Arp Header Format For Essay

If you learned about the OSI Model and encapsulation / decapsulation you know that when two computers on the LAN want to communicate with each other the following will happen:

  • An IP packet is created with a source and destination IP address carrying the data from an application.
  • The IP packet will be encapsulated in an Ethernet frame with a source and destination MAC address.

The sending computer will of course know its source MAC address but how does it know the destination MAC address? That’s where ARP comes into play. Let me show you an example:

In the picture above we have two computers, H1 and H2 and you can see their IP addresses and their MAC addresses.

We are sitting behind H1, open up a command prompt and type:

C:UsersH1>ping 192.168.1.2 Pinging 192.168.1.2 with 32 bytes of data: Reply from 192.168.1.2: bytes=32 time=15ms TTL=57 Reply from 192.168.1.2: bytes=32 time=15ms TTL=57 Reply from 192.168.1.2: bytes=32 time=14ms TTL=57 Reply from 192.168.1.2: bytes=32 time=17ms TTL=57 Ping statistics for 192.168.1.2: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 14ms, Maximum = 17ms, Average = 15ms

You know about the OSI-model and also know we have to go through all the layers.

Ping uses the ICMP protocol and IP uses the network layer (layer 3). Our IP packet will have a source IP address of 192.168.1.1 and a destination IP address of 192.168.1.2. Next step will be to put our IP packet in an Ethernet frame where we set our source MAC address AAA and destination MAC address BBB.

Now wait a second…how does H1 know about the MAC address of H2? We know the IP address because we typed it but there is no way for H1 to know the MAC address of H2. There is another protocol we have that will solve this problem for us, it’s called ARP (Address Resolution Protocol). Let me show you how it works:

C:UsersH1>arp -a Interface: 192.168.1.1 --- 0xb   Internet Address      Physical Address      Type   192.168.1.2          00-0c-29-63-af-d0     dynamic   192.168.1 .255       ff-ff-ff-ff-ff-ff     static   224.0.0.22            01-00-5e-00-00-16     static   224.0.0.252           01-00-5e-00-00-fc     static   239.255.255.250       01-00-5e-7f-ff-fa     static   255.255.255.255       ff-ff-ff-ff-ff-ff     static

In the example above you see an example of an ARP table on a H1. As you can see there is only one entry, this computer has learned that the IP address 192.168.1.2 has been mapped to the MAC address 00:0C:29:63:AF:D0.

Let’s take a more detailed look at ARP and how it functions:

In this example we have two computers and you can see their IP address and MAC address. We are sitting behind H1 and we want to send a ping to H2. The ARP table is empty so we have no clue what the MAC address of H2 is. The first thing that will happen is that H1 will send an ARP Request. This message basically says “Who has 192.168.1.2 and what is your MAC address?” Since we don’t know the MAC address we will use the broadcast MAC address for the destination (FF:FF:FF:FF:FF:FF). This message will reach all computers in the network.

 

H2 will reply with a message ARP Reply and is basically saying “that’s me! And this is my MAC address”. H1 can now add the MAC address to its ARP table and start forwarding data towards H2.

If you want to see this in action you can look at it in Wireshark:

Above you see the ARP request for H1 that is looking for the IP address of H2. The source MAC address is the MAC address of H1, the destination MAC address is “Broadcast” so it will be flooded on the network.

The second packet is the ARP reply. H2 will send its MAC address to H1. Here’s a detailed look:

