.  Source MAC Address: The DHCP server MAC address (m5) is replaced with the downlink MAC address of the DHCP relay agent (m2). IP Header  Source IP Address: The DHCP server IP address (100.1.1.1) is replaced with the downlink IP address of the DHCP relay agent (1.1.1.254).  Destination IP Address: The downlink IP address of the DHCP relay agent (giaddr=1.1.1.254) is replaced with the broadcast IP address (255.255.255.255). Note: In this example, as we assumed that the “Broadcast Flag” value is set to 1, the relay agent broadcasts the message. 12
  • 14. Understanding DHCP Relay Agents Netmanias Research and Consulting Scope 99 00 01 02 03 04 05 06 07 08 09 10 11 12 13 eMBMS/Mobile IPTV CDN/Mobile CDN Transparent Caching BSS/OSS Services Cable TPS Voice/Video Quality IMS Policy Control/PCRF IPTV/TPS LTE Mobile Network Mobile WiMAX Carrier WiFi LTE Backaul Data Center Migration Carrier Ethernet FTTH Wireline Network Data Center Metro Ethernet MPLS IP Routing Visit http://www.netmanias.com to view and download more technical documents. About NMC Consulting Group NMC Consulting Group is an advanced and professional network consulting company, specializing in IP network areas (e.g., FTTH, Metro Ethernet and IP/MPLS), service areas (e.g., IPTV, IMS and CDN), and wireless network areas (e.g., Mobile WiMAX, LTE and Wi-Fi) since 2002. Copyright © 2002-2013 NMC Consulting Group. All rights reserved. 0

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