network protocols handbook.pdf

114Protocols GuideSecurity and VPN - Secured Routing ProtocolsProtocol NameIPsec ESP: IPsec EncapsulatingSecurity PayloadProtocol DescriptionEncapsulating Security Payload (ESP), a key protocol in theIPsec (Internet Security) architecture, is designed to provide amix of security services in IPv4 and IPv6. The IP EncapsulatingSecurity Payload (ESP) seeks to provide confidentiality andintegrity by encrypting data to be protected and placing the encrypteddata in the data portion of the IP ESP. Depending on theuser’s security requirements, this mechanism may be used toencrypt either a transport-layer segment (e.g., TCP, UDP, ICMP,IGMP) or an entire IP datagram. Encapsulating the protecteddata is necessary to provide confidentiality for the entire originaldatagram.The ESP header is inserted after the IP header and before theupper layer protocol header (transport mode) or before an encapsulatedIP header (tunnel mode). The Internet AssignedNumbers Authority has assigned Protocol Number 50 to ESP.The header immediately preceding an ESP header will alwayscontain the value 50 in its Next Header (IPv6) or Protocol (IPv4)field. ESP consists of an unencrypted header followed by encrypteddata. The encrypted data includes both the protectedESP header fields and the protected user data, which is eitheran entire IP datagram or an upper-layer protocol frame (e.g.,TCP or UDP).ESP is used to provide confidentiality, data origin authentication,connectionless integrity, an anti-replay service, and limitedtraffic flow confidentiality. The set of services provided dependson options selected at the time of Security Association establishmentand on the placement of the implementation. Confidentialitymay be selected independent of all other services. However,use of confidentiality without integrity/authentication (either inESP or separately in AH) may subject traffic to certain forms ofactive attacks that could undermine the confidentiality service.Data origin authentication and connectionless integrity are jointservices and are offered as an option in conjunction with (optional)confidentiality. The anti-replay service may be selectedonly if data origin authentication is selected, and its election issolely at the discretion of the receiver.Protocol Structure16 24 32bitSecurity association identifier (SPI)Sequence NumberPayload data (variable length)Padding (0-255 bytes)Pad Length Next HeaderAuthentication Data (variable)• Security association identifier - a pseudo-randomvalue identifying the security association for this datagram.• Sequence Number – contains a monotonically increasingcounter value and is mandatory and is alwayspresent even if the receiver does not elect toenable the anti-replay service for a specific SA.• Payload Data - a variable-length field containing datadescribed by the Next Header field.• Padding – padding for encryption.• Pad length - indicates the number of pad bytes immediatelypreceding it.• Next header - identifies the type of data contained inthe Payload Data field, e.g., an extension header inIPv6 or an upper layer protocol identifier.• Authentication Data - a variable-length field containingan Integrity Check Value (ICV) computed over theESP packet minus the Authentication Data.Related protocolsIPsec, AH, DES, AES, IKE, DOI, HMAC, HMAC-MD5, HMAC-SHA, PKI, IP, IPv6, ICMPSponsor SourceESP is defined by IETF (http://www.ietf.org) in RFC 2406.Referencehttp://www.javvin.com/protocol/rfc2406.pdfIP Encapsulating Security Payload (ESP)