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EXPERIMENTAL

Internet Research Task Force (IRTF) RJ Atkinson Request for Comments: 6747 Consultant Category: Experimental SN Bhatti ISSN: 2070-1721 U. St Andrews November 2012 Address Resolution Protocol (ARP)for the Identifier-Locator Network Protocol for IPv4 (ILNPv4) Abstract This document defines an Address Resolution Protocol (ARP) extension to support the Identifier-Locator Network Protocol for IPv4 (ILNPv4). ILNP is an experimental, evolutionary enhancement to IP. This document is a product of the IRTF Routing Research Group. Status of This Memo This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation. This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Research Task Force (IRTF). The IRTF publishes the results of Internet-related research and development activities. These results might not be suitable for deployment. This RFC represents the individual opinion(s) of one or more members of the Routing Research Group of the Internet Research Task Force (IRTF). Documents approved for publication by the IRSG are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6747. Atkinson & Bhatti Experimental [Page 1]
RFC 6747 ILNPv4 ARP November 2012 Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. This document may not be modified, and derivative works of it may not be created, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction ....................................................31.1. ILNP Document Roadmap ......................................31.2. Terminology ................................................52. ARP Extensions for ILNPv4 .......................................52.1. ILNPv4 ARP Request Packet Format ...........................52.2. ILNPv4 ARP Reply Packet Format .............................72.3. Operation and Implementation of ARP for ILNPv4 .............83. Security Considerations .........................................94. IANA Considerations .............................................95. References .....................................................105.1. Normative References ......................................105.2. Informative References ....................................116. Acknowledgements ...............................................11Atkinson & Bhatti Experimental [Page 2]
RFC 6747 ILNPv4 ARP November 20121. Introduction This document is part of the ILNP document set, which has had extensive review within the IRTF Routing RG. ILNP is one of the recommendations made by the RG Chairs. Separately, various refereed research papers on ILNP have also been published during this decade. So, the ideas contained herein have had much broader review than the IRTF Routing RG. The views in this document were considered controversial by the Routing RG, but the RG reached a consensus that the document still should be published. The Routing RG has had remarkably little consensus on anything, so virtually all Routing RG outputs are considered controversial. At present, the Internet research and development community are exploring various approaches to evolving the Internet Architecture to solve a variety of issues including, but not limited to, scalability of inter-domain routing [RFC4984]. A wide range of other issues (e.g., site multihoming, node multihoming, site/subnet mobility, node mobility) are also active concerns at present. Several different classes of evolution are being considered by the Internet research and development community. One class is often called "Map and Encapsulate", where traffic would be mapped and then tunnelled through the inter-domain core of the Internet. Another class being considered is sometimes known as "Identifier/Locator Split". This document relates to a proposal that is in the latter class of evolutionary approaches. The Identifier Locator Network Protocol (ILNP) is a proposal for evolving the Internet Architecture. It differs from the current Internet Architecture primarily by deprecating the concept of an IP Address, and instead defining two new objects, each having crisp syntax and semantics. The first new object is the Locator, a topology-dependent name for a subnetwork. The other new object is the Identifier, which provides a topology-independent name for a node. 1.1. ILNP Document Roadmap This document describes extensions to ARP for use with ILNPv4. The ILNP architecture can have more than one engineering instantiation. For example, one can imagine a "clean-slate" engineering design based on the ILNP architecture. In separate documents, we describe two specific engineering instances of ILNP. The term ILNPv6 refers precisely to an instance of ILNP that Atkinson & Bhatti Experimental [Page 3]
RFC 6747 ILNPv4 ARP November 2012 is based upon, and backwards compatible with, IPv6. The term ILNPv4 refers precisely to an instance of ILNP that is based upon, and backwards compatible with, IPv4. Many engineering aspects common to both ILNPv4 and ILNPv6 are described in [RFC6741]. A full engineering specification for either ILNPv6 or ILNPv4 is beyond the scope of this document. Readers are referred to other related ILNP documents for details not described here: a) [RFC6740] is the main architectural description of ILNP, including the concept of operations. b) [RFC6741] describes engineering and implementation considerations that are common to both ILNPv4 and ILNPv6. c) [RFC6742] defines additional DNS resource records that support ILNP. d) [RFC6743] defines a new ICMPv6 Locator Update message used by an ILNP node to inform its correspondent nodes of any changes to its set of valid Locators. e) [RFC6744] defines a new IPv6 Nonce Destination Option used by ILNPv6 nodes (1) to indicate to ILNP correspondent nodes (by inclusion within the initial packets of an ILNP session) that the node is operating in the ILNP mode and (2) to prevent off-path attacks against ILNP ICMP messages. This Nonce is used, for example, with all ILNP ICMPv6 Locator Update messages that are exchanged among ILNP correspondent nodes. f) [RFC6745] defines a new ICMPv4 Locator Update message used by an ILNP node to inform its correspondent nodes of any changes to its set of valid Locators. g) [RFC6746] defines a new IPv4 Nonce Option used by ILNPv4 nodes to carry a security nonce to prevent off-path attacks against ILNP ICMP messages and also defines a new IPv4 Identifier Option used by ILNPv4 nodes. h) [RFC6748] describes optional engineering and deployment functions for ILNP. These are not required for the operation or use of ILNP and are provided as additional options. Atkinson & Bhatti Experimental [Page 4]
RFC 6747 ILNPv4 ARP November 20121.2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. ARP Extensions for ILNPv4 ILNP for IPv4 (ILNPv4) is merely a different instantiation of the ILNP architecture, so it retains the crisp distinction between the Locator and the Identifier. As with ILNPv6, only the Locator values are used for routing and forwarding ILNPv4 packets [RFC6740]. As with ILNP for IPv6 (ILNPv6), when ILNPv4 is used for a network-layer session, the upper-layer protocols (e.g., TCP/UDP pseudo-header checksum, IPsec Security Association) bind only to the Identifiers, never to the Locators [RFC6741]. However, just as the packet format for IPv4 is different to IPv6, so the engineering details for ILNPv4 are different also. While ILNPv6 is carefully engineered to be fully backwards-compatible with IPv6 Neighbor Discovery, ILNPv4 relies upon an extended version of the Address Resolution Protocol (ARP) [RFC826], which is defined here. While ILNPv4 could have been engineered to avoid changes in ARP, that would have required that the ILNPv4 Locator (i.e., L32) have slightly different semantics, which was architecturally undesirable. The packet formats used are direct extensions of the existing widely deployed ARP Request (OP code 1) and ARP Reply (OP code 2) packet formats. This design was chosen for practical engineering reasons (i.e., to maximise code reuse), rather than for maximum protocol design purity. We anticipate that ILNPv6 is much more likely to be widely implemented and deployed than ILNPv4. However, having a clear definition of ILNPv4 helps demonstrate the difference between architecture and engineering, and also demonstrates that the common ILNP architecture can be instantiated in different ways with different existing network-layer protocols. 2.1. ILNPv4 ARP Request Packet Format The ILNPv4 ARP Request is an extended version of the widely deployed ARP Request (OP code 1). For experimentation purposes, the ILNPv4 ARP Request OP code uses decimal value 24. It is important to note that decimal value 24 is a pre-defined, shared-use experimental OP code for ARP [RFC5494], and is not Atkinson & Bhatti Experimental [Page 5]
RFC 6747 ILNPv4 ARP November 2012 uniquely assigned to ILNPv4 ARP Requests. The ILNPv4 ARP Request extension permits the Node Identifier (NID) values to be carried in the ARP message, in addition to the node's 32-bit Locator (L32) values [RFC6742]. 0 7 15 23 31 +--------+--------+--------+--------+ | HT | PT | +--------+--------+--------+--------+ | HAL | PAL | OP | +--------+--------+--------+--------+ | S_HA (bytes 0-3) | +--------+--------+--------+--------+ | S_HA (bytes 4-5)|S_L32 (bytes 0-1)| +--------+--------+--------+--------+ |S_L32 (bytes 2-3)|S_NID (bytes 0-1)| +--------+--------+--------+--------+ | S_NID (bytes 2-5) | +--------+--------+--------+--------+ |S_NID (bytes 6-7)| T_HA (bytes 0-1)| +--------+--------+--------+--------+ | T_HA (bytes 3-5) | +--------+--------+--------+--------+ | T_L32 (bytes 0-3) | +--------+--------+--------+--------+ | T_NID (bytes 0-3) | +--------+--------+--------+--------+ | T_NID (bytes 4-7) | +--------+--------+--------+--------+ Figure 2.1: ILNPv4 ARP Request packet format In Figure 2.1, the fields are as follows: HT Hardware Type (*) PT Protocol Type (*) HAL Hardware Address Length (*) PAL Protocol Address Length (uses new value 12) OP Operation Code (uses experimental value OP_EXP1=24) S_HA Sender Hardware Address (*) S_L32 Sender L32 (* same as Sender IPv4 address for ARP) S_NID Sender Node Identifier (8 bytes) T_HA Target Hardware Address (*) T_L32 Target L32 (* same as Target IPv4 address for ARP) T_NID Target Node Identifier (8 bytes) Atkinson & Bhatti Experimental [Page 6]
RFC 6747 ILNPv4 ARP November 2012 The changed OP code indicates that this is ILNPv4 and not IPv4. The semantics and usage of the ILNPv4 ARP Request are identical to the existing ARP Request (OP code 2), except that the ILNPv4 ARP Request is sent only by nodes that support ILNPv4. The field descriptions marked with "*" should have the same values as for ARP as used for IPv4. 2.2. ILNPv4 ARP Reply Packet Format The ILNPv4 ARP Reply is an extended version of the widely deployed ARP Reply (OP code 2). For experimentation purposes, the ILNPv4 ARP Request OP code uses decimal value 25. It is important to note that decimal value 25 is a pre-defined, shared-use experimental OP code for ARP [RFC5494], and is not uniquely assigned to ILNPv4 ARP Requests. The ILNPv4 ARP Reply extension permits the Node Identifier (NID) values to be carried in the ARP message, in addition to the node's 32-bit Locator (L32) values [RFC6742]. 0 7 15 23 31 +--------+--------+--------+--------+ | HT | PT | +--------+--------+--------+--------+ | HAL | PAL | OP | +--------+--------+--------+--------+ | S_HA (bytes 0-3) | +--------+--------+--------+--------+ | S_HA (bytes 4-5)|S_L32 (bytes 0-1)| +--------+--------+--------+--------+ |S_L32 (bytes 2-3)|S_NID (bytes 0-1)| +--------+--------+--------+--------+ | S_NID (bytes 2-5) | +--------+--------+--------+--------+ |S_NID (bytes 6-7)| T_HA (bytes 0-1)| +--------+--------+--------+--------+ | T_HA (bytes 3-5) | +--------+--------+--------+--------+ | T_L32 (bytes 0-3) | +--------+--------+--------+--------+ | T_NID (bytes 0-3) | +--------+--------+--------+--------+ | T_NID (bytes 4-7) | +--------+--------+--------+--------+ Figure 2.2: ILNPv4 ARP Reply packet format Atkinson & Bhatti Experimental [Page 7]
RFC 6747 ILNPv4 ARP November 2012 In Figure 2.2, the fields are as follows: HT Hardware Type (*) PT Protocol Type (*) HAL Hardware Address Length (*) PAL Protocol Address Length (uses new value 12) OP Operation Code (uses experimental value OP_EXP2=25) S_HA Sender Hardware Address (*) S_L32 Sender L32 (* same as Sender IPv4 address for ARP) S_NID Sender Node Identifier (8 bytes) T_HA Target Hardware Address (*) T_L32 Target L32 (* same as Target IPv4 address for ARP) T_NID Target Node Identifier (8 bytes) The changed OP code indicates that this is ILNPv4 and not IPv4. The semantics and usage of the ILNPv4 ARP Reply are identical to the existing ARP Reply (OP code 2), except that the ILNPv4 ARP Reply is sent only by nodes that support ILNPv4. The field descriptions marked with "*" should have the same values as for ARP as used for IPv4. 2.3. Operation and Implementation of ARP for ILNPv4 The operation of ARP for ILNPv4 is almost identical to that for IPv4. Essentially, the key differences are: a) where an IPv4 ARP Request would use IPv4 addresses, an ILNPv4 ARP Request MUST use: 1. a 32-bit L32 value (_L32 suffixes in Figures 2.1 and 2.2) 2. a 64-bit NID value (_NID suffixes in Figures 2.1 and 2.2) b) where an IPv4 ARP Reply would use IPv4 addresses, an ILNPv4 ARP Reply MUST use: 1. a 32-bit L32 value (_L32 suffixes in Figures 2.1 and 2.2) 2. a 64-bit NID value (_NID suffixes in Figures 2.1 and 2.2) As the OP codes 24 and 25 are distinct from ARP for IPv4, but the packet formats in Figures 2.1 and 2.2 are, effectively, extended versions of the corresponding ARP packets. It should be possible to implement this extension of ARP by extending existing ARP implementations rather than having to write an entirely new implementation for ILNPv4. It should be emphasised, however, that OP codes 24 and 25 are for experimental use as defined in [RFC5494], and so it is possible that other experimental protocols could be using these OP codes concurrently. Atkinson & Bhatti Experimental [Page 8]
RFC 6747 ILNPv4 ARP November 20123. Security Considerations Security considerations for the overall ILNP architecture are described in [RFC6740]. Additional common security considerations applicable to ILNP are described in [RFC6741]. This section describes security considerations specific to the specific ILNPv4 topics discussed in this document. The existing widely deployed Address Resolution Protocol (ARP) for IPv4 is a link-layer protocol, so it is not vulnerable to off-link attackers. In this way, it is a bit different than IPv6 Neighbor Discovery (ND); IPv6 ND is a subset of the Internet Control Message Protocol (ICMP), which runs over IPv6. However, ARP does not include any form of authentication, so current ARP deployments are vulnerable to a range of attacks from on-link nodes. For example, it is possible for one node on a link to forge an ARP packet claiming to be from another node, thereby "stealing" the other node's IPv4 address. [RFC5227] describes several of these risks and some measures that an ARP implementation can use to reduce the chance of accidental IPv4 address misconfiguration and also to detect such misconfiguration if it should occur. This extension does not change the security risks that are inherent in using ARP. In situations where additional protection against on-link attackers is needed (for example, within high-risk operational environments), the IEEE standards for link-layer security [IEEE-802.1-AE] SHOULD be implemented and deployed. Implementers of this specification need to understand that the two OP code values used for these 2 extensions are not uniquely assigned to ILNPv4. Other experimenters might be using the same two OP code values at the same time for different ARP-related experiments. Absent prior coordination among all users of a particular IP subnetwork, different experiments might be occurring on the same IP subnetwork. So, implementations of these two ARP extensions ought to be especially defensively coded. 4. IANA Considerations This document makes no request of IANA. If in the future the IETF decided to standardise ILNPv4, then allocation of unique ARP OP codes for the two extensions above would be sensible as part of the IETF standardisation process. Atkinson & Bhatti Experimental [Page 9]
RFC 6747 ILNPv4 ARP November 20125. References5.1. Normative References [IEEE-802.1-AE] IEEE, "Media Access Control (MAC) Security", IEEE Standard 802.1 AE, 18 August 2006, IEEE, New York, NY, 10016, USA. [RFC826] Plummer, D., "Ethernet Address Resolution Protocol: Or Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware", STD 37, RFC 826, November 1982. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5227] Cheshire, S., "IPv4 Address Conflict Detection", RFC5227, July 2008. [RFC5494] Arkko, J. and C. Pignataro, "IANA Allocation Guidelines for the Address Resolution Protocol (ARP)", RFC 5494, April 2009. [RFC6740] Atkinson, R. and S. Bhatti, "Identifier Locator Network Protocol (ILNP) Architectural Description", RFC 6740, November 2012. [RFC6741] Atkinson, R. and S. Bhatti, "Identifier-Locator Network Protocol (ILNP) Engineering and Implementation Considerations", RFC 6741, November 2012. [RFC6742] Atkinson, R., Bhatti, S., and S. Rose, "DNS Resource Records for the Identifier-Locator Network Protocol (ILNP)", RFC 6742, November 2012. [RFC6745] Atkinson, R. and S. Bhatti, "ICMP Locator Update Message for the Identifier-Locator Network Protocol for IPv4 (ILNPv4)", RFC 6745, November 2012. [RFC6746] Atkinson, R. and S.Bhatti, "IPv4 Options for the Identifier-Locator Network Protocol (ILNP)", RFC6746, November 2012. Atkinson & Bhatti Experimental [Page 10]
RFC 6747 ILNPv4 ARP November 20125.2. Informative References [RFC4984] Meyer, D., Ed., Zhang, L., Ed., and K. Fall, Ed., "Report from the IAB Workshop on Routing and Addressing", RFC 4984, September 2007. [RFC6743] Atkinson, R. and S. Bhatti, "ICMPv6 Locator Update Message", RFC 6743, November 2012. [RFC6744] Atkinson, R. and S. Bhatti, "IPv6 Nonce Destination Option for the Identifier-Locator Network Protocol for IPv6 (ILNPv6)", RFC 6744, November 2012. [RFC6748] Atkinson, R. and S. Bhatti, "Optional Advanced Deployment Scenarios for the Identifier-Locator Network Protocol (ILNP)", RFC 6748, November 2012. 6. Acknowledgements Steve Blake, Stephane Bortzmeyer, Mohamed Boucadair, Noel Chiappa, Wes George, Steve Hailes, Joel Halpern, Mark Handley, Volker Hilt, Paul Jakma, Dae-Young Kim, Tony Li, Yakov Rehkter, Bruce Simpson, Robin Whittle, and John Wroclawski (in alphabetical order) provided review and feedback on earlier versions of this document. Steve Blake provided an especially thorough review of an early version of the entire ILNP document set, which was extremely helpful. We also wish to thank the anonymous reviewers of the various ILNP papers for their feedback. Roy Arends provided expert guidance on technical and procedural aspects of DNS issues. Atkinson & Bhatti Experimental [Page 11]
RFC 6747 ILNPv4 ARP November 2012 Authors' Addresses RJ Atkinson Consultant San Jose, CA, 95125 USA EMail: rja.lists@gmail.com SN Bhatti School of Computer Science University of St Andrews North Haugh, St Andrews, Fife KY16 9SX Scotland, UK EMail: saleem@cs.st-andrews.ac.uk Atkinson & Bhatti Experimental [Page 12]
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